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The Catoctin volcanic province represents a sequence of rift-related tholeiitic magmas erupted during the Late Neoproterozoic opening of the Iapetus Ocean basin. Geochemical data, coupled with stratigraphy for three sections in central Virginia, permit creation of a chemical stratigraphy. The stratigraphy can be divided into chemical subunits based on trends of relatively immobile elements. These trends can be modeled by fractional crystallization of plagioclase, clinopyroxene, and olivine, with few recognizable effects of crustal contamination. Discontinuities between chemical subunits imply replenishment of magma chambers, or contributions from new magma sources with slightly different chemical signatures. Bulk chemical characteristics of the metabasalts can be explained by upper mantle to midcrustal gabbroic fractionation from a mantle source. Chondrite-normalized plots of trace elements indicate that the magmas were derived from a fertile mantle source, in a model similar to those for other flood basalt sequences. Low Nb/Zr and Nb/Y ratios similar to those for magmas generated at the Hawaiian, Iceland, and Reunion hot spots, suggest a plume source. Relatively low Ce/Yb and Sm/Yb ratios imply that Yb was not retained by garnet in the mantle, so the zone of melting must have been above the garnet-spinel transition zone, at a depth of 40–70 km.

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