Asmae El Bakili, M. Corsini, Valérie Bosse, J. Lardeaux
This work presents petrologic data obtained from shielded mineral inclusions within large garnets from the Beni-Bousera metamorphic unit (internal Rif belt, northern Morocco) combined with in situ U‒Th‒Pb dating of monazite inclusions. In the considered Beni-Bousera metapelites, the occurrence of mineral inclusions of kyanite + rutile + plagioclase +K-feldspar + quartz ± graphite trapped in garnet cores is diagnostic of high-pressure granulite-facies metamorphism. Geothermobarometry combined with thermodynamic modelling yields minimal P‒T conditions of approximately 1.37-1.5 GPa and 780-795 °C, consistent with the development of palaeogradients during crustal thickening in collision belts. A concordant U/Pb age of 281 ± 3 Ma is obtained from associated monazite inclusions in the same garnet cores. These new petrochronological data attest to the lower Permian continental collision in northern Morocco. The obtained dataset is constrained with petrologic and geochronologic parameters available on other neighboring segments of the Variscan belt. Within the framework of recently unified full plate reconstruction models, the presented tectonic scenario suggests that all these orogenic segments were built during the late Carboniferous-early Permian crustal thickening at the northwestern Gondwana margin in response to the convergence of Laurentia and Gondwana during the late stages of Pangea construction.
{"title":"Metamorphic and chronological records of early Permian continental collision in Beni Bousera granulites (Internal Rif belt): implications for the tectonic evolution of northern Morocco during Pangea construction","authors":"Asmae El Bakili, M. Corsini, Valérie Bosse, J. Lardeaux","doi":"10.1144/jgs2024-018","DOIUrl":"https://doi.org/10.1144/jgs2024-018","url":null,"abstract":"This work presents petrologic data obtained from shielded mineral inclusions within large garnets from the Beni-Bousera metamorphic unit (internal Rif belt, northern Morocco) combined with in situ U‒Th‒Pb dating of monazite inclusions. In the considered Beni-Bousera metapelites, the occurrence of mineral inclusions of kyanite + rutile + plagioclase +K-feldspar + quartz ± graphite trapped in garnet cores is diagnostic of high-pressure granulite-facies metamorphism. Geothermobarometry combined with thermodynamic modelling yields minimal P‒T conditions of approximately 1.37-1.5 GPa and 780-795 °C, consistent with the development of palaeogradients during crustal thickening in collision belts. A concordant U/Pb age of 281 ± 3 Ma is obtained from associated monazite inclusions in the same garnet cores. These new petrochronological data attest to the lower Permian continental collision in northern Morocco. The obtained dataset is constrained with petrologic and geochronologic parameters available on other neighboring segments of the Variscan belt. Within the framework of recently unified full plate reconstruction models, the presented tectonic scenario suggests that all these orogenic segments were built during the late Carboniferous-early Permian crustal thickening at the northwestern Gondwana margin in response to the convergence of Laurentia and Gondwana during the late stages of Pangea construction.","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.6,"publicationDate":"2024-07-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141799674","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Various mélange types occur within the Franciscan accretionary Complex of western California. The largest mélange body, called the Central Belt Mélange (or similar names) served earlier as the type example for the orogen-long, subduction channel model. Yet, in the northwestern San Francisco Bay Area, the name does not accurately reflect the geology. The mélange designation was commonly applied where resistant exotic and native blocks of rock are scattered across a relatively smooth terrain. Detailed mapping shows that many blocks experienced post-accretion transport. Aerially large rock masses previously designated as Central Belt Mélange consist of multiple units and less than 30 percent of the tectonostratigraphy is mélange. Weakly metamorphosed sandstone-mudrock broken to dismembered formational units and similarly deformed sandstone-mudrock submarine fan facies dominate the tectonostratigraphy. Subordinate mélanges of the northwestern San Francisco Bay Area are of tectonic or sedimentary origin. The sedimentary bodies represent submarine mass flow deposits. Tectonic mélanges mark Mesozoic subduction zone faults or Cenozoic strike-slip faults. Discriminating among mélange types and their origins, and reconstructing tectonostratigraphic columns for major fault blocks, clarifies the primary accretionary complex architecture and reveals significant along strike variations in the Franciscan subduction accretionary Complex.
