Geochemical evolution of peraluminous plutons in southern Nova Scotia, Canada - a pegmatite-poor suite

Three southern Nova Scotia plutons crystallized rapidly at similar to 375 M a from magma containing both mantle and crustal components. Isotopic and ch emical data suggest that the crustal contribution included both lower crust al material and Cambro-Ordovician turbidites of the host Meguma Group. Desp ite local evidence of mixing and mingling of magmas, the bull, of the pluto ns evolved by assimilation and fractional crystallisation. Evolved portions of the plutons have compositions appropriate for development of rare-metal pegmatite fields, but pegmatites are relatively rare and little differenti ated. Like parental plutons, pegmatites fall into biotite + plagioclase and muscovite + potassium feldspar assemblages. The latter locally contain Mn- rich garnet + biotite, giving calculated P-T conditions of pegmatite crysta llisation of similar to 620 degrees C, 0.44 GPa under water-saturated condi tions. Host rocks at the time of emplacement experienced P-T conditions var ying from < 500 degrees to similar to 620 degrees C at 0.44 GPa. Beryl-bear ing pegmatites occur only where host rocks were below sillimanite-grade, an d pegmatites of any kind are scarce in sillimanite-grade host rocks. Rarity of pegmatites can be ascribed to a combination of insufficient amounts of F, Li, and B in the magma with rapid cooling which prevented extensive frac tionation and undercooling. Our data suggest rather that peraluminous suite s, produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks, and typified by biotite-rich, cordierite-bearing pluton s, are not favorable locales for major rare-metal-enriched pegmatite fields . Such fields appear to require relatively prolonged anatexis resulting fro m crustal thickening, or an areally extensive mafic underplate, (C) 1998 El sevier Science B.V. All rights reserved.