Paper No. 9
Presentation Time: 3:15 PM


GUEVARA, Victor E., Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, CADDICK, Mark J., Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061 and TRACY, Robert J., Geosciences, Virginia Tech, Blacksburg, VA 24061,

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.