Published U-Pb-Hf values for detrital zircons of Mesoproterozic age in Cambrian and Ordovician strata of the Argentina Precordillera and San Rafael Block of the Cuyania Terrane overlap extensively with values from Grenville/Appalachian detritus and the Granite-Rhyolite Province of Laurentia and with values from Sierra de Maz and Sierra de Pie de Palo of the Western Sierras Pampeanas (WSP) of western Argentina. These data have been interpreted as evidence of the Laurentian origin of Cuyania and the WSP. However, U-Pb and corresponding εHf values presented here for Mesoproterozoic zircons from Ediacaran sandstones deposited on the Río de la Plata Craton (RPC) are markedly similar to those of Mesoproterozoic zircons from Laurentia, Cuyania, and the WSP, thus contradicting the previous interpretation. Given the geotectonic history of southwestern Gondwana, the U-Pb-Hf values from the Mesoproterozoic zircons of the Piedras de Afilar and Cerro Largo Formations of the RPC are consistent with a proto-Andean Mesoproterozoic belt on the western margin of the RPC. This belt would have served as the basement and thus the provenance of Mesoproterozoic detrital zircons in Cuyania and the WSP. The existence of this Mesoproterozoic belt introduces a different paradigm for interpreting the late Neoproterozoic to early Paleozoic geotectonic history of the proto-Andean margin of Gondwana and renders Neoproterozoic–early Paleozoic transfers of a Sierra de Maz–Arequipa–Rio Apa Terrane and the Cuyania Terrane from Laurentia unnecessary.
已公布的阿根廷Precordillera和Cuyania地块寒武系和奥陶系中元古代碎屑锆石U-Pb-Hf值与Grenville/Appalachian碎屑岩和Laurentia花岗流纹岩省的U-Pb-Hf值以及阿根廷西部西帕米亚山脉(WSP)的Sierra de Maz和Sierra de Pie de Palo的U-Pb-Hf值有广泛的重叠。这些数据被解释为Cuyania和WSP在劳伦斯起源的证据。然而，Río de la Plata克拉通(RPC)埃迪卡拉系砂岩中元古代锆石的U-Pb和相应的εHf值与Laurentia, Cuyania和WSP的中元古代锆石的U-Pb和相应的εHf值明显相似，从而与之前的解释相矛盾。结合冈瓦纳西南部大地构造史，RPC的Piedras de Afilar组和Cerro Largo组中元古代锆石U-Pb-Hf值与RPC西缘的原安第斯中元古代带一致。该带可能是贵州和WSP中元古代碎屑锆石的基底和物源。该中元古代带的存在为冈瓦纳原安第斯边缘新元古代晚期至早古生代大地构造史的解释提供了一种不同的范式，使得Sierra de Maz-Arequipa-Rio Apa地体和Cuyania地体从Laurentia的新元古代至早古生代的转移变得不必要。
{"title":"Hf Isotopes in Detrital Zircon Point to a Mesoproterozoic Orogenic Belt Bordering the Western Margin of the Río de la Plata Craton","authors":"S. Finney, C. Gaucher","doi":"10.1086/717889","DOIUrl":"https://doi.org/10.1086/717889","url":null,"abstract":"Published U-Pb-Hf values for detrital zircons of Mesoproterozic age in Cambrian and Ordovician strata of the Argentina Precordillera and San Rafael Block of the Cuyania Terrane overlap extensively with values from Grenville/Appalachian detritus and the Granite-Rhyolite Province of Laurentia and with values from Sierra de Maz and Sierra de Pie de Palo of the Western Sierras Pampeanas (WSP) of western Argentina. These data have been interpreted as evidence of the Laurentian origin of Cuyania and the WSP. However, U-Pb and corresponding εHf values presented here for Mesoproterozoic zircons from Ediacaran sandstones deposited on the Río de la Plata Craton (RPC) are markedly similar to those of Mesoproterozoic zircons from Laurentia, Cuyania, and the WSP, thus contradicting the previous interpretation. Given the geotectonic history of southwestern Gondwana, the U-Pb-Hf values from the Mesoproterozoic zircons of the Piedras de Afilar and Cerro Largo Formations of the RPC are consistent with a proto-Andean Mesoproterozoic belt on the western margin of the RPC. This belt would have served as the basement and thus the provenance of Mesoproterozoic detrital zircons in Cuyania and the WSP. The existence of this Mesoproterozoic belt introduces a different paradigm for interpreting the late Neoproterozoic to early Paleozoic geotectonic history of the proto-Andean margin of Gondwana and renders Neoproterozoic–early Paleozoic transfers of a Sierra de Maz–Arequipa–Rio Apa Terrane and the Cuyania Terrane from Laurentia unnecessary.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"1 - 22"},"PeriodicalIF":1.8,"publicationDate":"2021-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42494804","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}
Yi Sun, Tao‐tao Wu, Long Xiao, M. Bai, Jingxuan Zhang, Longhui Lv, Jiang Lu
High-Mg diorites with high Sr/Y ratios are commonly associated with metal mineralization. But their petrogenesis still remains controversial. Here we present a case study of the Early Cretaceous high-Mg dioritic intrusions (Fushan intrusive complex) in the Handan-Xingtai district, central North China Craton (NCC). Samples from the Fushan Complex (123±1 Ma) have SiO2 of 53.3–62.6 wt%, relatively variable K2O of 0.5–3.6 wt%, and high Mg# values (44–63). They show enrichment in light rare earth elements and depletion in heavy rare earth elements and high field strength elements, with high Sr/Y ratios and insignificant Eu anomalies, similar to those of adakitic rocks. For the Sr-Nd isotopes, they have whole-rock initial 87Sr/86Sr ratios and εNd(t=123 Ma) values ranging from 0.706564 to 0.707546 and from −16.1 to −12.8, respectively, which are distinct from those in the lower continental crust but similar to those in the contemporaneous mafic rocks in the study area. Because of these characteristics, together with their zircon δ18O values of 6.6‰±0.4‰, we suggest that the Fushan high-Mg dioritic complex was most likely formed by factional crystallization of mantle-derived basaltic magma. The high Sr/Y ratios could be achieved by the fractionation of amphibole crystals plus significant volumes of feldspars from a mafic magma. We suggest that late Mesozoic strong lithospheric extension and thinning, caused by the subduction of the paleo-Pacific slab in the east of the NCC, might have triggered the melting of the metasomatic lithospheric mantle to generate these intrusive complexes in the Handan-Xingtai district.
