THE MESOZOIC BASALT FLOWS FROM DEEP DRILL CORES NEAR CHARLESTON, SC: A MAJOR- AND TRACE-ELEMENT GEOCHEMICAL STUDY

The Eastern North American (ENA) Mesozoic igneous province is part of the much larger Central Atlantic Magmatic Province (CAMP). The CAMP comprises basaltic dikes, sills, and flows in eastern North America, northern South America, western Europe and western Africa; CAMP volcanism is associated with the breakup of Pangea and with the major Triassic-Jurassic mass extinction of marine and terrestrial fauna. Early Mesozoic basalt flows sampled from the Clubhouse Crossroads (CC) cores near Charleston, SC, and a previously unstudied hydrological test core from northwest of Charleston, SC (the 'St. George' core), provide important information regarding the petrology, tectonic setting, and age of CAMP volcanism. Initial geochemical studies of the CC flows (Gottfried and associates) reported two distinct intercalating chemical types of 'high-Ti' quartz-normative (TiO₂ > 1 wt. %) and olivine-normative basalts that can be correlated with differing Mesozoic dike swarms in the Piedmont. However, the CC basalts can also be interpreted as quartz-normative tholeiites (SiO₂ is 52%-54%), and documented compositional variations appear largely an artifact of secondary alteration. In addition, previous workers identified multi-element patterns for quartz-normative tholeiitic dikes from Georgia and Alabama suggesting crustal contamination of the source magma. We have analyzed basalt from the St. George core, and re-sampled and analyzed basalt from the CC-cores. The trace element patterns show large ion lithophile enrichment with prominent troughs at Nb-Ta (La/Nb=1.7), Sr, P, and Ti (TiO₂=1.2 wt. %). The basalts also have a slight light REE enrichment (La/Yb=1.9). These patterns suggest that the source for the CC and St. George basalts was lithospheric, of an ancient island-arc type, and is interpreted to correspond with the subcontinental lithosphere. These observations strengthen correlations of the CC flows with N-S and NW-trending dikes within the southeastern USA, in that the flows show marked major- and trace-element geochemical similarities to the intermediate-Ti dikes of the CAMP province (Salters et al., in prep).