GEOCHEMISTRY AND PETROLOGY OF ASHE METAMORPHIC SUITE AMPHIBOLITES AT ELK KNOB STATE PARK, NORTH CAROLINA

Elk Knob State Park is located in the eastern Blue Ridge belt, ~15.3 km north of Boone, NC. Recent detailed geologic mapping of the Ashe Metamorphic Suite (AMS) at Elk Knob reveals three map-scale units, which includes (1) a structurally lower amphibolite member, overlain by (2) a middle pelitic schist member with minor amounts of amphibolite interlayered with ±muscovite+biotite semischistose gneiss, and (3) an upper amphibolite member. The purpose of this research is to discern the origin (igneous or sedimentary) for minor, interlayered amphibolite bodies within the pelitic schist member. This study compares the petrologic/geochemical characteristics of amphibolites within the schist to lower and upper member amphibolites from Elk Knob. The data are then compared to previous studies of AMS amphibolites. Elk Knob amphibolites show geochemical traits consistent with Misra and Conte’s (1991) intermediate-Ti, highly fractionated amphibolites (group II), which characterizes the majority of AMS amphibolites in northwestern N.C. Fractionation patterns are evident based on Zr variation diagrams that show strong trends of increasing Fe₂O₃, K₂O, P₂O₅, Sr, and Rb, and strong trends of decreasing CaO, MgO, V, and Ni. Plots of selected immobile trace elements for Elk Knob amphibolites display flat trends typical of N-type MORB, but vary to trends enriched in more incompatibe elements typical of E-type MORB. At least three trends are apparent in REE patterns with samples varying from depleted to enriched in LREE ([La/Yb]_N = 0.399 to 1.94), with one anomalous sample being highly enriched in LREE ([La/Yb]_N = 2.91). Slight to pronounced negative Eu anomalies for four samples suggest that plagioclase may have been a fractionating phase. An igneous origin is also supported by well-documented igneous rock fractionation trends, e.g., Niggli mg-(Al-alk)-ca values, in addition to tectonic discrimination diagrams that indicate N-MORB and E-MORB provenances.