2019 Vol. 10, No. 6
2019, 10(6): 1981-1992.
doi: 10.1016/j.gsf.2017.12.006
Abstract:
Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at 633.2±9.0 Ma (2σ). These magmas intrude the forearc Ad Dawadimi Basin, which is composed of metapelitic schists and greywacke along with ophiolitic mélanges 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 post-collisional A-type magmatism.
Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at 633.2±9.0 Ma (2σ). These magmas intrude the forearc Ad Dawadimi Basin, which is composed of metapelitic schists and greywacke along with ophiolitic mélanges 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 post-collisional A-type magmatism.
2019, 10(6): 1993-2006.
doi: 10.1016/j.gsf.2018.05.010
Abstract:
We present new U-Pb and Hf zircon isotope data on a suite of granitoids from a hitherto unstudied area of the Palaeoproterozoic Lupa terrane of the Ubendian belt in SW Tanzania. The major part of the area comprises a sequence of upper amphibolite grade paragneisses and migmatites. Subsequently, the field relations show a complex history of granitoid emplacement from early pre-tectonic strongly foliated diorite-tonalite-granodiorite orthogneiss to late tectonic, weakly deformed ellipsoidal granitoid plutons. These were followed by emplacement of undeformed K-feldspar rich porphyritic alkali granite. U-Pb zircon data on the granitoids show their emplacement ages span the period ~1925 Ma to 1890 Ma and constrain the culmination of the Ubendian orogeny in the Lupa terrane to about 1900 Ma. The undeformed K-feldspar rich Chimala granite, previously considered to be unconformably overlain by the Buanji Group volcano-sedimentary rocks (recently shown to be ~1675 Ma, not Neoproterozoic as originally thought) was dated at ~1407 Ma showing that it must intrude the sedimentary rocks, although the age relationships are not exposed in the mapping area. This is the first record of early Irumide/Kibaran-aged post-tectonic granites in this part of East Africa. Hf isotope data from the zircons show that the granitoids have a long crustal pre-history stretching back to as far as ~3.9 Ga, and showing that the Lupa terrane may form part of the "metacratonic" margin of an originally much more extensive Tanzania Craton.
We present new U-Pb and Hf zircon isotope data on a suite of granitoids from a hitherto unstudied area of the Palaeoproterozoic Lupa terrane of the Ubendian belt in SW Tanzania. The major part of the area comprises a sequence of upper amphibolite grade paragneisses and migmatites. Subsequently, the field relations show a complex history of granitoid emplacement from early pre-tectonic strongly foliated diorite-tonalite-granodiorite orthogneiss to late tectonic, weakly deformed ellipsoidal granitoid plutons. These were followed by emplacement of undeformed K-feldspar rich porphyritic alkali granite. U-Pb zircon data on the granitoids show their emplacement ages span the period ~1925 Ma to 1890 Ma and constrain the culmination of the Ubendian orogeny in the Lupa terrane to about 1900 Ma. The undeformed K-feldspar rich Chimala granite, previously considered to be unconformably overlain by the Buanji Group volcano-sedimentary rocks (recently shown to be ~1675 Ma, not Neoproterozoic as originally thought) was dated at ~1407 Ma showing that it must intrude the sedimentary rocks, although the age relationships are not exposed in the mapping area. This is the first record of early Irumide/Kibaran-aged post-tectonic granites in this part of East Africa. Hf isotope data from the zircons show that the granitoids have a long crustal pre-history stretching back to as far as ~3.9 Ga, and showing that the Lupa terrane may form part of the "metacratonic" margin of an originally much more extensive Tanzania Craton.
2019, 10(6): 2007-2019.
doi: 10.1016/j.gsf.2018.05.014
Abstract:
Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P-T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO) indicates peak metamorphic conditions of 850-960℃ and 6.9-7.7 kbar, and retrograde P-T conditions of <740℃ and <4.8 kbar, that define a clockwise P-T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514±33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P-T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.
Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P-T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO) indicates peak metamorphic conditions of 850-960℃ and 6.9-7.7 kbar, and retrograde P-T conditions of <740℃ and <4.8 kbar, that define a clockwise P-T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514±33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P-T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.
2019, 10(6): 2021-2044.
doi: 10.1016/j.gsf.2018.05.019
Abstract:
The Zambezi Belt in southern Africa has been regarded as a part of the 570-530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts. Here, we report new petrological, geochemical, and zircon U-Pb geochronological data of various metamorphic rocks (felsic to mafic orthogneiss, pelitic schist, and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe, and evaluate the timing and P-T conditions of the collisional event as well as protolith formation. Geochemical data of felsic orthogneiss indicate within-plate granite signature, whereas those of mafic orthogneiss suggest MORB, ocean-island, or within-plate affinities. Metamorphic P-T estimates for orthogneisses indicate significant P-T variation within the study area (700-780℃/6.7-7.2 kbar to 800-875℃/10-11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks. Zircon cores from felsic orthogneisses yielded two magmatic ages:2655±21 Ma and 813±5 Ma, which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism. Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic (ca. 2700-750 Ma). The Neoarchean (ca. 2700-2630 Ma) and Paleoproterozoic (ca. 2200-1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe, respectively. The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso-to Neoproterozoic (ca. 1500-950 Ma) and early Neoproterozoic (ca. 900-750 Ma) detrital zircons, respectively. Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic (744-670 Ma), and subsequently underwent high-grade metamorphism at 557-555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian. In contrast, 670-627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports, but consistent with ~680-650 Ma metamorphic ages reported from different parts of the Kuunga Orogen, suggesting Cryogenian thermal events before the final collision.
The Zambezi Belt in southern Africa has been regarded as a part of the 570-530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts. Here, we report new petrological, geochemical, and zircon U-Pb geochronological data of various metamorphic rocks (felsic to mafic orthogneiss, pelitic schist, and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe, and evaluate the timing and P-T conditions of the collisional event as well as protolith formation. Geochemical data of felsic orthogneiss indicate within-plate granite signature, whereas those of mafic orthogneiss suggest MORB, ocean-island, or within-plate affinities. Metamorphic P-T estimates for orthogneisses indicate significant P-T variation within the study area (700-780℃/6.7-7.2 kbar to 800-875℃/10-11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks. Zircon cores from felsic orthogneisses yielded two magmatic ages:2655±21 Ma and 813±5 Ma, which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism. Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic (ca. 2700-750 Ma). The Neoarchean (ca. 2700-2630 Ma) and Paleoproterozoic (ca. 2200-1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe, respectively. The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso-to Neoproterozoic (ca. 1500-950 Ma) and early Neoproterozoic (ca. 900-750 Ma) detrital zircons, respectively. Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic (744-670 Ma), and subsequently underwent high-grade metamorphism at 557-555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian. In contrast, 670-627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports, but consistent with ~680-650 Ma metamorphic ages reported from different parts of the Kuunga Orogen, suggesting Cryogenian thermal events before the final collision.
2019, 10(6): 2045-2061.
doi: 10.1016/j.gsf.2018.07.005
Abstract:
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 tectono-thermal 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 evolved εHf(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 and εHf(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.
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 tectono-thermal 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 evolved εHf(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 and εHf(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.
