Basalt Research Articles

Remote Mapping of Bedrock for Future Cosmogenic Nuclide Exposure Dating Studies in Unvisited Areas of Antarctica

Cosmogenic nuclide exposure dating is an important technique for reconstructing glacial histories. Many of the most commonly applied cosmogenic nuclides are extracted from the mineral quartz, meaning sampling of felsic (silica-rich) rock is often preferred to sampling of mafic (silica-poor) rock for exposure dating studies. Fieldwork in remote regions such as Antarctica is subject to time constraints and considerable logistical challenges, making efficient sample recovery critical to successful research efforts. Remote sensing offers an effective way to map the geology of large areas prior to fieldwork and expedite the sampling process. In this study, we assess the viability of multispectral remote sensing to distinguish felsic from mafic rock outcrops at visible-near infrared (VNIR) and shortwave infrared (SWIR) wavelengths using both the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and very high-resolution Worldview-3 (WV-3) imagery. We applied a combination of spectral mapping and ground truth from spectral measurements of 17 rock samples from Mount Murphy in the Amundsen Sea sector of West Antarctica. Using this approach, we identified four dominant rock types which we used as a basis for felsic–mafic differentiation: felsic granites and gneisses, and mafic basalts and fragmental hydrovolcanic rocks. Supervised classification results indicate WV-3 performs well at differentiating felsic and mafic rock types and that ASTER, while coarser, could also achieve satisfactory results and be used in concert with more targeted WV-3 image acquisitions. Finally, we present a revised felsic–mafic geological map for Mt Murphy. Overall, our results highlight the potential of spectral mapping for preliminary reconnaissance when planning future cosmogenic nuclide sampling campaigns in remote, unvisited areas of the polar regions.

Geochemical signatures and contamination levels of rare earth elements in soil profiles controlled by parent rock and soil properties.

Rare earth elements (REEs) are emerging contaminants rendering potential risks in soils to environmental quality and human health. The causation between their geochemical signatures and contamination levels with parent rocks and soil properties are critical for REEs risk assessments, which are urgently needed globally. Thus, this study aimed to elucidate cause-and-effect among hydrofluoric-acid-digested total and ethylenediaminetetraacetic acid extracted bioavailable soil REEs and their contamination degree evaluated by pollution indices in 268 soil layer (horizon) samples from 50 soil profiles derived from felsic, intermediate, mafic, ultramafic, and sedimentary rocks in Taiwan. The total REEs was 133 ± 61.9mg/kg and all individual REEs were classified as minimal contamination by the enrichment factor. The highest total soil REEs was from granite, followed by sandstone and shale, mafic rocks, andesite, and ultramafic rocks. All soils were ranked as significantly pollution risk by the pollution loading index. Furthermore, REEs were accumulated in all subsoils due to the co-translocation with soil colloids during the intensive leaching, corresponding with their higher geoaccumulation index value than those in surface soils. The contamination degree and bioavailable REEs significantly increased with the increasing total REEs. However, the stronger sequestration of clay and free Fe oxides for heavy REEs (HREEs) over light REEs (LREEs) mitigated their bioavailability, especially in the highly weathered soils like Ultisols and Oxisols. This study highlighted the dominance of soil properties in REEs fractionation, where the preferential fixation of soil colloids for HREEs throughout soil profiles reduced their uptake risk for biome even under the rising contamination potential.

Paleomagnetic data from volcanic rocks in the southern Central Andes of Argentina and their implications for tectonics and geomagnetic field behavior

