K. Seelos, Y. Rojas‐Agramonte, A. Kröner, T. Toulkeridis, Gillian Inderwies, Yvonne Buelow
The Galápagos Archipelago is the surface expression of an active hotspot or long-lived mantle plume. The Archipelago consists of a group of 13 main islands which are located in the eastern central Pacific Ocean about 1,000 km west of the northern edge of the South American continent, east of the East Pacific Rise and south of the Galápagos spreading center. Because of the large distance to the nearest continental land mass, Galapagos can be seen as an almost isolated sedimentary system. A provenance study conducted on samples collected from seventeen beaches on eleven islands, demonstrates that mineral grains and particles were derived from weathering of predominantly basaltic rocks and were transported within the islands, between the islands or inside the coastal area around the Archipelago. The exclusion of external sources allows advanced studies about erosion processes, transport pathways of particles and the accumulation of autochthonous sediments. The combined usage of optical particle size and shape analysis with RAMAN spectroscopy allows a successful spatial delimitation of host rocks and a reconstruction of transport pathways. The analyzed samples can be subdivided into three groups: 1) Type-A sediments: fine-grained and sampled on beaches of the oldest islands in the eastern part of Galápagos. The composition of volcanic minerals corresponds to the alkaline character of the basaltic source rocks. 2) Type-B: well sorted sediments characterized by medium-grained olivine, pyroxene, plagioclase and even a small amount of quartz grains. The islands of this group are located in the central region of the Archipelago. 3) Type-C samples: olivine and pyroxene are the predominant volcanic minerals. These samples indicate bimodal, coarse-grained size distributions and large proportions of pumice and are found in Floreana in the south and the youngest islands Isabela and Fernandina in the west of Galápagos.
{"title":"Composition and provenance analysis of beach sands in an almost isolated sedimentary system – A field study of the Galápagos Archipelago","authors":"K. Seelos, Y. Rojas‐Agramonte, A. Kröner, T. Toulkeridis, Gillian Inderwies, Yvonne Buelow","doi":"10.2475/05.2021.04","DOIUrl":"https://doi.org/10.2475/05.2021.04","url":null,"abstract":"The Galápagos Archipelago is the surface expression of an active hotspot or long-lived mantle plume. The Archipelago consists of a group of 13 main islands which are located in the eastern central Pacific Ocean about 1,000 km west of the northern edge of the South American continent, east of the East Pacific Rise and south of the Galápagos spreading center. Because of the large distance to the nearest continental land mass, Galapagos can be seen as an almost isolated sedimentary system. A provenance study conducted on samples collected from seventeen beaches on eleven islands, demonstrates that mineral grains and particles were derived from weathering of predominantly basaltic rocks and were transported within the islands, between the islands or inside the coastal area around the Archipelago. The exclusion of external sources allows advanced studies about erosion processes, transport pathways of particles and the accumulation of autochthonous sediments. The combined usage of optical particle size and shape analysis with RAMAN spectroscopy allows a successful spatial delimitation of host rocks and a reconstruction of transport pathways. The analyzed samples can be subdivided into three groups: 1) Type-A sediments: fine-grained and sampled on beaches of the oldest islands in the eastern part of Galápagos. The composition of volcanic minerals corresponds to the alkaline character of the basaltic source rocks. 2) Type-B: well sorted sediments characterized by medium-grained olivine, pyroxene, plagioclase and even a small amount of quartz grains. The islands of this group are located in the central region of the Archipelago. 3) Type-C samples: olivine and pyroxene are the predominant volcanic minerals. These samples indicate bimodal, coarse-grained size distributions and large proportions of pumice and are found in Floreana in the south and the youngest islands Isabela and Fernandina in the west of Galápagos.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"888 - 906"},"PeriodicalIF":2.9,"publicationDate":"2021-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45039372","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
L. Boschman, D. Hinsbergen, C. Langereis, K. Flores, P. Kamp, D. Kimbrough, H. Ueda, S. H. Lagemaat, Erik van der Wiel, W. Spakman
The Panthalassa Ocean, which surrounded the late Paleozoic-early Mesozoic Pangea supercontinent, was underlain by multiple tectonic plates that have since been lost to subduction. In this study, we develop an approach to reconstruct plate motions of this subducted lithosphere utilizing paleomagnetic data from accreted Ocean Plate Stratigraphy (OPS). We first establish the boundaries of the Panthalassa domain by using available Indo-Atlantic plate reconstructions and restorations of complex plate boundary deformation at circum-Panthalassa trenches. We reconstruct the Pacific Plate and its conjugates, the Farallon, Phoenix, and Izanagi plates, back to 190 Ma using marine magnetic anomaly records of the modern Pacific. Then, we present new and review published paleomagnetic data from OPS exposed in the accretionary complexes of Cedros Island (Mexico), the Santa Elena Peninsula (Costa Rica), the North Island of New Zealand, and Japan. These data provide paleolatitudinal plate motion components of the Farallon, Phoenix and Izanagi plates, and constrain the trajectories of these plates from their spreading ridges towards the trenches in which they subducted. For 83 to 150 Ma, we use two independent mantle frames to connect the Panthalassa plate system to the Indo-Atlantic plate system and test the feasibility of this approach with the paleomagnetic data. For times prior to 150 Ma, and as far back as Permian time, we reconstruct relative and absolute Panthalassa plate motions such that divergence is maintained between the Izanagi, Farallon and Phoenix plates, convergence is maintained with Pangean continental margins in Japan, Mexico and New Zealand, and paleomagnetic constraints are met. The reconstruction approach developed here enables data-based reconstruction of oceanic plates and plate boundaries in the absence of marine magnetic anomaly data or mantle reference frames, using Ocean Plate Stratigraphy, paleo-magnetism, and constraints on the nature of circum-oceanic plate boundaries. Such an approach is a crucial next step towards quantitative reconstruction of the currently largely unknown tectonic evolution of the Earth's oceanic domains in deep geological time.
