Pirometamorfismo em calcários da formação Jandaíra, bacia potiguar, nordeste do Brasil

The present study aims the characterization of thermally affected carbonate rocks from Jandaíra Formation in contact with Paleogene and Neogene basic intrusions in the region of the Pedro Avelino and Jandaíra municipalities (RN), northeastern Brazil. For this study, field, petrographic, x-ray diffraction, electron microprobe, and whole rock litogeochemistry data of carbonates were undertaken. The thermally unaffected limestones are classified like wackstones, grainstones and packstones. They may constitute carbonates grains of benthic foraminifera, echinoderm spines, ostracods, algae, corals, bivalves, gastropods, peloids and intraclasts. The porosities are classified like vug, intraparticle, interparticle, intercrystal and moldic types. The major minerals are calcite, ankerite and dolomite; the detrital are montmorillonite, pyrite, limonite, quartz and microcline. The thermally affected limestones are very coarse to very fine-grained and light to dark gray color. The fossiliferous components totally disappear, and the porosity tends to disappear. With the data obtained, it can be inferred that the carbonate protoliths would be calciferous to dolomitic limestones, both with small amount of clay minerals. Crystalline carbonates from dolomitic protolith have rhombohedral calcite and iron oxides / hydroxides, making the rocks much darker. The carbonates from calciferous protolith have a wide variation of grain size according to the recrystallization degree, increasing toward contact with the basic bodies. In this group, it was identified the minerals lizardite and spinel in weakly to moderately affected samples, and spinel and spurrite in strongly affected rocks, as well as calcite, that occur everywhere. The geological context (shallow level diabase intrusions), the crystallization of the pyrometamorphic minerals spurrite and olivine, and comparison with diagrams from the literature allow estimating temperatures and pressures around 1050-1200 °C and 0.5-1.0 kbar, respectively, for PTOTAL=PCO2. The post-intrusion cooling would have afforded the releasing of metasomatic / hydrothermal fluids, allowing the opening of the metamorphic system, with possible contribution of chemical elements from host units (sandstones, shales) and from basic intrusions. This would induce hydration of previous phases, allowing the formation of serpentine, chlorite and brucite. The results discussed here reveal the strong influence of the heat from basic intrusions within the sedimentary pile. Whereas in the offshore portion of the basin occur sills with up to 1000 m thickness, the understanding of pyrometamorphism might be useful for understanding and measuring the thermally affected rocks.

Expressão geofísica-estrutural do lineamento transbrasiliano na porção central da Bacia do Parnaíba (Maranhão-Piauí)

The objective of this study was to characterize the structural-geophysical expression of the Transbrasiliano Lineament (TBL) in the east-central portion of the Parnaíba Basin. The TBL corresponds to a major Neoproterozoic NE-trending shear zone related to the Brasiliano orogenic cycle, with dextral strike-slip kinematics, underlying (but also laterally exposed in the NE and SW basin edges) the sedimentary section of the Parnaíba Basin. In this study, the interpretation of gravity and magnetic anomaly maps is consistent with the TBL kinematics, the signature of the geophysical anomalies corresponding to the high (plastic behaviour) and subsequent declining temperature (ductile to brittle behaviour) stages during Brasiliano and late Brasiliano times. The pattern of residual gravity anomalies is compatible with an S-C dextral pair shaping the geological bodies of an heterogeneous basement, such as slices of gneisses and granulites (positive anomalies), granitic and low-medium grade metasedimentary rocks (negative anomalies). Such anomalies curvilinear trends, ranging from NNE (interpreted as S surfaces) to NE (C surfaces), correspond to flattening surfaces (S), while the NE rectilinear trend must represent a C band. The narrower magnetic anomalies also display NNE to NE (S surfaces) trends and should correspond to similar (although narrower and more discontinuous) sources in the equivalent anomaly patterns. Pre-Silurian pull-apart style grabens may contribute to the NE negative gravimetric anomalies, although this interpretation demands control by seismic data analysis. On the other hand, the curvilinear anomalies associated to contractional trends are incompatible with their interpretation as pre-Silurian graben, in both maps. In the (reduced to the pole) magnetic anomalies map, most of these are again associated to low-temperature shear zones (C planes) and faults, juxtaposing distinct blocks in terms of magnetic properties, or eventually filled with basic bodies. It is also possible that some isolated magnetic anomalies correspond to igneous bodies of late-Brasiliano or Mesozoic age. The basement late discontinuities pattern can be interpreted in analogy to the Riedel fractures model, with steep dipping surfaces and a sub-horizontal movement section. This study also explored 2D gravity modeling controlled by the interpretation of a dip seismic line as regards to the Transbrasiliano Lineament. The rock section equivalent to the Jaibaras Group occupying a graben structure (as identified in the seismic line) corresponds to a discrete negative anomaly superimposed to a gravimetric high, once again indicating a stronger influence of older crystalline basement rocks as gravimetric sources, mainly reflecting the heterogeneities and anisotropies generated at high temperature conditions and their subsequent cooling along the TBL, during the Brasiliano cycle.

