LA-ICP-MS zircon U-Pb dating and geochemical characteristics of Zhula intrusion in eastern Gangdise, Tibet

Field investigation has found that Zhula intrusion consists of biotite monzogranite with abundant dioritic enclaves, and there exists gradually transitional relationship between the enclaves and their host rock. Zircon LA-ICP-MS U-Pb dating of 3 samples from the granite yielded the ages of 64.6±0.8Ma，64.3± 0.8Ma and 63.9±0.5Ma respectively，with a surface age of 776Ma of inherited zircon. A dioritic enclave samples yielded the age of 66.1±0.3Ma. The dioritic enclaves have rich Fe and high alkalinity as well as poor Si, Mg and Ca, thus belonging to aluminous, high-K cale-alkaline series; they have Nb/Ta ratio of 15.9, Sr/Y ratio of 3.69, LREE/HREE ratio of 1.92, substantial negative Eu anomalies, higher LILE(Rb,K) and RHE(U,Th) concentrations, lower HFSE (Nb, Ta, Sr, Ti) concentrations relative to the primordial mantle, thus indicating that dioritic magma originated from partial melting of the mantle. The host rocks have rich Si, K, and poor P, belonging to meta-aluminous and aluminous and cale-alkaline to high-K cale-alkaline series. they have Nb/Ta ratio of 15.9, higher LILE(Rb, K), lower HFSE(Nb, Ta, Ba, P, Ti), similar to calc-alkaline rocks in island arc or active continental margin. The host rocks and enclaves might have been derived from the same magmatic processes. The enclave rocks originated from crystallization differentiation of mantle source magma assimilating and contaminating some crust materials, while the host rocks originated from crystallization differentiation of large quantities of crustal magma mixed with some mantle sourced magma. In combination with previous research results, the authors hold that the petrogenesis might have been related to the northward subduction of the Yarlung Zangbo Ocean.