The South Atlantic passive margin along the south-eastern Brazilian highlands exhibits a complex landscape,including a northern inselberg area and a southern elevated plateau,separated by the Doce River valley.This la...The South Atlantic passive margin along the south-eastern Brazilian highlands exhibits a complex landscape,including a northern inselberg area and a southern elevated plateau,separated by the Doce River valley.This landscape is set on the Proterozoic to early Paleozoic rocks of the region that once was the hot core of the Aracuai orogen,in Ediacaran to Ordovician times.Due to the break-up of Gondwana and consequently the opening of the South Atlantic during the Early Cretaceous,those rocks of the Araquai orogen became the basement of a portion of the South Atlantic passive margin and related southeastern Brazilian highlands.Our goal is to provide a new set of constraints on the thermo-tectonic history of this portion of the south-eastern Brazilian margin and related surface processes,and to provide a hypothesis on the geodynamic context since break-up.To this end,we combine the apatite fission track(AFT)and apatite(U-Th)/He(AHe)methods as input for inverse thermal history modelling.All our AFT and AHe central ages are Late Cretaceous to early Paleogene.The AFT ages vary between 62 Ma and90 Ma,with mean track lengths between 12.2μm and 13.6μm.AHe ages are found to be equivalent to AFT ages within uncertainty,albeit with the former exhibiting a lesser degree of confidence.We relate this Late Cretaceous-Paleocene basement cooling to uplift with accelerated denudation at this time.Spatial variation of the denudation time can be linked to differential reactivation of the Precambrian structural network and differential erosion due to a complex interplay with the drainage system.We argue that posterior large-scale sedimentation in the offshore basins may be a result of flexural isostasy combined with an expansion of the drainage network.We put forward the combined compression of the Mid-Atlantic ridge and the Peruvian phase of the Andean orogeny,potentially augmented through the thermal weakening of the lower crust by the Trindade thermal anomaly,as a probable cause for the uplift.展开更多
The Alto MoxotóTerrane 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 Neopro...The Alto MoxotóTerrane 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 negativeεNd(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 MoxotóTerrane,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.展开更多
The Ediacaran-Cambrian transition witnessed some of the most important biological,tectonic,climatic and geochemical changes in Earth's history.Of utmost importance for early animal evolution is the likely shift in...The Ediacaran-Cambrian transition witnessed some of the most important biological,tectonic,climatic and geochemical changes in Earth's history.Of utmost importance for early animal evolution is the likely shift in redox conditions of bottom waters,which might have taken place in distinct pulses during the late Ediacaran and early Paleozoic.To track redox changes during this transition,we present new trace element,total organic carbon and both inorganic and organic carbon isotopes,and the first iron speciation data on the Tamengo and Guaicurus formations of the CorumbáGroup in western Brazil,which record important paleobiological changes between 555 Ma to<541 Ma.The stratigraphically older Tamengo Formation is composed mainly of limestone with interbedded marls and mudrocks,and bears fragments of upper Ediacaran biomineralized fossils such as Cloudina lucianoi and Corumbella werneri.The younger Guaicurus Formation represents a regional transgression of the shallow carbonate platform and is composed of a homogeneous fine-grained siliciclastic succession,bearing meiofaunal bilateral burrows.The new iron speciation data reveal predominantly anoxic and ferruginous(non-sulfidic)bottom water conditions during deposition of the Tamengo Formation,with FeHR/FeT around 0.8 and FePy/FeHR below 0.7.The transition from the Tamengo to the Guaicurus Formation is marked by a stratigraphically rapid drop in FeHR/FeT to below 0.2,recording a shift to likely oxic bottom waters,which persist upsection.Redox-sensitive element(RSE)concentrations are muted in both formations,but consistent with non-sulfidic bottom water conditions throughout.We interpret the collected data to reflect a transition between two distinct paleoenvironmental settings.The Tamengo Formation represents an environment with anoxic bottom waters,with fragments of biomineralized organisms that lived on shallower,proba-bly mildly oxygenated surficial waters,and that were then transported down-slope.Similar to coeval suc-cessions(e.g.,the Nama Group in Namibia),our data support the hypothesis that late Ediacaran biomineralized organisms lived in a thin oxygenated surface layer above a relatively shallow chemocline.The Guaicurus Formation,on the other hand,records the expansion of oxic conditions to deeper waters during a sea level rise.Although the relationship between global biogeochemical changes and the activities of early bioturbators remains complex,these results demonstrate an unequivocal synchronous relationship between oxygenation of the Corumbábasin and the local appearance of meiofaunal bioturbators.展开更多
基金supported by the Special Research Fund of Ghent University (BOF 01N03915)
