FORMATION OF THE PEACH BOTTOM SLATE BY DYNAMIC RETROGRADE METAMORPHISM, PENNSYLVANIA

We propose that volumetrically significant slate can form by retrograde metamorphism of schist by dynamic recrystallization and grain size reduction in a major ductile shear zone. Although most slates form by low-grade prograde metamorphism of shale, minor occurrences of slaty phyllonite in high strain ductile shear zones do occur.

The Peach Bottom slate (PBs) was recognized as the hardest slate on Earth in the 19th century, and is still renown for durability and color retention. The PBs defines a 1-1.5 km wide belt that dips steeply, strikes NE and resides within a major Alleghanian strike-slip shear zone. The PBs is laterally bound by sheared portions of the Peters Creek Fm, but extends along strike into a belt of gray-black phyllonite (Drumore tectonite). Both across and along strike, the slate grades outward into silver-gray schistose rock, similar to rocks in the adjacent Peters Creek Fm. Whole rock, trace element, and REE chemistry are remarkably consistent across the slate belt and adjacent higher-grade pelitic lithologies. The transition from slate to phyllite across strike, occurs over a distance of about 100m, and petrographic analysis across this transition revealed 2nd generation muscovite and chlorite that formed from primary micas. SEM data across this transition revealed that micas in the phyllite are bladed- and tabular-shaped (~50-100 microns), whereas the slate contains planar aggregates of anhedral micas with individual grains (<10 microns). Rare large micas with "shredded" margins occur in the PBs. We interpret these textural variations to represent grain-size reduction of larger relict micas. Chloritoid and feldspar grains form porphyroclasts with asymmetric quartz-muscovite pressure fringes. Porphyroclasts of mica are abundant, in addition to shear bands and mica-fish. All these microstructures form, to some extent, by dynamic recrystallization and result in grain size reduction. Some portions of the slate belt contain abundant quartz veins that exhibit extreme dynamic recrystallization, grain size reduction, and preferred grain shape orientation. We conclude that the texture and fabric of the PBs are due to dynamic retrograde metamorphism.