Mantled Feldspars from the Granites of Wisconsin

Abstract

Abundant microcline-perthite megacrysts mantled by plagioclase () occur sporadically in Precambrian granites over a wide area in northeastern Wisconsin. Earlier workers offered the novel suggestion that this rapakivi texture is a unique variety of perthite formed during brecciation of the granite. The mineralogy and chemistry of these feldspars has been reinvestigated optically and with an electron microprobe. Many of the newer observations are inconsistent with the earlier theory. The mantling is older than the brecciation and independent of it. Besides mantling, the megacrysts exhibit multiple zoning and what are regarded as resorption and synneusis textures. The chemical composition of the two feldspar phases indicates that they equilibrated under relatively uniform, subsolvus conditions. It is concluded that this rapakivi texture is a primary magmatic feature which has been considerably modified by late-stage recrystallization and exsolution. The sequence of crystallization of feldspars deduced on textural grounds is: (1) alkali feldspar, (2) alkali feldspar + oligoclase, (3) mantling of alkali feldspar by oligoclase, (4) a second stage of alkali feldspar formation, (5) further mantling by oligoclase, and (6) late-stage alkali feldspar growth, exsolution, and ordering.