{"title":"Reconstructing lost plates of the panthalassa ocean through Paleomagnetic data from circum-pacific accretionary orogens","authors":"L. Boschman, D. Hinsbergen, C. Langereis, K. Flores, P. Kamp, D. Kimbrough, H. Ueda, S. H. Lagemaat, Erik van der Wiel, W. Spakman","doi":"10.2475/06.2021.08","DOIUrl":"https://doi.org/10.2475/06.2021.08","url":null,"abstract":"The Panthalassa Ocean, which surrounded the late Paleozoic-early Mesozoic Pangea supercontinent, was underlain by multiple tectonic plates that have since been lost to subduction. In this study, we develop an approach to reconstruct plate motions of this subducted lithosphere utilizing paleomagnetic data from accreted Ocean Plate Stratigraphy (OPS). We first establish the boundaries of the Panthalassa domain by using available Indo-Atlantic plate reconstructions and restorations of complex plate boundary deformation at circum-Panthalassa trenches. We reconstruct the Pacific Plate and its conjugates, the Farallon, Phoenix, and Izanagi plates, back to 190 Ma using marine magnetic anomaly records of the modern Pacific. Then, we present new and review published paleomagnetic data from OPS exposed in the accretionary complexes of Cedros Island (Mexico), the Santa Elena Peninsula (Costa Rica), the North Island of New Zealand, and Japan. These data provide paleolatitudinal plate motion components of the Farallon, Phoenix and Izanagi plates, and constrain the trajectories of these plates from their spreading ridges towards the trenches in which they subducted. For 83 to 150 Ma, we use two independent mantle frames to connect the Panthalassa plate system to the Indo-Atlantic plate system and test the feasibility of this approach with the paleomagnetic data. For times prior to 150 Ma, and as far back as Permian time, we reconstruct relative and absolute Panthalassa plate motions such that divergence is maintained between the Izanagi, Farallon and Phoenix plates, convergence is maintained with Pangean continental margins in Japan, Mexico and New Zealand, and paleomagnetic constraints are met. The reconstruction approach developed here enables data-based reconstruction of oceanic plates and plate boundaries in the absence of marine magnetic anomaly data or mantle reference frames, using Ocean Plate Stratigraphy, paleo-magnetism, and constraints on the nature of circum-oceanic plate boundaries. Such an approach is a crucial next step towards quantitative reconstruction of the currently largely unknown tectonic evolution of the Earth's oceanic domains in deep geological time.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"907 - 954"},"PeriodicalIF":2.9,"publicationDate":"2021-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48780446","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
X. Wang, Jian Zhang, C. Yin, Hai Zhou, Jin Liu, Xiaoguang Liu, Chen Zhao
Located in the Western Block of the North China Craton, the Khondalite Belt is one of the three Paleoproterozoic tectonic belts that were linked to the final assembly of the craton. At present, a popular model is that the Khondalite Belt was formed by the collision between the Yinshan and Ordos blocks at ∼1.95 Ga. However, the initiation of oceanic subduction and its related arc magmatism and accretionary process before the collisional event were poorly constrained. The Daqingshan Complex is located in the middle East part of the Khondalite Belt, and contains highly deformed and metamorphosed rock assemblages, and thus represents a key area to decipher the above issue. In this study, we carried out petrological, geochemical and geochronological analysis on the TTG granitoids of the Daqingshan Complex. Zircon U-Pb results from three typical TTG samples yielded upper intercept ages of 2545 ± 50 Ma, 2484 ± 68 Ma and 2452 ± 32 Ma, indicating that the TTG granitoids were emplaced in the late Neoarchean. Metamorphic zircons from two samples gave 207Pb/206Pb weighted mean ages of 1892 ± 53 Ma and 1906 ± 27 Ma, respectively, recording the timing of a continent-to-continent collisional event. Thirteen TTG granitoid samples are geochemically low-, medium- and high-K calc-alkaline, with metaluminous to peraluminous trends and are enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, La, Ce, Nd, and depleted in high field strength elements (HFSEs) such as Nb and Ta. Chondrite-normalized rare earth element (REE) patterns show fractionation with (La/Yb) N ratios ranging from 8.20 to 27.47, with weak Eu negative anomalies (δEu = 0.50 – 0.98). In addition, TTG granitoids of the Daqingshan Complex belong to I-type granitoids, and their igneous protoliths were intimately related to a subduction-related magmatic arc environment. New results of this study reveal that the initial oceanic lithosphere subduction operated since ∼2.55 Ga along the southern margin of the Yinshan Block, and generated the coeval arc-related TTG granitoids. Closure of the ocean led to the final collision between the Yinshan and Ordos blocks and the amalgamation of the Western Block at 1.95 to 1.85 Ga.
