| Citation: | Quanru Geng, Guitang Pan, Zhenmin Jin, Liquan Wang, Zhongli Liao. Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet. Journal of Earth Science, 2005, 16(4): 283-296.
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The Early Jurassic bimodal volcanic rocks in the Yeba Formation, situated between Lhasa, Dagzê and Maizhokunggar, composed of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia, are an important volcanic suite for the study of the tectonic evolution of the Gangdise magmatic arc and the Mesozoic Tethys.Based on systematic field investigations, we carried out geochemical studies on representative rock samples.Major and trace element compositions were analyzed for these rock samples by XRF and ICP-MS respectively, and an isotope analysis of Rb-Sr and Sm-Nd was carried out by a MAT 262 mass spectrograph.The results show that the SiO2 contents in lava rocks are 41%-50.4% and 64%-69%, belonging to calc-alkaline basalt and dacite.One notable feature of the basalt is its low TiO2 content, 0.66%-1.01%, much lower than those of continental tholeiite.The ΣREE contents of basalt and dacite are 60.3-135 μg/g and 126.4-167.9 μg/g respectively.Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly.The basalts have depleted Ti, Ta and Nb and slightly negative Nb and Ta anomalies, with Nb*=0.54-1.17 averaging 0.84.The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb*=0.74-1.06 averaging 0.86. Major and trace elemental and isotopic studies suggest that both basalt and dacite originated from the partial melting of the mantle wedge at different degrees above the subduction zone.The spinal lherzolite in the upper mantle is likely to be their source rocks, which might have been affected by the selective metasomatism of fluids with crustal geochemistry.The LILE contents of both rocks were affected by metamorphism at later stages.The Yeba bimodal volcanic rocks formed in a temporal extensional situation in a mature island arc resulting from the Indosinian Gangdise magmatic arc.
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