Ash deposits from short-lived vulcanian eruptions are ubiquitous at active volcanoes and are important for understanding the development of a volcanic edifice. In this study, we investigated the sedimentary system of ash deposits produced by long-term vulcanian activity from Showa crater of Sakurajima volcano, Japan, based on the grain size characteristics of ash from individual eruptions. We collected falling ash samples from 20 individual vulcanian eruptions between March 3 and 11, 2010. The grain size distributions of the ash samples were unimodal, bimodal, and trimodal, depending on the event. The median particle size of the main subpopulation decreases with distance from vent. The low standard deviation of the grain size of the main subpopulation implies deposition from transient volcanic plumes. For the finest subpopulation in the polymodal samples, the median grain size is very fine ash (<30 μm), indicating that this subpopulation was derived from ash aggregates. Overall, the sedimentary system of the long-term vulcanian activity at Sakurajima volcano is explained by accumulation of ash particles from repeated transient plumes and ash aggregation, and the latter controlled the behavior of the very fine ash.
{"title":"Sedimentary System of Ash Deposits from Long-Term Vulcanian Activity at Sakurajima Volcano, Japan","authors":"T. Miwa, F. Nanayama","doi":"10.1086/714175","DOIUrl":"https://doi.org/10.1086/714175","url":null,"abstract":"Ash deposits from short-lived vulcanian eruptions are ubiquitous at active volcanoes and are important for understanding the development of a volcanic edifice. In this study, we investigated the sedimentary system of ash deposits produced by long-term vulcanian activity from Showa crater of Sakurajima volcano, Japan, based on the grain size characteristics of ash from individual eruptions. We collected falling ash samples from 20 individual vulcanian eruptions between March 3 and 11, 2010. The grain size distributions of the ash samples were unimodal, bimodal, and trimodal, depending on the event. The median particle size of the main subpopulation decreases with distance from vent. The low standard deviation of the grain size of the main subpopulation implies deposition from transient volcanic plumes. For the finest subpopulation in the polymodal samples, the median grain size is very fine ash (<30 μm), indicating that this subpopulation was derived from ash aggregates. Overall, the sedimentary system of the long-term vulcanian activity at Sakurajima volcano is explained by accumulation of ash particles from repeated transient plumes and ash aggregation, and the latter controlled the behavior of the very fine ash.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"171 - 182"},"PeriodicalIF":1.8,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/714175","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49100240","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Malaviarachchi, M. Satish-Kumar, Toshiro Takahashi
The Vijayan Complex (VC) of Sri Lanka is a Neoproterozoic arc terrain having no convincingly proven counterpart elsewhere in the Gondwana supercontinent. Therefore, little is known about the nature of its pre-Gondwanan ancestry. We present here new elemental and Sr-Nd isotope systematics of orthogneisses to envisage the origin and evolution of the VC. Samples with high and variable Sr-initials and negative εNd > +4 are compatible with their derivation by melting of older rocks with a long crustal residence history. Most of the studied rocks show depleted-mantle-model ages (1.4–3.3 Ga) considerably older than the ∼1 Ga magmatic crystallization age of the VC. The rest of the samples show model ages of ∼0.9–1.1 Ga, coeval with the crystallization age of the VC, indicating separation from the mantle as juvenile inputs. Therefore, protolith sources of the majority of these rocks are mixtures of distinctive components formed much earlier than the time of predominant magmatism (∼1 Ga) in the VC. Major and trace elements and rare earth element ratios divide these samples into two categories. The “Depleted Group” represents protoliths similar to Proterozoic oceanic island basalt–like mantle materials that intensively interacted/mixed with different proportions of melts derived from juvenile and ancient lower crust. The “Enriched Group” represents mid-ocean ridge basalt–like mantle materials subsequently interacted/mixed with variable amounts of melts derived from Archean felsic and mafic crust. Geochemical-mixing modeling with 87Sr/86Sr and 143Nd/144Nd confirms the involvement of above multiple source components. Thus, our findings illustrate evidence for hybridization of mantle-derived melts from various juvenile and ancient crustal components. Therefore, this first finding of Paleoproterozoic to early Archean–aged components within the VC provides new insights to elucidate its pre-Gondwanan geodynamic setting, revising the previous understanding of it as an “exotic terrain” to Gondwana with only largely juvenile Neoproterozoic materials.
