ABSTRACT The Seve Nappe Complex in the Scandinavian Caledonides records a range of peak metamorphic conditions and timings. To better understand pressure-temperature-deformation-time differences throughout the complex and possible tectonic scenarios, metamorphosed mafic rocks within the Tarfala Valley of the Kebnekaise Massif (Sweden) were investigated using integrated petrologic and geochronologic techniques. Thermodynamic modeling of two samples using domainal and whole-rock compositions integrated with mineral chemistry, mineral textures, and titanite and zircon U-Pb geochronology constrained a portion of the pressure-temperature ( P-T ) path. Peak metamorphic conditions of 590–660 °C and 9.7–10.5 kbar were followed by near-isothermal decompression or a subsolidus clockwise P-T path. Amphibolite units in the valley record retrograde conditions at 450–550 °C at less than 7.5 kbar, although mineral modes and textures are most consistent with pressures <4 kbar. The majority of titanite growth occurred due to the introduction of hydrous fluids during cooling and following exhumation to midcrustal levels. U-Pb ages of retrograde titanite define a spread from ca. 480 to 449 Ma, and the oldest age is interpreted to constrain the timing of retrogression following exhumation. This interpretation is supported by a U-Pb zircon crystallization age of 481 ± 7 Ma for a metamorphosed intermediate to felsic synkinematic dike hosted in one of the amphibolite units. These results indicate that the Kebnekaise region records Early Ordovician deformation and metamorphism that was of lower grade compared to other Seve Nappe Complex locations to the south. The tectonic history of these rocks includes metamorphism and exhumation during the Cambrian–Ordovician pre-Scandian event, followed by thrusting of the Seve Nappe Complex and neighboring rocks onto Baltica during the Silurian Scandian orogeny.

Abstract Paleoproterozoic supracrustal rocks in the region near Big Thompson Canyon, northern Colorado, have long been recognized as a spectacularly exposed example of regionally zoned metamorphism, preserving an apparently complete sequence from biotite- to migmatite-zones. Due to its location and relatively easy access, the Big Thompson Metamorphic Suite has also provided a valuable field-based educational experience for universities and colleges all along the Front Range and from elsewhere. In addition to a number of other studies, the pioneering work of William Braddock and graduate students from the University of Colorado resulted in more than a dozen M.Sc. and Ph.D. theses from the 1960s to the 1990s. Despite the volume of ground-breaking science conducted on these rocks in the past, there remain a number of fundamental questions regarding the metamorphic history and overall tectonic significance of many of the observable features. Several lines of evidence suggest there is potential for a complex tectonometamorphic history that likely spans from ~1.8 to 1.4 Ga. These include: thermochronologic and geochronologic data supporting multiple thermal and magmatic episodes, structural evidence for multiple deformation events, multiple generations of typical Barrovian minerals (e.g., staurolite), and the widespread occurrence of minerals not commonly associated with a classic Barrovian sequence (e.g., andalusite, cordierite). One purpose of this fieldtrip is to foster new ideas and stimulate new research directions that will utilize the Big Thompson Metamorphic Suite, and the Colorado Rockies in general, as field laboratories for better understanding fundamental orogenic processes.

The Carthage-Colton mylonite zone is a major geothermochronological discontinuity across the northwest Adirondack Mountains of New York, a southern extension of the Grenville Province. A large syenitic gneiss body, the Diana syenite, occurs along most of the southern Carthage-Colton mylonite zone. The present study examined petrofabrics and magnetofabrics of oriented cores and accurately oriented thin- sections to investigate the sources of anisotropy of magnetic susceptibility (AMS) within a central portion of the Diana syenite. Three petrographic foliations, a petrographic lineation, and a magnetic intersection lineation were clearly distinguished. Two of the foliations appear to represent axial planar foliations of the second- and third-phases of regional folding as defined by Wiener (1983). The youngest foliation and the magnetic intersection lineation have not been previously described. This research suggests that folding identified in the Adirondack Lowlands can be traced to at least the southwest margin of the Diana syenite with no obvious discontinuity. Significant implications of this research suggest that: (1) the Adirondack Lowlands deformation likely includes some folding events associated with Ottawan orogen compression, and (2) the kinematics and style of deformation within the Carthage-Colton mylonite zone remain cryptic and cannot conclusively be connected to the fabrics explored in this research.