THE LOWER CRUST OF SE BELARUS: PETROLOGICAL, GEOPHYSICAL AND GEOCHEMICAL CONSTRAINTS FROM XENOLITHS

Abstract

Highly altered alkaline lamprophyre tuffs of Devonian age from SE Belarus contain lower crustal xenoliths that vary from feldspar-rich mafic granulites to rarer feldspar-poor eclogitic granulites, together with coarse-grained hornblendites. Textural evidence, mineral compositions and T/P data (700–800°C/0.9–1.1 GPa) of the garnet granulite xenoliths indicate that equilibration occurred in the lower crust and therefore we consider that these xenoliths represent samples of the in situ lower crust. They are highly altered, but the main primary minerals are almandine garnet, diopside and andesine plagioclase. K-feldspar, scapolite, biotite, apatite, ilmenite, Ti-magnetite and rutile are accessory minerals. A subsequent metasomatic event, post-dating the granulite facies metamorphism, has introduced widespread pargasitic amphibole. The granulites are basic in composition, are enriched in the most incompatible trace elements and have large negative Nb anomalies (LaN/NbN range from 3 to 12), inferring a subduction-related origin. REE patterns show variable LREE enrichment (La/Yb=6–27) and some positive Eu anomalies, indicating plagioclase accumulation. Trace element and REE patterns for one sample suggest a melt composition. The granulites may represent a gabbroic underplate of unknown age and may be petrogenetically related to the Osnitsk–Mikashevichi plutonic belt.