Petrology and geochemistry of Mesozoic granitic rocks from the Nansha micro-block, the South China Sea: Constraints on the basement nature

摘要

There are several micro-blocks dispersed in the South China Sea (SCS), e.g.. Xisha-Zhongsha block, Nansha block and Reed-Northeastern Palawan block, etc., but detailed petrological constraints on their basement nature were previously lacking. The magmatic ages for granitic rock samples from two dredge stations in the Nansha micro-block vary from 159 to 127 Ma, which are comparable to magmatic activities occurred in the northern margin (Pearl river mouth), HongKong and East China. Petrographic characteristics, major-, trace element and Sr-Nd isotopic data of nine samples from two dredged station performed in the Nansha micro-block, the SCS, are reported. Petrographically, these granitic rocks can be divided into two groups which underwent a complex history of magmatic process, i.e., tonalitic rock (Group I) and monzogranitic rock (Croup II). The Rittmann index (σ) for these rocks (1.9-3.1) suggest that they belong to calc-alkaline rocks. Croup 1 rocks which is of typical 1-type, have higher contents of TiO_2, Al_2O_3, FeO, MgO, CaO, Na_2O and P_2O_5, but lower values of SiO_2 and K_2O, when compared with those of Group 11 with 1-type characteristics. Group 1 rocks are produced by partial melting of older Precambrian basement with the variable influence of mantle-derived magma which results from the interaction of released fluids from the subducted slab and the overlying mantle wedge in a general convergent margin setting, and Group II rocks result from partial melting of lower crustal basic rocks (amphibolite) and/or further partial melting of the Group I rocks associated with the variable influence from the underplating mantle-derived magma resulting from lithospheric extensional regime. Both Croups I and II have undergone assimilation and fractional crystallization (AFC) processes during its petrogenesis. This study therefore demonstrates that there exists a continental basement within micro-blocks in the South China Sea. and further supports the idea that a Middle Jurassic to Mid-Cretaceous subduction zone existed across the temporary Taiwan, Palawan to Southern Vietnam, which was associated with westward to northestward convergence of the Pacific Plate during Late Mesozoic. We suggest that this subduction zone may have been connected with the paleo-Pacific plate subduction zone offshore eastern China during Mesozoic era. This study provides petrologic data for the pre-Cenozoic tectonic evolution of the South China Sea.