The Early Triassic Andean-type Halagatu granitoids pluton in the East Kunlun orogen, northern Tibet Plateau: Response to the northward subduction of the Paleo-Tethys Ocean

Abstract

Voluminous granitoids located in the East Kunlun orogen (EKO) play an important role in characterizing the crustal growth and northward subduction of the Paleo-Tethys oceanic lithosphere. This paper presents new petrological, whole-rock geochemical, zircon U-Pb isotope geochronological and Sr-Nd-Hf isotopic data for the Halagatu granitoids (HG) pluton. The HG pluton comprises several different textural and compositional types, i.e., medium- to fine-grained granodiorites (MG), porphyritic monzogranites (PMG), coarse-grained monzogranites (CMG), and dark mafic microgranular enclaves (ME). U-Pb dating of zircons from the HG pluton yielded a weighted mean 206Pb/238U age of 248–244Ma, which represents the crystallization age of the HG pluton. The whole-rock geochemical data indicate that the PMG, CMG and MG have similar geochemical features; for example, they are classified as high-K calc-alkaline, metaluminous to wildly peraluminous I-type granitoids, with negative Nb-Ta and Ti anomalies, low Mg# and Al2O3/(FeOT+MgO+TiO2) values, and high Th/Y ratios. These characteristics suggest the parent magmas were crustally derived. In contrast, the ME are characterized by medium-K calc-alkaline and metaluminous compositions, low Th/Y ratios and relatively high Mg#, Cr, and Ni values, likely suggesting these magmas were derived from a mantle source. Furthermore, the host granodiorites and microgranular enclaves have consistent Sr-Nd-Hf isotopic features, with εNd(t) values of −6.10 to −4.30 and zircon εHf(t) values of −7.8 to −0.4, indicating an enriched subcontinental lithospheric mantle (SCLM). On the basis of these values and the whole-rock geochemistry, the HG pluton magmas were the result of mixing between lithospheric mantle-derived and crust-derived magmas and that fractional crystallization (FC) played an important role in the formation of the HG pluton. The combination of these new data with previously published data reveals that the HG pluton was emplaced in a continental magmatic arc setting by the northward subduction of a branch of the Paleo-Tethys Ocean (Buqingshan Ocean) beneath the East Kunlun-Qaidam terrane (EKT) and that the Buqingshan Ocean began to subduct strongly northward in the late Permian (ca. 260Ma), with subduction terminating during the late Middle Triassic (ca. 240Ma).