2019 Vol. 10, No. 4
2019, 10(4): 1223-1254.
doi: 10.1016/j.gsf.2019.02.003
Abstract:
Crustal recycling at convergent plate boundaries is essential to mantle heterogeneity. However, crustal signatures in the mantle source of basaltic rocks above subduction zones were primarily incorporated in the form of liquid rather than solid phases. The physicochemical property of liquid phases is determined by the dehydration behavior of crustal rocks at the slab-mantle interface in subduction channels. Because of the significant fractionation in incompatible trace elements but the full inheritance in radiogenic isotopes relative to their crustal sources, the production of liquid phases is crucial to the geochemical transfer from the subducting crust into the mantle. In this process, the stability of specific minerals in subducting crustal rocks exerts a primary control on the enrichment of given trace elements in the liquid phases. For this reason, geochemically enriched oceanic basalts can be categorized into two types in terms of their trace element distribution patterns in the primitive mantle-normalized diagram. One is island arc basalts (IAB), showing enrichment in LILE, Pb and LREE but depletion in HFSE such as Nb and Ta relative to HREE. The other is ocean island basalts (OIB), exhibiting enrichment in LILE and LREE, enrichment or non-depletion in HFSE but depletion in Pb relative to HREE. In either types, these basalts show the enhanced enrichment of LILE and LREE with increasing their incompatibility relative to normal mid-ocean ridge basalts (MORB).
The thermal regime of subduction zones can be categorized into two stages in both time and space. The first stage is characterized by compressional tectonism at low thermal gradients. As a consequence, metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile, resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE, Pb and LREE but depleted in HFSE and HREE relative to normal MORB. This provides the crustal signature for the mantle sources of IAB. The second stage is indicated by extensional tectonism at high thermal gradients, leading to the partial melting of metamorphically dehydrated crustal rocks at subarc to postarc depths. This involves not only the breakdown of hydrous minerals such as amphibole, phengite and allanite in the stability field of garnet but also the dissolution of rutile into hydrous melts. As such, the hydrous melts can acquire the trace element composition that is significantly enriched in LILE, HFSE and LREE but depleted in Pb and HREE relative to normal MORB, providing the crustal signature for the mantle sources of OIB. In either case, these liquid phases would metasomatize the overlying mantle wedge peridotite at different depths, generating ultramafic metasomatites such as serpentinized and chloritized peridotites, and olivine-poor pyroxenites and hornblendites. As a consequence, the crustal signatures are transferred by the liquid phases from the subducting slab into the mantle.
Crustal recycling at convergent plate boundaries is essential to mantle heterogeneity. However, crustal signatures in the mantle source of basaltic rocks above subduction zones were primarily incorporated in the form of liquid rather than solid phases. The physicochemical property of liquid phases is determined by the dehydration behavior of crustal rocks at the slab-mantle interface in subduction channels. Because of the significant fractionation in incompatible trace elements but the full inheritance in radiogenic isotopes relative to their crustal sources, the production of liquid phases is crucial to the geochemical transfer from the subducting crust into the mantle. In this process, the stability of specific minerals in subducting crustal rocks exerts a primary control on the enrichment of given trace elements in the liquid phases. For this reason, geochemically enriched oceanic basalts can be categorized into two types in terms of their trace element distribution patterns in the primitive mantle-normalized diagram. One is island arc basalts (IAB), showing enrichment in LILE, Pb and LREE but depletion in HFSE such as Nb and Ta relative to HREE. The other is ocean island basalts (OIB), exhibiting enrichment in LILE and LREE, enrichment or non-depletion in HFSE but depletion in Pb relative to HREE. In either types, these basalts show the enhanced enrichment of LILE and LREE with increasing their incompatibility relative to normal mid-ocean ridge basalts (MORB).
The thermal regime of subduction zones can be categorized into two stages in both time and space. The first stage is characterized by compressional tectonism at low thermal gradients. As a consequence, metamorphic dehydration of the subducting crust prevails at forearc to subarc depths due to the breakdown of hydrous minerals such as mica and amphibole in the stability field of garnet and rutile, resulting in the liberation of aqueous solutions with the trace element composition that is considerably enriched in LILE, Pb and LREE but depleted in HFSE and HREE relative to normal MORB. This provides the crustal signature for the mantle sources of IAB. The second stage is indicated by extensional tectonism at high thermal gradients, leading to the partial melting of metamorphically dehydrated crustal rocks at subarc to postarc depths. This involves not only the breakdown of hydrous minerals such as amphibole, phengite and allanite in the stability field of garnet but also the dissolution of rutile into hydrous melts. As such, the hydrous melts can acquire the trace element composition that is significantly enriched in LILE, HFSE and LREE but depleted in Pb and HREE relative to normal MORB, providing the crustal signature for the mantle sources of OIB. In either case, these liquid phases would metasomatize the overlying mantle wedge peridotite at different depths, generating ultramafic metasomatites such as serpentinized and chloritized peridotites, and olivine-poor pyroxenites and hornblendites. As a consequence, the crustal signatures are transferred by the liquid phases from the subducting slab into the mantle.
2019, 10(4): 1255-1284.
doi: 10.1016/j.gsf.2018.12.007
Abstract:
We present a compilation of published data (field, petrography, ages and geochemistry) from 73 ophiolitic complexes of the Central Asian Orogenic Belt. The ophiolitic complexes, ranging in age from Neoproterozoic to Triassic, have been geochemically classified as subduction-related and subductionunrelated categories applying recent, well-established discrimination diagrams. The subductionunrelated category is further subdivided into Mid-Ocean Ridge type (MOR), a common rift-drift stage and Plume type, and the subduction-related category is subdivided into Backarc (BA), Forearc (FA), Backarc to Forearc (BA-FA) and Volcanic Arc (VA) types. The four subduction-related types define highly different geochemical features, with the BA and FA types defining end members showing subduction influence of 10%-100% and 90%-100% subduction influence, respectively, and the two other types (BAFA and VA) define values between the two end members. The subduction-related category comprises 79% of the examined ophiolites, of which the BA type ophiolites is by far the dominant group, followed by the BA-FA type, and with FA and VA types as subordinate groups. The Neoproterozoic and Ordovician complexes exhibit the highest, whereas those of Silurian age exhibit the lowest subduction-influence. Of the remaining 21% subduction-unrelated ophiolites, the MOR type dominates. Both the subductionrelated and subduction-unrelated types, in particular the latter, are commonly associated with alkaline basalts taken to represent ocean island magmatism. Harzburgite, dunite, gabbro and basalt are the common lithologies in all ophiolite types, whereas the BA-FA, FA and VA types generally contain intermediate to felsic rocks, and in the FA type boninites occur. The subduction-related ophiolites types generally show low metamorphic grade, whereas greenschist, amphibolite and blueschist grades occur in the subduction-unrelated and BA types. The highly different subduction contribution (from 0 to 100% in the MOR and FA, respectively), attest to variable dips of the subducting slab, as well as variable flux of subduction-related elements into the mantle above subducting slabs, from where the ophiolite magmas got their geochemical fingerprints. As most MOR ophiolites get subducted to the deep mantle, the subduction-related ophiolites have become a dominant ophiolitic type on Earth's surface through all times supporting the idea about the early start of Plate Tectonics.
We present a compilation of published data (field, petrography, ages and geochemistry) from 73 ophiolitic complexes of the Central Asian Orogenic Belt. The ophiolitic complexes, ranging in age from Neoproterozoic to Triassic, have been geochemically classified as subduction-related and subductionunrelated categories applying recent, well-established discrimination diagrams. The subductionunrelated category is further subdivided into Mid-Ocean Ridge type (MOR), a common rift-drift stage and Plume type, and the subduction-related category is subdivided into Backarc (BA), Forearc (FA), Backarc to Forearc (BA-FA) and Volcanic Arc (VA) types. The four subduction-related types define highly different geochemical features, with the BA and FA types defining end members showing subduction influence of 10%-100% and 90%-100% subduction influence, respectively, and the two other types (BAFA and VA) define values between the two end members. The subduction-related category comprises 79% of the examined ophiolites, of which the BA type ophiolites is by far the dominant group, followed by the BA-FA type, and with FA and VA types as subordinate groups. The Neoproterozoic and Ordovician complexes exhibit the highest, whereas those of Silurian age exhibit the lowest subduction-influence. Of the remaining 21% subduction-unrelated ophiolites, the MOR type dominates. Both the subductionrelated and subduction-unrelated types, in particular the latter, are commonly associated with alkaline basalts taken to represent ocean island magmatism. Harzburgite, dunite, gabbro and basalt are the common lithologies in all ophiolite types, whereas the BA-FA, FA and VA types generally contain intermediate to felsic rocks, and in the FA type boninites occur. The subduction-related ophiolites types generally show low metamorphic grade, whereas greenschist, amphibolite and blueschist grades occur in the subduction-unrelated and BA types. The highly different subduction contribution (from 0 to 100% in the MOR and FA, respectively), attest to variable dips of the subducting slab, as well as variable flux of subduction-related elements into the mantle above subducting slabs, from where the ophiolite magmas got their geochemical fingerprints. As most MOR ophiolites get subducted to the deep mantle, the subduction-related ophiolites have become a dominant ophiolitic type on Earth's surface through all times supporting the idea about the early start of Plate Tectonics.