2019, 10(6): 2063-2084.
doi: 10.1016/j.gsf.2018.07.007
Abstract:
The East African Orogen involves a collage of Proterozoic microcontinents and arc terranes that became wedged between older cratonic blocks during the assembly of Gondwana. The Ediacaran-Cambrian Ambalavao and Maevarano Suites in Madagascar were emplaced during the waning orogenic stages and consist of weakly deformed to undeformed plutonic rocks and dykes of mainly porphyritic granite but also gabbro, diorite and charnockite. U-Pb geochronological data date emplacement of the Ambalavao Suite to between ca. 580 Ma and 540 Ma and the Maevarano Suite to between ca. 537 Ma and 522 Ma. Major and trace element concentrations are consistent with emplacement in a syn-to post-collisional tectonic setting as A-type (anorogenic) suites. Oxygen (δ18O of 5.27‰-7.45‰) and hafnium (εHf(t) of -27.8 to -12.3) isotopic data from plutons in the Itremo and Antananarivo Domains are consistent with incorporation of an ancient crustal source. More primitive δ18O (5.27‰-5.32‰) and εHf(t) (+0.0 to +0.2) isotopic values recorded in samples collected from the Ikalamavony Domain demonstrate the isotopic variation of basement sources present in the Malagasy crust. The Hf isotopic composition of Malagasy zircon are unlike more juvenile Ediacaran-Cambrian zircon sources elsewhere in the East African Orogen and, as such, Madagascar represents a distinct and identifiable detrital zircon source region in Phanerozoic sedimentary provenance studies. Taken together, these data indicate that high-T crustal anatexis, crustal assimilation and interaction of crustal material with mantle-derived melts were the processes operating during magma emplacement. This magmatism was coeval with polyphase deformation throughout Madagascar during the amalgamation of Gondwana and magmatism is interpreted to reflect lithospheric delamination of an extensive orogenic plateau.
The East African Orogen involves a collage of Proterozoic microcontinents and arc terranes that became wedged between older cratonic blocks during the assembly of Gondwana. The Ediacaran-Cambrian Ambalavao and Maevarano Suites in Madagascar were emplaced during the waning orogenic stages and consist of weakly deformed to undeformed plutonic rocks and dykes of mainly porphyritic granite but also gabbro, diorite and charnockite. U-Pb geochronological data date emplacement of the Ambalavao Suite to between ca. 580 Ma and 540 Ma and the Maevarano Suite to between ca. 537 Ma and 522 Ma. Major and trace element concentrations are consistent with emplacement in a syn-to post-collisional tectonic setting as A-type (anorogenic) suites. Oxygen (δ18O of 5.27‰-7.45‰) and hafnium (εHf(t) of -27.8 to -12.3) isotopic data from plutons in the Itremo and Antananarivo Domains are consistent with incorporation of an ancient crustal source. More primitive δ18O (5.27‰-5.32‰) and εHf(t) (+0.0 to +0.2) isotopic values recorded in samples collected from the Ikalamavony Domain demonstrate the isotopic variation of basement sources present in the Malagasy crust. The Hf isotopic composition of Malagasy zircon are unlike more juvenile Ediacaran-Cambrian zircon sources elsewhere in the East African Orogen and, as such, Madagascar represents a distinct and identifiable detrital zircon source region in Phanerozoic sedimentary provenance studies. Taken together, these data indicate that high-T crustal anatexis, crustal assimilation and interaction of crustal material with mantle-derived melts were the processes operating during magma emplacement. This magmatism was coeval with polyphase deformation throughout Madagascar during the amalgamation of Gondwana and magmatism is interpreted to reflect lithospheric delamination of an extensive orogenic plateau.
2019, 10(6): 2093-2100.
doi: 10.1016/j.gsf.2019.01.007
Abstract:
In quartzo-feldspathic continental crust with moderate-to-high heat flow, seismic activity extends to depths of 10-20 km, bounded by isotherms in the 350-450℃ range. Fluid overpressuring above hydrostatic in seismogenic crust, is heterogeneous but tends to develop in the lower seismogenic zone (basal seismogenic zone reservoir=b.s.z. reservoir) where the transition between hydrostatically pressured and overpressured crust is likely an irregular, time-dependent, 3-D interface with overpressuring concentrated around active faults and their ductile shear zone roots.
The term Arterial Fault is applied to fault structures that root in portions of the crust where pore fluids are overpressured (i.e. at >hydrostatic pressure) and serve as feeders for such fluids and their contained solutes into overlying parts of the crust. While arterial flow may occur on any type of fault, it is most likely to be associated with reverse faults in areas of horizontal compression where fluid overpressuring is most easily sustained. Frictional stability and flow permeability of faults are both affected by the state of stress on the fault (shear stress, τ; normal stress, σn), the level of pore-fluid pressure, Pf, and episodes of fault slip, allowing for a complex interplay between fault movement and fluid flow. For seismically active faults the time dependence of permeability is critical, leading to fault-valve behaviour whereby overpressures accumulate at depth during interseismic intervals with fluid discharged along enhanced fault-fracture permeability following each rupture event. Patterns of mineralization also suggest that flow along faults is non-uniform, concentrating along tortuous pathways within the fault surface.
Equivalent hydrostatic head above ground level for near-lithostatic overpressures at depth (<1.65×depth of zone) provides a measure of arterial potential. Settings for arterial faults include fault systems developed in compacting sedimentary basins, faults penetrating zones of active plutonic intrusion that encounter overpressured fluids exsolved from magma, together with those derived from contact metamorphism of fluid-rich wallrocks, and/or from regional devolatilisation accompanying prograde metamorphism. Specially significant are active faults within accretionary prisms rooted into overpressured subduction interfaces, and steep reverse faults activated by high overpressures from b.s.z. reservoirs during compressional inversion.
In quartzo-feldspathic continental crust with moderate-to-high heat flow, seismic activity extends to depths of 10-20 km, bounded by isotherms in the 350-450℃ range. Fluid overpressuring above hydrostatic in seismogenic crust, is heterogeneous but tends to develop in the lower seismogenic zone (basal seismogenic zone reservoir=b.s.z. reservoir) where the transition between hydrostatically pressured and overpressured crust is likely an irregular, time-dependent, 3-D interface with overpressuring concentrated around active faults and their ductile shear zone roots.
The term Arterial Fault is applied to fault structures that root in portions of the crust where pore fluids are overpressured (i.e. at >hydrostatic pressure) and serve as feeders for such fluids and their contained solutes into overlying parts of the crust. While arterial flow may occur on any type of fault, it is most likely to be associated with reverse faults in areas of horizontal compression where fluid overpressuring is most easily sustained. Frictional stability and flow permeability of faults are both affected by the state of stress on the fault (shear stress, τ; normal stress, σn), the level of pore-fluid pressure, Pf, and episodes of fault slip, allowing for a complex interplay between fault movement and fluid flow. For seismically active faults the time dependence of permeability is critical, leading to fault-valve behaviour whereby overpressures accumulate at depth during interseismic intervals with fluid discharged along enhanced fault-fracture permeability following each rupture event. Patterns of mineralization also suggest that flow along faults is non-uniform, concentrating along tortuous pathways within the fault surface.
Equivalent hydrostatic head above ground level for near-lithostatic overpressures at depth (<1.65×depth of zone) provides a measure of arterial potential. Settings for arterial faults include fault systems developed in compacting sedimentary basins, faults penetrating zones of active plutonic intrusion that encounter overpressured fluids exsolved from magma, together with those derived from contact metamorphism of fluid-rich wallrocks, and/or from regional devolatilisation accompanying prograde metamorphism. Specially significant are active faults within accretionary prisms rooted into overpressured subduction interfaces, and steep reverse faults activated by high overpressures from b.s.z. reservoirs during compressional inversion.
2019, 10(6): 2101-2115.
doi: 10.1016/j.gsf.2019.02.005
Abstract:
Veins and dikes are often oriented subparallel to the axial surfaces of folds in the adjacent layered or foliated rocks. This implies an awkward situation, since veins would lay in planes close-to-parallel to the maximum stretching axis. A series of geometric models have been conceived in order to gain insight into the possible mechanisms for their formation. The models are based on the analysis of a varied selection of field structures and on the review of similar structures in the existing literature. A first categorization consists on distinguishing between axial-planar veins achieved by either progressive or polyphase deformation. Five models of axial-planar veins resulting from progressive deformation are described and discussed:(1) fold-related veins associated with the standard folding mechanisms, (2) fracture arrays localized along the short limbs of folds (asymmetric fold-related veins), (3) folds associated with rotation of extension veins (vein-related folds), (4) high strain and transposition of early veins, and (5) high strain and late veins parallel to axial planar foliations (axial planar foliation-related veins). The axial planar geometry is achieved through variable amounts of progressive rotational strain, except in model 5, in which the co-planarity is acquired at the time of vein intrusion. The possibility for axial-planar veins to have developed in two distinct phases in the context of polyphase or polyorogenic tectonics has also been explored and discussed. This study contributes to a better understanding of the intriguing interplays between deformation, metamorphic and magmatic processes in orogenic belts.
