“Black” Pyroxenites in Mantle Xenoliths Found in Volcanics of Some Regions in the East Asian Margin. The Evolution and Petrogenesis. Part 1. The Mineralogical Compositions, Conditions of Generation

This paper reports a study of the mineralogical composition with accompanying classification of the “black” suite in Pliocene–Quaternary volcanics sampled in SE Vietnam, Valovayam R. area (the southern Koryak Upland), and lava sheets in Bakening Volcano (central Kamchatka). Apart from monomineral varieties, we have identified pyroxenites that contain garnet, orthopyroxene, and olivine. The xenoliths of this suite might have been produced by melting of heads of mantle plumes as they were rising toward the lower lithosphere: the temperature regime of their generation is 100–150°C higher than that of the xenoliths in the hypothetical lithosphere, while the pressure is proper to the transition from the garnet facies to that of spinel lherzolites. Considering the evolutionary series of monomineral pyroxenites to websterite to olivine pyroxenites, one can see a directed variation in the composition of the clinopyroxenes due to their incongruous melting as the melts were rising toward the ground surface. The process involves the formation of secondary orthopyroxene and a melt where “daughter” phases were generated: spinellids, olivine, clinopyroxene, and plagioclase. The compositions of “black pyroxenite” clinopyroxenes as a possible relict phase are occasionally observed among a similar set of minerals in intraplate volcanics, which may be viewed as providing evidence of a genetic affinity between this type of xenolith and the volcanics.