A. Festa, F. Meneghini, G. Balestro, L. Pandolfi, P. Tartarotti, Y. Dilek, M. Marroni
The Jurassic ophiolites in the Northern Apennines and the Western Alps represent fossil mid-ocean ridge (MOR) oceanic lithosphere that formed in the Mesozoic Ligurian-Piedmont Ocean Basin (LPOB). Their sedimentary covers include chaotic rock units containing ophiolite-derived material. The processes of formation and the lithostratigraphic position of these chaotic units in the Western Alps remain a matter of debate, unlike their counterparts in the Northern Apennines. This is because of pervasive tectonic deformation and high-pressure metamorphism that affected their internal structure during collisional tectonics. A comparative analysis of these chaotic units in both mountain belts reveals the nature of processes involved in their formation. Chaotic deposits of gravitational origin occur both below and above the extrusive sequences in the ophiolites. They represent synextensional, hyperconcentrated deposits associated with the seafloor-spreading evolution of the LPOB lithosphere during Middle and Late Jurassic times. Mass transport deposits (MTDs) occur as intercalations within turbiditic sequences above the ophiolites. They represent syncontractional submarine slides that occurred on frontal accretionary prism slopes during the Late Cretaceous–Paleocene closure of the LPOB. The results of our comparative analysis imply that (1) the structure-stratigraphy of the chaotic deposits and MTDs of the Northern Apennines can be used as a proxy to better identify their metamorphosed and highly deformed counterparts in the Western Alps, (2) sedimentological processes associated with slow-spreading MOR tectonics and accretionary prism development in convergent-margin tectonics contributed to the sediment budgets of the cover sequences, and (3) magmatic, tectonic, and sedimentological processes that occurred during the formation of the Jurassic oceanic lithosphere and its sedimentary cover in the LPOB were remarkably uniform and synchronous.
{"title":"Comparative Analysis of the Sedimentary Cover Units of the Jurassic Western Tethys Ophiolites in the Northern Apennines and Western Alps (Italy): Processes of the Formation of Mass-Transport and Chaotic Deposits during Seafloor Spreading and Subduction Zone Tectonics","authors":"A. Festa, F. Meneghini, G. Balestro, L. Pandolfi, P. Tartarotti, Y. Dilek, M. Marroni","doi":"10.1086/716498","DOIUrl":"https://doi.org/10.1086/716498","url":null,"abstract":"The Jurassic ophiolites in the Northern Apennines and the Western Alps represent fossil mid-ocean ridge (MOR) oceanic lithosphere that formed in the Mesozoic Ligurian-Piedmont Ocean Basin (LPOB). Their sedimentary covers include chaotic rock units containing ophiolite-derived material. The processes of formation and the lithostratigraphic position of these chaotic units in the Western Alps remain a matter of debate, unlike their counterparts in the Northern Apennines. This is because of pervasive tectonic deformation and high-pressure metamorphism that affected their internal structure during collisional tectonics. A comparative analysis of these chaotic units in both mountain belts reveals the nature of processes involved in their formation. Chaotic deposits of gravitational origin occur both below and above the extrusive sequences in the ophiolites. They represent synextensional, hyperconcentrated deposits associated with the seafloor-spreading evolution of the LPOB lithosphere during Middle and Late Jurassic times. Mass transport deposits (MTDs) occur as intercalations within turbiditic sequences above the ophiolites. They represent syncontractional submarine slides that occurred on frontal accretionary prism slopes during the Late Cretaceous–Paleocene closure of the LPOB. The results of our comparative analysis imply that (1) the structure-stratigraphy of the chaotic deposits and MTDs of the Northern Apennines can be used as a proxy to better identify their metamorphosed and highly deformed counterparts in the Western Alps, (2) sedimentological processes associated with slow-spreading MOR tectonics and accretionary prism development in convergent-margin tectonics contributed to the sediment budgets of the cover sequences, and (3) magmatic, tectonic, and sedimentological processes that occurred during the formation of the Jurassic oceanic lithosphere and its sedimentary cover in the LPOB were remarkably uniform and synchronous.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"533 - 561"},"PeriodicalIF":1.8,"publicationDate":"2021-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/716498","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45378735","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
F. Haissen, M. Zaghloul, Y. Dilek, Oriol Gimeno‐Vives, G. Mohn, A. Cambeses, D. F. Lamotte, V. Bosse
We present new field evidence, geochemical and isotopic data, and age constraints on Lower Jurassic mafic rock suites within a >200-km-long curvilinear belt in the Rif orogenic belt in northern Morocco and show that these rock assemblages formed as part of the Central Atlantic Magmatic Province (CAMP). The CAMP represents a large igneous province that straddles the edges of the modern peri-Atlantic continents. It developed ~200 Ma, following the initiation of the breakup of Pangea. Main magmatic rocks in the Rif External Zone include basaltic lavas, massive dolerite, and isotropic and cumulate gabbros, all intruded by dolerite and trondhjemite dikes and sills. Available U-Pb zircon ages from dolerite, gabbro, and trondhjemite dike rocks are 200±4, 196±4, and 192±Ma, respectively. Based on their geochemical affinities and isotopic compositions, the analyzed rocks define basalt-dolerite and gabbro-cumulate gabbro-trondhjemite groups. The basalt-dolerite group samples are subalkaline in nature and have low TiO2 contents, whereas the gabbro-cumulate gabbro-trondhjemite group samples are alkaline and display high TiO2 values. Most samples are tholeiitic in character and show large-ion lithophile and light rare earth element enrichment and high field strength element depletion compared with normal mid-ocean ridge basalt (MORB). Samples of both groups display low 143Nd/144Nd201 Ma (0.51182–0.51262) and high 87Sr/86Sri ratios with ɛNd values ranging from −1.51 to +4.85. The basalt-dolerite group rocks have enriched MORB compositions, compatible with the low-Ti CAMP suites, whereas the gabbro-cumulate gabbro-trondhjemite group rocks have oceanic island basalt compositions reminiscent of high-Ti CAMP suites in other continents. Magmas of the gabbro-cumulate gabbro-trondhjemite group underwent differentiation through tholeiitic fractionation. Magmas of the rocks of both groups included melt components, originated from partial melting of a previously subduction-modified subcontinental lithospheric mantle. Our results indicate that the Early Jurassic CAMP magmatism in northern Morocco marked a major episode of continental magmatism before the opening of the Maghrebian Tethys between Africa and Iberia in the latest Jurassic.
