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Sample records for adamellite

  1. Lead isotope systematics of some igneous rocks from the Egyptian Shield

    NASA Technical Reports Server (NTRS)

    Gillespie, J. G.; Dixon, T. H.

    1983-01-01

    Lead isotope data on whole-rock samples and two feldspar separates for a variety of Pan-African (late Precambrian) igneous rocks for the Egyptian Shield are presented. It is pointed out that the eastern desert of Egypt is a Late Precambrian shield characterized by the widespread occurrence of granitic plutons. The lead isotope ratios may be used to delineate boundaries between Late Precambrian oceanic and continental environments in northeastern Africa. The samples belong to three groups. These groups are related to a younger plutonic sequence of granites and adamellites, a plutonic group consisting of older tonalites to granodiorites, and the Dokhan volcanic suite.

  2. Late Triassic syn-exhumation magmatism in central Qiangtang, Tibet: Evidence from the Sangehu adakitic rocks

    NASA Astrophysics Data System (ADS)

    Liu, Han; Wang, Bao-di; Ma, Long; Gao, Rui; Chen, Li; Li, Xiao-bo; Wang, Li-quan

    2016-12-01

    The geodynamic setting of Late Triassic magmatic activity along the Longmu Co-Shuanghu suture zone (LSSZ) in central Qiangtang, Tibet is a matter of debate. This paper presents zircon LA-ICP-MS U-Pb ages, zircon Hf isotopic compositions, and whole-rock geochemical data for the Sangehu (SGH) granitic intrusion in central Qiangtang, and addresses the petrogenesis of Late Triassic magmatism, and the history of collision between the northern and southern Qiangtang terranes. The SGH pluton consists mainly of biotite adamellite with mafic microgranular enclaves (MMEs), and small amounts of K-feldspar granite. The biotite adamellite, MMEs, and K-feldspar granite give ages of 207.8 ± 3.0 Ma, 212.4 ± 31 Ma, and 211.6 ± 3.8 Ma, respectively. The MMEs show magmatic textures and acicular apatite, and are coeval with the host biotite adamellite, suggesting they were produced by magma mixing. All samples from the SGH pluton show high Sr and low Y contents, and positive Eu anomalies, similar to adakitic rocks. The high K2O contents and low Mg#, Cr, and Ni contents, and enriched Hf isotopic characteristics of the zircons indicate that these magmas were derived from the partial melting of thickened crust. However, the whole-rock geochemical data and zircon Hf isotopic compositions also reveal heterogeneity at the source. The combined magmatic and metamorphic records suggest that Triassic magmatic activity in central Qiangtang was closely related to the collision of the northern and southern Qiangtang terranes. The large-scale Late Triassic (225-200 Ma) magmatic event in central Qiangtang may have resulted from the breakoff of the Longmu Co-Shuanghu Tethys Ocean lithospheric slab in the early Late Triassic (236-230 Ma). The Late Triassic magmatic rocks, including adakitic rocks, are coeval with retrograde high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in central Qiangtang, and show characteristics of syn-exhumation magmatism. The early adakitic rocks (>220 Ma

  3. Petrogenesis and tectonic setting of the early Mesozoic Xitian granitic pluton in the middle Qin-Hang Belt, South China: Constraints from zircon U-Pb ages and bulk-rock trace element and Sr-Nd-Pb isotopic compositions

    NASA Astrophysics Data System (ADS)

    Wu, Qianhong; Cao, Jingya; Kong, Hua; Shao, Yongjun; Li, Huan; Xi, Xiaoshuang; Deng, Xuantong

    2016-10-01

    The Xitian pluton in southeast Hunan province is one of the early Mesozoic (Indosinian) granitic plutons in the South China Block. It is composed of biotite adamellite with K-feldspar megacrysts, biotite adamellite, and biotite granite that have U-Pb zircon ages of 229.9 ± 1.4 Ma, 223.6 ± 1.3 Ma, and 224.0 ± 1.4 Ma, respectively. The Indosinian granitoids in the Xitian pluton belong to S-type granites, with highly radiogenic initial 87Sr/86Sr ratios (0.71397-0.71910), negative εNd(t) values ranging from -10.1 to -9.4, and old Nd model ages (1858-1764 Ma). They are enriched in radiogenic Pb isotopes, with (206Pb/204Pb)t ranging from 18.130 to 18.903, (207Pb/204Pb)t from 15.652 to 15.722, and (208Pb/204Pb)t from 38.436 to 39.037, respectively. These features indicate that the granitoidswithin the Xitian pluton were formed from magmas generated by remelting of metapelite and metapsammite of the Paleoproterozoic metamorphic basement at temperatures of ca. 800 °C, with low oxygen fugacity. The Sr-Nd isotopic compositions of the rocks from Xitian pluton indicate that the granitic magmas were mixed with less than 10% mantle-derived magmas. We suggest that the Xitian pluton was emplaced in an extensional tectonic setting related to release of compressional stresses within the thickened crust during the early Mesozoic.

