Susanna Eleonora Sichel

Geophysical and geochemical evidence for cold upper mantle beneath the Equatorial Atlantic Ocean

This paper presents geophysical and geochemical evidence for the possible existence of cold, subducted lithosphere beneath the Saint Paul Fracture Zone of the Equatorial Atlantic Ocean. The ocean floor along the fracture zone is characterized by a high percentage of abyssal peridotites. The abyssal peridotites were emplaced by tectonic uplift of the oceanic lithosphere. The top of the ridge is exposed at Saint Peter and Saint Paul islets. The Os isotopic composition of these abyssal peridotites indicate the presence of old depleted mantle material in this region with Re-depletion model ages (TRD) from 0.32 to 1.1Ga. Melt inclusions in plagioclase phenocrysts of the MORB close to this location have boninitic composition, suggesting that some basalts originated from low-degree mantle melting. The global tomography models show fast seismic velocities in the upper and lower mantle of the Equatorial Atlantic Ocean, consistent with the presence of cold subducted lithosphere. The range of Re-depletion model ages are consistent with paleo-reconstructions of plate motion, suggesting that the fossil subducted slab was formed during the closure of both the Iapetus and the Rheic oceans.

Avaliação de aspectos texturais e estruturais de corpos vulcânicos e subvulcânicos e sua relação com o ambiente de cristalização, com base em exemplos do Brasil, Argentina e Chile

Many volcanoes and subvolcanic rock bodies observed in Brazil, Argentina, and Chile represent didactic examples that explain the relation between textural and structural aspects of their constituent rocks and crystallization environment of the rock bodies. The volcanic bodies, such as lavas and pyroclastic flows, have large horizontal extension, small thickness, and sub-horizontal contacts plains. On the other hand, many subvolcanic bodies, such as vent-filling rock bodies have small horizontal extension, large vertical dimension, and sub-vertical contacts plains. The volcanic rock bodies are constituted by fine-grained rocks, however some subvolcanic bodies also are made up of similar rocks. The grain-size of igneous rocks is related mainly to magma-cooling rate, and not directly to their emplacement depth. Therefore, the interpretation of geological occurrence mode from the lithological and petrological description is often impossible, especially in case of pyroclastic rocks. Geological occurrence mode of an igneous body must be determined by means of field observations, especially of contact outcrops.

Taxa de soerguimento atual do arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial

The Saint Peter and Saint Paul Islets are situated in the Equatorial Atlantic Ocean at the top of a submarine ridge made up of mantle peridotite. It is 90 km long, 25 km wide, and 3,800 m high, and is called the Saint Peter Saint Paul Peridotite Ridge. The submarine morphology is highly rugged with hundreds-of-meters-high scarps, suggesting the occurrence of probably recent active tectonism. The summit level map shows two surfaces of wave-cut benches with respective altitudes of 4~5 m and 7~9 m. The wave-cut benches are observed on three major islets in the same altitude ranges, indicating that no tilting or differential uplifting between the islets has occurred. Considering the upper platform to be formed during the Flandrian Transgression and the lower one developed by a recent erosive process, the mean uplift rate in the last 6.000 years is calculated to be 1.2~1.5 mm/year. The 14C datings for the coral fossils collected from the Belmont Island indicate the uplift rate in the last 6600 years to be 1.5~1.8 mm/year. Therefore, a rate of 1.5 mm/year should be reasonable for the present-day uplifting. Interpreting the peridotite ridge as a tectonically deformed megamullion, the total uplift would be 1500~3000 m and the tectonism should have started at 1~2 Ma. If the Peridotite Ridge is originated from the subcrustal oceanic mantle, the total uplift would be approximately 9000 m and the tectonism would have started close to 6 Ma.

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.

