Petrological and geochemical evolution of the Kymi stock, a topaz granite cupola within the Wiborg rapakivi batholith, Finland

Abstract

The 6×3 km Kymi monzogranite stock represents the apical part of an epizonal late-stage pluton that was emplaced within the 1.65 to 1.63 Ga Wiborg rapakivi batholith. The stock has a well-developed zonal structure, from the rim to the center: stockscheider pegmatite, equigranular topaz granite, porphyritic topaz granite. The contact between the two granites is usually gradational within a few centimeters, but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later. Hydrothermal greisen and quartz veins, some of which contain genthelvite, beryl, wolframite, cassiterite, and sulfides, cut the granites of the stock and the surrounding country rocks. The equigranular granite contains 1 to 4 vol.% topaz, and its biotite is lithian siderophyllite; the porphyritic granite has 0 to 3 vol.% topaz, and the mica is siderophyllite. The equigranular granite is geochemically highly evolved with elevated Li, Rb, Ga, Ta, and F, and very low Ba, Sr, Ti, and Zr. The REE patterns show deep negative Eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid–rock interaction. The zonal structure of the stock is interpreted as a result of differentiation within the magma chamber. Internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved, volatile-rich magma in the apical part of the chamber. Crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite; the underlying crystal mush solidified as the porphyritic granite. Much of the crystallization took place from volatile-saturated melt, and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization.