Prograde pressure-temperature path of jadeite-bearing eclogites and associated high-pressure/low-temperature rocks from western Tianshan, northwest China

Jadeite-bearing eclogites and associated blueschists locally crop out in a green-schist facies area at Kuldkourla, near the Akeyazhi River in the western Chinese Tianshan region, northwestern China. Garnet in these metamorphic rocks shows prograde zoning with increasing Mg and decreasing Mn from the crystal center towards the rim, and is divided into Ca-poor/Fe-rich core and Ca-rich/Fe-poor mantle parts. The garnet cores include the assemblages of (i) jadeite/omphacite (X-jd = 0.34 - 0.96) + barroisite/taramite; and (ii) omphacite + barroisite/pargasite, with paragonite, epidote, rutile and quartz as major phases with rare albite. The garnet mantles rarely contain inclusions of omphacite, glaucophane, epidote, rutile and quartz. Major matrix phases of the pre-exhumation stage are omphacite, glaucophane, paragonite, rutile and quartz. These mineral parageneses give pressure (P)- temperature (T) conditions of 0.9 GPa/390 degrees C-1.4 GPa/560 degrees C for the stage of the garnet core formation, 1.8 GPa/520 degrees C for the stage of the garnet mantle formation, and 2.2 GPa/495 degrees C-2.4 GPa/535 degrees C for the peak eclogite facies assemblage in the matrix. The estimated P-T conditions and continuous changes of mineral parageneses imply a counterclockwise P-T path which is a combination of ( i) an early prograde stage of highpressure/low-temperature (HP/LT) blueschist facies and/or LP/LT eclogite facies; (ii) a later prograde stage involving compression with minimal heating; and (iii) a climax-of-subduction stage characterized by a slight decrease of temperature with increasing pressure. The negative dP/dT of the latest subduction stage is possibly a record of the following events after a continuous subduction and ridge approach: (i) material migration within the upper part of the subducting slab, which has an inverse thermal gradient caused by ductile flow and/or slab break during subduction; and/or (ii) temporary cooling of the wedge mantle - slab interface by continuous subduction of a relatively cold slab following subduction of a hotter ridge.