{"title":"Deconstruction of the Franciscan Complex central terrane mélange and re-evaluation of Franciscan mélanges and architecture of the northwestern San Francisco Bay area, California, USA","authors":"L. A. Raymond","doi":"10.1144/jgs2024-102","DOIUrl":"https://doi.org/10.1144/jgs2024-102","url":null,"abstract":"Various mélange types occur within the Franciscan accretionary Complex of western California. The largest mélange body, called the Central Belt Mélange (or similar names) served earlier as the type example for the orogen-long, subduction channel model. Yet, in the northwestern San Francisco Bay Area, the name does not accurately reflect the geology. The mélange designation was commonly applied where resistant exotic and native blocks of rock are scattered across a relatively smooth terrain. Detailed mapping shows that many blocks experienced post-accretion transport. Aerially large rock masses previously designated as Central Belt Mélange consist of multiple units and less than 30 percent of the tectonostratigraphy is mélange. Weakly metamorphosed sandstone-mudrock broken to dismembered formational units and similarly deformed sandstone-mudrock submarine fan facies dominate the tectonostratigraphy. Subordinate mélanges of the northwestern San Francisco Bay Area are of tectonic or sedimentary origin. The sedimentary bodies represent submarine mass flow deposits. Tectonic mélanges mark Mesozoic subduction zone faults or Cenozoic strike-slip faults. Discriminating among mélange types and their origins, and reconstructing tectonostratigraphic columns for major fault blocks, clarifies the primary accretionary complex architecture and reveals significant along strike variations in the Franciscan subduction accretionary Complex.","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.6,"publicationDate":"2024-07-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141801887","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The petrogenesis of contemporary igneous and metamorphic rocks is commonly explained by plate tectonics, but how far back in time does this relationship hold? Here we investigate whether the distinctive petrological features of recent ocean crust, subduction-related magmatism and regional metamorphism can be unambiguously identified in the Archean geological record. From an igneous perspective based on geological relationships and Th – Nb systematics, it is difficult to claim that any Archean ‘ophiolite’ was part of a global plate system rather than deriving from a plume ascending through attenuating lithosphere. Furthermore, the rarity of subduction-related rocks, particularly their plutonic equivalents which have good preservation potential, is consistent with the concept of local convergence and short-lived subduction. From a metamorphic perspective, the appearance of orogenic eclogites in the Paleoproterozoic, the widespread occurrence of blueschists and ultrahigh pressure metamorphic rocks since the late Neoproterozoic, and a change from a unimodal to a bimodal distribution of metamorphic T/P during the Proterozoic, are responses to secular cooling and the evolution of tectonics since the Archean. Our petrological perspective is that plate tectonics analogous to that on Earth today is probably a post Archean phenomenon.
{"title":"Is plate tectonics a post-Archean phenomenon? A petrological perspective","authors":"Michael Brown, Julian A. Pearce, Tim E. Johnson","doi":"10.1144/jgs2024-091","DOIUrl":"https://doi.org/10.1144/jgs2024-091","url":null,"abstract":"\u0000 The petrogenesis of contemporary igneous and metamorphic rocks is commonly explained by plate tectonics, but how far back in time does this relationship hold? Here we investigate whether the distinctive petrological features of recent ocean crust, subduction-related magmatism and regional metamorphism can be unambiguously identified in the Archean geological record. From an igneous perspective based on geological relationships and Th\u0000 –\u0000 Nb systematics, it is difficult to claim that any Archean ‘ophiolite’ was part of a global plate system rather than deriving from a plume ascending through attenuating lithosphere. Furthermore, the rarity of subduction-related rocks, particularly their plutonic equivalents which have good preservation potential, is consistent with the concept of local convergence and short-lived subduction. From a metamorphic perspective, the appearance of orogenic eclogites in the Paleoproterozoic, the widespread occurrence of blueschists and ultrahigh pressure metamorphic rocks since the late Neoproterozoic, and a change from a unimodal to a bimodal distribution of metamorphic\u0000 T/P\u0000 during the Proterozoic, are responses to secular cooling and the evolution of tectonics since the Archean. Our petrological perspective is that plate tectonics analogous to that on Earth today is probably a post Archean phenomenon.\u0000","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.6,"publicationDate":"2024-07-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141829825","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. J. Veevers, David C. Ray, Kenneth T. Ratcliffe, Alan T. Thomas
The Silurian Coralliferous Formation of Pembrokeshire comprises 94–143 m of shelfal sediments deposited on the southern margin of the Welsh Basin, UK. The succession begins with rudites (Renney Slip Member), which are overlain by interbedded silty mudstone and sandstone (Deadman's Bay Member). Biostratigraphical control is weak between the two principal sections (Renney Slip and Marloes Sands), but high-resolution chemostratigraphy permits 11 subdivisions to be distinguished. Some chemostratigraphical units can be traced between the sections, whereas others are locally absent. The correlation of the chemostratigraphical units has been compared with sea-level cycles identified by a study of the frequency, distribution and characteristics of storm-beds using proximality trend analysis. This suggests a synchroneity of traceable chemostratigraphical units within the Deadman's Bay Member, indicating that deposition began in the Renney Slip Section, and that a greater water depth was achieved there. A holistic assessment shows that the Coralliferous Formation onlaps an unconformity surface with a palaeo-relief of 57 m, over a current distance of 3 km. Further considerations of benthic assemblages, subsidence rates and global sea-level fluctuations support the presence of a late Telychian transgression with glacio-eustasy contributing between 40 and 70 m to that sea-level rise.
