Julio Cezar Mendes

Architecture of Late Orogenic Plutons in the Araçuaí-Ribeira Fold Belt, Southeast Brazil

Abstract Post-collisional to late orogenic magmatism (580 to 480 Ma) in the Aracuai-Ribeira Fold Belt, SE Brazil, is characterized by the predominance of high-K metaluminous, allanite-titanite-bearing granitoids. Small lenses of coronitic gabbro, anorthosite, pyroxenite and phlogopite-peridotite are also common in deeper exposed areas of this fold belt. In the region of southern Espirito Santo State, the deep erosional level associated with a steep topography reveals the internal architecture of the intrusions: a tendency to funnel-shaped bodies, with sub-vertical hemi-ellipsoidal/conical roots changing upwards to shallow-dipping tops. Associated stocks, sills and dykes of basic and acid rocks generally intrude the enclosing gneisses along the foliation planes, local ductile shear zones and parallel to fold hinges. The contact between intrusions and the enclosing rocks is sharp in deeply eroded plutons e.g., Santa Angelica, Venda Nova, Mimoso do Sul and Varzea Alegre, but at shallow levels e.g., Castelo, Pedra Azul and Conceicao de Muqui agmatic stoping zones occur along the borders. A magmatic foliation within the granitoids is usually well marked, but the schistosity in the surrounding gneisses wraps around the plutons. The intrusions have a bimodal chemical distribution and generally are reversely zoned with mafic cores (gabbro, diorite to tonalite) surrounded by a mingled (marble cake) zone where basic and acid rocks are interfingered, and an external zone of syenomonzonite and granite. Widespread evidence of mingling and mixing between contrasting magmas of gabbroic and granitic and/or syenomonzonitic compositions is characteristic for all intrusive complexes. We suggest that replacement of lithospheric mantle by hot asthenospheric mantle induced partial melting of the crust. The mantle exchange was due to lithospheric mantle delamination and slab breakoff following collisional orogenesis. The bimodal plutons result from interaction among the contrasting magmas. Ascent and emplacement followed older regional structures, such as regional fold hinges and ductile shear zones.

Marins Granite (MG/SP): petrography, geochemistry, geochronology, and geotectonic setting

The Marins Granite forms an ellipsoidal body that crops out along the border between Sao Paulo and Minas Gerais states. It intrudes meta-sedimentary and meta-igneous rocks of Neoproterozoic age, belonging to the Socorro Nappe and Embu Complex, in the interference zone between the Brasilia and Ribeira fold belts. Geochemical analyses indicate a high-K calc-alkaline signature, weakly peraluminous, type I character, poorly differentiated with restricted silica range and similar rare earth element patterns enriched in light rare earth elements. Two lithofacies were defined: (1) Marins facies, predominantly inside the body, isotropic, light gray with inequigranular porphyritic texture containing euhedral to subhedral microcline phenocrysts (< 1 cm), and (2) Mendanha facies, restricted to the border zone of the body, are pinkish with elongated microcline phenocrysts and aligned biotite grains, defining a deformational foliation. Neodymium isotopes point to a crustal signature, revealed by a eNd(0.6): -10.4. T DM ages indicate a paleoproterozoic source. U-Pb LA-ICPMS ages in zircon yielded concordia crystallization ages of 606.9 ± 1.9 Ma for the Mendanha facies and 603.7 ± 4.8 Ma for the Marins facies. These ages are consistent with the interpretation that the Marins Granite is a late to post-collisional body with respect to the collision of the southern Brasilia Belt, also reinforced by the location of the granite in the upper plate (Socorro Nappe), a few tens of kilometers away from the suture. The apparent lack of deformation related to collision in the Ribeira Belt (590 - 570 Ma), in the Marins facies, after crystallization of the body, is interpreted as the result of its rheological behavior, more resistant to deformation than the surrounding schists, similar to the behavior of a pre- or syntectonic porphyroblast.