{"title":"High-Mg Dioritic Magmas Generated via Fractional Crystallization: Insights from Early Cretaceous Complex in the Handan-Xingtai District, North China Craton","authors":"Yi Sun, Tao‐tao Wu, Long Xiao, M. Bai, Jingxuan Zhang, Longhui Lv, Jiang Lu","doi":"10.1086/718350","DOIUrl":"https://doi.org/10.1086/718350","url":null,"abstract":"High-Mg diorites with high Sr/Y ratios are commonly associated with metal mineralization. But their petrogenesis still remains controversial. Here we present a case study of the Early Cretaceous high-Mg dioritic intrusions (Fushan intrusive complex) in the Handan-Xingtai district, central North China Craton (NCC). Samples from the Fushan Complex (123±1 Ma) have SiO2 of 53.3–62.6 wt%, relatively variable K2O of 0.5–3.6 wt%, and high Mg# values (44–63). They show enrichment in light rare earth elements and depletion in heavy rare earth elements and high field strength elements, with high Sr/Y ratios and insignificant Eu anomalies, similar to those of adakitic rocks. For the Sr-Nd isotopes, they have whole-rock initial 87Sr/86Sr ratios and εNd(t=123 Ma) values ranging from 0.706564 to 0.707546 and from −16.1 to −12.8, respectively, which are distinct from those in the lower continental crust but similar to those in the contemporaneous mafic rocks in the study area. Because of these characteristics, together with their zircon δ18O values of 6.6‰±0.4‰, we suggest that the Fushan high-Mg dioritic complex was most likely formed by factional crystallization of mantle-derived basaltic magma. The high Sr/Y ratios could be achieved by the fractionation of amphibole crystals plus significant volumes of feldspars from a mafic magma. We suggest that late Mesozoic strong lithospheric extension and thinning, caused by the subduction of the paleo-Pacific slab in the east of the NCC, might have triggered the melting of the metasomatic lithospheric mantle to generate these intrusive complexes in the Handan-Xingtai district.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"130 1","pages":"45 - 62"},"PeriodicalIF":1.8,"publicationDate":"2021-12-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45256203","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}
Hydrodynamic equations are frequently employed in the study of coastal boulder movement to estimate the height of tsunami or storm waves responsible for initiating dislodgement. A recent review has challenged the use of such equations and, through a test using a data set of boulders that were moved during storms over winter 2013–14 in the Aran Islands (Ireland), concludes that the equations are flawed. To evaluate this claim, this study revisits the foundation of the equations and reflects on the recent review. We conclude that, although the review is timely and welcomed for the questions it prompts, it arguably does not provide grounds to pronounce the equations flawed. This is due to various considerations, including (1) the equations have been misapplied to physical settings other than as originally intended, (2) wave-type parameters for estimating tsunami and storm wave heights from boulder measurements appear reasonable and not flawed when used within the intended physical settings, (3) the Aran Islands may not be an appropriate location to test the equations because of their physical setting, (4) the exclusion of storm surge and wave setup effects in the test is likely to underestimate the calculated height attained by storm waves, and (5) for comparison, recently revised equations are used to recalculate the Aran Islands boulder data set, indicating that all but 33 boulders demonstrably moved by the 2013–14 storms may be explained by the maximum storm wave height proposed by the review and that all these moved boulders might be explained if the effects of storm surge are included. Analytical tools, such as provided by hydrodynamic equations for boulder dislodgement, are considered important in the contribution they make to coastal risk assessment and hazard management.
