Petrography and geochemistry of the topaz-bearing granite stocks in Artjärvi and Sääskjärvi, western margin of the Wiborg rapakivi granite batholith

Abstract

LUKKARI, SARI 2002. Petrography and geochemistry of the topaz-bearing granite stocks in Artjarvi and Saaskjarvi, western margin of the Wiborg rapakivi granite batholith. Bulletin of the Geological Society of Finland 74, Parts 1–2, 115– 132. The Artjarvi and Saaskjarvi granite stocks at the western margin of the Wiborg rapakivi batholith are multiphase rapakivi granite intrusions in which the most evolved phase is topaz-bearing granite. The Artjarvi stock is composed of porphyritic and even-grained biotite granite and even-grained topaz granite, and the Saaskjarvi stock comprises even-grained biotite granite and porphyritic topaz granite. The granites are metaluminous to peraluminous A-type granites, showing within-plate (WPG) geochemical characteristics. The topaz granites from the Artjarvi and Saaskjarvi stocks are petrographically and geochemically similar to other topaz-bearing rapakivi granites in Finland. The anomalous geochemistry of the topaz granite is essentially magmatic; postmagmatic reactions have only slightly modified its composition. Greisen veins, some of which are mineralized, are widely found associated with the Artjarvi and Saaskjarvi stocks. The most characteristic feature of the Artjarvi granite stock is a stockscheider at the roof contact of the topaz granite. The stockscheider is composed of schlieren layering and pegmatite layers parallel to the contact. The most probable mechanism for the formation of the schlieren layering is velocity-gradient sorting parallel to the flow, which led to accumulation of mafic minerals along the upper contact of the topaz granite. Cooling and contraction of the topaz granite formed fractures parallel to the roof contact and the residual pegmatite magmas were injected along the fractures forming pegmatite layers. Textures like graphic intergrowths, aplitic groundmass and fan-shaped alkali feldspar crystals associated with the Artjarvi porphyritic biotite granite and pegmatite indicate crystallization from undercooled magma. Miarolitic cavities found in the porphyritic biotite granite and topaz granite indicate at least local volatile saturation.