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 lherzolites (two-pyroxene temperature T = 1100-1250°C; orthopyroxene-garnet pressure P = 18-23 kbar); (2) spinel lherzolites and harzburgites of protogranular (2a) to tabular equigranular (2b) microstructures (T = 800-900°C; P = 10-15 kbar); (3) mosaic equigranular lherzolites (T = 710-750°C). 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% TiO2, 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°C) and low-temperature (T = 860-1000°C). 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.
|Number of pages||23|
|Journal||Geologiya i Geofizika|
|Publication status||Published - 2000|
- Alkali basalt
- Upper mantle