Le magmatisme basique filonien néoprotérozoïque de la boutonnière de Zenaga, Anti-Atlas central, Maroc: pétrologie, géochimie et signification géodynamiqueNeoproterozoic basic dykes of the Zenaga Inlier, central Anti-Atlas, Morocco: petrology, geochemistry and geodynamic significance

NASA Astrophysics Data System (ADS)

Hafid, A.; Sagon, J. P.; Julivert, M.; Arboleya, M. L.; Saquaque, A.; El-Boukhari, A.; Saidi, A.; Soler, J. M. F.

2001-05-01

Before the Pan-African Orogeny, the Palæoproterozoic basement and its Neoproterozoic cover (limestones and quartzites) of the Zenaga Inlier were cross-cut by a swarm of doleritic dykes. They are more or less altered. The primary mineral assemblage consists of plagioclase, clinopyroxene, very rare orthopyroxene, ilmenite, apatite, micropegmatite and sometimes hornblende and biotite. Mineralogical and geochemical studies indicate that the dolerites are continental tholeiites. Two groups of dykes have been distinguished. Accordingly, rare earth elements, P 2O 5, Zr, Th, Ba and Sr contents are higher in group I than in group II, which is richer in V. Group I comprises the north-south and northwest-southeast swarms, while group II corresponds to northeast-southwest and east-west swarms, which were emplaced later. These geochemical variations may be explained by a higher degree of melting of the mantle source for the later group II. Doleritic dykes of Zenaga had been emplaced during an extensional episode, prior to Pan-African folding.

Dyke-sill relationships in Karoo dolerites as indicators of propagation and emplacement processes of mafic magmas in the shallow crust

NASA Astrophysics Data System (ADS)

Coetzee, A.; Kisters, A. F. M.

2017-04-01

This paper describes the spatial and temporal relationships between Karoo-age (ca. 180 Ma) dolerite dykes and a regional-scale saucer-sill complex from the Secunda (coal mine) Complex in the northeastern parts of the Karoo Basin of South Africa. Unlike parallel dyke swarms of regional extensional settings, mafic dykes commonly show curved geometries and highly variable orientations, short strike extents and complex cross-cutting and intersecting relationships. Importantly, the dyke networks originate from the upper contacts of the first-order dolerite sill-saucer structure and are not the feeders of the saucer complex. Cross-cutting relationships indicate the largely contemporaneous formation of dykes and the inner sill and inclined sheets of the underlying saucer. Systematic dykes form a distinct boxwork-type pattern of two high-angle, interconnected dyke sets. The formation and orientation of this dyke set is interpreted to be related to the stretching of roof strata above elongated magma lobes that facilitated the propagation of the inner sill, similar to the "cracked lid" model described for large saucer complexes in Antarctica. Dyke patterns generally reflect the saucer emplacement process and the associated deformation of wall rocks rather than far-field regional stresses.

Early Jurassic mafic dykes from the Aigao uranium ore deposit in South China: Geochronology, petrogenesis and relationship with uranium mineralization

NASA Astrophysics Data System (ADS)

Zhang, Di; Zhao, Kui-Dong; Chen, Wei; Jiang, Shao-Yong

2018-05-01

Mafic dykes are abundant and widely distributed in many granite-hosted uranium ore deposits in South China. However, their geochronology, petrogenesis and relationship with uranium mineralization were poorly constrained. In this study, apatite U-Pb dating, whole-rock major and trace element and Sr-Nd-Pb isotope analysis were conducted for the dolerite dykes from the Aigao uranium ore deposit. Apatite U-Pb isotopic data indicate that the mafic dykes were emplaced at Early Jurassic (189 ± 4 Ma), which provides new evidence for the rarely identified Early Jurassic magmatism in South China. Pyroxene from the dykes is mainly augite, and plagioclase belongs to albite. The dolerite samples have relatively low SiO2 contents (45.33-46.79 wt%), relatively high total alkali contents (K2O + Na2O = 4.11-4.58 wt%) and Al2O3 contents (13.39-13.80 wt%), and medium MgO contents (4.29-5.16 wt%). They are enriched in Nb, Ta, Ti, rare earth elements and depleted in Rb, K, Sr, Th, showing the typical OIB-like geochemical affinity. All the dolerite samples show homogeneous Sr-Nd-Pb isotopic compositions, with (87Sr/86Sr)i varying from 0.706049 to 0.707137, εNd(t) from +4.6 to +5.2, 206Pb/204Pb from 19.032 to 19.126 and 207Pb/204Pb from 15.641 to 15.653. The mafic dykes in the Aigao deposit should be derived from the partial melting of the asthenospheric mantle and formed in a within-plate extensional environment. The emplacement age of the mafic dykes is older than the uranium mineralization age. Therefore, CO2 in ore-forming fluids couldn't originate from the basaltic magma as suggested by previous studies. The dolerite dykes might only provide a favorable reducing environment to promote the precipitation of uraninite from oxidize hydrothermal fluids.

Le volcanisme cambrien du Maroc central : implications géodynamiquesThe Central Morocco Cambrian volcanism: geodynamic implications

NASA Astrophysics Data System (ADS)

Ouali, Houssa; Briand, Bernard; Bouchardon, Jean-Luc; Capiez, Paul

2003-05-01

In southeastern Central Morocco, the Bou-Acila volcanic complex is considered of Cambrian age. In spite of low-grade metamorphic effect, initial volcanic texture and mineralogy can be recognized and volcanic rocks are dominated by dolerites and porphyric dolerites. The initial mineralogy is composed of plagioclases, pyroxenes and dark minerals. A secondary mineral assemblage is composed of albite, epidote, chlorite and calcite. According to their immobile elements compositions, the southeastern central Morocco metavolcanites are of within-plate continental tholeiites. This volcanism and those recognized in many other areas in Morocco confirm a Cambrian extensive episode within the Gondwana supercontinent. To cite this article: H. Ouali et al., C. R. Geoscience 335 (2003).To cite this article: H. Ouali et al., C. R. Geoscience 335 (2003).

The first paleomagnetic data on dolerites from Jeannette Island (New Siberian Islands, Arctic)

NASA Astrophysics Data System (ADS)

Zhdanova, A. I.; Metelkin, D. V.; Vernikovsky, V. A.; Matushkin, N. Yu.

2016-06-01

The first paleomagnetic data on dolerite dikes from the volcanogenic-sedimentary section of Jeannette Island (De Long Archipelago, New Siberian Islands) are discussed. The petromagnetic data and results of the baked contact and fold tests are used to substantiate the nature of the characteristic magnetization component, which in combination with the 40Ar/39Ar dates implies its likely Late Precambrian-Early Paleozoic age. The calculated paleomagnetic pole makes it possible to extend the trajectory of the apparent polar movement for the New Siberian Islands block and confirms the assumption that this structural element of the Arctic shelf evolved as a terrane. Two variants of paleotectonic interpretation of the obtained data and their consistency with the available data on the geology and tectonics of the New Siberian Islands are considered.

Fracturing of doleritic intrusions and associated contact zones: Implications for fluid flow in volcanic basins

NASA Astrophysics Data System (ADS)

Senger, Kim; Buckley, Simon J.; Chevallier, Luc; Fagereng, Åke; Galland, Olivier; Kurz, Tobias H.; Ogata, Kei; Planke, Sverre; Tveranger, Jan

2015-02-01

Igneous intrusions act as both carriers and barriers to subsurface fluid flow and are therefore expected to significantly influence the distribution and migration of groundwater and hydrocarbons in volcanic basins. Given the low matrix permeability of igneous rocks, the effective permeability in- and around intrusions is intimately linked to the characteristics of their associated fracture networks. Natural fracturing is caused by numerous processes including magma cooling, thermal contraction, magma emplacement and mechanical disturbance of the host rock. Fracturing may be locally enhanced along intrusion-host rock interfaces, at dyke-sill junctions, or at the base of curving sills, thereby potentially enhancing permeability associated with these features. In order to improve our understanding of fractures associated with intrusive bodies emplaced in sedimentary host rocks, we have investigated a series of outcrops from the Karoo Basin of the Eastern Cape province of South Africa, where the siliciclastic Burgersdorp Formation has been intruded by various intrusions (thin dykes, mid-sized sheet intrusions and thick sills) belonging to the Karoo dolerite. We present a quantified analysis of fracturing in- and around these igneous intrusions based on five outcrops at three individual study sites, utilizing a combination of field data, high-resolution lidar virtual outcrop models and image processing. Our results show a significant difference between the three sites in terms of fracture orientation. The observed differences can be attributed to contrasting intrusion geometries, outcrop geometry (for lidar data) and tectonic setting. Two main fracture sets were identified in the dolerite at two of the sites, oriented parallel and perpendicular to the contact respectively. Fracture spacing was consistent between the three sites, and exhibits a higher degree of variation in the dolerites compared to the host rock. At one of the study sites, fracture frequency in the surrounding host rock increases slightly toward the intrusion at approximately 3 m from the contact. We conclude by presenting a conceptual fluid flow model, showing permeability enhancement and a high potential for fluid flow-channeling along the intrusion-host rock interfaces.

Geochemical study of the Umkondo dolerites and lavas in the Chimanimani and Chipinge Districts (eastern Zimbabwe) and their regional implications

NASA Astrophysics Data System (ADS)

Munyanyiwa, Hubert

1999-02-01

The Umkondo Group is a supracrustal sequence cropping out in eastern Zimbabwe in the Nyanga, Chimanimani and Chipinge Districts. In these areas the sequence has been divided into a weakly metamorphosed and deformed unit of argillaceous, arenaceous and carbonate rocks (Zimbabwe facies) in the west, and a strongly deformed and medium- to high-grade metamorphosed sequence of mainly quartzites and metapelites (Mozambique facies) in the east. The two sequences were tectonically juxtaposed during the Neoproterozoic Pan-African Mozambique Belt deformation. The Zimbabwe facies sedimentary rocks are intruded by extensive dolerite sills and minor interlayered basalts flows. The mafic rocks are sub-alkaline continental tholeiites. They have low mg numbers associated with low Cr, Cu, Ni and Co, which indicate that the parental magma underwent some differentiation processes en route to the surface. They are LREE enriched with ( {La}/{Yb}N = 5.0-7.6 , high Ce/Yb (>10) and {La}/{Nb} (>0.5) values, and exhibit troughs at Nb, Sr, Ti and P on a MORB-normalised, multi-element spider diagram. These chemical characteristics, together with the large areal extent of the Umkondo dolerites and basalts, suggest that the Umkondo mafic igneous suite was once widespread and formed part of a continental flood basalt province. This is supported by the depositional environment (shallow water platform type setting) of the sedimentary sequence into which the mafic rocks were emplaced. The widespread occurrence of the Umkondo igneous event is further supported by the similarity in palæomagnetic poles of a number of mafic units in southern Africa.

Geochemistry and tectonic setting of the Paleoproterozoic metavolcanic rocks from the Chirano Gold District, Sefwi belt, Ghana

NASA Astrophysics Data System (ADS)

Senyah, Gloria A.; Dampare, Samuel B.; Asiedu, Daniel K.

2016-10-01

Major and trace elements, including rare earth elements (REEs) data are presented for metavolcanic rocks of the Paleoproterozoic Birimian Sefwi belt to determine the geochemical characteristics as well as the possible tectonic setting of emplacement of these rocks. In order to accomplish the aim of the study, the petrographical characteristics of the rocks were examined coupled with analysis of the rocks for their whole-rock major and trace elements contents by ICP-AES and ICP-MS methods respectively. The rocks have been classified as basalt/basaltic andesites and dolerites based on their textural and mineralogical compositions. It is observed that the rocks have suffered various degrees of alteration evident in the overprinting of primary minerals such as pyroxenes and plagioclase by chlorite, epidote, sericite and others. Generally, the rocks are moderately deformed and may have experienced at least greenschist metamorphism. The basalt/basaltic andesites are derivative magmas [Mg# (20-51), Cr (10-220 ppm) and Ni (5-137 ppm)], and show flat REE to fractionated REE patterns with (La/Sm)N = 1.36-3.90, (La/Yb)N = 1.17-5.32 and strong negative to non-existent Eu anomalies (Eu/Eu* = 0.51-1.03). N-MORB-normalised multi-element diagrams show that the rocks have geochemical patterns characterised by enrichment in LILE relative to HFSE and in LREE relative to HREE. The basalt/basaltic andesites exhibit characteristics of subduction zone-related magmas, such as pronounced negative Nb-Ta anomalies, slightly negative Hf and variable negative Ti anomalies. The dolerites do not vary much from the basalts and basaltic andesites. The MgO and Fe2O3 values of the dolerite range from 2.97 to 6.93 and 5.98 to 14.35 wt.% respectively, corresponding to Mg#s of 38-62. LREEs enrichment over HREEs with (La/Sm)N ranging from 0.61 to 4.61 and (Gd/Yb)N ranging from 0.99 to 2.91 is also typical of these rocks. The dolerites also exhibit quite invariable Eu anomalies (Eu/Eu* = 0.81-1.00) and display a pronounced Nb-Ta trough and a minor negative Ti anomaly, suggesting arc characteristics. The metavolcanic rocks from the study area generally exhibit subduction-related setting characteristics with evidence of a sub-lithospheric contamination.

3D modeling of a dolerite intrusion from the photogrammetric and geophysical data integration.

NASA Astrophysics Data System (ADS)

Duarte, João; Machadinho, Ana; Figueiredo, Fernando; Mira, Maria

2015-04-01

The aims of this study is create a methodology based on the integration of data obtained from various available technologies, which allow a credible and complete evaluation of rock masses. In this particular case of a dolerite intrusion, which deployed an exploration of aggregates and belongs to the Jobasaltos - Extracção e Britagem. S.A.. Dolerite intrusion is situated in the volcanic complex of Serra de Todo-o-Mundo, Casais Gaiola, intruded in Jurassic sandstones. The integration of the surface and subsurface mapping, obtained by technology UAVs (Drone) and geophysical surveys (Electromagnetic Method - TEM 48 FAST), allows the construction of 2D and 3D models of the study local. The combination of the 3D point clouds produced from two distinct processes, modeling of photogrammetric and geophysical data, will be the basis for the construction of a single model of set. The rock masses in an integral perspective being visible their development above the surface and subsurface. The presentation of 2D and 3D models will give a perspective of structures, fracturation, lithology and their spatial correlations contributing to a better local knowledge, as well as its potential for the intended purpose. From these local models it will be possible to characterize and quantify the geological structures. These models will have its importance as a tool to assist in the analysis and drafting of regional models. The qualitative improvement in geological/structural modeling, seeks to reduce the value of characterization/cost ratio, in phase of prospecting, improving the investment/benefit ratio. This methodology helps to assess more accurately the economic viability of the projects.

The Age and Geodynamic Evolution of the Metamorphic sole rocks from Izmir-Ankara-Erzıncan suture zone (Northern-Turkey)

NASA Astrophysics Data System (ADS)

Melih Çörtük, Rahmi; Faruk Çelik, Ömer; Özkan, Mutlu; Sherlock, Sarah C.; Marzoli, Andrea; Altıntaş, İsmail Emir; Topuz, Gültekin

2016-04-01

The İzmir-Ankara-Erzincan suture zone in northern Turkey is one of the major tectonic zones separating the Pontides to the North from the Anatolide-Tauride block and Kı rşehir Massif to the South. The accretionary complex of the İzmir-Ankara-Erzincan suture zone, near Artova, is composed mainly of peridotites with varying degree serpentinization, metamorphic rocks, basalt, sandstones, pelagic and neritic limestones. The metamorphic rocks are represented by amphibolite, garnet micaschit, calc-schist and marble. The metamorphic rocks were interpreted as the metamorphic sole rocks. Because; (i) They are tectonically located beneath the serpentinized peridotites. (ii) Foliation planes of both the amphibolites and mantle tectonites are parallel to each other. (iii) The metamorphic rocks are crosscut by non-metamorphic dolerite dikes which exhibite Nb and Ta depletion relative to Th enrichment on the N-MORB normalized multi-element spider diagram. The dolerite dikes display flat REE patterns (LaN/YbN=0.85-1.24). These geochemical signatures of the dolerite dikes are indicative of subduction component during their occurrences. Geochemical observations of the amphibolites suggest E-MORB- and OIB-like signatures (LaN/SmN= 1.39-3.14) and their protoliths are represented by basalt and alkali basaltic rocks. Amphiboles from the amphibolites are represented by calcic amphiboles (magnesio-hornblende, tchermakite and tremolite) and they yielded 40Ar-39Ar ages between 157.8 ± 3.6 Ma and 139 ± 11 Ma. These cooling ages were interpreted to be the intra-oceanic subduction/thrusting time of the İzmir-Ankara-Erzincan oceanic domain. This study was funded by TÜBİTAK (Project no: 112Y123).

Linking Orbital, Field, and Laboratory Analyses of Dolerites in the McMurdo Dry Valleys of Antarctica: Terrestrial Studies and Planetary Applications

NASA Astrophysics Data System (ADS)

Salvatore, M. R.; Mustard, J. F.; Head, J. W.; Marchant, D. R.; Wyatt, M. B.; Seeley, J.

2012-03-01

Primary igneous and secondary alteration signatures can be resolved using orbital spectroscopy over mafic regions of the McMurdo Dry Valleys. We assess the nature of these signatures and their link to surface stability and regional microclimates.

Mapping and characterization from aeromagnetic data of the Foum Zguid dolerite Dyke (Anti-Atlas, Morocco) a member of the Central Atlantic Magmatic Province (CAMP)

NASA Astrophysics Data System (ADS)

Bouiflane, Mustapha; Manar, Ahmed; Medina, Fida; Youbi, Nasrrddine; Rimi, Abdelkrim

2017-06-01

A high-resolution aeromagnetic survey was carried out in the Anti- Atlas, Morocco covering the main areas traversed by the Great CAMP Foum Zguid dyke (FZD). This ;doleritic; dyke belongs to the Central Atlantic Magmatic Province (CAMP), a Large Igneous Province which is associated with the fragmentation of the supercontinent Pangaea and the initial stages of rifting of the Central Atlantic Ocean. It also coincides in time with the mass extinction of the Triassic - Jurassic boundary. Based on the study of geological maps and Google Earth satellite images, it appears that the FZD is poorly exposed and, often covered by Quaternary deposits. This work proposes aeromagnetic modelling and interpretation of the FZD in order to better constrain its structural extent. The data have allowed (i) mapping of the dyke over great distances, under the Quaternary deposits and through areas where it was poorly characterized on the geological map; (ii) identifying major tectonic lineaments interpreted as faults; (iii) recognizing magnetic anomalies related to mafic intrusive bodies; and (iv) informing about regional structural context.

Geothermal Modeling of the Karoo Basin, South Africa, using Rock Magnetic Methods

NASA Astrophysics Data System (ADS)

Maré, L.; de Kock, M.; Cairncross, B.; Mouri, H.; Ferre, E. C.; Jackson, M. J.

2014-12-01

The viability of using magnetic fabric analyses as geothermometers in sedimentary basins was investigated and new constraints on regional-scale geotherms for the Karoo sedimentary Basin were identified. A comparative study of the variation in magnetic properties in eight boreholes located along a southwest to northeast traverse across the Karoo Basin was performed. The thermal impact of the intrusive Karoo Dolerite Suite on the surrounding sediments was determined using several magnetic experiments. The methods employed during the investigation include low field anisotropy of magnetic susceptibility, variation in magneto-stratigraphy using the classic baked contact test, the magnetic Alteration Index (A40) as well as looking at the possibility to use magnetite-pyrrhotite geothermometry.The experiments indicated the heating effect of the dolerite intrusions to be limited to short distances within the contact aureole. Boreholes that intersected dolerite sills indicated that the heating effect occurred no wider than half the sill thicknesses. However, due to the extensive network of sills and dykes in the Karoo Basin, an overall elevation in the geomagnetic temperatures of the stratigraphic sequence to temperatures above 150 °C was observed. This knowledge could have major implications for any potential shale-gas industry in South Africa.Temperatures calculated by the A40 method (minimum observed values) indicated a general increase from southwest to northeast in the thermal effect of intrusions on the Karoo sediments. This correlates with reported increased coal maturity from west to east. Several hypotheses exist for this geothermal variation including the influence that distance to magma source might have had. The most probable hypothesis however, relates to the different environmental settings that prevailed between the western and eastern parts of the basin during magma intrusion and the associated differences in thermal conductivity of low permeable marine shale in the west compared to the porous lacustrine sandstone and siltstone towards the east.

Magmatic context of Bou Skour copper deposit (Eastern Anti-Atlas, Morocco): Petrogrography, geochemistry and alterations

NASA Astrophysics Data System (ADS)

EL Azmi, Daoud; Aissa, M.; Ouguir, H.; Mahdoudi, M. L.; El Azmi, M.; Ouadjo, A.; Zouhair, M.

2014-09-01

The Bou Skour copper deposit is located in the western part of the Saghro massif (Eastern Anti-Atlas), about 50 km East of the city of Ouarzazate. It is subdivided into several areas that are, from North to South: “Panthère”, “Chaigne”, “Anne Marie”, “Chapeau de fer” and “Patte d'Oie”. The latter is economically the most important and is the object of this study. The “Patte d'Oie” district consists mainly of extrusive and intrusive igneous rocks. The extrusive rocks are represented by andesites spatially associated with pyroclastic terms (ignimbrites and pyroclastic breccias). This volcanic unit is intruded by a pink granite pluton and a I-type granodiorite with equigranular texture (Bou Skour granodiorite) showing to the border a microgranular facies (microgranodiorite). All these magmatic formations are intersected by rhyolitic dykes (NNE-SSW) and doleritic dykes (WNW-ESE to NW-SE). The granodiorite and andesite have undergone a polyphase hydrothermal alteration: (i) potassic alteration, (ii) phyllitic alteration, (iii) silicification, (iv) argillic alteration and (v) propylitic alteration. The analysis of geochemical data of granodiorite, granite, andesite and dolerite confirmed: (i) their petrographic natures, (ii) the medium-K calc-alkaline affiliation of andesite and granodiorite, which would have been set up into an active geotectonic environment, probably of island arc or collision, during the Pan-African orogeny, (iii) The high-K calc-alkaline character of granite indicating a post-collision development during the Pan-African orogeny and (iv) The alkaline affinity of the dolerite which is linked to an extensive post-orogenic setting (post-Pan-African). The copper mineralization of “Patte d'Oie” area is hosted, exclusively, in the andesitic and granodioritic facies. It is represented, essentially, by chalcopyrite and bornite minerals and is, probably, related to a porphyry system (disseminated and stockwork mineralization) remobilized late, in Hercynian structures (vein mineralization).

High-velocity frictional experiments on dolerite and quartzite under controlled pore pressure

NASA Astrophysics Data System (ADS)

Togo, T.; Shimamoto, T.; Ma, S.

2013-12-01

High-velocity friction experiments on rocks with or without gouge have been conducted mostly under dry conditions and demonstrated dramatic weakening of faults at high velocities (e.g., Di Toro et al., 2011, Nature). Recent experiments under wet conditions (e.g., Ujiie and Tsutsumi, 2010, GRL; Faulkner et al., 2011, GRL) revealed very different behaviors from those of dry faults, but those experiments were done under drained conditions. Experiments with controlled pore pressure Pp are definitely needed to determine mechanical properties of faults under fluid-rich environments such as those in subduction zones. Thus we have developed a pressure vessel that can be attached to our rotary-shear low to high-velocity friction apparatus (Marui Co Ltd., MIS-233-1-76). With a current specimen holder, friction experiments can be done on hollow-cylindrical specimens of 15 and 40 mm in inner and outer diameters, respectively, at controlled Pp to 35 MPa, at effective normal stresses of 3~9 MPa, and at slip rates of 60 mm/year to 2 m/s. An effective normal stress can be applied with a 100 kN hydraulic actuator. We report an outline of the experimental system and preliminary high-velocity experiments on Shanxi dolerite and a quartzite from China that are composed of pyroxene and plagioclase and of almost pure quartz, respectively. High-velocity friction experiments were performed on hollow-cylindrical specimens of Shanxi dolerite at effective normal stresses of 0.13~1.07 MPa and at slip rates of 1, 10, 100 and 1000 mm/sec. All experiments were conducted first with the nitrogen gas filling the pressure vessel (dry tests) and then with a controlled pore-water pressure (wet tests). In the dry tests an axial force was kept at 1 kN and the nitrogen gas pressure was increased in steps to 5 MPa to change an effective normal stress. In the wet tests the specimens were soaked in distilled water in the vessel and Pp was applied by nitrogen gas in a similar manner as in the dry tests. Nitrogen gas acted as buffer to prevent an abrupt changes in the pore-water pressure during experiments. The steady-state friction coefficient (μss) of dry dolerite increased from 0.3~0.35 at 10 mm/s to 0.55~0.8 at 100 mm/s and then decreased down to 0.2~0.6 at 1000 mm/s. The results are quite similar to those of dry granite tested under similar conditions (Reches and Lockner, 2010, Nature). However, the μss of dolerite under a pore-water pressure decreased monotonically from 0.4~0.8 at 1 mm/s to 0.3~0.5 at 1000 mm/s, and the strengthening from 10 to 100 mm/s disappeared with a pore-water pressure. Two experiments were conducted on solid-cylindrical specimens of quartzite at effective normal stresses of 1.39 MPa (a dry test with CO2 gas pressure of 6.22 MPa) and of 0.99 MPa (a wet test with pore-water pressure of 6.1 MPa, also applied with pressurized CO2 gas). In dry and wet tests, the friction coefficient decreases nearly exponentially from about 0.35 at the peak friction to around 0.05 (dry) and 0.03 (wet) at the steady state. A notable difference was that wet quartzite exhibit much more rapid slip weakening with the slip weakening distance Dc of several meters than the dry specimen with Dc of about 15 m. We plan to conduct more experiments with controlled pore-water pressure and to do textural and material analysis of specimens to gain insight on the weakening mechanisms.

Petrological and geochemical study of doleritic intrusions of Moatize area, Tete Province, Mozambique

NASA Astrophysics Data System (ADS)

Ilídio Mário, Rui; Mendes, Maria Helena; Francisco Santos, Jose; Ribeiro, Sara

2017-04-01

The dolerite samples studied in this work are part of drilling cores, obtained during exploration campaigns by the Ncondezi Coal Company, in the prospect area 805L, located at NE of Moatize, Tete Province, Mozambique. The dolerite bodies are intrusive into sedimentary formations of the Karoo Supergroup. The intrusions have a probable Jurassic age, around 180 Ma, based on a geochronological information (GTK Consortium, 2006) from a similar body cropping out in another area of the Tete Province. The studied rocks were affected by hydrothermal alteration, testified by the pervasive occurrence of the assemblage serpentine + chlorite + sericite + sphene + calcite ± epidote ± tremolite-actinolite, and by filling of vesicles and fractures by calcite, pyrite or calcite + pyrite ± quartz. However, the selected samples preserve igneous intergranular textures. Petrographic evidence suggests that the primary mineral associations included plagioclase, titanaugite, olivine, apatite, opaques, biotite and hornblende. These assemblages are variably preserved and, in the samples most intensely altered, the igneous minerals were almost totally replaced. Whole-rock major and trace element data, with particular emphasis on immobile elements, indicate that the analysed samples are basic and that they can be seen as cogenetic, belonging to the alkaline series and showing compositions similar to present-day intraplate basalts. The Rb-Sr and Sm-Nd data seem to confirm the cogenetic nature of the studied dolerites. In fact, in the least altered samples, both [87Sr/86Sr]180Ma and ɛNd180Ma define relatively small ranges: +0.7050 ≥ [87Sr/86Sr]180Ma ≥ 0.7038 +10 ≥ ɛSr180Ma ≥ -7 and +3.6 ≥ ɛNd180Ma ≥ +1.7. In addition, this clearly indicates that parental melts were generated in a mantle source and that magmas did not undergo significant crustal contamination during their ascent and emplacement. The described isotopic compositions, besides plotting in an area common to OIB, are similar to those found in igneous rocks related to the rifting process in Tanzania and Kenya. Samples that were more intensely affected by hydrothermal alteration display similar ɛNd180Ma values, but show more radiogenic Sr signatures (up to [87Sr/86Sr]180Ma = 0.7063). This indicates a significant crustal contribution in the aqueous fluids responsible for the hydrothermal processes. The whole set of obtained data is in agreement with a magmatic event related with the activity of a mantle plume which caused not only a thermal effect but also geochemical enrichment in the mantle source of the parental magmas of the studied rocks. Reference: GTK Consortium (2006). Map Explanation; Volume 1: Sheets 2032 - 2632. Direcção Nacional de Geologia de Moçambique, Maputo, 341 pp. Acknowledgments: Ncondezy Coal Company, for providing the samples; FCT (Portugal), through project GeoBioTec (UID/GEO/04035/2013), for the financial support.

Landscape Evolution in Polar Deserts: Alteration Rind Detachment via Thermal Fatigue Weathering in Antarctica

NASA Astrophysics Data System (ADS)

Lamp, J. L.; Marchant, D. R.; Mackay, S. L.; Head, J. W.

2012-12-01

Mechanical weathering at high latitudes is largely accomplished through the freeze and thaw of water and ice. However, in upland regions of the ice-free McMurdo Dry Valleys (MDV), Antarctica, the extreme hyper-arid conditions limit the role of water in landscape change. In these regions, secondary weathering processes, such as thermal fatigue, may take on relatively significant roles in mechanical weathering and landscape evolution. Here, we examine morphological changes at the surface of dolerite cobbles along a multi-million year soil chronosequence in Mullins and Beacon Valleys, Antarctica (78°S, 160°E). The data show that dolerite clasts with exposure histories >45,000 years exhibit disintegration via flaking of mm-scale surface fragments (altered crusts) at a rate of ~4 cm/Ma. To assess the likelihood of thermal fatigue in this process, we collected high-frequency (15-second interval) temperature data at the surface and at depth on multiple dolerite clasts. Allied meteorological data, also collected at these sites, include atmospheric temperature and relative humidity, wind-speed and direction, and solar intensity. Temperatures at the top and bottom of flakes change rapidly due to solar heating and convective cooling by wind. Vertical temperature gradients across the 1-to-2-mm thick flakes surpassed 8°C during the 28-day study interval (11/2010-12/2010) and maximum rates of surface temperature change exceeded 5°C/min. The latter value greatly exceeds the accepted value for producing thermal fracture in igneous rocks (Richter and Simmons, 1974). The field data are used as input to a 1-D thermal stress model which shows that stresses in the outer few millimeters of the rock approach the tensile strength of dolerite. In addition, the production of altered rinds in the upper millimeters of rock surfaces (Salvatore et al., in review) may modify thermal properties and help facilitate fracture at the interface between altered and unaltered material. Visual inspection of sediment surrounding weathered cobbles show that the detached flakes add to the surrounding regolith, increasing in abundance with inferred soil age. This process thus modifies clast shape and promotes self-burial, which in turn reduces the overall surface area exposed to solar radiation and provides a negative feedback to further erosion by this process. Our measurements imply that the detachment of altered material in the area represents a dynamic equilibrium process that may have important implications for rates of landscape evolution in the MDV. In addition, the findings can be applied toward the study of cosmogenic nuclide dating in the MDV. Assuming a typical, total weathering rate of ~15 cm/Ma for the region (Summerfield et al., 1999), our study suggests that as much as 30% of total rock degradation may be accomplished through thermal fatigue in extremely dry, upland regions of the MDV.

Mineralogy and geochemistry of picro-dolerite dykes from the central Deccan Traps flood basaltic province, India, and their geodynamic significance

NASA Astrophysics Data System (ADS)

Dongre, Ashish; Viljoen, K. S.; Rathod, A.

2018-04-01

Constituent mineral compositions and whole rock major element geochemistry of picro-dolerite dykes from the central part of the Deccan flood basalt province are presented and discussed. The dykes are characterized by an MgO content of about 13 wt%, coupled with 13-16 modal percents of olivine. A high whole rock molar Mg# value of 71 and the presence of magnesian olivine phenocrysts ( Fo78) are consistent with a primitive (i.e. unevolved) geochemistry. The nature and composition of clinopyroxene (augite and pigeonite), plagioclase feldspar (labradorite) and Fe-Ti oxides (mostly ilmenite and magnetite) are also discussed, with implications drawn with respect to the geodynamics. High MgO magmas and rocks such as picrites are generally considered to be indicative of plume magmatism, formed by high degrees of partial melting in, e.g. the high-temperature region of a plume head. Recent age data is consistent with a model in which the Deccan LIP picritic magmatism is associated with the main phase of Deccan Trap activity at 66 Ma, as a result of a syn- to post rifting phase associated with the impact of the Rèunion mantle plume. It is speculated that the differentiation of primary olivine basaltic magma of picritic composition, may have been the mechanism for the generation of alkalic basalts which occurs in the Deccan Trap basaltic sequence.

Thermal stress weathering and the spalling of Antarctic rocks

NASA Astrophysics Data System (ADS)

Lamp, J. L.; Marchant, D. R.; Mackay, S. L.; Head, J. W.

2017-01-01

Using in situ field measurements, laboratory analyses, and numerical modeling, we test the potential efficacy of thermal stress weathering in the flaking of millimeter-thick alteration rinds observed on cobbles and boulders of Ferrar Dolerite on Mullins Glacier, McMurdo Dry Valleys (MDV). In particular, we examine whether low-magnitude stresses, arising from temperature variations over time, result in thermal fatigue weathering, yielding slow crack propagation along existing cracks and ultimate flake detachment. Our field results show that during summer months clasts of Ferrar Dolerite experience large-temperature gradients across partially detached alteration rinds (>4.7°C mm-1) and abrupt fluctuations in surface temperature (up to 12°C min-1); the latter are likely due to the combined effects of changing solar irradiation and cooling from episodic winds. The results of our thermal stress model, coupled with subcritical crack growth theory, suggest that thermal stresses induced at the base of thin alteration rinds 2 mm thick, common on rocks exposed for 105 years, may be sufficient to cause existing cracks to propagate under present-day meteorological forcing, eventually leading to rind detachment. The increase in porosity observed within alteration rinds relative to unaltered rock interiors, as well as predicted decreases in rind strength based on allied weathering studies, likely facilitates thermal stress crack propagation through a reduction of fracture toughness. We conclude that thermal stress weathering may be an active, though undervalued, weathering process in hyperarid, terrestrial polar deserts such as the stable upland region of the MDV.

Assessing SPO techniques to constrain magma flow: Examples from sills of the Karoo Igneous Province, South Africa

NASA Astrophysics Data System (ADS)

Hoyer, Lauren; Watkeys, Michael K.

2015-08-01

Shape ellipsoids that define the petrofabrics of plagioclase in Jurassic Karoo dolerite sills in KwaZulu-Natal, South Africa are rigorously constrained using the long axis lengths of plagioclase crystals and ellipse incompatibility. This has been undertaken in order to determine the most effective technique to determine petrofabrics when using the SPO-2003 programme (Launeau and Robin, 2005). The technique of segmenting an image for analysis is scrutinised and as a process is found redundant. A grain size threshold is defined to assist with the varying grain sizes observed within and between sills. Where grains exceed the 0.2 mm size threshold, images should be acquired at a high magnification (i.e., 10 × magnification). Petrofabrics are determined using the foliation and the lineation of the ellipsoid as defined by the maximum and minimum principal axes (respectively) of the resultant ellipsoid. Samples with strongly prolate fabrics are isolated allowing further constraint on the petrofabric to be made. Once the efficacy of the petrofabric determination process has been determined, the resultant foliations (and lineations) then elucidate the most accurate petrofabric attainable. The most accurate petrofabrics will be determined by using the correct magnification when the images are obtained and to run the analyses without segmenting the image. The fabrics of the upper and lower contacts of the Karoo dolerite sills are analysed in detail using these techniques and the fabrics are used as a proxy for magma flow.

Disequilibrium dihedral angles in dolerite sills

USGS Publications Warehouse

Holness, Marian B.; Richardson, Chris; Helz, Rosalind T.

2012-01-01

The geometry of clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, measured by the median dihedral angle Θcpp, is created during solidification. In the solidifying Kilauea Iki (Hawaii) lava lake, the wider junctions between plagioclase grains are the first to be filled by pyroxene, followed by the narrower junctions. The final Θcpp, attained when all clinopyroxene-plagioclase-plagioclase junctions are formed, is 78° in the upper crust of the lake, and 85° in the lower solidification front. Θcpp in the 3.5-m-thick Traigh Bhàn na Sgùrra sill (Inner Hebrides) is everywhere 78°. In the Whin Sill (northern England, 38 m thick) and the Portal Peak sill (Antarctica, 129 m thick), Θcpp varies symmetrically, with the lowest values at the margins. The 266-m-thick Basement Sill (Antarctica) has asymmetric variation of Θcpp, attributed to a complex filling history. The chilled margins of the Basement Sill are partially texturally equilibrated, with high Θcpp. The plagioclase grain size in the two widest sills varies asymmetrically, with the coarsest rocks found in the upper third. Both Θcpp and average grain size are functions of model crystallization times. Θcpp increases from 78° to a maximum of ∼100° as the crystallization time increases from 1 to 500 yr. Because the use of grain size as a measure of crystallization time is dependent on an estimate of crystal growth rates, dihedral angles provide a more direct proxy for cooling rates in dolerites.

Sr-Nd-Hf-O isotope geochemistry of the Ertaibei pluton, East Junggar, NW China: Implications for development of a crustal-scale granitoid pluton and crustal growth

NASA Astrophysics Data System (ADS)

Tang, Gong-Jian; Wang, Qiang; Zhang, Chunfu; Wyman, Derek A.; Dan, Wei; Xia, Xiao-Ping; Chen, Hong-Yi; Zhao, Zhen-Hua

2017-09-01

To better understand the compositional diversity of plutonic complexes and crustal growth of the Central Asian Orogenic Belt (CAOB), we conducted an integrated study of the Ertaibei pluton, which obtained geochronological, petrological, geochemical, and isotopic (including whole rock Sr-Nd, in situ zircon Hf-O) data. The pluton (ca. 300 Ma) is composed of granodiorites that contain mafic microgranular enclaves (MMEs), dolerite dikes, and granite dikes containing quartz-tourmaline orbicules. The dolerite dikes were possibly generated by melting of an asthenospheric mantle source, with discrete assimilation of lower crustal components in the MASH (melting, assimilation, storage, and homogenization) zone. The MMEs originated from hybridization between mantle and crust-derived magmas, which spanned a range of melting depths (˜25-30 km) in the MASH zone and were episodically tapped. Melting of the basaltic lower crust in the core of the MASH zone generated magmas to form the granodiorites. The granite dikes originated from melting of an arc-derived volcanogenic sedimentary source with a minor underplated basaltic source in the roof of the MASH zone (˜25 km). The compositional diversity reflects both the magma sources and the degree of maturation of the MASH zone. Although having mantle-like radiogenic isotope compositions, the Ertaibei and other postcollisional granitoids show high zircon δ18O values (mostly between +6 and +9‰), indicating a negligible contribution to the CAOB crustal growth during the postcollisional period.

Timing of mafic magmatism in the Tapajós Province (Brazil) and implications for the evolution of the Amazon Craton: evidence from baddeleyite and zircon U Pb SHRIMP geochronology

NASA Astrophysics Data System (ADS)

Santos, João Orestes Schneider; Hartmann, Léo Afraneo; McNaughton, Neal Jesse; Fletcher, Ian Robert

2002-09-01

The precise timing and possible sources of the mafic rocks in the Amazon craton are critical for reconstruction of the Atlantica supercontinent and correlation of mafic magmatism worldwide. New SHRIMP U-Pb baddeleyite and zircon ages and the reinterpretation of 207 existing dates indicate one orogenic (Ingarana) and four postorogenic (Crepori, Cachoeira Seca, Piranhas, and Periquito) basaltic events in the Tapajós Province, south central Amazon craton. Orogenic gabbro dikes that host gold mineralization are 1893 Ma and interpreted as associated with the Ingarana gabbro intrusions of the bimodal calk-alkalic Parauari intrusive suite. The age of 1893 Ma can be used as a guide to discriminate older and mineralized orogenic dikes from younger and nonmineralized Crepori- and Cachoeira Seca-related mafic dikes. The baddeleyite U-Pb age of the postorogenic Crepori dolerite (gabbro-dolerite sills and dikes) is 1780±9 Ma, ˜150 my older than the ages provided by K-Ar. This value correlates well with the Avanavero tholeiitic intrusions in the Roraima group, in the northern part of the craton in Guyana, Venezuela, and Roraima in Brazil. Early Statherian tholeiitic magmatism was widespread not only in the Amazon craton, but also in the La Plata craton of southern South America, where it is known as the giant Piedra Alta swarm of Uruguay and the post-Trans-Amazonian dikes of Tandil in Argentina. The Cachoeira Seca troctolite represents laccoliths, Feixes, and São Domingos, whose baddeleyite U-Pb age is 1186±12 Ma, 120-150 my older than the known K-Ar ages. This age is comparable to other Stenian gabbroic rocks with alkalic affinity in the craton, such as the Seringa Formation in NE Amazonas and the basaltic flows of the Nova Floresta formation in Rondônia. Dolerite from the giant Piranhas dike swarm in the western Tapajós Province has a Middle Cambrian age (507±4 Ma, baddeleyite) and inherited zircons in the 2238-1229 Ma range. The Piranhas dikes fill extensional NNE and NE faults that are possibly related to an early rifting period before the Ordovician onset of the Amazon Basin sedimentation. Representative rocks of the Paleozoic Taiano magmatism of the northern Amazon craton were not detected in the Tapajós Province. Mesozoic dikes are widespread in the Amazon craton, related to Gondwana continental break-up with K-Ar ages in the 260-124 Ma range.

An Intensive Survey of Archaeological Resources in the Proposed Long Branch Reservoir. Volume 2B

DTIC Science & Technology

1977-01-01

GcGb - Gneissic Gabbro Hematite TGn - Talc Gneiss c - chipped ShGb - Schistic Gabbro a - scratched ShD - Schistic Dolerite f - flake FH/SS - Flint...Hill Sandstone g - ground Mss - Micaceous Sandstone fss - Ferruginous Sandstone A - Argillite c - chert Qtz - Quartz FGQtt - Fine-grained Quartzite Qtt...ARTIFACTS - LONG BRANCI RESERVOIR 41 0 Points Contracti~ng-@ taed, square-based points l a 23MC55 4-4 50 25 9 9.6g b 23MCSS 2-1 74 39 9 22.8g c

Electrical conductivity of Icelandic deep geothermal reservoirs: insight from HT-HP laboratory experiments

NASA Astrophysics Data System (ADS)

Nono, Franck; Gibert, Benoit; Loggia, Didier; Parat, Fleurice; Azais, Pierre; Cichy, Sarah

2016-04-01

Although the Icelandic geothermal system has been intensively investigated over the years, targeting increasingly deeper reservoirs (i.e. under supercritical conditions) requires a good knowledge of the behaviour of physical properties of the host rock in order to better interpret large scale geophysical observations. In particular, the interpretation of deep electrical soundings remains controversial as only few studies have investigated the influence of altered minerals and pore fluid properties on electrical properties of rocks at high temperature and pressure. In this study, we investigate the electrical conductivity of drilled samples from different Icelandic geothermal fields at elevated temperature, confining pressure and pore pressure conditions (100°C < T < 600°C, confining pressure up to 100 MPa and pore pressure up to 35 MPa). The investigated rocks are composed of hyaloclastites, dolerites and basalts taken from depths of about 800 m for the hyaloclastites, to almost 2500 m for the dolerites. They display different porosity structures, from vuggy and intra-granular to micro-cracked porosities, and have been hydrothermally alterated in the chlorite to amphibolite facies. Electrical conductivity measurements are first determined at ambient conditions as a function of pore fluid conductivity in order to establish their relationships with lithology and pore space topology, prior to the high pressure and temperature measurements. Cementation factor varies from 1.5 for the dolerites to 2.83 for the basalt, reflecting changes in the shape of the conductive channels. The surface conductivities, measured at very low fluid conductivity, increases with the porosity and is correlated with the cation exchange capacity. At high pressure and temperature, we used the two guard-ring electrodes system. Measurements have been performed in dry and saturated conditions as a function of temperature and pore pressure. The supercritical conditions have been investigated and temperature cycles have been performed systematically. Dry electrical conductivity measurements show for most of the samples irreversible changes when temperatures exceed 500°C. These changes are interpreted as destabilization/dehydration of alteration minerals that could lead to the presence of a conductive fluid phase in the samples. Very low and high salinity (NaCl) electrical conductivity measurements have been performed as a function of temperature. At supercritical conditions, electrical conductivity at low salinity is not pore pressure dependent and surface conduction is preponderant. At saturated conditions, the rock's electrical conductivity increases linearly (as a function of T-1) until 350°C. Above 350°C, the conductivity decreases. All rock types exhibit the same increasing rate. This work was funded by the of the EC project IMAGE (Integrated Methods for Advanced Geothermal Exploration, grant agreement No. 608553).

Frictional strength of ground dolerite gouge at a wide range of slip rates

NASA Astrophysics Data System (ADS)

Wada, Jun-ichi; Kanagawa, Kyuichi; Kitajima, Hiroko; Takahashi, Miki; Inoue, Atsuyuki; Hirose, Takehiro; Ando, Jun-ichi; Noda, Hiroyuki

2016-04-01

We conducted a series of rotary-shear friction experiments on ground dolerite gouges, in which the amount of adsorbed moisture increases with grinding time (tgr), at room temperature and humidity, a normal stress of 2 MPa, and constant equivalent slip rates (Veqs) ranging from 20 µm/s to 1.3 m/s. Their frictional strength changed with Veq and tgr in three different ways depending on Veq and the gouge temperature (T). At Veq ≤ 1.3 cm/s, T did not exceed 80°C, and the steady state friction coefficient (μss) ranged from 0.59 to 0.80. μss changes little with Veq, while μss at a given Veq systematically increases with tgr probably due to moisture-adsorbed strengthening of gouges. At Veq = 4 cm/s, T exceeded 100°C, and dehydration of gouges resulted in roughly the same μss values (0.60-0.66) among gouges with different periods of tgr. At Veq ≥ 13 cm/s, T reached 160-500°C, and μss dramatically decreases with Veq to 0.08-0.26 at Veq = 1.3 m/s, while μss at a given Veq systematically decreases with tgr. At these fast Veqs, dehydration of gouges likely occurred too fast for water vapor to completely escape out from the gouge layer. Therefore, faster dehydration at faster Veq possibly resulted in a larger pore pressure increase and lower frictional strength. In addition, because gouges with longer periods of tgr contain larger amounts of adsorbed moisture, they became weaker due to larger increases in pore pressure and hence larger amounts of reduction in frictional strength.

Accreted fragments of the Late Cretaceous Caribbean Colombian Plateau in Ecuador

NASA Astrophysics Data System (ADS)

Mamberti, Marc; Lapierre, Henriette; Bosch, Delphine; Jaillard, Etienne; Ethien, Raynald; Hernandez, Jean; Polvé, Mireille

2003-02-01

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous (≈123 Ma) oceanic plateau accreted around 85-80 Ma (San Juan-unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit). Picrites have LREE-depleted patterns, high ɛNd i and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Galápagos HIMU component; their ɛNd i are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower ɛNd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan-Multitud Unit by higher Pb ratios and lower ɛNd i. The Ecuadorian and Gorgona 88-86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92-86 Ma Mg-rich basalts of the Caribbean-Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOP. The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (≈68-65 Ma).

Failed Silurian continental rifting at the NW margin of Gondwana: evidence from basaltic volcanism of the Prague Basin (Teplá-Barrandian Unit, Bohemian Massif)

NASA Astrophysics Data System (ADS)

Tasáryová, Zuzana; Janoušek, Vojtěch; Frýda, Jiří

2018-06-01

The Silurian volcanic rocks of the Prague Basin represent within-plate, transitional alkali to tholeiitic basalts, which erupted in a continental rift setting through the thick Cadomian crust of the Teplá-Barrandian Unit (Bohemian Massif). Despite the variable, often intense alteration resulting in post-magmatic replacement of the basalt mass due to carbonatization, the geochemical signatures of Silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting. The studied interval of Silurian volcanic activity ranges from Wenlock (Homerian, 431 Ma) to late Ludlow (Gorstian, 425 Ma) with a distinct peak at the Wenlock/Ludlow boundary ( 428 Ma). Trace-element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source. Wenlock basalts are similar to alkaline OIB with depleted radiogenic Nd signature compared to Ludlow basalts, which are rather tholeiitic, EMORB-like with enriched radiogenic Nd signature. The correlation of petrogenetically significant trace-element ratios with Nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous, possibly two-component mantle source during the Wenlock-Ludlow melting. Lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills. The latter represent olivine-rich cumulates of basaltic magmas of probably predominantly Ludlow age. Meimechites with dolerites and, to a lesser extent, some lavas were subject to alteration due to wall-rock-fluid interaction. The trigger for the Wenlock-to-Ludlow (431-425 Ma) extension and related volcanism in the Prague Basin is related to far-field forces, namely slab-pull regime due to progressive closure of the Iapetus Ocean. The main stage of the Baltica-Laurentia collision then caused the Prague Basin rift failure at ca. 425 Ma that has never reached an oceanic stage.

Subduction Initiation Existed Along the Ancient Continent Margins? Evidence of U-Pb ages of zircons from the Bonin Trench, Japan

NASA Astrophysics Data System (ADS)

Li, Y. B.; Pearce, J. A.; Ryan, J. G.; Li, X. H.; Haraguchi, S.; Iizuka, T.; Kon, Y.; Yamamoto, S.; Sawaki, Y.; Ishii, T.; Maruyama, S.

2017-12-01

Although it is not cleanly known when and where the subduction initiation began on the Paleo-Izu-Bonin-Mariana (IBM) Trench, Jurassic and Cretaceous plutonic rocks, such as gabbroic, granitic and metamorphic rocks had been sampled from the Amami Plateau-Daito Ridge-Okidaito Ridge (ADO) in the Philippine Sea Plate. Furthermore, Mesozonic to Paleozonic ages zircons were obtained from volcaniclastic sandstones collected from northern Izu-Bonin forarc (Tani et al., 2012). We present U-Pb ages, Hf-O isotopes and trace element compositions of zircon grains separated from sediment, volcanic rock, dolerite and gabbro, collected from Chichijima Island and Bonin forearc seafloor (KH03-3, KT04-28 cruise of the University of Tokyo, IODP Leg 352). In the zircon age histogram, several age groups were identified. The age peaks are 0-3 Ma and 13 Ma (Hahajima Seamount: soft mud and volcanic tuff); 38 Ma (Oomachi Seamount: sandstone); 45 Ma (Chichijima Island: volcanic rock); 40 Ma, 48 Ma and 52 Ma (Hahajima Seamount: dolerite and gabbro); 45 Ma and 164-165 Ma (IODP Leg 352: volcanic rock), respectively. Zircon U-Pb ages ranging 0-52 Ma correspond well to the multi-stages of magmatism in the IBM. However, 164-165 Ma maybe represent the ages of zircon xenocryst including in forearc volcanic rock , which pre-existing in ancient continent crustal materials (SE China Continent Crust?) as the basement of Paleo-IBM. It seems reasonable to suppose that the subduction initiation of IBM existed along the ancient SE China Continent margins. The initiation of subduction zone is a consequence of lateral compositional buoyancy contrast within the lithosphere, that advocated by Niu et al. (2003, 2016).

Anisotropy of magnetic susceptibility of the large dolerite sills of the Angara-Taseeva depression (the Siberian Traps LIP) and the magma flow reconstructions

NASA Astrophysics Data System (ADS)

Latyshev, Anton; Ulyahina, Polina; Veselovskiy, Roman

2017-04-01

The Siberian Traps Large Igneous Province is considered to be the classic example of the continental LIP magmatism. Within the Angara-Taseeva depression (the southern part of the Siberian platform) the products of the Permian-Triassic magmatic activity represent huge dolerite sills intruding the Paleozoic sediments. The extension of the discrete intrusive bodies, their age and order of emplacement remain uncertain. Previously we performed the detailed paleomagnetic investigation revealing the essential magmatic events. Here we present the results of the detailed study of the anisotropy of magnetic susceptibility in the sills of the Angara-Taseeva depression. In 50% of the studied sites we found so-called "normal" magnetic fabric when the minimal axis of the AMS ellipsoid (K3) is normal to the contact (subvertical in sills) and the two other axes are shallow. In this case we interpreted the orientation of the maximal axis (K1) as the magma flow direction. 25% of the studied locations demonstrated the "inverse" magnetic fabric when K1 is normal to the contact. The other sites showed intermediate, diagonal or dispersed type of the AMS ellipsoid axis. In the inner part of the depression the normal magnetic fabric is predominant, and, in general, K1 axes of the AMS ellipsoid converge to the center. This fact confirms the suggestion that the magma feeder zone for the intrusions was located in the central part of the Angara-Taseeva depression. In addition, the pattern of K1 axis allows revealing the local centers of intruding, corresponding to the Padunskiy and Tulunskiy sills. In the periphery of the depression, on the contrary, the inverse magnetic fabric is the most common (in the Tolstomysovskiy sill chiefly). This study was funded by RFBR (projects № 16-35-60114) and the Ministry of Education and Science RF (project № 14.Z50.31.0017).

