Do Porphyroblast Inclusion Trails Provide Information About Regional Tectonism?

University Of Southern California, Los Angeles CA

Investigators

Abstract

The PI, Dr. Ron Vernon, was recently appointed a Research Professor at the University of Southern California. This proposal would support research by the PI and USC graduate student, Luke Jensen. This proposal outlines a project designed to examine several hypotheses regarding the significance of inclusion trails preserved within porphyroblasts and the timing and structural setting of porphyroblast growth. It has been suggested that porphyroblast inclusion trails may preserve information about regional orogenic events and possibly past plate motions. If this hypothesis is correct, then porphyroblast studies provide a powerful method of unraveling orogeny and past plate motions. However, the PI 's studies elsewhere suggest that structural and metamorphic heterogeneities are formed by various processes at all scales and can markedly affect inclusion trail patterns in porphyroblasts; that complex porphyroblast growth features and internal inclusion trail patterns can fon-n in potphyroblasts that grow during short time intervals in contact aureoles, indicating that local complexity in porphyroblasts does not imply regional comple xity, and that using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjustified. They hope to test these contradictory hypotheses by examining porphyroblasts in a prograde metamorphic sequence (chlorite to sillimanite zones),Cascades core, Washington in three settings: regional, contact aureoles, and in folds. They will combine modem techniques for studying poiphyroblast matrix relationships with cathode luminescence, neutron diffraction, and limited geochronologic studies.