2020 Vol. 11, No. 3

Focus Paper
A scale-integrated exploration model for orogenic gold deposits based on a mineral system approach
David I. Groves, M. Santosh, Liang Zhang
2020, (3): 719-738. doi: 10.1016/j.gsf.2019.12.007
Abstract(236) HTML PDF(7)
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Concept-based orogenic gold exploration requires a scale-integrated approach using a robust mineral system model. Most genetic hypotheses for orogenic gold deposits that involve near-surface or magmatic-hydrothermal fluids are now negated in terms of a global mineral system model. Plausible models involve metamorphic fluids, but the fluid source has been equivocal. Crustal metamorphic-fluid models are most widely-accepted but there are serious problems for Archean deposits, and numerous Chinese provinces, including Jiaodong, where the only feasible fluid source is sub-crustal. If all orogenic gold deposits define a coherent mineral system, there are only two realistic sources of fluid and gold, based on their syn-mineralization geodynamic settings. These are from devolatilization of a subducted oceanic slab with its overlying gold-bearing sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a subduction event, particularly adjacent to craton margins. In this model, CO2 is generated during decarbonation and S and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 C. This orogenic gold mineral system can be applied to conceptual exploration by first identifying the required settings at geodynamic to deposit scales. Within these settings, it is then possible to define the critical gold mineralization processes in the system: fertility, architecture, and preservation. The geological parameters that define these processes, and the geological, geophysical and geochemical proxies and responses for these critical parameters can then be identified. At the geodynamic to province scales, critical processes include a tectonic thermal engine and deep, effective, fluid plumbing system driven by seismic swarms up lithosphere-scale faults in an oblique-slip regime during uplift late in the orogenic cycle of a convergent margin. At the district to deposit scale, the important processes are fluid focussing into regions of complex structural geometry adjacent to crustal-scale plumbing systems, with gold deposition in trap sites involving complex conjugations of competent and/or reactive rock sequences and structural or lithological fluid caps. Critical indirect responses to defined parameters change from those generated by geophysics to those generated by geochemistry with reduction in scale of the mineral system-driven conceptual exploration.
Artificial intelligence in seismology: Advent, performance and future trends
Pengcheng Jiao, Amir H. Alavi
2020, (3): 739-744. doi: 10.1016/j.gsf.2019.10.004
Abstract(215) HTML PDF(12)
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Realistically predicting earthquake is critical for seismic risk assessment, prevention and safe design of major structures. Due to the complex nature of seismic events, it is challengeable to efficiently identify the earthquake response and extract indicative features from the continuously detected seismic data. These challenges severely impact the performance of traditional seismic prediction models and obstacle the development of seismology in general. Taking their advantages in data analysis, artificial intelligence (AI) techniques have been utilized as powerful statistical tools to tackle these issues. This typically involves processing massive detected data with severe noise to enhance the seismic performance of structures. From extracting meaningful sensing data to unveiling seismic events that are below the detection level, AI assists in identifying unknown features to more accurately predicting the earthquake activities. In this focus paper, we provide an overview of the recent AI studies in seismology and evaluate the performance of the major AI techniques including machine learning and deep learning in seismic data analysis. Furthermore, we envision the future direction of the AI methods in earthquake engineering which will involve deep learning-enhanced seismology in an internet-of-things (IoT) platform.
