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":null,"pages":null},"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":null,"pages":null},"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":null,"pages":null},"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}
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":null,"pages":null},"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}
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":null,"pages":null},"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}
A. Festa, F. Meneghini, G. Balestro, L. Pandolfi, P. Tartarotti, Y. Dilek, M. Marroni
The Jurassic ophiolites in the Northern Apennines and the Western Alps represent fossil mid-ocean ridge (MOR) oceanic lithosphere that formed in the Mesozoic Ligurian-Piedmont Ocean Basin (LPOB). Their sedimentary covers include chaotic rock units containing ophiolite-derived material. The processes of formation and the lithostratigraphic position of these chaotic units in the Western Alps remain a matter of debate, unlike their counterparts in the Northern Apennines. This is because of pervasive tectonic deformation and high-pressure metamorphism that affected their internal structure during collisional tectonics. A comparative analysis of these chaotic units in both mountain belts reveals the nature of processes involved in their formation. Chaotic deposits of gravitational origin occur both below and above the extrusive sequences in the ophiolites. They represent synextensional, hyperconcentrated deposits associated with the seafloor-spreading evolution of the LPOB lithosphere during Middle and Late Jurassic times. Mass transport deposits (MTDs) occur as intercalations within turbiditic sequences above the ophiolites. They represent syncontractional submarine slides that occurred on frontal accretionary prism slopes during the Late Cretaceous–Paleocene closure of the LPOB. The results of our comparative analysis imply that (1) the structure-stratigraphy of the chaotic deposits and MTDs of the Northern Apennines can be used as a proxy to better identify their metamorphosed and highly deformed counterparts in the Western Alps, (2) sedimentological processes associated with slow-spreading MOR tectonics and accretionary prism development in convergent-margin tectonics contributed to the sediment budgets of the cover sequences, and (3) magmatic, tectonic, and sedimentological processes that occurred during the formation of the Jurassic oceanic lithosphere and its sedimentary cover in the LPOB were remarkably uniform and synchronous.
{"title":"Comparative Analysis of the Sedimentary Cover Units of the Jurassic Western Tethys Ophiolites in the Northern Apennines and Western Alps (Italy): Processes of the Formation of Mass-Transport and Chaotic Deposits during Seafloor Spreading and Subduction Zone Tectonics","authors":"A. Festa, F. Meneghini, G. Balestro, L. Pandolfi, P. Tartarotti, Y. Dilek, M. Marroni","doi":"10.1086/716498","DOIUrl":"https://doi.org/10.1086/716498","url":null,"abstract":"The Jurassic ophiolites in the Northern Apennines and the Western Alps represent fossil mid-ocean ridge (MOR) oceanic lithosphere that formed in the Mesozoic Ligurian-Piedmont Ocean Basin (LPOB). Their sedimentary covers include chaotic rock units containing ophiolite-derived material. The processes of formation and the lithostratigraphic position of these chaotic units in the Western Alps remain a matter of debate, unlike their counterparts in the Northern Apennines. This is because of pervasive tectonic deformation and high-pressure metamorphism that affected their internal structure during collisional tectonics. A comparative analysis of these chaotic units in both mountain belts reveals the nature of processes involved in their formation. Chaotic deposits of gravitational origin occur both below and above the extrusive sequences in the ophiolites. They represent synextensional, hyperconcentrated deposits associated with the seafloor-spreading evolution of the LPOB lithosphere during Middle and Late Jurassic