Metasomatism and transformations of the upper mantle beneath the southern Baikal territory: Evidence from xenoliths of the Bartoy volcanic area

Pliocene-Pleistocene volcanic rocks of the Bartoy group of volcanoes in the southern Baikal area contain nodules of mantle peridotites, mantle and lower crustal pyroxenites, and a wide spectrum of megacrysts. The nodules are dominated by spinel lherzolites, with more than 50% of them containing amphibole and phlogopite. The Cr-diopside group of nodules is subdivided into (1a) anhydrous spinel lherzolites and harzburgites; (1b) garnet and spinel pyroxenites; and (1c) spinel lherzolites and websterites with interstitial phlogopite and amphibole. The Al-augite group is classified into (2a) spinel websterites, often with amphibole and phlogopite; (2b) garnet granulites; and (2c) garnet and spinel pyroxenites genetically related to the megacryst assemblage. The group of rocks of intermediate composition includes (3a) amphibole-phlogopite pyroxenites and amphibole-phlogopite rocks and (3b) high-Al garnet clinopyroxenites. The megacryst assemblage includes clinopyroxene, garnet, amphibole, biotite, ilmenite, titanomagnetite, and alkaline feldspar. The chemistry of minerals in the peridotites and water-bearing veins suggests two types of mantle metasomatic processes in the Bartoy area, which resulted in (1) the crystallization of interstitial amphibole and phlogopite in the lherzolites and pyroxenites and (2) the development of amphibole-phlogopite and amphibole-phlogopite-pyroxene veins in the lherzolites. The calculated P-T parameters of the Bartoy Cr-diopside pyroxenites indicate that these rocks were produced at 15-22 kbar and 1100-1200°C. The analogous values for the Al-augite pyroxenites are 10-14 kbar and 900-1200°C. The phlogopite-bearing lherzolites crystallized at higher temperatures (1050-1100°C than the anhydrous and amphibole-bearing varieties (900-1070°C). These P-T parameters are compatible with the experimentally determined stability fields of amphibole and phlogopite in lherzolites. Data on the Fe³⁺ contents in the spinel indicate that the Bartoy mantle is significantly oxidized which is consistent with widespread manifestations of hydrous metasomatism. The geotherm of the Bartoy xenoliths (nodules) is the highest temperature throughout the Baikal rift zone, which is in agreement with the idea of the southern Baikal hotspot and the maximum heating of the mantle beneath the southern Baikal area.