INVESTIGATING THE SOURCE AND RELATIVE TIMING OF MIXING FOR THE WHITSIDE COMPLEX: PANTHERTOWN VALLEY WESTERN NC

Examining syn-tectonic magmatism in the deeper crust helps in understanding the history and mechanical and petrologic evolution of convergent margins. The Alleghanian Blue Ridge Thrust Complex of western North Carolina exposes evidence of multiple Paleozoic felsic magmatic pulses emplaced into the deeper crust of the southern Appalachian orogen. Studies have largely focused on the timing of magmatic pulses with little investigation into magma source or evidence of magma mixing.

The granodioritic to trondjhemitic Whiteside felsic plutonic complex formed during the Ordovician Taconic Orogeny. Peak metamorphic conditions reported for Whiteside and adjacent high-grade metapelitic schists are consistent with upper amphibolite conditions (9 kbar and 700-775 °C). Most of the original intrusive field structures relationships and microstructures have been modified by ductile deformation, but locally, they are sufficiently preserved for study.

Part of the NE-most Whiteside complex and its margin is exposed in the Panthertown valley area of SW NC. Here, two-mica felsic gneiss structurally below micaceous schist and gneiss are folded into a km-scale, refolded, NE-trending open antiform. Detailed mapping, thin section petrography, and whole-rock Sr and Nd isotope analyses have defined two magmatic zones from the fold limbs into the fold core, transitions between which lack cross-cutting structures. The inner zone is dominantly trondjhemitic in composition with a small granodioritic component. This has a gradational contact with the outer migmatitic zone, which in turn grades into the micaceous schist and gneiss. Isotopic data across the transition between the inner and outer zones suggest that the trondhjemitic melt was not directly sourced from local amphibolite facies metapelitic schists, whereas its outer margins mixed with melt generated in a migmatitic contact aureole. Further, a gradient in foliation intensity from highest in the contact aureole and weakest in the structurally deeper parts of the trondjhemitic inner zone, and microstructures in quartz and feldspar, suggest deformation was coeval with migmatization and continued through cooling into the solid state.