Petrogenesis And Geological Significance Of Early Cretaceous Intermediate-acid Intrusive Rocks In Asa,Tibet

We report zircon U-Pb ages,Hf isotopic,and major and trace element data for the diorites and granite-porphyries in Asa area,Tibet.The two diorite samples yielded zircon U-Pb ages of 118 and 122Ma.The diorites are characterized by moderate SiO2 content(51.4?61.8wt.%),high MgO content(4.91?7.37wt.%),high Mg#values(57.5?72.6)values,and also high Cr and Ni(up to 217.6ppm and 145.lppm).These features indicate that the Asa diorites are similar to high-Mg andesite(HMA)and show a geochemical affinity with sanukitoidsin Japan,but have a lower content of elements such as La and Th.Zircon ?Hf(t)values range from +14.7 to +15.6,together with young zircon Hf model ages of 158?196Ma.The Asa high-Mg diorites originated from depleted mantle below the northern Gangdise subterrane that has been fluxed by slab-derived aqueous fluids and represent initial products after partial melting of mantle peridotite.The granite-porphyries yielded zircon U-Pb ages of 114?120Ma that are coeval with the diorites.Based on the relative degree of crystallization differentiation,the granite-porphyries can be divided into two groups featured with low and high differentiation.The lowly fractionated group havs high abundances of Sr,Ba,Zr and so on while the other generally have low element abundance sc learly shown in low Zr,Zr+Nb+Ce+Y,and Ga/Al.All granite-porphyries are characterized byhigh SiO2 content(74.9?78.5wt.%),Fe2O3Tcontent(1.47?3.39wt.%),and high ratios of FeOT/MgO(5.02?13.1),but depleted in Ba,Sr,P and Ti.These rocks display a peraluminous signature similar to aluminousA-type Granites from the Lachlan Fold Belt,Southeastern Australia.A slight variation is observed to zircon ?Hf(t)values(+0.3-+2.5).Our geochemical and isotopic data indicate that the Asa granite-porphyries were derived from a dioritic or granodioritic juvenile crust and experienced different degrees of crystallization differentiation in the northern Gangdise subterrane.Combined with previous research,the discovery of the Asa high-Mg diorites reveals that the middle and westerm part of Bangong-Nujiang Tethyan Ocean was notcompletelyclosed and still subducted southward in the Early Cretaceous.High temperature and extensional environment is required for the Asa A-type granite-porphyries and high-Mg diorites.During the subduction of the Bangong-Nujiang Tethyan Ocean,slab roll-back caused the upwelling of the asthenosphere that not only leaded to regional extension,but also provided the high temperature for magmatism.Under this background,underplating of mantle-derived basaltic magma was responsible for crustal growth and the formation of the Asa granite-porphyries in the northern Gangdise subterrane.Meanwhile,partial melting of mantle peridotite fluxed by slab-derived aqueous fluids also played an important role in the process of crustal growth.The "andesite model" represented by the Asa high-Mg dioritesdid not further separate promoting the crustal growth.