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

Several types of xenoliths occur in a Permian basanite sill in Fidra, easte rn central Scotland. One group consists of spinel lherzolites, which have g eochemical and isotopic characteristics similar to those of lithospheric up per mantle from elsewhere in western Europe, with both LREE-depleted and LR EE-enriched compositions. A separate group comprises pyroxenites and wehrli tes, some of which contain plagioclase; these have compositions and texture s that indicate that they are cumulates from mafic magmas. In terms of Sr a nd Nd isotope compositions, the pyroxenites closely resemble the host basan ite and most likely formed by high-pressure fractionation of Permo-Carbonif erous alkaline magmas at lower crustal depths. They also have mantle-like d elta O-18 values. A third group is composed of granulite xenoliths that var y between plagioclase-rich and clinopyroxene-rich compos it ions, some of w hich 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 an d Nd radiogenic isotope ratios similar to those of the host basanite, indic ating that they have formed from a similar magma. However, several of the g ranulites show more enriched isotopic compositions, including higher delta O-18 values, trending towards an older crustal component. Thus, the pyroxen ites and granulites are largely cogenetic and are mainly the product of a m afic underplating event that occurred during the widespread magmatism in ce ntral Scotland during Perino-Carboniferous times. (C) 2001 Elsevier Science BN. All rights reserved.