THE MIDDLE JURASSIC FLOOD BASALTS OF SOUTHEASTERN NORTH AMERICA

The central Atlantic magmatic province (CAMP) comprises dikes, sills and flows of tholeiitic basalt in west Africa, eastern North America and northern South America that formed close in time with the Triassic-Jurassic boundary (most magmatism at ca. 200-199 Ma; e.g., Nomade et al., 2007). The immense seaward dipping reflector sequences (SDRS) of the North American margin are correlated seismically with flood basalts of the South Georgia Rift basin, and each have been included with ca. 200 Ma reconstructions of the CAMP by many workers. Samples of these flood basalts from deep drill cores near Charleston, SC (the ‘Clubhouse Crossroads’ core, CC, and a state survey core in Dorchester County, DOR) have major, trace, and REE chemistry similar to the intermediate-titanium basalts of the CAMP. Laser incremental heating ⁴⁰Ar/³⁹Ar ages for CC-DOR basalt whole rock samples and plagioclase separates generally yield total gas and plateau ages younger than 180 Ma, with variations interpreted to reflect combinations of ⁴⁰Ar loss and uptake of extraneous argon. The most consistent results from each core were obtained for plagioclase separates from basalt sampled near flow tops: from DOR, 169.48±1.34 Ma and 172.35±0.80 Ma, and from CC, 169.1±2.0 Ma (2σ precision, each plateau with ~ 90% or more of ³⁹Ar_K released over 15 or more steps and MSWD=0.6-1.0). We interpret these to record crystallization of the ‘J-reflector’ flood basalt, as sampled in the CC-DOR cores, at ca. 170 Ma. This age result is consistent with original ⁴⁰Ar/³⁹Ar work on the CC cores by Lanphere (1983) and is the same as the mean paleomagnetic age determined by Phillips (1983). The new ages and correlation of the J-reflector to the SDRS are interpreted to indicate that the SDRS are also ca. 170 Ma (about 30 million years younger than the peak of CAMP magmatism). The new, younger ages are consistent with paleomagnetic and stratigraphic estimates for the age of the SDRS (Benson, 2003) and the earliest ocean crust of the North American and African margins. Thus, the new data in combination with a number of original studies indicate the J-reflector flood basalt of southeastern North America and the SDRS are not directly related to or part of the CAMP (though they may share a source with some CAMP basalts) but formed instead with continent separation and initial sea-floor spreading.