A conceptual hydrodynamic model of a geological discontinuity in hard rock aquifers: Example of a quartz reef in granitic terrain in South India

NASA Astrophysics Data System (ADS)

Dewandel, Benoît; Lachassagne, Patrick; Zaidi, Faisal K.; Chandra, Subash

2011-08-01

SummaryThe structure and hydrodynamic properties of geological discontinuities and of a deeply weathered granite aquifer near these structures are described on the basis of geological, geophysical and hydrodynamic investigations in two sites of South India located along a 20-40-m-wide quartz reef intruding a weathered Archean biotite granite. One of the two sites also comprises a metre-wide dolerite dyke. Weathering processes appear to be at the origin of fissures development and of a related enhanced local hydraulic conductivity, both in the quartz reef and in the surrounding granite. The weathering profile in the granite (saprolite and fissured layer) is characterized by an abrupt deepening of the weathered layers in the granite near the contact and in the quartz reef itself. Therefore, the weathering profile shows a 'U'-shape geometry with, among others, the verticalization of the granite's fissured layer. The hydraulic conductivity of this verticalized layer is on average 5 × 10 -6 m/s and storativity about 10 -3 (-). The hydraulic conductivity of the fissured quartz is 4-6 × 10 -6 m/s and its storativity about 3-5 × 10 -4 (-). Both media are also characterized by a matrix hydraulic conductivity (10 -7-10 -9 m/s) and by a significant heterogeneity in hydrodynamic properties that generates preferential flow paths along the sub-vertical fissures parallel to the reef axis. A special attention has been paid for characterizing this heterogeneity. The weathering of the dolerite dyke, however, results in a local low hydraulic conductivity, which consequently does not enhance either the thickness of weathered granite layers or its hydraulic conductivity. The obtained results complete the conceptual hydrogeological model developed for weathered granite aquifers in characterizing the relationships between weathering processes and hydrodynamic properties near geological discontinuities.

Magmatismes tholéiitique et alcalin des demi-grabens crétacés de Mayo Oulo Léré et de Babouri Figuil (Nord du Cameroun Sud du Tchad) en domaine d'extension continentaleTholeiitic and alkaline magmatisms of the Early-Cretaceous half-grabens of Mayo Oulo Léré and Babouri Figuil (Northern Cameroon Southern Chad) in extensional structural settings

NASA Astrophysics Data System (ADS)

Ngounouno, Ismaı̈la; Déruelle, Bernard; Guiraud, René; Vicat, Jean-Paul

2001-08-01

Two major dykes of basalts, microgabbros, olivine dolerites (continental tholeiites), and of camptonites and benmoreites (alkaline rocks) are respectively exposed in the Mayo Oulo-Léré and Babouri-Figuil Early Cretaceous half-grabens (Northern Cameroon-Southern Chad). The tholeiites were probably derived from an asthenospheric source in connection with a lithospheric thinning occurring between Santonian and Eocene times. In contrast, the alkaline rocks may be derived from a deeper metasomatized mantle source.

Characterisation of the hydrogeology of the Augustus River catchment, Western Australia

NASA Astrophysics Data System (ADS)

Wilkes, Shane M.; Clement, T. Prabhakar; Otto, Claus J.

Understanding the hydrogeology of weathered rock catchments is integral for the management of various problems related to increased salinity within the many towns of Western Australia. This paper presents the results of site characterisation investigations aimed at improving the overall understanding of the hydrogeology of the southern portion of the Augustus River catchment, an example of a weathered rock catchment. Site data have highlighted the presence of both porous media aquifers within the weathered profile and fractured rock aquifers within the basement rocks. Geophysical airborne surveys and other drilling data have identified a large number of dolerite dykes which crosscut the site. Fractured quartz veins have been found along the margins of these dolerite dykes. Detailed groundwater-level measurements and barometric efficiency estimates indicate that these dolerite dykes and fractured quartz veins are affecting groundwater flow directions, promoting a strong hydraulic connection between all aquifers, and also influencing recharge mechanisms. The hydrogeological significance of the dolerite dykes and fractured quartz veins has been assessed using a combination of high-frequency groundwater-level measurements (30-min sampling interval), rainfall measurements (5-min sampling interval) and barometric pressure fluctuations (30-min sampling interval). A conceptual model was developed for describing various hydrogeological features of the study area. The model indicates that fractured quartz veins along the margins of dolerite dykes are an important component of the hydrogeology of the weathered rock catchments. Comprendre l'hydrogéologie des bassins en roches altérées est essentiel pour la gestion de différents problèmes liés à l'augmentation de la salinité dans de nombreuses villes d'Australie occidentale. Cet article présente les résultats d'études de caractérisation de sites conduites pour améliorer la compréhension de l'hydrogéologie de la partie sud du bassin de la rivière Augustus, exemple de bassin en roches altérées. Les données concernant le site ont mis en évidence la présence simultanée d'aquifères poreux dans le profil d'altération et d'aquifères de roches fracturées dans le socle. Des campagnes de géophysique aéroportée et d'autres données de forages ont identifié de très nombreux dykes de dolérite traversant le site. Des veines de quartz fracturées ont été trouvées aux marges de ces dykes de dolérite. Des mesures détaillées de niveau des nappes et des estimations des effets barométriques indiquent que ces dykes de dolérite et les veines de quartz fracturées affectent les directions d'écoulement souterrain, favorisant une forte connexion hydraulique entre tous ces aquifères, et influençant également les mécanismes de recharge. La signification hydrogéologique des dykes de dolérite et des veines de quartz fracturées a été analysée en combinant des mesures à haute fréquence du niveau des nappes (toutes les 30 min), de la pluie (toutes les 5 min) et des variations de la pression barométrique (toutes les 30 min). Un modèle conceptuel a été établi pour décrire les différents phénomènes hydrogéologiques de la région étudiée. Ce modèle indique que les veines de quartz aux marges des dykes de dolérite sont une importante composante de l'hydrogéologie des bassins en roches altérées. Entender la hidrogeología de cuencas con rocas meteorizadas es esencial para gestionar diversos problemas relacionados con el incremento de salinidad en muchas ciudades de Australia Occidental. Este artículo presenta los resultados obtenidos en la caracterización de varios emplazamientos con el fin de mejorar el conocimiento general de la hidrogeología en la zona sur de la cuenca del Río Augustus, que sirve como ejemplo de cuenca en rocas meteorizadas. Los datos de campo resaltan la presencia tanto de medios acuíferos porosos dentro del perfil meteorizado como de acuíferos en rocas fracturadas dentro de la roca fresca. Los registros geofísicos aéreos y datos de las perforaciones han identificado un gran número de diques de dolerita que intersectan el emplazamiento. Se ha hallado venas de cuarzo fracturado a lo largo de los márgenes de los diques de dolerita. Medidas detalladas del nivel piezométrico y estimaciones de la eficiencia barométrica indican que los diques de dolerita y las venas de cuarzo fracturado afectan las direcciones del flujo de las aguas subterráneas, originando una fuerte conexión hidráulica entre todos los acuíferos e influenciando también a los mecanismos de recarga. Se ha establecido la importancia hidrogeológica de los diques de dolerita y de las venas de cuarzo fracturado mediante una combinación de medidas muy frecuentes del nivel piezométrico (cada 30 min), de la precipitación (cada 5 min) y de las fluctuaciones de la presión barométrica (cada 30 min). Se ha desarrollado un modelo conceptual para describir varias características hidrogeológicas del área de estudio. El modelo indica que las venas de cuarzo fracturado en los márgenes de los diques de dolerita constituyen un componente importante de la hidrogeología de cuencas con rocas meteorizadas.

Nature, geochemistry and petrogenesis of the syn-tectonic Amspoort suite (Pan-African Boundary Igneous Complex, Kaoko Belt, NW Namibia)

NASA Astrophysics Data System (ADS)

Janousek, Vojtech; Konopasek, Jiri; Ulrich, Stanislav

2010-05-01

Crucial information on the Neoproterozoic-Cambrian amalgamation of Western Gondwana is provided by studies of the large Pan-African collisional belt in central-northern Namibia. This so-called Damara Orogen (Miller, 1983) can be subdivided into two branches, the SW-NE trending Damara Belt and a roughly perpendicular, NNW-SSE trending Kaoko Belt further north. The Kaoko Belt consists of two principal crustal units. The easterly part has a Congo Craton affinity (a basement built mostly by ≥ 1.5 Ga granitic gneisses with Neoproterozoic metasedimentary cover), whereas the westerly Coastal Terrane consists of Neoproterozoic (c.850-650 Ma) metapsammites and minor metabasic bodies; no exposures of the basement were found. The at least 180 km long, NNW-SSE trending suture between both units was intruded by numerous syn-tectonic magmatic bodies with ages spanning the interval 580-550 Ma (Seth et al., 1998; Kröner et al., 2004) designated as the Boundary Igneous Complex by Konopásek et al. (2008). The most typical representatives of this syn-collision igneous association are c.550 Ma old K-feldspar-phyric, Bt ± Cam granites-granodiorites of the Amspoort suite, with minor Cpx gabbro and rare two-pyroxene dolerite bodies. The petrological character, whole-rock geochemistry and Sr-Nd isotopic signatures of the scarce Opx-Cpx-Bt dolerites indicate an origin from a CHUR-like mantle-derived melts (87Sr/86Sr550 ~ 0.7045, ɛNd550 ~ 0) modified by extensive (?Ol-) Cpx fractionation. The rest of the suite is interpreted as a product of a high-temperature anatexis of a heterogeneous lower crust, built mainly by immature metapsammites - rich in arc-derived detritus - with minor metabasite and intermediate metaigneous bodies. The most likely source appears to be the anatectic Coastal Terrane gneisses. Yet, partial melting of the so far little constrained Congo Craton cover, if formed by immature and youthful detritus unrelated to the basement, cannot be discounted. In any case, the rather primitive Sr-Nd isotopic compositions of the Amspoort suite (apart from dolerites; ɛNd550 = -3.4 to -5.3, 87Sr/86Sr550= 0.7063-0.711), rule out any major role for the Congo Craton-basement derived material. On a much broader scale, the contribution of Congo-derived material in the Pan-African granitoids in the Kaoko Belt seems to increase southwards from nearly nil in the studied suite, through minimal in Hoanib Valley (Seth et al., 2002) to maximum in southern extremity of the Belt (Jung et al., 2009 and, in particular, van de Flierdt et al., 2003). This research was financially supported by GAČR Project 205/07/1409 (to JK). Jung, S., et al., 2009. Lithos, 111: 220-235. Konopásek, J., et al., 2008. Journal of the Geological Society, 165, 153-165. Kröner, S., et al., 2004. South African Journal of Geology, 107, 455-476. Miller, R.M., 1983. In: Miller, R.M. (Ed.), Evolution of the Damara Orogen of South West Africa/Namibia. Geological Society of South Africa Special Publications 11, 431-515. Seth, B., et al., 1998. Precambrian Research, 92, 341-363. Seth, B., et al., 2002. South African Journal of Geology, 105, 179-192. van de Flierdt, T. , et al., 2003. Lithos, 67, 205-226.

Suprasubduction volcanic rocks of the Char ophiolite belt, East Kazakhstan: new geochemical and first geochronological data

NASA Astrophysics Data System (ADS)

Safonova, Inna; Simonov, Vladimir; Seltmann, Reimar; Yamamoto, Shinji; Xiao, Wenjiao

2016-04-01

The Char ophiolite belt is located in the western Central Asian Orogenic Belt, a world largest accretionary orogen, which has evolved during more than 800 Ma. The Char belt formed during Kazakhstan - Siberia collision. It has been known for hosting fragments of Late Devonian-Early Carboniferous oceanic crust, MORB, OPB and OIB, of the Paleo-Asian Ocean (Safonova et al., 2012). The Char is surrounded by two Paleozoic island-arc terranes: Zharma-Saur in the west and Rudny Altai in the east, however, until recent times, no island-arc units have been found within it. We were the first to find island-arc units as tectonic sheets occurring adjacent to those consisting of oceanic rocks. In places, island-arc andesites cut oceanic basalts. The Char volcanic and subvolcanic rocks of a probable suprasubduction origin are basalt, microgabbro, dolerite, andesite, tonalite and dacite. The mafic to andesitic volcanics possessing low TiO2 (0.85 wt.%av.) and show MgO vs. major elements crystallization trends suggesting two magma series: tholeiitic and calc-alkaline. The tholeiitic varieties are less enriched in incompatible elements then the calc-alkaline ones. Two samples are high-Mg and low-Ti andesibasalts similar to boninites. The rocks possess moderately LREE enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/Lapm = 0.14-0.47; Nb/Thpm = 0.7-1.6).The distribution of rare-earth elements (La/Smn = 0.8-2.3, Gd/Ybn = 0.7-1.9) and the results of geochemical modeling in the Nb-Yb system suggest high degrees of melting of a depleted harzburgite-bearing mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry indicates crystallization of melts at 1020-1180°C. Melt inclusion composition based numerical calculations show that primary melts were derived at 1350-1530°C and 14-26 kbar and crystallized at 1150-1190°C (Simonov et al., 2010). All these features are indicative of a supra-subduction origin of rocks. The age of gabbro, dolerite, andesite and tonalite was determined by LA ICP MS U-Pb zircon dating performed in the University of Kyoto, Japan. The andesites and tonalites yielded Carboniferous ages of ca. 322-336 Ma and the gabbro and dolerite appeared Devonian (387-395 Ma). Thus, the Char volcanic rocks possess geochemical signatures of supra-subduction magmas and could be derived at high degree melting of relatively shallow mantle sources. The volcanic units probably formed at one or two island-arcs or at an intra-oceanic arc and continental margin arc during the Middle Devonian - Mississippian. Later, the island-arc units were probably accreted to the active margin of the Kazakhstan continent. The work was supported by RFBR Project no. 16-05-00313. Contribution to IGCP#592 of UNESCO-IUGS. Safonova, I.Yu., Simonov V.A., Kurganskaya E.V., Obut O.T., Romer R.L., Seltmann R., 2012. Late Paleozoic oceanic basalts hosted by the Char suture-shear zone, East Kazakhstan: geological position, geochemistry, petrogenesis and tectonic setting. Journal of Asian Earth Sciences 49, 20-39. Simonov V.A., Safonova I.Yu., Kovyazin S.V., 2010. Petrogenesis of island-arc complexes of the Char zone, East Kazakhstan. Petrology 18, 59-72.

The ophiolite of the Eohellenic nappe in the island of Skyros, Greece: Geotectonic environment of formation and metamorphic conditions inferred by mineralogical and geochemical data

NASA Astrophysics Data System (ADS)

Karkalis, Christos; Magganas, Andreas; Koutsovitis, Petros

2014-05-01

The island of Skyros is located in the Sporades-Aegean region. It includes an ophiolitic mélange sequence consisting of serpentinites, gabbroic and doleritic rocks, and also lavas which mostly appear in massive form, but in rare cases as deformed pillows. The ophiolitic mélange sequence also includes rodingites, ophicalcites, as well as radiolarites. This formation belongs to the Eohellenic tectonic nappe, which encompasses marbles, sandstones and schists and was emplaced onto the Pelagonian Zone during Early Cretaceous [1, 2]. Serpentinites were most likely formed after serpentinization of harzburgitic protoliths and consist of serpentine, bastite, spinel and magnetite. The chemistry of spinels (TiO2=0.14-0.25 wt.%, Al2O3=35.1-35.21 wt.%, Cr#=37.38-38.87), shows that the harzburgitic protoliths plausibly resemble back-arc basin peridotites [3]. Gabbros and dolerites present mostly subophitic textures, between the hornblende/clinopyroxene and plagioclase grains. Based upon their petrography and on their mineral chemistry hornblendes have been distinguished into magmatic and metamorphic hornblendes, with the first occurring mostly in gabbroic rocks. Magmatic hornblendes exhibit relatively high TiO2 (1.42-1.62 wt.%), Al2O3 (5.11-5.86 wt.%) and Na2O (1.01-1.09 wt.%) contents, with their presence implying that the magma was at least to some degree hydrous. Lavas are tholeiitic basalts with relatively high FeOt≡12 wt.% and low K2O and Th contents, consisting mostly albite, altered clinopyroxene and devitrified glass. Tectonomagmatic discrimination diagrams [4, 5] illustrate that the studied gabbros and lavas of Skyros are most likely associated with SSZ processes. Gabbroic rocks, subvolcanic dolerites and lavas have been subjected to greenschist/subgreenschist metamorphic processes, as confirmed by the presence of secondary amphiboles (metamorphic hornblende, actinolite/tremolite), epidote, pumpellyite and chlorite in all of the studied samples. On the other hand, the occurrence of rodingites and ophicalcites clearly point to interaction of the gabbroic rocks and serpentinites with hydrothermal fluids, which most probably took place during the stage of exhumation and tectonic emplacement. Ophicalcites contain serpentine, calcite, magnetite, as well as rare pyroxene and spinel. Rodingites on their behalf include hydroandradite (Alm0.00Adr61.33-67.43Grs28.25-35.18Prp0.10-2.49Sps0.00-0.33Uv0.41-2.75), vesuvianite (MgO=2.78-3.33 wt.%; TiO2=0.02-0.59 wt.%) diopside neoblasts (En48.53-49.89Wo47.56-48.10Fs2.32-3.33; Mg#=93.96-96.28), chlorite and also accessory prehnite. Some small-sized Cr-bearing hydrogarnet crystals (Cr2O3=10.34 wt.%) were most likely formed at the expense of spinel. The types of hydrogarnet and vesuvianite crystals are highly indicative for the involvement of subduction-related fluids during the formation of the rodingites [6]. References: [1] Jacobshagen & Wallbrecher 1984: Geol. Soc., London, Sp. Pub. 17, 591-602, [2] Pe-Piper 1991: Ofioliti, 16, 111 - 120, [3] Kamenetsky Sobolev, Joron & Semet 2001: J Petrol 42, 655-671, [4] Agrawal, Guevara & Verma 2008: Intern. Geol. Rev. 50, 1057-1079, [5] Pearce & Cann 1973: Earth Plan. Sci. Lett. 19, 290-300, [6] Koutsovitis, Magganas, Pomonis & Ntaflos 2013. Lithos 172-173, 139-157.

Sulfur degassing due to contact metamorphism during flood basalt eruptions

NASA Astrophysics Data System (ADS)

Yallup, Christine; Edmonds, Marie; Turchyn, Alexandra V.

2013-11-01

We present a study aimed at quantifying the potential for generating sulfur-rich gas emissions from the devolatilization of sediments accompanying sill emplacement during flood basalt eruptions. The potential contribution of sulfur-rich gases from sediments might augment substantially the magma-derived sulfur gases and hence impact regional and global climate. We demonstrate, from a detailed outcrop-scale study, that sulfur and total organic carbon have been devolatilized from shales immediately surrounding a 3-m thick dolerite sill on the Isle of Skye, Scotland. Localized partial melting occurred within a few centimetres of the contact in the shale, generating melt-filled cracks. Pyrite decomposed on heating within 80 cm of the contact, generating sulfur-rich gases (a mixture of H2S and SO2) and pyrrhotite. The pyrrhotite shows 32S enrichment, due to loss of 34S-enriched SO2. Further decomposition and oxidation of pyrrhotite resulted in hematite and/or magnetite within a few cm of the contact. Iron sulfates were produced during retrogressive cooling and oxidation within 20 cm of the contact. Decarbonation of the sediments due to heating is also observed, particularly along the upper contact of the sill, where increasing δ13C is consistent with loss of methane gas. The geochemical and mineralogical features observed in the shales are consistent with a short-lived intrusion, emplaced in <5 h. The dolerite magma contains pervasive pyrite and localized sulfur concentrations greater than the sulfur concentration at sulfide liquid saturation, consistent with addition of sulfur (perhaps from sediments) at a late stage. Our study provides evidence for desulfurization, as well as decarbonation, of shales adjacent to an igneous intrusion. The liberated fluids, rich in sulfur and carbon, are likely to be focused along regions of low pore fluid pressure along the margins of the sill. The sulfur gases liberated from the sediments would have augmented the sulfur dioxide (and hydrogen sulfide) yield of the eruption substantially, had they reached the surface. This enhancement of the magmatic sulfur budget has important implications for the climate impact of large flood basalt eruptions that erupt through thick, volatile-rich sedimentary sequences.

Rapakivi texture formation via disequilibrium melting in a contact partial melt zone, Antarctica

NASA Astrophysics Data System (ADS)

Currier, R. M.

2017-12-01

In the McMurdo Dry Valleys of Antarctica, a Jurassic aged dolerite sill induced partial melting of granite in the shallow crust. The melt zone can be traced in full, from high degrees of melting (>60%) along the dolerite contact, to no apparent signs of melting, 10s of meters above the contact. Within this melt zone, the well-known rapakivi texture is found, arrested in various stages of development. High above the contact, and at low degrees of melting, K-feldspar crystals are slightly rounded and unmantled. In the lower half of the melt zone, mantles of cellular textured plagioclase appear on K-feldspar, and thicken towards the contact heat source. At the highest degrees of melting, cellular-textured plagioclase completely replaces restitic K-feldspar. Because of the complete exposure and intact context, the leading models of rapakivi texture formation can be tested against this system. The previously proposed mechanisms of subisothermal decompression, magma-mixing, and hydrothermal exsolution all fail to adequately describe rapakivi generation in this melt zone. Preferred here is a closed system model that invokes the production of a heterogeneous, disequilibrium melt through rapid heating, followed by calcium and sodium rich melt reacting in a peritectic fashion with restitic K-feldspar crystals. This peritectic reaction results in the production of plagioclase of andesine-oligoclase composition—which is consistent with not just mantles in the melt zone, but globally as well. The thickness of the mantle is diffusion limited, and thus a measure of the diffusive length scale of sodium and calcium over the time scale of melting. Thermal modeling provides a time scale of melting that is consistent with the thickness of observed mantles. Lastly, the distribution of mantled feldspars is highly ordered in this melt zone, but if it were mobilized and homogenized—mixing together cellular plagioclase, mantled feldspars, and unmantled feldspars—the result would be akin to rapakivi granites observed globally in Proterozoic systems. In essence, the melt zone is an embryonic rapakivi granite; not yet fully developed and displaying clear ties to its parental rock.

Tracing salt provenance in McMurdo Dry Valley soils by using magnesium isotopes

NASA Astrophysics Data System (ADS)

Cuozzo, N.; Sletten, R. S.; Hu, Y.; Teng, F. Z.

2016-12-01

The McMurdo Dry Valleys (MDV) are a hyper-arid polar desert that contain a rich geologic record in permafrost that has been preserved over millions of years. Soluble salts accumulate through the surface and subsurface of MDV soils. Sources of salt accumulation include mineral weathering, transport of marine aerosols, and possible glacial meltwater. This project seeks to study the provenance of these salts in a 30-meter ice cemented permafrost core collected in Beacon Valley. The ice-rich core was thawed and water extracted by centrifugation using a double bottom centrifuge tube. The extracted water was analyzed for ionic composition, pH, and Mg isotopes (δ26Mg), which are useful in interpreting provenance. The ionic and δ26Mg values show a disconformity at around 7 meters. Above 7 meters, δ26Mg values vary between -0.76 to -0.52, indicating rock-water interactions. These samples are isotopically heavier than the lower section of the core and can be explained by a slow accumulation of sediment and warmer near-surface temperatures that allow for greater chemical weathering of dolerite in the ice-cemented, debris-rich permafrost core. This interpretation is also supported by the more alkaline pH values (7.07 - 7.54) above 7 meters, which is consistent with chemical weathering of dolerite. In comparison, salt samples below 7 meters have δ26Mg values between -0.95 to -0.84, which overlaps with modern seawater (δ26Mg = -0.83 ± 0.09) and is fairly consistent throughout the rest of the core. Furthermore, below a depth of 7 meters, Mg/Na and Mg/K ratios are also similar to modern seawater. In summary, these results indicate differing sources of salts along the depth of the Dry Valley permafrost core, changing from a marine-dominant signature in the deeper section to a stronger weathered signal in the upper section. Additional work dating the sediment using cosmogenic nuclides provides a history for the burial of the sediments in the permafrost core and may provide broad scale paleoclimatic implications.

Strides in Preservation of Malawi's Natural Stone

NASA Astrophysics Data System (ADS)

Kamanga, Tamara; Chisenga, Chikondi; Katonda, Vincent

2017-04-01

The geology of Malawi is broadly grouped into four main lithological units that is the Basement Complex, the Karoo Super group, Tertiary to Quaternary sedimentary deposits and the Chilwa Alkaline province. The basement complex rocks cover much of the country and range in age from late Precambrian to early Paleozoic. They have been affected by three major phases of deformation and metamorphism that is the Irumide, Ubendian and The Pan-African. These rocks comprise gneisses, granulites and schists with associated mafic, ultramafic, syenites and granite rocks. The Karoo System sedimentary rocks range in age from Permian to lower Jurassic and are mainly restricted to two areas in the extreme North and extreme Alkaline Province - late Jurassic to Cretaceous in age, preceded by upper Karoo Dolerite dyke swarms and basaltic lavas, have been intruded into the Basement Complex gneisses of southern Malawi. Malawi is endowed with different types of natural stone deposits most of which remain unexploited and explored. Over twenty quarry operators supply quarry stone for road and building construction in Malawi. Hundreds of artisanal workers continue to supply aggregate stones within and on the outskirts of urban areas. Ornamental stones and granitic dimension stones are also quarried, but in insignificant volumes. In Northern Malawi, there are several granite deposits including the Nyika, which is the largest single outcrop occupying approximately 260.5 km2 , Mtwalo Amazonite an opaque to translucent bluish -green variety of microcline feldspar that occurs in alkali granites and pegmatite, the Ilomba granite (sodalite) occurring in small areas within biotite; apatite, plagioclase and calcite. In the Center, there are the Dzalanyama granites, and the Sani granites. In the South, there are the Mangochi granites. Dolerite and gabbroic rocks spread across the country, treading as black granites. Malawi is also endowed with many deposits of marble. A variety of other igneous, metamorphic and sedimentary rocks are also used as dimension stones. Discovery and preservation of more natural stone deposits through research is essential in the country .Natural stone preservation has not only the potential to generate significant direct and indirect economic benefits for Malawi but also to preserve its heritage .

Rates of Eolian Rock Abrasion in the Ice-Free Valleys, Antarctica

NASA Astrophysics Data System (ADS)

Hallet, B.; Malin, M. C.; Sletten, R. S.

2016-12-01

Eolian abrasion is a principal surface process in dry regions of Earth and Mars and there is evidence for wind processes active on Venus and Titan. Rock abrasion also has practical significance in diverse fields ranging from preservation of cultural material (artifacts, monuments) to damage of solar panels and windshields in arid regions. Despite its scientific and practical importance, and there have ben only few studies that define rates of rock abrasion quantitatively under natural conditions. Herein we report abrasion rates that have been exceptionally well characterized through a unique long-term (30+-year) field experiment in the ice-free McMurdo Dry Valleys, Antarctica. In 1983 and 1984, over 5000 rock targets of several lithologies (25.4 mm-diameter and 5 mm-thick disks of dolerite, basalt, tuff and sandstone) were installed at five heights (7,14, 21, 35, and 70 cm) facing the 4 cardinal directions at 10 locations (one additional site contains fewer targets). Sequential collections of rock targets exposed to abrasion enable definition of mass loss after 1, 5, 10, 30 and 31 years of exposure; the latter were retrieved during the 2014-2015 season. The abrasion rates generally show striking consistency for each lithology at any site; the multiple targets permit definition of intrinsic differences in mass loss. The rates vary considerably from site to site owing to differences in availability of transportable sediment, wind regime, and surface roughness, and at each site, owing to target orientation relative to the dominant winds and, secondarily, to height above the ground. For the hardest targets, basalt and dolerite, mass loss in 30+ years ranged from essentially zero at some sites to 1/3 of the deployed mass (2.59 g; equivalent to a rock thickness >1.8 mm) where abrasion was most active (Site 7, Central Wright Valley). The tuff targets showed the greatest mass loss, and in many cases were entirely abraded away by the end of the experiment.Current work is focused on understanding the spatial and directional variation in measured mass losses based on a wealth of information.

Geodynamic Setting of Proterozoic Dyke Swarms of the Leo-Man Craton, West Africa, Based on New U-Pb Dating and Geochemistry

NASA Astrophysics Data System (ADS)

Baratoux, L.; Jessell, M.; Söderlund, U.; Ernst, R. E.; Benoit, M.; Naba, S.; Cournede, C.; Perrouty, S.; Metelka, V.; Yatte, D.; Diallo, D. P.; Ndiaye, P. M.; Dioh, E.; Baratoux, D.

2016-12-01

Over 20 sets of dolerite dykes crosscutting Paleoproterozoic basement in West Africa were distinguished via the interpretation of regional and high-resolution airborne magnetic data available over the West African Craton. Some of the dykes reach over 300 km in length and are considered parts of much larger systems of mafic dyke swarms which form the plumbing system of Large Igneous Provinces (LIPs). Five different dyke swarms in Burkina Faso, Niger, Ghana and Senegal were investigated. In terms of petrography and composition, the mafic dykes correspond to tholeiitic basalts and are typically composed of plagioclase + clinopyroxene ± orthopyroxene ± olivine. They display a doleritic texture of variable grain size. Eleven ID-TIMS U-Pb ages obtained on baddeleyite define five generations of Proterozoic age. The N10 Libiri dyke swarm, found in western Niger, yielded an age of ca. 1790 Ma. The N40 Bassari swarm in Senegal was dated at ca. 1764 Ma, and is potentially linked to the 1790 Ma Libiri swarm, 1400 km away. The 300 by 400 km Korsimoro N100 dyke swarm transects central Burkina Faso and was dated at ca. 1575 Ma. Five ca. 1520 Ma ages were obtained for dykes of the Essakane swarm, three in Burkina Faso, one from Ghana (N130 orientation) and one from Senegal (E-W orientation), and document a large extent (600 km wide and 1500 km long) and short duration of dyke emplacement. The Manso N350 dyke swarm in southern Ghana, which is about 400 km long and about 200 km wide, yields a preliminary age of ca 870 Ma. A mantle plume origin is suggested for these swarms, especially the 1790-1765 Ma Libiri-Bassari swarm and the 1520 Ma Essakane swarms (which have lithosphere-contaminated E-MORB chemistry), whose scale is similar to largest giant radiating swarms (e.g. CAMP and Mackenzie). The 870 Ma Manso swarm has composition closer to OIB, consistent with a plume/hotspot origin. The 1575 Ma Korsimoro swarm has composition between EMORB and NMORB, which suggests a rift setting.

Crystal chemistry and oxidation state of Fe-rich prehnite from a hydrothermally altered dolerite

NASA Astrophysics Data System (ADS)

Nagashima, Mariko; Iwasa, Kiyoka; Akasaka, Masahide

2018-04-01

Fe-rich prehnite, Ca2(Al,Fe)(AlSi3)O10(OH)2, in a hydrothermally altered dolerite sill from Mitsu, Shimane Peninsula, Japan, was studied using 57Fe Mössbauer spectroscopy and X-ray Rietveld method to determine the oxidation state and distribution of Fe within the prehnite and to clarify its structural properties. Prehnite shows two modes of occurrence: a druse and vein mineral (prehnite I) associated with Fe-rich pumpellyite and laumontite and a replacement of primary plagioclase (prehnite II). The Fe contents of prehnite I and II are 0.33-0.44 and 0.01-0.46 Fe3+ atoms per formula unit, respectively. The Mössbauer spectrum of prehnite II consists of one doublet with isomer shift ( IS) = 0.364 mm/s and quadrupole splitting ( QS) = 0.284 mm/s assigned to Fe3+ at the octahedral M site. In contrast, the Mössbauer spectrum of prehnite I consists of two doublets assigned to Fe3+ at the M site ( IS = 0.369 mm/s and QS = 0.299 mm/s) and Fe2+ at the seven coordinated A site ( IS = 1.05 and QS = 2.78 mm/s). According to X-ray Rietveld refinements with Pmna and Pma2 space groups, the fitting with Pma2 gave more reduced reliability factors than those using Pmna for both specimens, implying ordering of Al and Si at the tetrahedral T2 sites. Determined T2-O bond lengths at the Al-rich and Si-rich T2 sites, 1.71-1.72 and 1.62-1.64 Å, respectively, also indicate the ordered arrangement of Al and Si at the T2 sites. Refined site occupancies at the A and M sites are represented as A (Ca0.993(9)Fe2 + 0.007) M (Al0.666(6)Fe3 + 0.334) for prehnite I, and A Ca1.0 M (Al0.865(5)Fe3 + 0.135) for prehnite II, respectively. The existence of Fe2+ in the A site filling Ca deficiency in prehnite I is consistent with the result from the Mössbauer analysis.

The Chara-Sina dyke swarm in the structure of the Middle Paleozoic Vilyui rift system (Siberian Craton)

NASA Astrophysics Data System (ADS)

Kiselev, A. I.; Konstantinov, K. M.; Yarmolyuk, V. V.; Ivanov, A. V.

2016-11-01

The formation of the Vilyui rift system in the eastern Siberian Craton was finished with breakdown of the continent and formation of its eastern margin. A characteristic feature of this rift system is the radial distribution of dyke swarms of basic rocks. This peculiarity allows us to relate it to the breaking processes above the mantle plume, the center of which was located in the region overlain in the modern structure by the foreland of the Verkhoyan folded-thrust belt. The Chara-Sina dyke swarm is the southern part of a large area of Middle Paleozoic basaltic magmatism in the eastern Siberian Craton. The OIB-like geochemical characteristics of dolerite allow us to suggest that the melting substrate for Middle Paleozoic basaltic magmatism was represented by a relatively homogeneous, mid-depleted mantle of the plume with geochemical parameters similar to those of OIB.

Petrology and metamorphic evolution of ultramafic rocks and dolerite dykes of the Betic Ophiolitic Association (Mulhacén Complex, SE Spain): evidence of eo-Alpine subduction following an ocean-floor metasomatic process

NASA Astrophysics Data System (ADS)

Puga, E.; Nieto, J. M.; Díaz de Federico, A.; Bodinier, J. L.; Morten, L.

1999-10-01

The Betic Ophiolitic Association, cropping out within the Mulhacén Complex (Betic Cordilleras), is made up of numerous metre- to kilometre-sized lenses of mafic and/or ultramafic and meta-sedimentary rocks. Pre-Alpine oceanic metasomatism and metamorphism caused the first stage of serpentinization in the ultramafic sequence of this association, which is characterized by local clinopyroxene (Cpx) breakdown and Ca-depletion, and complementary rodingitization of the basic dykes intruded in them. Subsequent eo-Alpine orogenic metamorphism developed eclogite facies assemblages in ultramafic and basic lithotypes, which were partly retrograded in Ab-Ep-amphibolite facies conditions during a meso-Alpine event. The heterogeneous development of the oceanic metasomatism in the ultramafic rock-types led to the patchy development of highly serpentinized Ca-depleted domains, without gradual transition to the host, and less serpentinized, Cpx-bearing ultramafites, mainly lherzolitic in composition. The high-pressure eo-Alpine recrystallization of these ultramafites in subduction conditions originated secondary harzburgites in the Ca-depleted domains, consisting of a spinifex-like textured olivine+orthopyroxene paragenesis, and a diopside+Ti-clinohumite paragenesis in the enclosing lherzolitic rocks. During the meso-Alpine event, secondary harzburgites were partly transformed into talc+antigorite serpentinites, whereas the diopside and clinohumite-bearing residual meta-lherzolites were mainly transformed into Cpx-bearing serpentinites. Relics of mantle-derived colourless olivine may be present in the more or less serpentinized secondary harzburgites. These relics are overgrown by the eo-Alpine brown pseudo-spinifex olivine, which contains submicroscopic inclusions of chromite, ilmenite and occasional halite and sylvite, inherited from its parental oceanic serpentine. The same type of mantle-derived olivine relics is also preserved within the Cpx-bearing serpentinites, although it has been partly replaced by the eo-Alpine Ti-clinohumite. The dolerite dykes included in the ultramafites were partly rodingitized in an oceanic environment. They were then transformed during the eo-Alpine event into meta-rodingites in their border zones and into eclogites towards the innermost, less-rodingitized portions. Estimated P- T conditions for the high-pressure assemblages in ultramafic and basic lithotypes range from 650 to 750°C and 16-25 kb.

Shoshonites and Associated Calc-Alkaline Rocks from the Eastern Sayan, Central Asian Orogenic Belt: Geochemistry and Tectonic Setting

NASA Astrophysics Data System (ADS)

Vernikovskaya, A. E.; Romanov, M. I.; Kadilnikov, P. I.; Matushkin, N. Y.; Romanova, I.

2017-12-01

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens in the world, which formation started in the Neoproterozoic giving rise to numerous assemblages of island arcs, ophiolites, continental fragments and sedimentary basins. The Eastern Sayan, located at the southwestern margin of the Siberian craton, is the key area in understanding the initiation of orogenic processes in the CAOB. Widely distributed mafic igneous rocks (dolerites, gabbro etc.) in the Eastern Sayan were previously considered as part of the Nersa igneous complex of the Neoproterozoic age, whereas tectonic setting of these rocks remained highly debatable. New geochemical and mineralogical data from igneous mafic rocks within the Eastern Sayan show presence of rocks with shoshonitic and high- and low-K calc-alkaline affinities and allowed us to refine the tectonic context of their formation in the southwestern margin of the Siberian craton.All studied intrusive and volcanic rocks in the Eastern Sayan showing OIB-like geochemical signatures. The high-K rocks contain orthoclase, olivine, diopside, augite, anorthite, various amphiboles, including edenite, cataphorite, Mg-cataphorite, anthophyllite-gedrite, Mg-Fe hornblende, biotites of the siderophyllite-eastonite-annite series, as well as zircon, baddeleyite, apatite, magnetite, ilmenite and Cr-spinel. The high-K rock type is characterised by high K2O contents (up to 9.2 wt. %), K2O/Na2O ratios over 90, lowered TiO2 and MgO and moderate FeO contents and negative P and Sr anomalies. In contrast, low-K rocks, characterised by moderate and increased TiO2 and MgO contents, contain augite, pigeonite, olivine, andesine and accessory minerals, such as rutile, titanite, ilmenite and apatite. Both rock types vary considerably in Nb and Ta concentrations, from OIB-like to E-MORB. Such geochemical signatures of calc-alkaline and shoshonitic igneous rocks are indicative of an active continental margin setting. Presence of the active continental margin setting in the southwestern margin of the Siberian craton during the late Neoproterozoic-early Cambrian time is in agreement with the U-Pb age of 511 Ma of high-K dolerites (Gladkochub et al., 2006) and the development of the coeval island arc assemblages in the northern part of the CAOB.

Jurassic metabasic rocks in the Kızılırmak accretionary complex (Kargı region, Central Pontides, Northern Turkey)

NASA Astrophysics Data System (ADS)

Çelik, Ömer Faruk; Chiaradia, Massimo; Marzoli, Andrea; Özkan, Mutlu; Billor, Zeki; Topuz, Gültekin

2016-03-01

The Kızılırmak accretionary complex near Kargı is tectonically bounded by the Jurassic and Early Cretaceous metamorphic massives of the Central Pontides. It consists mainly of serpentinite, serpentinized peridotite, gabbro, basalt, metabasite and deep-marine sedimentary rocks. The metabasites in the Kızılırmak accretionary complex are tectonically located within a serpentinite, radiolarian chert, spilitized basalt, gabbro association and commonly display a steep contact with serpentinites. Amphiboles from metabasites yielded robust 40Ar/39Ar plateau ages ranging between 159.4 ± 0.4 Ma and 163.5 ± 0.8 Ma. These are interpreted as cooling ages of the metabasites. The metabasites have 87Sr/86Sr(i) between 0.7035 and 0.7044 and 206Pb/204Pb(i) ranging between 18.18 and 18.92. The gabbros have higher 87Sr/86Sr(i) between 0.7044 and 0.7060 and 206Pb/204Pb(i) ranging between 17.98 and 18.43. Three basalt samples display 87Sr/86Sr(i) between 0.7040 and 0.7059. Their 206Pb/204Pb(i) are unrealistically low (15.42 and 15.62), suggesting, most likely, Pb loss which results in over-corrected values for decay through time. Pb-Sr-Nd isotopic compositions for all samples consistently plot between the fields of MORB or the Depleted MORB Mantle reservoirs and enriched mantle reservoirs (EMII rather than EMI). All the samples (except one dolerite dike) have negative ɛNdDM(t = 160 Ma) values, suggesting derivation from a reservoir more enriched than the depleted mantle. The protoliths of metabasites correspond to diverse sources (N-MORB, E-MORB, OIB and IAT) based on whole rock major and trace element composition. An IAT-like protolith for the metabasites indicates that the İzmir-Ankara-Erzincan ocean domain was subducting and the tectonic regime was compressional during Late Jurassic and before. The protoliths of these rocks were metamorphosed during the subduction/accretion processes, as observed in the metamorphic rocks located along the Balkan, Northern Turkey and Armenia/Iran ophiolites and/or accretionary complexes. IAT-like geochemistry for the gabbro/dolerites indicates that the non-metamorphosed basaltic rocks occurred in a supra-subduction tectonomagmatic environment and is in agreement with their radiogenic isotope compositions.

U-Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas Drâa Inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton

NASA Astrophysics Data System (ADS)

El Bahat, Abdelhakim; Ikenne, Moha; Söderlund, Ulf; Cousens, Brian; Youbi, Nasrrddine; Ernst, Richard; Soulaimani, Abderrahmane; El Janati, M'hamed; Hafid, Ahmid

2013-08-01

In the Bas-Drâa Inlier (Anti-Atlas, Morocco), the Paleoproterozoic basement which is cut by the Ediacaran Taourgha granite is also crosscut by numerous dykes of a variety of trends, mostly of uncertain age. Two doleritic dykes are dated by the ID-TIMS U-Pb method on baddeleyite and yield emplacement ages of 1381 ± 8 Ma (MSWD = 0.84) and 1384 ± 6 Ma (MSWD = 1.4) determined for a N135°E and a N40°E trending dyke, respectively. These dates represent the first geochronological evidence of a Mesoproterozoic magmatic event in the Anti-Atlas. This magmatic event falls in the previously considered ca 1.7-1.0 Ga (Mesoproterozoic) gap in geological activity in the Anti-Atlas. The poorly dated Taghdout and Taarotihate sequences could represent remnants of the ca. 1380 Ma magmatism and rift-related sedimentation. The Mesoproterozoic sedimentary succession of the Atar Group in the Taoudeni basin (Mauritania) could also represent a good candidate for rift-related sedimentation but it postdates the 1380 Ma magmatic event by 270 Ma. The dated 1380 Ma dykes are transitional to mildly alkaline basalts, not unlike some Hawaiian lavas. However, these dykes have a distinct negative Nb anomaly (a common features in many Large Igneous Provinces, LIPs), and this requires interaction with the lithosphere. This interaction may have occurred at the level of the lithospheric mantle or the crust. These newly dated 1380 Ma dykes may converge to the north, speculatively suggesting a magmatic center (associated with a 1380 Ma mantle plume?) along the northern margin, and possibly linked to rifting and possible breakup on that margin, and also to a regional uplift that largely removed the evidence of a 1380 Ma cover sequence. Contemporaneous 1380-1390 Ma magmatism is reported elsewhere on other crustal blocks, and that in northeastern Laurentia (northern Greenland), northern Siberia (Anabar shield), and Baltica (southern Urals) can be reconstructed with that of the Bas Drâa Inlier (Anti Atlas region of the West African Craton, WAC) into a single Large Igneous Province (LIP) extending over an area of > 1 million km2, and associated with the final fragmentation of the Columbia (Nuna) supercontinent.

An analysis of the chemical and microbiological quality of ground water from boreholes and shallow wells in Zimbabwe

NASA Astrophysics Data System (ADS)

Moyo, N. A. G.

Groundwater from boreholes and shallow wells is a major source of drinking water in most rural areas of Zimbabwe. The quality of groundwater has been taken for granted and the status and the potential threats to groundwater quality have not been investigated on a large scale in Zimbabwe. A borehole and shallow well water quality survey was undertaken between January, 2009 and February, 2010 to determine the chemical and microbial aspects of drinking water in three catchment areas. Groundwater quality physico-chemical indicators used in this study were nitrates, chloride, water hardness, conductivity, alkalinity, total dissolved solids, iron, magnesium, manganese, potassium, calcium, fluoride, sulphates, sodium and pH. The microbiological indicators were total coliforms, faecal coliforms and heterotrophs. Principal component analysis (PCA) showed that most of the variation in ground water quality in all catchment areas is accounted for by Total Dissolved Solids (TDS), electrical conductivity (EC), sodium, bicarbonate and magnesium. The principal dissolved constituents in ground water are in the form of electrically charged ions. Nitrate is a significant problem as the World Health Organization recommended levels were exceeded in 36%, 37% and 22% of the boreholes in the Manyame, Mazowe and Gwayi catchment areas respectively. The nitrate levels were particularly high in commercial farming areas. Iron and manganese also exceeded the recommended levels. The probable source of high iron levels is the underlying geology of the area which is dominated by dolerites. Dolerites weather to give soils rich in iron and other mafic minerals. The high level of manganese is probably due to the lithology of the rock as well as mining activity in some areas. Water hardness is a problem in all catchment areas, particularly in the Gwayi catchment area where a value of 2550 mg/l was recorded in one borehole. The problems with hard water use are discussed. Chloride levels exceeded the recommended levels in a few areas under irrigation. Most of the chloride is probably from agricultural activity particularly the application of potassium chloride. Fluoride levels were particularly elevated in the Gwayi catchment area and this is because of the geology of the area. There was no evidence of microbial contamination in all the boreholes sampled as the total coliform, faecal coliforms, heterotrophs count was nil. However, severe microbial contamination was found in the wells especially those in clay areas.

Origin of Magnetic High at Basalt-Ultramafic Hosted Hydrothermal Vent Field in the Central Indian Ridge

NASA Astrophysics Data System (ADS)

Fujii, M.; Okino, K.; Sato, T.; Sato, H.; Nakamura, K.

2014-12-01

Hydrothermal alteration processes can change crustal magnetization by destruction and creation of magnetic minerals. In the Yokoniwa hydrothermal vent field (YHVF), located at the NTO-massif in the Central Indian Ridge, a high magnetization zone (with ~12 A/m in ~200 m-scale) was discovered by previous deepsea AUV survey. Basalts and ultramafic rocks were found around the YHVF, however the origin of magnetic high and its relationship with hydrothermal activity are remains to be investigated. Therefore, we conducted additional magnetic field measurement, rock sampling, and geological observation using submersible Shinkai 6500 during the YK09-13 and YK13-03 cruises. Vector geomagnetic field were obtained along the dive tracks at an altitude of ~ 10 m. The crustal absolute magnetization is estimated using the 2D and 3D forward modeling technique. The values of magnetization show ~10 A/m just around the YHVF. This value is consistent with that of equivalent magnetization deduced from AUV data. Rock magnetic characters were measured for 8 basalts, 4 dolerites, 5 sulfides, and 30 serpentinized peridotites (SPs). The measurements of NRM, magnetic susceptibility, magnetic hysteresis, low (6-300K) and high (50-700°C) temperature magnetization curves were performed. The estimated magnetization values are 0.1-6 A/m in basalts, 0.2-0.6 A/m in dolerites, and <0.1 A/m in sulfides. The SPs show strong magnetization of 0.4-11 A/m. The magnetic grain sizes ranges over single domain to pseudo-single domain. The temperature-magnetization curves clearly show the Verway transition and Currie temperature of 580 °C, therefore magnetic carrier of SPs is supposed as pure magnetite, which is created during serpentinization process. Serpentinization degree (Sd) was also estimated by grain density measurement based on empirical formula from Oufi et al., 2002. Amount of magnetite was also estimated from saturation magnetization. The results show that the values of Sd vary in a range from 17 to 100 % and the highly serpentinized samples (>85%) have high magnetization (> 6 A/m). Consequently, we proposed the presence of highly SP bodies at the YHVF as the origin of the high magnetizaion zone. It was formed by locally alteration of upper mantle section due to the hydrothermal activity.

Mush Column Magma Chambers

NASA Astrophysics Data System (ADS)

Marsh, B. D.

2002-12-01

Magma chambers are a necessary concept in understanding the chemical and physical evolution of magma. The concept may well be similar to a transfer function in circuit or time series analysis. It does what needs to be done to transform source magma into eruptible magma. In gravity and geodetic interpretations the causative body is (usually of necessity) geometrically simple and of limited vertical extent; it is clearly difficult to `see' through the uppermost manifestation of the concentrated magma. The presence of plutons in the upper crust has reinforced the view that magma chambers are large pots of magma, but as in the physical representation of a transfer function, actual magma chambers are clearly distinct from virtual magma chambers. Two key features to understanding magmatic systems are that they are vertically integrated over large distances (e.g., 30-100 km), and that all local magmatic processes are controlled by solidification fronts. Heat transfer considerations show that any viable volcanic system must be supported by a vertically extensive plumbing system. Field and geophysical studies point to a common theme of an interconnected stack of sill-like structures extending to great depth. This is a magmatic Mush Column. The large-scale (10s of km) structure resembles the vertical structure inferred at large volcanic centers like Hawaii (e.g., Ryan et al.), and the fine scale (10s to 100s of m) structure is exemplified by ophiolites and deeply eroded sill complexes like the Ferrar dolerites of the McMurdo Dry Valleys, Antarctica. The local length scales of the sill reservoirs and interconnecting conduits produce a rich spectrum of crystallization environments with distinct solidification time scales. Extensive horizontal and vertical mushy walls provide conditions conducive to specific processes of differentiation from solidification front instability to sidewall porous flow and wall rock slumping. The size, strength, and time series of eruptive behavior of the system, coupled with these processes, define the fundamental compositional and dynamic diversity of the Mush Column. In some ways it functions like a complex musical instrument. Entrainment, transport, and sorting of cumulate crystals as a function of repose time and the local flux intensity also contribute to the basic compositional diversity of the system. In the Ferrar dolerite system, about 104 km3 of dolerite is distributed throughout a fir-tree like stack of 4 or 5 extensive 300-750 m thick sills. The lowest sill contains a vast tongue of entrained orthopyroxene (opx) cumulates emplaced with the sill itself. The bulk sill composition varies from 20 pc MgO in the tongue center to 7 pc in the leading tip and margins of the sill, which itself defines the compositional spectrum of the whole complex and is remarkably similar to that exhibited by Hawaii. Relative sorting of large (1-50 mm) opx and small (1-3 mm) plagioclase due to kinetic sieving in the tongue produces pervasive anorthosite stringers. Through local ponding this has culminated in the formation of a small, well-formed layered intrusion consisting of alternating layers of orthopyroxenite and anorthosite. Upwards in the system the sills become progressively depleted in MgO and temporally and spatially contiguous flood basalts are low MgO tholeiites with no sign of opx cumulates. The size, extent, number of sills, and the internal structure of individual sills suggest a rhythm of injection similar to that of volcanic episodes. The continued horizontal stretching of a system of this type would lead to processes as recorded by ophiolites, and the repeated injection into a single reservoir would undoubtedly lead to a massive layered intrusion or to a series of high-level nested plutons.

High resolution remote sensing information identification for characterizing uranium mineralization setting in Namibia

NASA Astrophysics Data System (ADS)

Zhang, Jie-Lin; Wang, Jun-hu; Zhou, Mi; Huang, Yan-ju; Xuan, Yan-xiu; Wu, Ding

2011-11-01

The modern Earth Observation System (EOS) technology takes important role in the uranium geological exploration, and high resolution remote sensing as one of key parts of EOS is vital to characterize spectral and spatial information of uranium mineralization factors. Utilizing satellite high spatial resolution and hyperspectral remote sensing data (QuickBird, Radarsat2, ASTER), field spectral measurement (ASD data) and geological survey, this paper established the spectral identification characteristics of uranium mineralization factors including six different types of alaskite, lower and upper marble of Rössing formation, dolerite, alkali metasomatism, hematization and chloritization in the central zone of Damara Orogen, Namibia. Moreover, adopted the texture information identification technology, the geographical distribution zones of ore-controlling faults and boundaries between the different strata were delineated. Based on above approaches, the remote sensing geological anomaly information and image interpretation signs of uranium mineralization factors were extracted, the metallogenic conditions were evaluated, and the prospective areas have been predicted.

Âge 40K/ 40Ar, Carbonifère inférieur, du magmatisme basique filonien du synclinal paléozoïque de Tin Serririne, Sud-Est du Hoggar (Algérie)

NASA Astrophysics Data System (ADS)

Djellit, Hamou; Bellon, Hervé; Ouabadi, Aziouz; Derder, Mohamed E. M.; Henry, Bernard; Bayou, Boualem; Khaldi, Allaoua; Baziz, Kamal; Merahi, Mounir K.

2006-07-01

Palaeozoic formations of the Tassilis Oua-n-Ahaggar (southeastern Hoggar) include magmatic rocks in the Tin Serririne syncline. Slight contact metamorphism of the overlying bed and studies of anisotropy of magnetic susceptibility of these rocks show that the latter correspond to sills and NW-SE or north-south dykes. 40K/ 40Ar dating of separated feldspars and whole rock for one sample and of whole rock for two other samples give a mean age of 347.6±16.2Ma (at the 2- σ level), thus corresponding to a Lower Carboniferous (Tournaisian) age. Taking into account both the age of this magmatism and the stratigraphic and structural data for this region suggests that dolerites were emplaced within distensive zones that are related to the reactivation of Panafrican faults. To cite this article: H. Djellit et al., C. R. Geoscience 338 (2006).

The Permian Whitehill Formation (Karoo Basin, South Africa): deciphering the complexity and potential of an unconventional gas resource

NASA Astrophysics Data System (ADS)

Götz, Annette E.

2014-05-01

A key energy policy objective of the South African government is to diversify its energy mix from coal which constitutes 85% of the current mix. Gas will play a key role in the future South African economy with demand coming from electricity generation and gas-to-liquids projects. A study on world shale reserves conducted by the Energy Information Agency (EIA) in 2011 concluded that there could be as much as 485 Tcf recoverable reserves of shale gas in the South African Karoo Basin. However, the true extent and commercial viability is still unknown, due to the lack of exploration drilling and modern 3D seismic. The present study compiles existing data from literature review and new data from outcrop analogue studies on the Permian Whitehill Formation, the main target formation for future shale gas production, including thickness, depth, maturity, TOC, lithologies, sedimentary and organic facies, and dolerite occurrence to provide a first reference dataset for further investigations and resource estimates.

