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

Abstract

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 Neoproterozoic ophiolites of the Khantaishir Formation, eclogite -bearing orthogneisses and micaschists of the Alag Khadny 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 muscovite, 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 °C 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 °C 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 °C 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.

Multiple Metamorphic Events Recorded within Eclogites of the Chandman District, SW Mongolia

“…We argue that older c. 600 Ma age for eclogites probably represent a cooling age of the HP subduction event. Whereas younger c. 540 Ma ages for both eclogites and garnet-chloritoid schists are probably represent a cooling age of MP collision event of metamorphism since the peak P-T conditions of MP event of eclogite (550-610 • C and 7-9 kbar) and garnet-chloritoid schist (560-590 • C; 10-11 kbar) [28] are comparable. It is possible that during the MP event, component of Ar in the phengite of eclogite close to garnet-chloritoid schist were re-equilibrated.…”

“…[37]. Ellipsoid gray colored polygon is P-T conditions of prograde and peak metamorphic stage of garnet-chloritoid schist [28]. -0.042 0.797 [37].…”

“…The distinguished MP event in amphibolitized eclogite is probably related with a collisional event [50] with emplacement of granitic magmatism and their subsequent metamorphism (orthogneiss formation). Javkhlan et al [28] suggested collisional metamorphism for garnet-chloritoid schists in the marbles of Maykhan Tsakhir Formation that are distributed close to the eclogite bodies in the Chandman district. The P-T path of pre-peak (500-510 • C; P = 7-8 kbar) and peak (560-590 • C; 10-11 kbar) stages of garnet-chloritoid schist characterize a Barrovian type metamorphism.…”

“…The single P-T loop have been identified in granulitized eclogites from the Ama Drime range, Eastern Himalayas [1], eclogites from Makbal district, N Tien-Shan [2], and Dabie-Sulu UHP rocks, Central China [3]. Two P-T loops have been unraveled in several orogenic belts-i.e, eclogites in Sambagawa belt [4], kyanite eclogite [8]); (b) Simplified tectonic sketch map of the Lake Zone (modified after Javkhlan et al [28]). [8]); (b) Simplified tectonic sketch map of the Lake Zone (modified after Javkhlan et al [28]).…”

“…Two P-T loops have been unraveled in several orogenic belts-i.e, eclogites in Sambagawa belt [4], kyanite eclogite [8]); (b) Simplified tectonic sketch map of the Lake Zone (modified after Javkhlan et al [28]). [8]); (b) Simplified tectonic sketch map of the Lake Zone (modified after Javkhlan et al [28]). [15]).…”