The Alisitos arc segment is the southernmost and only part of the western Peninsular Ranges batholith accreted during the Cretaceous. Collision-related deformation is concentrated along the northern and eastern margins of the arc segment. While shortening within the Alisitos arc produced similar amounts of crustal thickening throughout the arc, suppression of parts of the lower crust of the Alisitos arc due to throw across the terrane-bounding faults varies substantially. Geobarometric change across the Main Mártir thrust suggests that ~15 km of additional crust was thrust onto the central Alisitos arc. Geochemical and geochronologic data from intrusive rocks of the Alisitos arc indicate arc magmatism was active before, during, and after collision. The data suggest that all Peninsular Ranges batholith intrusive rocks within the Alisitos arc were derived from a broadly similar primitive source, lacking interaction with evolved continental lithologies. Postcollisional intrusions from the central Alisitos arc adjacent to the Main Mártir thrust yield trace elemental signatures suggesting melt derivation at depths where garnet would be a stable residual phase. The spatial and temporal coincidence of these intrusions with the Main Mártir thrust suggests that the increased pressure of anatexis inferred for the depth of generation of these melts was generated by displacement on this fault. Further, close temporal and spatial characteristics, and similar geochemical characteristics between the central Alisitos arc intrusions and La Posta intrusions east of the Main Mártir thrust suggest that the Alisitos arc intrusions represent precursors to the much larger flare-up event. This observation supports models suggesting collision as a cause of magmatic flare-ups in arcs.

The ca. 570 Ma Catoctin volcanics, exposed in the Blue Ridge of northern Virginia, include metamorphosed rift-related basalts extruded during breakup of the supercontinent Rodinia. Field relationships, petrography, and geochemistry are used to decipher the stratigraphy for two areas of the volcanics, one at the base of the formation, and the other near its top. Geochemical characteristics of sequential flows can be explained by fractional crystallization of minerals commonly occurring in basalts. Intervening flows with slightly different geochemical features that cannot be explained by fractional crystallization from magma corresponding to an underlying flow, or by crustal contamination, most likely represent new pulses of magma. Positive Pb and negative Nb anomalies, coupled with ε Nd values of +1.5 to +4.5 and model ages that exceed crystallization ages by over 500 m.y., suggest that an enriched component was added to the mantle prior to melting. This component resulted from devolatilization of subducting sediments prior to assembly of Rodinia, likely during Grenville orogenic events. Stratigraphic control shows that lower lavas in the sequence contain more of the enriched component than upper flows.