Late Palaeozoic-Mesozoic volcano-sedimentary rocks within the Rushan-Pshart Suture zone in the Pamir contain critical information on the subduction-accretion history of the Rushan-Pshart Ocean (Meso-Tethys) prior to the Central-South Pamir collision. In this article, we report new field, petrographic, geochronological, and geochemical data of the Permian to Triassic basic volcanic and sedimentary rocks from the Pshart area. Our field study unravels block-in-matrix components within some sedimentary mélanges, which, together with some previously defined ophiolitic mélanges, enables us to define the Pshart accretionary complex (AC), and thus for the first time to discuss the subduction-accretion history of the Rushan-Pshart Ocean and the growth of the Pshart AC. The youngest detrital U-Pb zircons suggest that deposition of sediments was in the Late Triassic (212 Ma). Detritus of Triassic age was primarily derived from the Triassic Karakul-Mazar Arc-AC (Northern Pamir) and the Bashgumbaz Magmatic Arc, which developed along the northern margin of South Pamir. The evidence of north-directed thrusts along the Kara Djilga-2 and Ken Djilga transverses confirms previous interpretations of southward subduction of the Rushan-Pshart oceanic lithosphere beneath the South Pamir. Geochemical OIB-type data of the Pshart alkaline basaltic rocks suggest formation in seamounts incorporated into the Pshart AC during the southward subduction of the Rushan-Pshart Ocean. The youngest Late Triassic deposition age is consistent with the coeval time of closure of the Palaeo-Tethys and Meso-Tethys oceans in the Pamir. The Pshart AC formed by subduction-accretion processes during the southward subduction of the Meso-Tethys Ocean along the northern South Pamir and the final docking of the Central and South Pamir (Cimmerian Blocks) may have occurred after the Late Triassic.

Identification of slab window process is important for understanding the nature of the accretionary orogenesis. In this study, we report detailed petrological, geochronological, geochemical, Sr–Nd isotopic, and mineral chemical data for two dyke-like gabbroic intrusions from the South Tianshan belt of Tajikistan, southwestern margin of the Central Asian Orogenic Belt. Both intrusions are composed of coarse- and fine-grained gabbros. U–Pb zircon dating shows that they were emplaced at 431±5 Ma. The gabbroic rocks show relatively large variation in elemental and isotopic compositions, with SiO2 of 40.62–53.97 wt.%, Sr of 333–1261 ppm, and εNdt of +2.5 to +5.8. Especially, the fine-grained gabbros show lower SiO2 and higher MgO but more evolved isotopes than the coarse-grained gabbros for each of the intrusions. All the rocks display OIB-like or transitional OIB-/E-MORB-like geochemical characteristics with no obvious Nb-Ta depletion, indicative of an intraplate affinity. Combined with their mineral chemical compositions, we suggest that these gabbroic rocks were generated by partial melting of asthenospheric mantle in the transitional spinel-garnet stability field, followed by different degrees of fractional crystallization of olivine, clinopyroxene, and plagioclase and mixing with carbonatitic melts. The available data indicate that roll-back of the subducting Turkestan oceanic slab occurred during the Late Ordovician to Early Silurian period. Asthenosphere upwelling due to the opening of slab window resulted from localized slab tearing during slab roll-back may have been responsible for the generation of the studied dyke-like gabbroic intrusions.

The Pamir orogen was formed by the subducted accretion and amalgamation of Cimmerian terranes from the northern margin of Gondwana with the southern margin of Eurasia. The Mesozoic magmatic rocks are widespread in Pamir and record the tectonic evolution in different stages. The Rushan–Pshart suture zone represents an ancient ocean between Central and Southern Pamir. This paper reports the petrography, geochronology, and geochemistry of Cretaceous granites and diabase dikes that intrude into the Pshart complex. The granites were emplaced between 124 and 118 Ma, based on their zircon U-Pb ages. These granites are characterized by high-K calc-alkaline, low magnesian, and high SiO2, A/CNK, and K2O+Na2O values. They also display strong depletion of Ba, Sr, Eu, and Ti and comparatively weak negative Nb anomalies in spidergrams. Thus, we proposed in this study that these are highly fractionated, strongly peraluminous S-type granites. They were generated by the partial melting of the metasedimentary rocks in the plagioclase stability field and underwent subsequent fractional crystallization during their ascent. The diabase dikes contain low SiO2, and high MgO levels and negative Nb and Ta anomalies, which were interpreted to form in an extensional environment. Late Jurassic–Early Cretaceous closure of the Rushan–Pshart Ocean and subsequent foundering of its oceanic lithosphere caused local extension and upwelling of the asthenospheric mantle. The underplating of mafic magma provided a heat source to melt the metasedimentary-derived granitic that formed in the initial post-collisional environment. The subsequent local extension caused the emplacement of diabase dikes. Based on our new data and combined with data from previous studies, we concluded that the Rushan–Pshart suture zone is the remnant of the Meso-Tethys Ocean and may represent the western continuation of the Bangong–Nujiang suture of the Tibetan Plateau.