{"title":"Hydrodynamic Equations for Coastal Boulder Movement: Reflections on a Recent Review","authors":"S. Haslett, Bernardine R. Wong","doi":"10.1086/717784","DOIUrl":"https://doi.org/10.1086/717784","url":null,"abstract":"Hydrodynamic equations are frequently employed in the study of coastal boulder movement to estimate the height of tsunami or storm waves responsible for initiating dislodgement. A recent review has challenged the use of such equations and, through a test using a data set of boulders that were moved during storms over winter 2013–14 in the Aran Islands (Ireland), concludes that the equations are flawed. To evaluate this claim, this study revisits the foundation of the equations and reflects on the recent review. We conclude that, although the review is timely and welcomed for the questions it prompts, it arguably does not provide grounds to pronounce the equations flawed. This is due to various considerations, including (1) the equations have been misapplied to physical settings other than as originally intended, (2) wave-type parameters for estimating tsunami and storm wave heights from boulder measurements appear reasonable and not flawed when used within the intended physical settings, (3) the Aran Islands may not be an appropriate location to test the equations because of their physical setting, (4) the exclusion of storm surge and wave setup effects in the test is likely to underestimate the calculated height attained by storm waves, and (5) for comparison, recently revised equations are used to recalculate the Aran Islands boulder data set, indicating that all but 33 boulders demonstrably moved by the 2013–14 storms may be explained by the maximum storm wave height proposed by the review and that all these moved boulders might be explained if the effects of storm surge are included. Analytical tools, such as provided by hydrodynamic equations for boulder dislodgement, are considered important in the contribution they make to coastal risk assessment and hazard management.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"725 - 733"},"PeriodicalIF":1.8,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42241434","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}
Steven E. Zhang, G. Nwaila, J. Bourdeau, H. Frimmel, Y. Ghorbani, Riham Elhabyan
Data-driven methods have increasingly been applied to solve geoscientific problems. Incorporation of data-driven methods with hypothesis testing can be effective to address some long-standing debates and reduce interpretation uncertainty by leveraging larger volumes of data and more objective data analytics, which leads to increased reproducibility. In this study, lithogeochemical data from regionally persistent Archean shale units were aggregated from literature, with special reference to the Kaapvaal Craton of South Africa—namely, shales from the Barberton, Witwatersrand, Pongola, and Transvaal Supergroups—and the Belingwe and Buhwa Greenstone Belts of the Zimbabwe Craton. We examine the feasibility of using machine-learning algorithms to produce a geochemical classification and demonstrate that machine learning is capable of accurately correlating stratigraphy at the formation, group, and supergroup levels. We demonstrate the ability to extract highly useful scientific findings through a data-driven approach, such as geological implications for the uniqueness of the sediment compositions of the Central Rand and West Rand Groups. We further demonstrate that when lithogeochemistry and machine-learning algorithms are used, only about 50 samples per geological unit are necessary to reach accuracy levels of around 80%–90% for our shale samples. Consequently, for many traditional tasks, such as rock identification and mapping, some expensive analyses and manual labor can be replaced by an abundance of cheaper data and machine learning. This approach could transform large-scale geological surveys by enabling more detailed mapping than currently possible, by vastly increasing the coverage rate and total coverage. In addition, the aggregation of historical data facilitates data reuse and open science. These results justify the need to bridge data- and hypothesis-driven techniques for the stratigraphic correlation and prediction of rock units, which can improve the accuracy of the inferred stratigraphic correlation and basin setting.
{"title":"Application of Machine-Learning Algorithms to the Stratigraphic Correlation of Archean Shale Units Based on Lithogeochemistry","authors":"Steven E. Zhang, G. Nwaila, J. Bourdeau, H. Frimmel, Y. Ghorbani, Riham Elhabyan","doi":"10.1086/717847","DOIUrl":"https://doi.org/10.1086/717847","url":null,"abstract":"Data-driven methods have increasingly been applied to solve geoscientific problems. Incorporation of data-driven methods with hypothesis testing can be effective to address some long-standing debates and reduce interpretation uncertainty by leveraging larger volumes of data and more objective data analytics, which leads to increased reproducibility. In this study, lithogeochemical data from regionally persistent Archean shale units were aggregated from literature, with special reference to the Kaapvaal Craton of South Africa—namely, shales from the Barberton, Witwatersrand, Pongola, and Transvaal Supergroups—and the Belingwe and Buhwa Greenstone Belts of the Zimbabwe Craton. We examine the feasibility of using machine-learning algorithms to produce a geochemical classification and demonstrate that machine learning is capable of accurately correlating stratigraphy at the formation, group, and supergroup levels. We demonstrate the ability to extract highly useful scientific findings through a data-driven approach, such as geological implications for the uniqueness of the sediment compositions of the Central Rand and West Rand Groups. We further demonstrate that when lithogeochemistry and machine-learning algorithms are used, only about 50 samples per geological unit are necessary to reach accuracy levels of around 80%–90% for our shale samples. Consequently, for many traditional tasks, such as rock identification and mapping, some expensive analyses and manual labor can be replaced by an abundance of cheaper data and machine learning. This approach could transform large-scale geological surveys by enabling more detailed mapping than currently possible, by vastly increasing the coverage rate and total coverage. In addition, the aggregation of historical data facilitates data reuse and open science. These results justify the need to bridge data- and hypothesis-driven techniques for the stratigraphic correlation and prediction of rock units, which can improve the accuracy of the inferred stratigraphic correlation and basin setting.