A tectonic mélange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen, Australia. The clastics are quartz-poor and are dominated by mafic volcanic and fragmented plagioclase clasts; felsic volcanic clasts are less common. They exhibit erosional bases, graded bedding, load structures, and lithologies ranging from laminated mudstones to pebbly sandstones. Based on these features, they are interpreted as turbidites. During subduction, these turbidites were deformed and metamorphosed under prehnite–pumpellyite and lower greenschist facies conditions. Geochemically, they have a calc-alkaline, intra-oceanic arc signature; show no recycling; and have been derived from a provenance dominated by mafic volcanic rocks of basaltic-andesite composition. Further, chemical index of alteration (44–69) and Index of Compositional Variability (0.8–2.4) data reveal they show little weathering and are immature. The lack of weathering of rocks in a location where tropical climatic conditions existed is attributed to extreme erosion associated with a dynamic setting resulting in rapid transportation of the sediments to the fore arc basin and subsequently to the trench. As a consequence, little time was available for weathering to take place. The detritus in the turbidites is thought to have been derived from Late Ordovician volcanics in the Macquarie Arc and fore arc basin sequences of the Murrawong Formation. The cherts with which they are associated record both a continental and oceanic arc geochemical signature.
{"title":"Source of Detritus in Subducted Turbidites, Tectonic Mélange, Port Macquarie Block, Southern New England Orogen, Australia—A Geochemical Perspective","authors":"R. Offler, R. Boyd","doi":"10.1086/713684","DOIUrl":"https://doi.org/10.1086/713684","url":null,"abstract":"A tectonic mélange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen, Australia. The clastics are quartz-poor and are dominated by mafic volcanic and fragmented plagioclase clasts; felsic volcanic clasts are less common. They exhibit erosional bases, graded bedding, load structures, and lithologies ranging from laminated mudstones to pebbly sandstones. Based on these features, they are interpreted as turbidites. During subduction, these turbidites were deformed and metamorphosed under prehnite–pumpellyite and lower greenschist facies conditions. Geochemically, they have a calc-alkaline, intra-oceanic arc signature; show no recycling; and have been derived from a provenance dominated by mafic volcanic rocks of basaltic-andesite composition. Further, chemical index of alteration (44–69) and Index of Compositional Variability (0.8–2.4) data reveal they show little weathering and are immature. The lack of weathering of rocks in a location where tropical climatic conditions existed is attributed to extreme erosion associated with a dynamic setting resulting in rapid transportation of the sediments to the fore arc basin and subsequently to the trench. As a consequence, little time was available for weathering to take place. The detritus in the turbidites is thought to have been derived from Late Ordovician volcanics in the Macquarie Arc and fore arc basin sequences of the Murrawong Formation. The cherts with which they are associated record both a continental and oceanic arc geochemical signature.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"49 - 62"},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713684","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47398629","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}
Up to 2 km of siliciclastic and chemical sedimentary rocks, metamorphosed to greenschist facies, are preserved in the Baraboo Hills of southern Wisconsin. The strata compose two sedimentary successions separated by an angular unconformity. The lower succession includes the Baraboo quartzite, Seeley slate, and Freedom Formation, and the upper includes the Dake quartzite and Rowley Creek slate. Despite being studied for more than 100 y, the exposed section is only recently subdivided into informal members based on identification of sedimentary lithofacies and geologic mapping. This study integrates detrital zircon and paleocurrent analyses with sedimentologic and stratigraphic observations derived from detailed field mapping of the Baraboo and Dake quartzites. Our purpose is twofold: (1) characterize lithofacies of the Baraboo quartzite to interpret the environments of deposition for this unit and (2) evaluate sediment provenance and constrain depositional age. Lithofacies, paleocurrent, and detrital zircon U-Pb analyses of the Baraboo quartzite record fluvial braid plain, eolian, and fluvial-, tide- and wave-influenced deltaic depositional environments with sediments sourced from the north and deposited after 1714 ± 17 Ma. Lithofacies compose two backstepping alluvial to marine successions separated by a thick, laterally continuous coastal dune deposit. North-directed paleocurrents and detrital zircon ages confirm the Dake quartzite is a distinct unit with maximum depositional age of less than 1630.1 ± 8.6 Ma. Deposition of the Dake quartzite reflects basement uplift that postdates deposition of the Baraboo quartzite. The hiatus represented by the unconformity beneath the Dake quartzite is unconstrained and may represent more than 100 My.
