M. P. Searle, R. Shail, J. M. Pownall, C. Jurkowski, A. B. Watts, Laurence J. Robb
The Permian Cornubian granite batholith (295−275 Ma) in SW England includes seven major plutons and numerous smaller stocks extending for ∼250 km from the Isles of Scilly in the WSW to Dartmoor in the ENE. The granites are peraluminous and classified as crustal melt S-type, predominantly two-mica granites, and biotite or tourmaline monzo- and syenogranites, with subordinate minor topaz granite and lithium mica granite. The granites and their host rocks are pervasively mineralized with tin (cassiterite), tungsten (wolframite, ferberite), copper (chalcopyrite, chalcocite, bornite), arsenic (arsenopyrite), and zinc (sphalerite) mineralized lodes. Quartz-muscovite selvedges (greisen-bordered) also contain enrichment of lithophile elements such as boron (tourmaline), fluorine (fluorite), and lithium (lithium-micas such as lepidolite and zinnwaldite). They are derived from both muscovite and biotite dehydration melting of pelitic-psammitic rocks and intruded from a common source along the length of the batholith. Pressure estimates from andalusite and cordierite-bearing hornfels in the contact metamorphic aureole (150 ± 100 MPa) show that the granites intruded to 3 km depth. Cupolas around the Land’s End and Tregonning granites show aplite-pegmatite dikes and tourmaline + quartz + muscovite veins (greisen) that are frequently mineralized. Synchronous intrusions of lamprophyre dikes suggest an additional heat source for crustal melting may have been from underplating of alkaline magmas. The lack of significant erosion means that the source region is not exposed. In an accompanying paper (Part 2; Watts et al., 2024), gravity modeling reveals possible solutions for the shape and depth of the granite and the structure of the lower crust. We present a new model for the Land’s End, Tregonning, and Carnmenellis granites showing a mid-crustal source composed of amphibolite facies migmatites bounded by prominent seismic reflectors, with upward expanding dikes feeding inter-connected granite laccoliths that show inflated cupolas with shallow contact metamorphism. The Cornubian granites intruded >90 m.y. after obduction of the Lizard ophiolite complex, and after Upper Devonian−Carboniferous Variscan compressional, and later extensional, deformation of the surrounding Devonian country rocks. Comparisons are made between the Cornubian batholith and the Patagonian batholith in Chile, the Himalayan leucogranites, and the Baltoro granite batholith along the Karakoram range in northern Pakistan.
{"title":"The Permian Cornubian granite batholith, SW England; Part 1: Field, structural, and petrological constraints","authors":"M. P. Searle, R. Shail, J. M. Pownall, C. Jurkowski, A. B. Watts, Laurence J. Robb","doi":"10.1130/b37457.1","DOIUrl":"https://doi.org/10.1130/b37457.1","url":null,"abstract":"The Permian Cornubian granite batholith (295−275 Ma) in SW England includes seven major plutons and numerous smaller stocks extending for ∼250 km from the Isles of Scilly in the WSW to Dartmoor in the ENE. The granites are peraluminous and classified as crustal melt S-type, predominantly two-mica granites, and biotite or tourmaline monzo- and syenogranites, with subordinate minor topaz granite and lithium mica granite. The granites and their host rocks are pervasively mineralized with tin (cassiterite), tungsten (wolframite, ferberite), copper (chalcopyrite, chalcocite, bornite), arsenic (arsenopyrite), and zinc (sphalerite) mineralized lodes. Quartz-muscovite selvedges (greisen-bordered) also contain enrichment of lithophile elements such as boron (tourmaline), fluorine (fluorite), and lithium (lithium-micas such as lepidolite and zinnwaldite). They are derived from both muscovite and biotite dehydration melting of pelitic-psammitic rocks and intruded from a common source along the length of the batholith. Pressure estimates from andalusite and cordierite-bearing hornfels in the contact metamorphic aureole (150 ± 100 MPa) show that the granites intruded to 3 km depth. Cupolas around the Land’s End and Tregonning granites show aplite-pegmatite dikes and tourmaline + quartz + muscovite veins (greisen) that are frequently mineralized. Synchronous intrusions of lamprophyre dikes suggest an additional heat source for crustal melting may have been from underplating of alkaline magmas. The lack of significant erosion means that the source region is not exposed. In an accompanying paper (Part 2; Watts et al., 2024), gravity modeling reveals possible solutions for the shape and depth of the granite and the structure of the lower crust. We present a new model for the Land’s End, Tregonning, and Carnmenellis granites showing a mid-crustal source composed of amphibolite facies migmatites bounded by prominent seismic reflectors, with upward expanding dikes feeding inter-connected granite laccoliths that show inflated cupolas with shallow contact metamorphism. The Cornubian granites intruded >90 m.y. after obduction of the Lizard ophiolite complex, and after Upper Devonian−Carboniferous Variscan compressional, and later extensional, deformation of the surrounding Devonian country rocks. Comparisons are made between the Cornubian batholith and the Patagonian batholith in Chile, the Himalayan leucogranites, and the Baltoro granite batholith along the Karakoram range in northern Pakistan.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"34 23","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140744535","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Huitong Yang, Wei Tan, Lei Wu, Yongshu Zhang, Bo Wang, Junyong Zhang, Xuebing Wei, Pengcheng Tang, Liguang Mao, Chuanwu Wang, Yan Chen, Jianchao Tang, Kai Huang, Ancheng Xiao, Xiubin Lin, Hanlin Chen