Abstract A paleomagnetic study of basaltic lava flows exposed in the northern Neuquén Cordillera, southernmost Central Andes, along the Antiñir-Copahue fault zone (ACFZ), involved 25 sites of the Cola de Zorro Formation (Pliocene–Early Pleistocene) along two different sections. The sites show exclusive normal polarity, corresponding to the Late Pliocene Gauss chron (3.6–2.6 Ma). The angular standard deviation of virtual geomagnetic poles (VGPs; ASD = 14.8°) is consistent with the expected values from recent geomagnetic models, in opposition to anomalously low dispersion found in previous studies in Pleistocene VGPs of reverse polarity from neighboring areas to our study zone. Mean paleomagnetic directions for Bella Vista (Dec = 0.0°, Inc = −50.0°, α₉₅ = 7.6°, K = 36.7, N = 11) and Río Huaraco sections (Dec = 354.9°, Inc = −57.0°, α₉₅ = 7.5°, K = 55.7, N = 8) do not show tectonic rotation around vertical axes. Combining and regrouping our and previous data by area confirmed the absence of tectonic rotations in the Huaraco-Trohunco block and a statistically significant clockwise rotation of 14.4° ± 10.3° of three adjacent tectonic blocks located south of our study locality in Pleistocene times. These results suggest that strike-slip deformation along some sections of the ACFZ was significant in the Pleistocene structural evolution of this region.

On-land evidence for subduction of the proto−South China Sea beneath northern Borneo and tectonic reconstruction of Southeast Asia during the early Permian–Oligocene

The development of the Mesozoic proto−South China Sea was closely related to the tectonic evolution of the Tethys and Paleo-Pacific domains and may have been a key driving mechanism for the opening of the South China Sea. Despite its importance, direct geologic evidence for the proto−South China Sea remains limited, and its development is debated between two primary models: intra-oceanic subduction and oceanic-continental subduction. Here, we present petrographic, geochronologic, and geochemical data from tuffs and Eocene−Pliocene sedimentary records in Sabah, northern Borneo. The results reveal that these tuffs, formed ca. 28 Ma, possess geochemical characteristics indicative of oceanic arc origins. Combined with changes in sediment provenance since the Oligocene and various subduction timelines around Borneo, we suggest that these tuffs resulted from the subduction of the proto−South China Sea. This oceanic arc, along with Eocene mid-ocean-ridge−like mafic rocks in Sabah, indicates that a younger oceanic basin had formed within the proto−South China Sea. This aligns with observations in Palawan, Philippines, illustrating the transition from an oceanic spreading center to a subduction zone and indicating the proto−South China Sea extended from eastern Sabah to Palawan. In addition, inherited zircons (ca. 280 Ma) in the tuffs exhibit geochemical signatures typical of continental arcs. Combined with the Permian−Cretaceous arc-related igneous rocks in Southeast Asia, this suggests that southern Sabah was part of the Paleo-Tethys Ocean before integrating into the Paleo-Pacific domain since the Triassic. By shedding light on these processes, our research provides critical insights into the existence, extent, and evolution of the proto−South China Sea and reconstructs the multiphase transitions between different tectonic domains in Southeast Asia.

Transition of CO2 from Emissions to Sequestration During Chemical Weathering of Ultramafic and Mafic Mine Tailings

Weather-enhanced sulphide oxidation accelerates CO2 release into the atmosphere. However, over extended geological timescales, ultramafic and mafic magmatic minerals may transition from being sources of CO2 emissions to reservoirs for carbon sequestration. Ultramafic and mafic mine tailings present a unique opportunity to monitor carbon balance processes, as mine waste undergoes instantaneous and rapid chemical weathering, which shortens the duration between CO2 release and absorption. In this study, we analysed 30 vanadium-titanium magnetite mine tailings ponds with varying closure times in the Panxi region of China, where ~60 years of mineral excavation and dressing have produced significant outcrops of mega-mine waste. Our analysis of anions, cations, saturation simulations, and 87Sr/86Sr; δ13C and δ34S isotopic fingerprints from mine tailings filtrates reveals that the dissolution load of mine tailings may depend significantly on early-stage sulphide oxidation. Despite the abundance of ultramafic and mafic minerals in tailings, dolomite dominates chemical weathering, accounting for ~79.2% of the cationic load. Additionally, due to sulphuric-carbonate weathering, the filtrates undergo deacidification along with sulphide depletion. The data in this study suggest that pristine V-Ti-Fe tailings ponds undergo CO2 emissions in the first two years but subsequently begin to absorb atmospheric CO2 along with the filtrates. Our results provide valuable insights into monitoring weathering transitions and carbon balance in ultramafic and mafic rocks.