{"title":"Revised Provenance, Depositional Environment, and Maximum Depositional Age for the Baraboo (","authors":"E. Stewart, L. Brengman, E. Stewart","doi":"10.1086/713687","DOIUrl":"https://doi.org/10.1086/713687","url":null,"abstract":"Up to 2 km of siliciclastic and chemical sedimentary rocks, metamorphosed to greenschist facies, are preserved in the Baraboo Hills of southern Wisconsin. The strata compose two sedimentary successions separated by an angular unconformity. The lower succession includes the Baraboo quartzite, Seeley slate, and Freedom Formation, and the upper includes the Dake quartzite and Rowley Creek slate. Despite being studied for more than 100 y, the exposed section is only recently subdivided into informal members based on identification of sedimentary lithofacies and geologic mapping. This study integrates detrital zircon and paleocurrent analyses with sedimentologic and stratigraphic observations derived from detailed field mapping of the Baraboo and Dake quartzites. Our purpose is twofold: (1) characterize lithofacies of the Baraboo quartzite to interpret the environments of deposition for this unit and (2) evaluate sediment provenance and constrain depositional age. Lithofacies, paleocurrent, and detrital zircon U-Pb analyses of the Baraboo quartzite record fluvial braid plain, eolian, and fluvial-, tide- and wave-influenced deltaic depositional environments with sediments sourced from the north and deposited after 1714 ± 17 Ma. Lithofacies compose two backstepping alluvial to marine successions separated by a thick, laterally continuous coastal dune deposit. North-directed paleocurrents and detrital zircon ages confirm the Dake quartzite is a distinct unit with maximum depositional age of less than 1630.1 ± 8.6 Ma. Deposition of the Dake quartzite reflects basement uplift that postdates deposition of the Baraboo quartzite. The hiatus represented by the unconformity beneath the Dake quartzite is unconstrained and may represent more than 100 My.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"1 - 31"},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713687","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44373895","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}
Numerous late Mesozoic (Late Jurassic to Early Cretaceous) granitic plutons are present along the southern margin of the North China Craton (s-NCC), which belongs to the northernmost tectonic unit of the eastern Qinling orogenic belt. This tectonic unit records multiple tectonothermal and metallogenic events that occurred after continental collision of the North and South China Cratons in the late Early to Middle Triassic. This study reports the zircon U-Pb ages and the whole-rock and apatite geochemical compositions of the Taishanmiao granite, one of the largest late Mesozoic plutons in eastern Qinling. Three phases of this granite, emplaced between ∼125 and ∼113 Ma, are characterized by uniformly high SiO2 and K2O but low CaO, TFe2O3, and MgO contents and enriched Nd isotopic composition with initial εNd values of −13.5 to −12.7. Geochemical features reveal that these are A-type granites derived from partial melting mainly of ancient crust. Temperatures obtained from Ti-in-zircon thermometry and oxygen fugacity (fO2) data indicate that the magma(s) formed at high crystallization temperatures (∼800°–840°C) and fO2 levels (fayalite–magnetite–quartz [FMQ] buffer −1.5 to +1.5). Comparison with neighboring granites, such as the ∼150–130 Ma adakitic Heyu granite and the ∼115 Ma Donggou ore-bearing granite porphyry, implies that the Heyu granite formed from thickened crust and that the Donggou granite porphyry and Taishanmiao granite likely originated from a consistent magma system during the crustal thinning that occurred ∼125–113 Ma, except the porphyry rock was more evolved. This finding implies that their uniform magma chamber could provide sufficient metals and associated materials and even energy for the Donggou Mo deposit.
