IS THERMODYNAMIC MODELING OF LOCAL MINERAL EQUILIBRIA REQUIRED TO ELUCIDATE HIGH TEMPERATURE P-T EVOLUTION?

Recent advances in petrologic techniques have resulted in increasingly higher resolution P-T paths of rocks. Such advances can partly be attributed to the advent and ubiquitous use of quantitative P-T pseudosection modeling. A key assumption involved in interpreting a P-T pseudosection is that the bulk-rock composition used is representative of that experienced during successive mineral reaction throughout the P-T evolution. However, local chemical heterogeneities intrinsic in metamorphic rocks may instead control the mineral equilibria in a rock, in essence being more appropriate for any given mineral texture than a single ‘bulk’ composition. This is particularly crucial to consider for cases in which the rock’s composition evolved significantly throughout its P-T history (e.g. during partial melting).

This study focuses on elucidating the P-T history of a single chemically and texturally heterogeneous hand sample of UHT granulite from the Gruf complex, Central Alps. Trace element thermometry and quantitative P-T modeling (using ‘local’ chemical compositions from texturally distinct rock domains) reveal a detailed multi-stage P-T history. Zr in rutile (rtl) thermometry of grains in resorbed, high-Ti biotite (bt) yields T of 710–780 °C. Textural association with high-T opx + grt suggests that this may record heating up to incomplete bt dehydration melting, and the formation of porphyroblastic opx with rtl inclusions (which yield T of 810–880 °C). Ti in quartz and Zr in rtl thermometry on grt inclusions yields T of 800-850 °C, suggesting coeval grt and opx growth. Rtl grains in texturally equilibrated bt or in matrix crd record late stage, low T crystallization at ~650 °C. P-T pseudosections calculated for compositions of different mineral textures in the rock suggest equilibration at peak conditions of 920-950 °C, 7-8 kbar, in broad agreement with results of previous studies, but suggesting lower P by at least 1 kbar.