Formation mechanism of boehmite and diaspore in karstic bauxites: trace element geochemistry in source materials using large sample geochemical dataset and random forest model

Abstract Boehmite and diaspore are the two economic ore minerals of karstic bauxites. Although their genesis has been studied from different perspectives, their formation mechanism is controversial. Random forest (RF) model of machine learning was employed to extract the combining characteristics of trace elements in boehmite-type bauxite (BTB) and diaspore-type bauxite (DTB). The BTB predominantly exhibits higher median concentrations of Co, Ni, V, and Cr, while the DTB exhibits a more significant enrichment in elements of U, Hf, Th, and Zr. Combining the characteristics of La/Yb and Ga/Al ratios, it is found that disparities between BTB and DTB are mirroring to those between basic rocks and intermediate-felsic rocks. Furthermore, the Zr-Cr-Ga diagram and Ni/Zr versus Cr/Zr plot were utilized to examine the correlation between karstic bauxite (BTB or DTB), lateritic bauxite, and their respective weathered parent rocks. It is found that BTB exhibits consistent characteristics with lateritic bauxite weathered from basic rock and its parent rocks. Similarly, DTB displays consistent characteristics with lateritic bauxite weathered from intermediate-felsic rock and its parent rocks. Through studying the relationship between Ni content and Fe3+/Fe2+ ratios, it has been discovered that the presence of trace elements like Ni in source materials can affect or regulate ore-forming process, ultimately driving the transformation of gibbsite into either boehmite or diaspore. Consequently, the formation of BTB and DTB is significantly influenced, or even governed, by the composition of their source materials. Our study initially highlights a novel insight into the significant impact of source material’s geochemical composition on the formation of boehmite and diaspore, which is associated with weathered igneous rocks categories.

Apatite Geochemical Perspectives on the Maturation of Continental Arc Crust via Mush‐Facilitated Processes During Magmatic Flare‐Up

AbstractIn volcanic arcs, magma evolves from basaltic to intermediate and felsic composition, resulting in arc crust maturation. It remains unclear whether processes involving mush during magmatic flare‐ups would enhance this evolution. This study revealed a temporal‐compositional evolution of plutonic rocks from mafic (∼94 Ma) to intermediate (∼92–88 Ma) to felsic (∼88 Ma) during a magmatic flare‐up event in the Gangdese arc, Tibet, with increasing radiogenic Sr–Nd isotope enrichment. Apatites in mafic and felsic rocks have εNd(t) values similar to their hosts, while intermediate rocks show higher values. The elemental composition of apatites in mafic and intermediate rocks is similar but differs from those in felsic rocks. Textural and compositional features indicate varying degrees of influence of mafic rock compositions by accumulation. Triangular and linear covariation relationships between apatite‐compatible (e.g., La) and ‐incompatible (e.g., Rb) elements with SiO2, respectively, for all plutonic rocks as a whole, confirm the incorporation of apatite‐rich mushes into the mixing process. These findings suggest that mafic magma crystallized into apatite‐rich mush, which was later remobilized and mixed with felsic magma to form intermediate magma. Felsic rocks represent end‐member magmas resulting from crustal anatexis and/or mafic magma differentiation. Thus, the Gangdese arc's maturation during the magmatic flare‐up progressed sequentially through mafic magma crystallization and mush formation, mush remobilization and mixing with felsic magma, and the eventual accumulation and segregation of felsic magma. This sequence of events during flare‐ups illustrates a common crustal maturation process in volcanic arcs, as also seen in the Andean Cordillera.

Effects of high temperature annealing and low temperature metamictization on Archean zircon: constraints from U–Pb isotopes, trace elements and Raman dating