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"647 - 672"},"PeriodicalIF":1.8,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43185272","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}
To date, all source-to-sink research has focused on the Phanerozoic and Proterozoic stratigraphic record. This study attempts to apply source-to-sink principles to the 2.8–3.0 Ga placer gold–hosting Witwatersrand Supergroup in South Africa, for which the source area is eroded such that a reconstruction of source parameters is based on the preserved stratigraphic record. The paradigm tested in this study is that bankfull channel depth is a function of drainage basin area and discharge and that an understanding of these parameters can constrain paleogeographic models including source area location and age. Bankfull depth estimates based on thicknesses of fluvial architectural elements are 3.5 and 6.2 m for the older Main and younger Mondeor Formations of the Central Rand Group, respectively. Drainage basin area and discharge estimates are based on empirical relationships developed for different climatic settings. Evidence from facies associations and mudstone geochemistry suggests a warm paleoclimate with relatively high rainfall that constrains drainage basin areas to 3900–19,000 km2 for the Main Formation and 17,000–106,000 km2 for the Mondeor Formation. Estimates for both formations imply an increase in drainage basin area through time, and detrital zircon age spectra reveal a significantly older source area component for the Mondeor than for the Main Formation. The likely sources of the older zircon grains are the Buhwa Quartzite and Tokwe Segment in southern Zimbabwe, some 500 km north of the preserved Witwatersrand sedimentary rocks, consistent with the larger drainage basin area estimate for the Mondeor Formation (106,000 km2). An enlarged drainage basin area is compatible with tectonic models that infer collision of the Kaapvaal and Zimbabwe cratons during the late stages of Witwatersrand sedimentation.
迄今为止，所有的源-汇研究都集中在显生界和元古代的地层记录上。本研究试图将源-汇原理应用于南非2.8 ~ 3.0 Ga含金砂矿Witwatersrand超群，该群的矿源区域受到侵蚀，因此可以根据保存的地层记录重建矿源参数。本研究验证的范式是，河岸河道深度是流域面积和流量的函数，对这些参数的理解可以约束包括源区位置和年龄在内的古地理模型。基于河流建筑元素厚度的河岸深度估计分别为3.5米和6.2米，分别为中央兰德群较老的Main和较年轻的Mondeor组。流域面积和流量估算是基于针对不同气候环境开发的经验关系。来自相组合和泥岩地球化学的证据表明，温暖的古气候和相对高的降雨量将流域面积限制在3900-19,000 km2(主要组)和17,000-106,000 km2 (Mondeor组)。对这两个组的估计表明，随着时间的推移，流域面积增加，碎屑锆石年龄谱显示，蒙德多组的源区成分明显比主组的源区成分更古老。较古老的锆石颗粒可能来自津巴布韦南部的Buhwa石英岩和Tokwe段，位于保存完好的Witwatersrand沉积岩以北约500公里处，与Mondeor组更大的流域面积估计(106,000平方公里)一致。扩大的流域面积与推断威特沃特斯兰德沉积晚期Kaapvaal和津巴布韦克拉通碰撞的构造模式相吻合。
{"title":"Source-to-Sink Analysis of the Gold-Hosting Mesoarchean Main and Mondeor Formations (Central Rand Group) in the Area South of Johannesburg, Witwatersrand Basin, Kaapvaal Province, South Africa","authors":"K. Eriksson, Wilson S. McClung","doi":"10.1086/716963","DOIUrl":"https://doi.org/10.1086/716963","url":null,"abstract":"To date, all source-to-sink research has focused on the Phanerozoic and Proterozoic stratigraphic record. This study attempts to apply source-to-sink principles to the 2.8–3.0 Ga placer gold–hosting Witwatersrand Supergroup in South Africa, for which the source area is eroded such that a reconstruction of source parameters is based on the preserved stratigraphic record. The paradigm tested in this study is that bankfull channel depth is a function of drainage basin area and discharge and that an understanding of these parameters can constrain paleogeographic models including source area location and age. Bankfull depth estimates based on thicknesses of fluvial architectural elements are 3.5 and 6.2 m for the older Main and younger Mondeor Formations of the Central Rand Group, respectively. Drainage basin area and discharge estimates are based on empirical relationships developed for different climatic settings. Evidence from facies associations and mudstone geochemistry suggests a warm paleoclimate with relatively high rainfall that constrains drainage basin areas to 3900–19,000 km2 for the Main Formation and 17,000–106,000 km2 for the Mondeor Formation. Estimates for both formations imply an increase in drainage basin area through time, and detrital zircon age spectra reveal a significantly older source area component for the Mondeor than for the Main Formation. The likely sources of the older zircon grains are the Buhwa Quartzite and Tokwe Segment in southern Zimbabwe, some 500 km north of the preserved Witwatersrand sedimentary rocks, consistent with the larger drainage basin area estimate for the Mondeor Formation (106,000 km2). An enlarged drainage basin area is compatible with tectonic models that infer collision of the Kaapvaal and Zimbabwe cratons during the late stages of Witwatersrand sedimentation.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"625 - 646"},"PeriodicalIF":1.8,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43911717","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}
