New U-Pb and Sm-Nd isotopic data for orthogneiss and granitoid rocks from the Neoproterozoic Goiá... more New U-Pb and Sm-Nd isotopic data for orthogneiss and granitoid rocks from the Neoproterozoic Goiás magmatic arc in western Goiás constrain the geological evolution of this juvenile crust in the western Brasília belt. Orthogneiss rock samples have U-Pb crystallization ages of 804G6, 669G3, 662G12, 634G8, 630G5, and 637G20 Ma and show 3 Nd (T) values between C2.8 and K15.1. Rock units with negative 3 Nd (T) are more frequent in the eastern part of the studied area to the south of Anicuns, which indicates the presence of older continental crust in that part of the arc. Metagranitoids have ages of 821G10, 810G10, 792G5, 790G12, 782G14, 748G4, and 614G5 Ma and 3 Nd (T) values between C5.1 and K3.7. The data presented here, combined with those in the literature, suggest that igneous activity in the Goiás magmatic arc took place in two episodes: between ca. 0.89 and 0.8 Ga, probably in intraoceanic settings, and between ca. 0.66 and 0.60 Ga, likely in an active continental margin at the end of the Brasiliano orogeny.
The Natividade-Cavalcanti crustal block is a Paleoproterozoic sialic terrain which forms the base... more The Natividade-Cavalcanti crustal block is a Paleoproterozoic sialic terrain which forms the basement of sedimentary sequences belonging to the Neoproterozoic Brasília Belt in central Brazil. Two main domains were recognized in this crustal block, the limits of which are not well constrained due to the lack of geological maps at an appropriate scale. The northeastern domain is characterized by narrow supracrustal belts which wrap around gneiss domes. At least three rock-forming events are recorded in the calc-alkaline, metaluminous basement gneisses of this domain: (i) in the Almas-Dianópolis area, granitoids were dated at c. 2.2 Ga, (ii) early Paleoproterozoic magmatism was dated at c. 2.3-2.4 Ga in the Natividade-Conceiç ão do Tocantins area, and (iii) younger additions were dated at 2144 ± 21 Ma. T DM model ages range between 2.24 and 3.11 Ga, and ε Nd (t) values range from +2 to close to zero, suggesting juvenile sources. Bordering this domain west-and southwards, extending from Cavalcante to Arraias, Paranã and Pindorama do Tocantins, mostly peraluminous granitoids of the Aurumina Suite are exposed. Geochronological data for Aurumina granitoid rocks of this sector point to magmatic events between 2.13 and 2.18 Ga, whereas T DM model ages range between 2.21 and 2.92 Ga with generally negative ε Nd (t) values. An exception is a largely undeformed tonalite sample from close to Arraias, dated at 2042 ± 12 Ma.
The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultrama... more The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe-and Al-rich metasedimentary rocks of the upper unit. Sm-Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00^0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show T DM model ages of mostly 2.33-2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite-garnet schist yields an age of 505^7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixás represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny.
In the Mara Rosa area, in the northern sector of the arc, the earlier magmatic activity was dated... more In the Mara Rosa area, in the northern sector of the arc, the earlier magmatic activity was dated at ca. 860 Ma, and is represented by juvenile rocks with island arc geochemical and isotopic signature (Viana et al., 1995; Pimentel et al., 1997). Syntectonic peraluminous granites and one diorite intrusion are in the age interval between 615 and 640 Ma. Sm-Nd garnet ages and titanite U-Pb ages of 610, 600, 630, and 760 Ma are indicative of the main metamorphic events (Junges et al., 2002a,b). A younger episode of juvenile magmatism, dated at ca. 600-640 Ma, is recorded by tonalites and amphibolites at the western part of the Mara Rosa arc (Junges et al., 2002a, 2003). The youngest ages, of ca. 570 Ma, refer to postorogenic granites. In this study we report new Sm-Nd isotopic data and U-Pb zircon ages of metavolcanic and metaplutonic rocks from Santa Terezinha de Goiás, ca. 100 km to the southwest of Mara Rosa, which indicate island arc-related volcanism and sedimentation at ca. 670 Ma and calc-alkaline intrusions of ca. 610-650 Ma.
