MINERALOGY OF MANTLE XENOLITHS FROM THE PLIOCENE BASANITES OF THE DZHILINDA RIVER (VITIM VOLCANIC FIELD)

Abstract

Abundant mantle xenoliths found in Pliocene basanites from the Dzhilinda River are divided into three groups. These are: (1) coarse-grained protogranular spinel and garnet spinel Iherzolites (two-pyroxene temperature T = 1100-1250 degreesC; orthopyroxene-garnet pressure P = 18-23 kbar); (2) spinel Iherzolites and harzburgites of protogranular (2a) to tabular equigranular (2b) microstructures (T = 800-900 degreesC; P = 10-15 kbar); (3) mosaic equigranular Iherzolites (T = 710-750 degreesC). Peridotites of groups 1 and 2a are primitive or slightly depleted mantle substrata of two depth levels corresponding to garnet-spinel transitional layer and spinel stability field. The 2b peridotites and related low-Al websterites result from low-temperature depleted mantle-basaltic melt interaction under oxidizing conditions (close to QFM buffer). Peridotites of group 3 are enriched in modal pyroxene and spinel. They characterize a unigue type of mantle-melt interaction resulting from percolation of Ti-Zn-bearing melt through extremely depleted peridotites. Minerals of these peridotites have a specific chemical composition. Clinopyroxenes contain 2-2.2% TiO(2), olivines 0.7% NiO, and spinel 0.6-0.7% NiO and 0.9-1.0% ZnO. Cr-diopside pyroxenites and peridotites from the Dzhilinda are subdivided into two groups: high-temperature (T = 1000-1230 degreesC) and low-temperature (T = 860-1000 degreesC). They are considered the products of interaction between basaltic melts and primitive peridotites at two depth levels - 60-80 and about 50 km. Al-augite pyroxenites are represented by spinel websterite and are compositionally similar to low-temperature Cr-diopside pyroxenites. The Al-augite pyroxenites make up the uppermost mantle or lower crust at a depth of 35-50 km. Clinopyroxene megacrysts are rich in Mg and are similar to Mg-rich garnet pyroxenites from Miocene picrite-basalts, Pressure estimates suggest that they form at a depth of 60-80 km. The skarn-like clinopyroxenites, most likely, form from volatile-rich low-Al melts in the middle-lower crust. The estimations based on fluid inclusions show that pressures were no lower than 5-6 kbar.