Mantle-derived lherzolite xenoliths and megacrysts from Permo-Triassic dykes,Sunnhordland, western Norway

Permo-Triassic alkaline dykes in the Sunnhordland region contain angular inclusions of spinellherzolite composed of olivine (Fo_89.5), high-alumina pyroxenes, and Al₂O₃-rich spinel. Mineral chemistry suggests that the lherzolites represent mantle material which equilibrated at temperatures in the ranges 1050 to 1150°C, compatible with an oceanic geotherm. Megacrysts of augite and spinel are associated with the xenoliths, and have compositions which differ from analogous phases occurring both as phenocrysts and as xenolith constituents. Systematic chemical variations in the clinopyroxene megacrysts may reflect magmatic fractionation processes. Megacrysts may represent disaggregated parts of coarsely crystalline polymineralic assemblages precipitated at high pressures, and temperatures above that indicated by the xenoliths, with subsequent exsolution at temperatures of about 1060°C.