The Neoproterozoic Brasõ Âlia Belt, in central Brazil, includes in its eastern part a thick pile ... more The Neoproterozoic Brasõ Âlia Belt, in central Brazil, includes in its eastern part a thick pile of sediments deposited and deformed along the western margin of the Sa Äo Francisco±Congo Craton. Several lithostratigraphic units are identi®ed (the Araõ Â, Paranoa Â, Serra da Mesa, Araxa Â, Ibia Â, Vazante, Canastra and Bambuõ  groups) and have been traditionally interpreted as part of a passive margin association ,1:2 Ga; with sediments being derived from Archaean or Paleoproterozoic continental sources to the north and east. Nd isotopic signatures of ®ne-grained detrital sediments of the several rock units of the belt were investigated in order to assess: (i) the nature and average crustal residence ages of the source areas, and (ii) the tectonic signi®cance of the different sedimentary units in respect to the evolution of the Brasõ Âlia Belt. T DM model ages of the ca. 1.2±0.9 Ga old Paranoa  and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3 Ga, suggesting relatively uniform Paleoproterozoic continental sources within the Sa Äo Francisco continent. The sediments of the detritic/ carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger T DM values (1.7±2.1 Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources. The Ibia  and Araxa  groups show a bimodal distribution of model age values, with a set of samples displaying T DM values between 1.8 and 2.1 Ga (similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3 Ga. Younger sources such as those represented by the Neoproterozoic Goia Âs Magmatic Arc (0.93±0.64 Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments (feldspathic micaschists) within the magmatic arc, in fact, have T DM model ages mostly between 1.0 and 1.2 Ga, indicating that the original sediments represent products of erosion of the arc itself. Therefore, part of the rock units that have been mapped as Araxa  and Ibia  groups can be deeper water equivalents of a passive margin sequence (sediments with Paleoproterozoic model ages). However, another part has been clearly deposited under the in¯uence of an island arc source, most probably in a back-arc basin. The Bambuõ  Group sediments ,0:8 Ga?; at the top of the sequence, also indicate the presence of a young source. T DM model ages vary between ca. 1.4 and 1.9 Ga. However, the distribution pattern of T DM values is more uniform, being intermediate between the Neoproterozoic juvenile source and the Paleoproterozoic continental source area. The Bambuõ  Group is interpreted as a post-inversion sequence, with detrital sediments being mostly derived from erosion of a mountain range in the west, after accretion of the Goia Âs Magmatic Arc.
INTRODUCTION Magnetic iron ore-hosted copper deposit occurs in layered mafic-ultramafic intrusion... more INTRODUCTION Magnetic iron ore-hosted copper deposit occurs in layered mafic-ultramafic intrusions in the low Rio São Francisco River valley, in the States of Bahia, Sergipe and Alagoas in Northeastern Brazil. They are nested in metavolcano-sedimentary sequences, which are considered as part of the South Alagoas Zone, Amorim (1995). Two layered mafic-ultramafic complexes are recognized in this part of the Sergipano Belt: the Canindé and the Serrote da Laje complexes. The age of the Serrote da Laje is uncertain, but it is roughly inferred to be older than 1,400 Ma, which was obtained via Rb-Sr dating of the younger Bela Auroratype granites (Amorim et al., 1990). This study is an exercise geared to assess the age relationships between the Serrote da Laje Complex and its metasedimentary host rocks using the Sm-Nd and PbPb systematics. The preliminary age determinations of the deformed and polimetamorphosed rocks of this fragment of the South Alagoas Zone also aim to contribute to the u...
