From a F-rich granite to a NYF pegmatite: Magmatic-hydrothermal fluid evolution of the Kymi topaz granite stock, SE Finland

Abstract

The hydrothermal evolution of the chemically highly evolved topaz granite Kymi stock in southeastern Finland was reconstructed from fluid inclusion petrography, microthermometry, and LA-ICP-MS analysis. The Kymi stock shows a clear zonation and intrusion sequence consisting of an early porphyritic granite, a subsequent equigranular granite, and a late marginal stockscheider pegmatite. Miarolitic cavities from different hydrothermal stages occur within all rock types of the stock, but they are more abundant near the contact between the porphyritic and equigranular granites. Three successive generations of miarolitic cavities and quartz from the stockscheider pegmatite provide the paragenetic and chronological framework for establishing the fluid evolution history. Based on petrographically established time relations, the microthermometric, and LA-ICP-MS data, fluids with different compositional trends were identified and linked to distinct fluid saturation events. The magmatic fluids from the Kymi stock contain high concentrations of elements that are characteristic of highly evolved granites and related pegmatites such as K, Li, B, Be, Rb, Tl, and Cs. Barium and Sr concentrations in fluid inclusions systematically decrease from the intermediately evolved miarolitic cavity to the stockscheider pegmatite and more evolved miarolitic cavities. Additionally, high As, Nb, and Ta concentrations were detected in fluid inclusions from the miarolitic cavities and stockscheider pegmatite. The fluids from the Kymi stock are significantly depleted in zinc, having low Zn/Pb ratios when compared to other magmatic fluids worldwide. This is interpreted to be caused by lower Zn availability in the melt resulting from the crystallization of Zn-enriched biotites in the early porphyritic granite. The most important fluid exsolution event between the crystallization of the porphyritic and the equigranular granite was key to generating the residual, highly evolved and strongly fluorine-enriched melt from which the later rocks including the stockscheider pegmatite crystallized.