Lin-jun Huang, Kongyou Wu, Yin Liu, Jian-guo Pan, Weijiang Yao, Bo Liu, Jian Cao
The Che-Guai region is located in the most strongly deformed area of the northwestern Junggar Basin (northwest China) and its deformation history is key to understanding the tectonic evolution of West Junggar and its associated hydrocarbon accumulation. However, the presence of complex multiphase structures makes it difficult to unravel the deformation history. In this study, we present comprehensive seismic and well log data, combined with geological observations from sedimentary and volcanic rocks, to reveal the structural deformation of the northwestern Junggar Basin. Based on our results, four zones of concentrated deformation are identified: (1) the Ke-Bai Fault Zone, (2) the Hongshanzui transition zone, (3) the Zhongguai uplifted zone, and (4) the Hong-Che Fault Zone. Based on the interpretation of seismic profiles, we suggest that Paleozoic strata were affected mainly by thrust faults, whereas Triassic and Jurassic strata of the Zhongguai uplifted zone were affected by strike-slip faults. Deformation in Cretaceous and Cenozoic strata is associated mainly with normal faults. Based on our results, the following tectonic history is proposed. The first stage involved initial basin formation during the Permian under the influence of northwestward subduction of the Junggar Ocean crust. The second stage, during the Triassic and Jurassic, involved transpressional deformation related to movement along the Darbute Fault. The third stage, during the Cretaceous and Cenozoic, involved moderate extensional deformation triggered by collision between the Indian and Eurasian plates. In the context of this tectonic history, hydrocarbon accumulation is proposed to be linked to fault terraces of the Hong-Che and Ke-Bai Fault Zones and the uplift of the Zhongguai Uplift.
{"title":"Deformation of the Northwestern Junggar Basin (Che-Guai Region, Northwest China) and Implications for Hydrocarbon Accumulation","authors":"Lin-jun Huang, Kongyou Wu, Yin Liu, Jian-guo Pan, Weijiang Yao, Bo Liu, Jian Cao","doi":"10.1086/706263","DOIUrl":"https://doi.org/10.1086/706263","url":null,"abstract":"The Che-Guai region is located in the most strongly deformed area of the northwestern Junggar Basin (northwest China) and its deformation history is key to understanding the tectonic evolution of West Junggar and its associated hydrocarbon accumulation. However, the presence of complex multiphase structures makes it difficult to unravel the deformation history. In this study, we present comprehensive seismic and well log data, combined with geological observations from sedimentary and volcanic rocks, to reveal the structural deformation of the northwestern Junggar Basin. Based on our results, four zones of concentrated deformation are identified: (1) the Ke-Bai Fault Zone, (2) the Hongshanzui transition zone, (3) the Zhongguai uplifted zone, and (4) the Hong-Che Fault Zone. Based on the interpretation of seismic profiles, we suggest that Paleozoic strata were affected mainly by thrust faults, whereas Triassic and Jurassic strata of the Zhongguai uplifted zone were affected by strike-slip faults. Deformation in Cretaceous and Cenozoic strata is associated mainly with normal faults. Based on our results, the following tectonic history is proposed. The first stage involved initial basin formation during the Permian under the influence of northwestward subduction of the Junggar Ocean crust. The second stage, during the Triassic and Jurassic, involved transpressional deformation related to movement along the Darbute Fault. The third stage, during the Cretaceous and Cenozoic, involved moderate extensional deformation triggered by collision between the Indian and Eurasian plates. In the context of this tectonic history, hydrocarbon accumulation is proposed to be linked to fault terraces of the Hong-Che and Ke-Bai Fault Zones and the uplift of the Zhongguai Uplift.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"45 - 68"},"PeriodicalIF":1.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/706263","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49456298","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}
T. Carley, C. Miller, C. Fisher, J. Hanchar, J. Vervoort, A. Schmitt, R. Economos, B. Jordan, A. J. Padilla, Tenley J. Banik
Iceland exemplifies the potential for generation of abundant silicic magma in the absence of mature island arc or preexisting continental crust. Zircon ages (U-Th and U-Pb) and isotope compositions (Hf and O), combined with whole-rock isotope data (Nd, Hf, Pb), provide insight into the petrogenesis and mantle heritage of these silicic magmas. Zircon and whole-rock samples represent the past 15 Ma of Iceland’s geologic evolution, geographic extent (marginal fjordlands to neovolcanic zones), and modern tectonic settings (on-rift, propagating-rift, off-rift). The generation of Icelandic silicic magma has been influenced by hydrothermally altered crust, via assimilation and/or anatexis, throughout Iceland’s history. This is shown by consistently depleted O isotopes in zircon (median δ18O +3.1‰; >98% below +5.3‰), and silicic rocks. Zircon δ18O values appear to have become lower and more diverse since ca. 0.7 Ma (median +1.9‰). This decrease may reflect lower δ18O of meteoric waters involved in hydrothermal alteration during the Pleistocene and/or more volumetrically significant contributions from low δ18O altered crust. Zircon O compositions from historically active volcanoes confirm that the role of altered crust is greater in on-rift than in off-rift settings; diversity in δ18O at volcanoes in propagating rift settings suggests highly variable contributions from altered crust. The silicic record (whole-rock and zircon) exhibits a correlation between geographic position and isotope composition that seems to be independent of local tectonic setting. Silicic samples of all ages collected above 65° N have more radiogenic whole-rock Hf and Nd isotopic compositions, and less radiogenic Pb, than samples collected in southern Iceland; published isotopic data for basalts suggest a similar time-independent latitudinal trend. The persistence of this trend through time suggests that northern Iceland has been underlain by a more depleted mantle source than southern Iceland throughout the island’s history.