times. Mass transport deposits (MTDs) occur as intercalations within turbiditic sequences above the ophiolites. They represent syncontractional submarine slides that occurred on frontal accretionary prism slopes during the Late Cretaceous–Paleocene closure of the LPOB. The results of our comparative analysis imply that (1) the structure-stratigraphy of the chaotic deposits and MTDs of the Northern Apennines can be used as a proxy to better identify their metamorphosed and highly deformed counterparts in the Western Alps, (2) sedimentological processes associated with slow-spreading MOR tectonics and accretionary prism development in convergent-margin tectonics contributed to the sediment budgets of the cover sequences, and (3) magmatic, tectonic, and sedimentological processes that occurred during the formation of the Jurassic oceanic lithosphere and its sedimentary cover in the LPOB were remarkably uniform and synchronous.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/716498","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45378735","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
F. Haissen, M. Zaghloul, Y. Dilek, Oriol Gimeno‐Vives, G. Mohn, A. Cambeses, D. F. Lamotte, V. Bosse
We present new field evidence, geochemical and isotopic data, and age constraints on Lower Jurassic mafic rock suites within a >200-km-long curvilinear belt in the Rif orogenic belt in northern Morocco and show that these rock assemblages formed as part of the Central Atlantic Magmatic Province (CAMP). The CAMP represents a large igneous province that straddles the edges of the modern peri-Atlantic continents. It developed ~200 Ma, following the initiation of the breakup of Pangea. Main magmatic rocks in the Rif External Zone include basaltic lavas, massive dolerite, and isotropic and cumulate gabbros, all intruded by dolerite and trondhjemite dikes and sills. Available U-Pb zircon ages from dolerite, gabbro, and trondhjemite dike rocks are 200±4, 196±4, and 192±Ma, respectively. Based on their geochemical affinities and isotopic compositions, the analyzed rocks define basalt-dolerite and gabbro-cumulate gabbro-trondhjemite groups. The basalt-dolerite group samples are subalkaline in nature and have low TiO2 contents, whereas the gabbro-cumulate gabbro-trondhjemite group samples are alkaline and display high TiO2 values. Most samples are tholeiitic in character and show large-ion lithophile and light rare earth element enrichment and high field strength element depletion compared with normal mid-ocean ridge basalt (MORB). Samples of both groups display low 143Nd/144Nd201 Ma (0.51182–0.51262) and high 87Sr/86Sri ratios with ɛNd values ranging from −1.51 to +4.85. The basalt-dolerite group rocks have enriched MORB compositions, compatible with the low-Ti CAMP suites, whereas the gabbro-cumulate gabbro-trondhjemite group rocks have oceanic island basalt compositions reminiscent of high-Ti CAMP suites in other continents. Magmas of the gabbro-cumulate gabbro-trondhjemite group underwent differentiation through tholeiitic fractionation. Magmas of the rocks of both groups included melt components, originated from partial melting of a previously subduction-modified subcontinental lithospheric mantle. Our results indicate that the Early Jurassic CAMP magmatism in northern Morocco marked a major episode of continental magmatism before the opening of the Maghrebian Tethys between Africa and Iberia in the latest Jurassic.
{"title":"Geochemistry and Petrogenesis of Lower Jurassic Mafic Rock Suites in the External Rif Belt, and Chemical Geodynamics of the Central Atlantic Magmatic Province (CAMP) in Northwest Morocco","authors":"F. Haissen, M. Zaghloul, Y. Dilek, Oriol Gimeno‐Vives, G. Mohn, A. Cambeses, D. F. Lamotte, V. Bosse","doi":"10.1086/716499","DOIUrl":"https://doi.org/10.1086/716499","url":null,"abstract":"We present new field evidence, geochemical and isotopic data, and age constraints on Lower Jurassic mafic rock suites within a >200-km-long curvilinear belt in the Rif orogenic belt in northern Morocco and show that these