There is a paucity of evidence preserved in the rock record regarding Earth’s earliest enriched crust and its complementary depleted mantle during the Hadean. In recent years, vestiges of these early reservoirs have been inferred by examination of Hf isotope systematics compiled from zircons. The Singhbhum craton of Eastern India, for example, preserves only the existence of an enriched (εHf <0) crustal reservoir during the Hadean−Eoarchean, with the notable absence of a depleted mantle reservoir signature (εHf >0) until ca. 3.5 Ga. Here we report a new Sm-Nd isochron for the Lower Lava greenstones of the western Iron Ore Group from the Singhbhum craton, confirming a 3.42 ± 0.14 Ga crystallization age with an initial εNd of +5.7 ± 2.5. This is the highest positive εNd value derived from an isochron of this age. We infer that this depleted mantle source is a vestige complementary to the primary crust following planetary differentiation. Furthermore, we present U-Pb zircon ages for a 3.39 ± 0.02 Ga tuff that lies stratigraphically above the Lower Lava and <30 cm below an extensive conformable banded iron formation (BIF). This age implies that the western Iron Ore Group’s BIF is the largest economic-grade iron formation of its Paleoarchean age, suggesting that free atmospheric oxygen existed as more than just whiffs at this time.
{"title":"Vestiges of Earth’s earliest depleted mantle reservoir","authors":"Jordan K. Wright, Asish R. Basu","doi":"10.1130/g51936.1","DOIUrl":"https://doi.org/10.1130/g51936.1","url":null,"abstract":"There is a paucity of evidence preserved in the rock record regarding Earth’s earliest enriched crust and its complementary depleted mantle during the Hadean. In recent years, vestiges of these early reservoirs have been inferred by examination of Hf isotope systematics compiled from zircons. The Singhbhum craton of Eastern India, for example, preserves only the existence of an enriched (εHf <0) crustal reservoir during the Hadean−Eoarchean, with the notable absence of a depleted mantle reservoir signature (εHf >0) until ca. 3.5 Ga. Here we report a new Sm-Nd isochron for the Lower Lava greenstones of the western Iron Ore Group from the Singhbhum craton, confirming a 3.42 ± 0.14 Ga crystallization age with an initial εNd of +5.7 ± 2.5. This is the highest positive εNd value derived from an isochron of this age. We infer that this depleted mantle source is a vestige complementary to the primary crust following planetary differentiation. Furthermore, we present U-Pb zircon ages for a 3.39 ± 0.02 Ga tuff that lies stratigraphically above the Lower Lava and <30 cm below an extensive conformable banded iron formation (BIF). This age implies that the western Iron Ore Group’s BIF is the largest economic-grade iron formation of its Paleoarchean age, suggesting that free atmospheric oxygen existed as more than just whiffs at this time.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"212 1‐2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140247304","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}
Éva Oravecz, A. Bálazs, Taras Gerya, Dave A. May, László Fodor
Structural inversion of rifted basins is generally associated with surface uplift and denudation of the sedimentary infill, reflecting the active contractional deformation in the crust. However, worldwide examples of inverted rifts show contrasting basin-scale subsidence and widespread sedimentation patterns during basin inversion. By conducting a series of three-dimensional coupled geodynamic and surface processes models, we investigated the dynamic controls on these subsidence anomalies during the successive stages of rifting and basin inversion, and we propose a new evolutionary model for this process. Our models show that the inherited thermo-rheological properties of the lithosphere influence the initial strain localization and subsequent migration of crustal deformation during inversion. The sense of the vertical movements (i.e., uplift or subsidence), however, is not directly linked to the underlying crustal stress patterns; rather, it reflects the balance among contraction-induced tectonic uplift, postrift thermal subsidence of the inherited lithosphere, and sediment redistribution. Based on the interplay among the competing differential vertical movements with different amplitudes and wavelengths, inversion of rifted basins may lead to the growth of intraplate orogens, or the contraction-driven localized uplift may be hindered by the thermal sag effects of the inherited shallow lithosphere-asthenosphere boundary, resulting in basin-scale subsidence. In such basins, dating the first erosional surfaces and other unconformities may not provide accurate timing for the onset of inversion.