The Qaidam Basin is the largest sedimentary basin within the Tibetan Plateau, with up to ∼15-km-thick deposits accumulated in the Cenozoic. Understanding how it deformed in response to the far-field effects of India-Eurasia collision is critical to improving our knowledge of the mechanism underlying northward plateau growth. Unlike typical compressional basins, where upper-crustal deformation concentrates at their margins, the Qaidam Basin features the development of many NW- to WNW-striking folds across the entire basin. Why crustal shortening occurred in the interior of Qaidam Basin, ∼100 km away from the margins, together with the underground geometries beneath these folds, remains unknown. Herein, based on newly acquired three- and two-dimensional (3-D and 2-D) seismic reflection data, borehole logging, and scaled physical analog modeling, we investigated the geometries, kinematics, and formation mechanisms of the folds within the interior of Qaidam Basin. For the first time, we reveal three local weak layers in the Lulehe, Upper Xiaganchaigou, and Shangyoushashan Formations, respectively. They consist mainly of mudstone intercalated with evaporites and limestones, and they have different spatial distributions that are likely confined by major faults and folds. These mechanically weak layers became locally thickened or thinned in response to tectonic loading and/or facilitated detachment slip to form many décollement folds that were observed at the surface. The shallow deformation above the weak layers is largely decoupled from underlying basement-involved faulting and folding, which mostly terminate upward in these weak layers. Analog modeling results suggest that the lowermost and widely distributed décollement layer in the Lulehe Formation likely facilitated long-distance rapid propagation of deformation into the basin interior. In sum, our study highlights the significance of multiple weak layers during Cenozoic deformation in the Qaidam Basin interior.
{"title":"Impact of multiple weak layers on deformation of the interior of Qaidam Basin, northern Tibetan Plateau","authors":"Huitong Yang, Wei Tan, Lei Wu, Yongshu Zhang, Bo Wang, Junyong Zhang, Xuebing Wei, Pengcheng Tang, Liguang Mao, Chuanwu Wang, Yan Chen, Jianchao Tang, Kai Huang, Ancheng Xiao, Xiubin Lin, Hanlin Chen","doi":"10.1130/b37299.1","DOIUrl":"https://doi.org/10.1130/b37299.1","url":null,"abstract":"The Qaidam Basin is the largest sedimentary basin within the Tibetan Plateau, with up to ∼15-km-thick deposits accumulated in the Cenozoic. Understanding how it deformed in response to the far-field effects of India-Eurasia collision is critical to improving our knowledge of the mechanism underlying northward plateau growth. Unlike typical compressional basins, where upper-crustal deformation concentrates at their margins, the Qaidam Basin features the development of many NW- to WNW-striking folds across the entire basin. Why crustal shortening occurred in the interior of Qaidam Basin, ∼100 km away from the margins, together with the underground geometries beneath these folds, remains unknown. Herein, based on newly acquired three- and two-dimensional (3-D and 2-D) seismic reflection data, borehole logging, and scaled physical analog modeling, we investigated the geometries, kinematics, and formation mechanisms of the folds within the interior of Qaidam Basin. For the first time, we reveal three local weak layers in the Lulehe, Upper Xiaganchaigou, and Shangyoushashan Formations, respectively. They consist mainly of mudstone intercalated with evaporites and limestones, and they have different spatial distributions that are likely confined by major faults and folds. These mechanically weak layers became locally thickened or thinned in response to tectonic loading and/or facilitated detachment slip to form many décollement folds that were observed at the surface. The shallow deformation above the weak layers is largely decoupled from underlying basement-involved faulting and folding, which mostly terminate upward in these weak layers. Analog modeling results suggest that the lowermost and widely distributed décollement layer in the Lulehe Formation likely facilitated long-distance rapid propagation of deformation into the basin interior. In sum, our study highlights the significance of multiple weak layers during Cenozoic deformation in the Qaidam Basin interior.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"2 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140744238","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The provenance of most basin systems today is interpreted based on radiogenic ages or the geochemical composition of detrital minerals, which has all but replaced the use of whole-rock geochemical approaches that can effectively complement provenance information inferred from detrital approaches. Here, we further investigate previous provenance models developed using detrital zircon U-Pb geochronology by applying whole-rock major and trace element geochemistry of fine-grained clastic rocks from the late Oligocene−middle Miocene Tyonek Formation, late Miocene Beluga Formation, and Pliocene Sterling Formation in the Cook Inlet Basin, Alaska, USA. When taken alone, our new geochemical data suggest solely intermediate igneous sediment sources to the basin. When paired with existing detrital zircon U-Pb data, however, significant mixing of felsic and mafic sediment sources is evident, which indicates that thorough mixing of geochemically distinct source terranes can mask the input from individual sources in whole-rock geochemical studies. Furthermore, we demonstrate that both weathering and provenance influence the major element chemistry of sediment source terranes as well as the resultant basinal strata. Our conclusions indicate that the combination of whole-rock geochemistry with other detrital approaches provides a robust interpretation of sedimentary basin provenance.
