Petrogenesis of albite-rich mid- to late Proterozoic tephra-fall deposits ('brown beds')

摘要

The discovery of volcanogenic 'brown beds' in Dalradian and Torridonian (Sleat Group and Diabaig Group) sequences in Scotland has raised questions about their origin and provenance. New discoveries of a grey facies cognate to the 'brown beds' have led to an hypothesis which proposes that these beds originated as tephra-fall deposits of intermediate to mafic composition. Subsequent prehnite-pumpellyite- and greenschist-facies metamorphism generated an albite-chlorite-muscovite-quartz-calcite assemblage. Recent sub-aerial weathering selectively dissolved interstitial calcite and oxidized iron, which left an albite-rich 'brown bed' with a porous saccharoidal texture. Field relationships and mineralogy support their origin as tephra-fall deposits and their widespread occurrence suggests they are not localized phenomena. Crucially, the weathered 'brown beds' point to the existence of otherwise cryptic grey metamorphosed tuffs which tend to blend in with their host metasediments.