IS THE MELT-IMPREGNATED SANTA LUZ CHROMITE PERIDOTITE, BAHIA-BRAZIL, AN EARLY STAGE OPHIOLITIC PERIDOTITE REMNANT OF THE RIO ITAPICURU GREENSTONE BELT EVOLUTION?

 

 

Oliveira, E.P.¹, Escayola, M.², Araújo, M.G.S³

 

^1. Instituto de Geociências - Unicamp. elson@ige.unicamp.br

^2. Instituto de Geociências – Universidade de Brasília. escayola@unb.br

^3. Petrobrás. marcelusglaucus@petrobras.com.br

 

 

ABSTRACT

The Santa Luz chromite-rich peridotite complex crops out in the Serrinha Block of the Itabuna-Salvador-Curaçá orogen aproximately between banded gneisses of the Archaean basement and metasupracrustals of the Paleoproterozoic Rio Itapicuru greenstone belt. It comprises serpentinites, serpentinised peridtotites, gabbroic patches, amphibolite dykes and massive and disseminated chromites. This rock association along with feldspathic peridotites at the dykes contact is interpreted as melt percolation and impregnation of mantle peridotite. Massive chromitites show major element chemistry and platinum group elements signature similar to ophiolite chromitites. Aplite and pegmatitic dykes intrude the peridotite complex. The complex is older than 2085 Ma and younger than 2983 Ma based on U-Pb ages obtained respectively on an aplite dyke and host banded gneiss. So far, Sm-Nd data failed to constrain the timing of melt impregnation. The Santa Luz peridotite complex might have its origin related either to the initial rifting stage of the Rio Itapicuru greenstone belt or to early extensional events in the Serrinha Block.

 

Keywords: Peridotite, melt percolation, chromitite, ophiolite, Brazil

 

Introduction

The discovery of ophiolitic remnants in the geological record is a subject of major interest not only for tectonic reconstruction of ancient orogenic belts but also to constrain how far back in time modern style plate tectonics has bee in operation.

Ophiolites are thought to have formed at mature mid-ocean ridges or back-arc basins  (e.g. Condie 2005), and their lithological succession generally comprises a layer of pillowed basalts, often with interlayered chemical sediments, followed downwards by sheeted dykes, noncumulate gabbro, cumulate gabbro and ultramafics, and tectonized mantle peridotite. Although relatively well-ordered ophiolite complexes are sometimes preserved (e.g. Oman, Troodos), most commonly they are considerably disrupted and occur in apparently chaotic terrains.

In contrast with the idealized section of the oceanic lithosphere suggested for several ophiolite complexes, the Alpine-Apennine ophiolites have anomalous lithological successions. Among other features (reviews in Rampone & Piccardo, 2001; Piccardo, 2003; Muntener & Piccardo, 2004), (i) MORB-type gabbroic rocks are intruded into mantle peridotites and (ii) serpentinized mantle peridotites are directly covered by MORB lava and oceanic sediments. These particular features are interpreted as steps of rifting during transition from passive lithosphere extension to active oceanic drifting.

Here, we comment on field relationships and geochemical signatures of chromite-rich peridotites and associated mafic rocks of the Santa Luz ultramafic complex, which render them comparable to Alpine peridotites. The implication for the early evolution of the Rio Itapicuru greenstone belt is also addressed. 

GEOLOGIC SETTING

The Santa Luz ultramafic complex (also known in the literature as the Pedra Preta basic-ultrasic complex; cf. Carvalho Filho et al., 1986; Oliveira & Knauer, 1993) crops out 2 km easterly of the Santa Luz city, state of Bahia. It is known for the high-quality refractory chromite that has been exploited since the Second World War. Currently, Magnesita S.A. holds the right of mineral exploitation.

The complex is a small body (circa 2.5 km long and 1.6 km wide) conformably interleaved with NW-trending, tightly folded, nearly-vertical banded gneisses (amphibolite and felsic gneiss) of the basement of the Paleoproterozoic Rio Itapicuru greenstone belt. These banded gneisses may be the end products of deformation of Archaean grey gneisses intruded by much younger mafic dykes, as deduced from field relationships farther west of the mine. The contact between the regional gneisses and the ultramafic body is tectonic and marked by narrow shear zones sometimes rich in biotite-phlogopite; the ultramafic rocks may have been folded along with the host gneisses to account for minor occurrences of serpentinite within the latter.   Although the rocks are highly weathered in the mine open-pit, the complex is chiefly composed of serpentinite and serpentinized harzburgite crosscut by quartz veins and leucocratic aplites and pegmatites. Fractures and shear zones are common and they are usually associated with alteration of the ultramafic rocks into biotitite and tremolitite. However, one of the most outstanding features of this complex is the occurrence of dykes, patches and irregular bodies of coarse- and medium-grained metagabbro and metadiabase (now converted into amphibolites) intrusive into the serpentines or in structural conformity with them Fig 1).

 

 

Figure. 1. Amphibolite dykes intrusive into serpentinised peridotite, Santa Luz peridotite complex, as

evidence for melt impregnation.

 

At the dykes contact, the peridotites show feldspar spots and veins. We interpret all these features as impregnation of peridotite by basaltic melts. Geochemical data presented below on the massive chromitites and amphibolite dykes further strength the model.   