Abstract Zircon is a common accessory mineral that can survive extreme temperature conditions in the crust, although U-Th series radioactive decay is responsible for its self-destruction over long time scales. Upon exhumation at lower temperatures, metamict zircon grains are then more susceptible to isotopic and trace element disturbance. In this work, we investigated the isotopic and trace element behavior of zircon through LA-ICP-MS, coupled with Raman spectroscopy, from two Archean mylonitic samples within the São José do Campestre Massif, northeastern Brazil. U–Pb in zircon dating indicates Paleoarchean (3.3 Ga) sedimentation followed by Mesoarchean (3.0–2.9 Ga) high temperature metamorphism in the mylonitic paragneiss sample, while the mylonitic amphibolite was emplaced during the late Neoarchean (2521 ± 22 Ma) and reached high-temperature conditions during shear zone development and deformation during the Paleoproterozoic (2.0 Ga). Inherited zircon grains in the amphibolite are consistent in morphology and ages with zircon grains from the host paragneiss. However, high temperature annealing and structure recovery upon entrainment of inherited zircon grains in the hot mafic magma led to homogenization of trace elements and preservation of internal textures. On the other hand, zircon grains from the host paragneiss accumulated radiation damage for a longer time, leading to self-destruction of the crystalline structure. Upon later low temperature fluid influx, non-formula elements were incorporated in metamict grains, as evidenced by the positive correlation between trace elements and radiation damage. Raman zircon dating indicates Cambrian to Late Paleozoic exhumation for these rocks, which agrees with U–Pb in zircon lower intercept ages. Although often avoided in geochronological studies, metamict zircon can provide important constraints on low temperature events. Collectively, these observations show a complex trans-crustal history of zircon recycling and exhumation from the Paleoarchean to the Paleozoic and the rock-dependent behavior of zircon upon fluid flow and deformation. The results have implications for the interpretation of trace 36 elements in naturally annealed zircon grains, especially those inherited in mafic rocks.

Revisiting the 10th‐Century Eldgjá Eruption: Modeling the Climatic and Environmental Impacts

AbstractThe 10th‐century Eldgjá eruption in Iceland is the largest basaltic flood lava eruption of the Common Era. However, the extent of its impacts is unclear due to limited historical records. Combining volcanology and ice‐core analyses from both recent and earlier studies, we present a gas emissions scenario for the Eldgjá eruption spanning 937 to 940 CE, co‐injecting volcanic sulfur and halogens. The combined tropospheric and stratospheric sulfur emissions are 3–8 times higher than those adopted for Eldgjá in existing paleoclimate simulations. Earth system modeling of this scenario under pre‐industrial conditions reveals a compound event with maximum northern extratropics surface cooling of C in summer‐autumn of 939 and 940 CE, prolonged Arctic sea ice growth, and large‐scale precipitation changes, concurrent with stratospheric ozone depletion and elevated pollution. These results imply that the combined climatic and environmental effects of the Eldgjá eruption may have significantly impacted human populations at the time.

Late Paleozoic potassic intrusions of the eastern part of the “Nikolaev Line” and associated W–Mo–Cu–Au mineralization: first isotopic U–Pb zircon data (LA-ICP-MS method) for rocks from the Adyrtor intrusions (Middle Tien Shan, Eastern Kyrgyzstan)

The paper presents first data on isotopic U–Pb study (LA-ICP-MS method) of zircon from intrusive rocks of the Adyrtor intrusions – a group of small intrusives situated in the easternmost part of the deep-seated fault system of the “Nikolaev Line”. Here, these intrusions cut through one of the oldest (1.8-2.6 Ga) complexes of the basement rocks of Tien Shan (metamorphic rocks of the “Kuilyu suite”). These intrusive rocks belong to the high-potassic calc-alkaline to shoshonitic series, with some geochemical signatures also of anorogenic (A-type) magmatism. These intrusions are accompanied by skarn and porphyry W–Mo–Cu–Au mineralization. Together with the other Au, W and Cu deposits and occurrences, they are parts of the extended metallogenic belt of Tien Shan. The concordant isotopic U–Pb age data for zircon autocrysts indicate the crystallization of quartz syenite (330.7±4.3 Ma) and quartz monzonite (329.5±5.8 Ma) in the latest Early Carboniferous. The distinct W–Mo “specialization” of ore mineralization related to the intrusions studied is in agreement with post-collisional tectonic setting during their emplacement, despite of their older or similar age compared to the ore-bearing plutons in more western segments of the Middle Tien Shan, which corresponded to subduction-related setting. This supports the possibility of the ‘scissor-like” (from east to west) closure of the Turkestan paleoocean that resulted in the earlier cessation of subduction in the eastern Tien Shan compared to the western Tien Shan. The rocks also contain zircon xenocrysts with older (in the order of 1.7–2.6 Ga) age that is in agreement with the age of the host metamorphic rocks of the Tarim craton basement. This highlights the possibility of magma chamber development in the continental crust, together with the magma chambers in the metasomatically-enriched subcontinental upper mantle, the latter being common for high-potassic magmatism.