{"title":"Competing effects of crustal shortening, thermal inheritance, and surface processes explain subsidence anomalies in inverted rift basins","authors":"Éva Oravecz, A. Bálazs, Taras Gerya, Dave A. May, László Fodor","doi":"10.1130/g51971.1","DOIUrl":"https://doi.org/10.1130/g51971.1","url":null,"abstract":"Structural inversion of rifted basins is generally associated with surface uplift and denudation of the sedimentary infill, reflecting the active contractional deformation in the crust. However, worldwide examples of inverted rifts show contrasting basin-scale subsidence and widespread sedimentation patterns during basin inversion. By conducting a series of three-dimensional coupled geodynamic and surface processes models, we investigated the dynamic controls on these subsidence anomalies during the successive stages of rifting and basin inversion, and we propose a new evolutionary model for this process. Our models show that the inherited thermo-rheological properties of the lithosphere influence the initial strain localization and subsequent migration of crustal deformation during inversion. The sense of the vertical movements (i.e., uplift or subsidence), however, is not directly linked to the underlying crustal stress patterns; rather, it reflects the balance among contraction-induced tectonic uplift, postrift thermal subsidence of the inherited lithosphere, and sediment redistribution. Based on the interplay among the competing differential vertical movements with different amplitudes and wavelengths, inversion of rifted basins may lead to the growth of intraplate orogens, or the contraction-driven localized uplift may be hindered by the thermal sag effects of the inherited shallow lithosphere-asthenosphere boundary, resulting in basin-scale subsidence. In such basins, dating the first erosional surfaces and other unconformities may not provide accurate timing for the onset of inversion.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"39 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140249075","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}
U. Anyiam, Jiawei Qian, Yuyang Tan, Haijiang Zhang
In the Changning region of the Sichuan Basin, China, which has experienced decades-long injection of freshwater for commercial salt mining, a Ms 6.0 earthquake occurred in June 2019, along with four Ms >5 aftershocks. Using data from local and regional seismic stations, we determine accurate locations for this earthquake sequence, velocity structures, and excess pore fluid pressures of the source region. Our results show that the Ms 6.0 earthquake is extremely shallow (∼1.6 km below mean sea level) and is associated with low Vs and high Vp/Vs values and excess fluid pressures, indicating a fluid-induced earthquake. The mainshock was likely the shallowest M >6.0 induced earthquake, and it was triggered by the combined effect of pore pressure increase from the diffusion of injected fluids, differential formation subsidence from salt caverns, and highly fractured slip-prone rocks enriched in quartz and silica content. Following Coulomb stress transfer from the mainshock, the sequence ruptured in a cascading manner involving preexisting oblique faults of varying dips.
中国四川盆地长宁地区经历了数十年的商业采盐淡水注入，2019 年 6 月发生了 Ms 6.0 地震，并伴有四次 Ms >5 余震。利用当地和区域地震台站的数据，我们确定了此次地震序列的准确位置、速度结构以及震源区域的过剩孔隙流体压力。我们的结果表明，Ms 6.0 地震震源极浅（低于平均海平面 1.6 千米），并伴有低 Vs 值、高 Vp/Vs 值和过剩流体压力，表明这是一次流体诱发地震。主震很可能是 M >6.0 的最浅诱发地震，它是由注入流体扩散造成的孔隙压力增加、盐洞造成的不同地层下陷以及富含石英和二氧化硅的高断裂易滑动岩石的共同作用引发的。在主震的库仑应力传递之后，岩层以级联的方式发生断裂，涉及先前存在的不同倾角的斜断层。
{"title":"Comprehensive seismic evidence for the inducing mechanism of extremely shallow 2019 Changning Ms 6.0 earthquake by solution salt mining, Sichuan Basin, China","authors":"U. Anyiam, Jiawei Qian, Yuyang Tan, Haijiang Zhang","doi":"10.1130/g51699.1","DOIUrl":"https://doi.org/10.1130/g51699.1","url":null,"abstract":"In the Changning region of the Sichuan Basin, China, which has experienced decades-long injection of freshwater for commercial salt mining, a Ms 6.0 earthquake occurred in June 2019, along with four Ms >5 aftershocks. Using data from local and regional seismic stations, we determine accurate locations for this earthquake sequence, velocity structures, and excess pore fluid pressures of the source region. Our results show that the Ms 6.0 earthquake is extremely shallow (∼1.6 km below mean sea level) and is associated with low Vs and high Vp/Vs values and excess fluid pressures, indicating a fluid-induced earthquake. The mainshock was likely the shallowest M >6.0 induced earthquake, and it was triggered by the combined effect of pore pressure increase from the diffusion of injected fluids, differential formation subsidence from salt caverns, and highly fractured slip-prone rocks enriched in quartz and silica content. Following Coulomb stress transfer from the mainshock, the sequence ruptured in a cascading manner involving preexisting oblique faults of varying dips.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"45 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140253128","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}
