Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous-Tertiary magmatism. U-Pb zircon geochronological data from small exposures of pre-Belt supergroup basement along the western side of the Wyoming craton, in southwestern Montana, reveal crystallization ages ranging from -2.4 to -1.8 Ga. Rock-forming events in the area as young as -1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous-Eocene granitoids. Most of this lithosphere is primitive, gives ages -1.7-1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions.

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The Nelson Batholith is a ca. 1,800 km 2 Jurassic intrusive body in southeastern British Columbia surrounded by a contact aureole, 0.7-1.8 km wide, developed in graphitic argillaceous rocks that show only minor variations in bulk composition. Contrasting prograde sequences of mineral assemblages are developed around the aureole in a regular pattern, reflecting different pressures of contact metamorphism. The following assemblages are seen going from lower to higher pressure (all assemblages contain muscovite + biotite + quartz -Mn-rich garnet): (1) cordierite-only assemblages, (2) mix of cordierite-only and cordierite + andalusite assemblages, locally with cordierite + K-feldspar and andalusite + K-feldspar assemblages at higher grade, (3) andalusite-only assemblages, with sillimanite + andalusite assemblages and locally sillimanite + K-feldspar assemblages at higher grade, (4) staurolite-only assemblages, (5) staurolite -andalusite assemblages, with sillimanite-bearing and locally sillimanite + K-feldspar assemblages at higher grade. The higher-pressure sequences with staurolite -andalusite are restricted to the aureole surrounding the east half of the batholith, whereas the lower-pressure cordierite -andalusite are restricted to the aureole surrounding the west half of the batholith and its northern and southern tips. The sequences of mineral assemblages correspond closely to the facies series of Pattison & Tracy (1991) and are interpreted to represent a series of approximately isobaric metamorphic field-gradients below the Al 2 SiO 5 triple point, providing an excellent opportunity to evaluate thermodynamically calculated low-pressure phase equilibria in the metapelitic system. The total difference in pressure represented by the contrasting assemblages is about 1.0 kbar, showing that they are a sensitive measure of small differences in pressure within the stability field of andalusite. Thermobarometry results from the aureole are moderately consistent with the mineral assemblage constraints, but carry pressure uncertainties larger than the total range of pressure represented by the aureole's assemblages. Pressures of the intrusive rocks derived from hornblende barometry are scattered, and many do not agree with the pressure constraints from the aureole. The mineral-assemblage constraints indicate down-to-the-west post-metamorphic tilting of the batholith and aureole of about 10 , interpreted to be due to a combination of eastward thrusting of the Nelson Batholith over crustal-scale ramps during Cretaceous-Paleocene shortening and Eocene east-side-down normal motion on the Slocan Lake -Champion Lakes fault system that forms the western boundary of the batholith. SOMMAIRE Le batholite jurassique de Nelson, couvrant une superficie d'environ 1,800 km 2 dans le sud-est de la Colombie-Britannique, est entouré d'une auréole de contact entre 0.7 et 1.8 km de large, développée dans une roche argileuse à graphite sans variations importantes en composition. Des contrastes dans les séquences d'assemblages progrades de mi...

[1] The Pioneer core complex (PCC) in central Idaho lies along a transition between Early Eocene and ca. ≤40 Ma core complexes to the north and south, respectively. Thus, the age of extensional development of the PCC is important in understanding the spatial-temporal patterns of core-complex development in the North American Cordillera. New results, including structural observations and U-Pb zircon (SHRIMP and ICPMS) geochronology, constrain the early extensional history of the footwall for the first time. High-temperature strain with a top-WNW shear-sense is pervasive throughout metamorphic rocks of the northwestern footwall. An isoclinally folded dike yields a crystallization age of $48-47 Ma, whereas a crosscutting dike yielded an age of 46 Ma. Metamorphic rocks are also intruded by the $50-48 Ma Pioneer intrusive suite (PIS), a W-dipping granodiorite sheet displaying a magmatic fabric. Northwest-trending lineations are locally visible and also defined by anisotropy of magnetic susceptibility, indicating that during emplacement, the PIS was undergoing similarly oriented extensional strain as the enclosing metamorphic rocks. Therefore, WNW-directed extension spanning this structural section occurred between $50 and 46 Ma. Following emplacement of crosscutting 46 Ma dikes, deformation was partitioned into the WNW-directed Wildhorse detachment. Motion on the detachment occurred between $38 and 33 Ma, as documented by previous 40 Ar/ 39 Ar thermochronology. It is not clear, however, whether extension was continuous through the interval between these two time periods. Although Early Eocene extension in the PCC was synchronous with extension in core complexes to the north, rates of footwall exhumation in central Idaho were much lower. This southward slowing is compatible with N-S differences in inferred subduction zone geometry/kinematics and in the internal character of the orogenic wedge.