North Kara and Vize–Ushakov Composite Tectono-Sedimentary Elements, Kara Sea

The North Kara Shelf and adjoining Severnaya Zemlya Archipelago represent a region of a complex tectonic interplay between two large Arctic fold-and-thrust belts: the Early Mesozoic Pay-Khoy–Novaya Zemlya–South Taimyr belt in the SW and the Late Paleozoic Taimyr–Severnaya Zemlya belt in the SSE and east; and an inferred early Carboniferous deformation zone in the north. Contractional deformations of different ages formed a series of basement highs and inverted sediment-cored swells that separate deeply subsided sedimentary basins. In the framework of this volume's terminology, this region consists of two composite tectono-sedimentary elements (CTSEs): the Vize–Ushakov CTSE and the North Kara CTSE. The principal difference between these elements is in the age of the underlying basement rocks, which, in turn, determines the stratigraphic span of the overlying sedimentary successions. In this chapter we characterize the geology of this remote Arctic region based on the available seismic data supported by published results and our own studies.

Delineating Structural Features Related to Hydrothermal Alterations for Possible Mineralization in Share Area, Kwara State Nigeria Using Aeromagnetic Data

Mineral deposits of significant economic value are abundant in the subsurface of Nigeria, presenting a promising alternative to the nations over dependence on petroleum revenues. This study interprets aeromagnetic data from Share, Kwara State, Nigeria, to delineate subsurface structural features associated with hydrothermal zones, which are key indicators for potential mineralization. The methodologies applied upward continuation, analytic signal, tilt derivative, and first vertical derivative (FVD). These offer insights into subsurface geology that can be broadly applied in geophysical exploration and mineral resource management. The results reveal structural trends predominantly in the NE–SW direction, with some NW–SE alignments, indicative of hydrothermal alterations linked to mineral deposits. The analytical signal map identified amplitude values ranging from 0.004 nT/m to 0.073 nT/m, with low and intermediate magnetic intensities linked to sediment-filled basement rocks and possible limestone and sandstone formations. High-gradient anomalies, 1.280 nT/m to 1.374 nT/m, were attributed to geological contacts, fractures, dykes, and hydrothermal vents. Depth estimates from the source parameter imaging map revealed hydrothermal and structural zones at depths ranging from 287.9 m to 1360.7 m, with deeper sources >1202.1 m indicating tectonic activity and mineralization potential. The FVD and Tilt Derivative maps further highlighted faulted zones, shear structures, and intrusive bodies with intensities between 0.031 nT/m and 0.041 nT/m, suggesting active tectonics. High magnetic anomalies in the central, northeastern, and southeastern regions were identified as prime targets for exploration, indicating magnetite-rich bodies, igneous intrusions, and hydrothermal zones. Integrated exploration strategies combining geophysical, geochemical, and structural data are recommended to refine anomaly delineation, prioritize field validation, and enhance mineralization discovery. These findings establish the Share area as a promising site for regional mineral exploration, supporting Nigeria’s diversification efforts toward sustainable resource development.

Investigating the Structural Framework Beneath Riyadh Region for Potential Geothermal Exploration Utilizing Geological and Geophysical Data

With growing electricity demand and an increasing focus on renewable energy, geothermal energy exploration is gaining prominence in Saudi Arabia. This study investigates the geothermal potential of the Riyadh region by integrating various geophysical data with geological information, providing a detailed image of the structural framework and thermal distribution. We generated a 2D density and susceptibility model using gravity and aeromagnetic data, constrained by well-logs information for sedimentary rocks. Distinct salt layers with low densities (2.15 g/cm3) were introduced in the model overlying the basement rocks, as well as unexposed faults, which may play a crucial role in the thermal regime in the study area. We produced a comprehensive thermal model extracted from the density and susceptibility model, along with temperature measurements from the Minjur Aquifer. The presence of the salt layer not only affects the geothermal gradient, but also suggests the potential for enhanced surface heat flow in specific areas. The results highlight the presence of a promising geothermal reservoir, the Minjur Formation, which exhibits significant potential for geothermal energy extraction due to its porosity, permeability, and sufficient thermal gradient ranging from 80 °C to 120 °C at depths of 2 to 3 km. These findings offer significant implications for Saudi Arabia’s transition to cleaner energy sources and support the future development of geothermal infrastructure in urban areas like Riyadh.

