14:15 〜 14:30
[SMP25-15] Newly discovered eclogite in the Khungui zone, Zavkhan terrane, Western Mongolia: P-T evolution and tectonic implication
キーワード：Eclogite, Metamorphic evolution , the Khungui zone, the CAOB
The Central Asian Orogenic belt (CAOB) is a significant site to understand the Paleo-subduction system (Neoproterozoic-Permian). The CAOB can be divided into southern and northern parts based on the distribution of ultrahigh-pressure and high-pressure rocks. Although the Paleo-subduction system in the south part of the CAOB has been studied in detail, the Paleo-subduction system in the northern part of the CAOB is not fully understood. A few details study has an occurrence of HP rock in the northern part of CAOB such as Chandman (Štípská et al., 2010), Gorny-Altai (Ota et al., 2007), and Kokecthav (Y et al., 2001). In this study, we present the P-T condition evolution, tectonic implication, and garnet micro-texture of the newly discovered Khungui eclogite from the Zavkhan terrane in western Mongolia, the northern part of the CAOB, and discuss its significance to the northern part of the CAOB. The geological character and metamorphic age of Khungui eclogite described by Naemure et al. (2020).
The seven eclogites collected from the Khungui zone, the Zavkhan terrane, western Mongolia were examined in detail. The geochemical signatures of eclogites manifest basaltic composition with a signature of continental arc basalt (Nb/La vs. La/Yb ratios). The eclogites are composed of garnet, omphacite, amphibole, Ti-rich phases (rutile, ilmenite, and sphene), phengite, quartz, epidote, plagioclase, and a small amount of zircon, chlorite, and feldspar. According to the modal abundance of minerals, the eclogites show two evident layers: Ti-rich phase-dominant and epidote-dominant layers. In Ti-rich phase-dominant layers, garnet occurs as the aggregates (>500 µm) composed of a large number of individual garnet grains (10-250 µm in size and xenomorphic shape), which are separated by high angle boundaries. In epidote-dominant layers, garnet appears as a euhedral crystal (>100 µm). In the Ti-rich phase-dominant layers, the individual garnet boundary is not controlled by compositional zoning. The garnet aggregate rim (bordered with the Ti-rich phase) is enriched in Mn. Garnet aggregate shows zoning core to rim: Alm0.62-0.57, Grs0.30-0.31, Prp0.06-0.11, Sps0.02-0.01 in the Ti-rich phase-dominant layers. The single euhedral garnet shows zoning core to rim: Alm0.71-0.59, Grs0.10-0.31, Prp0.16-0.09, Sps0.03-0.01 in the epidote-dominant layers. Jadeite content of the omphacite varies Xjd=0.32-0.38 in the matrix of the layers and Xjd=0.34-0.44 in the rim of garnets. The Si content of phengite is homogenous (3.23 - 3.49 a.p.f.u.) in a matrix of the layers and rim of garnets. Amphibole occurs as barroisite to taramite (core to rim) in the matrix of layers, hornblende in symplectite, and barroisite coexisted with the rim of garnets. Mineralogy of the eclogite suggests pre-peak (garnet core), peak (garnet rim), and retrograde (symplectite of hornblende + plagioclase) stages. The peak assemblage – garnet rim, omphacite, rutile, phengite, epidote, and barroisite, corresponds to T=445-610 °C and P=20-21 kbar. Taramite (barroisite rim), and the symplectite are interpreted as retrograde stages formed at T=541-615 °C and P=5-15 kbar.
The compositional and textural differences of garnets in the two layers suggest the aggregate formation of small garnets and heterogeneity of compositional zoning could be related to Ti- and Si-rich fluid. Garnet aggregate texture is not commonly observed in other northern parts of HP rocks of CAOB. Pressure-Temperature path based on mineralogy of the eclogites indicates clockwise metamorphism, which is similar to other HP rock in CAOB. The signature of continental arc basalt for eclogites in the Khungui zone (associated with orthogneisses) contrasts with other eclogite bodies (commonly related to meta-ophiolite or ophiolite sequences) signatures (MORB type) in the CAOB. Outstanding microtexture, P-T path, protolith, and the surrounding rock of the eclogites in the Khungui zone imply the Paleo-collision type which is contrary to the eclogite bodies recording paleo-subduction system in the northern part of the CAOB.
The seven eclogites collected from the Khungui zone, the Zavkhan terrane, western Mongolia were examined in detail. The geochemical signatures of eclogites manifest basaltic composition with a signature of continental arc basalt (Nb/La vs. La/Yb ratios). The eclogites are composed of garnet, omphacite, amphibole, Ti-rich phases (rutile, ilmenite, and sphene), phengite, quartz, epidote, plagioclase, and a small amount of zircon, chlorite, and feldspar. According to the modal abundance of minerals, the eclogites show two evident layers: Ti-rich phase-dominant and epidote-dominant layers. In Ti-rich phase-dominant layers, garnet occurs as the aggregates (>500 µm) composed of a large number of individual garnet grains (10-250 µm in size and xenomorphic shape), which are separated by high angle boundaries. In epidote-dominant layers, garnet appears as a euhedral crystal (>100 µm). In the Ti-rich phase-dominant layers, the individual garnet boundary is not controlled by compositional zoning. The garnet aggregate rim (bordered with the Ti-rich phase) is enriched in Mn. Garnet aggregate shows zoning core to rim: Alm0.62-0.57, Grs0.30-0.31, Prp0.06-0.11, Sps0.02-0.01 in the Ti-rich phase-dominant layers. The single euhedral garnet shows zoning core to rim: Alm0.71-0.59, Grs0.10-0.31, Prp0.16-0.09, Sps0.03-0.01 in the epidote-dominant layers. Jadeite content of the omphacite varies Xjd=0.32-0.38 in the matrix of the layers and Xjd=0.34-0.44 in the rim of garnets. The Si content of phengite is homogenous (3.23 - 3.49 a.p.f.u.) in a matrix of the layers and rim of garnets. Amphibole occurs as barroisite to taramite (core to rim) in the matrix of layers, hornblende in symplectite, and barroisite coexisted with the rim of garnets. Mineralogy of the eclogite suggests pre-peak (garnet core), peak (garnet rim), and retrograde (symplectite of hornblende + plagioclase) stages. The peak assemblage – garnet rim, omphacite, rutile, phengite, epidote, and barroisite, corresponds to T=445-610 °C and P=20-21 kbar. Taramite (barroisite rim), and the symplectite are interpreted as retrograde stages formed at T=541-615 °C and P=5-15 kbar.
The compositional and textural differences of garnets in the two layers suggest the aggregate formation of small garnets and heterogeneity of compositional zoning could be related to Ti- and Si-rich fluid. Garnet aggregate texture is not commonly observed in other northern parts of HP rocks of CAOB. Pressure-Temperature path based on mineralogy of the eclogites indicates clockwise metamorphism, which is similar to other HP rock in CAOB. The signature of continental arc basalt for eclogites in the Khungui zone (associated with orthogneisses) contrasts with other eclogite bodies (commonly related to meta-ophiolite or ophiolite sequences) signatures (MORB type) in the CAOB. Outstanding microtexture, P-T path, protolith, and the surrounding rock of the eclogites in the Khungui zone imply the Paleo-collision type which is contrary to the eclogite bodies recording paleo-subduction system in the northern part of the CAOB.