OXYGEN ISOTOPE THERMOMETRY AND THE EXTRACTION OF PROGRADE MINERAL GROWTH IN POLYMETAMORPHIC ROCKS, AN EXAMPLE FROM THE TRUCHAS MOUNTAINS, NEW MEXICO

Reconstructing prograde P-T-t paths of metamorphic terranes is necessary to understand crustal assembly, and requires that minerals preserve a record of their growth history. The completeness and accuracy of information preserved in a mineral, typically in the form of chemical zoning or isotopic compositions, and our ability to analyze and interpret it precisely, are critical. We take advantage of the refractory nature of the Al₂SiO₅ polymorphs kyanite, andalusite and sillimanite to gain insight into the prograde metamorphic history of the Proterozoic terrane in NM. Using quartz-Al₂SiO₅ polymorph oxygen isotope thermometry in quartzite, Al-rich pelitic schist, and fibrolite-bearing quartz veins, we are able to constrain accurately the prograde growth history of kyanite, andalusite, prismatic sillimanite and fibrolite in a suite of rocks from the Truchas Mountains. Stable isotope analyses show that growth of kyanite, andalusite, and prismatic sillimanite occurred at c. 580, 620 and 640^oC, respectively, and that two generations of fibrolite growth occurred at 585^oC and 690^oC. No correlation between rock type, locality or elevation of the sample exists for the different temperatures. Retrograde metamorphic effects for quartz are minimized due to the bimineralic nature of the quartzite studied, so that measured temperature estimates are interpreted as closely approximating actual temperatures of growth for each Al₂SiO₅ polymorph. Petrographic observations suggest the paragenetic sequence: kyanite ® andalusite + prismatic sillimanite ® fibrolite. Growth of kyanite occurred at pressures above the Al₂SiO₅ invariant point (M₁), whereas andalusite, prismatic sillimanite and fibrolite grew during M₂ at pressures below the Al₂SiO₅ invariant point. The ability of the Al₂SiO₅ polymorphs to retain their d18O value from their time of growth allows us to go beyond questions regarding peak metamorphic conditions to the more fundamental issues that include constraining actual reactions responsible for Al₂SiO₅ polymorph growth and the evolution of a metamorphic assemblage during orogenesis.