THE REGIONAL MANTLE PROCESSES IN NORTH-EASTERN BRAZIL: EVIDENCE OF INTERACTION BETWEEN THE CONTINENTAL MANTLE LITHOSPHERE AND THE FERNANDO DE NORONHA PLUME

We investigated new occurrences of mantle xenolith in the Cenozoic alkali basalts of north-eastern Brazil in order to constrain the regional mantle processes. Xenoliths have been divided in three groups, on the basis of their textural characteristics: G1, porphyroclastic; G2, protogranular; G3, transitional between G1 and G2. Clinopyroxenes from G1 peridotites have REE patterns varying from L-MREE-enriched convex-upward, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. G2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. Most of G3 clinopyroxenes show LREE-depleted patterns similar to the G2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the G1 clinopyroxenes form an array between DM and EMI-like components, both of them also present in the host basalts. Melts estimated to be in equilibrium with the G1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The G2 and G3 clinopyroxenes define two distinct compositional fields at higher 143Nd/144Nd values, correlated with their LREE composition. The isotopes of the G2 and G3 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 G3 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, G3 clinopyroxenes. G1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the G2 documents the shallower lithospheric section, with G3 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.