THERMAL AND HYDROTHERMAL HISTORY OF THE PRIEST PLUTON CONTACT AUREOLE, NEW MEXICO: APPLICATION OF OXYGEN AND HYDROGEN ISOTOPE METHODS

Proterozoic rocks exposed in the Manzano Mnts. of central New Mexico show evidence for regional 1.65 Ga metamorphism that is overprinted by contact metamorphism and hydrothermal alteration associated with intrusion of the 1.43 Ga Priest pluton. Previous studies estimated peak contact metamorphic temperatures using garnet-biotite thermometry. In this study, temperatures were calculated using oxygen isotope thermometry of quartz-Al₂SiO₅ mineral pairs, and fluid sources were evaluated from the hydrogen isotope dD compositions of micas in the country rock and pluton.

Andalusite porphyroblasts within 500 meters of the Priest pluton have abundant quartz and relict kyanite inclusions. Kyanite is also present as porphyroblasts (~10 cm) in lenticular quartz veins. d18O values of quartz included in andalusite and matrix quartz were determined separately. Quartz-andalusite (d18O=8.9 ‰) temperature estimates are 580°C using included quartz (d18O=12.0 ‰) and 720°C using matrix quartz (d18O=11.2‰). Temperature estimates from quartz-kyanite fractionations are 490°C for quartz included in andalusite and 625°C for matrix quartz (d18O_kyanite=8.1‰). The unreasonably hot temperature estimates calculated using the matrix quartz suggests disequilibrium between matrix quartz and andalusite due to later exchange between matrix quartz and hydrothermal fluids. Andalusite retains its primary d18O signature because of its refractory nature and inclusion quartz, armored by andalusite, could not exchange with hydrothermal fluids. Therefore, temperature estimates using quartz inclusions provides the best approximation to peak metamorphic temperatures. Evidence for a plutonic source for the hydrothermal fluids is supported by hydrogen isotope data. dDvalues of muscovite from the Priest pluton and country rock are unusually light and nearly identical (dD_pluton=-78‰, dD_{host rock} =-74‰), suggesting aqueous fluid communication between these rocks.

Temperature estimates determined using oxygen isotope thermometry are up to 80^oC greater than garnet-biotite thermometry estimates in the area. We believe this disparity could be due to retrograde resetting of biotite enhanced by hydrothermal alteration.