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
In the Paleoproterozoic of West Africa, there are two generations of volcanism which are up to
80 × 10
6
years apart c. 2185 and 2105 Ma and both contain tholeiitic as well as calc-alkaline
assemblages Hirdes et al., 1996. Several authors have proposed that ultramafic-mafic assemblages
are associated with tholeiitic basalts whereas granitic rocks are associated with calc-alkaline
suite e.g. Abouchami et al., 1990; Boher et al., 1992; Sylvester and Attoh, 1992. Now,
1. in several volcanic belts in Ghana and Coˆte d’Ivoire, synvolcanic granitoids occur within
tholeiitic piles, 2. in this study and in Ghana, in the Ashanti belt
Loh and Hirdes, 1996, the largest ultramafic bodies of the country occurs in a calc-alkaline
suite.
6
.
1
. Petrogenetic implications In the ultramafic-mafic assemblage, the crystal-
lization sequence inferred from the cumulates is olivine + chromite followed by clinopyroxene +
amphibole 9 orthopyroxene 9 biotite. Orthopy-
roxene is rare as discrete grains in the wehrlites, but is common as narrow exsolution lamellae in
Fig. 12. N-Type MORB-normalized multi-element diagram. Normalization values of Sun and Mc Donough 1989.
the more evolved gabbros Fig. 10. Plotting the data of incompatible elements e.g. U versus Th,
Fig. 11, we note that most of the points repre- senting mafic-ultramafic samples lie very close to
a straight line passing through the origin, consis- tent with fractional crystallization Joron and
Treuil, 1977, 1989. REE patterns Fig. 9 are flat 2 – 8 × chondritic abundance for the ultramafic
clinopyroxene. Plagioclase is never present in the wehrlites, and appears only in the gabbros. The
high Mg-number of both clinopyroxene 0.90 – 0.85 and orthopyroxene 0.88 – 0.84 reflects the
fact that they followed olivine very closely on the liquidus, before the magma was depleted in Mg
Elthon et al., 1982. Jaques and Green 1980 showed that the first phase crystallizing after
olivine in a cumulate sequence is determined by the degree of partial melting of the mantle
source:
plagioclase corresponding
to low,
clinopyroxene to medium and orthopyroxene to high degrees of partial melting. Ishiwatari 1985
showed also that high TiO
2
0.6 – 0.8 wt is found in clinopyroxenes in plagioclase-type cu-
mulates, moderate TiO
2
0.4 wt in clinopyrox- ene-type cumulates and low TiO
2
0.1 wt in those in orthopyroxene-type cumulates. The early
stage of crystallization of both pyroxenes and the moderate to low TiO
2
content of clinopyroxene average value of 0.22, st. dev. of 0.14 is consis-
tent with the parental magma being generated by a moderate to high degree of partial melting. The
comparatively low heavy REE, Y and Sc con- tents of the basaltic rocks Table 4 could thus
be a result of high degrees of melting of a de- pleted source Baker et al., 1994.
Evidence for relatively high crystallization pressures is suggested by the early crystallization
of aluminous chromite and clinopyroxene. Ac- cording to Dick and Bullen 1984, partition co-
efficients for Cr in spinel decrease significantly with increasing pressure, leading to lower Cr
contents CrCr + Al B 0.6 in spinels formed at high pressure. In addition, Irvine 1967 sug-
gested that, at pressures above the olivine-plagio- clase stability field \ 8 kbar, a basaltic liquid
will crystallize aluminous spinel and pyroxene rather than plagioclase. The low CrCr + Al ra-
tios 0.50 – 0.60 in the earliest crystallized spinels from Loraboue´ and the absence of olivine + pla-
gioclase assemblages may therefore be indicative of crystallization at elevated pressures. Moreover,
the presence of orthopyroxene exsolution lamel- lae in clinopyroxene of diopsidic composition,
the low Mg contents of chromite grains trapped in olivine and the absence of zonation in
chromite, olivine and pyroxene crystals are in- dicative of extensive subsolidus reequilibration
during slow cooling of the complex at high pres- sures.
Evidence of primary crystallization of brown amphibole
is demonstrated
by the
textural postcumulus phase enclosing olivine grains and
compositional rich in Cr, Ti and alkalis charac- teristics, similar to those reported in the layered
sequences of ophiolites in the Oman Lippard et al., 1986, in Halmahera, eastern Indonesia Bal-
lantyne, 1992, in amphibole gabbros, e.g. Ton- sina, Alaska Burns, 1985; DeBari and Coleman,
1989 and in picritic rocks, e.g. Scourie dykes, northwest Scotland Tarney and Weaver, 1987
as well as in lower crustal xenoliths from mature island arcs, e.g. the Aleutians DeBari et al.,
1987. So, the presence of primary magmatic am- phibole and biotite Green, 1982, and the late
crystallization of plagioclase Burnham, 1979; Al- lan and Carmichael, 1984 indicate relatively
high water contents in the parental magma com- patible with hydrous partial melting of a mantle
source affected by previous metasomatic events.
