Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology

New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma). This geological history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed the Western Amazon belt) that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guaporé suture zone) in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-Antofalla basement to the Amazonian craton

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

New constraints on the Mesozoic emplacement of diorite dykes in the Baldissero mantle peridotite massif (Ivrea-Verbano Zone)

The Baldissero peridotite is the main mantle massif occurring in the south-western part of the Ivrea zone. It is in contact at the East with the gabbros of the Ivrea-Verbano Mafic Complex and at the West it is sheared by the faults of the Insubric system and in tectonic contact with the terrains of the Austroalpine Domain (Canavese one). Another Alpine shear zone, the Cremosina line, limits at the North-West the Baldissero terrains from the rest of the Ivrea-Verbano Mafic Complex. Dioritic dykes in the Baldissero mantle peridotite have high Mg# values in bulk-rock and mafic phases which positively correlate with incompatile element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace element with respect to the LREE-depleted lherzolites typical of the Baldissero massif. The geochemical characteristics of the dykes are explained by flow, accompanied by crystallisation and reaction with the ambient peridotite, of a hydrous silica-saturated basaltic melt under crustal condition. The least evolved melts in this process are those that have the lowest incompatible trace element concentration and Mg#. New Internal Sm-Nd isochrons on two dyke samples (198±29, with Ndi = +5.6 and 204±31 Ma with Ndi = +6.0) and Re-Os data on the peridotite at the contact indicate late Triassic - lower Jurassic ages. Recently, Sm-Nd isochron ages published for the Mafic Complex in the Val Sesia and Val Sessera area has been interpreted to date the cooling of mineral assemblages below the blocking temperature for the Sm-Nd system (Peressini et al., 2007). However, the Mafic Complex in the Finero area show differences of about 10 Ma between Sm-Nd internal isochron ages (isochrons with 3 minerals, ranging from 223 to 231 Ma and having positive εNdi from +2.7 to + 5.3, Lu et al, 1997) and U/Pb zircon ages (232 ± 2 Ma; Peressini et al., 2005). Therefore, even if the ages obtained for the diorite dykes in Baldissero may be slightly younger with respect to the age of emplacement, this last should be, anyway, not older than the late Triassic. At this estimated age of dyke intrusion, the Ivrea-Verbano mantle peridotites were already emplaced in the crust. Thus, the low Mg# of the melt intruding the peridotite is explained by previous fractionation in the crust. Mesozoic igneous events are unknown in the southern Ivrea-Verbano region, but only in the northernmost Finero region. The Baldissero dykes, besides for their age, exhibit geochemical and isotopic similarities with the External Gabbro Unit of the Finero Mafic Complex, constituted by amphibole-bearing diorites. The Finero Mafic Complex is in tectonic contact, marked by a high-temperature ENE shear-zone, with the Permian relatively anhydrous mafic-ultramafic sequences occurring in Val Sesia and on the right side of the Val d'Ossola. Therefore, the evidences of Mesozoic magmatism in the westernmost sector of the Southern Alps are, presently, confined by tectonic lineaments to the southernmost and northernmost portion, respectively. Furthermore, the mantle phlogopite-bearing peridotite of Finero, show a metasomatic imprint of late Triassic age, which was attributed to a hydrous silica-saturated component of crustal origin, deriving from a subducting slab (Hartmann & Wedephol, 1993, Zanetti et al. 1999). The evidence of a Mesozoic magmatic activity in the westernmost sector of the Southern Alps, its particular location within the Ivrea-Verbano Zone and the nature of the involved parent melts are a potentially important mark in the geodynamic evolution of the Southern Alps

First lead isotopic data for cinnabar in the Almadén district (Spain): implications for the genesis of the mercury deposits

The Almadén district constitutes the largest and probably the most intriguing mercury concentration in the world. This paper reports the first lead isotope compositions of cinnabar from the district. Whole samples and stepwise leaching cinnabar aliquots display relatively homogeneous isotopic compositions (206Pb/204Pb = 18.112 - 18.460; 207Pb/204Pb = 15.635 - 15.705; 208Pb/204Pb = 38.531 - 38.826). Taken together with Jébrak et al. (2002) pyrite lead isotope results, the new cinnabar isotopic data defines a steep array trend on the 207Pb/204Pb - 206Pb/204Pb diagram, suggesting a mixed contribution from both ancient upper continental crust and (enriched mantle derived) magmatic sources for the ores of the Almadén Hg deposits

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