40Ar/ 39Ar dating of the emplacement of the Muslim Bagh ophiolite, Pakistan

NASA Astrophysics Data System (ADS)

Mahmood, Khalid; Boudier, Françoise; Gnos, Edwin; Monié, Patrick; Nicolas, Adolphe

1995-11-01

The obduction-related basal part of the Muslim Bagh ophiolite (Baluchistan, Pakistan) and the underlying metamorphic sequence were studied structurally which demonstrated a WSW-ENE-trending thrusting sequence for the initial obduction. 40Ar/ 39Ar measurements on amphiboles and plagioclase from the subophiolitic metamorphic rocks, and on plastically deformed and recrystallized dolerite samples from the base of the sheeted dyke complex give apparent ages between 70.7 ± 5.0 and 65.1 ± 4.1 Ma interpreted as cooling ages dating approximately the formation of the plastic deformation and obduction. The results indicate that the Muslim Bagh ophiolite represents a segment of ocean floor from the small and slow-spreading ocean branch of the Neo-Tethys located between the Indo-Pakistani and the Afro-Arabian plates. The WSW-ENE-oriented obduction of the Muslim Bagh ophiolite onto the Indo-Pakistani continental margin occurred with the convergence of the Neo-Tethys branch during the Late Cretaceous and before the Tertiary collision of the Indo-Pakistani plate with the Eurasian plate.

Preliminary observations and logs of BARB 1 and BARB 2: komatiites from the Tjakastad site

NASA Astrophysics Data System (ADS)

Coetzee, Grace; Arndt, Nicholas; Wilson, Allan

2013-04-01

The BARB 1 and BARB 2 cores intersect a suite of komatiite flows and komatiitic basalts as well as fragmental rocks of the Komati Formation of the Onverwacht Group, Barberton Greenstone Belt. The cores give important and previously unattainable information on the structures, textures and contact relationships between individual komatiite flows and different lithological units within the flows. BARB 1 was drilled at -48° on a 5° azimuth to a depth of 419.9 m. This core contains a unique volcanic tumulus succession in the stratigraphically lower 100 m and the rest of the core consists of about 59 flows of spinifex-textured komatiite (1-3 m thick), massive komatiite (0.5-10 m thick), komatiitic basalt (1-9 m thick) and a single basalt layer (10 m thick), intruded by gabbro (0.5-2 m thick) and a single dolerite dyke (18 m thick). BARB 2, approximately 50 m from BARB 1 and parallel to it, was drilled at -45°on an 8° azimuth to a depth of 431.5 m. This core contains approximately 39 flows of komatiite (0.5-10 m thick) and komatiitic basalt (2-23 m thick) which contain possible selvages of pillows. Basalt flows are more numerous (0.3-4 m thick) in BARB 2 whilst gabbro (0.6-7 m thick) is less prevalent. The dolerite dyke observed in BARB 1 does not occur in BARB 2. As the Barberton strata young towards the east, the cores intersected the stratigraphy in a reverse sequence. The cores were drilled such that there exists a 141 m overlap in stratigraphy between them. The section 141 m from the base of BARB 1 should theoretically correlate with the top 141 m of BARB 2. However, this overlap is not evident in the core or in the core logs. A single gabbro layer appears to be lithologically correlatable between both holes. There is no apparent correlation between the pattern of the komatiite flows leading to an initial conclusion that the komatiite flows were not laterally extensive or changed laterally in form over short distances. In both cores the proportion of komatiitic basalt appears to increase with depth. However, chemical analyses indicate that some of the units originally logged as komatiitic basalt are actually komatiite. The rocks have all undergone alteration to serpentine, and in extreme cases are carbonated together with carbonate veins. Despite the alteration, the original spinifex and olivine cumulate textures, as a well as primary volcanic structures, including spectacular hyaloclastite in the cumulus unit, are well preserved. To date 140 samples have been analysed for major and trace elements and controls by olivine and possibly orthopyroxene have been demonstrated.

Radiolarian biostratigraphic data from the Casiguran Ophiolite, Northern Sierra Madre, Luzon, Philippines: Stratigraphic and tectonic implications

NASA Astrophysics Data System (ADS)

Queano, Karlo L.; Marquez, Edanjarlo J.; Aitchison, Jonathan C.; Ali, Jason R.

2013-03-01

Results from the first detailed radiolarian biostratigraphic study conducted in Luzon are reported. The data were obtained from cherts associated with the Casiguran Ophiolite, a dismembered ophiolite mass consisting of serpentinized peridotites, gabbros, dolerite dikes and pillow basalts exposed along the eastern coast of the Northern Sierra Madre, Luzon, Philippines. Cherts and limestone interbeds conformably overlie the ophiolite. The radiolarian assemblages from the cherts constrain the stratigraphic range of the cherts to the Lower Cretaceous (upper Barremian-lower Aptian to Albian). This new biostratigraphic result is in contrast with the Upper Cretaceous stratigraphic range previously reported in the region. Radiolarian biostratigraphic results from the Casiguran Ophiolite provide additional evidence for the existence of Mesozoic oceanic substratum upon which Luzon and neighboring regions within the Philippine archipelago were likely built. Interestingly, the result closely resembles those reported for the ophiolite in southeastern Luzon as well as the oceanic crust of the Huatung Basin situated east of Taiwan and the ophiolites in eastern Indonesia. In light of this, along with previously gathered geochemical data from the ophiolites, a common provenance is being looked into for these crust-upper mantle sequences in the western Pacific region.

Structural mapping of Chikotra River basin in the Deccan Volcanic Province of Maharashtra, India from ground magnetic data

NASA Astrophysics Data System (ADS)

Anand, S. P.; Erram, Vinit C.; Patil, J. D.; Pawar, N. J.; Gupta, Gautam; Suryavanshi, R. A.

2016-03-01

Ground magnetic data collected over Chikotra River in the peripheral region of Deccan Volcanic Province (DVP) of Maharashtra located in Kolhapur district was analysed to throw light on the structural pattern and distribution of magnetic sources within the basin. In order to isolate the magnetic anomalies showing varying trend and amplitude, several transformation operations including wavelength filtering, and upward continuation has been carried out on the reduced to pole anomaly map. Qualitative interpretation of these products help identify the distribution of magnetic sources, viz., the Deccan basalts, dolerite intrusives and older greenstone and schist belts in the subsurface. Present study suggests that the Chikotra basin is composed of three structural units; a NE-SW unit superposed on deeper NW-SE unit with randomly distributed trap flows on the surface. One of the major outcome of the present study is the delineation of almost 900-m thick Proterozoic Kaladgi sediments below the Deccan trap flows. The NE-SW magnetic sources may probably represent intrusives into the Kaladgi sediments, while the deeper NW-SE trends are interpreted as the northward extension of the Dharwars, underneath the Deccan lava flows, that forms the basement for the deposition of Kaladgi sediments.

U-Pb geochronology and paleomagnetism of the Neoproterozoic St Simeon dolerite dykes, Quebec: an eastern Laurentian perspective of Ediacaran Rodinia breakup

NASA Astrophysics Data System (ADS)

Pisarevsky, Sergei; Murphy, Brendan; Hamilton, Mike; Söderlund, Ulf; Hodych, Joseph

2013-04-01

The St Simeon (SS) mafic dykes (150 km NE of Quebec City) are now dated at 548 ± 1 Ma (U-Pb; baddeleyite). This age is similar to a published LA-ICPMS zircon age of 550 ± 7 Ma for the Mt. St-Anselme (MS) basalts, which supports previous inferences of (i) a genetic relationship between them, (ii) the pene-contemporaneity of OIB-type mafic magmatism in East Laurentia and (iii) the existence of two late Ediacaran plumes that attended the final breakup of Rodinia and opening of the Iapetus Ocean and Tornquist Sea. Both the SS dykes and the MS basalts were sampled for paleomagnetic study. The paleomagnetic pole for SS is similar to the previously published pole for coeval basalts (Skinner Cove, SC) from Newfoundland. Unlike SC, the St Simeon pole represents rocks which are unambiguously coherent tectonically with the Laurentian Craton. This new pole is also coeval with high quality poles from the Winter Coast (Baltica) and provides paleomagnetic constraints on the history of the final breakup of Rodinia and opening of Eastern Iapetus and Tornquist Sea.

Identification of areas of recharge and discharge using Landsat-TM satellite imagery and aerial photography mapping techniques

NASA Astrophysics Data System (ADS)

Salama, R. B.; Tapley, I.; Ishii, T.; Hawkes, G.

1994-10-01

Aerial photographs (AP) and Landsat (TM) colour composites were used to map the geomorphology, geology and structures of the Salt River System of Western Australia. Geomorphic features identified are sand plains, dissected etchplain, colluvium, lateritic duricrust and rock outcrops. The hydrogeomorphic units include streams, lakes and playas, palaeochannels and palaeodeltas. The structural features are linear and curvilinear lineaments, ring structures and dolerite dykes. Suture lines control the course of the main river channel. Permeable areas around the circular granitic plutons were found to be the main areas of recharge in the uplands. Recharge was also found to occur in the highly permeable areas of the sandplains. Discharge was shown to be primarily along the main drainage lines, on the edge of the circular sandplains, in depressions and in lakes. The groundwater occurrence and hydrogeological classification of the recharge potential of the different units were used to classify the mapped areas into recharge and discharge zones. The results also show that TM colour composites provide a viable source of data comparable with AP for mapping and delineating areas of recharge and discharge on a regional scale.

Glacier advance during Marine Isotope Stage 11 in the McMurdo Dry Valleys of Antarctica

PubMed Central

Swanger, Kate M.; Lamp, Jennifer L.; Winckler, Gisela; Schaefer, Joerg M.; Marchant, David R.

2017-01-01

We mapped six distinct glacial moraines alongside Stocking Glacier in the McMurdo Dry Valleys, Antarctica. Stocking Glacier is one of several alpine glaciers in the Dry Valleys fringed by multiple cold-based drop moraines. To determine the age of the outermost moraine, we collected 10 boulders of Ferrar Dolerite along the crest of the moraine and analyzed mineral separates of pyroxene for cosmogenic 3He. On the basis of these measurements, the exposure age for the outermost moraine is 391 ± 35 ka. This represents the first documented advance of alpine glacier ice in the Dry Valleys during Marine Isotope Stage (MIS) 11. At this time, Stocking Glacier was ~20–30% larger than today. The cause of ice expansion is uncertain, but most likely it is related to increased atmospheric temperature and precipitation, associated with reduced ice extent in the nearby Ross Embayment. The data suggest complex local environmental response to warm climates in Antarctica and have implications for glacial response to Holocene warming. The study also demonstrates the potential for using alpine glacier chronologies in the Transantarctic Mountains as proxies for retreat of grounded glacier ice in the Ross Embayment. PMID:28139676

Organic matter and containment of uranium and fissiogenic isotopes at the Oklo natural reactors

USGS Publications Warehouse

Nagy, B.; Gauthier-Lafaye, F.; Holliger, P.; Davis, D.W.; Mossman, D.J.; Leventhal, J.S.; Rigali, M.J.; Parnell, J.

1991-01-01

SOME of the Precambrian natural fission reactors at Oklo in Gabon contain abundant organic matter1,2, part of which was liquefied at the time of criticality and subsequently converted to a graphitic solid3,4. The liquid organic matter helps to reduce U(VI) to U(IV) from aqueous solutions, resulting in the precipitation of uraninite5. It is known that in the prevailing reactor environments, precipitated uraninite grains incorporated fission products. We report here observations which show that these uraninite crystals were held immobile within the resolidified, graphitic bitumen. Unlike water-soluble (humic) organic matter, the graphitic bituminous organics at Oklo thus enhanced radionu-clide containment. Uraninite encased in solid graphitic matter in the organic-rich reactor zones lost virtually no fissiogenic lan-thanide isotopes. The first major episode of uranium and lead migration was caused by the intrusion of a swarm of adjacent dolerite dykes about 1,100 Myr after the reactors went critical. Our results from Oklo imply that the use of organic, hydrophobic solids such as graphitic bitumen as a means of immobilizing radionuclides in pretreated nuclear waste warrants further investigation. ?? 1991 Nature Publishing Group.

Strain localization in the lower crust: brittle precursors versus lithological heterogeneities (Musgrave Ranges, Central Australia)

NASA Astrophysics Data System (ADS)

Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Pennacchioni, Giorgio; Camacho, Alfredo

2016-04-01

The Davenport shear zone in Central Australia is a strike-slip ductile shear zone developed during the Petermann Orogeny (~ 550 Ma). The conditions of shearing are estimated to be amphibolite-eclogite facies (650 °C, 1.2 GPa). The up to seven kilometre thick mylonite zone encloses several large low strain domains with excellent exposure, thus allowing a thorough study of the initiation of shear zones. Quartzo-feldspathic gneisses and granitoids inherit a suite of lithological heterogeneities such as quartz-rich pegmatites, mafic layers and dykes. When in a favourable orientation to the shortening direction, these rheologically different pre-existing layers might be expected to localize deformation. However, with the singular exception of long, continuous and fine-grained dolerite dykes, this is not observed. Quartz-rich pegmatites are mostly unsheared, even if in a favourable orientation, and sometimes boudinaged or folded. There are instead many shear zones only a few mm to cm in width, extending up to tens of metres, which are in fact oriented at a very high angle to the shortening direction. Parallel to these, a network of little to moderately overprinted brittle fractures are observed, commonly marked by pseudotachylyte (pst) and sometimes new biotite. Shear reactivation of these precursor fractures is generally limited to the length of the initial fracture and typically re-uses and shears the pst. The recrystallized mineral assemblage in the sheared pst consists of Cpx+Grt+Fsp±Ky and is the same to that in the adjacent sheared gneiss, with the same PT estimates (650 °C, 1.2 GPa). In some cases, multiple generations of cross-cutting and sheared pst demonstrate alternating fracture and flow during progressive shear zone development and a clear tendency for subsequent pst formation to also localize in the existing shear zone. The latest pst may be both unsheared and unrecrystallized (no grt) and is probably related to a late stage, still localized within the same shear zone. The observation that pst is preferentially sheared indicates that it is weaker than the host rock, although their bulk compositions are about the same, suggesting that the governing factors for localization are the finer grain size and the elongate, nearly planar geometry of the original pst generation zone. The same may be true of the sheared dolerite dykes, which are long, narrow and generally finer grained than the surrounding gneiss or granite. Although quartz-rich pegmatites are not preferred sites of localization, quartzo-feldspathic mylonites are fully recrystallized with a relatively coarse grain size (typically > 50 microns) typical of rather low long-term flow stress. We therefore propose that localization in the lower crust only occurs on long planar layers with a finer grain size that can promote weakening by grain-size sensitive creep. Coarser-grained lithological layers and boundaries are not exploited during the initiation of a shear zone and, in particular, quartz-rich layers are not preferentially sheared.

Erosion-tectonics feedbacks in shaping the landscape: An example from the Mekele Outlier (Tigray, Ethiopia)

NASA Astrophysics Data System (ADS)

Sembroni, Andrea; Molin, Paola; Dramis, Francesco; Faccenna, Claudio; Abebe, Bekele

2017-05-01

An outlier consists of an area of younger rocks surrounded by older ones. Its formation is mainly related to the erosion of surrounding rocks which causes the interruption of the original continuity of the rocks. Because of its origin, an outlier is an important witness of the paleogeography of a region and, therefore, essential to understand its topographic and geological evolution. The Mekele Outlier (N Ethiopia) is characterized by poorly incised Mesozoic marine sediments and dolerites (∼2000 m in elevation), surrounded by strongly eroded Precambrian and Paleozoic rocks and Tertiary volcanic deposits in a context of a mantle supported topography. In the past, studies about the Mekele outlier focused mainly in the mere description of the stratigraphic and tectonic settings without taking into account the feedback between surface and deep processes in shaping such peculiar feature. In this study we present the geological and geomorphometric analyses of the Mekele Outlier taking into account the general topographic features (slope map, swath profiles, local relief), the river network and the principal tectonic lineaments of the outlier. The results trace the evolution of the study area as related not only to the mere erosion of the surrounding rocks but to a complex interaction between surface and deep processes where the lithology played a crucial role.

Aperçu de precambrien de côte d'Ivoire: geologie-metallogenie

NASA Astrophysics Data System (ADS)

Angoran, Y.; Kadio, E.

The Ivory Coast is situated at the southern limits of the West African Craton and constitute a part of the 'Dorsale de Man'. The precambrian rocks occupy 97% of the superficial area of the country and include rocks of two orogenic episodes: the Liberian (3000-2580 Ma) and the Eburnian of lower Proterozoic (2400-1550 Ma). Liberian Orogeny, which is the most ancient, consists of gneisses, amphibo-pyroxinites, fine-grained itabirites and coarse-grained ferruginous quartzites. The aluminous gneisses, amphibo-pyroxinites and ferruginous quartzites are supracrustals that have been transformed by a high grade Catasonal metamorphism resulting in highly folded rocks. The Liberian plutons are infracrustals consisting of complex basic and ultrabasic rocks, migmatites, charnockites and granites associated with magmatites. This Liberian complex is intruded by some dolerites (2200 Ma), and kimberlites with diamond (2210-2500 Ma) which have been eroded to produce Birrimian placer deposits of Tortiya and Birrim in Ghana. The eburnian geosyncline consists of alternating subparallel intrageosynclines and intrageanticlines. The volcano-sedimentary complexes were intruded by eburnian plutons of 2100-1550 Ma. About 20 different types of mineralisations are common within the Pre-Cambrian rocks of the Ivory Coast and they are of Archaen to lower Proterozoic age. Examples of these mineral concentrations are cited in this paper.

Seafloor expressions of tectonic structures in Isfjorden, Svalbard: implications for fluid migration

NASA Astrophysics Data System (ADS)

Roy, Srikumar; Noormets, Riko; Braathen, Alvar

2014-05-01

This study investigates the seafloor expressions of Isfjorden in western Svalbard, interlinked with sub-seafloor structures using a dense grid of 2D multichannel marine seismic and magnetic data integrated with high resolution multibeam bathymetric data. The underlying bedrock structures spans from Paleozoic carbonates and evaporates to Mesozoic and Paleogene sandstones and shales. This 4 to 6 km thick succession is truncated by structures linked to Eocene transpressional deformation that resulted in the formation of the West Spitsbergen Fold-and-Thrust Belt (WSFTB). The WSFTB divides into three major belts : (a) western zone characterized by a basement involved fold-thrust complex, (b) central zone consisting of three thin-skinned fold-thrust sheets with thrusts splaying from décollement layers and, east of a frontal duplex system, (c) eastern zone showing décollement in Mesozoic shales with some thrust splays, and with the décollement interacting with reactivated, steep and basement-rooted faults (Bergh et al., 1997). In the continuation, we discuss combined seafloor and bedrock observations, starting from the west. In the west, a 6.5 km long and 5 to 9 m high ridge demarcates the eastern boundary of the major basement involved fold complex, with thrusted and folded competent Cretaceous to Paleogene units reaching the seafloor. Three submarine slides originate from this ridge, possibly triggered by tectonic activities. In Central Isfjorden (central zone of the WSFTB), several NNW-SSE striking ridges with a relief of 5 to 25 m have been tied with shallow, steep faults and folds. In addition to the NNW-SSE striking ridges, a set of SW-NE striking ridges with relief of 2 to 5 m are observed in Nordfjorden. Based on the seismic data observations, these ridges can be linked to the surface expression of competent sandstones that are transported on splay-thrusts above a décollement in Triassic shales. Further, seafloor ridges with relief of 5 of 18 m, linked to high amplitude flat reflectors and high magnetic values have been interpreted as Cretaceous dolerite intrusions in Nordfjorden and central Isfjorden. In the eastern Isfjorden (eastern zone of WSFTB), a 10.5 km long N-S striking ridge in Billefjorden corresponds to the deep-seated Billefjorden Fault Zone, extending south across the mouth of Tempelfjorden where it is 8.5 km long. This composite ridge is bound by a steep east-dipping fault, placing competent Carboniferous and Permian carbonates at the seafloor. Overall, our study shows a distinct pattern of pockmarks concentrated along the identified ridges on the seafloor of Isfjorden. These ridges can be linked to fault-fold systems and dolerite intrusions in the bedrock, thereby suggesting various possible fluid migration pathways towards pockmarks: (i) along fracture networks associated with folds and intrusions, (ii) along décollement zones and faults acting as localized conduits, and (iii) directly from organic rich layers when exposed at the seafloor. Reference: Bergh, S. G., Braathen, A., and Andresen, A., 1997, Interaction of basement-involved and thin-skinned tectonism in the Tertiary fold-thrust belt of central Spitsbergen, Svalbard: AAPG Bulletin, v. 81, no. 4, p. 637-661.

Evolved Gas Analysis of Mars Analog Samples from the Arctic Mars Analog Svalbard Expedition: Implications for Analyses by the Mars Science Laboratory

NASA Technical Reports Server (NTRS)

McAdam, A.; Stern, J. C.; Mahaffy, P. R.; Blake, D. F.; Bristow, T.; Steele, A.; Amundsen, H. E. F.

2012-01-01

The 2011 Arctic Mars Analog Svalbard Expedition (AMASE) investigated several geologic settings on Svalbard, using methodologies and techniques being developed or considered for future Mars missions, such as the Mars Science Laboratory (MSL). The Sample Analysis at Mars (SAM) instrument suite on MSL consists of a quadrupole mass spectrometer (QMS), a gas chromatograph (GC), and a tunable laser spectrometer (TLS), which analyze gases created by pyrolysis of samples. During AMASE, a Hiden Evolved Gas Analysis-Mass Spectrometer (EGA-MS) system represented the EGA-QMS capability of SAM. Another MSL instrument, CheMin, will use x-ray diffraction (XRD) and x-ray fluorescence (XRF) to perform quantitative mineralogical characterization of samples. Field-portable versions of CheMin were used during AMASE. AMASE 2011 sites spanned a range of environments relevant to understanding martian surface materials, processes and habitability. They included the basaltic Sverrefjell volcano, which hosts carbonate globules, cements and coatings, carbonate and sulfate units at Colletth0gda, Devonian sandstone redbeds in Bockfjorden, altered basaltic lava delta deposits at Mt. Scott Keltie, and altered dolerites and volcanics at Botniahalvoya. Here we focus on SAM-like EGA-MS of a subset of the samples, with mineralogy comparisons to CheMin team results. The results allow insight into sample organic content as well as some constraints on sample mineralogy.

Pyroxene thermometry in the OPX Tongue of the Basement Sill, McMurdo Dry Valleys, Antarctica

NASA Astrophysics Data System (ADS)

Simon, A. C.; Marsh, B.

2005-12-01

Unusually large orthopyroxene (Opx) crystals (5 mm) in the 330 m thick doleritic Basement Sill, the basal sill of the Ferrar Dolerite sill complex, possess strong variations in modal abundance and size both across and along the sill length. Opx phenocrysts (with greatly subordinate cpx) are concentrated toward the middle of the sill, forming an extensive ultramafic tongue, which dominates the thickness of the sill in the feeder zone near Bull Pass and progressively thins with increasing distance in all directions away from the feeder zone. The chilled margins of the sill contain no Opx phenocrysts. Based on the physical position of the Opx tongue and the size, abundance and textural condition of these Opx crystals, it has been postulated that these Opx phenocrysts were entrained in the ascending magma after having been texturally equilibrated at a much deeper location than the present level of sill emplacement, approximately 3- 4 km paleodepth. Determining the ultimate provenance of these phenocrysts is an important step in understanding the formation and overall dynamics of this magmatic system. In a real sense these crystals are important dynamic tracers. To gain insight into the origin and history of transport of these crystals, we performed electron probe microanalyses of pyroxenes from samples collected at 15 m intervals through the entire Basement Sill near the feeder zone in West Bull Pass to quantify the compositions of pyroxenes both within the tongue and the bounding chilled margin in order to use crystal-chemical variations to infer crystal histories. Additionally, we calculated equilibrium temperatures for Opx - Cpx mineral pairs, using the QUILF projection scheme at 200 MPa, to determine if pyroxene crystals in the tongue reflect higher temperatures, indicative of crystallization at deeper levels of the sill-complex plumbing system. Despite some scatter in pyroxene compositions, the data indicate that the En (mole % MgSiO3) component of Opx and Cpx increases systematically with depth through the sill, reaching a plateau in the middle of the transect and then decreasing to the chilled margin at the base of the section. The range of En content of Cpx (27 En) is much less than that of Opx (40 En). The Di component of Cpx acts to some degree as a pivot for the varying En/Fs of Opx. The trend in the En component of Opx parallels roughly the trend in whole-rock MgO, the latter being a manifestation of the increase in abundance of En-rich Opx crystals (phenocrysts) in the central part of the sill. Intra-crystal compositional variation is minimal except for the (expected) decrease in En/Fs of grain margins, reflecting simply the iron enrichment of the residual melt with progressive crystallization. The concentrations of Cr and Al in Opx increase systematically with depth in the sill are highest in Opx of the tongue. Compositional tie-lines for Opx-Cpx pairs from the chilled margin and Opx-tongue, when plotted in the pyroxene quadrilateral, possess negative and positive slopes, respectively. Temperature data define a thermal maximum plateau that coincides with the position of the Opx-tongue; 2-pyroxene temperature increasing on the order of 200 degrees C between the chilled margin (900 to 1000 C)and the Opx-tongue (1070 to 1242 C). The pyroxene tongue represents a subsequent, higher temperature, intrusive event. The relatively high temperatures, trace element abundances, and textural information all suggest that the provenance of the crystals forming the Opx Tongue were entrained from a cumulate pile associated with an earlier magmatic event.

K-Ar chronology and geochemistry of the Miocene magmatism of Collo-Bougaroun and Edough-Cap de Fer areas (NE Algeria). Temporal constraints on geodynamic evolution of the Eastern Algerian margin between 6° and 8°E

NASA Astrophysics Data System (ADS)

Abbassene, F.; Bellon, H.; Chazot, G.; Ouabadi, A.

2013-12-01

The ''Petite Kabylie'' corresponds to the eastern Algerian coastal magmatic chain outcropping from Jijel to the west, up to the plain of Annaba to the east. In this area, the Collo-Bougaroun volcano-plutonic complex, of ca. 300 km2, comprises (1) granular rocks, mainly cordierite bearing peraluminous granites, (2) gabbros that occur at the northern and southern parts of Cap Bougaroun pluton where they are associated with ultramafic rocks and form the layered complex of Yadene?; (3) microgranular rocks, mainly microgranites, that outcrop at the eastern part of the Bougaroun pluton, in Collo basin and El Milia, microdiorites in Bouserdoum and some doleritic or microgabbroic metric veins at Cap Bougaroun and (4) of rhyolitic lava in Kef Cheraïa. The Bougaroun complex form a huge elliptical batholite along a major axis of 20km oriented ENE- WSW that intrudes serpentinized peridotites and kinzigites of the Bougaroun basement to the east. This granitic pluton gives time constraints as it induces deformation and contact metamorphism of the Oligo-Miocene Kabyle sediments of Collo-Oued Zhour basin in the south. These sediments reach the Upper Burdigalien which suggests that the lower limit of emplacement of this granite is coeval at least with this age. The majority of these magmatic rocks show subalkaline affinity with strong enrichment (0.13 to 4.13 %) in K2O during fractionation to calc-alkaline and high-K calc-alkaline affinity for the most differentiated rocks. The felsic rocks (granites, microgranites and rhyolites) are marked by a significant crustal contamination (ξNd = -10, I Sr = 0.720, δ18O = +12 ‰ [1], [2]) during their petrogenesis. However, the presence of basic rocks (gabbros and dolerites) that are depleted in K2O (0.13 to 0.44%) provides information on mantle composition and origin of magmas. The geochemical data on these rocks are discussed in the very particular geodynamic context of the northern Algerian margin.Twenty-four 40K-40Ar analyses were performed on whole rock and separated grain minerals (biotite, quartz and feldspar) from some granites. Grains were chosen in 150-300 μm separates. The obtained results from mineral separates from the granites and gabbros scatter between 21 and 16 Ma. These results appear older compared to field observations that fixe the age of pluton intrusion around 16-17 Ma. Several assumptions are made on the possible origin of the possible excess argon, particularly during crustal contamination of magmas and differentiation processes. Syn-late or post-magmatic hydrothermal alteration is also considered. The Chetaïbi-Cap Fer area shows mafic (gabbro, basalt), intermediate (diorite) and felsic rocks (microgranite and rhyolite) that were emplaced either as lava-flows, sills, dykes or laccoliths intruding Miocene sediments. 14 samples were dated by K/Ar whole rock method and in some cases biotite and quartz & feldspar separates. The results show three groups: between 16 and 15Ma, about 14Ma and about 13Ma. We consider that three distinct magmatic events are responsible for their emplacement.These results agree well with the overall geodynamic context of Algerian margin which was structured during three tangential tectonic events, dated respectively 17 Ma, 15 Ma and 9 Ma.

Vent Complexes above Dolerite Sills in Phanerozoic LIPs: Implications for Proterozoic LIPs and IOCG Deposits

NASA Astrophysics Data System (ADS)

Ernst, R. E.; Bleeker, W.; Svensen, H.; Planke, S.; Polozov, A. G.

2009-05-01

New insights into the origin of IOCG (iron oxide copper gold) deposits [e.g., 1, 2, 3] follow from recent studies of Phanerozoic Large Igneous Provinces (LIPs). Detailed seismic studies of the 62-55 Ma North Atlantic Igneous Province and complementary studies in the 183 Ma Karoo and 250 Ma Siberian LIPs reveal thousands of hydrothermal vent complexes (HVCs). Up to 5-10 km across at the paleosurface, these vents connect to underlying dolerite sills at paleodepths of up to 8 km [4, 5, 6, 7]. They originate from explosive release of gases generated when thick sills (>50 m) are emplaced into volatile-rich but low-permeability sedimentary strata. HVCs are phreatomagmatic in origin. Their architecture, economic potential for IOCG-type deposits, and effects on climate strongly depend on the type of host rocks (black shales at Karoo and evaporites at Siberian LIPs) and its fluid (brines) saturation at the time of emplacement. About 250 HVCs associated with the Siberian LIP are mineralized having magnetite in the matrix. Some are being mined for Fe (Korshunovskoe and Rudnogorskoe), but their economic potential for copper and gold mineralization is understudied. These observations from the Phanerozoic LIP record suggest that HVCs should also be an essential component of sill provinces associated with Proterozoic LIPs, with a potential for causing major climatic shifts and IOCG-type deposits, particularly if the host sediments include substantial evaporites. Two examples are discussed here. The 725 Ma Franklin LIP covers 1.1 Mkm2 in northern Canada [8]; in the Minto Inlier of Victoria Island, this event comprises volcanics, sills, and breccia pipes [9, 10]. The breccia pipes appear identical to HVCs and, furthermore, the presence of evaporites in the host sediments of the Shaler Supergroup suggests (based on the Siberian trap example) the potential for IOCG-type mineralization. Could 1.59 Ga sills, as exemplified by the exposed Western Channel Diabase sills on the eastern side of Great Bear Lake [11], be the cause of both the Wernecke Breccias of the Yukon (with their hematite, Cu, Co, U and Au mineralization) and the Olympic Dam giant IOCG deposit of the Gawler craton of Australia, which was probably an adjacent block at this time [12, 11]? A dramatic expansion of new targets for IOCGs potentially could be achieved via a systematic survey of sill provinces associated with Proterozoic and Paleozoic LIPs from around the world, with a special focus on those in which the host sediments are evaporate-rich, with the goal of identifying mineralized HVCs. [1] Hitzman, 2000, In: Porter (ed.) v.1; PGC Publishing; [2] Williams et al., 2005, Econ. Geol; [3] Corriveau, 2007, GAC Min. Dep. Div. Spec. Pub 5; [4] Jamtveit et al., 2004, In: Geol. Soc. London, Spec. Publ. 234; [5] Planke et al., 2005, In: Dore & Vining (eds) Geol. Soc. London; [6] Svensen et al., 2006, J. Geol. Soc. London; [7] Svensen et al., 2008, EPSL; [8] Buchan & Ernst, 2004 GSC. Map 2022A; [9] Jefferson et al., 1994, GSC. OF 2789; [10] Rainbird, 1998, GSC OF File 3671; [11] Hamilton & Buchan 2007, GSA Ann. Mtg ; [12] Thorkelson et al., 2001, Prec. Res.

Dyke emplacement at the incipient Namibian margin - structural and anisotropy of magnetic susceptibility (AMS) studies in the Henties Bay - Outjo Dyke Swarm

NASA Astrophysics Data System (ADS)

Wiegand, Miriam; Trumbull, Robert; Greiling, Reinhard O.

2010-05-01

During the Cretaceous breakup of western Gondwana, the conjugate Namibian and South American margins were the site of flood basalts, mafic dyke swarms and subvolcanic intrusive complexes which make up the South Atlantic Large Igneous Province and the volcanic margin of northwestern Namibia. This contribution presents data on internal fabrics in mafic dykes (mostly subalkaline tholeiitic dolerites) from the major Henties Bay-Outjo dyke swarm (HOD) in coastal and inland NW Namibia, which are discussed in terms of magma emplacement. The HOD is some 100 km wide and extends at least 500 km from the continental margin. The dykes were emplaced in Neoproterozoic (Panafrican) Damara mobile belt, which is bounded by the Angola/Congo craton on the north and the Kalahari craton on the south. Field relations and radiometric dates indicate Early Cretaceous emplacement ages for the dykes. In coastal exposures north of the HOD, dolerite dykes are mainly coast-parallel (NNW-SSE) and syn-tectonic with normal faults that offset Etendeka lavas. Coast-parallel dykes are also common within the HOD, but the great majority of dykes strike NE-SW. We observed the latter dykes to crosscut coast-parallel ones. But the opposite relationship is also found locally. The dominant NE-SW strike of HOD indicates the influence of the Damara Belt structural grain at a regional scale, but locally the dykes commonly crosscut basement foliations and lithologic contacts. Depending on dyke thickness, which varies in the HOD from a few cm to about 50 m), the dykes are variably fine grained with chilled margins. Vesiculation is seldom observed. Typical textures are intersertal to subophitic, with plagioclase, clinopyroxene and olivine being the main mineral phases. Common minor minerals include opaque oxides and acicular apatite. Linear dykes are composed of segments, 10 m to some km in length, which are connected by transfer zones. Often a minor horizontal displacement can be observed between these segments. Dyke margins are often offset at pre-existing fractures or they may follow suitably oriented ones. These observations imply a major horizontal principal stress parallel to the dykes, at a high angle with the coastline. Successive, multiple intrusions of subparallel dykes imply a crack-seal mechanism of emplacement. We analysed by the anisotropy of magnetic susceptibility (AMS) or magnetic fabric of dykes and sampled at 24 different stations in the area between Henties Bay at the coast and Outjo ca. 300 km inland. Most of the dykes were sampled on both margins and some also in the centre. This strategy allows inferring magma flow directions from the AMS results. Magnetic susceptibility is relatively high, around 20x10E-3 SI units. Microscopic studies and kappa-T measurements indicate magnetite and titanomagnetite as dominating magnetic minerals. The anisotropy of magnetic susceptibility is mostly low, indicating a primary fabric with little secondary overprint. A few samples show higher anisotropies and distinctly prolate fabrics, which are interpreted as the expression of a strong flow fabric. Steep long axes of such fabrics may be related to vertical melt emplacement at intrusion centres, and shallow plunges to horizontal flow along the dyke fractures. Oblate fabrics were observed in places, which define a flat-lying magnetic foliation, that we tentatively attribute to a rheologic disturbance or gravitational deformation at the top end of dykes, where the melt was prevented from flowing further upwards.

In-situ arc crustal section formed at the initial stage of oceanic island arc -Diving survey in the Izu-Bonin forearc-

NASA Astrophysics Data System (ADS)

Ishizuka, O.; Yuasa, M.; Tani, K.; Umino, S.; Reagan, M. K.; Kanayama, K.; Harigane, Y.; Miyajima, Y.

2009-12-01

The Bonin Ridge is an unusually prominent forearc massif in the Izu-Bonin arc that exposes early arc volcanic rocks on Bonin Islands. Submarine parts of the ridge, which could complement the record of volcanism preserved on the islands, had not been extensively investigated. In 2007, dredge sampling in the Izu-Bonin forearc brought us ample evidence of exposure of arc crustal section formed at initial stage of this arc along the landward slope of Izu-Ogasawara trench. Based on this discovery, we conducted Shinkai 6500 submersible survey in May, 2009. This expedition enabled us to obtain general understanding of the crustal section that formed when this oceanic arc began. We investigated 3 areas of the Bonin Ridge. Near 28o25’N, 4 dives were used to look at the lower to upper crustal section. The deepest dive observed both gabbro and basalt/dolerite, and appears to have passed over the boundary between the two. Lower slope is composed of fractured gabbro, whereas pillow lava was observed in the uppermost part of this dive track. Two dives surveyed up-slope of the previous dive found outcrop of numerous doleritic basalt dykes and fractured basaltic lava cut by dykes between water depth of 6000 and 5500m. The shallowest dive recovered volcanic breccia and conglomerate with boninitic and basaltic clasts. Combined with results from other dives and dredging, the members of forearc crustal section are from bottom to top: 1) gabbroic rocks, 2) a sheeted dyke complex, 3) basaltic lava flows, 4) volcanic breccia and conglomerate with boninitic and basaltic clasts, 5) boninite and tholeiitic andesite lava flows and dykes (on the Bonin Islands). In addition to this crustal section, dredge sampling and ROV Kaiko dives recovered mantle peridotite below the gabbro. These observations indicate that almost all of the forearc crust down to Moho has been preserved. Preliminary data indicate that basaltic rocks made of sheeted dykes and lava flows and lower gabbros are generally comagmatic. These basalts show chemical characteristics similar to MORB (i.e., with no slab signature). These basalts have lower Ti, LREE, LREE/HREE, Nb/Zr and Zr/Y than Philippine Sea MORB, but with comparable or slightly lower 143Nd/144Nd. Even though the likely source of these MORB-like basalts can be linked to an Indian Ocean-type mantle, the source for these basalt could be more depleted due to previous event of melt extraction. These basalts also have distinctly higher 87Sr/86Sr and 206Pb/204Pb than Philippine Sea MORB, which may imply the presence of lithospheric mantle with ancient enrichment. Age determination of basalt and gabbro by Ar/Ar and U-Pb methods has confirmed that these rocks predate boninite and could be older than 50Ma. Chemically and petrographically they are similar to tholeiites from the Mariana forearc that predate boninitic volcanism in that region that are considered to be related to subduction (Reagan et al., in prep). This strongly implies that MORB-like tholeiitic magmatism was associated with forearc spreading along the length of the Izu-Bonin-Mariana arc.

Geological and geophysical investigation of water leakage from two micro-dam reservoirs: Implications for future site selection, northern Ethiopia

NASA Astrophysics Data System (ADS)

Berhane, Gebremedhin; Amare, Mogos; Gebreyohannes, Tesfamichael; Walraevens, Kristine

2017-05-01

Water resources are essential to human development activities and to eradicate extreme poverty and hunger. Geological problems of two water harvesting Micro-Dam Reservoirs (MDRs) were evaluated from leakage perspectives in the northern part of Ethiopia, East Africa. Conventional geological mapping, discontinuity and weathering descriptions, test pits and geophysical methods were used to characterize the hydrogeological features of the MDRs. Vertical Electrical Sounding (VES) and Electrical Profiling (EP), were executed using Terrameter SAS (signal averaging system) 1000 manufactured by ABEM, Sweden, with Schlumberger and Wenner array configuration respectively. It was concluded that the foundations of both MDRs, except the right abutment for Adishuhu which is partly composed of dolerite, are pervious due to the presence of thin bedding planes, joints, weathered materials and fault. The presence of water in the downstream toe of the MDRs, at depressions, existing test pits and test pits excavated during the present study which lie within the seepage zone demarcated during surface geological mapping, correspond with the electrical resistivity study. The results of the electrical resistivity survey (EP and VES) were merged with the geological and structural mapping and the observation of seepage zones, for the delineation of weak zones responsible for leakage. Monitoring of the leakage (reservoir water and groundwater levels), both manually and using automatic divers, is recommended, along with monitoring of the stability of the embankments and the discharge or flow downstream of the MDRs.

Rb-Sr geochronology of the region between the Antarctic Peninsula and the Transantarctic Mountains: Haag nunataks and Mesozoic granitoids

NASA Astrophysics Data System (ADS)

Millar, I. L.; Pankhurst, R. J.

Seventy-two new Rb-Sr whole-rock analyses are reported for Haag Nunataks, Mount Woollard, the Whitmore Mountains, the Pirrit and Nash hills, and Pagano Nunatak. For Haag Nunataks, three isochrons for gneisses and later aplogranite and microgranite sheets establish the age of crustal formation as 1000-1100 Ma. No other basement rocks of this age are known from the Antarctic Peninsula or Ellsworth Land. Results from the migmatite-pegmatite complex at Mount Woollard are inconclusive but do not suggest that this represents Precambrian crystalline basement. Provisional results for the Whitmore Mountains granites are compatible with crystallization of all components within error of a 182±5 Ma isochron for fine-grained microgranite, but variation in initial 87Sr/86Sr from 0.707 for porphyritic granites to 0.722 for the microgranite rule out simple crystal fractionation models which require a common parental magma. The granites of the Ellsworth-Thiel mountains ridge are well dated as Middle Jurassic by the new data: Pirrit Hills 173±3 Ma, Nash Hills 175±8 Ma, and Pagano Nunatak 175±8 Ma. Initial 87Sr/86Sr ratios of 0.707, 0.712, and 0.716, respectively, confirm that these are intracratonic S-type granites with a large crustal component involved in magma generation. The dolerite of Lewis Nunatak is shown by its Rb, Sr, and 87Sr/86Sr composition to be a member of the Jurassic Ferrar Supergroup.

Mafic dikes at Kahel Tabelbala (Daoura, Ougarta Range, south-western Algeria): New insights into the petrology, geochemistry and mantle source characteristics

NASA Astrophysics Data System (ADS)

Mekkaoui, Abderrahmane; Remaci-Bénaouda, Nacéra; Graïne-Tazerout, Khadidja

2017-09-01

New petrological, geochemical and Sr-Nd isotopic data of the Late Triassic and Early Jurassic Kahel Tabelbala (KT) mafic dikes (south-western Algeria) offer a unique opportunity to examine the nature of their mantle sources and their geodynamic significance. An alkaline potassic Group 1 of basaltic dikes displaying relatively high MgO, TiO2, Cr and Ni, La/YbN ∼ 15, coupled with low 87Sr/86Sri ∼ 0.7037 and relatively high ɛNd(t) ∼ +3, indicates minor olivine and clinopyroxene fractionation and the existence of a depleted mantle OIB source. Their parental magma was generated from partial melting in the garnet-lherzolite stability field. A tholeiitic Group 2 of doleritic dikes displaying low MgO, Cr and Ni contents, La/YbN ∼ 5, positive Ba, Sr and Pb anomalies, the absence of a negative Nb anomaly coupled with moderate 87Sr/86Sri ∼ 0.7044 and low ɛNd(t) ∼ 0 (BSE-like), indicates a contamination of a mantle-derived magma that experienced crystal fractionation of plagioclase and clinopyroxene. This second group, similar to the low-Ti tholeiitic basalts of the Central Atlantic Magmatic Province (CAMP), was derived from partial melting in the peridotite source within the spinel stability field. Lower Mesozoic continental rifting could have been initiated by a heterogeneous mantle plume that supplied source components beneath Daoura, in the Ougarta Range.

Hydrocarbon source rock potential of the Karoo in Zimbabwe

NASA Astrophysics Data System (ADS)

Hiller, K.; Shoko, U.

1996-07-01

The hydrocarbon potential of Zimbabwe is tied to the Karoo rifts which fringe the Zimbabwe Craton, i.e. the Mid-Zambezi basin/rift and the Mana Pools basin in the northwest, the Cabora Bassa basin in the north and the Tuli-Bubye and Sabi-Runde basins in the south. Based on the geochemical investigation of almost one thousand samples of fine clastic Karoo sediments, a concise source rock inventory has been established showing the following features. No marine source rocks have been identified. In the Mid-Zambezi area and Cabora Bassa basin, the source rocks are gas-prone, carbonaceous to coaly mudstones and coal of Lower Karoo age. In the Cabora Bassa basin, similar gas-prone source rocks occur in the Upper Karoo (Angwa Alternations Member). These kerogen type III source rocks are widespread and predominantly immature to moderately mature. In the southern basins, the Lower Karoo source rocks are gas-prone; in addition some have a small condensate potential. Most of the samples are, however, overmature due to numerous dolerite intrusions. Samples with a mixed gas, condensate and oil potential (mainly kerogen types II and III) were identified in the Lower Karoo (Coal Measure and Lower Madumabisa Mudstone Formations) of the Mid-Zambezi basin, and in the Louver Karoo (Mkanga Formation) and Upper Karoo (Upper Angwa Alternations Member Formation) of the Cabora Bassa basin. The source rocks, with a liquid potential, are also immature to moderately mature and were deposited in swamp, paludal and lacustrine environments of limited extent.

Formation and tectonic evolution of the Cretaceous Jurassic Muslim Bagh ophiolitic complex, Pakistan: Implications for the composite tectonic setting of ophiolites

NASA Astrophysics Data System (ADS)

Khan, Mehrab; Kerr, Andrew C.; Mahmood, Khalid

2007-10-01

The Muslim Bagh ophiolitic complex Balochistan, Pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the Indian and Eurasian plates. These ophiolites were obducted onto the Indian continental margin around the Late Cretaceous, prior to the main collision between the Indian and Eurasian plates. The upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes. Each pluton has a transitional dunite-rich zone at its base, and new geochemical data suggest a similar mantle source region for both the plutons and dykes. In contrast, the lower nappe consists of pillow basalts, deep-marine sediments and a mélange of ophiolitic rocks. The rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid-ocean ridge. The basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the Indian plate. The two nappes of the Muslim Bagh ophiolitic complex are thus distinctively different in terms of their age, lithology and tectonic setting. The recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved.

Metallogeny of the nikolai large igneous province (LIP) in southern alaska and its influence on the mineral potential of the talkeetna mountains

USGS Publications Warehouse

Schmidt, J.M.; Rogers, R.K.

2007-01-01

Recent geologic mapping has identified areas of extrusive basalts of the Middle to Late Triassic Nikolai Greenstone within the Wrangellia terrane that extend at least 80 km southwest of their previously known extent. Abundant dolerite sills of similar composition intrude Paleozoic and Mesozoic stratigraphy below the Nikolai throughout the central Talkeetna Mountains. The Talkeetna Mountains, therefore, have newly identified potential for copper, nickel, and platinum-group elements (PGEs) as disseminated, net-textured, or massive magmatic sulfide deposits hosted in mafic and ultramafic sill-form complexes related to emplacement of the Nikolai. Because of their potential high grades, similar magmatic sulfide targets have been the focus of increasing mineral exploration activity over the last decade in the Amphitheater Mountains and central Alaska Range, 100-200 km to the northeast. The Nikolai Greenstone, associated intrusions, and their metamorphosed equivalents also have potential to host stratabound disseminated "basaltic copper" deposits. Sedimentary and metasedimentary rocks overlying the Nikolai have the potential to host stratabound, disseminated, or massive "reduced-facies" type Cu-Ag deposits. Ultramafic rocks have been identified only in the extreme northeastern Talkeetna Mountains to date. However, coincident gravity and magnetic highs along the leading (northwestern) edge of and within Wrangellia in the Talkeetna and Clearwater Mountains suggest several areas that are highly prospective for ultramafic rocks related to extrusion of Nikolai lavas. In particular, the distribution, geometry, and composition of sills within the pre-Nikolai stratigraphy and the structural and tectonic controls on intrusive versus extrusive rock distribution deserve serious examination. Copyright ?? 2007 The Geological Society of America.

The iron source in phreatomagmatic pipes in the Tunguska Basin (eastern Siberia): insights into hydrothermal-metasomatic leaching processes from Fe isotopes, microstructures, and mass balances.

NASA Astrophysics Data System (ADS)

John, Timm; Svensen, Henrik; Weyer, Stefan; Polozov, Alexander; Planke, Sverre

2010-05-01

The Siberian iron-bearing phreatomagmatic pipes represent world class Fe-ore deposit, and 5-6 are currently mined in eastern Siberia. The pipes formed within the vast Tunguska Basin, cutting thick accumulations of carbonates (dolostones) and evaporites (anhydrite, halite, dolostone). These sediments were intruded by the sub-volcanic part of the Siberian Traps at 252 Ma, and sills and dykes are abundant throughout the basin. The pipes formed during sediment-magma interactions in the deep parts of the basin, and the degassing is believed to have triggered the end-Permian environmental crisis. A major problem with understanding the pipe formation is related to the source of iron. Available hypotheses state that the iron was leached from a Fe-enriched magmatic melt that incorporated dolostones. It is currently unclear how the magmatic, hydrothermal, and sedimentary processes interacted to form the deposits, as there are no actual constraints to pin down the iron source. We hypothesize two end-member scenarios to account for the magnetite enrichment and deposition, which is testable by analyzing Fe-isotopes of magnetite: 1) Iron sourced from dolerite magma through leaching and metasomatism by chloride brines. 2) Leaching of iron from sedimentary rocks (shale, dolostone) during magma-sediment interactions. We focus on understanding the Fe-isotopic architecture of the pipes in order constrain the source of the Fe and the mechanism that caused this significant Fe redistribution. We further evaluate possible fractionation during fast metasomatic ore-forming process that took place soon after pipe formation.

Effect of volcanic dykes on coastal groundwater flow and saltwater intrusion: A field-scale multiphysics approach and parameter evaluation

NASA Astrophysics Data System (ADS)

Comte, J.-C.; Wilson, C.; Ofterdinger, U.; González-Quirós, A.

2017-03-01

Volcanic dykes are common discrete heterogeneities in aquifers; however, there is a lack of field examples of, and methodologies for, comprehensive in situ characterization of their properties with respect to groundwater flow and solute transport. We have applied an integrated multiphysics approach to quantify the effect of dolerite dykes on saltwater intrusion in a coastal sandstone aquifer. The approach involved ground geophysical imaging (passive magnetics and electrical resistivity tomography), well hydraulic testing, and tidal propagation analysis, which provided constraints on the geometry of the dyke network, the subsurface saltwater distribution, and the sandstone hydrodynamic properties and connectivity. A three-dimensional variable-density groundwater model coupled with a resistivity model was further calibrated using groundwater and geophysical observations. A good agreement of model simulations with tide-induced head fluctuations, geophysically derived pore water salinities, and measured apparent resistivities was obtained when dykes' hydraulic conductivity, storativity, and effective porosity are respectively about 3, 1, and 1 orders of magnitude lower than the host aquifer. The presence of the dykes results in barrier-like alterations of groundwater flow and saltwater intrusion. Preferential flow paths occur parallel to observed dyke orientations. Freshwater inflows from upland recharge areas concentrate on the land-facing side of the dykes and saltwater penetration is higher on their sea-facing side. This has major implications for managing groundwater resources in dyke-intruded aquifers, including in coastal and island regions and provides wider insights on preferential pathways of groundwater flow and transport in highly heterogeneous aquifer systems.

Recycling an uplifted early foreland basin fill: An example from the Jaca basin (Southern Pyrenees, Spain)

NASA Astrophysics Data System (ADS)

Roigé, M.; Gómez-Gras, D.; Remacha, E.; Boya, S.; Viaplana-Muzas, M.; Teixell, A.

2017-10-01

In the northern Jaca basin (Southern Pyrenees), the replacement of deep-marine by terrestrial environments during the Eocene records a main drainage reorganization in the active Pyrenean pro-wedge, which leads to recycling of earlier foreland basin sediments. The onset of late Eocene-Oligocene terrestrial sedimentation is represented by four main alluvial fans: Santa Orosia, Canciás, Peña Oroel and San Juan de la Peña, which appear diachronously from east to west. These alluvial fans are the youngest preserved sediments deposited in the basin. We provide new data on sediment composition and sources for the late Eocene-Oligocene alluvial fans and precursor deltas of the Jaca basin. Sandstone petrography allows identification of the interplay of axially-fed sediments from the east with transversely-fed sediments from the north. Compositional data for the alluvial fans reflects a dominating proportion of recycled rock fragments derived from the erosion of a lower to middle Eocene flysch depocentre (the Hecho Group), located immediately to the north. In addition, pebble composition allows identification of a source in the North Pyrenean Zone that provided lithologies from the Cretaceous carbonate flysch, Jurassic dolostones and Triassic dolerites. Thus we infer this zone as part of the source area, located in the headwaters, which would have been unroofed from turbidite deposits during the late Eocene-Oligocene. These conclusions provide new insights on the response of drainage networks to uplift and topographic growth of the Pyrenees, where the water divide migrated southwards to its present day location.

The Archaen volcanic facies in the Migori segment, Nyanza greenstone belt, Kenya: stratigraphy, geochemistry and mineralisation

NASA Astrophysics Data System (ADS)

Ichang'l, D. W.; MacLean, W. H.

The Migori segment is an 80 by 20 km portion of the Nyanza greenstone belt which forms the northern part of the Archean Tanzanian Craton in western Kenya, northern Tanzania and southeastern Uganda. It consists of two volcanic centres, each with central, proximal and distal volcanic facies, comprising the Migori Group, the Macalder and Lolgorien Subgroups, and eleven volcano-sedimentary formations. The centres are separated by a basin of tuffs and greywacke turbidites. The volcanics are bimodal mafic basalt and dolerite ( Zr/Y = 3.8 - 6.5, La N/Yb N = 1.0 - 2.4) , and felsic calc-alkaline dacite-rhyolite ( Zr/Y = 10 - 21, La N/Yb N = 19 - 42 ) and high-K dacite ( Zr/Y = 9 - 16, La N/Yb N = 21 - 22 ). Felsic units form approximately three-fourths of the volcanic stratigraphy. Basalts, calc-alkaline dacites and rhyolites were deposited in a submarine environment, but the voluminous high-K dacites were erupted subaerially. The turbidites contain units of iron-formations. Granitic intrusions are chemically continuous with the high-K dacites. The felsic volcanics are anologous to those found at modern volcanic arc subduction settings involving continental crust. The Macalder ZnCuAuAg volcanogenic massive sulphide deposits is in central facies basalts-greywacke-rhyolite. Gold mineralisation occurs in proximal facies tuffs and iron formation, and in oblique and semi-conformable quartz veins. Greenstones in the Nyanza belt are dominated by calc-alkaline felsic volcanics in constrast to the komatiite-tholeiitic basalt volcanism in the Kaapvaal Craton of South Africa, and a mixture of the two types in the Zimbabwe Craton.