Thus, a combination of high P
total
and P
H
2
O
would cause an extended period of fractionation of Al-poor minerals, such as olivine and pyroxe-
nes, yielding plagioclase-free ultramafic cumu- lates.
The compositional
continuity between
volcanic and plutonic calc-alkaline rocks Figs. 8 and 10 is consistent with the fractionation of
such Al-poor phases leading to a residual magma represented by basalt. In addition, the clinopy-
roxene
and amphibole
compositional trends
Figs. 4 and 7 are also consistent with such a differentiation. Distinct linear correlation be-
tween highly incompatible elements, as well as the subparallel REE patterns, lend support to
fractional crystallization as the major differentia- tion process in this suite, in accordance with
models derived from mineralogical and major-el- ement data. Hence, the ultramafic and mafic cu-
mulate rocks are genetically linked to the magma that formed the calc-alkaline volcanic pile. Their
mineralogy and composition strongly suggests that they represent deep fractional crystallization
products of magmas that produced the volcanic rocks.
6
.
2
. Implications for tectonic setting It is now generally accepted that the compo-
nents of individual greenstone belts probably rep- resent a variety of tectonic settings e.g. Polat et
al., 1998. Moreover, various models have been proposed for the crustal growth of the West
African craton at 2.1 Ga: intracontinental rift Ratomaharo et al., 1988; Alric, 1990; Leube et
al., 1990, oceanic plateaus such as in the Nauru basin Abouchami et al., 1990; Boher et al., 1992;
Pouclet et al., 1996, intraoceanic island arc Sylvester and Attoh, 1992; Ama-Salah et al.,
1996 and back-arc basin Vidal and Alric, 1994; Ratomaharo et al., 1988. Here, we discuss only
the tectonic significance of the volcanic and plu- tonic assemblage of Loraboue´ as part of the
Boromo greenstone belt, but not the emplacement environment of the Birimian mafic volcanics as a
whole.
The volcanic and plutonic assemblage of Loraboue´ could represent a part of the crustal
members of an ophiolitic suite blue schists, sheeted dykes and the tectonized, depleted mantle
rocks having not yet been identified. However, occurrences of ophiolite as old as 2.0 Ga are rare,
e.g. Purtuniq Ophiolite Complex in the Cape Smith belt Scott et al., 1991 and Outokumpu
and Jormua Ophiolite Complexes Kontinen, 1987; Vuollo and Piirainen, 1989. In the same
way, these igneous rocks may also represent lay- ered intrusions and their related volcanic rocks.
Several aspects of the mineral chemistry as well as the phase relationships in the cumulate rocks
indicate that they were not crystallized at pres- sures and temperatures typical of a shallow ig-
neous intrusion. For example, typical ultramafic cumulate rocks of oceanic crust crystallize at shal-
low depths, with plagioclase crystallizing early in the sequence, directly after olivine and spinel. In
the
same way,
the penecontemporaneous
Bushveld complex contains layered ultramafic- mafic rocks in which orthopyroxene is the major
phase even when clinopyroxene is the minor phase and the plagioclase is mostly an interstitial com-
ponent of the lower zone e.g. Hatton and Von Gruenewaldt, 1990. So, the Loraboue´ assemblage
do represent neither typical layered intrusion nor typical ocean crust ophiolite.
Many authors e.g. Irvine, 1967; Dick and Bul- len, 1984; Leblanc, 1985; Haggerty, 1991; Stowe,
1994; Zhou and Robinson, 1997 have proposed a crystallochemical model that discriminate spinels
occurring in different tectonic environments Fig. 13.
As demonstrated
above, post-magmatic
changes affect the Loraboue´ chromite in produc- ing chromite of lower Mg ratio and higher Cr
ratio = CrCr + Al. So, the possible original compositions of the Loraboue´ chromite can be
inferred from the compositional trend Fig. 13
Fig. 13. 100CrCr + Al vs. 100MgMg + Fe
2 +
plot. See Fig. 5 for symbols: field of MORB spinel 1; field of ophiolitic
peridotite spinel from 2 ridge, 3 transitional, 4 arc clas- sification of Dick and Bullen, 1984; 5 field of ophiolite spinel
from arc-related crust from New Caledonia, Leblanc, 1985; 6 field of spinel from stratiform ultramafic-mafic complexes
Irvine, 1967; shaded area representing the possible original compositions of the Loraboue´ chromite.
Fig. 14. Plot of CeNb vs. ThNb. Data sources from Saunders and Tarney 1991.
and used in this discriminative diagram. Although the Loraboue´ chromite compositions overlap
more than one tectonic setting, their possible orig- inal compositions are located in the field of ophio-
lites from arc-related crust New Caledonia, Leblanc, 1987 rather than from mid-ocean ridges.
