The New Caledonia ophiolite(Peridotite Nappe)consists primarily of harzburgites,locally overlain by mafic-ultramafic cumulates,and minor spinel and plagioclase lherzolites.In this study,a comprehensive geochemical dat...The New Caledonia ophiolite(Peridotite Nappe)consists primarily of harzburgites,locally overlain by mafic-ultramafic cumulates,and minor spinel and plagioclase lherzolites.In this study,a comprehensive geochemical data set(major and trace element,Sr-Nd-Pb isotopes)has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures,locally grading into protomylonitic textures.They exhibit a refractory nature,as attested by the notable absence of primary clinopyroxene,very high Fo content of olivine(91-93 mol.%),high Mg#of orthopyroxene(0.91-0.93)and high Cr#of spinel(0.44-0.71).The harzburgites are characterised by remarkably low REE concentrations(<0.1 chondritic values)and display"U-shaped"profiles,with steeply sloping HREE(DyN/YbN=0.07-0.16)and fractionated LREE-MREE segments(LaN/SmN=2.1-8.3),in the range of modern fore-arc peridotites.Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions(15%fractional melting),followed by hydrous melting in a subduction zone setting(up to 15%-18%).However,melting models fail to explain the enrichments observed for some FME(i.e.Ba,Sr,Pb),LREE-MREE and Zr-Hf.These enrichments,coupled with the frequent occurrence of thin,undeformed films of Al2 O3,and CaO-poor orthopyroxene(Al2O3=0.88-1.53 wt.%,CaO=0.31-0.56 wt.%)and clinopyroxene with low Na2 O(0.03-0.16 wt.%),Al2 O3(0.66-1.35 wt.%)and TiO2(0.04-0.10 wt.%)contents,point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subductionrelated melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic(-0.80<εNdi≤+13.32)and negatively correlate with Sr isotopes(0.70257≤87Sr/86Sr≤0.70770).Pb isotopes cover a wide range,trending from DMM toward enriched,sediment-like,compositions.We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.展开更多
The New Caledonia Ophiolite(Peridotite Nappe), represents about one third of the island’s surface(i.e. 5 500 km2). The ophiolite is composed of harzburgites, dunites, lherzolites, minor mafic-ultramafic cumulates, an...The New Caledonia Ophiolite(Peridotite Nappe), represents about one third of the island’s surface(i.e. 5 500 km2). The ophiolite is composed of harzburgites, dunites, lherzolites, minor mafic-ultramafic cumulates, and various dykes and sills. The mantle section underwent a polyphase evolution, which involved prominent depletion and re-fertilization. The oldest events are probably recorded by abyssal-type lherzolites of the northern massifs, which bear traces of moderate partial melting. Plagioclase lherzolites were formed by shallow entrapment of highly depleted MORB melt in residual spinel lherzolites. Nd isotope compositions are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion. This evolution was most likely accomplished during the late Cretaceous breakup of the eastern Australian margin. The harzburgite-dunite association, which forms the bulk of Peridotite Nappe was probably formed through a multistage magma-producing process. Harzburgites composition may have be obtained by a first phase of ~15% dry fractional melting, followed by 15%–18% hydrous melting in a supra-subduction zone setting. Variable εNd negatively correlate with 87Sr/86Sr, while Pb isotopes cover a wide range, trending from depleted mantle towards enriched, sediment-like, compositions. Such signatures likely reflect the evolution of a highly depleted forearc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction. The harzburgite-dunite set is overlain by a dunite transition zone ~300 m thick, in turn discontinuously covered by cumulate lenses consisting of layered pyroxenites, dunites, and wherlites at the base and gabbronorites/websterites on top. The mafic cumulates crystallized from primitive, ultra-depleted melts in the nascent lower fore-arc crust. In particular, FME enrichments and Nd-Pb isotopes support an origin from a refractory mantle source modified by slab fluids for the gabbronorite-forming melts. The Peridotite Nappe has been extensively serpentinized(40% to 100%) with extremely scarce occurrences of unserpentinized rocks. Lizardite, brucite, magnetite and minor chrysotile developed from joints and intra-granular cooling cracks in a near-static environment. Serpentine-coated joints and peridotite foliation have been thereafter reopened and injected by various felsic, mafic and ultramafic supra-subduction melts emplaced within a narrow time interval(55–50 Ma), immediately after subduction inception at 56 Ma, i.e. the age of granulite-facies metamorphic sole. The youngest magmatic event is represented by island-arc tholeiite dykes dated at 50 Ma. A widespread set of antigorite and tremolite-bearing veins crosscut all previous structures in a progressively cooling forearc environment. The former are synkinematic crack seals, which display highly radiogenic, sediment-like 87Sr/86Sr ratios suggesting direct derivation of fluids from the subduction zone, while the latter bear mantle-like isotopic signatures and probably originated from the interaction of wall rocks with Ca-rich fluids released by Eocene dykes or fluids that leached them. Finally, continental subduction and obduction occurred during the 44–34 Ma interval and were accompanied by the development of the HP-LT metamorphic belt of northern New Caledonia, which constrains the polarity of subduction.展开更多
