Abstract
The Boehls Butte anorthosite consists
predominantly of an unusual bimodal assemblage of
andesine and bytownite–anorthite. Oxygen isotope compositions
of the anorthosite were profoundly altered by
high temperature, retrograde interaction with meteorichydrothermal
fluids that varied in composition from isotopically
evolved to nearly pristine meteoric water. Oxygen
isotope ratios of bulk plagioclase separates are in the range
?7.0 to -6.2% V-SMOW, however, secondary ion mass
spectrometry indicates spot-sized isotope values as low as
-16%. Typical inter- and intra-plagioclase grain variability
is 3–6%, and extreme heterogeneity of up to 20%is
noted in a few samples. High-temperature hydrothermal
alteration of intermediate plagioclase is proposed to
explain the origin of bytownite–anorthite in the anorthosite
and creation of its unusual bimodal plagioclase
assemblage. The anorthite-forming reaction created retrograde
reaction-enhanced permeability which, together with
rapid decompression, extension, and unroofing of the
anorthosite complex, helped to accommodated influx of
significant volumes of meteoric-hydrothermal fluids into
the anorthosite.