Mafic-intermediate igneous rocks in the East Kunlun Orogenic Belt, northwestern China: Petrogenesis and implications for regional geodynamic evolution during the Triassic

摘要

Mafic-intermediate igneous rocks can provide important insights into the geodynamic evolution of orogenic belts. Here, we investigate the petrogenesis of mafic-intermediate intrusive igneous rocks in the East Kunlun Orogenic Belt (EKOB) to constrain regional geodynamic and magmatic evolution. These intrusive rocks are all located in the Permian-Triassic magmatic arc and parallel to the Paleo-Tethys ophiolite belt. Zircon U-Pb dating results indicate that these various igneous rocks crystallized during the Triassic. The 250-248 Ma appinites display low SiO2 contents, high MgO contents, and marked enrichments in large-ion lithophile elements and depletions in high-field-strength elements; combined with their enriched epsilon(Hf)(t) values, these appinites were considered to be derived from lithospheric mantle modified by subducted slab-derived fluids. The Harizha host granodiorites and MMES have indistinguishable crystallization ages of 235 Ma but distinct SiO2 contents and delta O-18(SMOW) values, which, together with enriched Sr-Nd-Hf-Pb isotopes, suggest that their primary magmas were derived from the Mesoproterozoic lower crust (MLC) and subduction-modified lithospheric mantle (SMLM), respectively. The 241 Ma Baidungou quartz diorite and the 218 Ma Huanglonggou diorite porphyry show characteristics of medium SiO2 contents, moderate Mg-# and epsilon(HF)(t) values relative to contemporaneous mafic and granitic rocks; therefore they probably represent the mixing products of MLC-derived felsic and SMLM-derived mafic magmas. Different from the Baidungou diorite and Harizha granodiorite, the Huanglonggou diorite porphyries have adakitic characteristics, which suggests crustal thickening during the Late Triassic. A synthesis of these new data with previous researches shows that the EKOB undergone three magmatic peaks corresponding to three extension events (slab-rollback, slab-breakoff, and delamination) during the Triassic evolution. (C) 2019 Elsevier B.V. All rights reserved.