MESOPROTEROZOIC THERMAL HISTORY OF THE MANZANO MNTS., NM WITH IMPLICATIONS FOR ALUMINUM SILICATE STABILITY USING OXYGEN ISOTOPES

The Priest pluton contact aureole in the Manzano Mountains, central New Mexico, preserves evidence for upper amphibolite contact metamorphism associated with intrusion of the 1.42 Ga Priest pluton that was followed by localized retrograde hydrothermal alteration. Quartz-garnet and quartz-sillimanite oxygen isotope fractionations in pelitic schists document an increase in the temperatures of metamorphism from ~540°C one kilometer from the pluton to 690°C within meters of the pluton. Comparison of temperature estimates to one-dimensional thermal modeling constrains background temperatures to no higher than ~300°C during intrusion of the Priest pluton. Fibrolite is found within 300 meters of the Priest pluton in pelitic and alkali schists metamorphosed to temperatures >580°C. Coexisting fibrolite and garnet in pelitic schists are in oxygen isotope equilibrium suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite-in isograd is coincident with the staurolite-out isograd in pelitic schists, and alkali feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite controls fibrolite growth in staurolite-bearing pelitic schists, not the second-sillimanite reaction. Muscovite-rich alkali schists locally preserve the triple-point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in apparent isotopic equilibrium in these schists. Co-nucleation of fibrolite and andalusite at 580°C, in the presence of muscovite and absence of potassium feldspar, suggests that univariant polymorphic transformation from andalusite to fibrolite occurred. Kyanite growth occurred nearly 80°C lower than andalusite and fibrolite. Quartz-muscovite fractionations in hydrothermally altered schists and quartzites are small or negative suggesting that muscovite, but not quartz, exchanged isotopically with externally derived fluids after peak metamorphism and that hydrothermal alteration occurred below the closure temperature of quartz (< 520°C).