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