Petrology of spinel peridotite xenoliths from northeastern Brazil: lithosphere with a high geothermal gradient imparted by Fernando de Noronha plume

https://doi.org/10.1016/S0895-9811(02)00032-9Get rights and content

Abstract

Spinel lherzolite, harzburgite, and clinopyroxenite xenoliths and pyroxene megacrysts in Tertiary alkalic basalts of northeastern Brazil (∼30–13 Ma; K–Ar ages) provide information about melting, metasomatism, and geothermal gradients in subcontinental lithosphere as related to magmatism in that region since the Atlantic opening. That magmatism includes the xenoliths' host basalts, which have origins with the Fernando de Noronha plume, and regional tholeiitic basalts emplaced during continental rifting beginning ∼200 Ma. Peridotite textures are largely protogranular, but some are porphyroclastic. Mineral compositions show correlations among Mg#s, Cr#s, and pyroxene Cr2O3, Al2O3, and Na2O which suggest an upper-mantle history of varying melting and basalt extraction. Pyroxene equilibration temperatures range from ∼800–1250 °C and represent a high geotherm, ∼70–80 mW/m2, or ∼12 °C/km across the spinel stability field. Porphyroclastic xenoliths have the highest equilibration temperatures, >1150 °C. The equilibration temperatures do not correlate with the peridotite melting indicators (e.g. Cr#s; Cr2O3). In addition, Fe and Ti enrichments in minerals of porphyroclastic xenoliths, and light rare-earth element (LREE) enrichments, greatest in protogranular xenoliths (e.g. La(n) 2–16), each identify a metasomatic history for northeastern Brazil lithosphere.

Several of these xenolith features, particularly the geotherm they represent, can be linked to the Fernando de Noronha hotspot during Tertiary. The high geotherm likely originated as northeastern Brazil ‘passed’ over the plume. It was overprinted on subcontinental lithospheric mantle with existing melting characteristics that were possibly acquired during the earlier magmatism (e.g. Mesozoic) that attended the opening of the central Atlantic. The clinopyroxenite and the pyroxene megacrysts coexisting with the peridotite xenoliths likely represent Fernando de Noronha plume-derived basaltic melts that veined deformed lithosphere near plume margins to locally metasomatize that peridotite (porphyroclastic) with Fe and Ti. The LREE enrichments are probably also largely attributable to the plume, from which small percentage melts metasomatized the lithosphere to varying degrees, particularly the ‘cooler’, shallower level (protogranular) peridotite.

Introduction

Spinel lherzolite and harzburgite xenoliths occur in seven Tertiary alkalic basalt centers of northeastern Brazil (Fig. 1). Fodor et al. (1998) described these basaltic centers and noted that they are of particular petrologic interest because their origins are likely tied to the Fernando de Noronha plume. Namely, South American plate movement during Tertiary placed northeastern Brazil in proximity of the Fernando de Noronha hotspot from about 35–15 Ma (Morgan, 1983, O'Connor and Duncan, 1990), which coincides with the K–Ar ages for the basaltic centers (Fodor et al., 1998). As described by Fodor et al. (1998), the alkalic basalts of northeastern Brazil largely represent asthenospheric melts from the plume that mixed with small amounts of lithospheric melts.

Given this tectonic and petrologic history, the peridotite xenoliths in the northeastern Brazil Tertiary lavas offer the opportunity to examine a portion of South American subcontinental lithosphere likely influenced by Fernando de Noronha plume. In addition, northeastern Brazil lithosphere experienced prior magmatism, namely Mesozoic dike-emplacement preceding and attending the central Atlantic breakup that led to the region becoming a passive margin. Both the Mesozoic and Cenozoic events potentially affected northeastern Brazil subcontinental lithosphere with elevated temperatures, partial melting, melt metasomatism, stress deformation, and re-equilibration of phase compositions or processes and features that may be manifested in xenolithic samples of subcontinental lithosphere.