GEOCHRONOLOGY

Five representative samples of amphibolite dykes, gabbro patches and feldspathic peridotite were analysed for Sm-Nd isotopes in order to constrain the timing of melt impregnation and possibly also the onset of rifting. Unfortunately, the data fit in no reasonable isochron and more data should be added in the future. Nevertheless, a minimum age for the complex can be estimated from U-Pb SHRIMP analyses of zircon grains extracted from an aplite dyke intrusive into the peridotite. The results indicate that most zircon grains are inherited from the basement (individual ages vary from 3020 Ma to 3145 Ma) with only two nearly-concordant grains giving a ²⁰⁷Pb/²⁰⁶Pb age of 2085±12 Ma. Therefore, we suggest that the peridotite complex is older than 2.08 Ga. A felsic portion of the host banded gneiss yields an Archaean age (2983 Ma, cf. Oliveira et al. 2004).

 

CHEMISTRY OF MASSIVE CHROMITITE

Massive and disseminated chromites are ubiquitous in the Santa Luz peridotite complex. The former occurs as disrupted and faulted pods (Carvalho Filho et al. 1986). Because the disseminated chromites are prone to chemical exchange with associated silicates, only the massive chromitites were investigated in this study. Microprobe data from the core of chromite grains and platinum group element (PGE) analyses of chromitites are shown in figures 2 and 3.

 

Figure 2. Major element data (after Araújo,1998) for massive-type chromites of the Santa Luz

peridotite complex.

 

In several diagrams like that of figure 2, the Santa Luz chromites plot in the ophiolite or Alpine peridotite field. The chondrite normalized PGE patterns (Fig. 3) are similar to chromitites with very low sulphide contents such as those from ophiolite complexes and from the lower layers of the Critical Zone (LG-1) of the Bushveld complex (Von Gruenewaldt & Merkle, 1995). The studied chromitites bear no similarity with chromitites with sulphide inclusions, as the UG-2 chromitites from the Bushveld complex.

 

 

Figure 3. Chondrite-normalized platinum group elements for massive-type chromitites from the

Santa Luz peridotite complex (diamonds) compared with chromitites from ophiolite complex

(filled square) and layered intrusion (Bushveld UG2 and LG1, open square). Data after Mathez & Peach (1989) and Von Gruenewaldt & Merkle (1995). Chondrite abundances after Naldrett & Duke (1980).

See text for explanation.

 

 

GEOCHEMISTRY OF THE MAFIC DYKES AND GABBROIC PATCHES

The amphibolites have silica (41.98-47.09 wt%) and TiO₂ (0.2-0.8 wt%) contents, Ni abundances (61-186 ppm), Zr/Y ratios (1.1-2.9) and Zr/Nb ratios (14.7-17.6) comparable with transition-type mid-ocean ridge basalts (T-MORB).

DISCUSSION  

Field relationships, rock association and possibly also the geochemical characteristics of massive chromitites and amphibolite dykes/patches support the suggestion that the Santa Luz peridotite complex might have an origin similar to that of Alpine peridotites. Although hydrothermal alteration (serpentinization) and weathering complicate our investigation, the occurrence of irregular amphibolite dykes, gabbroic patches and feldspar spots in peridotites close to the dykes are field evidence for melt percolation and impregnation of mantle-like peridotites These processes are often recognised in ophiolite complexes worldwide (e.g. Paixão & Nilson, 2002; Muntener & Piccardo, 2004), and take place during the rifting stage of a basin when the lithosphere extends and thins, and the upwelling asthenosphere undergoes partial melting by adiabatic decompression. The implications of our discovery for the regional evolution depend on the precise age dating of the Santa Luz peridotite complex. The timing of peridotite formation and melt percolation is bracket by the ages of intrusive aplites (2084 Ma) and regional banded gneisses (2983 Ma). Therefore, there is a possibility that the Santa Luz peridotite is related to the early evolution of the Rio Itapicuru greenstone belt, for which the basal pillowed basaltic unit is dated at 2.2 Ga (cf. Silva et al. 2001). Metabasalts from this greenstone belt are of two types: one has geochemical affinities with N-MORB and the other with continental tholeiites with a marked negative Nb anomaly and no positive Sr anomaly. The latter type could be representative of basalts erupted during the early opening stage of an ocean basin, whereas the former would have erupted as ocean-floor basalts. However, given the available geochronological data, another interpretation is also plausible. It takes into account that the peridotite complex might not be related to the amphibolite dykes, nor to ophiolite processes; in this case the dykes would be much younger. This is quite likely, because a series of mafic dykes, marbles and quartzites, mapped to the west of Santa Luz city, could well be associated with a rifting event younger than the peridotite emplacement. In this scenario, the mafic dykes would equally intrude all regional rocks, including the Santa Luz chromite-rich peridotite complex. Nevertheless, more study is needed to demonstrate whether the Santa Luz peridotite complex is an ophiolitic peridotite, in which case it has the potential to be South America’s oldest ophiolite remnant.

 

ACKNOWLEDGMENTS

FAPESP (02/07536-4) and CNPq (300845/91) are acknowledged for research grants to EPO. Magnesita S.A. is greatly thanked for kindly giving us permission to study the chromite mine at Santa Luz.

 

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