T96 Approaches for Extracting Shear Zone History from the Ductile Rock Record: Probing Their Initiation, Evolution, and Reactivation II

Tuesday, 27 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

RETROGRADE DEFORMATION AND FLUID FLOW AT THE BASE OF A BARROVIAN SEQUENCE, DUTCHESS COUNTY, NY

Paper number: T96-14
Program: Technical Sessions
Time: 7:35 PM – 7:45 PM

Authors

Abstract

The Taconic orogeny in eastern New York resulted in what has been called a classic Barrovian sequence of metamorphosed rocks ranging from shales to metapelitic gneisses. Layers of quartzite and marble separate the meta-pelitic rocks of the Barrovian sequence proper from the underlying Precambrian gneisses. Field observations of the direction of maximum extension and foliation curvature into shear zones in the quartzite indicate top to the west verging shear, consistent with the inferred emplacement of the Taconic Allochthon. EBSD and compositional mapping show that the quartzite is actually a meta-arkose with up to 40% K-feldspar, minor albite as well as trace phases. Preliminary investigation shows that overall the meta-arkose layer is highly deformed to grain sizes of 20 - 30 micron for both quartz and K-feldspar. The deformation grade as well as the proportions of quartz and K-feldspar are variable across the layer. While quartz has a weak fabric, consistent with inferred peak metamorphic conditions, K-feldspar likely deformed by brittle processes. Pressure solution in the presence of a free fluid resulted in phase mixing, with precipitation of quartz and K-feldspar in pressure shadows of larger grains. Near euhedral gedrite is Fe-rich in its core and becomes Fe-poor in the rim. Biotite is observed to crystallize interstitially and takes up Fe, suggesting that gedrite zoning is related to biotite crystallization. Quartz and K-feldspar grain boundaries are decorated with pores, further indicating fluid circulation. Quartz deformation transitions from ductile with macroscopic drag features at peak temperatures to brittle in localized, millimeter scale shear bands. The shear bands cut and offset parts of grains, indicating brittle conditions. Fe-rich oxides precipitated within shear bands and around grain boundaries indicate continued fluid infiltration post brittle deformation of quartz. The presence of chloride further indicates decreasing pressure. Overall the observations indicate retrograde, fluid present deformation at the base of the Barrovian sequence.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358918

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