{"title":"Geochemistry and Petrogenesis of Lower Jurassic Mafic Rock Suites in the External Rif Belt, and Chemical Geodynamics of the Central Atlantic Magmatic Province (CAMP) in Northwest Morocco","authors":"F. Haissen, M. Zaghloul, Y. Dilek, Oriol Gimeno‐Vives, G. Mohn, A. Cambeses, D. F. Lamotte, V. Bosse","doi":"10.1086/716499","DOIUrl":"https://doi.org/10.1086/716499","url":null,"abstract":"We present new field evidence, geochemical and isotopic data, and age constraints on Lower Jurassic mafic rock suites within a >200-km-long curvilinear belt in the Rif orogenic belt in northern Morocco and show that these rock assemblages formed as part of the Central Atlantic Magmatic Province (CAMP). The CAMP represents a large igneous province that straddles the edges of the modern peri-Atlantic continents. It developed ~200 Ma, following the initiation of the breakup of Pangea. Main magmatic rocks in the Rif External Zone include basaltic lavas, massive dolerite, and isotropic and cumulate gabbros, all intruded by dolerite and trondhjemite dikes and sills. Available U-Pb zircon ages from dolerite, gabbro, and trondhjemite dike rocks are 200±4, 196±4, and 192±Ma, respectively. Based on their geochemical affinities and isotopic compositions, the analyzed rocks define basalt-dolerite and gabbro-cumulate gabbro-trondhjemite groups. The basalt-dolerite group samples are subalkaline in nature and have low TiO2 contents, whereas the gabbro-cumulate gabbro-trondhjemite group samples are alkaline and display high TiO2 values. Most samples are tholeiitic in character and show large-ion lithophile and light rare earth element enrichment and high field strength element depletion compared with normal mid-ocean ridge basalt (MORB). Samples of both groups display low 143Nd/144Nd201 Ma (0.51182–0.51262) and high 87Sr/86Sri ratios with ɛNd values ranging from −1.51 to +4.85. The basalt-dolerite group rocks have enriched MORB compositions, compatible with the low-Ti CAMP suites, whereas the gabbro-cumulate gabbro-trondhjemite group rocks have oceanic island basalt compositions reminiscent of high-Ti CAMP suites in other continents. Magmas of the gabbro-cumulate gabbro-trondhjemite group underwent differentiation through tholeiitic fractionation. Magmas of the rocks of both groups included melt components, originated from partial melting of a previously subduction-modified subcontinental lithospheric mantle. Our results indicate that the Early Jurassic CAMP magmatism in northern Morocco marked a major episode of continental magmatism before the opening of the Maghrebian Tethys between Africa and Iberia in the latest Jurassic.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"563 - 593"},"PeriodicalIF":1.8,"publicationDate":"2021-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/716499","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42212361","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
E. Garzanti, G. Pastore, A. Resentini, G. Vezzoli, P. Vermeesch, L. Ncube, H. V. Niekerk, G. Jouet, M. Dall’asta
The Zambezi River rises at the center of southern Africa, flows across the low-relief Kalahari Plateau, meets Karoo basalt, plunges into Victoria Falls, follows along Karoo rifts, and pierces through Precambrian basement to eventually deliver its load onto the Mozambican passive margin. Reflecting its polyphase evolution, the river is subdivided into segments with different geological and geomorphological character, a subdivision finally fixed by man’s construction of large reservoirs and faithfully testified by sharp changes in sediment composition. Pure quartzose sand recycled from Kalahari desert dunes in the uppermost tract is next progressively enriched in basaltic rock fragments and clinopyroxene. Sediment load is renewed first downstream of Lake Kariba and next downstream of Lake Cahora Bassa, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzo-feldspathic in the lower tract, where most sediment is supplied by high-grade basements rejuvenated by the southward propagation of the East African rift. Feldspar abundance in Lower Zambezi sand has no equivalent among big rivers on Earth and far exceeds that in sediments of the northern delta, shelf, and slope, revealing that provenance signals from the upper reaches have ceased to be transmitted across the routing system after closure of the big dams. This high-resolution petrologic study of Zambezi sand allows us to critically reconsider several dogmas, such as the supposed increase of mineralogical “maturity” during long-distance fluvial transport, and forges a key to unlock the rich information stored in sedimentary archives, with the ultimate goal to accurately reconstruct the evolution of this mighty river flowing across changing African landscapes since the late Mesozoic.