{"title":"Petrogenesis of the Taishanmiao A-type Granite in the Eastern Qinling Orogenic Belt: Implications for Late Cretaceous Tectonic Transition and Mineralization","authors":"Jun He, Yue Qi, Xinjie Fan, Fukun Chen","doi":"10.1086/713726","DOIUrl":"https://doi.org/10.1086/713726","url":null,"abstract":"Numerous late Mesozoic (Late Jurassic to Early Cretaceous) granitic plutons are present along the southern margin of the North China Craton (s-NCC), which belongs to the northernmost tectonic unit of the eastern Qinling orogenic belt. This tectonic unit records multiple tectonothermal and metallogenic events that occurred after continental collision of the North and South China Cratons in the late Early to Middle Triassic. This study reports the zircon U-Pb ages and the whole-rock and apatite geochemical compositions of the Taishanmiao granite, one of the largest late Mesozoic plutons in eastern Qinling. Three phases of this granite, emplaced between ∼125 and ∼113 Ma, are characterized by uniformly high SiO2 and K2O but low CaO, TFe2O3, and MgO contents and enriched Nd isotopic composition with initial εNd values of −13.5 to −12.7. Geochemical features reveal that these are A-type granites derived from partial melting mainly of ancient crust. Temperatures obtained from Ti-in-zircon thermometry and oxygen fugacity (fO2) data indicate that the magma(s) formed at high crystallization temperatures (∼800°–840°C) and fO2 levels (fayalite–magnetite–quartz [FMQ] buffer −1.5 to +1.5). Comparison with neighboring granites, such as the ∼150–130 Ma adakitic Heyu granite and the ∼115 Ma Donggou ore-bearing granite porphyry, implies that the Heyu granite formed from thickened crust and that the Donggou granite porphyry and Taishanmiao granite likely originated from a consistent magma system during the crustal thinning that occurred ∼125–113 Ma, except the porphyry rock was more evolved. This finding implies that their uniform magma chamber could provide sufficient metals and associated materials and even energy for the Donggou Mo deposit.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"97 - 114"},"PeriodicalIF":1.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/713726","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49377974","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}
New investigations of the Morrison Formation in the western Oklahoma panhandle reveal that the formation is approximately 60 m thick near Kenton, considerably less than historical measurements. We provide a new isopach map of the Morrison Formation at the basin margin and divide the formation into three new members based on geological, geochemical, petrographic, and paleontological characteristics. Each member is defined by unique depositional facies recording a variable climatic signal and the eventual progradation of the distributive fluvial system to the basin margin. Analogous depositional facies and lithologies of these units suggest they are lateral facies successions of the well-established members of the Colorado Plateau. The Cimarron Member is composed of numerous small, ephemeral, clastic lakes at the distal margin of the alluvial braid plain of the Tidwell Member. The Boise Member consists of numerous perennial limestone lakes at the distal edge of the Salt Wash Member distributive fluvial system. At the top of the member, the lake coalesced into a large lake, herein termed Lake Stovall. The Kenton Member records the progradation of the Brushy Basin Member distributive fluvial system to the basin margin in Oklahoma. Kenton Member fluvial deposition was by small, isolated, anastomosing fluvial channels and their associated splays. The gradual progradation of facies to the basin margin signifies that deposition in the foreland basin was geographically extensive and protracted. Dinosaur fossils excavated during the 1930s are placed into a stratigraphic framework. All quarries lie in the basal 6 m of the Kenton Member. Depositional facies and taphonomic data imply the dinosaurs died during a series of severe droughts. Bone dispersal and burial resulted from ephemeral flood splay events.
