Petrology and pyroxene chemistry of Monteregian dykes: the origin of concentric zoning and green cores in clinopyroxenes from alkali basalts and lamprophyres

Abstract

The Mesozoic Monteregian alkaline province of southern Quebec includes mafic alnöite, monchiquite, basanite, camptonite, and alkali basalt dykes. Most carry phenocrysts of clinopyroxene that generally zone towards Ti–AlIV–Fe–Mn-rich and Mg–AlVI–Cr-poor rims. The zoning can best be explained through polybaric crystallization and differentiation during ascent from the upper mantle. In intermediate and leucocratic dykes, clinopyroxene AlIV–Ti contents decrease with the Mg/(Mg + ΣFe) ratio, probably reflecting concurrent fractionation of a Ti-rich phase. Pyroxene phenocrysts in Monteregian mafic dykes commonly have green clinopyroxene cores that are richer in Na and Fe and poorer in Mg and Cr than the enclosing titansalite phenocrysts. Some cores are euhedral and sector zoned, implying crystallization from a melt more evolved than their present hosts. The high AlVI contents of these cores imply high pressures of crystallization. The abundance of crustal xenoliths and evolved pyroxene cores indicates that the host magmas hybridized with felsic melts, cumulates, or metasomatites within the crust or an anomalously Fe–Na-rich upper mantle. This implies that the host dykes are not primary magmas but hybrids. Consequently, dyke chemistry cannot simply be inverted to determine the composition and mineralogy of the mantle source.