Magmatic Research Articles

Deep Tectonic Environment Analysis of the Lingshan Conjugate Earthquake within the Qinzhou Fold Belt, South China: Insights Derived from 3D Resistivity Structure Model

The Qinzhou fold belt, situated at the contact zone between the Yangtze and Cathaysia blocks in South China, was affected by the 1936 Lingshan M6¾ earthquake and the 1958 Lingshan M5¾ earthquake, both of which occurred within the conjugate structure. Understanding the deep seismogenic setting and causal mechanism of the Lingshan conjugate earthquake is of great significance for assessing the seismic disaster risk in the region. In this study, we utilized 237 magnetotelluric datasets and employed three-dimensional electromagnetic inversion to characterize the deep-seated three-dimensional resistivity structure of the Qinzhou fold belt and the Lingshan seismic zone. The results reveal that: (1) The NE-trending faults within the Qinzhou fold belt and adjacent areas are classified as trans-crustal faults. The faults exhibit crust-mantle ductile shear zones in their deeper sections, which are essential in governing regional tectonic deformation and seismic activity; (2) The electrical structure of the Qinzhou fold belt is in line with the tectonic characteristics of a composite orogenic belt, having experienced several phases of tectonic modification. The southeastern region is being influenced by mantle-derived magmatic activities originating from the Leiqiong area over a significant distance; (3) In the Lingshan seismic zone, the NE-trending Fangcheng-Lingshan fault is a trans-crustal fault and the NW-trending Zhaixu fault is an intra-crustal fault. The electrical structure pattern “two low, one high” in the zone has a significant impact on the deep tectonic framework of the area and influences the deformation behavior of shallow faults; and (4) The seismogenic structure of the 1936 Lingshan M6¾ earthquake was the Fangcheng-Lingshan fault. The earthquake’s genesis was influenced by the coupling effect of tectonic stress and deep thermal dynamics. The seismogenic structure of the 1958 Lingshan M5¾ earthquake was the Zhaixu fault. The earthquake’s genesis was influenced by tectonic stress and static stress triggering from the 1936 Lingshan M6¾ earthquake. The conjugate rupture mode in the Lingshan seismic zone is influenced by various factors, including differences in physical properties, rheology of deep materials, and the scale and depth of fault development.

Raw Materials in the Paste of Ceramics of the Kulaika Culture Surgut Variant (Based on Samples from Barsova Gora)

A multidisciplinary analysis of ceramics from six sites of the Surgut variant of the Kulaika culture at Barsova Gora was made. Technology was assessed using traceological, petrographic, and X-ray phase analyses. At all the sites, the potters used ferruginous clays tempered with grus, grog, sand, and organic material. Fragments in the clay were either rounded, as in sand, or coarse, as in grus. The sand was mainly represented by feldspar and quartz, suggesting that this type of raw material was extracted from nearby non-metallic mineral deposits. The grus consisted of fragments of basaltoids, amphiboles, and pyroxenes, evidencing that it came from igneous common rocks associated with the Surgut volcanic fi eld and spread over a large area. Rocks were probably mined near settlements, perhaps on the floodplain of the Ob. Grog in all the samples was similar to the basic clay in terms of its composition. Three groups of sites were identified, differing in the composition of the clay of which the ceramics were made. This may indicate the presence of several groups within the Iron Age Kulaika population, utilizing various sources of clay.

Deep Forest Modeling: An Interpretable Deep Learning Method for Mineral Prospectivity Mapping

AbstractAccurate mineral prediction is crucial for reducing costs and uncertainties in mineral discovery and extraction. The use of artificial intelligence and big data has advanced mineral prediction into intelligent forecasting. Machine learning methods have shown significant promise in enhancing outcomes. Currently, neural network‐based approaches dominate deep learning (DL), but they lack interpretability and have high modeling complexity, making them less effective for complex problems and time‐consuming. Deep Forest, an innovative DL paradigm, addresses these issues by dynamically adjusting complexity and providing importance assessments for predictive factors. This study focuses on the North American Cordillera, known for its rich geological data and potential for porphyry copper deposits (PCDs). Predictions are made using Deep Forest with factors like Euclidean distance between faults and magmatic rock, fault line density, gravity anomalies, and stream‐sediment geochemical data. Deep neural networks, random forest, convolutional neural networks, transformer model and graph convolutional networks are also used for comparison. Deep Forest shows high performance and can avoid the black box problem of DL without relying on other tools in DL, providing a new perspective for the development and application of other non‐neural network DL models for mineral prediction. Feature importance analysis shows that geological structure and magmatism significantly influence PCD prediction. Elevated levels of elements like Al, Co, and Cr in stream sediments help identify mineralization‐related alterations. These findings underscore Deep Forest's capability to accurately and efficiently guide mineral exploration, highlighting its potential as a promising approach for mineral prospecting.

