Comparative Amphibole Chemistry of the Monteregian and White Mountain Alkaline Suites, and the Origin of Amphibole Megacrysts in Alkali Basalts and Lamprophyres*

Jean H. Bédard
Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, U.K.
Present address: Geological Survey of Canada, 601 Booth St., Ottawa, Ontario, Canada K1A 0E8.
*Earth Sciences Contribution 981.

Abstract: Amphiboles in dykes from the dominantly silica-undersaturated Monteregian series range from pargasitic megacrysts and xenocrysts to kaersutitic, pargasitic and hastingsitic phenocrysts, groundmass prisms and reaction rims. Amphiboles in dykes and plutons from the silica-oversaturated White Mountain Magma Series range from kaersutite, through hornblende, hastingsite and edenite, to sodic-calcic and sodic varieties. This contrast between the amphiboles from the two series is probably a reflection of differing melt silica activity and is a useful petrologic discriminant. In most cases, pargasitic amphibole megacrysts from Monteregian monchiquites reflect the Mg numbers of their host rocks and are considered cognate. The megacrysts are lower in Ti and higher in Mg and Al than their phenocryst mantles and rims. This is probably a result of higher pressures of formation.

Keywords: amphiboles • basalts • lamprophyres • Monteregian Province • White Mountain • Quebec • New England

Mineralogical Magazine; March 1988 v. 52; no. 364; p. 91-103; DOI: 10.1180/minmag.1988.052.364.08
© 1988, The Mineralogical Society
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