Metamorphic Evolution of Mid-To-Deep Levels of the Continental Crust in Northern Idaho

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This proposal requests continued support for a research program devoted to understanding the thermal and tectonic history of the North American Cordillera as exposed in northern Idaho. Previous studies by the P.I. and graduate students at the University of Oregon, have demonstrated that three major metamorphic events are recorded in pelitic and calcareous rocks north of the Idaho Batholith. Structural, mineralogical and chemical evidence for these events provide important data hearing on the evolution of the continental crust. The propose research will be directed toward improving our understanding of the metamorphic history, and the relationships between metamorphism, deformation and plutonism. Geothermometry and geobarometry will be carried out using a new approach which solves all equilibria implied by a mineral paragenesis simultaneously. Zoned minerals, especially garnet, will be studied in order to estimate the P-T time path of prograde metamorphism, and the decompression path associated with uplift. Such calculations will be tested with a new method of calculating equilibrium mineral assemblages and compositions with a free energy minimization algorithm. Fluid- rock interactions will also be examined by careful studies of interlayered calcareous and pelitic compositions. The phase equilibrium relations in calcareous rocks, which equilibrated with very saline fluids, will be examined in the light of new data bearing on the thermodynamic properties of minerals and aqueous species in the system H2O-CO2-NaCl. A new model describing the compositional relations among the mineral scapolite and NaCl-rich fluid will be developed and applied to the natural occurrences. Stable isotopic analyses will be undertaken in order to more fully characterize the nature, source and amounts of fluid reacting with a variety of rock compositions. Isotopic age determinations will be made in an attempt to learn the timing of multiple metamorphism, and the relationship of metamorphism of intrusion of the Idaho Batholith.