EVIDENCE FOR ORDOVICIAN-SILURIAN LOW-PRESSURE REGIONAL METAMORPHISM OVERPRINTED BY AN ALLEGHANIAN-AGE CONTACT EVENT, SOUTHEASTERN VIRGINIA PIEDMONT

Two thin lenses of andalusite-bearing mica schist occur near the western margin of the Hollister shear zone in the southeastern Piedmont of Virginia. The shear zone, which otherwise consists of phyllonite, forms part of the Eastern Piedmont fault system, and is bordered on the west in this area by the Lawrenceville granite (previously undated, but assumed to be Alleghanian). Although andalusite (as chiastolite) is obvious in hand sample, in most rocks it is completely pseudomorphed by white mica and the matrix appears recrystallized, indicating pervasive hydration/retrogression. Monazites in most samples are substantially replaced by retrograde allanite-apatite-thorite symplectites. Initially we believed the andalusite-bearing rocks to be the result of contact metamorphism by the adjacent Lawrenceville granite. To test this, we analyzed partially retrograded monazite grains from several samples using in-situ electron microprobe Th-U-total Pb chemical dating, with ambiguous results. However, one less-retrograded sample with remaining partially intact andalusite and unretrograded monazite yielded a single monazite age peak at 444 ± 5 Ma (49 points). The smaller, more altered grains in other samples yielded a more diffuse age population down to 350 Ma. These age results may be interpreted three ways: 1) the initial hypothesis was correct, but the Lawrenceville pluton is older than assumed; 2) the andalusite-forming event was older than, and unrelated to, granite emplacement, but fluids from granite solidification caused pervasive retrogression at roughly 350 Ma; 3) the granite is indeed Alleghanian (ca. 300 Ma), and fluids perturbed the Th-U-Pb system in retrograded monazite, producing anomalously young ages in highly retrogressed samples. To test these alternate explanations, we obtained a U-Pb zircon age (TIMS) for the granite. Four concordant grains yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 302.5 ± 0.4 Ma (MSWD=0.6), indicating an Alleghanian crystallization age and confirming the last alternative as the best explanation. The significance of an original low-pressure regional metamorphic event that produced andalusite is unclear, but the monazite age of 444 Ma is consistent with Ar-Ar cooling ages in the Carolina zone and with a proposed age of docking of the Carolina Terrane (Hibbard, Geology, 2000).