61 D20. Structural Geology, Microstructures, and Geochronology

Sunday, 22 March 2020
1:30 PM - 5:30 PM
Lake Fairfax

REFINING THE METAMORPHIC CONDITIONS AND STRUCTURAL SIGNIFICANCE OF THE CHUNKY GAL MOUNTAIN FAULT, CENTRAL BLUE RIDGE, NORTH CAROLINA, BY INTEGRATED EVALUATION OF METAMORPHIC ASSEMBLAGES, STRUCTURAL FABRICS AND MONAZITE AGES

Paper number: 61-4
Program: Technical Sessions
Time: 2:30 PM – 2:50 PM

Authors

Abstract

The Chunky Gal Mountain Fault (CGMF) separates two lithologically-distinct Central Blue Ridge terranes and is one of the few faults in the region that preserves ductile shear zone fabrics. Aluminous rocks within and near the fault zone contain large garnets with distinct compositional zoning and low variance matrix and inclusion assemblages that suggest re-evaluation of isograd locations in the vicinity of the CGMF. CGMF shear zones are steep with apparent normal shear sense; however, microstructures and electron backscatter diffraction patterns in quartz indicate a shallow movement direction with varied kinematics. Large garnet porphyroclasts in aluminous protomylonites include Sil ± Ilm ± Rt (as inclusions and in matrix) with either Sil or Ky+Ms in the matrix. Lower aluminum mylonitic rocks with garnet and K-feldspar porphyroclasts locally have fine matrix muscovite. Two distinct monazite age populations are present; core domains (~457-497 Ma) and rim/matrix domains (~440-463 Ma). These ages overlap the regional Taconic metamorphic ages. Asymmetric tails on matrix monazites are consistent with other kinematic indicators in the slide.

Previous work shows amphibolite facies (~725oC; 0.6 GPa) conditions for CGMF rocks, with granulite facies conditions (850oC±20; 0.8 GPa) at Winding Stair Gap (WSG) ~ 10 km to the NE. The majority of garnets in rocks near CGMF and WSG lack compositional zoning, a feature interpreted to result from diffusion at high temperatures. We focus on aluminous rocks with low variance assemblages (Ky-Sil-Ilm-Rt) and large garnets that preserve evidence of prograde zoning. Pseudosection thermobarometry and P-T path estimates from our CGMF samples indicate peak conditions (750-825oC; 0.8 GPa) similar to WSG granulite facies conditions and to peak metamorphic conditions determined from nearby ultramafic rocks. Previous amphibolite-facies estimates for CGMF rocks may reflect retrograde rather than peak metamorphic conditions.

Large garnets in Ms-St schist collected ~1.5 km west of CGMF outcrops near the previously mapped 1st/2nd Sil isograd show strong core-rim zoning (incr. Mg, decr. Mn and Ca) more typical of greenschist facies. We propose that peak to post-peak movement along the CGMF may have carried Taconian granulite facies rocks over nearby greenschist facies rocks to the west.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-345345

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