{"title":"Stratigraphy and Sedimentology of the Morrison Formation in the Western Panhandle of Oklahoma with Reference to the Historical Stovall Dinosaur Quarries","authors":"D. Richmond, Tyler C. Hunt, R. Cifelli","doi":"10.1086/712368","DOIUrl":"https://doi.org/10.1086/712368","url":null,"abstract":"New investigations of the Morrison Formation in the western Oklahoma panhandle reveal that the formation is approximately 60 m thick near Kenton, considerably less than historical measurements. We provide a new isopach map of the Morrison Formation at the basin margin and divide the formation into three new members based on geological, geochemical, petrographic, and paleontological characteristics. Each member is defined by unique depositional facies recording a variable climatic signal and the eventual progradation of the distributive fluvial system to the basin margin. Analogous depositional facies and lithologies of these units suggest they are lateral facies successions of the well-established members of the Colorado Plateau. The Cimarron Member is composed of numerous small, ephemeral, clastic lakes at the distal margin of the alluvial braid plain of the Tidwell Member. The Boise Member consists of numerous perennial limestone lakes at the distal edge of the Salt Wash Member distributive fluvial system. At the top of the member, the lake coalesced into a large lake, herein termed Lake Stovall. The Kenton Member records the progradation of the Brushy Basin Member distributive fluvial system to the basin margin in Oklahoma. Kenton Member fluvial deposition was by small, isolated, anastomosing fluvial channels and their associated splays. The gradual progradation of facies to the basin margin signifies that deposition in the foreland basin was geographically extensive and protracted. Dinosaur fossils excavated during the 1930s are placed into a stratigraphic framework. All quarries lie in the basal 6 m of the Kenton Member. Depositional facies and taphonomic data imply the dinosaurs died during a series of severe droughts. Bone dispersal and burial resulted from ephemeral flood splay events.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"477 - 515"},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712368","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46992105","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}
Julien Michel, C. Lanteaume, Alexandre Lettéron, J. Kenter, M. Morsilli, J. Borgomano
The present study provides the baseline status of the spatial distribution of carbonate platforms for the Oligo-Miocene interval. The resulting global trend quantitatively shows the decreasing growth potential of shallow-marine carbonates toward higher paleolatitudes. Such a global trend provides a geological context and external constraints for local and regional interpretations of specific case studies. Furthermore, the direct relationship between carbonate accumulations and paleoclimatic regions shows that, using such a qualitative and quantitative data set for calibration, paleoceanographic models could be utilized for the prediction of the global distribution of carbonate stratigraphic architecture.
{"title":"Oligocene and Miocene Global Spatial Trends of Shallow-Marine Carbonate Architecture","authors":"Julien Michel, C. Lanteaume, Alexandre Lettéron, J. Kenter, M. Morsilli, J. Borgomano","doi":"10.1086/712186","DOIUrl":"https://doi.org/10.1086/712186","url":null,"abstract":"The present study provides the baseline status of the spatial distribution of carbonate platforms for the Oligo-Miocene interval. The resulting global trend quantitatively shows the decreasing growth potential of shallow-marine carbonates toward higher paleolatitudes. Such a global trend provides a geological context and external constraints for local and regional interpretations of specific case studies. Furthermore, the direct relationship between carbonate accumulations and paleoclimatic regions shows that, using such a qualitative and quantitative data set for calibration, paleoceanographic models could be utilized for the prediction of the global distribution of carbonate stratigraphic architecture.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"563 - 570"},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712186","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47083151","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}
Sayani Khan, T. Majumder, S. Patranabis‐Deb, D. Saha
Occurrence of slump folds and associated faults generated by soft-sediment deformation from the Paleoproterozoic Vempalle Formation, southwestern Cuddapah basin, India, is being reported here for the first time. The slump horizon is preserved within a more or less undeformed shallow to deep-water carbonate unit in the Cuddapah basin exposed near Parnapalle village, Andhra Pradesh, India. The stratigraphic framework includes the Gulcheru Quartzite, lowermost unit of the Cuddapah basin succession, deposited in an alluvial to shallow marine shelf environment, and the overlying Vempalle Formation, representing a ramp-type stromatolitic carbonate platform. The synsedimentary sliding along a steepened ramp is evidenced by northeast-verging kink-like folds with wavelength up to 400 m and an overprinting set of thrusts with ramp-flat geometry, fault-cored folds or small break-thrusts showing top-to-southwest displacement, and smaller congruent folds. From the isolated occurrence within a generally undeformed succession, association of structures, and the stratigraphic context, we suggest soft-sediment deformation at the toe of a large (kilometer-scale) slump, with the prevalent bedding-parallel anisotropy exploited for common flexural slip and ramp-flat geometry. In addition, the bedding-parallel slickensides in the Gulcheru Quartzite immediately below the Vempalle Formation indicate a top-to-east-northeast, normal sense of slip representing extensional slip at the slump head. Northeast-verging large folds in the Parnapalle slump horizon possibly represent structures formed during the translation phase of slumping. As the large northeast propagating slump was halted, the backthrust-like structures and associated folds developed at the slump toe.