{"title":"New Sr-Nd Isotope Data Record Juvenile and Ancient Crust-Mantle Melt Interactions in the Vijayan Complex, Sri Lanka","authors":"S. Malaviarachchi, M. Satish-Kumar, Toshiro Takahashi","doi":"10.1086/714172","DOIUrl":"https://doi.org/10.1086/714172","url":null,"abstract":"The Vijayan Complex (VC) of Sri Lanka is a Neoproterozoic arc terrain having no convincingly proven counterpart elsewhere in the Gondwana supercontinent. Therefore, little is known about the nature of its pre-Gondwanan ancestry. We present here new elemental and Sr-Nd isotope systematics of orthogneisses to envisage the origin and evolution of the VC. Samples with high and variable Sr-initials and negative εNd > +4 are compatible with their derivation by melting of older rocks with a long crustal residence history. Most of the studied rocks show depleted-mantle-model ages (1.4–3.3 Ga) considerably older than the ∼1 Ga magmatic crystallization age of the VC. The rest of the samples show model ages of ∼0.9–1.1 Ga, coeval with the crystallization age of the VC, indicating separation from the mantle as juvenile inputs. Therefore, protolith sources of the majority of these rocks are mixtures of distinctive components formed much earlier than the time of predominant magmatism (∼1 Ga) in the VC. Major and trace elements and rare earth element ratios divide these samples into two categories. The “Depleted Group” represents protoliths similar to Proterozoic oceanic island basalt–like mantle materials that intensively interacted/mixed with different proportions of melts derived from juvenile and ancient lower crust. The “Enriched Group” represents mid-ocean ridge basalt–like mantle materials subsequently interacted/mixed with variable amounts of melts derived from Archean felsic and mafic crust. Geochemical-mixing modeling with 87Sr/86Sr and 143Nd/144Nd confirms the involvement of above multiple source components. Thus, our findings illustrate evidence for hybridization of mantle-derived melts from various juvenile and ancient crustal components. Therefore, this first finding of Paleoproterozoic to early Archean–aged components within the VC provides new insights to elucidate its pre-Gondwanan geodynamic setting, revising the previous understanding of it as an “exotic terrain” to Gondwana with only largely juvenile Neoproterozoic materials.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"233 - 253"},"PeriodicalIF":1.8,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/714172","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42020852","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yongsheng Wang, Qiao Bai, Ziqiang Tian, Huirong Du
The Hong’an-Dabie-Sulu orogen in China exposes the largest area of ultrahigh-pressure (UHP) metamorphic rocks in the world. Postcollisional large-scale extension and denudation resulted in the removal of large volumes of high-pressure (HP) and UHP rocks from the Dabie orogen, making it difficult to reconstruct the original distribution of these rocks and analyze the mechanism of exhumation. The Early Jurassic to Cretaceous Hefei Basin formed in response to uplift of the Dabie orogen, and we used detrital zircon age dating to assess the provenance of its sediments. The Lower Jurassic Fanghushan Formation, the oldest rocks in the Hefei Basin, shows a more complex provenance, with source materials from the HP-UHP, Susong Complex, and early Paleozoic arc-magmatic rocks. The Middle Jurassic Sanjianpu Formation shows age clusters mainly in the Triassic and middle Neoproterozoic. By comparing the detrital zircon age frequencies in basins around the orogen, we found that the rocks widely exposed in the Dabie orogen were the Susong Complex in the Early Jurassic; the HP-UHP rocks might have been only locally exposed along the northern margin of the orogen, and extensive outcrops may have occurred after the Middle Jurassic. Rapid uplift occurred at ca. 128 Ma, which caused the HP-UHP rocks to be denuded in the northern Dabie orogen and contributed detrital material to the conglomerates of the Lower Cretaceous Fenghuangtai Formation. Controlled by intense normal movements on a series of faults in the Hefei Basin and northern Dabie orogen, the Hefei Basin extends southward to the north of the Xiaotian-Mozitan Shear Zone.
