Volcanic Arc Research Articles

Oceanic anoxic event 3 in Arctic Canada—Arc volcanism and ocean fertilization drove anoxia

The global extent of the Late Cretaceous oceanic anoxic event 3 (OAE 3) remains uncertain. It is not considered to have extended into the Boreal Realm. To test this, we examined Late Cretaceous organic- and metal-rich black mudstones of the Smoking Hills Formation in Arctic Canada. New high-precision U-Pb zircon ages indicate that deposition of the Smoking Hills Formation (88.535−78.230 Ma) was temporally coincident with OAE 3, indicating a much broader global expression of this event than previously thought. OAE 3 was likely manifest throughout the proto−Arctic Ocean (now Arctic Canada). Abundant bentonite layers and cryptotephra within the Smoking Hills Formation have rare earth element (REE) patterns that are consistent with ashfall derived from Cretaceous arc volcanism. Anomalously high organic matter content in the Smoking Hills Formation, as compared to underlying and overlying units, suggests that ocean fertilization led to enhanced productivity and metal drawdown. A peak in arc volcanism may have been a key driver of the OAE 3 event. We also explored the potential use of cadmium as a geochemical marker of volcanism and show that high volcanogenic metal loading could affect the use of Cd and other proxies for marine productivity (e.g., Zn, Cu).

Coexistence of Algoma- and Superior-type BIFs in back-arc systems: Evidence from Xincai, North China Craton

Superior-type and Algoma-type banded iron formations (BIFs) generally developed in distinct tectonic environments and have unique geological characteristics. Superior-type BIFs were deposited on passive continental shelves or intracratonic basins and are laterally extensive, whereas Algoma-type BIFs were formed near volcanic arcs and spreading centers. Here, we present a case where late Archean (∼2.5 Ga) Superior- and Algoma-type BIFs co-developed in a back-arc setting at Xincai, along the southern margin of the North China Craton. The Algoma-type BIFs at Xincai exhibit high Ge/Si ratios (4.84 to 8.84) and a broad range of εNd(t) values (−7.2 to +1.2). These geochemical signatures, along with similar εNd(t) values (+1.2) in contemporaneous amphibolites, indicate a significant influence of hydrothermal fluids associated with submarine volcanism. In contrast, the Superior-type BIFs at Xincai are characterized by positive La anomalies, low Ge/Si ratios (0.21 to 4.60) and narrow range of εNd(t) values (−6.1 to −1.0) suggesting a major contribution from terrigenous sources. The coexistence of Algoma- and Superior-type BIF types at Xincai is related to the interplay of multiple fluid systems in a back-arc environment. The Algoma-type BIFs formed from the confluence of hydrothermal fluids originating from enriched subcontinental lithospheric mantle (high Ge/Si and low εNd(t)) with those emanating from the adjacent back-arc spreading centers (depleted mantle like—high Ge/Si and εNd(t)). On the other hand, the Superior-type BIFs formed near the continental margins of the back-arc basins, where surface seawater carrying terrigenous substances (low Ge/Si and εNd(t)) mixed with hydrothermal fluids from the back-arc spreading centers. The distinct co-existence of Algoma- and Superior-type BIFs at Xincai underscores the dynamic nature of back-arc environments as crucibles for diverse BIF mineralization processes, offering new perspectives on the contributions of submarine volcanic activity and terrigenous inputs to the BIFs.

A record of magmatic differentiation in plutonic xenoliths from Santorini (Greece)

