2022 Vol. 13, No. 5
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2022, 13(5): 101131.
doi: 10.1016/j.gsf.2020.11.018
Abstract:
Iron formations are valuable archives of sedimentary conditions and post-depositional events. However, geochemical proxies commonly used to determine genetic characteristics can be variably modified during metamorphism and deformation, hampering their use as records of regional geological events. This work focuses on strongly reworked magnetite-quartz-rich rocks from the São José do Campestre Massif, one of the oldest fragments of preserved crust in South America. The genetic classification of these magnetite-quartz-rich rocks is not straightforward because primary assemblages and textures were variably modified by granulite facies metamorphism during a regional Paleoproterozoic migmatization event. To address genetic ambiguities, we analyzed their magnetite and pyroxene chemistry, whole-rock geochemistry, and Sm-Nd isotopes. Magnetite chemistry indicates that pyroxene-poor iron formations (Type B) are low in trace elements such as Ti, Al, V, and Mn, suggesting a chemical similarity to iron formations elsewhere. In contrast, magnetites from pyroxene-enriched Type A iron formations are rich in trace elements and more akin to magnetite crystallized from higher temperature systems, such as skarn and IOCG. The 147Sm/144Nd of these rocks show substantial variation even at the outcrop scale, indicating a locally-controlled, highly heterogeneous mixture of Archean, Paleoproterozoic, and Neoproterozoic sources. Therefore, our geochemical tools point out to heterogenous signatures of these magnetite-quartz rocks and proxies compatible with both low and high-temperature conditions and age of deposition spanning sources from the Archean to the Neoproterozoic. We interpret that the studied São José do Campestre magnetite-quartz rocks represent Archean iron formations with original magnetite chemistry and isotopic signatures variably modified by metamorphism and by at least one deformation-related hydrothermal event. These results contrast with similar examples from China and Greenland where iron formations either preserved the magnetite chemistry or the primary isotopic signatures. Our study indicates that metamorphism can selectively affect chemical proxies used to study iron formations and undermine the genetic classification of iron ores. Thus, these proxies should be carefully applied in the interpretation of syn-depositional environments of polydeformed belts.
Iron formations are valuable archives of sedimentary conditions and post-depositional events. However, geochemical proxies commonly used to determine genetic characteristics can be variably modified during metamorphism and deformation, hampering their use as records of regional geological events. This work focuses on strongly reworked magnetite-quartz-rich rocks from the São José do Campestre Massif, one of the oldest fragments of preserved crust in South America. The genetic classification of these magnetite-quartz-rich rocks is not straightforward because primary assemblages and textures were variably modified by granulite facies metamorphism during a regional Paleoproterozoic migmatization event. To address genetic ambiguities, we analyzed their magnetite and pyroxene chemistry, whole-rock geochemistry, and Sm-Nd isotopes. Magnetite chemistry indicates that pyroxene-poor iron formations (Type B) are low in trace elements such as Ti, Al, V, and Mn, suggesting a chemical similarity to iron formations elsewhere. In contrast, magnetites from pyroxene-enriched Type A iron formations are rich in trace elements and more akin to magnetite crystallized from higher temperature systems, such as skarn and IOCG. The 147Sm/144Nd of these rocks show substantial variation even at the outcrop scale, indicating a locally-controlled, highly heterogeneous mixture of Archean, Paleoproterozoic, and Neoproterozoic sources. Therefore, our geochemical tools point out to heterogenous signatures of these magnetite-quartz rocks and proxies compatible with both low and high-temperature conditions and age of deposition spanning sources from the Archean to the Neoproterozoic. We interpret that the studied São José do Campestre magnetite-quartz rocks represent Archean iron formations with original magnetite chemistry and isotopic signatures variably modified by metamorphism and by at least one deformation-related hydrothermal event. These results contrast with similar examples from China and Greenland where iron formations either preserved the magnetite chemistry or the primary isotopic signatures. Our study indicates that metamorphism can selectively affect chemical proxies used to study iron formations and undermine the genetic classification of iron ores. Thus, these proxies should be carefully applied in the interpretation of syn-depositional environments of polydeformed belts.
2022, 13(5): 101167.
doi: 10.1016/j.gsf.2021.101167
Abstract:
The Alto Moxotó Terrane of the Borborema Province presents a wide exposure of Paleoproterozoic crust, but unlike other continental blocks of South America, its orogenic history is strongly obliterated by late Neoproterozoic deformation. New isotopic and geochemical studies were conducted in mafic-ultramafic (Fazenda Carmo Suite) and granitic-gneissic rocks (Riacho do Navio Suite) within the terrane. The former present zircon U-Pb crystallization ages at ca. 2.13 Ga, whereas Sm-Nd data suggests a juvenile origin via melting of early Paleoproterozoic to Archean peridotitic sources. Geochemical data for these rocks are compatible with tholeiitic magmas with some degree of crustal contamination and trace element distribution points to a continental-arc related setting interpreted as remnants of the early stages of subduction. In contrast, the Riacho do Navio Suite was emplaced at ca. 2.08 Ga and has highly negative εNd(t) values indicating crustal reworking. The suite displays calc-alkali to alkali-calcic and ferroan geochemical signatures compatible with Cordilleran magmas. In addition, trace-element distribution as well as discriminant diagrams suggest that the precursor magmas were generated during the later stages of a continental arc or in a syn-collisional setting. Based on our results, we suggest that the studied units might represent missing pieces of a Paleoproterozoic accretionary orogen that formed the crustal framework of the Alto Moxotó Terrane, and that this represents a block associated with assembly of the Nuna/Columbia supercontinent, which is now largely hidden within the Neoproterozoic orogenic belts of West Gondwana.
The Alto Moxotó Terrane of the Borborema Province presents a wide exposure of Paleoproterozoic crust, but unlike other continental blocks of South America, its orogenic history is strongly obliterated by late Neoproterozoic deformation. New isotopic and geochemical studies were conducted in mafic-ultramafic (Fazenda Carmo Suite) and granitic-gneissic rocks (Riacho do Navio Suite) within the terrane. The former present zircon U-Pb crystallization ages at ca. 2.13 Ga, whereas Sm-Nd data suggests a juvenile origin via melting of early Paleoproterozoic to Archean peridotitic sources. Geochemical data for these rocks are compatible with tholeiitic magmas with some degree of crustal contamination and trace element distribution points to a continental-arc related setting interpreted as remnants of the early stages of subduction. In contrast, the Riacho do Navio Suite was emplaced at ca. 2.08 Ga and has highly negative εNd(t) values indicating crustal reworking. The suite displays calc-alkali to alkali-calcic and ferroan geochemical signatures compatible with Cordilleran magmas. In addition, trace-element distribution as well as discriminant diagrams suggest that the precursor magmas were generated during the later stages of a continental arc or in a syn-collisional setting. Based on our results, we suggest that the studied units might represent missing pieces of a Paleoproterozoic accretionary orogen that formed the crustal framework of the Alto Moxotó Terrane, and that this represents a block associated with assembly of the Nuna/Columbia supercontinent, which is now largely hidden within the Neoproterozoic orogenic belts of West Gondwana.
