Upper mantle beneath the Udokan volcanic field: Study of peridotite xenoliths in late Cenozoic basaltoids

Study was given to Cr-diopside group mantle xenoliths from Late Cenozoic basaltoids of the Udokan volcanic field located at the boundary of the Aldan Shield and Baikal-Vitim terrane. Slightly depleted lherzolites are predominant xenoliths in the central part of the field (Pliocene basanites of Lake Kuas), whereas depleted harzburgites prevail in its northern part. The composition of the Udokan peridotites suggests that they are components of the Phanerozoic oceanic mantle subducted beneath the Siberian craton rather than the Archean mantle of the Aldan Shield. Xenoliths of Lake Kuas are divided into two series: harzburgite-lherzolite and lherzolite-websterite. The latter series probably represents ancient mantle, whereas the former might have been resulted from the later interaction of peridotites with hypothetic silicate melt, which was probably accompanied by Na-amphibole metasomatism. The Kuas spinel harzburgites and dunites are characterized by higher equilibrium temperatures (1000-1050°C) and higher oxidation state (average Δlogf _O2^(QFM) = -1.0) than the other rocks. Lherzolites have T = 950-1050°C and average Δlogf_O2^(QFM) = -1.9, and websterites have T = 900-1000°C and average Δlogf_O2 ^(QFM) = -3.1. Hence, having the same geothermal gradient, the harzburgite-lherzolite association formed at a greater depth than the lherzolite-websterite one. Most of the northern Udokan peridotites are highly oxidized; peridotites of the Ingamakit and Munduzhyak Volcanoes have Δlogf_O2^(QFM) of -1 to +2. Two-pyroxene temperatures calculated for the Munduzhyak peridotites are higher (1050-1200°C) than those for the other mantle xenoliths, whereas the Ca-orthopyroxene temperatures are low for all xenoliths (800-900°C). Depleted harzburgite xenoliths of the Udokan field contain clinopyroxenes enriched in LREE and depleted in Nb, Zr, and Ti. These element patterns might be explained by the reaction of the peridotites with percolating silicate melt or fluid. The high contents of LREE and MREE reflect the final stage of the peridotite-melt interaction, when depleted peridotites reached an equilibrium with the melt (fluid).