Composition Of Volcanic Rocks Research Articles

Geochemical Signatures of Mafic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Mafic Magmatism

Abstract Greenstone belts are dominated by mafic volcanic rocks with geochemical characteristics that indicate a range of possible geodynamic influences. Many analogies with modern tectonic settings have been suggested. Increasing exploration of the modern oceans and comprehensive sampling of volcanic rocks from the sea floor are now providing unique opportunities to characterize different melt sources and petrogenesis that can be more closely compared to greenstone belts. In this study, we have compiled high-quality geochemical analyses of more than 2,850 unique samples of submarine mafic volcanic rocks (<60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical variability spanning the full range of compositions of basalts found in greenstone belts. This diversity is interpreted to be due to variable crustal thickness, dry melting versus wet melting conditions, mantle mixing, and contamination. In particular, different melting conditions have been linked to mantle heterogeneity, complex mantle flow regimes, and short-lived tectonic domains, such as those associated with diffuse spreading, overlapping spreading centers, and triple junctions. These are well documented in the microplate mosaics of the Western Pacific. Systematic differences in mafic volcanic rock compositions in modern oceanic settings are revealed by a combination of principal components analysis and unsupervised hierarchical clustering of the compiled data. Mafic volcanic rocks from most arc-back arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry such as large ion lithophile element (LILE) enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements (HREEs). This contrasts with mafic volcanic rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry, a less depleted mantle, less wet melting, and variable crustal contamination. The differences are interpreted to be the result of the lower mantle temperatures, thinner crust, and subduction-related processes of present-day settings. However, mafic rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, including the Lau basin, East Scotia ridge, Bransfield Strait, and Manus basin, which are characterized by fertile mantle sources, high heat flow, and complex spreading regimes typical of small-scale microplate mosaics. These types of settings are recognized as favorable for volcanogenic massive sulfide (VMS) deposits in modern and ancient greenstone belts, and therefore the particular geochemical signatures of the mafic volcanic rocks are potentially important for area selection in base metal exploration.

Delineation of shallow volcanic structures from audio-frequency magnetotelluric data beneath Ulleung Island, East Sea (Sea of Japan)

As volcanoes are closely related to the living environment of humans, in particular via natural hazards and geothermal energy, scientific studies on volcanic edifice structures are required. Ulleung Island is a Quaternary volcanic island located in the mid-western East Sea (Sea of Japan). In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to image the substructure of the Ulleung volcanic edifice. In addition, the electrical structure was interpreted from the geothermal system perspective, geochemical compositions of volcanic rocks, and the possibility of the presence of a magma reservoir. AMT data were obtained from 25 stations and processed using the remote reference technique. Then, the three-dimensional (3-D) approach was reasonably adopted according to dimensionality analysis. Before conducting the 3-D inversion, the effects on topography and ocean were analyzed using a simplified 3-D synthetic model, because Ulleung Island is surrounded by sea and the topography is undulating. Most AMT stations on Ulleung Island are distorted by topographical and oceanic effects; in particular, oceanic effects are significant at frequencies lower than 10 Hz. The 3-D inversion was conducted with full impedance components and vertical magnetic transfer functions in a frequency range of 10,000–1 Hz. The results show that the Ulleung volcanic edifice is characterized by two layers of electrical structures as follows: near-surface resistive anomalies and an underlying conductive layer. Considering the high geothermal gradient on Ulleung Island, we suggest that the conductive layer of the volcanic edifice is due to hydrothermal alteration of basaltic rocks, with a potential heat source underneath. Based on the geochemical characteristics of the Ulleung volcanic rocks, the possibility of heat transfer from a trachytic magma reservoir within the shallow crustal is suggested. In summary, this study presents a geological interpretation of shallow volcanic structures beneath Ulleung Island and the possibility of an active magma reservoir as a potential heat source for the volcanic edifice.Graphical