2022, 13(5): 101179.
doi: 10.1016/j.gsf.2021.101179
Abstract:
The late- to post-collisional stage in orogenic systems is characterized by the coeval existence of bimodal potassic to ultrapotassic magmatic activity related to partial melting of an enriched lithospheric mantle together with crustal derived melts. In this paper, we present new whole rock geochemical analyses combined with zircon and titanite U-Pb and zircon Hf isotopic data from potassic to ultrapotassic rocks from six plutons that occur within the Archean Itacambira-Monte Azul block (BIMA), to discuss their petrogenesis and the tectonic implications for the São Francisco paleocontinent. The new U-Pb ages range from ca. 2.06 Ga to 1.98 Ga and reveal long-lasting potassic magmatism within the BIMA, which is within the late- to- post-collisional stage of the São Francisco paleocontinent evolution. The ultrapotassic rocks are compatible with a fluid-related metasomatized mantle source enriched by previous subduction events, whereas the potassic rocks are bimodal and have a transitional shoshonitic to A-type affinity. These rocks have a hybrid nature, possible related to the mixing between the mafic potassic/ultrapotassic rocks and high temperature crustal melts of the Archean continental crust. Our results also show an increase of within-plate signature towards the younger potassic magmas. The participation of an important Archean crustal component in the genesis of these rocks is highlighted by the common and occasionally abundant occurrence of Archean inherited zircons. The Hf isotopic record shows that most of the zircon inheritance has dominantly subchondritic εHf(t) values, which fits a crustal reworking derivation from a similar Eo- to Paleoarchean precursor crust. However, the presence of juvenile 2.36 Ga zircon inheritance in an ultrapotassic sample reveal the existence of a hidden reservoir that is somewhat similar to the described for the Mineiro Belt in southern São Francisco paleocontinent.
The late- to post-collisional stage in orogenic systems is characterized by the coeval existence of bimodal potassic to ultrapotassic magmatic activity related to partial melting of an enriched lithospheric mantle together with crustal derived melts. In this paper, we present new whole rock geochemical analyses combined with zircon and titanite U-Pb and zircon Hf isotopic data from potassic to ultrapotassic rocks from six plutons that occur within the Archean Itacambira-Monte Azul block (BIMA), to discuss their petrogenesis and the tectonic implications for the São Francisco paleocontinent. The new U-Pb ages range from ca. 2.06 Ga to 1.98 Ga and reveal long-lasting potassic magmatism within the BIMA, which is within the late- to- post-collisional stage of the São Francisco paleocontinent evolution. The ultrapotassic rocks are compatible with a fluid-related metasomatized mantle source enriched by previous subduction events, whereas the potassic rocks are bimodal and have a transitional shoshonitic to A-type affinity. These rocks have a hybrid nature, possible related to the mixing between the mafic potassic/ultrapotassic rocks and high temperature crustal melts of the Archean continental crust. Our results also show an increase of within-plate signature towards the younger potassic magmas. The participation of an important Archean crustal component in the genesis of these rocks is highlighted by the common and occasionally abundant occurrence of Archean inherited zircons. The Hf isotopic record shows that most of the zircon inheritance has dominantly subchondritic εHf(t) values, which fits a crustal reworking derivation from a similar Eo- to Paleoarchean precursor crust. However, the presence of juvenile 2.36 Ga zircon inheritance in an ultrapotassic sample reveal the existence of a hidden reservoir that is somewhat similar to the described for the Mineiro Belt in southern São Francisco paleocontinent.
2022, 13(5): 101199.
doi: 10.1016/j.gsf.2021.101199
Abstract:
The Gurupi Belt (together with the São Luís cratonic fragment), in north-northeastern Brazil, has been described in previous studies that used extensive field geology, structural analysis, airborne geophysics, zircon U-Pb dating, and whole-rock Sm-Nd isotope and geochemical data as a polyphase orogenic belt, with the Rhyacian being the main period of crust formation. This was related to a 2240 Ma to 2140 Ma accretionary processes that produced juvenile crust, which has subsequently been reworked during a collisional event at 2100 ±20 Ma, with little evidence of Archean crust. In this study, we use Lu-Hf isotopic data in zircon from granitoids (including gneiss) of variable magmatic series, and amphibolite to improve the knowledge of this scenario, and investigate additional evidence of recycling of Archean basement. Pre-collisional high Ba-Sr and ferroan granitoids and amphibolite formed in island arc (2180-2145 Ma), show only zircons with suprachondritic εHf values (ca. +1 to +8) indicating the large predominance of juvenile magmas. Only 10% of the data show slightly negative εHf values (0 to -4), which have been observed in granodiorite-gneiss formed in continental arc (2170-2140 Ma), and in strongly peraluminous collisional granites (2125-2070 Ma), indicating the rework of older Paleoproterozoic to Archean components (HfTDM=2.11-3.69 Ga). A two-component mixing model using both Hf and published Nd isotope data are in line with this interpretation and indicate more than 90% of juvenile material, and less influence of Archean materials. Comparing with other Rhyacian terranes that are interpreted to have been close to Gurupi in a pre-Columbia configuration (ca. 2.0 Ga), our results differ from those of SE-Guiana Shield, which show strong influence of Archean protoliths, and are very similar to those of the central-eastern portion of the Baoulé-Mossi Domain of the West African Craton, which has also been formed largely by juvenile magmas in an accretionary-collisional orogen.
The Gurupi Belt (together with the São Luís cratonic fragment), in north-northeastern Brazil, has been described in previous studies that used extensive field geology, structural analysis, airborne geophysics, zircon U-Pb dating, and whole-rock Sm-Nd isotope and geochemical data as a polyphase orogenic belt, with the Rhyacian being the main period of crust formation. This was related to a 2240 Ma to 2140 Ma accretionary processes that produced juvenile crust, which has subsequently been reworked during a collisional event at 2100 ±20 Ma, with little evidence of Archean crust. In this study, we use Lu-Hf isotopic data in zircon from granitoids (including gneiss) of variable magmatic series, and amphibolite to improve the knowledge of this scenario, and investigate additional evidence of recycling of Archean basement. Pre-collisional high Ba-Sr and ferroan granitoids and amphibolite formed in island arc (2180-2145 Ma), show only zircons with suprachondritic εHf values (ca. +1 to +8) indicating the large predominance of juvenile magmas. Only 10% of the data show slightly negative εHf values (0 to -4), which have been observed in granodiorite-gneiss formed in continental arc (2170-2140 Ma), and in strongly peraluminous collisional granites (2125-2070 Ma), indicating the rework of older Paleoproterozoic to Archean components (HfTDM=2.11-3.69 Ga). A two-component mixing model using both Hf and published Nd isotope data are in line with this interpretation and indicate more than 90% of juvenile material, and less influence of Archean materials. Comparing with other Rhyacian terranes that are interpreted to have been close to Gurupi in a pre-Columbia configuration (ca. 2.0 Ga), our results differ from those of SE-Guiana Shield, which show strong influence of Archean protoliths, and are very similar to those of the central-eastern portion of the Baoulé-Mossi Domain of the West African Craton, which has also been formed largely by juvenile magmas in an accretionary-collisional orogen.
2022, 13(5): 101201.
doi: 10.1016/j.gsf.2021.101201
Abstract:
The Campos Gerais Domain (CGD) in southeastern Brazil is an approximately 180 km×35 km area of Archean-Proterozoic rocks located southwest of the São Francisco Craton (SFC). The Archean-Paleoproterozoic evolution of the CGD-alongside its potential correlation with the SFC or other cratonic blocks in the region-is currently poorly-constrained. We present the results of systematic petrography, bulk-rock geochemistry, mineral chemistry and geochronology for a suite of scarcely studied mafic-ultramafic rocks from the CGD. We also provide a compilation of previously reported bulk-rock geochemical and spinel group mineral chemical data for mafic-ultramafic rocks throughout the CGD, and geochronological information for various lithotypes in the region. The CGD records a protracted Mesoarchean to Statherian (3.1-1.7 Ga) crustal evolution, which we interpret to share a common history with the southern SFC and their related reworked segments, suggesting that it is a westward extension of this cratonic terrain. The metavolcano-sedimentary rocks of the Fortaleza de Minas and Alpinópolis segments represent a Mesoarchean greenstone belt that is stratigraphically and chemically comparable to Archean greenstone belts worldwide, and that is broadly coeval with a local suite of tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites. U-Pb SHRIMP zircon data from a subalkaline metagabbro yielded a concordia age of ca. 2.96 Ga, revealing a previously unrecognized phase of Archean magmatism in the CGD that can be chrono-correlated with metakomatiite and TTG generation elsewhere in the São Francisco paleocontinent. Our data contradict a hypothesis whereby the metavolcano-sedimentary rocks of the Jacuí-Bom Jesus da Penha and Petúnia segments represent an ophiolite, as previously suggested, instead presenting features that point to formation in association with a continental arc. Coupled with a U-Pb (SHRIMP) crystallization age of ca. 2.13 Ga recorded by zircon grains from a metaultramafic rock, these data highlight that a magmatic event was chrono-correlated with the main accretionary phase of the Minas Orogeny, and with the Pouso Alegre/Amparo and São Vicente complexes. Finally, a U-Pb (SHRIMP) concordia age of ca. 590 Ma-obtained from metamorphic-textured zircon grains from a metaultramafic rock-points to a late metamorphic overprint related to upper amphibolite conditions, brittle fault activation and the juxtaposition of crustal blocks in association with the latest stages of western Gondwana's assembly in the southern SFC, with later retrogression to greenschist-facies.
