ABSTRACT The Grenville Province is the largest and most structurally complex orogenic belt that formed in the Mesoproterozoic, representing the amalgamation of the supercontinent Rodinia. The Mattawa domain, part of the Algonquin domain in Ontario, Canada, hosts some of the most deeply buried metamorphic rocks of this orogen. This high-grade metamorphic terrane consists of large areas of felsic orthogneiss and kilometer-sized mafic boudins. Dark-colored metabasite cropping out near Mattawa, Ontario, Canada, contains relict mineral assemblages and decompression textures indicative of high-pressure eclogite. Garnet porphyroblasts surrounded by plagioclase coronae are hosted in fine-grained symplectic intergrowths of diopside + plagioclase ± amphibole, which compositionally reintegrated into an omphacite composition (Na/[Na + Ca] ~0.5). Phase equilibria analysis revealed an eclogite-facies peak pressure of ~2 GPa at 850 °C. This temperature is consistent with the zirconium contents of rutile inclusions in garnet (up to 1725 ppm Zr). Despite high-temperature metamorphism, garnet growth zonation is partially preserved. Diffusion modeling of representative garnet profiles yielded a time scale of <0.1 m.y. for decompression from the peak pressure to ~1.2 GPa, suggesting an average exhumation rate of several decimeters per year. Decompression was followed by fast cooling within hundreds of thousands of years. Such fast decompression and cooling rates contrast with the protracted metamorphic evolution recorded in most of the Grenville orogen and likely resulted from local extrusion of lower-crustal material in response to localized extension during the early Ottawan stage. Since very few examples of Precambrian short-duration regional metamorphism have been documented, the fast decompression documented in this study provides valuable constraints for the geodynamic transition to a modern plate-tectonic regime.

ABSTRACT The Scandinavian Caledonides have a complex latest Proterozoic–Early Devonian history, but they were finally assembled during the Silurian–Devonian (Scandian orogeny) collision between Baltica and Laurentia. Their dominant structural components are the Lower (Baltican margin), Middle (Baltican and farther outboard), Upper (Iapetan arcs), and Uppermost (Laurentian margin) Allochthons. This study examined the Blåhø Nappe, a complex unit of metamorphosed, intensely deformed igneous and sedimentary rocks assigned to the Middle Allochthon. Metamorphic grades are regionally amphibolite facies, but granulite- and eclogite-facies rocks are locally found. Although most metamorphic ages span a range from Middle Ordovician to Devonian, Blåhø eclogite and other high-pressure rock ages are exclusively Scandian. We analyzed 95 samples of Blåhø Nappe metamorphosed igneous rocks, which were mostly mafic rocks, composed of a minor arc-derived set and a major set transitional between arc and depleted to enriched mid-ocean-ridge basalt (MORB), a range characteristic of back-arc basins. Historically, the Blåhø Nappe has been assigned to the Seve Nappe Complex, the upper part of the Middle Allochthon as mapped in western Sweden and easternmost Norway. In contrast to the Blåhø Nappe, eclogites and other high-pressure rocks in the Seve Nappe Complex have yielded exclusively pre–Scandian orogeny Cambrian and Ordovician ages. Additionally, post–mid-Proterozoic igneous rocks of the Seve Nappe Complex are overwhelmingly dike swarms that were emplaced during the latest Proterozoic breakup of Rodinia, which have rift and MORB-type chemical signatures rather than arc and back-arc signatures, as has the Blåhø Nappe. We hypothesize that the Blåhø Nappe precursors formed on the upper plate, above a west-directed, late Cambrian to Ordovician subduction zone off the Baltican margin. Subduction of the Baltican margin, and possibly rifted fragments on the lower plate, produced the older Seve Nappe Complex eclogites and thrust the Blåhø and Seve Nappe Complex materials onto Baltica. This left the Blåhø Nappe and Seve Nappe Complex precursors on the lower plate during Scandian subduction and collision with Laurentia, allowing exclusively Scandian eclogite formation in the Blåhø Nappe. The Blåhø Nappe and Seve Nappe Complex thus seem to have distinct origins and should not be correlated with one another.