Zircon Ages of Metasedimentary Rocks in the Wuwamen Ophiolitic Mélange,Chinese South Tianshan:Implications for the Paleozoic Subduction-Accretion in the Southern Central Asian Orogenic Belt

High-temperature and high-pressure(high-grade)metamorphic complexes of variable ages are common in the Central Asian orogenic belt(CAOB),and their precise geochronology and origin are essential to unravel the orogenic architecture and crust-production rate of the CAOB.Hereby it is essential to differentiate between pre-orogenic Precambrian basement and Paleozoic arc-accretionary complexes.This study provides precise in situ zircon U-Pb ages for the metasedimentary rocks in the Wuwamen ophiolitic mélange,which is traditionally thought to represent the pre-orogenic basement of the southwestern CAOB.A meta-sandstone from the meta-flysch sequence revealed a widespread ca.1.8 Ga high-grade metamorphic overprint similar to that of the underlying orthogneisses and,thus,was interpreted to represent pre-orogenic basement fragments that occur as tectonic blocks in the Paleozoic ophiolitic mélange.In contrast,a schist from the mélange matrix is characterized by a ca.333 Ma high-grade metamorphic overprint,indicating that the northward subduction of the South Tianshan Ocean was still active at this time.The thrust-imbricated high-grade metamorphic rocks of contrasting origins in an accretionary complex have important implications for the understanding the accretionary history and crustal growth of the CAOB.Furthermore,the strongly deformed ophiolitic mélange was intruded by an undeformed granite dyke with an emplacement age younger than ca.294 Ma,thus providing a minimum age limit for the final closure of the South Tianshan Ocean.