Neoproterozoic mafic-ultramafic layered intrusion in Quruqtagh of northeastern Tarim Block, NW China: Two phases of mafic igneous activity with different mantle sources

In this paper we report zircon U–Pb age, chemical compositions of rock-forming minerals, and whole-rock elemental and Sr–Nd isotopic data for the No. II mafic-ultramafic intrusive complex (N2MC) in the Quruqtagh area at the northeastern margin of the Tarim Block, northwestern China to evaluate its petrogenesis and tectonic significance. The N2MC with an exposure area of ca. 12 km2 has a funnel-shaped cross-section and intruded the Paleoproterozoic basement. U–Pb zircon dating gives a crystallization age of 760 ± 6 Ma. Rock types of the N2MC include lherzolite, pyroxenite, gabbro and minor diorite. Major elements geochemistry of these rocks exhibits a tholeiitic trend with a wide range of SiO2 contents (38.8–60 wt.%). On the other hand, they are systematically enriched in LILE, LREE and depleted in HFSE and HREE, thus leading to low HFSE/LREE ratios (e.g., Nb/La ≈ 0.3). Isotopically, the studied rocks are characterized by negative whole-rock εNd(t) values (− 7.6 to − 2.8) and variable high (87Sr/86Sr)i (0.7095–0.7059). These features, together with chemical compositions of the rock-forming minerals and the presence of the primary phlogopite and hornblende, suggest that N2MC was likely formed via crystal fractionation/cumulation (with negligible crustal contamination) of a tholeiitic magma derived from a metasomatized subcontinental lithosphere mantle (SCLM) in an extensional environment. The enrichment of the mantle source could be ascribed to the metasomatism by subducted-slab-released fluids before partial melting. Overall, reported Neoproterozoic igneous rocks throughout the Tarim Block constitute two major phases of Neoproterozoic igneous activities, i.e., ca. 825–800 Ma and ca. 780–745 Ma, respectively. Similar to that of many other Rodinian continents, this feature is interpreted to be related to the two phases of Neoproterozoic mantle plume activity under the Rodinia. Furthermore, there exist two types of mafic-ultramafic complex at Quruqtagh, i.e., the ca. 820 Ma carbonatite-bearing and the ca. 760 Ma tholeiitic, which could reflect the presence of two different mantle sources.