rock assemblages formed as part of the Central Atlantic Magmatic Province (CAMP). The CAMP represents a large igneous province that straddles the edges of the modern peri-Atlantic continents. It developed ~200 Ma, following the initiation of the breakup of Pangea. Main magmatic rocks in the Rif External Zone include basaltic lavas, massive dolerite, and isotropic and cumulate gabbros, all intruded by dolerite and trondhjemite dikes and sills. Available U-Pb zircon ages from dolerite, gabbro, and trondhjemite dike rocks are 200±4, 196±4, and 192±Ma, respectively. Based on their geochemical affinities and isotopic compositions, the analyzed rocks define basalt-dolerite and gabbro-cumulate gabbro-trondhjemite groups. The basalt-dolerite group samples are subalkaline in nature and have low TiO2 contents, whereas the gabbro-cumulate gabbro-trondhjemite group samples are alkaline and display high TiO2 values. Most samples are tholeiitic in character and show large-ion lithophile and light rare earth element enrichment and high field strength element depletion compared with normal mid-ocean ridge basalt (MORB). Samples of both groups display low 143Nd/144Nd201 Ma (0.51182–0.51262) and high 87Sr/86Sri ratios with ɛNd values ranging from −1.51 to +4.85. The basalt-dolerite group rocks have enriched MORB compositions, compatible with the low-Ti CAMP suites, whereas the gabbro-cumulate gabbro-trondhjemite group rocks have oceanic island basalt compositions reminiscent of high-Ti CAMP suites in other continents. Magmas of the gabbro-cumulate gabbro-trondhjemite group underwent differentiation through tholeiitic fractionation. Magmas of the rocks of both groups included melt components, originated from partial melting of a previously subduction-modified subcontinental lithospheric mantle. Our results indicate that the Early Jurassic CAMP magmatism in northern Morocco marked a major episode of continental magmatism before the opening of the Maghrebian Tethys between Africa and Iberia in the latest Jurassic.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/716499","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42212361","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}
E. Garzanti, G. Pastore, A. Resentini, G. Vezzoli, P. Vermeesch, L. Ncube, H. V. Niekerk, G. Jouet, M. Dall’asta
The Zambezi River rises at the center of southern Africa, flows across the low-relief Kalahari Plateau, meets Karoo basalt, plunges into Victoria Falls, follows along Karoo rifts, and pierces through Precambrian basement to eventually deliver its load onto the Mozambican passive margin. Reflecting its polyphase evolution, the river is subdivided into segments with different geological and geomorphological character, a subdivision finally fixed by man’s construction of large reservoirs and faithfully testified by sharp changes in sediment composition. Pure quartzose sand recycled from Kalahari desert dunes in the uppermost tract is next progressively enriched in basaltic rock fragments and clinopyroxene. Sediment load is renewed first downstream of Lake Kariba and next downstream of Lake Cahora Bassa, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzo-feldspathic in the lower tract, where most sediment is supplied by high-grade basements rejuvenated by the southward propagation of the East African rift. Feldspar abundance in Lower Zambezi sand has no equivalent among big rivers on Earth and far exceeds that in sediments of the northern delta, shelf, and slope, revealing that provenance signals from the upper reaches have ceased to be transmitted across the routing system after closure of the big dams. This high-resolution petrologic study of Zambezi sand allows us to critically reconsider several dogmas, such as the supposed increase of mineralogical “maturity” during long-distance fluvial transport, and forges a key to unlock the rich information stored in sedimentary archives, with the ultimate goal to accurately reconstruct the evolution of this mighty river flowing across changing African landscapes since the late Mesozoic.