Veins and dikes are often oriented subparallel to the axial surfaces of folds in the adjacent layered or foliated rocks. This implies an awkward situation, since veins would lay in planes close-to-parallel to the maximum stretching axis. A series of geometric models have been conceived in order to gain insight into the possible mechanisms for their formation. The models are based on the analysis of a varied selection of field structures and on the review of similar structures in the existing literature. A first categorization consists on distinguishing between axial-planar veins achieved by either progressive or polyphase deformation. Five models of axial-planar veins resulting from progressive deformation are described and discussed:(1) fold-related veins associated with the standard folding mechanisms, (2) fracture arrays localized along the short limbs of folds (asymmetric fold-related veins), (3) folds associated with rotation of extension veins (vein-related folds), (4) high strain and transposition of early veins, and (5) high strain and late veins parallel to axial planar foliations (axial planar foliation-related veins). The axial planar geometry is achieved through variable amounts of progressive rotational strain, except in model 5, in which the co-planarity is acquired at the time of vein intrusion. The possibility for axial-planar veins to have developed in two distinct phases in the context of polyphase or polyorogenic tectonics has also been explored and discussed. This study contributes to a better understanding of the intriguing interplays between deformation, metamorphic and magmatic processes in orogenic belts.
2019, 10(6): 2117-2133.
doi: 10.1016/j.gsf.2019.02.006
Abstract:
In this work, we develop a multidisciplinary approach to investigate a geothermal system located at the volcanic arc of a subduction-related orogen and highlight the interplay between active tectonism, stress field and fluid migration. By using results of field investigations from the Tinguiririca geothermal field in the High Andes of Chile (35°S), empirical analysis, and numerical models of static stress variations, we proposed a geomechanical model for evaluating the distribution of hydrothermal manifestations in a seismically-active region. The present geomechanical model follows four major steps:(1) development of the 3D structural model of fault pattern; (2) estimation of the in-situ stress field; (3) calculation of the resolved-shear-to-normal-stress ratio (slip tendency) on each fault with varying geomechanical parameters (coefficient of friction, pore pressure and cohesion) as inputs; and (4) estimation of Coulomb static stress changes as a consequence of failure in a nearby fault. Through combination of all these analyses, we characterize in detail both the active deformation in the geothermal field and its relationship with hot fluid migration.
In this work, we develop a multidisciplinary approach to investigate a geothermal system located at the volcanic arc of a subduction-related orogen and highlight the interplay between active tectonism, stress field and fluid migration. By using results of field investigations from the Tinguiririca geothermal field in the High Andes of Chile (35°S), empirical analysis, and numerical models of static stress variations, we proposed a geomechanical model for evaluating the distribution of hydrothermal manifestations in a seismically-active region. The present geomechanical model follows four major steps:(1) development of the 3D structural model of fault pattern; (2) estimation of the in-situ stress field; (3) calculation of the resolved-shear-to-normal-stress ratio (slip tendency) on each fault with varying geomechanical parameters (coefficient of friction, pore pressure and cohesion) as inputs; and (4) estimation of Coulomb static stress changes as a consequence of failure in a nearby fault. Through combination of all these analyses, we characterize in detail both the active deformation in the geothermal field and its relationship with hot fluid migration.
2019, 10(6): 2135-2145.
doi: 10.1016/j.gsf.2019.05.008
Abstract:
In fractured reservoirs characterized by low matrix permeability, fracture networks control the main fluid flow paths. However, in layered reservoirs, the vertical extension of fractures is often restricted to single layers. In this study, we explored the effect of changing marl/shale thickness on fracture extension using comprehensive field data and numerical modeling.
The field data were sampled from coastal exposures of Liassic limestone-marl/shale alternations in Wales and Somerset (Bristol Channel Basin, UK). The vertical fracture traces of more than 4000 fractures were mapped in detail. Six sections were selected to represent a variety of layer thicknesses. Besides the field data also thin sections were analyzed. Numerical models of fracture extension in a two-layer limestone-marl system were based on field data and laboratory measurements of Young's moduli. The modeled principal stress magnitude σ3 along the lithological contact was used as an indication for fracture extension through marls. Field data exhibit good correlation (R2=0.76) between fracture extension and marl thickness, the thicker the marl layer the fewer fractures propagate through. The model results show that almost no tensile stress reaches the top of the marl layer when the marls are thicker than 30 cm. For marls that are less than 20 cm, the propagation of stress is more dependent on the stiffness of the marls. The higher the contrast between limestone and marl stiffness the lower the stress that is transmitted into the marl layer. In both model experiments and field data the critical marl thickness for fracture extension is ca. 15-20 cm.
This quantification of critical marl thicknesses can be used to improve predictions of fracture networks and permeability in layered rocks. Up-or downsampling methods often ignore spatially continuous impermeable layers with thicknesses that are under the detection limit of seismic data. However, ignoring these layers can lead to overestimates of the overall permeability. Therefore, the understanding of how fractures propagate and terminate through impermeable layers will help to improve the characterization of conventional reservoirs.
In fractured reservoirs characterized by low matrix permeability, fracture networks control the main fluid flow paths. However, in layered reservoirs, the vertical extension of fractures is often restricted to single layers. In this study, we explored the effect of changing marl/shale thickness on fracture extension using comprehensive field data and numerical modeling.
The field data were sampled from coastal exposures of Liassic limestone-marl/shale alternations in Wales and Somerset (Bristol Channel Basin, UK). The vertical fracture traces of more than 4000 fractures were mapped in detail. Six sections were selected to represent a variety of layer thicknesses. Besides the field data also thin sections were analyzed. Numerical models of fracture extension in a two-layer limestone-marl system were based on field data and laboratory measurements of Young's moduli. The modeled principal stress magnitude σ3 along the lithological contact was used as an indication for fracture extension through marls. Field data exhibit good correlation (R2=0.76) between fracture extension and marl thickness, the thicker the marl layer the fewer fractures propagate through. The model results show that almost no tensile stress reaches the top of the marl layer when the marls are thicker than 30 cm. For marls that are less than 20 cm, the propagation of stress is more dependent on the stiffness of the marls. The higher the contrast between limestone and marl stiffness the lower the stress that is transmitted into the marl layer. In both model experiments and field data the critical marl thickness for fracture extension is ca. 15-20 cm.
This quantification of critical marl thicknesses can be used to improve predictions of fracture networks and permeability in layered rocks. Up-or downsampling methods often ignore spatially continuous impermeable layers with thicknesses that are under the detection limit of seismic data. However, ignoring these layers can lead to overestimates of the overall permeability. Therefore, the understanding of how fractures propagate and terminate through impermeable layers will help to improve the characterization of conventional reservoirs.
2019, 10(6): 2147-2151.
doi: 10.1016/j.gsf.2019.06.001
Abstract:
The passage of our Solar System through the spiral arms has been implicated as a contributor to global environmental perturbations. The suggestion of a consistent structure within the arms, informed by density wave theory, raises the possibility of repeating patterns of events at each arm crossing. Here we test the hypothesis that the structure of the arms of our galaxy influences the stratigraphic record on Earth. We construct independent structural and temporal models and combine these to compare the timings of arm tracers, materials from the earliest Solar System and events on Earth, including the largest extinctions. We find that a recurring sequence of events across the four arms emerges with an average arm-passing time of 188 million years. We suggest that the multiple temporal overlaps of events across arms, and their alignment with arm tracers and the earliest Solar System, presents an opportunity for a greater understanding of both Earth-based phenomena and galactic structure.
The passage of our Solar System through the spiral arms has been implicated as a contributor to global environmental perturbations. The suggestion of a consistent structure within the arms, informed by density wave theory, raises the possibility of repeating patterns of events at each arm crossing. Here we test the hypothesis that the structure of the arms of our galaxy influences the stratigraphic record on Earth. We construct independent structural and temporal models and combine these to compare the timings of arm tracers, materials from the earliest Solar System and events on Earth, including the largest extinctions. We find that a recurring sequence of events across the four arms emerges with an average arm-passing time of 188 million years. We suggest that the multiple temporal overlaps of events across arms, and their alignment with arm tracers and the earliest Solar System, presents an opportunity for a greater understanding of both Earth-based phenomena and galactic structure.
