EARLY JURASSIC MAGMATISM OF THE CENTRAL ATLANTIC MAGMATIC PROVINCE: REGIONAL VARIATIONS IN CHEMICAL COMPOSITION

Early Jurassic basaltic rocks of the Central Atlantic Magmatic Province (CAMP) are distributed along the Atlantic margin of North America from Georgia to Nova Scotia. The basalts are dominantly tholeiitic and show variations in chemical composition. According to the Fe-Ti-Mg relationships and trace element ratios the rocks can be subdivided into three groups. The first group is characterized by high FeO_tot/TiO₂ (11-30), but low Zr/Y (<2.6), (La/Yb)_n(1-1.6) and (La/Y)_n(<1.58) ratios. The second group has low FeO_tot/TiO₂ (<11) and high Zr/Y (>3.4), (La/Yb)_n(>2.4) and (La/Y)_n (>2.6) ratios. The third group has intermediate characteristics (FeO_tot/TiO₂ ~11-30; (La/Y)_n ~1.5-2.6; (La/Y)_n ~1.3-2.6) of the other two groups. The three groups have distinct geographic distributions. The rocks of the high FeO_tot/TiO₂ group are from the southern part of the CAMP, predominantly from the Deep River Basin (>75% samples) of North Carolina; the rest of the samples are from Georgia, Virginia and South Carolina. The rocks of the low FeO_tot/TiO₂ but high (La/Yb)_n and (La/Y)_n group are restricted to the northern part of the province, stretching from the Culpepper (Virginia) and Gettysburg basins (Pennsylvania) to the Fundy Basin (Nova Scotia). The group with intermediate characteristics consists of rocks from basins that are located between the Deep River and Gettysburg basins; particularly the Danville, Farmville and Culpepper basins of North Carolina and Virginia. The variations of the element ratios along the Atlantic margin from the south to the northeast are suggestive of the influence of a mantle plume. However, the trace element and isotopic characteristics of many of the basalts, including the North Mountain basalts of Nova Scotia, are indicative of a subcontinental lithospheric mantle source.