From magmatic generation to UHP metamorphic overprint and subsequent exhumation: A rapid cycle of plate movement recorded by the supra-subduction zone ophiolite from the North Qaidam orogen

摘要

The rapid subduction and exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments. Herein, ophiolitic fragments, eclogites, and amphibolites, from a subduction channel exposed in the Dulan terrane, North Qaidam orogen were investigated. Garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation. New uranium-lead (U-Pb) zircon ages of the current study combined with previous rutile U-Pb ages from eclogites indicate a rapid cycle of events, starting with the generation of the magmatic protolith at ca. 446 Ma, its subsequent subduction reaching a depth of ca. 100-110 km at ca. 435 Ma, and its exhumation into shallow portions of the crust which occurred at ca. 416 Ma. Thus a rapid 30 million years (Myr) protolith generation-subduction-exhumation cycle could be recorded. According to an ophiolitic rock association with island arc basalt (LAB)-like and mid-ocean ridge basalt (MORB)- to ocean island basalt (OIB)-like trace element compositions, the slab-derived components determined by Sr-Nd-Hf isotopes and the short time span of only about 11 Myr between photolith generation and UHP metamorphism in eclogites, the protolith of eclogite is best explained as a supra-subduction zone ophiolite. This finding likely records the closure of the South Qilian ocean and the initiation of a continent-arc collision at ca. 446 Ma. Furthermore, starting with subduction erosion during continent-arc collision in the early Palaeozoic, ophiolitic and arc-related materials become subducted in the North Qaidam. It was inferred that such a rapid succession of events from protolith formation to UHP overprint and final exhumation, could be explained by movements of rock material in a subduction channel. Thus, this study provides a new view in understanding the mechanics of different episodes of subduction systems in North Qaidam UHP metamorphic belt. (C) 2019 Elsevier B.V. All rights reserved.