2019, 10(6): 2153-2166.
doi: 10.1016/j.gsf.2019.05.005
Abstract:
The Junggar Alatau forms the northern extent of the Tian Shan within the Central Asian Orogenic Belt (CAOB) at the border of SE Kazakhstan and NW China. This study presents the Palaeozoic-Mesozoic post-collisional thermo-tectonic history of this frontier locality using an integrated approach based on three apatite geo-/thermochronometers:apatite U-Pb, fission track and (U-Th)/He. The apatite U-Pb dates record Carboniferous-Permian post-magmatic cooling ages for the sampled granitoids, reflecting the progressive closure of the Palaeo-Asian Ocean. The apatite fission track (AFT) data record (partial) preservation of the late Palaeozoic cooling ages, supplemented by limited evidence for Late Triassic (~230-210 Ma) cooling and a more prominent record of (late) Early Cretaceous (~150-110 Ma) cooling. The apatite (U-Th)/He age results are consistent with the (late) Early Cretaceous AFT data, revealing a period of fast cooling at that time in resulting thermal history models. This Cretaceous rapid cooling signal is only observed for samples taken along the major NW-SE orientated shear zone that dissects the study area (the Central Kazakhstan Fault Zone), while Permian and Triassic cooling signals are preserved in low-relief areas, distal to this structure. This distinct geographical trend with respect to the shear zone, suggests that fault reactivation triggered the Cretaceous rapid cooling, which can be linked to a phase of slab-rollback and associated extension in the distant Tethys Ocean. Similar conclusions were drawn for thermochronology studies along other major NW-SE orientated shear zones in the Central Asian Orogenic Belt, suggesting a regional phase of Cretaceous exhumation in response to fault reactivation at that time.
The Junggar Alatau forms the northern extent of the Tian Shan within the Central Asian Orogenic Belt (CAOB) at the border of SE Kazakhstan and NW China. This study presents the Palaeozoic-Mesozoic post-collisional thermo-tectonic history of this frontier locality using an integrated approach based on three apatite geo-/thermochronometers:apatite U-Pb, fission track and (U-Th)/He. The apatite U-Pb dates record Carboniferous-Permian post-magmatic cooling ages for the sampled granitoids, reflecting the progressive closure of the Palaeo-Asian Ocean. The apatite fission track (AFT) data record (partial) preservation of the late Palaeozoic cooling ages, supplemented by limited evidence for Late Triassic (~230-210 Ma) cooling and a more prominent record of (late) Early Cretaceous (~150-110 Ma) cooling. The apatite (U-Th)/He age results are consistent with the (late) Early Cretaceous AFT data, revealing a period of fast cooling at that time in resulting thermal history models. This Cretaceous rapid cooling signal is only observed for samples taken along the major NW-SE orientated shear zone that dissects the study area (the Central Kazakhstan Fault Zone), while Permian and Triassic cooling signals are preserved in low-relief areas, distal to this structure. This distinct geographical trend with respect to the shear zone, suggests that fault reactivation triggered the Cretaceous rapid cooling, which can be linked to a phase of slab-rollback and associated extension in the distant Tethys Ocean. Similar conclusions were drawn for thermochronology studies along other major NW-SE orientated shear zones in the Central Asian Orogenic Belt, suggesting a regional phase of Cretaceous exhumation in response to fault reactivation at that time.
2019, 10(6): 2167-2175.
doi: 10.1016/j.gsf.2019.03.009
Abstract:
The sub-watershed prioritization is the ranking of different areas of a river basin according to their need to proper planning and management of soil and water resources. Decision makers should optimally allocate the investments to critical sub-watersheds in an economically effective and technically efficient manner. Hence, this study aimed at developing a user-friendly geographic information system (GIS) tool, Sub-Watershed Prioritization Tool (SWPT), using the Python programming language to decrease any possible uncertainty. It used geospatial-statistical techniques for analyzing morphometric and topo-hydrological factors and automatically identifying critical and priority sub-watersheds. In order to assess the capability and reliability of the SWPT tool, it was successfully applied in a watershed in the Golestan Province, Northern Iran. Historical records of flood and landslide events indicated that the SWPT correctly recognized critical sub-watersheds. It provided a cost-effective approach for prioritization of sub-watersheds. Therefore, the SWPT is practically applicable and replicable to other regions where gauge data is not available for each sub-watershed.
The sub-watershed prioritization is the ranking of different areas of a river basin according to their need to proper planning and management of soil and water resources. Decision makers should optimally allocate the investments to critical sub-watersheds in an economically effective and technically efficient manner. Hence, this study aimed at developing a user-friendly geographic information system (GIS) tool, Sub-Watershed Prioritization Tool (SWPT), using the Python programming language to decrease any possible uncertainty. It used geospatial-statistical techniques for analyzing morphometric and topo-hydrological factors and automatically identifying critical and priority sub-watersheds. In order to assess the capability and reliability of the SWPT tool, it was successfully applied in a watershed in the Golestan Province, Northern Iran. Historical records of flood and landslide events indicated that the SWPT correctly recognized critical sub-watersheds. It provided a cost-effective approach for prioritization of sub-watersheds. Therefore, the SWPT is practically applicable and replicable to other regions where gauge data is not available for each sub-watershed.
2019, 10(6): 2177-2188.
doi: 10.1016/j.gsf.2019.05.001
Abstract:
Colloform pyrite with core-rim texture is commonly deposited in carbonate platforms associated with the sulfide ores such as the Caixiashan Pb-Zn deposit. However, the genesis of colloform pyrite in Pb-Zn deposits, its growth controls and their geological implication are insufficiently understood. Integration of in-situ trace element and SIMS sulfur isotopes has revealed geochemical variations among these pyrite layers. These colloform pyrite occur as residual phases of core-rim aggregates, the cores are made up of very fine-grained anhedral pyrite particles, with some rims being made up of fine-grained and poorly-crystallized pyrite, while the other rims were featured with euhedral cubic pyrite, which are cemented by fine-grained calcite and/or dolomite with minor quartz. Sulfur isotope analysis shows that some well-preserved rims have negative δ34S values (-28.12‰ to -0.49‰), whereas most of the cores and rims have positive δ34S values (>0 to +44.28‰; peak at +14.91‰). Integrating with the methane and sulfate were observed in previous fluid inclusion study, we suggest that the 34S depleted rims were initially formed by bacteria sulfate reduction (BSR), whereas the positive δ34S values were resulted from the sulfate reduction driven by anaerobic methane oxidation (AOM). The well-developed authigenic pyrite and calcite may also support the reaction of AOM. Combined with petrographic observations, trace element composition of the colloform pyrite reveals the incorporation and precipitation behavior of those high abundance elements in the pyrite:Pb and Zn were present as mineral inclusion and likely precipitated before Fe, as supported by the time-resolved Pb-Zn signal spikes in most of the analyzed pyrite grains. Other metals, such as Hg, Co and Ni, may have migrated as chloride complexes and entered the pyrite lattice. Arsenic and Sb, generally influenced by complex-forming reactions rather than substitution ones, could also enter the pyrite lattice, or slightly predate the precipitation of colloform pyrite as mineral inclusions, which are controlled by their hydrolysis constant in the ore fluids. The colloform pyrite may have grown inward from the rims. The successive BSR reaction process would enrich H232S in the overlying water column but reduce the metal content, the nucleation of these pyrite rims was featured by strongly negative sulfur isotopes. The following AOM process should be activated by deformation like the turbidity sediment of the mudstone as the sulfide deposition are associated with fault activities that caused the emission of methane migration upward and simultaneously replenishing the metal in the column. The higher AOM reaction rate and the higher metal supply (not only Fe, but with minor other metals such as Pb and Zn) caused by sediment movement enhanced the metal concentration within the pyrite lattice.