2019, 10(4): 1285-1303.
doi: 10.1016/j.gsf.2018.12.005
Abstract:
Rare earth elements (REE) include the lanthanide series elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) plus Sc and Y. Currently these metals have become very critical to several modern technologies ranging from cell phones and televisions to LED light bulbs and wind turbines. This article summarizes the occurrence of these metals in the Earth's crust, their mineralogy, different types of deposits both on land and oceans from the standpoint of the new data with more examples from the Indian subcontinent. In addition to their utility to understand the formation of the major Earth reservoirs, multi-faceted updates on the applications of REE in agriculture and medicine including new emerging ones are presented. Environmental hazards including human health issues due to REE mining and large-scale dumping of e-waste containing significant concentrations of REE are summarized. New strategies for the future supply of REE including recent developments in the extraction of REE from coal fired ash and recycling from e-waste are presented. Recent developments in individual REE separation technologies in both metallurgical and recycling operations have been highlighted. An outline of the analytical methods for their precise and accurate determinations required in all these studies, such as, Xray fluorescence spectrometry (XRF), laser induced breakdown spectroscopy (LIBS), instrumental neutron activation analysis (INAA), inductively coupled plasma optical emission spectrometry (ICP-OES), glow discharge mass spectrometry (GD-MS), inductively coupled plasma mass spectrometry (including ICP-MS, ICP-TOF-MS, HR-ICP-MS with laser ablation as well as solution nebulization) and other instrumental techniques, in different types of materials are presented.
Rare earth elements (REE) include the lanthanide series elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) plus Sc and Y. Currently these metals have become very critical to several modern technologies ranging from cell phones and televisions to LED light bulbs and wind turbines. This article summarizes the occurrence of these metals in the Earth's crust, their mineralogy, different types of deposits both on land and oceans from the standpoint of the new data with more examples from the Indian subcontinent. In addition to their utility to understand the formation of the major Earth reservoirs, multi-faceted updates on the applications of REE in agriculture and medicine including new emerging ones are presented. Environmental hazards including human health issues due to REE mining and large-scale dumping of e-waste containing significant concentrations of REE are summarized. New strategies for the future supply of REE including recent developments in the extraction of REE from coal fired ash and recycling from e-waste are presented. Recent developments in individual REE separation technologies in both metallurgical and recycling operations have been highlighted. An outline of the analytical methods for their precise and accurate determinations required in all these studies, such as, Xray fluorescence spectrometry (XRF), laser induced breakdown spectroscopy (LIBS), instrumental neutron activation analysis (INAA), inductively coupled plasma optical emission spectrometry (ICP-OES), glow discharge mass spectrometry (GD-MS), inductively coupled plasma mass spectrometry (including ICP-MS, ICP-TOF-MS, HR-ICP-MS with laser ablation as well as solution nebulization) and other instrumental techniques, in different types of materials are presented.
2019, 10(4): 1305-1326.
doi: 10.1016/j.gsf.2019.04.002
Abstract:
In this study, seven isotopic databases are presented and analyzed to identify mantle and crustal episodes on a global scale by focusing on periodicity ranging from 70 to 200 million years (Myr). The databases are the largest, or among the largest, compiled for each type of data-with an objective of finding some samples from every region of every continent, to make each database as global as conceivably possible. The databases contain zircon Lu/Hf isotopic data, whole-rock Sm/Nd isotopic data, U/Pb detrital zircon ages, U/Pb igneous zircon ages, U/Pb non-zircon ages, whole-rock Re/Os isotopic data, and large igneous province ages. Part I of this study focuses on the periodicities of age histograms and geochemical averages developed from the seven databases, via spectral and cross-correlation analyses. Natural physical cycles often propagate in exact integer multiples of a fundamental cycle, referred to as harmonics. The tests show that harmonic geological cycles of~93.5 and~187 Myr have persisted throughout terrestrial history, and the cyclicities are statistically significant for U/Pb igneous zircon ages, U/Pb detrital zircon ages, U/Pb zircon-rim ages, large igneous province ages, mean εHf(t) for all samples, mean εHf(t) values for igneous-only samples, and relative abundance of mafic rocks. Equally important, cross-correlation analyses show these seven time-series are nearly synchronous (±7 Myr) with a model consisting of periodicities of 93.5 and 187 Myr. Additionally, the similarities between peaks in the 93.5 and 187 Myr mantle cycles and terminal ages of established and suspected superchrons provide a framework for predicting and testing superchron periodicity.
In this study, seven isotopic databases are presented and analyzed to identify mantle and crustal episodes on a global scale by focusing on periodicity ranging from 70 to 200 million years (Myr). The databases are the largest, or among the largest, compiled for each type of data-with an objective of finding some samples from every region of every continent, to make each database as global as conceivably possible. The databases contain zircon Lu/Hf isotopic data, whole-rock Sm/Nd isotopic data, U/Pb detrital zircon ages, U/Pb igneous zircon ages, U/Pb non-zircon ages, whole-rock Re/Os isotopic data, and large igneous province ages. Part I of this study focuses on the periodicities of age histograms and geochemical averages developed from the seven databases, via spectral and cross-correlation analyses. Natural physical cycles often propagate in exact integer multiples of a fundamental cycle, referred to as harmonics. The tests show that harmonic geological cycles of~93.5 and~187 Myr have persisted throughout terrestrial history, and the cyclicities are statistically significant for U/Pb igneous zircon ages, U/Pb detrital zircon ages, U/Pb zircon-rim ages, large igneous province ages, mean εHf(t) for all samples, mean εHf(t) values for igneous-only samples, and relative abundance of mafic rocks. Equally important, cross-correlation analyses show these seven time-series are nearly synchronous (±7 Myr) with a model consisting of periodicities of 93.5 and 187 Myr. Additionally, the similarities between peaks in the 93.5 and 187 Myr mantle cycles and terminal ages of established and suspected superchrons provide a framework for predicting and testing superchron periodicity.
2019, 10(4): 1327-1336.
doi: 10.1016/j.gsf.2019.03.005
Abstract:
Igneous and detrital zircons have six major U/Pb isotopic age peaks in common (2700 Ma, 1875 Ma, 1045 Ma, 625 Ma, 265 Ma and 90 Ma). For igneous rocks, each age peak is comprised of subpeaks with distinct geographic distributions and a subpeak age range per age peak ≤ 100 Myr. There are eight major LIP age peaks (found on ≥ 10 crustal provinces) of which only four are in common to major detrital zircon age peaks (2715 Ma, 1875 Ma, 825 Ma, 90 Ma). Of the whole-rock Re depletion ages, 58% have corresponding detrital zircon age peaks and 55% have corresponding LIP age peaks. Ten age peaks are found in common to igneous zircon, detrital zircon, LIP, and Re depletion age time series (3225 Ma, 2875 Ma, 2145 Ma, 2085 Ma, 1985 Ma, 1785 Ma, 1455 Ma, 1175 Ma, 825 Ma, and 90 Ma), and these are very robust peaks on a global scale as recorded in both crustal and mantle rocks. About 50% of the age peaks in each of these time series correspond to predicted peaks in a 94-Myr mantle cycle, including four of the ten peaks in common to all four time series (2875 Ma, 1785 Ma, 825 Ma and 90 Ma). Age peak widths and subpeak ranges per age peak suggest that mantle events responsible for age peaks are <100 Myr and many <50 Myr in duration. Age peak geographic distributions show three populations (≤ 1000 Ma, 2500 -1000 Ma, ≥ 2500 Ma), with the number of new provinces in which age peaks are represented decreasing with time within each population. The breaks between the populations (at 2.5 Ga and 1 Ga) fall near the onsets of two transitions in Earth history. The First Transition may represent a change from stagnant-lid tectonics into plate tectonics and the Second Transition, the onset of subduction of continental crust. The major factor controlling geographic distribution of age peaks is the changing locations of orogeny. Before~2 Ga, age subpeaks and peaks are housed in orogens within or around the edges of crustal provinces, mostly in accretionary orogens, but beginning at 1.9 Ga, collisional orogens become more important. The coincidence in duration between magmatic flare-ups in Phanerozoic arcs and duration of age subpeaks (10-30 Myr) is consistent with subpeaks representing periods of enhanced arcrelated magmatism, probably caused by increased subduction flux. The correlation of isotopic age peaks between time series supports a cause and effect relationship between mantle plume activity, continental magma production at convergent margins, and crustal deformation. Correlation of over half of the detrital zircon age peaks (and six of the nine major peaks) with Re depletion age peaks supports an interpretation of the zircon peaks as crustal growth rather than selective preservation peaks.
Igneous and detrital zircons have six major U/Pb isotopic age peaks in common (2700 Ma, 1875 Ma, 1045 Ma, 625 Ma, 265 Ma and 90 Ma). For igneous rocks, each age peak is comprised of subpeaks with distinct geographic distributions and a subpeak age range per age peak ≤ 100 Myr. There are eight major LIP age peaks (found on ≥ 10 crustal provinces) of which only four are in common to major detrital zircon age peaks (2715 Ma, 1875 Ma, 825 Ma, 90 Ma). Of the whole-rock Re depletion ages, 58% have corresponding detrital zircon age peaks and 55% have corresponding LIP age peaks. Ten age peaks are found in common to igneous zircon, detrital zircon, LIP, and Re depletion age time series (3225 Ma, 2875 Ma, 2145 Ma, 2085 Ma, 1985 Ma, 1785 Ma, 1455 Ma, 1175 Ma, 825 Ma, and 90 Ma), and these are very robust peaks on a global scale as recorded in both crustal and mantle rocks. About 50% of the age peaks in each of these time series correspond to predicted peaks in a 94-Myr mantle cycle, including four of the ten peaks in common to all four time series (2875 Ma, 1785 Ma, 825 Ma and 90 Ma). Age peak widths and subpeak ranges per age peak suggest that mantle events responsible for age peaks are <100 Myr and many <50 Myr in duration. Age peak geographic distributions show three populations (≤ 1000 Ma, 2500 -1000 Ma, ≥ 2500 Ma), with the number of new provinces in which age peaks are represented decreasing with time within each population. The breaks between the populations (at 2.5 Ga and 1 Ga) fall near the onsets of two transitions in Earth history. The First Transition may represent a change from stagnant-lid tectonics into plate tectonics and the Second Transition, the onset of subduction of continental crust. The major factor controlling geographic distribution of age peaks is the changing locations of orogeny. Before~2 Ga, age subpeaks and peaks are housed in orogens within or around the edges of crustal provinces, mostly in accretionary orogens, but beginning at 1.9 Ga, collisional orogens become more important. The coincidence in duration between magmatic flare-ups in Phanerozoic arcs and duration of age subpeaks (10-30 Myr) is consistent with subpeaks representing periods of enhanced arcrelated magmatism, probably caused by increased subduction flux. The correlation of isotopic age peaks between time series supports a cause and effect relationship between mantle plume activity, continental magma production at convergent margins, and crustal deformation. Correlation of over half of the detrital zircon age peaks (and six of the nine major peaks) with Re depletion age peaks supports an interpretation of the zircon peaks as crustal growth rather than selective preservation peaks.
