Multi-stage coronitic overgrowths on OPX in mafic granulite

This is a Ca map of an orthopyroxene core and its surrounding muti-generation mantle from the 2.6 Ga East Athabaska mylonite triangle located north of Stony Rapids, Saskatchewan. The complex coronitic sequence progresses from the original opx core in the lower right corner to a mantle of cpx+qtz, then to a second generation of opx, outward to a mote of plagioclase, a simplectitic intergrowth of opx+plag+magnetite, then to an outer shell of garnet, and finally into the matrix plagioclase in the upper left corner.

A proposed reaction history for this sample involves the prograde growth of a clinopyroxene+garnet+quartz assemblege at the expence of orthopyroxene +plagioclase, and retrograde growth of orthopyroxene+plagioclase+oxide from the peak assemblege. The map presented illustrates the complex distribution of Ca in all of the above phases.

The distributuion of Ca in plagioclase is particularly surprizing in this sample, given its optically homogenous and course character. The large matrix plagioclase in the upper left corner is here revealed as an aggregate of albitic cores with anorthitic overgrowths. The plagioclase mote is also zoned from anorthitic in contact with the simplectite to albitic in contact with the second generation opx. Note also the zonation in garnet at the top of the image, suggesting a high grossular core that was truncated by the late stage simplectite. This sample demonstrates the benefit, or even necessity, of compositional mapping when modeling complex reaction histories and interpreting quantitative analyses.

For more information about this image, contact Michael L. Williams