Dating and tectonic significance of plagiogranite from Guoganjianian Mountain，central Qiangtang， northern Tibet, China

Plagiogranite is firstly found in the ophiolite in Guoganjianian Mountain. It has great significance for the petrogenesis of ophiolite studies，especially for the precise dating of ophiolite formation and evolution. Based on the study of field geological characteristics, petrology，geochemistry and isotope geochronology, the authors suggest the plagiogranite has similar characteristics of adakite，which is high in Al2O3（14.84%～15.35%），Sr（338×10-6～356×10-6），Sr/Y（Sr/Y=46.94～55.45）and low in Y，Yb（Y=6.42×10-6～7.20×10-6，Yb=0.56×10-6～0.63×10-6）. Zircon SHRIMP U-Pb dating age is (348±9) Ma and it represents the subduction time of oceanic crust. The above geochemical characteristics indicate that plagiogranite in the Guoganjianian Mountain is the product of direct partial melting of subduction plate in high pressure，and these granites are likely produced in the environment of subduction zone（SSZ type）.