2019, 10(4): 1337-1357.
doi: 10.1016/j.gsf.2018.09.011
Abstract:
The origin of life on Earth remains enigmatic with diverse models and debates. Here we discuss essential requirements for the first emergence of life on our planet and propose the following nine requirements:(1) an energy source (ionizing radiation and thermal energy); (2) a supply of nutrients (P, K, REE, etc.); (3) a supply of life-constituting major elements; (4) a high concentration of reduced gases such as CH4, HCN and NH3; (5) dry-wet cycles to create membranes and polymerize RNA; (6) a non-toxic aqueous environment; (7) Na-poor water; (8) highly diversified environments, and (9) cyclic conditions, such as dayto-night, hot-to-cold etc.
Based on these nine requirements, we evaluate previously proposed locations for the origin of Earth's life, including:(1) Darwin's "warm little pond", leading to a "prebiotic soup" for life; (2) panspermia or Neo-panspermia (succession model of panspermia); (3) transportation from/through Mars; (4) a deepsea hydrothermal system; (5) an on-land subduction-zone hot spring, and (6) a geyser systems driven by a natural nuclear reactor. We conclude that location (6) is the most ideal candidate for the origin point for Earth's life because of its efficiency in continuously supplying both the energy and the necessary materials for life, thereby maintaining the essential "cradle" for its initial development. We also emphasize that falsifiable working hypothesis provides an important tool to evaluate one of the biggest mysteries of the universe-the origin of life.
The origin of life on Earth remains enigmatic with diverse models and debates. Here we discuss essential requirements for the first emergence of life on our planet and propose the following nine requirements:(1) an energy source (ionizing radiation and thermal energy); (2) a supply of nutrients (P, K, REE, etc.); (3) a supply of life-constituting major elements; (4) a high concentration of reduced gases such as CH4, HCN and NH3; (5) dry-wet cycles to create membranes and polymerize RNA; (6) a non-toxic aqueous environment; (7) Na-poor water; (8) highly diversified environments, and (9) cyclic conditions, such as dayto-night, hot-to-cold etc.
Based on these nine requirements, we evaluate previously proposed locations for the origin of Earth's life, including:(1) Darwin's "warm little pond", leading to a "prebiotic soup" for life; (2) panspermia or Neo-panspermia (succession model of panspermia); (3) transportation from/through Mars; (4) a deepsea hydrothermal system; (5) an on-land subduction-zone hot spring, and (6) a geyser systems driven by a natural nuclear reactor. We conclude that location (6) is the most ideal candidate for the origin point for Earth's life because of its efficiency in continuously supplying both the energy and the necessary materials for life, thereby maintaining the essential "cradle" for its initial development. We also emphasize that falsifiable working hypothesis provides an important tool to evaluate one of the biggest mysteries of the universe-the origin of life.
2019, 10(4): 1359-1370.
doi: 10.1016/j.gsf.2019.02.001
Abstract:
The dominant geodynamic processes that underpin the formation and evolution of Earth's early crust remain enigmatic calling for new information from less studied ancient cratonic nuclei. Here, we present U-Pb ages and Hf isotopic compositions of detrital zircon grains from~2.9 Ga old quartzites and magmatic zircon from a 3.505 Ga old dacite from the Iron Ore Group of the Singhbhum craton, eastern India. The detrital zircon grains range in age between 3.95 Ga and 2.91 Ga. Together with the recently reported Hadean, Eoarchean xenocrystic (up to 4.24 Ga) and modern detritus zircon grains from the Singhbhum craton, our results suggest that the Eoarchean detrital zircons represent crust generated by recycling of Hadean felsic crust formed at~4.3-4.2 Ga and~3.95 Ga. We observe a prominent shift in Hf isotope compositions at~3.6-3.5 Ga towards super-chondritic values, which signify an increased role for depleted mantle and the relevance of plate tectonics. The Paleo-, Mesoarchean zircon Hf isotopic record in the craton indicates crust generation involving the role of both depleted and enriched mantle sources. We infer a short-lived suprasubduction setting around~3.6-3.5 Ga followed by mantle plume activity during the Paleo-, Mesoarchean crust formation in the Singhbhum craton. The Singhbhum craton provides an additional repository for Earth's oldest materials.
The dominant geodynamic processes that underpin the formation and evolution of Earth's early crust remain enigmatic calling for new information from less studied ancient cratonic nuclei. Here, we present U-Pb ages and Hf isotopic compositions of detrital zircon grains from~2.9 Ga old quartzites and magmatic zircon from a 3.505 Ga old dacite from the Iron Ore Group of the Singhbhum craton, eastern India. The detrital zircon grains range in age between 3.95 Ga and 2.91 Ga. Together with the recently reported Hadean, Eoarchean xenocrystic (up to 4.24 Ga) and modern detritus zircon grains from the Singhbhum craton, our results suggest that the Eoarchean detrital zircons represent crust generated by recycling of Hadean felsic crust formed at~4.3-4.2 Ga and~3.95 Ga. We observe a prominent shift in Hf isotope compositions at~3.6-3.5 Ga towards super-chondritic values, which signify an increased role for depleted mantle and the relevance of plate tectonics. The Paleo-, Mesoarchean zircon Hf isotopic record in the craton indicates crust generation involving the role of both depleted and enriched mantle sources. We infer a short-lived suprasubduction setting around~3.6-3.5 Ga followed by mantle plume activity during the Paleo-, Mesoarchean crust formation in the Singhbhum craton. The Singhbhum craton provides an additional repository for Earth's oldest materials.
2019, 10(4): 1371-1381.
doi: 10.1016/j.gsf.2019.01.003
Abstract:
Detrital zircon U/Pb geochronology is a common tool used to resolve stratigraphic questions, inform basin evolution and constrain regional geological histories. In favourable circumstances, detrital zircon populations can contain a concomitant volcanic contribution that provides constraints on the age of deposition. However, for non-volcanic settings, proving isolated detrital zircon grains are from contemporaneous and potentially remote volcanism is challenging. Here we use same grain (U-Th)/He thermochronology coupled with U/Pb geochronology to identify detrital zircon grains of contemporary volcanic origin. (U-Th)/He ages from Cretaceous zircon grains in southern Australia define a single population with a weighted mean age of 104±6.1 Ma, indistinguishable from zircon U/Pb geochronology and palynology (~104.0-107.5 Ma). Detrital zircon trace-element geochemistry is consistent with a continental signature for parent rocks and coupled with detrital grain ages, supports derivation from a >2000 km distant early-to mid-Cretaceous Whitsunday Volcanic Province in eastern Australia. Thus, integration of biostratigraphy, single-grain zircon double-dating (geochronology and thermochronology) and grain geochemistry enhances fingerprinting of zircon source region and transport history. A distal volcanic source and rapid continental-scale transport to southern Australia is supported here.
Detrital zircon U/Pb geochronology is a common tool used to resolve stratigraphic questions, inform basin evolution and constrain regional geological histories. In favourable circumstances, detrital zircon populations can contain a concomitant volcanic contribution that provides constraints on the age of deposition. However, for non-volcanic settings, proving isolated detrital zircon grains are from contemporaneous and potentially remote volcanism is challenging. Here we use same grain (U-Th)/He thermochronology coupled with U/Pb geochronology to identify detrital zircon grains of contemporary volcanic origin. (U-Th)/He ages from Cretaceous zircon grains in southern Australia define a single population with a weighted mean age of 104±6.1 Ma, indistinguishable from zircon U/Pb geochronology and palynology (~104.0-107.5 Ma). Detrital zircon trace-element geochemistry is consistent with a continental signature for parent rocks and coupled with detrital grain ages, supports derivation from a >2000 km distant early-to mid-Cretaceous Whitsunday Volcanic Province in eastern Australia. Thus, integration of biostratigraphy, single-grain zircon double-dating (geochronology and thermochronology) and grain geochemistry enhances fingerprinting of zircon source region and transport history. A distal volcanic source and rapid continental-scale transport to southern Australia is supported here.
2019, 10(4): 1383-1396.
doi: 10.1016/j.gsf.2019.01.009
Abstract:
Prospectivity analyses are used to reduce the exploration search space for locating areas prospective for mineral deposits. The scale of a study and the type of mineral system associated with the deposit control the evidence layers used as proxies that represent critical ore genesis processes. In particular, knowledge-driven approaches (fuzzy logic) use a conceptual mineral systems model from which data proxies represent the critical components. These typically vary based on the scale of study and the type of mineral system being predicted. Prospectivity analyses utilising interpreted data to represent proxies for a mineral system model inherit the subjectivity of the interpretations and the uncertainties of the evidence layers used in the model. In the case study presented, the prospectivity for remobilised nickel sulphide (NiS) in the west Kimberley, Western Australia, is assessed with two novel techniques that objectively grade interpretations and accommodate alternative mineralisation scenarios. Exploration targets are then identified and supplied with a robustness assessment that reflects the variability of prospectivity value for each location when all models are considered.
