GEOCHEMISTRY OF THE MAFIC AND ULTRAMAFIC UNITS IN THE EASTERN BLUE RIDGE OF VIRGINIA AND NORTH CAROLINA: COMPARISON OF THE ASHE AND ALLIGATOR BACK METAMORPHIC SUITES AND THE LYNCHBURG FORMATION

Despite similar lithologies and occurrences, mafic and ultramafic units in the eastern Blue Ridge of Virginia and North Carolina have historically been interpreted as representing different tectonic origins. Mafic-ultramafic rocks in the Lynchburg Formation of northern and central Virginia have been described as mantle-derived dikes and sills within a rifted continental margin basin whereas those in the Ashe and Alligator Back Metamorphic Suites are regarded as ophiolitic fragments obducted in an accretionary mélange. Field relationships, mineral compositions, and major element analyses have been used in previous studies to separately support these interpretations, but direct comparisons are lacking.

We present new geochemistry on the mafic and-ultramafic units collected from the Ashe and Alligator Back Metamorphic Suites and the Lynchburg Formation using updated techniques of discrimination that leverage trace element and rare earth element geochemistry. Mafic-ultramafic rocks occur as linear to elliptical, locally folded, foliated bodies with long-axes parallel to regional strike. Contacts with surrounding metasedimentary rocks are generally concordant with the dominant foliation but may be faulted and/or truncate foliations. Mafic rocks contain the metamorphic assemblage of tremolite/hornblende ± actinolite ± chlorite ± quartz ± epidote/clinozoisite ± serpentine ± garnet ± titanite ± rutile and ultramafic rocks contain the metamorphic assemblage of tremolite + chlorite ± anthophyllite ± serpentine ± talc ± magnetite with few samples containing pyroxene and/or olivine. The geochemical data indicate significant local variability among the mafic-ultramafic rocks. One subset displays immobile and rare earth element concentrations typical of mid-ocean ridge basalts, while another shows enrichment in light rare earth elements and ratios of fluid mobile large ion lithophile elements to high field strength elements indicative of volcanic arc lavas. High concentrations of Ni and Cr along with highly depleted immobile elements in the ultramafic rocks suggest significant prior source depletion. Our data indicate that at least two suites of rocks with disparate origins occur in close association within the eastern Blue Ridge.