{"title":"The effects of weathering and sediment source mixing on whole-rock geochemical provenance studies, Cook Inlet forearc basin, south-central Alaska, USA","authors":"A. Kapolas, E. Finzel, L. Horkley, D.W. Peate","doi":"10.1130/b37418.1","DOIUrl":"https://doi.org/10.1130/b37418.1","url":null,"abstract":"The provenance of most basin systems today is interpreted based on radiogenic ages or the geochemical composition of detrital minerals, which has all but replaced the use of whole-rock geochemical approaches that can effectively complement provenance information inferred from detrital approaches. Here, we further investigate previous provenance models developed using detrital zircon U-Pb geochronology by applying whole-rock major and trace element geochemistry of fine-grained clastic rocks from the late Oligocene−middle Miocene Tyonek Formation, late Miocene Beluga Formation, and Pliocene Sterling Formation in the Cook Inlet Basin, Alaska, USA. When taken alone, our new geochemical data suggest solely intermediate igneous sediment sources to the basin. When paired with existing detrital zircon U-Pb data, however, significant mixing of felsic and mafic sediment sources is evident, which indicates that thorough mixing of geochemically distinct source terranes can mask the input from individual sources in whole-rock geochemical studies. Furthermore, we demonstrate that both weathering and provenance influence the major element chemistry of sediment source terranes as well as the resultant basinal strata. Our conclusions indicate that the combination of whole-rock geochemistry with other detrital approaches provides a robust interpretation of sedimentary basin provenance.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"14 16","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140745893","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Michigan Basin is composed of geological formations that contain brines and evaporites, and solutes from these geological sources have affected benthic sediment pore-water chemistry in Saginaw Bay (Lake Huron). We hypothesize that there exists similar potential for upward solute transport directly from the Michigan Basin into other Great Lakes areas. To test our hypothesis, we present here previously unpublished pore-water chemistry analyses from sediment cores collected during multiple Lake Michigan sampling events (spanning 1991−1999) and a new evaluation of previously published data. In several box cores, pore-water chloride concentrations increase with depth, and Cl:Br ratios are consistent with a geological formation brine source. In all gravity cores we collected from southern Lake Michigan, pore-water sodium concentrations increase with sediment depth. At one sample station, pore-water sodium concentrations exceed 2000 mg L−1 within 2 m of the sediment-water interface. Given the pore-water chemistry changes reported here, combined with information from previous studies of Lake Michigan bedrock geology, a Devonian formation brine is a plausible solute source. The presence of saline pore water within glaciolacustrine sediments underlying Lake Michigan indicates that this solute flux has been active during the past 10 k.y. However, the origins of this solute flux, including timing (onset) and contributions from advective and/or diffusive transport, are unknown. The specific geological source and solute transport process are important to resolve in order to evaluate potential effects of these Michigan Basin solute sources on the Great Lakes’ sediment biogeochemistry and water quality.