TheNeoproterozoicBrasÂõliaBelt,incentralBrazil,includesinitseasternpartathickpileofsedimentsdepos... more TheNeoproterozoicBrasÂõliaBelt,incentralBrazil,includesinitseasternpartathickpileofsedimentsdepositedanddeformedalongthe western margin of the SaÄo Francisco±Congo Craton. Several lithostratigraphic units are identi®ed the AraÂõ, ParanoaÂ, Serra da Mesa, Araxá, Ibiá, Vazante, Canastra and Bambuí groups) and have been traditionally interpreted as part of a passive margin association (<1,2 Ga) with sediments being derived from Archaean or Paleoproterozoic continental sources to the north and east. Nd isotopic signatures egraineddetritalsedimentsoftheseveralrockunitsofthebeltwereinvestigatedinordertoassess:i)the nature and average crustal residence ages of the source areas, and ii) the tectonic signi®cance of the different sedimentary units in respect to the evolution of the BrasÂõlia Belt. TDM model ages of the ca. 1.2±0.9 Ga old Paranoa and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3 Ga, suggesting relatively uniform Paleoproterozoic continental sources within the SaÄo Francisco continent. The sediments of the detritic/ carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger TDM values 1.7±2.1 Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources. The Ibia and Araxa groups show a bimodal distribution of model age values, with a set of samples displaying TDM values between 1.8 and 2.1 Ga similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3 Ga. Younger sources such as those represented by the Neoproterozoic GoiaÂs Magmatic Arc 0.93±0.64 Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments feldspathic micaschists) within the magmatic arc, in fact, have TDM model ages mostly between 1.0 and1.2Ga,indicatingthattheoriginalsedimentsrepresentproductsoferosionofthearcitself.Therefore,partoftherockunitsthathave been mapped as Araxa and Ibia groups can be deeper water equivalents of a passive margin sequence sediments with Paleoproterozoic model ages). However, another part has been clearly deposited under the in ̄uence of an island arc source, most probably in a back-arc basin. The BambuÂõ Group sediments D0X8 Ga?Y at the top of the sequence, also indicate the presence of a young source. TDM model ages vary between ca. 1.4 and 1.9 Ga. However, the distribution pattern of TDM values is more uniform, being intermediate between the Neoproterozoic juvenile source and the Paleoproterozoic continental source area. The BambuÂõ Group is interpreted as a post-inversion sequence, with detrital sediments being mostly derived from erosion of a mountain range in the west, after accretion of the GoiaÂs Magmatic Arc.
... BRAS LIA BELT, CENTRAL BRAZIL, BASED ON SHRIMP AND LA-ICPMS U-Pb SEDIMENTARY PROVENANCE DATA:... more ... BRAS LIA BELT, CENTRAL BRAZIL, BASED ON SHRIMP AND LA-ICPMS U-Pb SEDIMENTARY PROVENANCE DATA: A REVIEW M rcio M. Pimentel* - Instituto de Geoci ncias, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil, marcio.pimentel@ufrgs.br ...
The Mara Rosa arc constitutes the northern portion of the Goiás Magmatic Arc, an important tecton... more The Mara Rosa arc constitutes the northern portion of the Goiás Magmatic Arc, an important tectonic component of the Brasília Belt that represents the result of the convergence of the Amazonian and San Francisco cratons between 900 and 600 Ma. New in situ zircon U-Pb and Lu-Hf isotope data by LA-MC-ICP-MS on representative magmatic rocks permitted to characterize three Neoproterozoic magmatic events in the Mara Rosa arc. Zircons from a metavolcanic rock yielded a U-Pb crystallization age of 916 ± 5 Ma with positive ε Hf values (+ 8 to + 12) suggesting that the host magma derived from a depleted mantle. Two orthogneisses showed heterogeneous zircon populations: the youngest zircons from the two samples gave U-Pb crystallization ages of 792 ± 8 Ma and 811 ± 7 Ma respectively; the abundant inherited zircons (with Meso-to Paleoproterozoic U-Pb ages) showed, for both samples, negative ε Hf and old T DM values, suggesting an important contribution of Paleoproterozoic to Archean crust in the petrogenesis of the host magmas. Zircons from post-orogenic qz-diorite yielded a crystallization age of 638 ± 4 Ma, with a variation in ε Hf values (+1 to + 8) suggesting that the host magma involved both a juvenile and a recycled older crustal components.