{"title":"Petrogenesis and tectonic implications of TTG granitoids from the Daqingshan Complex of the Khondalite Belt, North China Craton","authors":"X. Wang, Jian Zhang, C. Yin, Hai Zhou, Jin Liu, Xiaoguang Liu, Chen Zhao","doi":"10.2475/06.2021.02","DOIUrl":"https://doi.org/10.2475/06.2021.02","url":null,"abstract":"Located in the Western Block of the North China Craton, the Khondalite Belt is one of the three Paleoproterozoic tectonic belts that were linked to the final assembly of the craton. At present, a popular model is that the Khondalite Belt was formed by the collision between the Yinshan and Ordos blocks at ∼1.95 Ga. However, the initiation of oceanic subduction and its related arc magmatism and accretionary process before the collisional event were poorly constrained. The Daqingshan Complex is located in the middle East part of the Khondalite Belt, and contains highly deformed and metamorphosed rock assemblages, and thus represents a key area to decipher the above issue. In this study, we carried out petrological, geochemical and geochronological analysis on the TTG granitoids of the Daqingshan Complex. Zircon U-Pb results from three typical TTG samples yielded upper intercept ages of 2545 ± 50 Ma, 2484 ± 68 Ma and 2452 ± 32 Ma, indicating that the TTG granitoids were emplaced in the late Neoarchean. Metamorphic zircons from two samples gave 207Pb/206Pb weighted mean ages of 1892 ± 53 Ma and 1906 ± 27 Ma, respectively, recording the timing of a continent-to-continent collisional event. Thirteen TTG granitoid samples are geochemically low-, medium- and high-K calc-alkaline, with metaluminous to peraluminous trends and are enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, La, Ce, Nd, and depleted in high field strength elements (HFSEs) such as Nb and Ta. Chondrite-normalized rare earth element (REE) patterns show fractionation with (La/Yb) N ratios ranging from 8.20 to 27.47, with weak Eu negative anomalies (δEu = 0.50 – 0.98). In addition, TTG granitoids of the Daqingshan Complex belong to I-type granitoids, and their igneous protoliths were intimately related to a subduction-related magmatic arc environment. New results of this study reveal that the initial oceanic lithosphere subduction operated since ∼2.55 Ga along the southern margin of the Yinshan Block, and generated the coeval arc-related TTG granitoids. Closure of the ocean led to the final collision between the Yinshan and Ordos blocks and the amalgamation of the Western Block at 1.95 to 1.85 Ga.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"680 - 707"},"PeriodicalIF":2.9,"publicationDate":"2021-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42218917","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
H. Robertson, Fiona Whitaker, Cathy Hollis, H. Corlett
{"title":"SOLUBILITY PRODUCT CONSTANTS FOR NATURAL DOLOMITE (0-200°C) THROUGH A GROUNDWATER-BASED APPROACH USING THE USGS PRODUCED WATER DATABASE – PART A","authors":"H. Robertson, Fiona Whitaker, Cathy Hollis, H. Corlett","doi":"10.31223/x5ps42","DOIUrl":"https://doi.org/10.31223/x5ps42","url":null,"abstract":"","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"1 1","pages":""},"PeriodicalIF":2.9,"publicationDate":"2021-05-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"69638375","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Sebastian Jimenez-Rodriguez, M. Dettinger, J. Quade, Kendra E. Murray
Establishing the timing of surface uplift in the Central Andes is essential for evaluating the geodynamic mechanisms responsible for mountain building and their role in the development of dry conditions along the western coasts of Peru and Chile. Here, we present new stable hydrogen isotopic values from stream waters and hydration water in volcanic glass from northern Chile (18.5–19.5°S) that show that the Western Cordillera was already elevated by the early Miocene. The hydrogen isotopic values of reconstructed surface waters obtained from ancient and modern volcanic glass indicate that the Western Cordillera in northern Chile attained modern elevations by at least 22.8 Ma. When combined with paleoaltimetric records from the Altiplano and northwestern Puna, these results demonstrate that surface uplift of the Andean plateau was a time-transgressive process that varied not just from west to east but also from north and south along the strike of the orogen. Our paleoaltimetry reconstruction also suggests that the Western Cordillera has blocked moisture coming from the east since at least the early Miocene, consistent with previously published evidence of arid-semiarid conditions in the Atacama Desert. However, hyperaridity on the western Andean slope developed later and appears to correspond with the timing of uplift in the Eastern Cordillera and Altiplano. Our results suggest that the growth of the Central Andean rain shadow relied not only on the elevation of the Western Cordillera but also on the widening of the plateau.