{"title":"Major ion pore-water chemistry evolution in Lake Michigan benthic sediments: Evidence for direct input from Michigan Basin saline groundwater","authors":"Jonathan J. Kolak, David T. Long","doi":"10.1130/b37143.1","DOIUrl":"https://doi.org/10.1130/b37143.1","url":null,"abstract":"The Michigan Basin is composed of geological formations that contain brines and evaporites, and solutes from these geological sources have affected benthic sediment pore-water chemistry in Saginaw Bay (Lake Huron). We hypothesize that there exists similar potential for upward solute transport directly from the Michigan Basin into other Great Lakes areas. To test our hypothesis, we present here previously unpublished pore-water chemistry analyses from sediment cores collected during multiple Lake Michigan sampling events (spanning 1991−1999) and a new evaluation of previously published data. In several box cores, pore-water chloride concentrations increase with depth, and Cl:Br ratios are consistent with a geological formation brine source. In all gravity cores we collected from southern Lake Michigan, pore-water sodium concentrations increase with sediment depth. At one sample station, pore-water sodium concentrations exceed 2000 mg L−1 within 2 m of the sediment-water interface. Given the pore-water chemistry changes reported here, combined with information from previous studies of Lake Michigan bedrock geology, a Devonian formation brine is a plausible solute source. The presence of saline pore water within glaciolacustrine sediments underlying Lake Michigan indicates that this solute flux has been active during the past 10 k.y. However, the origins of this solute flux, including timing (onset) and contributions from advective and/or diffusive transport, are unknown. The specific geological source and solute transport process are important to resolve in order to evaluate potential effects of these Michigan Basin solute sources on the Great Lakes’ sediment biogeochemistry and water quality.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"2 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140745960","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Felipe R. Ferroni, P. DeCelles, Jaime Oller Veramendi
The Sub-Andean retroarc region is a unique example of an active continental-scale retroarc foreland basin system. Heavily targeted for hydrocarbon exploration, the region hosts a large volume of subsurface data coupled to surface studies dedicated to refining its evolution in time and space. This paper presents a regional correlation of stratigraphic markers from seismic reflection and well logs across the Sub-Andean foothills at 23−21°S in southern Bolivia and northern Argentina, which reveals the contrasting along-strike history of Mesozoic to Cenozoic tectonics that preceded the foreland basin setting. Supported by published geochronological data and new zircon U-Pb maximum depositional ages, we describe the depositional transition from pre-Andean to Andean stratigraphy and discrete episodes of foreland basin subsidence and shortening. Based on interpreted stratigraphic breaks, we define the extent and stepwise evolution of this foreland basin, which was characterized by the progressive eastward migration of foreland basin depozones. Based on restored thickness profiles, we present flexural models of basin subsidence for the Sub-Andean foothills region. The modeling of discrete episodes of foreland basin subsidence refines the widely accepted bimodal elastic strength in the foreland basin at 21−23°S, which is weaker in the western ranges (∼20 km effective elastic thickness) and stronger eastward (>40 km). Modeling results also reveal minimum values of subsidence rates (up to 1.2 mm/yr) in the sequential foredeep depozones and suggest that the modeled tectonic load migration—as constrained by the record of syntectonic strata—probably increased over time through the incorporation of Sub-Andean rocks into the orogenic wedge.
{"title":"Neogene to modern foreland basin development in the Sub-Andean zone of southern Bolivia and northern Argentina, 21−23°S","authors":"Felipe R. Ferroni, P. DeCelles, Jaime Oller Veramendi","doi":"10.1130/b37206.1","DOIUrl":"https://doi.org/10.1130/b37206.1","url":null,"abstract":"The Sub-Andean retroarc region is a unique example of an active continental-scale retroarc foreland basin system. Heavily targeted for hydrocarbon exploration, the region hosts a large volume of subsurface data coupled to surface studies dedicated to refining its evolution in time and space. This paper presents a regional correlation of stratigraphic markers from seismic reflection and well logs across the Sub-Andean foothills at 23−21°S in southern Bolivia and northern Argentina, which reveals the contrasting along-strike history of Mesozoic to Cenozoic tectonics that preceded the foreland basin setting. Supported by published geochronological data and new zircon U-Pb maximum depositional ages, we describe the depositional transition from pre-Andean to Andean stratigraphy and discrete episodes of foreland basin subsidence and shortening. Based on interpreted stratigraphic breaks, we define the extent and stepwise evolution of this foreland basin, which was characterized by the progressive eastward migration of foreland basin depozones. Based on restored thickness profiles, we present flexural models of basin subsidence for the Sub-Andean foothills region. The modeling of discrete episodes of foreland basin subsidence refines the widely accepted bimodal elastic strength in the foreland basin at 21−23°S, which is weaker in the western ranges (∼20 km effective elastic thickness) and stronger eastward (>40 km). Modeling results also reveal minimum values of subsidence rates (up to 1.2 mm/yr) in the sequential foredeep depozones and suggest that the modeled tectonic load migration—as constrained by the record of syntectonic strata—probably increased over time through the incorporation of Sub-Andean rocks into the orogenic wedge.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"55 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140742048","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Although subduction is a continuous process, arc system behavior is non-steady-state, leading to uncertainty surrounding the composite spatial and temporal evolution of transcrustal arc magma plumbing systems. This study integrates field, geochronologic, and geochemical data sets from the central Sierra Nevada arc section to investigate the extent to which spatial inheritance is recorded in arc geochemical compositions, and how these signals may be modified by dynamic arc behaviors through time, from arc-wide flare-ups, migration, and crustal thickening to regional magma focusing. Geochemical patterns across Mesozoic arc rocks characterize persistent spatial signals of inheritance, whereas geochemical trends during Cretaceous arc activity provide the temporal component of simultaneous dynamic processes. Distinct bulk-rock isotopic signals define each of the three Mesozoic magmatic flare-ups, which, during Cretaceous arc magmatism, is coupled with eastward arc migration. Additionally, Cretaceous magmatic and tectonic thickening doubled the thickness of arc crust, and magmatism was focused toward a central zone, culminating in the formation of the ∼1100 km2 Tuolumne Intrusive Complex. During magma focusing, temporal signals of magma mixing outweighed the previously pervasive signal of spatial inheritance. Distinct dynamic behaviors effectively primed the arc by the Late Cretaceous, generating transcrustal hot zones of increased magma mixing, recycling, long-term storage, and homogenization. Non-steady-state behavior in the Sierra Nevada resulted in mountain building and voluminous continental crust formation by transforming the physical, thermal, and chemical properties of the lithosphere over tens of millions of years.