A comparative study of the elevated radioactivity in beach placer deposits of Bangladesh.

Beach sediments are mineral deposits formed through weathering and erosion of either igneous or metamorphic rocks. Among the rock constituent minerals are some natural radionuclides that contribute to ionizing radiation exposure on Earth. Kolatoli and Kuakata are the two major beaches with heavy mineral deposits and important tourist sites in Bangladesh. Natural radioactivity in Kolatoli and Kuakata beach sand deposits along the southern coast of Bangladesh was assessed and compared to identify the sources, causes, and possible environmental impact. Result shows most of the radionuclides have higher activity concentrations than the background level, and the activity varies with the sample locations. The dominant radionuclides were found to be the radionuclides of thorium series i.e. Th-232 and Ra-228 followed by uranium series and K-40. The radioactivity in Kolatoli beach sands was observed to be much higher than Kuakata beach due to the presence of a higher content of heavy minerals i.e. illmenite, rutile, zircon, garnet and monazite. Furthermore, monazite and zircon are the two radioactive minerals that are considered to be the main contributors to the radioactivity in Kolatoli beach sand. These minerals are dominated by the activity of thorium series radionuclides i.e. Th-232 and Ra-228 surpass the activity of all other radionuclides such as U-238, U-234, Th-230, Ra-226, Po-210, and K-40. However, major contribution of radioactivity in Kuakata beach sand comes from uranium series radionuclides such as U-238, U-234, Ra-226, and Po-210. Beach morphology, sedimentological, and geochemical evolution of those minerals might be important areas of further study for the radioactivity monitoring activity in those areas.

Geochemistry of Mafic Rocks From the Nagrota–Kathindi Section, Himachal Pradesh, Northwestern Himalaya: A Probable Example of Plume–Lithosphere Interaction

ABSTRACTThe Northwest Himalayan region has a record of several phases of mafic magmatic activity spanning from Precambrian to Cenozoic in a dynamic tectonic setting. Here, we studied detailed petrography and new whole‐rock geochemistry of mafic volcanic and dykes from the Nagrota–Kathindi Section (NKS), Himachal region of the NW Himalaya, to understand the petrogenesis and possible tectonic setting. Both rock types have comparable mineralogical compositions (clinopyroxene + plagioclase + actinolite‐tremolite + chlorite + iron oxides ± hornblende ± epidote ± quartz ± carbonates) overprinted by greenschist to lower amphibolite facies metamorphism. The mafic volcanic and dykes of NKS exhibit subalkaline basalts to basaltic andesites and a typical tholeiite compositional character. The chondrite‐normalized rare earth element pattern exhibits similar LREE‐enrichment and strong HREE‐fractionation, whereas primitive mantle‐normalized multi‐element patterns show pronounced LILE‐enrichment of Rb, Ba, Th, LREE, and HFSE depletion of Nb, K, P, and Ti. The Zr–Y–Nb–Th relationships indicate that both rock types were derived from the plume source, whereas low Nb/La (< 1), similar high large‐ion lithophile element concentrations, and pronounced negative Nb, Zr, P, and Ti anomalies suggest that components other than mantle plume must have been involved in the generation and evolution of both rock types, that is, most likely plume and subcontinental lithosphere mantle (SCLM) interaction. The genesis of parent magma for the NKS volcanic and dykes was derived by 4%–6% and 10%–20% partial melting from the spinel + garnet lherzolite stability field. The majority of the studied samples correspond to spinel + garnet peridotite melting on (Gd/Yb)N versus CaO/Al2O3 diagram, thereby corroborating residual garnet in the mantle restite. All the basalts and dykes from the NK section did erupt/intrude in an intracontinental rift setting based on geochemical discrimination. The key petro‐tectonic processes attributed to the formation of these rocks are as follows: (i) the melting of the ascending plume by adiabatic decompression; (ii) the partial melting of this plume–SCLM source in the melting regime, which produces basaltic magma with a tholeiitic composition; and (iii) the release of heat that provides the thermal condition for melting of SCLM and interaction between upwelling mantle plume and subduction metasomatized SCLM.

