Magmatic-hydrothermal evolution of the Kymi topaz granite stock, SE Finland: Mineral chemistry evidence for episodic fluid exsolution

Abstract

The Kymi stock is a highly evolved topaz-bearing granite stock that has intruded into rapakivi granites in the central part of the Wiborg batholith (southwest Finland). The stock shows a well-defined zonation and intrusion sequence consisting of early porphyritic granites, subsequent equigranular granite and a late marginal stockscheider pegmatite. Miarolitic cavities occur within all rock types of the stock, but they are more abundant near the contact between the porphyritic and equigranular granites. Related hydrothermal greisen alteration, barren and Pb-Zn mineralized fluorite-bearing quartz veins crosscut the host rapakivi granites close to the Kymi stock. The major and trace element chemistry of biotite, K-feldspar, plagioclase and fluorite tracks the magmatic to hydrothermal evolution of the system. The biotite composition ranges from Al-rich annite to Li-rich siderophyllite, and the biotite chemistry shows an increase in Si, Li, AlVI, F, Mn, Rb, Cs, Zn, Be, Ga, Tl, Ta, and F, and a decrease in Fe, AlIV, Mg, Ti, Ba, Sr, Nb, and Cl along the evolution of the system. The Nb/Ta ratios in biotites suggest that magmatic fractionation is responsible for the progressive Ta enrichment in early porphyritic granites. The subsequent decrease in Nb at rather constant Ta in the more evolved equigranular granite and stockscheider pegmatite cannot be explained by magmatic fractionation, but requires selective partitioning of Nb into an aqueous fluid phase. The halogen composition of the biotites changes drastically from the early porphyritic granites to the later more evolved equigranular granite and stockscheider pegmatite. The Cl/F data of biotite from the miarolitic cavities plots at the transition between the porphyritic and equigranular granites, and is interpreted in terms of major fluid exsolution occurring at this stage. The minor and trace element data of K-feldspar and plagioclase show similar compositional trends as the biotites. The REEY patterns of fluorites from the equigranular granite and the stockscheider pegmatite, compared with those from the earlier porphyritic granites, show pronounced tetrad effects and correlated Y anomalies. This suggests that the F enrichment in the equigranular granite and the stockscheider pegmatite, in conjunction with major fluid exsolution and REEY complexing as fluoride species at this stage, have been key to development of the tetrad effect and Y anomalies. The fluorite from hydrothermal greisen alteration and quartz veins shows different chondrite normalized REEY patterns, indicating contributions from other sources such as meteoric fluids and/or interaction with the surrounding host rocks. The major and trace element data from the biotites of the Kymi stock are distinct from those of the host rapakivi granites, suggesting that the parental melts have not been entirely derived from fractionated rapakivi granite melts.