{"title":"Fingerprinting the geochemical signals of episodic arc activity in the Sierra Nevada batholith in space and time","authors":"K. Ardill, Snir Attia, V. Memeti, S. Paterson","doi":"10.1130/b37266.1","DOIUrl":"https://doi.org/10.1130/b37266.1","url":null,"abstract":"Although subduction is a continuous process, arc system behavior is non-steady-state, leading to uncertainty surrounding the composite spatial and temporal evolution of transcrustal arc magma plumbing systems. This study integrates field, geochronologic, and geochemical data sets from the central Sierra Nevada arc section to investigate the extent to which spatial inheritance is recorded in arc geochemical compositions, and how these signals may be modified by dynamic arc behaviors through time, from arc-wide flare-ups, migration, and crustal thickening to regional magma focusing. Geochemical patterns across Mesozoic arc rocks characterize persistent spatial signals of inheritance, whereas geochemical trends during Cretaceous arc activity provide the temporal component of simultaneous dynamic processes. Distinct bulk-rock isotopic signals define each of the three Mesozoic magmatic flare-ups, which, during Cretaceous arc magmatism, is coupled with eastward arc migration. Additionally, Cretaceous magmatic and tectonic thickening doubled the thickness of arc crust, and magmatism was focused toward a central zone, culminating in the formation of the ∼1100 km2 Tuolumne Intrusive Complex. During magma focusing, temporal signals of magma mixing outweighed the previously pervasive signal of spatial inheritance. Distinct dynamic behaviors effectively primed the arc by the Late Cretaceous, generating transcrustal hot zones of increased magma mixing, recycling, long-term storage, and homogenization. Non-steady-state behavior in the Sierra Nevada resulted in mountain building and voluminous continental crust formation by transforming the physical, thermal, and chemical properties of the lithosphere over tens of millions of years.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"49 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140749626","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Guangyou Zhu, Kun Zhao, W. Ding, Ruimin Wang, Haoran Ma, X. Lang, Tingting Li, Chao Li, Bing Shen
The Cryogenian (ca. 717−635 Ma) snowball Earth glaciations ended with the precipitation of “cap” carbonate successions with negative carbon isotope (δ13Ccarb) values, which have been explained by the addition of various 13C-depleted carbon sources in the deglacial process. These arguments assumed that marine dissolved inorganic carbon (DIC) was enriched in 13C in the synglacial ocean. However, this assumption has not yet been tested, because the synglacial ocean chemistry is unknown. In this study, we carried out detailed analyses of the petrology, carbonate carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes, organic carbon (δ13Corg) isotopes, major and minor elemental compositions (Ca, Mg, Mn, Fe, Sr), and iron speciation (total Fe, highly reactive Fe, pyrite Fe) of the carbonate layers (also called synglacial carbonate layers) from the Nantuo Formation (ca. 650−635 Ma) on the Yangtze block, South China. Petrographic observations indicated that the synglacial carbonate comprises dolomicrite, mud-crystal powder dolomite, lime dolomite, and dolomitic limestone, supporting an authigenic carbonate origin, and thus, it potentially recorded the ocean chemistry during the Marinoan ice age. The synglacial carbonate is characterized by extreme Mn enrichment, low Fe/Mn ratios, and low δ13Ccarb (−7‰) values. High Mn contents and low Fe/Mn ratios imply marine redox conditions favoring Mn2+ accumulation and Fe2+ oxidation, while low δ13Ccarb values might be attributed to CO2 degassing of submarine volcanoes as well as low primary burial during the glaciation. Since the δ13CDIC value of the synglacial ocean was lower than the δ13Ccarb values of most cap carbonates, we infer the addition of 13C-enriched DIC or removal of 12C during cap carbonate precipitation, such as through carbonate weathering or organic carbon burial. These findings provide new insights into the nature of Cryogenian glaciation, the origin of cap carbonates, and the aftermath of global glaciation.