Expressão geofísica-estrutural do lineamento transbrasiliano na porção central da Bacia do Parnaíba (Maranhão-Piauí)

The objective of this study was to characterize the structural-geophysical expression of the Transbrasiliano Lineament (TBL) in the east-central portion of the Parnaíba Basin. The TBL corresponds to a major Neoproterozoic NE-trending shear zone related to the Brasiliano orogenic cycle, with dextral strike-slip kinematics, underlying (but also laterally exposed in the NE and SW basin edges) the sedimentary section of the Parnaíba Basin. In this study, the interpretation of gravity and magnetic anomaly maps is consistent with the TBL kinematics, the signature of the geophysical anomalies corresponding to the high (plastic behaviour) and subsequent declining temperature (ductile to brittle behaviour) stages during Brasiliano and late Brasiliano times. The pattern of residual gravity anomalies is compatible with an S-C dextral pair shaping the geological bodies of an heterogeneous basement, such as slices of gneisses and granulites (positive anomalies), granitic and low-medium grade metasedimentary rocks (negative anomalies). Such anomalies curvilinear trends, ranging from NNE (interpreted as S surfaces) to NE (C surfaces), correspond to flattening surfaces (S), while the NE rectilinear trend must represent a C band. The narrower magnetic anomalies also display NNE to NE (S surfaces) trends and should correspond to similar (although narrower and more discontinuous) sources in the equivalent anomaly patterns. Pre-Silurian pull-apart style grabens may contribute to the NE negative gravimetric anomalies, although this interpretation demands control by seismic data analysis. On the other hand, the curvilinear anomalies associated to contractional trends are incompatible with their interpretation as pre-Silurian graben, in both maps. In the (reduced to the pole) magnetic anomalies map, most of these are again associated to low-temperature shear zones (C planes) and faults, juxtaposing distinct blocks in terms of magnetic properties, or eventually filled with basic bodies. It is also possible that some isolated magnetic anomalies correspond to igneous bodies of late-Brasiliano or Mesozoic age. The basement late discontinuities pattern can be interpreted in analogy to the Riedel fractures model, with steep dipping surfaces and a sub-horizontal movement section. This study also explored 2D gravity modeling controlled by the interpretation of a dip seismic line as regards to the Transbrasiliano Lineament. The rock section equivalent to the Jaibaras Group occupying a graben structure (as identified in the seismic line) corresponds to a discrete negative anomaly superimposed to a gravimetric high, once again indicating a stronger influence of older crystalline basement rocks as gravimetric sources, mainly reflecting the heterogeneities and anisotropies generated at high temperature conditions and their subsequent cooling along the TBL, during the Brasiliano cycle.

Petrologia e geocronologia U-Pb do plúton granítico Serra da Rajada, porção central do domínio Rio Piranhas - Seridó, Província Borborema, NE do Brasil