Química mineral e geotermobarometria do Batólito Serra da Água Limpa, Nappe Socorro-Guaxupé, Faixa Brasília Meridional, Sudeste do Brasil

The Serra da Agua Limpa Batholith, that crops out along the boundary of Minas Gerais and Sao Paulo states, in Brazil, has been interpreted as part of a magmatic arc of the Paranapanema paleocontinent active margin, southern Brasilia belt. Locally igneous features are preserved, but tectonic foliation is common. Mineral chemistry microprobe data are presented for plagioclase, amphibole, biotite and K-feldspar. Crystallization temperatures were calculated by the whole rock zircon saturation method, yielding values between 863 and 1015oC. The pressure was calculated resulting in values in the range from 5 to 6 kbar. Thermometric data yielded two distinct temperature intervals: the cores of minerals rendered temperatures between 830 - 860oC, and the rims from 740 - 770oC. Three hypotheses may be considered: (A) the higher values obtained in the cores are related to magmatic crystallization, whereas the lower ones obtained in the rims represent metamorphism related to continental collision; (B) both cores and rims are metamorphic revealing a metamorphic peak temperature of 830 - 860oC with retrometamorphism in the range 740 - 770oC; (C) cores and rims of amphibole and plagioclase crystals retained their igneous composition and reflect igneous crystallization and subsequent cooling of the magma. The first interpretation seems to be the most likely because it is consistent with other data published for the region.

Superposition of structures in the interference zone between the southern Brasília belt and the central Ribeira belt in the region SW of Itajubá (MG), SE Brazil

The study area is localized in the Socorro nappe, part of the southern Brasilia belt, with a minor part in the Embu terrane, part of the central Ribeira belt. Three phases of deformation were detected, Dn-1, Dn and Dn+1. Sn-1 seems to be generally transposed into Sn, but in the northwestern part it is well preserved, dipping about 60o to W and SW, with a stretching and/or mineral lineation plunging down dip. The main foliation in most of the area is Sn, dipping about 70o to SSE. Dn folds are tight to isoclinal, with axes that plunge about 40o to SW. Quartz-feldspathic segregation veins are folded by Dn. The structures related to Dn-1 and Dn are cut and modified by four important shear zones ascribed to deformation phase Dn+1. Two samples of a granite that is elongated along the Caxambu shear zone, and also cut by it, were dated. One yielded a crystallization age of 575 ± 5 Ma, and the other one, from the shear zone, an age of 567 ± 8 Ma, interpreted either as representing the age of movement along the Caxambu shear zone, or as metamorphic growth.

Color electron microprobe cathodoluminescence of Bishunpur meteorite compared with the traditional optical microscopy method

Cathodoluminescence (CL) imaging is an outstanding method for sub classification of Unequilibrated Ordinary Chondrites (UOC) – petrological type 3. CL can be obtained by several electron beam apparatuses. The traditional method uses an electron gun coupled to an optical microscope (OM). Although many scanning electron microscopes (SEM) and electron microprobes (EPMA) have been equipped with a cathodoluminescence, this technique was not fully explored. Images obtained by the two methods differ due to a different kind of signal acquisition. While in the CL-OM optical photography true colors are obtained, in the CL-EPMA the results are grayscale monochromatic electronic signals. L-RGB filters were used in the CLEPMA analysis in order to obtain color data. The aim of this work is to compare cathodoluminescence data obtained from both techniques, optical microscope and electron microprobe, on the Bishunpur meteorite classified as LL 3.1 chondrite. The present study allows concluding that 20 KeV and 7 nA is the best analytical condition at EPMA in order to test the equivalence between CL-EPMA and CL-OM colour results. Moreover, the color index revealed to be a method for aiding the study of the thermal metamorphism, but it is not definitive for the meteorite classification.