Geochemistry of Neogene Nansatsu volcanic rocks in South Kyushu, Japan

ABSTRACT We report the whole-rock major and trace element and Sr – Nd – Pb isotopic compositions of volcanic rocks from the Nansatsu area in South Kyushu. We analysed samples from the Oldest stage (middle Miocene), Older stage (late Miocene), and Middle stage (early Pliocene) Nansatsu volcanic rocks, which are andesite to dacite in composition, with an island-arc trace element signature. The compositional variabilities within the Older and the Middle stage rocks are relatively small compared to those in the Oldest stage and Quaternary volcanic rocks. Isotopic characteristics suggested that the Oldest stage volcanic rocks had magma sources similar to regional ilmenite-series granitoids of later ages (Outer Zone granitoids), which incorporated a substantial amount of graphite-bearing accretionary sediments (Shimanto Supergroup). The sources for the Older and Middle stage magmas were similar to those of primary Quaternary basalts in South Kyushu, indicating that Pliocene and Quaternary volcanic rocks in South Kyushu share a common mantle source, despite the change in the nature of volcanic activity from andesite-dominated to more variable volcanism of basalt to rhyolite.

Geochemical comparison between oceanic and continental arc volcanic rocks: Insights to arc magmatism

Compositional differences between continental and oceanic arc volcanic rocks have long been recognized; however, our understanding of them is incomplete. This study presents a comprehensive geochemical comparison, including major, trace and Sr–Nd–Pb isotope compositions, between global continental and oceanic arc volcanic rocks. Our data compilation reveals that volcanic rocks from a single oceanic arc occupy basaltic to basaltic–andesitic average lithochemical compositions, except for volcanic rocks from the Lesser Antilles arc, which occupies an andesitic average lithochemical composition, similar to the bulk composition of volcanic rocks from single continental arcs. The composition of the average continental arc volcanic rocks, the average Lesser Antilles arc volcanic rocks and the average oceanic arc volcanic rocks are different in many ways, which span the entire compositional range from basalts to rhyolites, and are reflected in major, trace element and Sr–Nd–Pb isotope compositions. Qualitative geochemical analyses and semi‐quantitative modelling suggest that the compositional differences in mafic arc volcanic rocks can be accounted for by differences in source mixing between mantle wedge peridotite and slab‐derived crustal components. Slab melts‐peridotite source mixing governs most of the lithochemical features of the continental arc and the Lesser Antilles arc mafic volcanic rocks, which is especially true for Sr–Nd–Pb isotopes. On the other hand, such source mixing occurred to a lesser extent in the petrogenesis of oceanic arc mafic volcanic rocks. Magma evolution processes such as fractional crystallization may further modify the lithochemical composition of the arc volcanic rocks, especially the felsic ones, but this is not the governing factor.

SOSVA-LOZVA GRABEN – THE DEVONIAN-CARBONIFEROUS STRUCTURE OF EXTENSION IN THE REAR OF AN ACTIVE CONTINENTAL MARGIN ON THE EASTERN SLOPE OF THE NORTHERN URALS

Within the northern part of the Tagil paleo-island-arc structure in the Northern Urals, the Sosva-Lozva graben is identified. The graben is filled mainly with terrigenous and volcanic strata of the Devonian and Lower Carboniferous. The location of the graben to the west (in modern coordinates) of the supra-subduction magmatic complexes and the composition of volcanic rocks, which combines signs of igneous formations of convergent and divergent settings, suggest that this structure formed under extensional conditions in the rear of the active continental margin.

The Sosva–Lozva Graben: A Devonian–Carboniferous Extensional Structure at the Rear of an Active Continental Margin on the Eastern Slope of the Northern Urals

Within the northern part of the Tagil paleo-island-arc structure in the Northern Urals, the Sosva–Lozva graben is identified. The graben is filled mainly with Devonian and Lower Carboniferous terrigenous and volcanic strata. The location of the graben to the west (in modern coordinates) of the supra-subduction igneous complexes and the composition of volcanic rocks, which combines the signs of igneous formations of convergent and divergent settings, suggest that this structure formed under extensional conditions at the rear of the active continental margin.