{"title":"Deformation Structures in a Large Slump Horizon, Paleoproterozoic Vempalle Formation, Cuddapah Basin, Southern India","authors":"Sayani Khan, T. Majumder, S. Patranabis‐Deb, D. Saha","doi":"10.1086/712290","DOIUrl":"https://doi.org/10.1086/712290","url":null,"abstract":"Occurrence of slump folds and associated faults generated by soft-sediment deformation from the Paleoproterozoic Vempalle Formation, southwestern Cuddapah basin, India, is being reported here for the first time. The slump horizon is preserved within a more or less undeformed shallow to deep-water carbonate unit in the Cuddapah basin exposed near Parnapalle village, Andhra Pradesh, India. The stratigraphic framework includes the Gulcheru Quartzite, lowermost unit of the Cuddapah basin succession, deposited in an alluvial to shallow marine shelf environment, and the overlying Vempalle Formation, representing a ramp-type stromatolitic carbonate platform. The synsedimentary sliding along a steepened ramp is evidenced by northeast-verging kink-like folds with wavelength up to 400 m and an overprinting set of thrusts with ramp-flat geometry, fault-cored folds or small break-thrusts showing top-to-southwest displacement, and smaller congruent folds. From the isolated occurrence within a generally undeformed succession, association of structures, and the stratigraphic context, we suggest soft-sediment deformation at the toe of a large (kilometer-scale) slump, with the prevalent bedding-parallel anisotropy exploited for common flexural slip and ramp-flat geometry. In addition, the bedding-parallel slickensides in the Gulcheru Quartzite immediately below the Vempalle Formation indicate a top-to-east-northeast, normal sense of slip representing extensional slip at the slump head. Northeast-verging large folds in the Parnapalle slump horizon possibly represent structures formed during the translation phase of slumping. As the large northeast propagating slump was halted, the backthrust-like structures and associated folds developed at the slump toe.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"517 - 534"},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712290","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46174955","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}
Fei Liu, D. Lian, Guangying Feng, Zhao-li Li, Xiaolu Niu, Jingsui Yang
Ophiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the western part of Yarlung Zangbo suture zone (YZSZ) in Tibet, which in turn is tectonically thrust over a volcanic-sedimentary sequence. Geochemical data and radiolarian fauna of cherts and detrital zircon ages of litho-quartz sandstones in the sequence provide robust constraints to elucidate the stratigraphic and paleo-depositional environments in which these rocks formed. Eight cherts from Purang, Dongbo, Daba Qu, East Daba, and Labuzha massifs in the SB reveal Late Jurassic–Early Cretaceous radiolarians; they are coeval with minimum detrital zircon U-Pb ages of 132 and 149 Ma, respectively, from two litho-quartz sandstones in the northwestern part of Purang massif. Thirty chert samples from five massifs geochemically show that they have high SiO2 contents of 86.51–95.93 wt%, and high mean ratios of Al/(Al+Fe+Mn) ranging from 0.59 to 0.78, indicating a nonhydrothermal, biogenic, and terrigenous origin. Ce/Ce* ratios of cherts range from 0.93 to 1.52, combined with claystone interlayered with radiolarian chert sporadically overlying litho-quartz sandstone and quartzose sandstone, suggesting a continental slope setting. Given the structural and stratigraphic evidences of ophiolites associated with sedimentary strata and no arc-related magmatism in the SB, we propose that SB ophiolites and ophiolitic mélanges represent southward-thrust nappes from the NB.