The mid-Variscan suture in the Morais complex of Trás-os-Montes (Portugal): insights from geochemistry and geochronology of oceanic rocks

ABSTRACT The Middle Allochthon of the Galicia-Trás-os-Montes Zone (GTMZ) in NW Iberia is an assemblage of oceanic units representing the Variscan suture of one or more peri-Gondwanan oceans. This contribution brings new geochronological and geochemical data from the Middle Allochthon of the Morais Complex, in northern Portugal. There, the oceanic ensemble consists of a stack of five tectonic units which from bottom to top are the Junqueira, Pombais and Izeda oceanic supracrustals and the Remondes and Morais-Talhinhas ophiolitic units. The Junqueira and Pombais units consist of greenschists and metapelites. No age data are available for these two units, but a correlation is established with a Cambro-Ordovician unit in the Órdenes Complex of Galicia (NW Spain) dated at 500 Ma. The Izeda Unit consists of fine-grained, low-grade amphibolites transitional to epidote-amphibolites and greenschists, for which an age of 484 ± 3 Ma has been obtained for a metabasic tuff. In the ophiolitic ensemble, the Remondes and Morais-Talhinhas units consist of fine-grained amphibolites associated with deformed metagabbros, mafic cumulates and serpentinized ultramafics. In this study, two plagiogranite samples from the Remondes Unit yielded a complex spread of age data with Silurian ages representing the dominant population. This suggests that the newly defined Remondes Unit is younger than the Izeda Unit but older than the previously published age for these rocks, which are considered part of the overlying Morais-Talhinhas Unit, dated as Devonian (406–395 Ma) in a previous study. The geochemistry of the basic rocks in the five units is that of NMORB with a subduction-derived component that varies from weak to null. The ages, together with new and previously published whole rock geochemical data obtained from basic igneous samples, are interpreted to date and reflect the formation of igneous protoliths in oceanic ridge settings associated with the Mid-Variscan Ocean, a part of the Rheic oceanic realm that registered magmatic activity between the late Cambrian and the Lower Devonian.

From birth to death: The role of upper-crustal thermal maturation and volcanism in porphyry ore formation revealed in the Yerington district

The current race towards greener energy and its intertwined increase in Cu demand imposes the need to find mineralized centers that are economic despite greater depths. To this end, a better understanding of the processes controlling the metal tonnage of porphyry Cu deposits, the main source of Cu, is needed and the focus of current research and debate. This study provides a comprehensive view of the duration of upper-crustal magmatism and its significance in the degree of metal endowment in porphyry Cu deposits using the Yerington batholith, Nevada, USA, as a case study. The Yerington district constitutes an exceptional example of an exposed porphyry Cu system where the plutonic environment that fed the porphyritic dikes is accessible. Combining trace element geochemistry and high-precision geochronology of zircon from all exposed units, we propose that the thermal maturation of the upper crust by a continuum in magmatic activity without significant hiatuses is a key factor in enabling the formation of large porphyry Cu style mineralization. The Yerington magmatic system transitioned from a volcanically active environment with coeval plutonism to a growing upper-crustal magmatic reservoir that fed the porphyritic dikes and associated mineralized centers to then resume its volcanic activity terminating the ore-forming episode. A relatively high volcanic/plutonic ratio characterized the first expressions of Jurassic magmatism in the district from ∼170 Ma with the coeval eruption of the Artesia Lake volcanics and the protracted emplacement of the McLeod Hill pluton. Geochemically, this period recorded distinctive negative Eu anomalies and a gentle increase in Yb/Dy ratios in zircon with decreasing Ti contents over a large range of zircon crystallization temperatures. This persistent magmatism over ∼2 Myr thermally matured the upper crust, enabling the growth of magmatic reservoirs that formed the Bear and Luhr Hill plutons without known volcanic activity for ∼1.3 Myr. Zircon trace element compositions indicate that this transition occurred as a continuum in decreasing negative Eu anomalies and increasing Yb/Dy with decreasing Ti contents and age across the plutonic units, reaching the more evolved signatures and lowest crystallization temperatures in the zircon from porphyry dikes. An increase in the volcanic/plutonic ratio with the eruption of the Fulstone Spring sequence likely terminated the ore-forming potential of the district. Although many other studies have provided important views on the timing and duration of porphyry Cu deposit formation, the link to their plutonic roots has been limited due to their inaccessibility. Therefore, this study offers the first complete picture into the temporal and chemical evolution of an upper-crustal magmatic system and the roles that factors like prolonged magmatism and volcanism play before and after ore formation. We argue that without the thermal maturation of the upper crust, the construction of a long-lived magmatic reservoir would not have been possible, precluding the formation of porphyry Cu deposits. Under this premise, not only the duration of ore-related magmatic activity, but the occurrence of long-lived precursor magmatism might exercise crucial control on the final tonnage of the deposit and should be considered during exploration for new porphyry deposits.

