Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at633.2±9.0 Ma(2σ).These magnas intrude the forearc Ad Dawadimi Basin,which is composed of metapelitic schists and g...Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at633.2±9.0 Ma(2σ).These magnas intrude the forearc Ad Dawadimi Basin,which is composed of metapelitic schists and greywacke along with ophiolitic melanges of boninitic affinity which underwent inversion and deformation by^620 Ma.This adakitic magmatism,while intruding parts of the Ad Dawadimi Basin,predates this deformation,but is possibly coincident with basin closure.As adakitic magmatism requires melting of an amphibolite or eclogitic source,empirical and experimental constraints require anomalously hot supra-subduction zone mantle.Considering that this magmatism immediately predates basin inversion,these magmas possibly pinpoint the timing of the slab breaking,marking the terminal stages of arc magmatism,terrane accretion and the influx of hot oceanic asthenospheric mantle.This influx of hot asthenospheric mantle may also be responsible for postcolltsional A-type magmatism.展开更多
The Aegean Sea area is thought to be an actively extending back-arc region,north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean.The area shows extensive normal ...The Aegean Sea area is thought to be an actively extending back-arc region,north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean.The area shows extensive normal faulting,ductile‘extensional’shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone.In this paper,we question this interpretation,and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous-Eocene NE-dipping subduction zone that was responsible for continentcontinent collision between Eurasia and Adria-Apulia/Cyclades.Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift.We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension,herein called the Aegean Orogeny.This orogeny followed a classic Wilson cycle from early ophiolite obduction(ca.74 Ma)onto a previously passive continental margin,to attempted crustal subduction with HP eclogite and blueschist facies metamorphism(ca.54-45 Ma),through crustal thickening and regional kyanite-sillimanite grade Barroviantype metamorphism(ca.22-14 Ma),to orogenic collapse(<14 Ma).At least three periods of‘extensional’fabrics relate to:(1)Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE,base-SW fabrics,recording return flow along a subduction channel in a compressional tectonic setting(ca.50-35 Ma).(2)Extensional fabrics within the core complexes formed by exhumation of kyanite-and sillimanite gneisses showing thrust-related fabrics at the base and‘extensional’fabrics along the top(ca.18.5-14 Ma).(3)Regional ductile-brittle‘extensional’fabrics and low-angle normal faulting related to the North Cycladic Detachment(NCD)and the South(West)Cycladic Detachment(WCD)during regional extension along the flanks of a major NW-SE anticlinal fold along the middle of the Cyclades.Major low-angle normal faults and ductile shear zones show symmetry about the area,with the NE chain of islands(Andros,Tinos,Mykonos,Ikaria)exposing the NE-dipping NCD with consistent top-NE ductil e fabrics along 200 km of strike.In contrast,from the Greek mainland(Attica)along the SE chain of islands(Kea,Kythnos,Serifos)a SW-dipping low-angle normal fault and ductile shear zone,the WCD is inferred for at least 100 km along strike.Islands in the middle of the Cyclades show deeper structural levels including kyanite-and sillimanite-grade metamorphic core complexes(Naxos,Paros)as well as Variscan basement rocks(Naxos,Ios).The overall structure is an~100 km wavelength NW-SE trending dome with low-angle extensional faults along each flank,dipping away from the anticline axis to the NE and SW.Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes(Naxos,Paros,Ios,Sifnos)indicating a post-Miocene late phase of E-W shortening.展开更多