{"title":"Radiolarian Biochronology, Detrital Zircon Geochronological and Geochemical Constraints on Provenance and Depositional Environment of Cherts in the Southern Belt of the Western Yarlung Zangbo Suture Zone, Tibet","authors":"Fei Liu, D. Lian, Guangying Feng, Zhao-li Li, Xiaolu Niu, Jingsui Yang","doi":"10.1086/712185","DOIUrl":"https://doi.org/10.1086/712185","url":null,"abstract":"Ophiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the western part of Yarlung Zangbo suture zone (YZSZ) in Tibet, which in turn is tectonically thrust over a volcanic-sedimentary sequence. Geochemical data and radiolarian fauna of cherts and detrital zircon ages of litho-quartz sandstones in the sequence provide robust constraints to elucidate the stratigraphic and paleo-depositional environments in which these rocks formed. Eight cherts from Purang, Dongbo, Daba Qu, East Daba, and Labuzha massifs in the SB reveal Late Jurassic–Early Cretaceous radiolarians; they are coeval with minimum detrital zircon U-Pb ages of 132 and 149 Ma, respectively, from two litho-quartz sandstones in the northwestern part of Purang massif. Thirty chert samples from five massifs geochemically show that they have high SiO2 contents of 86.51–95.93 wt%, and high mean ratios of Al/(Al+Fe+Mn) ranging from 0.59 to 0.78, indicating a nonhydrothermal, biogenic, and terrigenous origin. Ce/Ce* ratios of cherts range from 0.93 to 1.52, combined with claystone interlayered with radiolarian chert sporadically overlying litho-quartz sandstone and quartzose sandstone, suggesting a continental slope setting. Given the structural and stratigraphic evidences of ophiolites associated with sedimentary strata and no arc-related magmatism in the SB, we propose that SB ophiolites and ophiolitic mélanges represent southward-thrust nappes from the NB.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"535 - 562"},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712185","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46924763","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}
Granulite facies metamorphism of the lower crust has decreased in scale since the Late Archean, but many of its definitive features have persisted: (1) Punctuated, sometimes relatively short-lived, episodes of high-grade metamorphism. These are recorded, in favorably simple cases, by discrete growth rims on zircons. (2) A consistent age gap of a few to several tens of millions of years between juvenile magmatism (crustal accretion) and high-temperature metamorphism. The secondary thermal pulse is an event distinct from primary crustal accretion. (3) Involvement of mineralizing pore fluids of lowered H2O activity, that is, with high CO2 and saline concentrations. Very high oxidation states of some granulites implicate sulfur as an important fluid component. (4) Transcurrent faulting as a conspicuous feature of synmetamorphic deformation. This gives rise to characteristic transposed foliation and lineation. (5) Emplacement of coeval postorogenic K-rich granites at midcrust levels. These features can be rationalized by concepts of modern plate tectonics. High-angle plate collision is succeeded by orogen-parallel transport. This change of plate motion necessarily detaches the underthrust portion of the lithosphere, liberating asthenospheric melts and/or fluids in a postorogenic resurgence. A generation of volatile-rich mafic magmas invades the continental margin; high CO2 and halogen contents cause outgassing and freezing of the magmas at depth. Liberated volatiles effect granulite facies metamorphism by leaching H2O and lithophile elements, importantly K, and transporting these components and heat upward. Extensive melting of the lower crust is inhibited by the low H2O activity of saline-carbonic pore fluids at high pressure. Melting of orthogneiss and supracrustal rocks occurs at midcrust levels by increase of H2O activity as pressure on alkali chloride solutions falls below 0.6–0.5 GPa. The foregoing hypothesis is an alternative to the classical view that granite results from fluid-absent partial melting of, and extraction from, the lower crust, leaving granulites.
{"title":"Young and Old Granulites: A Volatile Connection","authors":"R. C. Newton","doi":"10.1086/711026","DOIUrl":"https://doi.org/10.1086/711026","url":null,"abstract":"Granulite facies metamorphism of the lower crust has decreased in scale since the Late Archean, but many of its definitive features have persisted: (1) Punctuated, sometimes relatively short-lived, episodes of high-grade metamorphism. These are recorded, in favorably simple cases, by discrete growth rims on zircons. (2) A consistent age gap of a few to several tens of millions of years between juvenile magmatism (crustal accretion) and high-temperature metamorphism. The secondary thermal pulse is an event distinct from primary crustal accretion. (3) Involvement of mineralizing pore fluids of lowered H2O activity, that is, with high CO2 and saline concentrations. Very high oxidation states of some granulites implicate sulfur as an important fluid component. (4) Transcurrent faulting as a conspicuous feature of synmetamorphic deformation. This gives rise to characteristic transposed foliation and lineation. (5) Emplacement of coeval postorogenic K-rich granites at midcrust levels. These