Rushan-Pshart Paleo-Tethyan suture deduced from geochronological, geochemical, and Sr-Nd-Hf isotopic characteristics of granitoids in Pamir

Abstract

Correlation of blocks comprising the Pamir plateau with blocks in the Tibetan plateau is debated due to the complex tectonic history of amalgamated orogenic belts, followed by widespread Cenozoic intracontinental deformation within the Pamir plateau. Of particular interest is the Rushan-Pshart suture along the southern edge of the Central Pamir. It is uncertain if the suture zone is a Meso-Tethyan or a Paleo-Tethyan suture and if it is the western extension of the Bangong-Nujiang suture. This study presents new in-situ zircon U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotopes of three granitic plutons (Tahman, Tash, and Mingtie) along the southern edge of the Central Pamir block. Emplacement ages of the three plutons range from ~206 Ma to ~201 Ma. Samples from Tahman and Tash contain 67% to 76% SiO2, and yield A/CNK values between 0.92 and 1.17. They exhibit high LILE/HFSE ratios, negative Eu anomalies (Eu/Eu* = 0.41–0.77), pronounced negative Nb, Ta, P, and Ti anomalies, negative εNd(t) values (between −7.3 and −6.9), and negative zircon εHf(t) values (between −15.5 and −1.4). Geochemical data indicate that the Tahman and Tash samples represent metaluminous or peraluminous, I-type arc-related granitoids. The Tahman granites are inferred to have formed by partial melting of the Precambrian metaigneous and metasedimentary rocks in the lower crust, while the Tash pluton was formed by partial melting of the infracrustal medium-to-high K basaltic compositions within the garnet stability field of the lowermost crust. We suggest that these newly discovered Late Triassic granite plutons are related to the closure of the Rushan-Pshart suture. This is supported by recent geochronological, paleontological, and stratigraphic studies. Thus, the Rushan-Pshart suture is a Paleo-Tethyan suture. We further notice that the Central Pamir block is bounded by the Tanymas and Rushan-Pshart Paleo-Tethyan sutures to the north and south, respectively, which is structurally similar to the North Qiangtang and the Indochina blocks that are also bounded by two Paleo-Tethyan sutures from both sides. This indicates that the Central Pamir block is the western extension of the Tianshuihai-North Qiangtang block, both of which underwent oblique convergence during the closure of the Paleo-Tethys ocean.