Exploring Talc Deposits: Physical Characteristics, Geological Factors, and Formation in Ultramafic and Mafic Rocks—A Comprehensive Review

Exploring Talc Deposits: Physical Characteristics, Geological Factors, and Formation in Ultramafic and Mafic Rocks—A Comprehensive Review

Using coupled bulk-rock geochemistry and short-wave infrared (SWIR) spectral reflectance data as rapid exploration tools in metamorphosed VHMS deposits: insights from the King Zn deposit, Yilgarn Craton, Western Australia

AbstractBulk rock geochemistry and SWIR reflectance spectroscopy are widely used by companies for rapid and cost-effective exploration of volcanic-hosted massive sulfide (VHMS) deposits. However, few studies have integrated bulk-rock geochemistry with hyperspectral reflectance spectroscopy in greenstone belts that have undergone high-grade metamorphism. Here we present an extensive dataset combining bulk-rock geochemistry with chlorite and white mica SWIR spectral reflectance from the amphibolite-grade King VHMS deposit of the Yilgarn Craton, Western Australia. At King, the footwall stratigraphy is dominated by tholeiitic mafic rocks overlain by a sequence of calc-alkaline intermediate-felsic metavolcanic rocks. The hanging-wall stratigraphy is characterized by a thin metaexhalite layer, overlain by thick succession of interbedded metasedimentary and metavolcanic rocks. Chlorite spectral signatures are more Fe-rich in mafic lithologies and Mg-rich in felsic rocks, particularly where intense Mg-metasomatism occurred before metamorphism. In all units, Fe/Mg ratios of chlorite are strongly tied to bulk rock Fe/Mg ratios. White mica in the footwall is primarily muscovitic, with minor amounts of phengite in deep Fe-rich mafic rocks. By contrast, the hanging-wall sequence is dominated by phengitic signatures in both the Fe-rich metaexhalite, and weakly Ca-Mg altered volcanic rocks. This study concludes that chlorite SWIR reflectance is largely influenced by the bulk Fe/Mg composition of the host rock, whereas white mica reflectance correlates with the type and intensity of hydrothermal alteration prior to metamorphism. These findings underscore the potential of using chlorite and white mica spectral signatures to understand hydrothermal alteration patterns and detect new orebodies in metamorphosed VHMS systems.

A machine learning-based approach using clinopyroxene data to improve accuracy and efficiency in predicting tectonic settings: Implications for Rodinia supercontinent breakup triggered by mantle plume events

Abstract Basalts are ubiquitous mafic rocks found within diverse tectonic settings on Earth. Despite concerted efforts to distinguish tectonic settings through the chemical compositions of basalt, some features of these rocks related to tectonic processes can be obscured by weathering and erosion over geological time, making the discrimination results ambiguous. In this study, we utilized major and trace element data of clinopyroxene in basalts from five distinct tectonic settings: continental within-plate basalts (WPB), island arc basalts (IAB), ocean island basalts (OIB), oceanic floor basalts (OFB), and continental flood basalts (CFB). Employing three machine learning techniques—Multi-layer Perceptron (MLP), Support Vector Machine (SVM), and Random Forest (RF)—we aim to discriminate tectonic settings and magma affinities. Sparse Multinomial Regression (SMR) approach is used to quantitatively discern geochemical signatures that are distinctive of each tectonic setting. The outcomes reveal the efficacy of SVM, which attains an accuracy of 92.1% (major element-based) and 95.2% (major and trace elements-based) for tectonic discrimination. Furthermore, SVM achieves an accuracy of 92.9% (major elements-based) and 95.7% (major and trace elements-based) for magma affinity discrimination. Our study shows that the integration of electron microprobe data from clinopyroxene with machine learning techniques provides an effective approach to distinguish various tectonic settings and magma affinities. The classifier models have been applied to investigate the Neoproterozoic geodynamics of the Jiangnan Orogen. The models show the two pulses of mantle plume events (∼830 Ma and ∼785 Ma) triggering break-up of the Rodinia supercontinent.