{"title":"Petrogenesis of Silicic Magmas in Iceland through Space and Time: The Isotopic Record Preserved in Zircon and Whole Rocks","authors":"T. Carley, C. Miller, C. Fisher, J. Hanchar, J. Vervoort, A. Schmitt, R. Economos, B. Jordan, A. J. Padilla, Tenley J. Banik","doi":"10.1086/706261","DOIUrl":"https://doi.org/10.1086/706261","url":null,"abstract":"Iceland exemplifies the potential for generation of abundant silicic magma in the absence of mature island arc or preexisting continental crust. Zircon ages (U-Th and U-Pb) and isotope compositions (Hf and O), combined with whole-rock isotope data (Nd, Hf, Pb), provide insight into the petrogenesis and mantle heritage of these silicic magmas. Zircon and whole-rock samples represent the past 15 Ma of Iceland’s geologic evolution, geographic extent (marginal fjordlands to neovolcanic zones), and modern tectonic settings (on-rift, propagating-rift, off-rift). The generation of Icelandic silicic magma has been influenced by hydrothermally altered crust, via assimilation and/or anatexis, throughout Iceland’s history. This is shown by consistently depleted O isotopes in zircon (median δ18O +3.1‰; >98% below +5.3‰), and silicic rocks. Zircon δ18O values appear to have become lower and more diverse since ca. 0.7 Ma (median +1.9‰). This decrease may reflect lower δ18O of meteoric waters involved in hydrothermal alteration during the Pleistocene and/or more volumetrically significant contributions from low δ18O altered crust. Zircon O compositions from historically active volcanoes confirm that the role of altered crust is greater in on-rift than in off-rift settings; diversity in δ18O at volcanoes in propagating rift settings suggests highly variable contributions from altered crust. The silicic record (whole-rock and zircon) exhibits a correlation between geographic position and isotope composition that seems to be independent of local tectonic setting. Silicic samples of all ages collected above 65° N have more radiogenic whole-rock Hf and Nd isotopic compositions, and less radiogenic Pb, than samples collected in southern Iceland; published isotopic data for basalts suggest a similar time-independent latitudinal trend. The persistence of this trend through time suggests that northern Iceland has been underlain by a more depleted mantle source than southern Iceland throughout the island’s history.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"1 - 28"},"PeriodicalIF":1.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/706261","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41470923","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}
High amounts of Chattian-Langhian orogenic magmatism have generated volcaniclastic deposits that are interbedded within the penecontemporaneous sedimentary marine successions in several central-western peri-Mediterranean chains. These deposits are widespread in at least 41 units of different basins located in different geotectonic provinces: (1) the Africa-Adria continental margins (external units), (2) the basinal units resting on oceanic or thinned continental crust of the different branches of the western Tethys, (3) the European Margin (external units), and (4) the Western Sardinia zone (Sardinia Trough units). The emplacement of volcaniclastic material in marine basins was controlled by gravity flows (mainly turbidites; epiclastites) and fallout (pyroclastites). A third type comprises volcaniclastic grains mixed with marine deposits (mixed pyroclastic-epiclastic). Calc-alkaline magmatic activity is characterized by a medium- to high-potassium andesite-dacite-rhyolite suite and is linked to complex geodynamic processes that affected the central-western Mediterranean area in the ∼26 to 15 My range. The space/time distribution of volcaniclastites, together with a paleogeographic reconstructions, provide keys and constraints for a better reconstruction of some geodynamic events. Previous models of the central-western Mediterranean area were examined to compare their compatibility with main paleotectonic and paleogeographic constraints presented by the main results of the study. Despite the complexity of the topic, a preliminary evolutionary model based on the distribution of volcaniclastites and active volcanic systems is proposed.