{"title":"Synglacial carbonate records of snowball Earth ocean composition—Evidence from the Nantuo Formation, South China","authors":"Guangyou Zhu, Kun Zhao, W. Ding, Ruimin Wang, Haoran Ma, X. Lang, Tingting Li, Chao Li, Bing Shen","doi":"10.1130/b37227.1","DOIUrl":"https://doi.org/10.1130/b37227.1","url":null,"abstract":"The Cryogenian (ca. 717−635 Ma) snowball Earth glaciations ended with the precipitation of “cap” carbonate successions with negative carbon isotope (δ13Ccarb) values, which have been explained by the addition of various 13C-depleted carbon sources in the deglacial process. These arguments assumed that marine dissolved inorganic carbon (DIC) was enriched in 13C in the synglacial ocean. However, this assumption has not yet been tested, because the synglacial ocean chemistry is unknown. In this study, we carried out detailed analyses of the petrology, carbonate carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes, organic carbon (δ13Corg) isotopes, major and minor elemental compositions (Ca, Mg, Mn, Fe, Sr), and iron speciation (total Fe, highly reactive Fe, pyrite Fe) of the carbonate layers (also called synglacial carbonate layers) from the Nantuo Formation (ca. 650−635 Ma) on the Yangtze block, South China. Petrographic observations indicated that the synglacial carbonate comprises dolomicrite, mud-crystal powder dolomite, lime dolomite, and dolomitic limestone, supporting an authigenic carbonate origin, and thus, it potentially recorded the ocean chemistry during the Marinoan ice age. The synglacial carbonate is characterized by extreme Mn enrichment, low Fe/Mn ratios, and low δ13Ccarb (−7‰) values. High Mn contents and low Fe/Mn ratios imply marine redox conditions favoring Mn2+ accumulation and Fe2+ oxidation, while low δ13Ccarb values might be attributed to CO2 degassing of submarine volcanoes as well as low primary burial during the glaciation. Since the δ13CDIC value of the synglacial ocean was lower than the δ13Ccarb values of most cap carbonates, we infer the addition of 13C-enriched DIC or removal of 12C during cap carbonate precipitation, such as through carbonate weathering or organic carbon burial. These findings provide new insights into the nature of Cryogenian glaciation, the origin of cap carbonates, and the aftermath of global glaciation.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"82 8","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140077306","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Bo‐Chuan Zhang, Jian-Jun Fan, An‐Bo Luo, Si-Lin Sun, Jia-Wei Bai
Subduction erosion is crucial in crustal material recycling. However, subduction erosion caused by intra-oceanic arc subduction has not been sufficiently investigated. In this study, we provide new geological, geochronological, geochemical, and isotopic data from Dongco granodiorites in the central Bangong−Nujiang suture zone of central Tibet to explore subduction erosion caused by intra-oceanic arc subduction. Analysis shows that the ca. 158−155 Ma Dongco granodiorites originated from the subducted oceanic plate, and they were contaminated with accretionary wedge when they intruded the Dongco ophiolite. This suggests that the Dongco ophiolite was emplaced in the subducted accretionary wedge before the Late Jurassic. Based on the intra-oceanic arc affinity and lack of volcanic rocks of Middle Jurassic Dongco ophiolite and other regional data, we believe that the main body of the central intra-oceanic arcs and a portion of the western intra-oceanic arcs in the Meso-Tethys Ocean subducted beneath the southern Qiangtang terrane during the Middle−Late Jurassic. In addition, the different degree absence of the Jurassic accretionary wedge, forearc region, and arc magmatic rocks in the southern Qiangtang terrane indicate that the central and western parts of the southern Qiangtang terrane experienced both vigorous and relatively weak subduction erosion during the Middle−Late Jurassic, respectively. Thus, there is a significant spatiotemporal coupling between subduction erosion of the southern Qiangtang terrane and intra-oceanic arc subduction. Based on these studies and the research on subduction erosion, we suggest that subduction of the main body of the central intra-oceanic arcs and partial subduction of the western intra-oceanic arcs in the Meso-Tethys Ocean caused both vigorous and relatively weak subduction erosion of the southern Qiangtang terrane during the Middle−Late Jurassic, respectively. In addition, the increase in subduction rate also promoted Middle−Late Jurassic subduction erosion of the southern Qiangtang terrane.