{"title":"The application of chemostratigraphy and proximality trends to the Silurian Coralliferous Formation of SW Wales: rhythmical sedimentation during the transgression of a palaeo-shoreline","authors":"S. J. Veevers, David C. Ray, Kenneth T. Ratcliffe, Alan T. Thomas","doi":"10.1144/jgs2024-009","DOIUrl":"https://doi.org/10.1144/jgs2024-009","url":null,"abstract":"The Silurian Coralliferous Formation of Pembrokeshire comprises 94–143 m of shelfal sediments deposited on the southern margin of the Welsh Basin, UK. The succession begins with rudites (Renney Slip Member), which are overlain by interbedded silty mudstone and sandstone (Deadman's Bay Member). Biostratigraphical control is weak between the two principal sections (Renney Slip and Marloes Sands), but high-resolution chemostratigraphy permits 11 subdivisions to be distinguished. Some chemostratigraphical units can be traced between the sections, whereas others are locally absent. The correlation of the chemostratigraphical units has been compared with sea-level cycles identified by a study of the frequency, distribution and characteristics of storm-beds using proximality trend analysis. This suggests a synchroneity of traceable chemostratigraphical units within the Deadman's Bay Member, indicating that deposition began in the Renney Slip Section, and that a greater water depth was achieved there. A holistic assessment shows that the Coralliferous Formation onlaps an unconformity surface with a palaeo-relief of 57 m, over a current distance of 3 km. Further considerations of benthic assemblages, subsidence rates and global sea-level fluctuations support the presence of a late Telychian transgression with glacio-eustasy contributing between 40 and 70 m to that sea-level rise.","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.6,"publicationDate":"2024-07-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141831226","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Mei Luo, Glenn A. Brock, Yue Liang, Baopeng Song, Yazhou Hu, Fan Liu, Zhifei Zhang
Early Cambrian small shelly fossils (SSFs) represent some of the earliest biomineralized examples of metazoans and are often extremely abundant and diverse, so can be useful for regional and intercontinental correlation in pre-trilobitic carbonate or phosphorite deposits. New SSF assemblages chemically extracted from carbonates from the Cambrian Fortunian Stage are reported for the first time from the Yangjiagou Member in the Fucheng area, southern Shaanxi Province, China. The SSFs include anabaritids, protoconodonts, hyolitheminths, maikhanellids, Quadrapyrgites , Archaeooides , Olivooides , and Siphogonuchites . The SSFs recovered represent two previously identified SSF assemblage zones: the Anabarites trisulcatus – Protohertzina anabarica assemblage zone, and the succeeding Paragloborilus subglobosa – Purella squamulosa assemblage zone. The early Cambrian fossil assemblage zones described from this locality are correlated on a regional scale across the Yangtze Platform from eastern Yunnan, the Three Gorge region of the western Hubei, the northern Sichuan, the Ningqiang and Xixiang regions of the southern Shaanxi and the Nemakit–Daldynian Stages of Siberia. The results reveal that the Yangjiagou Member is provisionally equivalent to the Zhongyicun Member of Yunnan, the Yanjiahe Formation of Hubei, the Maidiping Formation of Sichuan, and the Kuanchuanpu Formation of Shaanxi. It offers new evidence and fossil data for the location of the Precambrian–Cambrian boundary in the Micangshan area of the South China.
