Petrology of Precambrian metaultramafites of the Gridino high-pressure complex (Karelia)

Along with eclogitized gabbro and gabbronorite bodies, boudinaged metaultramafites such as garnet-pyroxene rocks and orthopyroxenites were revealed in the Archean plagiogneiss strata of the Gridino complex. The garnet-pyroxene rock crope out as a boudin on Vysokii Island. The early stage of the rock evolution is documented by inclusions of diabantite (Fe-Si chlorite), a mineral that occurs in metasomatized peridotites. Diabantite was found in all rock-forming minerals in paragenesis with mineral phases enriched in REE (Ce, Nd, La, etc.), U, and Th. The confinement of ore phases to the inclusion rims and the development of two systems of cracks, radial and concentric, around the inclusions in pyroxenes point to the transformation of the inclusions after their trapping. Thermobarometric studies of the crystal cores revealed that the anhydrous paragenesis garnet + orthopyroxene + clinopyroxene, which replaced the chlorite-bearing rock, formed at ~690 ºC and ~17 kbar. The rims of the rock-forming minerals reflect isothermal decompression to ~12 kbar, which was followed by decompression cooling to ~650 ºC and ~9 kbar with the formation of regressive amphibole-garnet-pyroxene paragenesis. The giant-grained orthopyroxenites compose chains of boudinaged bodies on Izbnaya Luda Island. The orthopyroxene crystals host abundant amphibole, quartz, biotite, and pyrite inclusions pointing to amphibolite metamorphism at the early stage of the rock evolution. There are two types of amphibole: magnesian hornblende and anthophyllite. The hornblende is a primary mineral, whereas the low-temperature anthophyllite forming rims around the quartz inclusions was produced at the regressive stage of metamorphism. There are no indicators of the PT-conditions of the peak metamorphism in the orthopyroxenite. The reaction enstatite + quartz + H2O = anthophyllite allows deciphering water activity of anthophyllite formation, a ≤ 0.5.