MYLONITES ON THE MONROE FAULT IN KIRBY, VT

The Monroe Fault bounds the Bronson Hill sequence on the east and the Eastern Vermont sequence on the west in northeastern Vermont. A 30 meter thick zone of protomylonites is exposed in the bed of Kirby Brook where it crosses the Monroe Fault. Immediately east of the mylonitic zone is a coarse grained metagabbro of the Late Silurian Comerford Intrusive Complex and immediately west is the Early Devonian Meeting House Slate with cross laminae interpreted to be a primary sedimentary structure. The rocks mylonitized are interpreted to be mostly siliceous, argillaceous, and calcareous pelites of the Ordovician Ammonoosuc Volcanics. A narrow zone of foliated metagabbro borders the metagabbro to the east. We propose that the Monroe Fault is a Middle Devonian synmetamorphic thrust that is associated with a regional scale west facing recumbent syncline. Rankin, 1996, suggested the thrust was premetamorphic.

N20^oE-85^oE foliation in the mylonite is parallel to the foliation in the Meeting House Slate and nearby exposures of the Cambrian or Ordovician Albee Formation. Stretching lineation down the dip of the foliation is most distinguishable megascopically in the mylonites. Dynamic recrystallization in the mylonite took place during the development of the foliation under biotite-zone greenschist facies metamorphism sometime after the Early Devonian. Static recrystallization under biotite-zone metamorphism took place in Late Devonian. The static recrystallization and the low-grade of metamorphism made the identification of shear zone indicators difficult in thin section. Most sigma type quartz clasts, polycrystalline quartz porphyroclasts, and polycrystalline mantled quartz porphyroclasts indicate east side up shear sense in oriented thin sections cut perpendicular to the foliation and parallel to the stretching lineation.

Cross laminae in Meeting House Slate are exposed at a maximum distance of seven meters from the mylonite. Superimposed on these cross laminations is a zonal shear band cleavage (extensional crenulation cleavage), which may be related to cleavage refraction. The platy minerals in the assumed argillaceous laminae are oblique to the cross laminae and parallel to the foliation in the mylonite. The shear sense of this structure is east side up.