A newly identified Precambrian terrane at the Pamir Plateau: The Archean basement and Neoproterozoic granitic intrusions

Abstract A number of Precambrian terranes are distributed in the Tibetan Plateau, including the Lhasa, Sibumasu, Qiangtang, Qaidam, Qilian and Tianshuihai terranes. These terranes exhibit distinct features in rock associations and tectonothermal history. They are important for understanding the evolution of the Tethys Ocean and the assemblage and breakup of the Precambrian supercontinents. In this contribution, we identify a Precambrian terrane at the southeastern section of the Pamir Syntax and we name it as Mazar terrane. Systematic zircon U-Pb dating shows that the basement, mainly composed of metamorphic bimodal volcanic rocks and gneiss/schist, deposited at ca.2.5 Ga and was overprinted by ca.2.0 Ga amphibolite-facies metamorphism. The Neoproterozoic granitic intrusions into the basement, emplaced at 840–835 Ma. Geochemistry of the granitic intrusions defines two distinct types of granitic rocks, including the trondhjemite-tonalites and granodiorites. The sodic trondhjemite- tonalites show TTG-like or adakitic signatures such as enrichment of LREE and LILEs, depletion of HREE and HFSE, resulting in their high Sr/Y and (La/Yb) N ratios. The granodiorites exhibit typical signatures of the calc-alkaline I-type granites. Both trondhjemite-tonalites and granodiorites have coupled unradiogenic whole-rock Nd and zircon Hf isotope compositions. eNd(T) and eHf(T) values of the trondhjemite- tonalites range from −9.5 to −8.7 and from −14 to −10, respectively. The granodiorites have slightly more radiogenic Nd-Hf isotope compositions with eNd(T) values ranging from −5.5 to −4.4 and eHf(T) values ranging from −10 to −7. According to elemental and isotope signatures of the granitic intrusions, the trondhjemite-tonalites were derived from partial melting of a mafic crust at depth ≥30 km with a garnet amphibolite residue whereas the granodiorite was formed by partial melting of a mafic crust at a shallower level and mantle-derived magma could be involved in the genesis of the granodiorite. In line with their geochemistry and the fact that the granodiorite is slightly younger than that of the trondhjemite-tonalites, we propose that those granitic intrusions were most likely formed in an extensional environment, and that they could be part of the earliest igneous activity related to the initial breakup of the Rodinia supercontinent. Our study, in combination with previous works, suggests that the Mazar terrane, as well as the Qilian, Qaidam, Tianshuihai terranes, is most likely continental fragments drifted from the Yangtze Block during the breakup of the Rodinia supercontinent.