The Southern Irumide Belt(SIB)is an orogenic belt consisting of a number of lithologically varied Mesoproterozoic and Neoproterozoic terranes that were thrust upon each other.The belt lies along the southwest margin o...The Southern Irumide Belt(SIB)is an orogenic belt consisting of a number of lithologically varied Mesoproterozoic and Neoproterozoic terranes that were thrust upon each other.The belt lies along the southwest margin of the Archaean to Proterozoic Congo Craton,and bears a Neoproterozoic tectonothermal overprint relating to the Neoproterozoic-Cambrian collision between the Congo and Kalahari cratons.It preserves a record of about 500 million years of plate interaction along this part of the Congo margin.Detrital zircon samples from the SIB were analysed for U-Pb and Lu-Hf isotopes,as well as trace element compositions.These data are used to constrain sediment-source relationships between SIB terranes and other Gondwanan terranes such as the local Congo Craton and Irumide belt and wider afield to Madagascar(Azania)and India.These correlations are then used to interpret the Mesoproterozoic to Neoproterozoic affinity of the rocks and evolution of the region.Detrital zircon samples from the Chewore-Rufunsa and Kacholola(previously referred to as Luangwa-Nyimba)terranes of the SIB yield zircon U-Pb age populations and evolvedε(Hf)(t)values that are similar to the Muva Supergroup found throughout eastern Zambia,primarily correlating with Ubendian-Usagaran(ca.2.05-1.80 Ga)phase magmatism and a cryptic basement terrane that has been suggested to underlie the Bangweulu Block and Irumide Belt.These data suggest that the SIB was depositionally connected to the Congo Craton throughout the Mesoproterozoic.The more eastern Nyimba-Sinda terrane of the SIB(previously referred to as Petauke-Sinda terrane)records detrital zircon ages andε(Hf)(t)values that correlate with ca.1.1-1.0 Ga magmatism exposed elsewhere in the SIB and Irumide Belt.We ascribe this difference in age populations to the polyphase development of the province,where the sedimentary and volcanic rocks of the Nyimba-Sinda terrane accumulated in extensional basins that developed in the Neoproterozoic.Such deposition would have occurred following late-Mesoproterozoic magmatism that is widespread throughout both the Irumide and Southern Irumide Belts,presently considered to have occurred in response to collision between a possible microcontinental mass and the Irumide Belt.This interpretation implies a multi-staged evolution of the ocean south of the Congo Craton during the mid-Mesoproterozoic to late-Neoproterozoic,which ultimately closed during collision between the Congo and Kalahari cratons.展开更多
文摘Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at633.2±9.0 Ma(2σ).These magnas intrude the forearc Ad Dawadimi Basin,which is composed of metapelitic schists and greywacke along with ophiolitic melanges of boninitic affinity which underwent inversion and deformation by^620 Ma.This adakitic magmatism,while intruding parts of the Ad Dawadimi Basin,predates this deformation,but is possibly coincident with basin closure.As adakitic magmatism requires melting of an amphibolite or eclogitic source,empirical and experimental constraints require anomalously hot supra-subduction zone mantle.Considering that this magmatism immediately predates basin inversion,these magmas possibly pinpoint the timing of the slab breaking,marking the terminal stages of arc magmatism,terrane accretion and the influx of hot oceanic asthenospheric mantle.This influx of hot asthenospheric mantle may also be responsible for postcolltsional A-type magmatism.