{"title":"Geodynamic Implications of the Latest Chattian-Langhian Central-Western Peri-Mediterranean Volcano-Sedimentary Event: A Review","authors":"M. Martín-Martín, F. Guerrera, M. Tramontana","doi":"10.1086/706262","DOIUrl":"https://doi.org/10.1086/706262","url":null,"abstract":"High amounts of Chattian-Langhian orogenic magmatism have generated volcaniclastic deposits that are interbedded within the penecontemporaneous sedimentary marine successions in several central-western peri-Mediterranean chains. These deposits are widespread in at least 41 units of different basins located in different geotectonic provinces: (1) the Africa-Adria continental margins (external units), (2) the basinal units resting on oceanic or thinned continental crust of the different branches of the western Tethys, (3) the European Margin (external units), and (4) the Western Sardinia zone (Sardinia Trough units). The emplacement of volcaniclastic material in marine basins was controlled by gravity flows (mainly turbidites; epiclastites) and fallout (pyroclastites). A third type comprises volcaniclastic grains mixed with marine deposits (mixed pyroclastic-epiclastic). Calc-alkaline magmatic activity is characterized by a medium- to high-potassium andesite-dacite-rhyolite suite and is linked to complex geodynamic processes that affected the central-western Mediterranean area in the ∼26 to 15 My range. The space/time distribution of volcaniclastites, together with a paleogeographic reconstructions, provide keys and constraints for a better reconstruction of some geodynamic events. Previous models of the central-western Mediterranean area were examined to compare their compatibility with main paleotectonic and paleogeographic constraints presented by the main results of the study. Despite the complexity of the topic, a preliminary evolutionary model based on the distribution of volcaniclastites and active volcanic systems is proposed.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"29 - 43"},"PeriodicalIF":1.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/706262","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46117075","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}
Ambik Kumar, A. Prakash, L. Saha, F. Corfu, A. Bhattacharya
The northwestern domains of India record Proterozoic orogenies that reflect global cycles of convergence and extension. A garnet-biotite migmatitic orthogneiss hosted within the Agucha-Kekri Shear Zone sandwiched between the Bhilwara Belt and the North Delhi Fold Belt (NDFB) has two zircon populations yielding U-Pb ages of 1726 and 938 Ma. The older age is correlated with the intrusion of the migmatite protolith, consistent with the partial melting event recorded in gneisses in the southern margin of the Bhilwara Belt. The younger age is interpreted as the age of partial melting and migmatization. Petrographical observations and pressure-temperature (PT) pseudosection analyses indicate incongruent melting of biotite and plagioclase in the gneiss-produced garnet, potash feldspar, and melt under water-fluxed conditions. The peak conditions of ~9 kbar and ≥700°C estimated for the partial melting are similar to those of coeval migmatization recorded at the northwestern margin of the Bhilwara Belt, but lower than those in the adjacent NDFB. This is interpreted to indicate formation of a migmatitic front along the northwestern margin of the Bhilwara Belt while this was being underthrust under the NDFB. Migmatization under similar PT conditions and, at the same time, estimated for the Central Indian Tectonic Zone implies the presence of several loci of crustal amalgamation leading to the final architecture of peninsular India during Rodinia formation.