{"title":"Mesozoic Unroofing History of the Dabie Orogen, Eastern China: Evidence from Detrital Zircon Geochronology of Sediments in the Hefei Basin","authors":"Yongsheng Wang, Qiao Bai, Ziqiang Tian, Huirong Du","doi":"10.1086/714176","DOIUrl":"https://doi.org/10.1086/714176","url":null,"abstract":"The Hong’an-Dabie-Sulu orogen in China exposes the largest area of ultrahigh-pressure (UHP) metamorphic rocks in the world. Postcollisional large-scale extension and denudation resulted in the removal of large volumes of high-pressure (HP) and UHP rocks from the Dabie orogen, making it difficult to reconstruct the original distribution of these rocks and analyze the mechanism of exhumation. The Early Jurassic to Cretaceous Hefei Basin formed in response to uplift of the Dabie orogen, and we used detrital zircon age dating to assess the provenance of its sediments. The Lower Jurassic Fanghushan Formation, the oldest rocks in the Hefei Basin, shows a more complex provenance, with source materials from the HP-UHP, Susong Complex, and early Paleozoic arc-magmatic rocks. The Middle Jurassic Sanjianpu Formation shows age clusters mainly in the Triassic and middle Neoproterozoic. By comparing the detrital zircon age frequencies in basins around the orogen, we found that the rocks widely exposed in the Dabie orogen were the Susong Complex in the Early Jurassic; the HP-UHP rocks might have been only locally exposed along the northern margin of the orogen, and extensive outcrops may have occurred after the Middle Jurassic. Rapid uplift occurred at ca. 128 Ma, which caused the HP-UHP rocks to be denuded in the northern Dabie orogen and contributed detrital material to the conglomerates of the Lower Cretaceous Fenghuangtai Formation. Controlled by intense normal movements on a series of faults in the Hefei Basin and northern Dabie orogen, the Hefei Basin extends southward to the north of the Xiaotian-Mozitan Shear Zone.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"183 - 206"},"PeriodicalIF":1.8,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/714176","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42506382","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The ca. 280 Ma, postorogenic, S-type Dartmoor pluton was assembled from numerous sheets of granitic magma, emplaced into the shallow crust. The main magma source lies in the middle crust and is most probably Proterozoic metagreywackes, with minor metapelites and metavolcanic or plutonic rocks, possibly formed in a syncollisional environment. Partial melting of this source may have occurred under fluid-deficient conditions, and the magmas most likely had relatively high initial H2O contents. The pluton contains substantial, whole-rock-Sr and quartz-O isotope heterogeneities on scales down to a meter or less, and such small-scale heterogeneities are probably common in granitic intrusions derived from heterogeneous protoliths. Thus, variations in source terranes may not be fully captured with the sample numbers and scales commonly applied in studies of granitic plutons. The preservation of both large- and small-scale isotopic heterogeneities suggests that the Dartmoor magmas were never efficiently homogenized by flow-driven mechanical mixing. This implies a source terrane with lithological variations on scales of tens of meters or less. The granitic rocks form five texturally, chemically, and isotopically distinct groups, each of which had somewhat different sources or mixtures of sources. The main chemical variations cannot have been formed through fractionation of any combination of the major minerals in the rocks. Instead, entrainment of variable proportions of peritectic plagioclase, orthopyroxene, and ilmenite was responsible, together with local crystal fractionation. Low-density, late-magmatic melts and aqueous fluids produced patchy enrichment in light elements and extreme enrichment in some of the highly silicic, two-mica microgranites. However, although they are also enriched in light elements, the “aplites” were not produced through fractionation and seem to have had independent magmatic origins.
