Early Cretaceous granitoids in the Southern Pamir: Implications for the Meso-Tethys evolution of the Pamir Plateau

摘要

Although it is generally accepted that the Pamir was formed by long-term terrane drifting and accretion to the south margin of the Asia, the tectonic evolution during the Mesozoic remains unclear. The voluminous Cretaceous intrusions in the Southern Pamir-Karakorumprovide key information for understanding the Mesozoic evolution in this region. In this contribution, we report detailed petrography, ages, mineral geochemistry, whole-rock elemental and Sr-Nd geochemistry and in situ zircon Lu-Hf isotopic compositions of the Early Cretaceous granitoids from the Chinese Wakhan Corridor, Southern Pamir. These granitoids are composed primarily of granodiorite and minor monzogranite and host dioritic enclaves. The granitoids were emplaced at 107-102 Ma, based on their zircon U-Pb ages. They have 63-73 wt% SiO2, Mg-# between 39 and 53 and ACNK mostly lower than 1.1, and are I-type granites. The granitoids display arc-like geochemical signatures characterized by LREE-enriched patterns, variable negative Eu anomalies (La-N/Yb-N= 11.38-23.27; Eu/Eu*= 0.36-0.74), and a significant Nb-Ta trough (Nb/La= 0.24-0.41). Isotopically, the granitoids have negative whole-rock eNd (t) (-8.73-7.35) and zircon epsilon(Hf)(t) (-17.7-5.3) values. The coeval dioritic enclaves (ca. 104 Ma) contain 57-63 wt% SiO2, and have high Mg-# (46-57), lowCr (6.86-79.50 ppm), and Ni (2.88-10.00 ppm). These enclaves exhibit arc-like trace element patterns (Nb/La = 0.34-0.43) with variable negative Eu anomaly (Eu/Eu* = 0.52-0.87), uniform whole-rock epsilon(Nd)(t) (-4.74-4.73), and a large range of zircon epsilon(Hf)(t) values (-13.3-3.7). In line with their rock association and geochemistry, we suggest that the dioritic enclaveswere derived from melting of a phlogopite-bearing mantle sourcemodified by subduction-related process, followed by fractional crystallization and later assimilation by granitic hosts. These Early Cretaceous granitoids were most likely generated by partialmelting of the Precambrian lower crust with variable involvement of mafic magma parental to the dioritic enclaves. Our new data, combined with the previous studies, reveal that the widespread Early Cretaceous magmatism in the Southern Pamir-Karakorum were most likely generated by flat-slab subduction of the Meso-Tethys Ocean during the Cretaceous. (C) 2020 Elsevier B.V. All rights reserved.