{"title":"Correlation and stratigraphic implications of the lowermost Cambrian Small Shelly Fossils from new sites of South China","authors":"Mei Luo, Glenn A. Brock, Yue Liang, Baopeng Song, Yazhou Hu, Fan Liu, Zhifei Zhang","doi":"10.1144/jgs2023-231","DOIUrl":"https://doi.org/10.1144/jgs2023-231","url":null,"abstract":"\u0000 Early Cambrian small shelly fossils (SSFs) represent some of the earliest biomineralized examples of metazoans and are often extremely abundant and diverse, so can be useful for regional and intercontinental correlation in pre-trilobitic carbonate or phosphorite deposits. New SSF assemblages chemically extracted from carbonates from the Cambrian Fortunian Stage are reported for the first time from the Yangjiagou Member in the Fucheng area, southern Shaanxi Province, China. The SSFs include anabaritids, protoconodonts, hyolitheminths, maikhanellids,\u0000 Quadrapyrgites\u0000 ,\u0000 Archaeooides\u0000 ,\u0000 Olivooides\u0000 , and\u0000 Siphogonuchites\u0000 . The SSFs recovered represent two previously identified SSF assemblage zones: the\u0000 Anabarites trisulcatus\u0000 –\u0000 Protohertzina anabarica\u0000 assemblage zone, and the succeeding\u0000 Paragloborilus subglobosa\u0000 –\u0000 Purella squamulosa\u0000 assemblage zone. The early Cambrian fossil assemblage zones described from this locality are correlated on a regional scale across the Yangtze Platform from eastern Yunnan, the Three Gorge region of the western Hubei, the northern Sichuan, the Ningqiang and Xixiang regions of the southern Shaanxi and the Nemakit–Daldynian Stages of Siberia. The results reveal that the Yangjiagou Member is provisionally equivalent to the Zhongyicun Member of Yunnan, the Yanjiahe Formation of Hubei, the Maidiping Formation of Sichuan, and the Kuanchuanpu Formation of Shaanxi. It offers new evidence and fossil data for the location of the Precambrian–Cambrian boundary in the Micangshan area of the South China.\u0000","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141345626","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Geodynamic models for Pangea assembly require knowledge of Paleozoic mantle convection patterns. Application of basic geodynamic principles to Neoproterozoic–Paleozoic plate reconstructions yields Pangea in the incorrect configuration (predicting that Pangea should have formed by consumption of the exterior paleo-Pacific Ocean instead of Iapetus, Rheic, and Proto-Tethys oceans). We contend that the mantle legacy of Late Neoproterozoic–Cambrian amalgamation of Gondwana must be factored into models for Pangea amalgamation. Proxy data suggest that the mantle downwelling driving Pan-African collisions and Gondwana assembly evolved into a mantle upwelling as evidenced by the interplay between subduction-related and plume-related tectonics around the periphery of Gondwana. Orthoversion theory, whereby a supercontinent assembles ∼90° away from the centre of the previous supercontinent, suggests that Gondwana amalgamated above an intense downwelling along a meridional subduction girdle that bisected two antipodal sub-equatorial upwellings. Several processes beneath and around Gondwana reduced the intensity of the original downwelling, as evidenced by plume-related activity along its margins, initiation of subduction zone roll-back, and the export of terranes from Gondwana that collided with the margin of Laurentia–Baltica. As upwelling beneath it intensified, Gondwana migrated along the girdle until it collided with Laurentia–Baltica, resulting in the final assembly of Pangea.
