Discussion Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol101.Issue1.2000:

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