Evaluation of magma mixing in the subvolcanic rocks of Ghansura Felsic Dome of Chotanagpur Granite Gneiss Complex, eastern India

NASA Astrophysics Data System (ADS)

Gogoi, Bibhuti; Saikia, Ashima; Ahmad, Mansoor; Ahmad, Talat

2018-06-01

The subvolcanic rocks exposed in the Ghansura Felsic Dome (GFD) of the Bathani volcano-sedimentary sequence at the northern fringe of the Rajgir fold belt in the Proterozoic Chotanagpur Granite Gneiss Complex preserves evidence of magma mixing and mingling in mafic (dolerite), felsic (microgranite) and intermediate (hybrid) rocks. Structures like crenulated margins of mafic enclaves, felsic microgranular enclaves and ocelli with reaction surfaces in mafic rocks, hybrid zones at mafic-felsic contacts, back-veining and mafic flows in the granitic host imply magma mingling phenomena. Textural features like quartz and titanite ocelli, acicular apatite, rapakivi and anti-rapakivi feldspar intergrowths, oscillatory zoned plagioclase, plagioclase with resorbed core and intact rim, resorbed crystals, mafic clots and mineral transporting veins are interpreted as evidence of magma mixing. Three distinct hybridized rocks have formed due to varied interactions of the intruding mafic magma with the felsic host, which include porphyritic diorite, mingled rocks and intermediate rocks containing felsic ocelli. Geochemical signatures confirm that the hybrid rocks present in the study area are mixing products formed due to the interaction of mafic and felsic magmas. Physical parameters like temperature, viscosity, glass transition temperature and fragility calculated for different rock types have been used to model the relative contributions of mafic and felsic end-member magmas in forming the porphyritic diorite. From textural and geochemical investigations it appears that the GFD was a partly solidified magma chamber when mafic magma intruded it leading to the formation of a variety of hybrid rock types.

The Žermanice sill: new insights into the mineralogy, petrology, age, and origin of the teschenite association rocks in the Western Carpathians, Czech Republic

NASA Astrophysics Data System (ADS)

Matýsek, Dalibor; Jirásek, Jakub; Skupien, Petr; Thomson, Stuart N.

2018-04-01

The Žermanice locality represents the best-exposed example of the meta-basaltoid/meta-gabbroic rock type of the teschenite association. It forms a subhorizontal volcanic body (sill) 27-30 m thick. The subvolcanic rock is inhomogeneous and slightly differentiated. The predominant rock type is a basaltoid (diabase-dolerite), medium-grained, speckled, mesocratic rock exhibiting an evident subophitic texture. Miarolitic cavities are abundant in some places. The major rock constituents are albite, microcline, chlorite, and pyroxene, as well as analcime and plagioclase in places. The accessory magmatic phases are biotite, ilmenite, fluorapatite, sulphides, Ti-rich magnetite, Nb-rich baddeleyite, and chevkinite-(Ce) or perrierite-(Ce). A large extent of alteration is evident from the presence of chloritization, albitization of plagioclases, and zeolitization (analcimization). Geochemical analyses reveal an affinity for metaluminous igneous rocks. The best fit is with the within-plate basalts or the within-plate volcanic zones. The classification of this rock is problematic because of the mixed intrusive and extrusive features; the choice is between meta-alkali basalt and metadolerite (meta-microgabbro). 207Pb common lead-corrected U-Pb apatite dating yields a weighted mean age of 120.4 ± 9.6 Ma, which corresponds to the middle Aptian. The igneous body is at most ca. 10 Ma younger than the surrounding late Hauterivian sediments and might have been emplaced into unconsolidated or partly consolidated sediments. According to our research, it is evident that at least some teschenite association rocks are in fact low-grade metamorphic rocks.

The geology of the northern tip of the Arabian-Nubian Shield

NASA Astrophysics Data System (ADS)

Beyth, M.; Eyal, Y.; Garfunkel, Z.

2014-11-01

Recently, a detailed (1:50,000) geological map of the Elat area, southern Israel was published. Attached to this map is a stratigraphic table of the Neoproterozoic metamorphic-magmatic complex of the study area. The Neoproterozoic basement in the Elat area encapsulates the Arabian Nubian Shield (ANS) geologic evolution. Uranium-Lead and Lead-Lead zircon ages, included in previous studies and referred to in this paper, reveal that these rocks were formed during more than 300 million years of Neoproterozoic time. The major process controlling the formation of the ANS as part of the East African Orogen is the closure of the Mozambique Ocean. The first orogenic phase in the Elat area, represented by the metamorphic rocks, includes the development of an island arc, erosion of the islands and deposition, and metamorphism. This event took place between ∼950 Ma and 780-790 Ma. Elat Schist, the oldest metamorphic rock in the area, was deformed and then intruded by quartz dioritic and granitic plutons that were later deformed and metamorphosed. The amphibolite metamorphic rock facies indicate metamorphic conditions of up to 650 °C and between 4 and 5 kbar. The peak of the metamorphic event was most probably before 750 Ma. A gradual change from compressional to extensional stress regime is evidenced by emplacement andesitic magnesium-rich dykes dated to 705 Ma that were later metamorphosed to schistose dykes at a greenschist metamorphic facies. The second orogenic phase (terrane amalgamation, main shaping of crust) was associated with the emplacement of large volumes (>50% of area) of calc-alkaline intrusions in a post-collision setting. These very last stages of metamorphism and deformation are characterized by intrusion of ∼630 Ma granitoids exhibiting some foliation. Pluton emplacement continued also after the end of deformation. Exhumation and transition to an extensional regime is recorded by the intrusion of shallow alkaline granites in ∼608 Ma which were accompanied in ∼609 Ma by rhyolite, andesite and composite dykes. The last magmatic event in the Elat area is represented by the volcano-conglomeratic series comprising rhyolites, basalts, andesites, hypabyssal intrusions of monzonite and syenite and conglomerates. The conglomerates, dated to about 590 Ma, are the products of a major erosion phase in which about 12,000 m of the section were removed. These conglomerates were intruded by 585 Ma rhyolite, andesite and composite dykes. The Neoproterozoic basement is truncated by a peneplain whose age, post 532 Ma, is constrained by the age of the youngest eroded dolerite dykes. This Early Cambrian peneplain was associated with erosion of 2000 m of the section and by chemical weathering. Three major breaks in Neoproterozoic magmatic activity are recognized: the first, occurred in Cryogenian time, lasted ∼60 million years after the amphibolite facies metamorphism and before emplacement of the calc alkaline plutons, separating the first and the second orogenic phases; the second break between the orogenic and the extensional phases occurred in early Ediacaran time, encompassed ∼20 million years between the emplacement of the calc-alkaline and alkaline plutonic rocks and rhyolite, andesite and the composite dykes; and the third, ∼50 Ma break, occurred between the emplacement of the last felsic intrusions at ∼585 Ma and intrusion of the dolerite dykes in 532 Ma, before the Early Cambrian peneplain developed. The great lateral extension of the Cambrian to Eocene sedimentary rocks and their slow facies and thickness changes suggest a stable flat platform area at the northern tip of the ANS. Early Cambrian sedimentation began with fluviatile subarkoses of the Amudei Shlomo Formation. It was overlain by an Early to Middle Cambrian transgressive-regressive lagoonal cycle of dolostones, sandstones, and siltstones of the Timna Formation. Then Middle Cambrian subarkoses and siltstones of the Shehoret Formation and the quartz arenite of the Netafim Formation were deposited in a coastal, intertidal environment representing the southern transgression of a Cambrian ocean.

Overview of micro-dam reservoirs (MDR) in Tigray (northern Ethiopia): Challenges and benefits

NASA Astrophysics Data System (ADS)

Berhane, Gebremedhin; Gebreyohannes, Tesfamichael; Martens, Kristine; Walraevens, Kristine

2016-11-01

Water scarcity is a key factor in food security and sustainable livelihood in sub-Saharan Africa, particularly in East Africa. The problem is severe in many parts of Ethiopia where water plays a central role in the country's economy. To alleviate and curb water scarcity different water harvesting technologies were introduced in Ethiopia during the last two decades; nevertheless their sustainability and livelihood impacts are not well addressed. For the first time a complete and comprehensive inventory of micro-dam reservoirs (MDRs) in Tigray has been established including the geological background and currently observed problems. The inventory of 92 MDRs in Tigray was conducted using the direct field observational method, selected interviews and secondary data, to understand the overall situation of the schemes from engineering geological and geo-hydrological perspectives and its implication to sustainability and water availability. Analysis of the inventory shows that sustainability and livelihood impact of the water harvesting schemes are threatened by siltation, leakage, insufficient run-off, poor water management and structural damages on the dam body as well as on irrigation infrastructure and spillway. Basic statistical analysis showed that 61% of them are found to have siltation problems, 53% suffer from leakage, 22% from insufficient inflow, 25% have structural damages and 21% have spillway erosion problems. Furthermore, nearly 70% of the MDRs are founded on carbonate dominant sedimentary terrain at places with intrusion of dolerite sills/dykes and the problems of siltation and leakage are found to be extremely high in the MDRs located in such geological setting, as compared to those on crystalline metamorphic rocks. Lack of proper water management was observed in most of the reservoirs with irrigation practices. Future research and analysis on the causes of the bottlenecked problems and monitoring surveillance are recommended.

Formation of Si-Al-Mg-Ca-rich zoned magnetite in an end-Permian phreatomagmatic pipe in the Tunguska Basin, East Siberia

NASA Astrophysics Data System (ADS)

Neumann, Else-Ragnhild; Svensen, Henrik H.; Polozov, Alexander G.; Hammer, Øyvind

2017-12-01

Magma-sediment interactions in the evaporite-rich Tunguska Basin resulted in the formation of numerous phreatomagmatic pipes during emplacement of the Siberian Traps. The pipes contain magnetite-apatite deposits with copper and celestine mineralization. We have performed a detailed petrographic and geochemical study of magnetite from long cores drilled through three pipe breccia structures near Bratsk, East Siberia. The magnetite samples are zoned and rich in Si (≤5.3 wt% SiO2), Ca, Al, and Mg. They exhibit four textural types: (1) massive ore in veins, (2) coating on breccia clasts, (3) replacement ore, and (4) reworked ore at the crater base. The textural types have different chemical characteristics. "Breccia coating" magnetite has relatively low Mg content relative to Si, as compared to the other groups, and appears to have formed at lower oxygen fugacity. Time series analyses of MgO variations in microprobe transects across Si-bearing magnetite in massive ore indicate that oscillatory zoning in the massive ore was controlled by an internal self-organized process. We suggest that hydrothermal Fe-rich brines were supplied from basalt-sediment interaction zones in the evaporite-rich sedimentary basin, leading to magnetite ore deposition in the pipes. Hydrothermal fluid composition appears to be controlled by proximity to dolerite fragments, temperature, and oxygen fugacity. Magnetite from the pipes has attributes of iron oxide-apatite deposits (e.g., textures, oscillatory zoning, association with apatite, and high Si content) but has higher Mg and Ca content and different mineral assemblages. These features are similar to magnetite found in skarn deposits. We conclude that the Siberian Traps-related pipe magnetite deposit gives insight into the metamorphic and hydrothermal effects following magma emplacement in a sedimentary basin.

Unraveling the unusual morphology of the Cretaceous Dirck Hartog extinct mid-ocean ridge

NASA Astrophysics Data System (ADS)

Watson, S. J.; Whittaker, J. M.; Halpin, J.; Williams, S.; Milan, L. A.; Daczko, N. R.; Wyman, D. A.

2015-12-01

The Perth Abyssal Plain (PAP), offshore southwest Australia formed during Mesozoic East Gondwana breakup and Kerguelen plume activity. This study combines petrographic and geochemical data from the first samples ever to be dredged from the flanks of the Dirck Hartog Ridge (DHR), a prominent linear bathymetric feature in the central PAP, with new bathymetric profiles across the PAP to better constrain the formation of the early Indian Ocean floor. The DHR exhibits high relief and distinctive asymmetry that is unusual compared to most active or extinct spreading centres and likely results from compression and deformation of the recently extinct DHR during changes in relative motion of the Indian plate (110 - 100 Ma). Exhumation of gabbros in the southern DHR and an increase in seafloor roughness towards the centre of the PAP, likely result from a half spreading rate decrease from 35 mm/yr (based on magnetic reversals) to 24 mm/yr at ~114 Ma. The results support a slowdown of spreading prior to full cessation at ~102 Ma. The composition of basaltic samples varies along the DHR: from sub-alkaline dolerites with incompatible element concentrations most similar to depleted-to-normal mid-ocean ridge basalts in the south, to alkali basalts similar to ocean island basalts in the north. Therefore, magma sources and degrees of partial melting varied in space and time, a result supporting the interpretation that the DHR is an extinct spreading ridge rather than a pseudofault. The enriched alkali basalt signatures may be attributed to melting of a heterogeneous mantle or to the influence of the Kerguelen plume over distances greater than 1000 km. The results demonstrate the significance of regional tectonic plate motions on the formation and deformation of young ocean crust, and provide insight into the unique DHR morphology.

Relating Seismicity to Dike Emplacement, and the Conundrum of Dyke-Parallel Faulting

NASA Astrophysics Data System (ADS)

Dering, G.; Micklethwaite, S.; Cruden, A. R.; Barnes, S. J.; Fiorentini, M. L.

2016-12-01

Seismic monitoring shows that faulting and fracturing precede and accompany magma emplacement on timescales of hours and days. One outstanding problem is that the precision of earthquake hypocentre locations is typically limited to tens or hundreds of meters and cannot resolve whether the hypocentres relate to strain of wall rock fragments within the dikes, in a process zone around the intrusion or peripherally in the country rock. We examine a swarm of 19 dolerite dikes, near Albany, Western Australia using an unmanned aerial vehicle and Structure-from-Motion photogrammetry to obtain accurate, high resolution 3D reconstructions of outcrop and to digitally extract structural data. We find rare overprinting relationships indicate dike emplacement and faulting was coeval and that the number of faults/fractures increase into the dike swarm (2.2 ± 0.7 more fractures, per unit length in host rocks intruded by dikes relative to the background value). The faults are cataclasite-bearing and parallel to the dikes but intriguingly dike emplacement appears to have been accommodated by mode 1 extension. We further provide the first evidence that dike-parallel shear failure occurs in a damage zone associated with the dike swarm. These results support seismological observations of dike-parallel shear failure associated with some intrusion events, which contradict Mohr-Coulomb theory and numerical modelling of dike propagation in brittle-elastic rock, where shear failure is predicted to occur on faults oriented approximately 30° to the dyke plane. We suggest the dike swarm occupies a network of joints and fractures formed prior to swarm emplacement but then reactivated ahead of propagating dikes and remaining active during the early stages of emplacement.

Morphogenesis of Antarctic Paleosols: Martian Analogue

NASA Astrophysics Data System (ADS)

Mahaney, W. C.; Dohm, J. M.; Baker, V. R.; Newsom, Horton E.; Malloch, D.; Hancock, R. G. V.; Campbell, Iain; Sheppard, D.; Milner, M. W.

2001-11-01

Samples of horizons in paleosols from the Quartermain Mountains of the Antarctic Dry Valleys (Aztec and New Mountain areas) were analyzed for their physical characteristics, mineralogy, chemical composition, and microbiology to determine the accumulation and movement of salts and other soluble constituents and the presence/absence of microbial populations. Salt concentrations are of special interest because they are considered to be a function of age, derived over time, in part from nearby oceanic and high-altitude atmospheric sources. The chemical composition of ancient Miocene-age paleosols in these areas is the direct result of the deposition and weathering of airborne-influxed salts and other materials, as well as the weathering of till derived principally from local dolerite and sandstone outcrops. Paleosols nearer the coast have greater contents of Cl, whereas near the inland ice sheet, nitrogen tends to increase on a relative basis. The accumulation and vertical distribution of salts and other soluble chemical elements indicate relative amounts of movement in the profile over long periods of time, in the order of several million years. Four of the six selected subsamples from paleosol horizons in two ancient soil profiles contained nil concentrations of bacteria and fungi. However, two horizons at depths of between 3 and 8 cm, in two profiles, yielded several colonies of the fungi Beauveria bassiana and Penicillium brevicompactum, indicating very minor input of organic carbon. Beauveria bassiana is often reported in association with insects and is used commercially for the biological control of some insect pests. Penicillium species are commonly isolated from Arctic, temperate, and tropical soils and are known to utilize a wide variety of organic carbon and nitrogen compounds. The cold, dry soils of the Antarctic bear a close resemblance to various present and past martian environments where similar weathering could occur and possible microbial populations may exist.

Toward an understanding of disequilibrium dihedral angles in mafic rocks

USGS Publications Warehouse

Holness, Marian B.; Humphreys, Madeleine C.S.; Sides, Rachel; Helz, Rosalind T.; Tegner, Christian

2012-01-01

The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109˚ {plus minus} 2˚). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. Firstly, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78˚, rather than 60˚ which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intra-plutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts, associated with rounding of chadacrysts.

Origin of magnetic highs at ultramafic hosted hydrothermal systems: Insights from the Yokoniwa site of Central Indian Ridge

NASA Astrophysics Data System (ADS)

Fujii, Masakazu; Okino, Kyoko; Sato, Taichi; Sato, Hiroshi; Nakamura, Kentaro

2016-05-01

High-resolution vector magnetic measurements were performed on an inactive ultramafic-hosted hydrothermal vent field, called Yokoniwa Hydrothermal Field (YHF), using a deep-sea manned submersible Shinkai6500 and an autonomous underwater vehicle r2D4. The YHF has developed at a non-transform offset massif of the Central Indian Ridge. Dead chimneys were widely observed around the YHF along with a very weak venting of low-temperature fluids so that hydrothermal activity of the YHF was almost finished. The distribution of crustal magnetization from the magnetic anomaly revealed that the YHF is associated with enhanced magnetization, as seen at the ultramafic-hosted Rainbow and Ashadze-1 hydrothermal sites of the Mid-Atlantic Ridge. The results of rock magnetic analysis on seafloor rock samples (including basalt, dolerite, gabbro, serpentinized peridotite, and hydrothermal sulfide) showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of Yokoniwa. These observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite. Comparisons with the Rainbow and Ashadze-1 suggest that in ultramafic-hosted hydrothermal systems, strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite. After the completion of serpentinization and production of hydrogen, pyrrhotites convert into pyrite or nonmagnetic iron sulfides, which considerably reduces their levels of magnetization. Our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic-hosted hydrothermal systems. Furthermore, the results highlight the use of near-seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems.

Geochemistry and petrology of the indium-bearing polymetallic skarn ores at Pitkäranta, Ladoga Karelia, Russia

NASA Astrophysics Data System (ADS)

Valkama, M.; Sundblad, K.; Cook, N. J.; Ivashchenko, V. I.

2016-08-01

The historic mining district of Pitkäranta in the Ladoga region, Fennoscandian Shield, was exploited for Fe, Cu, Zn, Pb, Sn and Ag in the nineteenth to twentieth centuries. The Pitkäranta region is dominated by Palaeoproterozoic supracrustal rocks, which, together with gneissic Archaean dome structures, constitute an allochthonous terrane complex that amalgamated to the Archaean continent during the Svecokarelian orogeny at 1.9-1.8 Ga. This crustal complex was intruded by 1.8 Ga Late orogenic granites, 1.54 Ga anorogenic rapakivi granites and 1.45 Ga dolerites. The polymetallic skarn ores of Pitkäranta extend over a 25-km-long zone in Palaeoproterozoic supracrustal rocks and formed from hydrothermal solutions, which emanated from the anorogenic rapakivi granites and reacted with marble layers. Four major ore types are recognised after the dominating metal: Fe, Cu, Sn and Zn, respectively. These types are not restricted to individual mines or mine fields but represent end members in zonation patterns within each ore body. Pitkäranta was the second discovery site in the world for indium but has been without modern documentation for more than 75 years. The indium contents in the ores are up to 600 ppm, in most cases sphalerite-hosted. The only roquesite-bearing sample in this study had an indium grade of 291 ppm and an In/Zn ratio of 51 (close to the criteria for the limiting conditions for creating an In-rich mineral). The Pitkäranta ores have a potential for future small-scale exploitation, but all such plans are hampered by high contents if Bi, Cd and As.

Geological hazards associated with intense rain and flooding in Natal

NASA Astrophysics Data System (ADS)

Thomas, M. A.; van Schalkwyk, A.

1993-02-01

The combination of rugged topography and climate predisposes the province of Natal to severe floods. Information available since 1856 shows that bridge and slope failures have been recorded in twenty out of twenty-five flood episodes. Bridge failures are caused mostly by geological factors. The mechanism of failure can be classified broadly into foundation failures and changes of river course. Scour and debris build-up have led to failures of foundations located in rock and alluvial sediments. In preparing and replacing bridges the aims have been to increase the area of waterway, increase foundation depths to reach more competent strata and lay protection along banks and abutments to counteract scour. Historically, slope failures have not been well documented but following the 1987/88 storms 223 slope failures were recorded. The classification of the failures allowed the mechanisms of failure to be ascertained, and general design considerations to be reviewed. In areas adjacent to the Drakensberg Mountains slope failures are part of a natural erosion cycle which may be accelerated in periods of heavy rain. Throughout Natal, hummocky ground adjacent to dolerite intrusions reveals the on-going history of failure caused by water ingress and the generation of high pore water pressures on the slip planes. Classic flows occurred throughout the Greater Durban area where residual sandy soils of the Natal Group sandstone became supersaturated. Slumping was common on steep terrain underlain by granite-gneiss in the Kwa-Zulu area. Shales of the Pietermaritzburg Formation are notoriously unstable, yet few failures occurred during the summer storms of 1987/88. Inadequate drainage was responsible for many failures, this was particularly so along the railways.

Basalts erupted along the Tongan fore arc during subduction initiation: Evidence from geochronology of dredged rocks from the Tonga fore arc and trench

NASA Astrophysics Data System (ADS)

Meffre, Sebastian; Falloon, Trevor J.; Crawford, Tony J.; Hoernle, Kaj; Hauff, Folkmar; Duncan, Robert A.; Bloomer, Sherman H.; Wright, Dawn J.

2012-12-01

A wide variety of different rock types were dredged from the Tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 Boomerang voyage. 40Ar-39Ar whole rock and U-Pb zircon dating suggest that these fore arc rocks were erupted episodically from the Cretaceous to the Pliocene (102 to 2 Ma). The geochemistry suggests that MOR-type basalts and dolerites were erupted in the Cretaceous, that island arc tholeiites were erupted in the Eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time. The ages generally become younger northward suggesting that fore arc crust was created in the south at around 48-52 Ma and was extended northward between 35 and 28 Ma, between 9 and 15 Ma and continuing to the present-day. The episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the Bonin-Marianas arc. The Bonin-Marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 Ma at the beginning of subduction above a rapidly foundering west-dipping slab. Instead a model where the 52 Ma basalts that are presently in a fore arc position were created in the arc-back arc transition behind the 57-35 Ma Loyalty-Three Kings arc and placed into a fore arc setting after arc reversal following the start of collision with New Caledonia is proposed for the oldest rocks in Tonga. This is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time.

Assessing Planetary Habitability: Don't Forget Exotic Life!

NASA Astrophysics Data System (ADS)

Schulze-Makuch, Dirk

2012-05-01

With the confirmed detection of more than 700 exoplanets, the temptation looms large to constrain the search for extraterrestrial life to Earth-type planets, which have a similar distance to their star, a similar radius, mass and density. Yet, a look even within our Solar System points to a variety of localities to which life could have adapted to outside of the so-called Habitable Zone (HZ). Examples include the hydrocarbon lakes on Titan, the subsurface ocean environment of Europa, the near- surface environment of Mars, and the lower atmosphere of Venus. Recent Earth analog work and extremophile investigations support this notion, such as the discovery of a large microbial community in a liquid asphalt lake in Trinidad (as analog to Titan) or the discovery of a cryptoendolithic habitat in the Antarctic desert, which exists inside rocks, such as beneath sandstone surfaces and dolerite clasts, and supports a variety of eukaryotic algae, fungi, and cyanobacteria (as analog to Mars). We developed a Planetary Habitability Index (PHI, Schulze-Makuch et al., 2011), which was developed to prioritize exoplanets not based on their similarity to Earth, but whether the extraterrestrial environment could, in principle, be a suitable habitat for life. The index includes parameters that are considered to be essential for life such as the presence of a solid substrate, an atmosphere, energy sources, polymeric chemistry, and liquids on the planetary surface. However, the index does not require that this liquid is water or that the energy source is light (though the presence of light is a definite advantage). Applying the PHI to our Solar System, Earth comes in first, with Titan second, and Mars third.

Flexural bending-induced plumelets and their seamounts in accretionary (Japanese-style) and collisional (Tethyan-style) orogenic belts

NASA Astrophysics Data System (ADS)

Hirano, N.; Dilek, Y.

2015-12-01

Seamounts and seamount chains are common in both the upper and lower plates of active subduction zones. Their OIB-type volcanic products are distinctly different from suprasubduction zone (arc, forearc and backarc) generated volcanic rocks in terms of their compositions and mantle sources. Tectonic accretion of such seamounts into the Japanese archipelago in the NW Pacific and into subduction-accretion complexes and active margins of continents/microcontinents within the Tethyan realm during the Cretaceous played a significant role in continental growth. Seamount assemblages comprise alkaline volcanic rocks intercalated with radiolarian and hemipelagic chert, and limestone, and may also include hypabyssal dolerite and gabbro intrusions. In the Tethyan orogenic belts these seamount rocks commonly occur as km-scale blocks in mélange units beneath the late Jurassic - Cretaceous ophiolites nappes, whereas on the Japanese islands they form discrete, narrow tectonic belts within the late Jurassic - Cretaceous accretionary prism complexes. We interpret some of these OIB occurrences in the Japanese and Tethyan mountain belts as asperities in downgoing oceanic plates that formed in <10 million years before their accretion. Their magmas were generated by decompressional melting of upwelling asthenosphere, without any significant mantle plume component, and were brought to the seafloor along deep-seated brittle fractures that developed in the flexed, downgoing lithosphere as it started bending near a trench. The modern occurrences of these "petit-spot volcanoes" are well established in the northwestern Pacific plate, off the coast of Japan. The proposed mechanism of the formation of these small seamounts better explains the lack of hotspot trails associated with their occurrence in the geological record. Magmatic outputs of such flexural bending-induced plumelets should be ubiquitious in the accretionary (Japanese-style) and collisional (Tethyan-style) orogenic belts.

Comprehensive Pb-Sr-Nd-Hf isotopic, trace element, and mineralogical characterization of mafic to ultramafic rock reference materials

NASA Astrophysics Data System (ADS)

Fourny, Anaïs.; Weis, Dominique; Scoates, James S.

2016-03-01

Controlling the accuracy and precision of geochemical analyses requires the use of characterized reference materials with matrices similar to those of the unknown samples being analyzed. We report a comprehensive Pb-Sr-Nd-Hf isotopic and trace element concentration data set, combined with quantitative phase analysis by XRD Rietveld refinement, for a wide range of mafic to ultramafic rock reference materials analyzed at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. The samples include a pyroxenite (NIM-P), five basalts (BHVO-2, BIR-1a, JB-3, BE-N, GSR-3), a diabase (W-2), a dolerite (DNC-1), a norite (NIM-N), and an anorthosite (AN-G); results from a leucogabbro (Stillwater) are also reported. Individual isotopic ratios determined by MC-ICP-MS and TIMS, and multielement analyses by HR-ICP-MS are reported with 4-12 complete analytical duplicates for each sample. The basaltic reference materials have coherent Sr and Nd isotopic ratios with external precision below 50 ppm (2SD) and below 100 ppm for Hf isotopes (except BIR-1a). For Pb isotopic reproducibility, several of the basalts (JB-3, BHVO-2) require acid leaching prior to dissolution. The plutonic reference materials also have coherent Sr and Nd isotopic ratios (<50 ppm), however, obtaining good reproducibility for Pb and Hf isotopic ratios is more challenging for NIM-P, NIM-N, and AN-G due to a variety of factors, including postcrystallization Pb mobility and the presence of accessory zircon. Collectively, these results form a comprehensive new database that can be used by the geochemical community for evaluating the radiogenic isotope and trace element compositions of volcanic and plutonic mafic-ultramafic rocks.

Re-Os geochronology of a Mesoproterozoic sedimentary succession, Taoudeni basin, Mauritania: Implications for basin-wide correlations and Re-Os organic-rich sediments systematics

NASA Astrophysics Data System (ADS)

Rooney, Alan D.; Selby, David; Houzay, Jean-Pierre; Renne, Paul R.

2010-01-01

The exceptionally well-preserved sedimentary rocks of the Taoudeni basin, NW Africa represent one of the world's most widespread (> 1 M km 2) Proterozoic successions. Hitherto, the sedimentary rocks were considered to be Mid Tonian based on Rb-Sr illite and glauconite geochronology of the Atar Group. However, new Re-Os organic-rich sediment (ORS) geochronology from two drill cores indicates that the Proterozoic Atar Group is ˜ 200 Ma older (1107 ± 12 Ma, 1109 ± 22 Ma and 1105 ± 37 Ma). The Re-Os geochronology suggests that the Rb-Sr geochronology records the age of diagenetic events possibly associated with the Pan African collision. The new Re-Os geochronology data provide absolute age constraints for recent carbon isotope chemostratigraphy which suggests that the Atar Group is Mesoproterozoic and not Neoproterozoic. The new Re-Os ORS geochronology supports previous studies that suggest that rapid hydrocarbon generation (flash pyrolysis) from contact metamorphism of a dolerite sill does not significantly disturb the Re-Os ORS systematics. Modelled contact conditions suggest that the Re-Os ORS systematics remain undisturbed at ˜ 650 °C at the sill/shale contact and ≥ 280 °C 20 m from the sill/shale contact. Moreover, the Re-Os geochronology indicates that the West African craton has a depositional history that predates 1100 Ma and that ORS can be correlated on a basin-wide scale. In addition, the Re-Os depositional ages for the ORS of the Taoudeni basin are comparable to those of ORS from the São Francisco craton, suggesting that these cratons are correlatable. This postulate is further supported by identical Os i values for the Atar Group and the Vazante Group of the São Francisco craton.

Characterizing the nature of melt-rock reaction in peridotites from the Santa Elena Ophiolite, NW Costa Rica

NASA Astrophysics Data System (ADS)

Carr, D.; Loocke, M. P.; Snow, J. E.; Gazel, E.

2017-12-01

The Santa Elena Ophiolite (SEO), located on the northwestern coast of Costa Rica, consists primarily of preserved oceanic mantle and crustal rocks thrust above an accretionary complex. The SEO is predominantly characterized by mantle peridotites (i.e., primarily spinel lherzolite with minor amounts of harzburgite and dunite) cut and intruded by minor pegmatitic gabbros, layered gabbros, plagiogranites, and doleritic and basaltic dykes. Previous studies have concluded that the complex formed in a suprasubduction zone (SSZ) setting based on the geochemical nature of the layered gabbros and plagiogranites (i.e., depleted LREE and HFSE and enriched LILE and Pb), as well, as the peridotites (i.e., low-TiO2, Zr, and V, and high MgO, Cr, and Ni)(Denyer and Gazel, 2009). Eighteen ultramafic samples collected during the winter 2010/2011 field season (SECR11) exhibit abundant evidence for melt-rock reaction (e.g., disseminated plagioclase and plagioclase-spinel, clinopyroxene-spinel, and plagioclase-clinopyroxene symplectites) and provide a unique opportunity to characterize the textural and chemical nature of melt-rock reaction in the SEO. We present the results of a petrologic investigation (i.e., petrography and electron probe microanalysis) of 28 thin sections (19 spinel lherzolites, of which 14 are plagioclase-bearing, 4 pyroxenite veins, and 5 harzburgites) derived from the SECR11 sample set. The results of this investigation have the potential to better our understanding of the nature of melt generation and migration and melt-rock interaction in the SEO mantle section and shed further light on the complex petrogenetic history of the SEO. Denyer, P., Gazel, E., 2009, Journal of South American Earth Sciences, 28:429-442.

Applications of Cosmogenic He-3 and Ne-21 Dating to Glacial Moraines in Antarctica and California

NASA Astrophysics Data System (ADS)

Sams, S.; Morgan, D. J.; Balco, G.; Putkonen, J.; Bibby, T.

2015-12-01

The depositional age of moraines can be determined through cosmogenic nuclide exposure dating. These ages are useful in establishing a glacial history of an area and ascribing age constraints to transport processes. Be-10 is the most common nuclide used for exposure dating today, but this method is both expensive and time consuming because it requires analysis by accelerator mass spectrometry (AMS). He-3 and Ne-21 can be analyzed using noble gas mass spectrometry, which is more cost efficient than AMS and requires less chemical preparation. We collected samples from areas in Moraine Canyon, Antarctica (86.10° S, 157.75° W), which is a dry valley in the Transantarctic Mountains. Dolerite boulders along a transect of recessional moraines were sampled in the typical fashion of using a large piece of the boulder for analysis. Pyroxene minerals have been separated from these samples following the method of Bromley et al. (2014) using hydrofluoric acid. Exposure ages will be calculated from the He-3 concentrations in them. In the Mono Lake area of California, moraines were sampled from Bloody Canyon and McGee Creek sites. Instead of collecting a sample from an individual boulder, we collected approximately 25 granitic pebbles (1-3 cm) from 4-6 sites along the crest of the moraines following the method of Briner (2009). Each suite of pebbles was crushed together, and quartz minerals were separated from the agglomeration of pebbles. Cosmogenic Ne-21 will be measured from these samples to determine their exposure age. From these two field sites, we will use He-3 and Ne-21 to better understand the timing and extent of glaciation in these areas.

Pre-stack depth migration in an anisotropic crystalline environment at the COSC-1 borehole, central Sweden

NASA Astrophysics Data System (ADS)

Simon, H.; Buske, S.; Hedin, P.; Juhlin, C.; Krauß, F.; Giese, R.

2017-12-01

The Scandinavian Caledonides represent a well preserved deeply eroded Palaeozoic orogen, formed by the collision of the two palaeocontinents Baltica and Laurentia. Today, after four hundred million years of erosion along with uplift and extension during the opening of the North Atlantic Ocean, the geological structure in central western Sweden consists of allochthons, underlying autochthonous units, and the shallow west-dipping décollement that separates the two and is associated with Cambrian black shales. The project Collisional Orogeny in the Scandinavian Caledonides (COSC) aims to investigate these structures and their physical conditions with two approximately 2.5 km deep fully cored scientific boreholes in central Sweden. The first borehole COSC-1 was successfully drilled in 2014 and obtained a continuous cored section through the highly deformed Seve Nappe. After drilling was completed several surface and borehole based seismic experiments were conducted. The data from a multi-azimuthal walkaway VSP in combination with long offset surface lines was used to image the structures in the vicinity of the borehole. Clear differences in vertical and horizontal P-wave velocities made it necessary to also account for anisotropy. The resulting VTI velocity model provides the basis for subsequent application of seismic imaging approaches. An anisotropic eikonal solver was used to calculate the traveltimes needed for Kirchhoff-based pre-stack depth migration methods. The resulting images were compared to the corresponding migration results based on an isotropic velocity model. Both images are dominated by strong and clear reflections, however, they appear more continuous and better focused in the anisotropic result. Most of the dominant reflections originate below the bottom of the borehole and therefore they are probably situated within the Precambrian basement. They might represent dolerite intrusions or deformation zones of Caledonian or pre-Caledonian age.

Pitted rock surfaces on Mars: A mechanism of formation by transient melting of snow and ice

NASA Astrophysics Data System (ADS)

Head, James W.; Kreslavsky, Mikhail A.; Marchant, David R.

2011-09-01

Pits in rocks on the surface of Mars have been observed at several locations. Similar pits are observed in rocks in the Mars-like hyperarid, hypothermal stable upland zone of the Antarctic Dry Valleys; these form by very localized chemical weathering due to transient melting of small amounts of snow on dark dolerite boulders preferentially heated above the melting point of water by sunlight. We examine the conditions under which a similar process might explain the pitted rocks seen on the surface of Mars (rock surface temperatures above the melting point; atmospheric pressure exceeding the triple point pressure of H2O; an available source of solid water to melt). We find that on Mars today each of these conditions is met locally and regionally, but that they do not occur together in such a way as to meet the stringent requirements for this process to operate. In the geological past, however, conditions favoring this process are highly likely to have been met. For example, increases in atmospheric water vapor content (due, for example, to the loss of the south perennial polar CO2 cap) could favor the deposition of snow, which if collected on rocks heated to above the melting temperature during favorable conditions (e.g., perihelion), could cause melting and the type of locally enhanced chemical weathering that can cause pits. Even when these conditions are met, however, the variation in heating of different rock facets under Martian conditions means that different parts of the rock may weather at different times, consistent with the very low weathering rates observed on Mars. Furthermore, as is the case in the stable upland zone of the Antarctic Dry Valleys, pit formation by transient melting of small amounts of snow readily occurs in the absence of subsurface active layer cryoturbation.

Microbiology and Geochemistry of Antarctic Paleosols

NASA Astrophysics Data System (ADS)

Mahaney, W. C.; Malloch, D.; Hancock, R. G. V.; Campbell, I. B.; Sheppard, D.

2000-08-01

Samples of ancient soils from horizons in paleosols from the Quartermain Mountains (Aztec and New Mountain areas of the Antarctic Dry Valleys) were analyzed for their chemical composition and microbiology to determine the accumulation and movement of salts and other soluble constituents. The salt concentrations are of special interest because they are considered to be a function of age, derived in part from nearby oceanic and high altitude atmospheric sources. The geochemistry of ancient Miocene-age paleosols in these areas is the direct result of the deposition and weathering of till, derived principally from dolerite and sandstone source rock, in association with airborne-influxed salts. Paleosols nearer the coast have greater contents of chlorine, and farther inland near the Inland Ice Sheet, nitrogen tends to increase on a relative basis. The accumulation and vertical distribution of salts and other soluble chemical elements indicate relative amounts of movement in the profile over long periods of time, to the order of several million years. Iron, both in total concentration and in the form of various extracts, indicates it can be used as a geochronometer to assess the buildup of goethite plus hematite over time in the paleosols. Trends for ferrihydrite, a partially soluble Fe-hydroxide, shows limited profile translocation that might be related to the movement of salt. Six of the eight selected subsamples from paleosol horizons in three soil profiles contained nil concentrations of bacteria and fungi. However, two horizons at depths of between three to eight centimeters yielded several colonies of the fungi Beauveria bassiana and Penicillium spp., indicating some input of organic carbon. Beauveria bassiana is often reported in association with insects and is used commercially for the biological control of some insect pests. Penicillium species are commonly isolated from Arctic, temperate and tropical soils and are known to utilize a wide variety of organic carbon and nitrogen compounds.

Extraction of remanent magnetization from magnetization vector inversions of airborne full tensor magnetic gradiometry data

NASA Astrophysics Data System (ADS)

Queitsch, M.; Schiffler, M.; Stolz, R.; Meyer, M.; Kukowski, N.

2017-12-01

Measurements of the Earth's magnetic field are one of the most used methods in geophysical exploration. The ambiguity of the method, especially during modeling and inversion of magnetic field data sets, is one of its biggest challenges. Additional directional information, e.g. gathered by gradiometer systems based on Superconducting Quantum Interference Devices (SQUIDs), will positively influence the inversion results and will thus lead to better subsurface magnetization models. This is especially beneficial, regarding the shape and direction of magnetized structures, especially when a significant remanent magnetization of the underlying sources is present. The possibility to separate induced and remanent contributions to the total magnetization may in future also open up advanced ways for geological interpretation of the data, e.g. a first estimation of diagenesis processes. In this study we present the results of airborne full tensor magnetic gradiometry (FTMG) surveys conducted over a dolerite intrusion in central Germany and the results of two magnetization vector inversions (MVI) of the FTMG and a conventional total field anomaly data set. A separation of the two main contributions of the acquired total magnetization will be compared with information of the rock magnetization measured on orientated rock samples. The FTMG inversion results show a much better agreement in direction and strength of both total and remanent magnetization compared to the inversion using only total field anomaly data. To enhance the separation process, the application of additional geophysical methods, i.e. frequency domain electromagnetics (FDEM), in order to gather spatial information of subsurface rock susceptibility will also be discussed. In this approach, we try to extract not only information on subsurface conductivity but also the induced magnetization. Using the total magnetization from the FTMG data and the induced magnetization from the FDEM data, the full separation of induced and remanent magnetization should be enabled. First results this approach will be shown and discussed.

Paleomagnetism and magnetic fabric of the Triassic rocks from Spitsbergen

NASA Astrophysics Data System (ADS)

Dudzisz, K.; Szaniawski, R.; Michalski, K.; Manby, G.

2017-12-01

Understanding the origin and directions of the natural remanent magnetization and the tectonic deformation pattern reflected in magnetic fabric is of importance for investigation of the West Spitsbergen Fold and Thrust Belt (WSFTB) and its foreland. Previous research carried out on Triassic rocks from the study area concluded that these rocks record a composite magnetization of both, normal and reverse polarity, consisting of a primary Triassic remanence that is overlapped by a secondary post-folding component. Standard paleomagnetic procedures were conducted in order to determine the remanence components and a low-field AMS was applied to assess the degree and pattern of deformation. The AMS results from the WSFTB reveal a magnetic foliation that parallels the bedding planes and a dominantly NNW-SSE oriented magnetic lineation that is sub-parallel to the regional fold axial trend. These results imply a low to moderate degree of deformation and a maximum strain orientation parallel to that of the fold belt. These data are consistent with an orthogonal convergence model for the WSFTB formation. In turn, the magnetic fabric on the undeformed foreland displays a distinct NNE-SSW orientation that we attribute to the paleocurrent direction. Rock-magnetic analyses reveal that the dominant ferrimagnetic carriers are magnetite and titanomagnetite. The Triassic rocks are characterised by complicated NRM patterns often with overlapping unblocking temperature spectra of particular components. The dominant magnetisation is characterised, however, by a steep inclination of 70-80º. The derived paleomagnetic direction from the WSFTB falls on the Jurassic - recent sector of the apparent polar wander path (APWP) of Baltica after tectonic unfolding. These data imply that at least some of the identified secondary components could have originated before the Eurekan folding event (K/Pg), for example, in Early Cretaceous time which corresponds to the period of rifting events on Barents Sea and emplacement of dolerite intrusions. In contrast, paleomagnetic data from the foreland coincides with the APWP for Triassic - recent sector and partly matches previously published data.

Timing, mantle source and origin of mafic dykes within the gravity anomaly belt of the Taihang-Da Hinggan gravity lineament, central North China Craton

NASA Astrophysics Data System (ADS)

Liu, Shen; Feng, Caixia; Feng, Guangying; Xu, Mengjing; Coulson, Ian M.; Guo, Xiaolei; Guo, Zhuang; Peng, Hao; Feng, Qiang

2017-09-01

Six mafic dyke swarms crop out in Hebei Province within the Taihang-Da Hinggan gravity lineament magmatic belt, China, and were sampled. Here, we present new zircon laser ablation-inductively coupled plasma-mass spectrometry U-Pb age, whole rock geochemical, and Sr-Nd-Pb-Hf isotopic data for the six areas where these mafic dykes occur. The mafic (dolerite) dykes formed between 131.6 ± 1.6 and 121.6 ± 1.1 Ma, and are enriched in the light rare earth elements (LREE), some of the large ion lithophile elements (LILE; e.g., Rb, Ba, and Sr) and Pb, and are depleted in Th, U, Nb and Ta; some samples are also depleted in Eu. The dykes have high initial 87Sr/86Sr ratios (0.7055-0.7057), negative εNd (t) values (-12.5 to -11.9), relatively constant Pb isotopic ratios ((206Pb/204Pb)i = 16.45-16.51, (207Pb/204Pb)i = 15.44-15.51, (208Pb/204Pb)i = 36.49-36.53), negative εHf (t) values (-18.2 to -15.1), and old Nd (TNdDM2; 2.17-2.47 Ga) and Hf (THfDM2; 2.28-2.33 Ga) model ages. These geochronological, geochemical, and isotopic data indicate that the dykes were derived from magmas generated by low to moderate degree partial melting (1.0%-10%) of an EM1-like garnet lherzolite mantle source; these magmas fractionated olivine, clinopyroxene, and hornblende prior to emplacement, and assimilated minimal amounts of crustal material. Several possible models have previously been proposed to explain the origin of Mesozoic magmatism in this region. However, here we propose a foundering model for these studied mafic dykes, involving the foundering of eclogite from thickened lower crust due to the collision between the Siberian Craton and the North China Craon.

Permanent groundwater storage in basaltic dyke fractures and termite mound viability

NASA Astrophysics Data System (ADS)

Mège, Daniel; Rango, Tewodros

2010-04-01

Many basaltic dykes of the Ethiopian flood basalt province are observed in the northwestern Ethiopian lowlands. In this area, the termites preferentially build their epigeous mounds on the top of dolerite dykes. The relationship between termite mounds and dykes is investigated from the analysis of their distribution along one of these dykes, of thickness 2-5 m, that we could follow over 2000 m. Termite mounds are periodically spaced (mean distance 63 m, R2 = 0.995), and located exclusively where the topographic relief of the dyke is not more than 2 m above the surrounding area. From these observations and from the geological context, a hydrological circuit model is proposed in which (1) dykes are preferential conduits for groundwater drainage during the rainy season due to pervasive jointing, (2) during the dry season, the portion of the dyke forming a local topographic relief area dries up more quickly than the surroundings, the elevation difference between the dyke summit and the surroundings being a factor restricting termite mound development. For dyke topographic relief >2 m, drying is an obstacle for maintaining the appropriate humidity for the termite colony life. Periodic termite mound spacing is unlikely to be related to dyke or other geological properties. It is more likely related to termite population behaviour, perhaps to clay shortage, which restricts termite population growth by limiting the quantity of building material available for mound extension, and triggers exploration for a new colonization site that will be located along the dyke at a distance from the former colony that may be controlled by the extent of the zone covered by its trail pheromones. This work brings out the importance of dykes in channelling and storing groundwater in semiarid regions, and shows that dykes can store groundwater permanently in such settings even though the dry season is half the year long. It contributes also to shedding light on water supply conditions tolerated by termite populations, and factors governing termite mound distribution.

Recent Volcanism in the Northern Gulf of California: The Effects of Thick Deltaic Sedimentation in Magmatic Differentiation

NASA Astrophysics Data System (ADS)

Martin, A.; Hurtado, J. C.; Weber, B.; Schmitt, A. K.

2016-12-01

Quaternary volcanism in the northern Gulf of California provides a unique opportunity to characterize active crustal accretion under thick deltaic sedimentation from the Colorado River. Up to 17 volcanic seamounts are identified by high-resolution bathymetry and seismic reflexion profiles, principally in the Lower Delfin and the sheared peninsular margin north of Canal de Ballenas. Samples from eight subaereal and three submarine volcanoes are distinctively composed of andesite to rhyolite, and no basaltic eruptions are yet recognized, although dolerite sills and xenoliths of microphyric gabbro are reported in geothermal wells both, in the Salton and Cerro Prieto basins and saucer shape sills in the Lower Delfin basin indicate shallow mafic intrusions. Sr-Nd isotope data indicate that parent magma derives from partial melting of the Pacific mantle indicating that continental rupture is complete in the active rift basins. However, these rocks also demonstrate evidence of assimilation (eNd 1 to 5) and thus are compositionally modified as they are transported through the thick sequence of water rich sediments. Re-melting of hydrothermally altered mafic intrusives, crystal fractionation and variable (<20%) assimilation of continental crust and sediments produce the observed compositional spectra of Quaternary to Holocene eruptions. We explore the effects of thick, poorly consolidated sediments in the ascent of basaltic magma by means of a hydrostatic model that consider the crustal density structure in the Upper Delfin basin and sediment density logs from exploration wells. The hydrostatic model predicts that basaltic magma (2.68 g/cc) stalls 1 to 1.5 km beneath the seafloor and only andesite to rhyolite magmas reach shallower levels, where they exsolve volatiles and produce volcanic eruptions. We conclude that thick deltaic deposits promote magmatic differentiation and formation of a hybrid type of new crust in narrow rift basins in the northern Gulf of California and the Salton Trough.

Detachment Faulting, Serpentinization, Fluids and Life: Preliminary Results of IODP Expedition 357 (Atlantis Massif, MAR 30°N)

NASA Astrophysics Data System (ADS)

Fruh-Green, G. L.; Orcutt, B.; Green, S.; Cotterill, C.

2016-12-01

We present an overview of IODP Expedition 357, which successfully used two seabed rock drills to core 17 shallow holes at 9 sites across Atlantis Massif (Mid-Atlantic Ridge 30°N). A major goal of this expedition is to investigate serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration. The cores show highly heterogeneous rock type, bulk rock chemistry and alteration that reflect multiple phases of magmatism and fluid-rock interaction within the detachment fault zone. In cores along an E-W transect of the southern wall, recovered mantle peridotites are locally intruded by gabbroic and doleritic dikes and veins. The proportion of mafic rocks are volumetrically less than the amount of mafic rocks recovered previously in the central dome at IODP Site U1309, suggesting a lower degree of melt infiltration into mantle peridotite at the ridge-transform intersection. New technologies were developed and successfully applied for the first time: (1) an in-situ sensor package and water sampling system on each seabed drill measured real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential, temperature, and conductivity during drilling and took water samples after drilling; (2) a borehole plug system to seal the boreholes was successfully deployed at two sites to allow access for future sampling; and (3) delivery of chemical tracers into the drilling fluids for contamination testing. We will provide an overview of the drilling strategy and preliminary results of Expedition 357, and highlight the role of serpentinization in sustaining microbial communities in a region of active serpentinization and low temperature hydrothermal alteration.

Hydropedological interpretation of arid soilscapes, South Africa

NASA Astrophysics Data System (ADS)

Tinnefeld, Martin; Van Tol, Jacobus; Le Roux, Pieter

2017-04-01

Hydropedological investigations in arid regions are scarce due to the low the low contribution of these areas to water resources. Infrequent rainfall and few flow events also complicates measurements hydrological studies. Hydropedological studies, relating soil morphological properties and their spatial distribution to hydrological response, have been studied in detail in semi-arid, temperate, and sub-humid regions. In this paper, we investigated the relation between soil morphological properties and selected hydrological properties of soils in an arid landscape. We also studied the spatial distribution of the morphological properties to conceptualise the hydrological behaviour of different soilscapes in the area. A total of 806 soil profiles, covering an area of 4836 ha in the Northern Cape Province of South Africa were described and classified. The geology is dominated by Dwyka tillite overlain by aeolian sands with scattered Dolerite buttes. Thirteen modal profiles, representing the dominant soils types were selected, sampled at horizon level, and analysed for pH, CEC, iron, manganese, carbonate content. In situ measurements of saturated and near saturated (tension) hydraulic conductivity (Ks) were conducted to determine the water conducting macroporosity (WCM). Undisturbed cores were collected on which water retention characteristics were determined under laboratory conditions. Results indicate that dry soil colour, degree of structure development and the presence, absence, and abundance of carbonates as well as the degree of precipitation, are important indicators of hydrological response. For example; grey soils typically have lower Ks with higher storage capacity than soils dominated by red colours, whereas abundant carbonate precipitations in the soil matrix have lower WCM due to clogging of macropores. The dominant soil distribution pattern indicates that rapid vertical flow, through and out of the pedon, might contribute to recharge of an accumulative soil lateral flow at soil/rock interface on upper and midslope positions. Abundant carbonate precipitations decrease in consistency to valley bottom positions, indicating that this area serves as a periodic store of water during and after rain events.

U-Pb zircon geochronology of the Paleoproterozoic Tagragra de Tata inlier and its Neoproterozoic cover, western Anti-Atlas, Morocco

USGS Publications Warehouse

Walsh, G.J.; Aleinikoff, J.N.; Benziane, F.; Yazidi, A.; Armstrong, T.R.

2002-01-01

New U-Pb zircon data obtained by sensitive high resolution ion microprobe (SHRIMP) from the Tagragra de Tata inlier in the western Anti-Atlas, Morocco establish Paleoproterozoic ages for the basement schists, granites, and metadolerites, and a Neoproterozoic age for an ignimbrite of the Ouarzazate Series in the cover sequence. The age of interbedded felsic metatuff in the metasedimentary and metavolcanic sequence of the basement schists is 2072 ?? 8 Ma. This date represents: (1) the first reliable age from the metasedimentary and metavolcanic sequence; (2) the oldest reliable age for the basement of the Anti-Atlas; (3) the first date on the timing of deposition of the sediments on the northern edge of the Paleoproterozoic West African craton; (4) a lower age limit on deformation during the Eburnean orogeny; and (5) the first date obtained from the non-granitic Paleoproterozoic basement of Morocco. Ages of 2046 ?? 7 Ma (Targant granite) and 2041 ?? 6 Ma (Oudad granite) support earlier interpretations of a Paleoproterozoic Eburnean igneous event in the Anti-Atlas. The granites post-date the Eburnean D1 deformation event in the Paleoproterozoic schist sequence, and place a ???2046 Ma limit on short-lived Eburnean deformation in the area. Cross-cutting metadolerite is 2040 ?? 6 Ma; this is the first date from a metadolerite in the western Anti-Atlas. All of the dolerites in the area post-date emplacement of the two granites and the new age constrains the onset of late- or post-Eburnean extension. Ignimbrite of the Ouarzazate Series, immediately above the Paleoproterozoic basement is 565 ?? 7 Ma. This Neoproterozoic age agrees with ages of similar volcanic rocks elsewhere from the Ouarzazate Series. The date also agrees with the ages of associated hypabyssal intrusions, and marks the second and final stage of Pan-African orogenic activity in the western Anti-Atlas. ?? 2002 Elsevier Science B.V. All rights reserved.

