MAFIC AND ULTRAMAFIC ROCKS OF OLISTOSTROMAL ASSEMBLAGES OF THE BLUE RIDGE IN SW NC: INSIGHTS INTO CONVERGENT MARGIN PROCESSES OF THE PALEOZOIC

Earlier workers (see Raymond et al, 1989) identified a zone between the Brevard and Hayesville fault zones in the eastern Blue Ridge of the Southern Appalachians as a block-in-matrix terrane, possibly representing a subduction complex produced by Taconic collision. Mafic and ultramafic bodies are distinct constituents of this terrane, as they are recognizable despite high P-T metamorphism. Ongoing field and petrologic studies of several mafic/ultramafic associations, as part of the NSF REU Site research program run jointly by USF and WCU, provide insights into the origins of these units that place constraints on the evolution of a Paleozoic convergent margin in this region.

The westernmost Cullowhee olistostromal terrane includes interlayered mafic and ultramafic rocks as part of the Buck Creek, Lake Chatuge, and Carroll Knob complexes. Buck Creek and Lake Chatuge are similar lithologically (dunite-metatroctolite-amphibolite/gabbro), geochemically (signatures consistent with ultramafic/mafic crustal portions of "high Ti" ophiolites), and in terms of metamorphic P-T trajectories (deep burial at >12 kbars, ~800C). Both complexes are in close proximity to the Hayesville thrust and may represent fragments of deeply subducted ocean crust uplifted during collision. The Carroll Knob complex, further east of the Hayesville thrust, is dominated by amphibolites with much less extensive ultramafic rocks Although Carroll Knob amphibolites are lower in Ti, they are still consistent geochemically with mafic cumulate rocks in ophiolites.

To the NE in the Cullowhee terrane, mafic and ultramafic bodies are typically smaller, more isolated and less intimately associated with amphibolite. Lenses and layers of mafic and ultramafic rock range in scale from sub-meter to map scale. Mineral assemblages and chemical compositions differ from those in the west. The petrogenesis of these ultramafics has not yet been established, but existing data points to more residual compositions, consistent with upper mantle material incorporated via subduction erosion of the overriding plate. Our observations support interpretations of the Cullowhee terrane as a subduction complex assemblage developed in the early Paleozoic, with oceanic lower-plate rocks emplaced along the suture during Taconic collision.