Metamorphic pressure-temperature evolution of garnet-chloritoid schists from the Lake Zone, SW Mongolia

摘要

Garnet-chloritoid schists occur in the Chandman district of the Lake Zone, SW Mongolia, which is located within the central part of the Central Asian Orogenic Belt. The Chandman district is composed of Neoprotero-zoic ophiolites of the Khantaishir Formation, eclogite-bearing orthogneisses and micaschists of the Alag Khad-ny metamorphic complex, marbles of the Maykhan Tsakhir Formation, and the basement block of the Zamtyn Nuruu Formation. Garnet-chloritoid schists occur as lenses or layers within marbles of the Maykhan Tsakhir Formation, which lie in contact with eclogite bodies. They consist mainly of garnet, chloritoid, phengitic mus-covite, chlorite and quartz, with minor amounts of kyanite, rutile, ilmenite, zircon and carbonaceous matter. The texture and mineral chemistry of the garnet-chloritoid schists record prepeak and peak stages of high-pressure intermediate type metamorphism. The pre-peak stage of epidote-amphibolite facies conditions (T= 500-510 ℃ and P = 7-8 kbar) is deduced from cores of garnet containing mineral inclusions of chloritoid, Fe-rich chlorite, phengitic muscovite, and quartz. The peak mineral assemblage of the garnet-chloritoid schists defined by garnet rims containing chloritoid, phengitic muscovite, Mg-rich chlorite, kyanite and quartz as inclusions indicates T= 560-590 ℃ and P = 10-11 kbar, falling in transitional conditions between the epidote-amphibolite and the eclogites facies. The pressure conditions of the garnet-chloritoid schists are distinctly lower than those of the eclogites (T= 590-610 ℃ and P = 20-25 kbar), whereas temperature conditions are similar. The eclogites are considered to have formed by subduction of oceanic crust. In contrast, the metamorphism of the garnet-chloritoid schists was probably related to collisional tectonic event in the Central Asian Orogenic Belt.