{"title":"The Segmented Zambezi Sedimentary System from Source to Sink: 1. Sand Petrology and Heavy Minerals","authors":"E. Garzanti, G. Pastore, A. Resentini, G. Vezzoli, P. Vermeesch, L. Ncube, H. V. Niekerk, G. Jouet, M. Dall’asta","doi":"10.1086/715792","DOIUrl":"https://doi.org/10.1086/715792","url":null,"abstract":"The Zambezi River rises at the center of southern Africa, flows across the low-relief Kalahari Plateau, meets Karoo basalt, plunges into Victoria Falls, follows along Karoo rifts, and pierces through Precambrian basement to eventually deliver its load onto the Mozambican passive margin. Reflecting its polyphase evolution, the river is subdivided into segments with different geological and geomorphological character, a subdivision finally fixed by man’s construction of large reservoirs and faithfully testified by sharp changes in sediment composition. Pure quartzose sand recycled from Kalahari desert dunes in the uppermost tract is next progressively enriched in basaltic rock fragments and clinopyroxene. Sediment load is renewed first downstream of Lake Kariba and next downstream of Lake Cahora Bassa, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzo-feldspathic in the lower tract, where most sediment is supplied by high-grade basements rejuvenated by the southward propagation of the East African rift. Feldspar abundance in Lower Zambezi sand has no equivalent among big rivers on Earth and far exceeds that in sediments of the northern delta, shelf, and slope, revealing that provenance signals from the upper reaches have ceased to be transmitted across the routing system after closure of the big dams. This high-resolution petrologic study of Zambezi sand allows us to critically reconsider several dogmas, such as the supposed increase of mineralogical “maturity” during long-distance fluvial transport, and forges a key to unlock the rich information stored in sedimentary archives, with the ultimate goal to accurately reconstruct the evolution of this mighty river flowing across changing African landscapes since the late Mesozoic.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715792","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46877097","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}
Rajat Mazumder, Farah Bt Mohd Anthony, Basil Shung Say Teo, Subhajit Roy, Amal Al Hajri, T. Ohta, S. De, O. Catuneanu
Sedimentary successions developed at the destructive plate margin settings are extremely important as they bear valuable record of contemporary basin tectonics and consequent sedimentation. Intense deformation and metamorphism often obliterate the primary sedimentary texture and structures of the sedimentary successions formed at destructive plate margins. However, sedimentological analysis of young unmetamorphosed forearc successions provides rare opportunity to infer the interplay between tectonics and sedimentation. Therefore, a process-based sedimentological facies analysis and provenance studies of the Cretaceous Pedawan Formation in the Kuching Zone, Borneo, have been undertaken for the first time. The sandstones are compositionally and texturally immature. They are normally graded with sole marks and have erosional lower contacts with mudstones and generally have gradational upper contacts. The massive sandstones, parallel-laminated sandstones, and the combination of rippled sandstones with thin mudstones are interpreted as turbidites. The lack of wave-generated structures, including hummocky cross stratification, indicates that deposition took place below storm wave base, possibly in a shelf setting. The lower part of the Pedawan Formation is mudstone dominated, and the upper part progressively becomes sandstone dominated. The Pedawan Formation bears several penecontemporaneously deformed horizons sandwiched between laterally persistent undeformed beds. Deformation structures include folds with reclined to recumbent geometry and layer-confined normal and reverse faults. We have interpreted these deformed horizons as seismites. Modal analyses of the sandstones indicate recycled orogenic as well as arc provenance and thus indicate mixing of recycled orogenic sediments with arc-derived volcanic material. The sedimentary facies characteristics of the Pedawan Formation in combination with numerous penecontemporaneous deformation features at selected stratigraphic levels indicate that the turbidites formed in a seismically active deepwater depositional setting as part of a long-lived subduction complex in eastern Sundaland during which several crustal fragments were accreted to Borneo.