  4. The New England Batholith, eastern Australia: Geochemical variations in time and space

    NASA Astrophysics Data System (ADS)

    Shaw, S. E.; Flood, R. H.

    1981-11-01

    The New England Batholith, Australia, is part of the Upper Paleozoic New England Fold Belt, with most plutons intruded into the deformed trench-complex metasedimetary rocks in the southeast part of the Fold Belt. The Batholith was emplaced in two major periods of plutonism, the first during the Upper Carboniferous and the second during the Upper Permian and Triassic, with a major phase of metamorphism and deformation including westward overthrusting of the trench-complex sedimentary rocks between the two periods. On the basis of petrography, geochemistry and isotopic characteristics, the granitoids of the Batholith are subdivided into five named intrusive suites and a group of leucoadamellites. The differences between the six groups are considered to reflect differences in their source-rock types. The Carboniferous granitoids are peraluminous S-type and are divided into the Bundarra Plutonic Suite, a belt of very coarse-grained adamellites with cordierite ± garnet, and the Hillgrove Plutonic Suite, a belt of biotite-rich ± garnet deformed adamellites and granodiorites. Both suites have δ 18O greater than 10, negative δ34S, 87Sr/86Sr initial ratios about 0.706, are ilmenite-bearing and have low FeO3 ratios. The Bundarra Plutonic Suite, however, is consistently SiO2-rich (greater than 70%), contains cordierite and has higher δ18O than the Hillgrove Plutonic Suite. The two S-type suites are inferred to have formed by partial melting of the deepest parts of a wedge of trench-complex sedimentary rocks oceanwards of an `Andean' volcanic chain to the west, analagous to the lower Tertiary S-type plutons of the Sanak-Baranof plutonic belt of southern Alaska. Of the remaining four groups of plutons emplaced during the Upper Permian and Triassic, two are metaluminous I-type granitoids, one the Clarence River Plutonic Suite (new name), a group of K-poor granodiorites and tonalites with low 87Sr/86Sr initial ratios (average 0.7035), and the other the Moonbi Plutonic Suite

  5. Magma mixing in the Kalaqin core complex, northern North China Craton: Linking deep lithospheric destruction and shallow extension

    NASA Astrophysics Data System (ADS)

    Fu, Lebing; Wei, Junhao; Tan, Jun; Santosh, M.; Zhang, Daohan; Chen, Jiajie; Li, Yanjun; Zhao, Shaoqing; Peng, Lina

    2016-09-01

    The widespread Mesozoic magmatism in the North China Craton (NCC) has received considerable attention as a trigger for large scale lithospheric destruction. Here we investigate the Early Cretaceous Jiguanzi adamellite from the northern part of the NCC which is contemporaneous with shallow extensional deformation and deep lithospheric destruction. This intrusion emplaced at ca. 133 Ma is located in the foot wall of the Kalaqin metamorphic core complex (MCC), and occurs as a synextensional ring complex with numerous magmatic equigranular (Group 1) and porphyritic (Group 2) enclaves. Hornblende and plagioclase from the host adamellite and xenocrysts of Group 2 enclaves show distinct inverse zoning with Mg- and Ca-rich mantle. The Group 2 enclaves are characterized by plagioclase xenocrysts hosting hornblende, biotite and apatite inclusions, quartz ocelli with fine-grained rim enriched in biotite and hornblende, and poikilitic biotite surrounded by hornblende. Geochemically, the host intrusion is calc-alkaline to alkaline and metaluminous with variable contents of SiO2 (60.70-72.20 wt.%), Al2O3 (14.19-17.22 wt.%), Na2O + K2O (6.16-9.42 wt.%), and Mg# values (28.0-47.7), whereas the Group 2 enclaves exhibit low SiO2 (54.05-55.55 wt.%), high Fe2O3 (8.18-8.64 wt.%) and TiO2 (2.08-2.28 wt.%), and moderate Mg# (44.0-44.1). Both the host intrusion and Group 2 enclaves are enriched in large-ion lithophile and light rare earth elements, and depleted in high field strength elements and heavy rare earth elements except that the latter has lower Ba and high Nb, Ta and Ti contents. The major and trace element contents of the Group 1 enclaves are broadly similar to those of the host intrusion. Analyses of Sr-Nd-Hf isotopes in the host intrusion, and in Group 1 and Group 2 enclaves show (87Sr/86Sr)ihost = 0.70600-0.70618, εNd(t)host = - 8.2 to - 9.6, T2DM(Nd)host = 1592-1706 Ma, εHf(t)host = - 9.2 to - 12.0, (87Sr/86Sr)iGroup 1 = 0.70590-0.70635, εNd(t)Group 1 = - 9.6 to - 10

