Fig. 6. Ti versus Fe
3 +
in chromites.
SiO
2
plot Fig. 8, the Loraboue´ basaltic rocks are clearly distributed on both sides of the Miyashiro
1974 discriminant boundary. A first group made up of dolerites, isolated massive gabbros and
some basalts is tholeiitic, while the second group, made up of basaltic rocks, evolved gabbros and
rhyolites is calc-alkaline. The chronology can be established on the field, dykes of rhyolite cross-
cutting basalts of the first group Fig. 2A, anal. 14 in Table 4. Due to their cumulative nature, the
Fig. 7. Compositional variation of amphiboles. Al
IV
, Na + K and Ti in atoms p.f.u.
countered in the volcanic sequence in having higher TiO
2
1.5 – 3 wt and Cr
2
O
3
0.5 – 0.7 wt contents. Mn-rich ilmenite \ 5 wt MnO
partially replaced by titanite, apatite and zircon are common accessories in the more differentiated
gabbros Fig. 2G. Magnetite and Ni-Fe sulphide heazlewoodite and pentlandite are common by-
products of the serpentinization of olivine in ul- tramafic
cumulates and
gabbros. However,
Ti-magnetite is present as a primary phase in the isolated massive gabbros, dolerites and the vol-
canic sequence.
5. Geochemistry
Some thirty rocks exposed in the Loraboue´ prospect were analysed by XRF spectrometry for
major elements as well as Nb, Rb, Y and Zr, and by ICP-AES for Ba, Co, Cr, Ni and Sr at Chemex
Labs, Canada. The concentrations of the remain- ing elements were measured by ICP-MS at the
Laboratoire de Ge´ochimie, Toulouse. Fourteen representative analyses are reported in Table 4.
Whole-rock major and trace element analyses of cumulate rocks reflect their cumulus mineralogy;
for example, this is shown by the high MgO \ 36 wt, Cr \ 600 ppm and Ni \ 1000 ppm, as
well as the low Al
2
O
3
B 3 wt and CaO B 1 wt contents in the dunite. The moderate mg
MgMg + Fe = 0.40 – 0.66 and moderate levels of Cr, Ni and Co B 240, 110 and 50 ppm,
respectively of the basaltic rocks indicate moder- ately evolved basalts. In the FeOMgO versus
Fig. 8. FeOMgO vs. SiO
2
plot Miyashiro, 1974.
On the basis of chondrite-normalized rare earth element REE patterns Fig. 9, two types of
basaltic rocks can be also distinguished: a tholei- itic group with chondrite-normalized LaYb ratio
LaYb
N
in the range 2.1 – 3.1, similar to some tholeiitic basalts of Haute-Comoe´, Ivory-Coast
Group 2 from Pouclet et al., 1996, in the central part of Mako Sabodala region, N’Gom, 1995
and the Sirba region, Niger Subgroup 1b from Ama-Salah et al., 1996, and a calc-alkaline group
with LaYb
N
higher than 4 similar to the calc-al- kaline lavas of Haute-Comoe´, Sirba region, Diale´-
Dale´ma supergroups Bassot, 1987; Dioh, 1995 and of the Bouroum area, Burkina Faso Zonou,
1987. However, the tholeiitic group clearly differs from some of the tholeiitic Birimian basalts of
Haute-Comoe´ Group 1 from Pouclet et al., 1996, Mako Abouchami et al., 1990; N’Gom,
1995, the Sirba region Subgroup 1a from Ama Salah and al., 1996 and Bouroum Abouchami et
al., 1990; Zonou, 1987 which all exhibit flat to slightly light-REE-depleted patterns compatible
with an oceanic magmatic signature.
A plot of Al
2
O
3
versus their MgO content in the different lithological units shows a steady
trend of sharply increasing Al
2
O
3
from the plagio- clase-free Al-poor ultramafic cumulates through
the gabbroic and calc-alkaline basaltic rocks to
Fig. 9. Concentrations of the rare-earth elements REE for different lithological units of the Loraboue´ area C-1 chondrite
values of Sun and Mc Donough, 1989; rock numbers from Table 4.
Fig. 10. Abundances of Al
2
O
3
vs. MgO for suites of samples from the Loraboue´ area.
ultramafic rocks dunite, wehrlite and layered gabbro associated with the differentiated gabbros
plot in the tholeiitic field but appear clearly linked to the calc-alkaline suite. In the same way, the
highly carbonatized basalts e.g. anal. 12 in Table 4 plot in the same field because of the strong
decrease of SiO
2
induced by carbonate alteration.
Fig. 11. Abundances of U vs. Th for suites of samples from the Loraboue´ area. Note change in scale between upper and
lower parts of the figure.
cumulates, to light-REE-enriched 10 – 30 × chon- dritic abundance for basic rocks. With fractiona-
tion of basic magma, the REE abundances increase, the light-REE become fractionated, the
heavy-REE show a parallel rise, and the Eu anomaly decreases, becoming negative in the most
evolved gabbroic rocks. The latter present the strongest REE contents with 60 – 80 × chondritic
abundance.
If normalized to the composition of N-type MORB, the different types of Loraboue´ basalt, as
well as the dolerites and isolated massive gabbros, all show the classic features of arc magmatism,
namely large-ion lithophile element LILE en- richment and high field-strength element HFSE
depletion, with a large enrichment spike in Pb Fig. 12. Nevertheless, we note lowest contents,
notably for the LILE, in the tholeiitic samples.
6. Discussion
