Basalt Research Articles

Xenolith petrography for identification of substratum lithology of an active volcano: a case study of Lava Kekep, Sundoro Volcano, Central Java

Abstract Sundoro volcano is an active Quaternary stratovolcano in Central Java, located approximately 50 km west of the Merapi volcano. It is attributed to parasitic cones producing basaltic lava flow. We present our findings of sedimentary and igneous xenoliths embedded in the basaltic lava of the Kekep parasitic cone. Xenolith is one of the key materials to help identify various lithologies composing the substratum of a volcano. Twelve thin sections were analysed through detailed petrographic observation. We identified variations of xenolith lithology consisting of sedimentary rock, particularly siltstone, and igneous rocks such as dunite, pyroxenite and lherzolite. Detailed accounts of substratum lithologies comprises of (a) siltstone composed of >0.125-0.5 mm size plagioclase crystals and silicic fine-grained matrix, (b) pyroxenite consists of massive interlocking 1-5 mm size clinopyroxene and few <1 – 1 mm orthopyroxene, (c) dunite consists of 1 - 2 mm phenocryst of plagioclase, pyroxene, and olivine, embedded in a minor groundmass of plagioclase and pyroxene, (d) lherzolite consists of massive interlocking olivine crystals 1-5 mm in size. The analysis results identify gradual or groundmass and sharp contact between basaltic host rock and xenoliths (pyroxenite, lherzolite, dunite and siltstone). Other characteristics of the twelve samples includes (a) <1 mm minerals fragments embedded in silt to clay-grained lithic sandstone, (b) siltstone showing chilled-baked margin in hand sample specimen, (c) identified disequilibrium textures in basaltic lava, and contact between xenolith and host rock. Substratum lithology along the Java arc is likely composed of Tertiary sedimentary rock in the upper part, transitions of granitoid – gabbroic diorite crustal material in the lower part, with ultramafic crustal material under Sundoro.

Mineralogy of Lateritic Weathering Profiles Developed on the Rocks under a Sub-equatorial Monsoon Climate: Case of the Bambouto Mountains

ABSTRACT The main objective of this work is to make a comparative assessment of the mineralogy on weathering profiles resulting from the alteration of the different rocks in an equatorial microclimate of altitude. This was carried out in the lower flank of the southern slope of the Bambouto Mountains. Mineralogical analysis and normative restructuring have given very interesting results on the profiles, between the basement rocks and the surface/cover rocks. Given that the alteration pathways of the basement rocks are different, they are similar within the zone of containment and potential leaching. Meanwhile, the cover rocks vary from one domain to another: virtual weathering, potential confinement, potential and virtual weathering, and induration. The alterological activity is very intense in the profile on trachybasalt. The predominant weathering processes in all cases are allitization and monosialitization, which are marked by a singular phenomenon of ferrolysis. The characteristic minerals of this profile are kaolinite and diopside. The weathering profile on biotite and hornblende granitoid is not very differentiated and is particularly rich in quartz (max: 40 weight-%) and microcline (max: 40 weight-%). The soils developed on granitoid, adopt elastic-fragile deformation to elastoplastics with sharp break in relation to the load applied. The alteration profile on orthogneiss is poorly differentiated, and thin, with a low degree of alteration. Hydrolytic alteration by monosiallitization marked this profile. This is particularly rich in quartz (max: 100 weight-%) and kaolinite (max: 57 weight-%). The profile on anatexite is particularly rich in quartz (max: 66 weight-%) and kaolinite (max: 40 weight-%). The normative restructuring confirmed over 98 weight-% of the various minerals obtained and thus made it possible to better characterize the alteration and its application in geotechnics. These soils constitute potential deposits of chromatogenic minerals: kaolinite, hematite, goethite, gibbsite, magnetite, biotite.

