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

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

AbstractNew 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

The growth of large mafic intrusions: comparing Niquel\ue2ndia Mafic-Ultramafic and Ivrea Mafic Complexes or Niquel\ue2ndia and Ivrea Layered Complexes or IVrea and Niquel\ue2ndia Complexes

The Niquel\ue2ndia Complex, Brazil, is one of theworld's largestmafic\u2013ultramafic plutonic complexes. Like the Mafic Complex of the Ivrea-Verbano Zone, it is affected by a pervasive high-T foliation and shows hypersolidus deformation structures, contains significant inclusions of country-rock paragneiss, and is subdivided into a Lower and an Upper Complex. In this paper, we present new SHRIMP U\u2013Pb zircon ages that provide compelling evidence that the Upper and the Lower Niquel\ue2ndia Complexes formed during the same igneous event at ca. 790 Ma. Coexistence of syn-magmatic and high-T subsolidus deformation structures indicates that both complexes grew incrementally as large crystal mush bodies which were continuously stretched while fed by pulses of fresh magma. Syn-magmatic recrystallization during this deformation resulted in textures and structures which, although appearing metamorphic, are not ascribable to post-magmatic metamorphic event(s), but are instead characteristic of the growth process in huge and deep mafic intrusions such as both the Niquel\ue2ndia and Ivrea Complexes. Melting of incorporated country-rock paragneiss continued producing hybrid rocks during the last, vanishing stages of magmatic crystallization. This resulted in the formation of minor, late-stage hybrid rocks, whose presence obscures the record of the main processes of interaction between mantle magmas and crustal components, which may be active at the peak of the igneous events and lead to the generation of eruptible hybrid magmas

Petrology of metabasaltic dykes in the Diamantina region, Minas Gerais, Brazil.

A sequence of mafic dykes is found in the Diamantina region (Minas Gerais, Brazil) at the eastern border of the southern São Francisco Craton. The dykes have been distinguished into four groups on the basis of petrography and geochemical characteristics. Groups 1 to 3 cut the rock sequence older than Meso-Proterozoic, but do not cut the Neo-Proterozoic rocks, thus constraining their emplacement age. Group 1 is constituted by fine grained, sometimes foliated metabasites, which only exceptionally preserve relics of the primary mineral assemblage, and are located only in the lowest stratigraphic units of the Espinhaço Supergroup, suggesting an older emplacement age with respect to the other groups. Group 2 and 3 metabasites are better preserved than those of Group 1. Group 4 represents a single igneous body virtually non metamorphic. Its age is not constrained, but it is similar to Mesozoic dykes occurring further south in this region. In all groups, composition is basaltic with tholeiitic affinity. Metamorphic element mobility substantially affected only the LILE, whereas igneous variation trends are preserved for all the other elements. The various groups differ for their incompatible trace element composition and ratios. These ratios are more similar to OIB (Ocean Island Basalt) values rather than to any other magma type. Geochemical evidence rules out any important influence of crustal contamination, fractional crystallisation, or variable degrees of melting of a common source material as an explanation for the inter-group variability. It is inferred that the geochemistry of the different groups reflects complementary characteristics and differences of their mantle sources. These latter are attributed either to a metasomatic enrichment of a variably depleted premetasomatic mantle by a component with OIB characteristics, or, alternatively (our preferred interpretation), to the melting of the metasomatised mantle at different depths. The metasomatised sources underwent extensive melting, producing tholeiitic melts retaining OIB-like geochemical characteristics. By analogy with OIB, the metasomatic component may be plume-related. Dyke emplacement may be controlled by passive crustal rifting induced by plume-related mantle diapirs. In such a scenario, the Group 1 samples could be related to the initial rifting phase and plume impingement in the lithosphere, whereas Groups 2 and 3 could represent advanced stages of crustal thinning and melting of the plume head source. The youngest Group 4 dykes presumably represent a limited and local occurrence of the Mesozoic mafic magmatism which affects the area of the Serra do Espinhaçho

Petrology of the upper proterozoic mafic dikes in the Nico Perez region, Central Uruguay

Tholeiitic basaltic and basaltic andesite dikes of Brasiliano (or PanAfrican) age (similar to 600 Ma) intrude the basement of the Nice Perez region, Uruguay. Major and trace element geochemistry of the basalts indicates that they suffered fractionation in shallow magma chambers. The variation in element ratios, which remain virtually unchanged during fractionation (K/Rb, Rb/Ba, Ba/Nb, La/Nb, Zr/Nb and Ti/Zr), indicate that the dikes are not strictly comagmatic. However, they have certain features in common: LILE and LREE enrichment with respect to HFSE and HREE; high Rb/Ba(> 0.9) and Rb/Sr (> 0.08); low K/Rb ( 2; Ba/Nb > 22, Ti/Zr < 60). Sr-87/Sr-86 and Nd-143/Nd-144 at 665 Ma are in the range 0.7052 - 0.7119 and 0.51158 - 0.51177, respectively. The lack of correlations between isotope and trace element variations indicate that these characteristics are not controlled by crustal contamination of the melts. They are interpreted as being due to the melting of an enriched mantle (C1) under the influence of a fluid-rich component (C2) which stabilized a Nb-retaining titanate phase in the residuum. Although this process may be related to a subduction environment, it is also possible that it occurred in an ensialic region by the interaction of deep mantle fluids with the lithospheric continental mantle. The parent mantle underlying the early Proterozoic (1.8 Ga) Florida region had isotope and geochemical characteristics which could evolve to the values observed in the adjacent Nice Perez region. It is therefore proposed that mantle enrichment took place in the early Proterozoic and that this mantle melted under the influence of fluids in the late Proterozoic to derive the Nice Perez dikes