The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultrama... more The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe-and Al-rich metasedimentary rocks of the upper unit.
The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultrama... more The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe-and Al-rich metasedimentary rocks of the upper unit.
Resumo -O Complexo Máfico-Ultramáfico Mata Grande, localizado no município de São Sepé, porção NW... more Resumo -O Complexo Máfico-Ultramáfico Mata Grande, localizado no município de São Sepé, porção NW do Escudo Sul-rio-grandense, é uma intrusão acamadada com cerca de 5 km 2 que mantém contatos por meio de falhas normais com gnaisses do Complexo Cambaí ao SW, com os xistos magnesianos e serpentinitos do Complexo Arroio Lajeadinho a SE e com as rochas sedimentares da Bacia do Paraná a N. Neste trabalho foram descritas três unidades de rochas cumuláticas: Unidade Máfica (UM), Unidade Ultramáfica (UUM) e Unidade Transicional (UT). A principal estrutura primária identificada nessas unidades é um acamamento composicional/textural milimétrico a centimétrico e uma intercalação de camadas em escalas de afloramento e regional. As rochas da UM cristalizaram a partir da acumulação de cristais de plagioclásio e, em menor proporção, de olivina, além de fases minerais intercúmulus, que representam de 24 % a 41 % de líquido intersticial aprisionado nesta acumulação. Na UT, a acumulação de plagioclásio e olivina ocorreu em proporções muito próximas, com uma menor proporção do líquido aprisionado (cerca de 15 %). As amostras da UUM evidenciam uma acumulação principal de olivina com plagioclásio intercúmulus mais uma proporção do líquido intersticial (20 %). Todas as unidades do Complexo Mata Grande são afetadas pelo metamorfismo de contato causado pelo Granito São Sepé, sob condições de temperatura equivalentes às das fácies albita-epidoto hornfels e hornblenda hornfels. Feições de deformação subsolidus indicam um processo de compactação durante a fase final de cristalização e resfriamento. Palavras-chave: Cinturão Dom Feliciano, Terreno São Gabriel, Complexo Mata Grande, Magmatismo Máfico.
The Brasília belt borders the western margin of the São Francisco Craton and records the history ... more The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U-Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana.
New U-Pb and Sm-Nd isotopic data for orthogneiss and granitoid rocks from the Neoproterozoic Goiá... more New U-Pb and Sm-Nd isotopic data for orthogneiss and granitoid rocks from the Neoproterozoic Goiás magmatic arc in western Goiás constrain the geological evolution of this juvenile crust in the western Brasília belt. Orthogneiss rock samples have U-Pb crystallization ages of 804G6, 669G3, 662G12, 634G8, 630G5, and 637G20 Ma and show 3 Nd (T) values between C2.8 and K15.1. Rock units with negative 3 Nd (T) are more frequent in the eastern part of the studied area to the south of Anicuns, which indicates the presence of older continental crust in that part of the arc. Metagranitoids have ages of 821G10, 810G10, 792G5, 790G12, 782G14, 748G4, and 614G5 Ma and 3 Nd (T) values between C5.1 and K3.7. The data presented here, combined with those in the literature, suggest that igneous activity in the Goiás magmatic arc took place in two episodes: between ca. 0.89 and 0.8 Ga, probably in intraoceanic settings, and between ca. 0.66 and 0.60 Ga, likely in an active continental margin at the end of the Brasiliano orogeny.
