Antonio Carlos Galindo

Evolution of Brasiliano-age granitoid types in a shear-zone environment, Umarizal-Caraubas region, Rio Grande do Norte, northeast Brazil

A sequence of Brasiliano-age granitoid types is exposed in a small area near the cities of Umarizal and Caraubas in Rio Grande do Norte State, Northeast Brazil. Porphyritic K-alkali-calcic monzogranite is an important facies of the oldest Caraubas intrusion (RbSr whole rock isochron age of ca. 630 Ma), which suffered solid-state deformation due to movements on a major NE-trending shear zone. The intrusion of the Prado and part of the Quixaba bodies was probably controlled by the shear zone. These two bodies include mafic/intermediate rocks, some of which contain two pyroxenes, and have hybrid, partly alkaline and partly shoshonitic geochemical characteristics. Rock types and ages are similar to those of some Pan-African occurrences in southwestern Nigeria. The Tourao body, intruded at ca. 590 Ma, presents preferred mineral orientations which are probably largely magmatic, since little evidence is found for widespread solid-state deformation. On the other hand, its intrusion may have been facilitated by the presence of the shear-zone faults. The rocks form a monomodal felsic K-alkali-calcic suite. With the exception of the Quixaba body, all these earlier granitoids are magmatic epidote- and magnetite-bearing porphyritic monzogranites with trace element geochemical characteristics of modern syn-collisional granites. The latest intrusion at ca. 545 Ma is mainly represented by potassic quartz syenites and related rocks, some of which contain fayalite or ferrohypersthene. These rocks possess neither well developed mineral orientations of magmatic origin nor signs of solid-state deformation. They are mineralogically similar to, but younger than some of the “bauchites” of central Nigeria. Geochemical signatures are comparable with those of modern within-plate granites. All granitoids present high (87Sr/86Sr)i ratios which range from 0.708 to 0.712, and increase with decreasing age. Such ratios are compatible with important or dominant crustal contributions. On the other hand, the more mafic rocks are likely to have formed from enriched mantle.

The Umarizal Igneous Association, Borborema Province, NE Brazil: Implications for the Genesis of A-Type Granites

Abstract The Umarizal igneous association (NE Brazil) consists of three A-type plutons, intruded over an interval of about 15 Ma, during the post-collisional phase of the Brasiliano orogeny. All the plutons have Sr isotopic compositions which indicate important or dominant crustal contributions. Crystallization of the Umarizal sill, formed of fayalite quartz syenite and syenogranite, and of the Lagoa stocks, formed by mangerite and granite, commenced under water-undersaturated conditions at pressures around 700-800 MPa and temperatures around 900°C, and continued during magma rise with the crystallization of hornblende at about 480-570 MPa. Crystallization of the Acao stock, which contains a rapakivi-like facies, commenced at similar temperatures, and hornblende appeared at slightly less than 800°C and around 500 MPa. Different f O2 conditions controlled the compositions of the ferromagnesian phases and the nature of the oxide mineral assemblage. Simple fractional crystallization models from homogeneous parent magmas are insufficient to explain the chemical variation of the rocks suites.

Augen gnaisses riacianos no Domínio Rio Piranhas-Seridó - Província Borborema, Nordeste do Brasil

Augen gneisses occurring at the Fazenda Genezare in the Serra da Formiga east of Caico, at the border of the Sao Vicente-Florânia (RN) nucleus, and in exposures SW and NW of Sao Jose do Serido (RN) were studied by geologic mapping, petrography, lithogeochemistry and U-Pb dating. These rocks consist of biotite augen gneisses, and granitic/granodioritic orthogneisses, with porphyro-granoblastic textures and light gray to pink colors. Their mineralogy consists of perthitic microcline, oligoclase, quartz, besides biotite and amphibole, with epidote, titanite, chlorite, opaque minerals, apatite, zircon and allanite as accessory minerals. Petrography indicates that these rocks were affected by amphibolite facies metamorphism, and a later greenschist facies retrometamorphism. They have a prominent low-angle NW-SE foliation containing a high rake stretching lineation. Lithogeochemistry indicates high-K, metaluminous to peraluminous and calc-alkaline geochemical signatures related to a collisional environment. U-Pb dating of zircon (LA-ICP-MS) provided an upper intercept age of 2252 ± 17 Ma and 2171 ± 20 Ma, for the rocks found north of Florânia (RN), and east of Genezare, respectively. In the latter values of 2.84 Ga TDM and ƐNd(0) of -20.76 were also obtained. The data allow attribution of the augen gneisses to the Rhyacian period, and are consequently associated with to the magmatism which formed the Caico Complex.