{"title":"The assembly of Pangea: geodynamic conundrums revisited","authors":"J. B. Murphy, R. Nance, Ross N. Mitchell","doi":"10.1144/jgs2024-006","DOIUrl":"https://doi.org/10.1144/jgs2024-006","url":null,"abstract":"Geodynamic models for Pangea assembly require knowledge of Paleozoic mantle convection patterns. Application of basic geodynamic principles to Neoproterozoic–Paleozoic plate reconstructions yields Pangea in the incorrect configuration (predicting that Pangea should have formed by consumption of the exterior paleo-Pacific Ocean instead of Iapetus, Rheic, and Proto-Tethys oceans).\u0000 We contend that the mantle legacy of Late Neoproterozoic–Cambrian amalgamation of Gondwana must be factored into models for Pangea amalgamation. Proxy data suggest that the mantle downwelling driving Pan-African collisions and Gondwana assembly evolved into a mantle upwelling as evidenced by the interplay between subduction-related and plume-related tectonics around the periphery of Gondwana.\u0000 Orthoversion theory, whereby a supercontinent assembles ∼90° away from the centre of the previous supercontinent, suggests that Gondwana amalgamated above an intense downwelling along a meridional subduction girdle that bisected two antipodal sub-equatorial upwellings. Several processes beneath and around Gondwana reduced the intensity of the original downwelling, as evidenced by plume-related activity along its margins, initiation of subduction zone roll-back, and the export of terranes from Gondwana that collided with the margin of Laurentia–Baltica. As upwelling beneath it intensified, Gondwana migrated along the girdle until it collided with Laurentia–Baltica, resulting in the final assembly of Pangea.","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141348485","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Southeast Tibet has experienced significant deformation during the Late Cenozoic period, characterized by clockwise block rotation and separated by left-lateral strike-slip faults. Through our new field survey, structural analyses and microstructural observations, we have identified an early phase of ductile shear zones along three major faults: the Anninghe, Jinhe-Jinghe fault and Jinpingshan faults. New geochronological and thermochronological data collected from fault zones and carbonatites associated with a tear fault between two right-lateral oblique faults provides evidences that the slip occurred before ∼30 Ma. These new findings have led to a new conceptual model suggests that these NS-trending right-lateral oblique-slip faults, together with these NW-trending left-lateral strike-slip faults in Southeast Tibet, form a network of large conjugate fractures. The network was formed due to N-NE-trending contraction and played a crucial role in controlling the overall tectonic frame of Southeast Tibet during the Oligocene. By compiling and reviewing previous research with our latest findings, we propose that the region has undergone a transtion from oblique dextral to sinistral shearing since the Oligocene, providing new insights into the tectonic evolution of the East Tibet. Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change Supplementary material: https://doi.org/10.6084/m9.figshare.c.7262890
{"title":"Conjugatal extrusion with dextral deformation in the Southeast Tibet: New insight into the Oligo-Miocene deformation in the Southeast Tibet","authors":"Hailong Li, Yueqiao Zhang, Guihong Zhang, Qihang Yin, Yuntao Tian, Qiangmao Wang, Tianxiang Ren","doi":"10.1144/jgs2023-186","DOIUrl":"https://doi.org/10.1144/jgs2023-186","url":null,"abstract":"The Southeast Tibet has experienced significant deformation during the Late Cenozoic period, characterized by clockwise block rotation and separated by left-lateral strike-slip faults. Through our new field survey, structural analyses and microstructural observations, we have identified an early phase of ductile shear zones along three major faults: the Anninghe, Jinhe-Jinghe fault and Jinpingshan faults. New geochronological and thermochronological data collected from fault zones and carbonatites associated with a tear fault between two right-lateral oblique faults provides evidences that the slip occurred before ∼30 Ma. These new findings have led to a new conceptual model suggests that these NS-trending right-lateral oblique-slip faults, together with these NW-trending left-lateral strike-slip faults in Southeast Tibet, form a network of large conjugate fractures. The network was formed due to N-NE-trending contraction and played a crucial role in controlling the overall tectonic frame of Southeast Tibet during the Oligocene. By compiling and reviewing previous research with our latest findings, we propose that the region has undergone a transtion from oblique dextral to sinistral shearing since the Oligocene, providing new insights into the tectonic evolution of the East Tibet.\u0000 \u0000 Thematic collection:\u0000 This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at:\u0000 https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change\u0000 \u0000 \u0000 Supplementary material:\u0000 https://doi.org/10.6084/m9.figshare.c.7262890\u0000","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-06-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141267922","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jianqiang He, Yan Xia, Xisheng Xu, Hongyang Shi, Jiaqi Gu
