MELT INCLUSIONS IN BASALT FROM DEEP DRILL CORES NEAR CHARLESTON, SC: IMPLICATIONS FOR THE DYNAMIC MAGMATIC SYSTEM OF THE CAMP

The Central Atlantic Magmatic Province (CAMP) represents the most extensive Large Igneous Province (LIP) yet identified. Studies of basalts in the southeastern USA are critical for evaluating the CAMP, considering their proximity to the geographic center of the province and an inferred Triassic-Jurassic triple junction. This is particularly so for investigating the magmatic processes that preceded extrusion of CAMP basalt flows. In an effort to understand more of the pre-eruptive dynamic setting for the CAMP, we have undertaken an electron microprobe study of phenocryst and melt inclusion chemistry for basalt flows sampled in deep drill cores near Charleston, SC. The USGS 'Clubhouse Crossroads' cores and a previously unreported USGS hydrogeology test core from Dorchester County (the 'St. George' core) are the focus of this study. Phenocryst assemblages in these basalts typically include augite and intermediate plagioclase, with occasional orthopyroxene, and finer-grained magnetite. Olivine is very rare in these tholeiitic basalts. Magmatic inclusions are commonly preserved within individual plagioclase phenocrysts, and they also occur in augite. Most inclusions range from 10-40 microns wide, and can be up to about 1 mm long. The inclusions tend to be elongate, with a crystallographically-controlled orientation, or equant and concentrated in growth-zones of crystals. The whole-rock, major-element compositions of all basalts sampled is relatively consistent (e.g., SiO₂ ranges from 52-54 wt. % and K₂O+Na₂O ranges from 2.5-3.5 wt. %). In contrast to the relatively consistent bulk-rock composition, the melt inclusions occur in three distinct chemical groups: a low Si and high Mg group, a high Na+K and low Si group, and a high SiO₂ and low Na+K group. We find the high Na+K/low Si group inclusions only in the stratigraphically lower Clubhouse Crossroads basalt, and suggest that they might represent early eruptions from the top of a highly differentiated magma chamber. Other melt compositions probably reflect a combination of fractionation and country rock assimilation in a long-lived and dynamic magmatic system.