The Chinese Tianshan belt is a major part of the southern Central Asian Orogenic Belt, extending westward to Kyrgyzstan and Kazakhstan. Its Paleozoic tectonic evolution, crucial for understanding the amalgamation of C...The Chinese Tianshan belt is a major part of the southern Central Asian Orogenic Belt, extending westward to Kyrgyzstan and Kazakhstan. Its Paleozoic tectonic evolution, crucial for understanding the amalgamation of Central Asia, comprises two stages of subduction-collision. The first collisional stage built the Eo-Tianshan Mountains, before a Visean unconformity, in which all structures are verging north. It implied a southward subduction of the Central Tianshan Ocean beneath the Tarim active margin, that induced the Ordovician-Early Devonian Central Tianshan arc, to the south of which the South Tianshan back-arc basin opened. During the Late Devonian, the closure of this ocean led to a collision between Central Tianshan arc and the Kazakhstan-Yili-North Tianshan Block, and subsequently closure of the South Tianhan back-arc basin, producing two suture zones, namely the Central Tianshan and South Tianshan suture zones where ophiolitic melanges and HP metamorphic rocks were emplaced northward. The second stage included the Late Devonian-Carboniferous southward subduction of North Tianshan Ocean beneath the Eo-Tianshan active margin, underlined by the Yili-North Tianshan arc, leading to the collision between the Kazakhstan-Yili-NTS plate and an inferred Junggar Block at Late Carboniferous-Early Permian time. The North Tianshan Suture Zone underlines likely the last oceanic closure of Central Asia Orogenic Belt; all the oceanic domains were consumed before the Middle Permian. The amalgamated units were affected by a Permian major wrenching, dextral in the Tianshan. The correlation with the Kazakh and Kyrgyz Tianshan is clarified. The Kyrgyz South Tianshan is equivalent to the whole part of Chinese Tianshan （CTS and STS） located to the south of Narat Fault and Main Tianshan Shear Zone; the so-called Middle Tianshan thins out toward the east. The South Tianshan Suture of Kyrgyzstan correlates with the Central Tianshan Suture of Chinese Tianshan. The evolution of this southern domain remains similar from east （Gangou area） to west until the Talas-Ferghana Fault, which reflects the convergence history between the Kazakhstan and Tarim blocks.展开更多
The Altaid tectonic collage extends over Central Asia, exposing numerous accretionary orogens that can account for the Palaeozoic continental crust growth. A pluridisciplinary approach, using geochronological, geochem...The Altaid tectonic collage extends over Central Asia, exposing numerous accretionary orogens that can account for the Palaeozoic continental crust growth. A pluridisciplinary approach, using geochronological, geochemical, structural and palaeomagnetic tools was carried out to unravel the architecture and the evolution of West Junggar （Northwestern China）, a segment of the Altaid Collage. A polycyclic geodynamic evolution is inferred and includes： （1） an Early Palaeozoic cycle, characterized by the closure of two oceanic basins bounded by island-arc systems; （2） an Early Devonian subduction jamming resulting in a minor-scale collision documented by thrusting, syntectonic sedimentation and subsequent crutal thinning associ- ated with alkaline magmatism; （3） a Late Palaeozoic cycle, driven by the evolution of two opposite subduction zones devel- oped upon the Early Palaeozoic basement. Detailed structural analysis and paleomagnetic data provide constraints for the late evolution of Junggar in the frame of the development of the Late Palaeozoic Kazakh orocline, which led to oblique subduction and transpression in the West Junggar accretionary complex. Progressive buckling of the Kazakh orocline further resulted in Late Carboniferous to Permian wrench tectonics, and lateral displacement of lithotectonic units. Block rotations that continued after the Late Triassic are due to diachronous intraplate reactivation. This scenario mirrors the Palaeozoic geodynamics of the Altaid Collage. Multiple Early Palaeozoic collisions of intra-oceanic arcs and micro continents have contributed to the formarion of the Kazakhstan Microconrinent. Since the Late Palaeozoic, subductions formed around this microcontinent and the final oblique closure of oceanic domains resulted in the transcurrent collage of Tarim and Siberia cratons. Palaeozoic strike-slip faults were later reactivated during Mesozoic intracontinental tectonics.展开更多
基金supported by a Vinci grant (Italian-French University) and by Italian-PRIN prot.2015C5LN35
