Garnet pyroxenite xenoliths and pyropic megacrysts in Scottish alkali basalts

Abstract

Synopsis Pyropic garnet occurs within garnet pyroxenite xenoliths and as discrete crystals in Carboniferous–Permian alkali basaltic tuffs and hypabyssal intrusions. Eight localities are now known from the Midland Valley together with another in the NW Highlands. The xenoliths are inferred to have been derived from minor pyroxenitic intercalations in spinel lherzolite within the lithospheric mantle. Although pyrope crystals at Elie Ness are regarded as high-pressure phenocrysts cognate with the basanitic host, it is indeterminable whether discrete garnets at the other localities are phenocrysts or xenocrystal fragments derived from garnet pyroxenites. All, however, whether phenocrystic, xenocrystic or integral components of pyroxenitic xenoliths, are regarded as cognate in the sense of being products of high-pressure crystallization of late Palaeozoic basanite magmas. Kaersutite, phlogopite, hercynitic spinel, ilmenite and pseudomorphs after orthopyroxene occur as minor components in some of the garnet pyroxenite xenoliths. Mineral compositional data (electron- and ion-microprobe) are presented, with comparisons to xenolith-megacryst suites elsewhere. The garnets have strong similarities with each other and with megacryst populations from alkali basalts outside the U.K. It is concluded that the pyroxenites originally crystallized from basaltic magmas as high-pressure cumulates from depths of >50 km and represent some of the deepest samples from the Scottish lithosphere.