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

Abstract

Spinel lherzolite, harzburgite, and clinopyroxenite xenoliths and pyroxene megacrysts in Tertiary alkalic basalts of northeastern Brazil (∼30–13Ma; 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 ∼200Ma. 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, ∼70–80mW/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.