INVESTIGATING DEEP AND MIDDLE CRUSTAL PROCESSES IN ISOBARICALLY COOLED TERRANES: THE EAST ATHABASCA AREA OF CANADA AND THE PROTEROZOIC TERRANE OF SW-USA

Isobarically cooled (IBC) terranes provide information that is difficult to glean from remote data sources about compositional relationships and structural and metamorphic processes at specific levels of the crust. The granulite facies East Athabasca area, Saskatchewan, is an example of an isobarically cooled terrane that resided in the deep crust (1.0-1.2 GPa) from ~2.5 Ga to at least 1.9 Ga. Proterozoic rocks in the southwestern USA may have been in the middle crust (0.4-0.6 GPa) from at least 1.65 Ga to 1.4 Ga. The single overwhelming characteristic of both regions is the extreme heterogeneity, at all scales, in lithology, structure, and tectonic history. At the largest scale, both regions have a block architecture, with lithologically and structurally distinct blocks or domains. Domains in the granulite facies, East Athabasca area, are dominated by tonalite, charnockite/granite, felsic gneiss, or deformed migmatite. Locally abundant swarms of mafic dikes document heat and mass transfer from mantle to crust. Most rocks are restitic to some degree, showing evidence of melt loss to higher levels of the crust. Mid-crustal rocks in the SW-USA are dominated by amphibolite facies metasediments and granitoids. Mafic dikes are much less common, as are migmatites. Instead, most regions record the emplacement of large volumes of felsic magma (plutons, dikes, and pegmatites). Both terranes, deep crustal and mid-crustal, have a similar structural signature, with an early shallowly dipping fabric, overprinted by domains of intense upright fabric. The shallow fabric may reflect early collisional processes, or alternatively it could reflect crustal flow events that post-date continental assembly. Understanding the significance of the two fabrics at the two crustal levels is a major goal, with implications for a range of crustal processes and characteristics. In-situ monazite geochronology, combined with detailed petrologic and structural analysis is proving to be a powerful tool for deconvoluting the multi-phase P-T-t-D paths in these IBC terranes. Once distinguished, each part of the history can provide significant insight into the nature of modern continental crust and modern crustal processes.