{"title":"The Segmented Zambezi Sedimentary System from Source to Sink: 1. Sand Petrology and Heavy Minerals","authors":"E. Garzanti, G. Pastore, A. Resentini, G. Vezzoli, P. Vermeesch, L. Ncube, H. V. Niekerk, G. Jouet, M. Dall’asta","doi":"10.1086/715792","DOIUrl":"https://doi.org/10.1086/715792","url":null,"abstract":"The Zambezi River rises at the center of southern Africa, flows across the low-relief Kalahari Plateau, meets Karoo basalt, plunges into Victoria Falls, follows along Karoo rifts, and pierces through Precambrian basement to eventually deliver its load onto the Mozambican passive margin. Reflecting its polyphase evolution, the river is subdivided into segments with different geological and geomorphological character, a subdivision finally fixed by man’s construction of large reservoirs and faithfully testified by sharp changes in sediment composition. Pure quartzose sand recycled from Kalahari desert dunes in the uppermost tract is next progressively enriched in basaltic rock fragments and clinopyroxene. Sediment load is renewed first downstream of Lake Kariba and next downstream of Lake Cahora Bassa, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzo-feldspathic in the lower tract, where most sediment is supplied by high-grade basements rejuvenated by the southward propagation of the East African rift. Feldspar abundance in Lower Zambezi sand has no equivalent among big rivers on Earth and far exceeds that in sediments of the northern delta, shelf, and slope, revealing that provenance signals from the upper reaches have ceased to be transmitted across the routing system after closure of the big dams. This high-resolution petrologic study of Zambezi sand allows us to critically reconsider several dogmas, such as the supposed increase of mineralogical “maturity” during long-distance fluvial transport, and forges a key to unlock the rich information stored in sedimentary archives, with the ultimate goal to accurately reconstruct the evolution of this mighty river flowing across changing African landscapes since the late Mesozoic.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"343 - 369"},"PeriodicalIF":1.8,"publicationDate":"2021-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715792","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46877097","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Rajat Mazumder, Farah Bt Mohd Anthony, Basil Shung Say Teo, Subhajit Roy, Amal Al Hajri, T. Ohta, S. De, O. Catuneanu
Sedimentary successions developed at the destructive plate margin settings are extremely important as they bear valuable record of contemporary basin tectonics and consequent sedimentation. Intense deformation and metamorphism often obliterate the primary sedimentary texture and structures of the sedimentary successions formed at destructive plate margins. However, sedimentological analysis of young unmetamorphosed forearc successions provides rare opportunity to infer the interplay between tectonics and sedimentation. Therefore, a process-based sedimentological facies analysis and provenance studies of the Cretaceous Pedawan Formation in the Kuching Zone, Borneo, have been undertaken for the first time. The sandstones are compositionally and texturally immature. They are normally graded with sole marks and have erosional lower contacts with mudstones and generally have gradational upper contacts. The massive sandstones, parallel-laminated sandstones, and the combination of rippled sandstones with thin mudstones are interpreted as turbidites. The lack of wave-generated structures, including hummocky cross stratification, indicates that deposition took place below storm wave base, possibly in a shelf setting. The lower part of the Pedawan Formation is mudstone dominated, and the upper part progressively becomes sandstone dominated. The Pedawan Formation bears several penecontemporaneously deformed horizons sandwiched between laterally persistent undeformed beds. Deformation structures include folds with reclined to recumbent geometry and layer-confined normal and reverse faults. We have interpreted these deformed horizons as seismites. Modal analyses of the sandstones indicate recycled orogenic as well as arc provenance and thus indicate mixing of recycled orogenic sediments with arc-derived volcanic material. The sedimentary facies characteristics of the Pedawan Formation in combination with numerous penecontemporaneous deformation features at selected stratigraphic levels indicate that the turbidites formed in a seismically active deepwater depositional setting as part of a long-lived subduction complex in eastern Sundaland during which several crustal fragments were accreted to Borneo.