Structural setting of Fimiston- and Oroya-style pyrite-telluride-gold lodes, Paringa South mine, Golden Mile, Kalgoorlie: 1. Shear zone systems, porphyry dykes and deposit-scale alteration zones

NASA Astrophysics Data System (ADS)

Mueller, Andreas G.

2017-07-01

The Golden Mile in the 2.7 Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, has produced 385 million tonnes of ore at a head grade of 5.23 g/t gold (1893-2016). Gold-pyrite ore bodies (Fimiston Lodes) trace kilometre-scale shear zone systems centred on the D2 Golden Mile Fault, one of three northwest striking sinistral strike-slip faults segmenting upright D1 folds. The Fimiston shear zones formed as D2a Riedel systems in greenschist-facies (actinolite-albite) tholeiitic rocks, the 700-m-thick Golden Mile Dolerite (GMD) sill and the Paringa Basalt (PB), during left-lateral displacement of up to 12 km on the D2 master faults. Pre-mineralisation granodiorite dykes were emplaced into the D2 shear zones at 2674 ± 6 Ma, and syn-mineralisation diorite porphyries at 2663 ± 11 Ma. The widespread infiltration of hydrothermal fluid generated chlorite-calcite and muscovite-ankerite alteration in the Golden Mile, and paragonite-ankerite-chloritoid alteration southeast of the deposit. Fluid infiltration reactivated the D2 shear zones causing post-porphyry displacement of up to 30 m at principal Fimiston Lodes moving the southwest block down and southeast along lines pitching 20°SE. D3 reverse faulting at the southwest dipping GMD-PB contact of the D1 Kalgoorlie Anticline formed the 1.3-km-long Oroya Shoot during late gold-telluride mineralisation. Syn-mineralisation D3a reverse faulting alternated with periods of sinistral strike-slip (D2c) until ENE-WSW shortening prevailed and was accommodated by barren D3b thrusts. North-striking D4 strike-slip faults of up to 2 km dextral displacement crosscut the Fimiston Lodes and the barren thrusts, and control gold-pyrite quartz vein ore at Mt. Charlotte (2651 ± 9 Ma).

Reprocessing Seismic Data - Using Wits Seismic Exploration Data to Image the Karoo Basin

NASA Astrophysics Data System (ADS)

Webb, S. J.; Scheiber-Enslin, S. E.; Manzi, M. S.

2016-12-01

During the heyday of seismic exploration of the Witwatersrand Basin, Anglo American's Gold Division acquired several thousand kilometres of Vibroseis reflection seismic data. These data, acquired from 1983-1994, were collected with the goal of finding extensions to the Witwatersrand Basin. In a prescient move, over 500 line kilometres were collected at 16 s two way travel time (TWT), extending to depths of 50 -70 km and have provided critical insight into the formation of the Kaapvaal Craton. In addition to these deep seismic lines, Anglo American acquired an extensive network of heretofore unpublished seismic lines that were collected at 6 sec TWT extending well beyond the known limits of the Witwatersrand Basin. The South African government as part of the national geophysical program in the late 1980s acquired six research reflection seismic lines in varied geological settings accruing another 700 km of data. Many of these data are now hosted at the University of the Witwatersrand's newly established Seismic Research Centre and represent unprecedented coverage and research opportunities. With recent global interest in shale gas, attention focused on the Karoo Basin in South Africa. Early exploration seismic data acquired by Soekor in the 1970s has been lost; however, digitized paper records indicate clear reflection targets. Here we examine one of the AngloGold seismic lines that was acquired in the middle of the Karoo Basin just south of Trompsburg extending to the southeast towards Molteno. This 150 km long line crosses the edge of the Kaapvaal Craton and shows clear reflectors throughout the Karoo Basin. These include the well-defined base of the Karoo and a number of dolerite sills within it. Nearby gas escape structures have been identified on surface and it is likely that several disruptions along this line are related to these or to dykes associated with the sills.

Paleomagnetism of Devonian dykes in the northern Kola Peninsula and its bearing on the apparent polar wander path of Baltica in the Precambrian

NASA Astrophysics Data System (ADS)

Veselovskiy, Roman V.; Bazhenov, Mikhail L.; Arzamastsev, Andrey A.

2016-04-01

Mafic dykes and large alkaline and carbonatite intrusions of Middle-Late Devonian age are widespread on the Kola Peninsula in NE Fennoscandia. These magmatic rocks are well characterized with petrographic, geochemical and geochronological data but no paleomagnetic results have been reported yet. We studied dolerite dykes from the northern part of the Peninsula and isolated three paleomagnetic components in these rocks. A low-temperature component is aligned along the present-day field, while a major constituent of natural remanent magnetization is an intermediate-temperature component (Decl. = 79.6°, Inc. = 78.5°, α95 = 5,9°, N = 17 sites) that is present in most Devonian dykes but is found in some baked metamorphic rocks and Proterozoic dykes too. Finally, a primary Devonian component could be reliably isolated from two dykes only. Rock-magnetic studies point to presumably primary low-Ti titanomagnetite and/or pure magnetite as the main remanence carriers but also reveal alteration of the primary minerals and the formation of new magnetic phases. The directions of a major component differ from the Middle Paleozoic reference data for Baltica but closely match those for the 190 ± 10 Ma interval recalculated from the apparent polar wander path of the craton. We assume that this Early Jurassic component is a low-temperature overprint of chemical origin. The main impact of the new results is not to mid-Paleozoic or Early Mesozoic times but to much older epochs. Analysis of paleomagnetic data shows that the directionally similar remanences are present in objects with the ages ranging from 500 Ma to 2 Ga over entire Fennoscandia. Hence we argue that an Early Jurassic remagnetization is of regional extent but cannot link it to a certain process and a certain tectonic event. If true, this hypothesis necessitates a major revision of the APWP for Baltica over a wide time interval.

Pseudotachylyte formation vs. mylonitization - repeated cycles of seismic fracture and aseismic creep in the middle crust (Woodroffe Thrust, Central Australia)

NASA Astrophysics Data System (ADS)

Wex, Sebastian; Mancktelow, Neil; Hawemann, Friedrich; Camacho, Alfredo; Pennacchioni, Giorgio

2014-05-01

The Musgrave Ranges in Central Australia provide excellent exposure of the shallowly south-dipping Woodroffe Thrust, which placed ~1200 Ma granulites onto amphibolite facies gneisses. This ~400 km long E-W structure developed under mid-crustal conditions during the intracratonic Petermann Orogeny around 550 Ma. From field observations and measurements, the shortening direction is constrained to be N-S and the movement sense top-to-north. Ductile deformation during this process almost entirely localized in the footwall rocks, developing a zone of mylonites, ultramylonites and sheared pseudotachylytes, several hundred metres wide, with pseudotachylyte abundance rapidly decreasing further into the footwall. In contrast, the hanging wall behaved in a predominantly brittle manner, producing significant volumes of pseudotachylyte breccia and isolated veins, but was otherwise mostly unaffected and only weakly foliated. The difference in rheological behaviour is reflected in the pseudotachylyte fabric, which is dominantly sheared in the footwall and largely unsheared in the hanging wall. Low-strain domains in the footwall show that localized shearing initiated along pseudotachylyte veins and that shear zones and mylonitic foliations were in turn exploited by subsequent pseudotachylyte veins. Neither phyllonitization nor synkinematic growth of new muscovite is observed. In contrast to models with a simple brittle-to-viscous transition, these observations show that a continuous cycle of brittle fracturing and shearing is active in dry mid-crustal environments. The products of multiple earthquakes and ductile overprint, repeatedly exploiting the same structural discontinuity, are composite layers of sheared pseudotachylyte. In the Woodroffe Thrust, these layers are numerous and frequently observed parallel to the foliation in the footwall mylonites. The thickest of these sheared pseudotachylyte horizons (~15 m thick) mark the immediate contact to the hanging wall and almost entirely consist of pseudotachylyte matrix. Particularly in the footwall, but locally also in the hanging wall, shear strain can additionally be concentrated along the margins of dolerite dykes, whose mineral assemblages will be studied to determine the metamorphic conditions that were active during development of the Woodroffe Thrust.

Natural constraints on the rheology of the lower continental crust (Musgrave Ranges, Central Australia)

NASA Astrophysics Data System (ADS)

Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Camacho, Alfredo; Pennacchioni, Giorgio

2015-04-01

Current models and extrapolated laboratory data generally predict viscous flow in the lower continental crust and any localized brittle deformation at these depths has been proposed to reflect downward propagation of the frictional-viscous transition zone during short-term seismic events and related high strain rates. Better natural constraints on this proposed rheological behaviour can be obtained directly from currently exposed lower crust that has not been strongly overprinted during its exhumation. One of the largest and best preserved lower crustal sections is located in the Musgrave Ranges, Central Australia. The Petermann Orogeny (550 Ma) in this area is characterized by the development of localized shear zones on a wide range of scales, overprinting water-deficient granulites of Musgravian age (1.2 Ga) as well as younger granites and gabbros. Shearing is rarely localized on lithological inhomogeneities, but rather on precursor fractures and on commonly associated pseudotachylytes. The only exception is that older dolerite dykes are often exploited, possibly because they are planar layers of markedly smaller grain size. Sheared pseudotachylyte often appears caramel-coloured in the field and has a fine grained assemblage of Grt+Cpx+Fsp. Multiple generations of pseudotachylyte formed broadly coeval with shearing are indicated by clasts of sheared pseudotachylyte within pseudotachylyte veins that then themselves subsequently sheared. The ductile shear zones formed under sub-eclogitic conditions of ca. 650°C and 1.2 GPa, generally typical of the lower continental crust. However, the P-T conditions during pseudotachylyte formation cannot be readily determined using classical geothermobarometry, because of the fine grain sizes and possible disequilibrium. The software "Xmaptools" (by Pierre Lanari) allows the quantification of X-ray maps produced by EDS or WDS. It provides both very precise definition of local mineral compositions for exchange geothermobarometry on a statistical basis, and an estimate of the bulk pseudotachylyte composition for small areas, avoiding clasts and heterogeneous composition of the former melt. The combination with thermodynamic modelling using PerpleX is used to test the results from geothermobarometry. The estimated conditions are similar to the ductile shear zones and support evidence for synchronous action of brittle faulting and viscous shearing in the lower crust.

Eddy Flow during Magma Emplacement: The Basemelt Sill, Antarctica

NASA Astrophysics Data System (ADS)

Petford, N.; Mirhadizadeh, S.

2014-12-01

The McMurdo Dry Valleys magmatic system, Antarctica, forms part of the Ferrar dolerite Large Igneous Province. Comprising a vertical stack of interconnected sills, the complex provides a world-class example of pervasive lateral magma flow on a continental scale. The lowermost intrusion (Basement Sill) offers detailed sections through the now frozen particle macrostructure of a congested magma slurry1. Image-based numerical modelling where the intrusion geometry defines its own unique finite element mesh allows simulations of the flow regime to be made that incorporate realistic magma particle size and flow geometries obtained directly from field measurements. One testable outcome relates to the origin of rhythmic layering where analytical results imply the sheared suspension intersects the phase space for particle Reynolds and Peclet number flow characteristic of macroscopic structures formation2. Another relates to potentially novel crystal-liquid segregation due to the formation of eddies locally at undulating contacts at the floor and roof of the intrusion. The eddies are transient and mechanical in origin, unrelated to well-known fluid dynamical effects around obstacles where flow is turbulent. Numerical particle tracing reveals that these low Re number eddies can both trap (remove) and eject particles back into the magma at a later time according to their mass density. This trapping mechanism has potential to develop local variations in structure (layering) and magma chemistry that may otherwise not occur where the contact between magma and country rock is linear. Simulations indicate that eddy formation is best developed where magma viscosity is in the range 1-102 Pa s. Higher viscosities (> 103 Pa s) tend to dampen the effect implying eddy development is most likely a transient feature. However, it is nice to think that something as simple as a bumpy contact could impart physical and by implication chemical diversity in igneous rocks. 1Marsh, D.B. (2004), A magmatic mush column Rosetta stone: the McMurdo Dry Valleys of Antarcica. EOS, 85, 497-502. 2Petford, N. (2009), Which Effective Viscosity? Mineralogical Magazine, 73, 167-191. Fig. 1. Numerical simulation in the geometry showing magma flow field and eddy formation where circulating magma is trapped. Streamlines track particle orbits.

Subsurface deposition of Cu-rich massive sulphide underneath a Palaeoproterozoic seafloor hydrothermal system—the Red Bore prospect, Western Australia

NASA Astrophysics Data System (ADS)

Agangi, Andrea; Reddy, S. M.; Plavsa, D.; Vieru, C.; Selvaraja, V.; LaFlamme, C.; Jeon, H.; Martin, L.; Nozaki, T.; Takaya, Y.; Suzuki, K.

2018-02-01

The Proterozoic Bryah and Yerrida basins of Western Australia contain important base and precious metal deposits. Here we present microtextural data, trace element and S isotope analyses of massive sulphide mineralisation hosted in Palaeoproterozoic subvolcanic rocks (dolerite) recently discovered at Red Bore. The small-scale high-grade mineralisation, which extends from the sub-surface to at least 95 m down-hole, is dominated by massive chalcopyrite and contains minor pyrite and Bi-Te-(Se) phases. Massive sulphide mineralisation is surrounded by discontinuous brecciated massive magnetite, and a narrow (< 2 m) alteration halo, which suggests very focussed fluid flow. Laser ablation ICP-MS analyses indicate that chalcopyrite contains up to 10 ppm Au and in excess of 100 ppm Ag. Sulphur isotope analyses of pyrite and chalcopyrite indicate a narrow range of δ34SVCD (- 0.2 to + 4.6 ‰), and no significant mass-independent fractionation (- 0.1 < Δ33S < + 0.05 ‰). Re-Os isotope analyses yield scattered values, which suggests secondary remobilisation. Despite the geographical proximity and the common Cu-Au-Ag association, the mineralisation at Red Bore has significant differences with massive sulphide mineralisation at neighbouring DeGrussa, as well as other massive sulphide deposits around the world. These differences include the geometry, sub-volcanic host rocks, extreme Cu enrichment and narrow δ34S ranges. Although a possible explanation for some of these characteristics is leaching of S and metals from the surrounding volcanic rocks, we favour formation as a result of the release of a magmatic fluid phase along very focussed pathways, and we propose that mixing of this fluid with circulating sea water contributed to sea floor mineralisation similar to neighbouring VHMS deposits. Our data are permissive of a genetic association of Red Bore mineralisation with VHMS deposits nearby, thus suggesting a direct connection between magmatism and mineralising fluids responsible for VHMS deposition at surface. Therefore, the Red Bore mineralisation may represent the magmatic roots of a VHMS system.

Stable isotopic compositions of carbonates from the Mesoproterozoic Bangemall Group, northwestern Australia

NASA Technical Reports Server (NTRS)

Buick, R.; Des Marais, D. J.; Knoll, A. H.

1995-01-01

Marine carbonate rocks from the Mesoproterozoic Bangemall Group of northwestern Australia show little deviation (+/-1.3%) in whole-rock delta 13C(carb)-values about a mean of -0.5%. This narrow range persists despite close sampling (every 10-20 m) through long sections (up to 2500 m) that are geographically widespread (up to 250 km apart), over many depositional environments (supralittoral to outer shelf), sediment sources (stromatolitic bioherms to detrital calcilutites) and rock types (pure limestones to dolomitic shales). The only major excursions from the norm seem related to unusual environmental or post-depositional processes, as they are correlated with large enrichments (to -3%) or depletions (to -16%) in 18O. Relatively heavy delta 13C-values, up to +2.5%, occur in a single bed of brecciated ferruginous dolostone at a single locality; these abnormal values may result from local evaporitic conditions. Limey and shaley nodular dolostones have delta 13C-values as low as -4.3%, probably caused by remineralization of organic matter during late and patchy dolomitization. Most notably, sharp negative excursions in delta 13C, up to -8.4%, occur in bleached kerogen-free rocks with mineral assemblages of dolomite + quartz + calcite +/- tremolite + talc, reflecting isotopic re-equilibration in thick metamorphic aureoles around dolerite intrusions. General environmental variations are minor, with delta 13C-values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative. There are no strong secular trends, but subtle fluctuations within the range -2 to +l% can be correlated along the northwestern margin of the basin. This resembles the pattern seen in other Mesoproterozoic successions, but is markedly unlike the heavy background (> +5%) and extreme variations (up to l0%) in delta 13C evident in Neoproterozoic successions of similar thickness and environmental setting. Hence, in contrast to the Neoproterozoic, the global rate of organic carbon burial was probably fairly constant during deposition of the Bangemall Group, and perhaps generally during the Mesoproterozoic, as was the redox state of the atmosphere and hydrosphere.

Exploring Cumulates in Small, Shallow Parts of a Large Mafic Magma System to Provide Baseline Models for Crystallization in Larger Intrusions

NASA Astrophysics Data System (ADS)

Srogi, L.; Willis, K. V.; Lutz, T. M.; Plank, T. A.; Pollock, M.; Connolly, B.; Wood, A. M.

2016-12-01

Small, shallow portions of large magmatic systems cool more rapidly and potentially have less subsolidus overprinting than large mafic intrusions, but it is unclear whether they are small-scale analogs for the same crystallization processes. The Morgantown-Jacksonwald magmatic system (MJS), western Newark Basin, Pennsylvania, is part of the 201-Ma Central Atlantic Magmatic Province (CAMP) formed during Pangean rifting. The MJS consists of several interconnected intrusions exposed in cross-section from the Jacksonwald basalt at the paleosurface to 6 km depth (<0.2 GPa). Mg-rich orthopyroxene (opx) phenocrysts form crystal accumulations in some dikes and in basal and roof zones of sills in the MJS, in many CAMP intrusions, and in younger Ferrar dolerites, Antarctica. Some samples with opx phenocrysts have dm-scale modal layering. Despite ubiquitous occurrence, the opx is little-studied and our work tests most previous authors' assumption that opx was brought in from deeper intrusions. Opx cores with Mg/(Mg+Fe) of 80-77% yield mid-crustal pressures of 0.4-0.6 GPa (using method of Putirka, 2008). LA-ICPMS was used to obtain trace element concentrations in mm-size phenocrysts in a chill margin within 0.5m of the basal contact and cm-size phenocrysts from cumulate about 10m above. REE concentrations are similar in both samples: LREE-depleted cores (normalized La/Sm = 0.05-0.1); variably LREE-enriched rims; some negative Eu anomalies. REE patterns calculated for liquids in equilibrium with opx using published Kd values are roughly parallel to but significantly higher than REE in host chill margin diabase. CSDs of opx and matrix plagioclase from several samples within 10m of the basal contact will be used to evaluate models of crystal growth vs. mechanical sorting. Modes and mineral compositions are not consistent with MELTS fractionation models: opx crystallizes in place of pigeonite; pyroxenes are zoned in Ca not Fe-Mg; late-crystallizing quartz and K-feldspar are lacking. These features suggest crystallization buffered by earlier phases in the crystal mush with some melt migration, similar to processes that produce more extreme layering in large mafic intrusions.

From Carbonatite to Ikaite: How high-T carbonates are transformed into low-T carbonate minerals in SW Greenland

NASA Astrophysics Data System (ADS)

Stockmann, G. J.; Tollefsen, E.; Ranta, E.; Skelton, A.; Sturkell, E.; Lundqvist, L.

2015-12-01

The 1300 Ma Grønnedal-Íka igneous complex in southwest Greenland comprises nepheline syenites and carbonatites. It belongs to a suite of intrusions formed 1300-1100 Ma ago referred to as the Gardar period. In modern time (the last ca. 8000 years), fluid-rock interactions involving the nepheline syenites and carbonatites gives rise to about one thousand submarine columns made of the rare low-T mineral ikaite (CaCO3x6H2O). The columns are found in a shallow, narrow fjord named Ikka Fjord and their distribution clearly follows the outcrop of the Grønnedal-Íka complex. When meteoric water percolates through the highly fractured complex, a sodium carbonate solution of pH 10 is formed through hitherto unknown fluid-rock reactions. This basic solution seeps up through fractures at the bottom of Ikka Fjord and when mixed with seawater, the mineral ikaite is formed. As the seepage water has a lower density than seawater, there is an upwards flow that creates columns. What is peculiar about ikaite is its limited stability making it unstable above +6 °C. Isotopic studies of ikaite reveal a seawater origin for the Ca2+ ions, and the carbonatite being the most likely source for the CO32- ions. The carbonatite is mainly of søvite composition (CaCO3) with high contents of siderite and ankerite in certain areas. The nepheline syenites contain Na,K-rich minerals like nepheline, alkali-feldspar, aegirine-augite, katophorite and biotite. Nepheline is mainly replaced by muscovite, and aegirine-augite partly by chlorite, which could release sodium into solution. A dolerite dyke of unknown age prompted extensive mineralization of magnetite by activating hydrothermal fluid convection. The fluid interacted with the carbonatite, replacing siderite and ankerite by magnetite and later hematite. In a newly launched project at Stockholm University, we are trying to unravel the chemical reactions taking place inside the Grønnedal-Íka igneous complex leading to the formation of the sodium carbonate solution issuing in Ikka Fjord.

Lamprophyres from the Harohalli dyke swarm in the Halaguru and Mysore areas, Southern India: Implications for backarc basin magmatism

NASA Astrophysics Data System (ADS)

Lanjewar, Shubhangi; Randive, Kirtikumar

2018-05-01

The Bangalore and Harohalli dyke swarms occur in the eastern part of the Dharwar craton. The older Bangalore dyke swarm is made up of dolerites, trending east-west, and the younger contains alkaline dykes that trend approximately north-south. The lamprophyres of the Harohalli dyke swarm occur in the Halaguru and Mysore industrial areas where they are exposed as fresh porphyritic - panidiomorphic dykes, containing crustal xenoliths, and showing chilled contacts with the country rock charnokites. They are chiefly composed of amphiboles which form well-developed phenocrysts. Clinopyroxenes are present in some of the dykes. Compositional zoning is observed in clinopyroxenes and amphiboles; their zoning patterns indicate that the magma experienced cryptic variations and that fractional crystallization was a dominant process in the evolution of the Harohalli Lamprophyres (HRL). The HRL are calc-alkaline with shoshonitic affinity and exhibit a K2O/Na2O ratio of ∼1. They show primitive (MORB-like) trace-element characters. LILE and LREE both show marginally enriched patterns; whereas HFSE and HREE show strongly depleted patterns. In the regional geologic sense, HRL dykes are characterised by two major influences; namely, (i) primary source region characteristics, which are geochemically more primitive, roughly falling within fields of primitive - MORB and enriched- MORB and (ii) the continental lithosphere. The data points for the HRL distinctly show their proximity to N-MORB and scatter towards the continental crust. Moreover, features like xenolith assimilation might influence the trace-element characteristics of the HRL dykes. Such magmas with mixed characters can be formed in a backarc basin environment. Geochemical proxies such as Ba/Nb vs Nb/Yb, Ba/Th vs Th/Nb, and the water content of magmas; which have been effectively used for discriminating backarc basin magmas worldwide, also indicate that the HRL magmas were generated in a backarc environment with inputs from a shallow subduction component and interaction with carbonatite melt. This paper therefore presents a new provenance for the generation of calc-alkaline lamprophyres, which were so far known to occur in orogenic belts.

40Ar/39Ar geochronology of terrestrial pyroxene

NASA Astrophysics Data System (ADS)

Ware, Bryant; Jourdan, Fred

2018-06-01

Geochronological techniques such as U/Pb in zircon and baddeleyite and 40Ar/39Ar on a vast range of minerals, including sanidine, plagioclase, and biotite, provide means to date an array of different geologic processes. Many of these minerals, however, are not always present in a given rock, or can be altered by secondary processes (e.g. plagioclase in mafic rocks) limiting our ability to derive an isotopic age. Pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40Ar/39Ar has been met with little success so far. In this study, we analyzed pyroxene crystals from two different Large Igneous Provinces using a multi-collector noble gas mass spectrometer (ARGUS VI) since those machines have been shown to significantly improve analytical precision compared to the previous single-collector instruments. We obtain geologically meaningful and relatively precise 40Ar/39Ar plateau ages ranging from 184.6 ± 3.9 to 182.4 ± 0.8 Ma (2σ uncertainties of ±1.8-0.4%) and 506.3 ± 3.4 Ma for Tasmanian and Kalkarindji dolerites, respectively. Those data are indistinguishable from new and/or published U-Pb and 40Ar/39Ar plagioclase ages showing that 40Ar/39Ar dating of pyroxene is a suitable geochronological tool. Scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated. Numerical models of argon diffusion in plagioclase and pyroxene support these observations. However, we found that the viability of 40Ar/39Ar dating approach of pyroxene can be affected by irradiation-induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal, hence requiring careful petrographic observations before analysis. Finally, diffusion modeling show that 40Ar/39Ar of pyroxene can be used as a powerful tool to date the formation age of mafic rocks affected by greenschist metamorphism and will likely play an important role in high temperature thermochronology.

Provenance and metamorphic PT conditions of Cryogenian-Ediacaran metasediments from the Kid metamorphic complex, Sinai, NE Arabian-Nubian Shield: Insights from detrital zircon geochemistry and mineral chemistry

NASA Astrophysics Data System (ADS)

El-Bialy, Mohammed Z.; Ali, Kamal A.; Abu El-Enen, Mahrous M.; Ahmed, Ahmed H.

2015-12-01

The Malhaq and Um Zariq formations occupy the northern part of the Neoproterozoic Kid metamorphic complex of SE Sinai, NE Arabian-Nubian Shield. This study presents new mineral chemistry data and LA-ICP-MS analyses of the trace element concentrations on zircons separated from metapelites from these formations. The detrital zircons of Um Zariq Formation are more enriched in ΣREE, whereas Malhaq Formation zircons are markedly HREE-enriched with strongly fractionated HREE patterns. The quite differences in the overall slope and size of the Eu and Ce anomalies between REE patterns of the two zircon suites provide a robust indication of different sources. The Ti-in-zircon thermometer has revealed that the zircons separated from Malhaq Formation were crystallized within the 916-1018 °C range, while those from Um Zariq Formation exhibit higher range of crystallization temperatures (1084-1154 °C). The detrital zircons of Malhaq Formation were derived mainly from mafic source rocks (basalt and dolerite), whereas Um Zariq Formation zircons have varied and more evolved parent rocks. Most of the investigated zircons from both formations are concluded to be unaltered magmatic that were lately crystallized from a high LREE/HREE melt. All the studied detrital zircon grains show typical trace elements features of crustal-derived zircons. All of the Um Zariq Formation and most of Malhaq Formation detrital zircons are geochemically discriminated as continental zircons. Both formation metapelites record similar, overlapping peak metamorphic temperatures (537-602 °C and 550-579 °C, respectively), and pressures (3.83-4.93 kbar and 3.69-4.07 kbar, respectively). The geothermal gradient, at the peak metamorphic conditions, was quite high (37-41 °C/km) corresponding to metamorphism at burial depth of 14-16 km. The peak regional metamorphism of Um Zariq and Malhaq formations is concluded to be generated during extensional regime and thinning of the lithosphere in an island arc setting with heat flow from the underlying arc granitoids.

The 48 Ma Koko Guyot: Early Indications of Temporal Changes in the Composition of the Hawaiian Plume?

NASA Astrophysics Data System (ADS)

Thompson, P. M.; Kempton, P. D.; Saunders, A. D.

2002-12-01

The 48 Ma Koko Guyot is the youngest Emperor Seamount drilled during ODP Leg 197. Leg 197 drilled 278 m into a sequence of 15 lava flows and hyaloclastites, with subordinate amounts of volcaniclastic sandstone and limestone. The sampled lava flows are mainly tholeiitic to transitional basalts and dolerites, with some intercalated alkalic basalts. Thus, the lavas sampled at Koko Guyot resemble the late shield stage of a modern-day Hawaiian volcano, being dominantly tholeiitic in character. The alkalic basalts generally display higher Zr and TiO2 for a given MgO compared to the tholeiites. The degree of scatter for most incompatible elements when plotted against MgO implies that the lavas do not define one liquid line of descent: several parent magma compositions must therefore be invoked. The lavas from Koko have Sr, Nd, Pb and Hf isotope compositions that are the most Hawaiian-like of the Emperor Seamounts that have been studied, displaying similar ɛ Nd to Mauna Kea. Our new data are consistent with the suggestion from trace elements that several different source compositions are required in the genesis of the Koko lavas. The involvement of at least two components is suspected from the apparently linear array in ɛ Hf-ɛ Nd space, which is also indicated by Pb and Sr isotope data. This linear array in ɛ Hf-ɛ Nd space defines a steeper slope than that of Recent Hawaiian magma types, which suggests a fundamental source difference between Koko and modern-day Hawaii. The shallower slope of Hawaiian volcanoes is thought to indicate the involvement of recycled pelagic sediment in the genesis of Hawaiian lavas (Blichert-Toft et al., 1999). Thus, preliminary data from the Koko Guyot suggest that the composition of the Hawaiian plume has changed in composition over time. The causes of this temporal variation are unknown, but may result from changes in the amount of pelagic sediment recycled from the deep plume source. Blichert-Toft, J., Frey, F.A. and Albarède, F., 1999. Hf isotope evidence for pelagic sediments in the source of Hawaiian basalts. Science, 285, 879-882.

Strain localization on different scales and the importance of brittle precursors during deformation in the lower crust (Davenport Shear Zone, Central Australia)

NASA Astrophysics Data System (ADS)

Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Camacho, Alfredo; Pennacchioni, Giorgio

2014-05-01

High strain rocks in the Musgrave Ranges (Central Australia) provide a rather unique insight into the development of lower crustal shear zones during the 550 Ma Petermann Orogeny, allowing common models for lower crustal deformation to be critically evaluated. The observed structures in the study area are, from south to north: (1) The Mann Fault, which is poorly exposed but evident on airborne geomagnetic maps. This regional scale fault with a component of dextral shear shows a step-over resulting in the formation of a pull-apart basin. (2) The Davenport Shear Zone, accommodating the horizontal extension in a 7 km wide WNW-ESE-trending mylonitic zone developed under subeclogitic, lower crustal conditions. This high strain zone is bounded to the north by a more than 50 km long, continuous, sheared dolerite dyke. North of this dyke, the ~1200 Ma Musgravian fabric is still preserved, only slightly rotated and typically N-S trending. (3) The Woodroffe Thrust, marking the northern boundary of the Musgrave Ranges, brings these lower crustal rocks on top of amphibolite facies units, with a top-to-north sense of movement. Strain in the Davenport Shear Zone is very heterogeneously distributed, with localization and partitioning from the kilometre down to the millimetre scale. Pseudotachylyte is commonly associated with dykes, especially on the boundaries, and is often sheared. The orientation of sheared dykes and localized shear zones is typically at a high angle to either side of the shortening direction, resulting in a variable sense of shear and a major component of flattening, with a nearly horizontal extension direction. Detailed outcrop-scale mapping shows that compositional inhomogeneities, such as quartz veins, are generally not exploited, even when favourably oriented for shear reactivation. Ultramylonitic shear zones are sometimes only a few millimetres wide but extend for several metres and are generally oblique to the background foliation. Pseudotachylyte often predates or is coeval with localized shearing and fracturing clearly played a major role in the nucleation of mesoscale discrete shear zones. In order to constrain the conditions of pseudotachylyte formation, and to establish whether they developed under lower crustal subeclogitic conditions, garnet-bearing sheared pseudotachylytes were sampled for geothermobarometric analysis.

Paleomagnetism of Early Paleozoic Rocks from the de Long Archipelago and Tectonics of the New Siberian Islands Terrane

NASA Astrophysics Data System (ADS)

Metelkin, D. V.; Chernova, A. I.; Matushkin, N. Y.; Vernikovskiy, V. A.

2017-12-01

The De Long archipelago is located to the north of the Anjou archipelago as a part of a large group between the Laptev Sea and the East Siberian Sea - the New Siberian Islands and consists of Jeannette Island, Bennett Island and Henrietta Island. These islands have been shown to be part of a single continental terrane, whose tectonic history was independent of other continental masses at least since the Ordovician. Paleomagnetic and precise geological data for the De Long archipelago were absent until recently. Only in 2013 special international field trips to the De Long Islands could be organized and geological, isotope-geochronological and paleomagnetic studies were carried out.On Jeannette Island a volcanic-sedimentary sequence intruded by mafic dikes was described. The age of these dikes is more likely Early Ordovician, close to 480 Ma, as evidenced by the results of our 40Ar/39Ar and paleomagnetic investigations of the dolerites as well as the result from detrital zircons in the host rocks published before. On Bennett Island, there are widespread Cambrian-Ordovician mainly terrigenous rocks. Paleomagnetic results from these rocks characterize the paleogeographic position of the De Long archipelago at 465 Ma and perhaps at 530 Ma, although there is no evidence for the primary origin of magnetization for the latter. On Henrietta Island the Early Cambrian volcanic-sedimentary section was investigated. A paleomagnetic pole for 520 Ma was obtained and confirmed by new 40Ar/39Ar results. Adding to our previous paleomagnetic data for the Anjou archipelago the extended variant of the apparent polar wander path for the New Siberian Island terrane was created. The established paleolatitudes define its location in the equatorial and subtropical zone no higher than 40 degrees during the Early Paleozoic. Because there are no good confirmations for true polarity and related geographic hemisphere we present two possibilities for tectonic reconstruction. But both these solutions demonstrate a very close paleogeographic position between the New Siberian Island and the Siberian continent. The study was supported by Ministry of Education and Science of the Russian Federation (grant No 5.2324.2017), RSF (grant No 14-37-00030), RFBR (grant No 15-05-01428).

On the Serpentinization Degree (S) of IODP Expedition 357 Atlantis Massif Rocks: Insights from Rock Magnetic Properties and Microscopic Magnetic Mineralogy Study of six Sites

NASA Astrophysics Data System (ADS)

Herrero-Bervera, E.; Whattam, S. A.; Frederichs, T.

2016-12-01

We have studied the magnetic properties of 37 serpentinized samples recovered via drilling during IODP Expedition 357, Atlantis Massif. We have recovered various lithologies including ultamafic rocks (primarily extensively serpentinized), subordidate gabbros, dolerites (small-scale melt injections) and schists. We have conducted remanence and induced magnetic experiments on the samples to determine for instance the degree of serpentinization (S). Stepwise alternating field and thermal demagnetization experiments from 2.5 to 70 mT and from 28 to 700°C, respectively, yielded univectorial diagrams showing the removal of secondary components (e.g., VRM, IRM, CRM) by isolating a characteristic component (ChRM) at various fields and temperatures. The normalized intensity of demagnetization (J/Jo) shows that the decrease of the magnetization of the specimens where about 50% of the original magnetization and is lost at about 5 mT and 100°C (i.e., Median Destructive Field). The stereograms show magnetic stability of the specimens by determining the directional behavior after 4 demagnetization steps (from 7.5-10 mT fields and low temperatures). Induced magnetization such as SIRM's, hysteresis saturation loops, back-fields and FORC experiments were performed. Diagnostic values of Mrs/Ms and Brc/Bc determine the domain structure of a magnetic sample. The magnetic grain sizes were determined using the protocol of Dunlop [2000]. Most of the samples were distributed over the Single (SD), Pseudo-Single Domain (PSD) and a few over the Multi Domain (MD) ranges with a certain degree of clustering on the PSD range. Curie points were obtained by measuring their low-field susceptibility vs. temperature from 28°C up to 700°C in an Argon atmosphere showing a minimum of 1-4 magnetic mineral phases with temperatures ranging from 100°C up to 640°C. These phases are predominantly Ti-poor, Ti-rich magnetite, maghemite and magnetite as corroborated by microscopic analysis as well as the Verwey transition (Tv≈110-120K). Samples studied show appreciable variation in bulk susceptibility (77.8 x 10-3 to 0.31 x 10-3 SI units). The samples are characterized by low, intermediate and high degree of serpentinization based on the results of their magnetic properties (e,g, Kappa, density, magnetic stability and Mrs/Ms vs Bcr/Bc).

Exploring the Hydrothermal System in the Chicxulub Crater and Implications for the Early Evolution of Life on Earth

NASA Astrophysics Data System (ADS)

Kring, D. A.; Schmieder, M.; Tikoo, S.; Riller, U. P.; Simpson, S. L.; Osinski, G.; Cockell, C. S.; Coolen, M.; Gulick, S. P. S.; Morgan, J. V.

2017-12-01

Impact cratering, particularly large basin-size craters with diameters >100 km, have the potential to generate vast subsurface hydrothermal systems. There were dozens of such impacts during the Hadean and early Archean, some of which vaporized seas for brief periods of time, during which the safest niches for early life may have been in those subsurface hydrothermal systems. The Chicxulub crater can serve as a proxy for those events. New IODP-ICDP core recovered by Expedition 364 reveals a high-temperature (>300 degree C) system that may have persisted for more than 100,000 years. Of order 105 to 106 km3 of crust was structurally deformed, melted, and vaporized within about 10 minutes of the impact. The crust had to endure immense strain rates of 104/s to 106/s, up to 12 orders of magnitude greater than those associated with igneous and metamorphic processes. The outcome is a porous, permeable region that is a perfect host for hydrothermal circulation across the entire diameter of the crater to depths up to 5 or 6 km. The target rocks at Chicxulub are composed of an 3 km-thick carbonate platform sequence over a crystalline basement composed of igneous granite, granodiorite, and a few other intrusive components, such as dolerite, and metamorphic assemblages composed, in part, of gneiss and mica schist. Post-impact hydrothermal alteration includes Ca-Na- and K-metasomatism, pervasive hydration to produce layered silicates, and lower-temperature vug-filling zeolites as the system cycled from high temperatures to low temperatures. While the extent of granitic crust on early Earth is still debated and, thus, the direct application of those mineral reactions to the Hadean and early Archean can be debated, the thermal evolution of the system should be applicable to diverse crustal compositions. It is important to point out that pre-impact thermal conditions of Hadean and early Archean crust can affect the size of an impact basin and, in turn, the proportion of that basin that may experience hydrothermal conditions. The Chicxulub crater - as sampled by Expedition 364 - will provide the baseline needed to assess the spatial and temporal extent of Hadean and early Archean hydrothermal systems and their potential as crucibles for pre-biotic chemistry and the early evolution of microbial life.

The Paradox of the Axial Melt Lens: Petrology and Geochemistry of the Upper Plutonics at Hess Deep

NASA Astrophysics Data System (ADS)

Lissenberg, C. J.; Loocke, M. P.; MacLeod, C. J.

2014-12-01

The axial melt lens (AML) is a steady-state magma-rich body located at the dyke-gabbro transition at intermediate- and fast-spreading ridges. It is widely believed to be the reservoir from which mid-ocean ridge basalt (MORB) is erupted. The paradox of the axial melt lens is that the plutonic rocks that occur at this level are far too evolved to be in equilibrium with MORB, which is basaltic by definition; hence, the plutonic and volcanic records do not match. We explore this paradox by study of the first comprehensive sample suite of the uppermost plutonics of a fast-spreading ridge, taken by remotely-operated vehicle from the Hess Deep rift during cruise JC21. 23 samples (8 dolerites, 14 gabbronorites, and 1 gabbro) were collected from a section containing the transition from the uppermost gabbroic section into sheeted dykes. We present the results of a detailed petrographic and microanalytical investigation of these samples. They are dominated by evolved, varitextured (both in hand sample and thin section) oxide gabbronorites; olivine occurs in only one sample. A preponderance of the samples have positive Eu/Eu* and Sr/Sr*, indicating a cumulate origin. However, the minerals have evolved compositions, and are in equilibrium with melts significantly more evolved than East Pacific Rise MORB. Furthermore, the trace element contents of clinopyroxene differ significantly from clinopyroxene in equilibrium with MORB, being more enriched in incompatible elements. To account for both the evidence of derivation of MORB from the AML and the evolved nature of its rock record, we posit that the AML must be fed by melts on two different timescales: continual low-volume feeding by evolved interstitial melt from the cumulus pile below is augmented episodically by delivery of high volumes of more primitive melt. The latter episodes may trigger eruptions; hence the primitive melts are held in the magma chamber for only short periods, and erupt on the seafloor before significant crystallisation in the AML has taken place. This model for the feeding of the AML provides ample opportunity for mixing between the relatively primitive melts and the evolved, trace-element-rich melt, and accounts for the observed over-enrichment in incompatible elements of MORB.

Detrital Zircon U-Pb Analysis of the Liuqu Conglomerate Along the Yarlung-Zangbo Suture Zone, and Implications for the Mode and Timing of Collision Tectonics in Southern Tibet

NASA Astrophysics Data System (ADS)

Xie, Y.; Dilek, Y.

2016-12-01

The Liuqu Conglomerate (LQC) along the Yarlung-Zangbo suture zone (YZSZ) in Southern Tibet is a terrestrial deposit that provides significant spatial and temporal constraints for the timing and nature of collisional events in the tectonic evolution of the Tibetan-Himalayan orogenic belt. The 10-km-wide (N-S) LQC is exposed discontinuously for more than 1000 km in an E-W direction, and is tectonically overlain to the north by the Cretaceous Neotethyan oceanic lithosphere along a S-vergent thrust fault system and to the south by Triassic-Jurassic metamorphosed sedimentary-volcanic rocks of the Tethyan Himalaya along N-vergent reverse-thrust faults. The major facies of the LQC are the matrix-supported to clast-supported conglomerates. The matrix is poorly to moderate sorted red quartz sandstone, mudstone and sub-rounded pebble to cobble-sized clasts. The clast lithology present in central and southern parts includes dark red sandstone, siltstone and mudstone greyish-green shale, grey phyllite and slate with their provenance in the Triassic Tethyan Himalaya to the south. The clastic material making up its stratigraphy in the northern part of the LQC includes quartz sandstone, radiolarian chert, minor dolerite, gabbro and peridotite derived from the Cretaceous ophiolite. Here we report in-situ detrital zircon U-Pb age analysis of sandstone from the LQC near Liuqu area. 163 concordant U-Pb ages obtained from sample 22-LQ-15, 27-LQ-15 and 35-LQ-15 show the youngest age is 307±13 Ma with discordance of -17.02%, and the oldest zircon grain is 3362 ±51 Ma with discordance of 2.63%. Statistically, the age spectrum of these zircons from the three sandstone samples display a prominent peak centred in 935 Ma, a large peak around 516 Ma, and two small clusters around 2429 Ma and 2772 Ma. The zircon U-Pb results provide evidence of age clusters of the sandstone in LQC are consistent with the detrital U-Pb age signature of the sandstone in Tethyan Himalaya. Thus, the sediments in the LQC could be derived from the northern edge of the Indian margin and a late Jurassic-Cretaceous intra-oceanic island arc that lay within Thethys and developed prior to the final collision between India and Eurasia plates.

Permeability Changes in Reaction Induced Fracturing

NASA Astrophysics Data System (ADS)

Ulven, Ole Ivar; Malthe-Sørenssen, Anders; Kalia, Rajiv

2013-04-01

The process of fracture formation due to a volume increasing chemical reaction has been studied in a variety of different settings, e.g. weathering of dolerites by Røyne et al.[4], serpentinization and carbonation of peridotite by Rudge et al.[3] and replacement reactions in silica-poor igneous rocks by Jamtveit et al.[1]. It is generally assumed that fracture formation will increase the net permeability of the rock, and thus increase the reactant transport rate and subsequently the total reaction rate, as summarised by Kelemen et al.[2]. Røyne et al.[4] have shown that transport in fractures will have an effect on the fracture pattern formed. Understanding the feedback process between fracture formation and permeability changes is essential in assessing industrial scale CO2 sequestration in ultramafic rock, but little is seemingly known about how large the permeability change will be in reaction-induced fracturing under compression, and it remains an open question how sensitive a fracture pattern is to permeability changes. In this work, we study the permeability of fractures formed under compression, and we use a 2D discrete element model to study the fracture patterns and total reaction rates achieved with different permeabilities. We achieve an improved understanding of the feedback processes in reaction-driven fracturing, thus improving our ability to decide whether industrial scale CO2 sequestration in ultramafic rock is a viable option for long-term handling of CO2. References [1] Jamtveit, B, Putnis, C. V., and Malthe-Sørenssen, A., "Reaction induced fracturing during replacement processes," Contrib. Mineral Petrol. 157, 2009, pp. 127 - 133. [2] Kelemen, P., Matter, J., Streit, E. E., Rudge, J. F., Curry, W. B., and Blusztajn, J., "Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO2 Capture and Storage," Annu. Rev. Earth Planet. Sci. 2011. 39:545-76. [3] Rudge, J. F., Kelemen, P. B., and Spiegelman, M., "A simple model of reaction induced cracking applied to serpentinization and carbonation of peridotite," Earth Planet. Sci. Lett. 291, Issues 1-4, 2010, pp. 215 - 227. [4] Røyne, A., Jamtveit, B., and Malthe-Sørenssen, A., "Controls on rock weathering rates by reaction-induced hierarchial fracturing," Earth Planet. Sci. Lett. 275, 2008, pp. 364 - 369.

Experimental Determination of the Cosmogenic Ar Production Rate From Ca

NASA Astrophysics Data System (ADS)

Niedermann, S.; Schäfer, J. M.; Wieler, R.; Naumann, R.

2005-12-01

Cosmogenic 38Ar is produced in terrestrial surface rocks by spallation of target nuclides, in particular K and Ca. Though the presence of cosmogenic Ar in Ca-rich minerals has been demonstrated earlier [1], is has proven difficult to establish its production rate. To circumvent problems connected to 36Ar production by 35Cl neutron capture and different production rates from K and Ca, we have analyzed the noble gases in seven pyroxene separates (px) from the Antarctic Dry Valleys which are essentially free of Cl and K. The px were obtained from dolerite rocks, for which 3He and 21Ne exposure ages from 1.5 to 6.5 Ma have been reported [2]. The noble gases were extracted in two or three heating steps at GFZ Potsdam, yielding 38Ar/36Ar ratios up to 0.2283 ± 0.0008 (air: 0.1880). Ca (3.7-11.2 wt. %) is expected to be the only relevant target element for Ar production in the five pure px (<0.05% K); the production rate from Fe is at least two orders of magnitude lower than that from Ca [e.g. 3]. Assuming an 38Ar/36Ar production ratio of 1.5 ± 0.2, we obtain cosmogenic 38Ar concentrations between 130 and 530x106 atoms/g. The 38Ar production rate was calculated based on 21Ne exposure ages [2], corrected for elevated nuclide production in Antarctica due to prevailing low air pressure and for the revised 21Ne production rate from Si. We obtain values between 188 ± 17 and 243 +110/-24 atoms (g Ca)-1 a-1 at sea level and high (northern) latitudes for four out of the five pure px, while one yields a very high value of 348 ± 70 atoms (g Ca)-1 a-1. Values above 250 atoms (g Ca)-1 a-1 are also obtained from two less pure px containing 0.3 and 0.9% K and from one feldspar/quartz accumulate, indicating that the production rate from K may be higher than that from Ca. The weighted mean (excluding the outlier) of ~200 atoms (g Ca)-1 a-1 is in excellent agreement with Lal's [3] theoretical estimate. [1] Renne et al., EPSL 188 (2001) 435. [2] Schäfer et al., EPSL 167 (1999) 215. [3] Lal, EPSL 104 (1991) 424.

Quantitative Investigations of Polygonal Ground in Continental Antarctica: Terrestrial Analogues for Polygons on Mars

NASA Astrophysics Data System (ADS)

Sassenroth, Cynthia; Hauber, Ernst; Schmitz, Nicole; de Vera, Jean Pierre

2017-04-01

Polygonally fractured ground is widespread at middle and high latitudes on Mars. The latitude-dependence and the morphologic similarity to terrestrial patterned ground in permafrost regions may indicate a formation as thermal contraction cracks, but the exact formation mechanisms are still unclear. In particular, it is debated whether freeze-thaw processes and liquid water are required to generate the observed features. This study quantitatively investigates polygonal networks in ice-free parts of continental Antarctica to help distinguishing between different hypotheses of their origin on Mars. The study site is located in the Helliwell Hills in Northern Victoria Land ( 71.73°S/161.38°E) and was visited in the framework of the GANOVEX XI expedition during the austral summer of 2015/2016. The local bedrock consists mostly of sediments (sandstones) of the Beacon Supergroup and mafic igneous intrusions (Ferrar Dolerites). The surfaces are covered by glacial drift consisting of clasts with diverse lithologies. Thermal contraction cracks are ubiquitous. We mapped polygons in the northern part of Helliwell Hills in a GIS environment on the basis of high-resolution satellite images with a pixel size of 50 cm. The measured spatial parameters include polygon area, perimeter, length, width, circularity and aspect. We also analyzed the connectivity of enclosed polygons within a polygon network. The polygons do not display significant local relief, but overall the polygon centers are slightly higher than the bounding cracks (i.e. high-center polygons). Sizes of polygons can vary widely, dependent on the geographical location, between 10m2 and >900m2. In planar and level areas, thermal contraction cracks tend to be well connected as hexagonal or irregular polygonal networks without a preferred alignment. In contrast, polygonal networks on slopes form elongated, orthogonal primary cracks, which are either parallel or transverse to the steepest topographic gradient. During fieldwork, excavations were made in the center of polygons and across the bounding cracks. Typically, the uppermost 40 cm of regolith are dry and unconsolidated. Below that, there is commonly a sharp transition to ice-cemented material or very clear ice with no bubbles. Soil profiles were recorded, and sediment samples were taken and analyzed for their grain size composition with laser diffractometric measurement methods. External factors such as slope gradient and orientation, insolation and composition of surface and subsurface materials were included in the analysis.

Superhot Drilling in Iceland, the Experience of the Iceland Deep Drilling Project.

NASA Astrophysics Data System (ADS)

Elders, W. A.; Friðleifsson, G. Ó.; Zierenberg, R. A.; Fowler, A. P.

2017-12-01

The Iceland Deep Drilling Project aims to improve geothermal economics by producing supercritical fluids (www.iddp.is). Supercritical wells could yield an order of magnitude more usable energy than that from conventional geothermal wells because of higher enthalpy and enhanced flow properties. In 2009, the IDDP-1 well failed to reach supercritical conditions in the Krafla caldera in NE Iceland, after encountering rhyolite magma at only 2.1 km depth. The completed geothermal well became the world's hottest and produced superheated steam with a wellhead temperature of 452°C and flow sufficient to generate 35 MWe. The IDDP next moved SW to the Reykjanes Peninsula, the landward extension of the Mid-Atlantic Ridge, where it is possible to study an analog of the roots of a black smoker. Reykjanes is unique among Icelandic geothermal systems in being recharged by seawater, which has a critical point of 406°C at 298 bars. Drilling began by deepening an existing 2.5 km deep production well to 3 km depth, and then angling it towards the main upflow zone of the system, for a total slant depth of 4,659 m. Total circulation losses were encountered below 3 km that could not be cured by lost circulation materials or by multiple cement jobs. Accordingly, drilling continued to total depth without return of drill cuttings. We attempted 13 core runs below 3 km depth, only half of which recovered core. The cores are basalts and dolerites with alteration ranging from lower greenschist facies to lower amphibolite facies, suggesting formation temperatures >450°C. After the end of drilling in January 2017, following only six days of heating, supercritical conditions (426°C at 340 bars) were measured in the well at a depth of 4.5 km. The well has not yet been allowed to equilibrate to full in situ temperature. A perforated liner was inserted to 4,570 m, depth to facilitate temperature cycling to enhance permeability at depth through thermal cracking. In 2018 this will be followed by a flow test and eventual production of the well. The project is co-funded by the DEEPEGS project (EU H2020), HS Orka (the field operator), Statoil, the IDDP consortium, and the ICDP. Planning is underway to drill IDDP-3 at Hellisheidi.

Absolute Geomagnetic Paleointensity as Recorded by Mafic Dykes of the ~1.98 Ga Bundelkhand Swarm and ~0.75 Ga Malani Igneous Suite from Northern India

NASA Astrophysics Data System (ADS)

Piispa, E. J.; Smirnov, A. V.; Pandit, M. K.

2012-12-01

Determining the long-term behavior and configuration of the Precambrian geomagnetic field is crucial for understanding the origin and nature of Earth's early geodynamo. However, our knowledge about strength and morphology of the Precambrian geomagnetic field is extremely limited due to paucity of reliable data on the ancient field strength (paleointensity). Information correlating the strength and characteristics of Earth's ancient geomagnetic field can be gained by measuring the paleodirectional and paleointensity properties of Precambrian rocks. We investigated two Proterozoic mafic dyke swarms from the Indian subcontinent: the extensive NW-SE trending 1979±8 Ma (U-Pb) dyke swarm in the Bundelkhand craton and the N-S trending Malani mafic dyke swarm, the latter representing the third and final phase of magmatism in the Malani Igneous Suite. Malani rhyolites have been precisely dated at 771±2 to 751±3 Ma (U - Pb zircon ages). The Malani mafic dykes have been correlated with the ~750 Ma dolerite dykes of Seychelles based on geological and geochemical criteria. The mafic dykes of both studied swarms are vertical to sub-vertical and show little or no evidence of alteration. Detailed paleomagnetic studies, using both thermal and alternating field demagnetization, revealed the presence of stable dual-polarity magnetic component for both dyke swarms. The primary nature of the magnetization is supported by positive baked contact tests. Typically, the characteristic magnetization is single component with narrow unblocking temperature spectra between ~500°C and 550-570°C, with remanence carried by small PSD magnetite or low-Ti titanomagnetite. Absolute paleointensities of these dyke swarms were obtained by two different heating based methods: multiple specimen domain-state corrected (MSP-DSC) and Thellier double heating method with alternating infield-zerofield (IZ) and zerofield-infield (ZI) steps. The multiple sample protocol incorporated checks for alteration, additivity and reciprocity during heating. The magnetic stability of samples was checked using thermomagnetic curves and by monitoring the magnetic susceptibility changes through the paleointensity experiments. Magnetization measurements before and after low temperature demagnetization (LTD) in liquid nitrogen allowed comparison of the effectiveness of the LTD treatment with other experimental domain state corrections. We will discuss implications of our results for the Precambrian geomagnetic field evolution.