{"title":"940 Ma Anatexis in 1726 Ma Orthogneiss in the Northern Margin of the Bhilwara Belt and Significance for the Precambrian Evolution in Northwest India","authors":"Ambik Kumar, A. Prakash, L. Saha, F. Corfu, A. Bhattacharya","doi":"10.1086/705325","DOIUrl":"https://doi.org/10.1086/705325","url":null,"abstract":"The northwestern domains of India record Proterozoic orogenies that reflect global cycles of convergence and extension. A garnet-biotite migmatitic orthogneiss hosted within the Agucha-Kekri Shear Zone sandwiched between the Bhilwara Belt and the North Delhi Fold Belt (NDFB) has two zircon populations yielding U-Pb ages of 1726 and 938 Ma. The older age is correlated with the intrusion of the migmatite protolith, consistent with the partial melting event recorded in gneisses in the southern margin of the Bhilwara Belt. The younger age is interpreted as the age of partial melting and migmatization. Petrographical observations and pressure-temperature (PT) pseudosection analyses indicate incongruent melting of biotite and plagioclase in the gneiss-produced garnet, potash feldspar, and melt under water-fluxed conditions. The peak conditions of ~9 kbar and ≥700°C estimated for the partial melting are similar to those of coeval migmatization recorded at the northwestern margin of the Bhilwara Belt, but lower than those in the adjacent NDFB. This is interpreted to indicate formation of a migmatitic front along the northwestern margin of the Bhilwara Belt while this was being underthrust under the NDFB. Migmatization under similar PT conditions and, at the same time, estimated for the Central Indian Tectonic Zone implies the presence of several loci of crustal amalgamation leading to the final architecture of peninsular India during Rodinia formation.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"127 1","pages":"627 - 641"},"PeriodicalIF":1.8,"publicationDate":"2019-10-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705325","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43770330","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}
Youngest Toba Tuff ashes in the Purna alluvial basin of central India have been encountered at various places and are represented by light-gray, yellowish-brown, grayish-orange-pink, loosely packed, fine-grained material. These are preserved as pockets, lenticles, and discontinuous beds of 0.5–3.0-m thickness in the arenoargillaceous sediments of fluvial environments. Based on color, thickness, and sedimentological features, this distal material is divisible into primary ash and reworked ash. Microscopically, the ashes consist predominantly of glass shards with bubble-wall morphologies, minor minerals, unidentifiable fine-grain admixture, and, rarely, channel sediments. The glass shards are colorless, transparent, angular, unfractured, and unaltered in nature. A total of six morphotypes have been identified: (1) uniradiate, or two-bubble-wall, shards, (2) Y-shaped, or triradiate, shards, (3) platy shards, (4) blocky shards, (5) shards with elongated vesicles, and (6) pumice shards. Facies architecture of various successions shows broadly low-energy fluviolacustrine depositional environments. Major-element analysis of the glass shards shows a high percentage of silica, followed by alumina, potassium, and sodium. In general, high percentages of major ions in the glass shards reflect rhyolitic magma composition. The minerals in the ash, though minor in quantity, are represented by quartz, feldspar, biotite, amphibole, pyroxene, olivine, allanites, and Fe-Ti oxides. Preservations of various paleoclimatic proxies are also recorded from pretephra successions and are represented by rhizoliths, rhizolith balls, rhizosphere, ant traces, and vertebrate remains.
{"title":"Nature, Occurrence, and Lithological Setup of Youngest Toba Tuff Volcanic Ash, Purna Alluvial Basin, Central India","authors":"A. Srivastava, Ajab Singh","doi":"10.1086/705326","DOIUrl":"https://doi.org/10.1086/705326","url":null,"abstract":"Youngest Toba Tuff ashes in the Purna alluvial basin of central India have been encountered at various places and are represented by light-gray, yellowish-brown, grayish-orange-pink, loosely packed, fine-grained material. These are preserved as pockets, lenticles, and discontinuous beds of 0.5–3.0-m thickness in the arenoargillaceous sediments of fluvial environments. Based on color, thickness, and sedimentological features, this distal material is divisible into primary ash and reworked ash. Microscopically, the ashes consist predominantly of glass shards with bubble-wall morphologies, minor minerals, unidentifiable fine-grain admixture, and, rarely, channel sediments. The glass shards are colorless, transparent, angular, unfractured, and unaltered in nature. A total of six morphotypes have been identified: (1) uniradiate, or two-bubble-wall, shards, (2) Y-shaped, or triradiate, shards, (3) platy shards, (4) blocky shards, (5) shards with elongated vesicles, and (6) pumice shards. Facies architecture of various successions shows broadly low-energy fluviolacustrine depositional environments. Major-element analysis of the glass shards shows a high percentage of silica, followed by alumina, potassium, and sodium. In general, high percentages of major ions in the glass shards reflect rhyolitic magma composition. The minerals in the ash, though minor in quantity, are represented by quartz, feldspar, biotite, amphibole, pyroxene, olivine, allanites, and Fe-Ti oxides. Preservations of various paleoclimatic proxies are also recorded from pretephra successions and are represented by rhizoliths, rhizolith balls, rhizosphere, ant traces, and vertebrate remains.