José Ribeiro Aires

Reavaliação genética do modelo do Vulcão de Nova Iguaçu, RJ: origem eruptiva ou intrusão subvulcânica?

This paper re-examines the geologic structures that were considered to be of eruptive origin of the Mendanha Alkaline Complex, State of Rio de Janeiro, Brazil. The valley interpreted to be a crater is not underlain by pyroclastic agglomerate, but by massive trachyte. The alluvial deposit on this valley is a few meters thick and composed of regolitic materials. The talus deposit of large boulders suggestive of crater wall collapse is not observed. No morphologic elevation indicative of volcanic cone is found. The outcrops around the supposed crater show no volcanic stratification. The valley is not considered to be a volcanic crater but a landform resulted from erosion. The most abundant trachyte of this area is cut by the alkali syenite, and therefore, cannot be the lava flow that covers the syenite. The trachyte takes place up to the top of the massif, 600 m above the supposed crater. The columnar joints are originated from dikes, and the bread crust-like structure is attributed to weathering. These observations indicate that the trachyte is not constituent of lava flows, but of subvolcanic bodies. The authors conclude that the volcanic rocks in question are not constituent of volcanic edifice, but of subvolcanic vent-filling tuff breccia, and the hypothesis of the Nova Iguacu Volcano is implausible.

Brecha tectônica da área de Cabo Frio - RJ, intrudida por dique máfico do Eocretáceo: evidência do tectonismo rúptil do Pan-Africano?

This paper presents the field descriptions and microscopic observations of a tectonic breccia in the basement gneiss of the Cabo Frio and Arraial do Cabo areas, State of Rio de Janeiro, Brazil, and its intrusive contact with the Early Cretaceous mafic dyke. At the sea cliff close to the Ilha do Japones, there is an excellent contact outcrop between them. The tectonic breccia zone is 10 to 20m wide and has N30oE direction. The breccia clasts are angular and characterized by auto-brecciation texture, and composed of breccia with similar aspect of the host tectonic breccia. The matrix is firmly consolidated by hydrothermalism and following silicification. The mafic dyke is 7 to 10m wide and of N45oE direction. Along the contact, the dyke chilled margin featured by fine-grained basalt and prismatic joints can be observed. At the Conchas Beach and Arraial do Cabo city, there are four outcrops demonstrating the mafic dyke intrusion into the consolidated tectonic breccias. These outcrops prove that the tectonic breccias are older than the Early Cretaceous tholeiitic dykes. The fault breccias could have been formed during the brittle-phase tectonism of the last stage of the Pan-African Orogeny by hydrothermalism without magmatic activities, namely tectonic hydrothermalism. The existence of the clasts constituent of the breccia that are composed of breccia suggests that the fault movement and following hydrothermalism occurred repeatedly.

New method of semi-automatic modal analyses for phenocrysts of porphyritic rocks based on quantitative digital colour specification technique

This paper proposes a new method for modal analyses of centimetric phenocrysts and porphyroblasts using a digital rock image. The dot matrix image of the rock is imported into vectorial draw software as background. The outline of all of the phenocrysts is marked on the overlay by closed vectorial curves and their internal area is painted in black. After this operation, the rock image on the background is deleted and the background is painted in white. The vectorial objects composed of the background and the overlay are exported as a matrix image of BMP format. The percentage of the areas painted in black corresponds to modal abundance of the phenocrysts, and it is calculated by the Wilber colour specification software. This method has high precision in graphic processing. However the translucency of the groundmass causes uncertainness in vectorial phenocryst marking. Therefore, it is recommended for rocks with phenocrysts larger than 5 mm.

Terremoto fóssil evidenciado pela brecha tectônica silicificada da área de Cabo Frio, RJ, e sua relação com a gênese do movimento intermitente da falha e o hidrotermalismo amagmático associado