The Campos Gerais Domain (CGD) in southeastern Brazil is an approximately 180 km×35 km area of Archean-Proterozoic rocks located southwest of the São Francisco Craton (SFC). The Archean-Paleoproterozoic evolution of the CGD-alongside its potential correlation with the SFC or other cratonic blocks in the region-is currently poorly-constrained. We present the results of systematic petrography, bulk-rock geochemistry, mineral chemistry and geochronology for a suite of scarcely studied mafic-ultramafic rocks from the CGD. We also provide a compilation of previously reported bulk-rock geochemical and spinel group mineral chemical data for mafic-ultramafic rocks throughout the CGD, and geochronological information for various lithotypes in the region. The CGD records a protracted Mesoarchean to Statherian (3.1-1.7 Ga) crustal evolution, which we interpret to share a common history with the southern SFC and their related reworked segments, suggesting that it is a westward extension of this cratonic terrain. The metavolcano-sedimentary rocks of the Fortaleza de Minas and Alpinópolis segments represent a Mesoarchean greenstone belt that is stratigraphically and chemically comparable to Archean greenstone belts worldwide, and that is broadly coeval with a local suite of tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites. U-Pb SHRIMP zircon data from a subalkaline metagabbro yielded a concordia age of ca. 2.96 Ga, revealing a previously unrecognized phase of Archean magmatism in the CGD that can be chrono-correlated with metakomatiite and TTG generation elsewhere in the São Francisco paleocontinent. Our data contradict a hypothesis whereby the metavolcano-sedimentary rocks of the Jacuí-Bom Jesus da Penha and Petúnia segments represent an ophiolite, as previously suggested, instead presenting features that point to formation in association with a continental arc. Coupled with a U-Pb (SHRIMP) crystallization age of ca. 2.13 Ga recorded by zircon grains from a metaultramafic rock, these data highlight that a magmatic event was chrono-correlated with the main accretionary phase of the Minas Orogeny, and with the Pouso Alegre/Amparo and São Vicente complexes. Finally, a U-Pb (SHRIMP) concordia age of ca. 590 Ma-obtained from metamorphic-textured zircon grains from a metaultramafic rock-points to a late metamorphic overprint related to upper amphibolite conditions, brittle fault activation and the juxtaposition of crustal blocks in association with the latest stages of western Gondwana's assembly in the southern SFC, with later retrogression to greenschist-facies.
2022, 13(5): 101202.
doi: 10.1016/j.gsf.2021.101202
Abstract:
Despite representing one of the largest cratons on Earth, the early geological evolution of the Amazonia Craton remains poorly known due to relatively poor exposure and because younger metamorphic and tectonic events have obscured initial information. In this study, we investigated the sedimentary archives of the Carajás Basin to unravel the early geological evolution of the southeastern Amazonia Craton. The Carajás Basin contains sedimentary rocks that were deposited throughout a long period spanning more than one billion years from the Mesoarchean to the Paleoproterozoic. The oldest archives preserved in this basin consist of a few ca. 3.6 Ga detrital zircon grains showing that the geological roots of the Amazonia Craton were already formed by the Eoarchean. During the Paleoarchean or the early Mesoarchean (<3.1 Ga), the Carajás Basin was large and rigid enough to sustain the formation and preservation of the Rio Novo Group greenstone belt. Later, during the Neoarchean, at ca. 2.7 Ga, the southeastern Amazonia Craton witnessed the emplacement of the Parauapebas Large Igneous Province (LIP) that probably covered a large part of the craton and was associated with the deposition of some of the world largest iron formations. The emplacement of this LIP immediately preceded a period of continental extension that formed a rift infilled first by iron formations followed by terrigenous sediments. This major change of sedimentary regime might have been controlled by the regional tectonic evolution of the Amazonia Craton and its emergence above sea-level. During the Paleoproterozoic, at ca. 2.1 Ga, the Rio Fresco Group, consisting of terrigenous sediments from the interior of the Amazonia Craton, was deposited in the Carajás Basin. At that time, the Amazonian lithosphere could have either underwent thermal subsidence forming a large intracratonic basin or could have been deformed by long wavelength flexures that induced the formation of basins and swells throughout the craton under the influence of the growing Transamazonian mountain belt.
Despite representing one of the largest cratons on Earth, the early geological evolution of the Amazonia Craton remains poorly known due to relatively poor exposure and because younger metamorphic and tectonic events have obscured initial information. In this study, we investigated the sedimentary archives of the Carajás Basin to unravel the early geological evolution of the southeastern Amazonia Craton. The Carajás Basin contains sedimentary rocks that were deposited throughout a long period spanning more than one billion years from the Mesoarchean to the Paleoproterozoic. The oldest archives preserved in this basin consist of a few ca. 3.6 Ga detrital zircon grains showing that the geological roots of the Amazonia Craton were already formed by the Eoarchean. During the Paleoarchean or the early Mesoarchean (<3.1 Ga), the Carajás Basin was large and rigid enough to sustain the formation and preservation of the Rio Novo Group greenstone belt. Later, during the Neoarchean, at ca. 2.7 Ga, the southeastern Amazonia Craton witnessed the emplacement of the Parauapebas Large Igneous Province (LIP) that probably covered a large part of the craton and was associated with the deposition of some of the world largest iron formations. The emplacement of this LIP immediately preceded a period of continental extension that formed a rift infilled first by iron formations followed by terrigenous sediments. This major change of sedimentary regime might have been controlled by the regional tectonic evolution of the Amazonia Craton and its emergence above sea-level. During the Paleoproterozoic, at ca. 2.1 Ga, the Rio Fresco Group, consisting of terrigenous sediments from the interior of the Amazonia Craton, was deposited in the Carajás Basin. At that time, the Amazonian lithosphere could have either underwent thermal subsidence forming a large intracratonic basin or could have been deformed by long wavelength flexures that induced the formation of basins and swells throughout the craton under the influence of the growing Transamazonian mountain belt.
2022, 13(5): 101250.
doi: 10.1016/j.gsf.2021.101250
Abstract:
Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ33S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE; 2.45-2.2 Ga) are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ33S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763-2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ13Corg (<-30‰) and dampened Δ33S (0.4‰ to -0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ13Corg (-33.4‰ to -19.2‰) and expanded Δ33S (2.3‰ ±0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ13Corg (-30‰ to -25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ33S (-0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.
Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ33S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE; 2.45-2.2 Ga) are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ33S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763-2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ13Corg (<-30‰) and dampened Δ33S (0.4‰ to -0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ13Corg (-33.4‰ to -19.2‰) and expanded Δ33S (2.3‰ ±0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ13Corg (-30‰ to -25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ33S (-0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.