  6. Rare earth elements content in geological samples from eastern desert, Egypt, determined by instrumental neutron activation analysis.

    PubMed

    El-Taher, A

    2010-09-01

    Twenty representative geological samples (tonalite, granodiorite, adamellite, syenogranite, rapakivi syenogranite, alkali feldspar granite and monzogranite) were collected from G. Kattar area in Eastern Desert, Egypt, for analysis by instrumental neutron activation as a sensitive nondestructive analytical tool for the determination of 14 rare earth elements (REEs) and to find out the following: (1) what information could be obtained about the REEs and distribution patterns of REEs in geological samples under investigation, (2) to estimate the accuracy, reproducibility and detection limit of the INAA method in case of the given samples. The samples were properly prepared together with standard reference material and simultaneously irradiated in a neutron flux of 7x10(11)n/cm(2)s in the TRIGA Mainz research reactor facilities. The gamma spectra were collected by an HPGe detector and the analysis was done by means of a computerized multichannel analyzer. The choice of the nuclear reaction, irradiation and decay times, and of the proper gamma radiation in counting are presented and discussed. The results are found to be in good agreement with certified values.

  7. Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

    NASA Astrophysics Data System (ADS)

    Betts, Peter G.; Valenta, Rick K.; Finlay, Jim

    2003-05-01

    Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S 1). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north-south shortening (D 2), open to isoclinal south-southeast-oriented F 2 folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D 2 Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D 2 history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.

  8. Behaviour of thermal waters through granite rocks based on residence time and inorganic pattern

    NASA Astrophysics Data System (ADS)

    Delgado-Outeiriño, I.; Araujo-Nespereira, P.; Cid-Fernández, J. A.; Mejuto, J. C.; Martínez-Carballo, E.; Simal-Gándara, J.

    2009-07-01

    SummaryThermal waters are certainly a substantial asset of the Galicia region of Spain. They can be regarded as worth developing because of their human health implications and, if thermal tourism is promoted, their importance to the local economy. In this paper the chemistry of major and trace inorganic elements in about 45 thermal springs and wells discharging in the same hydrographical system are presented and discussed. For handling the results of all measurements, graphical representations of B/Li vs.SO42-/Cl - ratios, Hill-Piper diagram, discriminant analysis (DA) and principal component analysis (PCA) were employed. All this with the intention to classify, based on their inorganic pattern, both thermal springs and wells waters, but also waters circulating through adamellite and granodiorite rocks. The results of the hydrogeochemistry analysis showed three main water families: sulphated, chlorinated and bicarbonated waters. The results show also the presence of saline materials with chloride influence in the deeper aquifer, allowing its classification in deeper and younger/shallow waters.

  9. Numerical modeling of mullions in the Taili high deformation zone, North China: Implications for the rheology of granitic rocks

    NASA Astrophysics Data System (ADS)

    Li, Zhiyong; Zeng, Zuoxun; Mohammed, Adil S.

    2017-05-01

    This paper presents a combined field measurement and finite element modeling analysis of the mullions occurring on the contact of two granitic rocks with different grain size in the Taili High-Strain Deformation Zone (THDZ), West Liaoning of North China. All of the field data are located in the plot zone of the modeling results. Numerical modeling results indicate that: (1) The inter-angle between the tangent lines cross the cusp point and the ratio R of amplitude and width of mullions are the most effective parameters to describe the geometric shape and evolution of mullions, as well as useful indicators of the rheology of rocks. (2) The competence contrast controls the growth rate of mullions under shortening. It determines the possible ratio R of final mullions. Moreover, decreasing of the cusp angle in high competence contrast materials is faster than that in low competence contrast model. (3) The initial disturbance is an essential factor for the generation of mullions. Those contacts with higher initial disturbance will develop into mullions more easily and have a high growth rate during the same shortening deformation regime. (4) The rheology and deformation behavior of the granitic rocks in the study area are primarily controlled by the grain sizes of quartz and feldspar. The effective viscosity ratio of biotite adamellite and granitic gneisses is about 0.01-0.5. The deformation mechanisms of these granitic rocks should be dominated by a grain-size-sensitive diffusion creep.

