Textural analyses of classical rapakivi granites: Texture formation through coarsening, size-selective replacement, and stirring

Abstract

The well-known rapakivi texture has been extensively studied through a geochemical lens. Presented here is a detailed textural analysis of two classical rapakivi granites from the Proterozoic: The Wiborg Batholith, Finland and the Wolf River Batholith, USA. Crystal size distributions (CSDs) of mantled and unmantled ovoid populations indicate that protracted textural coarsening was important prior to mantling. An analysis of mantle thicknesses, coupled with a model replicating slices through mantled feldspars, indicates that mantling occurred in a semi-selective manner: mantle thickness on smaller K-feldspar ovoids range from 0 to 100% of the ovoid, and the variability of mantling decreases with increasing size. Selective mantling could be accomplished within a replacement front operating over a thermal gradient, whereby melting augments the ovoid population and generates the mantles, but with partial overlap of these two processes. CSDs of mantled and unmantled feldspars are exceptionally similar between randomly selected samples; suggesting stirring post-mantling and prior to emplacement was an efficient process. Results indicate that classical rapakivi granites are texturally complex and cannot be generated through a single, down temperature process. It is suggested that coarsening of ovoids, followed by mantling and homogenization, may be efficiently generated in the sub-caldera setting, where there are repeated pulses of mantle-derived magmas, or high-temperature anatectic melts acting as an intermediary for mantle heat.