Discovery of garnet amphibolite in Zaheba ophiolitic melange, eastern Junggar, NW China
东准噶尔扎河坝蛇绿混杂岩中的石榴角闪岩

Following the report on garnet pyroxenite and quartz-magnesitite with the ultra-high pressure (UHP) origin in the Zaheba region, the authors further discovered there the garnet amphibolite in the same origin. The garnet amphibolite is mainly composed of amphibole, garnet, pyroxene and albite. It contains not only the high pressure minerals, such as jadeite and phengite, but also the relicts of supersilicic garnet and supersilicic pyroxene with UHP genetic features. Based on the field occurrence and mineral assemblage, the garnet amphibolite can be sub-divided into two groups: the jadeite-bearing and the phengite-bearing, and their primitive rocks are all oceanic basalts. The identification of garnet amphibolite further confirmed that there is an exhumed oceanic crust after ultradeep subduction in the Zaheba ophiolitic mélange, and the composition of the oceanic crust is more complicated, including sedimentary rocks, gabbros and oceanic basalts. The existence of UHP metamorphic rocks, such as garnet amphibolite, quartz-magnesitite and garnet pyroxenite illustrated that the Zaheba ophiolitic belt was an ultradeep subduction zone of the oceanic slab, and it connected with the Keramay ophiolitic belt at its western section, and then extended westwards out of the national boundary, forming an EW stricking ultradeep subduction zone along the northern Junggar margin. The discovery of this ultradeep subduction offers a new idea in the study on the process and mechanism of ocean-continent transition during Early Paleozoic, and also has a significant enlightenment in the investigations of crust-mantle interaction at the northern margin of the Junggar basin and the southern margin of the Altai mountain during Early Paleozoic, and the geodynamic mechanism of the formation of Central Asian orogen.