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Mineralogical and Chemical aiability plus a Three-AMphibole Occurence in an Outcrop of the Chopawamsic Formation, Cenral Piedmont Province, Virginia
Dill, Rebecca A.
Dill, Rebecca A.
Abstract
A spectacular ~1000 m? 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 and is characterized by abundant garnet porphyroblasts. We interpret this finer-grained rock to represent one or more deformed dikes. The occurrence of garnet in these rocks appears to be a function of slight differences in bulk composition, specifically, relative abundances of Fe and Al. A rare three-amphibole assemblage consisting of hornblende, gedrite and cummingtonite 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).
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2001-01-01
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Geology