{"title":"Cretaceous Forearc Sedimentation and Contemporary Basin Tectonics in Northwestern Borneo: New Sedimentological Insights from Pedawan Formation, Kuching Zone, East Malaysia","authors":"Rajat Mazumder, Farah Bt Mohd Anthony, Basil Shung Say Teo, Subhajit Roy, Amal Al Hajri, T. Ohta, S. De, O. Catuneanu","doi":"10.1086/715790","DOIUrl":"https://doi.org/10.1086/715790","url":null,"abstract":"Sedimentary successions developed at the destructive plate margin settings are extremely important as they bear valuable record of contemporary basin tectonics and consequent sedimentation. Intense deformation and metamorphism often obliterate the primary sedimentary texture and structures of the sedimentary successions formed at destructive plate margins. However, sedimentological analysis of young unmetamorphosed forearc successions provides rare opportunity to infer the interplay between tectonics and sedimentation. Therefore, a process-based sedimentological facies analysis and provenance studies of the Cretaceous Pedawan Formation in the Kuching Zone, Borneo, have been undertaken for the first time. The sandstones are compositionally and texturally immature. They are normally graded with sole marks and have erosional lower contacts with mudstones and generally have gradational upper contacts. The massive sandstones, parallel-laminated sandstones, and the combination of rippled sandstones with thin mudstones are interpreted as turbidites. The lack of wave-generated structures, including hummocky cross stratification, indicates that deposition took place below storm wave base, possibly in a shelf setting. The lower part of the Pedawan Formation is mudstone dominated, and the upper part progressively becomes sandstone dominated. The Pedawan Formation bears several penecontemporaneously deformed horizons sandwiched between laterally persistent undeformed beds. Deformation structures include folds with reclined to recumbent geometry and layer-confined normal and reverse faults. We have interpreted these deformed horizons as seismites. Modal analyses of the sandstones indicate recycled orogenic as well as arc provenance and thus indicate mixing of recycled orogenic sediments with arc-derived volcanic material. The sedimentary facies characteristics of the Pedawan Formation in combination with numerous penecontemporaneous deformation features at selected stratigraphic levels indicate that the turbidites formed in a seismically active deepwater depositional setting as part of a long-lived subduction complex in eastern Sundaland during which several crustal fragments were accreted to Borneo.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"391 - 415"},"PeriodicalIF":1.8,"publicationDate":"2021-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715790","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47968328","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Our study of a banded charnockite complex of the Mühlig-Hofmannfjella in Dronning Maud Land, Antarctica, illustrates how the combination of high-temperature (re-)crystallization processes, melts, and volatile fluids leads to complex intrusive, metasomatic, and structural relationships. The igneous complex consists of gently dipping sets of charnockite interlayered with dolerite and leucogranite. The banded complex formed primarily by magmatic processes, but with superimposed modifications by metasomatism. The charnockite has a ferroan composition and contains both orthopyroxene (Fs80–84) and olivine (Fa94–96). Zircon U-Pb dates the emplacement of charnockite at 515 Ma, and inherited zircon cores and negative εNd values of −3 to −5 indicate that the age of the source of the magma was about 1100 Ma. Neodymium isotopes were not homogenized during the Cambrian magmatic event, which suggests that the generation and emplacement of the magma took place in separate batches during construction of the banded complex. By contrast, the Rb-Sr system in the charnockite was extensively homogenized, likely because of the superimposed late-magmatic fluid activity, which also produced the bands and networks of leucogranites. These events occurred during the late stages of the assembly of Gondwana, with postcollisional extension and mantle upwelling maintaining a high heat flow.
{"title":"Banded Charnockite: The Result of Crustal Magma Generation, Piecemeal Emplacement, and Fluid-Driven Mineral Replacement in High-Grade Crust (Central Dronning Maud Land, Antarctica)","authors":"A. Engvik, F. Corfu, I. Kleinhanns, S. Elvevold","doi":"10.1086/715789","DOIUrl":"https://doi.org/10.1086/715789","url":null,"abstract":"Our study of a banded charnockite complex of the Mühlig-Hofmannfjella in Dronning Maud Land, Antarctica, illustrates how the combination of high-temperature (re-)crystallization processes, melts, and volatile fluids leads to complex intrusive, metasomatic, and structural relationships. The igneous complex consists of gently dipping sets of charnockite interlayered with dolerite and leucogranite. The banded complex formed primarily by magmatic processes, but with superimposed modifications by metasomatism. The charnockite has a ferroan composition and contains both orthopyroxene (Fs80–84) and olivine (Fa94–96). Zircon U-Pb dates the emplacement of charnockite at 515 Ma, and inherited zircon cores and negative εNd values of −3 to −5 indicate that the age of the source of the magma was about 1100 Ma. Neodymium isotopes were not homogenized during the Cambrian magmatic event, which suggests that the generation and emplacement of the magma took place in separate batches during construction of the banded complex. By contrast, the Rb-Sr system in the charnockite was extensively homogenized, likely because of the superimposed late-magmatic fluid activity, which also produced the bands and networks of leucogranites. These events occurred during the late stages of the assembly of Gondwana, with postcollisional extension and mantle upwelling maintaining a high heat flow.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"371 - 390"},"PeriodicalIF":1.8,"publicationDate":"2021-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715789","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42545882","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
To understand the protracted accretionary evolution along the Central Indian Tectonic Zone (CITZ), we investigated garnet-staurolite schists and associated lithologies from the central Mahakoshal Belt (MB). Mesoscale structures, porphyroblast growth, garnet zoning, and pseudosection modeling were coupled with U-Th-Pb monazite dating to reconstruct a clockwise pressure-temperature-time (P-T-t) path for the garnet- and staurolite-bearing schists. The prograde path is characterized by near isobaric heating conditions, which initiates at pressure-temperature (P-T) conditions of 4.5–5.0 kbar and 550°–560°C and attains peak metamorphism at 5.5–6.0 kbar and 610°–620°C. The peak metamorphism was contemporaneous with the emplacement of southern-margin granitoids at ~1.70 Ga, resulting in the perturbation of geotherms in the collision setting. The retrograde arm (MR) of the P-T path passes through isobaric cooling at ~5.0 kbar and ~500°C. This late Paleoproterozoic P-T-t path is overprinted by hitherto uncharacterized tectonism that coincides with the Sausar orogeny and provides evidence for the northward extension of 0.95–0.85 Ga orogenic activity within the CITZ. The 1.80–1.55 and 0.95–0.85 Ga tectonothermal events identified in this study support that crustal evolution in the CITZ involved a mosaic of domains that were accreted together in the Neoproterozoic time.