The Natividade-Cavalcanti crustal block is a Paleoproterozoic sialic terrain which forms the base... more The Natividade-Cavalcanti crustal block is a Paleoproterozoic sialic terrain which forms the basement of sedimentary sequences belonging to the Neoproterozoic Brasília Belt in central Brazil. Two main domains were recognized in this crustal block, the limits of which are not well constrained due to the lack of geological maps at an appropriate scale. The northeastern domain is characterized by narrow supracrustal belts which wrap around gneiss domes. At least three rock-forming events are recorded in the calc-alkaline, metaluminous basement gneisses of this domain: (i) in the Almas-Dianópolis area, granitoids were dated at c. 2.2 Ga, (ii) early Paleoproterozoic magmatism was dated at c. 2.3-2.4 Ga in the Natividade-Conceiç ão do Tocantins area, and (iii) younger additions were dated at 2144 ± 21 Ma. T DM model ages range between 2.24 and 3.11 Ga, and ε Nd (t) values range from +2 to close to zero, suggesting juvenile sources. Bordering this domain west-and southwards, extending from Cavalcante to Arraias, Paranã and Pindorama do Tocantins, mostly peraluminous granitoids of the Aurumina Suite are exposed. Geochronological data for Aurumina granitoid rocks of this sector point to magmatic events between 2.13 and 2.18 Ga, whereas T DM model ages range between 2.21 and 2.92 Ga with generally negative ε Nd (t) values. An exception is a largely undeformed tonalite sample from close to Arraias, dated at 2042 ± 12 Ma.
The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultrama... more The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe-and Al-rich metasedimentary rocks of the upper unit. Sm-Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00^0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show T DM model ages of mostly 2.33-2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite-garnet schist yields an age of 505^7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixás represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny.
In the Mara Rosa area, in the northern sector of the arc, the earlier magmatic activity was dated... more In the Mara Rosa area, in the northern sector of the arc, the earlier magmatic activity was dated at ca. 860 Ma, and is represented by juvenile rocks with island arc geochemical and isotopic signature (Viana et al., 1995; Pimentel et al., 1997). Syntectonic peraluminous granites and one diorite intrusion are in the age interval between 615 and 640 Ma. Sm-Nd garnet ages and titanite U-Pb ages of 610, 600, 630, and 760 Ma are indicative of the main metamorphic events (Junges et al., 2002a,b). A younger episode of juvenile magmatism, dated at ca. 600-640 Ma, is recorded by tonalites and amphibolites at the western part of the Mara Rosa arc (Junges et al., 2002a, 2003). The youngest ages, of ca. 570 Ma, refer to postorogenic granites. In this study we report new Sm-Nd isotopic data and U-Pb zircon ages of metavolcanic and metaplutonic rocks from Santa Terezinha de Goiás, ca. 100 km to the southwest of Mara Rosa, which indicate island arc-related volcanism and sedimentation at ca. 670 Ma and calc-alkaline intrusions of ca. 610-650 Ma.
The Neoproterozoic Brasõ Âlia Belt, in central Brazil, includes in its eastern part a thick pile ... more The Neoproterozoic Brasõ Âlia Belt, in central Brazil, includes in its eastern part a thick pile of sediments deposited and deformed along the western margin of the Sa Äo Francisco±Congo Craton. Several lithostratigraphic units are identi®ed (the Araõ Â, Paranoa Â, Serra da Mesa, Araxa Â, Ibia Â, Vazante, Canastra and Bambuõ  groups) and have been traditionally interpreted as part of a passive margin association ,1:2 Ga; with sediments being derived from Archaean or Paleoproterozoic continental sources to the north and east. Nd isotopic signatures of ®ne-grained detrital sediments of the several rock units of the belt were investigated in order to assess: (i) the nature and average crustal residence ages of the source areas, and (ii) the tectonic signi®cance of the different sedimentary units in respect to the evolution of the Brasõ Âlia Belt. T DM model ages of the ca. 1.2±0.9 Ga old Paranoa  and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3 Ga, suggesting relatively uniform Paleoproterozoic continental sources within the Sa Äo Francisco continent. The sediments of the detritic/ carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger T DM values (1.7±2.1 Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources. The Ibia  and Araxa  groups show a bimodal distribution of model age values, with a set of samples displaying T DM values between 1.8 and 2.1 Ga (similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3 Ga. Younger sources such as those represented by the Neoproterozoic Goia Âs Magmatic Arc (0.93±0.64 Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments (feldspathic micaschists) within the magmatic arc, in fact, have T DM model ages mostly between 1.0 and 1.2 Ga, indicating that the original sediments represent products of erosion of the arc itself. Therefore, part of the rock units that have been mapped as Araxa  and Ibia  groups can be deeper water equivalents of a passive margin sequence (sediments with Paleoproterozoic model ages). However, another part has been clearly deposited under the in¯uence of an island arc source, most probably in a back-arc basin. The Bambuõ  Group sediments ,0:8 Ga?; at the top of the sequence, also indicate the presence of a young source. T DM model ages vary between ca. 1.4 and 1.9 Ga. However, the distribution pattern of T DM values is more uniform, being intermediate between the Neoproterozoic juvenile source and the Paleoproterozoic continental source area. The Bambuõ  Group is interpreted as a post-inversion sequence, with detrital sediments being mostly derived from erosion of a mountain range in the west, after accretion of the Goia Âs Magmatic Arc.