Plutonic xenoliths from volcanic arcs provide unique insights into transcrustal magmatic systems in subduction zone settings. At Santorini volcano in the Central Aegean Volcanic Arc (Greece), plutonic xenoliths occur throughout a sequence of lavas and pyroclastic rocks erupted within the last ~360 ka. They are mineralogically variable, ranging from troctolites to olivine gabbros, gabbros, gabbronorites, and diorites. Thermobarometric calculations based on mineral and melt inclusion compositions indicate equilibration over a range of temperatures (1100 to 750 °C) at shallow to mid-crustal depths (P <400 MPa), but there is no evidence for crystallisation at lower crustal depths. Oxygen isotope data of mineral separates and calculated δ18O melt values are in line with extensive closed-system fractional crystallisation at magmatic temperatures, without a requirement for extensive assimilation of the subvolcanic continental basement. The xenolith minerals compositionally overlap with phenocrysts from the volcanic rocks, but they also contain evidence for the presence of highly evolved melt compositions in the form of melt inclusions with extremely silica-rich compositions (up to 82 wt.% SiO2) and high enrichments of incompatible trace elements coupled with increasing negative Eu anomalies in clinopyroxenes. Since these characteristics correlate systematically with differentiation indices and rock type, they are interpreted to reflect melt evolution via fractional crystallisation as the dominant differentiation process with no significant role of reactive porous flow. These observations highlight that trapped melt fractions can influence mineral compositional variations in the plutonic xenoliths, and in turn the mineral compositions demonstrate a melt compositional variability not preserved in the volcanic rock record.

Arc building and maturation of the Lombok Island, East Sunda Arc

Intra-oceanic subduction zones are major sites of crust–mantle material exchange and crustal growth; however, the processes responsible for the magmatic evolution and transformation of nascent arcs into mature oceanic arcs with thicker crust are debated. We present mineral chemistry, zircon UPb age and Hf isotopic data, and whole-rock chemical and Sr–Nd–Hf–Pb isotopic data for Cenozoic volcanic rocks on Lombok in the East Sunda Arc, Indonesia, to investigate their petrogenesis and arc maturation. SIMS zircon UPb analyses of lavas from southern Lombok yield an early Oligocene age of ∼30 Ma. The lavas can be divided into three groups based on their ages and geochemical characteristics: group I (late Eocene) and II (∼30 Ma) lavas from southern Lombok, and group III lavas (∼3–0 Ma) from northern Lombok. The lavas are characterized by a progressive enrichment in K2O and incompatible element contents, and NdHf isotopic compositions with time. Group I lavas are tholeiitic basalts with low K2O contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and NdHf isotopic compositions (εNd = +7.4; εHf = +14.5), enrichment in large-ion lithophile elements (e.g., Rb, U, and K), and typical forearc basalt (FAB) geochemical characteristics. Group II lavas are low-K basaltic andesites to dacites that yield flat chondrite-normalized REE patterns [(La/Sm)N = 0.9–1.4], uniform initial 87Sr/86Sr ratios (0.7038–0.7042), and positive εNd(t) (+5.9 to +6.8) and εHf(t) (+13.9 to +14.4) values. Group III lavas are typically medium- to high-K calc-alkaline basalt to andesite and are enriched in light REEs [(La/Sm)N = 1.6–3.3] with uniform initial 87Sr/86Sr ratios (0.7038–0.7042) and slightly low εNd(t) (+3.9 to +5.4) and εHf(t) (+10.5 to +12.5) values. Progressive enrichment in incompatible element contents and NdHf isotopic compositions over time, from FAB to high-K calc-alkaline lavas, reflect the maturation of the East Sunda Arc. Sr–Nd–Pb–Hf isotopic mixing models, along with trace element ratios (e.g., Ba/Th, La/Yb, Th/Yb), suggest that increasing amounts of sediments were incorporated into the mantle sources of the magmas from group I (∼0%) to group II (<0.5%) and group III (0.5%–1.0%) lavas. Partial melting models show that the formation of group I and II lavas can be reproduced by high-degree (∼10%) partial melting of spinel peridotite, whereas the group III lavas were formed by low-degree (∼5%) partial melting of spinel peridotite. We propose that the evolution of Lombok arc magmas was controlled mainly by the fluxes of subducted slab material, along with decreasing degrees of partial melting as a result of crustal thickening over time.