Research Paper
Garnet fractionation, progressive melt loss and bulk composition variations in anatectic metabasites: Complications for interpreting the geodynamic significance of TTGs
Jillian Kendrick, Chris Yakymchuk
2020, (3): 745-763. doi: 10.1016/j.gsf.2019.12.001
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Tonalite-trondhjemite-granodiorite (TTG) suites constitute a large proportion of the Archean geological record; however, the geodynamic processes that generated them, and Archean continental crust in general, remain a subject of debate. The concentrations and ratios of Sr, Y, La, Yb, Nb, and Ta in TTGs are commonly used to determine the depth of melting of their metabasic sources. The trace element composition of melt produced by metabasic source rocks during anatexis is strongly affected by the presence and abundance of pressure-sensitive minerals, such as plagioclase (Sr-bearing), garnet (Y- and HREE-bearing), and rutile (Nb- and Ta-bearing). Elevated Sr/Y and La/Yb ratios and low concentrations of Nb and Ta in TTGs are generally considered to indicate melting at high pressures (2.0 GPa). The depth of melting is a key factor in determining the origin of TTGs as this provides critical information on the tectonic setting of their generation. We use phase equilibrium and trace element modelling to explore the effects of three potential influences on TTG trace element compositions: fractionation of trace elements into peritectic garnet cores, progressive melt loss from the source, and source bulk composition. We model three different compositions of Archean basalts along thermal gradients of 500 C/GPa, 750 C/GPa, and 1000 C/GPa. The models produce major and trace element melt compositions that are generally consistent with measured compositions of TTGs. Although Sr/Y, La/Yb, Nb, and Ta exhibit pressure-dependent behaviour, other factors also affect these values. Garnet fractionation causes Sr/Y and La/Yb to reach much greater values and in this scenario, the values also increase with increasing temperature. Source bulk composition has an effect in all scenarios and most strongly influences La/Yb, Nb, and Ta. Overall, these results show that Sr/Y, La/Yb, Nb, and Ta can reach values generally considered to be indicative of high pressure melting at a range of P–T conditions including P < 2.0 GPa. Consequently, trace element compositions of TTGs alone may provide a misleading impression of the depth of melting of metabasites and the geodynamic environment of Archean crustal growth and reworking
Identifying the leucogranites in the Ailaoshan-Red River shear zone: Constraints on the timing of the southeastward expansion of the Tibetan Plateau
Xiaohu He, Shucheng Tan, Jiaxi Zhou, Zheng Liu, Zhifang Zhao, Siqi Yang, Yahui Zhang
2020, (3): 765-781. doi: 10.1016/j.gsf.2019.07.008
Abstract:
The uplift of the Tibetan Plateau significantly affected the global climate system. However, the timing of its uplift and the formation of its vast expanse are poorly understood. The occurrence of two types of leucogranites (the two-mica leucogranites and garnet-bearing leucogranites) identified in the Ailaoshan-Red River (ASRR) shear zone suggests an extension event in the southeastern Tibetan Plateau. The age of these leucogranites could be used to constrain the timing of uplift and southeastward expansion of the plateau. Petrography, geochronology and geochemistry investigations, including Sr–Nd isotope analysis, were conducted on the two-mica leucogranites and garnet-bearing leucogranites from the ASRR shear zone. LA-ICP-MS zircon U–Pb dating indicates that these rocks were emplaced at ~27 Ma, implying that the Tibetan Plateau had already achieved maximum uplift prior to the late Oligocene. It subsequently started to expand southeastward as a result of crustal flow. Compared to classic metapelite-derived leucogranites from Himalaya, the two-mica leucogranites show high K2O/Na2O (1.31–1.92), low Rb/Sr, CaO, lower 87Sr/86Sr ratios (0.7089–0.7164) and higher εNd(t) (8.83 to 3.10). This whole-rock geochemical characteristics likely indicates a mixing source origin, composed predominantly of amphibolite with subordinated metapelite, which is also evidenced by 87Sr/86Sr vs. εNd(t) diagram. However, The garnetbearing leucogranites with high SiO2 contents (72.25–74.12 wt.%) have high initial 87Sr/86Sr ratios (0.7332–0.7535) and low εNd(t) (16.36 to 18.98), indicating that they are derived from the source comprised of metapelite and results of fluexed muscovite melting under lower crustal level, which is also evidenced by the Rb–Sr–Ba systematics. These leucogranites formed from partial melting of the thickened lower crust, which resulted in the formation of granitic melt that weakened the crust. The weakened crust aided the left-lateral strikeslip movement of the ASRR shear zone, triggering the escape of the Indochina terrane in the southeastern Tibetan Plateau during the late Oligocene.
Assessment of geochemical and sedimentological characteristics of atmospheric dust in Shiraz, southwest Iran
Amir Karimian Torghabeh, Biswajeet Pradhan, Ashkan Jahandari
2020, (3): 783-792. doi: 10.1016/j.gsf.2019.08.004
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Geogenic dust is commonly believed to be one of the most important environmental problems in the Middle East. The present study investigated the geochemical characteristics of atmospheric dust particles in Shiraz City (south of Iran). Atmospheric dust samples were collected through a dry collector method by using glass trays at 10 location sites in May 2018. Elemental composition was analysed through inductively coupled plasma optical emission spectrometry. Meteorological data showed that the dustiest days were usually in spring and summer, particularly in April. X-ray diffraction analysis of atmospheric dust samples indicated that the mineralogical composition of atmospheric dust was calcite þ dolomite (24%)>palygorskite (18%)>quartz (14%)>muscovite (13%)>albite (11%)>kaolinite (7%)>gypsum (7%)>zircon ¼ anatase (3%). The high occurrence of palygorskite (16%–23%) could serve as a tracer of the source areas of dust storms from the desert of Iraq and Saudi Arabia to the South of Iran. Scanning electron microscopy indicated that the sizes of the collected dust varied from 50 μm to 0.8 μm, but 10 μm was the predominant size. The atmospheric dust collected had prismatic trigonal– rhombohedral crystals and semi-rounded irregular shapes. Moreover, diatoms were detected in several samples, suggesting that emissions from dry-bed lakes, such as Hoor Al-Azim Wetland (located in the southwest of Iran), also contributed to the dust load. Backward trajectory simulations were performed at the date of sampling by using the NOAA HYSPLIT model. Results showed that the sources of atmospheric dust in the study area were the eastern area of Iraq, eastern desert of Saudi Arabia, Kuwait and Khuzestan Province. The Ca/Al ratio of the collected samples (1.14) was different from the upper continental crust (UCC) value (UCC ¼ 0.37), whereas Mg/ Al (0.29), K/Al (0.22) and Ti/Al (0.07) ratios were close to the UCC value (0.04). This condition favours desert calcisols as the main mineral dust sources. Analysis of the crustal enrichment factor (EFcrustal) revealed geogenic sources for V, Mo, Pb, Sr, Cu and Zn (<2), whereas anthropogenic sources affected As, Cd, Cr and Ni.