Unique intraplate volcano eruptions and westward volcano migration since the Oligocene have been observed in northeast China, where an overriding continental zone is tectonically controlled by the subduction of the northwestern Pacific plate and the opening of Japan Sea. Interestingly, these intraplate magmatic events occur around a subsiding basin (the Songliao Basin), but no volcanic activity has been observed within the Songliao Basin. The geodynamic mechanism responsible for these volcanoes remains unclear. To address the geodynamic process beneath northeast China, we conducted numerical models constrained by data from regional reconstruction and seismic and volcanic studies. The vertical velocity field of mantle convection and lithospheric partial melting structures derived from our models show that mantle upwelling and melting centers migrate from east to west in northeast China with the westward propagation of the sub-horizontal slab in the transition zone, leading to the observed volcano migration. Also, with the subduction retreat of the northwestern region of the Pacific plate and the opening of the Japan Sea, significant lithospheric thickness differences developed between the Changbaishan-Mudanjiang region and the Songliao Basin, leading to lithospheric unstable dripping. This dripping structure prevents the partial melting of the lithosphere but facilitates the subsidence of the Songliao Basin. Moreover, the lithospheric dripping model successfully predicts upper mantle structures consistent with the proposed tomography model, the observed Moho depth, and surface topography variations. Thus, lithospheric dripping induced by lithospheric thickness differences and the retreating subduction of the Pacific slab provides a robust mechanism for the unique geodynamic processes in northeast China.
{"title":"An ongoing lithospheric dripping process beneath northeast China and its impact on intraplate volcanism","authors":"Feiyu Lin, Liang Qi, Nan Zhang, Zhen Guo","doi":"10.1130/g51861.1","DOIUrl":"https://doi.org/10.1130/g51861.1","url":null,"abstract":"Unique intraplate volcano eruptions and westward volcano migration since the Oligocene have been observed in northeast China, where an overriding continental zone is tectonically controlled by the subduction of the northwestern Pacific plate and the opening of Japan Sea. Interestingly, these intraplate magmatic events occur around a subsiding basin (the Songliao Basin), but no volcanic activity has been observed within the Songliao Basin. The geodynamic mechanism responsible for these volcanoes remains unclear. To address the geodynamic process beneath northeast China, we conducted numerical models constrained by data from regional reconstruction and seismic and volcanic studies. The vertical velocity field of mantle convection and lithospheric partial melting structures derived from our models show that mantle upwelling and melting centers migrate from east to west in northeast China with the westward propagation of the sub-horizontal slab in the transition zone, leading to the observed volcano migration. Also, with the subduction retreat of the northwestern region of the Pacific plate and the opening of the Japan Sea, significant lithospheric thickness differences developed between the Changbaishan-Mudanjiang region and the Songliao Basin, leading to lithospheric unstable dripping. This dripping structure prevents the partial melting of the lithosphere but facilitates the subsidence of the Songliao Basin. Moreover, the lithospheric dripping model successfully predicts upper mantle structures consistent with the proposed tomography model, the observed Moho depth, and surface topography variations. Thus, lithospheric dripping induced by lithospheric thickness differences and the retreating subduction of the Pacific slab provides a robust mechanism for the unique geodynamic processes in northeast China.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"142 20","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140078293","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}
Micrometer-size magnetite crystals with peculiar morphologies, such as spearhead, spindle, needle, and giant bullet, known as giant magnetofossils, were previously identified in marine sediments mainly during the Eocene epoch. The origin of these unusual magnetite crystals remains unclear because no known modern analogues have been found, and data about their spatiotemporal distribution are sparse. Here, using electron microscope observations, we performed a large-scale spatiotemporal search for these mysterious magnetite crystals. We report the occurrence of giant magnetofossils in variable marine sedimentary environments, including the first report in modern South Atlantic and Indian Ocean environments and the oldest occurrence at ca. 93 Ma in the North Atlantic Ocean. Grain-size data for the giant magnetofossils in the Southwest Pacific and North Atlantic Oceans suggest that the dimension of spearheads is sensitive to regional environments during similar warming periods but is insensitive to environmental conditions across the distinct geological periods at the same locality. The grain-size distributions of needles and giant bullets do not show significant changes in diverse environments. These observations greatly expand the known temporal and geographic distribution of giant magnetofossils, shedding new light on their likely biological origin.
