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The Origin and Significance of Metamorphosed Ultramafic Rocks in the Chopawamsic Terrane, Buckner Quadrangle, Virginia
Vaka, Nicholas
Vaka, Nicholas
Abstract
Recent geologic mapping of the Buckner quadrangle near Mineral, Virginia in the Piedmont province has identified 14 previously unknown lensoid bodies of metamorphosed ultramafic rocks. These rock bodies occur within the Ta River Metamorphic Suite, a rock unit comprised of amphibolite, amphibolite gneiss, and granite hypothesized to be the oceanward facies of a volcanic arc. In this study, I report field and petrographic observations, and whole-rock major and trace element compositions for six metamorphosed ultramafic rock bodies. Three broad rock types are present: amphibole-chlorite rock, actinolite-chlorite schist, and chlorite-actinolite-olivine rock. Mg\# are high for all rocks (76.67-81.27), and the rocks display a narrow range in SiO\textsubscript{2} (43.38 to 52.40 wt.\%), Al\textsubscript{2}O\textsubscript{3} (4.99 to 11.94 wt.\%), and CaO (2.3 to 6.4 wt.\%). All rocks contain relatively high concentrations of Ni (up to 1014 ppm) and Cr (up to 2942 ppm). Rare-earth element (REE) chondrite-normalized plots show slight light REE-enrichment and flat patterns with slight negative Eu-anomalies. Trace element primitive mantle-normalized plots show flat patterns with troughs at lithophile elements. The actionlite-chlorite schist has the most unique textural, mineralogical, and chemical properties of all samples and is likely a metasomatic alteration boundary rock. Normative mineralogy indicates orthopyroxene-rich protoliths for all samples. Major element oxide compositions, REE chondrite-normalized and trace element primitive mantle-normalized plot patterns, highly abundant normative orthopyroxene, and field relations far from faults suggest that these rocks have a likely origin as ultramafic cumulates from the Ta River Metamorphic Suite mafic rocks and not as ultramafic portions of ophiolites.
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2021-04-01
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Geology