POLYPHASE DEFORMATION AND METAMORPHISM IN THE LOCH RAVEN SCHIST, MARYLAND PIEDMONT: EVIDENCE FOR A POST-TECTONIC THERMAL EVENT

Mid-Paleozoic progressive deformation and metamorphism of the Wissahickon group produced two distinct generations of garnet in the central Maryland Piedmont, one syntectonic and one post-tectonic. Metapelites of the Cambro-Ordovician Loch Raven Schist containing garnet+staurolite±kyanite± sillimanite were examined across a southwest-plunging Taconian F2b structure that folds the kyanite isograd. The pervasive foliation in these metapelites, S2b, is folded by meso- and megascopic F2c folds. Almandine-rich garnets (M1) grew throughout the progressive D2 event. M1 garnets are usually several millimeters in diameter with large poikiloblastic cores of elongate inclusions of the S1 foliation and narrow inclusion-free rims. Compositional profiles of M1 garnets display patterns typical of Barrovian metamorphism, with cores rich in Ca and Mn and increasing concentrations of Mg and Fe toward the rims.

Later pyropic garnets (M2) occur immediately east of the kyanite isograd and are not found with M1 garnets in this area. They are smaller, idioblastic, relatively free of inclusions, and essentially unzoned. M2 garnet boundaries grow across both the pervasive S2b foliation and hinges of F2c crenulations, clearly making them post-tectonic. This later metamorphic assemblage also includes fine-grained prismatic kyanite and sillimanite that are randomly oriented, also indicating post-tectonic growth.

Preliminary estimates of peak temperature and pressure for M1 garnet in the Loch Raven schist are 625ºC and 6.5 kb, consistent with findings of Lang (1999) for the underlying Setters Formation approximately two kilometers away. Tentative values for peak M2 conditions for the growth of pyropic garnet in the Loch Raven Schist are 575ºC and 5 kb. One or more late or post-Taconian heating events may be related to either the emplacement of pegmatites across this region ca. 470 Ma or the intrusion of the Ellicott City granodiorite at 458 Ma, approximately 30 km to the southwest. In either case, the post-deformation growth of magnesian M2 garnets in the Loch Raven Schist suggests the Paleozoic thermal history of the Maryland Piedmont may be more complex than previously thought.