{"title":"Proterozoic High-Temperature–Low-Pressure Metamorphism in the Mahakoshal Belt, Central Indian Tectonic Zone (India): Structure, Metamorphism, U-Th-Pb Monazite Geochronology, and Tectonic Implications","authors":"T. Deshmukh, N. Prabhakar, A. Bhattacharya","doi":"10.1086/715791","DOIUrl":"https://doi.org/10.1086/715791","url":null,"abstract":"To understand the protracted accretionary evolution along the Central Indian Tectonic Zone (CITZ), we investigated garnet-staurolite schists and associated lithologies from the central Mahakoshal Belt (MB). Mesoscale structures, porphyroblast growth, garnet zoning, and pseudosection modeling were coupled with U-Th-Pb monazite dating to reconstruct a clockwise pressure-temperature-time (P-T-t) path for the garnet- and staurolite-bearing schists. The prograde path is characterized by near isobaric heating conditions, which initiates at pressure-temperature (P-T) conditions of 4.5–5.0 kbar and 550°–560°C and attains peak metamorphism at 5.5–6.0 kbar and 610°–620°C. The peak metamorphism was contemporaneous with the emplacement of southern-margin granitoids at ~1.70 Ga, resulting in the perturbation of geotherms in the collision setting. The retrograde arm (MR) of the P-T path passes through isobaric cooling at ~5.0 kbar and ~500°C. This late Paleoproterozoic P-T-t path is overprinted by hitherto uncharacterized tectonism that coincides with the Sausar orogeny and provides evidence for the northward extension of 0.95–0.85 Ga orogenic activity within the CITZ. The 1.80–1.55 and 0.95–0.85 Ga tectonothermal events identified in this study support that crustal evolution in the CITZ involved a mosaic of domains that were accreted together in the Neoproterozoic time.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"417 - 444"},"PeriodicalIF":1.8,"publicationDate":"2021-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715791","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49489923","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}
Diverse origins have been proposed for continental adakites, with great emphasis on high-pressure melting of the lower crust. However, the source composition is usually ignored in interpreting the generation of the adakitic geochemical signature (e.g., high Sr/Y and La/Yb) and thus may affect our understanding of the petrogenesis and tectonic settings. Here, we present geochronological, mineralogical, geochemical, and isotopic data for the Late Jurassic to Early Cretaceous adakitic volcanic rocks from the Yangyuan area in central North China Craton (NCC) to constrain their petrogenesis and tectonic implications. These adakitic rocks are trachyandesites and dacites with an eruption age of 146–141 Ma. They are characterized by high Sr/Y (80.6–97.5) and (La/Yb)N (60.6–67.6) ratios but low MgO (0.85–1.43 wt%) contents, with negligible Eu anomalies (Eu/Eu*=0.93–1.08). Isotopically, the samples have high initial 87Sr/86Sr ratios (0.707146–0.707207) and enriched whole-rock Nd (εNd(t)=−14.0 to −13.8) and zircon Hf (εHf(t)=−19.4 to −15.8) values that are consistent with Mesozoic lower-crust-derived adakitic rocks and lower-crustal xenoliths in the NCC. Trace-element modeling results suggest that they were more likely derived by partial melting of a continental lower crust with a thickness of <40 km, although the high (Gd/Yb)N ratios (6.16–7.35) indicate the presence of residual garnet in the magma source. The geochemical characteristics, combined with published data in the central and eastern NCC, reveal that the “adakitic” signature of these rocks is mainly controlled by source rock compositions. This study suggests that continental adakitic rocks are not necessarily produced by high-pressure melting, and thus their use as an indicator of thickened or foundered lower crust should be treated with caution.