Relação entre cisalhamento e alojamento de magmas graníticos: zona de cisalhamento Remígio-Pocinhos, maciço São José do Campestre, NE do Brasil

Based on mineralogical, geochemical and textural characteristics, a variety of granite types was identified amongst the intrusions emplaced during the tectonic activity along the Remigio-Pocinhos shear zone in NE Brazil during the latest stages of the Brasiliano orogeny. They include peraluminous granites with I-type mineralogical compositions as well as transitional-to-alkaline geochemical characteristics. These granites contain mafic to intermediate microgranular enclaves. Overall, they are very similar to many other Brasiliano-age plutons in the extreme NE of Brazil. Metaluminous alkaline granites with aegirine-augite usually contain andradite as a minor phase, and titanite as an important accessory mineral. Several aspects of their trace element geochemistry repeat features of syn to late- tectonic alkaline granites in other parts of NE Brazil and in the rest of the world. The shear zone has deep roots, and the heat flow during granite genesis was probably high. A variety of lower crustal source rocks was probably involved in the genesis of these granites.

U-Pb e Sm-Nd em Ortognaisses Riacianos e Estaterianos nos Terrenos Rio Piranhas-Seridó e Jaguaribeano, Província Borborema, Brasil

A zona de cisalhamento Portalegre separa os terrenos geologicos Rio Piranhas-Serido e Jaguaribeano do Dominio Setentrional da Provincia Borborema, com gnaisses bandados dos Complexos Caico e Jaguaretama representando os respectivos embasamentos destes terrenos. Analises U-Pb forneceram idades de 2.193 ± 16 Ma para os ortognaisses do Complexo Jaguaretama, resultados muito similares as determinadas para o Complexo Caico. Idades T DM Sm-Nd nesses Complexos forneceram valores neoarqueanos (ca. 2,5 Ga), com eNd(t) muito proximos ao CHUR. Intrudindo as rochas do Complexo Jaguaretama, destacam-se corpos alongados, foliados e com porfiros estirados, denominados de Suite Serra do Deserto. Estes forneceram idades de cristalizacao a 1.777 ± 8 Ma, ratificando um periodo extensional que ocorreu no Estateriano, bem representado no terreno Jaguaribeano por sequencias vulcanossedimentares e plutonicas Tipo-A. Augen gnaisses com texturas similares, denominados de Suite Poco da Cruz, sao encontrados intrudindo o Complexo Caico no terreno Rio Piranhas-Serido, porem, as idades registradas por todo este dominio sao predominantemente Riacianas (ca. 2,2 Ga) e de afinidades calcio-alcalinas, indicando que estes augen gnaisses possuem diferentes assinaturas e estao bem separados no tempo. Alem disso, alem dos dados geofisicos previos que sugerem esta divisao de Terrenos marcada pela Zona de Cisalhamento Portalegre, os Complexos Caico e Jaguaretama exibem diferencas composicionais marcantes. Corpos de anfibolitos alongados e concordantes a subconcordantes com o bandamento do Complexo Caico e importante contribuicao sedimentar na constituicao do Complexo Jaguaretama com importantes intercalacoes de marmores sao algumas destas diferencas.

Petrographic Characterization of Waste Rocks: Applicability as Concrete Aggregates

Petrographic characterization showing mineralogical, textural and structural aspects of rocks is an essential technique in the study of the performance of concrete aggregates, specifically on the investigation of the alkali-aggregate reaction (AAR). The assessment of this deleterious reaction, considered as one of the most important pathologies, is a required test in the waste utilization programs. In this paper, waste rock samples of a feldspar mine were studied by optical microscopy, approaching aspects related to morphology, texture and alteration degree to determine the potential alkali reactivity for use as aggregates in cementitious mixtures. The rock studied shows characteristics that make it susceptible to be a reactive aggregate, due to, mainly, the strained quartz and the microcrystalline quartz. However, these results must be confirmed by standard mechanical test methods.