2022, 13(5): 101252.
doi: 10.1016/j.gsf.2021.101252
Abstract:
Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U-Pb-Hf-O isotopes of zircon and multiple sulfur isotopes (32S, 33S, 34S, and 36S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ±25 Ma old and has zircon with εHf(t)= -2.47 to -1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ18O=+3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ±22 Ma. These detrital zircons have εHf(t)=-5.40 to -0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ18O=+4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ33S between -1.3‰ to +1.4‰) and a Δ36S/Δ33S slope of -0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ34S range (0 to +2‰), Δ33S~0‰, and Δ36S~0‰. A carbonaceous schist shows positive δ34S (2.1‰-3.5‰) and elevated Δ33S (1.2‰-1.4‰) values, with corresponding negative Δ36S between -1.2‰ to -0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ34S, Δ33S, and Δ36S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.
Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U-Pb-Hf-O isotopes of zircon and multiple sulfur isotopes (32S, 33S, 34S, and 36S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ±25 Ma old and has zircon with εHf(t)= -2.47 to -1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ18O=+3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ±22 Ma. These detrital zircons have εHf(t)=-5.40 to -0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ18O=+4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ33S between -1.3‰ to +1.4‰) and a Δ36S/Δ33S slope of -0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ34S range (0 to +2‰), Δ33S~0‰, and Δ36S~0‰. A carbonaceous schist shows positive δ34S (2.1‰-3.5‰) and elevated Δ33S (1.2‰-1.4‰) values, with corresponding negative Δ36S between -1.2‰ to -0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ34S, Δ33S, and Δ36S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.
2022, 13(5): 101289.
doi: 10.1016/j.gsf.2021.101289
Abstract:
Field observations and CA-LA-ICP-MS U-Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic-ultramafic intrusions to the south of the BHC.
Field observations and CA-LA-ICP-MS U-Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic-ultramafic intrusions to the south of the BHC.
2022, 13(5): 101292.
doi: 10.1016/j.gsf.2021.101292
Abstract:
The southern São Francisco Paleocontinent (SFP) comprises Archean nuclei and Paleoproterozoic complexes encompassing magmatic arcs juxtaposed during a Rhyacian to Orosirian orogenic event. The Juiz de Fora Complex (JFC) represents an imbricated thrust system that comprises orthogranulites with a wide compositional range formed in an intra-oceanic setting during the Siderian to the Orosirian and later accreted to the southeastern margin of the SFP. Here we report new petrological, geochemical, whole-rock Nd and Sr data, as well as zircon U-Pb ages from felsic and mafic orthogranulites from the JFC. The new data is combined with a regional compilation that enables an evaluation of the interaction between magmatism and orogenetic episodes in the context of the consolidation of São Francisco Paleocontinent during the Rhyacian-Orosirian. Pre collisional Island Arc tholeiites (IAT), Tonalites-Trondhjemites-Granodiorites (TTGs) and sanukitoid magmatism occurred from 2200 Ma to 2085 Ma. This was followed by post-collisional magmatism, which is represented by hybrid granitoids coeval with the emplacement of E-MORB basic rocks. Crustal signatures for the Rhyacian to Orosirian evolution are highlighted by the dominance of negative εNd(t) associated with Meso- to Neoarchean Nd TDM model ages as well as inherited zircon grains from the hybrid granitoids. The JFC is extensively highlighted in the literature as a primitive intra-oceanic arc, but here we propose the reworking or recycling of ancient crustal segments within the mature arc stage of the JFC, suggesting a Mesoarchean crustal source involved in the JFC evolution.
The southern São Francisco Paleocontinent (SFP) comprises Archean nuclei and Paleoproterozoic complexes encompassing magmatic arcs juxtaposed during a Rhyacian to Orosirian orogenic event. The Juiz de Fora Complex (JFC) represents an imbricated thrust system that comprises orthogranulites with a wide compositional range formed in an intra-oceanic setting during the Siderian to the Orosirian and later accreted to the southeastern margin of the SFP. Here we report new petrological, geochemical, whole-rock Nd and Sr data, as well as zircon U-Pb ages from felsic and mafic orthogranulites from the JFC. The new data is combined with a regional compilation that enables an evaluation of the interaction between magmatism and orogenetic episodes in the context of the consolidation of São Francisco Paleocontinent during the Rhyacian-Orosirian. Pre collisional Island Arc tholeiites (IAT), Tonalites-Trondhjemites-Granodiorites (TTGs) and sanukitoid magmatism occurred from 2200 Ma to 2085 Ma. This was followed by post-collisional magmatism, which is represented by hybrid granitoids coeval with the emplacement of E-MORB basic rocks. Crustal signatures for the Rhyacian to Orosirian evolution are highlighted by the dominance of negative εNd(t) associated with Meso- to Neoarchean Nd TDM model ages as well as inherited zircon grains from the hybrid granitoids. The JFC is extensively highlighted in the literature as a primitive intra-oceanic arc, but here we propose the reworking or recycling of ancient crustal segments within the mature arc stage of the JFC, suggesting a Mesoarchean crustal source involved in the JFC evolution.
2022, 13(5): 101293.
doi: 10.1016/j.gsf.2021.101293
Abstract:
We document new U-Pb detrital zircon LA-MC-ICP-MS data for seven metavolcanic-sedimentary successions and metasedimentary sequences and reassess additional dates of five siliciclastic samples toward their tectonic significance in the context of the Mineiro belt, Southern São Francisco Craton. This belt represents a crustal segment of the 2.47-2.00 Ga Minas Orogen, classically known by its Siderian and Rhyacian juvenile rocks with important implications in the Earth's geodynamics. The new and compiled detrital provenance constraints unravel the long-lived magmatic and sedimentary history of the studied basins, lasting ca. 230-220 Myr. The maximum depositional dates around 2.1 Ga reflect the renewed sediment budget with the subsequent metamorphic episode ca. 2.0 Ga. Most of the unmixed relative probability diagrams are consistent with sourcing from the Siderian and Rhyacian arcs of the Mineiro belt, determining a detrital provenance change in time and space for the precursor basins. Alternative potential sources could be the youngest rocks of the Mantiqueira and Juiz de Fora terranes that constitute the other segments of the Minas Orogen, given the age match. The overall detrital fingerprints determine the study basins resumed mainly in Rhyacian fore-arc and/or back-arc settings, i.e., akin to a subduction-related system that evolved to a collisional (foreland) environment. Few samples show fingerprints of primary extensional settings, determined by major Archean detrital populations sourced from areas outside the Mineiro belt beside the Paleoproterozoic detritus. The working model considers the collage between the Mineiro belt and the ancient foreland around 2.10 Ga and eventual interaction with other crustal segments of the Minas Orogen, generating the ca. 2.0 Ga metamorphism over the metasedimentary samples. The more complete isotopic repository in detrital and igneous zircon grains for the studied supracrustal successions and the associated rocks allows new insights into the Rhyacian-Orosirian dynamics of the Minas orogeny. In a broader perspective, the juvenile nature of the Mineiro belt reinforces the paradigm of uninterrupted continental growth during the Paleoproterozoic Earth.
We document new U-Pb detrital zircon LA-MC-ICP-MS data for seven metavolcanic-sedimentary successions and metasedimentary sequences and reassess additional dates of five siliciclastic samples toward their tectonic significance in the context of the Mineiro belt, Southern São Francisco Craton. This belt represents a crustal segment of the 2.47-2.00 Ga Minas Orogen, classically known by its Siderian and Rhyacian juvenile rocks with important implications in the Earth's geodynamics. The new and compiled detrital provenance constraints unravel the long-lived magmatic and sedimentary history of the studied basins, lasting ca. 230-220 Myr. The maximum depositional dates around 2.1 Ga reflect the renewed sediment budget with the subsequent metamorphic episode ca. 2.0 Ga. Most of the unmixed relative probability diagrams are consistent with sourcing from the Siderian and Rhyacian arcs of the Mineiro belt, determining a detrital provenance change in time and space for the precursor basins. Alternative potential sources could be the youngest rocks of the Mantiqueira and Juiz de Fora terranes that constitute the other segments of the Minas Orogen, given the age match. The overall detrital fingerprints determine the study basins resumed mainly in Rhyacian fore-arc and/or back-arc settings, i.e., akin to a subduction-related system that evolved to a collisional (foreland) environment. Few samples show fingerprints of primary extensional settings, determined by major Archean detrital populations sourced from areas outside the Mineiro belt beside the Paleoproterozoic detritus. The working model considers the collage between the Mineiro belt and the ancient foreland around 2.10 Ga and eventual interaction with other crustal segments of the Minas Orogen, generating the ca. 2.0 Ga metamorphism over the metasedimentary samples. The more complete isotopic repository in detrital and igneous zircon grains for the studied supracrustal successions and the associated rocks allows new insights into the Rhyacian-Orosirian dynamics of the Minas orogeny. In a broader perspective, the juvenile nature of the Mineiro belt reinforces the paradigm of uninterrupted continental growth during the Paleoproterozoic Earth.
