VOLCANISM IN THE CENTRAL ATLANTIC MAGMATIC PROVINCE

J. Gregory McHone, GLSP, Wesleyan University, Middletown, CT 06459-0519

    Mesozoic basins that preserve extrusive basalts of the 200 Ma Central Atlantic Magmatic Province (CAMP) may total about 300,000 km2. However, dikes and sills of CAMP that also fed basin basalts are spread over at least 10 million km2 within four continents, centered upon but extending far outside of the initial Pangaean rift zone. In addition, basalts of the East Coast margin igneous province (ECMIP) of North America, which cause the East Coast Magnetic Anomaly, covered about 60,000 km2 with perhaps 1.3 million km3 of extrusive lavas. If only half of the continental CAMP area was originally covered by 200 m of lava, the total volume of CAMP and ECMIP basalt exceeded 2.4 million km3 and may be the largest known subaerial flood basalt event.



    Tholeiitic basalts comprise nearly all CAMP magmas, divided among three general type groups.  The most common type is an intermediate TiO2 andMgO quartz tholeiite, which occurs in several variants through most of the province.  Low TiO2/high MgO olivine tholeiite is abundant in the western portion (southern USA), and high TiO2/low MgO tholeiite is common in the central part of the province (northern Brazil and SW coastal Africa).

    These types are well displayed by a triangular Mg-Fe-Ti oxide diagram, which also shows trends due to crystal fractionation or partial mantle melting. Very large dike systems (200 to 800 km long) in the province each define distinct variants of mantle melts.  Same-magma lavas from several of the large dike systems are found in several widely-separate Mesozoic basins in northeastern North America, indicating flows that originally may have exceeded 250,000 km2.

    Averages of volatile contents of eastern North American CAMP tholeiitic dikes and sills, in weight %, are: CO2 = 0.066; S = 0.046; F = 0.032; and Cl = 0.064. Atmospheric emissions of volatiles from extrusive basalts can be reasonably estimated as half of the volatile content of their comagmatic intrusive sources, mainly as gaseous plumes from lava curtains at the erupting fissures.  Volcanic emissions of these gases therefore ranged between 1.13 x 1012 and 2.33 x 1012 metric tons, which likely had major world-wide environmental effects.  Radiometric and stratigraphic ages indicate most of the magmatic activity was brief, widespread, and close to the Tr-J boundary, which is also marked by a profound mass extinction. A causal connection between these great lava flows and the mass extinction has not been demonstrated within Mesozoic basins, and it may depend on how precisely new radiometric dates for the basalts bracket the Tr-J boundary, and on new geochemical and fossil studies.