Regional patterns of mineralogical and isotopic changes in metamorphic rocks (Vermont)

Regional patterns of mineralogical and isotopic changes in metamorphic rocks can be exploited to provide information about metamorphic processes that is difficult to obtain from local-scale studies alone. In this thesis, an inverse technique is presented for estimating a three-dimensional fluid flux field that is consistent with regional patterns of tracer measurements from metamorphic rocks. The inverse technique is validated with the results of an analytic forward model of fluid-rock interaction and it is applied to patterns of prograde changes in whole-rock CO2, delta 18O, and delta13C changes from the Waits River Formation, southeastern Vermont. Resolution tests indicate that the field sampling density was too coarse to accurately capture flux variability much below the terrain scale. However, the pilot field study shows that the geometry of terrain-scale reactive flow was largely controlled by regional structure, and flow was sourced primarily from tectonically-driven fluids that entered the study area horizontally from the east.; In addition, this thesis documents regional patterns in the occurrence of monazite and allanite in pelitic schists from Barrovian, Buchan, and contact metamorphic terrains. In the regional metamorphic terrains, monazite disappears from rocks at the biotite isograd, and allanite takes over as the primary mineralogical REE host. In all three terrains, allanite disappears from rocks at the first Al2SiO5 (andalusite or kyanite) isograd, and anhedral grains of monazite appear. Both the allanite- and monazite-forming reactions seem to be chain reactions that accompany the growth of major minerals during metamorphism. 208Pb/208Th age measurements from monazite in low-grade and high-grade rocks document a detrital component of some monazite grains at all grades. Monazite grains in rocks at or just above the Al2SiO5 isograd in each terrain return 208Pb/208Th ages that are statistically consistent with a single date of the formation of the Al2SiO5 isograd.