2022, 13(5): 101366.
doi: 10.1016/j.gsf.2022.101366
Abstract:
The composition and formation of the Earth's primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America's primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex:Eoarchean (ca. 3.65-3.60 Ga), early Paleoarchean (ca. 3.55-3.52 Ga), middle-late Paleoarchean (ca. 3.49-3.33 Ga) and Neoarchean (ca. 2.74-2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle-late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.
The composition and formation of the Earth's primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America's primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex:Eoarchean (ca. 3.65-3.60 Ga), early Paleoarchean (ca. 3.55-3.52 Ga), middle-late Paleoarchean (ca. 3.49-3.33 Ga) and Neoarchean (ca. 2.74-2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle-late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.
2022, 13(5): 101372.
doi: 10.1016/j.gsf.2022.101372
Abstract:
The generation of the continental crust is widely accepted to have taken place predominantly in the Archean, when TTG magmatism associated with greenstone-belt supracrustal succession development was typically followed by emplacement of high-K granites before crustal stabilization. This study focuses on the Campos Gerais complex (CGC), which is an Archean granite-greenstone belt lithological association in a tectonic window located in the southwesternmost portion of the São Francisco craton (SFC). The CGC is an important segment of Paleo- to Mesoarchean continental crust to be integrated into paleogeographic reconstructions prior to the transition into the Paleoproterozoic. This investigation reports field relationships, 28 major and trace element compositions, U-Pb (zircon) geochronological results, and Hf and Sm-Nd isotope data for orthogneiss and amphibolite samples. The results indicate that the CGC records a complex Archean crustal evolution, where voluminous 2.97 Ga TTG tonalites and trondhjemites (εNd(t)=-4.7; TDM=3.24 Ga) were followed by 2.89 Ga sanukitoid tonalite production (εNd(t)=-1.9; TDM=3.02 Ga), broadly coeval with the development of the Fortaleza de Minas and Pitangui greenstone-belts. These events are interpreted to represent the initial stage of an important subduction-accretion tectonic cycle, which ended with the emplacement of 2.82-2.81 Ga high-K leucogranites and migmatization of the TTG-sanukitoid crust, with hybrid and two-mica, peraluminous compositions (εNd(t)=-8.0 to-8.6; TDM=3.57-3.34 Ga). The presence of inherited zircons with 207Pb/206Pb ages of 3.08 Ga, 3.29 Ga, 3.55 Ga and 3.62 Ga indicates that the Mesoarchean tectonic processes involved reworking of Meso- to Eo-archean crust. Renewed TTG magmatism took place at ca. 2.77 Ga represented by juvenile tonalite stocks (εNd(t)=+1.0 to-1.5; TDM=2.80-2.88 Ga) which intrude the TTG-greenstone belt association. Crustal stabilization was attained by 2.67 Ga, allowing for the emplacement of within-plate tholeiitic amphibolites (εNd(t)=-3.1; TDM=2.87 Ga). The CGC shows important tectonic diachronism with respect to other Archean terrains in the southern São Francisco craton, including an independent Meso- to Neoarchean crustal evolution.
The generation of the continental crust is widely accepted to have taken place predominantly in the Archean, when TTG magmatism associated with greenstone-belt supracrustal succession development was typically followed by emplacement of high-K granites before crustal stabilization. This study focuses on the Campos Gerais complex (CGC), which is an Archean granite-greenstone belt lithological association in a tectonic window located in the southwesternmost portion of the São Francisco craton (SFC). The CGC is an important segment of Paleo- to Mesoarchean continental crust to be integrated into paleogeographic reconstructions prior to the transition into the Paleoproterozoic. This investigation reports field relationships, 28 major and trace element compositions, U-Pb (zircon) geochronological results, and Hf and Sm-Nd isotope data for orthogneiss and amphibolite samples. The results indicate that the CGC records a complex Archean crustal evolution, where voluminous 2.97 Ga TTG tonalites and trondhjemites (εNd(t)=-4.7; TDM=3.24 Ga) were followed by 2.89 Ga sanukitoid tonalite production (εNd(t)=-1.9; TDM=3.02 Ga), broadly coeval with the development of the Fortaleza de Minas and Pitangui greenstone-belts. These events are interpreted to represent the initial stage of an important subduction-accretion tectonic cycle, which ended with the emplacement of 2.82-2.81 Ga high-K leucogranites and migmatization of the TTG-sanukitoid crust, with hybrid and two-mica, peraluminous compositions (εNd(t)=-8.0 to-8.6; TDM=3.57-3.34 Ga). The presence of inherited zircons with 207Pb/206Pb ages of 3.08 Ga, 3.29 Ga, 3.55 Ga and 3.62 Ga indicates that the Mesoarchean tectonic processes involved reworking of Meso- to Eo-archean crust. Renewed TTG magmatism took place at ca. 2.77 Ga represented by juvenile tonalite stocks (εNd(t)=+1.0 to-1.5; TDM=2.80-2.88 Ga) which intrude the TTG-greenstone belt association. Crustal stabilization was attained by 2.67 Ga, allowing for the emplacement of within-plate tholeiitic amphibolites (εNd(t)=-3.1; TDM=2.87 Ga). The CGC shows important tectonic diachronism with respect to other Archean terrains in the southern São Francisco craton, including an independent Meso- to Neoarchean crustal evolution.
2022, 13(5): 101410.
doi: 10.1016/j.gsf.2022.101410
Abstract:
This work presents isotopic data for the non-traditional isotope systems Fe, Cu, and Zn on a set of Chicxulub impactites and target lithologies with the aim of better documenting the dynamic processes taking place during hypervelocity impact events, as well as those affecting impact structures during the post-impact phase. The focus lies on material from the recent IODP-ICDP Expedition 364 Hole M0077A drill core obtained from the offshore Chicxulub peak ring. Two ejecta blanket samples from the UNAM 5 and 7 cores were used to compare the crater lithologies with those outside of the impact structure. The datasets of bulk Fe, Cu, and Zn isotope ratios are coupled with petrographic observations and bulk major and trace element compositions to disentangle equilibrium isotope fractionation effects from kinetic processes. The observed Fe and Cu isotopic signatures, with δ56/54Fe ranging from -0.95‰ to 0.58‰ and δ65/63Cu from -0.73‰ to 0.14‰, mostly reflect felsic, mafic, and carbonate target lithology mixing and secondary sulfide mineral formation, the latter associated to the extensive and long-lived (>105 years) hydrothermal system within Chicxulub structure. On the other hand, the stable Zn isotope ratios provide evidence for volatility-governed isotopic fractionation. The heavier Zn isotopic compositions observed for the uppermost part of the impactite sequence and a metamorphic clast (δ66/64Zn of up to 0.80‰ and 0.87‰, respectively) relative to most basement lithologies and impact melt rock units indicate partial vaporization of Zn, comparable to what has been observed for Cretaceous-Paleogene boundary layer sediments around the world, as well as for tektites from various strewn fields. In contrast to previous work, our data indicate that an isotopically light Zn reservoir (δ66/64Zn down to -0.49‰), of which the existence has previously been suggested based on mass balance considerations, may reside within the upper impact melt rock (UIM) unit. This observation is restricted to a few UIM samples only and cannot be extended to other target or impact melt rock units. Light isotopic signatures of moderately volatile elements in tektites and microtektites have previously been linked to (back-)condensation under distinct kinetic regimes. Although some of the signatures observed may have been partially overprinted during post-impact processes, our bulk data confirm impact volatilization and condensation of Zn, which may be even more pronounced at the microscale, with variable degrees of mixing between isotopically distinct reservoirs, not only at proximal to distal ejecta sites, but also within the lithologies associated with the Chicxulub impact crater.
