柴北缘乌北地体早古生代变质岩年代学、地球化学及其构造意义OA

The North Wulan Terrane,which represents the overriding plate of the Early Paleozoic subduction-collision system in the North Qaidam Orogen,is key to understanding the composition of the lower crust beneath the arc-back-arc system and the tectonic evolution associated with the subduction of the South Qilian Proto-Tethyan oceanic crust.In this paper,we present detailed petrological,geochronological,and geochemical data,together with metamorphic P-T conditions,for representative mafic to felsic metamorphic rocks from the North Wulan Terrane.The results reveal that the protolith ages and tectonic settings of the three studied metamorphic rock types differ significantly.The amphibolites exhibit compositional characteristics similar to E-MORB,but they do not preserve protolith age information,which limits further discussion of their geodynamic significance.The biotite-amphibole gneisses display geochemical signatures of a continental arc,with protoliths formed at 471±3 Ma,representing magmatic records of a continental marginal arc during subduction of the Proto-Tethyan oceanic crust.The granitic mylonites exhibit geochemical features resembling arc granites and formed at 965±6 Ma,indicating that they represent arc magmatism in an active continental margin setting related to the assembly of the Neoproterozoic Rodinia supercontinent.Combined with previous studies,it is proposed that the lower part of the crust beneath the arc-back-arc system,represented by the North Wulan Terrane,is primarily composed of Precambrian basement(including various types and ages of metasedimentary and meta-igneous rocks)and Early Paleozoic continental arc magmatic rocks.These rocks underwent widespread Middle to Late Ordovician HT/LP metamorphism,with corresponding ages concentrated between ca.475～440 Ma.This period is notably confined to the late stage of oceanic subduction recorded by arc magmatism(ca.510～450 Ma),and is nearly synchronous with the eclogite-facies metamorphism associated with subduction of oceanic crust documented in the Dulan UHP unit.Integrating these data with previous studies on metamorphic P-T evolution,we suggest that the HT/LP metamorphism occurred within an extensional environment at the continental marginal arc,and was triggered by slab rollback during the late stage of plate convergence.This highlights the critical role of studying the metamorphism in overriding plates within subduction zones to reconstruct the tectonic evolution of oceanic subduction.