The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil

doi:10.1016/j.lithos.2006.06.012

Lithos

Volume 94, Issues 1–4, March 2007, Pages 111-131

https://doi.org/10.1016/j.lithos.2006.06.012 Get rights and content

Abstract

New xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher ¹⁴³Nd/¹⁴⁴Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.

The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest ¹⁴³Nd/¹⁴⁴Nd values at any given ⁸⁷Sr/⁸⁶Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.

Introduction

Cenozoic alkali basalt centres in north-eastern Brazil are believed to represent the surface track of the passage of the craton over the Fernando de Noronha plume (O'Connor and Duncan, 1990). Gerlach et al. (1987) and Fodor et al. (1998) recognised in the Fernando de Noronha lavas three isotopic components: DM, EMII and HIMU (mantle end-members from Zindler and Hart, 1986). Fodor et al. (1998) emphasised that the Rio Grande do Norte alkali basalts derived from mixing of asthenospheric melts, largely of HIMU and DM characteristics, and EMI lithospheric components. At present, isotopic data on xenoliths representing the mantle lithosphere are available for the Fernando de Noronha Island and only one locality, Pico Cabugi, in north-eastern Brazil and show consistently different isotopic arrays, trending from DM towards an EMII and an EMI component, respectively (Rivalenti et al., 2000). The effects of the Fernando de Noronha plume impingement on the continental lithosphere remains, therefore, at present largely unconstrained.

Thus, in order to better constrain the effect of the plume on the lithosphere in north-eastern Brazil and add further evidence on the processes which affected the continental mantle lithosphere, we provide in this paper new geochemical and isotope data on the xenoliths of four unstudied Brazilian localities. We will show that the lithosphere had a steep geothermal gradient and was geochemically heterogeneous. The higher temperature regions record Cenozoic, melt-assisted, near-fractional melting induced by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics. The colder lithospheric regions, composed of fertile protogranular spinel-lherzolites, have higher ¹⁴³Nd/¹⁴⁴Nd isotope values and geochemically record ancient mantle processes, probably related to the Mesozoic tholeiitic events. The transition between these two regions underwent percolation of small melt volumes, which produced chromatographic enrichment of highly incompatible elements and high ⁸⁷Sr/⁸⁶Sr.

Section snippets

Analytical methods

Bulk-rock xenolith analyses were made on powders from cores only of samples having average diameters > 8 cm. These samples were cut into 1-cm thick slabs. Slabs were crushed to 2-mm size in a hardened steel mortar and 100 g of the sample thus obtained was ground in an agate mortar.

Major elements, Cr and Ni in bulk-rock xenoliths and basalts were analysed on glass discs at the Instituto de Geociências, University of São Paulo, according to Mori et al. (1999), using a Philips PW2400 XRF

Xenolith description and petrography

Although most Tertiary alkali basalts in north-eastern Brazil contain small mantle xenoliths (Fodor et al., 1998), only four new localities have xenoliths exceeding 5 cm in diameter, suitable for study purposes. These localities (Fig. 1) have similar petrographic characteristics and are distinguished on the basis of the sample label (SA = Serra Aguda, BO = Bodò, GR = Fazenda Geroncio and SV = Serra Verde). Host lavas are alkali basalts and basanite, whose main petrographic and geochemical

Bulk-rock

Only seven xenoliths from the new localities had dimensions suitable for bulk analysis (Supplementary Table 2, Appendix A). Simple mass balance calculation shows that the presence of < 1% by volume infiltrated basalt has a negligible effect on the peridotite major element concentration, while it may greatly influence that of incompatible minor and trace elements, as well as their isotopic composition. For this reason, only major elements are here considered, whereas complete analyses, including

Temperature and pressure

Equilibrium temperatures of the xenoliths were calculated according to Brey and Köhler (1990), with the exception of wehrlite SV14, for which equilibrium temperature was estimated according to the single clinoproxene geothermometer of Mercier et al. (1984). As for pressure, there is no reliable geobarometer for spinel-facies peridotites, except perhaps the Ca-in-olivine geobarometer of Köhler and Brey (1990). This latter, however, requires high precision estimates of the Ca concentration in

Discussion

The temperature–pressure variations of the three xenolith groups indicate they may represent a mantle section where the porphyroclastic group 1 and the protogranular group 2 were located in the hotter (possibly deeper) and colder (possibly shallower) regions, respectively, and the group 3 xenoliths were the transition. Xenolith textures vary with all the geochemical features, such as the solid–solid element partitioning, as well as the trace element and isotope composition of the

Conclusions

The xenoliths from the new localities of north-eastern Brazil examined in this study are samples of a mantle section ranging from a deep, high T, P region (group 1, porphyroclastic xenoliths) where lithosphere thermal and chemical erosion occurred, to a shallow and relatively cool region (group 2, protogranular xenoliths) that mainly records geochemical characteristics inherited from earlier mantle processes. The transition between the deepest and the shallower lithospheric levels is

Acknowledgements

J-.L. Bodinier and G.M. Yaxley are greatly thanked for their constructive critical review of the manuscript. J-.L. Bodinier is also thanked for having kindly provided the Plate Model software. The authors are grateful to H. Downes and H. H. G. J. Ulbrich for reviewing the manuscript and to P. Mori, M. Sugano and S. Andrade for the XRF and ICP-MS analyses on major element composition of the xenoliths and trace element concentration in the basalts. This research was financially supported by the

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Citation Excerpt :
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