FIELD AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF THE PANTHERTOWN VALLEY FELSIC COMPLEX: WESTERN, NC

The timing and depth that melting occurs during collisional tectonics is not well understood, although it is a fundamental process during mountain building. The Alleghanian Blue Ridge Thrust Complex of western North Carolina contains evidence of multiple felsic magmatic pulses exposed in deep crustal slices. Previous work has focused on the timing of magmatism with little interpretation into melt source locations and flow paths.

The ~466 Ma, Whiteside igneous complex has been interpreted to have formed by magma percolating from the deeper crust during the Taconic orogeny via magma buoyancy. However, regional T-t constraints, and peak metamorphic conditions reported for Whiteside and adjacent gneisses, are also consistent with anatexis of mid-lower crustal, high-grade metasedimentary rocks. Original magmatic and intrusive structures have been modified by ductile deformation making geochronology and field studies alone insufficient to constrain melt origin.

Part of the Whiteside complex exposes a two-mica felsic gneiss in the Panthertown valley conservation area. This forms the core of a km-scale, refolded, NE-trending open antiform whose limbs contain metasedimentary gneisses into which the magmatic protolith is thought to have intruded. The fold has been incised along fracture zones offering > 80 m of structural relief across the fold. Geologic mapping in conjunction with a NW-SE sampling traverse across the fold for petrologic analysis and bulk rock Nd and Sr radiogenic isotope compositions seek to better constrain the melt source. Outcrop relationships across the ~10 m transitional contact between the felsic and metasedimentary protoliths include the absence of cross-cutting structures and are defined by gradational garnet bearing quartz-feldspathic leucosomes parallel to gneissic foliation. These suggest either segregation of localized partial melt from the adjacent metasedimentary rocks or melt percolation into adjacent rocks from a magma sourced from the lower crust. Isotopic analysis of samples from the transitional contact, the inner and outer valley igneous body, and adjacent high-grade metasedimentary rocks will further test these hypotheses, including looking for evidence of magma mixing.