The ediacaran plutonic activity related to the Brasilian/Pan-African orogeny is one of the most important geological features in the Borborema Province, represented along its extension by numerous batholiths, stocks, and dikes.The object of this study, the Serra Rajada Granitic Pluton (SRGP), located in the central portion of the Piranhas-Seridó River Domain is an example of this activity. This pluton has been the subject of cartographic, petrographic, geochronological and lithogeochemical studies and its rocks were characterized by two facies. First, the granitic facies were described as monzogranites consisting of K-feldspar, plagioclase (oligoclase - An23-24%), quartz and biotite (main mafic) and opaque minerals such as titanite, allanite, apatite, and zircon as accessories. Alteration minerals are chlorite, white mica and carbonate. Second, the dioritic facies consist of rocks formed by quartz diorite containing plagioclase (dominant mineral phase), quartz and K-feldspar. Biotite and amphibole are the dominant mafic minerals; and titanite, opaque minerals, allanite, zircon and apatite are the accessories. However, previous geological mapping work in the region also identified the presence of other lithostratigraphic units. These were described as gneisses and migmatites with undifferentiated amphibolite lenses related to the Caicó Complex (Paleoproterozoic) and metasedimentary rocks of the Seridó Group (Neoproterozoic) composed of paragneiss with calc-silicate lenses, muscovite quartzite and biotite schist (respectively, the Jucurutu formations, Equador and Seridó), the host rocks for the SRGP rocks. Leucomicrogranite and pegmatite dikes have also been identified, both related to the end of the Ediacaran magmatism and colluvial- eluvial and alluvial deposits related to Neogene and Quaternary, respectively. Lithogeochemical data on the SRGP granite facies, highlighted quite evolved rocks (SiO2 69% to 75%), rich in alkalis (Na2O+K2O ≥ 8.0%), depleted of MgO (≤ 0.45%), CaO (≤ 1.42%) and TiO2 (≤ 0.36%) and moderate levels of Fe2O3t (2.16 to 3.53%). They display transitional nature between metaluminous and peraluminous (predominance of the latter) with sub-alkaline/monzonitic (High K calcium-alkali) affinity. Harker diagrams show negative correlations for Fe2O3t, MgO, and CaO, indicating mafic and plagioclase fractionation. REE spectrum shows enrichment of LREE relative to heavy REE (LaN/YbN = 23.70 to 10.13), with negative anomaly in the Eu (Eu/Eu* = 0.70 to 0.23), suggesting fractionation or accumulation in the feldspars source (plagioclase). Data integration allows to correlate the SRGP rocks with those described as Calcium-Alkaline Suite of equigranular High K. The crystallization conditions of the SRGP rocks were determined from the integration of petrographic and lithogeochemical data. These data indicated intermediate to high conditions of ƒO2 (mineral paragenesis titanite + magnetite + quartz), parent magma saturated in H2O (early biotite crystallization), tardi-magmatic processes of fluids rich in ƒCO2, H2O and O2 causing part of the mineral assembly to change (plagioclase carbonation and saussuritization, biotite chloritization and opaques Sphenitization). Thermobarometrical conditions were estimated based on geochemical parameters (Zr and P2O5) and CIPW normative minerals, with results showing the liquidus minimum temperature of about800°C and the solidus temperature of approximately 700°C. The final/minimum crystallization pressure are suggested to be between 3 and 5 Kbar. The presence of zoned minerals (plagioclase and allanite) associated with lithogeochemical data in bi-log diagrams for Rb vs. Ba and Rb vs. Sr suggest the role of fractional crystallization as the dominant process in the magmatic evolution of SRGP. U-Pb Geochronological and Sm-Nd isotope studies indicated, respectively, the crystallization age of biotite monzogranite as 557 ± 13 Ma, with TDM model age of 2.36 Ga, and εNd value of -20.10 to the crystallization age, allowing to infer paleoproterozoic crustal source for the magma.

Petrologia e geocronologia U-Pb do plúton granítico Serra da Rajada, porção central do domínio Rio Piranhas - Seridó, Província Borborema, NE do Brasil

