Abstract

Cathodoluminescence (CL) petrography and luminescence spectra of coexisting plagioclases from the regionally metamorphosed Boehls Butte anorthosite permit rapid visual distinction of complex plagioclase intergrowths and fine-scale, retrograde fluid flow features that are nearly invisible in transmitted light. Calcic plagioclase (An87–97) exhibits bright yellow luminescence, whereas intermediate plagioclase (An39-65) is green. Luminescence spectra indicate that red emissions are sensitive to anorthite content, shifting from 687 to 744 nm over the range An94–-An38. Green peak positions are less sensitive, with shorter wavelength emissions in anorthite (540–548 nm), compared with coexisting andesine (549–561 nm). The Mn2+ contents correlate well with the intensities of the peak at 540–560 nm, suggesting that Mn2+ is an effective CL activator in plagioclase at levels as low as 19 ppm. Fe2+ acts as neither a sensitive activator nor important quencher. Very weak Fe3+-activated peaks (680–745 nm) but variable Fe3+ abundance (23–2504 ppm) suggest that most Fe3+ is present in a minute, coexisting phase such as hematite. Anorthite inclusions in andesine megacrysts have significantly different CL peak positions and intensities than granoblastic, polygonal anorthite forming thin laminae in the anorthosite. Plagioclase occurring in association with the laminae shows CL evidence of dissolution and replacement and of grain fragmentation accommodating dilation. The laminae are inferred to have been foci of retrograde fluid circulation through the anorthosite that resulted from rapid uplift of the Boehls Butte region after the peak of regional metamorphism.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.