REE distributions in a high-grade Archaean gneiss complex in Scotland: Implications for the genesis of ancient sialic crust

Abstract

Seventeen rocks from the Lewisian Gneiss of the Inner Hebrides of Scotland, which represent three distinct lithological types at granulite to greenschist facies of metamorphism show rare-earth element patterns which seem not to have been disturbed by their complex metamorphic history. Some indication of their origin can be obtained by simple geochemical models. Three tonalitic pyroxene gneisses are characterized by: (1) light REE enrichment and heavy REE depletion; (2) low total REE contents; (3) moderate Eu enrichment. Their REE chemistry can be approximated by a model involving 10% partial melting of various garnet-bearing basaltic source materials. Alternatively, they may be some form of crystal cumulate, preserving their original anhydrous mineralogy, representing 30% crystallization of a parent tonalitic magma. Three tonalitic to granodioritic hornblende-biotite gneisses are characterized by: (1) light REE enrichment and heavy REE depletion; (2) significantly higher total REE contents than the pyroxene gneisses; (3) moderate Eu depletion. Their REE patterns can be approximated by a residual silicic melt in a model involving 30% fractional crystallization of solids with the modal mineralogy of the pyroxene gneisses or 40% removal of pure anorthosite from a parent dacitic magma. Two strongly metasomatised diopside-actinolite gneisses and one highly sheared epidote-chlorite gneiss have REE patterns which are not significantly different from the hornblende-biotite gneisses which were their precursors before metasomatism and late greenschist-facies shearing. This suggests that strong alteration has not enciphered the REE systematics of the gneisses. Basic gneisses of quartz tholeiite composition occurring as early dykes, which shared the same metamorphic history as the tonalitic to granodioritic gneisses, are characterised by: (1) slight enrichment in light REE relative to heavy REE; (2) variable total REE contents; (3) little difference between granulite and amphibolite facies types. Their REE patterns can be matched by models involving 5–15% partial melting of ultrabasic mantle with 3 times chondritic REE abundances, leaving a residue of olivine and orthopyroxene.