Aaron L. Hantsche, G. L. Farmer, Inocente Guadalupe Espinoza Maldonado, C. Fedo, C. Siddoway
In situ zircon U-Pb and Hf isotopic data from ∼1.1 Ga intrusive igneous rocks in Mexico and from Grenvillian (0.9–1.3 Ga) detrital zircons in sandstones from the southern midcontinent of Laurentia were used to refine provenance determinations for the Grenvillian detrital zircons delivered to southwestern Laurentia from the Neoproterozoic to the Cambrian and to address the reduction in the Grenvillian detrital zircon abundances documented in Cambrian sandstones from this region. Igneous zircons from Mesoproterozoic anorthosites and granites in northern Sonora have low εHf(0) values (<−22) and could not have been sources of the higher-εHf(0) (>−22), ∼1.1 Ga detrital zircons characteristic of Ediacaran to Terreneuvian sandstones in southwestern Laurentia. Abundant Grenvillian detrital zircons in Cryogenian sandstone injectites from central Colorado have U-Pb ages and high εHf(0) values (>−22) similar to those of zircons in Ediacaran to Terreneuvian sandstones throughout southwestern Laurentia. These zircons were derived from Mesoproterozoic rocks in the Llano uplift and vicinity in Texas and were fluvially transported across southwestern Laurentia from the Cryogenian to the Terreneuvian. In contrast, Cambrian glauconitic sandstones in the subsurface of east-central Colorado and from the Sawatch Sandstone in central Colorado have low Grenvillian zircon abundances, as observed in Cambrian sandstones exposed farther west in Laurentia. The low abundances of Grenvillian detrital zircons in sandstones found both east (Colorado) and west of the strike of a proposed Cambrian “transcontinental arch” suggest that this feature did not disrupt the supply of Llano uplift–derived Grenvillian zircons to southwestern Laurentia. The low Grenvillian zircon abundances instead coincide with the development of marine conditions in south-central Laurentia, suggesting that the progressive encroachment of the Sauk Sea into the continental interior provided a sink for fluvial sediments derived from the Llano uplift that restricted their transport farther to the north and west in the continent.
{"title":"U-Pb and Hf Isotopic Evidence on the Sources and Sinks of Grenvillian Detrital Zircons in Early Laurentia","authors":"Aaron L. Hantsche, G. L. Farmer, Inocente Guadalupe Espinoza Maldonado, C. Fedo, C. Siddoway","doi":"10.1086/716965","DOIUrl":"https://doi.org/10.1086/716965","url":null,"abstract":"In situ zircon U-Pb and Hf isotopic data from ∼1.1 Ga intrusive igneous rocks in Mexico and from Grenvillian (0.9–1.3 Ga) detrital zircons in sandstones from the southern midcontinent of Laurentia were used to refine provenance determinations for the Grenvillian detrital zircons delivered to southwestern Laurentia from the Neoproterozoic to the Cambrian and to address the reduction in the Grenvillian detrital zircon abundances documented in Cambrian sandstones from this region. Igneous zircons from Mesoproterozoic anorthosites and granites in northern Sonora have low εHf(0) values (<−22) and could not have been sources of the higher-εHf(0) (>−22), ∼1.1 Ga detrital zircons characteristic of Ediacaran to Terreneuvian sandstones in southwestern Laurentia. Abundant Grenvillian detrital zircons in Cryogenian sandstone injectites from central Colorado have U-Pb ages and high εHf(0) values (>−22) similar to those of zircons in Ediacaran to Terreneuvian sandstones throughout southwestern Laurentia. These zircons were derived from Mesoproterozoic rocks in the Llano uplift and vicinity in Texas and were fluvially transported across southwestern Laurentia from the Cryogenian to the Terreneuvian. In contrast, Cambrian glauconitic sandstones in the subsurface of east-central Colorado and from the Sawatch Sandstone in central Colorado have low Grenvillian zircon abundances, as observed in Cambrian sandstones exposed farther west in Laurentia. The low abundances of Grenvillian detrital zircons in sandstones found both east (Colorado) and west of the strike of a proposed Cambrian “transcontinental arch” suggest that this feature did not disrupt the supply of Llano uplift–derived Grenvillian zircons to southwestern Laurentia. The low Grenvillian zircon abundances instead coincide with the development of marine conditions in south-central Laurentia, suggesting that the progressive encroachment of the Sauk Sea into the continental interior provided a sink for fluvial sediments derived from the Llano uplift that restricted their transport farther to the north and west in the continent.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"673 - 693"},"PeriodicalIF":1.8,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47709224","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}
Aditya Kharya, H. Sachan, C. Spencer, K. Sen, D. Prakash, Shashi Ranjan Rai, Vikash Kumar
Quartz-calcite veins in the Zildat ophiolitic mélange (ZOM) and Shergol ophiolitic mélange (SOM) of the Indus Suture Zone preserve a diversity of fluid activity in the late stages of ophiolitic mélange formation. This article presents fluid-inclusion and isotope geochemistry of these veins to understand their source and evolution in terms of pressure and temperature. The microstructures of quartz and calcite veins indicate deformation temperatures between 200° and 400°C. The δ13C and δ18O values of calcite veins from the ZOM and SOM are within the mixing hyperbolas of marine and primitive-mantle fields in the mixing model. The Sr and Pb isotopic values of calcite veins from the ZOM suggest a mid-ocean ridge basalt (MORB) fluid source of vein formation that was radiogenically enriched by metasomatism in a suprasubduction zone. For the SOM, fluids may have originated from the enriched-mantle (EM) and the depleted-MORB-mantle rocks. It is inferred that the carbonic fluids were derived from ultramafic lithologies and oceanic crust that formed the ophiolitic mélange rocks, which also host these veins. These source rocks have EM and MORB geochemical signatures that are also obtained in the quartz-calcite veins, as characterized by their C-O-Sr-Pb isotopic ratios. The magmatic saline fluid is inferred to have formed in the early stages of vein formation and to have been subsequently diluted, as exemplified by the presence of low-saline secondary aqueous inclusions. The microthermometry fluid pressure-temperature estimation of veins from the studied sections suggests that the maximum depth of emplacement of veining fluid was about 24.5 MPa (corresponding to ∼2.5 km) at 336°C. The vein-forming fluids (calcareous and siliceous) were introduced into the fractures that developed in the host as a result of deformation.