Prediction of thermal diffusivity of volcanic rocks using machine learning and genetic algorithm hybrid strategy

Thermal diffusivity, a major determinant of heat transfer, has a substantial effect on rocks thermal properties. This investigation employed artificial intelligence to predict the thermal diffusivity of volcanic rocks. Models of machine learning with Genetic Algorithm are established to have a hollistic insight of thermal diffusivity analysis and prediction using elemental composition of volcanic rocks, and properties including temparature, density, specific gravity and apparent porosity. The support vector machine, ensembled learning tree, Gaussian process regression, and decision tree machine learning models are combined with a genetic algorithm to predict the thermal diffusivity of volcanic rocks. Using the genetic algorithm, feature selection and hyper-parameter optimization of machine learning methods improved the accuracy of model predictions. The SVM-GA predicts more accurately than other machine learning models, with a coefficient of determination of 0.99 and a root mean square error of 0.0046. In addition, the partial dependence plots analysis illustrates the potential effect of each parameter on thermal diffusivity. Finally, a graphical user interface based on the SVM-GA model was developed to predict thermal diffusivity without extensive machine learning knowledge. The difference between a graphical user interface and experimental yield was less than 1%.

Integrating global geochemical volcano rock composition with eruption history datasets

The major element composition of volcanic rocks carries important information about the source and differentiation processes affecting the magma, the physical properties that allow it to erupt, and its eruptive style. Although global rock geochemical databases exist, these are not linked to volcanic eruption history which hampers our global understanding of the relationship between magma composition and eruption dynamics. Here, we integrate two global databases, the Geochemistry of Rocks of the Oceans and Continents (GEOROC) and the Holocene volcanoes of the world of the Global Volcanism Program (VOTW-GVP). The integration is based on matching the location name, geographic position and eruption time, which is automated by a tool called DashVolcano. The tool is open-source, accessible at https://github.com/feog/DashVolcano, and gives access to the integrated datasets via an interactive dashboard. DashVolcano is based on more than 138,000 volcanic rock samples and provides the basis for the identification of global relationships between eruption styles, volcano types, and rock composition for more than 700 volcanoes and their eruptions for the last 10,000 years. The combined record of the eruptive history and its corresponding geochemical rock composition that DashVolcano provides can be used for characterizing global geochemical differences between volcanoes, and should also prove useful for improved long-term hazard and risk evaluations.

Excess of Naturally Occurring Fluoride in Groundwater Discharge in Macaronesia: Brava Island, Cape Verde

A high concentration of fluoride in groundwater poses a risk to human health. One of the best-known problems related to excess fluoride in drinking water is dental fluorosis. The characteristic composition of volcanic rocks in oceanic islands tends to present high concentrations of fluoride in groundwater discharge in springs. This study investigates fluoride content in two springs (Encontro and Ferreiros) on the island of Brava (Cape Verde), with the objective of monitoring the operational control production and distribution of drinking water. Using a spectrophotometric method, the level of fluoride found in the water discharged from the Encontro spring was found to be in the range of 4.8 to 6.5 mg/L, whereas at the Ferreiros spring, it was 5.8 to 6.2 mg/L. These results are far above the threshold limit established in the decree No. 5 of 2017 of the Cape Verdean legislation, whose legal threshold value for fluoride is 1.5 mg/L, the same value established by the World Health Organization (WHO). Due to geological conditions and consequences for the health of the population, the water is not adequate for consumption without proper treatment.

Late Mesozoic volcanism in the southern Xiong'er Mountains, eastern Qinling: Partial melting of the subducted Qinling crust underneath North China

