CRYSTALLIZATION OF THE KYMI TOPAZ GRANITE STOCK WITHIN THE WIBORG RAPAKIVI GRANITE BATHOLITH, FINLAND: EVIDENCE FROM MELT INCLUSIONS

Abstract

The crystallization of the late-stage F-enriched Kymi topaz granite, in southeastern Finland, has been investigated by remelting and analyzing crystallized melt inclusions in quartz and topaz grains from porphyritic and equigranular topaz granites and marginal pegmatite. The conditions of rehomogenization were 300 MPa and 700°C for the granites, and 100 MPa and 700°C for pegmatite, as well as 1 atm and 900°C for all phases of the stock. After rehomogenization, quartz from the granites and pegmatite contains four types of melt inclusion: two coexisting types of melt inclusion varying in vapor content, extremely high-F melt inclusions and melt inclusions with very low F contents. The compositions of the melt inclusions show that the topaz granites and the stockscheider pegmatite of the Kymi stock crystallized from very F- and H2O-rich melts, and that Li and B were present in the granitic melt at high concentrations. They also confirm that the equigranular granite and pegmatite crystallized from a more evolved melt than the porphyritic granite. The melt inclusions in quartz and topaz grains from the granites and pegmatite contain indirect indications of melt separation and formation of a minor peralkaline melt fraction in addition to the prevailing peraluminous melt at the final stages of the crystallization. Combined petrological and field observations as well as melt-inclusion studies of the Kymi stock indicate that the zoned structure of the stock is a result of the migration of a highly evolved residual melt from the deeper parts of the magma chamber to the carapace of the chamber. This migration may be related to an opening of the contact between the surrounding rapakivi granite bedrock and residual crystal mush of the porphyritic granite, and to a migration of the residual melt into the carapace along contacts and fractures, or to a migration of highly evolved interstitial magma and fluid through the crystallizing porphyritic granite and along the walls of the magma chamber to the carapace.