Zoned Pyroxenes and Amphiboles from Camptonites Near Gran, Oslo Region, Norway

TLDR: In this paper, a mechanism of alternate crystallization of pyroxene and kaersutite in response to changes in PH~O, with some partial resorption of the non-crystallizing phase, is suggested to account for the observed petrographic features and oscillatory zoning in both minerals.

Abstract: CAMPTONITE sills of Permian age, intruded into Cambrosilurian sediments, are present in the northern part of the Oslo region, immediately to the north of Gran, Hadeland. They have been considered, along with associated dykes of camptonite and other minor intrusions of maenaite, to be related to the predominantly gabbroic rocks comprising the Oslo-essexite plugs (Br6gger, I894). Porphyritic varieties of carni)tbriite are the most common, containing phenocrysts of concentrically zoned clinopyroxene and concentrically zoned brown amphibole; but, aphyric varieties with abundant brown amphibole also occur. A second amphibole (actinolite) forms thin, patchy, green overgrowths to some phenocryst or matrix brown amphiboles. The pyroxenes range in composition from diopside to sahlite. They contain up to 0.7 % Cr203 in the most Mg-rich zones and up to 0.6 % Na20 in the most Fe-rich zones. TiO2 and Al~O3 are in the ranges 0.5-3.8 % and 3.o-9.2 % respectively. These compositions are similar to pyroxenes from other camptonites and alkali basaltic and alkali gabbroic rocks. 'Normal ' zoned phenocrysts, in which an outer pinkish zone surrounds a colourless core in thin section, have more Fe, Ti, and A1, and less Mg and Si in the outer zone. In 'reversed' zoned crystals, the slightly pinkish core contains more Fe, Ti, and A1, and less Mg and Si. Oscillatory zoning, restricted to three concentric zones (pinkish-colourless-pinkish) show the same chemical changes with colour variation. In terms of end-member molecules, substitutions involving CaTiA1206, CaAI2SiO6 plus CaFeSi206 replacing CaMgSi206 are responsible for the zoning. The majority of the zoned brown amphiboles are kaersutite following the criteria of Leake (1978). Those zones with Ti 940 ~ and at probably greater depths (7-i 5 km) than crystallization of nearby exposed gabbroic plugs (Oslo essexites). A mechanism Of alternate crystallization of pyroxene and kaersutite in response to changes in PH~O, with some partial resorption of the non-crystallizing phase, is suggested to account for the observed petrographic features and oscillatory zoning in both minerals. The actinolite is a product of late-stage metasomatic activity.