Origin of Carboniferous intra-oceanic arc granitoids from the eastern Pamir and implications for the Paleo-Tethyan ocean

Abstract

The expanding and diminishing of the Paleo-Tethyan ocean recorded in the Pamir, and relationships between different segments of the Paleo-Tethyan suture from Pamir to Tibet are poorly constrained. Here we present new zircon U–Pb ages and geochemical data of the Waqia granitoid pluton from the eastern Pamir. Both the host granitoids and gabbroic to dioritic enclaves have similar zircon U–Pb ages of 323–318 Ma, being contemporaneous in origin. These rocks are metaluminous, calc-alkaline, tholeiitic series. They exhibit low K2O, low Sr/Y ratios, low REE contents with depleted to flat LREE patterns, slight enrichment in Rb, Ba, Zr, Hf, and negative Nb, Ta, P, and Ti anomalies. They have similar initial 87Sr/86Sr ratios of 0.7040 to 0.7045, positive εNd(t) (+4.63 to +5.02) and εHf(t) (+9.9 to +14.7) values. Detailed elemental and isotopic data indicate that these coeval rocks in the Waqia pluton were cognate and formed by hydrous partial melting of a juvenile mafic crust within an intra-oceanic arc setting. This study is the first to report reliable isotopic ages of the early Paleo-Tethyan subduction in the eastern Pamir. Further, the Tanymas-Mazha Paleo-Tethyan suture in the eastern Pamir should be the western extension of the Jinshanjiang Paleo-Tethyan suture in the central to eastern Tibetan plateau. An integration of published results leads us to conclude that closure of the Jinshanjiang Paleo-Tethyan ocean was diachronous along its 3000–4000 km length, with closure times of ca 240 Ma in the Pamir and ca 220 Ma in the Sanjiang region.