Petrogenesis of the diamondiferous Pipe-8 ultramafic intrusion from the Wajrakarur kimberlite field of Southern India and its relation to the worldwide Mesoproterozoic (~1.1 Ga) magmatism of kimberlite and related rocks
Ashish Dongre, K.S. Viljoen, G. Belyanin, Petrus Le Roux, M. Malandkar
2020, (3): 793-805. doi: 10.1016/j.gsf.2019.07.010
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Detailed mineralogy, bulk rock major, trace and Sr–Nd isotope compositions, and 40Ar/39Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India, is reported. Based on the presence of Ti-rich phlogopite, high Na/K content in amphibole, Al- and Ti-rich diopside, a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass, the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre (i.e., aillikite). An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr–Nd isotope geochemistry. Sr–Nd isotope data are consistent with a common, moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India. A phlogopite-rich groundmass 40Ar/39Ar plateau age of 1115.8  7.9 Ma (2σ) for the Pipe-8 intrusion falls within a restricted 100 Ma time bracket as defined by the 1053–1155 Ma emplacement ages of kimberlites and related rocks in India. The presence of ultramafic lamprophyres, carbonatites, kimberlites, and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle. The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic (ca. 1.1 Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari, Australian, West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly. In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia. This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia. It is speculated that the presence of a large plume at or close to 1.1 Ga within the Rodinian supercontinent, with the Indian block located on its periphery, could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic, volatile rich rocks in India due to comparatively low thermal effects from the distant plume.
Evolutionary biogeography on Ophiocordyceps sinensis: An indicator of molecular phylogeny to geochronological and ecological exchanges
Yongdong Dai, Changkui Wu, Feng Yuan, Yuanbing Wang, Luodong Huang, Zihong Chen, Wenbo Zeng, Yao Wang, Zhuliang Yang, Pusheng Zeng, Paul Lemetti, Xuanxue Mo, Hong Yu
2020, (3): 807-820. doi: 10.1016/j.gsf.2019.09.001
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The fungus Ophiocordyceps sinensis is endemic to the vast region of the Qinghai-Tibetan plateau (QTP). The unique and complex geographical environmental conditions have led to the “sky island” distribution structure of O. sinensis. Due to limited and unbalanced sample collections, the previous data on O. sinensis regarding its genetic diversity and spatial structure have been deemed insufficient. In this study, we analyzed the diversity and phylogeographic structures of O. sinensis using internally transcribed spacer region (ITS) and 5-locus datasets by a large-scale sampling. A total of 111 haplotypes of ITS sequences were identified from 948 samples data of the fungus O. sinensis, with representing high genetic diversity, and 8 phylogenetic clades were recognized in O. sinensis. Both the southeastern Tibet and the northwestern Yunnan were the centers of genetic diversity and genetic differentiation of the fungus, and they were inferred as the glacial refugia in the Quaternary. Three distribution patterns were identified to correspond to the 8 clades, including but not limited to the coexistence of widely and specific local distributive structures. It also revealed that the differentiation pattern of O. sinensis did not fit for the isolation-by-distance model. The differentiation into the 8 clades occurred between 1.56 Myr and 6.62 Myr. The ancestor of O. sinensis most likely originated in the late Miocene (6.62 Myr) in the northwestern Yunnan, and the Scene A–C of the Qinghai–Tibetan movements may have played an important role in the differentiation of O. sinensis during the late Miocene–Pliocene periods. Our current results provide a much clearer and detailed understanding of the genetic diversity and geographical spatial distribution of the endemic alpine fungus O. sinensis. It also revealed that the geochronology resulting from paleogeology could be cross-examined with biomolecular clock at a finer scale.
