The Early Silurian Gabbro in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Constraints on the Proto-Tethyan Ocean Closure

Abstract

The Early Paleozoic is a crucial period in the formation and evolution of the Eastern Kunlun Orogenic Belt (EKOB), which is of great significance for understanding the evolutionary history of the Proto-Tethyan Ocean. This paper presents new petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopic research on the Yuejingshan gabbro from the eastern segment of the EKOB. Zircons U-Pb data suggests the gabbro formed in the Early Silurian (435 ± 2 Ma). All samples have relatively low TiO2 contents (0.45-2.97%), widely varying MgO (6.58-8.41%) and Mg# (58-65) contents, rich in large ion lithophile elements (LILE such as Rb, Ba, Th, and U) and light rare earth elements (LREE),which indicate it has a similar geochemical composition than island arc basalt. The major elements features indicate that the formation of this gabbro underwent fractional crystallization of clinopyroxene, olivine, and plagioclase. The depletion of high field strength elements (HFSE such as Nb, Ta, and Ti), and feature a slightly enriched Hf isotope composition (with εHf (t) ranging from 1.13 to 2.45) may be related to the partial melting of spinel-bearing peridotite, led by the slab fluid metasomatism. The significantly earlier than the metamorphic age (428 Ma) of the collision-related eclogite facies and the peak metamorphic age (427 Ma) of medium pressure (epidotization) amphibolite facies in the Eastern Kunlun Orogenic Belt indicates that its formation is still closely related to the subduction of the Eastern Kunlun oceanic crust. The gabbro likely represents magmatic records of the latest period of the Early Paleozoic oceanic crust subduction in the Eastern Kunlun. Therefore, the final closure of the Proto-Tethyan Ocean and the beginning of collisional orogeny, occurred before the Early Silurian.