Exhumation of deeply subducted continental crust along contrasting pathways: Examples from the North Qaidam ultrahigh pressure metamorphic terrane, North Tibet

Abstract

Deeply buried continental terranes now exposed in the core of mountain belts are characterized by dominant felsic gneisses enclosing eclogite and garnet peridotite. Some medium–low temperature eclogites can preserve peak mineral assemblages. In contrast, some eclogites and their country rocks have been turned to granulite-facies assemblages. The apparent discrepancy between the two types of eclogites may reflect different exhumation pathways and mechanisms. Based on detailed field investigation and geological mapping, two ultrahigh pressure (UHP) metamorphic units are recognized in the western North Qaidam Mountains. The Yuka-Luofengpo UHP metamorphic unit (YKU) is characterized by medium–low temperature eclogites and metapelites, whereas the Luliangshan UHP metamorphic unit (LLU) primarily consists of granulitized eclogites and pelitic granulites. The two units are separated by an early Paleozoic arc magmatic-metamorphic unit.The disparate metamorphic evolutions of the UHP metamorphic rocks from the YKU and LLU indicate that the two adjacent metamorphic units underwent different exhumation pathways. We combine our data with published geodynamic numerical modeling to infer that the YKU UHP metamorphic rocks detached from the subducted slab at different depths and exhumed within a cold continental subduction channel. In contrast, the granulitized eclogites and pelitic granulites of the LLU were vertically exhumed from the subduction channel through the hot mantle wedge into the overriding crustal plate by diapirism (relamination) and experienced a prolonged granulite facies overprint prior to exhumation to the shallow crust.