Distribution of selenium and arsenic in differentiated multicellular eukaryotic fossils and their significance
Jie Long, Shixi Zhang, Kunli Luo
2020, (3): 821-833. doi: 10.1016/j.gsf.2019.08.005
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We present variation patterns of trace elements within different sequences of the Ediacaran Doushantuo Formation (DST, 635–551 million years ago), and inside the cells of the earliest differentiated multicellular eukaryotic fossils of the Weng’an biota in the Weng’an County of Guizhou Province. The results showed that selenium is the most enriched and significantly varied trace element among the 22 trace elements throughout the DST, followed by arsenic. The highest selenium and arsenic content sequences are consistent with the first appeared sequence of the earliest differentiated multicellular eukaryotic fossils Megasphaera at the middle to upper parts of the DST. Nanoman secondary ion mass spectrometry analyses show that selenium and arsenic have an inhomogeneous and punctate distribution in the nucleus and cytoplasm. The nucleus has anomalously enriched levels of selenium and arsenic among the organelles. The selenium and arsenic concentrations exhibit a positive correlation with the diversity of fossilized Megasphaera. These new findings give us a clue that the anomalous enrichment of selenium and arsenic might contributes to the cell differentiation in Ediacaran Doushantuo period.
Provenance bias between detrital zircons from sandstones and river sands: A quantification approach using 3-D grain shape, composition and age
V. Markwitz, C.L. Kirkland, K. Gessner
2020, (3): 835-842. doi: 10.1016/j.gsf.2019.09.002
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Preservation bias may significantly impact the application of detrital zircon geochronology in reconstructing Earth surface processes. Here we compare detrital zircons from the actively eroding Murchison River channel in Western Australia with Ordovician fluvial sediments that have drained similar source rocks along the western margin of the West Australian Craton. In addition to standard analysis of detrital zircon age spectra we apply multivariate statistics to test the relation between 3-D grain shape, U-content and U–Pb ages, with the objective to quantify differences between both sample groups and track preservation along the transport pathway of the Murchison River. Our results show that zircon grains in modern river sands display an upstream trend toward larger surface areas, volume equivalent diameters and grain widths, as well as toward higher U-contents and lower apparent grain densities. 3-D grain shape, size and age spectra of Murchison River zircons evolve consistently downstream, but even at the river outlet remain distinct from the Ordovician samples, as a less mature representation of source. We interpret Ordovician river zircons to represent a significantly depleted subset from which up to 22% of the zircon population may have been lost compared to the actively transported detrital load. This discrepancy between the characteristics of detrital zircons in modern active rivers and ancient fluvial Ordovician sandstones demonstrates a bias that could be relevant for other source-sink detrital transport systems throughout Earth history.
Strategies towards robust interpretations of in situ zircon Lu–Hf isotope analyses
C.J. Spencer, C.L. Kirkland, N.M.W. Roberts, N.J. Evans, J. Liebmann
2020, (3): 843-853. doi: 10.1016/j.gsf.2019.09.004
Abstract(153) HTML PDF(5)
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The combination of U–Pb and Lu–Hf compositions measured in zircon crystals is a remarkably powerful isotopic couplet that provides measures on both the timing of mineral growth and the radiogenic enrichment of the source from which the zircon grew. The U–Pb age documents the timing of zircon crystallization/recrystallization and Hf isotopes inform on the degree to which the host melt was derived from a radiogenic reservoir (e.g. depleted mantle) versus an unradiogenic reservoir (e.g. ancient continental crust), or some mixture of these sources. The ease of generating large quantities of zircon U–Pb and Lu–Hf data has been in large part facilitated by instrument advances. However, the dramatic increase in time constrained zircon Lu–Hf analyses in the Earth science community has brought to the fore the importance of careful data collection and reduction workflows, onto which robust geological interpretations may be based. In this work, we discuss the fundamentals of Lu–Hf isotopes in zircon, which then allows us to provide a robust, accessible, methodology for the assessment of data quality. Additionally, we discuss some novel techniques for: data visualization — that facilitates better transparency of data interpretation; integration of geographic information—that may reveal spatial trends where temporal trends were only apparent before; and some novel statistical evaluation tools — that may provide more rigorous interand intra-sample comparisons.
