Zoned pyrozenes and amphiboles from camptonites near Gran, Oslo region, Norway
Authors: Scott Peter W. 
Published: December 2006

Synopsis

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 (Brögger, 1894). Porphyritic varieties of camptonite 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% Cr2O3 in the most Mg-rich zones and up to 0.6% Na2O in the most Fe-rich zones. TiO2 and Al2O3 are in the ranges 0.5–3.8% and 3.0–9.2% 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 Al, and less Mg and Si in the outer zone. In ‘reversed’ zoned crystals, the slightly pinkish core contains more Fe, Ti, and Al, 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 CaTiAl2O6, CaAl2SiO6 plus CaFeSi2O6 replacing CaMgSi2O6 are responsible for the zoning.

The majority of the zoned brown amphiboles are kaersutite following the criteria of Leake (1978). Those zones with Ti < 0.5 atoms per formula unit range from titanian pargasite and titanian ferroan pargasite to titanian ferro-pargasite. These compositions are similar to kaersutites crystallizing from other camptonites and as inclusions, or as megacrysts and phenocrysts in alkali basalts and derivative rocks. A simple concentric zoning with more Fe and less Mg in the outer zones occurs; but, an oscillatory zoning pattern, with an intermediate lighter brown zone (containing more Mg and less Fe) between two concentric darker zones, is more common. Other elements do not show consistent variations between zones, but Ti generally increases with increase in Aliv.

It is postulated that the earliest pyroxene zones crystallized from an alkali basalt magma at approximately 20 km depth. Crystallization of the remainder of the pyroxene phenocrysts and the kaersutite took place at high temperatures (> 940 °C) and at probably greater depths (7–15 km) than crystallization of nearby exposed gabbroic plugs (Oslo essexites). A mechanism of alternate crystallization of pyroxene and kaersutite in response to changes in PH2O, 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.