Noble gases in shocked igneous rocks from the 380 Ma-old Siljan impact structure (Sweden): A search for paleo-atmospheric signatures

Finding direct records of the ancient Earth's atmosphere is key to understand the evolution of the entire planet. Nevertheless, such records are scarce, and only a few geological samples with preserved paleo-atmospheric signatures are available today. In this study, we analyzed noble gases contained in shocked granitic and volcanic rocks collected across the 380 Ma-old Siljan impact structure (in Sweden) to investigate the potential of shocked quartz crystals with planar deformation features (PDFs), often decorated by fluid inclusions, to preserve atmospheric signatures.Our results reveal that most of the investigated samples show relative elemental abundances of noble gases falling between those of air and meteoric water values. Atmospheric gases are present in the samples but excesses in radiogenic, nucleogenic, and fissiogenic noble gas isotopes highlight interactions of fluids with crustal rocks during the impact-generated hydrothermal circulation of fluids after the impact. Such fluid circulation event(s) could also have remobilized older fluids originally present in fluid inclusions preserved in the mineral lattices. Furthermore, results obtained on the two pure quartz fractions highlight a stronger atmospheric signature correlated with the presence of PDFs. The atmospheric component detected in this study could represent either a paleo-atmospheric signature trapped shortly after the Siljan impact event, or it may result from modern atmospheric contamination. Future studies of noble gases contained within shocked quartz crystals from other impact structures will unveil if such samples present advantages compared to other paleo-atmospheric proxies.

Microscale δ34S and δ18O variations of barite as an archive for fluid mixing and microbial sulphur metabolisms in igneous rock aquifers

ABSTRACT The stable isotope compositions of sulphur (δ34S) and oxygen (δ18O) in barite are frequently used as proxies for microbial sulphate reduction (MSR) in diverse environments, such as in relation to anaerobic oxidation of methane in marine cold seeps. There, isotopically heavy barite is used as a marker for MSR from a sulphate pool that has undergone semi-closed system conditions. Closed-system MSR is also a commonly observed feature in igneous rock hosted fracture aquifers, as shown by extremely 34S-enriched pyrite. What is less well-constrained is whether δ34S in barite can be used as a proxy for MSR in such systems. Here we explore the microscale heterogeneity of δ34S and δ18O via secondary ion mass spectrometry and the trace element Sr via LA-ICP-MS maps in barite precipitated in granite-hosted boreholes during a 17-year experiment, at Äspö, Sweden. We compare it with δ18Osulfate, δ34Ssulfate, and δ34Ssulfide of the fracture fluids and with paragenetic pyrite with δ34S values reflecting closed system MSR. The δ18O values in barite (+9.4 to +16.9 ‰) represent two generations of barite, one with low values and one with high values. The latter are likely impacted by sulphur disproportionating or -oxidizing bacteria. The barite reflects a much smaller span in δ34S (+14.5 to +28.6 ‰) than the pyrite (−47.2 to +53.3 ‰). This lack of extremely high δ34Sbarite values is proposed to be due to that barite saturation only occurred in the early parts of the Rayleigh cycle. Additionally, fluid migration has affected the δ34S values to lower values, accompanied by higher Sr concentrations. Taken together, barite δ34S values cannot be regarded as a reliable independent proxy for MSR in deep sulphate-poor igneous rock hosted aquifers. However, the relation between the δ34S values of coeval barite and pyrite is regarded as a useful proxy for MSR-related fractionation during early stages of MSR.

Clay mineralogy in mylonite weathering products from Njimom (west Cameroon): origin and terracotta suitability

This study investigated the mineralogical assemblage of weathering products developed on mylonite rocks in west Cameroon. Smectite and vermiculite occurrence was studied, particularly to emphasize its origin and the potential of the samples in terracotta production. Samples characteristics were determined through X-Ray Diffraction (XRD) and X-ray Fluorescence (XRF) methods. Weathering products were observed along road trenches of 450 m long and 50 m height on a syenitic basement rock intruded by magmatic rock veins. Clay minerals are mainly composed of kaolinite and illite. 2:1 swelling clay minerals (vermiculite, montmorillonite) occur in some samples mainly originated from the contact between weathering materials and quartzo feldspatic bands. The main oxides are SiO2 (46–72%), Al2O3 (16–31%), Fe2O3 (2–16%) and K2O (0.1–6%). Vermiculite may be formed by fluid-mineral interactions during alteration of the chlorite in the plasmic microsystems of the soil. Structural changes in the upper part of the saprolite leading to the transformation of vermiculite into smectite under supergene alteration control, which predominates in humid tropical zones. Heavy rainfall leads to the formation of kaolinite at the expense of 2:1 clay minerals. Empirical classification revealed that the studied weathering products can be used to produce common bricks.