Dating long thrust systems on Mercury: New clues on the thermal evolution of the planet
L. Giacomini, M. Massironi, V. Galluzzi, S. Ferrari, P. Palumbo
2020, (3): 855-870. doi: 10.1016/j.gsf.2019.09.005
Abstract(119) HTML PDF(1)
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The global tectonics of Mercury is dominated by contractional features mainly represented by lobate scarps, high relief ridges, and wrinkle ridges. These structures are the expression of thrust faults and are linear or arcuate features widely distributed on Mercury. Locally, these structures are arranged in long systems characterized by a preferential orientation and non-random spatial distribution. In this work we identified five thrust systems, generally longer than 1000 km. They were named after the main structure or crater encompassed by the system as: Thakur, Victoria, Villa Lobos, Al-Hamadhani, and Enterprise. In order to gain clues about their formation, we dated them using the buffered crater counting technique, which can be applied to derive the ages of linear landforms such as faults, ridges and channels. To estimate the absolute age for the end of the thrust system’s activity, we applied both Le Feuvre and Wieczorek Production Function and Neukum Production Functions. Moreover, to further confirm the results obtained with the buffered crater counting method, the classic stratigraphic approach has been adopted, in which a faulted and an unfaulted craters were dated for each system. The results gave consistent ages and suggested that the most movements along major structures all over Mercury most likely ended at about 3.6–3.8 Ga. This gives new clues to better understand the tectonics of the planet and, therefore, its thermal evolution. Indeed, the early occurrence of tectonic activity in the planet’s history, well before than predicted by the thermophysical models, coupled with the orientation and spatial distribution of the thrust systems, suggests that other processes beside global contraction, like mantle downwelling or tidal despinning, could have contributed to the first stage of the planet’s history.
How do machine learning techniques help in increasing accuracy of landslide susceptibility maps?
Yacine Achour, Hamid Reza Pourghasemi
2020, (3): 871-883. doi: 10.1016/j.gsf.2019.10.001
Abstract(226) HTML PDF(8)
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Landslides are abundant in mountainous regions. They are responsible for substantial damages and losses in those areas. The A1 Highway, which is an important road in Algeria, was sometimes constructed in mountainous and/or semi-mountainous areas. Previous studies of landslide susceptibility mapping conducted near this road using statistical and expert methods have yielded ordinary results. In this research, we are interested in how do machine learning techniques help in increasing accuracy of landslide susceptibility maps in the vicinity of the A1 Highway corridor. To do this, an important section at Ain Bouziane (NE, Algeria) is chosen as a case study to evaluate the landslide susceptibility using three different machine learning methods, namely, random forest (RF), support vector machine (SVM), and boosted regression tree (BRT). First, an inventory map and nine input factors were prepared for landslide susceptibility mapping (LSM) analyses. The three models were constructed to find the most susceptible areas to this phenomenon. The results were assessed by calculating the receiver operating characteristic (ROC) curve, the standard error (Std. error), and the confidence interval (CI) at 95%. The RF model reached the highest predictive accuracy (AUC ¼ 97.2%) comparatively to the other models. The outcomes of this research proved that the obtained machine learning models had the ability to predict future landslide locations in this important road section. In addition, their application gives an improvement of the accuracy of LSMs near the road corridor. The machine learning models may become an important prediction tool that will identify landslide alleviation actions.
Proterozoic tectonothermal processes imaged with magnetotellurics and seismic reflection in southern Australia
Tom Wise, Stephan Thiel
2020, (3): 885-893. doi: 10.1016/j.gsf.2019.09.006
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Over the last two decades, co-located seismic and magnetotelluric (MT) profiles provided fundamental geophysical data sets to image the Australian crust. Despite their complimentary nature, the data are processed and often interpreted separately without common processes in mind. We here qualitatively compare 2D resistivity inversion models derived from MT and seismic reflection profiles across a region of Archean–Proterozoic Australia to address the causes of variations in seismic response and anomalous conductivity in the crust. We find that there exists a spatial association between regions of low reflectivity in seismic sections and low resistivity in co-located 2D MT modelled sections. These relationships elucidate possible signatures of past magmatic and fluid-related events. Depending on their diffuse or discrete character, we hypothesize these signatures signify fossil melting of the crust due to mafic underplating, magma movement or hydrothermal fluid flow through the crust. The approach discussed herein is a process-oriented approach to interpretation of geophysical images and a significant extension to traditional geophysical methods which are primarily sensitive to a singular bulk rock property or state.
Late Mesozoic magmatism in the Jiaodong Peninsula, East China: Implications for crust–mantle interactions and lithospheric thinning of the eastern North China Craton
Yayun Liang, Xuefei Liu, Qingfei Wang, Rui Zhao, Yao Ma
2020, (3): 895-914. doi: 10.1016/j.gsf.2019.09.008
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A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula, East China, containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions, was chosen to investigate the lithospheric evolution of the eastern North China Craton (NCC). Zircon U–Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic (~145 Ma) and late Early Cretaceous (112–107 Ma), respectively; high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at ~139 Ma and ~118 Ma, and 125–145 Ma and 115–120 Ma, respectively. The geochemical data, including whole-rock major and trace element compositions and Sr–Nd–Pb isotopes, imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab. Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC, while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts. Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous. We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic. Slab rollback of the plate from ~160 Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism.