  10. Ages and Sr isotope systematics of Archean basement rocks from the south-central Beartooth Mountains

    SciTech Connect

    Montgomery, C.W. ); Gray, B.A.

    1989-07-01

    The Precambrian basement rocks of the Beartooth Mountains north of Cooke City, Montana are dominated by granitic gneisses. Consistent intrusive relationships indicate that the pink gneiss, of granitic-to-adamellitic composition, is the younger. It yields a well-defined Rb-Sr whole-rock ages of 2,745 plus/minus 37 Ma with primitive initial {sup 87}Sr/{sup 86}Sr = .7005 plus/minus .0006 (all errors 2 standard deviations). The low initial Sr ratio is consistent with either a mantle source or a granulitic lower-crustal source. The older, tonalitic-to-granitic grey gneiss cannot be dated precisely by Rb-Sr methods, but does contain systematically more radiogenic Sr for given Rb/Sr ratio than does the pink gneiss. These data, combined with previously published zircon data suggest two alternative interpretations for the age of the grey gneiss: either it is up to several hundred million years older than the pink gneiss; or the two are nearly contemporaneous, but the grey gneiss has inherited some radiogenic Sr from assimilated pre-existing crustal material. No evidence has been found for earlier Archean rocks in this region, in contrast to portions of the eastern Beartooth Plateau block, where 3.4-Ga granulites and possible 3.9-Ga detrital zircons have been identified.

  11. Structures, microfabrics, fractal analysis and temperature-pressure estimation of the Mesozoic Xingcheng-Taili ductile shear zone in the North China craton

    NASA Astrophysics Data System (ADS)

    Liang, Chenyue; Neubauer, Franz; Liu, Yongjiang; Jin, Wei; Zeng, Zuoxun; Bernroider, Manfred; Li, Weimin; Wen, Quanbo; Han, Guoqing; Zhao, Yingli

    2014-05-01

    The ductile shear zone in Xingcheng-Taili area (western Liaoning Province in China) is tectonically located in the eastern section of the northern margin of the North China craton, and dominantly comprises deformed granitic rocks of Neoarchean and Triassic to Late Jurassic age, which were affected by shearing within middle- to low-grade metamorphic conditions. Because a high-temperature metamorphic overprint is lacking, microstructures attesting to low-temperature ductile deformation are well preserved. However, the rocks and its structures have not been previously analyzed in detail except by U-Pb zircon dating and some geochemistry. Here, we describe the deformation characteristics and tectonic evolution of the Xingcheng-Taili ductile shear zone, in order to understand the mode of lithosphericscale reactivation, extension and thinning of the North China craton. The ductile deformation history comprises four successive deformation phases: (1) In the Neoarchean granitic rocks, a steep gneissosity and banded structures trend nearly E-W (D1). (2) A NE-striking sinistral structure of Upper Triassic rocks may indicate a deformation event (D2) in Late Triassic times, which ductile deformation structures superimposed on Neoarchean granitic rocks. (3) A gneissose structure with S-C fabrics as well as an ENE-trending sinistral strike-slip characteristic (D3) developed in Upper Jurassic biotite adamellite and show the deformation characteristics of a shallow crustal level and generated mylonitic fabrics superimposed on previous structures. (4) Late granitic dykes show different deformational behavior, and shortening with D4 folds. The attitude of the foliation S and mineral stretching lineation of three main types of rocks shows remarkable differences in orientation. The shapes of recrystallized quartz grains from three main types of granitic rocks with their jagged and indented boundaries were natural records of deformation conditions (D1to D3). Crystal preferred

  12. Radioelement distributions in the Proterozoic granites and associated pegmatites of Gabal El Fereyid area, Southeastern Desert, Egypt.