{"title":"Paleoaltimetry of the Western Andes in Northern Chile (∼18.5–19.5°S)","authors":"Sebastian Jimenez-Rodriguez, M. Dettinger, J. Quade, Kendra E. Murray","doi":"10.2475/05.2021.01","DOIUrl":"https://doi.org/10.2475/05.2021.01","url":null,"abstract":"Establishing the timing of surface uplift in the Central Andes is essential for evaluating the geodynamic mechanisms responsible for mountain building and their role in the development of dry conditions along the western coasts of Peru and Chile. Here, we present new stable hydrogen isotopic values from stream waters and hydration water in volcanic glass from northern Chile (18.5–19.5°S) that show that the Western Cordillera was already elevated by the early Miocene. The hydrogen isotopic values of reconstructed surface waters obtained from ancient and modern volcanic glass indicate that the Western Cordillera in northern Chile attained modern elevations by at least 22.8 Ma. When combined with paleoaltimetric records from the Altiplano and northwestern Puna, these results demonstrate that surface uplift of the Andean plateau was a time-transgressive process that varied not just from west to east but also from north and south along the strike of the orogen. Our paleoaltimetry reconstruction also suggests that the Western Cordillera has blocked moisture coming from the east since at least the early Miocene, consistent with previously published evidence of arid-semiarid conditions in the Atacama Desert. However, hyperaridity on the western Andean slope developed later and appears to correspond with the timing of uplift in the Eastern Cordillera and Altiplano. Our results suggest that the growth of the Central Andean rain shadow relied not only on the elevation of the Western Cordillera but also on the widening of the plateau.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"491 - 533"},"PeriodicalIF":2.9,"publicationDate":"2021-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42959206","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The dissolved chemistry of rivers has been extensively studied to elucidate physical and climatic controls of chemical weathering at local to global spatial scales, as well as the impacts of chemical weathering on climate over short to geologic temporal scales. Within this effort, mixing models with Monte Carlo uncertainty propagation are a common tool for inverting measurements of dissolved river chemistry to distinguish among contributions from end-members with distinct elemental and/or isotopic compositions. However, the methods underlying prior river inversion models have typically been opaque. Here we present Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), a set of MATLAB scripts that enable highly customizable inversion of dissolved river chemistry with Monte Carlo propagation of uncertainty. First, we present an overview of the mathematics underlying MEANDIR. This overview includes, among other topics, derivation of equations for mass balance, implementation of chlorine critical values, construction of cost functions, normalization to the sum of dissolved variables, quantification of river sulfate sourced from pyrite oxidation, resolution of petrogenic organic carbon oxidation, representation of secondary phase formation with isotopic fractionation, and calculation of the impact of weathering on atmospheric carbon dioxide. Second, we apply MEANDIR to five previously published datasets to demonstrate the sensitivity of results to parameter choices. We invert data from two global compilations of river chemistry (Gaillardet and others, 1999; Burke and others, 2018), the major element chemistry and sulfate sulfur isotope ratios of rivers in the Peruvian Amazon (Torres and others, 2016), the major element chemistry of Icelandic rivers (Gíslason and others, 1996), and the major and trace element chemistry of water samples from the Mackenzie River (Horan and others, 2019). MEANDIR and its user guide are freely available online.
{"title":"Presentation and applications of mixing elements and dissolved isotopes in rivers (MEANDIR), a customizable MATLAB model for Monte Carlo inversion of dissolved river chemistry","authors":"P. Kemeny, Mark A. Torres","doi":"10.2475/05.2021.03","DOIUrl":"https://doi.org/10.2475/05.2021.03","url":null,"abstract":"The dissolved chemistry of rivers has been extensively studied to elucidate physical and climatic controls of chemical weathering at local to global spatial scales, as well as the impacts of chemical weathering on climate over short to geologic temporal scales. Within this effort, mixing models with Monte Carlo uncertainty propagation are a common tool for inverting measurements of dissolved river chemistry to distinguish among contributions from end-members with distinct elemental and/or isotopic compositions. However, the methods underlying prior river inversion models have typically been opaque. Here we present Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), a set of MATLAB scripts that enable highly customizable inversion of dissolved river chemistry with Monte Carlo propagation of uncertainty. First, we present an overview of the mathematics underlying MEANDIR. This overview includes, among other topics, derivation of equations for mass balance, implementation of chlorine critical values, construction of cost functions, normalization to the sum of dissolved variables, quantification of river sulfate sourced from pyrite oxidation, resolution of petrogenic organic carbon oxidation, representation of secondary phase formation with isotopic fractionation, and calculation of the impact of weathering on atmospheric carbon dioxide. Second, we apply MEANDIR to five previously published datasets to demonstrate the sensitivity of results to parameter choices. We invert data from two global compilations of river chemistry (Gaillardet and others, 1999; Burke and others, 2018), the major element chemistry and sulfate sulfur isotope ratios of rivers in the Peruvian Amazon (Torres and others, 2016), the major element chemistry of Icelandic rivers (Gíslason and others, 1996), and the major and trace element chemistry of water samples from the Mackenzie River (Horan and others, 2019). MEANDIR and its user guide are freely available online.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"579 - 642"},"PeriodicalIF":2.9,"publicationDate":"2021-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44008045","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In the northernmost segment of the Arabian–Nubian Shield, a post-collisional high-K calc-alkaline volcanic sequence is exposed along Wadi Abu Ma’amel, Eastern Desert of the Nubian Shield. It comprises a series of intermediate to silicic volcanics and associated pyroclastics that include the Imperial Porphyry and calc-alkaline volcanics typical of the Dokhan Volcanics. The Imperial Porphyry occurs as subvolcanic sill-like intrusions forming the young member of the Dokhan Volcanics. The volcanic sequence extruded through synorogenic granite and was intruded by post-collisional granite, which also caused thermal contact metamorphism. The red and purple colors of the Imperial Porphyry reflect hydrothermal alterations, which resulted in the formation of dispersed flakes of hematite, epidote, and piemontite. The entire high-K calc-alkaline volcanic sequence, ranging from andesite through dacite and rhyodacite, exhibits post-collisional geochemical characteristics. Most samples of the Imperial Porphyry and some of the typical Dokhan Volcanics have characteristics of adakitic rocks, including high Sr (694–889 ppm), low Y (10.6–18.8 ppm), high Sr/Y (41.1–83.8), (La/Yb)n (8.6–15.6), and low (Yb)n (5.4–9.0). The mostly calc-alkaline character and other traits of the studied volcanics that were previously interpreted to indicate arc magmatism reflect, instead, remelting of earlier (pre-collisional) arc-related material. The formation of Wadi Abu Ma'amel volcanics resulted from upwelling of hot asthenospheric material during thinning of the previously thickened lithosphere as a consequence of lithospheric delamination. The parental magma was generated by partial melting of mafic lower crust that mixed with upper-crust-derived magma. It evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe–Ti oxides in the more-evolved dacitic and rhyodacitic rocks.
