Polymetamorphism and metastability in Paleozoic schists of the central Appalachian Baltimore Terrane, USA

Metapelitic assemblages are often considered highly reactive during prograde metamorphism, such that they effectively record a history of tectono-metamorphic processes. Across eastern North America, metamorphosed rift-to-drift stratigraphy has been central to disentangling the terminal history of Appalachian convergence. We report results from a single metapelitic outcrop in the Loch Raven Schist of Maryland's Baltimore Terrane, where regional metamorphism is historically interpreted to have derived only from the Ordovician Taconic orogeny. An aluminum-rich litho-horizon alludes to two phases of metamorphism: an early, low/medium-pressure (P) sillimanite-stabilizing phase largely overprinted by a subsequent medium/high-P kyanite-stabilizing event, terminating at c. 391–383 Ma. This provides evidence of substantial crustal disturbances in the central Appalachians during Avalonian collision of comparable timing to equivalent events in the north. In a subordinate and aluminum-poor litho-horizon, large (2–3.5 cm) garnet provides geochronological evidence only for a protracted phase of metamorphism at 440–424 Ma. We interpret this age as recording the early, low/medium-P phase of metamorphism that either: (a) extends the duration of the Taconic event; or (b) provides new evidence for metamorphism associated with Silurian tectonism and/or extension in the Central Appalachians. However, this assemblage appears to have been metamorphically unresponsive during Devonian overprinting, despite peak metamorphic conditions of ∼670 °C and ∼8 kbar. We suggest that a potential combination of the paucity of fluid, limited strain accumulation, and coarse refractive assemblages stabilized a kinetically sluggish and metastable composition.