Cathodoluminescence studies of feldspars and apatites from the Coldwell Alkaline Complex

Abstract

The Coldwell alkaline Complex is located on the north shore of Lake Superior, and consists of three Centers representing three magmatic episodes. Center I consists of gabbro, and layered and unlayered ferroaugite syenite. Center II consists of alkalic biotite gabbro, miaskitic nepheline syenite, amphibole nepheline syenite, perthitic nepheline syenites and recrystallized nepheline syenites. Center III consists of magnesio-hornblende syenite, ferro-edenite syenite, contaminated ferroedenite syenite, and quartz syenite. The textures and compositions of the feldspars and apatite from these three Centers were examined using cathodoluminescence (CL) and the scanning electron microscope (SEM). All of the colours described in the text refer to cathodoluminescence colours. The feldspars in Center I consist of light blue to light violet blue, optically homogeneous alkali feldspar; braid microperthite and incipient perthi te; and irregular vein or patch perthite. The irregular vein and patch perthite contain light violet blue cryptoperthitic patches, light blue exsolved albite and dull blue exsolved K-feldspar. During late-stage fluid-induced alteration, these feldspars were replaced by violet secondary albite and purple secondary K-feldspar; and were coarsened by a deuteric antiperthitic rim. The Fe-rich antiperthitic rim consists of deep red secondary albite and brown secondary K-feldpsar. The later dominates in the upper series of the layered and unlayered syenites. The homogeneous alkali feldspar crystallized at high temperatures :t!700°C, exsolved into microperthite at 650-550°C and formed irregular vein and patch perthite at 520-420°C.