DIACHRONOUS TACONIAN DUCTILE DEFORMATION AND METAMORPHISM IN THE WESTERN PIEDMONT OF PENNSYLVANIA

The western Piedmont of PA is mostly underlain by siliciclastic metasedimentary rocks that experienced a complex history of deformation and metamorphism through the early to late Paleozoic. Earlier workers attributed the regional deformation and metamorphism to either Taconian or Alleghanian tectonism. The Taconian event was described as a northwestward episode of ductile flow that resulted in multiple phases of folding and the regional metamorphism in the Octoraro and Peters Creek Formations. Metamorphic zones were mapped in the Octoraro Fm and a broad garnet zone that occurs in the core of the Tucquan antiform is symmetrically mantled by pairs of biotite and chlorite zones, both to the north and south. The metamorphic zones are roughly parallel to the penetrative schistosity in the Octoraro Fm. These relationships demonstrat that the Tucquan antiform is younger than the regional metamorphism, but it also demonstrates that the regional metamorphism increases in grade in the structurally downward direction. The western Piedmont is geographically extensive and the peak metamorphism at any location may not have been synchronous with the peak of deformation. The Octoraro Fm contains albite porphyroblasts and a systematic study of the timing of porphyroblast growth relative to ductile fabric development was undertaken to see if a sensible pattern exists. The NW part of the Octoraro Fm (structurally highest) contain albite porphyroblasts with straight inclusion trails that are wrapped by the dominant schistosity. This suggest that albite growth predated the peak deformation. In the core of the Tucquan antiform, the albite (and garnet) porphyroblasts contain sigmoidal inclusion trails suggesting that grain growth began prior to deformation, but continued while deformation ensued. The southern part of the Octoraro Fm contains some of the most robust albite porphyroblasts, and in some cases small folds and shear bands are preserved as inclusion trail patterns within aggregates. But, the aggregates are incorporated into the regional schistosity. Although the albite aggregates began to grow when deformation was well underway, the ductile flow outlasted the grain growth. The overall pattern of albite growth relative to deformation in the Octoraro Fm is one of regional NW directed ductile flow through an upward migrating thermal pulse.