INTRODUCTION Magnetic iron ore-hosted copper deposit occurs in layered mafic-ultramafic intrusion... more INTRODUCTION Magnetic iron ore-hosted copper deposit occurs in layered mafic-ultramafic intrusions in the low Rio São Francisco River valley, in the States of Bahia, Sergipe and Alagoas in Northeastern Brazil. They are nested in metavolcano-sedimentary sequences, which are considered as part of the South Alagoas Zone, Amorim (1995). Two layered mafic-ultramafic complexes are recognized in this part of the Sergipano Belt: the Canindé and the Serrote da Laje complexes. The age of the Serrote da Laje is uncertain, but it is roughly inferred to be older than 1,400 Ma, which was obtained via Rb-Sr dating of the younger Bela Auroratype granites (Amorim et al., 1990). This study is an exercise geared to assess the age relationships between the Serrote da Laje Complex and its metasedimentary host rocks using the Sm-Nd and PbPb systematics. The preliminary age determinations of the deformed and polimetamorphosed rocks of this fragment of the South Alagoas Zone also aim to contribute to the u...
TheNeoproterozoicBrasÂõliaBelt,incentralBrazil,includesinitseasternpartathickpileofsedimentsdepos... more TheNeoproterozoicBrasÂõliaBelt,incentralBrazil,includesinitseasternpartathickpileofsedimentsdepositedanddeformedalongthe western margin of the SaÄo Francisco±Congo Craton. Several lithostratigraphic units are identi®ed the AraÂõ, ParanoaÂ, Serra da Mesa, Araxá, Ibiá, Vazante, Canastra and Bambuí groups) and have been traditionally interpreted as part of a passive margin association (<1,2 Ga) with sediments being derived from Archaean or Paleoproterozoic continental sources to the north and east. Nd isotopic signatures egraineddetritalsedimentsoftheseveralrockunitsofthebeltwereinvestigatedinordertoassess:i)the nature and average crustal residence ages of the source areas, and ii) the tectonic signi®cance of the different sedimentary units in respect to the evolution of the BrasÂõlia Belt. TDM model ages of the ca. 1.2±0.9 Ga old Paranoa and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3 Ga, suggesting relatively uniform Paleoproterozoic continental sources within the SaÄo Francisco continent. The sediments of the detritic/ carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger TDM values 1.7±2.1 Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources. The Ibia and Araxa groups show a bimodal distribution of model age values, with a set of samples displaying TDM values between 1.8 and 2.1 Ga similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3 Ga. Younger sources such as those represented by the Neoproterozoic GoiaÂs Magmatic Arc 0.93±0.64 Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments feldspathic micaschists) within the magmatic arc, in fact, have TDM model ages mostly between 1.0 and1.2Ga,indicatingthattheoriginalsedimentsrepresentproductsoferosionofthearcitself.Therefore,partoftherockunitsthathave been mapped as Araxa and Ibia groups can be deeper water equivalents of a passive margin sequence sediments with Paleoproterozoic model ages). However, another part has been clearly deposited under the in ̄uence of an island arc source, most probably in a back-arc basin. The BambuÂõ Group sediments D0X8 Ga?Y at the top of the sequence, also indicate the presence of a young source. TDM model ages vary between ca. 1.4 and 1.9 Ga. However, the distribution pattern of TDM values is more uniform, being intermediate between the Neoproterozoic juvenile source and the Paleoproterozoic continental source area. The BambuÂõ Group is interpreted as a post-inversion sequence, with detrital sediments being mostly derived from erosion of a mountain range in the west, after accretion of the GoiaÂs Magmatic Arc.