Hydration of the Atlantis Massif: Halogen, Noble Gas and In-Situ δ18O Constraints

NASA Astrophysics Data System (ADS)

Williams, M. J.; Kendrick, M. A.; Rubatto, D.

2017-12-01

A combination of halogen (Cl, Br, I), noble gases (He, Ne, Ar, Kr and Xe) and in situ oxygen isotope analysis have been utilized to investigate the fluid-mobile element record of hydration and alteration processes at the Atlantis Massif (30°N on the Mid-Atlantic Ridge). The sample suite investigated includes serpentinite, talc-amphibole ± chlorite schist and hydrated gabbro recovered by seafloor drilling undertaken at sites on a transect across the Atlantis Massif during IODP Expedition 357. Serpentine mesh and veins analysed in-situ by SHRIMP SI exhibit δ18O from 6‰ down to ≈0‰, suggesting serpentinization temperatures of 150 to >280°C and water/rock ratios >5. Differences of 1.5-2.5‰ are observed between adjacent generations of serpentine, but the δ18O range is similar at each investigated drilling site. Halogen and noble gas abundances in serpentinites, talc-amphibole schist and hydrated gabbro have been measured by noble gas mass spectrometry of both irradiated and non-irradiated samples. Serpentinites contain low abundances of halogens and noble gases (e.g. 70-430 ppm Cl, 4.7-12.2 x 10-14 mol/g 36Ar) relative to other seafloor serpentinites. The samples have systematically different Br/Cl and I/Cl ratios related to their mineralogy. Serpentinites retain mantle-like Br/Cl with a wide variation in I/Cl that stretches toward seawater values. Talc-amphibole schists exhibit depletion of Br and I relative to Cl with increasing Cl abundances, suggesting tremolite exerts strong control on halogen abundance ratios. Serpentinites show no evidence of interaction with halogen-rich sedimentary pore fluids. Iodine abundances are variable across serpentinites, and are decoupled from Br and Cl; iodine enrichment (up to 530 ppb) is observed within relatively oxidised and clay-bearing samples. Serpentinized harzburgites exhibit distinct depletion of Kr and Xe relative to atmospheric 36Ar in seawater. Oxygen isotope compositions and low abundances of both halogens and noble gases are consistent with high water/rock ratios. Successive generations of serpentine have δ18O trends suggesting exposure to higher W/R ratios during exhumation and deformation of the massif. Low noble gas abundances of may also be influenced by thermal loss related to impregnation and intrusion of the Massif by gabbros and dolerites.

Transient versus long-term strength of the "dry" lower continental crust (Musgrave Ranges, Central Australia)

NASA Astrophysics Data System (ADS)

Mancktelow, Neil; Hawemann, Friedrich; Wex, Sebastian; Pennacchioni, Giorgio; Camacho, Alfredo

2017-04-01

One-dimensional yield strength envelope or "Christmas tree" models for the strength of the continental lithosphere assume homogeneous deformation at constant strain-rate and generally predict that felsic lower crust should be viscous and relatively weak. Over the longer term, distributed flow of this supposedly weak lower crust should tend to flatten any irregularities in the Moho. However, these model predictions are in direct contradiction to observations from the well-exposed lower-crustal Fregon Subdomain in the Musgrave Ranges, Central Australia. This unit underwent dehydrating granulite facies metamorphism during the ca. 1200 Ma Musgravian Orogeny. During the subsequent Petermann Orogeny (ca. 550 Ma), these effectively "dry" rocks were very heterogeneously deformed under sub-eclogitic, lower-crustal conditions (ca. 650°C, 1.2 GPa). Shear zones localized over a wide range of thickness and length scales, from mm to km. Widespread and repeated fracturing and pseudotachylyte generation also occurred during the same deformation event, providing weak and approximately planar precursors on which viscous shear zones subsequently localized. On the lithospheric scale, the present day Moho still preserves an offset on the order of 20 km that was caused by the Petermann Orogeny. Brittle fracturing of dry rocks and related pseudotachylyte formation at pressures of ca. 1.2 GPa imply high differential stresses on the order of 1 GPa, if the Mohr-Coulomb yield criterion is still approximately correct at such high confining pressure. High stresses, at least transiently, are also implied by the observed local fracturing of granulite-facies garnets in the vicinity of pseudotachylytes. However, the stress associated with slower crystal-plastic flow appears to be much less, on the order of 10's of MPa, as indicated by the dynamically recrystallized grain size of quartz. Several other observations also indicate that the long-term viscous strength could not have been maintained at GPa levels: (1) viscous reactivation of fractures that are highly misoriented, with planes at a large angle to the shortening direction; (2) the lack of any discernible pressure difference between doleritic dykes oriented at varying angles to the shortening direction (i.e. no tectonic overpressure or underpressure effects); and (3) the lack of evident long-term shear heating on major shear zones. The implication is that the high differential stress must have occurred as transient pulses, causing repeated seismic fracturing of lower crustal rocks that on the longer term were deforming by crystal-plastic viscous creep at much lower differential stress.

Triassic deposits of the Chukotka Arctic continental margin (sedimentary implications and detrital zircon data)

NASA Astrophysics Data System (ADS)

Tuchkova, Marianna; Sokolov, Sergey; Verzhbitsky, Vladimir

2013-04-01

Triassic clastic deposits of Chukotka are represented by rhythmic intercalation of sandstones, siltstones and mudstones. During the Triassic, sedimentation was represented by continental slope progradation. Detrital zircons from Triassic sedimentary rocks were collected for constrain its paleogeographic links to source terranes. Zircons populations from three Chukotka's samples are very similar, and youngest zircon ages show peaks at 236-255 Ma (Miller et al., 2006). Lower Triassic sandstones from the Chaun subterrane do not contain the young population 235-265 Ma that is characteristic of the Upper Triassic rocks from the Anyui subterrane and Wrangel Island. The young zircon population is missing also from the coeval Sadlerochit Group (Alaska) and Blind Fiord Formation of the Sverdrup basin (Miller et al., 2006; Omma et al., 2011). Our data of Triassic sandstones of Wrangel island demonstrate detrital zircons ages dominated by Middle Triassic (227-245 Ma), Carboniferous (309-332 Ma) and Paleoproterozoic (1808-2500 Ma) ages. The new data on Chukotka show that populations of detrital zircons from Chukotka, the Sverdrup basin, and Alaska, the Sadlerochit Mountains included, demonstrate greater similarity than it was previously thought. Consequently, it may be assumed that they originate from a single source situated in the north. The data on zircon age of gabbro-dolerite magmatism in eastern Chukotka (252 Ma. Ledneva et al., 2011) and K-Ar ages obtained for sills and small intrusive bodies (Geodynamics…, 2006) in Lower Triassic deposits allow the local provenance. The presence of products of synchronous magmatism and shallow-water facies in the Lower Triassic sequences confirm this assumption. At the same time, coeval zircons appear only in the Upper Triassic strata. It is conceivable that the young zircon population originates from intrusive, not volcanic rocks, which were subjected to erosion only in the Late Triassic. In our opinion, the assumption of the local source with synchronous magmatism is consistent with the evolution of the petrological-mineralogical and geochemical compositions in the Triassic sandstones of Chukotka. Similar zircon peaks in Triassic rocks of northern Wrangel Island, Sverdrup basin, and Alaska indicate the same provenances for the Triassic periods. It is possible that all obtained data may indirectly support existence of the hypothetical "Hyperborean Platform" or Crockerland-Arctida microcontinent Work was supported by RBRR projects 11-05-00787, 11-05-00074, Scientific school # NSh-5177.2012.5, kontrakts 01/14/20/11; and we are extremely grateful to the TGS company for execution of a joint research project "Geological history of Wrangel Island".

Petrology, geochronology and emplacement model of the giant 1.37 Ga arcuate Lake Victoria Dyke Swarm on the margin of a large igneous province in eastern Africa

NASA Astrophysics Data System (ADS)

Mäkitie, Hannu; Data, Gabriel; Isabirye, Edward; Mänttäri, Irmeli; Huhma, Hannu; Klausen, Martin B.; Pakkanen, Lassi; Virransalo, Petri

2014-09-01

A comprehensive description of the petrography, geochemical composition, Sm-Nd data and intrinsic field relationships of a giant arcuate Mesoproterozoic mafic dyke swarm in SW Uganda is presented for the first time. The swarm is ∼100 km wide and mainly hosted in the Palaeoproterozoic Rwenzori Belt between the Mesoproterozoic Karagwe-Ankole Belt and the Archaean Uganda Block. The dykes trend NW-SE across Uganda, but can be correlated across Lake Victoria to another set of arcuate aeromagnetic anomalies that continue southwards into Tanzania, resulting in a remarkably large semi-circular swarm with an outer diameter of ∼500 km. We propose that this unique giant dyke structure be named the Lake Victoria Dyke Swarm (LVDS). The dykes are tholeiites with Mg numbers between 0.69 and 0.44, and with inherited marked negative Nb and P anomalies in spider diagrams. Two dykes provide Sm-Nd mineral ages of 1368 ± 41 Ma and 1374 ± 42 Ma, with initial εNd values of -2.3 and -3.2, and 87Sr/86Sr ratios of ∼0.706-0.709. Geotectonic discrimination diagrams for the swarm exhibit more arc type than within-plate tectonic signatures, but this is in accordance with systematic enrichments in LREE, U and Th in the dolerites, more likely due to the involvement of the continental lithosphere during their petrogenesis. The LVDS is coeval with a regional ∼1375 Ma bimodal magmatic event across nearby Burundi, Rwanda and NW Tanzania, which can collectively be viewed as a large igneous province (LIP). It also indicates that the nearby Karagwe-Ankole Belt sequences - bracketed between 1.78 and 1.37 Ga and assumed by some to have been deposited within intracratonic basins - were capped by flood basalts that have subsequently been removed by erosion. Different geochemical signatures (e.g. LaN/SmN) suggest that most of the arcuate swarm was derived from an enriched SCLM, whereas related intrusions in the centre of this semi-circular segment have more or less enriched asthenospheric mantle source signatures. A model of how the LIP configuration formed, and especially its giant arcuate swarm, requires fortuitous pre-existing structures, an unusually large sub-crustal magma chamber, and/or some very intrinsic rift process. The LIP is apparently related to a global 1.4-1.2 Ga rifting event that led to the break-up of the Columbia/Nuna supercontinent.

Debris flow hazard mapping, Hobart, Tasmania, Australia

NASA Astrophysics Data System (ADS)

Mazengarb, Colin; Rigby, Ted; Stevenson, Michael

2015-04-01

Our mapping on the many dolerite capped mountains in Tasmania indicates that debris flows are a significant geomorphic process operating there. Hobart, the largest city in the State, lies at the foot of one of these mountains and our work is focussed on identifying areas that are susceptible to these events and estimating hazard in the valley systems where residential developments have been established. Geomorphic mapping with the benefit of recent LiDAR and GIS enabled stereo-imagery has allowed us to add to and refine a landslide inventory in our study area. In addition, a dominant geomorphic model has been recognised involving headward gully retreat in colluvial materials associated with rainstorms explains why many past events have occurred and where they may occur in future. In this paper we will review the landslide inventory including a large event (~200 000m3) in 1872 that affected a lightly populated area but since heavily urbanised. From this inventory we have attempted volume-mobility relationships, magnitude-frequency curves and likelihood estimates. The estimation of volume has been challenging to determine given that the area of depletion for each debris flow feature is typically difficult to distinguish from the total affected area. However, where LiDAR data exists, this uncertainty is substantially reduced and we develop width-length relationships (area of depletion) and area-volume relationships to estimate volume for the whole dataset exceeding 300 features. The volume-mobility relationship determined is comparable to international studies and in the absence of reliable eye-witness accounts, suggests that most of the features can be explained as single event debris flows, without requiring more complex mechanisms (such as those that form temporary debris dams that subsequently fail) as proposed by others previously. Likelihood estimates have also been challenging to derive given that almost all of the events have not been witnessed, some are constrained by aerial photographs to decade precision and many predate regional photography (pre 1940's). We have performed runout modelling, using 2D hydraulic modelling software (RiverFlow2D with Mud and Debris module), in order to calibrate our model against real events and gain confidence in the choice of parameters. Runout modelling was undertaken in valley systems with volumes calibrated to existing flood model likelihoods for each catchment. The hazard outputs from our models require developing a translation to hazard models used in Australia. By linking to flood mapping we aim to demonstrate to emergency managers where existing mitigation measures may be inadequate and how they can be adapted to address multiple hazards.

Geochronology and geochemistry of the granites from the Zhuxi W-Cu ore deposit in South China: Implication for petrogenesis, geodynamical setting and mineralization

NASA Astrophysics Data System (ADS)

Pan, Xiaofei; Hou, Zengqian; Zhao, Miao; Chen, Guohua; Rao, Jianfeng; Li, Yan; Wei, Jin; Ouyang, Yongpeng

2018-04-01

The giant Zhuxi tungsten deposit is located in the Taqian-Fuchun Ore Belt in northeastern Jiangxi province, and genetically associated with the Zhuxi granitic stocks and dykes. Three mineralization-related granites including granite porphyry dykes (GP), biotite granitic stocks (BG), and white granitic dykes (WG), were identified in the Zhuxi deposit. SHRIMP zircon U-Pb analysis for the three granitic rocks present ages ranging from 153.5 ± 1.0 Ma to 150.4 ± 1.0 Ma. The BG mainly contains quartz, microcline, albite, biotite and muscovite with minor accessory minerals including zircon, apatite, monazite, Ti/Fe oxides, and dolerite. However, the WG is mainly composed of quartz, microcline and albite with minor muscovite and accessory minerals. The GP is a medium-grained porphyritic granite and its phenocrysts include quartz, alkali feldspar, muscovite and plagioclase. All the Zhuxi granites have high SiO2 content (71.97 wt%-81.19 wt%) and total alkali (3.25 wt%-9.42 wt%), and their valid aluminum saturation index (ASI) values show a wide range of 1.03 to 2.49. High Rb/Sr ratios, low Sr content (<50 ppm) and markedly negative Eu anomalies of GP, WG and BG demonstrated that the Zhuxi granites are highly fractioned and intensive crystal differentiated. Because they display the features of both I- and S-types granites, they were confirmed to be I-S transform-type granites. Whole rock εNd(t) and zircon εHf(t) values fall into the ranges of -6.98 to -11.97, and -3.1 to -11.5, and the Nd (TDM2) and Hf two-stage model ages (TDMc) are 1.51-1.92 Ga and 1.42-2.01 Ga, respectively. Geochemical and isotopic data suggest that these highly fractionated I-S transform-type granites were originated from magmas which showed affinity with the Proterozoic continent and the Shuangqiaoshan Group and little mantle contribution was involved during the generation of Zhuxi granitic rocks. Extreme fractional crystallization resulted in further enrichment of tungsten in the evolved granitic magma. New data, presented together with previously published data, suggest that the Zhuxi granitic complex was likely to be formed during lithospheric compression setting during the late Jurassic to early Cretaceous. The biotite granite stock predominately contributed to the production of skarn alteration and mineralization, followed by the white granite dyke; the granite porphyry dykes have little effect.

The Iceland Deep Drilling Project 4.5 km deep well, IDDP-2, in the seawater-recharged Reykjanes geothermal field in SW Iceland has successfully reached its supercritical target

NASA Astrophysics Data System (ADS)

Friðleifsson, Guðmundur Ó.; Elders, Wilfred A.; Zierenberg, Robert A.; Stefánsson, Ari; Fowler, Andrew P. G.; Weisenberger, Tobias B.; Harðarson, Björn S.; Mesfin, Kiflom G.

2017-11-01

The Iceland Deep Drilling Project research well RN-15/IDDP-2 at Reykjanes, Iceland, reached its target of supercritical conditions at a depth of 4.5 km in January 2017. After only 6 days of heating, the measured bottom hole temperature was 426 °C, and the fluid pressure was 34 MPa. The southern tip of the Reykjanes peninsula is the landward extension of the Mid-Atlantic Ridge in Iceland. Reykjanes is unique among Icelandic geothermal systems in that it is recharged by seawater, which has a critical point of 406 °C at 29.8 MPa. The geologic setting and fluid characteristics at Reykjanes provide a geochemical analog that allows us to investigate the roots of a mid-ocean ridge submarine black smoker hydrothermal system. Drilling began with deepening an existing 2.5 km deep vertical production well (RN-15) to 3 km depth, followed by inclined drilling directed towards the main upflow zone of the system, for a total slant depth of 4659 m ( ˜ 4.5 km vertical depth). Total circulation losses of drilling fluid were encountered below 2.5 km, which could not be cured using lost circulation blocking materials or multiple cement jobs. Accordingly, drilling continued to the total depth without return of drill cuttings. Thirteen spot coring attempts were made below 3 km depth. Rocks in the cores are basalts and dolerites with alteration ranging from upper greenschist facies to amphibolite facies, suggesting that formation temperatures at depth exceed 450 °C. High-permeability circulation-fluid loss zones (feed points or feed zones) were detected at multiple depth levels below 3 km depth to bottom. The largest circulation losses (most permeable zones) occurred between the bottom of the casing and 3.4 km depth. Permeable zones encountered below 3.4 km accepted less than 5 % of the injected water. Currently, the project is attempting soft stimulation to increase deep permeability. While it is too early to speculate on the energy potential of this well and its economics, the IDDP-2 is a milestone in the development of geothermal resources and the study of hydrothermal systems. It is the first well that successfully encountered supercritical hydrothermal conditions, with potential high-power output, and in which on-going hydrothermal metamorphism at amphibolite facies conditions can be observed. The next step will be to carry out flow testing and fluid sampling to determine the chemical and thermodynamic properties of the formation fluids.

High-Resolution Physical Properties Logging of the AND-1B Sediment Core - Opportunity for Detecting High-Frequency Signals of Paleoenvironmental Changes

NASA Astrophysics Data System (ADS)

Niessen, F.; Magens, D.; Kuhn, G.; Helling, D.

2008-12-01

Within the ANDRILL-MIS Project, a more than 1200 m long sediment core, dating back to about 13 Ma, was drilled beneath McMurdo Ice Shelf near Ross Island (Antarctica) in austral summer 2006/07 with the purpose of contributing to a better understanding of the Late Cenozoic history of the Antarctic Ice Sheet. One way to approach past ice dynamics and changes in the paleoenvironment quantitatively, is the analysis of high- resolution physical properties obtained from whole-core multi-sensor core logger measurements in which lithologic changes are expressed numerically. This is especially applicable for the repeating sequences of diatomites and diamictites in the upper half of the core with a prominent cyclicity between 140-300 mbsf. Rather abrupt high-amplitude variations in wet-bulk density (WBD) and magnetic susceptibility (MS) reflect a highly dynamic depositional system, oscillating between two main end-member types: a grounded ice sheet and open marine conditions. For the whole core, the WBD signal, ranging from 1.4 kg/cu.m in the diatomites to 2.3 kg/cu.m in diamictites from the lower part of the core, represents the influence of three variables: (i) the degree of compaction seen as reduction of porosities with depth of about 30 % from top to bottom, (ii) the clast content with clasts being almost absent in diatomite deposits and (iii) the individual grain density (GD). GD itself strongly reflects the variety of lithologies as well as the influence of cement (mainly pyrite and carbonate) on the matrix grain density. The calculation of residual porosities demonstrates the strong imprint of glacial loading for especially diamictites from the upper 150 m, pointing to a significant thickness of the overriding Pleistocene ice sheet. MS on the other hand mainly documents a marine vs. terrestrial source of sediments where the latter can be divided into younger local material from the McMurdo Volcanic Province and basement clasts from the Transantarctic Mountains. Values range over several orders of magnitude from <10 (10-5 SI) in the diatomites to 8000 (10-5 SI) in single clasts (mainly dolerite). Synchronous minima and maxima in both WBD and MS support dramatic changes in the depositional environment, driven by oscillations in ice extent in response to global climate fluctuations on orbital timescales. Superimposed on this, small-amplitude variations of high frequency are found within diatomite units. A rhythmic pattern of probably millennial to centennial pacing proposes an additional non-orbital forcing as control on system dynamics, at least during interglacials.

Iron-oxide Magnetic, Morphologic, and Compositional Tracers of Sediment Provenance and Ice Sheet Extent in the ANDRILL AND-1B Drill Core, Ross Sea, Antarctica (Invited)

NASA Astrophysics Data System (ADS)

Brachfeld, S. A.; Pinzon, J.; Darley, J. S.; Sagnotti, L.; Kuhn, G.; Florindo, F.; Wilson, G. S.; Ohneiser, C.; Monien, D.; Joseph, L. H.

2013-12-01

The first drilling season of the Antarctic Drilling Program (ANDRILL) recovered a 13.57 million year Miocene through Pleistocene record of paleoclimate change (core AND-1B) within the Ross Sea. The magnetic mineral assemblage records the varying contributions of biological productivity, changing sediment sources, the emergence of volcanic centers, and post-depositional diagenesis. Characterization of bedrock samples from the McMurdo Volcanic Group (MVG) and Transantarctic Mountain (TAM) lithologic units allows us to construct fingerprints for the major source rocks bordering the Ross Sea, and identify their signatures within the AND-1B sediment. Key parameters that can be traced from source rock to sediment for the MVG-derived sediment include a 100-200 C order-disorder transition, titanomaghemite grains with homogenous textures but with substantial Al and Mg content, Fe-spinels with substantial Al, Cr, Mg, and Ti content, and titanomagnetite host grains with 1-3 swarms of ilmenite lamellae (both with variable amounts of oxidation). Distinctive signatures in TAM lithologies include low S-ratios in Koettlitz Group gneisses and Fe-sulfides with magnetite intergrowths in Byrd Glacier basement samples. The Cambrian Granite Harbor Intrusive Complex is characterized by coarse, homogeneous Mn-bearing ilmenite and nearly pure magnetite. The Jurassic dolerites and basalts of the Ferrar Group contain pseudo single domain to stable single domain-sized Fe-oxides with low-Ti content and homogeneous textures. Cu-Fe sulfides are also present in the Ferrar Group. Diamictites in the Pliocene-Pleistocene section of the AND-1B drill core contains Fe-oxide assemblages with MVG-type rock magnetic and textural characteristics, while the Miocene diamictites contain TAM-type signatures. These observations can be explained by increased ice flow from the west during the Miocene and/or the absence of MVG volcanic centers, which had not yet reached a significant size. During the Pliocene and Pleistocene, ice flow was from the south, entraining sediment from MVG volcanic centers south of the drill site. This work demonstrates the utility of using the combination of rock magnetic and electron microscopy signatures of Fe-oxides and Fe-sulfides to serve as provenance tracers in both ice proximal and distal sedimentary units, aiding in the study of ice sheet dynamics, and the identification of ice rafted debris sources and dispersal patterns in the Ross Sea sector of Antarctica.

Petrological evolution of subducted rodingite from seafloor metamorphism to dehydration of enclosing antigorite-serpentinite (Cerro del Almirez massif, southern Spain)

NASA Astrophysics Data System (ADS)

Laborda-López, Casto; López Sánchez-Vizcaíno, Vicente; Marchesi, Claudio; Gómez-Pugnaire, María Teresa; Garrido, Carlos J.; Jabaloy-Sánchez, Antonio; Padrón-Navarta, José Alberto

2016-04-01

Rodingites are common rocks associated with serpentinites in exhumed terrains that experienced subduction and high pressure metamorphism. However, the response of these rocks to devolatilization and redox reactions in subduction settings is not well constrained. In the Cerro del Almirez ultramafic massif (southern Spain) rodingites constitute about 1-2% of the total volume of exposed rocks. Metarodingites are enclosed in antigorite-serpentinite and chlorite-harzburgite separated by a transitional zone that represents the front of prograde serpentinite-dehydration in a paleo-subduction setting (Padrón-Navarta et al., 2011). Metarodingites occur as boudin lenses, 1 to 20 m in length and 30 cm to 2 m in thickness. During serpentinization of peridotite host rocks, dolerites and basalts precursor of rodingites underwent intense seafloor metasomatism, causing the enrichment in Ca and remobilization of Na and K. Subsequent metamorphism during subduction transformed the original igneous and seafloor metamorphic mineralogy into an assemblage of garnet (Ti-rich hydrogrossular), diopside, chlorite, and epidote. During prograde metamorphism, garnet composition changed towards higher andradite contents. High-pressure transformation of enclosing antigorite-serpentinite to chlorite-harzburgite released fluids which induced breakdown of garnet to epidote in metarodingites. Ti liberation by this latter reaction produced abundant titanite. Released fluids also triggered the formation of amphibole by alkalis addition. Highly recrystallized metarodingites in chlorite-harzburgite present a new generation of idiomorphic garnet with composition equal to 10-30% pyrope, 30-40% grossular and 35-55% almandine + spessartine. This garnet has titanite inclusions in the core and rutile inclusions in the rim. The contact between metarodingites and ultramafic rocks consists of a metasomatic zone (blackwall) with variable thickness (7 to 40 cm) constituted by chlorite, diopside, and titanite. Close to the contact with the blackwall, antigorite-serpentinite is very rich in diopside, olivine and Ti-clinohumite. In this study we present a thermodynamic model of phase relationships in rodingites and transitional blackwalls during their metamorphic history. We mainly aim to establish the evolution of P-T conditions experienced by metarodingites during subduction and the influence of fluids in the formation of mineral assemblages at different metamorphic stages. REFERENCES Padrón-Navarta, J.A., López Sánchez-Vizcaíno, V., Garrido, C.J., Gómez-Pugnaire, M.T., (2011): Metamorphic record of high-pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting (Cerro Del Almirez, Nevado-Filábride Complex, Southern Spain). Journal of Petrology, 52, 2047-2078.

New 40ar/39ar Radiometric, Geochemistry And Structural Data On The Giant Okavango Mafic Dike Swarm And Lava-flows From The Karoo Province In Botswana: Implications For Gondwana Break-up.

NASA Astrophysics Data System (ADS)

Jourdan, F.; Tshoso, G.; Féraud, G.; Bertrand, H.; Legall, B.; Tiercelin, J. J.; Kampunzu, A. B.

The lower Jurassic Karoo magmatism represents one of the most important conti- nental flood basalt (CFB) provinces of the Phanerozoic. It is dominated by tholeiites occurring as traps and apparently radiating giant dike swarms and is associated with the disruption of Gondwana and the opening of the Indian Ocean. The Karoo volcanic province located at the South-East of the African continent, covers a surface of about 3x106 km2. Whereas most of the geochronological and geochemical studies were per- formed in the Southern part of the province, very few data are available in the NW. This is particularly the case for lava flows and for the N110 oriented, 1500 km long and 100 km wide giant Okavango Dike Swarm (ODS) of Botswana. Lava-flows were sampled in a 800 m deep borehole located in the SE of Botswana and consist in low- Ti tholeiites. ODS dolerites are characterized mainly by augite and plagioclase with remnants of olivine and are high UTi tholeiites (TiO2> 2 wt%) enriched in LREE relative to HREE (La/Ybn = 3.5-9.7). 40Ar/39Ar plateau ages ranging from 177.3 s´ 2.1 (2 sigma) Ma (-58m deep) to 178.0 s´ 2.2 Ma (-719m deep), and from 178.3 s´ 1.1 Ma to 179.3 s´ 1.2 Ma have been obtained on pure plagioclase separates for the lava-flows and the ODS, respectively. No significant age variation could be identified along the 661m thick lava-flow section, but these lava-flows are slightly younger than both ODS dikes and high-Ti lava-flows from Zimbabwe (Jones et al., 2000, GC, v.2, p110). However, all these basaltic events (both low- and high- Ti) from the north- ern Karoo sub-province appear significantly younger than the southern low-Ti Karoo formations, particularly if we consider 40Ar/39Ar dates obtained only on plagioclase separates, yielding ages which range between 180.3 s´ 1.8 and 184.7 s´ 0.7 Ma (Duncan et al., 1997, Jour. Geoph. Res., v. 102, p18127). Therefore, a time-related northwards migration of the magmatism is suggested. Moreover, one dated ODS dike yields Pro- terozoïc age, suggesting that this ODS branch of the so-called triple junction structure, generally attributed to the Karoo mantle plume, may be at least partly due to injection and rejuvenation of inherited Proterozoïc basement structures.

Volcanism, mantle exhumation and spreading at the axial zone of a fossil slow spreading ocean

NASA Astrophysics Data System (ADS)

Chalot-Prat, F.; Coco, E.

2003-04-01

Within an axial zone of a slow spreading ocean, the mechanisms checking together volcano emplacement, mantle exhumation and ocean enlargement are poorly known. In order to better assess how they could be linked , a detailed mapping of a fossil ocean-floor structure, preserved from alpine tectonic and metamorphism, was performed in the Chenaillet unit (Franco-Italian Alps)(Chalot-Prat &Coco, submit.). The detailed 3D geometry of the ophiolite evidences that from its dimensions, topography, morphology, and the architecture of the volcanic cover at different scales, the Chenaillet unit is a witness of an axial zone of Atlantic type. The basement (serpentinized peridotites and gabbros), below and in the prolongation of the volcanic cover (le50 m), is capped by a tectonic breccias horizon (Chalot-Prat and Manatschal, 2002), underlining detachment faults responsible for its exhumation at the seafloor. Clasts of dolerite, found within the fault zone, indicate that basement exhumation had to be active during and even after volcano emplacement. Stair- and comb-type volcanic systems check the distribution of individual volcanoes; the higher the edifice, the younger it is relative to the others. In the stair-type (up to 600 m of height difference between base and top), each step is formed with a pillow and tube tongue stacking fed from fissural conduits located at the root of each step. This system formed by uplift, step by step fracturation of an already exhumed basement, and magma injection along the fissures once formed. The comb-type (up to 200 m of height difference between base and top) consists in well-defined alignments of pillow and tube conic edifices. Their central feeder dykes are emplaced on the crossing of two types of fractures, oblique (tooth) and parallel (line) to the main branch of the comb. Along a same line, eruptions are coeval as proved by rhythmic variations of major and trace element contents of basalts from one line to another. The comb formation needed initial basement fracturation, then uplift and exhumation of a new basement along the fracture which also controlled magma injection and is materialised by the main branch of the comb. Once formed, volcanoes were then dragged away and down on the travelator to give place to new volcanoes and so on. The building of comb systems was synchronous with an enlargement of the basement surface, the top of which was underlined by a detachment fault at the scale of the system. The pseudo-symmetry of most comb structures evidences that the exhumation process occurred synchronously, but not at the same rate, in opposite directions, as observed at any mid-oceanic ridge axis .

Homogeneous impact melts produced by a heterogeneous target?. Sr-Nd isotopic evidence from the Popigai crater, Russia

NASA Astrophysics Data System (ADS)

Kettrup, B.; Deutsch, A.; Masaitis, V. L.

The 35.7 ± 0.2 Ma old Popigai crater, Siberia, with a diameter of about 100 km is one of the best preserved large terrestrial impact structures. The heterogeneous target at the impact site consists of Archean to Lower Proterozoic metamorphic rocks of the crystalline basement, Upper Proterozoic quartzites and other clastic deposits, as well as Cambrian to Cretaceous clastic sediments and sedimentary rocks, including carbonate rocks. Moreover, Proterozoic and Permo-Triassic dolerite dykes are found in the target area. We report major element, Sr and Nd isotope data for 13 of these target rocks and for various types of impactites. The 15 analysed impactite samples include tagamites (impact melt rocks), suevites and impact glass from small veins. Furthermore, two impact breccias and two impact glass-coated gneiss bombs were analysed. We discuss the relation of these impactites to the target lithologies, and evaluate on the basis of literature data the relation of microkrystites (and associated microtektites) in Upper Eocene sediments to the Popigai event. The impactites have SiO 2 abundances ranging from 59 to 66 wt.% and show significant variations in the content of Fe, Ca, and Ti. They have present day 87Sr/ 86Sr ratios between 0.7191 and 0.7369. Their Sr model ages T SrUR range from 1.9 to 2.3 Ga. The 143Nd/ 144Nd ratios for the impactite samples cluster between 0.5113 and 0.5115. The Nd model ages T NdCHUR range from 1.9 to 2.1 Ga. In an ɛ CHUR(Nd)-ɛ UR(Sr) diagram, the impactites and Upper Eocene microkrystites (and associated microtektites) plot in a field delimited by Popigai target lithologies. The impactites are restricted to the field of crystalline basement rocks and Upper Proterozoic quartzites, but they show different isotopic signatures in different crater sectors. Impactites and Upper Eocene microkrystites plot in different, only partly overlapping clusters. The leucocratic microkrystites and microtektites have a higher affinity to the post-Proterozoic rocks in the target area than the impactites. Seemingly, the melanocratic microkrystites originated mostly from crystalline basement. This data alignment supports the assumption that Popigai is the source crater for all three types of ejecta. For the first time, clear relations are established of the geochemically variable Upper Eocene microkrystites and associated microtektites to specific target lithologies at Popigai crater. Finally, the observed range in Sr and Nd isotope parameters determined for impact melt lithologies that originated during the Popigai event show a much higher variability than known from other craters. This result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias, was incomplete at the time of ejecta dispersal.

Palaeointensity determinations on rocks from the Achaean- Paleoproterozoic dykes from the Karelian craton

NASA Astrophysics Data System (ADS)

Shcherbakov, Valera; Shcherbakova, Valentina; Lubnina, Natalia; Zhidkov, Grigory; Tsel'movich, Vladimir

2017-04-01

The Karelian craton was a fragment of either an earlier late Archean supercontinent, sometimes referred to as Kenorland. Now the craton is a large Archean composite granite-greenstone terrane in the eastern part of the Fennoscandian Shield bounded by Paleoproterozoic Svecofennian orogen in the south-west and by Lapland-Kola orogen in the north-east and Belomorian province in the east-north-east. Mafic dykes, volcanic rocks, sills, and layered intrusions with ages of ca. 2.51-2.45 Ga and ca. 2.06-1.95 Ga are widespread and well-studied in the Karelian Craton. Paleointensity (Banc) results obtained on the Shala dike (age of 2504 Ma by U-Pb, ID TIMS) tracked near vl. Shala and on the Deda island are discussed here. Eighteen block samples of gabbronotires were collected in two sites in the Shala quarry. Stepwise thermal demagnetization (≤ 20 steps, up to 600 C) and stepwise AF demagnetization were done. To monitor possible mineralogical changes during thermal cleaning, magnetic susceptibility was measured after each heating step. Intensive rock magnetic investigations and thermal palaeointensity experiments using the Thellier-Coe (with check-points) and Wilson procedures were carried out. Electronic microscopy study of two samples was made too. For the exception of a viscous component some specimens from the contact zone of the gabbronorite dyke with thin dolerite dyke show two distinct components. The first E-NE intermediate-down direction component was separated at fields up to 50-60 mT and unblocking temperatures up to 520-540 C. The other S-SE low-down direction component is separated at fields from 60 to 100 mT and unblocking temperatures from 540 to 590-600 C. Based on the positive contact tests for the gabbronorite dyke, the S-SE shallow inclination remanence (I = -5.7 degrees) is interpreted to be of primary origin. Reliable palaeointensity determinations Banc fitting a set of selection criteria were determined on 13 samples from 2 sites carrying well-identified S-SE high-temperature components. Mean values of Banc = 49 µT and 48 µT by the Thellier-Coe and Wilson procedures, correspondingly with mean VDM = 12.6x10^22 Am2 what considerably exceeds modern VDM = 7.8x10^22 Am^2). Our finding is in line with the previously reported high VDM  8.4x10^22 Am2 determined on the same Karelian craton for the Burakovka intrusion from mafic dikes of very close age 2450 Ma (Tarduno et al., 2003) thus supporting the idea of high Paleoproterozoic/NeoAchaean geomagnetic field intensity suggested by Tarduno et al. (2006) and Biggin et al. (2009).

AfricaArray International Geophysics Field School: Applications of Near Surface Geophysics to challenges encountered in mine planning

NASA Astrophysics Data System (ADS)

Webb, S. J.; Jones, M. Q.; Durrheim, R. J.; Nyblade, A.; Snyman, Q.

2012-12-01

Hard rock exploration and mining presents many opportunities for the effective use of near surface geophysics. For over 10 years the AfricaArray international geophysics field school has been hosted at a variety of mines in South Africa. While the main objective of the field school is practical training for the next generation of geophysicists, being hosted at a mine has allowed us to investigate applications of near surface geophysics in the early stages of mine planning and development as geophysics is often cheaper and faster than drilling. Several applications include: detailed delineation of dykes and stringer dykes, physical property measurements on drill core for modeling and marker horizons, determination of overburden thickness, locations of water and faults. Dolerite dykes are usually magnetic and are associated with loss of ground (i.e. where the dyke replaces the ore and thus reduces the amount of ore available) and safety/stability concerns. Thus the accurate mapping of dykes and narrow stringers that are associated with them are crucial to the safe planning of a mine. We have acquired several case studies where ground magnetic surveys have greatly improved on the resolution and detail of airborne magnetic surveys in regions of complicated dyke swarms. In many cases, thin stringer dykes of less than 5 cm have been detected. Physical property measurements of these dykes can be used to distinguish between different ages of dykes. It is important to accurately determine overburden thickness when planning an open pit mine as this directly affects the cost of development. Depending on the nature of the overburden, both refraction seismic and or DC resistivity can provide continuous profiling in the area of interest that fills in gaps between boreholes. DC resistivity is also effective for determining water associated with dykes and structures that may affect mine planning. The field school mainly addresses the training of a variety of students. The core students are the geophysics Honours students (~4th year undergraduates). In addition, up to 8 students from all over Africa are included in the program to help address practical training in Africa. The final cohort are minority students from the USA. Participants spend a week planning and costing out surveys, a week in the field collecting data using different methods including: gravity, DGPS, magnetics, resistivity, refraction seismic, EM methods, core logging and physical property measurements. The final week is spent interpreting and integrating their results. Graduate students are given the opportunity to instruct on the field school and manage the logistics for a particular method. The field school is unique in Africa and satisfies a need for practical training with limited resources, with a rare blend of cultural interactions!

A reappraisal of the age, origin and structural setting of sulphide mineralisation in the UK North Pennines Orefield

NASA Astrophysics Data System (ADS)

Holdsworth, Bob; Dempsey, Eddie; Selby, David; Le Cornu, Chris; Young, Brian

2015-04-01

The North Pennines Orefield (NPO) is centred on the Alston block, a structural high of fractured Carboniferous sedimentary rocks that unconformably overlie a Devonian age (ca. 399 Ma) granite pluton buried at shallow depths (<0.5 km). The orefield has long been considered to be a classic example of a Mississippi Valley Type (MVT) deposit where the source of the metals and sulphur are derived by hydrothermal leaching of the host sedimentary (carbonate-rich) rocks. The vein-hosted part of the orefield consists of linked systems of shear and tensile fractures with a variety of regionally recognised orientations (ESE-WNW Quarter Point, NE-SW, NW-SE Cross Veins). These are associated with lead (galena), iron (pyrite, pyrrhotite, marcasite), copper (chalcopyrite), zinc (sphalerite), fluorite, barite and quartz mineralization. New Rhenium-Osmium (Re-Os) isotope geochemical analysis of the vein-hosted pyrite mineralization suggests that: (i) the metalliferous ores of the NPO formed ca. 294Ma (earliest Permian); and (ii) that they carry an initial Os ratio indicative of a mantle source similar to that indicated by the initial Os ratio of the Whin Sill dolerite suite (emplacement ages ca. 297-294 Ma). New field observations and stress inversion analyses show that at least two regional deformation events are recognised in the Carboniferous host rocks of the NPO. A initial phase of Late Carboniferous ('Variscan') N-S compression pre-dates mineralisation and leads to formation of the NW-SE fractures, initiation of the Burtreeford Disturbance as a N-S fault and compressional reactivation of the previously extensional E-W Lunedale Fault. A later phase of dextral transtension (NNE-SSW extension, ESE-WNW compression) leads to the formation of the ESE-WNW and NE-SW veins, together with compressional reactivation of the Burtreeford Disturbance and Lunedale Fault. Field and microstructural analyses show that the transtensional deformation is synchronous with the main phases of NPO mineralisation and also with emplacement of the Whin Sill and associated intrusions. We conclude that: (i) the main phase of NPO mineralization occurred synchronously with regional dextral transtension during the earliest Permian; (ii) that mineralization is genetically linked to a mantle source and (iii) that the genesis of the NPO is closely linked to that of the broadly penecontemporaneous Whin Sill and associated intrusions in northern England. Our new findings are consistent with structural histories recognised in adjacent regions (e.g. Dent-Pennine Fault systems; Northumberland Basin) and point to a major regional phase of mantle-sourced mineralization, igneous intrusion and transtensional deformation in the early Permian. Previous models suggesting that the NPO is a classic example of a MVT mineral deposit or that the mineralizing fluids are related to the influx of Mesozoic brines are largely incorrect.

The McMurdo Dry Valleys Magmatic Laboratory Workshop of 2005 in Antarctica

NASA Astrophysics Data System (ADS)

Marsh, B. D.; Simon, A.; Charrier, A. D.; Hersum, T. G.; Eschholz, E.

2005-12-01

In January of 2005, twenty-five petrologists, volcanologists, geochemists, structural geologists, and magma dynamicists spent two weeks studying and discussing the Magmatic Mush Column represented by the 180 Ma Ferrar Dolerites of the McMurdo Dry Valleys, Antarctica. This exceptionally well-exposed system shows a series of massive interconnected sills culminating in a capping of regional flood basalts. The lowermost sill, the Basement Sill, contains a massive ultramafic tongue of large phenocrysts of orthopyroxene (Opx) with subordinate Cpx and much smaller plagioclase. The 3-D distribution of this Opx Tongue serves as a tracer for the filling dynamics and local motion of the magma. Ponding of the Basement Sill has resulted in a small (500 m), but exceedingly diversified and extensively layered ultramafic intrusion, the Dais Intrusion. Because of the relatively rapid cooling time of this body, the Dais textures have been preserved before extensive annealing, which presents the possibility of using these textures to understand those of much larger, slowly cooled bodies. The combination of seeing in detail a wide variety of exceptional field relations depicting layering, sill emplacement mechanics, internal ordering and crystal sorting in the Opx Tongue, dike and fissure distributions, wall rock thermal effects, and many other first order features of central interest to understanding magmatic processes and performing research in real time was a new challenge to all involved. Facilities were set up at McMurdo Station for rock cutting, thin-section making, map making, GIS analysis, petrographic analysis, and computer modeling using existing chemical and physical data on a spectrum of the representative rock types. At any one time half the group was housed in the field in Bull Pass near Wright Valley and the remaining group was shuttled in by helicopter each day. The principal groups were switched about every three days. Areas for daily field-work were decided upon by each individual through daily sign up sheets and field parties were shuttled to these locations each morning either from camp or directly from McMurdo. Even though this system presented unusually challenging logistical efforts, the system worked smoothly and routinely and the maximum possible science was carried out in the available time. Many new scientific insights were made into the formation, operation, petrogenesis, and dynamics of this magmatic system, which are directly applicable to planetary magmatism in general. Moreover, students were able to see in real time the process of research by groups of seasoned professionals. Hats off to the National Science Foundation, to the helicopter Pilots of PHI, to the support teams in McMurdo and Bull Pass, and to the participants for a safe and enormously productive scientific expedition.

Insights from analog gelatin experiments on the effect of bedding dip on sill morphology and crystal load

NASA Astrophysics Data System (ADS)

Currier, R. M.; Marsh, B. D.; Mittal, T.

2010-12-01

The profusion of sills the world over offers a wide spectrum of geologic conditions under which to study emplacement mechanisms and the establishment of the initial conditions governing the subsequent magmatic evolution. Many diabase/dolerite sills are featureless bodies whose only record of solidification is contained in the variation of crystal size. But other sills formed of magma containing crystals entrained from earlier crystallization episodes often show a rich history of interaction between settling crystals and solidification fronts such that the physical history of differentiation can be readily observed. This work explores this aspect of sills using visco-elastic gelatin as country rock, molten wax as magma and tiny particles as phenocrysts. Magmatic sills form mechanically, when an ascending dike encounters a more rigid layer, is diverted laterally, and systematically inflates as guided along by the interface. In this manner, sills grow about the injection site, and can do so symmetrically or asymmetrically. The degree of asymmetry is affected by the dip angle of the interface. An angled interface implies a directional pressure gradient, and magma flows preferentially in the direction of decreasing pressure, in this case, up tilt. So, the greater the tilt, the greater the asymmetry. By experimentally producing sills in layered, tilted, media, we have investigated the influence of bed dip on sill morphology. Experiments were performed by injecting wax and particles into gelatin where the layers were poured at set angles to mimic tilted bedding. In addition to its visco-elastic properties, gelatin also has the added benefit of transparency, allowing for direct observation during the experiment and can be washed away later to reveal the exact details of the remaining solid. To emulate magma as a multi-phase slurry, a magmatic analog was used consisting of a mixture of molten wax near its liquidus and ultrafine glitter. Wax solidifies in response to thermal loss, as does the liquid portion of magma, affecting emplacement characteristics itself and preserving transient features that would otherwise be lost. The particles act as crystal cargo, and track magmatic flow within the sill. Surprisingly, even though the injection process is a single pulse, the wax-particle combination intimately records several internal lobes that might otherwise be interpreted as a multiple-pulse style emplacement. The end product is a handheld magmatic plumbing system that can be examined in full detail. There are many cases of sills in the geologic record where the original host rock bedding dip at the time of emplacement is unknown due perhaps to subsequent tectonism. In addition to the experiments providing insights on the magmatic evolution of slurries, they may thus also prove useful in inferring the original bed orientation.

Impact of terrain attributes, parent material and soil types on gully erosion

NASA Astrophysics Data System (ADS)

Chaplot, Vincent

2013-03-01

Gully erosion is a worldwide matter of concern because of the irreversible losses of fertile land, which often have severe environmental, economic and social consequences. While most of the studies on the gullying process have investigated the involved mechanisms (either overland flow incision, seepage or piping erosion), only few have been conducted on the controlling factors of gully wall retreat, an important, if not the dominant, land degradation process and sediment source in river systems. In a representative 4.4 km2 degraded area of the Drakensberg foothills (South Africa) the main objective of this study was to evaluate the relationship between the rate of gully bank retreat (GBR) and parent material, soil types and selected terrain attributes (elevation, specific drainage area, mean slope gradient, slope length factor, stream power index, compound topographic index and slope curvatures). The survey of gully bank retreat was performed during an entire hydrological year, from September 2007 to September 2008, using a network of pins (n = 440 from 110 pits). Both the gully contours and pin coordinates were determined, using a GPS with a 0.5 m horizontal accuracy (n = 20,120). The information on the parent material and the soil types was obtained from field observations complemented by laboratory analysis, while terrain attributes were extracted from a 20 m DEM generated from 5 m interval contour lines. The average GBR value for the 6512 m of gully banks found in the area was 0.049 ± 0.0013 m y- 1, which, considering bank height and soil bulk density, corresponded to an erosion rate of 2.30 ton ha- 1 y- 1. There was no significant difference in GBR between sandstone and dolerite and between Acrisols and Luvisols. Despite a weak one-to-one correlation with the selected terrain attributes (r < 0.2), a principal component analysis (PCA), the first two axes of which explained 68% of the data variability, pointed out that GBR was the highest at hillslope inflexion points (profile and plan slope curvatures close to zero), in the vicinity of the head cuts and for drainage areas up to 500 m2, as both situations experience a high removal rate of the soil material produced from the gully bank collapse and protecting gullies from laterally retreating. These results could be used to digitally map the more active gully banks for the improved implementation of preventive measures of gully growth, if high resolution DEMs are available. There remained, however, a certain amount of unexplained variability in the data, that further research studies on the mechanisms and associated factors of control of GBR could help to address.

Hydrothermal modification of host rock geochemistry within Mo-Cu porphyry deposits in the Galway Granite, western Ireland

NASA Astrophysics Data System (ADS)

Tolometti, Gavin; McCarthy, Will

2016-04-01

Hydrothermal alteration of host rock is a process inherent to the formation of porphyry deposits and the required geochemical modification of these rocks is regularly used to indicate proximity to an economic target. The study involves examining the changes in major, minor and trace elements to understand how the quartz vein structures have influenced the chemistry within the Murvey Granite that forms part of the 380-425Ma Galway Granite Complex in western Ireland. Molybdenite mineralisation within the Galway Granite Complex occurred in close association with protracted magmatism at 423Ma, 410Ma, 407Ma, 397Ma and 383Ma and this continues to be of interest to active exploration. The aim of the project is to characterize hydrothermal alteration associated with Mo-Cu mineralisation and identify geochemical indicators that can guide future exploration work. The Murvey Granite intrudes metagabbros and gneiss that form part of the Connemara Metamorphic complex. The intrusion is composed of albite-rich pink granite, garnetiferous granite and phenocrytic orthoclase granite. Minor doleritic dykes post-date the Murvey Granite, found commonly along its margins. Field mapping shows that the granite is truncated to the east by a regional NW-SE fault and that several small subparallel structures host Mo-Cu bearing quartz veins. Petrographic observations show heavily sericitized feldspars and plagioclase and biotite which have undergone kaolinization and chloritisation. Chalcopyrite minerals are fine grained, heavily fractured found crystallized along the margins of the feldspars and 2mm pyrite crystals. Molybdenite are also seen along the margins of the feldspars, crystallized whilst the Murvey Granite cooled. Field and petrographic observations indicate that mineralisation is structurally controlled by NW-SE faults from the selected mineralization zones and conjugate NE-SW cross cutting the Murvey Granite. Both fault orientations exhibit quartz and disseminated molybdenite mineralization. Extensive hydrothermal alteration is observed within 75 meters of veins that exhibit prominent disseminated mineralisation. To investigate associated geochemical alteration 24 samples were selected along two traverses that cross cut two distinct vein structures. XRF analysis results show that calcium decreases from 1.8 - 0.2 wt% and sulphur increases from 0.2 - 0.9 wt% moving away from the mineralized zones which is to be expected due to their mobile nature. Unexpectedly, minor element data shows no fluctuation in Cu concentrations moving away from either vein structures, despite chalcopyrite found greatest near the vein structures. XRF data analysis is underway to compare the non-mobile and mobile elements to investigate the extent of the decreasing and increasing trends moving proximal to the mineralization zones. The relative decrease in calcium may be caused by the exchange of ion end members between feldspars and this will be tested using a WDS electron micro-probe.

Discovery of kimberlite in a magnetically noisy environment: a case study of the Syferfontein and Goedgevonden kimberlites (Invited)

NASA Astrophysics Data System (ADS)

Webb, S. J.; Van Buren, R.

2013-12-01

Airborne geophysical methods play an important role in the exploration for kimberlites. As regions become more intensively explored, smaller kimberlites, which can be extremely difficult to find, are being targeted. These smaller kimberlites, as evidenced by the M-1 Maarsfontein pipe in the Klipspringer cluster in South Africa, can be highly profitable. The Goedgevonden and Syferfontein pipes are small kimberlites (~0.2 ha) ~25 km NNE of Klerksdorp in South Africa. The Goedgevonden pipe has been known since the 1930s and is diamondiferous, but not commercially viable due to small stone size and low quality of stones. In the early 1990s, Gold Fields used this pipe as a typical kimberlite to collect example geophysical data. The nearby (~1 km to the east) Syferfontein pipe is not diamondiferous but was discovered in 1994 as part of a speculative airborne EM survey conducted by Gold Fields and Geodass (now CGG) as part of their case study investigations. Both kimberlites have had extensive ground geophysical survey data collected and have prominent magnetic, gravity and EM responses that aided in the delineation of the pipes. These pipes represent a realistic and challenging case study target due to their small size and the magnetically noisy environment into which they have been emplaced. The discovery of the Syferfontein pipe in 1994 stimulated further testing of airborne methods, especially as the surface was undisturbed. These pipes are located in a region that hosts highly variably magnetized Hospital Hill shales, dolerite dykes and Ventersdorp lavas, a 2-3 m thick resistive ferricrete cap and significant cultural features such as an electric railroad and high tension power line. Although the kimberlites both show prominent magnetic anomalies on ground surveys, the airborne data are significantly noisy and the pipes do not show up as well determined targets. However, the clay-rich weathered zone of the pipes provides an ideal target for the EM method, and both pipes have significant responses on the DIGHEM system. The HELITEM, helicopter borne time domain EM system also clearly mapped both pipes as did the TEMPEST time domain system from a fixed wing platform. Although there are other EM anomalies in the area, these are easily associated with dykes in the area based on joint interpretation with the aeromagnetic data. The lack of kimberlite exploration in this region is likely due to the lack of alluvial diamonds, the magnetically noisy environment, and the well-developed ferricrete and calcrete layers on surface which trap and resorb heavy minerals such as garnet, traditionally used in soil sampling. In this challenging environment, airborne EM combined with magnetic and ground geophysical methods for ground truthing are viable exploration methods.