    PubMed

    Abd El-Naby, H H; Saleh, G M

    2003-10-01

    Lithologically, the rock types in the Gabal El Fereyid area are dominantly granites with minor amounts of pegmatites. The granites range in composition from tonalite to granite-adamallite with minor acidic dikes, quartz and pegmatite veins. The granite-adamallite is peraluminous and formed as a result of partial melting of amphibole-bearing rocks at depths of approximately 24-30 km and at temperatures of 800-950 degrees C. Among the different rock types, the muscovite-rich pegmatites had the highest U and Th contents (66 and 38 ppm on average, respectively). The high level of radioactivity in pegmatites is attributed to the presence of the radioactive minerals thorianite, uranophane and allanite as confirmed by XRD analysis. Binary relations of Zr/U, Zr/Th, Ce/U and Ce/Th against either U or Th in the granite-adamellite exhibit significant negative correlations indicating that both elements are not preferentially hosted in the accessory minerals phases such as zircon and monazite, but could be associated with major forming minerals such as biotite, muscovite, plagioclase and quartz, or U is situated within labile sites within granite. The uranium and thorium enrichment in the pegmatites is a two-stage process. The primary stage is magmatic whereas the secondary enrichment is from hydrothermal concentration. The magmatic U and Th are indicated by the presence of thorianite and allanite, whereas evidence of hydrothermal mineralization is the alteration of rock-forming minerals such as feldspar and the formation of secondary minerals such as uranophane and pyrite.

  13. Nature of Phyllosilicates and Magma Types in the Plutonic Complexes of NE Egypt

    NASA Astrophysics Data System (ADS)

    Abdel-Rahman, A.

    2007-05-01

    The Late Proterozoic plutonic complexes of northeastern Egypt belong to a number of magma types of variable tectonic regimes, and exhibit a variety of phyllosilicate minerals. The results show that the phyllosilicates define three compositionally distinct groups. Phyllosilicates in the anorogenic peralkaline granites, which were developed from a hypersolvus felsic melt, are restricted to iron-rich siliceous annites and belong to type A-biotite (referring to biotite in anorogenic alkaline A-type suites). Such Fe-enriched, Al-depleted mica compositions suggest that the substitutions Fe3+ = Al, 3Fe2+ = 2Al, Fe2+ = Mg and (Fe2+ + Si) = 2Al are vital in producing annitic compositions in the peralkaline granites. In sharp contrast, phyllosilicates in the trondhjemitic pluton that crystallized from a peraluminous felsic melt, are siderophyllitic in composition and belong to type P-biotite (referring to biotite in peraluminous, including S-type suites). The substitutions Fe2+ = Mg, along with the Tschermaks substitution (Fe2+, Mg) + Si = 2Al are vital in producing siderophyllitic compositions in the trondhjemite pluton. The substitution 2Al = 3Mg (or Fe2+) also plays a role when referring to a possible solid solution between phlogopite (or annite) end-member and muscovite end-member. This pluton also contains late- to post-magmatic muscovite. Phyllosilicates in the diorite-tonalite and the voluminous granodiorite-adamellite orogenic complexes (crystallized from calcic to calc-alkaline subsolvus magmas), cover a wide range of composition, and belong to type C-biotite (referring to biotite in calcic and calc- alkaline, mostly I-type suites). The nature of micas and the magmatic stages of their development were controlled by the various physiochemical conditions (including the behavior of volatiles) prevailed during the crystallization of these hypersolvus, dry, anorogenic magmas versus subsolvus, wet, orogenic magmatic systems.

  14. Pan-African alkali granites and syenites of Kerala as imprints of taphrogenic magmatism in the South Indian shield

    NASA Technical Reports Server (NTRS)

    Santosh, M.; Drury, S. A.; Iyer, S. S.

    1988-01-01

    Granite and syenite plutons with alkaline affinities ranging in age from 550 to 750 Ma sporadically puncture the Precambrian granulites of the Kerala region. All the bodies are small (20 to 60 sq km), E-W to NW-SE elongated elliptical intrusives with sharp contacts and lie on or close to major late Proterozoic lineaments. Geochemical plots of A-F-M and An-Ab-Or relations show an apparent alkali enrichment trend on the former, but the plutons define relatively distinct fields on the latter. Most of the plutons are adamellitic to granitic by chemistry. The variations of SiO2 with log sub 10 K2O/MgO (1) brings out the distinct alkaline nature of the plutons. Some of the granites are extremely potassic, like the Peralimala pluton, which shows up to 11.8 percent K2O. On a SiO2-Al2O3-Na2O+K2O (mol percent) plot, the plutons vary from peraluminous to peralkaline, but none are nepheline normative. Low MgO, low to moderate CaO and high Fe2O3/FeO values are other common characteristics. Among trace elements, depletion of Ba, Sr and Rb with high K/Ba and K/Rb values are typical. Overall, the plutons show a trend of decreasing K/Rb ratio with increasing K content. Individual plutons show more clearly defined trends similar to those from granitic masses characterized by plagioclase fractionation.