{"title":"Origins and Scales of Compositional Variations in Crustally Derived Granitic Rocks: The Example of the Dartmoor Pluton in the Cornubian Batholith of Southwest Britain","authors":"J. Clemens, P. Helps, G. Stevens, N. Petford","doi":"10.1086/714174","DOIUrl":"https://doi.org/10.1086/714174","url":null,"abstract":"The ca. 280 Ma, postorogenic, S-type Dartmoor pluton was assembled from numerous sheets of granitic magma, emplaced into the shallow crust. The main magma source lies in the middle crust and is most probably Proterozoic metagreywackes, with minor metapelites and metavolcanic or plutonic rocks, possibly formed in a syncollisional environment. Partial melting of this source may have occurred under fluid-deficient conditions, and the magmas most likely had relatively high initial H2O contents. The pluton contains substantial, whole-rock-Sr and quartz-O isotope heterogeneities on scales down to a meter or less, and such small-scale heterogeneities are probably common in granitic intrusions derived from heterogeneous protoliths. Thus, variations in source terranes may not be fully captured with the sample numbers and scales commonly applied in studies of granitic plutons. The preservation of both large- and small-scale isotopic heterogeneities suggests that the Dartmoor magmas were never efficiently homogenized by flow-driven mechanical mixing. This implies a source terrane with lithological variations on scales of tens of meters or less. The granitic rocks form five texturally, chemically, and isotopically distinct groups, each of which had somewhat different sources or mixtures of sources. The main chemical variations cannot have been formed through fractionation of any combination of the major minerals in the rocks. Instead, entrainment of variable proportions of peritectic plagioclase, orthopyroxene, and ilmenite was responsible, together with local crystal fractionation. Low-density, late-magmatic melts and aqueous fluids produced patchy enrichment in light elements and extreme enrichment in some of the highly silicic, two-mica microgranites. However, although they are also enriched in light elements, the “aplites” were not produced through fractionation and seem to have had independent magmatic origins.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"131 - 169"},"PeriodicalIF":1.8,"publicationDate":"2021-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/714174","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"60722398","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
F. Pazzaglia, H. Malenda, Matthew L. McGavick, Cody Raup, M. Carter, C. Berti, S. Mahan, M. Nelson, T. Rittenour, R. Counts, J. Willenbring, D. Germanoski, S. C. Peters, W. D. Holt
We show that long-recognized seismicity in the central Virginia seismic zone of the eastern North American intraplate setting arises primarily from tectonic processes predicted by new, fully coupled plate tectonic geodynamic models. The study leverages much new geophysical and geologic data following the 2011 Mineral, Virginia, earthquake that ruptured a steeply dipping, northwest-verging reverse fault traversed by the South Anna River. The data are primarily assembled from a flight of six fluvial terrace geomorphic markers identified and correlated on texture, relative weathering, and numeric ages including one terrestrial cosmogenic nuclide (TCN) profile and 30 luminescence dates. Terrace thickness, stratigraphic age models, and incision rates downstream and upstream of the 2011 rupture are different. Long-term river incision rates of ∼25–30 m/My are superimposed on regional TCN-determined erosion rates of ∼8.5 m/My; however, there are at least 10 m of tectonically driven incision in the epicentral region at rates of ∼30–94 m/My. The inferred deformation resembles a hanging wall anticline above a blind reverse fault with a diffuse overlying carapace of minor brittle faults, an interpretation supported by seismology as well as bedrock and saprolite mapped across the epicentral region. These results are further supported by channel metrics that show nonuniform channel steepness (ksn) and a predicted steady-state channel elevation different from the actual channel elevation across the epicentral region. If all of the observed deformation is a consequence of the fault that ruptured in 2011, the recurrence interval of Mineral-sized events would be ∼5.5 ky.
{"title":"River Terrace Evidence of Tectonic Processes in the Eastern North American Plate Interior, South Anna River, Virginia","authors":"F. Pazzaglia, H. Malenda, Matthew L. McGavick, Cody Raup, M. Carter, C. Berti, S. Mahan, M. Nelson, T. Rittenour, R. Counts, J. Willenbring, D. Germanoski, S. C. Peters, W. D. Holt","doi":"10.1086/712636","DOIUrl":"https://doi.org/10.1086/712636","url":null,"abstract":"We show that long-recognized seismicity in the central Virginia seismic zone of the eastern North American intraplate setting arises primarily from tectonic processes predicted by new, fully coupled plate tectonic geodynamic models. The study leverages much new geophysical and geologic data following the 2011 Mineral, Virginia, earthquake that ruptured a steeply dipping, northwest-verging reverse fault traversed by the South Anna River. The data are primarily assembled from a flight of six fluvial terrace geomorphic markers identified and correlated on texture, relative weathering, and numeric ages including one terrestrial cosmogenic nuclide (TCN) profile and 30 luminescence dates. Terrace thickness, stratigraphic age models, and incision rates downstream and upstream of the 2011 rupture are different. Long-term river incision rates of ∼25–30 m/My are superimposed on regional TCN-determined erosion rates of ∼8.5 m/My; however, there are at least 10 m of tectonically driven incision in the epicentral region at rates of ∼30–94 m/My. The inferred deformation resembles a hanging wall anticline above a blind reverse fault with a diffuse overlying carapace of minor brittle faults, an interpretation supported by seismology as well as bedrock and saprolite mapped across the epicentral region. These results are further supported by channel metrics that show nonuniform channel steepness (ksn) and a predicted steady-state channel elevation different from the actual channel elevation across the epicentral region. If all of the observed deformation is a consequence of the fault that ruptured in 2011, the recurrence interval of Mineral-sized events would be ∼5.5 ky.