CONDITIONS, RATES, AND TIMING OF METAMORPHISM IN BALTIMORE CITY: A CORRELATIVE RECORD OF ACADIAN OROGENESIS BETWEEN CENTRAL AND NORTHERN APPALACHIA?

Along the eastern margin of the USA and Canada, Mesoproterozoic Grenvillian basement rocks crop out as a series of high-grade inliers. In Maryland’s Baltimore Terrane and Vermont’s Connecticut Valley Trough, these gneiss domes represent some of the most outboard components of eastern Laurentia. Both are unconformably overlain by successions of quartzite, marble, and schist that are interpreted to represent Iapetan rift-to-drift stratigraphy that was metamorphosed at least once since Taconic orogenesis in the late Ordovician. In rural Vermont, these units have been the focus of numerous petrological, geochronological, and structural studies. However, despite greater accessibility and excellent exposure, equivalent studies are absent in central Maryland, where the rocks are found in urbanized environs.

In Baltimore City, the youngest of this stratigraphy is exposed as the metapelitic Loch Raven Schist. Typically, these staurolite- and kyanite- bearing schists contain relatively coarse (1–4 mm) garnet porphyroblasts with inclusion-free rims and abundant fluid inclusions in core regions. Locally, however, porphyroblast diameter reaches 4 cm and inclusions are demonstrably pre-tectonic relative to schistosity. Over 400 miles to the north, in Vermont, it has been demonstrated that similarly large garnet in the comparable Townsend Dam Schist grew in response to Acadian metamorphism over c. 4 Myr (see Gatewood et al., 2015 for details). However, in the central Appalachians, where the Avalon Terrane is not (thought to be) exposed, evidence for the timing of this event—indeed, whether these rocks even record an Acadian signature—remains elusive. Here, we report on phase equilibria approaches and U–Pb zircon and Sm–Nd garnet geochronology to assess the conditions, timing, and rate of metamorphism in the Baltimore Terrane’s Loch Raven Schist. These results provide the opportunity to address the prospect of synchroneity/diachroneity of tectono-thermal events along the Appalachian margin.