Petrology and geochronology of an inverted metamorphic sequence in the western Cape Breton Highlands, Nova Scotia
University of New Brunswick
In the western Cape Breton Highlands of Nova Scotia, medium- and high-grade metasedimentary rocks of the Jumping Brook Metamorphic Sequence and structurally overlying Pleasant Bay Complex comprise an inverted sequence that formed in the Late Silurian-Early Devonian in response to terrane accretion on Cape Breton Island. Using numerical models and core-rim major element zoning in garnet, tight clockwise pressure-temperature paths of metamorphism were determined for the garnet and staurolite zones of the inverted sequence. Systematic discrepancies between thermodynamic model predictions and geochemical observations led to the discovery of ilmenites that sequestered Mn during prograde metamorphism, causing garnet core isopleth intersections to be decoupled from garnet-in curves. A multi-chronometer dataset revealed a significant discontinuity between the medium- and high-grade rocks of the inverted sequence. Detrital zircon geochronology indicates that the Jumping Brook Metamorphic Suite was deposited in the Silurian and derived from local sources in Ganderia, whereas the Pleasant Bay Complex was deposited in the Cambrian-Ordovician and derived from sources in Ganderia and Amazonia. Garnet and monazite geochronology constrain prograde metamorphism in the Jumping Brook Metamorphic Suite and Pleasant Bay Complex to a 7 Myr period between 395 and 388 M and a 22 Myr period between 418 and 396 Ma respectively. Despite differences in provenance, depositional age, and prograde metamorphic histories, apatite cooling ages indicate that the Jumping Brook Metamorphic Suite and Pleasant Bay Complex cooled contemporaneously in the Late Devonian. The geochronological dataset and tight clockwise pressure-temperature paths are consistent with a tectonic model in which volcanic arc material was accreted, transposed, and rapidly exhumed in the western Cape Breton Highlands along west-directed ductile shear zones as outboard terranes were accreted. An alternative Lu-Hf garnet geochronology method was developed using a staurolite zone schist from the Jumping Brook Metamorphic Suite. A population of garnet porphyroblasts were liberated from the sample and sorted into eight size classes. Lutetium-Hf ages for the individual size classes decrease with garnet grain size, consistent with diachronous nucleation during heating. Ages for the largest and smallest size classes define a minimum timescale of 7 Myr for prograde metamorphism, identical to independent constraints.