Composition and processes of the mantle lithosphere in northeastern Brazil and Fernando de Noronha: evidence from mantle xenoliths

Spinel-peridotite facies mantle xenoliths in Cenozoic alkali basalts of the Pico Cabuji volcano (Rio Grande do Norte State. Northeast Brazil) and the adjacent South Atlantic oceanic island of Fernando de Noronha are studied for: (1) the information they provide on the composition of the lithospheric component in the erupted basalt geochemistry, and (2) to check the effects of the Fernando de Noronha plume track on the mantle lithosphere. Xenoliths from Pico Cnbuji are protogranular lherzolites and porphyroclastic harzburgites recording average equilibrium temperatures of 825 +/- 116 and 1248 +/- 19 degrees C, respectively. Pressure in the porphyroclastic xenoliths ranges from 1.9 to 2.7 GPa (Ca-in-olivine geobarometer), Both groups show major element chemical variation trends in whole-rock and Ti and HREE (Er. Yb) variations in clinopyroxene consistent with fractional melting and basalt extraction. REE (rare earth element) profiles of clinopyroxenes vary from LREE (La, Ce) enriched (spoon shaped) to LREE depleted in the protogranular group, whereas they are slightly convex upward in most porphyroclastic clinopyroxenes, HFSE (Ti and Zr) negative anomalies are in general modest in the clinopyroxenes of both groups. Xenoliths from Fernando de Noronha have textural variations similar to those of Pico Cabuji. Protogranular and porphyroclastic samples have similar temperature (1035 +/- 80 degrees C) and thr pressure is 1-1.9 and 2.3 GPa, respectively. Whole-rock chemical variation trends overlap and extend further than those of Pico Cabuji. The trace element profiles of the clinopyroxenes of the porphyroclastic xenoliths are enriched in La up to 30 X PM and are smoothly fractionated from LREE to HREE. with deep, negative, Zr and Ti anomalies, The geochemical heterogeneities of the xenoliths: from both localities are interpreted in terms of reactive porous percolation. The porphyroclastic xenoliths from Pico Cabuji represent the lower Dart of a mantle column (the head of a mantle diapir. at the transition conductive-adiabatic mantle), where OIB infiltration triggers melting, and the protogranular xenoliths the top of the mantle column: chromatographically enriched by percolation at a low melt/rock ratio. This interpretation may also apply for Fernando de Noronha, but the different geochemical signature recorded by the clinopyroxenes requires a different composition of the infiltrated melt. Nd and Sr isotopes of the: Pico Cabuji porphyroclastic clinopyroxenes (Nd-143/Nd-144 = (0.51339-0.51255. Sr-87/Sr-86 = 0.70275-0.70319) and of Fernando de Noronha (Nd-143/Nd-144 = 0.51323-0.51285, Sr-87/Sr-86 = 0.70323-0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to low Nd-low Sr (EMI) and low Nd-high Sr (EMII), respectively. Correlation of the isotope variation with geochemical parameters indicates that the isotopic variation was induced by the metasomatic component, of EMI type at Pico Cabuji and of EMII type at Fernando de Noronha. These different components, enriched a lithosphere isotopically similar to DMM (depleted MORE mantle) at both localities, At Fernando de Noronha, the isotopic signature of the metasomatic component is similar to that of the similar to 8 Ma old lavas of the Remedios Formation suggesting that this is the age of metasomatism, At Pico Cabuji, the mantle xenoliths do not record the high Sr-87/Sr-86 component present in the basalts. We speculate that the EMII component derives from a lithospheric reservoir, which was not thermally affected during mantle metasomatism at Pico Cabuji, but was mobilized by the hotspot thermal influence at Fernando de Noronha. This interpretation provides a plausible explanation for the presence of distinct metasomatic components at the two localities, which would be difficult to reconcile with their genetic relationship with the same plume

Early Proterozoic dike swarms from western Uruguay: Geochemistry, Sr-Nd isotopes and petrogenesis

Early Proterozoic (1.86 +/- 0.12 Ga), unmetamorphosed basic dikes intrude the Precambrian Rio de la Plata Craton of western Uruguay. The dikes define two distinct swarms of tholeiitic andesitic basalt and andesite composition, respectively. Major- and trace-element geochemistry indicates that these two magma types are related via gabbro fractionation. The incompatible trace-element patterns favour a derivation of the parent melts from melting of a garnet peridotite source. The high LILE and LREE content is discussed in terms of crustal contamination of the melts or of the source, or of mantle metasomatism. Mixing calculations do not support important crustal contamination of the melts or of the source. Mantle metasomatism is a preferred process, which operated slightly before or during melting. An anorogenic ensialic environment of emplacement is inferred