[SMP38-P02] Field evidence and mineral assemblage of eclogite from the Khungui zone, Zavkhan terrane, Western Mongolia
Keywords:Garnet, Eclogite, Buduun formation, Central Asian Orogenic Belt
Zavkhan terrane in western Mongolia represents a northwest 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. Khungui zone is dominated by gneiss, quartzite, and amphibolite of Paleoproterozoic Buduun metamorphic formation, the basement of Zavkhan cratonal terrane.
We focus on 5 garnet amphibolites collected from Buduun formation of Khungui zone, Zavkhan terrane. Bulk composition of amphibolites shows on average 50.7 % SiO2, 9.3% CaO, 2.5% Na2O and 0.84% K2O, trace-elements patterns are similar to those of N-MORB, except for the element of Th. All amphibolites shows schistosity and have garnet (Grt), amphibole (Amp), quartz (q), clinopyroxene (Cpx), rutile, epidote (epi), plagioclase (Pl), muscovite (mus). It is noted that omphacite occurs as Grt inclusions in the most of samples.
Grts occur as two types: euhedral grain and aggregate. Compositionally, garnets can be divided into three types: garnet 1 (Grt1), garnet 2 (Grt2), and transitional garnet (TGrt). Grt1 is observed as cluster (400 µm) of garnet grains but its composition shows one garnet grain. Grt1 contains few inclusions (omphacite and q) and has composition zoning from core (Alm60.8Grs28.2Prp9.6Sps 1.4) to rim (Alm57.53Grs32.75Prp8.89Sps0.82). Omphacite is observed in rim of Grt1. Grt2 (190 µm) contains numerous inclusions (core: q, amphibole, epi, and rarely mus, and rim: omphacite) and has composition zoning from core (Alm51.3Grs34.17Sps8.62, Prp5.84) to rim (Alm57.0Grs 35.91Prp5.69Sps1.32). TGrt1 (40-210 µm) has very rare inclusions (q and rarely amp) and homogenous composition (Alm 59.26Grs 32.07Prp 7.43Sps1.22). Cpx (omphacite) occurs in matrix (Xjd=0.32-0.38) and as inclusion (Xjd=0.34-0.44) of Grt rim (Grt1 and Grt 2). Cpx is replaced by Pl, amp, and q forming symplectite. Calcic to sub calcic amp occur in matrix and as inclusions of garnets. In matrix, amp has compositional zoning from barroisite at core to hornblende at rim whereas amp in grt has hornblendic composition.
The Grt amphibolites from Buduun formation, Khungui zone, Zavkhan terrane were suffered from eclogite facies metamorphism. Based on minerology and mineral chemistry, 3 metamorphic stages can be determined: pre-Peak (M1), Peak (M2), and retrograde (M3). M1 is marked by inclusions (Amp (calcium), Epi, Mus, and q) in Grt2 core. M2 is represented by omphacite (in Grt1 and Grt2 rims) and composition of Grt1 (402-466 °C) and Grt2 (411-525 °C) rims. M3 (retrograde) is traced by transformation of omphacite into amp-Pl-q symplectite and hornblende. The new point of eclogite facies rocks is important to reconstruct tectonic evolution of PAO, especially from the Chandman area to northern Mongolia.