Tectonics of the Grenville orogen and micropetrology in metamorphic rocks

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A long-standing problem in metamorphic petrology has been the inability to unequivocally link the P-T estimates of metamorphic rocks (determined by geothermobarometry) to the timing of the metamorphism (determined by geochronology). The problem arises because the minerals generally used for geothermobarometry (e.g., major silicate phases such as garnet, pyroxene, plagioclase) are not the same as those typically used for geochronology (e.g., zircon, monazite, titanite, rutile), and because in conventional dating methods the rock is crushed, thereby destroying the spatial relationships between mineral species and any zoning within them. In the last few years, new analytical approaches have yielded significant advances towards the resolution of this problem. These include: (i) imaging the locations of individual minerals used in geochronology with respect to the various components of the metamorphic fabric and the minerals used in geothermobarometry, (ii) imaging and correlation of zoning patterns of minerals used in geothermobarometry and geochronology, thereby demonstrating contemporaneous growth or dissolution, and (iii) in situ microprobe methods of geochronology by EPMA, LA ICPMS, or ion microprobe that reduce or eliminate the need to crush the rock, thereby retaining important spatial information. We propose to use and develop these methods for application to Precambrian (Grenville, Slave) and Paleozoic (Appalachian) orogens. Coupling the data obtained from studies of this kind with regional structural information provides insight into metamorphic processes, as well as permitting robust reconstructions of the tectonic evolution of the orogenic crust.