Colloform pyrite with core-rim texture is commonly deposited in carbonate platforms associated with the sulfide ores such as the Caixiashan Pb-Zn deposit. However, the genesis of colloform pyrite in Pb-Zn deposits, its growth controls and their geological implication are insufficiently understood. Integration of in-situ trace element and SIMS sulfur isotopes has revealed geochemical variations among these pyrite layers. These colloform pyrite occur as residual phases of core-rim aggregates, the cores are made up of very fine-grained anhedral pyrite particles, with some rims being made up of fine-grained and poorly-crystallized pyrite, while the other rims were featured with euhedral cubic pyrite, which are cemented by fine-grained calcite and/or dolomite with minor quartz. Sulfur isotope analysis shows that some well-preserved rims have negative δ34S values (-28.12‰ to -0.49‰), whereas most of the cores and rims have positive δ34S values (>0 to +44.28‰; peak at +14.91‰). Integrating with the methane and sulfate were observed in previous fluid inclusion study, we suggest that the 34S depleted rims were initially formed by bacteria sulfate reduction (BSR), whereas the positive δ34S values were resulted from the sulfate reduction driven by anaerobic methane oxidation (AOM). The well-developed authigenic pyrite and calcite may also support the reaction of AOM. Combined with petrographic observations, trace element composition of the colloform pyrite reveals the incorporation and precipitation behavior of those high abundance elements in the pyrite:Pb and Zn were present as mineral inclusion and likely precipitated before Fe, as supported by the time-resolved Pb-Zn signal spikes in most of the analyzed pyrite grains. Other metals, such as Hg, Co and Ni, may have migrated as chloride complexes and entered the pyrite lattice. Arsenic and Sb, generally influenced by complex-forming reactions rather than substitution ones, could also enter the pyrite lattice, or slightly predate the precipitation of colloform pyrite as mineral inclusions, which are controlled by their hydrolysis constant in the ore fluids. The colloform pyrite may have grown inward from the rims. The successive BSR reaction process would enrich H232S in the overlying water column but reduce the metal content, the nucleation of these pyrite rims was featured by strongly negative sulfur isotopes. The following AOM process should be activated by deformation like the turbidity sediment of the mudstone as the sulfide deposition are associated with fault activities that caused the emission of methane migration upward and simultaneously replenishing the metal in the column. The higher AOM reaction rate and the higher metal supply (not only Fe, but with minor other metals such as Pb and Zn) caused by sediment movement enhanced the metal concentration within the pyrite lattice.
2019, 10(6): 2189-2202.
doi: 10.1016/j.gsf.2019.02.007
Abstract:
Apatite fission-track analysis and thermochronologic statistical modeling of Precambrian-Oligocene plutonic and metamorphic rocks from the Lesser Caucasus resolve two discrete cooling episodes. Cooling occurred during incremental crustal shortening due to obduction and continental accretion along the margins of the northern branch of the Neotethys. (1) The thermochronometric record of a Late Cretaceous (Turonian-Maastrichtian) cooling/exhumation event, coeval to widespread ophiolite obduction, is still present only in a relatively small area of the upper plate of the Amasia-Sevan-Akera (ASA) suture zone, i.e. the suture marking the final closure of the northern Neotethys during the Paleogene. Such area has not been affected by significant later exhumation. (2) Rapid cooling/exhumation occurred in the Early-Middle Miocene in both the lower and upper plates of the ASA suture zone, obscuring previous thermochronologic signatures over most of the study area. Miocene contractional reactivation of the ASA suture zone occurred contemporaneously with the main phase of shortening and exhumation along the Bitlis suture zone marking the closure of the southern branch of the Neotethys and the ensuing Arabia-Eurasia collision. Miocene collisional stress from the Bitlis suture zone was transmitted northward across the Anatolian hinterland, which was left relatively undeformed, and focused along preexisting structural discontinuities such as the eastern Pontides and the ASA suture zone.
Apatite fission-track analysis and thermochronologic statistical modeling of Precambrian-Oligocene plutonic and metamorphic rocks from the Lesser Caucasus resolve two discrete cooling episodes. Cooling occurred during incremental crustal shortening due to obduction and continental accretion along the margins of the northern branch of the Neotethys. (1) The thermochronometric record of a Late Cretaceous (Turonian-Maastrichtian) cooling/exhumation event, coeval to widespread ophiolite obduction, is still present only in a relatively small area of the upper plate of the Amasia-Sevan-Akera (ASA) suture zone, i.e. the suture marking the final closure of the northern Neotethys during the Paleogene. Such area has not been affected by significant later exhumation. (2) Rapid cooling/exhumation occurred in the Early-Middle Miocene in both the lower and upper plates of the ASA suture zone, obscuring previous thermochronologic signatures over most of the study area. Miocene contractional reactivation of the ASA suture zone occurred contemporaneously with the main phase of shortening and exhumation along the Bitlis suture zone marking the closure of the southern branch of the Neotethys and the ensuing Arabia-Eurasia collision. Miocene collisional stress from the Bitlis suture zone was transmitted northward across the Anatolian hinterland, which was left relatively undeformed, and focused along preexisting structural discontinuities such as the eastern Pontides and the ASA suture zone.
2019, 10(6): 2203-2218.
doi: 10.1016/j.gsf.2019.03.006
Abstract:
Shallow-platform settings with marked differences in paleoplatform bottom physiography influence the degree of connection with oceanic waters and overall circulation patterns, even when sharing the same palaeoclimatic conditions. Two Kimmeridgian shallow-marine settings have been explored to test the sensitivity and reliability of carbonate chemostratigraphy to detect such differences. An integrated overview of the obtained elemental trends depicted four major facies, shared along specific stratigraphic intervals of both depositional records. Diagenesis obliterated original geochemical signals only throughout the siliciclastics-rich interval, corresponding to the most landward setting. For the remaining facies, elemental features could be attributed to the differential action of forcing mechanisms operating along the south-Iberian paleomargin during Kimmeridgian times. The highest degree of continental influence can be recognized by a strong relationship between Fe and Mn for the most proximal setting, which fades out along the mixed carbonate-fine siliciclastic rhythmic deposition in more open settings. A characteristic geochemical signature of progressively more positive δ13C values and significantly higher Sr content is identified for the interval dominated by biogenic sponge buildups. Such a local response is related to local forcing by upwelling in the surroundings of a coral fringe. The geochemical signature of a hydrothermal origin can be clearly differentiated from the influence of mere terrigenous pulses. Accordingly, the decoupling of Fe and Mn along marginal settings is the clue to detecting major events of palaeogeographic restructuring. Observed temporal variations in Mg content along both studied sections are attributed to tectonic activity influencing nearshore/coastal water masses. By integrating chemostratigraphic information and complementary evidence, the palaeoenvironmental mechanisms promoting differentiated sedimentary records along ancient subtropical, shallow, coastal settings can be disentangled.
Shallow-platform settings with marked differences in paleoplatform bottom physiography influence the degree of connection with oceanic waters and overall circulation patterns, even when sharing the same palaeoclimatic conditions. Two Kimmeridgian shallow-marine settings have been explored to test the sensitivity and reliability of carbonate chemostratigraphy to detect such differences. An integrated overview of the obtained elemental trends depicted four major facies, shared along specific stratigraphic intervals of both depositional records. Diagenesis obliterated original geochemical signals only throughout the siliciclastics-rich interval, corresponding to the most landward setting. For the remaining facies, elemental features could be attributed to the differential action of forcing mechanisms operating along the south-Iberian paleomargin during Kimmeridgian times. The highest degree of continental influence can be recognized by a strong relationship between Fe and Mn for the most proximal setting, which fades out along the mixed carbonate-fine siliciclastic rhythmic deposition in more open settings. A characteristic geochemical signature of progressively more positive δ13C values and significantly higher Sr content is identified for the interval dominated by biogenic sponge buildups. Such a local response is related to local forcing by upwelling in the surroundings of a coral fringe. The geochemical signature of a hydrothermal origin can be clearly differentiated from the influence of mere terrigenous pulses. Accordingly, the decoupling of Fe and Mn along marginal settings is the clue to detecting major events of palaeogeographic restructuring. Observed temporal variations in Mg content along both studied sections are attributed to tectonic activity influencing nearshore/coastal water masses. By integrating chemostratigraphic information and complementary evidence, the palaeoenvironmental mechanisms promoting differentiated sedimentary records along ancient subtropical, shallow, coastal settings can be disentangled.
