Wladyslaw B. BetkowskiJohn RakovanDaniel E. Harlov
15页
查看更多>>摘要：Llallagua is one of the world's biggest tin deposits and part of the metallogenic Bolivian Tin Belt, which occurs in the inner arc of the Central Andes. The Llallagua deposit formed through emplacement of a subvolcanic porphyry-stock, of intermediate dacitic to rhyodacitic composition, metasomatism, and hydrothermal mineralization. The deposit is the subject of a well-established geochronologic controversy that includes contradictory ages (40 and 20 Ma) from multiple geochronometers. Geochronological characterization of phosphate minerals from the metasomatized igneous porphyry and hydrothermal vein assemblages, along with detailed petrography and chemical analyses, are used to reconcile the age controversy. The new interpretation is further supported by existing textural and geochemical data. The U-Pb ages of unaltered igneous fluorapatite (e.g. 21.2 ± 2.9 Ma), monazite (e.g. 21.22 + 0.80/-0.66 Ma), and zircon (e.g. 21.15 ± 0.39 Ma) from the Llallagua porphyry are all equivalent withn error at 20 Ma, and are interpreted to represent the age of porphyry stock emplacement. Ages determined from altered portions of these minerals (e.g. apatite, 18.8 ± 8.0 Ma) are within error the same as unaltered portions, suggesting alteration of the porphyry soon after emplacement. U-Pb ages from unaltered fluorapatite (e.g. 21.4 ± 6.7 Ma), altered and unaltered monazite (e.g. 19.4 ± 1.4 Ma and 20.29 ± 0.30 Ma respectively), and unaltered xenotime (e.g. 19.32 ± 0.67 Ma) from the hydrothermal veins are also 20 Ma, indicating a very short hiatus or temporal continuity between stock emplacement and hydrothermal vein formation. The early Miocene age for tin mineralization at Llallagua is consistent with neighboring mineralization centers at the Morococala, Colquechaca, Japo, and Santa Fe mines, and with regional trends observed across the Bolivian Tin Belt, including stratigraphy, magmatic, and tectonic history of the Andean Eastern Cordillera.
查看更多>>摘要：This work presents new major and trace element geochemical data and the first Sm-Nd and Lu-Hf isotopic data for the mafic suite of the Buem structural unit (BSU) in Ghana in order to infer its origin and geodynamic setting, relationship to the Pharusian suture zone and significance in the Dahomeyide belt. The BSU, which occurs about 50 km to the west of the Pharusian suture zone, comprises well-preserved, weakly metamorphosed, mafic volcanic and mafic-ultramafic plutonic rocks. Two lava types can be distinguished: Type I and Type II lavas that are spatially separated and geochemically distinct. The Type I lavas are subalkaline and show trace element patterns akin to mid-ocean ridge basalt (MORB). Their eNdsoowia values of +5.3 to +6.2 and εHf_(800Ma) values of +11.1 to +17.2 are indications of juvenile magma derived from a depleted mantle source in the spinel stability field. The Type II lavas have alkaline signatures with trace element patterns that resemble ocean island basalts. They formed by smaller degrees of disequilibrium partial melting at depth in which garnet likely played a role as a residual phase having εHf_(800Ma) and εNd_(800Ma) values of+3.2 to +5.1 and + 4.1 to +6.3 respectively. The gabbros show similar trace elements concentration and isotopic ratios as the BSU lavas, an indication of a similar mantle source and environment of formation. Because the geochemical features of the BSU mafic suites are comparable to the MORB-type high-pressure rocks of the Pharusian suture zone, both represent the remnant of the Pharusian oceanic crust, formed during the breakup of Rodinia. The BSU corresponds to the true oceanic crust of the West Gondwana Orogen continental margin-type ophiolite, sharing similar lithological and geochemical features to other continental margin-type ophiolites in the world.
查看更多>>摘要：Ocean-ridge subduction and the induced slab windows are important to understand the generation of arc magmatism and growth of juvenile crust both in present and ancient subduction zones. The physical conditions of oceanic ridges control the compositions of magmas during ridge subduction, and therefore magmatic composition can act as a marker of ridge subduction in ancient orogenic belts. Here we present the petrological, geochemical and geochronological studies for sodic rhyodacites and a concurrent ophiolite complex (basalt/ dolerite-gabbro and harzburgite) within the North Qilian orogen, NW China. The sodic rhyodacites are low Sr/Y tonalite-trondhjemite with depleted Sr-Nd isotopic compositions, suggesting derivation from melting of normal mid-ocean-ridge basalts (N-MORB) at 449 ± 2.9 Ma. Phase equilibrium and geochemical modeling confirm that the sodic rhyodacites formed via the partial melting of N-MORB sources at 3-5 kbar/780-840 °C. The basalts/ dolerites exhibit similar geochemical characteristics to typical N-MORB rocks, and the gabbro gives zircon U-Pb age of 450 ± 2.5 Ma. The petrogenesis of the basalts/dolerites is similar to the ''blow-torch'' effect often seen during ocean-ridge subduction. The harzburgites are similar to abyssal peridotites in mineral compositions and have equilibrium temperatures of 1230-1460 °C, suggesting the obduction of a hot asthenospheric mantle on the continental margin. Our results show the integrated melting processes of the upwelling mantle peridotite and the subducted oceanic crust within the slab window during ridge subduction of the Proto-Tethys Ocean. These results are also significant in understanding the petrogenesis of low Sr/Y tonalite-trondhjemites, which are widely preserved in Archean to modem terranes.
查看更多>>摘要：The present study discusses the geochemistry of the Early Cretaceous Shyok and Nubra volcanics from the Shyok Suture Zone (SSZ), Trans-Himalaya. Geochemically, the studied volcanic rocks belong to calc-alkaline magma series and vary in composition from basalt-basaltic andesite-andesite-dacite. The SSZ rocks are enriched in light rare earth elements (LREE: La, Ce, Nd) and large ion lithophile elements (LILE: Rb, Ba, U, K, Pb and Sr) while depleted in high field strength elements (HFSE: Nb, P and Ti), analogous to subduction zone magmatism. Their perturbed LILE concentrations coupled with higher ~(87)Sr/~(86)Sr ratios reflect possible influence of post-crystallization processes. Trace element modelling reflects variable degrees of partial melting (-5-30%) of a metasomatised spinel peridotite mantle source followed by fractional crystallization of clinopyroxene and amphibole along with olivine and plagioclase. The Shyok and Nubra volcanics depict positive ε_(Nd)(t =110 Ma) values (+2.3 to +5.2), slightly lower than the contemporary Chalt fore-arc boninites (+6.3 to +8.0) and adjoining intra-oceanic island arc rocks along the Indus Tsangpo Suture Zone (ITSZ) such as Nidar ophiolite (+6.8 to +8.7), Shergol gabbros (+5.1 to +8.4) and Dras volcanics (+5.8 to +7.0), therefore reflecting depleted mantle characteristics. In contrast to the ITSZ rocks, the Shyok and Nubra volcanics have enriched LREE-LILE characteristics, indicating their derivation from isotopically depleted and elementally (LREE-LILE) enriched mantle wedge sources. Thus, based on the geochemical and isotopic signatures of the SSZ rocks and their association with continentally derived sediments, we suggest an Andean-type arc system as opposed to ITSZ rocks and associated oceanic sediments, representing intra-oceanic arc system.
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