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"595 - 624"},"PeriodicalIF":1.8,"publicationDate":"2021-01-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712636","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47417885","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Past geological and geochemical changes suggest the existence of two transitions in Earth history at 2.5–2 and 1–0.5 Ga. Twenty-one changes during Transition One and eight during Transition Two signify global-scale shifts in terrestrial tectono-thermal systems. Transition-One changes include the onset of major zircon age peaks, increases in average plate speeds, increases in the number of collisional orogens and associated metamorphism, and continental sedimentation. In addition, changes in rock abundances and chemical composition of continents, of detrital sediments, and in seawater composition reflect an increase in volume of felsic component in the continental crust as well as increasing continental emergence. Changes in composition of oceanic basalts during this time reflect mantle cooling, oxidation, and the appearance of an enriched component in mantle magma sources. A rise in large igneous province (LIP) frequency may reflect enhanced mantle-plume activity responding to thermal-compositional disturbance in the deep mantle due to sinking slabs. Transition Two is characterized by the appearance and establishment of blueschists, ultrahigh-pressure (UHP) metamorphism, and greatly enhanced ophiolite preservation beginning at 750–600 Ma, and this transition may signal the first subduction of continental crust. Each transition is proposed to reflect a thermal threshold during which the lithosphere strengthens. Transition One is interpreted to record the onset, propagation, and eventual establishment of plate tectonics, and Transition Two the onset and establishment of subduction of a strong continental lithosphere.
{"title":"Two Major Transitions in Earth History: Evidence of Two Lithospheric Strength Thresholds","authors":"K. Condie","doi":"10.1086/711141","DOIUrl":"https://doi.org/10.1086/711141","url":null,"abstract":"Past geological and geochemical changes suggest the existence of two transitions in Earth history at 2.5–2 and 1–0.5 Ga. Twenty-one changes during Transition One and eight during Transition Two signify global-scale shifts in terrestrial tectono-thermal systems. Transition-One changes include the onset of major zircon age peaks, increases in average plate speeds, increases in the number of collisional orogens and associated metamorphism, and continental sedimentation. In addition, changes in rock abundances and chemical composition of continents, of detrital sediments, and in seawater composition reflect an increase in volume of felsic component in the continental crust as well as increasing continental emergence. Changes in composition of oceanic basalts during this time reflect mantle cooling, oxidation, and the appearance of an enriched component in mantle magma sources. A rise in large igneous province (LIP) frequency may reflect enhanced mantle-plume activity responding to thermal-compositional disturbance in the deep mantle due to sinking slabs. Transition Two is characterized by the appearance and establishment of blueschists, ultrahigh-pressure (UHP) metamorphism, and greatly enhanced ophiolite preservation beginning at 750–600 Ma, and this transition may signal the first subduction of continental crust. Each transition is proposed to reflect a thermal threshold during which the lithosphere strengthens. Transition One is interpreted to record the onset, propagation, and eventual establishment of plate tectonics, and Transition Two the onset and establishment of subduction of a strong continental lithosphere.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"455 - 473"},"PeriodicalIF":1.8,"publicationDate":"2021-01-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/711141","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48500551","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Mukhopadhyay, R. Armstrong, J. Gutzmer, M. de Kock, N. Beukes
The Bailadila Group of the Bastar Craton, India, is host to a 200-m-thick banded iron formation (BIF). We document the lithostratigraphic context for the BIF, informally referred to as the Bose iron formation, and provide radiometric constraints for its depositional age. Field evidence illustrates that the BIF was deposited on an inner-shelf succession with a quartz arenite that grades upward into the BIF through storm-dominated offshore shelf deposits. The quartz arenite to BIF transition records a relative sea level rise from transgressive to highstand systems tract when the BIFs were deposited in a starved outer continental shelf. U-Pb SHRIMP analyses of zircons from the basement of the Bailadila Group yielded mostly highly discordant U-Pb SHRIMP ages. However, the ages fall on well-defined discordia lines from which concordia intercept ages could be determined. These ages, in combination with the ages of a few zircons that are less than 6% discordant, indicate that the granitoid basement crystallized at 3500–3550 Ma. The maximum depositional age of the Bailadila Group is constrained from the weighted mean 207Pb/206Pb SHRIMP age of 2725±57 Ma from detrital zircons from the basal arenites. A well-constrained weighted mean 207Pb/206Pb SHRIMP age of 2733±53 Ma for zircons from a unit that unconformably overlies the Bailadila Group is within error of that age. Stratigraphic relationships suggest that the Bailadila succession is unconformably overlain by the ~2.5 Ga Kotri and Dongargarh Supergroups. The depositional age of the Bailadila Group is well constrained between ~2.7 and 2.5 Ga. In contrast to most other Archean Algoma-type iron formations of peninsular India, which are closely related to volcanic rocks in greenstone belts, the Bose iron formation is associated with siliciclastic shelf succession. It thus is considered a Superior-type iron formation that represents the oldest known one of its kind in India.