features can be rationalized by concepts of modern plate tectonics. High-angle plate collision is succeeded by orogen-parallel transport. This change of plate motion necessarily detaches the underthrust portion of the lithosphere, liberating asthenospheric melts and/or fluids in a postorogenic resurgence. A generation of volatile-rich mafic magmas invades the continental margin; high CO2 and halogen contents cause outgassing and freezing of the magmas at depth. Liberated volatiles effect granulite facies metamorphism by leaching H2O and lithophile elements, importantly K, and transporting these components and heat upward. Extensive melting of the lower crust is inhibited by the low H2O activity of saline-carbonic pore fluids at high pressure. Melting of orthogneiss and supracrustal rocks occurs at midcrust levels by increase of H2O activity as pressure on alkali chloride solutions falls below 0.6–0.5 GPa. The foregoing hypothesis is an alternative to the classical view that granite results from fluid-absent partial melting of, and extraction from, the lower crust, leaving granulites.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"395 - 413"},"PeriodicalIF":1.8,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/711026","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45773286","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}
Geochronologic data combined with a geographic information system-based spatial analysis of plutons in the Bohemian Massif, Variscan belt, allow us to describe the apparent magma fluxes and episodic temporal pattern of plutonism, different from the cyclic pattern of continental margin arcs. The former is interpreted as reflecting a secular thermotectonic evolution of the orogen and magma sources changing from mantle-dominated to intracrustal, paralleled by increasing significance of crustal recycling at the expense of crustal growth. The analysis also suggests that crustal thickening, commonly regarded as the main cause of plutonism in collisional orogens, may be of relatively lower significance than late-stage mantle delamination.
{"title":"Magmatic Tempos in Large Hot Orogens in Comparison with Continental Margin Arcs","authors":"J. Trubač, J. Žák, L. Kondrová","doi":"10.1086/711346","DOIUrl":"https://doi.org/10.1086/711346","url":null,"abstract":"Geochronologic data combined with a geographic information system-based spatial analysis of plutons in the Bohemian Massif, Variscan belt, allow us to describe the apparent magma fluxes and episodic temporal pattern of plutonism, different from the cyclic pattern of continental margin arcs. The former is interpreted as reflecting a secular thermotectonic evolution of the orogen and magma sources changing from mantle-dominated to intracrustal, paralleled by increasing significance of crustal recycling at the expense of crustal growth. The analysis also suggests that crustal thickening, commonly regarded as the main cause of plutonism in collisional orogens, may be of relatively lower significance than late-stage mantle delamination.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"465 - 475"},"PeriodicalIF":1.8,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/711346","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41956915","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}
J. V. L. Filho, R. Fuck, A. Ruiz, E. Dantas, J. Rodrigues, J. Scandolara
The Rio Apa Block, a crustal fragment of 20,000 km2 in the southern portion of the Amazonian Craton, is a Paleoproterozoic terrain, making up the basement of the Neoproterozoic Paraguay Belt. The block is not often considered in the evolution models of the Columbia supercontinent. Different hypotheses of the evolution of the Rio Apa Block consider this segment as part of the Amazonian Craton, and its correlation with other Paleoproterozoic accretionary systems remains controversial. We present new U-Pb and Sm-Nd data that suggest that the Rio Apa Block constitutes a continental arc system built around 1.8–1.7 Ga that can be a correlation element in supercontinent models. Recorded orogenic accretionary events and continental crust reworking divide the block into three main sectors. The western and central sectors comprise parts of the 1.88–1.71 Ga Amoguijá arc. The eastern sector consists of Orosirian high-K calc-alkaline and postcollisional A-type granites. The western segment comprises Orosirian banded orthogneisses of the Porto Murtinho Complex, intruded by initial phase granitoids of the Amoguijá arc. The central sector is characterized by weakly to moderately deformed granites of the Amoguijá Suite, Serra da Bocaina Formation metavolcanic rocks, Serra da Alegria gabbro-anorthosite suite, and Morro do Triunfo gabbro, overlain by the Amolar Group sedimentary rocks. The eastern segment comprises the backarc basin generated Alto Tererê Group, intruded by late to postorogenic granites of the Rio Apa Complex, including A-type granites. Mafic dikes and sill swarms related to a late extensional event intrude the preceding rocks. Two important metamorphic-deformational events are recorded in the opening of the Rb-Sr system in biotite around 1670 Ma, followed by thrust deformation and metamorphism probably related to the ca. 1300 Ma Rondonian-San Ignácio event, as indicated by Ar-Ar muscovite and U-Pb monazite ages.
