Origin of S-type granites in the forearc accretionary complex of the East Kunlun Orogenic Belt, northern Tibetan Plateau

A forearc environment is usually characterised by a relatively low geothermal gradient and hence little magmatic activity occurs. However, S-type granites were discovered within the forearc accretionary complex of the East Kunlun Orogenic Belt. The S-type granites intruded into an upper amphiolite facies partially migmatitic crystalline basement in form of dikes and sills at ca. 440 Ma which corresponds to the transition of the Proto-Tethyan to the Paleo-Tethyan realm in the northern Tibetan Plateau. The observed granites contain either garnet + biotite + muscovite or garnet + muscovite: (1) muscovite granite is strongly peraluminous with an aluminous saturation index (ASI) of more than 1.1 (ASI = molar [Al₂O₃/(Na₂O+K₂O+CaO]) and has high-K calc-alkaline characteristics, low Sr/Y (1.9–16.1) and La_N/Yb_N (1.85–13.2) ratios. (2) Two-mica granite is moderately peraluminous (ASI = 1.02–1.09), has high Ca and low K contents as well as high Sr/Y (16.8–67.7) and La_N/Yb_N(10.9–33.3) ratios. Other trace element contents and their ratios also show striking differences with high Sr (207–324 ppm) content and CaO/Na₂O (0.47–0.96) ratio, and a low Rb/Sr (0.04–0.32) ratio for two-mica granite, but low Sr (63–126 ppm) content and CaO/Na₂O (0.08–0.20) ratio, and a high Rb/Sr (0.56–2.53) ratio for muscovite granite. The observed differences are due to different protolith chemistries and melting mechanisms. Based on melting experiments of metasedimentary rocks (Patiño Douce and Harris, 1998), muscovite granite was most likely produced by dehydration melting of a metapelitic source and the two-mica granite by H₂O-fluxed melting of a metagreywacke. Zircon Hf isotopes of the two S-type granites have ε_Hf(440 Ma) values of -6.85 to +12.02 indicating the involvement of a mantle-derived magma which probably triggered the anatexis of supracrustal rocks deposited in a forarc regime. Coveal adakites with a younging westward trend as well as mafic rocks have been reported in this accretionary complex, which together with anatexis and metamorphism of accreted material support the occurrence of a slab window beneath the forearc accretionary complex of the East Kunlun Orogenic Belt during subduction of the Tethyan oceanic slab.

 

References

Patiño Douce, A.E., Harris, N., 1998. Experimental constraints on Himalayan anatexis. Journal of Petrology 39, 689–710.