New geological data of New Siberian Archipelago

NASA Astrophysics Data System (ADS)

Sobolev, Nikolay; Petrov, Evgeniy

2014-05-01

The area of New Siberian Archipelago (NSA) encompasses different tectonic blocks is a clue for reconstruction of geological structure and geodynamic evolution of East Arctic. According to palaeomagnetic study two parts of the archipelago - Bennett and Anjou Islands formed a single continental block at least from the Early Palaeozoic. Isotope dating of De Long Islands igneous and sedimentary rocks suggests Neoproterozoic (Baikalian) age of its basement. The De Long platform sedimentary cover may be subdivided into two complexes: (1) intermediate of PZ-J variously deformed and metamorphosed rocks and (2) K-KZ of weakly lithified sediments. The former complex comprises the Cambrian riftogenic volcanic-clastic member which overlain by Cambrian-Ordovician turbiditic sequence, deposited on a continental margin. This Lower Palaeozoic complex is unconformably overlain by Early Cretaceous (K-Ar age of c.120 Ma) basalts with HALIP petrochemical affinities. In Anjou Islands the intermediate sedimentary complex encompasses the lower Ordovician -Lower Carboniferous sequence of shallow-marine limestone and subordinate dolomite, mudstone and sandstone that bear fossils characteristic of the Siberian biogeographic province. The upper Mid Carboniferous - Jurassic part is dominated by shallow-marine clastic sediments, mainly clays. The K-KZ complex rests upon the lower one with angular unconformity and consists mainly of coal-bearing clastic sediments with rhyolite lavas and tuffs in the bottom (117-110 Ma by K-Ar) while the complexe's upper part contains intraplate alkalic basalt and Neogene-Quaternary limburgite. The De-Long-Anjou block's features of geology and evolution resemble those of Wrangel Island located some 1000 km eastward. The Laptev Sea shelf outcrops in intrashelf rises (Belkovsky and Stolbovoy Islands) where its geology and structure may be observed directly. On Belkovsky Island non-dislocated Oligocene-Miocene sedimentary cover of littoral-marine coal-bearing unconformably overlies folded basement. The latter encompasses two sedimentary units: the Middle Devonian shallow-marine carbonate and Late-Devonian-Permian olistostrome - flysch deposited in transitional environment from carbonate platform to passive margin. Dating of detrital zircons suggests the Siberian Platform and Taimyr-Severnaya Zemlya areas as the most possible provenance. The magmatic activity on Belkovsky Island resulted in formation of Early Triassic gabbro-dolerite similar to the Siberian Platform traps. Proximity of Belkovsky Island to the north of Verkhoyansk foldbelt allows continuation of the latter into the Laptev Sea shelf. The geology of Bolshoy Lyakhovsky Island is discrepant from the rest of the NSA. In the south of Bolshoy Lyakhovsky Island the ophiolite crops complex out: it is composed of tectonic melange of serpentinized peridotite, bandedf gabbro, pillow-basalt, and pelagic sediments (black shales and cherts). All the rocks underwent epidot - amphibolite, glaucophane and greenschist facies metamorphism. The ophiolite is intruded by various in composition igneous massifs - from gabbro-diorite to leuco-granite, which occurred at 110-120 Ma. The Bolshoy Lyakhovsky Island structure is thought to be a westerly continuation of the South Anui suture of Chukchi.

Reconstructing late Quaternary palaeosol development and landscape connectivity from combined soil magnetic, geochemical and micromorphological analyses: insights from the Wilgerbosch River, Great Karoo, South Africa.

NASA Astrophysics Data System (ADS)

Oldknow, Chris; Hooke, Janet; Oldfield, Frank

2017-04-01

The characteristics, spatial and temporal patterns and drivers of Quaternary climate change across South Africa remain contentious principally due to the paucity of dated proxy records. River and fan terrace palaeosols may offer an important addition to analysis of spatially pervasive geomorphological landforms (palaeolake shorelines, terraces, dunes) conventionally used to reconstruct patterns of palaeoenvironmental change. In the Great Karoo, alluvial and colluvial exposures in the Wilgerbosch River and several of its tributaries have revealed up to five late Quaternary terraces of varying thickness, extent and pedogenic overprinting which may be of palaeoenvironmental significance. A combination of geochronology (OSL and radiocarbon), soil micromorphology, geochemistry (XRF, XRD) and soil magnetic proxies on both bulk (0-63 μm) and particle size extracts (0-4 and 32-63 μm) from major terrace palaeosols was used in order to establish: 1) changing sediment fluxes and pathways; 2) the characteristics and drivers of palaeosol formation; and 3) evaluate the suitability of these terrace palaeosols as indicators of palaeoenvironmental change. The results are used to test a conceptual model of landscape connectivity. Colluvial (slopewash, head deposits) sedimentation on the valley floors occurred around the time of the LGM due to enhanced regolith production through physical weathering. Soils overprinting these deposits (T1) are goethite-rich attesting to reducing conditions. Incision into T1 resulted in an extensive channel network forming. High concentrations of mafic minerals (Fe, Cr, Ca, Ti) and enhanced ferrimagnetism (XLF > 80) in the 32-63 μm fraction indicated connectivity with slope colluvium proximal to weathering dolerite tors. Fluvial aggradation (T2) occurred as a complex response to this phase of connectivity and terminated in the Late Glacial period (around 17±2.5 ka). The development of a rhizogenic calcrete overprinting T2 indicated an elevated water table and wetlands on the valley floors. Inset clay coatings and calcite hypocoatings attest to fluctuating groundwater levels. The calcrete acted to blanket T1 and T2 reducing extent of connectivity in subsequent phases of terrace development. Sediment in inset fills (T3-4) exhibited diminished primary mineral concentrations and ferrimagnetism (XLF = 20-60) due to recycling of earlier deposits (T1/T2), disconnectivity with 1st order tributaries and episodic gleying and dissolution of organic matter due to fluctuating groundwater level. The terrace palaeosols in the Wilgerbosch catchment are therefore polygenetic reflecting both changing sources of sediment (slope or channel) and multiple phases of soil development under varied palaeohydrological conditions. Caution is therefore urged attempting to relate conventional magnetic (XLF, XARM, XFD) and geochemical (CIA, CIW) metrics of pedogenesis in morphologically similar catchments to palaeoclimatic records. The approach taken in this study is presently useful for reconstructing extent of connectivity between landscape components.

Structural evolution of the Mount Wall region in the Hamersley province, Western Australia and its control on hydrothermal alteration and formation of high-grade iron deposits

NASA Astrophysics Data System (ADS)

Dalstra, Hilke J.

2014-10-01

The discovery of two relatively small but high-grade iron ore deposits near Mt Wall, an intensely faulted part of the southwestern Hamersley province provides unique insights into the structural control on ore formation in this region. The deposits have many geological features typical of the high grade microplaty hematite group which also contains the much larger Mt Tom Price, Paraburdoo and Mt Whaleback deposits. The deposits are structurally controlled along early normal faults and contain abundant microplaty hematite and martite, and are largely confined to the Dales Gorge member of the Brockman Iron Formation. In addition to the microplaty hematite-martite ore, there are martite-goethite ores and rare magnetite-goethite or magnetite-hematite ores. Below the modern weathering surface, hydrothermally altered zones in wallrock BIF from the Lower Dales Gorge member contain magnetite, hematite and carbonate/talc bearing mineral assemblages. A staged ore genesis model involving early extension and fluid circulation along normal faults, hypogene silica leaching and carbonate alteration, followed by deep meteoric oxidation with microplaty hematite formation and finally weathering can explain most features of the Mt Wall deposits. The role of deformation was to provide pathways for mineralising fluids and initiate the seed points for the mineralised systems. High grade iron in the Wellthandalthaluna deposit is situated between the NW to NNW trending Boolgeeda Creek fault and a synthetic joining splay, the Northern fault. Both are high angle normal faults and formed during early extension in this part of the province. Faults are characterised by localised small scale deformation and brecciation, deep carbonate alteration and oxidation. Recent weathering has penetrated deeply into the fault zones, converting the carbonate-rich assemblages into goethite. Mineralisation in the Arochar deposit is situated in the overlap or relay zone between two segments of the Mt Wall fault zone, a moderately to steeply southerly dipping normal fault system which at Arochar is intruded by dolerite dykes. At both locations, the ore controlling faults are offset by later NW trending dextral and normal faults. Fault relay zones or fault splay zones were likely zones of increased permeability and fluid flow during fault development or reactivation and may also have been important in initiating mineralisation in larger deposits such as Mt Tom Price and Mt Whaleback. However structural controls on the largest iron ore deposits are often obscured due to the intensity and scale of ore development, whereas they are better preserved in the smaller deposits. Recognition that carbonate bearing protores at Mt Wall survived for nearly two billion years until intense recent weathering converted them to martite-goethite or magnetite-goethite ores may imply that more of the giant hematite-goethite deposits of the Hamersley province had hydrothermal precursors and were not formed by supergene processes alone.

Impact of a fixed Siberian Traps mantle plume on the tectonics of the Arctic

NASA Astrophysics Data System (ADS)

Lawver, L. A.; Norton, I. O.; Gahagan, L.

2012-12-01

Eruption of the Siberian Traps at the Permo-Triassic boundary [~250 Ma] produced more than 3 x 106 km3 of rapidly emplaced magma throughout a region ~2.5 x 106 km2 in extent. Dates from the New Siberian Islands of 252 ± 2 Ma (Kuzmichev & Pease, 2007) indicate that Siberian Trap-related magmas are found ~500 km to the east of where they are generally shown to terminate to the west of the Lena River. Cenozoic opening of the Eurasian Basin would account for some of this discrepancy. A Siberian Trap mantle plume in an absolute reference frame fixed to the present day location of the Iceland hot spot, tracks through time across the Taimyr Peninsula region during the Late Triassic period and then to north of the Severnaya Zemlya archipelago by the end of the Middle Jurassic. With the exception of some Middle Triassic dates from the Taimyr Peninsula there is no apparent expression of a hot spot track during the this period. Motion of Laurasia in a paleomagnetically controlled reference frame has the Franz Josef Land archipelago over the fixed hotspot from about 155 Ma to 147 Ma prior to the early phase of the High Arctic Large Igneous Province [HALIP], generally taken to be 130 Ma to 120 Ma. Campsie et al (1988) have one date of 145 Ma from samples collected by Fridthof Nansen in 1895-1896 on Solsberi Island. Dibner et al (1988) have a dozen ages from dolerite samples from various islands spanning the period 175 ±12 Ma to 138 ±10 Ma with five of them between 158 Ma to 144 Ma. During the Late Jurassic into the earliest Cretaceous the track of the fixed hotspot follows the future margin of the Barents Shelf just inboard of a reconstructed Lomonosov Ridge. By the end of the Valanginian, the hotspot tracks curves slightly, mimicking the southern curve of the Lomonosov Ridge off North America. The early phase of the HALIP moves the region of the northern Ellesmere Island over the hotspot while forming the Mendeleev and Alpha ridges. By middle Albian time, the Siberian Traps/Iceland hotspot is no longer impacting the Arctic Ocean region. Campsie, J., Rasmussen, M.H., Hansen, N., Liebe, C.J., Laursen, J., Brochwicz-Levinski, W. and L. Johnson, 1988. K-Ar ages of basaltic rocks collected during a traverse of the Frans Josef Land Archipelago (1895-1896). Polar Research 6(2) 173-177. Dibner, V.D., Andreichev, V.L., Tarakhovsky, A.n., and I.V. Shkola, 1988. Timing of Plateau-basalt magmatism. In: V.D. Dibner (Ed.) Geology of Franz Josef Land, Meddelelser 146, Norsk Polarinstitutt, Oslo, 131-137. Kuzmichev, A.B., and V.L. Pease, 2007. Siberian trap magmatism on the Sew Siberian Islands: consraints for Arcti Mesozoic plate tectonic reconstructions Journal of the Geological Society 164, 959 - 968.

Geochemistry of the Bela Ophiolite, Pakistan

NASA Astrophysics Data System (ADS)

Khan, M.; Nicholson, K. N.; Mahmood, K.

2008-12-01

The Bela ophiolite complex of Balochistan, Pakistan has been the subject of several geochemical and tectonic studies in the past. However until now there has never been a combined structural, geochemical and tectonic assimilation study which adequately explains the observed geochemistry and structural geology in a global tectonic framework. Here we present the geochemical findings of our work. The Bela ophiolite complex consists of two major units: the basal section or Lower Unit, and the Upper Unit, between the two is a mélange zone. The Lower Unit is relatively homogeneous and consists almost entirely of flow basalts and pillow basalts. The base of the Upper Unit is the metamorphic sole which is overlain by a sequence of massive basalts flows and intrusions of gabbro and granites. The entire Upper Unit is cut by doleritic dykes and sills. Geochemically the Lower Unit is comprised of basaltic lavas with E-MORB affinities. These lavas are tholeiitic, low-K series lavas with trace element signatures of E-type MORB. For example ratios such as V/Ti, Zr/Y, Nb/Th, Th/La and Nb/U all suggest these lavas are E-MORB. Previous workers have suggested these lavas are back-arc basin (BAB) however the samples lack the characteristic signatures of subduction modified MORB. This conclusion is supported by chondrite and N-MORB normalized spider diagrams where the Lower Unit lavas are enriched in the LILE with respect to the HFSE. The Upper Unit of the Bela Ophiolite sequence has a slightly more complex history. The older lavas sequences, the massive basalt flows, gabbros and granites, all formed in an oceanic arc environment. These lavas exhibit classic arc signatures such as a negative Nb and Ti anomalies, are enriched in LILE and LREE relative to HSFE, and plot in the volcanic arc and island arc fields in classic ternary plots such as 2Nb- Zr/4-Y and Y/15-La/10-Nb/8. The younger sequence of intrusions found in the Bela ophiolite appear to have BAB signatures. These lavas have relatively flat MORB normalized plots, are slightly depleted in the LILE relative the HFSE, and have a very small negative Nb anomaly. Source characteristics for both units have been determined using trace element data. This work suggests that the E-MORB lavas are derived from partial melting of enriched mantle. The lavas found in the Upper Unit have all been sourced from depleted or N-MORB mantle which has been modified by subducting fluids. It is possible that the younger BAB samples have a slightly more enriched source than the corresponding arc lavas which might indicate movement of the subduction zone allowing the influx of new mantle material below the wedge. In conclusion, our new geochemical work shows that the Bela ophiolite contains three distinct magmatic sequences: a lower E-MORB sequence over lain by a series of volcanic arc lavas which are cut by BAB-type sills and dykes.

The Nokomis Cu-Ni-PGE Deposit, Duluth Complex: A sulfide-bearing, crystal-laden magmatic slurry

NASA Astrophysics Data System (ADS)

Peterson, D. M.

2009-12-01

Duluth Metals Limited’s Nokomis deposit is the most recently discovered Cu-Ni-PGE deposit in the 1.1 Ga. Duluth Complex, Minnesota. The deposit was discovered utilizing a genetic ore deposit model that identified and back-tracked channelized magma flow within the basal zone of the South Kawishiwi intrusion (SKI). The model led to exploratory drilling in 2006, deposit discovery and initial resource estimation in 2007, and significant resource expansion in 2008, all in a period of 18 months. The deposit’s updated 2008 NI 43-101 compliant Resource Estimate, based on 108 holes drilled by Duluth Metals and 52 historic drill holes on and off the property, contains 449 million tonnes of Indicated Resources grading 0.624% copper, 0.199% nickel, and 0.600 grams per tonne of total precious metals (TPM = Platinum+Palladium+Gold), and an additional 284 million tonnes of Inferred Resources grading 0.627% copper, 0.194% nickel, and 0.718 grams per tonne of TPM. The combined Indicated and Inferred Resources contain approximately 10 billion lbs Cu, 3.1 billion lbs Ni, 165 million lbs Co, 4 million ounces Pt, 9 million ounces Pd, and 2 million ounces of Au. Within these NI 43-101 resources are large tonnages of higher grade material, and the company has commenced an internal research program to identify the geologic controls on the formation nickel-rich and PGE-rich mineralization in the SKI, as well as copper-PGE rich mineralization in the footwall Archean rocks. To date, Duluth Metals has drilled more than 500,000 Ft. (~155,000 m) of core in 155 holes into the deposit, and has only drilled about half of the property. The ore deposit model was developed in cooperation with researchers from the Natural Resources Research Institute of the University of Minnesota, Duluth. As well, research and collaboration with faculty and students at Johns Hopkins University on the Ferrar Dolerites of the Antarctic Dry Valleys has played a key role in developing the magmatic model for the deposit. A fundamental aspect of the ever-developing ore deposit model is an understanding of the initial conditions of the magmatic system - its crystallinity, sulfur capacity, geochemistry, and geometry - and how the sulfur saturated SKI magma lived, worked, and died. Such understanding includes the realization that the magma was a crystal-liquid (silicate and sulfide liquids) slurry and the identification of magma channelways and sub-channels and their associated thermal anomalies. In addition, the SKI magmas locally melted the footwall granitoid rocks, and such melts have been incorporated into the sulfide-bearing troctolitic melts of the SKI. In the end, hard work (>16,000 outcrops mapped, ~20,000 geochemical analyses completed, and >155,000 meters of core drilled) and intellectual geologic thought has been used to identify one of the world’s largest resources of Cu-Ni-PGEs.

Microstructural variation in the transport direction of a large-scale mid-crustal thrust (Woodroffe Thrust, Central Australia)

NASA Astrophysics Data System (ADS)

Wex, Sebastian; Mancktelow, Neil S.; Hawemann, Friedrich; Pennacchioni, Giorgio; Camacho, Alfredo

2016-04-01

The over ˜600 km long E-W trending mid-crustal Woodroffe Thrust is one the most prominent structures of a range of large-scale shear zones that developed in the Musgrave Ranges region in Central Australia. During the Petermann Orogeny around 550 Ma the Woodroffe Thrust placed 1.2 Ga granulites onto similarly-aged amphibolite and granulite facies gneisses along a south-dipping plane with a top-to-north shear sense. Due to late-stage open folding of the thrust plane, a nearly continuous N-S profile of 60 km length in the direction of thrusting could be studied for variation in microstructure. The regional P/T variations in the mylonitized footwall (600 to 500 °C at ~ 0.8 GPa from S to N) indicate that the original angle of dip was shallow (~ 10°) towards the south. Along the profile, evidence for fluid-present conditions are effectively absent in the more southerly areas and only present on a local scale in the north, characterizing the regional conditions to be "dry". This is indicated by: 1) only rare syntectonic quartz veins in the footwall; 2) very little sericitization of plagioclase; 3) breakdown of plagioclase to kyanite + garnet, rather than kyanite + clinozoisite; and 4) variable presence of hydrothermally introduced calcite. These changes in P/T conditions and fluid availability are associated with corresponding changes in mineral assemblage and microstructure. Mylonitized dolerites consists of a syn-kinematic assemblage (decreasing modal amounts from left to right) of Pl + Cpx + Grt + Ky + Rt + Ilm ± Opx ± Amp ± Qz in the central/southern areas and Pl + Bt + Amp + Chl + Ilm ± Kfs ± Mag ± Ap in the north. The amount of newly grown garnet decreases towards the north and garnet is generally absent in the northernmost exposures of the Woodroffe Thrust. Mylonitized felsic granulites and granitoids consist of syn-kinematic assemblages of Qz + Pl + Kfs + Grt + Cpx + Ky + Ilm + Rt ± Bt ± Amp ± Opx ± Ap in the south and Qz + Pl + Kfs + Bt + Czo + Grt + Ilm ± Mag + Ttn ± Ms ± Amp ± Ap in the north. Plagioclase and K-feldspar dynamically recrystallized (grainsize < 10 μm) along the entire 60 km N-S transect, but with an increasing degree in the more southern exposures. Over the entire area dynamically recrystallized quartz aggregates in mylonites show polygonal, strain-free, equigranular grains, with a morphology indicating SGR recrystallization, under temperatures usually considered typical for GBM, which could potentially be due to the relatively "dry" conditions. Quartz grainsizes are on average 24 μm and 44 μm in the southern and northern areas, respectively. The increase in grain size towards the north correlates well with the increasing influx of fluids, but is in contrast to the trend of higher metamorphic grade towards the south. This suggests that fluid, rather than temperature, may be the main factor controlling the rheology of such "dry" middle crust.

Re-evaluation of the petrogenesis of the Proterozoic Jabiluka unconformity-related uranium deposit, Northern Territory, Australia

NASA Astrophysics Data System (ADS)

Polito, Paul A.; Kurt Kyser, T.; Thomas, David; Marlatt, Jim; Drever, Garth

2005-11-01

The world class Jabiluka unconformity-related uranium deposit in the Alligator Rivers Uranium Field, Australia, contains >163,000 tons of contained U3O8. Mineralization is hosted by shallow-to-steeply dipping basement rocks comprising graphitic units of chlorite-biotite-muscovite schist. These rocks are overlain by flat-lying coarse-grained sandstones belonging to the Kombolgie Subgroup. The deposit was discovered in 1971, but has never been mined. The construction of an 1,150 m decline into the upper eastern sector of the Jabiluka II deposit combined with closely spaced underground drilling in 1998 and 1999 allowed mapping and sampling from underground for the first time. Structural mapping, drill core logging and petrographic studies on polished thin sections established a detailed paragenesis that provided the framework for subsequent electron microprobe and X-ray diffraction, fluid inclusion, and O-H, U-Pb and 40Ar/39Ar isotope analysis. Uranium mineralization is structurally controlled within semi-brittle shears that are sub-conformable to the basement stratigraphy, and breccias that are developed within the hinge zone of fault-related folds adjacent to the shears. Uraninite is intimately associated with chlorite, sericite, hematite ± quartz. Electron microprobe and X-ray diffraction analysis of syn-ore illite and chlorite indicates a mineralization temperature of 200°C. Pre- and syn-ore minerals extracted from the Kombolgie Subgroup overlying the deposit and syn-ore alteration minerals in the Cahill Formation have δ18Ofluid and δ D fluid values of 4.0±3.7 and -27±17‰, respectively. These values are indistinguishable from illite separates extracted from diagenetic aquifers in the Kombolgie Subgroup up to 70 km to the south and east of the deposit and believed to be the source of the uraniferous fluid. New fluid inclusion microthermometry data reveal that the mineralising brine was saline, but not saturated. U-Pb and 207Pb/206Pb ratios of uraninite by laser-ablation ICP-MS suggest that massive uraninite first precipitated at ca. 1,680 Ma, which is coincident with the timing of brine migration out from the Kombolgie Subgroup as indicated by 40Ar/39Ar ages of 1,683±11 Ma from sandstone-hosted illite. Unmineralized breccias cemeted by chlorite, quartz and sericite cross-cut the mineralized breccias and are in turn cut by straight-sided, high-angle veins of drusy quartz, sulphide and dolomite. U-Pb and 207Pb/206Pb ratios combined with fluid inclusion and stable isotope data indicate that these post-ore minerals formed when mixing between two fluids occurred sometime between ca. 1,450 and 550 Ma. Distinct 207Pb/206Pb age populations occur at ca. 1,302±37, 1,191±27 and 802±57 Ma, which respectively correlate with the intrusion of the Maningkorrirr/Mudginberri phonolitic dykes and the Derim Derim Dolerite between 1,370 and 1,316 Ma, the amalgamation of Australia and Laurentia during the Grenville Orogen at ca. 1,140 Ma, and the break-up of Rodinia between 1,000 and 750 Ma.

Geomicrobiology of a Supraglacial Stream on the Cotton Glacier, Victoria Land, Antarctica.

NASA Astrophysics Data System (ADS)

Foreman, C. M.; Morris, C. E.; Cory, R. M.

2006-12-01

The Cotton Glacier lies in the Transantarctic Mountains north of Cape Roberts and has a limited catchment area in the Clare and St. Johns ranges, but receives a large amount of sedimentary deposits from surrounding areas. The bedrock geology of the area is dominated by basement granite and Ferrar dolerite sills, with minor amounts of amphibolite and schist sandwiched between granite bodies. A unique fluvial system forms on the Cotton Glacier as a result of its location in the Transantarctic Mountains. The prevailing winds converge and deposit debris on the Cotton Glacier, warming up the surface and increasing meltwater production. During the austral summer of 2004-2005 we sampled a braided stream that flowed from mid glacier into a series of crevasses downstream. While low in dissolved organic carbon (44-47 μM C) and nutrients the supraglacial stream on the Cotton Glacier is capable of sustaining life, with bacterial cell abundances from 2.7 - 8.2 x 104 cells ml-1, and bacterial production ranging from 58.84 - 293.18 ng C d-1. Isolates recovered from the Cotton Glacier produced a rainbow of pigment colors and were similar to those recovered from other icy systems (Cytophaga- Flavobateria-Bacteroides and β-Proteobacteria lineages), suggesting that the occurrence of these related phylotoyes from diverse environs is due to similar survival strategies allowing them to remain active at sub- zero temperatures and survive multiple freeze-thaw events. Two isolates from the Cotton Glacier have been shown to possess ice nucleating activity. These bacteria can catalyze ice formation at -3.5°C and colder temperatures and likely possess Type I ice nuclei proteins. The fluorescence and absorbance spectra of the filtered Cotton Glacier water were analyzed to characterize the dissolved organic matter (DOM). The absorbance spectra of the Cotton Glacier sample exhibited a peak around 270 nm, which disappeared over time in the dark at 4°C. Analysis of excitation-emission matrices (EEMs) from the Cotton Glacier differed from typical DOM EEMs. The Cotton Glacier EEMs showed almost no fluorescence in the region of EEMs where peaks are normally present and have been attributed to the presence of humic and fulvic organic matter (Ex/Em 240/450 and 330/450). In contrast to most DOM EEMs, the Cotton Glacier EEMs were dominated by peaks in the amino-acid region (Ex/Em 240-270/300-350 nm). Thus, both the initial peak in absorbance at 270 nm and the EEM peaks in the amino-acid region suggest that the DOM present in this system is predominately proteinaceous material likely of microbial origin. This study demonstrates the presence of an active microbial assemblage in a supraglacial stream from a pristine Antarctic glacier. Recent discoveries over the past decade have shown that glacial ice contains an important record of microorganisms on our planet that theoretically could be used to assess biogeochemical processes and habitat types that occurred during past glacial and interglacial periods. This record may also contain information on microbial evolution and physiology, and provide new biotechnological innovations. Supraglacial streams provide an important example of contemporary microbial processes on the glacier surface.

To what extent can we attribute accelerated landscape change to human activity? A cautionary tale from the drylands of the South African interior

NASA Astrophysics Data System (ADS)

Tooth, Stephen; Lyons, Richard; Duller, Geoff; McCarthy, Terence

2013-04-01

Across many parts of interior South Africa, alluvial and colluvial sediments are currently subject to widespread erosion by rivers, dongas (gullies), sheetwash and wind. This creates an impression of accelerated landscape change that is commonly attributed to factors such as poor land management by European settlers (mid 18th century onwards) or indigenous peoples, possibly in combination with decadal-scale climatic fluctuations and/or susceptible soil characteristics. Many resources are devoted to managing degrading lands, but effective conservation and restoration efforts are contingent on correctly identifying the underlying causes of erosion. Across South Africa, varied population densities, and diverse climates and soil types, mean that the causes of erosion are likely to be complex and to vary regionally. In some regions, examples of accelerated erosion resulting from vegetation clearance, overburning, overstocking, artificial drainage or land abandonment can be demonstrated. In other regions, however, our geomorphological, sedimentological and geochronological investigations provide an alternative 'geological' perspective on this erosion 'problem' by demonstrating that erosion may be a recurring, natural process linked to late Quaternary climate change and/or longer term landscape denudation. In particular, luminescence chronologies for hillslopes, alluvial fans and river floodplains/terraces at various locations across interior South Africa have enabled comparison with other regional/global palaeoenvironmental records. These comparisons reveal that climatically-controlled changes in runoff and sediment supply, mediated through vegetation cover changes, resulted in shifts between sedimentation (relative aridity), soil formation (relative humidity) and minor channel/donga erosion (semiaridity) from at least 40 kyr until the late Holocene. By contrast, major erosion involving sustained channel incision and associated donga formation appears to have been initiated during the last few thousand years, at some sites apparently corresponding with rapid climatic fluctuations associated with the Mediaeval Warm Period and Little Ice Age. In some instances, channel incision depth has been controlled by the stability of downstream resistant rock barriers (e.g. dolerite sills and dykes) that form local baselevels in river long profiles; upstream of stable barriers, incision has been restricted but where barriers have been partially or fully breached, then deep incision into bedrock is characteristic. These findings provide evidence that: 1) during the late Quaternary, erosional phases have occurred independently of human activity, in different climatic settings, and across different soil types; and 2) in many regions, even the present phase of deep channel incision and donga formation predates the advent of European settlement or indigenous population expansion. These 'geological' perspectives on the age and history of major erosional features demonstrate that accelerated landscape change in South Africa is not necessarily a consequence of human activities, and cannot be assumed to represent an unequivocal signature of the Anthropocene. These perspectives also have implications for land management. Where erosion is indisputably occurring as a result of land mismanagement, then alternative land use practices and erosion-control measures may succeed in slowing or reversing erosion, but where erosion results from natural climatic or denudational processes, then such schemes are unlikely to succeed in the medium- or long-term.

Evaluating the temporal link between Siberian Traps magmatism and the end-Permian mass extinction (Invited)

NASA Astrophysics Data System (ADS)

Burgess, S. D.; Bowring, S. A.

2013-12-01

Interest in Large Igneous Provinces as agents for massive climatic and biological change is steadily increasing, though the temporal constraints on both are seldom precise enough to allow detailed testing of a causal relationship. The end-Permian mass extinction is one of the most biologically important and intensely studied events in Earth history and has been linked to many possible trigger mechanisms, from voluminous volcanism to bolide impact. Proposed kill mechanisms range from acidic and/or anoxic oceans to a cocktail of toxic gases, although the link between trigger and kill mechanisms is unconstrained due to the lack of a high-precision timeline. Critical to assessing the plausibility of different trigger and kill mechanisms is an accurate age model for the biotic crisis and the perturbations to the global carbon cycle and ocean chemistry. Recent work using the EARTHTIME U/Pb tracer solution has refined the timing of the onset and duration of the marine mass extinction event and the earliest Triassic recovery at the GSSP for the Permian-Triassic boundary in Meishan, China. This work constrains the mass extinction duration to less than 100 kyr and provides an accurate and precise time point for the onset of extinction, against which the timing of potential trigger mechanisms may be compared. For more than two decades, eruption and emplacement of the Siberian traps has been implicated as a potential trigger of the end-Permian extinction. In this scenario, magmatism drives the biotic crisis through mobilization of volatiles from the sedimentary rock with which intruding and erupting magmas interact. Massive volatile release is believed to trigger major changes in atmospheric chemistry and temperature, both of which have been proposed as kill mechanisms. Current temporal constrains on the timing and duration of the Siberian magmatism are an order of magnitude less precise than those for the mass extinction event and associated environmental perturbations, limiting detailed testing of a causal relationship. We present new high-precision U/Pb geochronology on zircon crystals isolated from a suite of shallowly intruded dolerites in the Noril'sk region and two welded tuffs in the Maymecha river-valley. These two sections are the most extensively studied in the magmatic province and although there are thick exposures of lava and volcaniclastic rock elsewhere, the Noril'sk and Maymecha-Kotuy sections are thought to be representative of the entire extrusive stratigraphy. Our dates suggest that intrusive and extrusive magmatism began within analytical uncertainty of the onset of mass extinction, permitting a causal connection with age precision at the ~ × 0.06 Ma level. The new dates also allow projection of the extinction interval and associated chemostratigraphy onto the Siberian trap stratigraphy, which suggests that ~300m of volcanicalstic rocks and ~1800m of lavas in the Maymecha-Kotuy section were erupted just prior to the onset of mass extinction. Comparison of a detailed eruption history to biological and chemical records over the extinction and recovery intervals allows for better evaluation of plausible kill mechanisms.

Superplumes and single plumes: their magmatic trails on moving lithospheric plates.

NASA Astrophysics Data System (ADS)

Puchkov, Victor

2017-04-01

Single plumes and superplumes have, in principle, the same nature and source: they are thought to be upward-directed mantle convective flows, heated and fluid-enriched. They are born in LLSVP (Large low-shear-velocity provinces), otherwise called superswells, situated within the D? layer. They represent a paleomagnetically supported "reference frame for movements in and on the Earth" [Torsvik et al., 2014]. Arriving to asthenosphere and then lithosphere, they induce melting, which results in magmatism of various kinds and volume at the earth's surface. However these two types of plumes are very different in details. Superwells generate at the earth's surface Large Igneous Provinces (LIPs) with the volumes of erupted and intruded magmatic rocks varying between 0.1 and 10 ? km3 and areas between 0.1 to 10 ? km^2. They are characterized by short impulses of activity, usually from 0.5 to several Ma; in case of several impulses, their general duration may grow to 20 Ma, and very rare- more than 40 [Ernst, 2014]. The main magmatic component of the eruptive parts of LIPs are flood basalts of typical chemistry connected with dolerite dikes, representing their plumbing system; alkaline basalts, carbonatites, kimberlites may be present as subsidiary phases; in the upper parts of the sections continental LIPs include rhyolites and granites. In continents, the plateaus of flood volcanos are combined with volcanos of active rift systems. In the oceans, the LIPs form vast volcanic plateaus; the thickness of their crust is greater than normal by several times. According to seismic data, the crust of the plateaus may consist of three parts (from below): underplated basites, pre-plume crust and basalt eruptions. As for single plumes, they are born predominantly at the periphery of LLSVPs and form single volcanos or their small clusters, OIB type (LREE-enriched), arranged in regular "time-progressive volcanic chains". Author [Puchkov, 2009] compiled an upgraded version of their distribution in the world. The last years gave an additional information which supports previous conclusions concerning the vectors of recent plate movements, induced from both the volcanic chains and space geodesy [Altamimi et al., 2016], which are in a good accordance. Therefore time-progressive volcanic chains are trails of single plumes at the earth's surface. Superplume trails are more problematic. The superswells spawn superplumes periodically, with interruptions, from different active parts of them. Therefore they form dashed, irregular, split trails (if any). There are two antipodal superswells (African, or Tuzo, and Pacific, or Jason). Their trails are preserved differently. LIPs of the Jason superswell were superimposed predominately on the oceanic crust and therefore Early Mesozoic and older Jason LIPS are mainly subducted or accreted and are present in ophiolites. As for Tuzo-born, they are traced through pre-, post-Pangean and probably -Rodinian continents [Kuzmin et al., 2011, 2013; Puchkov, 2016], though some oceanic areas are also affected. The trails of plumes of both types are locally interrupted by superimposed spreading oceanic basins (gaps). Supported by RSF grant 16-17-10192.

Lower Devonian paleomagnetic dating of a large mafic sill along the western border of the Murzuq cratonic basin (Saharan metacraton, SE Algeria).

NASA Astrophysics Data System (ADS)

El-M. Derder, Mohamed; Maouche, Said; Liégeois, Jean-Paul; Henry, Bernard; Amenna, Mohamed; Ouabadi, Aziouz; Bellon, Hervé; Bruguier, Olivier; Bayou, Boualem; Bestandji, Rafik; Nouar, Omar; Bouabdallah, Hamza; Ayache, Mohamed; Beddiaf, Mohamed

2017-04-01

The Murzuq basin located in central North Africa, in Algeria, Libya and Niger is a key area, delineating a relictual cratonic area within the Saharan metacraton (Liégeois et al., 2013). On its western border, we discovered a very large sill ("Arrikine" sill), with a thickness up to 250m and a minimum length of 35 km. It is made of mafic rocks and is interbedded within the Silurian sediments of the Tassilis series. In the vicinity, the only known post-Pan-African magmatism is the Cenozoic volcanism in the In Ezzane area. Further south in Niger, also along the SW border of the Murzuq basin, large Paleozoic dolerite (Carte géologique du Sahara central, 1962) are probably related to the "Arrikine" sill magmatism, as they are in the same stratigraphical position. Several hundred kilometers westward and southwestward of Arrikine, Paleozoic magmatic products are known: Carboniferous basic intrusives (346 Ma; Djellit et al., 2006) are located in the Tin Serririne basin and Devonian ring complexes (407 Ma; Moreau et al, 1994) in the Aïr Mountains. For the Arrikine sill, K/Ar data gave a rejuvenation age (326 Ma) related to a K-rich aplitic phase and the LA-ICP-MS U-Pb method on zircon showed that only inherited zircons are present (0.6 to 0.7, 2.0 and 2.7 Ga ages), pointing to ages from the underlying basement corresponding to the Murzuq craton covered by Pan-African sediments (Derder et al., 2016). By contrast, a well-defined paleomagnetic pole yielded an age of 410-400 Ma by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age, similar to that reported for the Aïr complexes (Moreau et al., 1994), can be correlated with the deep phreatic eruption before Pragian time thought to be at the origin of sand injections, which gave circular structures observed on different borders of the Murzuq basin (Moreau et al,. 2012). This Lower Devonian magmatism had therefore a regional extension and can be related to a "Caledonian" transtensive reactivation of the western metacratonic boundary of the Murzuq craton related to distant events along the northern Gondwana margin. References Carte géologique du Sahara central, from "carte géologique du nord ouest de l'Afrique", Centre National de la Recherche Scientifique (CNRS) edition, 1/2.000.000, 1962. Derder M.E.M., Maouche S., Liégeois J.P., Henry B., Amenna M., Ouabadi A., Bellon H., Bruguier O., Bayou B., Bestandji R., Nouar O., Bouabdallah H., Ayache M., Beddiaf M., 2016. Journal of African Earth Sciences 115, 159-176 Djellit, H., Bellon, H., Ouabadi, A., Derder, M.E.M., Henry, B., Bayou, B., Khaldi, A., Baziz, K., Merahi, M.K., 2006. Comptes Rendus Geosciences. 338, 624-631. Liégeois, J.P., Abdelsalam, M.G., Ennih, N., Ouabadi, A., 2013. Gondwana Research, 23, 220-237 Moreau, C., Demaiffe, D., Bellion, Y. and Boullier, A.M., 1994. Tectonophysics, 234, 129-146. Moreau, J., Ghienne, J.F., Hurst, A., 2012. Sedimentology, 59: 1321-1344.

Building The Bigger Picture - Using a field study geology programme to link and contextualise classroom topics.

NASA Astrophysics Data System (ADS)

Allen, Lesley

2013-04-01

Cornwall, UK, has been designated a World Heritage Site for industrial heritage, based on the extensive mining history built around hydrothermal deposits of tin and copper suite deposits. These deposits are found in a very varied and complex geological setting. The tectonic activity which emplaced the deposits also produced intense folding and faulting of Carboniferous and Devonian marine sediments, major and minor igneous intrusions, regional and contact metamorphism, and the emplacement of an ophiolite sequence on the Lizard peninsula. The region is targeted by college and university geology student groups from across the UK. It is also the home of the world famous Camborne School of Mines, part of the University of Exeter. We have developed a comprehensive series of field visits to cover all these geological aspects. We also use the history of mining as a context within which to teach the social, environmental and economic aspects of the geology curriculum at A Level. By this means we can reveal how disparate geological topics link together through 3 physical dimensions plus time. Field visits motivate students; they enthuse and excite them and help them to understand the large-scale and 3D visualisation aspects of geology, the timescales involved, and also assist students in developing observational and practical field and mapping skills. The series of visits also helps to bring many aspects of the curriculum together into a more complete picture. Site 1 - Bude. Intense folding of marine sediments with tectonic and sedimentary structures in cyclical sands and shales and some turbidites. Competent and incompetent rocks, axial planar cleavage, etc. Site 2 - Praa Sands. Minor porphyritic intrusion with baked & chilled margins and flow aligned feldspars. (Also incidental raised beach due to post-glacial isostatic rebound.) Site 3 - Rinsey Cove. Contact zone where 'roof' of major granitic intrusion intrudes slates. Pegmatites, classic marginal features, xenoliths, stoping and faulting visible. Site 4 - Polurrian Cove. Western end of boundary thrust fault between metamorphosed subterranean lava flows (hornblende schists)above and crustal Devonian slates below can be observed. Site 5 - Coverack Cove to Godrevy Cove. The beach rocks change from olivine rich serpentinised peridotite to gabbro as you move across the Moho boundary zone. Further along the coast at Dean Point, dolerite dykes are quarried and basalt dykes are found at neighbouring Godrevy Cove - the complete ophiolite sequence in a few miles! Site 6 - Holmans Mine. Camborne School of Mines' training mine where students can experience blasting and see methods of rock stabilisation. They can also see mineral veins in situ underground and investigate mining techniques. Site 7 - Carnon Valley. Once known as the 'richest square mile in the World' due to the amount of tin and copper being produced. The Wheal Jane pollution incident (1992) caused widespread alarm and was dealt with by chemical and biological treatment systems still in operation. Historical and current tailings dams can be seen and compared, and the ecology of environmentally damaged ground observed. Mine dumps still yield mineral specimens and the streams are still acidic after heavy rain.

Palaeomagnetic, rock-magnetic and mineralogical investigations of metadolerites from Western Svalbard : A preliminary report

NASA Astrophysics Data System (ADS)

Michalski, Krzysztof; Nejbert, Krzysztof; Domańska-Siuda, Justyna; Manby, Geoffrey

2014-05-01

A group of 42 independently oriented palaeomagnetic samples from 7 sites located in central part of the West Spitsbergen Thrust and Fault Belt has been investigated. The samples were collected from 5 distinct metadolerite sheets intruded into the Proterozoic - Lower Paleozoic metamorphic complex of Western Oscar II Land (Western Svalbard Caledonian Terrane - Harland, 1997 division). All analyzed metadolerite samples were metamorphosed under greenschist facies metamorphism. The metamorphic assemblage consist of hornblende, biotite, actinolite, chlorite, epidote, stilpnomelane, titanite, albite, and quartz. Calcite, associated with pyrrhotite, pyrite chalcopyrite, sphalerite, and covellite, that occurs as irregular intergrowths or thin veins, document high activity of H2O-CO2-rich fluids during metamorphism. Primary magmatic phases represented by clinopyroxene occur rarely, and only in thick metadolerite dykes. Accessory oxides change their mineralogical and chemical composition during metamorphism. In all examined samples primary Ti-magnetite and oxy-exsolved hematite break-down completely into titanite or have been dissolved. The ilmenite are also replaced by titanite, but in metadolerites at contact with host metapelites, slightly altered ilmenite grains with preserved hematite exsolution were documented. Basing on mineralogical observations it should be expected that metamorphic processes have almost completely reset the paleomagnetic data record from the time of dolerite crystallization. This stage can document only rare hematite oxy-exsolution preserved within ilmenite, and presumably small inclusion of magnetite still preserved within unaltered clinopyroxene. The paleomagnetic record of metamorphic stage is probably recorded by pyrrhotite, hematite, goethite, and late Ti-free magnetite that can grow during breakdown of pyrrhotite to pyrite (Ramdohr. 1980). The NRM (Natural Remanent Magnetisation) intensities of the palaeomagnetic samples exceed the minimum 10 mA/m. The first AF/thermal demagnetizations have revealed a stable NRM structure. ChRM (Characteristic Remanent Magnetisation) components can be extracted precisely from Zijderveld diagrams (precision parameter - ASD max. 10º). The following magnetic procedures have been applied to identify the ferromagnetic carriers of the samples: SIRM (saturation isothermal remanent magnetization) decay curves (procedure after Kadziałko-Hofmokl & Kruczyk, 1976) and the three-component IRM (isothermal remanent magnetization) procedure described by Lowrie (1990). Experimental work has revealed the dominance of pyrrhotite and magnetite phases as carriers with soft-medium coercivity (samples are saturated in 0.2-0.4 T) and distinct unblocking temperatures around 320-350 ºC and 575-600 ºC respectively. Presented study is the part of PALMAG project 2012-2015: "Integration of palaeomagnetic, isotopic and structural data to understand Svalbard Caledonian Terranes assemblage" (see also Michalski et al. 2012), funded by Polish National Science Centre. References: HARLAND,W.B. 1997. The Geology of Svalbard. Geological Society of London, Memoir 17, 521 pp. KĄDZIOŁKO-HOFMOKL,M. & KRUCZYK,J. 1976. Complete and partial self-reversal of natural remanent magnetization of basaltic rocks from Lower Silesia, Poland. Pure and Applied Geophysics 110, 2031-40. LOWRIE,W. 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophysical Research Letters 17, 159-62. MICHALSKI,K., LEWANDOWSKI,M., MANBY,G.M. 2012. New palaeomagnetic, petrographic and 40Ar/39Ar data to test palaeogeographic reconstructions of Caledonide Svalbard. Cambridge University Press. Geological Magazine 149 (4), 696-721. RAMDOHR,P. 1980. The ore minerals and their intergrowths. Pergamon Press, Oxford.

Mapping three-dimensional geological features from remotely-sensed images and digital elevation models

NASA Astrophysics Data System (ADS)

Morris, Kevin Peter

Accurate mapping of geological structures is important in numerous applications, ranging from mineral exploration through to hydrogeological modelling. Remotely sensed data can provide synoptic views of study areas enabling mapping of geological units within the area. Structural information may be derived from such data using standard manual photo-geologic interpretation techniques, although these are often inaccurate and incomplete. The aim of this thesis is, therefore, to compile a suite of automated and interactive computer-based analysis routines, designed to help a the user map geological structure. These are examined and integrated in the context of an expert system. The data used in this study include Digital Elevation Model (DEM) and Airborne Thematic Mapper images, both with a spatial resolution of 5m, for a 5 x 5 km area surrounding Llyn Cow lyd, Snowdonia, North Wales. The geology of this area comprises folded and faulted Ordo vician sediments intruded throughout by dolerite sills, providing a stringent test for the automated and semi-automated procedures. The DEM is used to highlight geomorphological features which may represent surface expressions of the sub-surface geology. The DEM is created from digitized contours, for which kriging is found to provide the best interpolation routine, based on a number of quantitative measures. Lambertian shading and the creation of slope and change of slope datasets are shown to provide the most successful enhancement of DEMs, in terms of highlighting a range of key geomorphological features. The digital image data are used to identify rock outcrops as well as lithologically controlled features in the land cover. To this end, a series of standard spectral enhancements of the images is examined. In this respect, the least correlated 3 band composite and a principal component composite are shown to give the best visual discrimination of geological and vegetation cover types. Automatic edge detection (followed by line thinning and extraction) and manual interpretation techniques are used to identify a set of 'geological primitives' (linear or arc features representing lithological boundaries) within these data. Inclusion of the DEM data provides the three-dimensional co-ordinates of these primitives enabling a least-squares fit to be employed to calculate dip and strike values, based, initially, on the assumption of a simple, linearly dipping structural model. A very large number of scene 'primitives' is identified using these procedures, only some of which have geological significance. Knowledge-based rules are therefore used to identify the relevant. For example, rules are developed to identify lake edges, forest boundaries, forest tracks, rock-vegetation boundaries, and areas of geomorphological interest. Confidence in the geological significance of some of the geological primitives is increased where they are found independently in both the DEM and remotely sensed data. The dip and strike values derived in this way are compared to information taken from the published geological map for this area, as well as measurements taken in the field. Many results are shown to correspond closely to those taken from the map and in the field, with an error of < 1°. These data and rules are incorporated into an expert system which, initially, produces a simple model of the geological structure. The system also provides a graphical user interface for manual control and interpretation, where necessary. Although the system currently only allows a relatively simple structural model (linearly dipping with faulting), in the future it will be possible to extend the system to model more complex features, such as anticlines, synclines, thrusts, nappes, and igneous intrusions.

Gold mineralisation throughout about 45 Ma of Archaean orogenesis: protracted flux of gold in the Golden Mile, Yilgarn craton, Western Australia

NASA Astrophysics Data System (ADS)

Bateman, Roger; Hagemann, Steffen

2004-10-01

The Golden Mile deposit was discovered in 1893 and represents today the largest Archaean orogenic lode gold system in the world (50 M oz produced gold). The Golden Mile deposit comprises three major styles of gold mineralisation: Fimiston, Oroya and Charlotte styles. Fimiston-style lodes formed at 250 to 350 °C and 100 to 200 MPa and are controlled by brittle ductile fault zones, their subsidiary fault zone and vein networks including breccias and open-cavity-infill textures and hydrothermally altered wall rock. Fimiston lodes were formed late D1, prior to D2 regional upright folding. Hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite, an increase in sericite and in Fe content of carbonates. Lodes contain siderite, pyrite, native gold, 17 different telluride minerals (Au Ag tellurides contain ~25% of total gold), tourmaline, haematite, sericite and V-rich muscovite. Oroya-style lodes formed at similar P T conditions as the Fimiston lodes and are controlled by brittle ductile shear zones, associated dilational jogs that are particularly well developed at the contact between Paringa Basalt and black shale interflow sedimentary rocks and altered wall rock. The orebodies are characterised by micro-breccias and zones of intense shear zone foliation, very high gold grades (up to 100,000 g/t Au) and the common association of tellurides and vanadian mica (green leader). Oroya lodes crosscut Fimiston lodes and are interpreted to have formed slightly later than Fimiston lodes as part of one evolving hydrothermal system spanning D1 and D2 deformation (ca. 2,675 2,660 Ma). Charlotte-style lodes, exemplified by the Mt Charlotte deposit, are controlled by a sheeted vein (stockwork) complex of north-dipping quartz veins and hydrothermally altered wall rock. The Mt Charlotte orebody formed at 120 to 440 °C and 150 to 250 MPa during movement along closely spaced D4 (2,625 Ma) and reactivated D2 faults with the quartz granophyre in the Golden Mile Dolerite exerting a strong lithological control on gold mineralisation. Veins consist of quartz carbonate minor scheelite, and wall-rock alteration comprises chlorite destruction and growth of ferroan carbonate sericite pyrite native gold. Pyrite pyrrhotite is zoned on the scale of vein haloes and of the entire mine, giving a vertical temperature gradient of 50 100 °C over 1,000 vertical metres. The structural hydrothermal model proposed consists of four major stages: (1) D1 thrusting and formation of Fimiston-style lodes, (2) D2 reverse faulting and formation of Oroya-style lodes, (3) D3 faulting and dissecting of Fimiston- and Oroya-style lodes, and (4) D4 faulting and formation of Mt Charlotte-style sheeted quartz vein system. The giant accumulation of gold in the Golden Mile deposit was formed due to protracted gold mineralisation throughout episodes of an Archaean orogeny that spanned about 45 Ma. Fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing, efficiency and duration. In addition to the long-lasting fluid plumbing system, the wide variety of transient structural and geochemical traps, multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world.

Fluid transport in reaction induced fractures

NASA Astrophysics Data System (ADS)

Ulven, Ole Ivar; Sun, WaiChing; Malthe-Sørenssen, Anders

2015-04-01

The process of fracture formation due to a volume increasing chemical reaction has been studied in a variety of different settings, e.g. weathering of dolerites by Røyne et al. te{royne}, serpentinization and carbonation of peridotite by Rudge et al. te{rudge} and replacement reactions in silica-poor igneous rocks by Jamtveit et al. te{jamtveit}. It is generally assumed that fracture formation will increase the net permeability of the rock, and thus increase the reactant transport rate and subsequently the total rate of material conversion, as summarised by Kelemen et al. te{kelemen}. Ulven et al. te{ulven_1} have shown that for fluid-mediated processes the ratio between chemical reaction rate and fluid transport rate in bulk rock controls the fracture pattern formed, and Ulven et al. te{ulven_2} have shown that instantaneous fluid transport in fractures lead to a significant increase in the total rate of the volume expanding process. However, instantaneous fluid transport in fractures is clearly an overestimate, and achievable fluid transport rates in fractures have apparently not been studied in any detail. Fractures cutting through an entire domain might experience relatively fast advective reactant transport, whereas dead-end fractures will be limited to diffusion of reactants in the fluid, internal fluid mixing in the fracture or capillary flow into newly formed fractures. Understanding the feedback process between fracture formation and permeability changes is essential in assessing industrial scale CO2 sequestration in ultramafic rock, but little is seemingly known about how large the permeability change will be in reaction-induced fracturing. In this work, we study the feedback between fracture formation during volume expansion and fluid transport in different fracture settings. We combine a discrete element model (DEM) describing a volume expanding process and the related fracture formation with different models that describe the fluid transport in the fractures. This provides new information on how much reaction induced fracturing might accelerate a volume expanding process. Jamtveit, B, Putnis, C. V., and Malthe-Sørenssen, A., ``Reaction induced fracturing during replacement processes,'' Contrib. Mineral Petrol. 157, 2009, pp. 127 - 133. Kelemen, P., Matter, J., Streit, E. E., Rudge, J. F., Curry, W. B., and Blusztajn, J., ``Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO2 Capture and Storage,'' Annu. Rev. Earth Planet. Sci. 2011. 39:545 - 76. Rudge, J. F., Kelemen, P. B., and Spiegelman, M., ``A simple model of reaction induced cracking applied to serpentinization and carbonation of peridotite,'' Earth Planet. Sc. Lett. 291, Issues 1-4, 2010, pp. 215 - 227. Røyne, A., Jamtveit, B., and Malthe-Sørenssen, A., ``Controls on rock weathering rates by reaction-induced hierarchial fracturing,'' Earth Planet. Sc. Lett. 275, 2008, pp. 364 - 369. Ulven, O. I., Storheim, H., Austrheim, H., and Malthe-Sørenssen, A. ``Fracture initiation during volume increasing reactions in rocks and applications for CO2 sequestration'', Earth Planet. Sc. Lett. 389C, 2014, pp. 132 - 142, doi:10.1016/j.epsl.2013.12.039. Ulven, O. I., Jamtveit, B., and Malthe-Sørenssen, A., ``Reaction-driven fracturing of porous rock'', J. Geophys. Res. Solid Earth 119, 2014, doi:10.1002/2014JB011102.

Lavas and Sills in the Ferrar Large Igneous Province: Field and Geochemical Evidence for the Order of Emplacement.

NASA Astrophysics Data System (ADS)

Elliot, D. H.; Fleming, T. H.

