Garnet-biotite thermometry investigations were carried out at migmatic garnet-biotite gneisses from the Priolchonje area at the western shore of Lake Baikal to gain insight into thermal conditions during migmatization.
The Baikal area marks the suture zone between the Siberian platform in the west and several (micro)continents in the east and south, such as the the Barguzin and Tuva-Mongolia microcontinents (Vernikovsky et al. 2004) and the Mongolia-North China continent (Zorin 1999). Consequently, the region was affected by orogenic processes probably from the Archean to the Mesozoic. The investigated gneisses belong to the Mukhor suite of the Olkhon series which experienced amphibolite to granulite facies metamorphism (Letnikov et al. 1984, Kuklej 1985) of unknown age; age estimates reach from the Archean to the Early Paleozoic (Fedorovsky 1995, Fedorovsky et al. 1995, Makrygina et al. 2000). The Olkhon series is variegated in composition and encompasses quarzites, paragneisses, amphibolites, marbles and calcsilicate rocks which are arranged in NE-SW trending folds (Kozhevnikov and Tezkan 1998). The Mukhor suite itself consists mainly of amphibolites accompanied by garnet-biotite gneisses and marbles. According to Petrova and Makrygina (1994) the educts of the garnet-biotite gneisses are terrigeneous graywackes.
The investigated garnet-biotite gneisses contain two types of garnet: 2000-4000 μm in diameter, inclusion-rich crystals and 500-1000 μm big, inclusion-free crystals. Both types of garnet are almandine rich (almandine component: 0.69 to 0.75) with broad isochemical plateaus in the garnet cores which are slightly enriched in pyrope and slightly depleted in spessartine relative to the crystal rims. This compositional pattern is indicative for diffusional zoning as it is typical for garnets which have experienced temperatures above 600°C (Waters 2007). Nevertheless, the bigger garnet crystals have higher Mg-contents within their cores than the smaller ones. Consequently, temperatures derived from garnet-biotite thermometry (Perchuk and Lavrent'eva 1983) depend also on the size of the individual garnet crystals. For temperature calculation microprobe measurements from garnet cores and rims in combination wit matrix biotite were used. The largest garnet crystal (3690 μm diameter) yielded temperatures between 716-741°C (core) and 652-690°C (rim), while the smallest garnet crystal (550 μm diameter) gave temperatures of 662-670°C (core) and 640-660°C (rim). The crystal size dependency of the pyrope content of the investigated garnets points to a continous and relatively slow cooling of the garnet-biotite gneisses from the Mukhor suite (Röbl et al. 2007), possibly in response to slow exhumation of the Olkhon series.