具有特殊形态（如矛头状、纺锤状、针状和巨型子弹状）的微米级磁铁矿晶体被称为巨磁化石，以前主要在始新世时期的海洋沉积物中发现过。这些不寻常的磁铁矿晶体的起源仍不清楚，因为没有发现已知的现代类似物，有关其时空分布的数据也很稀少。在这里，我们利用电子显微镜观察，对这些神秘的磁铁矿晶体进行了大规模的时空搜索。我们报告了在不同的海洋沉积环境中出现的巨磁石化石，包括首次在现代南大西洋和印度洋环境中出现的巨磁石化石，以及在北大西洋约 93 Ma 处出现的最古老的巨磁石化石。93 Ma）。西南太平洋和北大西洋巨型磁化石的粒度数据表明，矛头的尺寸对类似变暖时期的区域环境很敏感，但对同一地点不同地质时期的环境条件不敏感。针状物和巨型弹丸的粒度分布在不同的环境中没有明显的变化。这些观察结果极大地扩展了巨型磁化石的已知时间和地理分布，为其可能的生物起源提供了新的线索。
{"title":"Spatiotemporal distribution of giant magnetofossils holds clues to their biological origin","authors":"Pengfei Xue, Liao Chang","doi":"10.1130/g51809.1","DOIUrl":"https://doi.org/10.1130/g51809.1","url":null,"abstract":"Micrometer-size magnetite crystals with peculiar morphologies, such as spearhead, spindle, needle, and giant bullet, known as giant magnetofossils, were previously identified in marine sediments mainly during the Eocene epoch. The origin of these unusual magnetite crystals remains unclear because no known modern analogues have been found, and data about their spatiotemporal distribution are sparse. Here, using electron microscope observations, we performed a large-scale spatiotemporal search for these mysterious magnetite crystals. We report the occurrence of giant magnetofossils in variable marine sedimentary environments, including the first report in modern South Atlantic and Indian Ocean environments and the oldest occurrence at ca. 93 Ma in the North Atlantic Ocean. Grain-size data for the giant magnetofossils in the Southwest Pacific and North Atlantic Oceans suggest that the dimension of spearheads is sensitive to regional environments during similar warming periods but is insensitive to environmental conditions across the distinct geological periods at the same locality. The grain-size distributions of needles and giant bullets do not show significant changes in diverse environments. These observations greatly expand the known temporal and geographic distribution of giant magnetofossils, shedding new light on their likely biological origin.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"95 13","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140079641","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}
Dylan A. Vasey, J. Naliboff, Eric Cowgill, Sascha Brune, A. Glerum, Frank Zwaan
Although many collisional orogens form after subduction of oceanic lithosphere between two continents, some orogens result from strain localization within a continent via inversion of structures inherited from continental rifting. Intracontinental rift-inversion orogens exhibit a range of structural styles, but the underlying causes of such variability have not been extensively explored. We use numerical models of intracontinental rift inversion to investigate the impact of parameters including rift structure, rift duration, post-rift cooling, and convergence velocity on orogen structure. Our models reproduce the natural variability of rift-inversion orogens and can be categorized using three endmember styles: asymmetric underthrusting (AU), distributed thickening (DT), and localized polarity flip (PF). Inversion of narrow rifts tends to produce orogens with more localized deformation (styles AU and PF) than those resulting from wide rifts. However, multiple combinations of the parameters we investigated can produce the same structural style. Thus, our models indicate no unique relationship between orogenic structure and the conditions prior to and during inversion. Because the style of rift-inversion orogenesis is highly contingent upon the rift history prior to inversion, knowing the geologic history that preceded rift inversion is essential for translating orogenic structure into the processes that produced that structure.
