Variations in chemical and Sr isotopic signatures of a series of basaltic flows have been correlated with newly mapped stratigraphy sections in the upper Precambrian Catoctin flood basalts in the central Appalachians. Element mobility through fluid interaction is commonly observed in flows with a well-developed fabric, but some samples with more massive textures (commonly in the cores of thicker flows) still retain relict clinopyroxene and igneous textures and show only minor element redistribution. Five whole-rock samples with relict igneous textures and a clinopyroxene mineral separate yield an isochron with an age of 570±36 Ma (2σ) and a strontium inital ratio of 0.7035±1. This age is consistent with recent paleontological evidence for a latest Precambrian time for Catoctin volcanism and, along with isotopic age determinations of 680∼710 Ma for other rift-related magmatism, suggests either a two-stage or a very long, protracted episode of continental rifting.
Paleogeographic reconstruction indicates that the original site of eruption for the lavas must have been at least 56 km to the east, in the same geographic location from which the Mesozoic Appalachian tholeiite terrane evolved. Variations in Sr isotopic signatures and differences in major-element abundances between these two sequences suggest mantle heterogeneities that must have developed beneath the eastern North American craton during the interim 400 m.y.