The first technique grades the strength of structural interpretations on an individual line-segment basis. Gradings are obtained from an objective measure of feature evidence, which is the quantification of specific patterns in geophysical data that are considered to reveal underlying structure. Individual structures are weighted in the prospectivity model with grading values correlated to their feature evidence. This technique allows interpreted features to contribute prospectivity proportional to their strength in feature evidence and indicates the level of associated stochastic uncertainty.
The second technique aims to embrace the systemic uncertainty of modelling complex mineral systems. In this approach, multiple prospectivity maps are each generated with different combinations of confidence values applied to evidence layers to represent the diversity of processes potentially leading to ore deposition. With a suite of prospectivity maps, the most robust exploration targets are the locations with the highest prospectivity values showing the smallest range amongst the model suite. This new technique offers an approach that reveals to the modeller a range of alternative mineralisation scenarios while employing a sensible mineral systems model, robust modelling of prospectivity and significantly reducing the exploration search space for Ni.
Prospectivity analyses are used to reduce the exploration search space for locating areas prospective for mineral deposits. The scale of a study and the type of mineral system associated with the deposit control the evidence layers used as proxies that represent critical ore genesis processes. In particular, knowledge-driven approaches (fuzzy logic) use a conceptual mineral systems model from which data proxies represent the critical components. These typically vary based on the scale of study and the type of mineral system being predicted. Prospectivity analyses utilising interpreted data to represent proxies for a mineral system model inherit the subjectivity of the interpretations and the uncertainties of the evidence layers used in the model. In the case study presented, the prospectivity for remobilised nickel sulphide (NiS) in the west Kimberley, Western Australia, is assessed with two novel techniques that objectively grade interpretations and accommodate alternative mineralisation scenarios. Exploration targets are then identified and supplied with a robustness assessment that reflects the variability of prospectivity value for each location when all models are considered.
The first technique grades the strength of structural interpretations on an individual line-segment basis. Gradings are obtained from an objective measure of feature evidence, which is the quantification of specific patterns in geophysical data that are considered to reveal underlying structure. Individual structures are weighted in the prospectivity model with grading values correlated to their feature evidence. This technique allows interpreted features to contribute prospectivity proportional to their strength in feature evidence and indicates the level of associated stochastic uncertainty.
The second technique aims to embrace the systemic uncertainty of modelling complex mineral systems. In this approach, multiple prospectivity maps are each generated with different combinations of confidence values applied to evidence layers to represent the diversity of processes potentially leading to ore deposition. With a suite of prospectivity maps, the most robust exploration targets are the locations with the highest prospectivity values showing the smallest range amongst the model suite. This new technique offers an approach that reveals to the modeller a range of alternative mineralisation scenarios while employing a sensible mineral systems model, robust modelling of prospectivity and significantly reducing the exploration search space for Ni.
2019, 10(4): 1397-1408.
doi: 10.1016/j.gsf.2019.02.002
Abstract:
Postulated extreme sea-level rise of up to 1-1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious, as is the great rapidity (<104 yr) of the sea-level rise. Such extreme glacioeustatic events, if real, would have been global and affected all continents. In South Australia, up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin. The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise. In the central Adelaide Rift Complex, diamictite with faceted and striated clasts occurs at the top of the Elatina Formation <1-2 m beneath the early Ediacaran Nuccaleena Formation ‘cap carbonate’. One hundred kilometres to the south,~30 m of siltstone and sandstone followed by~6 m of clast-poor diamictite with clasts 10 + cm long occur between tidal rhythmites and the cap carbonate. Three hundred kilometres further south,~70 m of siltstone, dolomitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata. Hence the rhythmites were deposited during a high stand (interstadial or interglacial), not during post-glacial sea-level rise. Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition, and water depth and other palaeoenvironmental factors are uncertain, casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition. The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations. A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate, and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 105-106 yr. This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California, and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates. It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia.
Postulated extreme sea-level rise of up to 1-1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious, as is the great rapidity (<104 yr) of the sea-level rise. Such extreme glacioeustatic events, if real, would have been global and affected all continents. In South Australia, up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin. The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise. In the central Adelaide Rift Complex, diamictite with faceted and striated clasts occurs at the top of the Elatina Formation <1-2 m beneath the early Ediacaran Nuccaleena Formation ‘cap carbonate’. One hundred kilometres to the south,~30 m of siltstone and sandstone followed by~6 m of clast-poor diamictite with clasts 10 + cm long occur between tidal rhythmites and the cap carbonate. Three hundred kilometres further south,~70 m of siltstone, dolomitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata. Hence the rhythmites were deposited during a high stand (interstadial or interglacial), not during post-glacial sea-level rise. Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition, and water depth and other palaeoenvironmental factors are uncertain, casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition. The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations. A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate, and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 105-106 yr. This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California, and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates. It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia.
2019, 10(4): 1409-1420.
doi: 10.1016/j.gsf.2018.11.003
Abstract:
Reactivation of metasomatized mantle lithosphere may occur during continental extension, which is an important component of plate tectonics. The lower most part of the metasomatized domains in the subcontinental mantle lithosphere can be locally enriched in CO2. Therefore, partial melting of these metasomatized domains may play a crucial role in the global carbon cycle. However, little is known about this process and up until now few numerical constraints are available. Here we address this knowledge gap and use a 2-D high resolution petrological-thermomechanical model to assess lithospheric rifting, CO2 degassing and melting. We test 4 lithospheric thicknesses:90, 110, 130 and 200 km with a 10 km thick metasomatized layer at the base using CO2 of 2 wt.% in the bulk composition. The carbonate enriched layer is stable below~3 GPa (>110 km) for a temperature of 1300℃; therefore, we only observe degassing patterns for lithospheric models that are 130 km and 200 km thick. The metasomatized layer for the 130 km thick lithosphere mostly comprises carbonatite melting, whereas in the 200 km thick scenario propagation of melt development from kimberlites to carbonatites occurs as the metasomatic mantle is exhumed during extension. The numerical models fit well into natural rifting zones of the European Cenozoic Rift System for young (shallow) and of the North Atlantic Rift for old (thick) lithosphere.
Reactivation of metasomatized mantle lithosphere may occur during continental extension, which is an important component of plate tectonics. The lower most part of the metasomatized domains in the subcontinental mantle lithosphere can be locally enriched in CO2. Therefore, partial melting of these metasomatized domains may play a crucial role in the global carbon cycle. However, little is known about this process and up until now few numerical constraints are available. Here we address this knowledge gap and use a 2-D high resolution petrological-thermomechanical model to assess lithospheric rifting, CO2 degassing and melting. We test 4 lithospheric thicknesses:90, 110, 130 and 200 km with a 10 km thick metasomatized layer at the base using CO2 of 2 wt.% in the bulk composition. The carbonate enriched layer is stable below~3 GPa (>110 km) for a temperature of 1300℃; therefore, we only observe degassing patterns for lithospheric models that are 130 km and 200 km thick. The metasomatized layer for the 130 km thick lithosphere mostly comprises carbonatite melting, whereas in the 200 km thick scenario propagation of melt development from kimberlites to carbonatites occurs as the metasomatic mantle is exhumed during extension. The numerical models fit well into natural rifting zones of the European Cenozoic Rift System for young (shallow) and of the North Atlantic Rift for old (thick) lithosphere.
2019, 10(4): 1421-1435.
doi: 10.1016/j.gsf.2018.11.002
Abstract:
The calculation of a maximum depositional age (MDA) from a detrital zircon sample can provide insight into avariety of geological problems. However, the impact of sample size and calculation method on the accuracy of a resulting MDA has not been evaluated. We use large populations of synthetic zircon dates (N ≈ 25,000) to analyze the impact of varying sample size (n), measurement uncertainty, and the abundance of neardepositional-age zircons on the accuracy and uncertainty of 9 commonly used MDA calculation methods. Furthermore, a new method, the youngest statistical population is tested. For each method, 500 samples of n synthetic dates were drawn from the parent population and MDAs were calculated. The mean and standard deviation of each method over the 500 trials at each n-value (50-1000, in increments of 50) were compared to the known depositional age of the synthetic population and used to compare the methods quantitatively in two simulation scenarios. The first simulation scenario varied the proportion of near-depositional-age grains in the synthetic population. The second scenario varied the uncertainty of the dates used to calculate the MDAs. Increasing sample size initially decreased the mean residual error and standard deviation calculated by each method. At higher n-values (>~300 grains), calculated MDAs changed more slowly and the mean residual error increased or decreased depending on the method used. Increasing the proportion of near-depositional-age grains and lowering measurement uncertainty decreased the number of measurements required for the calculated MDAs to stabilize and decreased the standard deviation in calculated MDAs of the 500 samples. Results of the two simulation scenarios show that the most successful way to increase the accuracy of a calculated MDA is by acquiring a large number of low-uncertainty measurements (300 < n < 600). This maximizes the number of near-depositional-age grains that are dated. Ideally, a lowuncertainty (1%-2%, 2σ), large-n (n > 300) approach is used if the calculation of accurate MDAs are key to research goals. Other acquisition methods, such as high-to moderate-precision measurement methods (e.g., 1%-5%, 2σ) acquiring low-to moderate-n datasets (50 < n < 300), will typically calculate MDAs with larger residual error and higher variance between samples.