  15. Acoustic Emission During Heating of Granitoids

    NASA Astrophysics Data System (ADS)

    Nasimov, R. M.; Diaur, N. I.; Petrov, V. A.; Poluektov, V. V.

    Studies of acoustic emission (AE) activity (A) during heating of cubic (20x20x20 mm) samples of granitoids (gneisses, adamellites, leucogranites, tonalites, monzodi- orites and quartz diorites) collected from two wells within the Nizhnekansky granitoid massif were carried out. The investigation was made in connection with the problem to design and construct an underground facility for long-term storage of radioactive waste. All samples were heated with the same rate in two consecutive cycles (heating- cooling). Maximum temperatures were 125-130 C in first cycle and 250-260 C in the second one. The observation was carried out at ultrasonic range 0.2-2 MHz. Acous- tic events threshold crossing fixed level in amplitude were accumulated in computer using special interface card and software. The acoustic events exceeded the threshold were ignored as a negligible. Maximum accumulated number in first cycle was 50-500 while in second one was until 15000. Using accumulative curve (N-T) under heating and cooling the maximum and average values of an activity (A = dN/dt, where t is time) were calculated. Analysis of obtained data indicated that determinate tempera- ture of AE activation was existed for every rock group and after that the activity had significant increase. Generally in second cycle this temperature had greater value. It was found that accumulative curve N-T had two evolution stages, they were linear and exponential growths due to temperature. The obtained N and A data were analyzed simultaneously with VP data before and after heating in the same samples in both cycles. By means of high resolution computer scanning of sample surfaces a growth of number visible fractures was observed as a result of heating. Analysis of mineral composition and texture of rock samples was made as well. Ternary diagrams (% pla- gioclase content - % potash feldspar content - number of AE events under heating) were analyzed.

  16. Au-Skarn Mineralization: Constraints from LA-ICP-MS U-Pb Zircon Dating

    NASA Astrophysics Data System (ADS)

    Gaspar, M.; Vervoort, J. D.; Meinert, L. D.

    2003-12-01

    In situ U-Pb zircon geochronology by laser ablation ICP-MS was performed on samples from the intrusive rocks of the Buckhorn Mountain, Washington in order to constrain the age of the Crown Jewel Au-skarn deposit. The analyses were conducted at Washington State University using a ThermoFinnigan Element2 single collector, high resolution magnetic sector ICP-MS, and a New Wave UP 213 Nd-YAG (213 nm) laser ablation system. The analytical parameters included a repetition rate of 10 Hz, a 40 microns spot size, and a total analysis time of 30 seconds per spot. A 94-35 cascade standard was used to bracket the samples. Two distinct 206Pb/238 U ages were obtained, 52.3 Ma +/- 1.6, 165.0 Ma +/- 5.9, 163.4 +/- 5.7, and 169.8 +/- 12.1 (all errors are 2 sigma). These ages represent distinct magmatic events occuring during the two main tectonic periods in the region. The oldest age ( ˜165 Ma) is associated with the accretion of the Quesnel terrain during the Middle Jurassic while the younger age ( ˜52 Ma) represents the magmatism during the Eocene extension that is well represented by the Challis Volcanics in the vicinity. The Jurassic ages were obtained in one sample from a deformed granodiorite dike and two samples from the main granodiorite intrusive facies that, based on the skarn mineralogy zonation, is spatially associated with the skarn. The Eocene intrusive unit was intercepted in one drill core but is clearly distinct from the granodiorites both geochemically and mineralogically. This facies is more an adamellite, with a distinctive pinkish colour due to a K-spar alteration. It is higher in silica, and depleted in compatible elements (Ca, Fe, Ti, Mg, P, Y, and V) relative to the granodiorite. Geochemically the granodiorite has a composition typical of plutons associated with Au-skarns worldwide. Because Au mineralization is erratically distributed and does not correlate with any of the skarn mineralogy, a question can be raised. Does the Au mineralization have any