Indian ocean-type mantle domain in the Carboniferous Qushiman ophiolite in the Northern Pamir: Significance for the intra-oceanic subduction in the Paleo-Tethys Ocean

The opening, oceanic subduction, and final closure of several branches of the Paleo-Tethys Ocean provide significant constraints on reconstruction of the Central Asian blocks in Pangea. Although it has been generally agreed that the Triassic Tanymas ophiolite represents the relic of the Paleo-Tethys oceanic crust, the initiation of subduction and final closure of the Paleo-Tethys Ocean in the Pamir Plateau remain equivocal, which hampers further understanding of the Tethys tectonic domain. Here we present field investigations, zircon UPb ages, and whole-rock geochemistry and Sr-Nd-Pb-Hf isotopic data of the newly identified Qushiman supra-subduction zone (SSZ)-type ophiolite in the Northern Pamir in order to address the integrated evolution of the Paleo-Tethys Ocean in this less known Tethyan. The Qushiman ophiolite mainly consists of serpentinite, gabbro, basalt, diorite, plagiogranite, amphibolite, granulite, and schist. Zircons UPb dating of the gabbro yielded ages of 320–319 Ma. Both the gabbros and amphibolites exhibit typical fore-arc basalts (FAB) affinities with depleted Sr-Nd-Hf isotopic compositions, high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios, flat chondrite-normalized rare earth element (REE) patterns but remarkable low Nb/La ratios (0.21–0.29) for the gabbros. Their higher ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios are comparable to those of Indian MORB, indicating the presence of the DUPAL anomaly in the Pamir Plateau of the Paleo-Tethys domain. These geochemical features argue that the gabbros and amphibolite were likely derived from partial melting of depleted mantle wedge source with involvements of 2 % pelagic sediments. These FAB-like gabbros and amphibolites, together with magmatic flare-up at 320 Ma in the Pamir Plateau archived initial subduction of the Paleo-Tethys Ocean. The basalt samples exhibited ocean island basalt (OIB)-like geochemical signatures with enrichment of light rare earth element (LREE), positive anomalies of Nb, Ta, Zr, and Hf, and markedly enriched SrNd isotopic compositions, indicating partial melting of enriched mantle wedge. The diorites characterized by high MgO contents were likely derived from mantle wedge peridotite metasomatized by fluids derived from subducting oceanic slab. These magmatic assemblages of FAB-like gabbros and amphibolites, OIB-like basalts and high-Mg diorites, as well as geochemical transitions from normal mid-ocean ridge basalt (N-MORB)-like to arc-style affinities in the Qushiman ophiolite are analogous to those of the modern Izu-Bonin-Mariana intra-oceanic arc, suggesting tectonic transformation from oceanic expansion to initial intra-oceanic subduction of the Paleo-Tethys Ocean. The variety of magmatic compositions highlights the contributions of subduction-related pelagic sediments and fluids to their mantle wedge sources. The 184–179 Ma metamorphic age of amphibolites is consistent with the Early Jurassic regional amphibolite-facies metamorphism of the Cambrian Bulunkuole Group, recording final closure of the Paleo-Tethys Ocean and associated collision processes in the Northern Pamir. Integrating previous studies, we elucidate initial subduction, magma evolution, and extinction of the intra-oceanic arc system within the missing Paleo-Tethys Ocean in the Northern Pamir.