In general, the most successful and accurate methods tested are:the youngest single grain (YSG), youngest detrital zircon (YDZ), and the weighted average of the youngest three grains (Y3Z). These methods, however, are liable to calculate MDAs younger than the true depositional age if derived from populations with abundant near-depositional ages, or from large-n datasets (n > 300). Additionally, they are most susceptible to producing erroneous MDAs due to contamination in the field or laboratory, or through disturbances of the youngest zircon's U-Pb systematics (e.g., lead loss). More conservative methods that still produce accurate MDAs and are less susceptible to contamination or lead loss include:youngest grain cluster at 1σ uncertainty (YGC 1σ), youngest grain cluster at 2σ uncertainty (YGC 2σ), and youngest statistical population (YSP). The ages calculated by these methods may be more useful and appealing when fitting calculated MDAs in to pre-existing chronostratigraphic frameworks, as they are less likely to be younger than the true depositional age. From the results of our numerical models we illustrate what geologic processes (i.e., tectonic or sedimentary) can be resolved using MDAs derived from strata of different ages.
The calculation of a maximum depositional age (MDA) from a detrital zircon sample can provide insight into avariety of geological problems. However, the impact of sample size and calculation method on the accuracy of a resulting MDA has not been evaluated. We use large populations of synthetic zircon dates (N ≈ 25,000) to analyze the impact of varying sample size (n), measurement uncertainty, and the abundance of neardepositional-age zircons on the accuracy and uncertainty of 9 commonly used MDA calculation methods. Furthermore, a new method, the youngest statistical population is tested. For each method, 500 samples of n synthetic dates were drawn from the parent population and MDAs were calculated. The mean and standard deviation of each method over the 500 trials at each n-value (50-1000, in increments of 50) were compared to the known depositional age of the synthetic population and used to compare the methods quantitatively in two simulation scenarios. The first simulation scenario varied the proportion of near-depositional-age grains in the synthetic population. The second scenario varied the uncertainty of the dates used to calculate the MDAs. Increasing sample size initially decreased the mean residual error and standard deviation calculated by each method. At higher n-values (>~300 grains), calculated MDAs changed more slowly and the mean residual error increased or decreased depending on the method used. Increasing the proportion of near-depositional-age grains and lowering measurement uncertainty decreased the number of measurements required for the calculated MDAs to stabilize and decreased the standard deviation in calculated MDAs of the 500 samples. Results of the two simulation scenarios show that the most successful way to increase the accuracy of a calculated MDA is by acquiring a large number of low-uncertainty measurements (300 < n < 600). This maximizes the number of near-depositional-age grains that are dated. Ideally, a lowuncertainty (1%-2%, 2σ), large-n (n > 300) approach is used if the calculation of accurate MDAs are key to research goals. Other acquisition methods, such as high-to moderate-precision measurement methods (e.g., 1%-5%, 2σ) acquiring low-to moderate-n datasets (50 < n < 300), will typically calculate MDAs with larger residual error and higher variance between samples.
In general, the most successful and accurate methods tested are:the youngest single grain (YSG), youngest detrital zircon (YDZ), and the weighted average of the youngest three grains (Y3Z). These methods, however, are liable to calculate MDAs younger than the true depositional age if derived from populations with abundant near-depositional ages, or from large-n datasets (n > 300). Additionally, they are most susceptible to producing erroneous MDAs due to contamination in the field or laboratory, or through disturbances of the youngest zircon's U-Pb systematics (e.g., lead loss). More conservative methods that still produce accurate MDAs and are less susceptible to contamination or lead loss include:youngest grain cluster at 1σ uncertainty (YGC 1σ), youngest grain cluster at 2σ uncertainty (YGC 2σ), and youngest statistical population (YSP). The ages calculated by these methods may be more useful and appealing when fitting calculated MDAs in to pre-existing chronostratigraphic frameworks, as they are less likely to be younger than the true depositional age. From the results of our numerical models we illustrate what geologic processes (i.e., tectonic or sedimentary) can be resolved using MDAs derived from strata of different ages.
2019, 10(4): 1437-1447.
doi: 10.1016/j.gsf.2018.08.001
Abstract:
GeoPyTool is an open source application developed for geological calculations and plots, such as geochemical classification, parameter calculation, basic statistical analysis and diagrams for structural geology. More than acting as a link from raw data stored in Microsoft Excel® (MS Excel) files to vector graphic files, GeoPyTool includes recently developed routines that have not been included in previous software, such as the calculation of the Ce(IV)/Ce(III) ratio for zircons as a method to examine the temporal evolution of oxygen fugacity in the magmatic source for igneous rocks, and the temperature calculator with titanium in zircon and zirconium in rutile. Besides these routines, GeoPyTool also allows users to load any figure from articles or books as a base map. As a Python-based crossplatform program, GeoPyTool works on Windows® MacOS X® and GNU/Linux. GeoPyTool can do the whole process from data to results without the dependence of Microsoft Excel®, CorelDraw® and other similar software. It takes Excel® XLSX and CSV (Comma Separated Value) as the formats of both the input data source files and the output calculation results files. The figures generated by GeoPyTool can be saved as portable network graphics (PNG), scalable vector graphics (SVG) or portable document format (PDF). Another highlight of GeoPyTool is the multilingual support, the official version of GeoPyTool supports both Chinese and English, and additional languages can be loaded through interface files. GeoPyTool is still in the development stage and will be expanded with further geochemical and structural geology routines. As an open source project, all source code of GeoPyTool are accessible on Github (https://github.com/GeoPyTool/GeoPyTool). Users with Python experience can join in the development team and build more complex functions expanding the capabilities of GeoPyTool.
GeoPyTool is an open source application developed for geological calculations and plots, such as geochemical classification, parameter calculation, basic statistical analysis and diagrams for structural geology. More than acting as a link from raw data stored in Microsoft Excel® (MS Excel) files to vector graphic files, GeoPyTool includes recently developed routines that have not been included in previous software, such as the calculation of the Ce(IV)/Ce(III) ratio for zircons as a method to examine the temporal evolution of oxygen fugacity in the magmatic source for igneous rocks, and the temperature calculator with titanium in zircon and zirconium in rutile. Besides these routines, GeoPyTool also allows users to load any figure from articles or books as a base map. As a Python-based crossplatform program, GeoPyTool works on Windows® MacOS X® and GNU/Linux. GeoPyTool can do the whole process from data to results without the dependence of Microsoft Excel®, CorelDraw® and other similar software. It takes Excel® XLSX and CSV (Comma Separated Value) as the formats of both the input data source files and the output calculation results files. The figures generated by GeoPyTool can be saved as portable network graphics (PNG), scalable vector graphics (SVG) or portable document format (PDF). Another highlight of GeoPyTool is the multilingual support, the official version of GeoPyTool supports both Chinese and English, and additional languages can be loaded through interface files. GeoPyTool is still in the development stage and will be expanded with further geochemical and structural geology routines. As an open source project, all source code of GeoPyTool are accessible on Github (https://github.com/GeoPyTool/GeoPyTool). Users with Python experience can join in the development team and build more complex functions expanding the capabilities of GeoPyTool.
2019, 10(4): 1449-1458.
doi: 10.1016/j.gsf.2018.10.003
Abstract:
The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe0 system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by:6MgCO3 + 19Fe=8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe0 or Fe0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D"region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.
The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe0 system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by:6MgCO3 + 19Fe=8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe0 or Fe0-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D"region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.
2019, 10(4): 1459-1476.
doi: 10.1016/j.gsf.2018.09.010
Abstract:
Exceptionally preserved fossils, such as those from Cambrian Burgess Shale-type fossil-Lagerstätten are critical because of their unique contributions to knowledge of the phylogenetic radiation and palaeoecological expansion of metazoans during the Cambrian explosion. Critically, these deposits provide information that is usually unobtainable from shelly and skeletonized fossils alone. The Guanshan Biota (Cambrian Series 2, Stage 4) in the Yunnan Province of South China, has produced abundant and diverse, exquisitely preserved fossils that often retain soft tissues and organs. To date, most fossils from the Guanshan Biota have been collected from localities such as Gaoloufang and Gangtoucun, which have become inaccessible due to new urban expansions and constructions of residential buildings. Here we present the first report of soft bodied fossils from a new section at Kanfuqing, close to the Wulongqing village in Malong County, approximately 3 km east of the Wulongqing Formation stratotype section. Fossils retain soft morphology, and include brachiopods with delicate marginal setae, priapulids with well-preserved sclerites and vetulicolians with entire sections of body. In addition, this fauna includes rare occurrences of trilobites preserved with soft tissues replicated as pyrite pseudomorphs after weathering. This discovery represents an important palaeogeographical extension of soft-bodied fossils of the Guanshan fauna to the east of the Xiaojiang Fault (related to Tsinning tectonic movements ca. 700 Ma). The fauna from the new Kanfuqing section is similar to that reported from the Wulongqing Formation west of the Xiaojiang Fault, and thus has significant implications for early Cambrian palaeogeography, faunal successions and palaeoenvironments of eastern Yunnan.
Exceptionally preserved fossils, such as those from Cambrian Burgess Shale-type fossil-Lagerstätten are critical because of their unique contributions to knowledge of the phylogenetic radiation and palaeoecological expansion of metazoans during the Cambrian explosion. Critically, these deposits provide information that is usually unobtainable from shelly and skeletonized fossils alone. The Guanshan Biota (Cambrian Series 2, Stage 4) in the Yunnan Province of South China, has produced abundant and diverse, exquisitely preserved fossils that often retain soft tissues and organs. To date, most fossils from the Guanshan Biota have been collected from localities such as Gaoloufang and Gangtoucun, which have become inaccessible due to new urban expansions and constructions of residential buildings. Here we present the first report of soft bodied fossils from a new section at Kanfuqing, close to the Wulongqing village in Malong County, approximately 3 km east of the Wulongqing Formation stratotype section. Fossils retain soft morphology, and include brachiopods with delicate marginal setae, priapulids with well-preserved sclerites and vetulicolians with entire sections of body. In addition, this fauna includes rare occurrences of trilobites preserved with soft tissues replicated as pyrite pseudomorphs after weathering. This discovery represents an important palaeogeographical extension of soft-bodied fossils of the Guanshan fauna to the east of the Xiaojiang Fault (related to Tsinning tectonic movements ca. 700 Ma). The fauna from the new Kanfuqing section is similar to that reported from the Wulongqing Formation west of the Xiaojiang Fault, and thus has significant implications for early Cambrian palaeogeography, faunal successions and palaeoenvironments of eastern Yunnan.
