Genetic relationships between metabasalts and related gabbroic rocks: an example from the Fore-Sudetic Block, SW Poland

Abstract

The metabasalts occurring within the gneisses of the eastern part of the Góry Sowie gneiss block, have either enriched or depleted LREE patterns. However, these two types of metabasic rock are indistinguishable in terms of their major elements and many trace elements. Their intimate association indicates that their parent magmas erupted nearly contemporaneously. They originated through the decompressional, two-stage incremental melting of a mantle diapir source. The LRRE enriched variety of the basalts was formed from a spinel/garnet peridotite melt mixture which was followed by spinel peridotite melts. From the latter melts, cumulate gabbros crystallized and extracted portions of these melts provided the LRRE depleted variety of basalts. This strongly suggests that the metabasalts are compatible in age with the gabbros.
Both of the metabasalts varieties developed mainly by AFC processes involving mantle source melts and lower continental crust components. From a comparison of these metabasalts with those in adjacent areas, it is possible to draw the conclusion that volcanic activity in the whole region had the same time-span.
The metabasalts were metamorphosed to LP hornblende granulites, synchronously with the surrounding amphibolite facies gneisses of the margins of the Góry Sowie Block, which were metamorphosed to LP-HT cordierite gneisses. They originated due to the transition from high-grade amphibolite to granulite facies conditions associated with a near-isothermal decompression, during the time of the late Variscan (Carboniferous).