Electrical Resistivity Imaging of the Northeast Carpathian Volcanic Arc With 3‐D Magnetotellurics Reveals Shallow Hydrothermal System

Electrical Resistivity Imaging of the Northeast Carpathian Volcanic Arc With 3‐D Magnetotellurics Reveals Shallow Hydrothermal System

Volatile systematics in terrestrial igneous apatite: from microanalysis to decoding magmatic processes

Apatite has been recognized as a robust tool for the study of magmatic volatiles in terrestrial and extraterrestrial systems due to its ability to incorporate various volatile components and its common occurrence in igneous rocks. Most previous studies have utilized apatite to study individual magmatic systems or regions. However, volatile systematics in terrestrial magmatic apatite formed under different geological environments has been poorly understood. In this study, we filtered a large compilation of data for apatite in terrestrial igneous rocks (n > 20,000), categorized the data according to tectonic settings, rock types, and bulk-rock compositions, and conducted statistical analyses of the F–Cl–OH–S–CO2 contents (~ 11,000 data for halogen and less for other volatiles). We find that apatite from volcanic arcs preserves a high Cl signature in comparison to other tectonic settings and the median Cl contents differ between arcs. Apatite in various types and compositions of igneous rocks shows overlapping F–Cl–OH compositions and features in some rock groups. Specifically, apatite in kimberlite is characterized as Cl-poor, whereas apatite in plutonic rocks can contain higher F and lower Cl contents than the volcanic counterparts. Calculation using existing partitioning models indicates that apatite with a high OH (or F) content does not necessarily indicate a H2O-rich (or H2O-poor) liquid because it could be a result of high (or low) magma temperature. Our work may provide a new perspective on the use of apatite to investigate volatile behavior in magma genesis and evolution across tectonic settings, volatile recycling at subduction zones, and the volcanic-plutonic connection.

ORIGIN OF VOLCANIC-HOSTED MAGNETITE AT THE LAGUNA DEL MAULE COMPLEX, CHILE: A NEW EXAMPLE OF ANDEAN IRON OXIDE-APATITE MINERALIZATION

Abstract Iron oxide-apatite (IOA) deposits, also known as magnetite-apatite or Kiruna-type deposits, are a major source of iron and potentially of rare earth elements and phosphorus. To date, the youngest representative of this group is the Pleistocene (~2 Ma) El Laco deposit, located in the Andean Cordillera of northern Chile. El Laco is considered a unique type of IOA deposit because of its young age and its volcanic-like features. Here we report the occurrence of similarly young IOA-type mineralization hosted within the Laguna del Maule Volcanic Complex, an unusually large and recent silicic volcanic system in the south-central Andes. We combined field observations and aerial drone images with detailed petrographic observations, electron microprobe analysis (EMPA), and 40Ar/39Ar dating to characterize the magnetite mineralization—named here “Vetas del Maule”—hosted within andesites of the now extinct La Zorra volcano (40Ar/39Ar plateau age of 1.013 ± 0.028 Ma). Five different styles of magnetite mineralization were identified: (1) massive magnetite, (2) pyroxene-actinolite-magnetite veins, (3) magnetite hydrothermal breccias, (4) disseminated magnetite, and (5) pyroxene-actinolite veins with minor magnetite. Field observations and aerial drone imaging, coupled with microtextural and microanalytical data, suggest a predominantly hydrothermal origin for the different types of mineralization. 40Ar/39Ar incremental heating of phlogopite associated with the magnetite mineralization yielded a plateau age of 873.6 ± 30.3 ka, confirming that the emplacement of Vetas del Maule postdated that of the host andesite rocks. Our data support the hypothesis that the magnetite mineralization formed in a volcanic setting from Fe-rich fluids exsolved from a magma at depth. Ultimately, Vetas del Maule provides evidence that volcanic-related IOA mineralization may be more common than previously thought, opening new opportunities of research and exploration for this ore deposit type in active volcanic arcs.

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.