里约热内卢阿帕地块是位于亚马逊克拉通南部的一块面积为20,000 km2的地壳碎片，是一个古元古代的地形，构成了新元古代巴拉圭带的基底。该块体在哥伦比亚超大陆的演化模式中不常被考虑。关于里约热内卢Apa地块演化的不同假说认为该板块是亚马逊河克拉通的一部分，其与其他古元古代增生系统的对比仍然存在争议。我们提出了新的U-Pb和Sm-Nd数据，表明里约热内卢Apa地块构成了一个建立在1.8-1.7 Ga的大陆弧系统，可以作为超大陆模式的一个相关元素。有记录的造山增生事件和大陆地壳改造将地块划分为三个主要板块。西部和中部地区包括1.88-1.71 Ga amoguij弧的一部分。东段由奥陶系高钾钙碱性花岗岩和后碰撞a型花岗岩组成。西段为Murtinho港杂岩的Orosirian带状正长岩，被amoguij弧的初始期花岗岩侵入。中部地区发育Amolar群沉积岩覆盖的amoguij套、Serra da Bocaina组变质火山岩、Serra da Alegria辉长岩-斜长岩套、Morro do Triunfo辉长岩等弱-中度变形花岗岩。东段为弧后盆地生成的Alto Tererê群，被里约热内卢Apa杂岩的晚-后造山花岗岩侵入，包括a型花岗岩。与晚期伸展事件有关的基性岩脉和岩床群侵入了前面的岩石。1670 Ma前后，黑云母Rb-Sr体系的开启记录了两次重要的变质-变形事件，随后的逆冲变形和变质作用可能与约1300 Ma Rondonian-San Ignácio事件有关，由Ar-Ar白云母和U-Pb独辉石年龄表明。
{"title":"Rio Apa Block: A Juvenile Crustal Fragment in the Southwest Amazonian Craton and Its Implications for Columbia Supercontinent Reconstitution","authors":"J. V. L. Filho, R. Fuck, A. Ruiz, E. Dantas, J. Rodrigues, J. Scandolara","doi":"10.1086/710999","DOIUrl":"https://doi.org/10.1086/710999","url":null,"abstract":"The Rio Apa Block, a crustal fragment of 20,000 km2 in the southern portion of the Amazonian Craton, is a Paleoproterozoic terrain, making up the basement of the Neoproterozoic Paraguay Belt. The block is not often considered in the evolution models of the Columbia supercontinent. Different hypotheses of the evolution of the Rio Apa Block consider this segment as part of the Amazonian Craton, and its correlation with other Paleoproterozoic accretionary systems remains controversial. We present new U-Pb and Sm-Nd data that suggest that the Rio Apa Block constitutes a continental arc system built around 1.8–1.7 Ga that can be a correlation element in supercontinent models. Recorded orogenic accretionary events and continental crust reworking divide the block into three main sectors. The western and central sectors comprise parts of the 1.88–1.71 Ga Amoguijá arc. The eastern sector consists of Orosirian high-K calc-alkaline and postcollisional A-type granites. The western segment comprises Orosirian banded orthogneisses of the Porto Murtinho Complex, intruded by initial phase granitoids of the Amoguijá arc. The central sector is characterized by weakly to moderately deformed granites of the Amoguijá Suite, Serra da Bocaina Formation metavolcanic rocks, Serra da Alegria gabbro-anorthosite suite, and Morro do Triunfo gabbro, overlain by the Amolar Group sedimentary rocks. The eastern segment comprises the backarc basin generated Alto Tererê Group, intruded by late to postorogenic granites of the Rio Apa Complex, including A-type granites. Mafic dikes and sill swarms related to a late extensional event intrude the preceding rocks. Two important metamorphic-deformational events are recorded in the opening of the Rb-Sr system in biotite around 1670 Ma, followed by thrust deformation and metamorphism probably related to the ca. 1300 Ma Rondonian-San Ignácio event, as indicated by Ar-Ar muscovite and U-Pb monazite ages.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"128 1","pages":"415 - 444"},"PeriodicalIF":1.8,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/710999","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45399636","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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