Variscan basement tectonics and Alpine shear zones in the external Balkanides: Structural data from the Vezhen Massif, Central Stara Planina Mts., Bulgaria

The Alpine Balkanides in Bulgaria are developed over Variscan orogenic fragments, which are variously affected by late-stage thick-skinned thrust tectonics. Often, these fragments hold both Variscan and Alpine structural records. А key area to shed light on important aspects of the tectonic evolution of the external parts of the Alpine Balkanide orogen is the Vezhen Massif in the Central Stara Planina Mountains. Although it is built exclusively of Paleozoic crystalline rocks, its structure was long considered as an example of intense mid Eocene to early Oligocene (Late Alpine) shortening. Detailed fieldwork in the area shows a more complicated tectonic framework and provides evidence of a polyphase structural evolution, involving an important stage of Variscan metamorphism and deformation and two stages of north-vergent Alpine shortening. The Stargel-Boluvanya Tectonic Zone that affects the western Vezhen Massif is one of the most important Variscan structures in the Balkan fold-and-thrust belt. Structural data indicate that the metamorphic basement records initial top-to-north thrusting, followed by further shortening, which led to folding and localized strike-slip deformation. The Variscan syn-metamorphic fabric is cross-cut by several post-kinematic igneous bodies of late Carboniferous to Permian ages. The tectonic history continues with a development of a network of greenschist facies north-vergent mylonitic zones for which an Early Alpine (post-late Permian but pre-Late Cretaceous) age is assumed. Late Alpine north-vergent thrusting is evident only on the eastern and northern flanks of the Vezhen Massif. In a broad structural context, the documented record in the basement rocks of the Vezhen Massif is comparable with those of the external massifs of the Alps and Iberia.

A Trachytic Monogenetic Volcano in the Changbaishan Volcanic Field, Xitudingzi Volcano: Petrogenesis, Magma Plumbing System and Implications for Non-basaltic Monogenetic Volcanism

Abstract The Changbaishan volcanic field is the largest Quaternary volcanic field in China and includes the most dangerous polygenetic volcano, Tianchi volcano. In addition to the Tianchi volcano, the Changbaishan volcanic field contains two other polygenetic volcanoes (Wangtian’e volcano and Namphothe volcano) and at least 150 monogenetic volcanoes. The compositions of these monogenetic volcanoes range from basalt to rhyolite. Basaltic monogenetic volcanoes are the most abundant and spread throughout the volcanic field, whereas intermediate to evolved monogenetic volcanoes are found only within the Touxi subfield. We know very little about these non-basaltic monogenetic volcanoes, especially the magmatic petrogenesis, volcanological origins and structure of the magma plumbing system beneath them. Therefore, in this study, the Xitudingzi volcano within the Touxi subfield is used as an example to study the petrogenesis of silicic magma and explore the structure of the magma system by combining new 40Ar/39Ar dating, whole-rock geochemistry, Sr-Nd isotopes, and mineral chemistry. The 40Ar/39Ar chronology results showed that the Xitudingzi volcano erupted at 1.17 Ma; this volcano is younger than the other monogenetic volcanoes in the Touxi subfield. The geochemical results indicated that the eruption products of the Xitudingzi volcano are trachytic and formed from the assimilation fractional crystallization (AFC) process of an alkaline basaltic parental magma. The results from the Rhyolite-MELTS and thermobarometry indicated that the magma plumbing system of the Xitudingzi volcano contained two magma chambers at depths of 8 km and 19 km, respectively. The trachytic magma of Xitudingzi was produced by the AFC process from the alkaline basaltic magmas in the deep magma chamber and then transported to the shallow magma chamber. Furthermore, differences in the trace element concentrations between the different units of the Xitudingzi volcano indicated that the shallow pre-eruptive magma chamber was compositionally zoned. Based on the results from our study, the upper crustal magma plumbing system of the Xitutingzi volcano existed independently of the magma plumbing system of the cone-construction stage of the Tianchi volcano, even though the two volcanoes had similar ages of activity as well as the same chemical composition of eruptive products. Based on these results and other existing studies, a relatively thick crust and the developed faults are likely the key factors for the formation of intermediate to evolved monogenetic volcanos in the Touxi volcanic field.

Subsurface Microbial Colonization at Mineral-Filled Veins in 2-Billion-Year-Old Mafic Rock from the Bushveld Igneous Complex, South Africa

Recent advances in subsurface microbiology have demonstrated the habitability of multi-million-year-old igneous rocks, despite the scarce energy supply from rock-water interactions. Given the minimal evolution coupled with exceedingly slow metabolic rates in subsurface ecosystems, spatiotemporally stable igneous rocks can sustain microbes over geological time scales. This study investigated a 2-billion-year-old mafic rock in the Bushveld Igneous Complex, South Africa, where ultradeep drilling is being executed by the International Continental Scientific Drilling Program (ICDP). New procedures were successfully developed to simultaneously detect indigenous and contaminant microbial cells in a drill core sample. Precision rock sectioning coupled with infrared, fluorescence, and electron microscopy imaging of the rock section with submicron resolution revealed microbial colonization in veins filled with clay minerals. The entry and exit of microbial cells in the veins are severely limited by tight packing with clay minerals, the formation of which supplies energy sources for long-term habitability. Further microbiological characterization of drilled rock cores from the Bushveld Igneous Complex will expand the understanding of microbial evolution in deep igneous rocks over 2 billion years.