Позднемеловые-раннепалеогеновые вулканические породы Восточной Камчатки (особенности состава и изотопно-геохимическая гетерогенность)

New and previously published isotope-geochemical material on the Upper Cretaceous-Lower Paleogene volcanic complexes of the Ozernovsky-Valaginsky segment of the Achaivayam-Valaginsky paleoarc are presented. It is found that composition of rocks composing separate volcanic fields within the Achayvayam-Valagin paleoarc (terrane) is different. On Karaginsky Island, volcanics of the moderate-potassium calc-alkaline and shoshonite series are widespread. On Ozernaya Peninsula, low-potassium basalts of the tholeiitic series are predominantly developed. The rocks of the Kumroch Ridge include low-potassium basalt-basaltic andesite, medium-potassium calc-alkaline and high-potassium shoshonitic. Volcanic rocks of the Tumrok Ridge are represented by low-potassium picrites and high-potassium trachybasalt-trachyandesites. In the Valaginsky Ridge, low-potassium tholeiitic, moderate-potassium calc-alkaline and shoshonite-latite series, as well as picrites and lamproitoids are distinguished. Isotopic (Sr, Nd and Pb) and geochemical composition of the rocks indicate their formation from a depleted mantle source within an ensimatic volcanic arc. It is shown that in the absence of significant differences in the material composition of rocks, regional differences are manifested. It is suggested that large-ion lithophile elements enrichment of volcanics may relate to fluids introduced into the melts during the melting of the supra-subduction mantle wedge. Issues of geodynamics of the region are considered.

Permian tectonic evolution of the proto-Japan and East Asia: Insights from detrital zircon geochronology and crystal morphology

Combination analyses of detrital zircon ages and morphology from Permian sandstones in Japan provide evidence of orogenic evolution caused by collision in East Asia and reveal their paleogeographic setting. This study compares published and newly measured data from Permian fore-arc accretionary complexes (Akiyoshi Belt) and back-arc basin deposits (Maizuru Belt) in the Inner Zone of Southwest Japan. The fore-arc deposits and lower part of the back-arc deposits consist mainly of approximately 300–250 Ma zircon grains that are close to the depositional ages and are generally angular to subangular. These sediments formed around the Late Paleozoic volcanic arc isolated from the older continental crust, and their source rocks were exposed immediately prior to their erosion and transport and were transported directly to the depositional site in a relatively short time. In contrast, upper part of the back-arc deposits generally contains approximately 2,700–1,800 and 500–250 Ma zircon grains, which are commonly older than their depositional ages and are rounded. These grains were supplied from multiple sources, such as the Paleozoic volcanic arc and Precambrian continental block, and were affected by long-distance transport and a strong abrasion effect during transport. Comprehensive detrital zircon data emphasize that the oceanic arc–back-arc basin–continental margin system formed along the eastern margin of East Asia, and that the volcanic arc collided with the eastern margin of the Sino-Korean Block during the Permian. The variation in detrital zircon ages and shapes of back-arc deposits provides a constraint for modeling the East Asian Permian collision. Detritus derived from continental crust was captured by a back-arc basin and blocked by the uplift zone of the volcanic arc. However, a small amount of detritus originating from the continental crust was supplied to the fore-arc.

Disputed tectonic setting for the late Silurian–early Carboniferous development of the northern New England Orogen: detrital zircon ages suggest a continental margin arc association

Devonian successions in the Calliope and Yarrol provinces of the northern New England Orogen in central to northern Queensland have a lithological assemblage and depositional environments indicative of a close association with a subduction-related volcanic arc. These rocks have been interpreted as related to island arc and backarc settings based on geochemical characteristics of magmatic rocks. We present new detrital zircon ages from the Devonian to lower Carboniferous successions of the Calliope and Yarrol province successions. Most samples have no or relatively few zircon grains extracted except for samples of quartz-rich sandstone in the Middle Devonian Erebus beds of Hunter Island, located 160 km north-northwest of Rockhampton and another sample in the Rockhampton district. The samples with abundant zircon, and some with less common zircon, have prominent peaks of mid Cambrian to Ordovician ages and are consistent with derivation from the northern Thomson Orogen where igneous rocks of this age are documented in the Charters Towers Province. These data indicate a connection between the Devonian arc-flank deposits of the northern New England Orogen and the Thomson Orogen, which had previously been well established for Carboniferous deep-marine sandstones of the subduction complex (e.g. Shoalwater Formation). These results favour an east-facing continental margin arc setting in the Devonian contra to intra-oceanic island arc settings previously proposed. Detrital zircon age spectra from sampled early Carboniferous units of northern New England Orogen association indicate a pulse of felsic magmatism of that age.