{"title":"Middle−Late Jurassic subduction erosion caused by intra-oceanic arc subduction in central Tibet","authors":"Bo‐Chuan Zhang, Jian-Jun Fan, An‐Bo Luo, Si-Lin Sun, Jia-Wei Bai","doi":"10.1130/b37149.1","DOIUrl":"https://doi.org/10.1130/b37149.1","url":null,"abstract":"Subduction erosion is crucial in crustal material recycling. However, subduction erosion caused by intra-oceanic arc subduction has not been sufficiently investigated. In this study, we provide new geological, geochronological, geochemical, and isotopic data from Dongco granodiorites in the central Bangong−Nujiang suture zone of central Tibet to explore subduction erosion caused by intra-oceanic arc subduction. Analysis shows that the ca. 158−155 Ma Dongco granodiorites originated from the subducted oceanic plate, and they were contaminated with accretionary wedge when they intruded the Dongco ophiolite. This suggests that the Dongco ophiolite was emplaced in the subducted accretionary wedge before the Late Jurassic. Based on the intra-oceanic arc affinity and lack of volcanic rocks of Middle Jurassic Dongco ophiolite and other regional data, we believe that the main body of the central intra-oceanic arcs and a portion of the western intra-oceanic arcs in the Meso-Tethys Ocean subducted beneath the southern Qiangtang terrane during the Middle−Late Jurassic. In addition, the different degree absence of the Jurassic accretionary wedge, forearc region, and arc magmatic rocks in the southern Qiangtang terrane indicate that the central and western parts of the southern Qiangtang terrane experienced both vigorous and relatively weak subduction erosion during the Middle−Late Jurassic, respectively. Thus, there is a significant spatiotemporal coupling between subduction erosion of the southern Qiangtang terrane and intra-oceanic arc subduction. Based on these studies and the research on subduction erosion, we suggest that subduction of the main body of the central intra-oceanic arcs and partial subduction of the western intra-oceanic arcs in the Meso-Tethys Ocean caused both vigorous and relatively weak subduction erosion of the southern Qiangtang terrane during the Middle−Late Jurassic, respectively. In addition, the increase in subduction rate also promoted Middle−Late Jurassic subduction erosion of the southern Qiangtang terrane.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"140 29","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140078337","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. D. Buryak, Alberto V. Reyes, Christopher K. West, Britta J.L. Jensen, S. A. Dufrane, Joshua H. F. L. Davies, Yan Luo, Jennifer M. Galloway, P. Siver, J. Westgate, J. Reimink, D. G. Pearson, Alexander P. Wolfe
The Wombat and Giraffe kimberlite pipes in the Lac de Gras kimberlite field (64°N, 110°W) of the Northwest Territories, Canada, preserve unique post-eruptive lacustrine and paludal sedimentary records that offer rare insight into high-latitude continental paleoclimate. However, depositional timing—a key datum for atmospheric CO2 and paleoclimatic proxy reconstructions—of these maar infills remains ambiguous and requires refinement because of the large range in the age of kimberlites within the Lac de Gras kimberlite field. Existing constraints for the Giraffe pipe post-eruptive lacustrine and paludal maar sedimentary facies include a maximum Rb-Sr age of ca. 48 Ma (Ypresian, Eocene) based on kimberlitic phlogopite and a glass fission-track age of ca. 38 Ma (Bartonian, Eocene). The age of the Wombat pipe lacustrine maar sediments remains unclear, with unpublished pollen-based biostratigraphy suggesting deposition in the Paleocene (66−56 Ma). In this study, we examine distal rhyolitic tephra beds recovered from exploration drill cores intersecting the Wombat and Giraffe maar facies. We integrate zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) geochronology, glass fission-track dating, palynology, and tephra glass geochemistry to refine chronological frameworks for these sedimentary deposits. The Giraffe maar CA-ID-TIMS tephra zircon U-Pb dating yielded a Bayesian model age of 47.995 ± 0.082|0.087 Ma (Ypresian) for the upper portion of the lacustrine sediments, while a single zircon grain from tephra in the lowermost lacustrine sediments had an age of 48.72 ± 0.29|0.30 Ma. The revised geochronology for the Giraffe maar provides a working age model for the ∼50 m record of lacustrine silt and indicates an age ∼10 m.y. older than previously thought. The Wombat maar LA-ICP-MS zircon U-Pb dating yielded an age of 80.9 ± 1.0 Ma (Campanian), which indicates deposition during the Late Cretaceous. This first radiometric age for the Wombat maar deposits is substantially older than earlier biostratigraphic inferences of a Paleocene age. This new age suggests that the Wombat maar sediments preserve evidence of some of the oldest known freshwater diatoms and synurophytes and provide key constraints for the paleogeography of the Western Interior Seaway during the Late Cretaceous.
{"title":"Tephra zircon U-Pb geochronology of kimberlite maar sedimentary fills in subarctic Canada: Implications for Eocene paleoclimate and Late Cretaceous paleogeography","authors":"S. D. Buryak, Alberto V. Reyes, Christopher K. West, Britta J.L. Jensen, S. A. Dufrane, Joshua H. F. L. Davies, Yan Luo, Jennifer M. Galloway, P. Siver, J. Westgate, J. Reimink, D. G. Pearson, Alexander P. Wolfe","doi":"10.1130/b37276.1","DOIUrl":"https://doi.org/10.1130/b37276.1","url":null,"abstract":"The Wombat and Giraffe kimberlite pipes in the Lac de Gras kimberlite field (64°N, 110°W) of the Northwest Territories, Canada, preserve unique post-eruptive lacustrine and paludal sedimentary records that offer rare insight into high-latitude continental paleoclimate. However, depositional timing—a key datum for atmospheric CO2 and paleoclimatic proxy reconstructions—of these maar infills remains ambiguous and requires refinement because of the large range in the age of kimberlites within the Lac de Gras kimberlite field. Existing constraints for the Giraffe pipe post-eruptive lacustrine and paludal maar sedimentary facies include a maximum Rb-Sr age