{"title":"Source Composition Controls the Petrogenesis of Jurassic-Cretaceous Adakitic Volcanic Rocks in the Central North China Craton","authors":"Liang Zhou, Yuping Su, Jianping Zheng, Q. Ma, Jian Wang, Xiahui Zhang, X. Bian","doi":"10.1086/715241","DOIUrl":"https://doi.org/10.1086/715241","url":null,"abstract":"Diverse origins have been proposed for continental adakites, with great emphasis on high-pressure melting of the lower crust. However, the source composition is usually ignored in interpreting the generation of the adakitic geochemical signature (e.g., high Sr/Y and La/Yb) and thus may affect our understanding of the petrogenesis and tectonic settings. Here, we present geochronological, mineralogical, geochemical, and isotopic data for the Late Jurassic to Early Cretaceous adakitic volcanic rocks from the Yangyuan area in central North China Craton (NCC) to constrain their petrogenesis and tectonic implications. These adakitic rocks are trachyandesites and dacites with an eruption age of 146–141 Ma. They are characterized by high Sr/Y (80.6–97.5) and (La/Yb)N (60.6–67.6) ratios but low MgO (0.85–1.43 wt%) contents, with negligible Eu anomalies (Eu/Eu*=0.93–1.08). Isotopically, the samples have high initial 87Sr/86Sr ratios (0.707146–0.707207) and enriched whole-rock Nd (εNd(t)=−14.0 to −13.8) and zircon Hf (εHf(t)=−19.4 to −15.8) values that are consistent with Mesozoic lower-crust-derived adakitic rocks and lower-crustal xenoliths in the NCC. Trace-element modeling results suggest that they were more likely derived by partial melting of a continental lower crust with a thickness of <40 km, although the high (Gd/Yb)N ratios (6.16–7.35) indicate the presence of residual garnet in the magma source. The geochemical characteristics, combined with published data in the central and eastern NCC, reveal that the “adakitic” signature of these rocks is mainly controlled by source rock compositions. This study suggests that continental adakitic rocks are not necessarily produced by high-pressure melting, and thus their use as an indicator of thickened or foundered lower crust should be treated with caution.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"319 - 341"},"PeriodicalIF":1.8,"publicationDate":"2021-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715241","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48246613","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}
Sand composition of large African rivers has been increasingly studied, but identification of source rocks often remains hypothetical, and mineral signatures of the Archean to Proterozoic basement are weakly constrained. We analyzed heavy minerals of headwater streams on the East African Plateau draining characteristic lithotectonic units with respect to modern single-grain techniques in provenance research. Despite strong tropical weathering, the target minerals are well conserved and widely distributed. We could distinguish four lithotectonic units from river sand. (1) The high-grade metamorphic rocks of the Neoarchean North Uganda Terrane deliver high-almandine, low-spessartine garnet with two groups of varying grossular contents. Rutile is mainly of metapelitic origin and exhibits amphibolite/eclogite facies to granulite facies. The Neoarchean age is confirmed by zircon U-Pb ages. (2) Rivers that mainly erode metasedimentary rocks of the Paleoproterozoic Rwenzori Fold Belt are identified by almandine-spessartine garnet, mainly amphibolite/eclogite facies rutile, and detrital zircon ages that correspond to the Eburnian orogenic cycle. (3) The Neoproterozoic Bunyoro Group in Uganda delivers mixed detritus with rutile assemblages similar to the underlying North Uganda Terrane and a wide range of zircon ages. (4) Although the basement block of the extremely uplifted Rwenzori Mountains contains rocks of both the Rwenzori Fold Belt and the North Uganda Terrane, some distinct provenance indicators could be identified: extremely high proportions of amphibole, exclusive dominance of spessartine-rich garnet, strong dominance of amphibolite to granulite facies rutile, a pronounced population of rutile with high Nb concentration, and pinkish zircon grains. Zircon age, however, was not distinctive. We show that single-grain analysis of heavy minerals in river sand can characterize different basement units under tropical climatic conditions. Detrital zircon age spectra confirmed known orogenic cycles in East Africa but could not distinguish specific Cenozoic fault blocks of the East African Rift. This could be solved by additional single-grain techniques.