Pleistocene alkaline rocks of Martin Vaz volcano, South Atlantic: low-degree partial melts of a CO2-metasomatized mantle plume

ABSTRACT Several alkaline massifs on inland southeastern Brazil extend offshore, roughly parallel to ~20° S, through a seamount chain of the Vitoria-Trindade ridge. This paper presents the first extensive work on the Martin Vaz volcano through whole-rock and Sr and Nd isotopic composition of volcanic and subvolcanic lithotypes from the Martin Vaz Island, located at the easternmost of this volcanic chain. These alkaline rocks were generated during the Plio-Pleistocene (~0.47 My, 40Ar/39Ar dating in whole-rock) and represent the crystallization of sodic magmas of nephelinitic composition that evolved through fractional crystallization towards phonolites. Calculations from P–TLiquidus using PELE software show temperatures of 1045°C and 818°C, viscosity of 2.47 log Poise and 5.02 log Poise, and densities of 2.57 g/cm3 and 2.26 g/cm3 for nephelinite and phonolite, respectively. Like in Trindade Island, the nephelinitic volcanism in Martin Vaz may represent a Strombolian and/or Hawaii-type eruption due to low viscosity magma according to its physical properties whereas phonolitic intrusions present higher viscosity characteristics forming lava domes. The 87Sr/86Sr (~ 0.703800) and 143Nd/144Nd (~ 0.512750) ratios of lavas from the seamounts and Martin Vaz do not vary significantly, pointing to partial melting process from a homogeneous mantle source showing isotope signature close to HIMU. Beside the restrict variation on these isotopic ratios, a conspicuous enrichment in incompatible trace elements, mainly LREE, indicates that metasomatism is a recent process and not a long-term source characteristic. Non-modal partial melting models (fractional melting and batch melting) suggest that the source of the Martin Vaz magmatism is consistent with the garnet-lherzolite mantle stability field (>90 km depth; Tb/Yb >0.7), generated about 3.0 GPa by very small degree of partial melting of an enriched wet mantle source (F = 0.03–0.04) with 2.5 wt. % of CO2.

The Buritizal meteorite: classification of a new Brazilian chondrite

On August 14, 1967, the reporter Saulo Gomes, working at TV Tupi, went to a small city in the State of São Paulo called Buritizal to investigate reports of a meteorite fall and write a newspaper report. He actually recovered three fragments of the meteorite at a small farm. In 2014, he donated one of the fragments to the Museu Nacional of the Universidade Federal do Rio de Janeiro (MN/UFRJ). We named this meteorite Buritizal and studied its petrology, geochemistry, magnetic properties and cathodoluminescence with the intent to determine the petrologic classification of the meteorite. In this manner, the Buritizal meteorite is classified as an ordinary chondrite LL 3.2 breccia (as indicated by lithic fragments). The meteorite consists of ~ 2% of metallic Fe,Ni and many well-defined chondrules with ~ 0.8 mm in average diameter. An ultramafic ferromagnesian mineralogy is predominant in the meteorite, represented by olivine, orthopyroxene, clinopyroxene, Fe-Ni alloy, troilite and glass. The total iron content was calculated as 20.88 wt%. Furthermore, the meteorite was classified as weathering grade W1 and shock stage S3. Buritizal is the 25th observed meteorite fall recovered in Brazil, of 70 meteorites known from Brazil. Thus, the study of the Buritizal meteorite is very important and relevant for the Brazilian scientific community.

Assinatura isotópica de Sr e Nd do magmatismo cálcio-alcalino de alto-K na Faixa Ribeira central: o exemplo do Granito São Pedro em Lumiar, RJ

Sr and Nd isotopic signature of the high-K calc-alkaline magmatism of the central Ribeira belt: the São Pedro Granite in Lumiar, RJ. In the central-northern Ribeira belt there are many granitic to granodioritic bodies showing varied shape and size, characterizing a lateto post-collisional Ca-alkaline, cordilleran I-type province. The São Pedro Granite occurs in the mountain region of Rio de Janeiro State as small post-collisional bodies. It presents isotropic fabric, equigranular to seriate inequigranular texture, as well as local concentration of allanite, which gives discrete composition and texture variation to the rock. The granite has a high-K calcalkaline to alkali-calcic character and weakly peraluminous nature. Despite its short geochemical variation, high Ba, Zr and Th contents besides low concentrations of MgO and CaO are noticeable. High REE contents are associated with fractionated REE patterns showing strong negative Eu anomalies. A crustal origin for the granite can be assumed by its very negative and positive ЄNd and ЄSr values, respectively, as well as by 87Sr/86Sr initial ratios ranging from 0,718 to 0,740. TDM ages point to paleoproterozoic source, which agrees with geological time of intensive crust generation.