Eclogitization of transient crust of the Aktyuz Complex during Late Palaeozoic plate collisions in the Northern Tianshan of Kyrgyzstan

Abstract

The Aktyuz Metamorphic Complex in the western Tianshan Mountains of Kyrgyzstan comprises high-pressure (HP) granitic gneisses and metasediments hosting mafic HP lithologies including eclogite. Geological relationships and geochemical-isotopic data of the different rock types of the Aktyuz Complex have been interpreted as a continental crustal fragment and the mafic rocks as former dykes. This study combines Sm–Nd and 40Ar/39Ar mineral ages, and P–T pseudosection modelling for a single retrogressed eclogite sample in order to shed light on the P–T–t evolution of the mafic HP rocks. The eclogite experienced a clockwise prograde P–T path with peak metamorphic conditions of ca. 2.1GPa at ca. 670°C, corresponding to a burial depth of ca. 70km. The post-peak metamorphic P–T conditions suggest isothermal decompression and moderate initial uplift of the eclogite to ca. 40km depth. A Sm–Nd isochron age of 462±7Ma for garnet, omphacite, and whole-rock is interpreted as time of cooling and retrogression of the eclogite sample below 650 to 600°C at pressures <1GPa. We suggest that the HP rocks of the Aktyuz Complex represent deeply subducted continental crust of one of the Palaeo-Kazakhstan terranes. During exhumation of the metasediments (now paragneiss), the mafic rocks were juxtaposed at variable depth and at low temperature (<400°C, and in the case of the reported sample after ca. 462Ma) with the more buoyant paragneisses in the subduction channel. This channel flow exhumation model can account for the different high-temperature petrological and geochronological histories of paragneiss and eclogite.