The ediacaran plutonic activity related to the Brasilian/Pan-African orogeny is one of the most important geological features in the Borborema Province, represented along its extension by numerous batholiths, stocks, and dikes.The object of this study, the Serra Rajada Granitic Pluton (SRGP), located in the central portion of the Piranhas-Seridó River Domain is an example of this activity. This pluton has been the subject of cartographic, petrographic, geochronological and lithogeochemical studies and its rocks were characterized by two facies. First, the granitic facies were described as monzogranites consisting of K-feldspar, plagioclase (oligoclase - An23-24%), quartz and biotite (main mafic) and opaque minerals such as titanite, allanite, apatite, and zircon as accessories. Alteration minerals are chlorite, white mica and carbonate. Second, the dioritic facies consist of rocks formed by quartz diorite containing plagioclase (dominant mineral phase), quartz and K-feldspar. Biotite and amphibole are the dominant mafic minerals; and titanite, opaque minerals, allanite, zircon and apatite are the accessories. However, previous geological mapping work in the region also identified the presence of other lithostratigraphic units. These were described as gneisses and migmatites with undifferentiated amphibolite lenses related to the Caicó Complex (Paleoproterozoic) and metasedimentary rocks of the Seridó Group (Neoproterozoic) composed of paragneiss with calc-silicate lenses, muscovite quartzite and biotite schist (respectively, the Jucurutu formations, Equador and Seridó), the host rocks for the SRGP rocks. Leucomicrogranite and pegmatite dikes have also been identified, both related to the end of the Ediacaran magmatism and colluvial- eluvial and alluvial deposits related to Neogene and Quaternary, respectively. Lithogeochemical data on the SRGP granite facies, highlighted quite evolved rocks (SiO2 69% to 75%), rich in alkalis (Na2O+K2O ≥ 8.0%), depleted of MgO (≤ 0.45%), CaO (≤ 1.42%) and TiO2 (≤ 0.36%) and moderate levels of Fe2O3t (2.16 to 3.53%). They display transitional nature between metaluminous and peraluminous (predominance of the latter) with sub-alkaline/monzonitic (High K calcium-alkali) affinity. Harker diagrams show negative correlations for Fe2O3t, MgO, and CaO, indicating mafic and plagioclase fractionation. REE spectrum shows enrichment of LREE relative to heavy REE (LaN/YbN = 23.70 to 10.13), with negative anomaly in the Eu (Eu/Eu* = 0.70 to 0.23), suggesting fractionation or accumulation in the feldspars source (plagioclase). Data integration allows to correlate the SRGP rocks with those described as Calcium-Alkaline Suite of equigranular High K. The crystallization conditions of the SRGP rocks were determined from the integration of petrographic and lithogeochemical data. These data indicated intermediate to high conditions of ƒO2 (mineral paragenesis titanite + magnetite + quartz), parent magma saturated in H2O (early biotite crystallization), tardi-magmatic processes of fluids rich in ƒCO2, H2O and O2 causing part of the mineral assembly to change (plagioclase carbonation and saussuritization, biotite chloritization and opaques Sphenitization). Thermobarometrical conditions were estimated based on geochemical parameters (Zr and P2O5) and CIPW normative minerals, with results showing the liquidus minimum temperature of about800°C and the solidus temperature of approximately 700°C. The final/minimum crystallization pressure are suggested to be between 3 and 5 Kbar. The presence of zoned minerals (plagioclase and allanite) associated with lithogeochemical data in bi-log diagrams for Rb vs. Ba and Rb vs. Sr suggest the role of fractional crystallization as the dominant process in the magmatic evolution of SRGP. U-Pb Geochronological and Sm-Nd isotope studies indicated, respectively, the crystallization age of biotite monzogranite as 557 ± 13 Ma, with TDM model age of 2.36 Ga, and εNd value of -20.10 to the crystallization age, allowing to infer paleoproterozoic crustal source for the magma.

Análise de sedimentos associados à atividade de perfuração na plataforma continental externa, Bacia Potiguar - Brasil

This study was developed in an area located on the outer shelf in the Potiguar Basin, Brazilian equatorial margin; this tropical shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system. Field sampling was carried out during 3 cruises surrounding a shallow-water exploratory well to compare sediment properties of the seafloor, including grainsize, texture, mineral composition, carbonate content, and organic matter. Cruise 1 (C1) was carried prior to drilling, while Cruise 2(C2) and 3 (C3) respectivelly 3 and 12 months after drilling. The sample grid used had 16 stations located along 4 radials from 50 m the well up to a distance of 500 m. Sediments were analyzed in the first 0-2 cm, and 0-10 cm layers. The results show that sedimentary cover around the well is poor to very poorly sorted, with the particle size predominantly in the medium to coarse sand fraction, followed by gravel and mud. The content of calcium carbonate is greater than 96%, associated to bioclastic sediments, and the content of organic matter is less than 12%. Only minor sedimentological variations occured in the area affected by drilling operations. Mainly observed during the second cruise, in terms of a change in grain size distribution associated to an increase in siliciclastic content, This impact occurred in the most surficial sediment (0-2 cm), in the radial closest to the well (50 m), and could suggest the effects of drilling. However, in the third cruise, one year after drilling, the sediments return to show the same characteristics as in the first cruise. These results show no significant sedimentological variations due to drilling activity, and indicate that ocean dynamics in this area was high enough to recover the environment original characteristics.

