GEOCHEMISTRY OF PANGEAN AND RODINIAN
CONTINENTAL FLOOD BASALTS
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.
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