Highly differentiated magmas are closely related to the formation of tin deposits. The Lianhuashan Basin and Baiyunzhang Basin, surrounded by multiple coeval tin deposits, developed various types of volcanic and subvolcanic rocks. The diagenetic connections between different types of rocks within the two basins will provide new insight on the silicic magma systems evolution and the impact of highly differentiated magmas on tin mineralization. Both basins have consistent zircon U-Pb ages (143 ∼ 138 Ma) and similar whole-rock Nd isotopes ( ε Nd ( t ) = -5.7 ∼ -3.4) and zircon Hf-O isotopes ranges ( ε Hf ( t ) = -9.0 ∼ -2.5; δ 18 O = 5.1 ∼ 7.9), suggesting both basins originate from the same deep-level magma reservoir, followed by different degrees of crystal differentiation and several episodes of crystal-melt separation in their respective shallow-level magma reservoirs. The multiple pulses of magma extraction eventually produced different volcanic rocks, granite porphyry and rhyolite porphyry, while the remaining crystal mush consolidated in situ to form quartz monzonite porphyry. Further studies show that the tectonic regime changed from a compressive to an extensional environment at ∼140 Ma. Consequently, mantle-derived magmas with low oxygen fugacity injected shallow magma reservoirs that able to evolve to a high degree of differentiation through multiple recharge, thus favoring the formation of the Sn deposit. Supplementary material: https://doi.org/10.6084/m9.figshare.c.7258282
高分异性岩浆与锡矿床的形成密切相关。莲花山盆地和白云漳盆地被多个共生锡矿床包围，发育有不同类型的火山岩和次火山岩。两个盆地内不同类型岩石之间的成岩联系将为硅质岩浆系统演化以及高分异性岩浆对锡成矿的影响提供新的见解。两个盆地具有一致的锆石U-Pb年龄（143 ∼ 138 Ma）和相似的全岩钕同位素（ε Nd ( t ) = -5.7 ∼ -3.4）和锆石Hf-O同位素范围（ε Hf ( t ) = -9.0 ∼ -2.5; δ 18 O = 5.1 ∼ 7.9），表明这两个盆地起源于同一深部岩浆储层，随后在各自的浅部岩浆储层中发生了不同程度的晶体分异和多次晶体-熔体分离。岩浆的多次萃取最终形成了不同的火山岩--花岗斑岩和流纹斑岩，而剩余的晶泥则在原地固结形成了石英斑岩。进一步的研究表明，在 ∼140 Ma 时，构造体系从压缩环境转变为延伸环境。因此，低氧富集的地幔岩浆注入浅层岩浆储层，通过多次补给，演化出高度分异的岩浆，从而有利于硒矿床的形成。 补充材料：https://doi.org/10.6084/m9.figshare.c.7258282
{"title":"Pulsatile extractions of Lower Cretaceous silicic volcanic plumbing system beneath the Baiyunzhang and Lianhuashan Basins, eastern Guangdong: silicic magma evolution and related mineralization","authors":"Jianqiang He, Yan Xia, Xisheng Xu, Hongyang Shi, Jiaqi Gu","doi":"10.1144/jgs2024-049","DOIUrl":"https://doi.org/10.1144/jgs2024-049","url":null,"abstract":"\u0000 Highly differentiated magmas are closely related to the formation of tin deposits. The Lianhuashan Basin and Baiyunzhang Basin, surrounded by multiple coeval tin deposits, developed various types of volcanic and subvolcanic rocks. The diagenetic connections between different types of rocks within the two basins will provide new insight on the silicic magma systems evolution and the impact of highly differentiated magmas on tin mineralization. Both basins have consistent zircon U-Pb ages (143 ∼ 138 Ma) and similar whole-rock Nd isotopes (\u0000 ε\u0000 Nd\u0000 (\u0000 t\u0000 ) = -5.7 ∼ -3.4) and zircon Hf-O isotopes ranges (\u0000 ε\u0000 Hf\u0000 (\u0000 t\u0000 ) = -9.0 ∼ -2.5;\u0000 δ\u0000 18\u0000 O = 5.1 ∼ 7.9), suggesting both basins originate from the same deep-level magma reservoir, followed by different degrees of crystal differentiation and several episodes of crystal-melt separation in their respective shallow-level magma reservoirs. The multiple pulses of magma extraction eventually produced different volcanic rocks, granite porphyry and rhyolite porphyry, while the remaining crystal mush consolidated in situ to form quartz monzonite porphyry. Further studies show that the tectonic regime changed from a compressive to an extensional environment at ∼140 Ma. Consequently, mantle-derived magmas with low oxygen fugacity injected shallow magma reservoirs that able to evolve to a high degree of differentiation through multiple recharge, thus favoring the formation of the Sn deposit.\u0000 \u0000 \u0000 Supplementary material:\u0000 https://doi.org/10.6084/m9.figshare.c.7258282\u0000","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-06-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141271432","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