Análise de sedimentos associados à atividade de perfuração na plataforma continental externa, Bacia Potiguar - Brasil

This study was developed in an area located on the outer shelf in the Potiguar Basin, Brazilian equatorial margin; this tropical shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system. Field sampling was carried out during 3 cruises surrounding a shallow-water exploratory well to compare sediment properties of the seafloor, including grainsize, texture, mineral composition, carbonate content, and organic matter. Cruise 1 (C1) was carried prior to drilling, while Cruise 2(C2) and 3 (C3) respectivelly 3 and 12 months after drilling. The sample grid used had 16 stations located along 4 radials from 50 m the well up to a distance of 500 m. Sediments were analyzed in the first 0-2 cm, and 0-10 cm layers. The results show that sedimentary cover around the well is poor to very poorly sorted, with the particle size predominantly in the medium to coarse sand fraction, followed by gravel and mud. The content of calcium carbonate is greater than 96%, associated to bioclastic sediments, and the content of organic matter is less than 12%. Only minor sedimentological variations occured in the area affected by drilling operations. Mainly observed during the second cruise, in terms of a change in grain size distribution associated to an increase in siliciclastic content, This impact occurred in the most surficial sediment (0-2 cm), in the radial closest to the well (50 m), and could suggest the effects of drilling. However, in the third cruise, one year after drilling, the sediments return to show the same characteristics as in the first cruise. These results show no significant sedimentological variations due to drilling activity, and indicate that ocean dynamics in this area was high enough to recover the environment original characteristics.

Evolução estratigráfica da sequência neocarbonífera-eotriássica da Bacia do Parnaíba, NE do Brasil

The Parnaíba Basin consists in an intracratonic basin whose sucession of rocks is arranged in five supersequences. The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle and corresponds to Balsas Group, which is divided into four units: Piauí Formation, Pedra de Fogo Formation, Motuca Formation and Sambaíba Formation, from base to top. Different interpretations have been made by several authors in recent decades to interpreted the depositional system and environments related to each unit that belongs to this sequence. In general way, it is described as a thick pack of siliciclastic sediments deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work underwent a stratigraphic analysis of the Upper Carboniferous-Lower Triassic deposits by applying modern concepts of the sequence stratigraphy based on well and seismic database. Three main depositional sequences of higher frequency were identified in each well analyzed. The sequence 1 corresponds to rocks initially deposited by a fluvial system with braided channel characteristics which evolved to shallow marine with coastal sabkha conditions related to a transgressive stage, that later evolved to a deltaic system. The Sequence 2 corresponds to rocks deposited in a lacustrine/desert environment associated with sabkha generated during a period of increased aridity in which the area occupied by the Parnaíba Basin had been suffering. The registration of a major regressive phase is shown in Sequence 2 which evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphy analyses allow to define a series of stratigraphic surfaces and related genetic units, as well as to infer its lateral expression. Seismic facies associated with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting the sedimentary rate was relatively constant during deposition.

Evolução estratigráfica da sequência neocarbonífera-eotriássica da Bacia do Parnaíba, NE do Brasil

