Evidence from Mössbauer spectroscopy for intense localization of metamorphic fluid flow during the formation of spaced cleavage

Abstract

ABSTRACTArenites in the low‐grade metasedimentary rocks of the Stirling‐Barren Group, Western Australia, contain discrete spaced cleavages. These in part are pure white mica, but up to 30% of the cleavage zones consists of fine‐grained quartz resulting from crystal plastic deformation. A modified Gresens analysis shows that the cleavage domains formed by a 50% volume loss associated primarily with SiO2 and MgO concentration decreases of 58% and 37%, respectively, and a 12% decrease in Fe2O3. Mössbauer spectroscopy shows that the Fe2+: Fe3+ ratio from lithon to cleavage changed from 50:SO to 15:85. This substantial difference cannot be accounted for by loss of iron. The data are best explained by the focusing of a highly oxidizing fluid through the cleavage domains where volumetric fluid rock ratios around 150 to 1 are indicated. In this example advectic processes appear to be important in low‐grade regional metamorphism.