The Material Evidence Of Oceanic Crust Melting During Continental Collision And Continental Crust Growth

It has been well established for the bulk composition of continental crust after painstaking effort built on works of generations,yet our knowledge remains limited on its origin,its way and rate of growth.To overcome the shortcomings of the standard “island arc” model for continental crust growth,Niu and co-authors proposed a novel hypothesis that continental collision zones are primary sites of net continental crustal growth on the basis of their studies on the India-Asia syn-collisional(~55 Ma)andesitic rocks in southern Tibet.The abundant syn-collisional granitoids(and equivalent)produced and preserved at northern margin of the Tibetan Plateau is considered to provide an exceptional window into syn-collisional felsic magmatism and continental crust growth.Here we present the results of a comprehensive study of the syncollisional granitoids and their enclosed mafic magmatic enclaves(MMEs)on plutons from North Qilian orogenic belt(NQOB).Collectively,the main achievements of this study can be summarized as follows: 1)The MMEs and their host granitoids share many characteristics in common,including identical crystallization age(~430 Ma),same mineralogy,similar mineral chemistry and whole-rock isotopic compositions,indicating the MMEs being of cumulate origin formed at earlier stages of the same magmatic systems rather than representing mantle melt required by the popular and alleged magma mixing model;Accordingly,we established a cumulate model for the formation of orogenic MMEs.2)By accepting our model for the petrogenesis of the MMEs,we have proposed a new model for the origin of adakitic rocks in syn-collisonal sittings after thorough assessment of existing models,e.g.,the QMS adakitic granodiorite resulted from fractional crystallization dominated by mineral assemblages represented by the MMEs.3)The two types of MMEs and the LHS granitoid host are coeval and have indistinguishable bulk-rock Sr-Nd-Hf isotopic compositions,futher suggesting that the MMEs consistent with being earlier crystallized cumulate of the same magmatic systems;The syn-collsional granitoids in the NQOB presented here are material evidence of melting ocean crust and sediments;4)The remarkable compositional complementarity between the LHS granitoid pluton(MMEs and the host granitoid)and the model bulk continental crust(in terms of major and trace elements,and some key incompatible trace element ratios)indicate that the syn-collisional magmatism in the NQOB makes net contribution to the mass of continental crust,and that the way of continental growth in the Phanerozoic through syn-collisional felsic magmatism(production and preservation)is straightforward and does not require any additional processes.5)Incomplete sample digestion,isobaric interferences and instrumental drift remain obvious problems that must be overcome in order to obtain precise and accurate analysis of trace elements in rocks using the ICP-MS technique.For this reason,we have done many experiments and developed a set of simple,costeffective and practical methods widely applicable for precise and rapid determination of trace element abundances in geological materials using ICP-MS.