Metamorphic P–T evolution of garnet–kyanite–staurolite schist and garnet amphibolite from Bodonch area, western Mongolian Altai: Geothermobarometry and mineral equilibrium modeling

We report the first detailed petrological data from the pelitic schists and amphibolites of Bodonch area, southwestern Mongolia, which occupies a significant part of the Paleozoic history of the Altai Orogen in the southwestern margin of the Central Asian Orogenic Belt (or Altaids), and discuss pressure–temperature (P–T) evolution of the area. The dominant mineral assemblages of pelitic schist in Bodonch area are garnet + kyanite + staurolite + biotite + paragonite, garnet + biotite + staurolite + cordierite, and garnet + biotite + sillimanite with quartz, plagioclase, and ilmenite, while amphibolite contains calcic amphibole + quartz + plagioclase + garnet + ilmenite assemblage. Application of geothermobarometers and mineral equilibrium modeling on kyanite-bearing pelitic schists yielded peak P–T conditions of 640–690 °C/6.3–10.7 kbar. Slightly lower conditions were obtained from sillimanite-bearing schist (580–600 °C/3.6–4.5 kbar) and amphibolite (560–570 °C/3.0–3.5 kbar). The peak high-pressure amphibolite-facies condition and clockwise P–T evolution of Bodonch area estimated for the first time in this study is consistent with available reports of P–T conditions from other localities in the Altai Orogeny outside Mongolia. For instance, P–T conditions of 630 °C/8.7 kbar estimated for kyanite-bearing pelitic schists (garnet + biotite + staurolite + kyanite + quartz) from Xinjiang in Chinese Altai, which is about 340 km WNW from our locality along the regional structure, are comparable with our results. We suggest both Bodonch and Xinjiang areas in the Altai Orogeny underwent a similar metamorphism possibly related to regional accretionary and collisional evolution of the Central Asian Orogenic Belt.

► Phase equilibrium modeling of pelitic schists (grt+ky+st+bt+pl) yield P-T conditions of 640-690°C and 6.3-10.7 kbar.
► Post peak decompressional cooling along a clockwise P-T path.
► Contrasting P-T paths suggest discrete crustal blocks in the region.