Geochemistry of mafic and ultramafic xenoliths from Fidra (Southern Uplands, Scotland): implications for lithospheric processes in Permo-Carboniferous times

Abstract

Several types of xenoliths occur in a Permian basanite sill in Fidra, eastern central Scotland. One group consists of spinel lherzolites, which have geochemical and isotopic characteristics similar to those of lithospheric upper mantle from elsewhere in western Europe, with both LREE-depleted and LREE-enriched compositions. A separate group comprises pyroxenites and wehrlites, some of which contain plagioclase; these have compositions and textures that indicate that they are cumulates from mafic magmas. In terms of Sr and Nd isotope compositions, the pyroxenites closely resemble the host basanite and most likely formed by high-pressure fractionation of Permo-Carboniferous alkaline magmas at lower crustal depths. They also have mantle-like δ 18O values. A third group is composed of granulite xenoliths that vary between plagioclase-rich and clinopyroxene-rich compositions, some of which probably form a continuum with the pyroxenites and wehrlites. They are all LREE-enriched and most have positive Eu anomalies; thus, they are also mostly cumulates from mafic magmas. Many of the granulites also have Sr and Nd radiogenic isotope ratios similar to those of the host basanite, indicating that they have formed from a similar magma. However, several of the granulites show more enriched isotopic compositions, including higher δ 18O values, trending towards an older crustal component. Thus, the pyroxenites and granulites are largely cogenetic and are mainly the product of a mafic underplating event that occurred during the widespread magmatism in central Scotland during Permo-Carboniferous times.