THE ORIGIN OF RAPAKIVI TEXTURE

The mantling of large ovoids of K-feldspar by a rim of plagioclase has been investigated in the rapakivi granites from the Mid-Proterozoic Wiborg batholith of SE Finland. The formation of rapakivi texture, in this the type area, was examined using a variety of techniques including isotopic analyses of mineral separates from specific textural sites. Cathodoluminescence combined with microprobe analysis points to the pulsed development of the mantles involving growth of successive plagioclases of composition An30, An25, and An3, the last being in optical continuity with perthitic plagioclase exsolved from the K-feldspar. Plagioclase mantles have high deltaO-18 and Sr-87/Sr-86 signatures relative to K-feldspar, which indicate the presence of a late, low-temperature component thought to represent albite exsolved from the K-feldspar and redistributed onto the ovoid margin. Oligoclase components of the mantles are formed by a similar, although higher-temperature magmatic process. This involves the subsolvus re-equilibration of alkali feldspar compositions with evolving melt conditions. Redistribution of the exsolved plagioclase from the alkali feldspar phenocrysts is linked to high fluorine contents of rapakivi-type magmas, and this major reconstruction of the feldspar phenocrysts generates their distinctive ovoidal shape.