Mercury isotopes of the Late Ordovician to Middle Triassic tuff layers in South China link the fate of ancient volcanism and the mass extinction

Mercury (Hg) anomalies in sedimentary records are a newly developed proxy of large volcanism in the geological past. Tuff layers host abundant volcanic ash and record key information on the type of volcanic emission (e.g., arc volcanism and large igneous province eruptions). Here, we measured the Hg isotopic compositions of several tuff layers in South China. Tuff samples in the Late Ordovician Wufeng Formation and the Middle Triassic Guanling Formation mostly show positive Δ199Hg values of – 0.01 to 0.10‰ and – 0.06 to 0.16‰, respectively, suggesting arc volcanism occurred during these two periods. Tuff samples in the Middle Permian Dachang Layer mostly show near-zero Δ199Hg values (– 0.08 to 0.00‰), suggesting volcanism was driven by the Emeishan large igneous province eruption. Results of this study verify Hg isotopes as a useful proxy in revealing the type of volcanism in geological history.

Late Neoproterozoic tholeiitic plutonism in far-east Avalonia (the Dirgine Batholith, Istanbul Zone): Exposed mid-crustal section of an oceanic arc

Avalonia, a large composite terrane, extends from the northeastern regions of America and Canada to southern Britain and Belgium/Netherlands, continuing on to Poland. It was accreted to Laurentia and Baltica during early Paleozoic. The late Neoproterozoic basement of the Istanbul Zone (NW Turkey), which is considered the far-east extension of Avalonia, is poorly known. This study deals with the petrology and age of the Dirgine Batholith, the largest component of the late Neoproterozoic basement. The batholith, ∼70 km long and ∼12 km wide, was emplaced into Neoproterozoic low-grade mafic to felsic metavolcanic and metapyroclastic rocks. Tonalite and quartz diorite form the bulk of the batholith. Trondhjemite, granodiorite, and anorthositic gabbro occur in minor amounts. Overall, rock types resemble low-Al tonalite-trondhjemite-granodiorite series and trondhjemites in ophiolite sequences. The batholith formed over a narrow time interval (562–574 Ma, late Ediacaran), as inferred by U–Pb zircon geochronology. In-situ Lu-Hf analyses of igneous zircons reveal a limited range of significantly positive initial εHf values (9.47 ± 1.46, 2σ) and young Hf depleted mantle model ages (0.78 ± 0.06 Ga), indicating the relatively juvenile character of original magmas. All rock types have typical subduction-related trace-element signatures such as depletion in high-field-strength elements and enrichment in several fluid-mobile elements as well as nearly flat chondrite-normalized REE patterns. Geochemical characteristics indicate that crustal assimilation was insignificant, and the batholith formed by fractional crystallization of mantle-derived low-K tholeiitic magmas involving mainly clinopyroxene, hornblende and ± plagioclase. The Dirgine Batholith and metamorphic country rocks probably represent the mid-crustal section of a late Neoproterozoic oceanic arc. The interpretation of an oceanic rather than continental arc is based on the significantly lower concentrations of incompatible elements relative to continental arcs, nearly flat REE patterns of the rock types, and the juvenile nature of the primary magmas. These data, in concert with those in the literature, reveal that late Neoproterozoic oceanic arcs constitute an important component of far-east Avalonian terranes.