Timing and origin of gold mineralization in the Neoproterozoic Xixano Complex, Mozambique Belt, northeastern Mozambique: Case study of the Nanlia and Makorongo prospects

The metallogeny of gold in northeastern Mozambique is still debated, with the main point of contention being the timing of mineralization with respect to regional tectonism, metamorphism, and magmatism. In this study, we applied Re–Os sulfide geochronology to constrain the age of gold mineralization in the Nanlia and Makorongo prospects hosted by the Xixano Complex, in the southern part of the Mozambique Belt. In addition, chemical analyses of host rocks and sulfides complement the discussion on the genesis of mineralization. Single-mineral Re–Os model ages of pyrite and pyrrhotite in quartz veins constrain the occurrence of gold mineralization in the Nanlia and Makorongo prospects between ca. 580 and ca. 560 Ma. This mineralization age interval postdated the timing of peak metamorphism of the host rocks established at ca. 631–607 Ma and is contemporaneous with the metamorphism of basement rocks between ca. 607 and ca. 564 Ma, during the thrusting of tectonic nappes in the last stage of the Pan-African orogeny. In addition, the strike of the veins in the Nanlia and Makorongo prospects is parallel to the local orientation of the thrust fault that separated the overlying rocks from the basement rocks and the dominant S1 foliation of the host rocks. These geochronological and structural data indicate a close relationship between auriferous veins in the study area and the Pan-African thrust fault associated with the emplacement of the tectonic nappes. We propose a metamorphic model for the gold mineralization in the Nanlia and Makorongo prospects, in which the mineralizing fluids were sourced mainly from dehydration of basement rocks, as a result of prograde metamorphism triggered by the juxtaposition of the Neoproterozoic tectonic nappes, including the Xixano Complex, during the last stage of the Pan-African orogeny. The mineralizing fluids ascended through the Pan-African thrust faults and structures to the overlying rocks, where the gold was deposited along the S1 foliation.

Distribution of Radiation-Induced Defects in Quartz at the ACKIO Uranium Prospect, Athabasca Basin, Saskatchewan: Tracing Uranium-Bearing Fluids

Abstract Quartz grains from the ACKIO uranium prospect, located near the eastern margin of the Athabasca Basin, Saskatchewan, have been investigated using powder X-band electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of quartz separates from samples systematically collected from the Athabasca Supergroup sandstones and the underlying metamorphosed basement reveal a suite of silicon-vacancy hole centers, which were formed by the bombardment of alpha particles emitted from the radioactive decay of uranium, thorium, and their unstable progeny. The differences in EPR signal intensities of these hole centers indicate that quartz grains received different accumulative doses of alpha particle irradiation in different locations within the ACKIO prospect. For example, quartz extracted from a mineralized sample has the highest EPR signal intensity due to the presence of disseminated uraninite. For quartz sampled at distance from uranium mineralization, the elevated intensities of the silicon-vacancy hole centers most likely indicate a temporary source of radiation, such as ancient uranium-bearing fluids. The median EPR intensities of quartz from the basement rocks at ACKIO are an order of magnitude higher than those of its counterpart from the Athabasca sandstones. Also, the EPR intensities of quartz along the sandstone–basement contact at ACKIO differ by more than two orders of magnitude, suggesting limited migration of uranium-bearing fluids along this contact in the study area. Instead, anomalously high EPR intensities in quartz close to the sandstone–basement contact and elsewhere are restricted to fault gouges and brecciated areas, suggesting that they are the structural pathways for channelized migration of ancient uranium-bearing fluids. A three-dimensional distribution model of the EPR signal intensities has been constructed to define conduits for the migration of ancient uranium-bearing fluids at ACKIO and suggest favorable targets for further exploration. These findings demonstrate the power of EPR spectroscopy in delineating pathways of uranium-bearing fluids and predicting potential mineralization targets.

The distribution of vesicular structures in Sukadana Basalt, East Lampung, Indonesia, and its significance for the geological structure interpretation

Abstract The characteristics of the geological structures controlling the presence of Sukadana basalt on the Earth’s surface are still an enigma. Therefore, this study was conducted to determine the probable types of geological structures and their orientations that could control the occurrence of Sukadana Basalt lava on the Earth’s surface. To seek answers to this problem, this study utilized vesicular structure data from Sukadana Basalt based on 13 rock samples from 13 observation points. We measured the diameter of the vesicular and the sum of the megascopic vesicular per cm2 for every sample. We used a scale of 1-5, where 1-2 indicated as small-low intensity vesicular, 3 indicated as medium-middle intensity vesicular, and 4-5 indicated as large-high intensity vesicular. it was found that there are three groups of vesicular that were developed in Sukadana Basalts: small-low, medium-middle, and large-high groups. This study gives information that there are three fracturures including two normal faults at the northern and southern parts of Sukadana Basalt and one fisurre in the middle of Sukadana basalt. The normal fault at the southern part of Sukadana Basalt has the widest fault core spacing and the fissure at the middle of Sukadana Basalt has the narrowest fissure core spacing. All groups show northwest-southeast striking fractures. This study will provide a new point of view that in interpreting subsurface geological structures can use primary structure characteristics data, especially vesicular structures.