{"title":"Stratigraphy, Depositional Setting, and SHRIMP U-Pb Geochronology of the Banded Iron Formation–Bearing Bailadila Group in the Bacheli Iron Ore Mining District, Bastar Craton, India","authors":"J. Mukhopadhyay, R. Armstrong, J. Gutzmer, M. de Kock, N. Beukes","doi":"10.1086/713683","DOIUrl":"https://doi.org/10.1086/713683","url":null,"abstract":"The Bailadila Group of the Bastar Craton, India, is host to a 200-m-thick banded iron formation (BIF). We document the lithostratigraphic context for the BIF, informally referred to as the Bose iron formation, and provide radiometric constraints for its depositional age. Field evidence illustrates that the BIF was deposited on an inner-shelf succession with a quartz arenite that grades upward into the BIF through storm-dominated offshore shelf deposits. The quartz arenite to BIF transition records a relative sea level rise from transgressive to highstand systems tract when the BIFs were deposited in a starved outer continental shelf. U-Pb SHRIMP analyses of zircons from the basement of the Bailadila Group yielded mostly highly discordant U-Pb SHRIMP ages. However, the ages fall on well-defined discordia lines from which concordia intercept ages could be determined. These ages, in combination with the ages of a few zircons that are less than 6% discordant, indicate that the granitoid basement crystallized at 3500–3550 Ma. The maximum depositional age of the Bailadila Group is constrained from the weighted mean 207Pb/206Pb SHRIMP age of 2725±57 Ma from detrital zircons from the basal arenites. A well-constrained weighted mean 207Pb/206Pb SHRIMP age of 2733±53 Ma for zircons from a unit that unconformably overlies the Bailadila Group is within error of that age. Stratigraphic relationships suggest that the Bailadila succession is unconformably overlain by the ~2.5 Ga Kotri and Dongargarh Supergroups. The depositional age of the Bailadila Group is well constrained between ~2.7 and 2.5 Ga. In contrast to most other Archean Algoma-type iron formations of peninsular India, which are closely related to volcanic rocks in greenstone belts, the Bose iron formation is associated with siliciclastic shelf succession. It thus is considered a Superior-type iron formation that represents the oldest known one of its kind in India.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"115 - 130"},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713683","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43476614","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Christopher A. Vanderlip, R. Cox, D. Larsen, Jeff Mitchell, J. B. Harris, C. Cearley
A newly recognized thrust and nearby asymmetric anticline crop out 40 km north of Memphis, Tennessee, and they deform Eocene through Quaternary strata. These east–west-striking, south-verging structures are peripheral to the New Madrid seismic zone (NMSZ) of central North America, the source of M7+ earthquakes in 1811–1812. The thrust dips ∼20° N and has 55 m of throw in Eocene strata. An angular intraformational unconformity indicates most deformation was Eocene. The anticline’s limbs dip 7° N and 22° S and fold Eocene and Pleistocene strata. Pleistocene sediments are dropped at least 4 m into a graben along the fold axis. Holocene sediment is ponded upstream from the fold axis, suggesting Holocene activity. Based on outcrops, well logs, and seismic reflection, we interpret the anticline as a fault-tip fold above a splay of the thrust fault. We interpret these thrusts in the context of a previously published sandbox model of a restraining bend uplift, which we apply here to the Reelfoot Rift fault complex. Using the eastern rift margin as the strike-slip fault of the sandbox model, the periphery of the model uplift has an east–west-striking, south-verging oblique-slip thrust where the actual thrust and anticline crop out. These results suggest that young thrust faults may be common along the periphery of the NMSZ and similar active intraplate restraining bends, that the eastern margin of the Reelfoot Rift may have been a principal strike-slip fault of the restraining bend, and that the seismic zone was active as early as Eocene.