This work presents isotopic data for the non-traditional isotope systems Fe, Cu, and Zn on a set of Chicxulub impactites and target lithologies with the aim of better documenting the dynamic processes taking place during hypervelocity impact events, as well as those affecting impact structures during the post-impact phase. The focus lies on material from the recent IODP-ICDP Expedition 364 Hole M0077A drill core obtained from the offshore Chicxulub peak ring. Two ejecta blanket samples from the UNAM 5 and 7 cores were used to compare the crater lithologies with those outside of the impact structure. The datasets of bulk Fe, Cu, and Zn isotope ratios are coupled with petrographic observations and bulk major and trace element compositions to disentangle equilibrium isotope fractionation effects from kinetic processes. The observed Fe and Cu isotopic signatures, with δ56/54Fe ranging from -0.95‰ to 0.58‰ and δ65/63Cu from -0.73‰ to 0.14‰, mostly reflect felsic, mafic, and carbonate target lithology mixing and secondary sulfide mineral formation, the latter associated to the extensive and long-lived (>105 years) hydrothermal system within Chicxulub structure. On the other hand, the stable Zn isotope ratios provide evidence for volatility-governed isotopic fractionation. The heavier Zn isotopic compositions observed for the uppermost part of the impactite sequence and a metamorphic clast (δ66/64Zn of up to 0.80‰ and 0.87‰, respectively) relative to most basement lithologies and impact melt rock units indicate partial vaporization of Zn, comparable to what has been observed for Cretaceous-Paleogene boundary layer sediments around the world, as well as for tektites from various strewn fields. In contrast to previous work, our data indicate that an isotopically light Zn reservoir (δ66/64Zn down to -0.49‰), of which the existence has previously been suggested based on mass balance considerations, may reside within the upper impact melt rock (UIM) unit. This observation is restricted to a few UIM samples only and cannot be extended to other target or impact melt rock units. Light isotopic signatures of moderately volatile elements in tektites and microtektites have previously been linked to (back-)condensation under distinct kinetic regimes. Although some of the signatures observed may have been partially overprinted during post-impact processes, our bulk data confirm impact volatilization and condensation of Zn, which may be even more pronounced at the microscale, with variable degrees of mixing between isotopically distinct reservoirs, not only at proximal to distal ejecta sites, but also within the lithologies associated with the Chicxulub impact crater.
2022, 13(5): 101411.
doi: 10.1016/j.gsf.2022.101411
Abstract:
High-silica (SiO2 > 70 wt.%) granites (HSGs) are the main source of W, Sn, and rare metals. However, abundant HSGs, temporally, spatially, and genetically associated with Pb-Zn mineralization, in the Lhasa terrane (LT), provided an ideal opportunity to study the key factors responsible for Pb-Zn enrichment, instead of W-Sn enrichment. Here we contribute to this topic through U-Pb dating of zircon and garnet, and whole-rock and Sr-Nd-Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb-Zn and W deposits in the LT. The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U-Pb ages of 77.3 ±0.9 Ma and hydrothermal garnet U-Pb ages of 75.7 ±4.8 Ma, which is 10-15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb-Zn polymetallic mineralization in the LT. The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere, which occurred as a result of the collision of the Indian and Asian plates. These HSGs related to Pb-Zn mineralization, with high total-alkalis and low magnesian contents, are enriched in Ba, Th, and Rb, but depleted in Ti, Eu, Sr, and P. They belong to either the S-type, or I-type granites. The Sr-Nd-Hf isotopic compositions of the Pb-Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input. This was consistent with the postulated source of W enrichment in the LT. The Pb-Zn and W related granites have similar zircon-Ti-saturation temperatures, comparable low whole-rock Fe2O3/FeO ratios, and zircon oxygen fugacity. This indicated that the Pb-Zn-W enrichment in the high-silica magma system could be attributed to a relatively reduced magma. The Pb-Zn related HSGs, abundant quartz and feldspar phenocrysts, and weak fractionation of twin-elements in whole-rock analysis, can be used to reconstruct a model of the magma reservoir. We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir, with a loss of magmatic fluids. The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb-Zn related magma; however, different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in whole-rock are formed by a magmatic-hydrothermal transition in highly evolved system. As the source and oxygen fugacities of the Pb-Zn and W related magmas are similar, the absence of a giant W-Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation. This implies that the rocks that originated as highly evolved silicate-rich parent magmas, with a high degree of magmatic-hydrothermal alteration, would need to be targeted for W-Sn mineral exploration in the LT. In summary, our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb-Zn and W enrichment. This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.
High-silica (SiO2 > 70 wt.%) granites (HSGs) are the main source of W, Sn, and rare metals. However, abundant HSGs, temporally, spatially, and genetically associated with Pb-Zn mineralization, in the Lhasa terrane (LT), provided an ideal opportunity to study the key factors responsible for Pb-Zn enrichment, instead of W-Sn enrichment. Here we contribute to this topic through U-Pb dating of zircon and garnet, and whole-rock and Sr-Nd-Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb-Zn and W deposits in the LT. The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U-Pb ages of 77.3 ±0.9 Ma and hydrothermal garnet U-Pb ages of 75.7 ±4.8 Ma, which is 10-15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb-Zn polymetallic mineralization in the LT. The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere, which occurred as a result of the collision of the Indian and Asian plates. These HSGs related to Pb-Zn mineralization, with high total-alkalis and low magnesian contents, are enriched in Ba, Th, and Rb, but depleted in Ti, Eu, Sr, and P. They belong to either the S-type, or I-type granites. The Sr-Nd-Hf isotopic compositions of the Pb-Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input. This was consistent with the postulated source of W enrichment in the LT. The Pb-Zn and W related granites have similar zircon-Ti-saturation temperatures, comparable low whole-rock Fe2O3/FeO ratios, and zircon oxygen fugacity. This indicated that the Pb-Zn-W enrichment in the high-silica magma system could be attributed to a relatively reduced magma. The Pb-Zn related HSGs, abundant quartz and feldspar phenocrysts, and weak fractionation of twin-elements in whole-rock analysis, can be used to reconstruct a model of the magma reservoir. We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir, with a loss of magmatic fluids. The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb-Zn related magma; however, different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in whole-rock are formed by a magmatic-hydrothermal transition in highly evolved system. As the source and oxygen fugacities of the Pb-Zn and W related magmas are similar, the absence of a giant W-Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation. This implies that the rocks that originated as highly evolved silicate-rich parent magmas, with a high degree of magmatic-hydrothermal alteration, would need to be targeted for W-Sn mineral exploration in the LT. In summary, our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb-Zn and W enrichment. This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.
2022, 13(5): 101424.
doi: 10.1016/j.gsf.2022.101424
Abstract:
A new method is proposed to analyze the pore-scale mechanisms and characterization of light oil storage in shale nanopores, which is based on the Hydrocarbon Vapor Adsorption (HVA) and Pore Calculation Model (PCM). First, the basic principle of the HVA-PCM method is introduced, and the experimental/mathematical analysis processes are given. Then, the HVA-PCM method is applied to shale samples to analyze the mechanisms and characterization of light oil storage in shale nanopores. The results provide insights into the pore-scale oil storage mechanisms, oil storage structure, oil film thickness, oil distribution within different sized pores, and the oil storage state. Finally, the advantages and limitations of the HVA-PCM method are discussed, and suggestions for further improvement are proposed. Overall, the HVA-PCM method is a powerful tool for extracting quantitative information on the light oil storage in shale nanopores.