The Parnaíba Basin consists in an intracratonic basin whose sucession of rocks is arranged in five supersequences. The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle and corresponds to Balsas Group, which is divided into four units: Piauí Formation, Pedra de Fogo Formation, Motuca Formation and Sambaíba Formation, from base to top. Different interpretations have been made by several authors in recent decades to interpreted the depositional system and environments related to each unit that belongs to this sequence. In general way, it is described as a thick pack of siliciclastic sediments deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work underwent a stratigraphic analysis of the Upper Carboniferous-Lower Triassic deposits by applying modern concepts of the sequence stratigraphy based on well and seismic database. Three main depositional sequences of higher frequency were identified in each well analyzed. The sequence 1 corresponds to rocks initially deposited by a fluvial system with braided channel characteristics which evolved to shallow marine with coastal sabkha conditions related to a transgressive stage, that later evolved to a deltaic system. The Sequence 2 corresponds to rocks deposited in a lacustrine/desert environment associated with sabkha generated during a period of increased aridity in which the area occupied by the Parnaíba Basin had been suffering. The registration of a major regressive phase is shown in Sequence 2 which evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphy analyses allow to define a series of stratigraphic surfaces and related genetic units, as well as to infer its lateral expression. Seismic facies associated with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting the sedimentary rate was relatively constant during deposition.

Crescimento urbano e seus impactos no sistema de drenagem de uma bacia em Natal/RN

The uncontrolled growth of most Brazilian cities is not accompanied by the development of urban infrastructure. With increasing soil sealing, runoff and decreased infiltration volume, impacts on water resources and on population of the areas affected by urban growth are inevitable. This study aims to evaluate the use and occupation of a watershed and analyze the drainage system in order to control the impact using tools to integrate urban development with the drainage of rainwater in an important watershed in the Natal City, Rio Grande do Norte State. The study involved the characterization of the basin XII.4 on the land use and occupation, for the years 2005 and 2014. With the application of SWMM model was possible to analyze the impacts caused by the urbanization process in the existing drainage system, showing the two years analyzed have their areas very close to percentage saturation of impervious areas. Although the region is still predominantly single family residential, suffers increasing verticalization of mainly commercial buildings. The drainage system is inefficient for the area's needs by the year 2005. The drainage system was also tested for four variations of land use by developing scenarios. Scenario 1 is related to the year 2014, considered current. Scenario 2 was adopted the maximum rate of 80% for land use, allowed for the Natal City. Scenario 3 gives the critical condition of land use, with the area 100% impervious. Scenario 4 is applied to the existence of LID (Low Impact Device). The scenarios analysis showed that all indicate deficiency at some point of the drainage system as a result of the high degree of occupation of the area that generate higher flows than the initial drainage system capacity. With the study it became clear that the adoption of non-structural tools are effective in reducing flooding and improving the drainage system capacity.

Crescimento urbano e seus impactos no sistema de drenagem de uma bacia em Natal/RN

The uncontrolled growth of most Brazilian cities is not accompanied by the development of urban infrastructure. With increasing soil sealing, runoff and decreased infiltration volume, impacts on water resources and on population of the areas affected by urban growth are inevitable. This study aims to evaluate the use and occupation of a watershed and analyze the drainage system in order to control the impact using tools to integrate urban development with the drainage of rainwater in an important watershed in the Natal City, Rio Grande do Norte State. The study involved the characterization of the basin XII.4 on the land use and occupation, for the years 2005 and 2014. With the application of SWMM model was possible to analyze the impacts caused by the urbanization process in the existing drainage system, showing the two years analyzed have their areas very close to percentage saturation of impervious areas. Although the region is still predominantly single family residential, suffers increasing verticalization of mainly commercial buildings. The drainage system is inefficient for the area's needs by the year 2005. The drainage system was also tested for four variations of land use by developing scenarios. Scenario 1 is related to the year 2014, considered current. Scenario 2 was adopted the maximum rate of 80% for land use, allowed for the Natal City. Scenario 3 gives the critical condition of land use, with the area 100% impervious. Scenario 4 is applied to the existence of LID (Low Impact Device). The scenarios analysis showed that all indicate deficiency at some point of the drainage system as a result of the high degree of occupation of the area that generate higher flows than the initial drainage system capacity. With the study it became clear that the adoption of non-structural tools are effective in reducing flooding and improving the drainage system capacity.