This paper shows present the present state of the geologic and petrographic studies of the silicified tectonic breccia in the Cabo Frio area, State of Rio de Janeiro, Brazil. Based on the results, the authors propose a model for the stick-slip fault displacement. The tectonic breccia zones are 50 cm to 20 m wide and of NE-SW general strike. They are cut by early Cretaceous mafic dykes. The clasts are angular and characterized by auto-brecciation texture. The matrix is firmly consolidated by the silicification with the dissemination of hematite, carbonates, and sericite. The breccia zones are distributed randomly in a wade area of the State of Rio de Janeiro. This observation affirms that the silicification cannot be related to the local magmatism of Cretaceous to early Cenozoic alkaline intrusions. An alternative idea is the hydrothermalism originated form geothermal gradient. According to the normal continental geothermal gradient, of 25 to 30oC/km, the temperature attributed to the depth from 6 to 8 km is 180 to 220oC. In this condition, the H2O present there behaves as hydrothermal liquid. This type of hydrothermalism, without participation of magmas, is called amagmatic hydrothermalism. When a earthquake occurs, a cataclastic zone is formed. The surface water migrated down through it and arrived at the depth of 6 to 8 km transforms into amagmatic hydrothermal liquid. According to the stress adjustment and consequent accommodation of the crustal rock blocks, the hydrothermal liquid is compressed and forced to rise up along the cataclastic zone. During the ascension and cooling of the hydrothermal liquid, the cataclastic zone is silicified and consolidated. The silicified tectonic breccia observed in the studied area can represent this phenomenon which took place in the geologic time. By means of the silicification and consolidation, the fault plane transforms form a weak zone into a mechanically firm zone. This transformation enables the accumulation of tectonic energy in form of elastic deformation. That is, during and just after the earthquake, the fault plane is a weak zone. However, in the period between earthquakes it is a firm zone. The clasts of the fault breccia are composed of silicified cataclastic rock. This observation indicates that the fault plane rupture and its hydrothermal consolidation took place repeatedly, which implies repeated earthquakes along the same fault. The authors propose this process as an additional mechanism for stick-slip movement of the faults. The amagmatic hydrothermalism can be a geologic representation of asperity. It is a seismologic conception that corresponds to the specific locality on the fault plane in which the fault movement is stagnated and tectonic energy is accumulated in form of elastic deformation. A new earthquake occurs by means of asperity rupture.

Cristalização fracionada e assimilação da crosta continental pelos magmas de rochas alcalinas félsicas do estado do Rio de Janeiro

This paper presents a general model for the magmatic evolution for the felsic alkaline rocks of the State of Rio de Janeiro, Brazil. The rocks are meta-alkaline nepheline syenite, alkaline syenite, phonolite, and trachyte of potassic and ultrapotassic series. The compositional variation diagrams indicate the fractionation crystallisation of clinopyroxene, amphibole, titanite, ilmenite, and apatite. The magmatic fractionation for the mafic and intermediate alkaline rocks is characterised by the crystallisation of leucita and alkaline feldspar, and for the felsic alkaline rocks, of leucita and alkaline feldspar. By means of fractionation crystallisation, the felsic alkaline magma changes its composition from meta-alkaline to peralkaline and from potassic to sodic. The SSI diagram (silica saturation index) for the felsic alkaline rocks presents a linear trend crossing over the thermal divide, pointing out a strong effect of continental crust assimilation. The magmatic evolution occurs in three stages: 1) Leucite crystallisation; 2) Alkaline feldspar and nepheline crystallisation; 3) Continental crust assimilation and consequent transformation of the silica-undersaturated composition into oversaturated one. The assimilation rate is high, reaching up to 54%. The assimilation events took place during the Stage 2, which suggests magma super-reheating and consequent wall rock partial melting, proposing the origin of alkaline syenite and trachyte magmas of thermodynamically unstable composition.

Análises geomorfológicas do Platô de Teresópolis e da Serra do Mar, RJ, com o auxílio de seppômen e ASTER GDEM e sua relação aos tectonismos cenozoicos

This paper presents geomorphological analyses of the Teresopolis Plateau, the Serra do Mar Cliff, and the adjacent areas, State of Rio de Janeiro, Brazil, with the help of summit level technique using ASTER GDEM satellite topographic data, and its relation to the Cenozoic tectonism. According to the summit level map of the grid of 4 km, the studied area is divided into three geomorphologic domains, Rio de Janeiro Lowland (RJL), Serra do Mar Scarp (SMS), and Teresopolis Highland (TRH). The Rio de Janeiro Lowland, which corresponds to the Guanabara Graben area, is constituted by alluvial deposit planes and the areas of low hills that expose the metamorphic basement. Their limits are sharp and linear suggesting existence of small faults and grabens. The tops of the low hills are almost at the same altitude, constituting a summit level surface with approximate altitude of 100 m. The summit level maps of the grid of 4 km and 2 km show that the top of the Serra do Mar Scarp is of approximate altitude of 1300 m, and therefore, this normal fault has displacement of about 1200 m. The scarp strikes N75oW and divided into the segments of 15 km to 30 km in length. At some localities on the halfway of the scarp, mega-landslide morphology is observed. The Teresopolis highland is tilted to N30oW with mean dip of 0.9o. The drainage systems on the plateau shows three orientations, N60oE, N10oE e N75oW. Along the Paraiba do Sul River, there is a morphologic depression of 20 km wide and 300 m deep, suggesting presence of a shallow graben. On the Teresopolis Highland, there are three alignments of morphologic protrusions oriented to N50oE, with relative height up to 900 m. The height distribution diagrams for the summit level surfaces suggest that they should be remnant inselbergs before the fault movement of the Serra do Mar Scarp. The Cenozoic normal faults and the fracture systems of this area are often discordant with the basement structure. This phenomenon is explained by the model of the reactivation of the structures of the lithospheric mantle, and not of the continental crust.