Extreme geochemical fractionation during mantle melting: Insights from Hf-Nd isotopically ultra-depleted eclogite

Extreme fractionation of elements and isotopes in mafic igneous rocks is abnormal in deciphering the source nature and melting conditions of mafic magmatism. Especially, identification of geochemically ultra-depleted mafic melts and their mantle sources has great bearing on the property of crust-mantle differentiation at plate margins. This is illustrated by extreme Hf-Nd isotope fractionation in ultrahigh-pressure eclogites from the Sulu orogen in east-central China. In addition to the previous finding of ultrahigh εNd(0) values, we report here new data of whole-rock trace elements and Lu-Hf isotopes in eclogites and related rocks from the same region. The present results show extremely high Lu/Hf ratios and abnormally high εHf(0) values of up to 576 for the eclogites, significantly different from the garnet amphibolites and other eclogite-facies metamorphic rocks in the same orogen. This feature is coupled with the ultrahigh εNd(0) values as well as the severe depletion of light rare earth elements (LREE) and high field strength elements (HFSE). Because HFSE and LREE are immobile in aqueous solutions and the effect of melt extraction is insignificant during the continental deep subduction, the extreme fractionation of Lu/Hf and Sm/Nd indicate their origination from a geochemically ultra-depleted mantle source. These eclogites have the depleted mantle Hf model ages of 1.27 Ga to 1.61 Ga, similar to the depleted mantle Nd model ages of 1.39 Ga to 1.67 Ga as previously reported. This suggests that the protolith of the extremely high εHf-εNd eclogites was a kind of mafic igneous rocks derived from fractional crystallization of geochemically ultra-depleted mafic melts, which were produced by partial melting of the highly refractory lithospheric mantle during a series of seafloor spreading initiation-failure cycles at a divergent plate margin after the breakup of supercontinent Columbia in the Early Mesoproterozoic. The mafic igneous rocks were located in a passive continental margin in the Late Paleozoic and experienced deep subduction and exhumation in the Triassic, giving rise to the presently studied eclogites. The ancient geochemical signatures were retained without considerable influence by mantle convection, providing insights into the nature of crust-mantle differentiation during the tectonic transition from supercontinental breakup to seafloor spreading beneath the sub-ridge lithospheric mantle.

Tin-polymetallic mineralization and granitic rocks in the Nanling daping mining area: Geochemical and geochronological constraints

The Daping polymetallic deposit in the Nanling is a recently discovered deposit with significant potential in the Dayishan area. High-precision geochronological studies indicate that the emplacement ages of two fine-grained porphyritic (containing tourmaline) biotite granite units are 153.2 ± 0.9 Ma and 152.3 ± 1.0 Ma, representing products of the fifth magmatic activity in the Dayishan intrusion. The cassiterite U-Pb ages from the residual skarn type and greisen-type tin ores are 153.4 ± 3.6 Ma and 150.22 ± 0.37 Ma, respectively, and they are consistent within the error range, both belonging to the Yanshanian Early period. Petrogeochemical analyses of the granite reveal characteristics of high silica content (73.56 ∼ 75.91 %), alkali (K2O = 4.04 ∼ 5.58 %, Na2O = 2.22 ∼ 3.67 %), low calcium (0.42 ∼ 0.80 %), low magnesium (0.075 ∼ 0.29 %), and peraluminous nature (A/CNK = 1.26 ∼ 1.35). The rare earth element (REE) contents range from 35.78 × 10-6 to 111.57 × 10-6, with δEu values between 0.006 and 0.026, LREE/HREE ratios of 2.23 ∼ 3.41, and (La/Yb)N values of 1.20 ∼ 2.42. The granite is enriched in elements such as Rb, K, Th, U, Nd, Hf, and depleted in Ba, Sr, P, Nb, Ti, indicating strong crystal differentiation in the area. Sr, Nd, and Hf isotope studies show that the granite samples have ISr values of 0.67612–0.70040, εNd(t) values of −5.83 ∼ -8.23, εHf(t) values of −6.1 ∼ -1.9, (Nd)TDM2 values ranging from 1361 to 1573 Ma, and (Hf)TDM2 values ranging from 1147 to 1408 Ma. It is proposed that the Dayishan intrusion originated from the mixing of a mid-Paleoproterozoic mafic rock and a metamorphic sedimentary source region, with the latter being predominant. The detachment of source rocks from the mantle reservoir occurred in the mid-Paleoproterozoic. Combining lithological, petrogeochemical, and comparative analyses, it is classified as a highly fractionated A-type granite formed in a local extensional setting related to the subduction of the Pacific Plate. Additionally, this paper constructs a ore-forming model for the Daping tin deposit, aiming to provide reference for regional rare metal ore exploration.