Zircon U-Pb and Lu-Hf isotope constraints on Archean crustal evolution in Southeastern Guyana Shield
2019, 10(4): 1477-1506.
doi: 10.1016/j.gsf.2018.09.012
Abstract:
The southeastern Guyana Shield, northeast Amazonian Craton, in the north of Brazil, is part of a widespread orogenic belt developed during the Transamazonian orogenic cycle (2.26-1.95 Ga) that includes a large Archean continental landmass strongly reworked during the Transamazonian orogeny, named Amapá Block. It consists mainly of a high-grade metamorphic granulitic-migmatitic-gneiss complex, of Meso-to Neoarchean age and Rhyacian granitoids and supracrustal sequences. For the first time, coupled U-Pb and Lu-Hf isotope data were obtained on zircon by LA-ICP-MS from five tectono-stratigraphic units of the Archean basement and one Paleoproterozoic intrusive rock, in order to investigate the main episodes of crustal growth and reworking. Whole-rock Sm-Nd isotope data were compared to the zircon Lu-Hf data. Three main magmatic episodes were defined by U-Pb zircon dating, two in the Mesoarchean (~3.19 Ga and 2.85 Ga) and one in the Neoarchean (~2.69-2.65 Ga). Subchondritic εHf(t) values obtained for almost all investigated units indicate that crustal reworking processes were predominant during the formation of rocks that today make up the Amapá Block. Hf-TDMC model ages, ranging from 2.99 Ga to 3.97 Ga, indicate that at least two important periods of mantle extraction and continental crust formation occurred during the Archean in southeastern Guyana Shield, an older one in the Eoarchean (~4.0 Ga) and a younger one in the Mesoarchean (~3.0-3.1 Ga). The latter is recognized as an important period of crustal accretion worldwide. The recognition of an Eoarchean episode to the southeastern most part of the Guyana Shield is unprecedented and was not recorded by whole-rock Sm-Nd data, which were restricted to the Meso-Paleoarchean (2.83 Ga to 3.51 Ga). This finding reveals that continental crust generation in the Amazonian Craton began at least 500 Ma earlier than previously suggested by the SmNd systematics.
The southeastern Guyana Shield, northeast Amazonian Craton, in the north of Brazil, is part of a widespread orogenic belt developed during the Transamazonian orogenic cycle (2.26-1.95 Ga) that includes a large Archean continental landmass strongly reworked during the Transamazonian orogeny, named Amapá Block. It consists mainly of a high-grade metamorphic granulitic-migmatitic-gneiss complex, of Meso-to Neoarchean age and Rhyacian granitoids and supracrustal sequences. For the first time, coupled U-Pb and Lu-Hf isotope data were obtained on zircon by LA-ICP-MS from five tectono-stratigraphic units of the Archean basement and one Paleoproterozoic intrusive rock, in order to investigate the main episodes of crustal growth and reworking. Whole-rock Sm-Nd isotope data were compared to the zircon Lu-Hf data. Three main magmatic episodes were defined by U-Pb zircon dating, two in the Mesoarchean (~3.19 Ga and 2.85 Ga) and one in the Neoarchean (~2.69-2.65 Ga). Subchondritic εHf(t) values obtained for almost all investigated units indicate that crustal reworking processes were predominant during the formation of rocks that today make up the Amapá Block. Hf-TDMC model ages, ranging from 2.99 Ga to 3.97 Ga, indicate that at least two important periods of mantle extraction and continental crust formation occurred during the Archean in southeastern Guyana Shield, an older one in the Eoarchean (~4.0 Ga) and a younger one in the Mesoarchean (~3.0-3.1 Ga). The latter is recognized as an important period of crustal accretion worldwide. The recognition of an Eoarchean episode to the southeastern most part of the Guyana Shield is unprecedented and was not recorded by whole-rock Sm-Nd data, which were restricted to the Meso-Paleoarchean (2.83 Ga to 3.51 Ga). This finding reveals that continental crust generation in the Amazonian Craton began at least 500 Ma earlier than previously suggested by the SmNd systematics.
2019, 10(4): 1507-1520.
doi: 10.1016/j.gsf.2018.09.013
Abstract:
40Ar/39Ar and zircon U-Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt, Central China, constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle. These volcanic rocks yield hornblende or whole-rock 40Ar/39Ar plateau ages of 128.3-129.7 Ma and zircon U-Pb age of 131.3±1.3 Ma. They exhibit SiO2 of 56.86-66.86 wt.%, K2O of 0.99-2.46 wt.% and MgO of 1.03-4.47 wt.%, with Mg# of 42-56. They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE. All the samples have enriched SreNd isotopic compositions with initial 87Sr/86Sr ratios ranging from 0.7112 to 0.7149 and εNd(t) values from -10.2 to -6.3. Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithospherederived magma followed by the assimilation and fractional crystallization (AFC) process. The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt. The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.
40Ar/39Ar and zircon U-Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt, Central China, constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle. These volcanic rocks yield hornblende or whole-rock 40Ar/39Ar plateau ages of 128.3-129.7 Ma and zircon U-Pb age of 131.3±1.3 Ma. They exhibit SiO2 of 56.86-66.86 wt.%, K2O of 0.99-2.46 wt.% and MgO of 1.03-4.47 wt.%, with Mg# of 42-56. They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE. All the samples have enriched SreNd isotopic compositions with initial 87Sr/86Sr ratios ranging from 0.7112 to 0.7149 and εNd(t) values from -10.2 to -6.3. Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithospherederived magma followed by the assimilation and fractional crystallization (AFC) process. The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt. The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.
2019, 10(4): 1521-1533.
doi: 10.1016/j.gsf.2018.09.014
Abstract:
The northwest region of the Iberian Peninsula is home to a unique ecosystem of bogs, which are particularly sensitive to projected climate change. In this context, the rate of carbon (C) accumulation in Chao de Veiga Mol, an intact raised bog, was analysed. Changes in the accumulation rate over the past 10 millennia were determined in a peat core of 847 cm in depth, with a high mean rate of peat growth (11 yr cm-1, 0.09 cm yr-1). An age-depth model was generated from 22 14C dates and fallout radionuclides. Chronological, stratigraphical and physico-chemical data confirmed the existence of a single cycle of peat formation throughout the Holocene and the formation of ombrotrophic peat 9500 years ago. The total mean C content was 50.2%, and over 10 millennia 583 kg C m-2 accumulated at a mean rate of 35.3 g C m-2 yr-1, with a long-term (apparent) rate of carbon accumulation in the catotelm of 59.9 g C m-2 yr-1. These values are much higher than reported for other Iberian peatlands and are amongst the highest documented for peatlands in the northern hemisphere. The dynamics of C accumulation and other measured parameters reveals important variations throughout the Holocene. They could be associated with the main climatic events described in the northern hemisphere and are highly consistent with models established for northern latitudes. The Chao de Veiga Mol raised bog is unique and of great potential value for carrying out high resolution palaeoenvironmental studies, especially in relation to regional and local modulations in southern Europe.
The northwest region of the Iberian Peninsula is home to a unique ecosystem of bogs, which are particularly sensitive to projected climate change. In this context, the rate of carbon (C) accumulation in Chao de Veiga Mol, an intact raised bog, was analysed. Changes in the accumulation rate over the past 10 millennia were determined in a peat core of 847 cm in depth, with a high mean rate of peat growth (11 yr cm-1, 0.09 cm yr-1). An age-depth model was generated from 22 14C dates and fallout radionuclides. Chronological, stratigraphical and physico-chemical data confirmed the existence of a single cycle of peat formation throughout the Holocene and the formation of ombrotrophic peat 9500 years ago. The total mean C content was 50.2%, and over 10 millennia 583 kg C m-2 accumulated at a mean rate of 35.3 g C m-2 yr-1, with a long-term (apparent) rate of carbon accumulation in the catotelm of 59.9 g C m-2 yr-1. These values are much higher than reported for other Iberian peatlands and are amongst the highest documented for peatlands in the northern hemisphere. The dynamics of C accumulation and other measured parameters reveals important variations throughout the Holocene. They could be associated with the main climatic events described in the northern hemisphere and are highly consistent with models established for northern latitudes. The Chao de Veiga Mol raised bog is unique and of great potential value for carrying out high resolution palaeoenvironmental studies, especially in relation to regional and local modulations in southern Europe.
2019, 10(4): 1535-1541.
doi: 10.1016/j.gsf.2018.10.006
Abstract:
The stratigraphic division and sequence of the Upper Cretaceous sediments in eastern Heilongjiang Province, China, have been ambiguous and controversial, mainly due to a lack of biostratigraphically useful fossils and related radiometric dating. A new species of angiospermous fossil plant, Platanus heilongjiangensis sp. nov., from Qitaihe in eastern Heilongjiang has been found in sediments conformably above which zircons from a rhyolitic tuff has been dated by U-Pb radiometric methods as 96.2±1.7 Ma, indicating that the Upper Houshigou Formation is of Cenomanian age. This discovery not only provides new data to improve our stratigraphic understanding of the Houshigou Formation, but also shows that Platanus flourished in the early Late Cretaceous floras of the region. This new study also indicates active volcanism taking place in the eastern Heilongjiang region during the Cenomanian of the Late Cretaceous.