Middle Jurassic deposits of Ulakhan-Sis Ridge (Sakha Republic): biostratigraphy, lithological features and depositional environments

The age and sedimentation conditions of the Middle Jurassic terrigenous deposits crowning the Mesozoic section of the northeastern flank of the Nagondzha terrane within the Ulakhan-Sis Ridge are discussed. Based on our own finds of retrocerams and analysis of biostratigraphic data of predecessors, we accept the age of the deposits as Bajocian–Middle Bathonian. Three lithologically different strata are identified in the section. Sandstones from the lower unit belong to feldspathic litharenites, and from the upper strata – to litharenites with a predominance of extrabasinal carbonate clastics (calklithites). Analysis of textural and structural features, mineralogical, petrographic composition and distribution of trace elements in litharenites suggests that all these strata had been accumulated in the shelf part of the paleobasin associated with an active volcanic island arc, which presumably existed from the Early Jurassic on the Omulevka craton terrane until the completion of the formation of Kolyma-Omolon microcontinent.

Why are oceanic arc basalts Ca-rich and Ni-poor? Insights from olivine-hosted melt inclusions from Kibblewhite Volcano in the Kermadec arc

Ankaramites, which are clinopyroxene-rich basalts with primitive whole-rock compositions (Mg# >65), are common in oceanic arcs and are characterized by high whole-rock CaO/Al2O3 (>1.0) ratios and olivine crystals with anomalously low nickel contents (<0.2 wt% NiO). These geochemical characteristics cannot be explained by the melting of ordinary mantle peridotite. However, their origin is critical for understanding the formation of primary magmas in oceanic arcs. Here, we investigated olivine-hosted melt inclusions (MIs) from ankaramites and magnesian andesites of the Kibblewhite Volcano in the Kermadec arc. The MIs from the ankaramites have similar major and trace element characteristics to the host rocks, indicating that the ankaramites did not result from an accumulation of mafic minerals but rather represent the primary magma in the Kibblewhite Volcano. The MIs from the magnesian andesites were hosted in forsteritic olivine xenocrysts with a wide range of NiO contents (Fo90–92; 0.13–0.39 wt% NiO) and have similar major element compositions to the ankaramites but exhibit a wide range of CaO/Al2O3 (0.85–1.54). The trace element characteristics of the MIs from the magnesian andesites do not match those of the host rocks, indicating that they are not primary melts of the magnesian andesites but primitive basaltic melts generated before the magnesian andesites formed.Interestingly, the CaO/Al2O3 ratio of MIs from the magnesian andesites was negatively correlated with the NiO content of their host olivines. This correlation suggests that the composition of the primary basaltic magmas of the Kibblewhite Volcano changed continuously from peridotite-derived to ankaramitic. This correlation could not be explained by grain-scale process, crustal anatexis, or contribution of slab-derived carbonate-rich fluids. Instead, we propose that this correlation can be explained by the interaction of the ascending primary basaltic melts with the lithospheric mantle. During melt-mantle interaction, the assimilation of clinopyroxene and fractionation of olivine and orthopyroxene caused the CaO/Al2O3 ratio to increase in the melt and the Ni content to decrease. Furthermore, because the magnesian andesites have low CaO/Al2O3 ratios and could be derived from a clinopyroxene-poor mantle lithology, the interaction between the melt and mantle may also be closely related to the origin of the magnesian andesites at Kibblewhite Volcano. This interpretation provides a new perspective on the origin of the oceanic arc ankaramites and why primary andesitic and basaltic magmas coexist in the Kibblewhite Volcano.

Volcanic arc rigidity variations illuminated by coseismic deformation of the 2011 Tohoku-oki M9.

Rock strength has long been linked to lithospheric deformation and seismicity. However, independent constraints on the related elastic heterogeneity are missing, yet could provide key information for solid Earth dynamics. Using coseismic Global Navigation Satellite Systems (GNSS) data for the 2011 M9 Tohoku-oki earthquake in Japan, we apply an inverse method to infer elastic structure and fault slip simultaneously. We find compliant material beneath the volcanic arc and in the mantle wedge within the partial melt generation zone inferred to lie above ~100 km slab depth. We also identify low-rigidity material closer to the trench matching seismicity patterns, likely associated with accretionary wedge structure. Along with traditional seismic and electromagnetic methods, our approach opens up avenues for multiphysics inversions. Those have the potential to advance earthquake and volcano science, and in particular once expanded to InSAR type constraints, may lead to a better understanding of transient lithospheric deformation across scales.