Detrital clay mineral input reconstructed based on weathering records and its influence on organic matter enrichment: A case study of the Paleogene Shahejie Formation in the Dongpu Sag, Bohai Bay Basin

As major components of shales, clay minerals are important for investigating unconventional petroleum geology. However, there are few quantitative methods for investigating possible inputs of clay minerals during sedimentary processes. In this study, shales from the Shahejie Formation in the Dongpu Sag, Bohai Bay Basin, were chosen for investigating clay mineral inputs. The shales are abundant in a variety of clay minerals. The shales, uniformly derived from felsic igneous rocks, underwent mild K metasomatism. Thus, the detrital clay mineral input was primarily influenced by weathering intensity. Accordingly, representative samples subjected to varying weathering intensities were selected to identify detrital and diagenetic clay minerals. Kaolinite and chlorite are of detrital origin. Illite and illite/smectite mixed layers (I/S) have both detrital and diagenetic origins. The precursor minerals of diagenetic minerals can be identified based on their morphological and elemental characteristics. After quantifying the area proportions of various clay minerals, the detrital clay mineral compositions of selected samples were calculated. Further analysis indicated that the detrital illite content and the detrital chlorite content are closely related, while the detrital smectite content correlates well with the chemical index of alteration (CIA). Based on the preceding analysis, the fitting formulas were established to quantitatively characterize the input of detrital clay minerals. The shales from the north sag had a high input of detrital smectite, whereas the shales from the south sag had a high input of detrital illite. Consequently, the shales from the north sag contain more organic matter than those from the south sag.

Dyke emplacement under mixed loading conditions: Insights from the Dharwar Craton, India

Dykes are intrusive igneous bodies that play crucial role in the supply and ascent of magma to the Earth’s crust. Magma can intrude along pre-existing anisotropies such as fractures or foliations present within the host rock or it may create its own path by fracturing the host rock. In the latter scenario, when fractures are formed by the pressure exerted by the invading magma, a dyke’s outcrop shape and geometry are diagnostic of the conditions under which it evolved. Here, we report mafic dykes emplaced within the younger granites of Dharwar Craton, peninsular India. Outcrop attributes of these dykes are characteristic of emplacement under conditions of mixed mode loading. We discuss different discrete modes of fracture formation and their possible combinations to understand the generation and eventual emplacement of dykes under mixed mode loading. This leads to the development of a comprehensive sequence of progressive dyke evolution under mixed mode I-III loading and thereby distinguishing incremental orders of dyke horn formation. We further apply this knowledge along with collected field evidence on dyke body geometries to propose an evolutionary model of dyke formation and emplacement within the Chitradurga granite under varying regional stress fields of the Chitradurga Schist Belt, Western Dharwar Craton, India. We infer, that the dykes initiated as extensional fractures within an earlier NE-SW directed compressive stress field and were subsequently sheared sinistrally by the effect of the adjacent Chitradurga Shear Zone on account of a later E-W to ESE-WNW directed compression.

Ti- and Ba-rich phlogopitic micas in alkaline basic and upper mantle igneous rocks; stoichiometry, stability, and Fe valence estimation reassessed and rationalised

