HETTANGIAN FLOOD BASALTS ACROSS THE INITIAL 
PANGAEAN RIFT

	McHONE, J. Gregory, Graduate Liberal Studies Program, 	
		Wesleyan University,  Middletown, CT  06459-0519
	PUFFER, John H., Department of Geology, Rutgers 	
		University,  Newark, NJ  07102

The original extent of Hettangian (earliest Jurassic) rift basalts can be 
estimated from maps of their source dikes and of Mesozoic basins that 
contain remnants of the basalts.  At least two major provinces were 
formed, including a northern province of relatively high-TiO2 quartz-
normative tholeiites and a southern province of lower TiO2 quartz and 
olivine tholeiites.  A correlation of dikes and basalts on both sides of 
the Initial Pangaean Rift has been accomplished by new radiometric 
dates, by stratigraphic studies of associated basin sediments, and by 
comparisons of characteristic Ti, Mg, K and other large-ion elements. 
	The northern flood basalt province has relatively fewer but 
larger source dikes, and its HTQ-type basalts still remain in basins in 
northeastern North America and Morocco.  The northern province 
stretched from present-day northern Virginia (Culpeper basin) 
northeast-ward across Atlantic Canada to include the Avalon dike of 
Newfoundland, and eastward across much of modern Morocco, 
Algeria, Portugal, and Spain.  An area of at least 1.5 x 106 km2 is 
indicated.  Dikes and basalts of the northern province probably 
formed within a short time span (c.6 x 105 years) between 196 and 
202 Ma.  Gaps in the lava sheets may be due to highs in the 
Hettangian topography, and their relation to the younger ocean-crust 
magmatism remains unclear.
	The southern province is poorly constrained because most of 
its exposed basalts have been removed by erosion.  An initial basalt 
cover of possibly 8 x 105 km2 was spread across the present 
southeastern U.S. and conjugate areas of western Africa.  An age of 
c.196 Ma is likely for at least some dikes of the southern province, 
although a younger age near 180 Ma has been suggested for flood 
basalts beneath the South Carolina coastal plain and continental shelf.  
The latter section may overlap with a portion of the ³seaward dipping 
reflector² basalt and basaltic wedge at the Atlantic Ocean - North 
America margin, and it should thus be almost contemporaneous with 
the formation of the initial ocean crust.
	Environmental problems from such enormous areas and 
volumes of basalt include acidic rains and both cooling and 
greenhouse effects from the liberation of up to 3 x 1012 metric tons of 
CO2 and 2 x 1011 tons of SO4 aerosols, as well as considerable 
amounts of halides and ash.  Although the Initial Pangaean Rift 
basalts appear to be slightly younger than the Tr-J mass extinction, 
their possible role in such catastrophes should be assessed.