Abstract Volcanism along the continental margins is a powerful tool with which to probe the orogenic processes and crustal components underlying orogenic belts. In this study, we report the zircon ages and geochemical compositions of volcanic rocks exposed in the Luanchuan area, along the southern margin of the North China block. In previous studies, these volcanic rocks were reported to be products of Paleoproterozoic (ca. 1.75 Ga) volcanism related to rifting in the Xiong'er Mountains. Our zircon age data reveal that most volcanic rocks contain late Mesozoic zircon grains of magmatic origin, suggesting that Late Jurassic to Early Cretaceous (ca. 150–120 Ma) volcanism must have occurred in the southern Xiong'er Mountains, coeval with emplacement of large granitoid plutons along the southern margin of the North China block. These late Mesozoic volcanic rocks mainly consist of trachyandesite, andesite, dacite, and rhyolite. They vary in their major-element compositions but are relatively uniform in their normalized patterns of trace and rare earth elements and have consistently enriched Nd isotopic compositions. Numerous inherited and/or xenocrystic zircon grains of mostly Paleoproterozoic and occasionally Neoproterozoic to early Mesozoic ages were found in all of the volcanic rocks, indicating complex magma sources and/or diverse crustal rocks overlying the magma chambers. The Paleoproterozoic zircon grains originated from the Paleoproterozoic volcanic and basement rocks in the southern North China block. Notably, some volcanic rocks contained early Mesozoic, early Paleozoic, and Neoproterozoic zircon grains of both metamorphic and magmatic origin that indicate derivation from the North and South Qinling blocks, which recorded thermo-tectonic events during the closure of the Shangdan and Mianlue oceans in the early Paleozoic and early Mesozoic. Therefore, we propose that these late Mesozoic volcanic rocks originated from partial melting of crustal rocks and are composed partly of the subducted basement of the Qinling orogenic belt underneath the southern Xiong'er Mountains due to asthenospheric upwelling in an extensional setting.

Geochemistry and Sr–Nd–Hf–Pb isotope systematics of late Carboniferous sanukitoids in northern West Junggar, NW China: Implications for initiation of ridge-subduction

The study of Carboniferous magmatism in northern West Junggar plays an important role in understanding the tectonic evolution of that part of the Central Asian Orogenic Belt. In this study, we present petrology, zircon U–Pb geochronology, mineral and whole-rock geochemistry, and the Sr–Nd–Hf–Pb isotope compositions of volcanic rocks from the Hamutusi area of northern West Junggar. LA–ICP–MS zircon U–Pb analysis of a representative andesite yielded an early to late Carboniferous age of 324.4 ± 6.9 Ma. The volcanic rocks are calc-alkaline, with high SiO2 (54.2–57.8 wt%), MgO (5.97–8.33 wt%), Mg# (60–69), Cr (147–448 ppm), and Ni (29.1–120 ppm) contents, and are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE), these characteristics are similar to those of typical sanukitoids within the Setouchi volcanic belt in Japan. All samples have radiogenic initial Sr and Pb isotopic compositions, and low εNd(t) and εHf(t) values, indicating the sanukitoids were generated by partial melting of subducting sediments in which the melts interacted with the mantle. Geochemical modeling calculations indicate a proportion of 3–10% sediment melt and slab-derived fluids were mixed with the depleted mantle to produce the bulk of the Hamutusi rocks. We conclude that the studied rocks from north West Junggar record the initiation of ridge subduction related to a transition from normal subduction to subduction of young and hot oceanic lithosphere between the early and late Carboniferous.

Iron, copper, and zinc isotopic fractionation in seafloor basalts and hydrothermal sulfides

Studies of the Fe, Cu, and Zn isotopic compositions of volcanic rocks and sulfides provide an important tool for understanding magmatic, hydrothermal, and alteration processes, thereby enabling the determination of both transition metal sources and the quantification of the petrologic environmental impacts of hydrothermal activities. In this study, the δ56Fe and δ57Fe values of the mid-ocean ridge basalts (MORBs) are higher than those of the seafloor hydrothermal fluids, while the reverse is true for the δ66Zn and δ68Zn values, suggesting that basalt-fluid interactions preferentially incorporate isotopically light Fe and heavy Zn into the fluids, resulting in the relative enrichment of heavier Fe and lighter Zn isotopes in altered basaltic rocks. Most of the δ56Fe values (−1.96 to +0.11‰) of the sulfide minerals are within the range of the vent fluids, but they are significantly lower than those of the MORBs and back-arc basin basalts (BABBs), suggesting that the Fe in the sulfides was mainly derived from the fluids. However, the majority of the chalcopyrite δ56Fe and δ57Fe values are higher than those of the sphalerite and pyrite. This suggests that high-temperature sulfide minerals are enriched in 56Fe and 57Fe, whereas medium- and low-temperature sulfides are depleted in 56Fe and 57Fe. Moreover, the δ65Cu (−0.88 to −0.16‰) and δ66Zn (−0.39 to −0.03‰) values of the sulfide minerals are significantly lower than those of the MORBs, BABBs, and fluids, suggesting that 63Cu and 64Zn were preferentially removed from the fluids and incorporated into the chalcopyrite and sphalerite, respectively. Consequently, vent fluid injection and deposition can cause the heavier Cu and Zn isotopic compositions of hydrothermal plumes, seawater, and sediments.