When did Plate Tectonics begin on Earth, and what preceded it? Published thermo-mechanical mantle evolution models imply that the early history of planets with a composition and size similar to Earth and Venus should be characterized by periodic mantle overturns of 30-100 million years duration, separated by stable lid phases of 100-300 My. I argue this is best described as an Unstable Stagnant Lid, because this term captures the Jekyll-and-Hyde duality of such worlds, which alternate between a Stagnant Lid ss phase between mantle overturns, and a Mobile Lid phase during overturns. Mantle overturn upwelling zones would rework and resurface large tracts of pre-existing Hadean crust and basalt-dominated Archean-Style Oceanic Lithosphere (ASOL). Basal anatexis of ASOL ∼40-50 km thick (or melting within down-drips) could generate tonalite-trondhjemite melts (TTGs) and create proto-continental nuclei, while garnet pyroxenite restites delaminate into the mantle. With further reworking, low-K tonalitic rocks would remelt to produce granodiorite and granite, completing the transfer of radioactive elements out of the lower crust. Mantle overturns would generate large-scale lateral currents in the upper mantle that would push against Archaean-aged sub-continental lithospheric mantle keels, causing continental drift and orogenesis despite the absence of plate-boundary forces like slab pull. The validity of this is corroborated by the observed displacement of Lakshmi Planum (>1000 Km) on Venus, a planet with no arcs or ridges. Recent models suggest the Abitibi Greenstone Belt formed as an oceanic tract behind a detached ribbon continent during partial breakup of the Southern Superior craton; and represents a possible sample of Archaean oceanic lithosphere. The Abitibi has ∼50 km of apparent stratigraphy composed of 2-10 My mafic-felsic bimodal volcanic cycles that follow assimilation-fractionation trends indicating contamination of mantle-derived basalts with TTG-like anatectites derived from older basalts. ASOL of this type would be difficult to subduct because of its weakness and buoyancy, but would be fertile and could generate large amounts of second-stage melts. There are no sheeted dykes, precluding a seafloor-spreading model, while the absence of basal cumulates or attached mantle means this type of ASOL should not be called an ophiolite. Archaean/Proterozoic unconformities are followed by deposition of Fe-formations, clastic and volcanic rocks that are only rarely affected by sagduction. The increase in siliciclastic input and decreasing sagduction reflect near-global late Archean emergence from the water of stiffening granitic continents due to secular cooling and intra-continental differentiation. Albeit associated with continent-derived siliciclastic debris, many Paleo-Proterozoic volcanic (and plutonic) rocks resemble Archaean ones geochemically. The similarity of magmatic rocks and hot orogenic styles in the Archaean and Paleo-Proterozoic co
{"title":"A gradual Proterozoic transition from an Unstable Stagnant Lid to the modern Plate Tectonic system","authors":"Jean H. Bédard","doi":"10.1144/jgs2024-023","DOIUrl":"https://doi.org/10.1144/jgs2024-023","url":null,"abstract":"\u0000 When did Plate Tectonics begin on Earth, and what preceded it? Published thermo-mechanical mantle evolution models imply that the early history of planets with a composition and size similar to Earth and Venus should be characterized by periodic mantle overturns of 30-100 million years duration, separated by stable lid phases of 100-300 My. I argue this is best described as an Unstable Stagnant Lid, because this term captures the Jekyll-and-Hyde duality of such worlds, which alternate between a Stagnant Lid\u0000 ss\u0000 phase between mantle overturns, and a Mobile Lid phase during overturns. Mantle overturn upwelling zones would rework and resurface large tracts of pre-existing Hadean crust and basalt-dominated Archean-Style Oceanic Lithosphere (ASOL). Basal anatexis of ASOL ∼40-50 km thick (or melting within down-drips) could generate tonalite-trondhjemite melts (TTGs) and create proto-continental nuclei, while garnet pyroxenite restites delaminate into the mantle. With further reworking, low-K tonalitic rocks would remelt to produce granodiorite and granite, completing the transfer of radioactive elements out of the lower crust. Mantle overturns would generate large-scale lateral currents in the upper mantle that would push against Archaean-aged sub-continental lithospheric mantle keels, causing continental drift and orogenesis despite the absence of plate-boundary forces like slab pull. The validity of this is corroborated by the observed displacement of Lakshmi Planum (>1000 Km) on Venus, a planet with no arcs or ridges. Recent models suggest the Abitibi Greenstone Belt formed as an oceanic tract behind a detached