The stratigraphic division and sequence of the Upper Cretaceous sediments in eastern Heilongjiang Province, China, have been ambiguous and controversial, mainly due to a lack of biostratigraphically useful fossils and related radiometric dating. A new species of angiospermous fossil plant, Platanus heilongjiangensis sp. nov., from Qitaihe in eastern Heilongjiang has been found in sediments conformably above which zircons from a rhyolitic tuff has been dated by U-Pb radiometric methods as 96.2±1.7 Ma, indicating that the Upper Houshigou Formation is of Cenomanian age. This discovery not only provides new data to improve our stratigraphic understanding of the Houshigou Formation, but also shows that Platanus flourished in the early Late Cretaceous floras of the region. This new study also indicates active volcanism taking place in the eastern Heilongjiang region during the Cenomanian of the Late Cretaceous.
2019, 10(4): 1543-1560.
doi: 10.1016/j.gsf.2018.10.007
Abstract:
The volcanic rocks of the Xiong'er Group are situated in the southern margin of the North China Craton (NCC). Research on the Xiong'er Group is important to understand the tectonic evolution of the NCC and the Columbia supercontinent during the Paleoproterozoic. In this study, to constrain the age of the Xiong'er volcanic rocks and identify its tectonic environment, we report zircon LA-ICP-MS data with Hf isotope, whole-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopes of the volcanic rocks of the Xiong'er Group. The Xiong'er volcanic rocks mainly consist of basaltic andesite, andesite, dacite and rhyolite, with minor basalt. Our new sets of data combined with those from previous studies indicate that Xiong'er volcanism should have lasted from 1827 Ma to 1746 Ma as the major phase of the volcanism. These volcanics have extremely low MgO, Cr and Ni contents, are enriched in LREEs and LILEs but depleted in HFSEs (Nb, Ta, and Ti), similar to arc-related volcanic rocks. They are characterized by negative zircon εHf(t) values of -17.4 to -8.8, whole-rock initial 87Sr/86Sr values of 0.7023 to 0.7177 and εNd(t) values of -10.9 to -6.4, and Pb isotopes (206Pb/204Pb=14.366-16.431, 207Pb/204Pb=15.106 -15.371, 208Pb/204Pb=32.455-37.422). The available elemental and Sr-Nd-Pb-Hf isotope data suggest that the Xiong'er volcanic rocks were sourced from a mantle contaminated by continental crust. The volcanic rocks of the Xiong'er Group might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by oceanic subduction in the Archean. Thus, we suggest that the subduction-modified lithospheric mantle occurred in an extensional setting during the breakup of the Columbia supercontinent in the Late Paleoproterozoic, rather than in an arc setting.
The volcanic rocks of the Xiong'er Group are situated in the southern margin of the North China Craton (NCC). Research on the Xiong'er Group is important to understand the tectonic evolution of the NCC and the Columbia supercontinent during the Paleoproterozoic. In this study, to constrain the age of the Xiong'er volcanic rocks and identify its tectonic environment, we report zircon LA-ICP-MS data with Hf isotope, whole-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopes of the volcanic rocks of the Xiong'er Group. The Xiong'er volcanic rocks mainly consist of basaltic andesite, andesite, dacite and rhyolite, with minor basalt. Our new sets of data combined with those from previous studies indicate that Xiong'er volcanism should have lasted from 1827 Ma to 1746 Ma as the major phase of the volcanism. These volcanics have extremely low MgO, Cr and Ni contents, are enriched in LREEs and LILEs but depleted in HFSEs (Nb, Ta, and Ti), similar to arc-related volcanic rocks. They are characterized by negative zircon εHf(t) values of -17.4 to -8.8, whole-rock initial 87Sr/86Sr values of 0.7023 to 0.7177 and εNd(t) values of -10.9 to -6.4, and Pb isotopes (206Pb/204Pb=14.366-16.431, 207Pb/204Pb=15.106 -15.371, 208Pb/204Pb=32.455-37.422). The available elemental and Sr-Nd-Pb-Hf isotope data suggest that the Xiong'er volcanic rocks were sourced from a mantle contaminated by continental crust. The volcanic rocks of the Xiong'er Group might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by oceanic subduction in the Archean. Thus, we suggest that the subduction-modified lithospheric mantle occurred in an extensional setting during the breakup of the Columbia supercontinent in the Late Paleoproterozoic, rather than in an arc setting.
2019, 10(4): 1561-1576.
doi: 10.1016/j.gsf.2018.09.015
Abstract:
Concentration of elements or element groups in a geological body is the result of multiple stages of rockforming and ore-forming geological processes. An ore-forming element group can be identified by PCA (principal component analysis) and be separated into two components using BEMD (bi-dimensional empirical mode decomposition):(1) a high background component which represents the ore-forming background developed in rocks through various geological processes favorable for mineralization (i.e. magmatism, sedimentation and/or metamorphism); (2) the anomaly component which reflects the oreforming anomaly that is overprinted on the high background component developed during mineralization. Anomaly components are used to identify ore-finding targets more effectively than ore-forming element groups. Three steps of data analytical procedures are described in this paper; firstly, the application of PCA to establish the ore-forming element group; secondly, using BEMD on the ore-forming element group to identify the anomaly components created by different types of mineralization processes; and finally, identifying ore-finding targets based on the anomaly components. This method is applied to the Tengchong tin-polymetallic belt to delineate ore-finding targets, where four targets for Sn (W) and three targets for Pb-Zn-Ag-Fe polymetallic mineralization are identified and defined as new areas for further prospecting. It is shown that BEMD combined with PCA can be applied not only in extracting the anomaly component for delineating the ore-finding target, but also in extracting the residual component for identifying its high background zone favorable for mineralization from its oreforming element group.
Concentration of elements or element groups in a geological body is the result of multiple stages of rockforming and ore-forming geological processes. An ore-forming element group can be identified by PCA (principal component analysis) and be separated into two components using BEMD (bi-dimensional empirical mode decomposition):(1) a high background component which represents the ore-forming background developed in rocks through various geological processes favorable for mineralization (i.e. magmatism, sedimentation and/or metamorphism); (2) the anomaly component which reflects the oreforming anomaly that is overprinted on the high background component developed during mineralization. Anomaly components are used to identify ore-finding targets more effectively than ore-forming element groups. Three steps of data analytical procedures are described in this paper; firstly, the application of PCA to establish the ore-forming element group; secondly, using BEMD on the ore-forming element group to identify the anomaly components created by different types of mineralization processes; and finally, identifying ore-finding targets based on the anomaly components. This method is applied to the Tengchong tin-polymetallic belt to delineate ore-finding targets, where four targets for Sn (W) and three targets for Pb-Zn-Ag-Fe polymetallic mineralization are identified and defined as new areas for further prospecting. It is shown that BEMD combined with PCA can be applied not only in extracting the anomaly component for delineating the ore-finding target, but also in extracting the residual component for identifying its high background zone favorable for mineralization from its oreforming element group.
2019, 10(4): 1577-1595.
doi: 10.1016/j.gsf.2018.09.016
Abstract:
High resolution shallow seismic data was acquired from inner continental shelf of Goa, west coast of India to map underlying stratigraphic and buried geomorphic features of shelf strata. Seismic data revealed characteristic channel incisions beneath 4-15 m thick sediment layer and corresponds to multi cycle incisions. Stratigraphic analysis of these incision signatures reveals three prominent subaerial unconformities S6, S7 and S9. These unconformities were exposed during the last glacial, penultimate glacial (MIS-6) and prior to penultimate glacial (MIS-8) periods. On the basis of interpreted age of subaerial unconformities and differences in their morphological features, observed channel incisions have been divided grossly into three phases of incision. Phase-1 incisions are older than~330 kyr BP, whereas, incisions of Phase-2 and Phase-3 correspond to~320-125 kyr BP and~115-10 kyr BP respectively. Plan form of these incisions varied from a straight channel type to ingrown meander and then to anastomosing channel types. These channels meet at the confluence of present-day Mandovi and Zuari rivers. The confluence point has varied in due course of time because of cyclic incision and burial with repeated sea level fluctuations. The preserved main channel width varies from~100 m to 1000 m, and maximum channel depth reaches up to~35 m. Comparison of quantitative and qualitative morphologic results of different phases of incisions suggest that Phase-2 channels had~33% more mean bank full discharge than that of the Phase-3 channels. Phase-2 incisions had been carved in higher hydraulic energy condition as compared to Phase-3 incisions implying that the Indian summer monsoon was better during formative stages of Phase-2 incisions.
High resolution shallow seismic data was acquired from inner continental shelf of Goa, west coast of India to map underlying stratigraphic and buried geomorphic features of shelf strata. Seismic data revealed characteristic channel incisions beneath 4-15 m thick sediment layer and corresponds to multi cycle incisions. Stratigraphic analysis of these incision signatures reveals three prominent subaerial unconformities S6, S7 and S9. These unconformities were exposed during the last glacial, penultimate glacial (MIS-6) and prior to penultimate glacial (MIS-8) periods. On the basis of interpreted age of subaerial unconformities and differences in their morphological features, observed channel incisions have been divided grossly into three phases of incision. Phase-1 incisions are older than~330 kyr BP, whereas, incisions of Phase-2 and Phase-3 correspond to~320-125 kyr BP and~115-10 kyr BP respectively. Plan form of these incisions varied from a straight channel type to ingrown meander and then to anastomosing channel types. These channels meet at the confluence of present-day Mandovi and Zuari rivers. The confluence point has varied in due course of time because of cyclic incision and burial with repeated sea level fluctuations. The preserved main channel width varies from~100 m to 1000 m, and maximum channel depth reaches up to~35 m. Comparison of quantitative and qualitative morphologic results of different phases of incisions suggest that Phase-2 channels had~33% more mean bank full discharge than that of the Phase-3 channels. Phase-2 incisions had been carved in higher hydraulic energy condition as compared to Phase-3 incisions implying that the Indian summer monsoon was better during formative stages of Phase-2 incisions.