... BRAS LIA BELT, CENTRAL BRAZIL, BASED ON SHRIMP AND LA-ICPMS U-Pb SEDIMENTARY PROVENANCE DATA:... more ... BRAS LIA BELT, CENTRAL BRAZIL, BASED ON SHRIMP AND LA-ICPMS U-Pb SEDIMENTARY PROVENANCE DATA: A REVIEW M rcio M. Pimentel* - Instituto de Geoci ncias, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil, marcio.pimentel@ufrgs.br ...
The Mara Rosa arc constitutes the northern portion of the Goiás Magmatic Arc, an important tecton... more The Mara Rosa arc constitutes the northern portion of the Goiás Magmatic Arc, an important tectonic component of the Brasília Belt that represents the result of the convergence of the Amazonian and San Francisco cratons between 900 and 600 Ma. New in situ zircon U-Pb and Lu-Hf isotope data by LA-MC-ICP-MS on representative magmatic rocks permitted to characterize three Neoproterozoic magmatic events in the Mara Rosa arc. Zircons from a metavolcanic rock yielded a U-Pb crystallization age of 916 ± 5 Ma with positive ε Hf values (+ 8 to + 12) suggesting that the host magma derived from a depleted mantle. Two orthogneisses showed heterogeneous zircon populations: the youngest zircons from the two samples gave U-Pb crystallization ages of 792 ± 8 Ma and 811 ± 7 Ma respectively; the abundant inherited zircons (with Meso-to Paleoproterozoic U-Pb ages) showed, for both samples, negative ε Hf and old T DM values, suggesting an important contribution of Paleoproterozoic to Archean crust in the petrogenesis of the host magmas. Zircons from post-orogenic qz-diorite yielded a crystallization age of 638 ± 4 Ma, with a variation in ε Hf values (+1 to + 8) suggesting that the host magma involved both a juvenile and a recycled older crustal components.
The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultrama... more The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe-and Al-rich metasedimentary rocks of the upper unit.
The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultrama... more The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brasília belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe-and Al-rich metasedimentary rocks of the upper unit.
Resumo -O Complexo Máfico-Ultramáfico Mata Grande, localizado no município de São Sepé, porção NW... more Resumo -O Complexo Máfico-Ultramáfico Mata Grande, localizado no município de São Sepé, porção NW do Escudo Sul-rio-grandense, é uma intrusão acamadada com cerca de 5 km 2 que mantém contatos por meio de falhas normais com gnaisses do Complexo Cambaí ao SW, com os xistos magnesianos e serpentinitos do Complexo Arroio Lajeadinho a SE e com as rochas sedimentares da Bacia do Paraná a N. Neste trabalho foram descritas três unidades de rochas cumuláticas: Unidade Máfica (UM), Unidade Ultramáfica (UUM) e Unidade Transicional (UT). A principal estrutura primária identificada nessas unidades é um acamamento composicional/textural milimétrico a centimétrico e uma intercalação de camadas em escalas de afloramento e regional. As rochas da UM cristalizaram a partir da acumulação de cristais de plagioclásio e, em menor proporção, de olivina, além de fases minerais intercúmulus, que representam de 24 % a 41 % de líquido intersticial aprisionado nesta acumulação. Na UT, a acumulação de plagioclásio e olivina ocorreu em proporções muito próximas, com uma menor proporção do líquido aprisionado (cerca de 15 %). As amostras da UUM evidenciam uma acumulação principal de olivina com plagioclásio intercúmulus mais uma proporção do líquido intersticial (20 %). Todas as unidades do Complexo Mata Grande são afetadas pelo metamorfismo de contato causado pelo Granito São Sepé, sob condições de temperatura equivalentes às das fácies albita-epidoto hornfels e hornblenda hornfels. Feições de deformação subsolidus indicam um processo de compactação durante a fase final de cristalização e resfriamento. Palavras-chave: Cinturão Dom Feliciano, Terreno São Gabriel, Complexo Mata Grande, Magmatismo Máfico.
