Triassic igneous activities in the east flank of the East Kunlun orogenic belt: the Daheba complex example

Abstract

ABSTRACT The Triassic igneous complexes in the east flank of the East Kunlun orogenic belt (E-KOB) provide a window through which to investigate the Mesozoic magmatism in the E-KOB. Here, we report the precise timing of Triassic magmatism and magma petrogenesis of different rock types in the east flank of the E-KOB based on the petrology, mineral chemistry, geochemistry, zircon geochronology and Lu-Hf isotopes. Zircon U-Pb dating reveals that the Daheba complex was crystallized in 242–239 Ma. The rocks display enrichment in large-ion lithophile elements (LILEs) and LREEs and depletion in high field strength elements (HFSEs) and HREEs, which are typical of subduction-related magmatic rocks. The fine-grained granites (FGs) are characterized by a high SiO2 content (72.25–74.28 wt. %), a high differentiation index (DI = 95) and marked depletion in Sr, Ba and Eu, which indicates that the FGs are highly fractionated I-type granitoids. The zircon εHf(t) values from −3.6 to −1.1 with two-stage model ages (TDM2) of 1496–1340 Ma, which suggests that they were derived from an ancient Mesoproterozoic lower crust. The similarity in the REE patterns of the mafic microgranular enclaves (MMEs) and host granites (HGs) indicates that the mixing between mafic and felsic magmas could have contributed to the chemical variation of the MMEs. The mafic melts were derived from the partial melting of the mantle that had been metasomatized by slab-derived fluids. The MMEs show typical igneous textures, acicular apatites, sieve texture of plagioclase and reverse zoning of plagioclase, which correspond to magma mixing during petrogenesis. Our geochemical and isotopic data suggest that the Daheba complex formed in an active continental margin setting closely related to the northward subduction of the Palaeo-Tethys Ocean Plate beneath the E-KOB.