{"title":"Impact of rift history on the structural style of intracontinental rift-inversion orogens","authors":"Dylan A. Vasey, J. Naliboff, Eric Cowgill, Sascha Brune, A. Glerum, Frank Zwaan","doi":"10.1130/g51489.1","DOIUrl":"https://doi.org/10.1130/g51489.1","url":null,"abstract":"Although many collisional orogens form after subduction of oceanic lithosphere between two continents, some orogens result from strain localization within a continent via inversion of structures inherited from continental rifting. Intracontinental rift-inversion orogens exhibit a range of structural styles, but the underlying causes of such variability have not been extensively explored. We use numerical models of intracontinental rift inversion to investigate the impact of parameters including rift structure, rift duration, post-rift cooling, and convergence velocity on orogen structure. Our models reproduce the natural variability of rift-inversion orogens and can be categorized using three endmember styles: asymmetric underthrusting (AU), distributed thickening (DT), and localized polarity flip (PF). Inversion of narrow rifts tends to produce orogens with more localized deformation (styles AU and PF) than those resulting from wide rifts. However, multiple combinations of the parameters we investigated can produce the same structural style. Thus, our models indicate no unique relationship between orogenic structure and the conditions prior to and during inversion. Because the style of rift-inversion orogenesis is highly contingent upon the rift history prior to inversion, knowing the geologic history that preceded rift inversion is essential for translating orogenic structure into the processes that produced that structure.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"36 13","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140084544","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}
Jonathan Wolf, Mingming Li, Anne A. Haws, Maureen D. Long
The Yellowstone region (western United States) is a commonly cited example of intraplate volcanism whose origin has been a topic of debate for several decades. Recent work has suggested that a deep mantle plume, rooted beneath southern California, is the source of Yellowstone volcanism. Seismic anisotropy, which typically results from deformation, can be used to identify and characterize mantle flow. Here, we show that the proposed plume root location at the base of the mantle is strongly seismically anisotropic. This finding is complemented by geodynamic modeling results showing upwelling flow and high strains in the lowermost mantle beneath the Yellowstone region. Our results support the idea that the Yellowstone volcanism is caused by a plume rooted in the deepest mantle beneath southern California, connecting dynamics in the deepest mantle with phenomena at Earth’s surface.
{"title":"Strong seismic anisotropy due to upwelling flow at the root of the Yellowstone mantle plume","authors":"Jonathan Wolf, Mingming Li, Anne A. Haws, Maureen D. Long","doi":"10.1130/g51919.1","DOIUrl":"https://doi.org/10.1130/g51919.1","url":null,"abstract":"The Yellowstone region (western United States) is a commonly cited example of intraplate volcanism whose origin has been a topic of debate for several decades. Recent work has suggested that a deep mantle plume, rooted beneath southern California, is the source of Yellowstone volcanism. Seismic anisotropy, which typically results from deformation, can be used to identify and characterize mantle flow. Here, we show that the proposed plume root location at the base of the mantle is strongly seismically anisotropic. This finding is complemented by geodynamic modeling results showing upwelling flow and high strains in the lowermost mantle beneath the Yellowstone region. Our results support the idea that the Yellowstone volcanism is caused by a plume rooted in the deepest mantle beneath southern California, connecting dynamics in the deepest mantle with phenomena at Earth’s surface.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"4 9","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140083283","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}
Xinyue Xu, W. Bain, F. Tornos, J. Hanchar, Hector M. Lamadrid, Bernd Lehmann, Xiaochun Xu, Jeffrey A. Steadman, Ralph S. Bottrill, Majid Soleymani, A. Rajabi, Peng Li, Xuehai Tan, Shihong Xu, A. Locock, M. Steele‐MacInnis
The origins of magnetite-apatite deposits are controversial, and the crux of the debate is what types of fluids form these rocks. We present evidence from 20 magnetite-apatite deposits worldwide showing ubiquitous involvement of molten salts. The studied deposits are distributed globally, from various tectonic settings, and from Precambrian to Quaternary in age. In every case, water-poor polycrystalline melt inclusions in ore-stage minerals are dominated by sulfate, chloride, and carbonate components plus variable proportions of calc-silicates, phosphates, and iron ± titanium oxides that re-melt between 285 °C and 1100 °C. These fluids are very different from what is generally expected in most geologic settings, but their ubiquitous presence in magnetite-apatite rocks indicates that molten salts are widespread and essential to the formation of these deposits.