A new method is proposed to analyze the pore-scale mechanisms and characterization of light oil storage in shale nanopores, which is based on the Hydrocarbon Vapor Adsorption (HVA) and Pore Calculation Model (PCM). First, the basic principle of the HVA-PCM method is introduced, and the experimental/mathematical analysis processes are given. Then, the HVA-PCM method is applied to shale samples to analyze the mechanisms and characterization of light oil storage in shale nanopores. The results provide insights into the pore-scale oil storage mechanisms, oil storage structure, oil film thickness, oil distribution within different sized pores, and the oil storage state. Finally, the advantages and limitations of the HVA-PCM method are discussed, and suggestions for further improvement are proposed. Overall, the HVA-PCM method is a powerful tool for extracting quantitative information on the light oil storage in shale nanopores.
2022, 13(5): 101425.
doi: 10.1016/j.gsf.2022.101425
Abstract:
Multi-hazard susceptibility prediction is an important component of disasters risk management plan. An effective multi-hazard risk mitigation strategy includes assessing individual hazards as well as their interactions. However, with the rapid development of artificial intelligence technology, multi-hazard susceptibility prediction techniques based on machine learning has encountered a huge bottleneck. In order to effectively solve this problem, this study proposes a multi-hazard susceptibility mapping framework using the classical deep learning algorithm of Convolutional Neural Networks (CNN). First, we use historical flash flood, debris flow and landslide locations based on Google Earth images, extensive field surveys, topography, hydrology, and environmental data sets to train and validate the proposed CNN method. Next, the proposed CNN method is assessed in comparison to conventional logistic regression and k-nearest neighbor methods using several objective criteria, i.e., coefficient of determination, overall accuracy, mean absolute error and the root mean square error. Experimental results show that the CNN method outperforms the conventional machine learning algorithms in predicting probability of flash floods, debris flows and landslides. Finally, the susceptibility maps of the three hazards based on CNN are combined to create a multi-hazard susceptibility map. It can be observed from the map that 62.43% of the study area are prone to hazards, while 37.57% of the study area are harmless. In hazard-prone areas, 16.14%, 4.94% and 30.66% of the study area are susceptible to flash floods, debris flows and landslides, respectively. In terms of concurrent hazards, 0.28%, 7.11% and 3.13% of the study area are susceptible to the joint occurrence of flash floods and debris flow, debris flow and landslides, and flash floods and landslides, respectively, whereas, 0.18% of the study area is subject to all the three hazards. The results of this study can benefit engineers, disaster managers and local government officials involved in sustainable land management and disaster risk mitigation.
Multi-hazard susceptibility prediction is an important component of disasters risk management plan. An effective multi-hazard risk mitigation strategy includes assessing individual hazards as well as their interactions. However, with the rapid development of artificial intelligence technology, multi-hazard susceptibility prediction techniques based on machine learning has encountered a huge bottleneck. In order to effectively solve this problem, this study proposes a multi-hazard susceptibility mapping framework using the classical deep learning algorithm of Convolutional Neural Networks (CNN). First, we use historical flash flood, debris flow and landslide locations based on Google Earth images, extensive field surveys, topography, hydrology, and environmental data sets to train and validate the proposed CNN method. Next, the proposed CNN method is assessed in comparison to conventional logistic regression and k-nearest neighbor methods using several objective criteria, i.e., coefficient of determination, overall accuracy, mean absolute error and the root mean square error. Experimental results show that the CNN method outperforms the conventional machine learning algorithms in predicting probability of flash floods, debris flows and landslides. Finally, the susceptibility maps of the three hazards based on CNN are combined to create a multi-hazard susceptibility map. It can be observed from the map that 62.43% of the study area are prone to hazards, while 37.57% of the study area are harmless. In hazard-prone areas, 16.14%, 4.94% and 30.66% of the study area are susceptible to flash floods, debris flows and landslides, respectively. In terms of concurrent hazards, 0.28%, 7.11% and 3.13% of the study area are susceptible to the joint occurrence of flash floods and debris flow, debris flow and landslides, and flash floods and landslides, respectively, whereas, 0.18% of the study area is subject to all the three hazards. The results of this study can benefit engineers, disaster managers and local government officials involved in sustainable land management and disaster risk mitigation.
2022, 13(5): 101427.
doi: 10.1016/j.gsf.2022.101427
Abstract:
Populations and metropolitan centers are accumulated in coastal areas around the world. In view of the fact that they are geographically adjacent to coasts and intense anthropogenic activities, increasing global offshore pollution has been an important worldwide concern over the past several decades and has become a very serious problem that needs to be addressed urgently. Due to offshore pollution, various geological disasters occur in high frequency, including intensified erosion and salinization of coastal soils, frequent geological collapses and landslides and increasing seismic activities. Moreover, offshore pollution shows increasingly serious impacts on the topography and geomorphology of offshore and coastal areas, including coastal degradation, retreating coastlines and estuary delta erosion. Offshore sedimentation processes are strongly influenced by the pH changes of terrestrial discharges, and sedimentary dynamics have become extremely acute and complex due to offshore pollution. The seabed topography and hydrodynamic environment determine the fate and transport of pollutants entering offshore regions. Coastal estuaries, port basins and lagoons that have relatively moderate ocean currents and winds are more likely to accumulate pollutants. Offshore regions and undersea canyons can be used as conduits for transporting pollutants from the continent to the seabed. It is particularly noteworthy that the spatial/temporal distribution of species, community structures, and ecological functions in offshore areas have undergone unprecedented changes in recent decades. Due to increasing offshore pollution, the stable succession and development trend of marine ecosystems has been broken. It is thus important to identify and regulate the quantity, composition and transportation of pollutants in offshore regions and their behavior in marine ecosystems. In particular, crucial actions for stabilizing marine ecosystems, including increasing species and biodiversity, should be implemented to enhance their anti-interference capabilities. This review provides an overview of the current situation of offshore pollution, as well as major trends of pollutant fate and transportation from continent to marine ecosystems, transformation of pollutants in sediments, and their bioaccumulation and diffusion. This study retrospectively reviews the long-term geological evolution of offshore pollution from the perspective of marine geology, and analyses their long-term potential impacts on marine ecosystems. Due to ecological risks associated with pollutants released from offshore sediments, more research on the influence of global offshore pollution based on marine geology is undoubtedly needed.
Populations and metropolitan centers are accumulated in coastal areas around the world. In view of the fact that they are geographically adjacent to coasts and intense anthropogenic activities, increasing global offshore pollution has been an important worldwide concern over the past several decades and has become a very serious problem that needs to be addressed urgently. Due to offshore pollution, various geological disasters occur in high frequency, including intensified erosion and salinization of coastal soils, frequent geological collapses and landslides and increasing seismic activities. Moreover, offshore pollution shows increasingly serious impacts on the topography and geomorphology of offshore and coastal areas, including coastal degradation, retreating coastlines and estuary delta erosion. Offshore sedimentation processes are strongly influenced by the pH changes of terrestrial discharges, and sedimentary dynamics have become extremely acute and complex due to offshore pollution. The seabed topography and hydrodynamic environment determine the fate and transport of pollutants entering offshore regions. Coastal estuaries, port basins and lagoons that have relatively moderate ocean currents and winds are more likely to accumulate pollutants. Offshore regions and undersea canyons can be used as conduits for transporting pollutants from the continent to the seabed. It is particularly noteworthy that the spatial/temporal distribution of species, community structures, and ecological functions in offshore areas have undergone unprecedented changes in recent decades. Due to increasing offshore pollution, the stable succession and development trend of marine ecosystems has been broken. It is thus important to identify and regulate the quantity, composition and transportation of pollutants in offshore regions and their behavior in marine ecosystems. In particular, crucial actions for stabilizing marine ecosystems, including increasing species and biodiversity, should be implemented to enhance their anti-interference capabilities. This review provides an overview of the current situation of offshore pollution, as well as major trends of pollutant fate and transportation from continent to marine ecosystems, transformation of pollutants in sediments, and their bioaccumulation and diffusion. This study retrospectively reviews the long-term geological evolution of offshore pollution from the perspective of marine geology, and analyses their long-term potential impacts on marine ecosystems. Due to ecological risks associated with pollutants released from offshore sediments, more research on the influence of global offshore pollution based on marine geology is undoubtedly needed.
