ORIGINS AND EVOLUTION OF AMPHIBOLITES FROM THE GLADE GAP EXPOSURES OF THE CHUNKY GAL/BUCK CREEK MAFIC-ULTRAMAFIC COMPLEX, WESTERN NORTH CAROLINA

The Glade Gap roadcut exposures along US 64 in Clay County, NC, are mapped as part of amphibolite sequences within the Buck Creek mafic-ultramafic suite. However, these amphibolites differ from the others in the Buck Creek suite in that they include thin ‘pink’ horizons of Mn-rich garnet and associated potassium feldspar (Klute and Ryan 2006), and they have been intruded in places by fine-grained felsic igneous rocks. We have analyzed a suite of Glade Gap amphibolites and their associated felsic intrusives to try and evaluate models for their origin and metamorphic evolution.

Our examined Glade Gap amphibolites are strongly foliated with primary hornblende and plagioclase, varying from amphibole-rich to “salt and pepper” textures. Minor minerals in our samples include both rutile and titanite, as well as epidote and abundant biotite. In terms of major element chemistry our amphibolites have basaltic compositions, but do not show the elevated Mn and K seen in the garnet-bearing samples. The felsic dikes are tonalitic/trondjhemitic in character, with abundant quartz and sodic plagioclase, bulk Na₂O/K₂O ratios of 6-12, and bulk SiO₂ ≈ 70%. These intrusions both crosscut the amphibolite fabric as well as extend along foliation planes.

There is considerable fine-scale variability in the Glade Gap exposures, with dike-rich zones and zones with “pink’ garnet schlieren separated by no more than 1-10 meters. Our new samples confirm a basaltic protolith for these amphibolites, as had been previously suggested, but do not record anomalous concentrations of Mn or K which might be suggestive of marine hydrothermal sediments. The presence of epidote and biotite in amphibolites near the dikes may point to metamorphic re-equilibration of Glade Gap rocks associated with this intrusive activity.