Continental arc-derived eclogite in the Zavkhan Terrane, western Mongolia: Implications for the suture zone in the northern part of the Central Asian Orogenic Belt

Abstract

We report here our investigations into the petrology and geochemistry of the Khungui eclogites and metamorphosed continental crust in the Central Asian Orogenic Belt (CAOB), Western Mongolia. The eclogites consist mainly of garnet, omphacite, amphibole, Ti-bearing minerals, phengite, quartz, epidote, and plagioclase. Two types of garnet occur: Grt1 in aggregates > 500 µm and discrete Grt2 crystals > 100 µm in size. The omphacite occurs in the matrix and as inclusions in the rims of the garnets and has a jadeite content (Xjd) ranging from 0.32 to 0.44. The phengite also occurs both in the matrix and as inclusions in the garnet rims, and its Si content is homogeneous. Amphibole in the matrix shows compositional zoning from barroisite to taramite. The mineral assemblage of the eclogite-metamorphism is found in the garnet rims and includes omphacite, rutile, phengite, epidote, and barroisite. Geothermobarometry indicates eclogite P–T conditions of 2.1–2.2 GPa and 580–610 °C. In contrast, the matrix mineral assemblage of taramite, epidote, and symplectite (hornblende + plagioclase) was formed during decompression at T = 575–635 °C and P = 0.1–0.5 GPa. Interestingly, the Khungui eclogite shows a geochemical signature of continental arc basalt (Nb/La vs La/Yb ratios), which contrasts with the MORB compositions of most other eclogite terranes in the CAOB. The geochemical and petrological features of the Khungui eclogite indicate that the collision of the continental and island arcs causes Precambrian continental crust thickening in the CAOB.