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Figure 8. Plot of 100(MgO/MgO+FeOt) versus TiO2 for mafic (metabasalt, metadiabase, and diabase) dikes of inferred late Neoproterozoic (25 samples, filled squares) and Early Jurassic (4 samples, filled circles) age that intrude basement rocks within the field trip area. Data for samples of two greenstone dikes (squares with diagonal crosses) that intrude basement are also plotted. Data from Hackley (1999), Wilson and Tollo (2001), and Tollo and others (in press b). Compositional fields for (a) relatively unaltered lava flows from the Neoproterozoic Catoctin Formation (10 samples; Badger, 1989), (b) mafic dikes associated with and assumed to be contemporaneous with the Catoctin Formation (20 samples; Badger, 1989), and (c) Early Jurassic Mount Zion Church Basalt from the nearby Culpeper basin (18 samples; Tollo and others, 1988) are plotted for comparison. |