{"title":"Garnet and Rutile Mineral Chemistry and Zircon U-Pb Ages of Modern River Sand along the Western East African Rift (Albertine Rift, Uganda)","authors":"S. Schneider, M. Hinderer","doi":"10.1086/715295","DOIUrl":"https://doi.org/10.1086/715295","url":null,"abstract":"Sand composition of large African rivers has been increasingly studied, but identification of source rocks often remains hypothetical, and mineral signatures of the Archean to Proterozoic basement are weakly constrained. We analyzed heavy minerals of headwater streams on the East African Plateau draining characteristic lithotectonic units with respect to modern single-grain techniques in provenance research. Despite strong tropical weathering, the target minerals are well conserved and widely distributed. We could distinguish four lithotectonic units from river sand. (1) The high-grade metamorphic rocks of the Neoarchean North Uganda Terrane deliver high-almandine, low-spessartine garnet with two groups of varying grossular contents. Rutile is mainly of metapelitic origin and exhibits amphibolite/eclogite facies to granulite facies. The Neoarchean age is confirmed by zircon U-Pb ages. (2) Rivers that mainly erode metasedimentary rocks of the Paleoproterozoic Rwenzori Fold Belt are identified by almandine-spessartine garnet, mainly amphibolite/eclogite facies rutile, and detrital zircon ages that correspond to the Eburnian orogenic cycle. (3) The Neoproterozoic Bunyoro Group in Uganda delivers mixed detritus with rutile assemblages similar to the underlying North Uganda Terrane and a wide range of zircon ages. (4) Although the basement block of the extremely uplifted Rwenzori Mountains contains rocks of both the Rwenzori Fold Belt and the North Uganda Terrane, some distinct provenance indicators could be identified: extremely high proportions of amphibole, exclusive dominance of spessartine-rich garnet, strong dominance of amphibolite to granulite facies rutile, a pronounced population of rutile with high Nb concentration, and pinkish zircon grains. Zircon age, however, was not distinctive. We show that single-grain analysis of heavy minerals in river sand can characterize different basement units under tropical climatic conditions. Detrital zircon age spectra confirmed known orogenic cycles in East Africa but could not distinguish specific Cenozoic fault blocks of the East African Rift. This could be solved by additional single-grain techniques.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"297 - 317"},"PeriodicalIF":1.8,"publicationDate":"2021-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715295","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42360105","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 study includes the first set of muscovite, biotite, feldspar, and whole-rock 40Ar/39Ar ages from phyllonite belts of the Southern Rocky Mountain Trench (SRMT) and the western Rocky Mountain fold-thrust belt in the southern Canadian Cordillera. Eleven samples from the northern segment of the SRMT indicate two Early Cretaceous (135 and 125 Ma), two mid-Cretaceous (111 and 96 Ma), and two Late Cretaceous (85 and 75–72 Ma) peaks of transpressional tectonism and define the Southern Rocky Mountain shear zone. Four samples from the adjacent strike-slip Walker Creek fault zone yielded late Valanginian (133 Ma), early Aptian (124 Ma), and Albian-Cenomanian (111–96 Ma) ages, with Late Cretaceous (85–68 Ma) overprint. The oblique compression Bear Foot thrust (72 Ma) and its footwall, the Valemount strain zone, yielded consistent, late Campanian to earliest Maastrichtian plateau ages. Poorly recrystallized phyllonite samples from the Monarch, Moose Pass, and Chatter Creek thrust faults in the Rockies yielded ages consistent with a late Valanginian (134–131 Ma) tectonic event. The new structural and geochronological data indicate apparently distinct pulses of protracted orogen-parallel Cretaceous tectonism along the western margin of the Rockies and the eastern margin of the Omineca belt and document the kinematic link between the Foreland belt and its hinterland through a wide zone of transpression. These ages are also consistent with previously reported ages of thrusts in the Rockies accompanied by significant depositional changes in the foreland basin. The Early Cretaceous transpression (~135 and ~125 Ma) propagated into the western Rockies as previously unrecognized, out-of-sequence thrusting pulses that may have triggered the development of the forebulge responsible for the vast sub-Cadomin–sub-Manville unconformity in the foreland basin.
{"title":"40Ar/39Ar Dating of Phyllonite in the Southern Rocky Mountain Trench and Adjacent Rocky Mountains Unravels Kinematic Links between the Omineca and Foreland Belts of the Southern Canadian Cordillera","authors":"D. Pană","doi":"10.1086/715243","DOIUrl":"https://doi.org/10.1086/715243","url":null,"abstract":"This study includes the first set of muscovite, biotite, feldspar, and whole-rock 40Ar/39Ar ages from phyllonite belts of the Southern Rocky Mountain Trench (SRMT) and the western Rocky Mountain fold-thrust belt in the southern Canadian Cordillera. Eleven samples from the northern segment of the SRMT indicate two Early Cretaceous (135 and 125 Ma), two mid-Cretaceous (111 and 96 Ma), and two Late Cretaceous (85 and 75–72 Ma) peaks of transpressional tectonism and define the Southern Rocky Mountain shear zone. Four samples from the adjacent strike-slip Walker Creek fault zone yielded late Valanginian (133 Ma), early Aptian (124 Ma), and Albian-Cenomanian (111–96 Ma) ages, with Late Cretaceous (85–68 Ma) overprint. The oblique compression Bear Foot thrust (72 Ma) and its footwall, the Valemount strain zone, yielded consistent, late Campanian to earliest Maastrichtian plateau ages. Poorly recrystallized phyllonite samples from the Monarch, Moose Pass, and Chatter Creek thrust faults in the Rockies yielded ages consistent with a late Valanginian (134–131 Ma) tectonic event. The new structural and geochronological data indicate apparently distinct pulses of protracted orogen-parallel Cretaceous tectonism along the western margin of the Rockies and the eastern margin of the Omineca belt and document the kinematic link between the Foreland belt and its hinterland through a wide zone of transpression. These ages are also consistent with previously reported ages of thrusts in the Rockies accompanied by significant depositional changes in the foreland basin. The Early Cretaceous transpression (~135 and ~125 Ma) propagated into the western Rockies as previously unrecognized, out-of-sequence thrusting pulses that may have triggered the development of the forebulge responsible for the vast sub-Cadomin–sub-Manville unconformity in the foreland basin.