Moreover, they have Cr ratios 50 – 60 and Ti contents B 0.05 cation units lower than those of
layered intrusions such as Bushveld where Cr Cr + Al = 70 – 85 Irvine, 1967 and Ti = 0.1 – 0.3
cation units Stowe, 1994.
LILE Ba, Th, U and HFSE Nb, Ta, Zr, Hf, Ti, Y element ratios combined with light-REE
abundance have been widely used to identify the original tectonic environment. In this study, we
do not include alkalis K, Rb and Cs because of their mobility during secondary alteration e.g.
Ludden et al., 1982. On the contrary, Th, Ba, Ce and the HFSE are generally considered as immo-
bile in even strongly altered basalt, and therefore can be assumed to represent magmatic values with
reasonable confidence. For example, Saunders and Tarney 1991 proposed a CeNb versus Th
Nb Fig. 14 plot. This latter also allows compari- sons of the distribution of trace element data in
various basaltic rocks: ocean ridge basalt N-type and E-type MORB, island arc basalt IAB,
back-arc basalt BAB and intraplate basalt OIB. The high ratios of calc-alkaline types are
similar to those of IAB, consistent with the previ- ous light-REE and LILE enrichments and the
depletion in HFSE, even when the lowest ThNb ratios of the tholeiitic types bring them nearer the
BAB.
Recently, Albare`de 1998 focussing especially on
the Birimian
terranes of
West Africa
Abouchami et al., 1990; Boher et al., 1992, pointed out the importance of accreting oceanic
plateaus in crustal growth processes e.g. Stein and Hofmann, 1994; White et al., 1999. There-
fore, we have thought it necessary to plot our samples in the CeNb versus Ce diagram Fig. 15,
previously used by Abouchami et al. 1990 in order to show the similarity between the tholeiitic
basalts of West Africa and the Nauru basin. In this diagram, the CeNb ratios of the Loraboue´
tholeiitic rocks are significantly higher than those reported by Abouchami et al. 1990 and plot
separately from the field of the Nauru basalts. These results are in agreement with the existence
of two geodynamic environments for the Birimian tholeiitic rocks, either an intraoceanic island-arc
site Zonou, 1987; Dia, 1988; Sylvester and Attoh, 1992; Ama-Salah et al., 1996; this study or an
oceanic plateau site Abouchami et al., 1990; Bo- her et al., 1992; Pouclet et al., 1996.
As with most of the Birimian greenstone belts in the West African craton, we recognize both
tholeiitic and calc-alkaline lavas in the Loraboue´ prospect, but their chemical signature on the
whole are compatible with an island-arc environ- ment. So, we consider that all the volcanics of the
Loraboue´ area were generated in a same mag- matic environment. In reference to some modern
suites e.g. the Honshu arc, Gust et al., 1997 which include both tholeiitic and calc-alkaline
types, the presence of both tholeiitic and calc-al- kaline lavas could indicate a progressive change of
the primitive magma type, reflecting the variable contribution of a slab-derived fluid component to
the mantle wedge and the corresponding melting processes. There are some striking similarities in
composition and mineralogy between plutonic rocks in island arcs and the rocks of the Loraboue´
area. Thus, the rocks of Loraboue´, part of the Boromo greenstone belt, contain minerals and
mineral compositions that appear to be character- istic of cumulate rocks from the roots of island
arcs, in contrast to rift-related layered intrusions, and probably reflect the deeper levels of crystal-
lization in arcs generally, e.g. the Alaskan Tal- keetna Burns, 1985; DeBari and Coleman, 1989;
DeBari, 1997 and Kohistan Bard et al., 1980 island arcs.
The presence of massive Zn-Ag sulphide with accessory barite deposit at Perkoa, embedded in
the Birimian volcano-sedimentary sequence from the same Boromo belt, suggests an arc-related
depositional basin. Although albitization takes place in a wide variety of geological settings, the
occurrence
of gold
disseminated albitite
in Loraboue´ Be´ziat et al., 1998 and in the neigh-
bouring Larafella area Bamba et al., 1997 also hints at an arc-related geological setting. These
occurrences are similar to the gold-bearing al- bitized rocks of Alleghany, California Bo¨hlke,
1989, Bardoc-Kalgoorlie area, Western Australia Witt, 1992 and Sudbury-Wanapitei Lake area,
Ontario Schandl et al., 1994, respectively inter- preted as coeval with regional arc magmatic activ-
ity or associated with a continent-arc collision. In the same way, Bassot 1997 pointed out that
intense albitization has affected locally all the members of the Dale´ma calc-alkaline suite Pale-
oproterozoic of Eastern Senegal.
7. Conclusions