2005-12-01

Many large igneous provinces, particularly those associated with Gondwana break-up, include major sill complexes as well as flood basalt fields. In the Ferrar province, radiometric dates of lavas and sills are indistinguishable. Nevertheless, in north Victoria Land (NVL) field evidence suggests the lavas had to have been erupted first in order to create the overburden needed for emplacement at shallow depths of thick sills, lacking vesicles, in a thin (100 m) Upper Triassic sedimentary sequence overlying basement. Elsewhere in the Transantarctic Mountains sills occur almost exclusively in a thick (2-2.5 km) Devonian-Triassic sedimentary sequence (Beacon Supergroup) that was possibly capped by 500+m of lavas before sill emplacement. For south Victoria Land (SVL), Marsh (2004) proposed that the most evolved rocks were erupted first as lavas, and sills were emplaced at progressively greater depth as increasingly more magnesian magmas and crystal mushes were injected into supracrustal and finally basement rocks. In NVL most lavas have MgO between 6-8% with a few as low as 4.5% MgO, whereas analyzed chilled margins of sills range from 3.7-5.6% MgO. In the Prince Albert Mountains (PAM), SVL, lava and sill compositions overlap (3.9-7.3% MgO). In the greater Dry Valleys region (SVL) lavas at Carapace Nunatak range from 3.6-6.7% MgO; chilled margins of Dry Valleys sills range from about 4.2 to 7.2% MgO. In the Queen Alexandra Range, central Transantarctic Mountains (CTM), lavas are predominantly 2.6-5.7% MgO; sills in the region range from 4.5% to 10.7% MgO. In the Otway Massif region (head of the Shackleton Glacier, CTM) most lavas are strongly evolved (2.7-3.4% MgO); sills in the Shackleton Glacier region range from 4.3-7.3% MgO. Nowhere do lavas show unequivocal systematic temporal change in MgO, and notably in CTM the initial flows are the most mafic (7.5-8.0% MgO). Olivine dolerite sills (chilled margins: -9% MgO) tend to occur low in the stratigraphic section. Except for NVL where Beacon strata exposures are limited, sills are thicker (100-200 m) and more regular in lower stratigraphic levels. Sills with orthopyroxene crystal-mush tongues are not known outside the Dry Valleys except perhaps the Warren Range (SVL). No province-wide systematic relationship is apparent between compositions of lavas and sill chilled margins. Nevertheless, in CTM most lavas are significantly more evolved than the sills; within the sills there is no clear relationship between MgO and stratigraphy, and some less evolved compositions occur at relatively high stratigraphic levels. In SVL compositional overlap is almost complete; locally, cross-cutting relations show more mafic sills and sheets cutting less mafic compositions. In NVL the chemical relations between lavas and sills are opposite from those that have been advocated for SVL. Interpretation is compounded by sills that exchange stratigraphic position or climb stratigraphically. Factors affecting magma emplacement include magma density, lithostatic pressure, overpressures required for lateral emplacement, and rock physical properties; when and where the evolving source was tapped may play an equal role in the emplacement order. Further, detailed work on the sills will show whether crystallization might have yielded lower density residual liquids that could have migrated and formed distal fingers of sills or migrated to higher stratigraphic levels.

The Generation of Oceanic Lithosphere in an Embryonic Oceanic Crust : the Example of the Chenaillet Ophiolite in the Western Alps

NASA Astrophysics Data System (ADS)

Masini, E.; Manatschal, G.; Muntener, O.

2007-12-01

The Chenaillet Ophiolite exposed in the Franco-Italian Alps represents a well-preserved ocean-floor sequence that was only weakly affected by later Alpine convergence. Based on the similarity between rock types and structures reported from ultraslow spreading ridges and those observed in the Chenaillet Ophiolite, it may represent a field analogue for slow to ultraslow spreading ridges such as the Gakkel Ridge or the Southwest Indian Ridge. Mapping of the Chenaillet Ophiolite enabled to identify an oceanic detachment fault that extends over a surface of about 16 km2 capping exhumed mantle and gabbros onto which clastic sediments have been deposited. The footwall of the detachment is formed by mafic and ultramafic rocks. The mantle rocks are strongly serpentinized lherzolites and subordinate harzburgites and dunites. Microstructures reminiscent of impregnation, and cpx major and trace element chemistry indicate that spinel peridotite is (locally) replaced by plagioclase-bearing assemblages. Pyroxene thermometry on primary minerals indicates high temperatures of equilibration ( max 1200°C) for the mantle rocks. Gabbros range from troctolite and olivine-gabbros to Fe-Ti gabbros and show clear evidence of syn-magmatic deformation, partially obliterated by retrograde amphibolite and low-grade metamorphic conditions. In sections perpendicular to the detachment within the footwall, syn-tectonic gabbros and serpentinized peridotites grade over some tens of meters into cataclasites that are capped by fault gouges. Petro-structural investigations of the fault rocks reveal a syn-tectonic retrograde metamorphic evolution. Clasts of dolerite within the fault zone suggest that detachment faulting was accompanied by magmatic activity. Hydrothermal alteration is indicated by strong mineralogical and chemical modifications. Gabbro and serpentinized peridotite, together with serpentinite cataclasites occur as clasts in tectono-sedimentary breccias overlying directly the detachment fault. Across the whole Chenaillet Ophiolite, volcanic rocks directly overlie either the detachment fault or the sediments. In several places, N-S trending high-angle normal faults have been mapped. These faults truncate and displace the detachment fault leading to small domino-like structures. The basins, limited by these high-angle faults, are some hundreds to a few kilometres wide and few tens to some hundreds of meters deep. Because these high- angle faults are sealed locally by basalts and obliterated by volcanic structures, we interpret them as oceanic structures being active during the emplacement of the basalts. The alignment of porphyritic basaltic dykes parallel to, and their increasing abundance towards the high-angle faults suggest that they may have served as feeder channels for the overlying volcanic rocks. The complex poly-phase tectonic and magmatic processes observed in the Chenaillet Ophiolite are reminiscent of those reported from slow to ultraslow spreading ridges. The key result from our study is that mantle exhumation along detachment faults is followed by syn-magmatic normal faulting resulting in the emplacement of laterally variable, up to 300 meters thick massive lavas and pillow basalts covering the exhumed detachment fault. This implies that off-axis processes are more important as previously assumed and that large-scale detachment faults may be buried under massive volcanic sequences suggesting that detachment faulting is presumably more common than suggested by dredging or morpho-structural investigations of ultra- to slow- spreading oceanic crust.

A palaeomagnetic study of the lower part of the Palaeoproterozoic Waterberg Group, South Africa

NASA Astrophysics Data System (ADS)

Maré, L. P.; Eriksson, P. G.; Améglio, L.

2006-01-01

Pole positions from previous palaeomagnetic work in the late Palaeoproterozoic Waterberg Group, South Africa [Jones, D.L., McElhinny, M.W., 1967. Stratigraphic interpretation of paleomagnetic measurements on the Waterberg Red Beds of South Africa. Journal of Geophysical Research 72, 4171-4179] seemed to indicate that Waterberg Group sedimentation commenced during emplacement of the ca. 2.06 Ga Bushveld Complex, and continued intermittently through numerous tectonic events affecting the preserved Transvaal Basin to just before the ca. 1.1 Ga Umkondo thermal event of southern Africa. However, from these studies no consistent directions could be determined. Instead, a pattern was identified and interpreted in terms of apparent polar wander during the deposition and consolidation of the Waterberg sediments within South Africa and Botswana. The Swaershoek Formation, the basal unit of the Waterberg Group in the Nylstroom Protobasin, has been tentatively correlated with the Wilge River Formation, which is the only unit in the Middelburg basin. A new palaeomagnetic study on the Swaershoek and Wilge River Formations is reported here in an attempt to re-determine the palaeomagnetic pole positions for these two formations and to confirm their correlation. A total of 49 sites across both basins were sampled, both within the sedimentary succession and in the associated diabase intrusions. The calculated anti-poles for the Swaershoek Formation (18.9°N, 7.7°E, A95 = 21.9°) and the Wilge River Formation (16.9°N, 0.2°E, A95 = 22.4°) cannot be distinguished at the 95% level of confidence, although scattering is high. This either confirms the geological correlation or indicates simultaneous magnetic overprinting. The results from the two basins are provisionally combined to present a mean pole position for the lower-Waterberg Group. The mean anti-pole for the lower-Waterberg Group is 17.9°N, 3.9°E, A95 = 16.2°. The diabase in both basins was sampled to test for thermal overprinting of the magnetic direction of the sedimentary rocks. Although the calculated anti-pole for the diabase in the Nylstroom Protobasin (63.3°N, 53.2°E and A95 = 35.7°) is poorly determined, its circle of 95% confidence includes the confidence region of the diabase in the Middelburg basin (anti-pole 69.3°N, 28.5°E and A95 = 14.2°). These two poles cannot be distinguished from the results of previous studies on post-Waterberg diabase [Jones, D.L., McElhinny, M.W., 1966. Paleomagnetic correlation of basic intrusions in the Precambrian of Southern Africa. Journal of Geophysical Research 71, 543-552] nor from results of the Umkondo diabase [McElhinny, M.W., Opdyke, N.D., 1964. The palaeomagnetism of the Precambrian dolerites of eastern South Rhodesia. An example of geologic correlation by rock magnetism. Journal of Geophysical Research 69, 2465-2475]. The pole positions from the Waterberg sediment and associated diabase are sufficiently displaced from each other to rule out any overprinting by these intrusions. It does not, however, rule out magnetic overprinting at some earlier or later age. The calculated mean pole position for the lower-Waterberg Group is tentatively interpreted to represent a direction of magnetic overprinting due to the ˜1.88 Ga Palaeoproterozoic intraplate magmatism associated with the Eburnean Event, which means that the cumulative of palaeomagnetic data can no longer connect the onset of Waterberg sedimentation with the emplacement of the Bushveld Complex.

Tectonic/climatic control on sediment provenance in the Cape Roberts Project core record (southern Victoria Land, Antarctica): A pulsing late Oligocene/early Miocene signal from south revealed by detrital thermochronology

NASA Astrophysics Data System (ADS)

Olivetti, V.; Balestrieri, M. L.; Rossetti, F.; Talarico, F. M.

2012-04-01

The Mesozoic-Cenozoic West Antarctic Rift System (WARS) is one of the largest intracontinental rift on Earth. The Transantarctic Mountains (TAM) form its western shoulder, marking the boundary between the East and West Antarctica. The rifting evolution is commonly considered polyphase and involves an Early Cretaceous phase linked to the Gondwana break-up followed by a major Cenozoic one, starting at c. 50-40 Ma. This Cenozoic episode corresponds to the major uplift/denudation phase of the TAM, which occurred concurrently with transition from orthogonal to oblique rifting. The Cenozoic rift reorganization occurred concurrently with a major change in the global climate system and a global reorganization of plate motions. This area thus provide an outstanding natural laboratory for studying a range of geological problems that involve feedback relationships between tectonics and climate. A key to address the tectonic/climate feedback relations is to look on apparent synchronicity in erosion signal between different segments, and to compare these with well-dated regional and global climatic events. However, due to the paucity of Cenozoic rock sequences exposed along the TAM front, a few information is available about the neotectonics of the rift and rift-flank uplift system. The direct physical record of the tectonic/climate history of the WARS recovered by core drillings along the western margin of the Ross sea (DSDP, CIROS, Cape Roberts and ANDRILL projects) provides an invaluable tool to address this issue. Twenty-three samples distributed throughout the entire composite drill-cored stratigraphic succession of Cape Roberts were analyzed. Age probability plots of eighteen detrital samples with depositional ages between 34 Ma and the Pliocene were decomposed into statistically significant age populations or peaks using binomial peak-fitting. Moreover, three granitic pebbles, one dolerite clast and one sample of Beacon sandstones have been dated. From detrital samples, three peaks are detected reflecting different bedrock provenance areas. Two peaks older than 40 Ma (P2 and P3) are compatible with thermochronological data from TAM bedrock that underwent a stepwise denudation in Cretaceous times. A Peak younger than 40 Ma (P1) has been detected occasionally, recording the signal of a source area exhumed during late Oligocene /early Miocene with a constant denudation rate of 0.4 mm/yr (constant lag-time up-section), but absent in the onshore portion of the proximal TAM. Indeed, when compared with AFT data from ANDRILL cores, the relatively young P1 ages, suggest that part of sediments in the Cape Robert Rift basin have a provenance from source regions probably located far away in the south (i.e. Skelton-Byrd glaciers region) where bedrock experienced compatible thermal histories. This provenance would imply glacial systems with main flow patterns from south to the north, therefore orthogonal to the orientation of present-day drainage. We thus infer that the post-Eocene glacial and erosional history of the TAM front was significantly controlled by the N-S-trending transtensional regime that affected the western Ross Sea margin during transition from orthogonal to oblique rifting in the region. The appearance and disappearance of P1 along the drill-cored stratigraphic succession seems to be linked to the oscillation in the extent of the ice sheet.

Archean Arctic continental crust fingerprints revealing by zircons from Alpha Ridge bottom rocks

NASA Astrophysics Data System (ADS)

Sergeev, Sergey; Petrov, Oleg; Morozov, Andrey; Shevchenko, Sergey; Presnyakov, Sergey; Antonov, Anton; Belyatsky, Boris

2015-04-01

Whereas thick Cenozoic sedimentary cover overlapping bedrock of the Arctic Ocean, some tectonic windows were sampled by scientific submarine manipulator, as well as by grabbing, dredging and drilling during «Arctic-2012» Russian High-Arctic expedition (21 thousands samples in total, from 400-km profile along Alpha-Mendeleev Ridges). Among others, on the western slope of Alpha Ridge one 10x10 cm fragment without any tracks of glacial transportation of fine-layered migmatitic-gneiss with prominent quartz veinlets was studied. Its mineral (47.5 vol.% plagioclase + 29.6% quartz + 16.6% biotite + 6.1% orthoclase) and chemical composition (SiO2:68.2, Al2O3:14.9, Fe2O3:4.44, TiO2:0.54, MgO:2.03, CaO:3.13, Na2O:3.23, K2O:2.16%) corresponds to trachydacite vulcanite, deformed and metamorphozed under amphibolite facies. Most zircon grains (>80%) from this sample has an concordant U-Pb age 3450 Ma with Th/U 0.8-1.4 and U content of 100-400 ppm, epsilon Hf from -4 up to 0, and ca 20% - ca 3.3 Ga with Th/U 0.7-1.4 and 90-190 ppm U, epsilon Hf -6.5 to -4.5, while only 2% of the grains show Proterozoic age of ca 1.9 Ga (Th/U: 0.02-0.07, U~500 ppm, epsilon Hf about 0). No younger zircons were revealed at all. We suppose that magmatic zircon crystallized as early as 3450 Ma ago during acid volcanism, the second phase zircon crystallization from partial melt (or by volcanics remelting) under amphibolite facies metamorphism was at 3.3 Ga ago with formation of migmatitie gneisses. Last zircon formation from crustal fluids under low-grade metamorphic conditions was 1.9 Ga ago. There are two principal possibilities for the provenance of this metavolcanic rock. The first one - this is ice-rafted debris deposited by melted glacial iceberg. However, presently there are no temporal and compositional analogues of such rocks in basement geology of peri-oceanic regions, including Archean Itsaq Gneiss Complex, Lewisian Complex and Baltic Shield but these regions are far from the places of nowadays iceberg formation. Moreover, no Grenvillian-age zircons were revealed in studied sample. The nearest areas of Paleo- and Mesoarchean rocks appearance are deeply inland territories like Acasta Gneiss Complex, Uivak Gneisses, the Superior Province, Anabar and Aldan Shields, Northern China craton (Anshan complex), thus, a only possibility to bring some fragments of basement rocks from these areas to oceanic coast is transcontinental river transportation. The second possibility - this gneissic clast has a local provenance and has undergone a submarine weathering, shallow marine avalanche, proximal transportation by alongside ocean flows, tidal waves abrasion, and as so, reflects local bedrock geology, i.e. adjacent Alpha Ridge rock composition. Additional confirmation of this can be seen in a wide distribution of Qu-sandstones with Paleo-Mezoarchean zircons, and finds of similar allochtonous zircons in dolerites along Alpha-Mendeleev Ridge profile. The studied fragment is a unique evidence for the possible existence of Paleoarchean continental crust within the submarine Alpha-Mendeleev Ridge in Arctic Ocean.

ICDP supported coring in IDDP-2 at Reykjanes - the DEEPEGS demonstrator in Iceland - Supercritical conditions reached below 4.6 km depth.

NASA Astrophysics Data System (ADS)

Ómar Friðleifsson, Guðmundur; Elders, Wilfred A.; Zierenberg, Robert; Steafánsson, Ari; Sigurðsson, Ómar; Gíslason, Þór; Weisenberger, Tobias B.; Harðarson, Björn S.; Mesfin, Kiflom G.

2017-04-01

The Iceland Deep Drilling Project (IDDP) is exploring the technical and economic feasibility of producing supercritical geothermal resources. The IDDP-2 well is located in the Reykjanes saline geothermal system in SW Iceland, on the landward extension of the Mid-Atlantic Ridge, where we are probing the analog of the root zone of a black smoker. In 2009, Phase 1 of the IDDP was unsuccessful in reaching supercritical conditions in the Krafla volcanic caldera in NE Iceland, when the IDDP-1 drill hole unexpectedly encountered 900°C rhyolite magma at only 2.1 km depth. The completed well produced superheated steam with a well head temperature of 453°C with an enthalpy and flow rate sufficient to generate 35 MWe. Drilling the IDDP-2 began by deepening an existing 2.5 km deep production well (RN-15) to 3 km depth, casing it to 2941m depth and drilling it to 4626m. Total circulation losses which were encountered below 3 km depth, could not be cured by LCM and multiple cement jobs. Accordingly, drilling continued "blind" to total depth, without return of drill cuttings. We attempted 12 core runs below 3 km depth, half of which recovered some core. The cores are basalts and dolerites with alteration ranging from upper greenschist facies to amphibolite facies, suggesting formation temperatures >450°C. After a final report from the on-site science team, expected mid-year 2017, detailed petrological, petrophysical, and geochemical analyses of cores will be undertaken by the IDDP science team and collaborators and published in a special issue of a main-stream scientific journal. The drilling of the IDDP-2 was funded by the field operator HS Orka, and by Statoil, and the IDDP industry consortium. The coring was funded by ICDP and the science program of the IDDP. Deepening the RN-15 began 11th August 2016, and was completed to 4626m, 17th December 2016. A perforated liner was inserted to 4,571m and the well subsequently logged for temperature, pressure and injectivity, after 6 days partial heating-up. The injectivity index proved to be 1.7 (kg/s)/bar. Supercritical conditions were measured at the bottom, 427°C at 340 bar pressure. The T-log showed the main permeable zones to be at around 3360m, 4200m, 4370m and 4550m depth. Estimates suggest that 30% of 40 L/s injected into the well are received by the three deepest feed zones. This can possibly be enhanced by massive soft stimulation, which is a part of the DEEPEGS plan to be executed later this year. The DEEPEGS project is a demonstration project, supported by the European Commission, Horizon 2020. The goal is to demonstrate the feasibility of enhanced geothermal systems (EGS) for delivering energy from renewable resources in Europe. It is a four-year project coordinated by HS Orka, Iceland, in cooperation with partners from Iceland, France, Germany, Italy, and Norway. The project will demonstrate advanced technologies in three types of geothermal reservoirs, (i) in high enthalpy resource beneath existing hydrothermal field at Reykjanes with temperature up to 550°C, and (ii) in two very deep hydrothermal reservoirs in France with temperatures up to 220°C.

Pressure - temperature estimates of a phengite eclogite from the Grapesvåre Nappe, Norrbotten, Swedish Caledonides

NASA Astrophysics Data System (ADS)

Bukała, Michał; Majka, Jarosław; Walczak, Katarzyna; Barnes, Christopher; Klonowska, Iwona

2017-04-01

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides has well documented history of high pressure (HP) and ultra-high pressure (UHP) metamorphism (e.g. Klonowska et al. 2014). Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well studied and provide evidence for the Late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the Late Cambrian/Early Ordovician, have not been studied in detail, especially in terms of pressure-temperature (P-T) conditions of their formation. Within the SNC in Norrbotten, eclogites are limited to two tectonic lenses - Vaimok and Tsäkkok (e.g. Albrecht, 2000). Within the Vaimok Lens three nappes have been distinguished: (1) the eclogite-free Lower Seve Nappe, (2) the Grapesvåre Nappe and (3) the Maddåive Nappe. The two latter nappes are eclogite-bearing. For this study eclogites were collected from the lowermost part of the Grapesvåre Nappe (from the highly heterogeneous Daunasvagge unit dominated by garnet-bearing mica schists, quartzites and marbles). Eclogite boudins (former dolerite dikes and sills) are usually highly altered due to retrogressive recrystallization. Rare fresh eclogites occur within large boudins (>5m in diameter) and display only minor alteration limited to the scarce veinlets composed of amphibole + feldspar + garnet + zoisite + biotite + rutile + titanite. Metamorphic peak conditions mineral assemblage consists of garnet + omphacite + phengite + quartz + rutile. For P-T estimates the geothermobarometric method of Ravna & Terry (2004) has been used. The garnet-clinopyroxene Fe2+-Mg exchange thermometer and the net-transfer reaction barometer based on the garnet-phengite-omphacite equilibrium yielded a maximum pressure of 26.7 kbar and temperature of 677°C. The obtained temperature might be underestimated due to uncertainties in Fe2+/Fe3+ ratio in pyroxene. Therefore Zr-in-rutile geothermometer by Tomkins et al. (2007) has also been used and calculcated temperatures are in a 715-762°C range. The obtained results are somewhat similar to scarce former P-T estimates of eclogites from Norrbotten provided by Santallier (1988) (i.e. T=690-730°C and Pmin=18.5-19.5 kbar) and Albrecht (2000) (i.e. T=650-720°C and P=18.9-27.5 kbar). However, as Albrecht (2000) claimed, pressure values might have been largely underestimated due to limitation of the used methods. Thus our study provides for the first time an evidence for near UHP metamorphism recorded by eclogites of the Grapesvåre Nappe, hence shedding a new light on evolution of the SNC in Norrbotten. We speculate that maximum pressures of metamorphism might have been higher (even approaching coesite stability field), but further studies are required to pinpoint the maximum PT conditions. This work is financially supported by the NCN "CALSUB" research project no. 2014/14/E/ST10/00321. References: Albrecht L.G. (2000) PhD thesis, Lund University. Klonowska I., Majka J., Janák M., Gee D.G., Ladenberger A. (2014) New Perspectives on the Caledonides of Scandinavia and Related Areas. Geological Society, London, Special Publications, 390: 321-336. Ravna E.J., Terry M.P. (2004) Journal of Metamorphic Geology 22: 579-592. Santallier D.S. (1988) Geologiska Föreningen i Stockholm Förhandlingar 110: 89-98. Tomkins H.S., Powell R., Ellis J.D. (2007) Journal of Metamorphic Geology 25: 703-713.

Age and isotopic marks of K-rich Manning Massif trachybasalts: an evidence for Lambert-Amery rift-system initiation (East Antarctica)

NASA Astrophysics Data System (ADS)

Leitchenkov, German; Belyatsky, Boris; Lepekhina, Elena; Antonov, Anton; Krymsky, Robert; Andronikov, Alex; Sergeev, Sergey

2017-04-01

Volcanic rocks from the Manning Massif, which is situated in the western flank of the Paleozoic-Late Mesozoic Lambert Rift (East Antarctica), belong to a rare type of alkaline magmatism within the Precambrian East Antarctic Craton. K-rich olivine trachybasalts compose some flows resting upon a surface of Precambrian granulite terrain, each flow of 2.5-7 m in thickness and total section not less than 30 m. Each flow sequence comprises of glassy chilled base with vitroporphyritic texture, fine-plated vesicular basalt with interstitial texture, massive fine-grained basalt with porphyritic microlitic texture, amigdaloidal aphanitic basalt with poikilophytic texture, and vesicular mandelstone of slag crust with vitroporphyritic texture [Andronikov et al., 1998]. Rb-Sr and K-Ar isotopic age of this eruption was estimated as 40-50 Ma and the main reason for this Cenozoic continental volcanism was supposed the post-rift tectonic activity [Andronikov et al., 1998]. But the isotopic characteristics of these trachybasalts are very similar to those obtained for the part of spinel lherzolite and spinel-garnet lherzolite xenoliths from the Mesozoic alkaline picrite of the adjacent Jetty Peninsula region. That could be evidence of the trachybasalt mantle source in long-lived enriched upper mantle beneath the region, either under the lowermost levels of spinel lherzolite facies or on the highest levels of garnet lherzolite facies conditions. To reveal tectonic position of these enigmatic volcanics, we have studied 16 samples from different parts of basaltic flows for U-Pb geochronology and Pb-Sr-Nd-Os isotopic characteristics. U-Pb SIMS SHRIMP-II analysis was performed for 68 apatite grains from 5 samples. All obtained data-points are approximated by discordia line (MSWD=1.6) on Tera-Wasserburg diagram, corresponding to the age of 346±46 Ma. Common Pb isotope composition of these apatites differs from the model by increased 206Pb/204Pb (19.8) and 207Pb/204Pb (18.3) that means the source of contamination was an ancient material (> 2.4 Ga) and/or with high μ (26.5). The initial isotope characteristics of the studied basalts are the same for different individual flows: ɛNd=-3.4±0.4; 87Sr/86Sri=0.7061±0.0003, 206Pb/204Pbi=18.421±0.001; 207Pb/204Pbi=15.667±0.001; 208Pb/204Pbi=39.845±0.001; 187Os/186Osi = 0.2012±0.0004 and reflect minimal influence of host-rock contamination during or after melts crystallization and correspond to enriched mantle source signatures akin to plume-like. Thus the Manning Massif K-rich basalts correlate with the time of formation of the Late-Paleozoic coal-bearing sediments of the Lambert Glacier Rift and basic dykes of Jetty Peninsula [Mikhalsky, Sheraton, 1993] and can be interpreted to mark the earliest, Lower Carboniferous stage of the rifting. This event corresponds to the initial intracontinental stretching in the Eastern Gondwana wich was previously detected only in Perth Basin of western Australia. The research was done under financial support by RSF grant N 16-17-10139. References: Andronikov A.V., Foley S.F., Beliatsky B.V. 1998. Sm-Nd and Rb-Sr isotopic systematics of the East Antarctic Manning Massif alkaline trachybasalts and the development of the mantle beneath the Lambert-Amery rift. Mineral. Petrol. 63. 243-261. Mikhalsky E.V., Sheraton J.W. 1993. Association of dolerite and lamprophyre dykes, Jetty Peninsula (Prince Charles Mountains, East Antarctica). Antarctic Sciences. 5(3). 297-307.

Geochemical characteristics of Antarctic magmatism connected with Karoo-Maud and Kerguelen mantle plumes

NASA Astrophysics Data System (ADS)

Sushchevskaya, Nadezhda; Krymsky, Robert; Belyatsky, Boris; Antonov, Anton; Migdisova, Natalya

2013-04-01

Emplacement (130-115 m.y. ago) of dikes and sills of alkaline-ultrabasic composition within Jetty oasis (East Antarctica) is suggested as a later appearance of plume magmatism within the East-Antarctic Shield [Andronikov et al., 1993, 2001; Laiba et al., 1987]. This region is located opposite Kerguelen Islands and possibly could be properly connected with activity of the Kerguelen-plume [Foley et al., 2001, 2006]. Jurassic-Cretaceous dykes, stocks and sills of alkaline-ultrabasic rocks, relatively close to kimberlite-type, are exposed within Jetty oasis and on the southern shore of the Raddock Lake. This alkaline-ultrabasic magmatism has appeared to be connected with the main Mesozoic stage of the evolution of the Lambert and Amery glaciers riftogenic structure [Kurinin et al., 1980, 1988]. The alkaline-ultrabasic dikes and sills within Jetty oasis cut the rocks of the Beaver complex, Permo-Triassic terrigeneous successions of the Amery complex, and late Paleozoic low-alkaline basic dikes as well. Dashed chain of 6 stock bodies spread out on 15 km along the eastern shore of the Beaver Lake, marked their allocation with submeridianal zone of the deep cracks, boarded of the eastern side of the Beaver Lake trough. At the same time, new data upon Quaternary magmatism of the mountain Gaussberg has confirmed the unique features of ultra-potassium alkaline magmatism (up to 14-17% K2O) formed under exclusively continental conditions [Murphy et al., 2002]. Volcanic cone is located at the continuation of Gaussberg rift zone which is possibly a part of Lambert fracture zone. Its formation is connected with the early stages of Gondwana development, perhaps, reactivated in different Precambrian events and according to numerous data is a single rift zone which is traced Indian inland (Indrani graben, [Golynsky, 2011]). The time of lamproitic magmas eruption is estimated at 56000±5000 yeas ago [Tingey et al., 1983]. Earlier it had been shown the Mesozoic (about 170 Ma) basaltic dykes of the Schirmacher Oasis and basalts and dolerites of the Queen Maud Land (180 Ma) are identical in petrology and geochemistry terms and supposedly could be interpreted as the manifestation of the Karoo-Maud plume activity in Antarctica [Sushchevskaya et al., 2012]. The spatial distribution of the dikes indicates the eastward spreading of the plume material from DML to the Schirmacher Oasis within at least 10 Ma (up to ~35 Ma, taking into account the uncertainty of age determination). On the other hand, the considerable duration and multistage character of plume magmatism related to the activity of the Karoo-Maud plume in Antarctica and Africa [Leat et al., 2007; Luttinen et al., 2002] may indicate that the Mesozoic dikes of the oasis correspond to a single stage of plume magmatism. On the basis of obtained isotopic data it has been determined two magmatic melt evolution trends for basalts from: Queen Maud Land - Kerguelen Archipelago - Afanasy Nikitin Rise (Indian Ocean) and Jetty - Schirmacher oasises which mantle sources are quite different. Thus the Jetty - Schirmacher oasises magmatic melt sources are characterized by prevalence of the matter of moderately enriched or primitive chondritic mantle source and lithospheric mantle of Proterozoic ages but the substances of depleted mantle source similar to MORB-type and ancient mantle are absent. New data obtained on Nd, Sr, Pb isotopic and lithophile elements compositions of the alkaline-ultrabasic rocks from the Jetty oasis and Gaussberg volcano completed imagine of the Kerguelen-plume evolution. It has been confirmed unique character of the alkaline lamproiites of the Gaussberg volcano enrichments. Highly radiogenic Sr and Pb isotope ratios of these lamproiites reflect melting of the ancient sublithospheric depleted mantle which was stored from the Archean till nowadays unaffected by metasomatic-enrichment processes. During modern melting of this mantle part there is input of additional substances (crustal fluid of sediment origins, subducted sediments etc.) with high Rb/Sr ratio.

Ophiolites in ocean-continent transitions: From the Steinmann Trinity to sea-floor spreading

NASA Astrophysics Data System (ADS)

Bernoulli, Daniel; Jenkyns, Hugh C.

2009-05-01

Before the theory of plate tectonics took hold, there was no coherent model for ocean-continent transitions that included both extant continental margins and fragmentary ancient examples preserved in orogenic belts. Indeed, during the early 1900, two strands of thought developed, one relying on the antiquity and permanence of continents and oceans, advocated by the mainstream of the scientific community and one following mobilist concepts derived from Wegener's hypothesis (1915) of continental drift. As an illustration of the prevailing North-American view, the different composition and thickness of continental and oceanic crust and the resulting isostatic response showed "how improbable it would be to suppose that a continent could founder or go to oceanic depth or that ocean floor at ± 3000 fathoms could ever have been a stable land area since the birth of the oceans" [H.H. Hess, Trans. R. Soc. London, A 222 (1954) 341-348]. Because of the perceived permanence of oceans and continents, mountain chains were thought to originate from narrow, elongated, unstable belts, the geosynclines, circling the continents or following "zones of crustal weakness" within them, from which geanticlines and finally mountain belts would develop. This teleological concept, whereby a geosyncline would inevitably evolve into a mountain chain, dominated geological interpretations of orogenic belts for several decades in the mid-twentieth century. However, the concept of permanence of oceans and continents and the concept of the geosyncline had already met with the critiques of Suess and others. As early as 1905, Steinmann considered the association of peridotite, "diabase" (basalt/dolerite) and radiolarite (a typical ocean-continent transition assemblage), present in the Alps and Apennines, as characteristic of the deep-ocean floor and Bailey (1936) placed Steinmann's interpretation into the context of continental drift and orogeny. Indeed, in both authors' writings, the concept of ophiolites as ocean crust is apparent. Between 1920 and 1930, the stage was thus potentially set for modern mobilist concepts that were, however, to prove attractive to only a small circle of Alpine and peri-Gondwanian geologists. After the Second World War, the 1950s saw the rapid progress of the geophysical and geological exploration of oceans and continental margins that provided the data for a reevaluation of the geosynclinal concept. Actualistic models now equated the former preorogenic miogeosyncline of Stille (1940) and Kay (1951) with passive continental margins [C.L. Drake, M. Ewing, G.H. Sutton, Continental margin and geosynclines: the east coast of North America, north of Cape Hatteras, in: L. Ahrens, et al. (Eds.), Physics and Chemistry of the Earth 3, Pergamon Press, London, 1959, pp. 110-189], the (American version of the) eugeosyncline and its igneous rocks with "collapsing continental rises" [R.S. Dietz, J. Geol. 71 (1963) 314-333] and the ophiolites, the Steinmann Trinity, of the (European) eugeosyncline with fragments of oceanic lithosphere [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599-620]. The concept of sea-floor spreading [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599-620; H.H. Hess, Mid-oceanic ridges and tectonics of the sea-floor, in: W.F. Whittard, R. Bradshaw (Eds), Submarine Geology and Geophysics, Colston Papers 17, Butterworths, London, 1965, pp. 317-333] finally eliminated the weaknesses in Wegener's hypothesis and, with the coming of the "annus mirabilis" of 1968, the concept of the geosyncline could be laid to rest. Ocean-continent transitions of modern oceans, as revealed by seismology and deep-sea drilling, could now be compared with the remnants of their ancient counterparts preserved in the Alps and elsewhere.

The Morsárjökull rock avalanche in the southern part of the Vatnajökull glacier, south Iceland

NASA Astrophysics Data System (ADS)

Sæmundsson, Şorsteinn; Sigurősson, Ingvar A.; Pétursson, Halldór G.; Decaulne, Armelle; Jónsson, Helgi P.

2010-05-01

On the 20th of March 2007 a large rock avalanche fell on Morsárjökull, one of the outlet glaciers from the southern part of the Vatnajökull ice cap, in south Iceland. This is considered to be one of the largest rock avalanches which have occurred in Iceland during the last decades. It is believed that it fell in two separate stages, the main part fell on the 20th of March and the second and smaller one, on the 17th of April 2007. The Morsárjökull outlet glacier is about 4 km long and surrounded by up to 1000 m high valley slopes. The outlet glacier is fed by two ice falls which are partly disconnected to the main ice cap of Vatnajökull, which indicates that the glacier is mainly fed by ice avalanches. The rock avalanche fell on the eastern side of the uppermost part of the Morsárjökull outlet glacier and covered about 1/5 of the glacier surface, an area of about 720,000 m2. The scar of the rock avalanche is located on the north face of the headwall above the uppermost part of the glacier. It is around 330 m high, reaching from about 620 m up to 950 m, showing that the main part of the slope collapsed. It is estimated that about 4 million m3 of rock debris fell on the glacier, or about 10 million tons. The accumulation lobe is up to 1.6 km long, reaching from 520 m a.s.l., to about 350 m a.s.l. Its width is from 125 m to 650 m, or on average 480 m. The total area which the lobe covers is around 720.000 m2 and its mean thickness 5.5 m. The surface of the lobe shows longitudinal ridges and grooves and narrow flow-like lobes, indicating that the debris mass evolved down glacier as a mixture of a slide and debris flow. The debris mass is coarse grained and boulder rich. Blocks over 5 to 8 m in diameter are common on the edges of the lobe up to 1.6 km from the source. No indication was observed of any deformation of the glacier surface under the debris mass. The first glaciological measurements of Morsárjökull outlet glacier were carried out in the year 1896 and it is evident that since that time the glacier has retreated considerably and during the last decade the melting has been very rapid. It is thought that undercutting of the mountain slope by glacial erosion and the retreat of the glacier are the main contributing factors leading to the rock avalanche. The glacial erosion has destabilized the slope, which is mainly composed of palagonite and dolerite rocks, affected by geothermal alteration. Hence a subsequent fracture formation has weakened the strength of the bedrock. However the exact triggering factor is not known. No seismic activity or meteorological signal such as heavy rainfall or intensive snowmelt recorded prior to the rock avalanche which could be interpreted as triggering factors. From 2007 considerable changes have been observed on the glacier. The ice-front has retreated considerably and the debris lobe of the rock avalanche has moved downward along with the glacier ice about 90-100 m per year. The rocky material, by insulating the ice, has reduced its melting, leading to a relative "thickening" of the ice beneath the rock avalanche debris up to 11-15 m per year. After three melting seasons the debris mass was about 29 m above the surrounding ice surface.

Platinum mineralization in the Kapalagulu Intrusion, western Tanzania

NASA Astrophysics Data System (ADS)

Wilhelmij, Harry R.; Cabri, Louis J.

2016-03-01

Low-grade copper and nickel mineralization was found near the eastern shore of Lake Tanganyika at Kungwe Bay in the early part of the twentieth century. The mineralization occurs in harzburgite at the base of a layered gabbro complex known as the Kapalagulu Intrusion, emplaced between the Paleoproterozoic Ubendian basement and overlying Neoproterozoic Itiaso Group metasediments. Several mining and exploration companies continued the geophysical and drilling exploration for base metals throughout the last century culminating in the discovery of high-grade platinum-group element (PGE) mineralization associated with chromitite and sulfide-bearing harzburgite within the southeastern extension of the Kapalagulu Intrusion (known as the Lubalisi Zone) that is covered by a layer of nickel-rich laterite regolith. The poorly layered southeastern harzburgite forms part of the >1500 m-thick Lower Ultramafic Sequence and resembles a dike-like body that flares upwards into a succession of well-layered gabbroic rocks of the Upper Mafic Sequence. No PGE mineralization has been found in the layered gabbro; all the mineralization is associated with the chromite- and sulfide-rich harzburgite of the Lower Ultramafic Sequence and the laterite regolith overlying the mineralized harzburgite. The Lubalisi Zone harzburgite is underlain by basal dunite and overlain by an interval of layered harzburgite and troctolite and this ultramafic sequence is folded into a syncline that plunges towards the northwest that has been modified by major dolerite-filled faults orientated subparallel to the fold axial surface. Extensive deep drilling in the Lubalisi Zone of the Kapalagulu Intrusion shows that the folded harzburgite can be subdivided into a lower feldspathic harzburgite, a harzburgite containing chromitite seams and intervals of sulfide and chromite mineralization known as the Main Chromite Sulfide Succession (MCSS), an overlying sulfide-rich harzburgite, and an upper feldspathic harzburgite. Impersistent, stratiform PGE mineralized horizons occur within the MCSS harzburgite from which drill core samples were taken for platinum-group mineral (PGM) characterization from two drill holes. Where the PGE reefs reach the surface there is residual PGE mineralization within the laterite regolith from which drill core samples were taken from various laterite lithological units for PGM characterization. As the harzburgite PGE reefs contain significant concentrations of both sulfide and chromite (including chromitite seams) they resemble the PGE-rich chromitite seams of the Bushveld Complex rather than the PGE-bearing Main Sulfide Zone of the Great Dyke and Main Sulfide Layer of the Munni Munni Complex. The dominant Pd PGM in three PGE reef samples varies, ranging ( n = 164, relative wt%) from bismuthides (63 %), bismuthtellurides (19 %), and tellurides (6 %), to tellurides (39 %), bismuthtellurides (24 %), stannides (14 %), and alloys (13 %), and to antimon-arsenides (33 %), stannides (21 %), bismuthides (17 %), tellurides (13 %), and alloys (10 %). From 13.5 % to 21.0 % of the total Pd occurs as a solid solution in pentlandite. The three samples have similar Pt PGM modal distributions ( n = 172, relative wt%); the dominant Pt mineral is sperrylite (79, 58, and 47 %) followed by tellurides (15, 17, 21 %), alloys (2, 1, 1 %), and sulfides (2, 1, 0 %). Comparison of Pd/Pt ratios from assays to those calculated from minerals show that the data for the Pt and Pd PGM are very robust, confirming the concentration methodology and characterization. Study of samples from a shallow drill hole penetrating the laterite regolith shows that the primary Pd mineralization has not survived oxidation, is mainly dispersed, but some was reconstituted to form secondary minerals: cabriite, unnamed tellurides, a selenide, a Pd-Te-Hg mineral, alloys and Pd-bearing secondary sulfides (millerite and heazlewoodite). The primary Pt minerals are more resistant to oxidation and dissolution, especially sperrylite and isoferroplatinum, but it is likely that other Pt alloys (tetraferroplatinum and tulameenite) are of secondary origin after dissolution of Pt tellurides.

Geophysical studies of the West Antarctic Rift System

NASA Astrophysics Data System (ADS)

Behrendt, J. C.; Lemasurier, W. E.; Cooper, A. K.; Tessensohn, F.; TréHu, A.; Damaske, D.

1991-12-01

The West Antarctic rift system extends over a 3000 × 750 km, largely ice covered area from the Ross Sea to the base of the Antarctic Peninsula, comparable in area to the Basin and Range and the East African rift system. A spectacular rift shoulder scarp along which peaks reach 4-5 km maximum elevation marks one flank and extends from northern Victoria Land-Queen Maud Mountains to the Ellsworth-Whitmore-Horlick Mountains. The rift shoulder has maximum present physiographic relief of 5 km in the Ross Embayment and 7 km in the Ellsworth Mountains-Byrd Subglacial Basin area. The Transantarctic Mountains part of the rift shoulder (and probably the entire shoulder) has been interpreted as rising since about 60 Ma, at episodic rates of ˜1 km/m.y., most recently since mid-Pliocene time, rather than continuously at the mean rate of 100 m/m.y. The rift system is characterized by bimodal alkaline volcanic rocks ranging from at least Oligocene to the present. These are exposed asymmetrically along the rift flanks and at the south end of the Antarctic Peninsula. The trend of the Jurassic tholeiites (Ferrar dolerites, Kirkpatric basalts) marking the Jurassic Transantarctic rift is coincident with exposures of the late Cenozoic volcanic rocks along the section of the Transantarctic Mountains from northern Victoria Land to the Horlick Mountains. The Cenozoic rift shoulder diverges here from the Jurassic tholeiite trend, and the tholeiites are exposed continuously (including the Dufek intrusion) along the lower- elevation (1-2 km) section of Transantarctic Mountains to the Weddell Sea. Widely spaced aeromagnetic profiles in West Antarctica indicate the absence of Cenozoic volcanic rocks in the ice covered part of the Whitmore-Ellsworth-Mountain block and suggest their widespread occurrence beneath the western part of the ice sheet overlying the Byrd Subglacial Basin. A German Federal Institute for Geosciences and Natural Resources (BGR)-U.S. Geological Survey (USGS) aeromagnetic survey over the Ross Sea continental shelf indicates rift fabric and suggests numerous submarine volcanoes along discrete NNW trending zones. A Bouguer anomaly range of approximately 200 (+50 to -150) mGal having 4-7 mGal/km gradients where measured in places marks the rift shoulder from northern Victoria Land possibly to the Ellsworth Mountains (where data are too sparse to determine maximum amplitude and gradient). The steepest gravity gradients across the rift shoulder require high density (mafic or ultramafic?) rock within the crust as well as at least 12 km of thinner crust beneath the West Antarctic rift system in contrast to East Antarctica. Sparse land seismic data reported along the rift shoulder, where velocities are greater than 7 km/s, and marine data indicating velocities above 7 km/s beneath the Ross Sea continental shelf support this interpretation. The maximum Bouguer gravity range in the Pensacola Mountains area of the Transantarctic Mountains is only about 130 mGal with a maximum 2 mGal/km gradient, which can be explained solely by 8 km of crustal thickening. Large offset seismic profiles over the Ross Sea shelf collected by the German Antarctic North Victoria Land Expedition V (GANOVEX V) combined with earlier USGS and other results indicate 17-21 km thickness for the crust beneath the Ross Sea shelf which we interpret as evidence of extended rifted continental crust. A regional positive Bouguer anomaly (0 to +50 mGal), the width of the rift, extends from the Ross Sea continental shelf throughout the Ross Embayment and Byrd Subglacial Basin area of the West Antarctic rift system and indicates that the Moho is approximately 20 km deep tied to the seismic results (probably coincident with the top of the asthenosphere) rather than the 30 km reported in earlier interpretations. The interpretation of horst and graben structures in the Ross Sea, made from marine seismic reflection data, probably can be extended throughout the rift (i.e., the Ross Ice shelf and the Byrd Subglacial Basin areas). The near absence of earthquakes in the West Antarctic rift system probably results from a combination of primarily sparse seismograph coverage and, secondarily, suppression of earthquakes by the ice sheet (e.g., Johnston, 1987) and very high seismicity shortly after deglaciation in the Ross Embayment followed by abnormally low seismicity at present (e.g., Muir Wood, 1989). The evidence of high temperatures at shallow depth beneath the Ross Sea continental shelf and adjacent Transantarctic Mountains is supportive of thermal uplift of the mountains associated with lateral heat conduction from the rift and can possibly also explain the volcanism, rifting, and high elevation of the entire rift shoulder to the Ellsworth-Horlick-Whitmore Mountains. We infer that the Gondwana breakup and the West Antarctic rift are part of a continuously propagating rift that started in the Jurassic when Africa separated from East Antarctica (including the failed Jurassic Transantarctic rift). Rifting proceeded clockwise around East Antarctica to the separation of New Zealand and the Campbell Plateau about 85-95 Ma and has continued (with a spreading center jump) to its present location in the Ross Embayment and West Antarctica. The Cenozoic activity of the West Antarctic rift system appears to be continuous in time with rifting in the same area that began only in the late Mesozoic. Although the mechanism for rifting is not completely explained, we suggest a combination of the flexural rigidity model (Stem and ten Brink, 1989) proposed for the Ross Embayment and the thermal plume or hot spot concepts. The propagating rift may have been "captured" by the thermal plume.

Did clockwise rotation of Antarctica cause the break-up of Gondwanaland? An investigation in the 'deep-keeled cratons' frame for global dynamics

NASA Astrophysics Data System (ADS)

Osmaston, M. F.

2012-04-01

Introduction. The 'deep-keeled cratons' frame for global dynamics is the result of seeking Earth-behaviour answers to the following outside-the-box proposition:- "If cratons have tectospheric keels that reach or approach the 660 km discontinuity, AND the 660 level is an effective barrier to mantle circulation, then obviously (i) when two cratons separate, the upper mantle to put under the nascent ocean must arrive by a circuitous route and, conversely, (ii) if they approach one another, the mantle volume that was in between them must get extruded sideways." Surprisingly it has turned out [1 - 4] that Earth dynamical behaviour for at least the past 150 Ma provides persuasive affirmation of both these expectations and that there is a rational petrological explanation for the otherwise-unexpected immobility of subcratonic material to such depths [5 - 7]. Clockwise rotation of Antarctica? This contribution greatly amplifies my original plate dynamical arguments for suggesting [8] that such rotation is ongoing. Convection is unsuited to causing rotation about a pole within the plate so, as noted then, a gearwheel-like linkage to Africa at the SWIR would provide its clearly CCW (Biscay-Caucasus) relationship to the Mediterranean belt for the past 100 Ma, also seen in its separation from South America. Gearwheel-like linkage of motion requires the presence of some kind of E-W restraint further north. In that case it was the N Africa/Arabia involvement in the Alpide belt, but the earlier opening of the central Atlantic by the eastward motion of Africa, suggests its rigid Gondwanan attachment to Antarctica rotation at that time, with little constraint in the north. Further east, the seafloor data show that Australia-Antarctica separation involved no such opposite rotational linkage, so, with no E-W mechanical constraint in the north by Indonesia, they must have rotated together, as is recorded by Australia's eastward motion to generate the Mesozoic seafloor at its western side. Moving east again, the sigmoidal fracture-zone pattern between W Antarctica and Tonga Trench seems consistent with a gearwheel-linked relative rotation of the Pacific plate by about 35o CCW since about 120 Ma, so about half that (clockwise) by Antarctica. The triangular Cocos plate is then in the position where the two gearwheels separate. Further north, the dextral slip on the San Andreas Fault and the opening of the Gorda Ridge are broadly consistent with such rotation. Note that with our two-layer mantle all reference to 'absolute', lower mantle-related, positions is inappropriate. Our sole concern now is with relative motions of plates. Driving torque on the cratonic keel of East Antarctica. I maintain here my suggestion [8] that this keel, in actual contact with the lower mantle at its boundary, is picking up an electromagnetically generated torque, transmitted up from the polar zone of the CMB through the higher viscosity lower mantle. The reality of the rotation now invites more attention to this mechanism. The involvement of the cratonic keel is supported, as noted [8], by the apparent absence of rotational effects in the Arctic, where there is no keel in the polar position, although a similar CMB coupling to the lower mantle seems likely. The involvement of geomagnetism is supported by the sharp changes in central Pacific fracture zone orientation and the onset of the Ontong Java magmatism, correlating with the start and end of the Cretaceous long normal geochron [8, 9]. Such a change is also seen at M0 time in the Weddell Sea. Presumably the speed of Antarctica rotation was affected. Gondwanaland break-up. In view of these abundant tectonic effects attributable to Antarctica rotation, I propose that this was what broke up Gondwanaland, not a plume, as no such things are recognized in this thick-plate, two-layer mantle, version of the Earth-function paradigm. In this version, magmas with apparently lower mantle chemical signatures can be sourced within the upper mantle [10] and flood basalts can be generated by splitting cratons [11]. So the ~176 Ma age of the Ferrar Dolerite in Antarctica is a record of one of those splits. Gaps in the PalaeoPacific rim. If we restore Australia both westward to before the spreading at its western side and southward to its position against Antarctica, the Pacific rim was a fair approximation to a great circle, so it covered a hemisphere. Spreading of the other oceans, initiated by Gondwanaland break-up, must have been at the expense of the size of the Pacific, so it must formerly have covered much more than a hemisphere, and had a periphery correspondingly rather shorter than a great circle. Thus we have the surprising result that reducing the area of the Pacific actually required that its rim be made longer, by making gaps between the previously defining cratonic keels. A further result was that now-excess upper mantle material from below the Pacific had to flow through those gaps to put beneath the widening 'new' oceans. For all four of the obvious gaps - Caribbean, Scotia, Australia-Antarctica, Bering - there is evidence to support the presence of that outflow, and in two of the cases there is evidence that motions to open the gaps began very soon after Gondwana began to break up. Subduction and a two-layer mantle? In another contribution at this meeting (GD5.1) I explain that, in the thick-plate frame adopted here, subduction is neither a motivating player (for break-up purposes) in plate dynamics nor does it breach significantly our 2-layer mantle picture. The underlying reason is that oceanic 'tectosphere' is actually thicker for the same reason [5 - 7] as that of cratons, giving it ex-LVZ heat content which transforms the subduction picture. Three Conclusions. (1) The thick-plate, 2-layer mantle version of the earth-function paradigm [1 - 7] is alive and well. (2) The break-up of Gondwanaland was caused by Antarctica's clockwise rotation. (3) Such rotation is now to be considered a major agent in plate motion dynamics for the period during which East Antarctica, or any other sufficiently deep-keeled craton previously, was located at one of the Earth's poles. [1] Osmaston M. F. (2006) Global tectonic actions emanating from Arctic opening in the circumstances of a two-layer mantle and a thick-plate paradigm involving deep cratonic tectospheres: the Eurekan (Eocene) compressive motion of Greenland and other examples. In Proc. ICAM IV, 2003 (ed. R. Scott & D. Thurston). OCS Study MMS 2006-003, p.105-124: Also at: http://www.mms.gov/alaska/icam. [2] Osmaston M. (2005) Interrelationships between large-scale plate motions as indicators of mantle structure: new constraints on mantle modelling and compositional layout. In 3rd Workshop on "Earth's mantle composition, structure and phase transitions" St Malo, France. http://deep.earth.free.fr/participants.php. [3] Osmaston M. F. (2007) Cratonic keels and a two-layer mantle tested: mantle expulsion during Arabia-Russia closure linked to westward enlargement of the Black Sea, formation of the Western Alps and subduction of the Tyrrhenian (not the Ionian) Sea. XXIV IUGG, Session JSS 011, Abstr #2105 http://www.iugg2007perugia.it/webbook/. [4] Osmaston M. F. (2009) Deep cratonic keels and a 2-layer mantle? Tectonic basis for some far-reaching new insights on the dynamical properties of the Earth's mantle: example motions from Mediterranean, Atlantic-Arctic and India. Geophys. Res. Abstr. 11, EGU2009-6359 (Solicited). [5] Karato S. (1986) Does partial melting reduce the creep strength of the upper mantle? Nature 319, 309-310. [6] Hirth G. & Kohlstedt D. L. (1996) Water in the oceanic upper mantle: implication for rheology, melt extraction, and the evolution of the lithosphere. EPSL 144, 93-108. [7] Osmaston M. F. (2010) On the actual variety of plate dynamical mechanisms and how mantle evolution affected them through time, from core formation to the Indian collision. Geophys. Res. Abstr. 12, EGU2010-6101. [8] Osmaston M. F. (2003) What drives plate tectonics? Slab pull, ridge push or geomagnetic torque from the CMB? A new look at the old players vis-a-vis an exciting new one. In XXIII IUGG 2003, B129, Abstr no 016795-2. [9] Atwater T., Sclater J., Sandwell D., Severinghaus J., & Marlow M. S. (1993) Fracture zone traces across the North Pacific Cretaceous quiet zone and their tectonic implications. In The Mesozoic Pacific: geology, tectonics and volcanism, (ed. Pringle, Sager, Sliter, & Stein) AGU Geophys. Monogr. 77, 137-154. [10] Osmaston M. F. (2000) An upper mantle source for plumes and Dupal; result of processes and history that have shaped the Earth's interior from core to crust. Goldschmidt 2000, J. Conf. Abstr. 5 (2), 763. [11] Osmaston M. F. (2008) Extra-thick plates: basis for a single model of mantle magmagenesis, all the way from MORB to kimberlite. Goldschmidt 2008. Geochim.Cosmochim. Acta 72(12S), A711.