Subvolcanic vent-filling welded tuff breccia of Cabo Frio Island, State of Rio de Janeiro, Brazil

At the southwestern corner of Cabo Frio Island, State of Rio de Janeiro, Brazil, a pyroclastic body is present in an area of 500 x 600 m, intruding into the host orthogneiss with subvertical contact. Between the orthogneiss and pyroclastic body, there is a trachyte intercalation, 5 to 10 m wide. The pyroclastic rocks have a clast-matrix supported structure and the clast size ranges from 1 mm to 40 cm. Larger than 10 cm clasts are semi-rounded and those that are less than 5 cm are angular. They are composed mainly of trachyte, subordinately of orthogneiss, and eventually of welded tuff. The texture is heterogeneous and no grain-size sorting or volcanic layering of the clasts was observed. Microscopic observations have revealed hydrothermal alteration featured by calcite dissemination, alkaline feldspar sericitization, and mafic mineral decomposition. The matrix is filled by angular fragments of alkaline feldspar, quartz, plagioclase, and opaque minerals, smaller than 0.6 mm. The existence of the welded tuff clasts indicates that explosive eruptions took place repeatedly. The subvertical intrusive contact, small exposure area, rounded clasts, heterogeneous clast size, and absence of volcanic layering indicate that the pyroclastic rock is subvolcanic vent-filling welded tuff breccia, and not a constituent of a subaerial eruptive deposit.

Zona de fratura de Vitória-Trindade no Oceano Atlântico sudeste e suas implicações tectônicas

The integration of geological and geophysical data allowed a better understanding of the inter-relation between the oceanic and continental structure. Tectonic and magmatic phenomena of the Meso-Cenozoic were probably related to the prolongation of oceanic fracture zones into the Brazilian Southeast continental margin. This work will focus on the behaviour of the Vitoria-Trindade Fracture Zone (VTFZ). The Vitoria-Trindade Fracture Zone (VTFZ), located at 18o40S in the Mid-Atlantic Ridge (MAR), was defined by seismic, gravity and magnetic isochrones. At the MAR axis the VTFZ corresponds to a second-order discontinuity, with a 6 km axial offset. Away from the ridge axis, potential field data shows a gravimetric depression in E-W direction, which represents the asseismic portion of the VTFZ. Towards the Brazilian Continental Margin there is a series of topographic highs defining the Vitoria-Trindade Chain, which is divided into distinct sedimentary thickness and tectonic domains. This area is cut by sills and dikes, indicating tectonic instability, remobilization of faults and strike-slip tectonics which has uplifted the entire sedimentary column, including the oceanic bottom. All of these observations suggest recent oceanic crust reactivation with alternations of compressive and extensional stress associated with volcanism and normal faulting. Geological and geophysical studies, in addition to mapping the earthquakes epicenters, suggest that the continuation of the VTFZ that occurs onland is related to the Vitoria High and to tectonic and volcanic activities, which are expressed by the Trindade-Paxoreu /Alto do Paranaiba Alkalic Magmatic and Seismic Lineaments. We propose that the VTFZ acts as a pathway for the magma generated by the Trindade Mantle Plume.

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.

Geochemical behaviour of trace elements during fractional crystallization and crustal assimilation of the felsic alkaline magmas of the state of Rio de Janeiro, Brazil

This paper presents geochemical behaviour of trace elements of the felsic alkaline rocks of the state of Rio de Janeiro, Brazil, with special attention of fractional crystallization and continental crust assimilation. Fractionation of leucite and K-feldspar increases Rb/K and decreases K2O/(K2O+Na2O). Primitive nepheline syenite magmas have low Zr/TiO2, Sr, and Ba. On the Nb/Y vs. Zr/TiO2 diagram, these rocks are projected on the field of alkaline basalt, basanite, and nephelinite, instead of phonolite. Well-fractionated peralkaline nepheline syenite has high Zr/TiO2 but there are no zircon. The diagrams of silica saturation index (SSI) distinguish the trends originated form fractional crystallization and crustal assimilation. In the field of SSI<-200, Zr/TiO2 and Ba/Sr have negative correlations to SSI in consequence of fractional crystallization. In the field of SSI>-200, they show positive correlations due to continental crust assimilation. Total REEs (Rare Earth Elements) is nearly 10 times that of granitic rocks, but LaN/SmN and LaN/YbN are similar. REE trend is linear and Eu anomaly is irrelevant. The pegmatitic liquid generated by country rock partial melting is SiO2-oversaturated and peraluminous with high Ba, Sr, Ba/Sr, Zr/TiO2, and SSI, with high content of fluids. This model justifies the peraluminous and SiO2-oversaturated composition of the rocks with relevant effects of continental crust assimilation.