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"255 - 281"},"PeriodicalIF":1.8,"publicationDate":"2021-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/715243","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45769938","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}
C. Rosière, J. Santos, F. Braga, A. Hensler, V. K. Rolim, A. Bekker
Large-scale, hypogene iron mineralization systems developed recurrently in the São Francisco craton in association with two orogenies. During the ca. 2.1–2.0 Ga Trans-Amazonian orogeny, low-temperature and low-to-moderate-salinity metamorphic fluids resulted in carbonatization-related iron enrichment of the early Paleoproterozoic Cauê Iron Formation (IF) of the Minas Supergroup (MSG) as well as short-distance iron mobilization. Monazite from the iron-oxide veins yielded a SHRIMP 232Th/208Pb date corresponding to fluid circulation coeval with the migmatization of the Archean tonalite-trondhjemite-granodiorite crust. During the Ediacaran to Cambrian Brasiliano orogeny, when the São Francisco craton was consolidated in its present configuration, hydrothermal fluids repeatedly mineralized the Cauê IF and the younger IFs of the lower Espinhaço Supergroup. Far-field hydrothermal alteration associated with distinct Fe mineralization episodes widely affected Meso- to Neoproterozoic sequences as well as Archean to Paleoproterozoic terranes of the cratonic core. Circulation of hydrothermal fluids during the Brasiliano orogeny caused desilicification, iron mobilization, and widespread alteration of the rocks. The alteration events are dated with monazite and xenotime intergrown with hematite, anatase, and recrystallized rims of detrital zircons from interlayered quartzites and of igneous zircons from pegmatites, granites, acidic volcanic rocks, and orthogneisses, yielding U-Pb dates between 542 and 493 Ma. U-Pb dates point to five Cambrian hydrothermal events from the waning stages to the aftermath of the Brasiliano orogeny. These hydrothermal fluid circulation events correspond to the emplacement ages of four granite suites and pegmatites that lasted from the final stages of the collision to the collapse of the Araçuaí-West Congo orogen. Episodic circulation of fluids continued for approximately 2 billion years after IF deposition on the eastern São Francisco craton with the formation of high-grade iron ore deposits and resetting of the IF trace-element inventory.
{"title":"Multiple Hydrothermal Iron-Formation Upgrading Events in Southeastern São Francisco Craton","authors":"C. Rosière, J. Santos, F. Braga, A. Hensler, V. K. Rolim, A. Bekker","doi":"10.1086/715242","DOIUrl":"https://doi.org/10.1086/715242","url":null,"abstract":"Large-scale, hypogene iron mineralization systems developed recurrently in the São Francisco craton in association with two orogenies. During the ca. 2.1–2.0 Ga Trans-Amazonian orogeny, low-temperature and low-to-moderate-salinity metamorphic fluids resulted in carbonatization-related iron enrichment of the early Paleoproterozoic Cauê Iron Formation (IF) of the Minas Supergroup (MSG) as well as short-distance iron mobilization. Monazite from the iron-oxide veins yielded a SHRIMP 232Th/208Pb date corresponding to fluid circulation coeval with the migmatization of the Archean tonalite-trondhjemite-granodiorite crust. During the Ediacaran to Cambrian Brasiliano orogeny, when the São Francisco craton was consolidated in its present configuration, hydrothermal fluids repeatedly mineralized the Cauê IF and the younger IFs of the lower Espinhaço Supergroup. Far-field hydrothermal alteration associated with distinct Fe mineralization episodes widely affected Meso- to Neoproterozoic sequences as well as Archean to Paleoproterozoic terranes of the cratonic core. Circulation of hydrothermal fluids during the Brasiliano orogeny caused desilicification, iron mobilization, and widespread alteration of the rocks. The alteration events are dated with monazite and xenotime intergrown with hematite, anatase, and recrystallized rims of detrital zircons from interlayered quartzites and of igneous zircons from pegmatites, granites, acidic volcanic rocks, and orthogneisses, yielding U-Pb dates between 542 and 493 Ma. U-Pb dates point to five Cambrian hydrothermal events from the waning stages to the aftermath of the Brasiliano orogeny. These hydrothermal fluid circulation events correspond to the emplacement ages of four granite suites and pegmatites that lasted from the final stages of the collision to the collapse of the Araçuaí-West Congo orogen. Episodic circulation of fluids continued for approximately 2 billion years after IF deposition on the eastern São Francisco craton with the formation of high-grade iron ore deposits and resetting of the IF trace-element inventory.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":"129 1","pages":"283 - 296"},"PeriodicalIF":1.8,"publicationDate":"2021-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48243994","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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