2019, 10(4): 1597-1612.
doi: 10.1016/j.gsf.2018.10.008
Abstract:
The Tashisayi nephrite deposit is located in South Altyn Tagh, in Qiemo County, Xinjiang Province, northwest China. It is a recent discovery in the vast, well-known Kunlun-Altyn nephrite belt distributed along the south of the Tarim Basin, producing more than half of the nephrite from the whole belt in 2017. Field investigations revealed that it is a dolomitic marble-related (D-type) nephrite deposit, but little is known about its age of formation and relationships between the granites and marble. Here we report field investigations, petrography of the nephrite, as well as petrography, geochemistry, geochronology of the zoisite-quartz altered intrusive rock and adjacent granites. An A-type granite is identified with a SHRIMP U-Pb zircon age of 926±7 Ma, suggesting it was emplaced in an extensional tectonic environment at that time. The altered intrusive rock has a cluster of U-Pb zircon age of 433±10 Ma, with similar trace element features to the A-type granite, suggesting it was formed in an extensional regime at this later time. Nephrite formed because of the metasomatism of dolomite marble by hydrothermal fluids. It is inferred that Ca2+ was released from the dolomitic marble by metasomatism forming Ca-rich fluids, which caused alteration of both the intrusive rocks (6.00-8.22 wt.% CaO) and granite (1.76 -3.68 wt.% CaO) near the nephrite ore bodies. It is also inferred that Fe2+ from the granite migrated towards the dolomite marble. The fluids gave rise to the formation of Ca-minerals, such as zoisite, in the nephrite and altered intrusive rock, and epidote in the granite. Based on the contact relationships, similarity in hydrothermal processes, and consumption of Ca2+, the Tashisayi nephrite is considered to have formed at the same time as the alteration of the intrusive rocks, i.e.~433 Ma. The geochronological similarity (~926 Ma, 433 Ma) of South Altyn and North Qaidam may suggest that tectonically they belong to one single complex in the past, which was offset by the Altyn Tagh fault (ATF). The similar formation ages of the nephrites from Altyn Tagh (433 Ma) and the previously studied areas of West Kunlun (378-441 Ma) and East Kunlun (416 Ma) indicate that these nephrites formed during the closure of Proto-Tethys and in the accompanying post-collisional, extensional environment.
The Tashisayi nephrite deposit is located in South Altyn Tagh, in Qiemo County, Xinjiang Province, northwest China. It is a recent discovery in the vast, well-known Kunlun-Altyn nephrite belt distributed along the south of the Tarim Basin, producing more than half of the nephrite from the whole belt in 2017. Field investigations revealed that it is a dolomitic marble-related (D-type) nephrite deposit, but little is known about its age of formation and relationships between the granites and marble. Here we report field investigations, petrography of the nephrite, as well as petrography, geochemistry, geochronology of the zoisite-quartz altered intrusive rock and adjacent granites. An A-type granite is identified with a SHRIMP U-Pb zircon age of 926±7 Ma, suggesting it was emplaced in an extensional tectonic environment at that time. The altered intrusive rock has a cluster of U-Pb zircon age of 433±10 Ma, with similar trace element features to the A-type granite, suggesting it was formed in an extensional regime at this later time. Nephrite formed because of the metasomatism of dolomite marble by hydrothermal fluids. It is inferred that Ca2+ was released from the dolomitic marble by metasomatism forming Ca-rich fluids, which caused alteration of both the intrusive rocks (6.00-8.22 wt.% CaO) and granite (1.76 -3.68 wt.% CaO) near the nephrite ore bodies. It is also inferred that Fe2+ from the granite migrated towards the dolomite marble. The fluids gave rise to the formation of Ca-minerals, such as zoisite, in the nephrite and altered intrusive rock, and epidote in the granite. Based on the contact relationships, similarity in hydrothermal processes, and consumption of Ca2+, the Tashisayi nephrite is considered to have formed at the same time as the alteration of the intrusive rocks, i.e.~433 Ma. The geochronological similarity (~926 Ma, 433 Ma) of South Altyn and North Qaidam may suggest that tectonically they belong to one single complex in the past, which was offset by the Altyn Tagh fault (ATF). The similar formation ages of the nephrites from Altyn Tagh (433 Ma) and the previously studied areas of West Kunlun (378-441 Ma) and East Kunlun (416 Ma) indicate that these nephrites formed during the closure of Proto-Tethys and in the accompanying post-collisional, extensional environment.
2019, 10(4): 1613-1622.
doi: 10.1016/j.gsf.2018.09.017
Abstract:
Common base and noble metals represent an important economic factor in the actual industrial development. For instance the world resources for copper are actually estimated for about the next 30 years only. The situation requires rethinking the way major ore deposits form, leading to new guides for exploration. The present paper briefly examines the processes leading to ore formation, in relation with granitic or granodioritic intrusions. It identifies the importance of metal enrichment during the magmatic stage. Within the magma chamber that forms porphyry intrusions, metals may incorporate to first formed crystals, becoming inert; concentrate into the residual melt of a mush; or segregate by diffusion into the exsolved magmatic volatile phase (MVP) into which they are transported and precipitated. A competition results between elements partitioning and diffusivity. Hence, a specific Péclet number for each metal (Cu, Au, Ag, Mo, W, Sn, and REE) controls the ratio between the diffusive and the advective flux. Metals diffusivity in the melt shows differential behavior relative to a threshold of about 10-13 m2/s. Metals with slower diffusivity values (e.g. As) will not concentrate. Conversely, fast diffusive metals (Au, Ag, Cu) may rapidly incorporate the MVP, provided an adequate component (halogens or S) is attractive for metals. The chemistry of the MVP escaping the magma induces different alteration patterns. Their relative content in F, Cl or S, attested by the composition of biotites and apatites, links with the preferential content of metals in ore deposits, representing a valuable tool for exploration. Finally the model is replaced in a set of coupled mechanical-chemical instabilities, within a three phase material.
Common base and noble metals represent an important economic factor in the actual industrial development. For instance the world resources for copper are actually estimated for about the next 30 years only. The situation requires rethinking the way major ore deposits form, leading to new guides for exploration. The present paper briefly examines the processes leading to ore formation, in relation with granitic or granodioritic intrusions. It identifies the importance of metal enrichment during the magmatic stage. Within the magma chamber that forms porphyry intrusions, metals may incorporate to first formed crystals, becoming inert; concentrate into the residual melt of a mush; or segregate by diffusion into the exsolved magmatic volatile phase (MVP) into which they are transported and precipitated. A competition results between elements partitioning and diffusivity. Hence, a specific Péclet number for each metal (Cu, Au, Ag, Mo, W, Sn, and REE) controls the ratio between the diffusive and the advective flux. Metals diffusivity in the melt shows differential behavior relative to a threshold of about 10-13 m2/s. Metals with slower diffusivity values (e.g. As) will not concentrate. Conversely, fast diffusive metals (Au, Ag, Cu) may rapidly incorporate the MVP, provided an adequate component (halogens or S) is attractive for metals. The chemistry of the MVP escaping the magma induces different alteration patterns. Their relative content in F, Cl or S, attested by the composition of biotites and apatites, links with the preferential content of metals in ore deposits, representing a valuable tool for exploration. Finally the model is replaced in a set of coupled mechanical-chemical instabilities, within a three phase material.
2019, 10(4): 1623-1633.
doi: 10.1016/j.gsf.2018.09.018
Abstract:
Proto-Adamastor ocean bathed Rodinia and successor continental fragments from 1.0-0.9 Ga up to 0.75 Ga, and evolved into world Adamastor Ocean at 0.75-0.60 Ga. Mesoproterozoic oceanic crust is poorly preserved on continents, only indirect evidence registered in Brasiliano Orogen. We report first evidence of ophiolite originated in proto-Adamastor. We use multi-technique U-Pb-Hf zircon and δ11B tourmaline isotopic and elemental compositions. The host tourmalinite is enclosed in metaserpentinite, both belonging to the Bossoroca ophiolite. Zircon is 920 Ma-old, εHf(920 Ma)=+12, HfTDM=1.0 Ga and has ‘oceanic’ composition (e.g., U/Yb < 0.1). Tourmaline is dravite with δ11B=+1.8‰ (Tur 1), 0‰ (Tur 2), -8.5‰ (Tur 3). These characteristics are a novel contribution to Rodinia and associated world ocean, because a fragment of proto-Adamastor oceanic crust and mantle evolved at the beginning of the Brasiliano Orogen.
Proto-Adamastor ocean bathed Rodinia and successor continental fragments from 1.0-0.9 Ga up to 0.75 Ga, and evolved into world Adamastor Ocean at 0.75-0.60 Ga. Mesoproterozoic oceanic crust is poorly preserved on continents, only indirect evidence registered in Brasiliano Orogen. We report first evidence of ophiolite originated in proto-Adamastor. We use multi-technique U-Pb-Hf zircon and δ11B tourmaline isotopic and elemental compositions. The host tourmalinite is enclosed in metaserpentinite, both belonging to the Bossoroca ophiolite. Zircon is 920 Ma-old, εHf(920 Ma)=+12, HfTDM=1.0 Ga and has ‘oceanic’ composition (e.g., U/Yb < 0.1). Tourmaline is dravite with δ11B=+1.8‰ (Tur 1), 0‰ (Tur 2), -8.5‰ (Tur 3). These characteristics are a novel contribution to Rodinia and associated world ocean, because a fragment of proto-Adamastor oceanic crust and mantle evolved at the beginning of the Brasiliano Orogen.
