Late Devonian postcollisional magmatism in the ultrahighpressure metamorphic belt, Xitieshan terrane, NW China

Abstract

Postcollisional magmatism in the North Qaidam ultrahigh-pressure metamorphic belt provides important information on crustal reworking, crust-mantle interaction, and related geodynamic processes. Here, we report zircon U-Pb ages and Hf isotopic compositions, as well as whole-rock major- and trace-element and Sr-Nd isotopic compositions for a suite of gabbros and granitoids from the Xitieshan terrane in the North Qaidam ultrahigh-pressure metamorphic belt. These rocks formed concurrently at ca. 372 Ma (Late Devonian), ∼50 m.y. after the continental collision between the Qaidam and Qilian blocks. The gabbro samples have low- to medium-K tholeiitic composition, with high FeOT and TiO2 contents. They are enriched in large ion lithophile elements, and depleted in Nb and Ta, similar to the lithospheric mantle source, which is also indicated by their Zr/Nb, La/Nb, and Nb/Ta values. In addition, these rocks have relatively enriched isotopic signatures compared to mid-ocean ridge basalt. This implies that the mantle source may have been metasomatized by subduction-related melts, which is further confirmed by the signatures of trace elements. The coeval felsic rocks are high-K calc-alkaline, slightly peraluminous I-type granites. They are characterized by high light rare earth element/heavy rare earth element ratios, with a negative Eu anomaly, strong depletion in Ba, Sr, and high field strength elements (e.g., Nb, Ta, Ti, and P), and enrichment in Rb, Th, U, and Pb, resembling the upper continental crust. The Sr-Nd-Hf isotopic signatures of these granitoids, however, show remarkable differences from upper continental crust–derived melts and North Qaidam basements, but they are generally consistent with previously emplaced syncollisional granite in the Xitieshan terrane. Therefore, the granitoids are best interpreted as the products of melting of the syncollisional granite. Meanwhile, the involvement of coeval mantle-derived melts is suggested by the relatively depleted isotopic signatures and mafic magmatic enclaves hosted by the granitoids. Additionally, some of the inherited zircons in the granitoids display negative εHf(t) values and Paleoproterozoic depleted mantle model ages (TDM2), suggesting that the source also involved ancient crustal materials. Based on the temporal and spatial associations of the gabbro-granitoid suite, in conjunction with their petrogenetic features, we propose a geodynamic model involving convective removal of the lithosphere and resultant asthenospheric upwelling to explain the crustal reworking, crust-mantle interaction, and related postcollisional geodynamic processes in the North Qaidam ultrahigh-pressure metamorphic belt.