{"title":"Newly Recognized Quaternary Surface Faulting and Folding Peripheral to the New Madrid Seismic Zone, Central United States, and Implications for Restraining Bend Models of Intraplate Seismic Zones","authors":"Christopher A. Vanderlip, R. Cox, D. Larsen, Jeff Mitchell, J. B. Harris, C. Cearley","doi":"10.1086/713686","DOIUrl":"https://doi.org/10.1086/713686","url":null,"abstract":"A newly recognized thrust and nearby asymmetric anticline crop out 40 km north of Memphis, Tennessee, and they deform Eocene through Quaternary strata. These east–west-striking, south-verging structures are peripheral to the New Madrid seismic zone (NMSZ) of central North America, the source of M7+ earthquakes in 1811–1812. The thrust dips ∼20° N and has 55 m of throw in Eocene strata. An angular intraformational unconformity indicates most deformation was Eocene. The anticline’s limbs dip 7° N and 22° S and fold Eocene and Pleistocene strata. Pleistocene sediments are dropped at least 4 m into a graben along the fold axis. Holocene sediment is ponded upstream from the fold axis, suggesting Holocene activity. Based on outcrops, well logs, and seismic reflection, we interpret the anticline as a fault-tip fold above a splay of the thrust fault. We interpret these thrusts in the context of a previously published sandbox model of a restraining bend uplift, which we apply here to the Reelfoot Rift fault complex. Using the eastern rift margin as the strike-slip fault of the sandbox model, the periphery of the model uplift has an east–west-striking, south-verging oblique-slip thrust where the actual thrust and anticline crop out. These results suggest that young thrust faults may be common along the periphery of the NMSZ and similar active intraplate restraining bends, that the eastern margin of the Reelfoot Rift may have been a principal strike-slip fault of the restraining bend, and that the seismic zone was active as early as Eocene.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"77 - 95"},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713686","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43760228","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
N. Trọng, K. Zong, Yongsheng Liu, Yu Yuan, P. T. Hieu, L. Dung, P. Minh
Southeast Asia plays a key role in the evolution of the eastern Tethys, which is characterized by accretion and amalgamation of numerous terranes since early Paleozoic. In the present study, an early Paleozoic granitoid sequence from the Kontum massif in the central Indochina block was investigated to reveal the early Paleozoic accretionary orogenesis of the eastern Tethys. Early Paleozoic Kontum granitoids include diorites and granites. Early Ordovician (485–473 Ma) Ben Giang diorites show high Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as positive zircon ɛHf(t) values (+6.2 to +10), probably reflecting the melting of a metasomatized mantle wedge. Late Ordovician (457–453 Ma) Dien Binh hornblende-bearing diorites and granites exhibit low Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as negative zircon ɛHf(t) values (−8.7 to −4.2), which is interpreted to reflect the reworking of ancient continental mafic crust during subduction. In contrast, the Silurian (422 Ma) Dai Loc muscovite-bearing granites are characterized by relatively high A/CNK and heavy rare earth element contents, and a negative Eu anomaly, corresponding to a typical S-type granite affinity. Their negative zircon ɛHf(t) values (−6.0 to +0.6) suggest that a petrogenesis linked to reworking of crustal sediments. In combination with the widespread occurrence of coeval high-pressure metamorphism, we suggest that an advancing accretionary orogenesis was responsible for the generation of the early Paleozoic magmatic arc in the Kontum massif. Such magmatic arc systems dominated the early Paleozoic evolution of the northern Gondwana margin during the subduction of the Proto-Tethys.