Petrologia e geoquímica do magmatismo ediacarano Serra do Caramuru, Rio Grande do Norte, NE do Brasil

The Serra do Caramuru and Tapuio stocks, located in the extreme NE of Rio Piranhas-Seridó Domain (RN), are representative of the Ediacaran-Cambrian magmatism, an important magmatic feature of the Brasilian / Panafrican orogeny of the Borborema Province. These bodies are lithologically similar, intrusive in paleoproterozoic gneiss embasement, being separated by a thin belt of mylonitic orthogneiss. The field relations show a magmatic stratigraphy initiated by dioritic facies that coexists with the porphyritic granitic and equigranular granitic I facies, and less frequently with equigranular granitic II facies. These rocks are crosscut by late granitic dykes and sheets with NE-SW / NNE-SSW orientation. The dioritic facies (diorite, quartz diorite, quartz monzodiorites, tonalite and granodiorite) is leucocratic to melanocratic, rich in biotite and hornblende. The granitic facies are hololeucocratic to leucocratic, and have biotite ± hornblende. Petrographic and geochemical (whole rock) data, especially from Serra do Caramuru pluton, suggest fractionation of zircon, apatite, clinopyroxene (in diorites), opaque minerals, titanite, biotite, hornblende, allanite, plagioclase, microcline and garnet (in dykes). The behavior of trace elements such as Zr, La and Yb indicates that the dioritic magma does not constitute the parental magma for the granitic facies. On the other hand, the granitic facies seems to be cogenetic to each other, displaying differentiation trends and very similar rare earth elements (REE) spectra [12.3≤(La/Yb)N≤190.8; Eu/Eu*=0.30-0.68]. Field relationships and REE patterns [6.96≤(La/Yb)N≤277.8; Eu/Eu*=0.18-0.58] demonstrate that the granitic dykes and sheets are not cogenetically related to the Serra do Caramuru magmatism. The dioritic facies is metaluminous (A/CNK = 0.88-0.74) and shoshonitic, whereas the granitic ones are metaluminous to peraluminous (A/CNK = 1.08-0.93) and high potassium calc-alkaline. Dykes and sheets are strictly peraluminous (A/CNK = 1.01-1.04). Binary diagrams relating compatible and incompatible trace elements and microtextures indicate the fractional crystallization as the dominant mechanism of magmatic evolution of the various facies. The Serra do Caramuru and Tapuio stocks have well preserved magmatic fabric, do not show metamorphic minerals and are structurally isotropic, showing crosscutting contact with the ductile fabric of the basement. These observations lead to interpretate a stage of relative tectonic stability, consistent with the orogenic relaxation period of the Brasiliano / Pan-African orogeny. Chemical plots involving oxides and trace elements indicate late to post-collisional emplacement. In this context, the assumed better mechanism to describe the stocks emplacement within an extensional T Riedel joint, with ENE-WSW extensional vector. The U-Pb zircon age of 553 ± 10 Ma allows correlating the Serra do Caramuru magmatism to the group of post-collisional bodies, equigranular high potassium calc-alkaline granites of the NE of Rio Piranhas-Seridó Domain.

Petrologia e geoquímica do magmatismo ediacarano Serra do Caramuru, Rio Grande do Norte, NE do Brasil