Our approach to assessing this passive margin lithosphere through xenoliths was to examine 32 samples from the seven Tertiary alkalic basaltic centers for their petrography, mineral compositions, and, where samples were large enough, whole-rock compositions, including rare-earth elements (REE). We examine our data inclusive of previous studies (e.g. Rivalenti et al., 2000), but our emphasis for this xenolith- and basalt-rich province in northeastern Brazil is on how its lithosphere may manifest specific events related to the Fernando de Noronha plume and the earlier Atlantic opening. We also use a broad geographic coverage of sample sites and incorporate the pyroxene megacrysts and pyroxenite xenoliths that coexist with the peridotite xenoliths as part of the lithosphere's magmatic history. Above all, however, mineral compositional data are of interest because they reveal a wide range in pyroxene equilibration temperatures (∼450 °C). This equilibration temperature range, which is larger than that generally observed in xenolith provinces, is feasibly the signature of a high geothermal gradient imparted by a ‘passing’ asthenospheric plume.

Section snippets

Background

The seven northeastern Brazil alkalic basaltic centers are in Rio Grande do Norte, except for one in neighboring Pernambuco (Fig. 1). The peridotite xenoliths in some of these centers were first reported in Sial (1977), and some xenoliths were later included in a study by Comin-Chiaramonti et al. (1986). More recently, Princivalle et al. (1994) examined some xenoliths for pyroxene equilibration temperatures and pressures in view of xenolith textures. Also, Rivalenti et al. (2000) reported

Petrography

The xenoliths at each site are small, generally 1–3 cm, but some reach 6–8 cm. Their abundances at the sites vary from sparse to relatively common. Three of the sites also have clinopyroxenite xenoliths, some up to ∼6 cm, and cm-sized pyroxene megacrysts.

Most of the peridotites have protogranular textures, but four are porphyroclastic (Table 1). The protogranular textures have the characteristic (e.g. Mercier and Nicholas, 1975) curvilinear grain boundaries, grain sizes up to 5 mm, and rare kink

Analytical techniques

We used an ARL-SEMQ electron microprobe at North Carolina State University (NCSU) to obtain mineral compositions. Reference minerals were olivine, pyroxenes, and spinel from the Smithsonian Institution as well as a Ni-doped diopside. Phi–rho–Z matrix corrections were used. For CaO in olivine, we used 20-s peak and background count times per spot, rather than the ‘normal’ 10-s times. Whole-xenolith major element compositions plus Ni were obtained by X-ray fluorescence spectrometry at NCSU.

Mineral compositions

Average compositions for olivine, orthopyroxene, clinopyroxene, and spinel in peridotite xenoliths are in Table 2, Table 3, Table 4, Table 5. Abundances of the mineral major-elements Fe, Mg, Ca, and Al vary little within each xenolith, (standard deviations calculated for FeO, MgO, and CaO after 10–20 spot-analyses are listed in Table 2, Table 3, Table 4). For example, FeO and MgO in olivine and orthopyroxene and CaO in clinopyroxene generally vary <2% of the amounts present. In contrast, CaO in

Geothermometry and geobarometry

We calculated xenolith equilibration temperatures (Teq) from clinopyroxene and orthopyroxene compositions according to the two-pyroxene methods of Wells, 1977, Brey and Kohler, 1990, and from olivine and spinel according to Ballhaus et al. (1991). Results from the two pyroxene methods correlate well over the large temperature range that the xenoliths yield, ∼850 to ∼1250 °C, Wells (1977), and ∼800 to ∼1250 °C Brey and Kohler (1990), as shown by the nearly linear relationship in Fig. 6a. Pyroxene

Whole-xenolith compositions

Of the ten xenoliths analyzed, seven are protogranular and three are porphyroclastic. Compositions appear in Table 6, Table 7 and in MgO variation diagrams of Fig. 8. To express how the xenoliths compare to peridotite xenoliths elsewhere, Fig. 8 includes the compositional fields for a lherzolite–harzburgite suite from French Massif Central. Correlations among the northeastern Brazil xenoliths include the generally increasing SiO2, TiO2, Al2O3, and CaO abundances with decreasing MgO, and an