Ti- and Ba-rich tri-octahedral micas occur in fractionated basic igneous rocks, metasomatized mantle peridotites, metamorphosed pelites/carbonates, and hydrothermally altered mineral deposits. Electron microprobe analyses (EMP), with all iron reported as FeO, were widely used in the 1970/80s to interpret Ti and Ba substitution mechanisms, based on 22 O2– unit cell calculations, implying that cation vacancies occur in octahedral and/or intersheet sites. In 1996 EMP with chemical and physical analyses for ferric and total Fe, H2O, (OH), and element-specific Fe X-ray Absorption Spectroscopy (both K and L-edges) established valence states for Fe and Ti and cation site occupancies, that ∼50 % O replaces (OH) molecules, and that 24 anion cell formulae show the absence of cation vacancies. Cell formula calculation protocol for phlogopitic micas is refined here and results tested against the stoichiometric formula for vacancy-free phlogopite, XIIK2VIMg6IV[Si6Al2]O20(OH)4. Hypothetical sheet silicate compositions, calculated with fixed contents of vacancies linked to particular mixed-valence element substitutions, confirm that reliable unit cell formulae for natural mica solids require that each stoichiometric vacancy must be accounted for. If reliable estimates for ‘excess O’ (denoted WO2−) are assigned to EMP analyses, the proportion of the oxy-mica component in a mica solid solution can be defined. This approach is tested using published analyses for Ti- and Ba-rich biotites from fractionated basic and ultramafic volcanic igneous rocks (oxymica range 2.5–45 %; TiO2 up to 14 %; BaO up to 23 %), upper mantle peridotites (equivalent values 7–18 %; 6 %; 0.7 %), and metasomatised upper mantle (2–37 %; 9 %; 23 %). Enrichments of Ti and Ba in micas are clearly linked to the extra oxygen charge required to neutralise the more highly charged Ba2+ and Ti4+ replacing K+ and Mg2+.Substitution mechanisms involving Ba, Ti, and Fe3+ in ideal phlogopite involve coupled inter-site / inter-valence interactions so both site-ordering and valence-balance between the different cation sites must be accounted for. The cation exchange 2XIIK+ + 4VI(Mg2+ + Fe2+) + 4IVSi4+ ↔ XIIBa2+ + 3VITi4+ + 4(Altotal + Fe3+ + Crtotal)3+ is used here so that valence and site order can be considered together. Compositional variations show well-defined trends towards high-oxymica content and vacancy-free Ti and Ba mica end-members, rather than to oxy-free end-members with essential vacancies. Average compositions for different source rock micas are used to assess compositional trends; molecular WO2− values are refined by relating initial estimates directly to the wt.% TiO2 contents; and refined WO2− and stoichiometrically calculated Fe3+/Fetotal ratios ranging ∼0.1–0.5) are correlated with possible primary magmatic values.

Petrogenesis of episodic volcanic-intrusive rocks in SE China: Crystal-melt segregation and magma mixing

Crystal-melt segregation and magma mixing are key magmatic processes that generate compositional variabilities in igneous rocks. Distinguishing these processes can provide insight into the evolution of volcanic calderas and the driving force of plate subduction. Intensive episodic magmatism occurred in Southeast China during the Cretaceous, producing a series of volcanic-intrusive rocks during different magmatic stages in chain-like calderas. We conducted comprehensive petrological, geochemical, isotopic, and zircon trace element analyses on two representative calderas (Xiaoxiong and Shiniushan).The results show that the magmatism produced multistage volcanic and coexisting intrusive rocks regarding their crystallization ages (102–88 Ma) and NdHf isotopic compositions (zircon ɛHf(t) = −11.7 to −1.8, whole-rock εNd(t) = −8.3 to −2.4). The volcanic and intrusive rocks are related by fractional crystallization.In general, the erupted silicic rhyolitic rocks are characterized by distinctly negative Eu anomalies (δEu = 0.10–0.71) and depletions of Ba, Sr, P, and Ti, while the coexisting intrusive rocks show distinct or complementary geochemical signatures, such as neglectable Eu anomalies (δEu = 0.92–1.09) and positive Ba anomalies. Furthermore, the whole-rock samples from the calderas have variable δ41K values (from −0.14 ‰ to −0.59 ‰) that show strong correlations with indices of fractional crystallization such as Eu anomaly, Sr content, and K2O/Na2O ratios, indicating a key role of plagioclase during crystal-melt segregation. This is also supported by petrographic evidence for crystal accumulation such as the synneusis of plagioclase phenocrysts. Mafic microgranular enclaves in the volcanic-intrusive rocks indicate that magma recharge events occurred in the magma reservoir. Synthesizing these lines of evidence, we argue the multiple magmatism in plate subduction zone can be sustained by recharge events in the magma reservoir along with crystal-melt segregation processes. Particularly, the volcanic rocks tend to have isotopically lighter K than intrusive rocks, and strongly support that the volcanic rocks represent the extracted melt that is complementary to the cumulative intrusive rocks.

Particle Concentrations and Sizes for the Onset of Settling‐Driven Gravitational Instabilities: Experimental Validation and Application to Volcanic Ash Clouds