{"title":"Tracing Late-Stage Fluid Sources and Vein Formation within Ophiolitic Mélanges from the Indus Suture Zone, Ladakh Himalaya","authors":"Aditya Kharya, H. Sachan, C. Spencer, K. Sen, D. Prakash, Shashi Ranjan Rai, Vikash Kumar","doi":"10.1086/716964","DOIUrl":"https://doi.org/10.1086/716964","url":null,"abstract":"Quartz-calcite veins in the Zildat ophiolitic mélange (ZOM) and Shergol ophiolitic mélange (SOM) of the Indus Suture Zone preserve a diversity of fluid activity in the late stages of ophiolitic mélange formation. This article presents fluid-inclusion and isotope geochemistry of these veins to understand their source and evolution in terms of pressure and temperature. The microstructures of quartz and calcite veins indicate deformation temperatures between 200° and 400°C. The δ13C and δ18O values of calcite veins from the ZOM and SOM are within the mixing hyperbolas of marine and primitive-mantle fields in the mixing model. The Sr and Pb isotopic values of calcite veins from the ZOM suggest a mid-ocean ridge basalt (MORB) fluid source of vein formation that was radiogenically enriched by metasomatism in a suprasubduction zone. For the SOM, fluids may have originated from the enriched-mantle (EM) and the depleted-MORB-mantle rocks. It is inferred that the carbonic fluids were derived from ultramafic lithologies and oceanic crust that formed the ophiolitic mélange rocks, which also host these veins. These source rocks have EM and MORB geochemical signatures that are also obtained in the quartz-calcite veins, as characterized by their C-O-Sr-Pb isotopic ratios. The magmatic saline fluid is inferred to have formed in the early stages of vein formation and to have been subsequently diluted, as exemplified by the presence of low-saline secondary aqueous inclusions. The microthermometry fluid pressure-temperature estimation of veins from the studied sections suggests that the maximum depth of emplacement of veining fluid was about 24.5 MPa (corresponding to ∼2.5 km) at 336°C. The vein-forming fluids (calcareous and siliceous) were introduced into the fractures that developed in the host as a result of deformation.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"695 - 723"},"PeriodicalIF":1.8,"publicationDate":"2021-08-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49113533","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}
We have gained significant insights into the dynamic evolution of the earth, causes of geohazards, processes of continental growth, feedback mechanisms between tectonics, topography and climate, and rates of deformation and uplift in collision zones since the initial formulation of the plate tectonics theory 50 years ago. The articles in this special issue present some new concepts, data, and interpretations on various aspects of the plate tectonics paradigm. The onset of plate tectonics may have occurred in a major transition period (2.5–2.0 Ga) in Earth history during which short-lived, geographically limited subduction events resulted in a regime switch from stagnant-lid to plate tectonics. The 1.90–1.87 Ga granulites in the continental lower crust of the Siberian craton contain significant amount of water in the form of structural OH in nominally anhydrous minerals, indicating that the water content of the Precambrian granulites might have been considerably higher than that of the Phanerozoic granulites and the lower crust. The major mountain-building episode (vertical thickening, topographic buildup, and normal faulting) in the evolution of the continental collision zone of the Alps was driven by buoyant, solid-state emplacement of subducted oceanic and crustal material in a subduction return flow from depths more than 100 km. Chaotic rock deposits and mass transport deposits in the sedimentary covers of some ophiolites may represent synextensional and syncontractional submarine slides during the opening and closure stages of ancient ocean basins, respectively. The Lower Jurassic (201–190 Ma) mafic rock assemblages in the Rif orogenic belt of NW Morocco are part of the Central Atlantic Magmatic Province (CAMP), a major Phanerozoic large igneous province (LIP). The mantle melt source of the CAMP was a subduction-metasomatized mantle lithosphere of supercontinent Pangea and was not associated with mantle-plume activities, as was the case for the development of many other LIPs in Earth history. The Pleistocene seismicity and earthquake event (i.e., August 2011 Mineral earthquake in Virginia, Mw=5.7) in the eastern North American intraplate setting resulted from the release of accumulated strain associated with the state of regional compression. Based on the combined river terrace paleogeodetic data and modeled coseismic deformation of the 2011 Mineral earthquake, the estimated recurrence interval for similar-sized earthquakes in the region is 5.5 ky, significant information regarding the potential seismic hazard for an intraplate tectonic setting, where there is no historical record of seismic activity.