在阿拉伯-努比亚地盾的最北端，沿努比亚地盾东部沙漠Wadi Abu Ma 'amel暴露出一个碰撞后的高钾钙碱性火山序列。它由一系列中硅酸火山岩和伴生火山碎屑组成，其中包括帝国斑岩和多汗火山岩的典型钙碱性火山岩。帝国斑岩是次火山岩状侵入体，形成了多汗火山的年轻成员。火山层序在同造花岗岩中挤出，并被后碰撞花岗岩侵入，也引起了热接触变质作用。皇斑岩的红色和紫色反映了热液蚀变，这导致了赤铁矿、绿帘石和片铁矿的分散薄片的形成。整个高钾钙碱性火山序列，从安山岩到英安岩和流纹石，呈现出碰撞后的地球化学特征。大部分御斑岩样品和部分典型多罕火山岩样品具有高Sr (694 ~ 889 ppm)、低Y (10.6 ~ 18.8 ppm)、高Sr/Y(41.1 ~ 83.8)、(La/Yb)n(8.6 ~ 15.6)、低(Yb)n(5.4 ~ 9.0)的埃达质岩石特征。所研究的火山的主要钙碱性特征和其他特征先前被解释为表明弧岩浆作用，相反，反映了早期(碰撞前)与弧相关的物质的重熔。Wadi Abu Ma'amel火山的形成是由于岩石圈剥离导致先前增厚的岩石圈变薄期间，热软流圈物质上涌而成。母岩浆是由基性下地壳部分熔融与上地壳衍生岩浆混合形成的。它主要通过斜辉石和斜长石的分选演化而来，在较演化的英安岩和流纹岩中伴以磷灰石和铁钛氧化物。
{"title":"Origin and magmatic evolution of late Neoproterozoic post-accretion high-K calc-alkaline adakitic volcanics in the northern Arabian–Nubian Shield","authors":"Bassam A. Abuamarah, M. Azer, Heba S. Mubarak","doi":"10.2475/05.2021.02","DOIUrl":"https://doi.org/10.2475/05.2021.02","url":null,"abstract":"In the northernmost segment of the Arabian–Nubian Shield, a post-collisional high-K calc-alkaline volcanic sequence is exposed along Wadi Abu Ma’amel, Eastern Desert of the Nubian Shield. It comprises a series of intermediate to silicic volcanics and associated pyroclastics that include the Imperial Porphyry and calc-alkaline volcanics typical of the Dokhan Volcanics. The Imperial Porphyry occurs as subvolcanic sill-like intrusions forming the young member of the Dokhan Volcanics. The volcanic sequence extruded through synorogenic granite and was intruded by post-collisional granite, which also caused thermal contact metamorphism. The red and purple colors of the Imperial Porphyry reflect hydrothermal alterations, which resulted in the formation of dispersed flakes of hematite, epidote, and piemontite. The entire high-K calc-alkaline volcanic sequence, ranging from andesite through dacite and rhyodacite, exhibits post-collisional geochemical characteristics. Most samples of the Imperial Porphyry and some of the typical Dokhan Volcanics have characteristics of adakitic rocks, including high Sr (694–889 ppm), low Y (10.6–18.8 ppm), high Sr/Y (41.1–83.8), (La/Yb)n (8.6–15.6), and low (Yb)n (5.4–9.0). The mostly calc-alkaline character and other traits of the studied volcanics that were previously interpreted to indicate arc magmatism reflect, instead, remelting of earlier (pre-collisional) arc-related material. The formation of Wadi Abu Ma'amel volcanics resulted from upwelling of hot asthenospheric material during thinning of the previously thickened lithosphere as a consequence of lithospheric delamination. The parental magma was generated by partial melting of mafic lower crust that mixed with upper-crust-derived magma. It evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe–Ti oxides in the more-evolved dacitic and rhyodacitic rocks.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"534 - 578"},"PeriodicalIF":2.9,"publicationDate":"2021-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44121077","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A thermodynamic model is formulated for (Ca,Na)2(Mg,Fe2+,Al,Fe3+)T1 (Al,Fe3+,Si)2T2O7 melilites. It employs the compositional vertices: åkermanite (Ca2MgSi2O7, 1), gehlenite (Ca2Al2SiO7, 2), iron åkermanite (Ca2Fe2+Si2O7, 3), ferrigehlenite (Ca2Fe23 + SiO7, 4), sodium melilite (NaCaAlSi2O7, 5), and the convergent ordering variables: s = XAl3 + T2a – XAl3+T2b and t = XFe3 + T2a – XFe3 + T2b to describe the distribution of Al3+, Fe3+ and Si4+ between T2 subsites T2a and T2b. It is calibrated for åkermanite–gehlenite melilites based on the calorimetric data of Charlu and others (1981), the assumption that the synthetic samples of Charlu and others approached “equilibrium” states of Al-Si tetrahedral ordering at 970 K, and analogy with the Al2(MgSi) − 1 substitution in CaMgSi2O6 – CaMg1/2Ti1/2AlSiO6 – CaAl2SiO6 fassaites (for example, Sack and Ghiorso, 2017). In this model gehlenite has a disordered Al-Si distribution on T2 sites above 1443 K (1170 °C), consistent with the crystallographic data on c/a ratios of lattice parameters as a function of annealing temperature (Woodhead and Waldbaum, 1974) and the high-temperature heat capacities inferred from drop calorimetric data (Pankratz and Kelley,1964). However, above this critical temperature a partially ordered Al-Si distribution persists between T2a and T2b sites in åkermanite – gehlenite solid solutions with intermediate X2 (for example, 0.19 < X2 < 0.89 at 1573 K). To a first approximation activity-composition relations of the gehlenite component approximate those of ideal mixing (that is, ai = Xi), particularly in gehlenite-rich compositions, but those of åkermanite component display pronounced temperature dependence in intermediate compositions. Enthalpies of formation of åkermanite and gehlenite from the elements at 298.15 K, ΔH¯f 298.15o AK and ΔH¯f 298.15o GEHL, consistent with the experimental brackets on decarbonation equilibria of Walter (1963), Hoschek (1974), and Shmulovich (1974), the thermodynamic model for