ribbon continent during partial breakup of the Southern Superior craton; and represents a possible sample of Archaean oceanic lithosphere. The Abitibi has ∼50 km of apparent stratigraphy composed of 2-10 My mafic-felsic bimodal volcanic cycles that follow assimilation-fractionation trends indicating contamination of mantle-derived basalts with TTG-like anatectites derived from older basalts. ASOL of this type would be difficult to subduct because of its weakness and buoyancy, but would be fertile and could generate large amounts of second-stage melts. There are no sheeted dykes, precluding a seafloor-spreading model, while the absence of basal cumulates or attached mantle means this type of ASOL should not be called an ophiolite. Archaean/Proterozoic unconformities are followed by deposition of Fe-formations, clastic and volcanic rocks that are only rarely affected by sagduction. The increase in siliciclastic input and decreasing sagduction reflect near-global late Archean emergence from the water of stiffening granitic continents due to secular cooling and intra-continental differentiation. Albeit associated with continent-derived siliciclastic debris, many Paleo-Proterozoic volcanic (and plutonic) rocks resemble Archaean ones geochemically. The similarity of magmatic rocks and hot orogenic styles in the Archaean and Paleo-Proterozoic co","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-05-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141107718","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Giant sand injection complexes and localized swarms of sandstone intrusions are common in Upper Cretaceous to Miocene sedimentary successions of the Central and Northern California within a distance of less than 100 km from the Pacific margin of the North America Plate. One of the best preserved and extensively exposed injection complexes is the late Eocene Tumey Giant Injection Complex. The emplacement of sand injectites was driven by overpressure generated by thermal diagenesis of biosiliceous and smectite-rich mudstone host-rocks. The orientation and size distribution of sandstone intrusions was controlled by stress in which σ 1 and σ 3 were horizontal and, respectively, parallel and perpendicular to the present trace of the San Andreas Fault, and σ 2 was vertical. A strike-slip tectonic regime is inferred. Our analysis documents margin orthogonal extension and draws support for a late Eocene phase of increase of strain, and possibly active slip, along a syn-subduction strike-slip fault zone. Comparison with other injection complexes in the region indicates that the near-field maximum principal stress rotated through time, from normal to parallel with respect to the plate margin, probably in relation to variations of the relative motion vector of the converging plates.
{"title":"Controls on the Genesis of a Giant Sand Injection Complex. Insights on the Palaeogene Evolution of the Stress of Northern and Central California","authors":"M. Vigorito, A. Grippa, R. H. T. Callow","doi":"10.1144/jgs2024-001","DOIUrl":"https://doi.org/10.1144/jgs2024-001","url":null,"abstract":"\u0000 Giant sand injection complexes and localized swarms of sandstone intrusions are common in Upper Cretaceous to Miocene sedimentary successions of the Central and Northern California within a distance of less than 100 km from the Pacific margin of the North America Plate. One of the best preserved and extensively exposed injection complexes is the late Eocene Tumey Giant Injection Complex. The emplacement of sand injectites was driven by overpressure generated by thermal diagenesis of biosiliceous and smectite-rich mudstone host-rocks. The orientation and size distribution of sandstone intrusions was controlled by stress in which\u0000 \u0000 \u0000 σ\u0000 \u0000 \u0000 1\u0000 and\u0000 \u0000 \u0000 σ\u0000 \u0000 \u0000 3\u0000 were horizontal and, respectively, parallel and perpendicular to the present trace of the San Andreas Fault, and\u0000 \u0000 \u0000 σ\u0000 \u0000 \u0000 2\u0000 was vertical. A strike-slip tectonic regime is inferred. Our analysis documents margin orthogonal extension and draws support for a late Eocene phase of increase of strain, and possibly active slip, along a syn-subduction strike-slip fault zone. Comparison with other injection complexes in the region indicates that the near-field maximum principal stress rotated through time, from normal to parallel with respect to the plate margin, probably in relation to variations of the relative motion vector of the converging plates.\u0000","PeriodicalId":17320,"journal":{"name":"Journal of the Geological Society","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140968892","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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