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

Abstract

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 UPb 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 εHf(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 δ18OSMOW 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 εHf(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.