基金Natural Environment Research Council grant NE/L0021612/1NERC Isotope Geoscience Laboratory grant IP-1597-1115 for funding。
文摘The Aegean Sea area is thought to be an actively extending back-arc region,north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean.The area shows extensive normal faulting,ductile‘extensional’shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone.In this paper,we question this interpretation,and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous-Eocene NE-dipping subduction zone that was responsible for continentcontinent collision between Eurasia and Adria-Apulia/Cyclades.Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift.We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension,herein called the Aegean Orogeny.This orogeny followed a classic Wilson cycle from early ophiolite obduction(ca.74 Ma)onto a previously passive continental margin,to attempted crustal subduction with HP eclogite and blueschist facies metamorphism(ca.54-45 Ma),through crustal thickening and regional kyanite-sillimanite grade Barroviantype metamorphism(ca.22-14 Ma),to orogenic collapse(<14 Ma).At least three periods of‘extensional’fabrics relate to:(1)Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE,base-SW fabrics,recording return flow along a subduction channel in a compressional tectonic setting(ca.50-35 Ma).(2)Extensional fabrics within the core complexes formed by exhumation of kyanite-and sillimanite gneisses showing thrust-related fabrics at the base and‘extensional’fabrics along the top(ca.18.5-14 Ma).(3)Regional ductile-brittle‘extensional’fabrics and low-angle normal faulting related to the North Cycladic Detachment(NCD)and the South(West)Cycladic Detachment(WCD)during regional extension along the flanks of a major NW-SE anticlinal fold along the middle of the Cyclades.Major low-angle normal faults and ductile shear zones show symmetry about the area,with the NE chain of islands(Andros,Tinos,Mykonos,Ikaria)exposing the NE-dipping NCD with consistent top-NE ductil e fabrics along 200 km of strike.In contrast,from the Greek mainland(Attica)along the SE chain of islands(Kea,Kythnos,Serifos)a SW-dipping low-angle normal fault and ductile shear zone,the WCD is inferred for at least 100 km along strike.Islands in the middle of the Cyclades show deeper structural levels including kyanite-and sillimanite-grade metamorphic core complexes(Naxos,Paros)as well as Variscan basement rocks(Naxos,Ios).The overall structure is an~100 km wavelength NW-SE trending dome with low-angle extensional faults along each flank,dipping away from the anticline axis to the NE and SW.Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes(Naxos,Paros,Ios,Sifnos)indicating a post-Miocene late phase of E-W shortening.
基金funded by Australian Research Council Future Fellowship#FT120100340 to A.Collinssupported by a Research Training Program scholarship
文摘The Southern Irumide Belt(SIB)is an orogenic belt consisting of a number of lithologically varied Mesoproterozoic and Neoproterozoic terranes that were thrust upon each other.The belt lies along the southwest margin of the Archaean to Proterozoic Congo Craton,and bears a Neoproterozoic tectonothermal overprint relating to the Neoproterozoic-Cambrian collision between the Congo and Kalahari cratons.It preserves a record of about 500 million years of plate interaction along this part of the Congo margin.Detrital zircon samples from the SIB were analysed for U-Pb and Lu-Hf isotopes,as well as trace element compositions.These data are used to constrain sediment-source relationships between SIB terranes and other Gondwanan terranes such as the local Congo Craton and Irumide belt and wider afield to Madagascar(Azania)and India.These correlations are then used to interpret the Mesoproterozoic to Neoproterozoic affinity of the rocks and evolution of the region.Detrital zircon samples from the Chewore-Rufunsa and Kacholola(previously referred to as Luangwa-Nyimba)terranes of the SIB yield zircon U-Pb age populations and evolvedε(Hf)(t)values that are similar to the Muva Supergroup found throughout eastern Zambia,primarily correlating with Ubendian-Usagaran(ca.2.05-1.80 Ga)phase magmatism and a cryptic basement terrane that has been suggested to underlie the Bangweulu Block and Irumide Belt.These data suggest that the SIB was depositionally connected to the Congo Craton throughout the Mesoproterozoic.The more eastern Nyimba-Sinda terrane of the SIB(previously referred to as Petauke-Sinda terrane)records detrital zircon ages andε(Hf)(t)values that correlate with ca.1.1-1.0 Ga magmatism exposed elsewhere in the SIB and Irumide Belt.We ascribe this difference in age populations to the polyphase development of the province,where the sedimentary and volcanic rocks of the Nyimba-Sinda terrane accumulated in extensional basins that developed in the Neoproterozoic.Such deposition would have occurred following late-Mesoproterozoic magmatism that is widespread throughout both the Irumide and Southern Irumide Belts,presently considered to have occurred in response to collision between a possible microcontinental mass and the Irumide Belt.This interpretation implies a multi-staged evolution of the ocean south of the Congo Craton during the mid-Mesoproterozoic to late-Neoproterozoic,which ultimately closed during collision between the Congo and Kalahari cratons.
