Conversion of tectonic regimes from compression to extension in the Bangong Lake area,Xizang during the Late Cretaceous: Evidence from magmatic rocks

The Late Cretaceous intermediate to acidic rocks in the ophiolite mélanges from the Bangong Lake area in Xizang are dominantly made up of plagiogranite porphyry and granodiorite,which give the zircon U-Pb ages of 97.4±1.1 Ma and 91.94±0.78 Ma,respectively. These plagiogranite porphyry and granodiorite have the features of adakitic rocks,and are characterized by high SiO₂,Al₂O₃ and Sr,low Y and Yb,Sr/Y>35,highly differentiation in LREE and HREE,depletion of Nb,Ta and Ti,and very low contents of Cr and Ni,implying that they were formed by the partial melting of thickened lower crust caused by the underplating of the basaltic magmas. The pyroxene diorite dated at the ages of 80 to 76 Ma in the Rutog granites in the southern part of the Bangong Lake area displays the characteristics of the depletion of the high field strength (HFSE)such as Zr,Hf,Ti and Y, enrichment of the large-ion lithophile elements (LILE),and higher contents of Sc,Y,Cr,Co and HREE,and higher Mg# values (> 40),indicating a upper mantle origin of pyroxene diorite. The sustained collision and mountain building still took place in the Bangong Lake area during the periods of 97 to 92 Ma. After that,there were conversions of tectonic regimes from the compression during the collision stages to the post-collision intraplate extension. Finally the further extension persisted from 80 Ma to 76 Ma.