磁铁矿-磷灰石矿床的起源颇具争议，争论的焦点在于形成这些岩石的流体类型。我们展示了来自全球 20 个磁铁矿-磷灰石矿床的证据，表明熔盐的参与无处不在。所研究的矿床分布在全球各地，来自不同的构造环境，年代从前寒武纪到第四纪。在每一种情况下，矿石阶段矿物中的贫水多晶熔融包裹体都以硫酸盐、氯化物和碳酸盐成分为主，外加不同比例的钙硅酸盐、磷酸盐和铁±钛氧化物，在 285 °C 至 1100 °C 之间重新熔融。这些流体与大多数地质环境中的预期流体大相径庭，但它们在磁铁矿-磷灰石岩石中的普遍存在表明，熔盐在这些矿床的形成过程中非常普遍和重要。
{"title":"Magnetite-apatite ores record widespread involvement of molten salts","authors":"Xinyue Xu, W. Bain, F. Tornos, J. Hanchar, Hector M. Lamadrid, Bernd Lehmann, Xiaochun Xu, Jeffrey A. Steadman, Ralph S. Bottrill, Majid Soleymani, A. Rajabi, Peng Li, Xuehai Tan, Shihong Xu, A. Locock, M. Steele‐MacInnis","doi":"10.1130/g51887.1","DOIUrl":"https://doi.org/10.1130/g51887.1","url":null,"abstract":"The origins of magnetite-apatite deposits are controversial, and the crux of the debate is what types of fluids form these rocks. We present evidence from 20 magnetite-apatite deposits worldwide showing ubiquitous involvement of molten salts. The studied deposits are distributed globally, from various tectonic settings, and from Precambrian to Quaternary in age. In every case, water-poor polycrystalline melt inclusions in ore-stage minerals are dominated by sulfate, chloride, and carbonate components plus variable proportions of calc-silicates, phosphates, and iron ± titanium oxides that re-melt between 285 °C and 1100 °C. These fluids are very different from what is generally expected in most geologic settings, but their ubiquitous presence in magnetite-apatite rocks indicates that molten salts are widespread and essential to the formation of these deposits.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"109 51","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140089430","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}
Jing-Yuan Zhang, Kun-Feng Qiu, Runsheng Yin, Zheng-Yu Long, Yue-Chuan Feng, Hao-Cheng Yu, Zi-Yue Gao, Jun Deng
The subcontinental lithospheric mantle (SCLM) is now suggested by many workers to play a role in the formation of orogenic gold deposits in active continental margins, given that the gold and volatiles may be introduced into the SCLM during plate subduction. The giant Cretaceous Jiaodong gold province within the North China block occurs in a convergent margin setting where there is no terrane accretion, thus eliminating from consideration the commonly accepted metal and fluid source reservoir for orogenic gold—the metamorphosed upper crust of an active orogenic belt. Thus, the auriferous fluids were released below the high-grade Archean cratonic rocks from either enriched SCLM or directly from the subducted oceanic slab. Mercury (Hg) isotopes allow fingerprinting of isotopically distinct reservoirs, discriminating between the two possibilities. We present Hg isotope data for a set of pyrite grains, native gold, and lamprophyres from six main gold districts in the Jiaodong gold province. These samples dominantly yielded near-zero Δ199Hg (0‰ ± 0.1‰), within uncertainty of the mantle Δ199Hg value. The results provide novel evidence that the SCLM predominantly controlled the Hg budget of the Jiaodong gold deposits, implying that the SCLM acted as a buffer for the crustal mass-independent fractionation Hg signatures and a storage reservoir for gold and volatiles. Consequently, during slab subduction, gold and volatiles were recycled and stored within the SCLM and were later released from the mantle during uplift and relaxation of compressional tectonics.