AbstractSettling‐driven gravitational instabilities (SDGIs) can form at the base of buoyant particle‐laden suspensions, modulating particle sedimentation in various settings such as meteorological and volcanic clouds, fluvial plumes, magma chambers, submarine hydrothermal plumes, or industrial emissions. These instabilities result in the formation of rapidly descending currents called ‘fingers’ within which fine particles settle faster collectively than individually. This study investigates SDGI triggering conditions underneath volcanic ash clouds through analogue experiments considering sedimentation from aqueous particle suspensions. We confirm that the conditions for which SDGIs develop are controlled by two dimensionless numbers: Bf (ratio of the characteristic finger velocity to the individual particle settling velocity); and Bi (ratio of timescale for individual particle settling to that for collective settling controlled by inertial drag). SDGIs are triggered for values of Bf and Bi > 1 for which particles are fully coupled with the flow within fingers. Using these parameters, we produce a regime diagram for the 2010 eruption of Eyjafjallajökull (Iceland) that describes particle settling as a function of particle concentration and size. More studies are needed to produce a general regime diagram accounting for the evolution of SDGIs properties with eruption and atmospheric parameters. Nonetheless, our study confirms that fingers affect sedimentation from volcanic clouds with high ash volume fractions above 10−6 vol.%. Our validation of criteria predicting the onset of fingers due to SDGIs constitutes a step forward toward the incorporation of these collective settling processes in volcanic ash transport and dispersion models.

Age and provenance relationships between the basal Great Valley Group and its underlying basement: implications for initiation of the Great Valley forearc basin, California, U.S.A.

ABSTRACT The Great Valley forearc (GVf) basin, California, records deposition along the western margin of North America during active oceanic subduction from Jurassic through Paleogene time. Along the western GVf, its underlying basement, the Coast Range Ophiolite (CRO), is exposed as a narrow outcrop belt. CRO segments are overlain by the Great Valley Group (GVG), and locally, an ophiolitic breccia separates the CRO from basal GVG strata. New stratigraphic, petrographic, and geochronologic data (3865 detrital and 68 igneous zircon U-Pb ages) from the upper CRO, ophiolitic breccia, and basal GVG strata clarify temporal relationships among the three units, constrain maximum depositional ages (MDAs), and identify provenance signatures of the ophiolitic breccia and basal GVG strata. Gabbroic rocks from the upper CRO yield zircon U-Pb ages of 168.0 ± 1.3 Ma and 165.1 ± 1.2 Ma. Prominent detrital-zircon age populations of the ophiolitic breccia and GVG strata comprise Jurassic and Jurassic–Early Cretaceous ages, respectively, with pre-Mesozoic ages in both that are consistent with sources of North America affinity. Combined with petrographic modal analyses that show abundant volcanic grains (> 50%), we interpret the breccia to be mainly derived from the underlying CRO, with limited input from the hinterland of North America, and the basal GVG to be derived from Mesozoic igneous and volcanic rocks of the Sierra Nevada–Klamath magmatic arc and hinterland. Analysis of detrital-zircon grains from the lower and upper ophiolitic breccia yields MDAs of ∼ 166 Ma and ∼ 151 Ma, respectively. Along-strike variation in Jurassic and Cretaceous MDAs from basal GVG strata range from ∼ 148 to 141 Ma, which are interpreted to reflect diachronous deposition in segmented depocenters during early development of the forearc. The ophiolitic breccia was deposited in a forearc position proximal to North America < 4 Myr before the onset of GVG deposition. A new tectonic model for early development of the GVf highlights the role of forearc extension coeval with magmatic arc compression during the earliest stages of basin development.

Detrital zircon provenance record of the northern shelf in the Rakhine Basin, Bay of Bengal

The Cenozoic strata in the Rakhine Basin play a crucial role in hydrocarbon exploration; however, the provenance of these sedimentary rocks remains poorly investigated. In this study, we employed detrital zircon geochronology and geochemistry in the northern Rakhine shelf to investigate the provenance of sedimentary rocks and, combined with the previous studies, to identify their spatial and temporal variations and to elucidate the provenance evolution since the middle Miocene. The geochemical results of the zircons indicate that the primary source rocks of the sedimentary rocks are magmatic and metasedimentary rocks, particularly those of continental arc and post-collision granites. The origin of these rocks can be attributed to the Gangdese arc magmatic belt in the north, as well as the Indo-Burma Ranges in the east. The sedimentary rocks in the northern Rakhine yield zircon U-Pb age peaks primarily at 0-150 Ma, 400-600 Ma, 600-1000 Ma, and 1600-2100 Ma, with minor populations at 150-400 Ma and >2100 Ma. The detrital zircon provenance study shows that the zircons in the Miocene sedimentary rocks were primarily derived from the paleo-Brahmaputra River, with contributions ranging from 68.7% to 77.0%. During the Pliocene, the paleo-Brahmaputra River and the Chittagong-Tripura Fold Belt were the most significant sediment suppliers. Since the early Pleistocene, the northern Rakhine coastal region has played a significant role in their composition. The Brahmaputra River was the predominant contributor to upper Pleistocene rocks, and the Rakhine coastal region’s contribution to the shelf intensified throughout the Pleistocene. In conjunction with regional tectonic evolution, our study underscores the importance of the Indian Plate subduction as a pivotal factor influencing the provenance transitions in the northeastern Bay of Bengal.