Late Mesozoic intracontinental reactivation of the southern Altai, Central Asia

The Altai orogenic belt is a main constituent of the Central Asian Orogenic Belt, and serves as a crucial site for studying strain propagation from the Meso-Cenozoic plate margins to the Eurasian interior. The ranges of the Altai Mountains have undergone multiple reactivation events during the Mesozoic and Cenozoic, but the full extent of these events is not yet fully understood. To constrain the thermo-tectonic history of the southern Altai orogenic belt of Northwest China, apatite fission-track (AFT) and apatite (U-Th)/He (AHe) thermochronological methods were used to study 29 pre-Mesozoic basement rocks from several key localities, including the Altay, Xibodu, Fuyun, and Qinghe regions. Late Jurassic to Early Cretaceous AFT and AHe ages were found in the low-elevation Xibodu region, which has been characterized by slow-to-moderate rock cooling since the Jurassic. However, rock samples from all other regions investigated are characterized by comparable Late Cretaceous AFT and AHe ages. Inverse thermal history modeling results reveal moderate-to-rapid upper crustal cooling in the mid- to Late Cretaceous (ca. 110−70 Ma), which is interpreted to be related to distant plate-margin processes (e.g., the Cimmerian collisions). These findings, combined with previously published data, indicate that Late Cretaceous exhumation was widespread in the western Altai orogenic belt, including in the Chinese and Siberian (Russian) parts of the Altai region. As in many other areas of Central Asia, no Cenozoic low-temperature thermochronological signal was detected in this study. We propose that Cenozoic deformation indeed occurred in the southern Altai, but the magnitude of associated denudation was insufficient to have replaced the Cretaceous cooling signals.

Rudny Altai VMS-polymetallic belt (Russia, Kazakhstan) and its formation factors

The paper presents a modern metallogenic overview of Rudny Altai and the results of the study of the volcanic rocks associated with contrasting basalt-rhyolite formation, manifested as a consequence of riftogenic processes. There are two linear metallogenic subzones within the Rudny Altai polymetallic belt that extend in a northwestern direction. The Zmeinogorsko-Zyryanovskaya subzone is the main one: it contains 2/3 of the belt's deposits, 3/4 of Zn, Pb, Cu, and 4/5 of Au and Ag, which are associated with Emsian-Givetian basalt-rhyolite formation. The Irtysh metallogenic subzons extends along the Irtysh Shear Zone, and is mainly composed of the Eifelian – Early Famennian basalt-rhyolite formation. Devonian bimodal volcanism occurred against a background of extension deformation with the formation of pull-apart basins. Taking into account the structure-kinematic characteristics of faults, the Devonian architecture of the Rudny Altai block can be considered as a 'negative flower' (tulip) structures. Based on the trace element characteristics of initial basic rocks, the original magmas were the product of partial melting of metasomatised lithospheric mantle. This is confirmed by Pb-Pb studies of galena monofractions from the Rudny-Altai volcanogenic massive sulfide (VMS-type) deposits. The magma source of the subsequent major phases probably corresponded to the asthenosphere, which may have risen to the depth of the preceding melting region. The generation of significant volumes of felsic volcanic series, to which the main VMS-type deposits are genetically related, was most likely associated with large-scale melting of thick terrigenous strata of the pre-Devonian palaeoshelf under the influence of mantle magmas. The sequence of Devonian mineralization types is considered to be a consequence of the change in the type of volcanism initiated by transtension tectonics. This is consistent with the concept that the formation of ore-forming systems VMS type is associated with periods of hydrothermal activity during the mantle upwelling under extensive tectonic settings. For this region, the antidromic nature of magmatic series caused a specific evolution trend of its metallogeny, expressed in the change of barite-polymetallic and polymetallic deposits of the Emsian-Eifelian stage (Zyryanovskoe, Tishinka, Ridder-Sokolnoe), pyrite-polymetallic at the Givetian stage (Belousovskoe, Talovskoe) and then copper-pyritic at the Frasnian-Early Famennian stage (Kamyshinskoe, Nikolaevskoe). The results obtained are consistent with the model of evolution of the marginal arc – back arc system, in which mantle uplift is associated with basin extension and plate rollback.