文摘The South Atlantic passive margin along the south-eastern Brazilian highlands exhibits a complex landscape,including a northern inselberg area and a southern elevated plateau,separated by the Doce River valley.This landscape is set on the Proterozoic to early Paleozoic rocks of the region that once was the hot core of the Aracuai orogen,in Ediacaran to Ordovician times.Due to the break-up of Gondwana and consequently the opening of the South Atlantic during the Early Cretaceous,those rocks of the Araquai orogen became the basement of a portion of the South Atlantic passive margin and related southeastern Brazilian highlands.Our goal is to provide a new set of constraints on the thermo-tectonic history of this portion of the south-eastern Brazilian margin and related surface processes,and to provide a hypothesis on the geodynamic context since break-up.To this end,we combine the apatite fission track(AFT)and apatite(U-Th)/He(AHe)methods as input for inverse thermal history modelling.All our AFT and AHe central ages are Late Cretaceous to early Paleogene.The AFT ages vary between 62 Ma and90 Ma,with mean track lengths between 12.2μm and 13.6μm.AHe ages are found to be equivalent to AFT ages within uncertainty,albeit with the former exhibiting a lesser degree of confidence.We relate this Late Cretaceous-Paleocene basement cooling to uplift with accelerated denudation at this time.Spatial variation of the denudation time can be linked to differential reactivation of the Precambrian structural network and differential erosion due to a complex interplay with the drainage system.We argue that posterior large-scale sedimentation in the offshore basins may be a result of flexural isostasy combined with an expansion of the drainage network.We put forward the combined compression of the Mid-Atlantic ridge and the Peruvian phase of the Andean orogeny,potentially augmented through the thermal weakening of the lower crust by the Trindade thermal anomaly,as a probable cause for the uplift.
基金support from Australian Research Council grant FL160100168supported by the National Institute of Science and Technology for Tectonic Studies(INCT)of Brazil。
文摘The Alto MoxotóTerrane 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 negativeεNd(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 MoxotóTerrane,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.
基金supported by CNPq-Brazil through grants nb.408815/2021-3 and 304509/2021-3by Instituto Serrapilheira through Project“MOBILE:Mountain Belts and the Inception of Complex Life on Earth(geolifemobile.com)”,grant no.Serra-1912-31510,and by the Worldwide Universities Network-WUN through their Research Development Fund(RDF 2022)+3 种基金The first author is part of Instituto GeoAtlântico,a National Institute of Science and Technology,CNPq-Brazil process nb.405653/2022-0Most of the data presented here was acquired during a sevenmonth visiting research appointment of FAC at the Stanford Doerr School of Sustainability supported by CAPES-Brazil through their PRINT program(88887.682318/2022-00)EAS and some geochemical analyses at Stanford were supported by U.S.National Science Foundation grant EAR-2143164MG was supported by the Research Foundation of the Federal District(FAPDF)-process no.0193.001609/2017。
文摘The Ediacaran-Cambrian transition witnessed some of the most important biological,tectonic,climatic and geochemical changes in Earth's history.Of utmost importance for early animal evolution is the likely shift in redox conditions of bottom waters,which might have taken place in distinct pulses during the late Ediacaran and early Paleozoic.To track redox changes during this transition,we present new trace element,total organic carbon and both inorganic and organic carbon isotopes,and the first iron speciation data on the Tamengo and Guaicurus formations of the CorumbáGroup in western Brazil,which record important paleobiological changes between 555 Ma to<541 Ma.The stratigraphically older Tamengo Formation is composed mainly of limestone with interbedded marls and mudrocks,and bears fragments of upper Ediacaran biomineralized fossils such as Cloudina lucianoi and Corumbella werneri.The younger Guaicurus Formation represents a regional transgression of the shallow carbonate platform and is composed of a homogeneous fine-grained siliciclastic succession,bearing meiofaunal bilateral burrows.The new iron speciation data reveal predominantly anoxic and ferruginous(non-sulfidic)bottom water conditions during deposition of the Tamengo Formation,with FeHR/FeT around 0.8 and FePy/FeHR below 0.7.The transition from the Tamengo to the Guaicurus Formation is marked by a stratigraphically rapid drop in FeHR/FeT to below 0.2,recording a shift to likely oxic bottom waters,which persist upsection.Redox-sensitive element(RSE)concentrations are muted in both formations,but consistent with non-sulfidic bottom water conditions throughout.We interpret the collected data to reflect a transition between two distinct paleoenvironmental settings.The Tamengo Formation represents an environment with anoxic bottom waters,with fragments of biomineralized organisms that lived on shallower,proba-bly mildly oxygenated surficial waters,and that were then transported down-slope.Similar to coeval suc-cessions(e.g.,the Nama Group in Namibia),our data support the hypothesis that late Ediacaran biomineralized organisms lived in a thin oxygenated surface layer above a relatively shallow chemocline.The Guaicurus Formation,on the other hand,records the expansion of oxic conditions to deeper waters during a sea level rise.Although the relationship between global biogeochemical changes and the activities of early bioturbators remains complex,these results demonstrate an unequivocal synchronous relationship between oxygenation of the Corumbábasin and the local appearance of meiofaunal bioturbators.
