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TitlePetrology and pyroxene chemistry of Monteregian dykes: the origin of concentric zoning and green cores in clinopyroxenes from alkali basalts and lamprophyres
AuthorBédard, J H; Francis, D M; Ludden, J
SourceCanadian Journal of Earth Sciences vol. 25, no. 12, 1988 p. 2041-2058, Open Access logo Open Access
Alt SeriesGeological Survey of Canada, Contribution Series 13988
PublisherCanadian Science Publishing
Mediapaper; on-line; digital
File formatpdf
NTS21E; 31G/01; 31G/08; 31G/09; 31G/16; 31H
AreaMonteregian Hills; Oka; île Cadieux; Bizard; Mount Royal; mont St-Bruno; mont St-Hillaire; mont Rougemont; mont Yamaska; mont Shefford; mont Brome; lac Memphrémagog; mont Mégantic; White Mountains; Vermont; Barber Hill; Cuttingsville; Ascutney; New Hampshire; Pliny; Ossipee; Belknap; Pawtuckaway; Maine; New York State; Massachussetts; Canada; United States of America
Lat/Long WENS -74.2500 -70.0000 46.0000 42.6667
Subjectsigneous and metamorphic petrology; igneous rocks; intrusive rocks; dykes; mafic rocks; alnoites; monchiquites; camptonites; basalts; phenocrysts; clinopyroxene; zoning; crystallization; differentiation; pyroxene; pressure-temperature conditions; xenoliths; magmas; metasomatism; lamprophyres; petrogenesis; models; ultramafic rocks; electron probe analyses; mineralogical analyses; fractional crystallization; source rocks; Monteregian alkaline province; Monteregian magma series; White Mountain magma series; White Mountain Batholith; upper mantle; basanites; fractionation; titansalite; crust; magma hybridization; primary melts; cumulates; felsic melts; metasomatites; melt composition; polybaric crystallization; geochemical signatures; intermediate dykes; leucocratic dykes; xenocrysts; Mesozoic; Cretaceous
Illustrationsgeological sketch maps; tables; photomicrographs; plots; ternary diagrams; profiles
ProgramNSERC Natural Sciences and Engineering Research Council of Canada
ProgramFCAC - Fonds pour la Formation de Chercheurs et Action Concertée
AbstractThe Mesozoic Monteregian alkaline province of southern Quebec includes mafic alnà¶ite, 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.

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