2019, 10(6): 2219-2238.
doi: 10.1016/j.gsf.2019.03.008
Abstract:
The fluvial-tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FTT generally consists of three broad zones:(1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial-tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FTT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley ("A" Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial-tidal channels.
Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north.
Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the "A" Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FTT zone appears to have extended for several hundred kilometers from south to north.
Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems.
The fluvial-tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FTT generally consists of three broad zones:(1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial-tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FTT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley ("A" Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial-tidal channels.
Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north.
Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the "A" Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FTT zone appears to have extended for several hundred kilometers from south to north.
Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems.
2019, 10(6): 2239-2249.
doi: 10.1016/j.gsf.2019.04.001
Abstract:
Magmatic pulses in intraplate sedimentary basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin. The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe. Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation, Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma (Late Carboniferous) and 227 Ma (Triassic). Further, these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian, Panjal Traps, Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature. During the Phanerozoic Eon, the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.
Magmatic pulses in intraplate sedimentary basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin. The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe. Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation, Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma (Late Carboniferous) and 227 Ma (Triassic). Further, these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian, Panjal Traps, Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature. During the Phanerozoic Eon, the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.
2019, 10(6): 2251-2263.
doi: 10.1016/j.gsf.2019.03.007
Abstract:
In the northeastern (NE) Arabian Sea, the fluctuation in terrestrial and freshwater runoff directly depends on southwest monsoon (SWM) precipitation as well as the meltwater flux provided by Indus River. Therefore, analysis of multi-proxy spectral signatures was carried out to trace the high-resolution SWM periodicities and their influence on the productivity, regional sea level fluctuations and depositional processes in the NE Arabian Sea. The time series data of stable isotopes of oxygen (δ18OG.ruber), carbon (δ13CG.ruber and δ13Corg) and nitrogen (δ15N), Total Organic Carbon (TOC), planktic-benthic foraminiferal ratio (P/B ratio) and >63 μm coarse fraction (CF) were used from two coastal sedimentary cores located offshore Saurashtra, NE Arabian Sea (Core SK-240/485 having 88 m water depth; Core GC/SK-240/496 having 174 m water depth). The REDFIT based spectral analysis recorded significant periodicities (>90% significance) in δ18O time series centered at ~1609, ~667, ~525, ~296, ~290 and ~256 years. Further, the significant periodicities recorded in carbon isotopes time series (δ13CG.ruber and δ13Corg) centered at ~681, ~512, ~471, ~452, ~438, ~360, ~292, ~275, ~269, ~245 and ~209 years. The significant periodicities in TOC include ~471 and ~322 years whereas δ15N time series recorded significant periodicity centered at ~360 years. The significant periodicities in P/B ratio time series centered at ~512, ~388, ~304, ~250, ~235, ~217, ~152, ~139 and ~135 years while CF recorded ~268, ~216, ~209, ~198, ~188, ~173 and ~140 years significant periodicities. The observed periodicities in the multi-proxy record consist of similar cycles (within the radiocarbon dating error) which also match with previously reported solar insolation influenced SWM and other global and regional cycles. Further, the stationarity of the data has been verified using wavelet analysis and shows similar periodicities as observed in REDFIT analysis. Thereafter, the depositional behaviour was studied using correlation analysis of the common periods of δ18O time series of both the cores. The result suggests that the depositional behaviour was different for both the core sites during the early Holocene and became similar during the middle Holocene. The correlation analysis of Total Solar Index (TSI) with δ18O time series reveals a significant correlation with the core SK-240/485 whereas an insignificant correlation with the core GC/SK-240/496. These observations suggest that the solar insolation has been a leading factor responsible for the SWM trends during the Holocene which may have further influenced the productivity, regional sea level fluctuations and depositional conditions in the NE Arabian Sea. However, these trends are better preserved in shallow marine sediments as compared to the deeper marine sediments.
In the northeastern (NE) Arabian Sea, the fluctuation in terrestrial and freshwater runoff directly depends on southwest monsoon (SWM) precipitation as well as the meltwater flux provided by Indus River. Therefore, analysis of multi-proxy spectral signatures was carried out to trace the high-resolution SWM periodicities and their influence on the productivity, regional sea level fluctuations and depositional processes in the NE Arabian Sea. The time series data of stable isotopes of oxygen (δ18OG.ruber), carbon (δ13CG.ruber and δ13Corg) and nitrogen (δ15N), Total Organic Carbon (TOC), planktic-benthic foraminiferal ratio (P/B ratio) and >63 μm coarse fraction (CF) were used from two coastal sedimentary cores located offshore Saurashtra, NE Arabian Sea (Core SK-240/485 having 88 m water depth; Core GC/SK-240/496 having 174 m water depth). The REDFIT based spectral analysis recorded significant periodicities (>90% significance) in δ18O time series centered at ~1609, ~667, ~525, ~296, ~290 and ~256 years. Further, the significant periodicities recorded in carbon isotopes time series (δ13CG.ruber and δ13Corg) centered at ~681, ~512, ~471, ~452, ~438, ~360, ~292, ~275, ~269, ~245 and ~209 years. The significant periodicities in TOC include ~471 and ~322 years whereas δ15N time series recorded significant periodicity centered at ~360 years. The significant periodicities in P/B ratio time series centered at ~512, ~388, ~304, ~250, ~235, ~217, ~152, ~139 and ~135 years while CF recorded ~268, ~216, ~209, ~198, ~188, ~173 and ~140 years significant periodicities. The observed periodicities in the multi-proxy record consist of similar cycles (within the radiocarbon dating error) which also match with previously reported solar insolation influenced SWM and other global and regional cycles. Further, the stationarity of the data has been verified using wavelet analysis and shows similar periodicities as observed in REDFIT analysis. Thereafter, the depositional behaviour was studied using correlation analysis of the common periods of δ18O time series of both the cores. The result suggests that the depositional behaviour was different for both the core sites during the early Holocene and became similar during the middle Holocene. The correlation analysis of Total Solar Index (TSI) with δ18O time series reveals a significant correlation with the core SK-240/485 whereas an insignificant correlation with the core GC/SK-240/496. These observations suggest that the solar insolation has been a leading factor responsible for the SWM trends during the Holocene which may have further influenced the productivity, regional sea level fluctuations and depositional conditions in the NE Arabian Sea. However, these trends are better preserved in shallow marine sediments as compared to the deeper marine sediments.
2019, 10(6): 2265-2280.
doi: 10.1016/j.gsf.2019.04.003
Abstract:
We carried out SHRIMP zircon U-Pb dating on A-type granitic intrusions from the Namaqua-Natal Province, South Africa, Sverdrupfjella, western Dronning Maud Land, Antarctica and the Nampula Province of northern Mozambique. Zircon grains in these granitic rocks are typically elongated and oscillatory zoned, suggesting magmatic origins. Zircons from the granitoid intrusions analyzed in this study suggest ~1025-1100 Ma ages, which confirm widespread Mesoproterozoic A-type granitic magmatism in the Namaqua-Natal (South Africa), Maud (Antarctica) and Mozambique metamorphic terrains. No older inherited (e.g., ~2500 Ma Achean basement or ~1200 Ma island arc magmatism in northern Natal) zircon grains were seen. Four plutons from the Natal Belt (Mvoti Pluton, Glendale Pluton, Kwalembe Pluton, Ntimbankulu Pluton) display 1050-1040 Ma ages, whereas the Nthlimbitwa Pluton in northern Natal indicates older 1090-1080 Ma ages. A sample from Sverdrupfjella, Antarctica has ~1091 Ma old zircons along with ~530 Ma metamorphic rims. Similarly, four samples analysed from the Nampula Province of Mozambique suggest crystallization ages of ~1060-1090 Ma but also show significant discordance with two samples showing younger ~550 Ma overgrowths. None of the Natal samples show any younger overgrowths. A single sample from southwestern Namaqualand yielded an age of ~1033 Ma.
