Mineral chemistries and textures in mafic garnet-pyroxene rock within the Lost Creek Gneiss in the Purdy Hill quadrangle, Mason County, Texas, record three distinct P-T events, two of them at pressures higher than any previously documented for these Grenville-age rocks. The earliest metamorphic episode produced an eclogitic assemblage of pyrope-rich almandine + sodian augite + orthopyroxene + pargasitic amphibole. Some orthopyroxene cores record pressures greater than 15 kbar even for the most conservative estimated temperature of 500 °C. Garnet and orthopyroxene rims and matrix clinopyroxene and amphibole record a second event at ∼750 °C and 8 kbar. Symplectites of clinopyroxene + plagioclase and coronas around garnet of plagioclase + amphibole + magnetite formed in response to reheating and hydration associated with postkinematic intrusion at the amphibolite facies conditions (600-650 °C, 3.5-5 kbar) commonly recorded in the region. Recognition of early high-pressure metamorphic episodes markedly strengthens the correlation between these rocks and other Grenville-age exposures along the continental margin and provides petrologic support for models invoking Proterozoic subduction accompanying the Llano orogeny.

First Page Preview

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