Redox evolution of western Tianshan subduction zone and its effect on deep carbon cycle
Renbiao Tao, Lijuan Zhang, Lifei Zhang
2020, (3): 915-924. doi: 10.1016/j.gsf.2019.09.007
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Knowing the phase relations of carbon-bearing phases at high-pressure (HP) and high-temperature (HT) condition is essential for understanding the deep carbon cycle in the subduction zones. In particular, the phase relation of carbon-bearing phases is also strongly influenced by redox condition of subduction zones, which is poorly explored. Here we summarized the phase relations of carbon-bearing phases (calcite, aragonite, dolomite, magnesite, graphite, hydrocarbon) in HP metamorphic rocks (marble, metapelite, eclogite) from the Western Tianshan subduction zone and high-pressure experiments. During prograde progress of subduction, carbonates in altered oceanic crust change from Ca-carbonate (calcite) to Ca,Mg-carbonate (dolomite), then finally to Mgcarbonate (magnesite) via Mg–Ca cation exchange reaction between silicate and carbonate, while calcite in sedimentary calcareous ooze on oceanic crust directly transfers to high-pressure aragonite in marble or amorphous CaCO3 in subduction zones. Redox evolution also plays a significant effect on the carbon speciation in the Western Tianshan subduction zone. The prograde oxygen fugacity of the Western Tianshan subduction zone was constrained by mineral assemblage of garnet-omphacite from FMQ  1.9 to FMQ  2.5 at its metamorphic peak (maximum P-T) conditions. In comparison with redox conditions of other subduction zones, Western Tianshan has the lowest oxygen fugacity. Graphite and light hydrocarbon inclusions were ubiqutously identified in Western Tianshan HP metamorphic rocks and speculated to be formed from reduction of Fe-carbonate at low redox condition, which is also confirmed by high-pressure experimental simulation. Based on petrological observation and high-pressure simulation, a polarized redox model of reducing slab but oxidizing mantle wedge in subduction zone is proposed, and its effect on deep carbon cycle in subduction zones is further discussed.
Lead isotope evolution of the Central European upper mantle: Constraints from the Bohemian Massif
Simona Krmíčková, Lukáš Krmíček, Rolf L. Romer, Jaromír Ulrych
2020, (3): 925-942. doi: 10.1016/j.gsf.2019.09.009
Abstract(127) HTML PDF(1)
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The Pb isotope composition of the upper mantle beneath Central Europe is heterogeneous due to the subduction of regionally contrasting material during the Variscan and Alpine orogenies. Late Variscan to Cenozoic mantlederived melts allow mapping this heterogeneity on a regional scale for the last ca. 340 Myr. Late Cretaceous and Cenozoic anorogenic magmatic rocks of the Bohemian Massif (lamprophyres, volcanic rocks of basanite/ tephrite and trachyte/phonolite series) concentrate mostly in the Eger Rift. Cretaceous ultramafic lamprophyres yielded the most radiogenic Pb isotope signatures reflecting a maximum contribution from metasomatised lithospheric mantle, whereas Tertiary alkaline lamprophyres originated from mantle with less radiogenic 206Pb/204Pb ratios suggesting a more substantial modification of lithospheric source by interaction with asthenosphericderived melts. Cenozoic volcanic rocks of the basanite/tephrite and trachyte/phonolite series define a linear mixing trend between these components, indicating dilution of the initial lithospheric mantle signature by upwelling asthenosphere during rifting. The Pb isotope composition of Late Cretaceous and Cenozoic magmatic rocks of the Bohemian Massif follows the same Pb growth curve as Variscan orogenic lamprophyres and lamproites that formed during the collision between Laurussia, Gondwana, and associated terranes. This implies that the crustal Pb signature in the post-Variscan mantle is repeatedly sampled by younger anorogenic melts. Most Cenozoic mantle-derived rocks of Central Europe show similar Pb isotope ranges as the Bohemian Massif.