Currently available chronological data suggest magmatism took place in the Namaqua-Natal-Maud-Mozambique (NNMM) belt between ~1025 Ma and ~1100 Ma with two broad phases between ~1060-1020 Ma and 1100-1070 Ma respectively, with peaks at between ~1030-1040 Ma and ~1070-1090 Ma. The age data from the granitic intrusions from Namaqualand, combined with those from Natal, Antarctica and Mozambique suggest a crude spatial-age relationship with the older >1070 Ma ages being largely restricted close to the eastern and western margins of the Kalahari Craton in northern Natal, Mozambique, Namaqualand and WDML Antarctica whereas the younger <1060 Ma ages dominate in southern Natal and western Namaqualand and are largely restricted to the southern and possibly the western margins of the Kalahari Craton. The older ages of magmatism partially overlap with or are marginally younger than the intracratonic Mkondo Large Igneous Province intruded into or extruded onto the Kalahari Craton, suggesting a tectonic relationship with the Maud Belt. Similar ages from granitic augen gneisses in Sri Lanka suggest a continuous belt stretching from Namaqualand to Sri Lanka in a reconstituted Gondwana, formed during the terminal stages of amalgamation of Rodinia and predating the East African Orogen. This contiguity contributes to defining the extent of Rodinia-age crustal blocks, subsequently fragmented by the dispersal of Rodinia and Gondwana.
We carried out SHRIMP zircon U-Pb dating on A-type granitic intrusions from the Namaqua-Natal Province, South Africa, Sverdrupfjella, western Dronning Maud Land, Antarctica and the Nampula Province of northern Mozambique. Zircon grains in these granitic rocks are typically elongated and oscillatory zoned, suggesting magmatic origins. Zircons from the granitoid intrusions analyzed in this study suggest ~1025-1100 Ma ages, which confirm widespread Mesoproterozoic A-type granitic magmatism in the Namaqua-Natal (South Africa), Maud (Antarctica) and Mozambique metamorphic terrains. No older inherited (e.g., ~2500 Ma Achean basement or ~1200 Ma island arc magmatism in northern Natal) zircon grains were seen. Four plutons from the Natal Belt (Mvoti Pluton, Glendale Pluton, Kwalembe Pluton, Ntimbankulu Pluton) display 1050-1040 Ma ages, whereas the Nthlimbitwa Pluton in northern Natal indicates older 1090-1080 Ma ages. A sample from Sverdrupfjella, Antarctica has ~1091 Ma old zircons along with ~530 Ma metamorphic rims. Similarly, four samples analysed from the Nampula Province of Mozambique suggest crystallization ages of ~1060-1090 Ma but also show significant discordance with two samples showing younger ~550 Ma overgrowths. None of the Natal samples show any younger overgrowths. A single sample from southwestern Namaqualand yielded an age of ~1033 Ma.
Currently available chronological data suggest magmatism took place in the Namaqua-Natal-Maud-Mozambique (NNMM) belt between ~1025 Ma and ~1100 Ma with two broad phases between ~1060-1020 Ma and 1100-1070 Ma respectively, with peaks at between ~1030-1040 Ma and ~1070-1090 Ma. The age data from the granitic intrusions from Namaqualand, combined with those from Natal, Antarctica and Mozambique suggest a crude spatial-age relationship with the older >1070 Ma ages being largely restricted close to the eastern and western margins of the Kalahari Craton in northern Natal, Mozambique, Namaqualand and WDML Antarctica whereas the younger <1060 Ma ages dominate in southern Natal and western Namaqualand and are largely restricted to the southern and possibly the western margins of the Kalahari Craton. The older ages of magmatism partially overlap with or are marginally younger than the intracratonic Mkondo Large Igneous Province intruded into or extruded onto the Kalahari Craton, suggesting a tectonic relationship with the Maud Belt. Similar ages from granitic augen gneisses in Sri Lanka suggest a continuous belt stretching from Namaqualand to Sri Lanka in a reconstituted Gondwana, formed during the terminal stages of amalgamation of Rodinia and predating the East African Orogen. This contiguity contributes to defining the extent of Rodinia-age crustal blocks, subsequently fragmented by the dispersal of Rodinia and Gondwana.
2019, 10(6): 2281-2285.
doi: 10.1016/j.gsf.2019.04.005
Abstract:
A rare occurrence of a microspherule has been found in the infratrappean sediments, encountered below 338 m thick Deccan volcanic cover in KLR-1 scientific borehole, drilled in the epicentral zone of the 1993 Killari earthquake (Maharashtra, India). Palynological studies of the sediments indicate their age as Early Permian (Asselian, 298-295 Ma) for deposition. Transmission electron microscope studies reveal that the spherule from the infratrappeans, is having a similar composition to that of the Neoarchean amphibolite to granulite facies mid crustal basement. The spherule is non-spherical in nature, containing mostly FeO (10.70±0.20 wt.%), CaO (13.8±0.5 wt.%), Al2O3 (7.78±0.30 wt.%), MgO (6.47±0.3 wt.%), SiO2 (47.46±0.50 wt.%), TiO2 (2.47±0.3 wt.%), K2O (1.89±0.20 wt.%), and Cl (0.33±0.05 wt.%). Since the Fe composition of the spherule is almost same as the basement rock (10.5 wt.%), and the chlorine content is also in the same range as the basement (0.04-0.24 wt.%), it would suggest possibility of an extraterrestrial impact over the Indian terrain during the erstwhile Gondwana sedimentation period that may be associated with the Permian-Triassic mass extinction, the most severe one in the Earth's history.
A rare occurrence of a microspherule has been found in the infratrappean sediments, encountered below 338 m thick Deccan volcanic cover in KLR-1 scientific borehole, drilled in the epicentral zone of the 1993 Killari earthquake (Maharashtra, India). Palynological studies of the sediments indicate their age as Early Permian (Asselian, 298-295 Ma) for deposition. Transmission electron microscope studies reveal that the spherule from the infratrappeans, is having a similar composition to that of the Neoarchean amphibolite to granulite facies mid crustal basement. The spherule is non-spherical in nature, containing mostly FeO (10.70±0.20 wt.%), CaO (13.8±0.5 wt.%), Al2O3 (7.78±0.30 wt.%), MgO (6.47±0.3 wt.%), SiO2 (47.46±0.50 wt.%), TiO2 (2.47±0.3 wt.%), K2O (1.89±0.20 wt.%), and Cl (0.33±0.05 wt.%). Since the Fe composition of the spherule is almost same as the basement rock (10.5 wt.%), and the chlorine content is also in the same range as the basement (0.04-0.24 wt.%), it would suggest possibility of an extraterrestrial impact over the Indian terrain during the erstwhile Gondwana sedimentation period that may be associated with the Permian-Triassic mass extinction, the most severe one in the Earth's history.
2019, 10(6): 2287-2300.
doi: 10.1016/j.gsf.2019.04.007
Abstract:
The Sulu orogenic belt (SOB) separates the North and South China blocks in East Asia and formed during Triassic continent-continent collision. However, late Mesozoic post-collisional exhumation is poorly understood due to lack of surface evidence for Paleo-Pacific subduction and associated effects. This paper interprets the tectonic history of the SOB using detrital zircon age data from Early Cretaceous sedimentary units along with previously published geochronologic and geochemical data to reconstruct sedimentological and tectonic history. Detrital zircon age distributions obtained from sedimentary units include a 2.0 Ga subpopulation that appears only in turbidite units to the southeast. This sediment probably derived from the Yangtze Block. Terrestrial facies from the Jiao-Lai basin to the northwest appear to derive from the North China Block. Geochronologic and geochemical data indicate that Early Cretaceous, post-collisional volcanism was compositionally bimodal (mafic-felsic) with associated intrusive activity that peaked at 120 Ma. Seismic images of northerly regions of the study area indicate this occurred in an extensional setting. Sedimentary facies and field structural analyses revealed an unconformity interpreted to reflect rapid uplift with NW-SE compression to the south. Given observed sinistral movement along the Tan-Lu fault, we interpret northwest and southeast regions of the SOB as experiencing transtensional and transpressional tectonics, respectively, driven by continuous subduction of the Paleo-Pacific Plate. Intrusion of the Late Yanshannian granitoids marked the final formational stage of this unique tectonic setting.
