Petrogenesis and tectonic dynamics of Early Jurassic magmatic rock in the Taizhao area, southern Tibet

The Gangdese batholith in southern Tibet preserves various magmatic rocks that can be used to infer tectono-magmatic processes associated with the northward subduction of the Neo-Tethyan oceanic lithosphere and with the Indian -Eurasian continental collision. The granitic pluton in Taizhao area in this batholith consists of two groups of granites, i. e., Group I with high Na/K ratios and Group II with low Na/K ratios, respectively. Furthermore, a set of gabbro and diorite developed in Basum Co area, several kilometers to the east of Taizhao area. The Group II granites are treated as the main object in this paper to the studies for comparing with the other two ones. Zircon U-Pb dating results indicate that Group II granites formed at 190 similar to 194Ma, which is coeval with Group I granites. The Group II granites are characterized by slightly more enriched Hf (epsilon(Hf)( t) = 7. 2 similar to -2. 5) and Nd(epsilon(Nd)(t) = 4. 1 3. 7) and lower Sr (Sr-87/Sr-86( t) = 0. 7061 - 0. 7070) isotope compositions than those in Group I granites, which is similar to the isotope compositions of intermediate-mafic rocks. Such features indicate that Group II granites represent the product derived from fractional crystallization of intermediate-mafic magmas. The intermediate-mafic magmas originated from melting of melt -modified sub -arc mantle. The contents of SiO2 of Group II granites display negative correlations with Al2O3 CaO, TiO2, FeOT, MgO, Sr and Sc, but positive correlations with Na2O, Rb, Nb and Ta. These geochemical features suggest the important role of hornblende and plagioclase and potassium feldspar as fractional crystallization phases during the differentiation of more -primitive mafic magma and the formation of high -Si granites, respectively. Both of them possibly represent the contemporaneous magmatism during the northward subduction of the Neo-Tethyan Oceanic lithosphere.