Global influence of mantle temperature and plate thickness on intraplate volcanism

The thermochemical structure of lithospheric and asthenospheric mantle exert primary controls on surface topography and volcanic activity. Volcanic rock compositions and mantle seismic velocities provide indirect observations of this structure. Here, we compile and analyze a global database of the distribution and composition of Neogene-Quaternary intraplate volcanic rocks. By integrating this database with seismic tomographic models, we show that intraplate volcanism is concentrated in regions characterized by slow upper mantle shear-wave velocities and by thin lithosphere (i.e. <100 km). We observe a negative correlation between shear-wave velocities at depths of 125–175 km and melt fractions inferred from volcanic rock compositions. Furthermore, mantle temperature and lithospheric thickness estimates obtained by geochemical modeling broadly agree with values determined from tomographic models that have been converted into temperature. Intraplate volcanism often occurs in regions where uplifted (but undeformed) marine sedimentary rocks are exposed. Regional elevation of these rocks can be generated by a combination of hotter asthenosphere and lithospheric thinning. Therefore, the distribution and composition of intraplate volcanic rocks through geologic time will help to probe past mantle conditions and surface processes.

Geological features and the first isotopic data of volcanic rocks in the Iset river basin, East-Urals megazone

Relevance of the work. The Iset river basin contains the most extensive outcrops of volcanogenic formations of the Beklenishchevsky complex of the East Ural megazone, the age of which is determined as Early Carboniferous by the ratio of volcanic rocks with faunistically characterized sedimentary deposits. Volcanics here compose flows of andesite-basaltic and andesitic lavas and lava breccias. There are no geochronological dates specifying the age of the rocks, which makes it difficult to assess their role in the formation of the megazone. Therefore, isotopic dating of these formations is very important. Methods. The U – Pb age and data on the geochemistry of zircons were obtained by laser ablation (LA – ICP – MS). Purpose of the research is to study the features of the geological structure, the material composition of volcanic rocks in the Iset river basin, the geochemistry of zircons from andesites and the determination of their isotopic age. Results of the work and the scope of their application. Lava flows of andesites and basaltic andesites with minor amounts of basalts and dacites have tectonic contact with sedimentary rocks of the Early Carboniferous age. The distribution of rare elements in volcanics is typical of supra-subduction formations. Zircons in andesites are represented by prismatic and isometric crystals. Prismatic differences in the nature of the distribution of REE and the content of Li, Ti, Sr, Th, U refer to zircons of magmatic genesis, isometric – to “hydrothermal”. According to the U / Yb – Y ratios, the former correspond to the zircons of the ocean floor, while the latter are related to the continental ones. Isotopic dating of zircons from andesites was carried out for the first time. Their age was 311 million years. The data can be used in geological mapping, as well as in the compilation of large-scale geodynamic maps and diagrams. Conclusions. Volcanic rocks in the Iset river basin were formed in supra-subduction continental-marginal geodynamic conditions that took place in the Urals in the Carboniferous. The obtained value of the age of zircons from andesites, possibly, fixes the stage of their transformation. Keywords: East-Ural megazone, volcanic rocks, zircon, isotopic age.

Chemical classification of common volcanic rocks based on degree of silica saturation and CaO/K2O ratio.

Modal classifications of common volcanic rocks are expensive, difficult, or impossible to attain. As a consequence, these rocks are classified using the chemical composition. However, existing classificatory diagrams are unable to identify all 16 families of common volcanic rocks; the most used is the total alkali-silica (TAS) diagram that identifies six families. Rocks not in the TAS diagram are misclassified with other rock names; their names are evolving to extinction, e.g. latite and rhyodacite. Some diagrams use Na2O, which is a complicating element rather than discriminant. Na2O is present both in alkali feldspar and plagioclase making difficult the separation of the amount associated to either feldspar. Silicon, potassium, and calcium are the three major elements with highest variations among volcanic rocks. They are selected for use in two new diagrams confronting CaO/K2O ratio with K2O content (KCK diagrams). One diagram is designed for saturated (intermediate) rocks (quartz <5 vol.%) and the other for oversaturated (acid) volcanic rocks (quartz >5%). These diagrams are tested using compositions of volcanic rocks from Uatumã magmatism in the Amazon Craton and also by plotting compositions of rocks from type-localities and the world averages. The proposed limits between rock families agree with plotted rock compositions. The KCK diagrams are an alternative to existing diagrams to classify volcanic rocks.

