THE CLUBHOUSE CROSSROADS FLOOD BASALTS IN THE CONTEXT OF THE CAMP AND THE SDRS

Flood basalt flows beneath the Coastal Plain of South Carolina, part of the South Georgia Rift basin (SGR), record a critical magmatic stage in the breakup of Pangea and development of the North American passive margin. These flows have broad compositional affinities with basaltic dikes, sills, and other flows developed at ca. 200 Ma on each of the circum-Atlantic continental margins that constitute the Central Atlantic Magmatic Province (CAMP). SGR basalts correlate seismically with the extensive wedge of basalt flows in the continental slope of eastern North America known as Seaward Dipping Reflector Sequences (SDRS). The “Clubhouse Crossroads cores” (obtained near Charleston, South Carolina) presently provide the most important and complete samples of the SGR basalts as correlated with the SDRS. We re-collected samples from the Clubhouse Crossroads cores, and an associated core, to further investigate their geochemical nature and test their association with the CAMP and SDRS. Among these samples, major-element basalt composition standardized to 8% MgO, Na8 ranges from 1.6-2.3%, Si8 - 49.5-51.6%, Ti8 - 0.98-1.10%, and (Ca8/Al8) - 0.75-0.82%. The range for Ca8/Al8 ratio is relatively high for Mesozoic basalts of eastern North America and consistent with shallow melting and a high degree of partial melting. Variations in K/Ti among these basalts suggest the source region was relatively heterogeneous. Also, the Na8 values for these basalt flows are distinctly lower than the Na8 values for the abundant Mesozoic dikes of the Piedmont. The Clubhouse Crossroads basalts seem quite similar in composition to the intermediate-titanium (ITi) CAMP tholeiites. Collectively, the data are interpreted to indicate that the tholeiitic flows of the South Georgia Rift basin near Charleston, South Carolina formed through extensive melting of a heterogeneous lithospheric source that was markedly shallower than that associated with dikes in the SC-GA-AL Piedmont. Such a melting regime is consistent with extreme lithospheric attenuation during late stages of Pangean rifting. The seismic correlations of these SGR flood basalts suggest that tholeiitic basalts derived from a relatively shallow, lithospheric source are a significant component of the SDRS along the U.S. East Coast.