Comparative amphibole chemistry of the Monteregian and White Mountain alkaline suites, and the origin of amphibole megacrysts in alkali basalts and lamprophyres

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 Montereglan monchiquites reflect the Mg numbers of their host rocks and are considered cognate. The megacrysts are lower in Ti and higher in Mg and AI than their phenocryst mantles and rims. This is probably a result of higher pressures of formation.

Petrogenetic significance of ocellar camptonite dykes in the Ditrău Alkaline Massif, Romania

“…%; Al 2 O 3 <13.6 wt. %), similar to the amphiboles of the Monteregian (Bédard, 1988), Tamazert (Bouabdli et al, 1988), Pyrenean (Azambré et al, 1992), Moravian-Silesian, Mecsek-Alföld (Harangi et al, 2003) and Catalonian (Ubide et al, 2012) lamprophyres. Compositional trends for kaersutites are consistent with early crystallisation from Si-undersaturated magma (Bédard, 1988;Giret et al, 1980 (Adam and Green, 1994).…”

“…The texture of the lamprophyres (Fig. 3D) indicates that after about 40-50 % crystallisation, the magmas (of camptonite-II) consisted of euhedral ferromagnesian minerals and interstitial syenomonzonitic to phonolitic melt (Bédard et al, 1988;Bédard, 1994).…”

Post-collisional lamprophyric event in Sierra Norte, Córdoba, Argentina: Mineralogical, geochemical and isotopic characteristics

“…Experimental evidence in basalts indicate that the stability field of plagioclase is reduced by increasing water pressure (Bédard, 1988); some previous contributions (e.g. Lange et al, 1993;Carmichael et al, 1996) consider that in magmas with SiO 2 concentrations close to 60 wt.%, the abundance of primary amphibole and the nearly absent phenocrystic plagioclase could be evidence of melts with high water contents.…”

“…Additional evidence of primary equilibrium crystallization of amphiboles from the melt is from computed Mg# for the amphiboles using Bédard's (1988) equation (Mg# amphibole = 0.253125 * Mg# whole rock + 0.5425). The calculated values correspond closely to actual Mg#s of the amphiboles in the studied spessartites (Table 1).…”

The camptonites in the multiple intrusion of Platja Fonda (Girona, NE Spain): mechanisms of intrusion and geochemistry

“…In addition, if ilmenite had crystallized simultaneously with kaersutite, the incorporation of Ti in the latter would have decreased. Both earlier kaersutite and ilmenite crystals would have been resorbed later, due to rapid magma ascent or volatile exsolution (Bédard 1988). The thermometer of Otten (1984) provides temperatures for the phenocryst cores very similar in all camptonites (1031 ± 7 °C).…”