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.

We report new petrological data for granulites from the Central Zone of the Limpopo Complex, southern Africa, and construct a prograde P-T path that traverses from high-pressure granulite-facies metamorphism to peak ultrahigh-temperature (UHT) metamorphism by rapid decompression, which was followed by further decompression and cooling. Mg-rich ( X Mg ~0.58) staurolite enclosed within poikiloblastic garnet in an Mg-Al-rich rock from the Beit Bridge area is rarely mantled by a sapphirine + quartz corona, suggesting the progress of the prograde dehydration reaction: staurolite + garnet → sapphirine + quartz + H 2 O. The symplectic sapphirine + quartz developed around staurolite probably implies decompression from P >14 kbar toward the stability of sapphirine + quartz at T ~1000 °C along a clockwise P-T path. The orthopyroxene + sillimanite + quartz assemblage mantled by cordierite aggregates in a pelitic granulite from the same area also suggests extreme metamorphism and subsequent further decompression. Various corona textures such as kyanite + sapphirine, sapphirine + cordierite, and orthopyroxene + cordierite were probably formed as a result of decompression cooling events. The prograde high-pressure metamorphism and the following UHT event relate to the collisional tectonics of the Zimbabwe and Kaapvaal Cratons, which are associated with the amalgamation of microcontinents during the Neoarchean.

We conducted a detailed study of a unique metapelite from the ultrahigh-pressure (UHP) Brossasco-Isasca unit (Dora-Maira Massif). It consists of pre-Alpine porphyro-blastic garnet (Grt1), Alpine-age idioblastic garnet (Grt2), phengite, and kyanite. The pre-Alpine garnet contains inclusions of staurolite, kyanite, paragonite, chloritoid, chlorite, quartz, ilmenite, and rutile. The metapelite also includes quartz aggregates after former coesite and fine-grained paragonite aggregates after jadeite. Late chlori-toid and staurolite idioblasts, first reported from a Brossasco-Isasca unit metapelite, are also present in the rock matrix: they grew randomly across the main Alpine UHP foliation defined by phengite. The decompression pressure-temperature ( P - T ) path of the unit was constructed employing a P - T pseudosection calculated for the model MnNKFMASH (MnO-Na 2 O-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O) system in the pressure interval 0.6–1.8 GPa, i.e., below the coesite-quartz transition and within the albite stability field. The absence of albite and biotite, together with the compositions of staurolite, chloritoid, chlorite, and phengite allow us to constrain the 580–590 °C portion of the retrograde P - T path from P = 1.12–0.96 GPa. The results are in good agreement with previously estimated P - T paths, mostly inferred from retrograde assemblages in whiteschist and eclogite of the Brossasco-Isasca unit.