{"title":"Lithospheric mantle as a metal storage reservoir for orogenic gold deposits in active continental margins: Evidence from Hg isotopes","authors":"Jing-Yuan Zhang, Kun-Feng Qiu, Runsheng Yin, Zheng-Yu Long, Yue-Chuan Feng, Hao-Cheng Yu, Zi-Yue Gao, Jun Deng","doi":"10.1130/g51871.1","DOIUrl":"https://doi.org/10.1130/g51871.1","url":null,"abstract":"The subcontinental lithospheric mantle (SCLM) is now suggested by many workers to play a role in the formation of orogenic gold deposits in active continental margins, given that the gold and volatiles may be introduced into the SCLM during plate subduction. The giant Cretaceous Jiaodong gold province within the North China block occurs in a convergent margin setting where there is no terrane accretion, thus eliminating from consideration the commonly accepted metal and fluid source reservoir for orogenic gold—the metamorphosed upper crust of an active orogenic belt. Thus, the auriferous fluids were released below the high-grade Archean cratonic rocks from either enriched SCLM or directly from the subducted oceanic slab. Mercury (Hg) isotopes allow fingerprinting of isotopically distinct reservoirs, discriminating between the two possibilities. We present Hg isotope data for a set of pyrite grains, native gold, and lamprophyres from six main gold districts in the Jiaodong gold province. These samples dominantly yielded near-zero Δ199Hg (0‰ ± 0.1‰), within uncertainty of the mantle Δ199Hg value. The results provide novel evidence that the SCLM predominantly controlled the Hg budget of the Jiaodong gold deposits, implying that the SCLM acted as a buffer for the crustal mass-independent fractionation Hg signatures and a storage reservoir for gold and volatiles. Consequently, during slab subduction, gold and volatiles were recycled and stored within the SCLM and were later released from the mantle during uplift and relaxation of compressional tectonics.","PeriodicalId":503125,"journal":{"name":"Geology","volume":" 549","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140092534","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}
F.M. Rey, M. Malkowski, J. Fosdick, S.C. Dobbs, M. Calderón, M. Ghiglione, S.A. Graham
U-Pb zircon geochronology and isotopic records have played an influential role in our understanding of convergent margin dynamics. Orogenic cyclicity models link tectonic regimes with magmatic isotopic signatures in advancing orogens, relating compressional regimes with evolved signatures and extension with juvenile signatures; however, such frameworks may not apply for retreating orogens, which commonly produce substantial crustal heterogeneities during backarc rifting and ocean spreading. We explore the Mesozoic to Cenozoic Patagonian Andes tectonic evolution, combining U-Pb zircon ages, bulk rock εNd, and new detrital zircon εHf from the retroarc basin to understand the associated magmatic arc evolution during retreat and advance of the margin. Our results reveal a protracted phase of isotopically juvenile magmatism between 150 and 80 Ma, which began during backarc extension and persisted long after the margin switched to a contractional regime. We propose that the prolonged juvenile isotopic trend started mainly due to trenchward migration of the arc during backarc extension (150−120 Ma) and persisted due to partial melting of underthrusted juvenile attenuated and oceanic crust during backarc basin closure (120−80 Ma). This interpretation implies that tectonic stress alone does not predict isotopic trends, and factors like assimilation or the composition of underthrusted crust are important controls on magmatic isotopic composition, especially in retreating and transitional orogens.
{"title":"Detrital isotopic record of a retreating accretionary orogen: An example from the Patagonian Andes","authors":"F.M. Rey, M. Malkowski, J. Fosdick, S.C. Dobbs, M. Calderón, M. Ghiglione, S.A. Graham","doi":"10.1130/g51918.1","DOIUrl":"https://doi.org/10.1130/g51918.1","url":null,"abstract":"U-Pb zircon geochronology and isotopic records have played an influential role in our understanding of convergent margin dynamics. Orogenic cyclicity models link tectonic regimes with magmatic isotopic signatures in advancing orogens, relating compressional regimes with evolved signatures and extension with juvenile signatures; however, such frameworks may not apply for retreating orogens, which commonly produce substantial crustal heterogeneities during backarc rifting and ocean spreading. We explore the Mesozoic to Cenozoic Patagonian Andes tectonic evolution, combining U-Pb zircon ages, bulk rock εNd, and new detrital zircon εHf from the retroarc basin to understand the associated magmatic arc evolution during retreat and advance of the margin. Our results reveal a protracted phase of isotopically juvenile magmatism between 150 and 80 Ma, which began during backarc extension and persisted long after the margin switched to a contractional regime. We propose that the prolonged juvenile isotopic trend started mainly due to trenchward migration of the arc during backarc extension (150−120 Ma) and persisted due to partial melting of underthrusted juvenile attenuated and oceanic crust during backarc basin closure (120−80 Ma). This interpretation implies that tectonic stress alone does not predict isotopic trends, and factors like assimilation or the composition of underthrusted crust are important controls on magmatic isotopic composition, especially in retreating and transitional orogens.","PeriodicalId":503125,"journal":{"name":"Geology","volume":"25 22","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140419568","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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