Review: Andesitic aquifers—hydrogeological conceptual models and insights relevant to applied hydrogeology

Research on the hydrogeology of andesitic volcanic aquifers in subduction areas is reviewed. Andesitic aquifers are of high interest in volcanic arc islands and subduction zones, where they constitute a strategic water resource. This review gathers a compilation of worldwide results and case studies to propose a generic hydrogeological conceptual model (GHCM). It is based on the geological conceptual model splitting the volcanic edifice, from upstream to downstream, into central, proximal, medial and distal zones. In this geological structure, the GHCM identifies where the main aquifer types (fractured lava, pyroclastic flows, and the volcano-sedimentary basins downstream) and the typical aquitards (lahars, fine pyroclastic falls and surges, indurated pyroclastic flow, and weathered rocks) are structured and organized. To integrate the evolution of volcanoes and some specific volcanic activities, a specific GHCM for old andesitic volcanoes or andesitic shield volcanoes is detailed. The paper also describes how the GHCM results are of use to hydrogeologists in terms of scale (from the lithological units to the regional scale), to effectively site water wells, and to sustainably manage groundwater resources in such aquifers. Among these various scales, the volcanic “flank continuum” is presented as the most adapted to support groundwater resources management. Several ways to improve this GHCM are suggested, notably to better consider the geological complexity of these aquifers.

Devonian to Mississippian strata of the Shine Jinst region revisited: Facies development and stratigraphy in southern Mongolia (Gobi Altai Terrane)

AbstractThis report provides new stratigraphical and facies data from Devonian and Carboniferous rocks in the Shine Jinst region (Trans Altai Zone, southern Mongolia) with a special focus on the Lower Devonian Chuluun Formation, the Middle Devonian Tsagaankhalga Formation, and the Upper Devonian to Mississippian Heermorit Member of the Indert Formation. Facies development in the Shine Jinst region exhibits a fundamental break in the carbonate platform evolution in the Lower Devonian as reef building organisms were affected by a major regression and deposition of several metres-thick conglomerates at the base of the Tsakhir Formation (Lower Devonian). The overlying Hurenboom Member of the Chuluun Formation is composed of fossiliferous limestones. Reef building organisms, such as colonial corals and stromatoporoids show low diversity and exhibit limited vertical growth and lateral extension of individuals. Thus, they do not represent a real reef as proposed in previous publications but biostromal limestones instead. One reason might be the isolated position of the Shine Jinst region between an unknown continent and a volcanic arc in the early Middle Devonian that hampered the successful colonization in shallow-water areas. Bivalves of the Alatoconchid family were once grouped into reef builders or biostrome builders and they are known only from Permian rocks. The found bivalve biostomes in Mongolia may represent precursors, which would document the oldest record of Alatoconchids found in the world. Remarkable thicknesses of massive crinoidal grainstones (“encrinites”) are documented in many parts of the succession, which suggest rather stable environmental conditions of a carbonate ramp setting at different times. The occurrence of thick-bedded conglomerates in the Shine Jinst section is not restricted to the Lochkovian to Pragian interval (Tsakhir Formation), but also occurs in the Eifelian. A thick-bedded conglomerate, which is interpreted to represent braided fluvial or fan-delta to shallow-marine deposits occurs at the base of the Tsagaankhaalga Formation. A steep relief associated with uplift and volcanism seems to be a realistic scenario for deposition of these sediments. This succession points to a remarkable tectonic uplift or sea-level fall in the Middle Devonian. Conodont findings of the studied section confirm the occurrence of time-equivalent strata of the Choteč Event, the Dasberg Crisis, and the Hangenberg Event found elsewhere in the world, which are described from Mongolia for the first time. Sedimentological descriptions, revised biostratigraphical data, and U-Pb dating by LA ICP-MS of some volcaniclastic rocks from the Chuluun Formation are presented in this report. The studied section records a complex interaction of sedimentation, regional tectonics, sea-level changes and coeval volcanism, which is very similar to other regions in Mongolia. The new data provide the background for further scientific studies in this region. This is a contribution to the Special Series on “The Central Asian Orogenic Belt (CAOB) during Late Devonian: New insights from southern Mongolia”, published in this journal.

