Chemical characteristics of plagioclase and pyroxene megacrysts and their significance to the petrogenesis of the Nain anorthosites

Unmetamorphosed anorthosites of the Nain Plutonic Suite in Labrador are divided into a noritic, pale facies to the west and south, and a troctolitic, dark facies to the east and north. Two large intrusions along Tikkoatokhakh Bay (TIK) are taken as representative of the noritic suite, and massif anorthosites at Kikkertavak Island (KIK) and Port Manvers Run (PMR) represent the troctolitic suite in this study. Plagioclase and pyroxene megacrysts from the TIK bodies are distinguished from those of the KIK and PMR intrusions by many element concentrations and ratios. Most TIK plagioclase megacrysts ( An 42-54 ) have lower An values than found at KIK-PMR ( An 51-61 ). Associated pyroxene megacrysts have lower X _Mg in the TIK bodies than in KIK-PMR. Continuous variations of element concentrations and ratios support fractionation within each suite, but a discontinuity between TIK and KIK-PMR argues against fractionation from one suite to another and favors different magma compositions instead. Different pyroxene megacrysts suggest that PMR (with CPX) and KIK (with OPX) crystallized from different troctolitic magmas. The plagioclase megacrysts resemble their enclosing rocks in composition and are, therefore, consanguineous. Reliable discriminants between plagioclases of noritic and troctolitic suites appear in plots of Fe-An, Fe-Ti, (Fe/Ti)-An, K-Rb, and in values of K and Sr. The dark color of the troctolitic facies plagioclase is accompanied by a higher Fe content and higher Fe/Ti, suggesting incorporation of tetrahedral ferrous iron at relatively reduced conditions. K/Rb is highly scattered between 1200 and 4000, and Ba/ K is scattered between 5 and 8. Sr/Ca correlates strongly with K in a linear array, in contrast to the Kiglapait trend, which is concave-up at low values of K. The difference between the two trends is attributed to hypersthene saturation at depth in the Nain liquids. The plagioclase REE patterns are highly enriched in LREEs, with (Ce/Yb) _N around 36 in the TIK bodies and 12-27 in the KIK-PMR samples; the least evolved pattern (from PMR) matches a plagioclase from the Lower Zone of the nearby Kiglapait Intrusion. High-alumina hypersthene megacrysts occur in the TIK intrusions but not in the KIK-PMR bodies. The presence of a chilled margin on the west edge of the TIK bodies suggests that the high alumina contents are probably due to rapid growth in place rather than decompression from great depth. The most aluminous OPX megacrysts are also enriched in both incompatible Zr and compatible Cr, but not compatible Ni, in conflicting testimony bearing on the rapid growth hypothesis. The megacryst compositions can be used to discriminate and characterize magma types that produced massif anorthosite. With adequate partition coefficients, still lacking for many important elements, they can also be used to calculate magma compositions explicitly by inversion, within limits imposed by the scatter.