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"127 1","pages":"593 - 610"},"PeriodicalIF":1.8,"publicationDate":"2019-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705326","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43871640","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Nuweibi albite granite (NAG) is a postcollisional intrusion emplaced as a high-level magmatic cupola into metamorphic and syntectonic calc-alkaline country rocks. It consists of two cogenetic intrusive bodies: the western, nonporphyritic, albite granite was emplaced deeper than the eastern, fine-grained, porphyritic, albite granite. In places the nonporphyritic phase crosscuts the earlier porphyritic phase, but the occurrence of gradational contacts between the phases implies a near coincidence in time, with the nonporphyritic phase emplaced before crystallization of the porphyritic phase was complete. The steeply dipping slope of the western contact of the Nuweibi pluton against country rocks, in contrast to the gently dipping contacts above the eastern and northeastern parts, indicates the probable location of the cupola apex in the eastern part of the pluton. The NAG intrusion is highly evolved meta- to peraluminous leucocratic rare-metal albite granite. The NAG intrusion is chemically zoned, with upward increases of Al2O3, Na2O, Sr, Ga, and Ta concentrations, alongside upward decreases in SiO2, K2O, Rb, Nb, Zn, Zr, Th, Sn, and rare earth element concentrations. These trends are interrupted by a compositional gap with discontinuities in evolutionary trends of both compatible and incompatible elements, suggesting multiple pulses of intrusion. The NAG was generated via partial melting of a juvenile crust that had undergone extensive fractional crystallization combined with late-magmatic fluid overprint. Accumulation of residual volatile-rich melt and exsolved fluids in the apical part of the magmatic cupola produced stockscheider pegmatite, greisen, and quartz veins that cut the peripheries of the NAG pluton. Metasomatic activity by late- to postmagmatic fluids drove changes in the bulk composition of the cupola, removing K and driving the alkali feldspars toward pure albite.
{"title":"Geochemistry and Petrogenesis of Late Ediacaran Rare-Metal Albite Granite of the Nubian Shield: Case Study of Nuweibi Intrusion, Eastern Desert, Egypt","authors":"M. Azer, Khaled M. Abdelfadil, A. A. Ramadan","doi":"10.1086/705328","DOIUrl":"https://doi.org/10.1086/705328","url":null,"abstract":"The Nuweibi albite granite (NAG) is a postcollisional intrusion emplaced as a high-level magmatic cupola into metamorphic and syntectonic calc-alkaline country rocks. It consists of two cogenetic intrusive bodies: the western, nonporphyritic, albite granite was emplaced deeper than the eastern, fine-grained, porphyritic, albite granite. In places the nonporphyritic phase crosscuts the earlier porphyritic phase, but the occurrence of gradational contacts between the phases implies a near coincidence in time, with the nonporphyritic phase emplaced before crystallization of the porphyritic phase was complete. The steeply dipping slope of the western contact of the Nuweibi pluton against country rocks, in contrast to the gently dipping contacts above the eastern and northeastern parts, indicates the probable location of the cupola apex in the eastern part of the pluton. The NAG intrusion is highly evolved meta- to peraluminous leucocratic rare-metal albite granite. The NAG intrusion is chemically zoned, with upward increases of Al2O3, Na2O, Sr, Ga, and Ta concentrations, alongside upward decreases in SiO2, K2O, Rb, Nb, Zn, Zr, Th, Sn, and rare earth element concentrations. These trends are interrupted by a compositional gap with discontinuities in evolutionary trends of both compatible and incompatible elements, suggesting multiple pulses of intrusion. The NAG was generated via partial melting of a juvenile crust that had undergone extensive fractional crystallization combined with late-magmatic fluid overprint. Accumulation of residual volatile-rich melt and exsolved fluids in the apical part of the magmatic cupola produced stockscheider pegmatite, greisen, and quartz veins that cut the peripheries of the NAG pluton. Metasomatic activity by late- to postmagmatic fluids drove changes in the bulk composition of the cupola, removing K and driving the alkali feldspars toward pure albite.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"127 1","pages":"665 - 689"},"PeriodicalIF":1.8,"publicationDate":"2019-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705328","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44516779","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}
Minglong Li, Jingchun Tian, Lin Chen, Hui Xu, Deshun Zheng, Weiqing Liu
The evolution of the paleoclimate and the duration of the two glacial periods during the Cryogenian in the Yangtze Block are of great geological significance for understanding “Snowball Earth.” We studied the evolution of the paleoclimate during the Cryogenian, using chemical weathering, and determined the warming time of the interglacial paleoclimate, using zircon U-Pb dating in the Yangtze region. A new laser ablation ICP-MS zircon U-Pb age of 659.9±3.2 Ma is reported from the middle Datangpo Formation. The chemical index of alteration (CIA) for the Cryogenian fine clastic rock samples from a drill core shows that the climate in the provenance area during the Gucheng and Nantuo glacial periods was dry and cold and that the chemical weathering was weak; the average CIA values were 63.3 and 64.2, respectively. In the early stage of the Datangpo period, the climate in the provenance area was still dry and cold, with a CIA average of 60.7, compatible with that in the two glacial periods; in the middle and late stages, the climate was warm and humid, chemical weathering was enhanced, and the average CIA rose to 77.4, compatible with the 76.9 of the Liantuo period before the Cryogenian. The evolution of other chemical weathering (paleoclimate) proxies, such as the chemical index of weathering, the plagioclase index of alteration, and Rb/Sr, is compatible with that of the CIA. The warming time of the interglacial paleoclimate during the Cryogenian in the Yangtze Block was ca. 660 Ma, roughly consistent with the ending time of the Sturtian in Australia and Mongolia, indicating that the lower part of the Datangpo Formation may still represent Sturtian glacial deposits, thus further suggesting the global consistency of the end of the Sturtian.