of ca. 48 Ma (Ypresian, Eocene) based on kimberlitic phlogopite and a glass fission-track age of ca. 38 Ma (Bartonian, Eocene). The age of the Wombat pipe lacustrine maar sediments remains unclear, with unpublished pollen-based biostratigraphy suggesting deposition in the Paleocene (66−56 Ma). In this study, we examine distal rhyolitic tephra beds recovered from exploration drill cores intersecting the Wombat and Giraffe maar facies. We integrate zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) geochronology, glass fission-track dating, palynology, and tephra glass geochemistry to refine chronological frameworks for these sedimentary deposits. The Giraffe maar CA-ID-TIMS tephra zircon U-Pb dating yielded a Bayesian model age of 47.995 ± 0.082|0.087 Ma (Ypresian) for the upper portion of the lacustrine sediments, while a single zircon grain from tephra in the lowermost lacustrine sediments had an age of 48.72 ± 0.29|0.30 Ma. The revised geochronology for the Giraffe maar provides a working age model for the ∼50 m record of lacustrine silt and indicates an age ∼10 m.y. older than previously thought. The Wombat maar LA-ICP-MS zircon U-Pb dating yielded an age of 80.9 ± 1.0 Ma (Campanian), which indicates deposition during the Late Cretaceous. This first radiometric age for the Wombat maar deposits is substantially older than earlier biostratigraphic inferences of a Paleocene age. This new age suggests that the Wombat maar sediments preserve evidence of some of the oldest known freshwater diatoms and synurophytes and provide key constraints for the paleogeography of the Western Interior Seaway during the Late Cretaceous.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"6 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140083211","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ying‐De Jiang, Tan Shu, I. Soejono, R. Nádaskay, Karel Schulmann, Jun Ning, Jian Zhang, Lingzhu Kong
Sedimentological and geochronological data from late Paleozoic strata located between the East Junggar and Chinese Altai regions in NW China were examined, aiming to decipher the tectono-sedimentary evolution of this important tectonic boundary. Carboniferous sediments on the East Junggar side show arc-proximal depositional characteristics of the proximal Heishantou and Nanmingshui Formations and distal Beitashan and Yundukala Formations, while the Erqis complex on the Chinese Altai side is characterized by continental margin affinity. Lithological analysis revealed the dominant input of arc-related detritus for all these sequences and a uniform transition from volcaniclastic to siliciclastic components in their respective upper sections. The investigated East Junggar strata are dominated by Carboniferous zircons with positive εHf(t) values, sourced exclusively from the southerly Yemaquan-Jiangjunmiao arc domain, whereas the Erqis complex received detritus from the same arc domain but also evolved components from the northerly Chinese Altai. Combined with regional data, the examined strata are interpreted to have developed in a back-arc basin with regard to an arc that developed above the north-dipping Kalamaili subduction system. In contrast, the unmetamorphosed Lower Permian Tesibahan Formation, unconformably overlying the Erqis complex, received detritus mainly from the Chinese Altai. These sediments were deposited in an intracontinental piggyback or synformal basin following closure of the back-arc basin. The late Paleozoic sedimentation records support the interpretation that the Chinese Altai and East Junggar domains evolved from the same suprasubduction system prior to the Carboniferous rather than as independent terranes mutually juxtaposed during Permian lateral translation, as previously proposed.
{"title":"Late Paleozoic sedimentation recording back-arc basin evolution in response to Chinese Altai−East Junggar convergence in Central Asia","authors":"Ying‐De Jiang, Tan Shu, I. Soejono, R. Nádaskay, Karel Schulmann, Jun Ning, Jian Zhang, Lingzhu Kong","doi":"10.1130/b37247.1","DOIUrl":"https://doi.org/10.1130/b37247.1","url":null,"abstract":"Sedimentological and geochronological data from late Paleozoic strata located between the East Junggar and Chinese Altai regions in NW China were examined, aiming to decipher the tectono-sedimentary evolution of this important tectonic boundary. Carboniferous sediments on the East Junggar side show arc-proximal depositional characteristics of the proximal Heishantou and Nanmingshui Formations and distal Beitashan and Yundukala Formations, while the Erqis complex on the Chinese Altai side is characterized by continental margin affinity. Lithological analysis revealed the dominant input of arc-related detritus for all these sequences and a uniform transition from volcaniclastic to siliciclastic components in their respective upper sections. The investigated East Junggar strata are dominated by Carboniferous zircons with positive εHf(t) values, sourced exclusively from the southerly Yemaquan-Jiangjunmiao arc domain, whereas the Erqis complex received detritus from the same arc domain but also evolved components from the northerly Chinese Altai. Combined with regional data, the examined strata are interpreted to have developed in a back-arc basin with regard to an arc that developed above the north-dipping Kalamaili subduction system. In contrast, the unmetamorphosed Lower Permian Tesibahan Formation, unconformably overlying the Erqis complex, received detritus mainly from the Chinese Altai. These sediments were deposited in an intracontinental piggyback or synformal basin following closure of the back-arc basin. The late Paleozoic sedimentation records support the interpretation that the Chinese Altai and East Junggar domains evolved from the same suprasubduction system prior to the Carboniferous rather than as independent terranes mutually juxtaposed during Permian lateral translation, as previously proposed.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"114 17","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140090880","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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