Anomalous cobalt enrichment in pyrite from an altered mafic dike in the Linglong gold deposit, Jiaodong Peninsula, China: Mechanisms and implications

Traditionally, gold deposits in the Jiaodong gold province of China have been regarded as gold-only deposits lacking economic base metals. However, recent studies found some cobalt (Co) enrichment in those deposits. The mechanisms of this Co enrichment remain poorly understood. In this study, we identified a type of pyrite highly enriched in Co (concentrations up to 32800 ppm) at the Linglong gold deposit, Jiaodong gold province. The pyrite was collected from a hydrothermally altered mafic dike contacting with gold ore veins. To decipher the enrichment mechanisms of Co, in-situ textural, elemental and sulfur isotopic analyses were conducted on the pyrite. The pyrite typically exhibits core-mantle-rim textures. The cores (Py-m1) have high Co concentrations (up to 5950 ppm) with homogeneous texture and elemental distribution, which could be the primary pyrite formed by pyritization of the mafic dike. The mantles (Py-m2) have the highest Co concentrations (up to 32800 ppm) and contain abundant mineral inclusions (chalcopyrite, galena, and sphalerite) and pores, showing the formation through coupled dissolution-reprecipitation (CDR) reactions. The rims (Py-m3) have low concentrations of Co (0.14–559 ppm) and show cubic shapes, forming directly from hydrothermal fluids. The δ34S values increase from the cores (4.2 ‰ to 9.1 ‰, mean of 8.0 ‰) to the mantles (8.9 ‰ to 11.9 ‰), but vary between the above two in the rims (4.2 ‰ to 12.2 ‰, mean of 9.0 ‰). The sulfur isotopic compositions of the Py-m are similar to those of pyrite (4.8 ‰ to 9.5 ‰) from the neighboring auriferous quartz-sulfide veins at Linglong in previous study. Combined with the geological and textural features, it is inferred that hydrothermal fluids during gold mineralization extracted abundant Co from the mafic dikes, leading to the primary enrichment of Co in the Py-m1. The Py-m1 was dissolved and re-precipitated during subsequent hydrothermal activities, forming Py-m2 with enhanced re-enrichment of Co. Basically, the Co-rich mafic dike provided the initial enrichment of Co while the gold-bearing hydrothermal fluids contributed to concentrate Co further. Considering the widespread mafic rocks and extensive hydrothermal activities during gold mineralization throughout the Jiaodong gold province, there might be a considerable Co resource formed through the above processes.

Crustal structure beneath the Western Dharwar Craton segment of Western Ghats: Insights from ambient noise correlation technique

The Western Ghats (WG) comprise three major tectonic blocks: the Deccan Volcanic Province (DVP), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT) from north to south. Among these, the WDC region encompasses the oldest basement rocks, including greenstone belts and calc-alkaline plutons dating back to ∼3.0 to 3.4 Ga. While the crustal structure of the DVP and SGT segments of the WG has been extensively studied, knowledge regarding the crustal structure beneath the WDC segment of the WG is limited due to the scarcity of seismic stations. To address this gap, we utilized continuous waveform data from a newly established network of seismic stations by the National Centre for Earth Science Studies (NCESS), in addition to other available stations and deciphered the shallow crustal shear-wave velocity structure along a profile beneath the WDC segment of the WG using ambient noise cross-correlation and surface wave dispersion. The results reveal a continuous upper layer characterized by a high shear wave velocity of ∼3.6 km/s with notable variation in thickness along the profile. Furthermore, a thick middle layer with velocities reaching up to ∼3.9 km/s and a lower layer with velocities of ≥4.2 km/s are observed. Intriguingly, variations in the thickness of the upper layer, ranging from 5 km to 20 km, are noted beneath the study region. The thickness of this layer exhibits changes in relation to topographical variations, with this dependency diminishing for deeper layers. Notably, high shear wave velocity perturbations (∼10 %) are observed in the upper layer beneath low-elevation regions, possibly indicating the presence of Tonalite–Trondhjemite–Granodiorite (TTG)-type gneiss − a significant rock type in the WDC region, alongside greenstone belts and late calc-alkaline to potassic plutons. These findings suggest that the uneven topography of the WG may result from long-term erosion, exposing deeper and higher-density rocks and leading to higher shear wave velocities at shallow depths. This study serves as a foundation for further exploration into the influence of the upper crustal layer and its correlation with the evolution of the WG.

