Petrogenesis of the Triassic andesites in the East Kunlun Orogen, East Tethys: implications for crustal maturation within an extensional setting

Abstract

The tectonic and crustal evolution of the East Kunlun Orogen in the Triassic remains unclear. We investigated the Triassic andesites (235 and 216 Ma) to investigate these issues. The Mid- and Late Triassic andesites are calc-alkaline with enriched Sr–Nd isotopic compositions: ( 87 Sr/ 86 Sr) i = 0.709210–0.709758 and ε Nd ( t ) = −8.0 to −6.9. The Mid-Triassic andesites have higher Mg, Cr and Ni contents and more depleted Hf isotopic compositions ( ε Hf ( t ) = −4.2 to 1.3) than the Late Triassic andesites ( ε Hf ( t ) = −7.4 to −5.8). These features are explained by the derivation of magma from partial melting of the mafic lower continental crust, although minor lithospheric mantle materials are also involved in the Mid-Triassic andesites. The andesites were generated in extensional settings related to slab break-off in the Mid-Triassic and lithospheric delamination in the Late Triassic, respectively. The Late Triassic magmatic suites are higher in K and represent a more mature continental crust, indicating that crustal reworking in extensional settings contributes to crustal maturation. In combination with the regional magmatic, structural and sedimentary records, we propose that the East Kunlun Orogen evolved from oceanic subduction in the Late Permian to Mid-Triassic through continental collision in the Mid-Triassic to post-collision in the Late Triassic.