THE WIDESPREAD UNIFORM RIFT-RELATED EXTRUSIVES, AND SOME LOCAL DIVERSE INTRUSIVES, OF THE CENTRAL ATLANTIC MAGMATIC PROVINCE

The chemical compositions of the initial rift-related CAMP flood basalt flows are remarkably uniform with province-wide TiO₂ contents averaging 1.1 percent with a STDEV of only 0.07. These initial flows were extracted quickly from their source through rifts and probably represent a primary magma. A second batch of similar magma from a slightly depleted source was locally extruded onto eastern North America, but compositional variation within the interior of these secondary flows is largely confined to post -extrusive differentiation. However, in contrast to the initial flows, the dike and sill compositions described as part of the CAMP are highly diverse. This diversity is due in part to a combination of factors including: 1) processes such as lateral fractionation and viscosity segregation that can lead to variable sill and dike compositions, particularly those intruded during the waning stages of CAMP activity after most magma had already extruded; 2) contamination and hydrothermal alteration of CAMP intrusions, particularly chloritization of thin dikes; and 3) possible misidentification of some dikes due to less stratigraphic control than possessed by the flows. Some dike populations may need to be radiometerically dated before they can be confirmed as part of CAMP. Viscosity segregation of late magma batches can result in dike composition variations related to depth. This may explain the occurrence of some olivine normative dikes exposed at deep erosional levels throughout southeastern North America that are genetically related to late or drift stage LTQ magma. In addition, chloritization can result in compositions resembling olivine normative diabase such as the contact zone of the Laurel Hill intrusion of New Jersey. We, therefore, propose that CAMP magmatism was dominated by a widespread chemically uniform but short lived event triggered by the initiation of extensional plate motion and constrained by the flux content of an arc-type source. The degree of chemical diversity throughout CAMP is, however, skewed by local alteration of some early dikes and by the occurrence of local late-stage melts, intruded at relatively sluggish rates, that were susceptible to differentiation.