Quartz chemistry in polygeneration Sveconorwegian pegmatites, Froland, Norway

Abstract

Element concentrations in quartz, feldspar and biotite of Sveconorwegian (1.13–0.9 Ga) granitic pegmatites in Froland, Norway, were analysed by LA-ICP-MS, EPMA and XRF, respectively, in order to determine chemical variations between different pegmatite types and within individual pegmatitic bodies. A refined classification of the syn-, late- and post-orogenic granitic pegmatites of Froland is presented basing on the pegmatite structure, bulk composition and mineral chemistry. Syn-orogenic pegmatites (1.13–1.06 Ga) are relative primitive with respect to granite differentiation. Late-orogenic pegmatites linked to the Herefoss pluton (0.93 Ga) have the most primitive composition and contain Fe phlogopite. Post-orogenic zinnwaldite pegmatites (< 0.93 Ga) are the most evolved. Pegmatitic quartz has an astonishingly consistent trace element signature between and within syn-orogenic pegmatites. Average concentrations are in the range of 6–10 μgg−1 for Li, 34–44 μgg−1 for Al, 4–8 μgg−1 for Ti, and 0.9–1.8 μgg−1 for Ge. Al, Li, Fe, Ge, and Ti in quartz of late- and post-orogenic and contact-metamorphosed syn-orogenic pegmatites are more variable. Micro-mylonitisation and contact metamorphism caused the lowering of Li and Al and the increase of Ti and Ge in pegmatitic quartz of some syn-orogenic granites. Several generations of secondary quartz replaced pegmatitic quartz at the micro scale (< 1 mm) during retrograde fluid-driven overprint. Secondary quartz is depleted in Al, Ti and Li compared to the host quartz. In contrast to quartz, the feldspar and biotite chemistry depends largely on the differentiation degree of the pegmatites and varies significantly within structurally-zoned pegmatite bodies. Feldspar and biotite chemistry reflects changes in melt composition within pegmatites, which includes a decrease of Mg and Sr and increase of Li, Rb, and Ba. The syn-orogenic pegmatites were formed during the crustal accretion on the western margin of Fennoscandia under constant PTX -conditions causing the homogeneous trace element signature of quartz.