Journal of Petrology
C. Coulon , P. Vidal, C. Dupuy, P. Baudin, M. Popoff, H. Maluski, D. Hermitte
ABSTRACT
The Benue Trough is a continental rift related to the opening of the equatorial
domain of the South Atlantic which was initiated in Late Jurassic-Early Cretaceous
times. Highly diversified and volumetrically restricted Mesozoic to Cenozoic
magmatic products are scattered throughout the rift. Three periods of magmatic
activity have been recognized on the basis of 40Ar-39Ar ages: 147-106 Ma, 97-81
Ma and 68-49 Ma. Trace element and Sr, Nd and Pb isotope determinations, performed
on selected basaltic samples allow two groups of basaltic rocks to be identified:
(1) a group with a tholeiitic affinity, with Zr/Nb=7-11.1; La/Nb=0.77-1; 87Sr/86Sri=0.7042-0.7065
143Nd/144Ndi = 0.5125-0.5127; 206Pb/204Pbi =17.59-18.48; (2) a group with an
alkaline affinity, with Zr/N=3.6-6.8; La/Nb=0.53-0.66; 87Sr/86Sri=0.7029-0.7037;
143Nd/144Ndi=0.5126-0.5129; 206Pb/204Pbi =18.54-20.42. The geochemical data
lead to the conclusion that three types of mantle sources were involved in the
genesis of the Mesozoic to Cenozoic basaltic rocks of the Benue, without significant
crustal contamination: (1) an enriched subcontinental lithospheric mantle from
which the tholeiitic basalts were derived; (2) a HIMU-type (plume) component
from which the alkaline basaltic rocks originated ; (3) a depleted asthenospheric
mantle (N.MORB-type source), which was involved in the genesis of the alkaline
basaltic magmas. According to : (1) the postulated location of the St Helena
hot spot in the Equatorial Atlantic at about 130 Ma and, (2) the isotopic composition
of the alkaline basaltic rocks of the Benue Trough and their geochemical similarity
with the basalts of St Helena island, we conclude that the St Helena plume was
involved in the genesis of the alkaline magmatism of the Benue at the time of
opening of the Equatorial Atlantic. Moreover, the geochemical similarity between
the alkaline magmatism of the Benue Trough and that of the Cameroon Line suggests
that both magmatic provinces were related to the St Helena plume. Finally, the
temporal change of the mantle sources observed in the Benue Trough can be accounted
for by the recent models of plume dynamics, in the general framework of opening
of the Equatorial Atlantic.
Keywords:
Benue Trough / Mesozoic to Cenozoic magmatism / Equatorial
Atlantic / Mantle sources / St Helena plume
Return to header
Current Contents
................
J. Petrology Editor's Page
This page maintained by P Stuart and
J Rosenbaum
©1996 last updated 29 May 1996