2022, 13(5): 101429.
doi: 10.1016/j.gsf.2022.101429
Abstract:
Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite, Pimenta Bueno Kimberlitic Field, which is located on the southwestern border of the Amazonian Craton. We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite. Mantle xenoliths are mainly clinopyroxenites and garnetites. Some of the clinopyroxenites were classified as GPP-PP-PKP (garnet-phlogopite peridotite, phlogopite-peridotite, phlogopite-K-richterite peridotite) suites, and two clinopyroxenites (eclogites) and two garnetites are relicts of an ancient subducted slab. Temperature and pressure estimates yield 855-1102℃ and 3.6-7.0 GPa, respectively. Clinopyroxenes are enriched in light rare earth elements (LREE) (LaN/YbN=5-62; CeN/SmN=1-3; where N=primitive mantle normalized values), they have high Ca/Al ratios (10-410), low to medium Ti/Eu ratios (742-2840), and low Zr/Hf ratios (13-26), which suggest they were formed by metasomatic reactions with CO2-rich silicate melts. Phlogopite with high TiO2 (>2.0 wt.%), Al2O3 (>12.0 wt.%), and FeOt (5.0-13.0 wt.%) resemble those found in the groundmass of kimberlites, lamproites and lamprophyres. Conversely, phlogopite with low TiO2 (<1.0 wt.%) and lower Al2O3 (<12.0 wt.%) are similar to those present in GPP-PP-PKP, and in MARID (mica-amphibole-rutile-ilmenite-diopside) and PIC (phlogopite-ilmenite-clinopyorxene) xenoliths. The GPP-PP-PKP suite of xenoliths, together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton. The Sr-Nd isotopic ratios of pyrope xenocrysts (G3, G9 and G11) from the Carolina kimberlite are characterized by high 143Nd/144Nd (0.51287-0.51371) and εNd (+4.55 to +20.85) accompanied with enriched 87Sr/86Sr (0.70405-0.71098). These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites. Based on Sr-Nd whole-rock compositions, the Carolina kimberlite has affinity with Group 1 kimberlites. The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9 ±5.4 Ma (2 σ), which represents the cooling age after the proto-kimberlite melt metasomatism. Therefore, we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle (garnetites and eclogites); with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.
Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite, Pimenta Bueno Kimberlitic Field, which is located on the southwestern border of the Amazonian Craton. We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite. Mantle xenoliths are mainly clinopyroxenites and garnetites. Some of the clinopyroxenites were classified as GPP-PP-PKP (garnet-phlogopite peridotite, phlogopite-peridotite, phlogopite-K-richterite peridotite) suites, and two clinopyroxenites (eclogites) and two garnetites are relicts of an ancient subducted slab. Temperature and pressure estimates yield 855-1102℃ and 3.6-7.0 GPa, respectively. Clinopyroxenes are enriched in light rare earth elements (LREE) (LaN/YbN=5-62; CeN/SmN=1-3; where N=primitive mantle normalized values), they have high Ca/Al ratios (10-410), low to medium Ti/Eu ratios (742-2840), and low Zr/Hf ratios (13-26), which suggest they were formed by metasomatic reactions with CO2-rich silicate melts. Phlogopite with high TiO2 (>2.0 wt.%), Al2O3 (>12.0 wt.%), and FeOt (5.0-13.0 wt.%) resemble those found in the groundmass of kimberlites, lamproites and lamprophyres. Conversely, phlogopite with low TiO2 (<1.0 wt.%) and lower Al2O3 (<12.0 wt.%) are similar to those present in GPP-PP-PKP, and in MARID (mica-amphibole-rutile-ilmenite-diopside) and PIC (phlogopite-ilmenite-clinopyorxene) xenoliths. The GPP-PP-PKP suite of xenoliths, together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton. The Sr-Nd isotopic ratios of pyrope xenocrysts (G3, G9 and G11) from the Carolina kimberlite are characterized by high 143Nd/144Nd (0.51287-0.51371) and εNd (+4.55 to +20.85) accompanied with enriched 87Sr/86Sr (0.70405-0.71098). These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites. Based on Sr-Nd whole-rock compositions, the Carolina kimberlite has affinity with Group 1 kimberlites. The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9 ±5.4 Ma (2 σ), which represents the cooling age after the proto-kimberlite melt metasomatism. Therefore, we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle (garnetites and eclogites); with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.
2022, 13(5): 101406.
doi: 10.1016/j.gsf.2022.101406
Abstract:
2022, 13(5): 101428.
doi: 10.1016/j.gsf.2022.101428
Abstract:
Until the middle of the 20th century, the continental crust was considered to be dominantly granitic. This hypothesis was revised after the Second World War when several new studies led to the realization that the continental crust is dominantly made of metamorphic rocks. Magmatic rocks were emplaced at peak metamorphic conditions in domains, which can be defined by geophysical discontinuities. Low to medium-grade metamorphic rocks constitute the upper crust, granitic migmatites and intrusive granites occur in the middle crust, and the lower crust, situated between the Conrad and Moho discontinuities, comprises charnockites and granulites. The continental crust acquired its final structure during metamorphic episodes associated with mantle upwelling, which mostly occurred in supercontinents prior to their disruption, during which the base of the crust experienced ultrahigh temperatures (>1000℃, ultrahigh temperature granulite-facies metamorphism). Heat is provided by underplating of mantle-derived mafic magmas, as well as by a massive influx of low H2O activity mantle fluids, i.e. high-density CO2 and high-salinity brines. These fluids are initially stored in ultrahigh temperature domains, and subsequently infiltrate the lower crust, where they generate anhydrous granulite mineral assemblages. The brines can reach upper crustal levels, possibly even the surface, along major shear zones, where granitoids are generated through brine streaming in addition to those formed by dehydration melting in upper crustal levels.
Until the middle of the 20th century, the continental crust was considered to be dominantly granitic. This hypothesis was revised after the Second World War when several new studies led to the realization that the continental crust is dominantly made of metamorphic rocks. Magmatic rocks were emplaced at peak metamorphic conditions in domains, which can be defined by geophysical discontinuities. Low to medium-grade metamorphic rocks constitute the upper crust, granitic migmatites and intrusive granites occur in the middle crust, and the lower crust, situated between the Conrad and Moho discontinuities, comprises charnockites and granulites. The continental crust acquired its final structure during metamorphic episodes associated with mantle upwelling, which mostly occurred in supercontinents prior to their disruption, during which the base of the crust experienced ultrahigh temperatures (>1000℃, ultrahigh temperature granulite-facies metamorphism). Heat is provided by underplating of mantle-derived mafic magmas, as well as by a massive influx of low H2O activity mantle fluids, i.e. high-density CO2 and high-salinity brines. These fluids are initially stored in ultrahigh temperature domains, and subsequently infiltrate the lower crust, where they generate anhydrous granulite mineral assemblages. The brines can reach upper crustal levels, possibly even the surface, along major shear zones, where granitoids are generated through brine streaming in addition to those formed by dehydration melting in upper crustal levels.
