PETROGENESIS OF A MIDPROTEROZOIC ANORTHOSITE-MANGERITE-CHARNOCKITE-GRANITE (AMCG) COMPLEX - ISOTOPIC AND CHEMICAL EVIDENCE FROM THE NAIN PLUTONIC SUITE

The Nain Plutonic Suite (NPS), about 1.30 Ga, underlies some 19,000 km(2), intruding the boundary between Middle to Late Archean rocks in the east (Nain Province) and Late Archean and Early Proterozoic rocks in the west (Churchill Province). Nd isotopic compositions in most igneous units of the NPS reflect their geographic position relative to the inferred boundary between the Nain and Churchill Provinces. Rare exceptions are local intrusions of olivine-bearing basic rocks with highest epsilon(Nd) @ 1.3 Ga (near -3.0), present in both eastern and western sectors of the NPS. Anorthositic rocks have variable and significant contributions of crustal Nd but notably lower I,, than granitoid rocks. Ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids. Aluminous orthopyroxene megacrysts in NPS anorthositic rocks indicate crystallization pressures of 6.2 to 11.0 kbar, consistent with lower crustal to upper mantle depths. The granitoid rocks crystallized from magmas derived in large part by crustal partial melting. These rocks show the influence of sources similar to the basement gneisses, but were augmented by materials with much shorter crustal residence time, offering support for contemporaneous basaltic underplating of the lower crust. Nd and Sr isotope data, trace element geochemistry, and mineral compositions in major rock units of the NPS, together with knowledge of the timing of intrusive events, permits assessment of the essential features of petrogenesis and development of a paradigm that may have wider application. Separation of early granitoid melts left depleted, hot, plagioclase-pyroxene granulite crustal residues assimilated by mantle-derived basaltic magmas. That process developed large volumes of anorthositic magma in upper levels of deep crustal to uppermost mantle magma chambers by plagioclase flotation. Concurrently, fractionation moved residual liquids toward ferrodiorite compositions. Buoyant upward movement of anorthositic magmas followed crustal paths preheated by earlier passage of granitoid magmas. Only small Volumes of high density, Fe-rich, ferrodiorite melts were expelled to higher crustal levels.