文摘The New Caledonia ophiolite(Peridotite Nappe)consists primarily of harzburgites,locally overlain by mafic-ultramafic cumulates,and minor spinel and plagioclase lherzolites.In this study,a comprehensive geochemical data set(major and trace element,Sr-Nd-Pb isotopes)has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures,locally grading into protomylonitic textures.They exhibit a refractory nature,as attested by the notable absence of primary clinopyroxene,very high Fo content of olivine(91-93 mol.%),high Mg#of orthopyroxene(0.91-0.93)and high Cr#of spinel(0.44-0.71).The harzburgites are characterised by remarkably low REE concentrations(<0.1 chondritic values)and display"U-shaped"profiles,with steeply sloping HREE(DyN/YbN=0.07-0.16)and fractionated LREE-MREE segments(LaN/SmN=2.1-8.3),in the range of modern fore-arc peridotites.Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions(15%fractional melting),followed by hydrous melting in a subduction zone setting(up to 15%-18%).However,melting models fail to explain the enrichments observed for some FME(i.e.Ba,Sr,Pb),LREE-MREE and Zr-Hf.These enrichments,coupled with the frequent occurrence of thin,undeformed films of Al2 O3,and CaO-poor orthopyroxene(Al2O3=0.88-1.53 wt.%,CaO=0.31-0.56 wt.%)and clinopyroxene with low Na2 O(0.03-0.16 wt.%),Al2 O3(0.66-1.35 wt.%)and TiO2(0.04-0.10 wt.%)contents,point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subductionrelated melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic(-0.80<εNdi≤+13.32)and negatively correlate with Sr isotopes(0.70257≤87Sr/86Sr≤0.70770).Pb isotopes cover a wide range,trending from DMM toward enriched,sediment-like,compositions.We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.
文摘The New Caledonia Ophiolite(Peridotite Nappe), represents about one third of the island’s surface(i.e. 5 500 km2). The ophiolite is composed of harzburgites, dunites, lherzolites, minor mafic-ultramafic cumulates, and various dykes and sills. The mantle section underwent a polyphase evolution, which involved prominent depletion and re-fertilization. The oldest events are probably recorded by abyssal-type lherzolites of the northern massifs, which bear traces of moderate partial melting. Plagioclase lherzolites were formed by shallow entrapment of highly depleted MORB melt in residual spinel lherzolites. Nd isotope compositions are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion. This evolution was most likely accomplished during the late Cretaceous breakup of the eastern Australian margin. The harzburgite-dunite association, which forms the bulk of Peridotite Nappe was probably formed through a multistage magma-producing process. Harzburgites composition may have be obtained by a first phase of ~15% dry fractional melting, followed by 15%–18% hydrous melting in a supra-subduction zone setting. Variable εNd negatively correlate with 87Sr/86Sr, while Pb isotopes cover a wide range, trending from depleted mantle towards enriched, sediment-like, compositions. Such signatures likely reflect the evolution of a highly depleted forearc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction. The harzburgite-dunite set is overlain by a dunite transition zone ~300 m thick, in turn discontinuously covered by cumulate lenses consisting of layered pyroxenites, dunites, and wherlites at the base and gabbronorites/websterites on top. The mafic cumulates crystallized from primitive, ultra-depleted melts in the nascent lower fore-arc crust. In particular, FME enrichments and Nd-Pb isotopes support an origin from a refractory mantle source modified by slab fluids for the gabbronorite-forming melts. The Peridotite Nappe has been extensively serpentinized(40% to 100%) with extremely scarce occurrences of unserpentinized rocks. Lizardite, brucite, magnetite and minor chrysotile developed from joints and intra-granular cooling cracks in a near-static environment. Serpentine-coated joints and peridotite foliation have been thereafter reopened and injected by various felsic, mafic and ultramafic supra-subduction melts emplaced within a narrow time interval(55–50 Ma), immediately after subduction inception at 56 Ma, i.e. the age of granulite-facies metamorphic sole. The youngest magmatic event is represented by island-arc tholeiite dykes dated at 50 Ma. A widespread set of antigorite and tremolite-bearing veins crosscut all previous structures in a progressively cooling forearc environment. The former are synkinematic crack seals, which display highly radiogenic, sediment-like 87Sr/86Sr ratios suggesting direct derivation of fluids from the subduction zone, while the latter bear mantle-like isotopic signatures and probably originated from the interaction of wall rocks with Ca-rich fluids released by Eocene dykes or fluids that leached them. Finally, continental subduction and obduction occurred during the 44–34 Ma interval and were accompanied by the development of the HP-LT metamorphic belt of northern New Caledonia, which constrains the polarity of subduction.
