Eclogite-Like Metagabbro of the Olkhon Terrane, West Baikal Area

Abstract

Eclogite-like rocks of the Olkhon terrane correspond to two types according to the mode of occurrence and mineral compositions, although both types developed after the same type of gabbro of a single complex. The rocks of the first type are composed of garnet, clinopyroxene, pargasite (± scapolite, ± anorthite, ± spinel) and occur as thin veins or patches in the marginal parts of several small gabbro massifs of the Tonta Zone. The garnet is of almandine–pyrope–grossular composition and contains up to 40% of the pyrope end member. The rocks of the second type have been found in the Zamogoi Island in the Maloe More Strait. The island is composed of metasomatically altered gabbro cut by numerous granite and syenite veins. Carbonate and carbonate–silicate rocks compose dismembered veins and veinlets, as well as matrix embedding variably sized fragments of metasomatized gabbro and eclogite-like rocks in the southern part of the island. Based on the mode of occurrence and the presence of calcite–anorthite symplectite in some of the rocks, we suggest that the gabbro was penetrated by carbonate fluid or fluid–melt in a tectonic zone. The zone contains veins of garnet–clinopyroxene composition (varying from monomineralic garnetite to monomineralic pyroxenite) with minor pargasite, zoisite, anorthite, scapolite, and titanite. The garnet of Zamogoi Island has a grossular–almandine composition with a minor andradite concentration (about 10%). The compositions of minerals of the Zamogoi massif differ from the compositions of minerals in Tonta Zone, and the former are much more similar to skarn minerals.