Rf. Emslie et Jar. Stirling, RAPAKIVI AND RELATED GRANITOIDS OF THE NAIN PLUTONIC SUITE - GEOCHEMISTRY, MINERAL ASSEMBLAGES AND FLUID EQUILIBRIA, Canadian Mineralogist, 31, 1993, pp. 821-847
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.