åkermanite-gehlenite melilites developed here, the thermodynamic properties of the other phases in these reactions tabulated by Berman (1988), and the revised estimates for C¯p and S¯298.15o of diopside of Sack and Ghiorso (2017), are roughly 1 and 3 (kJ/gfw) more positive than those estimated by Berman (1988). More positive standard enthalpies of formation of both endmembers, together with a decrease in the vibrational heat capacity of gehlenite and less negative deviations from ideal mixing compared with previous calibrations, all contribute to reducing the stability of melilites in this model. Together these effects will decrease the predicted temperature of condensation of melilite from nebular vapors, bringing calculated temperatures of melilite condensation into closer alignment with those of MgAl2O4 spinel than the 80 to 100 K separating their appearances in previous calculations (for example, Yoneda and Grossman, 1995; Petaev and Wood,1998; Ebel and Grossman,2000). These eff
{"title":"Thermochemistry of melilites I. Towards resolving an inconsistency in nebular condensation calculations","authors":"R. Sack","doi":"10.2475/04.2021.02","DOIUrl":"https://doi.org/10.2475/04.2021.02","url":null,"abstract":"A thermodynamic model is formulated for (Ca,Na)2(Mg,Fe2+,Al,Fe3+)T1 (Al,Fe3+,Si)2T2O7 melilites. It employs the compositional vertices: åkermanite (Ca2MgSi2O7, 1), gehlenite (Ca2Al2SiO7, 2), iron åkermanite (Ca2Fe2+Si2O7, 3), ferrigehlenite (Ca2Fe23 + SiO7, 4), sodium melilite (NaCaAlSi2O7, 5), and the convergent ordering variables: s = XAl3 + T2a – XAl3+T2b and t = XFe3 + T2a – XFe3 + T2b to describe the distribution of Al3+, Fe3+ and Si4+ between T2 subsites T2a and T2b. It is calibrated for åkermanite–gehlenite melilites based on the calorimetric data of Charlu and others (1981), the assumption that the synthetic samples of Charlu and others approached “equilibrium” states of Al-Si tetrahedral ordering at 970 K, and analogy with the Al2(MgSi) − 1 substitution in CaMgSi2O6 – CaMg1/2Ti1/2AlSiO6 – CaAl2SiO6 fassaites (for example, Sack and Ghiorso, 2017). In this model gehlenite has a disordered Al-Si distribution on T2 sites above 1443 K (1170 °C), consistent with the crystallographic data on c/a ratios of lattice parameters as a function of annealing temperature (Woodhead and Waldbaum, 1974) and the high-temperature heat capacities inferred from drop calorimetric data (Pankratz and Kelley,1964). However, above this critical temperature a partially ordered Al-Si distribution persists between T2a and T2b sites in åkermanite – gehlenite solid solutions with intermediate X2 (for example, 0.19 < X2 < 0.89 at 1573 K). To a first approximation activity-composition relations of the gehlenite component approximate those of ideal mixing (that is, ai = Xi), particularly in gehlenite-rich compositions, but those of åkermanite component display pronounced temperature dependence in intermediate compositions. Enthalpies of formation of åkermanite and gehlenite from the elements at 298.15 K, ΔH¯f 298.15o AK and ΔH¯f 298.15o GEHL, consistent with the experimental brackets on decarbonation equilibria of Walter (1963), Hoschek (1974), and Shmulovich (1974), the thermodynamic model for åkermanite-gehlenite melilites developed here, the thermodynamic properties of the other phases in these reactions tabulated by Berman (1988), and the revised estimates for C¯p and S¯298.15o of diopside of Sack and Ghiorso (2017), are roughly 1 and 3 (kJ/gfw) more positive than those estimated by Berman (1988). More positive standard enthalpies of formation of both endmembers, together with a decrease in the vibrational heat capacity of gehlenite and less negative deviations from ideal mixing compared with previous calibrations, all contribute to reducing the stability of melilites in this model. Together these effects will decrease the predicted temperature of condensation of melilite from nebular vapors, bringing calculated temperatures of melilite condensation into closer alignment with those of MgAl2O4 spinel than the 80 to 100 K separating their appearances in previous calculations (for example, Yoneda and Grossman, 1995; Petaev and Wood,1998; Ebel and Grossman,2000). These eff","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"424 - 457"},"PeriodicalIF":2.9,"publicationDate":"2021-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43854564","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Detailed bedrock mapping, structural geology, meta-igneous whole rock geochemistry, and U-Pb geochronology from rocks sampled along a portion of a complexly deformed tectonic boundary between the Ordovician peri-Gondwanan Liberty-Orrington belt and Silurian syn-orogenic strata of the Fredericton trough (a.k.a. the Dog Bay Line) in mid-coastal Maine aid in deciphering the Silurian-Devonian tectonic evolution of the region. The new results provide constraints on several key events. First, initial terrane juxtapositioning