Shannon Entropy as an Indicator for Sorting Processes in Hydrothermal Systems.

Hydrothermal processes modify the chemical and mineralogical composition of rock. We studied and quantified the effects of hydrothermal processes on the composition of volcanic rocks by a novel application of the Shannon entropy, which is a measure of uncertainty and commonly applied in information theory. We show here that the Shannon entropies calculated on major elemental chemical composition data and short-wave infrared (SWIR) reflectance spectra of hydrothermally altered rocks are lower than unaltered rocks with a comparable primary composition. The lowering of the Shannon entropy indicates chemical and spectral sorting during hydrothermal alteration of rocks. The hydrothermal processes described in this study present a natural mechanism for transforming energy from heat to increased order in rock. The increased order is manifest as the increased sorting of chemical elements and SWIR absorption features of the rock, and can be measured and quantified by the Shannon entropy. The results are useful for the study of hydrothermal mineral deposits, early life environments and the effects of hydrothermal processes on rocks.

Inhomogeneous thinning of a cratonic lithospheric keel by tectonic extension: The Early Cretaceous Jiaodong Peninsula–Liaodong Peninsula extensional provinces, eastern North China craton

Abstract The mechanisms of lithospheric thinning and craton destruction have been hotly debated in the last decades. The Early Cretaceous Jiaodong and Liaodong extensional provinces (JEP and LEP, respectively) of the eastern North China craton are typical areas where the cratonic Archean lithosphere has been intensely extended and thinned. Various extensional structures, e.g., metamorphic core complexes (MCCs), low-angle detachment faults, and extensional basins, characterize the Early Cretaceous crustal deformation of the two provinces. However, profound differences exist in structural development and related magmatic activities between the two provinces. Distributed small-scale extensional basins were formed in association with exhumation of the Liaonan and Wanfu MCCs in the LEP, whereas the major Jiaolai Basin was developed coevally with exhumation of the Wulian, Queshan, and Linglong MCCs in the JEP. Sr-Nd isotope compositions of volcanic rocks from the basins of the two provinces are compatible with syntectonic magmatic activities of evolving magma sources in the LEP, but multiple and hybrid magma sources in the JEP. It is shown, from variations in structural styles, plutonic and volcanic activities, and thermal evolution of the two extensional provinces, that two stages (ca. 135–120 Ma and 120–100 Ma) of tectonic extension affected the JEP and LEP in the Early Cretaceous. We demonstrate that regional tectonic extension (parallel extension tectonics, or PET) is responsible for the formation of major extensional structures and the occurrence of the magmatic associations. Progressive wide rifting by coupled crust-mantle detachment faulting of a hot LEP lithosphere was accompanied by evolving magma sources from dominant ancient crust and enriched mantle to juvenile crust. Two stages of narrow rifting of a cold JEP lithosphere led to early crustal detachment faulting transitioning to late crust-mantle faulting, which resulted in intense magmatic activity from hybrid to multiple magma sources. These processes contributed to destruction of the craton, with thinning of its lithospheric keel and local transformation of the nature of the lithospheric mantle. It is expected that such a model is also applicable to interpretation of tectonic extension of contiguous areas of the North China craton and the remobilization of other cratons.