The Sulu orogenic belt (SOB) separates the North and South China blocks in East Asia and formed during Triassic continent-continent collision. However, late Mesozoic post-collisional exhumation is poorly understood due to lack of surface evidence for Paleo-Pacific subduction and associated effects. This paper interprets the tectonic history of the SOB using detrital zircon age data from Early Cretaceous sedimentary units along with previously published geochronologic and geochemical data to reconstruct sedimentological and tectonic history. Detrital zircon age distributions obtained from sedimentary units include a 2.0 Ga subpopulation that appears only in turbidite units to the southeast. This sediment probably derived from the Yangtze Block. Terrestrial facies from the Jiao-Lai basin to the northwest appear to derive from the North China Block. Geochronologic and geochemical data indicate that Early Cretaceous, post-collisional volcanism was compositionally bimodal (mafic-felsic) with associated intrusive activity that peaked at 120 Ma. Seismic images of northerly regions of the study area indicate this occurred in an extensional setting. Sedimentary facies and field structural analyses revealed an unconformity interpreted to reflect rapid uplift with NW-SE compression to the south. Given observed sinistral movement along the Tan-Lu fault, we interpret northwest and southeast regions of the SOB as experiencing transtensional and transpressional tectonics, respectively, driven by continuous subduction of the Paleo-Pacific Plate. Intrusion of the Late Yanshannian granitoids marked the final formational stage of this unique tectonic setting.
2019, 10(6): 2301-2312.
doi: 10.1016/j.gsf.2019.04.008
Abstract:
The Tonggou Cu polymetallic deposit in the Bogda Orogenic Belt, Eastern Tianshan shows evidence for three stages of hydrothermal mineralization:early pyrite veins (Stage 1), polymetallic sulfide±epidote-quartz (Stage 2), and late-stage pyrite-calcite veins (Stage 3). Fluid inclusion petrography and microthermometry analyses indicate that the liquid-rich aqueous inclusions (L), vapour-rich aqueous inclusions (V), and NaCl daughter mineral-bearing three phase inclusions (S) formed during the main stage of mineralization, and that the ore fluids represent high-temperature and high-salinity H2O-NaCl hydrothermal fluids that underwent boiling. Stable isotope (H, O) data indicate that the ore fluids of the Tonggou deposit were originally derived from magmatic water in Stage 2 and subsequently mixed with local meteoric water during Stage 3. Sulphur isotope compositions (6.7‰ to 10.9‰) are consistent with the δ34S values of pyrite from the Qijiaojing Formation sandstone, indicating the primary source of the sulphur ore. Furthermore, chalcopyrite grains separated from the chalcopyrite-rich ore samples yield an isochron age of 303±12 Ma (MSWD=1.2). These results indicate that the Tonggou deposit is a transition between high-sulfidation and porphyry deposits which formed in the Late Carboniferous. It also suggests an increased likelihood for the occurrence of Cu (Au, Mo) in the Bogda Orogenic Belt, especially at locations where the Cu-Zn deposits are thicker; further deep drilling and exploration are encouraged in these areas.
The Tonggou Cu polymetallic deposit in the Bogda Orogenic Belt, Eastern Tianshan shows evidence for three stages of hydrothermal mineralization:early pyrite veins (Stage 1), polymetallic sulfide±epidote-quartz (Stage 2), and late-stage pyrite-calcite veins (Stage 3). Fluid inclusion petrography and microthermometry analyses indicate that the liquid-rich aqueous inclusions (L), vapour-rich aqueous inclusions (V), and NaCl daughter mineral-bearing three phase inclusions (S) formed during the main stage of mineralization, and that the ore fluids represent high-temperature and high-salinity H2O-NaCl hydrothermal fluids that underwent boiling. Stable isotope (H, O) data indicate that the ore fluids of the Tonggou deposit were originally derived from magmatic water in Stage 2 and subsequently mixed with local meteoric water during Stage 3. Sulphur isotope compositions (6.7‰ to 10.9‰) are consistent with the δ34S values of pyrite from the Qijiaojing Formation sandstone, indicating the primary source of the sulphur ore. Furthermore, chalcopyrite grains separated from the chalcopyrite-rich ore samples yield an isochron age of 303±12 Ma (MSWD=1.2). These results indicate that the Tonggou deposit is a transition between high-sulfidation and porphyry deposits which formed in the Late Carboniferous. It also suggests an increased likelihood for the occurrence of Cu (Au, Mo) in the Bogda Orogenic Belt, especially at locations where the Cu-Zn deposits are thicker; further deep drilling and exploration are encouraged in these areas.
2019, 10(6): 2313-2327.
doi: 10.1016/j.gsf.2019.05.003
Abstract:
As one of the pivotal Gondwana-derived blocks, the kinematic history of the northern Qiangtang Block (in the Tibetan Plateau) remains unclear, mainly because quantitative paleomagnetic data to determine the paleoposition are sparse. Thus, for this study, we collected 226 samples (17 sites) from Triassic sedimentary rocks in the Raggyorcaka and Tuotuohe areas of the northern Qiangtang Block (NQB). Stepwise demagnetization isolated high temperature/field components from the samples. Both Early and Late Triassic datasets passed field tests at a 99% confidence level and were proved to be primary origins. Paleopoles were calculated to be at 24.9°N and 216.5°E with A95=8.2°(N=8) for the Early Triassic dataset, and at 68.1°N, 179.9°E with A95=5.6° (N=37) for the Late Triassic, the latter being combined with a coeval volcanic dataset published previously. These paleopoles correspond to paleolatitudes of 14.3°S±8.2° and 29.9°N±5.6°, respectively. Combining previously published results, we reconstructed a three-stage northward drift process for the NQB. (1) The northern Qiangtang Block was located in the subtropical part of the southern hemisphere until the Early Triassic; (2) thereafter, the block rapidly drifted northward from southern to northern hemispheres during the Triassic; and (3) the block converged with the Eurasian continent in the Late Triassic. The ~4800 km northward movement from the Early to Late Triassic corresponded to an average motion rate of ~11.85 cm/yr. The rapid drift of the NQB after the Early Triassic led to a rapid transformation of the Tethys Ocean.
As one of the pivotal Gondwana-derived blocks, the kinematic history of the northern Qiangtang Block (in the Tibetan Plateau) remains unclear, mainly because quantitative paleomagnetic data to determine the paleoposition are sparse. Thus, for this study, we collected 226 samples (17 sites) from Triassic sedimentary rocks in the Raggyorcaka and Tuotuohe areas of the northern Qiangtang Block (NQB). Stepwise demagnetization isolated high temperature/field components from the samples. Both Early and Late Triassic datasets passed field tests at a 99% confidence level and were proved to be primary origins. Paleopoles were calculated to be at 24.9°N and 216.5°E with A95=8.2°(N=8) for the Early Triassic dataset, and at 68.1°N, 179.9°E with A95=5.6° (N=37) for the Late Triassic, the latter being combined with a coeval volcanic dataset published previously. These paleopoles correspond to paleolatitudes of 14.3°S±8.2° and 29.9°N±5.6°, respectively. Combining previously published results, we reconstructed a three-stage northward drift process for the NQB. (1) The northern Qiangtang Block was located in the subtropical part of the southern hemisphere until the Early Triassic; (2) thereafter, the block rapidly drifted northward from southern to northern hemispheres during the Triassic; and (3) the block converged with the Eurasian continent in the Late Triassic. The ~4800 km northward movement from the Early to Late Triassic corresponded to an average motion rate of ~11.85 cm/yr. The rapid drift of the NQB after the Early Triassic led to a rapid transformation of the Tethys Ocean.
2019, 10(6): 2085-2092.
doi: 10.1016/j.gsf.2019.07.001
Abstract:
2019, 10(6): 2329-2329.
doi: 10.1016/j.gsf.2019.06.002
Abstract:
