Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

Abstract

Calc-alkaline batholiths of the Archaean Minto block, northeastern INTRODUCTION Superior Province, Canada, have pyroxeneand hornblende-bearing Since Holland’s (1900) initial description of south Indian mineral assemblages inferred to have crystallized from hot, watercharnockites as orthopyroxene-bearing rocks of granitic undersaturated magmas at 2·729–2·724 Ga. A regional amcomposition, the term charnockite has been applied to phiboliteto granulite-facies tectonothermal event at 2·70 Ga rocks of widely divergent origin: granitic rocks metaresulted in mild to negligible metamorphic effects on the dominantly morphosed to the granulite facies (metamorphic chargranodioritic units. Geochemical, textural and thermobarometric nockites); and rocks whose pyroxene crystallized studies define the crystallization history in compositions ranging directly from a magma (igneous charnockites). Unmetafrom cumulate pyroxenite through quartz diorite, granodiorite, granite, morphosed, discrete plutons such as the Barrington Tops and syn-magmatic gabbroic dykes. Early magmatic assemblages batholith (Eggins & Hensen, 1987), Kleivan granite (Peinclude orthopyroxene, clinopyroxene, plagioclase, biotite, Fe–Ti tersen, 1980), Mawson charnockite (Young & Ellis, 1991; oxides and ternary feldspar, indicating crystallization from magmas Young et al., 1997; Zhao et al., 1997) and Ballachulish containing 100°C than at present, producing large volumes of distinguish igneous and metamorphic origins of pyroxenewater-deficient magma. bearing granitoids. Field observations such as dykes of