What is the source of magnesium in hydrothermal dolomites? New insights from coupling δ26Mg - ∆47 isotopes

The occurrence of fault-controlled hydrothermal dolomitization (HTD) is ubiquitous across stratigraphic records and has been extensively studied due to its association with economic resources. The origin of HTD is often evaluated by combining carbonate geochemistry, fluid inclusion thermometry and reactive transport modelling. However, multiple diagenetic overprinting events can obscure original geochemical signatures. Here, we demonstrate the applicability of two combined isotope systems, magnesium and carbonate clumped isotopes (δ26Mg - ∆47), to trace the source of fluid and magnesium in basin-scale HTD from the Western Canadian Sedimentary Basin (WCSB) and Southern China. Extensive studies in these regions, providing tectono-stratigraphic information and dolomitization models, furnish a robust scenario to evaluate the new isotopic data. Our findings reveal that while the previous geochemical data (δ18Ofluid and 87Sr/86Sr) are partly compatible with dolomitization from seawater, the elevated δ26Mg and δ18Ofluid values (up to -0.3‰ and +11‰, respectively) and the high temperatures (up to 320 °C) are not consistent with dolomitization from seawater alone. Considering this, the uniform mean δ26Mg values of silicate rocks (-0.25‰) and the occurrence of basement-rooted faults in the study areas, the hypothesis from prior work that dolomitization was initially driven by seawater that progressively mixed with crustal fluids sourced from underlying basement rocks, was supported. As the δ26Mg values may reflect the isotopic signature of the fluid and the leached host rocks, when combined with ∆47-derived temperatures, it results in an effective tracer of fluid and Mg sources during dolomitization in tectonically complex basins that are otherwise challenging to unravel. A comparison of our results paints a global picture of isotopic imprints in hydrothermal dolomite and further demonstrate the ability of Mg isotopes to differentiate between dolomitization from seawater versus evaporated brines and also through Mg-rich crustal fluids.

Weathering, redox proxies and carbon isotope data from the Maokou Formation, Upper Yangtze, South China: Implications for the Guadalupian P3 glaciation of the Late Paleozoic Ice Age

Late Paleozoic Ice Age (LPIA) was a key climate event of the deep-time earth evolution system. Although the sedimentary successions from eastern Australia have well constrained the timeframe of the glacial-nonglacial intervals, the link between these glaciations and coeval geological events is still being explored. The Guadalupian (Middle Permian) P3 glaciation was the prelude to the upcoming end of LPIA, which is a key window to examine the uncertainty of the climatic transition from icehouse to greenhouse and linkages with the geological and biological events. This study focuses on the marine sedimentary succession of the Shangsi outcrop located in the low-latitude Yangtze block, investigating the carbon/strontium isotopes, redox proxy, and chemical index of alteration (CIA). Multiple weathering indices revealed climatic fluctuations during the Guadalupian, corresponding to the onset and demise of the high-latitude P3 glaciation. The high level of CIA following the Kungurian-age Large Igneous Provinces (LIPs) event indicates that the weathering of basic rocks caused the positive 13C drift, which was the main factor for the onset of the P3 glaciation. Then the breakup of Pangea, enhanced ocean oxidation, and a series of volcanic activities including Emeishan LIP could contribute to the accumulation of CO2 and consequently cause the demise of the P3 glaciation in middle Capitanian. Enhanced continental weathering was synchronous with the eruption surge of the volcanic activities, indicating a following global warming event, which may play a crucial role in triggering the Mid-Capitanian extinction event.

