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Paper No. 28-11
Presentation Time: 7:30 PM

METAMORPHIC EVOLUTION OF THE ECLOGITE FROM THE KHUNGUI ZONE, ZAVKHAN TERRANE, WESTERN MONGOLIA


BAYARBOLD, Manzshir1, OKAMOTO, Atsushi2, DANDAR, Otgonbayar2, UNO, Masaoki2 and TSUCHIYA, Noriyoshi3, (1)Sendai, 981 000, Japan, (2)Graduate School of Environmental Studies, Tohoku University, Aramaki-Aza-Aoba 6-6-20, Sendai, 980-0845, Japan, (3)Graduate School of Environmental Studies, Tohoku University, Aramaki-Aza-Aoba 6-6-20, Sendai, 980-8579, Japan

The Central Asian Orogenic belt (CAOB) is a long-lived orogenic belt (Neoproterozoic-Permian) and the best place to understand the Paleo-subduction system. However, the central part of the Paleo-ocean suture is unknown because of a lack of detailed studies. Grt-amphibolite located in Zavkhan terrane in western Mongolia representing a central segment of CAOB. Zavkhan terrane is a Proterozoic cratonic fragment and divided into three subzones, which are the Urgamal, Hutul, and Khungui zones from west to east. The study area is situated south of the Khungui zone and composed of gneiss, quartzite, and Grt-amphibolite of Buduun metamorphic formation.

We present a new occurrence of eclogite in the Khungui zone, Zavkhan terrane, western Mongolia, and report the preliminary results of petrological and geochemical analyses of Grt-amphibolite (originally eclogite).

The Grt-amphibolite shows schistosity texture and has Amp, Grt, Ep, Qz, Omp, Ab, Ph, Ilm, Rt, Ttn, with accessory carbonate mineral, and Zr, indicating the eclogite-facies metamorphism. Bulk composition of Grt-amphibolite shows basalt-andesitic basalt, and trace-elements patterns are similar to those of Island arc basalt.

Three metamorphic stages are recognized: Stage I is characterized by inclusions in Grt core, - Amp (taramite), Ep, and Qz, whereas stage II is defined by inclusions of Omp (Xjd=0.34-0.44), Ph, Amp (barrosite) in Grt rim. This is consistent with the normal zoning of Grt from core (Alm 60.8%, Grs 28.2%, Prp 9.6 %, Sps 1.4%) to rim (Alm 57.5%, Grs 32.8%, Prp 8.9%, Sps 0.8%). At stages III, Omp was broken from the symplectite of Hb and Ab. Therefore, Stage I, II, and III are epidote-amphibolite facies, eclogite-facies, and amphibolite-facies, respectively.

Thus, the discovery of the Khungui eclogite represents a significant finding to understand the shape of the central Paleo-ocean suture zone.

Keywords: Metamorphic evolution, Eclogite, Western Mongolia, CAOB

Session No. 28

D18. Petrology, Igneous/Metamorphic
Monday, 26 October 2020: 5:30 PM-8:00 PM
Geological Society of America Abstracts with Programs. Vol 52, No. 6
doi: 10.1130/abs/2020AM-354013
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