{"title":"New Zircon U-Pb Age and Its Restriction on the Warming Time of the Interglacial Paleoclimate during the Cryogenian in the Yangtze Block","authors":"Minglong Li, Jingchun Tian, Lin Chen, Hui Xu, Deshun Zheng, Weiqing Liu","doi":"10.1086/705327","DOIUrl":"https://doi.org/10.1086/705327","url":null,"abstract":"The evolution of the paleoclimate and the duration of the two glacial periods during the Cryogenian in the Yangtze Block are of great geological significance for understanding “Snowball Earth.” We studied the evolution of the paleoclimate during the Cryogenian, using chemical weathering, and determined the warming time of the interglacial paleoclimate, using zircon U-Pb dating in the Yangtze region. A new laser ablation ICP-MS zircon U-Pb age of 659.9±3.2 Ma is reported from the middle Datangpo Formation. The chemical index of alteration (CIA) for the Cryogenian fine clastic rock samples from a drill core shows that the climate in the provenance area during the Gucheng and Nantuo glacial periods was dry and cold and that the chemical weathering was weak; the average CIA values were 63.3 and 64.2, respectively. In the early stage of the Datangpo period, the climate in the provenance area was still dry and cold, with a CIA average of 60.7, compatible with that in the two glacial periods; in the middle and late stages, the climate was warm and humid, chemical weathering was enhanced, and the average CIA rose to 77.4, compatible with the 76.9 of the Liantuo period before the Cryogenian. The evolution of other chemical weathering (paleoclimate) proxies, such as the chemical index of weathering, the plagioclase index of alteration, and Rb/Sr, is compatible with that of the CIA. The warming time of the interglacial paleoclimate during the Cryogenian in the Yangtze Block was ca. 660 Ma, roughly consistent with the ending time of the Sturtian in Australia and Mongolia, indicating that the lower part of the Datangpo Formation may still represent Sturtian glacial deposits, thus further suggesting the global consistency of the end of the Sturtian.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"127 1","pages":"691 - 701"},"PeriodicalIF":1.8,"publicationDate":"2019-10-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705327","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45311921","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}
Discriminating among tectonic settings by the chemical composition of igneous rocks is a feasible method in geochemistry. In this study, the feasibility of using gabbroic rocks to discriminate among tectonic settings is analyzed, and a mathematical model based on Gaussian copula and Bayesian theory is set up to discriminate among three tectonic settings: island arc, ocean island, and mid-oceanic ridge. The derivation of the model includes three steps: (1) determine the probability density functions (PDFs) of the elements in different tectonic settings, (2) determine the joint PDFs of the geochemical components of the rocks from different tectonic settings using copula functions, and (3) determine the tectonic settings of rocks using Bayesian theory. The optimal parameters of the mathematical model are calculated using a genetic algorithm, and finally the definitive form of the model is determined with nine basic elements: TiO2, Al2O3, FeOT, CaO, MnO, K2O, Na2O, Ni, and Sr. An experiment shows that the success rates of the mathematical model on the three tectonic settings are 84.03%, 95.48%, and 91.84%, respectively. The average percent success rate is 92.13%, which is significantly higher than using discrimination diagrams and the naive Bayes algorithm. Such an ideal result indicates that using gabbroic rocks to determine the types of tectonic settings is feasible. Moreover, this study can provide support for the application of machine learning and mathematical methods in the field of geochemistry.