New Data on the Age, Material Composition, and Geological Structure of the Central Kamchatka Depression (CKD). Part 2. The Mineralogical Composition of Volcanic Rocks and Mantle Xenoliths. Toward a Petrologic Model

This study is concerned with the mineralogical composition of volcanic ejecta in the Central Kamchatka Depression: the plateau basalts of the Ozernaya Kamchatka, Kunch, and Nikolka Stratovolcano. These data were compared with rock compositions for some volcanoes in the Klyuchevskoi Group, the plateau basalts of Bakening Volcano, and data on xenoliths of two types, viz., the “island-arc” type (Klyuchevskoi, Kharchinsky, and Sheveluch volcanoes) and the “intraplate” type (the plateau of Bakening Volcano). We have identified complementary vulcanite–xenolith pairs for which we propose a concept of dynamic evolution that envisages the ascent of melts to the ground surface accompanied by nearly synchronous change in the mineralogical composition of rocks in both pairs, while we are merely able to detect some fragments of the process. Mineralogically, the CKD riftogenic vulcanites differ from the island-arc rocks of the Klyuchevskoi Group in (1) a greater iron content of the olivines and greater concentrations of MnO and CaO in these, (2) greater titanium concentrations (over 1 wt % in the former and below 1 wt % in the latter) in clinopyroxenes, their high sodium concentration (over 0.5 wt % in the former and below 0.5 wt % in the latter), and greater (occasionally reaching 9 wt %) alumina content, and (3) the appearance of high titanium chromous spinellids, anorthoclase, and sanidine. These differences are controlled by differences in alkalinities of the melts, in the depth, composition, and the degree of melting in the mantle substrate. The compositions of the CKD rocks that we have studied, considering that the depression is a riftogenic feature, could arise in connection with a low degree of melting in the mantle sources similar to those which gave rise to intraplate subalkaline basalts in the Bakening plateau, which we treated as the standard of riftogenic volcanic occurrence.

Diachronous initiation of post-collisional magmatism in the Arabia-Eurasia collision zone

The continental collision between Arabia and Eurasia which gave rise to the Caucasus-Iran-Anatolia (CIA) volcanic province provides a unique opportunity for understanding collisional zone magmatism. This study reports a comprehensive dataset of ages and geochemical compositions of volcanic rocks formed during the initial phase of post-collisional magmatism in the CIA province. The age data indicate a diachronous onset of volcanism that began ~17 Ma in SE Anatolia, and propagated northward from ~11 to 9 Ma toward NE Anatolia and NW Iran. The rocks are characteristically bimodal, with dominantly basic (SiO2 = 48–52 wt%) and silicic (SiO2 = 58–71 wt%) components that feature significant isotopic variations (εNd = +6 to −5), suggesting two principal magma sources: (1) a juvenile mantle-derived component, and (2) an older continental crust component. We therefore attribute the volcanic initiation to a migrating post-collisional extension regime caused by the successive breakoff of subducted Neo-Tethyan slabs. Subsequent volcanism that began from ~6.5 Ma resulted in a wide spectrum of calc-alkaline and alkaline rocks, with mafic to felsic lithologies in the entire province. From ~2 Ma, volcanism ceased in the western CIA province, and started propagating eastward and southeastward to SE Iran, following the Urumieh-Dokhtar magmatic belt, consistent with the notion of an oblique continental collision between Arabia and Eurasia.

ИЗОТОПНО-ГЕОХИМИЧЕСКАЯ НЕОДНОРОДНОСТЬ ПЛИОЦЕН-ЧЕТВЕРТИЧНЫХ ВУЛКАНИТОВ КАМЧАТКИ И ПРОБЛЕМА АСТЕНОСФЕРНОГО ДИАПИРИЗМА

Isotope-geochemical material for Pliocene-Quaternary volcanoes of the Kamchatka region is generalized on a cartographic basis. The Sr-isotope anomalies of moderate and elevated radiogenicity, geochemically confirmed, are spatially conjugated. This made it possible to interpret these anomalies not only as a reflection of mantle plume material in the composition of volcanic rocks, but also of its hybrid environment, as a consequence of plum-lithosphere remobilization. The presence of multi-directional geochemical trends made it possible to propose the concept of moving boundary values for the composition of indicator rocks of the intraplate type and adakites, which significantly expanded the possibilities of their diagnostics. The isotope-geochemical heterogeneity of basaltoids of the region is generally determined by the peculiarities of concentration of rocks with intraplate and adakite geochemical characteristics, which allows considering the asthenospheric diapirism as the main factor of petrogenesis of Pliocene-Quaternary volcanism in Kamchatka.