On the Feasibility of Deep Geothermal Wells Using Numerical Reservoir Simulation

This study examines the geothermal energy extraction potential from the basement rock within the Denver–Julesburg Basin, focusing on the flow performance and heat extraction efficiency of different geothermal well configurations. It specifically compares U-shaped, V-shaped, inclined V-shaped, and pipe-in-pipe configurations against enhanced geothermal system setups. Through numerical modeling, we evaluated the thermal behavior of these systems under various operational scenarios and fracture conditions. The results suggest that while closed-loop systems offer moderate temperature increases, Enhanced geothermal system configurations show substantial potential for high-temperature extraction. This underscores the importance of evaluating well configurations in complex geological settings. The insights from this study aid in strategic geothermal energy planning and development, marking significant advancements in geothermal technology and setting a foundation for future explorations and optimizations.

Assessment of the Crustal Thickness within Southern Bida Basin and Surrounding Basement Rocks, North Central Nigeria, using Gravity Data

The crustal thickness within the Southern Bida Basin has been determined using Bouguer Gravity Data. This study focuses on a region situated in the southern part of the Bida Basin, a significant inland sedimentary basin in North Central Nigeria. Two distinct methods were employed to estimate crustal thickness from Bouguer anomaly values: (i) an empirical relationship between crustal thickness and Bouguer anomaly, and (ii) spectral analysis of the radial wave number. The average crustal thickness was derived by averaging results obtained from both methods. Additionally, a linear equation (HC = 34.39 – 0.17BG) was formulated for the study area, enabling straightforward determination of crustal thickness from any given Bouguer anomaly value. The estimated crustal thickness in the study area ranges from 32 km to 40 km, aligning closely with the overall crustal thickness of Nigeria as reported by Daniyan et al. (2002). The tectonic stability of the study area, situated on the stable African plate, further underscores its significance within the broader geological context of Nigeria.

Fluid-mediated Cu and Zn isotope fractionation in subduction zones and implications for arc volcanism: Constraints from high pressure veins within eclogites in the Dabie Orogen

High-pressure (HP) veins in eclogites are the products of fluid-rock interaction and provide insight into the composition and evolution of fluids in subduction zones. We here present the Cu and Zn isotope data for different types of HP veins and their host eclogites from Ganghe and Hualiangting in the Dabie Orogen to reveal the behavior of Cu and Zn isotopes during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the Omp-Ep vein formation. The early Omp-Ep veins have variably lower δ65Cu (−1.32 to −1.12‰ at Ganghe, −0.70 to −0.66‰ at Hualiangting) but higher δ66Zn (0.36 to 0.38‰ at Ganghe, 0.40 to 0.41‰ at Hualiangting) relative to the host eclogites (δ65Cu: −0.71 vs. 1.84‰, δ66Zn: 0.22 vs. 0.33‰), indicating CuZn isotope fractionation during slab dehydration in subduction zones with the lighter Cu and heavier Zn isotopes preferentially entering the vein-forming fluids from the eclogites. Systematic increase of δ65Cu from the Omp-Ep (−0.70 to −0.66‰) through Ep-Qtz (0.01‰) to Ky-Ep-Qtz veins (0.46 to 0.95‰) can be attributed to isotope fractionation induced by redox changes during the evolution of metamorphic fluids, as revealed by the negative correlations of δ65Cu with redox-sensitive ratios of Fe3+/ΣFe and (Eu/Eu*)N in those veins. Additionally, the higher δ66Zn of the Omp-Ep veins relative to the Ky-Ep-Qtz veins can be explained by equilibrium isotope fractionation between crystallized minerals and evolved metamorphic fluids. Our results thus demonstrate that CuZn isotope fractionation occurred during the evolution of slab-derived metamorphic fluids in subduction zones. Binary mixing calculations show that fluids derived from dehydration of mafic rocks in subducted slabs, represented by the multistage HP veins in the present study, cannot account for the heavier Cu and lighter Zn isotope compositions in most arc magmas than in mid-ocean ridge basalts. This offset can be resolved by addition of forearc serpentinite-derived fluids enriched in heavy Cu and light Zn isotopes into the mantle source of arc magmas.