Melting and equilibration temperature histories

Some of the compositional relationships among the minerals comprising the xenoliths as well as some between minerals and whole-xenoliths indicate that the peridotite suite represents mantle that underwent varying degrees of melting and basalt extraction. Such relationships were first observed for northeastern Brazil xenoliths by Comin-Chiaramonti et al. (1986) and, as shown in Fig. 5, include the general increasing of spinel Cr#s with increasing silicate-phase Mg#s and Cr2O3 abundances and

Conclusions

The main thrust of this study is that a suite of 32 spinel lherzolite and harzburgite xenoliths from seven Tertiary basaltic centers in northeastern Brazil reflect what appears to be lithosphere with a geotherm elevated over that for stable subcontinental lithosphere. We attribute the geothermal gradient recorded in the xenoliths to thermal perturbation from the Fernando de Noronha asthenospheric plume, beginning about 40 Ma, which generated the Tertiary basaltic magmas that entrained the

Acknowledgements

This work is supported in part by NSF grant OCE8509894 (RF). We are grateful to personnel at the Oregon State University radiation center for providing instrumental neutron activation analyses for several trace elements under the US Department of Energy reactor sharing grant. We acknowledge constructive critiques by E. Piccirillo, W. Griffin, G. Kurat, C. Neal, and C. Stern.

References (52)

  • H.G. Wilshire et al.

    Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States: structural and textural relationships

    Phys. Chem. Earth

    (1975)
  • N.A. Zangana et al.

    Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from Ray Pic (French Massif Central): implications for the composition of the shallow lithospheric mantle

    Chem. Geol.

    (1999)
  • P. Affaton et al.

    The Dahomeyide Orogen: tectonothermal evolution and relationships with the Volta Basin

  • C. Ballhaus et al.

    High pressure experimental calibration of the olivine-orthopryoxene-spinel oxygen geobarometer: implications for the oxidiation state of the upper mantle

    Contrib. Mineral Petrol.

    (1991)
  • V.G. Batanova et al.

    Compositional heterogeniety in subduction related mantle peridodite, Troodos massif, Cypress

    Geology

    (2000)
  • J.L. Bodinier et al.

    Mechanism of mantle metasomatism: geochemcal evidence from the Lherz orogenic peridotite

    J. Petrol.

    (1990)
  • G.P. Brey et al.

    Geothermobarometry in four-phase lherzolite II. New thermobarometers, and practical assessment of existing thermobarometers

    J. Petrol.

    (1990)
  • D.L. Castro et al.

    Relevo da interface crossta-manto no Nordeste setentrional do Brasil: comparacao entre vinculos de isostasia e suavidade

    Fifth Congr. Int. Soc. Bras. Geofisica, São Paulo, Abs.

    (1997)
  • D.L. Castro et al.

    Mapa gravimetrico do Nordeste setentrional do Brasil e margem continental adjacente

    Fifth Congr. Int. Soc. Bras. Geofisica, São Paulo, Abs.

    (1997)
  • D.S. Chapman

    Thermal gradients in the continental crust

  • R.D. Dallmeyer et al.

    The West African Orogens and Circum-Atlantic Correlatives

    (1991)
  • R.M. Darros de Matos

    The northeast Brazilian rift system

    Tectonics

    (1992)
  • H. Downes

    Shear zones in the upper mantle—relation between geochemical enrichment and deformation in mantle peridotites

    Geology

    (1990)
  • Q. Fan et al.

    The mineral chemistry of ultramafic xenoliths of eastern China: implications for upper mantle composition and the paleogeotherms

    J. Petrol.

    (1989)
  • R.V. Fodor et al.

    Petrology, isotope characteristics, and K–Ar ages of the Maranhão, northern Brazil, Mesozoic basalt province

    Contrib. Mineral. Petrol.

    (1990)
  • L. Franz et al.

    Thermal evolution of the mantle underneath the Mid-German crystalline rise: evidence from mantle xenoliths from the Rhön area (Central Germany)

    Mineral. Petrol.

    (1997)
  • Cited by (0)

    View full text