O Stock Flores: Exemplo de magmatismo granítico tipo-A no Domínio Rio Piranhas-Seridó, NE da Província Borborema

The Flores granitic stock intrudes paleoproterozoic gneissic related to Caico Complex, in Rio Piranhas-Serido Domain, North part of the Borborema Province, NE Brazil. This work focused on the petrological and geochemical characterization of its monzogranites. The Flores Stock comprises equigranular, leucocratic monzogranites, which are divide in two petrografic facies: i) medium grained, pink monzogranites; and ii) fine grained, gray to pink monzogranites. Locally we can find in this type dioritic enclaves displaying magmas coexistence features (mixing and migling), and ellipsoidal schlieren formed by magmatic convection. Mineralogically, the Stock is constitued by biotite as its main mafic; titanite, allanite, apatite, zircon, opaque, and as late minerals, fluorite, chlorite and saussurite, these late ones are interpreted as related to fluids in subsolidus stage. The studied monzogranites show high SiO 2 contents (> 71% wt%) and alkaline affinity with Na 2 O + K 2 O ~ 9,0%. Tectonic discriminant diagrams of tectonic environment suggest a typology of A-type granite related to a post-collisional environment, which is corroborated by U-Pb zircon crystallization age of 553±4 Ma. Thus, the Flores Stock is interpreted as emplaced in a late to post-tectonic stage of the Brasiliana / Pan-African Orogeny in the Borborema Province.

Definição de suítes magmáticas em corpos ediacaranos no extremo NE da Província Borborema (estado do Rio Grande do Norte): Plútons Pitombeira, Taipu e Gameleira

The Pitombeira, Taipu and Gameleira plutons, located in the northeasternmost portion of the Sao Jose do Campestre Domain within the Borborema Province (NE, Brazil), are subjects of lithogeochemical studies in the present work. The Pitombeira Pluton comprises a monzo- to syenogranitic facies defined by K-feldspar phenocrysts, and a quartz-dioritic to tonalitic facies, mainly as enclaves, with laths of plagioclase showing compositional zoning and chilled rims. The Taipu Pluton is made of equigranular, locally porphyritic, monzo- to syenogranites, whereas the Gamaleira Pluton encompasses tonalitic and granodioritic rocks with porphyritic textures and compositionally zoned plagioclase crystals. Geochemical discriminant diagrams reveal that the rocks forming the Gameleira Pluton are metaluminous, with a calc-alkaline signature and magnesian character, contrasting with the subalkaline (high-K calc-alkaline) signature and ferroan character of the Taipu and Pitombeira Plutons, where the former is highly peraluminous and the latter is metaluminous to slightly peraluminous. Rare Earth Elements patterns are similar to all plutons and register negative Eu anomalies and a relative enrichment of LREE over HREE, with LaN/YbN ratios between 9.38 to 16.20 (Gamaleira Pluton), 17.99 to 31.39 (granitic facies of the Pitombeira Pluton), 14.15 to 21.81 (dioritic facies of the Pitombeira Pluton) and 15.17 to 175.41 (Taipu Pluton). Based on the combined investigation of structural, textural and geochemical data, a late- to post-collisional tectonic environment is suggested for the plutons here studied.

Condutividade térmica enrochas silicáticas cristalinas, com ênfase a rochas graníticas da província borborema, NE do Brasil, e sua correlação com parâmetros petrográficos e texturais

This work focuses on rock thermal conductivity (λ) and its correlation with petrographic and textural aspects especially in granitic rocks. We demonstrate the role of these variables in order to shed some light in the behavior of conductive heat transfer in rocks. Our results demonstrate clear correlation between λ and abundance of quartz and mafic minerals. We show that there is a positive linear correlation between λ and quartz content. On the other hand, we show there a decrease on λ with mafic minerals increase. We found that the relation between λ and quartz content may be divided into two sets. The first set are mostly comprised of samples with more than 20% of quartz exhibiting systematically larger λ values ranging from 2,5 W/mK; the second set comprising samples with less than 20% quartz content have an average λ below 2,5 W/mK. Regarding textural aspects, we verified that rocks considered thick/porphyry textures generally showed better correlations between λ and mineral content (quartz) when compared to rocks considered thin/medium. Therefore, our results clearly indicate that the associated usage of petrophysical measurements such as λ with petrographic and textural information can be used to characterize rocks.