{"title":"Some New Concepts in the Plate Tectonics Paradigm Fifty Years after Its Inception","authors":"Y. Dilek","doi":"10.1086/716515","DOIUrl":"https://doi.org/10.1086/716515","url":null,"abstract":"We have gained significant insights into the dynamic evolution of the earth, causes of geohazards, processes of continental growth, feedback mechanisms between tectonics, topography and climate, and rates of deformation and uplift in collision zones since the initial formulation of the plate tectonics theory 50 years ago. The articles in this special issue present some new concepts, data, and interpretations on various aspects of the plate tectonics paradigm. The onset of plate tectonics may have occurred in a major transition period (2.5–2.0 Ga) in Earth history during which short-lived, geographically limited subduction events resulted in a regime switch from stagnant-lid to plate tectonics. The 1.90–1.87 Ga granulites in the continental lower crust of the Siberian craton contain significant amount of water in the form of structural OH in nominally anhydrous minerals, indicating that the water content of the Precambrian granulites might have been considerably higher than that of the Phanerozoic granulites and the lower crust. The major mountain-building episode (vertical thickening, topographic buildup, and normal faulting) in the evolution of the continental collision zone of the Alps was driven by buoyant, solid-state emplacement of subducted oceanic and crustal material in a subduction return flow from depths more than 100 km. Chaotic rock deposits and mass transport deposits in the sedimentary covers of some ophiolites may represent synextensional and syncontractional submarine slides during the opening and closure stages of ancient ocean basins, respectively. The Lower Jurassic (201–190 Ma) mafic rock assemblages in the Rif orogenic belt of NW Morocco are part of the Central Atlantic Magmatic Province (CAMP), a major Phanerozoic large igneous province (LIP). The mantle melt source of the CAMP was a subduction-metasomatized mantle lithosphere of supercontinent Pangea and was not associated with mantle-plume activities, as was the case for the development of many other LIPs in Earth history. The Pleistocene seismicity and earthquake event (i.e., August 2011 Mineral earthquake in Virginia, Mw=5.7) in the eastern North American intraplate setting resulted from the release of accumulated strain associated with the state of regional compression. Based on the combined river terrace paleogeodetic data and modeled coseismic deformation of the 2011 Mineral earthquake, the estimated recurrence interval for similar-sized earthquakes in the region is 5.5 ky, significant information regarding the potential seismic hazard for an intraplate tectonic setting, where there is no historical record of seismic activity.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"445 - 454"},"PeriodicalIF":1.8,"publicationDate":"2021-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/716515","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47986990","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}
Water in the lower crust plays a critical role in rheological layering of the continental lithosphere. Sixteen granulite xenoliths were collected from the Late Devonian Udachnaya and Komsomolskaya kimberlites in the Siberian Craton. Mafic granulite samples experienced pressures of 0.6–1.0 GPa and temperatures of 549°–800°C using the Grt-Cpx (garnet-clinopyroxene) Fe-Mg thermometer, which are consistently lower than equilibrium temperatures of 737°–899°C from the REE-in-Grt-Cpx thermobarometer. Compared with pseudosection calculations, our samples experienced continuous cooling since the last granulite facies metamorphism. Moderate to high water content was measured in clinopyroxene (334–977 ppm H2O), garnet (23–149 ppm H2O), and plagioclase (157–779 ppm H2O), resulting in the bulk water content of 267–707 ppm H2O in granulite samples. Given the very limited later metasomatism and hydrogen loss, water content in granulite xenoliths probably represents in situ water-rich lower crust of the Siberian Craton from 1.8 Ga to the Late Devonian. Clinopyroxene and plagioclase show weak crystallographic preferred orientations, whereas garnet has random orientation. Compared with previous studies, the Precambrian lower crust in stable cratons contains comparable or less water than Phanerozoic lower crust in orogenic belts. Magma underplating in cratons can trigger partial melting of ancient water-rich granulites and produce heterogeneous water distribution in the lower crust.