The 2.03–2.02 Ga arc–back-arc mafic magmatism from the Zhongxiang Complex, northern Yangtze Block: Implications for Columbia supercontinent assembly

To better understand Paleoproterozoic tectonic processes in the Yangtze Block during the assembly of the Columbia supercontinent, an integrated geochronological and geochemical study of amphibolite facies mafic rocks exposed in the Zhongxiang Complex in the northern Yangtze Block is presented. Based on trace element and rare earth element (REE) characteristics, these mafic rocks can be divided into two coeval groups. The MORB-like mafic rocks show flat REE patterns and are enriched in large-ion lithophile elements (e.g. Rb, Ba, K, Sr). Their zircon ɛHf(t) values range from + 3.43 to + 4.90. The other mafic rocks are arc-like and have elevated REE contents and display relatively light REE enriched patterns, as well as depletion in high field strength elements (e.g. Nb, Ta, Ti). The zircon ɛHf(t) values of the latter group range from + 0.17 to + 2.68. Zircon U-Pb dating yielded a weighted mean 207Pb/206Pb age of 2029 ± 15 Ma for the MORB-like mafic rocks, and 2023 ± 14 Ma for the arc-like mafic rocks, respectively. The elemental and isotopic characteristics indicate that the MORB-like mafic rocks may be derived from a depleted asthenospheric mantle source, whereas its counterpart with arc-like signatures originated from a lithospheric mantle source. The low (La/Yb)CN (1.10–2.84) and Sm/Yb (0.99–1.29) ratios indicate that the mafic rocks are formed by partial melting of spinel lherzolite mantle. Combined with other geological observations, these mafic rocks are inferred to constitute part of a mid-Paleoproterozoic continental arc–back-arc basin system. The 2.03–2.02 Ga mafic rocks from the northern Yangtze Block indicate that the continental margin experienced subduction in response to assembly of the Columbia supercontinent.

A geochemical model for the transformation of gabbro into vesuvianite-bearing rodingite

Rodingite is a Ca-rich and Si-poor metasomatic rock commonly occurring in association with serpentinites. This rock is characterized by specific mineral assemblages consisting of hydrated garnet, diopside, vesuvianite, epidote-zoisite, chlorite, or prehnite. However, natural rodingites are significantly heterogeneous in mineral composition and vesuvianite occurs only in some extensively rodingitized rocks. Major factors controlling the mineral diversity as well as details on fluid-rock interactions leading to the evolution of mineral and chemical composition during rodingitization have not yet been fully constrained. In this work, we use PHREEQC software to present a geochemical model for the transformation of a mafic rock into vesuvianite-bearing rodingite at a temperature of 300 °C. Through these simulations, we investigate the effect of fluid composition and progress of the metasomatic process on rodingite formation. Our results show that rodingitization requires an open system with a high input of hydrothermal fluid. Additionally, a decrease in the Si/Ca ratio in the metasomatized rock is correlated to an increase in the volume of incoming fluid. Whole rock chemical and mineral composition in natural rodingites are well reproduced by the model. Furthermore, the diversity of mineral parageneses results mainly from different degrees of transformation and only to a lesser extent to the chemical composition of hydrothermal fluid or protolith. The hydrothermal fluid doesn't need to be especially rich in calcium to transform a mafic rock into rodingite, but it must be low in magnesium, silicon, and have a high pH, which is naturally controlled by serpentinization of surrounding ultramafic rocks.

Newly identified small vulcanian eruptions during the caldera-forming stage of Towada Volcano, Northeast Japan

Understanding the detailed eruptive history and conditions during the buildup to catastrophic caldera-forming eruptions is essential for understanding the evolution of caldera volcanoes and predicting eruptive hazards. Towada Volcano is an active caldera volcano in the northern part of the Northeast Japan Arc. At least two catastrophic caldera-forming eruptions (eruptive episodes N and L) occurred during its caldera-forming stage (61–15.7 ka). A detailed geological survey identified three small vulcanian tephra layers that were erupted during the caldera-forming stage. The tephra layers are blue–gray ash fall deposits that consist mainly of fresh blocky dacite–rhyolite fragments. In ascending order, these deposits are assigned to eruptive episodes O′, N′, and M′. Each eruption had a volume of <0.11 km3, which is much smaller than that of other eruptions during the caldera-forming stage. The eruptive ages of episodes O′, N′, and M′ are estimated to be ca. 40, 23.1, and 17.2 ka, respectively, based on new 14C ages and paleosol thicknesses data. At least three cycles of a medium- to large-scale (3–20 km3) explosive eruption, preceded by a small vulcanian eruption (<0.11 km3), occurred during the latter half of the caldera-forming stage. The small vulcanian eruptions (episodes O′, N′, and M′) occurred after relatively long periods of quiescence (4000–14,000 years) and were followed by medium- to large-scale explosive eruptions (episodes N, M, and L) 1500–4000 years later. This cyclic activity probably reflects changes in overpressure in the magma reservoir. As the overpressure increased, episodes O′, N′, and M′ probably occurred when the overpressure had not increased sufficiently to trigger a medium- to large-scale explosive eruption, and episodes N, M, and L occurred at higher overpressures. These cycles characterize the fermentation phase of Towada Volcano, and terminated with the formation of the Towada Caldera during episode L at 15.7 ka. The magma composition and eruption frequency changed abruptly after episode L, and a small stratovolcano was formed through intermittent eruptions of mafic magma. This change suggests that the caldera collapse during episode L changed the entire shallow magmatic system that existed during the fermentation phase, and the system shifted to a recovery (post-caldera) phase. Based on eruptive volumes and frequencies, the present Towada Volcano has not yet reached the conditions that existed during the late caldera-forming stage and is therefore unlikely to produce a catastrophic caldera-forming eruption in the near future.