基金supported by National Basic Research Program of China (Grant No. 2007CB411301)the Bureau of China Geological Survey (Grant No. 1212010611806)ISTO
文摘The Chinese Tianshan belt is a major part of the southern Central Asian Orogenic Belt, extending westward to Kyrgyzstan and Kazakhstan. Its Paleozoic tectonic evolution, crucial for understanding the amalgamation of Central Asia, comprises two stages of subduction-collision. The first collisional stage built the Eo-Tianshan Mountains, before a Visean unconformity, in which all structures are verging north. It implied a southward subduction of the Central Tianshan Ocean beneath the Tarim active margin, that induced the Ordovician-Early Devonian Central Tianshan arc, to the south of which the South Tianshan back-arc basin opened. During the Late Devonian, the closure of this ocean led to a collision between Central Tianshan arc and the Kazakhstan-Yili-North Tianshan Block, and subsequently closure of the South Tianhan back-arc basin, producing two suture zones, namely the Central Tianshan and South Tianshan suture zones where ophiolitic melanges and HP metamorphic rocks were emplaced northward. The second stage included the Late Devonian-Carboniferous southward subduction of North Tianshan Ocean beneath the Eo-Tianshan active margin, underlined by the Yili-North Tianshan arc, leading to the collision between the Kazakhstan-Yili-NTS plate and an inferred Junggar Block at Late Carboniferous-Early Permian time. The North Tianshan Suture Zone underlines likely the last oceanic closure of Central Asia Orogenic Belt; all the oceanic domains were consumed before the Middle Permian. The amalgamated units were affected by a Permian major wrenching, dextral in the Tianshan. The correlation with the Kazakh and Kyrgyz Tianshan is clarified. The Kyrgyz South Tianshan is equivalent to the whole part of Chinese Tianshan （CTS and STS） located to the south of Narat Fault and Main Tianshan Shear Zone; the so-called Middle Tianshan thins out toward the east. The South Tianshan Suture of Kyrgyzstan correlates with the Central Tianshan Suture of Chinese Tianshan. The evolution of this southern domain remains similar from east （Gangou area） to west until the Talas-Ferghana Fault, which reflects the convergence history between the Kazakhstan and Tarim blocks.
基金the National Basic Research Program of China(Grant Nos.2009CB825008&2007CB411301)Chinese National S&T Major Project(Grant No.2008ZX05008)+1 种基金the project‘‘Paleomagnetic study on the tectonic and paleogeographic evolution of northwest of China’’funded by SINOPECco-sponsored by the National Natural Science Foundation of China(Grant Nos.40821002&40802043)
文摘The Altaid tectonic collage extends over Central Asia, exposing numerous accretionary orogens that can account for the Palaeozoic continental crust growth. A pluridisciplinary approach, using geochronological, geochemical, structural and palaeomagnetic tools was carried out to unravel the architecture and the evolution of West Junggar （Northwestern China）, a segment of the Altaid Collage. A polycyclic geodynamic evolution is inferred and includes： （1） an Early Palaeozoic cycle, characterized by the closure of two oceanic basins bounded by island-arc systems; （2） an Early Devonian subduction jamming resulting in a minor-scale collision documented by thrusting, syntectonic sedimentation and subsequent crutal thinning associ- ated with alkaline magmatism; （3） a Late Palaeozoic cycle, driven by the evolution of two opposite subduction zones devel- oped upon the Early Palaeozoic basement. Detailed structural analysis and paleomagnetic data provide constraints for the late evolution of Junggar in the frame of the development of the Late Palaeozoic Kazakh orocline, which led to oblique subduction and transpression in the West Junggar accretionary complex. Progressive buckling of the Kazakh orocline further resulted in Late Carboniferous to Permian wrench tectonics, and lateral displacement of lithotectonic units. Block rotations that continued after the Late Triassic are due to diachronous intraplate reactivation. This scenario mirrors the Palaeozoic geodynamics of the Altaid Collage. Multiple Early Palaeozoic collisions of intra-oceanic arcs and micro continents have contributed to the formarion of the Kazakhstan Microconrinent. Since the Late Palaeozoic, subductions formed around this microcontinent and the final oblique closure of oceanic domains resulted in the transcurrent collage of Tarim and Siberia cratons. Palaeozoic strike-slip faults were later reactivated during Mesozoic intracontinental tectonics.