{"title":"Water Content and Deformation of the Lower Crust beneath the Siberian Craton: Evidence from Granulite Xenoliths","authors":"T. Jin, Qin Wang, V. Shatsky, Yue Liao","doi":"10.1086/716514","DOIUrl":"https://doi.org/10.1086/716514","url":null,"abstract":"Water in the lower crust plays a critical role in rheological layering of the continental lithosphere. Sixteen granulite xenoliths were collected from the Late Devonian Udachnaya and Komsomolskaya kimberlites in the Siberian Craton. Mafic granulite samples experienced pressures of 0.6–1.0 GPa and temperatures of 549°–800°C using the Grt-Cpx (garnet-clinopyroxene) Fe-Mg thermometer, which are consistently lower than equilibrium temperatures of 737°–899°C from the REE-in-Grt-Cpx thermobarometer. Compared with pseudosection calculations, our samples experienced continuous cooling since the last granulite facies metamorphism. Moderate to high water content was measured in clinopyroxene (334–977 ppm H2O), garnet (23–149 ppm H2O), and plagioclase (157–779 ppm H2O), resulting in the bulk water content of 267–707 ppm H2O in granulite samples. Given the very limited later metasomatism and hydrogen loss, water content in granulite xenoliths probably represents in situ water-rich lower crust of the Siberian Craton from 1.8 Ga to the Late Devonian. Clinopyroxene and plagioclase show weak crystallographic preferred orientations, whereas garnet has random orientation. Compared with previous studies, the Precambrian lower crust in stable cratons contains comparable or less water than Phanerozoic lower crust in orogenic belts. Magma underplating in cratons can trigger partial melting of ancient water-rich granulites and produce heterogeneous water distribution in the lower crust.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"475 - 498"},"PeriodicalIF":1.8,"publicationDate":"2021-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/716514","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47277607","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}
Mesoscopic structural measurements near the top and bottom of the Pennine Zone in the Central Alps of eastern Switzerland indicate multiple, spatially heterogeneous directions of Tertiary movement relative to the Austroalpine allochthon above and the Helvetic zone below. At the top of the Pennine Zone in the Oberhalbstein Valley, motion varies mainly from top-E to top-SSE. At the bottom of the Pennine Zone in the Val Lumnezia area, Ultrahelvetic units exhibit distributed top-NW and top-N shear overprinted by relatively brittle top-NE shear localized just beneath the contact with Penninic units in the Peidener shear zone, which we interpret largely to postdate juxtaposition of Penninic and Helvetic units. Where observed in the Chur Rhine Valley, just 35 km ENE of Val Lumnezia, movement within the basal Pennine units is exclusively top-N. The contrast in movement directions, from top-N to top-NW at the base, to top-E to -SSE at the top, supports the interpretation, drawn from thermochronological data, that the Pennine Zone was tectonically interposed between Adria and Europe as a 20-km-thick “piston” or “mega-pip” from ca. 29 to 18 Ma, driven by its buoyancy contrast with surrounding deep crust and mantle. Emplacement occurred after “docking” of Adria with cratonic Europe at ca. 35 Ma (i.e., continent-continent collision), raising the question of whether the formation of Alpine nappe structure, high Alpine topography, and the peripheral Molasse and Lombardy basins require significant coeval plate convergence.
{"title":"Plate Tectonics and the Alpine Orogeny: Implications of Thermometric and Kinematic Analyses of the Upper and Lower Boundaries of the Pennine Zone in the Central Alps","authors":"J. Price, B. Wernicke","doi":"10.1086/716497","DOIUrl":"https://doi.org/10.1086/716497","url":null,"abstract":"Mesoscopic structural measurements near the top and bottom of the Pennine Zone in the Central Alps of eastern Switzerland indicate multiple, spatially heterogeneous directions of Tertiary movement relative to the Austroalpine allochthon above and the Helvetic zone below. At the top of the Pennine Zone in the Oberhalbstein Valley, motion varies mainly from top-E to top-SSE. At the bottom of the Pennine Zone in the Val Lumnezia area, Ultrahelvetic units exhibit distributed top-NW and top-N shear overprinted by relatively brittle top-NE shear localized just beneath the contact with Penninic units in the Peidener shear zone, which we interpret largely to postdate juxtaposition of Penninic and Helvetic units. Where observed in the Chur Rhine Valley, just 35 km ENE of Val Lumnezia, movement within the basal Pennine units is exclusively top-N. The contrast in movement directions, from top-N to top-NW at the base, to top-E to -SSE at the top, supports the interpretation, drawn from thermochronological data, that the Pennine Zone was tectonically interposed between Adria and Europe as a 20-km-thick “piston” or “mega-pip” from ca. 29 to 18 Ma, driven by its buoyancy contrast with surrounding deep crust and mantle. Emplacement occurred after “docking” of Adria with cratonic Europe at ca. 35 Ma (i.e., continent-continent collision), raising the question of whether the formation of Alpine nappe structure, high Alpine topography, and the peripheral Molasse and Lombardy basins require significant coeval plate convergence.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"499 - 531"},"PeriodicalIF":1.8,"publicationDate":"2021-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47814228","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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