{"title":"Early Paleozoic Arc Magmatism and Accretionary Orogenesis in the Indochina Block, Southeast Asia","authors":"N. Trọng, K. Zong, Yongsheng Liu, Yu Yuan, P. T. Hieu, L. Dung, P. Minh","doi":"10.1086/713727","DOIUrl":"https://doi.org/10.1086/713727","url":null,"abstract":"Southeast Asia plays a key role in the evolution of the eastern Tethys, which is characterized by accretion and amalgamation of numerous terranes since early Paleozoic. In the present study, an early Paleozoic granitoid sequence from the Kontum massif in the central Indochina block was investigated to reveal the early Paleozoic accretionary orogenesis of the eastern Tethys. Early Paleozoic Kontum granitoids include diorites and granites. Early Ordovician (485–473 Ma) Ben Giang diorites show high Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as positive zircon ɛHf(t) values (+6.2 to +10), probably reflecting the melting of a metasomatized mantle wedge. Late Ordovician (457–453 Ma) Dien Binh hornblende-bearing diorites and granites exhibit low Mg#, Cr and Ni contents, and negative Nb-Ta and positive Pb anomalies as well as negative zircon ɛHf(t) values (−8.7 to −4.2), which is interpreted to reflect the reworking of ancient continental mafic crust during subduction. In contrast, the Silurian (422 Ma) Dai Loc muscovite-bearing granites are characterized by relatively high A/CNK and heavy rare earth element contents, and a negative Eu anomaly, corresponding to a typical S-type granite affinity. Their negative zircon ɛHf(t) values (−6.0 to +0.6) suggest that a petrogenesis linked to reworking of crustal sediments. In combination with the widespread occurrence of coeval high-pressure metamorphism, we suggest that an advancing accretionary orogenesis was responsible for the generation of the early Paleozoic magmatic arc in the Kontum massif. Such magmatic arc systems dominated the early Paleozoic evolution of the northern Gondwana margin during the subduction of the Proto-Tethys.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"33 - 48"},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713727","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42245499","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xin Zhou, J. Ji, Jing Zhou, Lhamo Yungchen, Wuxun Quan, J. Tu
The Jehol Biota is famous for its diversity of Early Cretaceous fossils preserved under large amounts of pyroclastic materials. However, the precise determination on the ages of volcanoclastic strata is challenging, because of their complex provenance. The solution to this problem may lie in the greatly improved geochronological methods, such as the 40Ar/39Ar geochronology. In this study, we use the laser fusion 40Ar/39Ar dating method to date volcanoclastic samples and apply statistical analyses of the acquired apparent ages to determine the volcanic eruption periods. From the 12 volcanoclastic samples analyzed in this study, we obtained 262 apparent ages that vary from 85.5 to 153.6 Ma. By fitting these apparent ages with a Gaussian mixture distribution model, we find three age clusters that are normally distributed, with each cluster statistically representing an eruption period. The mean values of the three normal distributions are 90.37±2.72 (1σ), 109.89±4.94, and 125.26±10.01 Ma. A comparison of our results with a statistical analysis of literature data indicates that volcanic activities occurred over a wide spatial scale across western Liaoning Province during the two older periods, whereas during the last time period, eruptions occurred more locally and belonged to the final stage of volcanic eruptions in the basin. The method and results reported in this study provide a good example of (1) dating volcanoclastic materials using the 40Ar/39Ar method to determine the time periods of volcanic activities and (2) investigating the stratigraphic correlation in complex volcanoclastic basins.
{"title":"A New 40Ar/39Ar Analysis Method of Volcanoclastic Strata to Determine Eruption Periods—Example of Xintaimen, China","authors":"Xin Zhou, J. Ji, Jing Zhou, Lhamo Yungchen, Wuxun Quan, J. Tu","doi":"10.1086/713685","DOIUrl":"https://doi.org/10.1086/713685","url":null,"abstract":"The Jehol Biota is famous for its diversity of Early Cretaceous fossils preserved under large amounts of pyroclastic materials. However, the precise determination on the ages of volcanoclastic strata is challenging, because of their complex provenance. The solution to this problem may lie in the greatly improved geochronological methods, such as the 40Ar/39Ar geochronology. In this study, we use the laser fusion 40Ar/39Ar dating method to date volcanoclastic samples and apply statistical analyses of the acquired apparent ages to determine the volcanic eruption periods. From the 12 volcanoclastic samples analyzed in this study, we obtained 262 apparent ages that vary from 85.5 to 153.6 Ma. By fitting these apparent ages with a Gaussian mixture distribution model, we find three age clusters that are normally distributed, with each cluster statistically representing an eruption period. The mean values of the three normal distributions are 90.37±2.72 (1σ), 109.89±4.94, and 125.26±10.01 Ma. A comparison of our results with a statistical analysis of literature data indicates that volcanic activities occurred over a wide spatial scale across western Liaoning Province during the two older periods, whereas during the last time period, eruptions occurred more locally and belonged to the final stage of volcanic eruptions in the basin. The method and results reported in this study provide a good example of (1) dating volcanoclastic materials using the 40Ar/39Ar method to determine the time periods of volcanic activities and (2) investigating the stratigraphic correlation in complex volcanoclastic basins.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"63 - 76"},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713685","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46412520","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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