Metasomatism-induced wehrlite formation in the upper mantle beneath the Nograd-G€om€or Volcanic Field (Northern Pannonian Basin): Evidence from xenoliths
Levente Patk o, N ora Liptai , L aszl o El}od Aradi , Rita Kl ebesz , Eszter Sendula, Robert J. Bodnar , Istv an J anos Kov acs , K aroly Hidas , Bernardo Cesare , Attila Nov ak , Bal azs Tr asy i, Csaba Szab
2020, (3): 943-964. doi: 10.1016/j.gsf.2019.09.012
Abstract(127) HTML PDF(1)
Abstract:
Clinopyroxene-enriched upper mantle xenoliths classified as wehrlites are common (~20% of all xenoliths) in the central part of the Nograd-G€om€or Volcanic Field (NGVF), situated in the northern margin of the Pannonian Basin in northern Hungary and southern Slovakia. In this study, we thoroughly investigated 12 wehrlite xenoliths, two from each wehrlite-bearing occurrence, to determine the conditions of their formation. Specific textural features, including clinopyroxene-rich patches in an olivine-rich lithology, orthopyroxene remnants in the cores of newlyformed clinopyroxenes and vermicular spinel forms all suggest that wehrlites were formed as a result of intensive interaction between a metasomatic agent and the peridotite wall rock. Based on the major and trace element geochemistry of the rock-forming minerals, significant enrichment in basaltic (Fe, Mn, Ti) and high field strength elements (Nb, Ta, Hf, Zr) was observed, compared to compositions of common lherzolite xenoliths. The presence of orthopyroxene remnants and geochemical trends in rock-forming minerals suggest that the metasomatic process ceased before complete wehrlitization was achieved. The composition of the metasomatic agent is interpreted to be a mafic silicate melt, which was further confirmed by numerical modelling of trace elements using the plate model. The model results also show that the melt/rock ratio played a key role in the degree of petrographic and geochemical transformation. The lack of equilibrium and the conclusions drawn by using variable lherzolitic precursors in the model both suggest that wehrlitization was the last event that occurred shortly before xenolith entrainment in the host mafic melt. We suggest that the wehrlitization and the Plio–Pleistocene basaltic volcanism are related to the same magmatic event.
Organo-petrographic and geochemical characteristics of Gurha lignite deposits, Rajasthan, India: Insights into the palaeovegetation, palaeoenvironment and hydrocarbon source rock potential
Runcie Paul Mathews , Bhagwan D. Singh , Vikram Partap Singh, Alpana Singh, Hukam Singh, Mahesh Shivanna, Suryendu Dutta , Vinod A. Mendhe , Rimpy Chetia
2020, (3): 965-988. doi: 10.1016/j.gsf.2019.10.002
Abstract(164) HTML PDF(8)
Abstract:
The sedimentary sequence containing lignite deposits in Gurha quarry of the Bikaner-Nagaur Basin (Rajasthan) has been investigated. The samples from lignite and allied shale horizons were evaluated for petrographical, palynological, palynofacies and organic geochemical inferences, to depict the source flora and to reconstruct the palaeodepositional conditions prevailed during the sedimentation. An assessment for the hydrocarbon generation potential of these deposits has also been made. The results revealed the dominance of huminite macerals and phytoclasts organic matter (OM) indicating the existence of forested vegetation in the vicinity of the depositional site. A relatively high terrigenous/aquatic ratio (TAR) and the carbon preference index (CPI) are also suggesting the contribution of higher plants in the peat formation. However, the n-alkane distributions, maximizing at n-C17 and n-C29, showed inputs from the algal communities along with the higher plant derived organic matters. Recovered palynomorphs of the families Onagraceae, Meliaceae, Arecaceae, Rhizophoraceae, Rubiaceae, Ctenolophonaceae, etc. together with oleanene and ursane types of triterpenoids suggest the contribution from angiosperms source vegetation. Interestingly, the presence of Araucareaceae and Podocarpaceae pollen grains shows the existence of gymnosperms vegetation. Further, the presence of tetracyclic diterpanes; demethylated entbeyerane, sandaracopimarane, pimarane, and Kaurane type of compounds confirms the contribution of conifers. The variation in the values of the coefficient of non-equality (H: 0.68%–7.56%), the standard deviation (δ: 0.04%–0.16%) and the coefficient of variability (V: 16.10%–46.47%), also shows the heterogeneity in the source organic matter. The various petrographical indices, palynological entities, and geochemical parameters indicate that the peatforming vegetation was accumulated under a mixed environment and fluctuating hydrological settings. The interpretation of palynofacies data on APP (Amorphous organic matter-Phytoclast-Palynomorphs) diagram suggests that the accumulation of organic matter occurred in a dysoxic-suboxic condition in a proximal (to land) setting with the shift to an anoxic condition in distal setting towards the termination of sedimentation. The huminite (ulminite) reflectance (Rr) values (av. 0.28%) showed a good relationship with average Tmax value (414 C), suggesting the immaturity. The TOC content ranges of 13–59 wt.%, and HI values vary between 101 and 546 mg HC/g TOC in the studied samples. Collectively, the studied lignite and shale samples have the admixed kerogens (Type III–II) and exhibit the ability to generate the gaseous to oil hydrocarbons upon maturation.
