PUFFER, John H., Dept. of Geology, Rutgers University, 	
		Newark, NJ, 07102

Although  the rifting of Pangea during the Mesozoic was a rare 
geologic event, it was preceded by the rifting of Rodinia during the 
late Proterozoic (Rogers, l996). In both cases, continental flood 
basalts were extruded along major portions of intracratonic rifts.  The 
quantity of geochemical analytical data on a world-wide basis has 
reached the point where it is becoming possible to compare Pangean 
and Rodinian basalts with each other and with flood basalts not 
associated with the fragmentation of supercontinents.  For purposes of 
comparison the most useful data are derived from the initial 
outpourings which are typically among the thickest, most widespread, 
and least fractionated flows; although contamination or alteration at 
the base of basalt sections is locally a complicating factor.  Both of the 
initial supercontinent flow groups are tholeiites that contain little 
normative olivine or quartz and are both more compositionally 
uniform than most initial non-supercontinent flood basalts.
	There are, however, major differences in the composition of 
Pangean basalts compared to Rodinian basalts.  The Pangean basalts 
are dominated by a chemical population that is typified by the 
Talcott/Orange Mountain/Mount Zion Church flows  of eastern North 
America with narrow ranges of TiO2 (1.0 to 1.2  %), MgO (7 to 8 %) 
and P2O5 (0.1 to 0.2 %).  The Rodinian basalts are dominated by a 
chemical population that is typified by the Cloud Mountain, Tibbit 
Hill, and Catoctin flows of eastern North America with 1.5 to 4 % 
TiO2, 6 to 7 % MgO, and 0.2 to 1.0 % P2O5.  The contrasting 
chemistries  may be due to contrasting heat flow conditions and 
continental crustal plate thicknesses, but probably not to differences in 
the degree of fractionation or crustal contamination.  Isotopic data 
suggests that the magmas were derived from either undepleted or 
enriched mantle sources but it is unlikely that isolated hot spot sources 
or enrichment due to previous subduction can sufficiently explain the 
contrasting chemistries.

Rogers, J.J.W., l996, A history of continents in the past three billion years: 
Journal of Geology, v. 104, p. 91-107.