ORIGIN OF COEXISTING PLAGIOCLASES (ANDESINE-ANORTHITE) IN THE BOEHLS BUTTE ANORTHOSITE, NORTHERN IDAHO

The origin of the unusual bimodal mineralogy (andesine An_34-48 and anorthite An_92-98) of the Boehls Butte anorthosite (BBA) has been debated for many years. Hypotheses proposed for its origin include (1) metasomatic alteration of sedimentary or igneous rocks; (2) subsolidus exsolution, shear differentiation or metamorphic coarsening of intermediate plagioclase, and; (3) magmatic crystallization, inclusion of metasedimentary restite and hydrothermal alteration; each of these hypotheses is problematic. We have previously reported O-isotope evidence of a complex history of retrograde interaction of the BBA with meteoric-hydrothermal fluids. In situ, SIMS O-isotope analyses indicate inter- and intra-plag d¹⁸O variability of 3-6‰, with localized plag values as low as -16‰ V-SMOW, and preservation of very steep, fine-scale gradients in d¹⁸O (up to 20‰ over 600 mm; Mora et al. 1999). Correspondence between the modal % of anorthite and low w.r. d¹⁸O suggest a hydrothermal mechanism for producing the unusual plag assemblage. CL petrography further supports interpretation of anorthite grains as the foci of retrograde fluid flow. Experimental studies by Orville (1972) and others have noted the rapid anorthitization of more-sodic plag at high T in the presence of a Ca-bearing brine, where altered plag may retain its shape, twin boundaries, etc. A similar reaction is invoked to explain the unusual textural and isotopic characteristics of the BBA. Removal of quartz in the hydrothermal fluid creates a large (~26%) void volume and significant reaction-enhanced permeability in the BBA. This provides a mechanism, in addition to brittle failure, to accommodate rapid influx and variable and widespread transport of meteoric- hydrothermal fluids through the BBA during late stage decompression, extension, and rapid unroofing of the BBA complex. The fractional conversion of andesine to anorthite and ¹⁸O shifts in plag can be used to constrain fluid-rock ratios and, with other constraints, to estimate the fluid fluxes responsible for isotopic resetting. The calculated values are much larger than typical for regional metamorphic systems, but not unreasonable for those estimated for hydrothermal circulation associated with intrusive systems.