Scattering and absorption imaging of a highly fractured fluid-filled seismogenetic volume in a region of slow deformation
Ferdinando Napolitano, Luca De Siena, Anna Gervasi, Ignazio Guerra, Roberto Scarpa, Mario La Rocca
2020, (3): 989-998. doi: 10.1016/j.gsf.2019.09.014
Abstract(161) HTML PDF(2)
Abstract:
Regions of slow strain often produce swarm-like sequences, characterized by the lack of a clear mainshockaftershock pattern. The comprehension of their underlying physical mechanisms is challenging and still debated. We used seismic recordings from the last Pollino swarm (2010–2014) and nearby to separate and map seismic scattering (from P peak-delays) and absorption (from late-time coda-wave attenuation) at different frequencies in the Pollino range and surroundings. High-scattering and high-absorption anomalies are markers of a fluid-filled fracture volume extending from SE to NW (1.5–6 Hz) across the range. With increasing frequency, these anomalies approximately cover the area where the strongest earthquakes occurred from the sixteenth century until 1998. In our interpretation, the NW fracture propagation ends where carbonates of the Lucanian Apennines begin, as marked by a high-scattering and low-absorption area. At the highest frequency (12 Hz) the anomalies widen southward in the middle of the range, consistently marking the faults active during the recent Pollino swarm. Our results suggest that fracture healing has closed small-scale fractures across the SE faults that were active in the past centuries, and that the propagation of fluids may have played a crucial role in triggering the 2010–2014 Pollino swarm. Assuming that the fluid propagation ended at the carbonates barrier in the NW direction, fractures opened new paths to the South, favoring the nucleation of the last Pollino swarm. Indeed, the recently active faults in the middle of the seismogenic volume are marked by a high-scattering and highabsorption footprints. Our work provides evidence that attenuation parameters may track shape and dynamics of fluid-filled fracture networks in fault areas.
3D model of Svecofennian Accretionary Orogen and Karelia Craton based on geology, reflection seismics, magnetotellurics and density modelling: Geodynamic speculations
Michael V. Mints , Victor N. Glaznev , Olga M. Muravina , Elena Yu Sokolova
2020, (3): 999-1023. doi: 10.1016/j.gsf.2019.10.003
Abstract:
A 3D model of deep crustal structure of the Archaean Karelia Craton and late Palaeoproterozoic Svecofennian Accretionary Orogen including the boundary zone is presented. The model is based on the combination of data from geological mapping and reflection seismic studies, along profiles 1-EU, 4B, FIRE-1-2a-2 and FIRE-3-3a, and uses results of magnetotelluric soundings in southern Finland and northern Karelia. A seismogeological model of the crust and crust–mantle boundary is compared with a model of subhorizontal velocity-density layering of the crust. The TTG-type crust of the Palaeoarchaean and Mesoarchaean microcontinents within the Karelia Craton and the Belomorian Province are separated by gently dipping greenstone belts, at least some of which are palaeosutures. The structure of the crust was determined mainly by Palaeoproterozoic tectonism in the intracontinental settings modified by a strong collisional compression at the end of the Palaeoproterozoic. New insights into structure, origin and evolution of the Svecofennian Orogen are provided. The accretionary complex is characterized by inclined tectonic layering: the tectonic sheets, ~15 km thick, are composed of volcanic-sedimentary rocks, including electro-conductive graphite-bearing sedimentary rocks, and electro-resistive granitoids, which plunge monotonously and consecutively eastward. Upon reaching the level of the lower crust, the tectonic sheets of the accretionary complex lose their distinct outlines. In the seismic reflection pattern they are replaced by a uniform acoustically translucent medium, where separate sheets can only be traced fragmentarily. The crust–mantle boundary bears a diffuse character: the transition from crust to mantle is recorded by the disappearance of the vaguely drawn boundaries of the tectonic sheets and in the gradual transition of acoustically homogeneous and translucent lower crust into transparent mantle. Under the effect of endogenic heat flow, the accretionary complex underwent high-temperature metamorphism and partial melting. Blurring of the rock contacts, which in the initial state created contrasts of acoustic impedance, was caused by partial melting and mixing of melts. The 3D model is used as a starting point for the evolutionary model of the Svecofennian Accretionary Orogen and for determination of its place in the history of the Palaeoproterozoic Lauro-Russian intracontinental orogeny, which encompassed a predominant part of the territory of Lauroscandia, a palaeocontinent combining North American and East European cratons. The model includes three stages in the evolution of the Lauro-Russian Orogen (~2.5, 2.2–2.1 and 1.95–1.87 Ga). The main feature of the Palaeoproterozoic evolution of the accretionary Svecofennian Orogen and Lauroscandia as a whole lay in the causal link with evolution of a superplume, which initiated plate-tectonic events. The Svecofennian–Pre-Labradorian palaeo-ocean originated in the superplume axial zone; the accretionary orogens were formed along both continental margins due to closure of the palaeo-ocean.