The Brasília belt borders the western margin of the São Francisco Craton and records the history ... more The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U-Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana.
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Papers by S. Junges
Nd isotopic signatures egraineddetritalsedimentsoftheseveralrockunitsofthebeltwereinvestigatedinordertoassess:i)the nature and average crustal residence ages of the source areas, and ii) the tectonic signi®cance of the different sedimentary units in respect to the evolution of the BrasÂõlia Belt.
TDM model ages of the ca. 1.2±0.9 Ga old Paranoa and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3 Ga, suggesting relatively uniform Paleoproterozoic continental sources within the SaÄo Francisco continent. The sediments of the detritic/ carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger TDM values 1.7±2.1 Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources.
The Ibia and Araxa groups show a bimodal distribution of model age values, with a set of samples displaying TDM values between 1.8 and 2.1 Ga similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3 Ga. Younger sources such as those represented by the Neoproterozoic GoiaÂs Magmatic Arc 0.93±0.64 Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments feldspathic micaschists) within the magmatic arc, in fact, have TDM model ages mostly between 1.0 and1.2Ga,indicatingthattheoriginalsedimentsrepresentproductsoferosionofthearcitself.Therefore,partoftherockunitsthathave been mapped as Araxa and Ibia groups can be deeper water equivalents of a passive margin sequence sediments with Paleoproterozoic model ages). However, another part has been clearly deposited under the in ̄uence of an island arc source, most probably in a back-arc basin.
The BambuÂõ Group sediments D0X8 Ga?Y at the top of the sequence, also indicate the presence of a young source. TDM model ages vary between ca. 1.4 and 1.9 Ga. However, the distribution pattern of TDM values is more uniform, being intermediate between the Neoproterozoic juvenile source and the Paleoproterozoic continental source area. The BambuÂõ Group is interpreted as a post-inversion sequence, with detrital sediments being mostly derived from erosion of a mountain range in the west, after accretion of the GoiaÂs Magmatic Arc.
Nd isotopic signatures egraineddetritalsedimentsoftheseveralrockunitsofthebeltwereinvestigatedinordertoassess:i)the nature and average crustal residence ages of the source areas, and ii) the tectonic signi®cance of the different sedimentary units in respect to the evolution of the BrasÂõlia Belt.
TDM model ages of the ca. 1.2±0.9 Ga old Paranoa and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3 Ga, suggesting relatively uniform Paleoproterozoic continental sources within the SaÄo Francisco continent. The sediments of the detritic/ carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger TDM values 1.7±2.1 Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources.
The Ibia and Araxa groups show a bimodal distribution of model age values, with a set of samples displaying TDM values between 1.8 and 2.1 Ga similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3 Ga. Younger sources such as those represented by the Neoproterozoic GoiaÂs Magmatic Arc 0.93±0.64 Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments feldspathic micaschists) within the magmatic arc, in fact, have TDM model ages mostly between 1.0 and1.2Ga,indicatingthattheoriginalsedimentsrepresentproductsoferosionofthearcitself.Therefore,partoftherockunitsthathave been mapped as Araxa and Ibia groups can be deeper water equivalents of a passive margin sequence sediments with Paleoproterozoic model ages). However, another part has been clearly deposited under the in ̄uence of an island arc source, most probably in a back-arc basin.
The BambuÂõ Group sediments D0X8 Ga?Y at the top of the sequence, also indicate the presence of a young source. TDM model ages vary between ca. 1.4 and 1.9 Ga. However, the distribution pattern of TDM values is more uniform, being intermediate between the Neoproterozoic juvenile source and the Paleoproterozoic continental source area. The BambuÂõ Group is interpreted as a post-inversion sequence, with detrital sediments being mostly derived from erosion of a mountain range in the west, after accretion of the GoiaÂs Magmatic Arc.