{"title":"A Mathematical Model Based on Bayesian Theory and Gaussian Copula for the Discrimination of Gabbroic Rocks from Three Tectonic Settings","authors":"Shuai Han, Mingchao Li, Qi Zhang, Heng Li","doi":"10.1086/705413","DOIUrl":"https://doi.org/10.1086/705413","url":null,"abstract":"Discriminating among tectonic settings by the chemical composition of igneous rocks is a feasible method in geochemistry. In this study, the feasibility of using gabbroic rocks to discriminate among tectonic settings is analyzed, and a mathematical model based on Gaussian copula and Bayesian theory is set up to discriminate among three tectonic settings: island arc, ocean island, and mid-oceanic ridge. The derivation of the model includes three steps: (1) determine the probability density functions (PDFs) of the elements in different tectonic settings, (2) determine the joint PDFs of the geochemical components of the rocks from different tectonic settings using copula functions, and (3) determine the tectonic settings of rocks using Bayesian theory. The optimal parameters of the mathematical model are calculated using a genetic algorithm, and finally the definitive form of the model is determined with nine basic elements: TiO2, Al2O3, FeOT, CaO, MnO, K2O, Na2O, Ni, and Sr. An experiment shows that the success rates of the mathematical model on the three tectonic settings are 84.03%, 95.48%, and 91.84%, respectively. The average percent success rate is 92.13%, which is significantly higher than using discrimination diagrams and the naive Bayes algorithm. Such an ideal result indicates that using gabbroic rocks to determine the types of tectonic settings is feasible. Moreover, this study can provide support for the application of machine learning and mathematical methods in the field of geochemistry.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"127 1","pages":"611 - 626"},"PeriodicalIF":1.8,"publicationDate":"2019-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705413","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46769410","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}
It is well known that the rare earth elements (REEs) incorporated in the crystal lattice of sedimentary apatite are commonly derived from ambient seawater. This study documents, for the first time, the possibility of mixed origin of apatite REEs present in the Egyptian Western Desert phosphorites, known as the Abu-Tartur phosphorites, one of the most important accumulations of sedimentary phosphorites in the Middle East and North Africa. Shale-like patterns of REEs, negative Ce anomalies, and a (La/Sm)N−(La/Yb)N binary diagram of the studied phosphorites indicate that the incorporation of REEs into apatite crystal lattices has occurred from the ambient seawater by substitution during late-stage diagenesis. The second origin is attributed to REE-bearing supergene fluids, which resulted in the occurrence of sedimentary britholite as green rims and patches containing about 14.93 wt% total REEs in places where the black phosphorites are gradually oxidized into the brown variety. For instance, the intensive chemical weathering induces the crystal lattice of carbonate fluorapatite to preferentially release Ca2+ and CO32− ion species to solution, resulting in the formation of a carbonate-depleted layer in which REEs, particularly heavy REEs, are incorporated into the preferential Ca2+ sites inside the altered apatite lattice, leading to britholite formation.
{"title":"Possibility of Mixed Origin of Rare Earth Elements in Sedimentary Marine Apatites: A Case Study from Phosphorites in the Cretaceous (Campanian-Maastrichtian) Duwi Formation, Abu-Tartur Plateau, Egypt","authors":"G. El-Habaak, M. Askalany, M. Abdel-Hakeem","doi":"10.1086/705414","DOIUrl":"https://doi.org/10.1086/705414","url":null,"abstract":"It is well known that the rare earth elements (REEs) incorporated in the crystal lattice of sedimentary apatite are commonly derived from ambient seawater. This study documents, for the first time, the possibility of mixed origin of apatite REEs present in the Egyptian Western Desert phosphorites, known as the Abu-Tartur phosphorites, one of the most important accumulations of sedimentary phosphorites in the Middle East and North Africa. Shale-like patterns of REEs, negative Ce anomalies, and a (La/Sm)N−(La/Yb)N binary diagram of the studied phosphorites indicate that the incorporation of REEs into apatite crystal lattices has occurred from the ambient seawater by substitution during late-stage diagenesis. The second origin is attributed to REE-bearing supergene fluids, which resulted in the occurrence of sedimentary britholite as green rims and patches containing about 14.93 wt% total REEs in places where the black phosphorites are gradually oxidized into the brown variety. For instance, the intensive chemical weathering induces the crystal lattice of carbonate fluorapatite to preferentially release Ca2+ and CO32− ion species to solution, resulting in the formation of a carbonate-depleted layer in which REEs, particularly heavy REEs, are incorporated into the preferential Ca2+ sites inside the altered apatite lattice, leading to britholite formation.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"127 1","pages":"643 - 663"},"PeriodicalIF":1.8,"publicationDate":"2019-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/705414","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43528885","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}
