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/0.9–1.1GPa) 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.Hornblendite xenoliths are relatively unaltered and consist of pargasite or pargasite plus magnetite and apatite. Amphibole compositions are very homogeneous and resemble those in the granulite xenoliths. The 40Ar/39Ar plateau age is 381±2Ma. LREE-enriched patterns, low 87Sr/86Sr ratios and textural constraints suggest that these hornblendites precipitated from a magma formed from a small degree partial melt of a garnet-bearing mantle. They may be petrogenetically related to the host lamprophyres, possibly as a cumulate from an earlier flux of magma and related to the metasomatic event that affected the granulites.Whole rock Vp values calculated from modal mineralogies of the granulite xenoliths range from 6.7 to 7.5km/s, assuming equilibration at 800°C/1.0GPa. They are compatible with seismic velocity measurements of the lower crust beneath SE Belarus, which reach 7.2km/s. Xenoliths with the least modal plagioclase and to some extent the most modal clinopyroxene and garnet give the highest whole rock Vp values. Calculated Vp values for hornblendite xenoliths (∼6.75km/s at 800°C/1.0GPa) suggest that identification of hornblendite in the lower crust from Vp values alone would be difficult.