{"title":"Cretaceous Forearc Sedimentation and Contemporary Basin Tectonics in Northwestern Borneo: New Sedimentological Insights from Pedawan Formation, Kuching Zone, East Malaysia","authors":"Rajat Mazumder, Farah Bt Mohd Anthony, Basil Shung Say Teo, Subhajit Roy, Amal Al Hajri, T. Ohta, S. De, O. Catuneanu","doi":"10.1086/715790","DOIUrl":"https://doi.org/10.1086/715790","url":null,"abstract":"Sedimentary successions developed at the destructive plate margin settings are extremely important as they bear valuable record of contemporary basin tectonics and consequent sedimentation. Intense deformation and metamorphism often obliterate the primary sedimentary texture and structures of the sedimentary successions formed at destructive plate margins. However, sedimentological analysis of young unmetamorphosed forearc successions provides rare opportunity to infer the interplay between tectonics and sedimentation. Therefore, a process-based sedimentological facies analysis and provenance studies of the Cretaceous Pedawan Formation in the Kuching Zone, Borneo, have been undertaken for the first time. The sandstones are compositionally and texturally immature. They are normally graded with sole marks and have erosional lower contacts with mudstones and generally have gradational upper contacts. The massive sandstones, parallel-laminated sandstones, and the combination of rippled sandstones with thin mudstones are interpreted as turbidites. The lack of wave-generated structures, including hummocky cross stratification, indicates that deposition took place below storm wave base, possibly in a shelf setting. The lower part of the Pedawan Formation is mudstone dominated, and the upper part progressively becomes sandstone dominated. The Pedawan Formation bears several penecontemporaneously deformed horizons sandwiched between laterally persistent undeformed beds. Deformation structures include folds with reclined to recumbent geometry and layer-confined normal and reverse faults. We have interpreted these deformed horizons as seismites. Modal analyses of the sandstones indicate recycled orogenic as well as arc provenance and thus indicate mixing of recycled orogenic sediments with arc-derived volcanic material. The sedimentary facies characteristics of the Pedawan Formation in combination with numerous penecontemporaneous deformation features at selected stratigraphic levels indicate that the turbidites formed in a seismically active deepwater depositional setting as part of a long-lived subduction complex in eastern Sundaland during which several crustal fragments were accreted to Borneo.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715790","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47968328","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}
Our study of a banded charnockite complex of the Mühlig-Hofmannfjella in Dronning Maud Land, Antarctica, illustrates how the combination of high-temperature (re-)crystallization processes, melts, and volatile fluids leads to complex intrusive, metasomatic, and structural relationships. The igneous complex consists of gently dipping sets of charnockite interlayered with dolerite and leucogranite. The banded complex formed primarily by magmatic processes, but with superimposed modifications by metasomatism. The charnockite has a ferroan composition and contains both orthopyroxene (Fs80–84) and olivine (Fa94–96). Zircon U-Pb dates the emplacement of charnockite at 515 Ma, and inherited zircon cores and negative εNd values of −3 to −5 indicate that the age of the source of the magma was about 1100 Ma. Neodymium isotopes were not homogenized during the Cambrian magmatic event, which suggests that the generation and emplacement of the magma took place in separate batches during construction of the banded complex. By contrast, the Rb-Sr system in the charnockite was extensively homogenized, likely because of the superimposed late-magmatic fluid activity, which also produced the bands and networks of leucogranites. These events occurred during the late stages of the assembly of Gondwana, with postcollisional extension and mantle upwelling maintaining a high heat flow.
{"title":"Banded Charnockite: The Result of Crustal Magma Generation, Piecemeal Emplacement, and Fluid-Driven Mineral Replacement in High-Grade Crust (Central Dronning Maud Land, Antarctica)","authors":"A. Engvik, F. Corfu, I. Kleinhanns, S. Elvevold","doi":"10.1086/715789","DOIUrl":"https://doi.org/10.1086/715789","url":null,"abstract":"Our study of a banded charnockite complex of the Mühlig-Hofmannfjella in Dronning Maud Land, Antarctica, illustrates how the combination of high-temperature (re-)crystallization processes, melts, and volatile fluids leads to complex intrusive, metasomatic, and structural relationships. The igneous complex consists of gently dipping sets of charnockite interlayered with dolerite and leucogranite. The banded complex formed primarily by magmatic processes, but with superimposed modifications by metasomatism. The charnockite has a ferroan composition and contains both orthopyroxene (Fs80–84) and olivine (Fa94–96). Zircon U-Pb dates the emplacement of charnockite at 515 Ma, and inherited zircon cores and negative εNd values of −3 to −5 indicate that the age of the source of the magma was about 1100 Ma. Neodymium isotopes were not homogenized during the Cambrian magmatic event, which suggests that the generation and emplacement of the magma took place in separate batches during construction of the banded complex. By contrast, the Rb-Sr system in the charnockite was extensively homogenized, likely because of the superimposed late-magmatic fluid activity, which also produced the bands and networks of leucogranites. These events occurred during the late stages of the assembly of Gondwana, with postcollisional extension and mantle upwelling maintaining a high heat flow.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2021-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715789","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42545882","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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