The Serra do Caramuru and Tapuio stocks, located in the extreme NE of Rio Piranhas-Seridó Domain (RN), are representative of the Ediacaran-Cambrian magmatism, an important magmatic feature of the Brasilian / Panafrican orogeny of the Borborema Province. These bodies are lithologically similar, intrusive in paleoproterozoic gneiss embasement, being separated by a thin belt of mylonitic orthogneiss. The field relations show a magmatic stratigraphy initiated by dioritic facies that coexists with the porphyritic granitic and equigranular granitic I facies, and less frequently with equigranular granitic II facies. These rocks are crosscut by late granitic dykes and sheets with NE-SW / NNE-SSW orientation. The dioritic facies (diorite, quartz diorite, quartz monzodiorites, tonalite and granodiorite) is leucocratic to melanocratic, rich in biotite and hornblende. The granitic facies are hololeucocratic to leucocratic, and have biotite ± hornblende. Petrographic and geochemical (whole rock) data, especially from Serra do Caramuru pluton, suggest fractionation of zircon, apatite, clinopyroxene (in diorites), opaque minerals, titanite, biotite, hornblende, allanite, plagioclase, microcline and garnet (in dykes). The behavior of trace elements such as Zr, La and Yb indicates that the dioritic magma does not constitute the parental magma for the granitic facies. On the other hand, the granitic facies seems to be cogenetic to each other, displaying differentiation trends and very similar rare earth elements (REE) spectra [12.3≤(La/Yb)N≤190.8; Eu/Eu*=0.30-0.68]. Field relationships and REE patterns [6.96≤(La/Yb)N≤277.8; Eu/Eu*=0.18-0.58] demonstrate that the granitic dykes and sheets are not cogenetically related to the Serra do Caramuru magmatism. The dioritic facies is metaluminous (A/CNK = 0.88-0.74) and shoshonitic, whereas the granitic ones are metaluminous to peraluminous (A/CNK = 1.08-0.93) and high potassium calc-alkaline. Dykes and sheets are strictly peraluminous (A/CNK = 1.01-1.04). Binary diagrams relating compatible and incompatible trace elements and microtextures indicate the fractional crystallization as the dominant mechanism of magmatic evolution of the various facies. The Serra do Caramuru and Tapuio stocks have well preserved magmatic fabric, do not show metamorphic minerals and are structurally isotropic, showing crosscutting contact with the ductile fabric of the basement. These observations lead to interpretate a stage of relative tectonic stability, consistent with the orogenic relaxation period of the Brasiliano / Pan-African orogeny. Chemical plots involving oxides and trace elements indicate late to post-collisional emplacement. In this context, the assumed better mechanism to describe the stocks emplacement within an extensional T Riedel joint, with ENE-WSW extensional vector. The U-Pb zircon age of 553 ± 10 Ma allows correlating the Serra do Caramuru magmatism to the group of post-collisional bodies, equigranular high potassium calc-alkaline granites of the NE of Rio Piranhas-Seridó Domain.

Pirometamorfismo provocado por intrusões básicas cenozoicas em rochas da Bacia Potiguar, RN: integração de dados geológicos e petrofísicos

The Potiguar Basin is located in the Brazilian Equatorial Margin and presents sedimentary rocks affected by Cenozoic basic igneous intrusions, known as Macau Magmatism. The most prominent effect related to these intrusions is the formation of buchites, pyrometamorphic rocks that occur at very high temperatures and very low pressures in the sanidinite metamorphic facies. Through literature review, field observations, petrographic and petrophysical data, accessing the database of previous studies and results from this research, it was possible to characterize and estimate the effects produced in the thermal aureole of some hypabyssal bodies in the basin. The most relevant features associated with the intrusions are: compactation, hydraulic fracturing, partial melting and recrystallization of country rocks. According to the observed mineral occurrences, temperature of 800 to 1200 °C and pressure below 0,5 kbar were estimated at the contacts of the igneous bodies. The thermal modeling of the São João plug indicates thermal effects extending up to 150 m away from the contact and cooling time of approximately 265,000 years. After the peak of temperature, followed a cooling phase registered by remobilization and precipitation of minerals at low-temperature in faults, fractures and geodes, interpreted as derived from reactions with sedimentary rocks and metasomatic / hydrothermal fluids with abundant carbonatization and silicification.

Pirometamorfismo provocado por intrusões básicas cenozoicas em rochas da Bacia Potiguar, RN: integração de dados geológicos e petrofísicos

The Potiguar Basin is located in the Brazilian Equatorial Margin and presents sedimentary rocks affected by Cenozoic basic igneous intrusions, known as Macau Magmatism. The most prominent effect related to these intrusions is the formation of buchites, pyrometamorphic rocks that occur at very high temperatures and very low pressures in the sanidinite metamorphic facies. Through literature review, field observations, petrographic and petrophysical data, accessing the database of previous studies and results from this research, it was possible to characterize and estimate the effects produced in the thermal aureole of some hypabyssal bodies in the basin. The most relevant features associated with the intrusions are: compactation, hydraulic fracturing, partial melting and recrystallization of country rocks. According to the observed mineral occurrences, temperature of 800 to 1200 °C and pressure below 0,5 kbar were estimated at the contacts of the igneous bodies. The thermal modeling of the São João plug indicates thermal effects extending up to 150 m away from the contact and cooling time of approximately 265,000 years. After the peak of temperature, followed a cooling phase registered by remobilization and precipitation of minerals at low-temperature in faults, fractures and geodes, interpreted as derived from reactions with sedimentary rocks and metasomatic / hydrothermal fluids with abundant carbonatization and silicification.