occurred along the east-verging Boothbay thrust fault (D1). This tectonism occurred prior to 423 Ma and is associated with the accretion of the Ganderian microcontinent to the Laurentian margin (that is, the Salinic orogeny). Subsequently, intrusion of an ultra-potassic magma, the protolith of the Edgecomb Gneiss, occurred at ca. 413 Ma. Its distinctive whole rock geochemical signature allows for correlation with rocks of similar composition and age along a relatively narrow 140 kilometer long distance on the northwestern margin of the Fredericton trough. This restricted area of ultra-potassic magma generation is attributed to the breakoff of the descending Salinic oceanic slab that triggered decompression melting of a previously metasomatized mantle wedge region beneath the accreted Ganderian microcontinent. Early thrust faults (D1) and the ca. 413 Edgecomb Gneiss igneous protolith were overprinted by an episode of upright folding (D2) and low-pressure amphibolite facies metamorphism associated with the Early to Middle Devonian Acadian orogeny. Zircon overgrowths in the Edgecomb Gneiss dated at ca. 399 Ma grew during this tectonic episode. Comparisons with previous geochronological studies across the region suggest this dominant phase of Acadian deformation and metamorphism was long-lived (ca. 40 m.y.) and associated with the outboard accretion of the Avalonian microcontinent. Dextral shear structures represent the final phase of deformation (D3) superimposed on this terrane boundary and are associated with the Norumbega fault and shear zone system that was active in Middle Devonian-Carboniferous time.
{"title":"Silurian-Devonian tectonic evolution of mid-coastal Maine, U.S.A.: Details of polyphase orogenic processes","authors":"D. P. West, E. Peterman, Jessica Chen","doi":"10.2475/04.2021.03","DOIUrl":"https://doi.org/10.2475/04.2021.03","url":null,"abstract":"Detailed bedrock mapping, structural geology, meta-igneous whole rock geochemistry, and U-Pb geochronology from rocks sampled along a portion of a complexly deformed tectonic boundary between the Ordovician peri-Gondwanan Liberty-Orrington belt and Silurian syn-orogenic strata of the Fredericton trough (a.k.a. the Dog Bay Line) in mid-coastal Maine aid in deciphering the Silurian-Devonian tectonic evolution of the region. The new results provide constraints on several key events. First, initial terrane juxtapositioning occurred along the east-verging Boothbay thrust fault (D1). This tectonism occurred prior to 423 Ma and is associated with the accretion of the Ganderian microcontinent to the Laurentian margin (that is, the Salinic orogeny). Subsequently, intrusion of an ultra-potassic magma, the protolith of the Edgecomb Gneiss, occurred at ca. 413 Ma. Its distinctive whole rock geochemical signature allows for correlation with rocks of similar composition and age along a relatively narrow 140 kilometer long distance on the northwestern margin of the Fredericton trough. This restricted area of ultra-potassic magma generation is attributed to the breakoff of the descending Salinic oceanic slab that triggered decompression melting of a previously metasomatized mantle wedge region beneath the accreted Ganderian microcontinent. Early thrust faults (D1) and the ca. 413 Edgecomb Gneiss igneous protolith were overprinted by an episode of upright folding (D2) and low-pressure amphibolite facies metamorphism associated with the Early to Middle Devonian Acadian orogeny. Zircon overgrowths in the Edgecomb Gneiss dated at ca. 399 Ma grew during this tectonic episode. Comparisons with previous geochronological studies across the region suggest this dominant phase of Acadian deformation and metamorphism was long-lived (ca. 40 m.y.) and associated with the outboard accretion of the Avalonian microcontinent. Dextral shear structures represent the final phase of deformation (D3) superimposed on this terrane boundary and are associated with the Norumbega fault and shear zone system that was active in Middle Devonian-Carboniferous time.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"458 - 489"},"PeriodicalIF":2.9,"publicationDate":"2021-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43300847","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Q. Chang, M. Hren, A. Lin, C. Tabor, Shun Yu, Y. Eley, G. Harris