Geochemical and geochronological evidence of a Neoproterozoic island-arc on the easternmost Sierras Pampeanas. New U-Pb LA-ICP-MS data from amphibolites and metacarbonate depositional sequences

The first basement outcrops west of the Río de La Plata craton are metabasaltic and metasedimentary units of the Sierra Chica belt, part of the Eastern Sierras Pampeanas. These are orthoamphibolites and subordinate garnet-biotite gneisses and metacarbonate sequences that are intruded by Cambrian calc-alkaline and peraluminous rocks of the Pampean Orogeny. In accordance with one of the leading tectonic models for the region, new provenance data from this research suggest that the metabasaltic-metasedimentary association represents the proximal deposits of a Late Neoproterozoic island-arc. This contribution brings new geochemical and geochronological evidence from orthoamphibolites located on different sites along the Sierra Chica belt, which are interbedded with metacarbonate and gneisses outcrops, showing tholeiitic compositions, island-arc related tectonic settings, and Cryogenian to Ediacaran crystallization ages. Moreover, silicate-rich metacarbonates interbedded within the same sequences yielded a maximum depositional age of 569 Ma, reinforcing the assumed age for the sequence. All things considered, these rocks are interpreted as old basic lava flows and gabbro dikes linked to an island-arc activity, interspersed within the carbonatic and siliciclastic shelf sediments, together constituting the Neoproterozoic Córdoba Arc from the Sierra Chica belt. A depositional analogy with the present-day Caribbean island-arcs deposits suggests that these sequences could have been part of Oceanic Arc Depositional Systems (OADS) during the diachronous closure of the Goiás ocean and the final assembly of West Gondwana. Overlapping thermo-tectonic processes associated with the Pampean Orogeny are ubiquitous throughout all the studied outcrops, as evidenced by the petrography, the zircon morphology, and the U-Pb results, indicating a complex scenario towards a Late Cambrian slab break-off event.

Sedimentary characteristics of deep-marine gravity flows influenced by island–arc volcanism: A case study of Carboniferous sedimentary successions in the Junggar Basin, NW China

The Carboniferous sedimentary successions of the north-western and southern margins of the Junggar Basin in northwest China show a gradual vertical transition from deep- to shallow-marine depositional environments. Stratigraphically, these successions are composed of sedimentary and igneous lithologies (volcaniclastic), which record the closure and subduction of the remnants of the western Junggar and northern Tianshan oceanic basins that formed branches of the Paleo-Asian Ocean. In the current study, 16 facies and 7 facies associations related to sediment gravity flows were identified. These lithofacies associations collectively reflect five depositional elements, which include mass transport deposits, gravity flow channel deposits, lobe complex deposits, submarine volcanic ridge deposits, and submarine volcanic turbidite fans. Submarine and subaerial arc volcanism are shown to have controlled the characteristics of gravity flow sedimentation recorded in these successions. Island–arc volcanic material in the Junggar Basin is divided into three sources: erosion of the arc by fluvial systems, island–arc eruption materials, and submarine volcanism. Volcaniclastic deposits were transported into the deep-marine basin by submarine sediment gravity flows. These flows included turbidity currents, supercritical flows, gravel-rich debris flows, sandy debris flows, and muddy debris flows. Submarine vents generated lava flows that were deposited onto the abyssal plain and triggered subaqueous pyroclastic density flows. This study and its analysis of the characteristics and likely genesis of these gravity flow sediments improve the understanding of the dynamic relationship between volcanic activity and gravity flow sedimentation in near-arc basins. These findings provide a useful case-example of sediment gravity flow systems in island–arcs, which can be compared with and used as a relevant analog for other deposits in similar settings globally.