RAPAKIVI AND RELATED GRANITOIDS OF THE NAIN PLUTONIC SUITE - GEOCHEMISTRY, MINERAL ASSEMBLAGES AND FLUID EQUILIBRIA

Granitoid rocks associated with massif anorthosites in the Nain Pluton ic Suite (NPS) of central Labrador display distinctive characteristics in their mineral assemblages, mineral chemistry, and whole-rock chemi stry. The principal granitoid plutons of NPS, despite wide areal dispe rsion, are remarkably similar in major- and trace-element chemistry an d display relatively moderate degrees of differentiation. Estimated co nditions of late-stage crystallization ranged from about 750-degrees t o 800-degres-C, with f(O2) 1 to 3 log units below FMQ buffer at a tota l pressure of 3.5 kbar. Low fugacities of water, in the range 250 to 9 00 bars, estimated from biotite reactions, tend to reflect near-solidu s upper limits for the magmas, because petrographic evidence indicates that biotite typically crystallized late. The composition of the magm a, rich in Or relative to Ab components, seems to have resulted from r educed activities of water during partial melting of the crustal sourc es. Chemical and physical properties of magmas and minerals are used t o constrain estimates of the conditions [T, P, f(H2O), viscosity, cool ing rate] under which the partial melts initially formed and subsequen tly evolved, and the nature of source materials and crustal residues. The classic rapakivi texture of plagioclase-mantled perthite is promin ent only in the Makhavinekh pluton which, however, is not chemically o r otherwise texturally or mineralogically unique. Physical conditions and processes attending emplacement and crystallization were probably decisive factors in development of the texture, Extraction of large vo lumes of granitic partial melt from the crust left geochemically deple ted, hot residues of plagioclase - pyroxene granulite. These residues formed optimal contaminants with which to derive anorthositic magmas t hrough assimilation by contemporary proximal basic magma that supplied the heat.