Fluvial sediments are important archives of paleoenvironments. However, variations in sediment production and transport processes greatly influence sediment geochemistry and resultant interpretations of ancient conditions. Tectonically-active tropical regions are particularly sensitive to climate feedbacks because these areas are often characterized by high precipitation rates, rapid erosion and short sediment residence times. We analyzed the hydrogen and carbon isotope composition of plant-derived n-alkanes (δ2Hn-alkane and δ13Cn-alkane) in sediment cores along the Gaoping River-submarine canyon system in southwestern Taiwan to examine climatic and geomorphic controls on isotope geochemical signatures of fluvial sedimentary archives. These records span the last ∼26 kyr and provide critical insight into the temporal and spatial variations in sedimentary biomarker isotopes within a source-to-sink system. Isotope data are coupled with new results from an iCESM 1.2 Earth System Model of precipitation isotopes during the last glacial-interglacial cycle. Biomarker isotope and modeling results support two important conclusions. First, biomarker isotope values change by ∼10 to 15‰ in δ2Hn-alkane and ∼1 to 2‰ δ13Cn-alkane in offshore SW Taiwan through the late Quaternary deglaciation. These shifts are consistent with iCESM predictions and other records from the South China Sea and are best explained by a shift in isotope hydrology due to regional warming and biologic responses to increased atmospheric pCO2. Second, the δ2Hn-alkane of biomarkers preserved in onshore sediments proximal to the mountain range is ∼15 to 20‰ more negative than biomarkers deposited in offshore sites, and the temporal change in carbon isotopes exceeds that observed in the offshore deposits. The onshore core locality is proximal to the orogen and characterized by a mean elevation > 1 km compared to the offshore site, which has a mean catchment elevation of ∼500 m. These data show that depositional setting and catchment hypsometry strongly bias the geochemical signature of sediments transported through the river system. The magnitude of isotopic variability generated by catchment geometry and sediment integration greatly exceeds the change associated with warming during deglaciation. This result suggests that catchment integration processes may play a similar or larger role in shaping fluvial geochemical records in tropical mountain systems than climatic factors.
{"title":"Terrestrial biomarker isotope records of late Quaternary climate and source-to-sink sediment transport processes in southwestern Taiwan","authors":"Q. Chang, M. Hren, A. Lin, C. Tabor, Shun Yu, Y. Eley, G. Harris","doi":"10.2475/04.2021.01","DOIUrl":"https://doi.org/10.2475/04.2021.01","url":null,"abstract":"Fluvial sediments are important archives of paleoenvironments. However, variations in sediment production and transport processes greatly influence sediment geochemistry and resultant interpretations of ancient conditions. Tectonically-active tropical regions are particularly sensitive to climate feedbacks because these areas are often characterized by high precipitation rates, rapid erosion and short sediment residence times. We analyzed the hydrogen and carbon isotope composition of plant-derived n-alkanes (δ2Hn-alkane and δ13Cn-alkane) in sediment cores along the Gaoping River-submarine canyon system in southwestern Taiwan to examine climatic and geomorphic controls on isotope geochemical signatures of fluvial sedimentary archives. These records span the last ∼26 kyr and provide critical insight into the temporal and spatial variations in sedimentary biomarker isotopes within a source-to-sink system. Isotope data are coupled with new results from an iCESM 1.2 Earth System Model of precipitation isotopes during the last glacial-interglacial cycle. Biomarker isotope and modeling results support two important conclusions. First, biomarker isotope values change by ∼10 to 15‰ in δ2Hn-alkane and ∼1 to 2‰ δ13Cn-alkane in offshore SW Taiwan through the late Quaternary deglaciation. These shifts are consistent with iCESM predictions and other records from the South China Sea and are best explained by a shift in isotope hydrology due to regional warming and biologic responses to increased atmospheric pCO2. Second, the δ2Hn-alkane of biomarkers preserved in onshore sediments proximal to the mountain range is ∼15 to 20‰ more negative than biomarkers deposited in offshore sites, and the temporal change in carbon isotopes exceeds that observed in the offshore deposits. The onshore core locality is proximal to the orogen and characterized by a mean elevation > 1 km compared to the offshore site, which has a mean catchment elevation of ∼500 m. These data show that depositional setting and catchment hypsometry strongly bias the geochemical signature of sediments transported through the river system. The magnitude of isotopic variability generated by catchment geometry and sediment integration greatly exceeds the change associated with warming during deglaciation. This result suggests that catchment integration processes may play a similar or larger role in shaping fluvial geochemical records in tropical mountain systems than climatic factors.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":"321 1","pages":"393 - 423"},"PeriodicalIF":2.9,"publicationDate":"2021-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47592052","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"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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