Results of gravity modeling of the central part of the Korsun-Novomyrhorod pluton (Ukrainian Shield)

Three-dimensional gravity modeling of the Gorodishchen and Smilyan gabbro-anorthosite massifs, located in the central part of the Korsun’-Novomirgorod pluton (Ukrainian shield), was performed. A three-dimensional model of the upper crust of the research area was developed using maps of the anomalous gravity field of 1:200 000 scale, taking into account the results of detailed seismic studies by the methods of the RWM (reflected wave method) and CDP (common depth point). Differences in the structure of the intrusive complex of the anorthosite-rapakivi granite formation and the gneisses of the Ingulo-Ingulets series surrounding it were reflected in the seismic wave fields, which made it possible to determine the boundaries of the entire intrusive. For separating the basic rocks and rapakivi granites that differ in their density, three-dimensional gravity modeling was performed using computer technology of automated interpretation of geophysical data based on the trial and error method. For the geological objects parameterization, an approximation model is proposed, which is represented by a set of three-dimensional rod bodies. In the process of solving the inverse problem, various criteria for local optimization of gravitaty field sources were implemented. Three different functionalswere calculated during the iterative process. It is proved that the joint use of the functionals allows to reduce various types of noise in the observed gravity data. During the solving the inverse problem we found out that using of various types of functionals in the algorithms of trial and error methods is quite appropriate. Three-dimensional gravity modeling made it possible to identify and outline gabbro-anorthosite bodies in the upper part of the section with maximum thickness of up to 4—5 km, to clarify the shape and dimensions of the rapakivi granites, and to study the contacts of the intrusive complex with the gneisses surrounding it. The obtained model, which takes into account all the available information on density and geometric parameters of the anomaly forming objects, could be used to obtain additional reliable geological information about the structure of the Gorodishchensk and Smilyan gabbro-anorthosite massifs. The reliability of this algorithm for three-dimensional trial and error gravity method, using an approximation model in the form of a three-dimensional rods construction, allows us to recommend it for the study of similar gabbro-anorthosite massifs of the Ukrainian Shield, primarily the Korosten pluton.

Geochronology and geochemistry characteristics of Neoproterozoic gabbros in Western Yangtze Block and its implication for an injection of mafic magma into the active continental margin

Preserved numerous intermediate to acid igneous rocks with minor basic to ultrabasic rocks, the Western Yangtze Block (WYB) is the window to see the evolutional process of South China Craton during Precambrian era. Here, we report geochronology and geochemistry data from gabbros in the south of Dechang County in SW Sichuan Province. Field investigation and zircon U–Pb age from a gabbro sample suggest that these mafic rocks were emplaced during the early Neoproterozoic era (831.8 ± 2.6 Ma). These gabbros show low contents of SiO2, K2O, and Na2O. They are enriched in Th, and LREEs but depleted in Nb, Ta, Zr, and Ti, showing right-inclined primitive mantle-normalized Rare Earth Elements (REE) patterns with negative to positive weak Eu anomalies. Trace-elemental geochemical features, multiple linear regression, and ratio-ratio plots point to the idea that the primary magma of these gabbros originated from the partial melting of a mantle metasomatized by subduction fluid and contaminated by subducted sediments. The geotectonic discrimination diagrams suggest that these Neoproterozoic gabbros formed at an island arc above a subduction zone in the active continental margin. In addition, we found that the arc-related ~ 830 Ma gabbros in WYB could be divided into two groups. The tholeiite Guandaoshan gabbros showing a flat REE pattern is chemically discerned from the calc-alkaline Dechang and Shimian gabbros having right-inclined REE patterns. We, therefore, thought that a slab window beneath WYB caused Guandaoshan gabbros before it migrated northward to form the Mianning and Shimian A-type granites later.