This article reports the existence of subduction-related tholeiitic, normal mid-ocean ridge basalt (N-MORB)-type mafic intrusives emplaced within the Sapi-Shergol ophiolitic mélange of the Indus Suture Zone, western Ladakh. The Shergol mafic intrusives show Fe enrichment with basalt to basaltic-andesite composition. Based on their mineralogy and textures, these mafic rocks can be identified as fine- to medium-grained gabbros that have undergone greenschist-grade metamorphism. These rocks have N-MORB-type geochemical characteristics, exhibiting nearly flat to depleted light rare earth element patterns ((La/Sm)N=0.66–1.05). Petrogenetic modeling suggests <20% partial melting of a depleted MORB-type mantle source, within the spinel peridotite stability. The presence of slightly negative anomalies of high field strength elements like Nb, Zr, and Ti in multielement patterns reflect the influence of subduction zone magmatism. The presence of low-Ti clinopyroxene (En38–50Fe11–25Wo31–43; enstatite-ferrosilite-wollastonite), Ca-rich plagioclase (An2–36; andesine), and pargasitic amphibole also reflects their subduction-related depleted-mantle origin. The MORB–island arc tholeiite signature displayed by the Shergol gabbros intrusive in Shergol peridotites reflect their generation in a mantle wedge associated with the Early Cretaceous intraoceanic subduction within the Neo-Tethys Ocean. They are similar to Spongtang ophiolite gabbros intrusive in the Spongtang ophiolite mantle peridotites from south Ladakh, ophiolitic mélange gabbros from southern Tibet, and Muslim Bagh ophiolite gabbros from Pakistan. Based on this study, we offer a geodynamic model suggesting that the Sapi-Shergol ophiolitic slice was intruded by mafic intrusive rocks that represent the relict of the intraoceanic substratum of the Cretaceous Dras arc complex.
{"title":"Petrology and Geochemistry of Mafic Intrusive Rocks from the Sapi-Shergol Ophiolitic Mélange, Indus Suture Zone, Western Ladakh: Constraints on Petrogenesis and Tectonic Setting","authors":"I. Bhat, T. Ahmad, D. S. Rao","doi":"10.1086/704254","DOIUrl":"https://doi.org/10.1086/704254","url":null,"abstract":"This article reports the existence of subduction-related tholeiitic, normal mid-ocean ridge basalt (N-MORB)-type mafic intrusives emplaced within the Sapi-Shergol ophiolitic mélange of the Indus Suture Zone, western Ladakh. The Shergol mafic intrusives show Fe enrichment with basalt to basaltic-andesite composition. Based on their mineralogy and textures, these mafic rocks can be identified as fine- to medium-grained gabbros that have undergone greenschist-grade metamorphism. These rocks have N-MORB-type geochemical characteristics, exhibiting nearly flat to depleted light rare earth element patterns ((La/Sm)N=0.66–1.05). Petrogenetic modeling suggests <20% partial melting of a depleted MORB-type mantle source, within the spinel peridotite stability. The presence of slightly negative anomalies of high field strength elements like Nb, Zr, and Ti in multielement patterns reflect the influence of subduction zone magmatism. The presence of low-Ti clinopyroxene (En38–50Fe11–25Wo31–43; enstatite-ferrosilite-wollastonite), Ca-rich plagioclase (An2–36; andesine), and pargasitic amphibole also reflects their subduction-related depleted-mantle origin. The MORB–island arc tholeiite signature displayed by the Shergol gabbros intrusive in Shergol peridotites reflect their generation in a mantle wedge associated with the Early Cretaceous intraoceanic subduction within the Neo-Tethys Ocean. They are similar to Spongtang ophiolite gabbros intrusive in the Spongtang ophiolite mantle peridotites from south Ladakh, ophiolitic mélange gabbros from southern Tibet, and Muslim Bagh ophiolite gabbros from Pakistan. Based on this study, we offer a geodynamic model suggesting that the Sapi-Shergol ophiolitic slice was intruded by mafic intrusive rocks that represent the relict of the intraoceanic substratum of the Cretaceous Dras arc complex.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"127 1","pages":"543 - 566"},"PeriodicalIF":1.8,"publicationDate":"2019-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/704254","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44549140","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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