MINERALOGICAL AND CHEMICAL VARIABILITY PLUS A THREE-AMPHIBOLE OCCURRENCE IN AN OUTCROP OF THE CHOPAWAMSIC FORMATION, CENTRAL PIEDMONT PROVINCE, VIRGINIA

A spectacular ~1000 m² outcrop of the otherwise poorly exposed Chopawamsic Formation (Ordovician) occurs along the Willis River in Buckingham County, Virginia. Two main rock types occur here: 1) a medium-grained, somewhat heterogeneous amphibole gneiss that makes up the bulk of the outcrop; and 2) a finer-grained, darker rock that is typically in sharp contact with the gneiss. We interpret this finer-grained rock to represent one or more deformed dikes. Although both rocks are amphibolites dominated by hornblende and plagioclase, they show striking local differences in mineralogy. The dike rock is characterized by abundant garnet porphyroblasts, whereas much of the gneiss is garnet-poor, and this obvious mineralogical contrast prompted our initial study. The occurrence of garnet in these rocks appears to be a function of slight differences in bulk composition. Specifically, the garnet-bearing dike rock is lower in Mg# and higher in absolute amounts of Fe₂O₃(T) and Al₂O₃ at a given level of SiO₂ relative to the gneiss. Collectively, six samples from various places in this outcrop display a wide range in major element composition [SiO₂: 45-60 wt%; MgO: 1.9-6.9 wt%; Fe₂O₃(T): 10-21 wt%; CaO: 2.6-10.4 wt%; Na₂O 3-5.7 wt%]. Dike rocks are more evolved (qtz normative, Mg# ~36) than the typical gneiss (ol normative, Mg# ~46), although the gneiss includes the most Si-poor and Si-rich compositions. A rare three amphibole assemblage consisting of hornblende (X_Mg=0.50, ~15.4 wt% Al₂O₃) + gedrite (X_Mg=0.53, ~13.6 wt% Al₂O₃, ~1 wt% Na₂O) + cummingtonite (X_Mg=0.58) occurs with abundant garnet in the most mafic and Ca-poor variety of the gneiss. All combinations of mutual contacts among the three amphiboles are present, which we interpret as evidence for equilibrium. The variable composition of the gneiss over relatively short distances probably reflects original protolith heterogeneity. This heterogeneity might represent eruption of compositionally distinct lavas, but could be due to alteration by sub-volcanic hydrothermal fluids, for which there is abundant evidence in this region (e.g., massive sulfide deposits).