Geophysical Exploration Institute of Heilongjiang Province, Harbin 150036, China

The shoshonites in Zaxue area of Tibet, including the hypabyssal intrusive quartz-monzonite porphyry and the volcanic rocks of Pana Formation, distribute in the central part of Gangdese magmatic belt. The quartz-monzonite porphyry intrusived in the Pre-Ordovician Songduo Group and Carboniferous-Permian Laigu Formation, unconformably covered by the volcanic rocks of Pana Formation. The K-Ar age of quartz-monzonite porphyry in shoshonites is 54.42 Ma, the K-Ar age of trachyandesite in Pana Formation volcanic rocks is 41.46�� 54.42 Ma, which indicates the Eocene in age. Their petrological and lithogeochemical characteristics show of typical shoshonites. The REE pattern displays LREE is rich, HREE is dep letion and there is weak-medium Eu negative anomaly. The curve of trace element distribution patterns normalized by the primitive mantle demonstrates Rb, Th, Hf as a wave peak and Ba, Sr as wave trough, indicating the shoshonites were formed in the island-arc setting ( IAT). The richment of LILE ( Ba, K) and HFSE ( Hf, Zr) reflects the feature of plate shoshonites. Zaxue shoshonites have both geochemical characteristics of IAT and plate igneous rocks, which is consistent with the post-collision igneous rocks in Tibet. The characteristics of REE and trace element show the shoshonites formed in continental crust thickening and partial melting setting, is a product of the post-collision between Indo and Eurasia Plates.