Tectonic affinity and geological significance of the Feidong Complex in the northeastern margin of the Yangtze Craton: Evidence from zircon U–Pb and Lu–Hf isotopes of the basement rocks

Tectonic affinity and geological significance of the Feidong Complex in the northeastern margin of the Yangtze Craton: Evidence from zircon U–Pb and Lu–Hf isotopes of the basement rocks

The Middle Triassic Intermediate to Basic Rocks in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Implication for the Paleo-Tethyan Ocean Closure

The Middle Triassic Intermediate to Basic Rocks in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Implication for the Paleo-Tethyan Ocean Closure

Hydrothermal alteration and its geochemistry of the Xiadian gold deposit, Jiaodong Peninsula, China: Implications for fluid-rock interaction processes and mineral exploration

The interaction between hydrothermal fluids and host rocks, along with associated alteration geochemistry within the Mesozoic Jiaodong gold system, presents significant gaps in our understanding. This study systematically examines the petrographic, mineralogical, and geochemical characteristics of alteration at the Xiadian gold deposit, including comprehensive whole-rock geochemistry analyses, mass balance calculations, and phase equilibria modeling. The alteration history of Xiadian could be divided into four stages. Amid Stage 1, the predominant process involves the replacement of plagioclase by K-feldspar-albite, accompanied by the simultaneous bleaching of amphibolite and associated significant depletion of MgO (ΔCi/CiO = −21.2 %) and Fe2O3 (−27.2 %) owing to the decomposition of mafic minerals. Stage 2a involves sericitization of albite and continued mafic mineral decomposition, resulting in the lowest recorded MgO (avg. 0.23 wt%) and Fe2O3 (avg. 0.92 wt%) contents in all alteration lithologies. Stage 2b is characterized by abundant chlorite vein formation, corresponding to the proximal enrichment of MgO (158.1 %) and Fe2O3 (108.2 %), accompanied by aMg2+/(aH+)2 decreased in the fluid. Stage 3, the main stage of gold mineralization, involves chlorite substitution with sericite during quartz-sericite-pyrite (QSP) alteration, suggesting mild acidification of the ore-forming fluid. Insights from phase equilibrium modeling suggest that sulfidation has contributed to gold precipitation, leading to increased fO2 and reduced fH2S (extending into quartz-carbonate alteration of stage 4). Mass balance calculations indicate gold is enriched compared to protolith at all stages, with the most intense enrichment in stage 3. This suggests that hydrothermal fluids at Xiadian were auriferous throughout their evolution. The process may induce fluid modifications, particularly through the dissolution of mafic minerals, more so in mafic mineral-rich rocks (e.g., amphibolite) than in granite, promoting the incorporation of hydrothermal chlorite into the Au system to contribute to sulfidation. Our results provide insights into lithology controls on gold mineralization at Xiadian, indicating that felsic rocks (Linglong granite and quartz porphyry) with lower initial CaO contents often develop K-metasomatism and Au-bearing QSP alteration, while mafic rocks (amphibolite and lamprophyre) with higher CaO contents develop carbonization. This is likely due to the influence of lower CaO contents on alteration minerals, which promote rock porosity, affecting rock deformation, pore-fluid flow, and reacting with wall rocks. Principal component analysis (PCA) results indicate that elements including Au, Ag, As, Bi, W, and Cu are proxies for Au mineralization, suggesting their mobilization and deposition together during the gold mineralization process. This highlights their significance as a proxy for gold exploration.