Submarine Volcanism Research Articles

Geodynamic experiments suggest that mantle plume caused Late Permian Emeishan Large Igneous Province in Southern China

ABSTRACT Stratigraphic, petrological and geophysical studies suggest that the Late Permian (~ 260 Ma) Emeishan Large Igneous Province in southern China may be formed by mantle plume activity. However, the plume impingement hypothesis remains controversial since interpretations based on volcano-stratigraphic analyses around plume induced domal uplift/inner zone suggest that the volcanism occurred under submarine environment rather than elevated sub-aerial (above sea level) conditions, usually associated with the dynamic topography effects of the ascending mantle plumes. Here, 2-D numerical and 3-D scaled laboratory (analogue) plume experiments are used to explore the coupled dynamics of plume-mantle-lithosphere interaction and their evolution of surface topography characteristics. Experimental results show that the initial (plume incubation) phase is characterized by rapid, transient domal uplift above the plume axis, subsequently, as plume head flattens, there is short wavelength topographic variation (ie. subsidence and uplift occurs synchronously) due to the shear stress imposed onto the base of the lithosphere and loss of gravitational potential energy. The surface depressions predicted by the plume models, next to the plume axial/inner/uplift zone, may explain the deposition of submarine volcanics at Lake Erhai, Dali in the western side and Xiluo and Daqiao in the eastern side, which may resolve the plume controversy for the formation of Emeishan Large Igneous Province. Notably, while experimental results from these two different techniques show some differences, (e.g much bigger plume head for the laboratory experiment), the overall characteristics of the predictions have robust similarities. For instance, the extension above the plume axis may explain the enigmatic cause of the Panxi rift system, in the middle of the inner zone where giant dyke swarms radiate from, and mafic magma underplatings in the lower crust has been described by seismological studies.

Bryozoans from Chella Bank (Seco de los Olivos), with the description of a new species and some new records for the Mediterranean Sea

Chella Bank (also known as Seco de los Olivos seamount) is a volcanic submarine elevation (76–700 m deep) located ca. 16 km off the southern coast of Spain, within the Alboran Sea, in the Atlantic-Mediterranean transition zone. It represents a biodiversity hotspot for Europe, with more than 600 species listed to date, and contains several habitats included in the EU Habitats Directive. During three ship-based expeditions, several areas of Chella Bank were surveyed and sampled in a depth range of 95–729 m, resulting in new records that improve our knowledge on poorly studied phyla, such as bryozoans. In 14 of the 21 samples examined during this study, 43 bryozoan taxa could be identified. Among these, one species is described as new to science (Buskea medwaves sp. nov.) and three other ones are reported for the first time from the Mediterranean Sea, namely Terminoflustra barleei (Busk, 1860), Marguetta pulchra Jullien in Jullien & Calvet, 1903, and Schizomavella (Schizomavella) linearis profunda Harmelin & d’Hondt, 1992a. Some species were abundant in the samples, such as Adeonellopsis distoma (Busk, 1859), B. medwaves sp. nov., Entalophoroecia cf. deflexa, and Reteporella pelecanus Lopez de la Cuadra & Garcia-Gomez, 2001. The highest species richness was detected in rhodolith beds and on coral rubble bottoms (especially exposed above the sediments) compared with other bottom types and habitats such as sandy bottoms and muddy bottoms. The finding highlights the importance of these environments for bryozoans.

Episodic ferruginous conditions associated with submarine volcanism led to the deposition of a Late Carboniferous iron formation

Iron formations (IFs) are iron-rich chemical sediments that were widespread throughout the Precambrian. The deposition of IFs has provided key evidence about the prevalence of anoxic and ferruginous marine conditions throughout Earth’s history. Here, we describe the Shikebutai IF, hosted in a Late Carboniferous volcano-sedimentary sequence in the Western Tianshan region of northwest China. The IF is largely composed of hematite, jasper, and siderite. The chemical composition of the Shikebutai IF is dominated by Fe2O3 and SiO2, and very low concentrations of Al2O3, TiO2, Th, Nb, and Sc were observed, indicative of an authigenic origin. High Fe/Ti, Fe/Al, Co/Zn, and Ni/Zn ratios, along with positive Eu anomalies and εNd(t) values point to a significant submarine volcanic and hydrothermal contribution. The absence of negative Ce anomalies and heterogeneous δ56Fe values suggest progressive oxidation of hydrothermally sourced Fe(II) and deposition in an anoxic to low oxygen basin. Low P/Fe ratios in the Shikebutai IF, comparable to those found in the Precambrian, are consistent with Si inhibition of P sorption and P drawdown with high hydrothermal inputs. The intimate association of this Late Carboniferous IF with submarine volcanic activity suggests an origin that is analogous to Precambrian Algoma-type IFs. Further, the Shikebutai IF provides evidence for an episode of basin-scale anoxic, ferruginous conditions coinciding with the generally well-oxygenated deep oceans of the Phanerozoic, with deposition driven by enhanced submarine volcanic and hydrothermal activity in the Western Tianshan region during the Late Carboniferous. This demonstrates how, even with near modern surface oxygen levels, enhanced hydrothermal activity can shape the marine redox landscape and cause the development of ferruginous conditions without quantitative sulfate drawdown.

Continuous evolution of oceanic crustal structure following an eruption at Axial Seamount, Juan de Fuca Ridge

Abstract We present the first continuous observations of the temporal evolution of oceanic crustal shear velocity beneath Axial Seamount, a submarine volcano on the Juan de Fuca Ridge (offshore northwestern North America). Weekly values of seafloor compliance, the periodic deformation of the seafloor under ocean waves, were estimated over the time period between December 2014 and May 2018 using data from two cabled broadband ocean-bottom seismometers with collocated absolute pressure sensors. We inverted these measurements for shear-wave velocity within the volcano beneath the two stations as a function of depth and time. Our results, combined with estimates of seismic compressional wave velocity, suggest that the shallow melt reservoir and the lower crust beneath the central caldera contain melt fractions of 14% and at least 4%, respectively. The eruption of April 2015 induced a dramatic drop in shear velocities beneath the central station, primarily in the lower crust, which could have been caused by an increase in melt fraction, a change in small-scale melt geometry, or both. The absence of such a change beneath the eastern flank of the caldera indicates that there is a lower-crustal conduit beneath the caldera center, which is much narrower in cross section (<1 km2) than the overlying melt reservoir (≥42 km2). Our study demonstrates the promise of using continuous data to understand submarine volcanism and crustal accretionary processes.

Volcanogenic talc‐copper deposits of Darhib‐Abu Jurdi area, Egypt: Petrogenesis and remote sensing characterization

Economic talc deposits were developed as an integral part of a volcanogenic massive sulfide system in Darhib‐Abu Jurdi bimodal‐mafic type submarine volcanics in the South Eastern Desert of Egypt. These Pan‐African metavolcanics were erupted in a rift‐related environment and experienced low‐grade greenschist metamorphism. They host talc‐malachite mineralization, mainly within E–W trending alteration zones along the major latitudinal shear planes. These mineralized alteration zones contain economic talc deposits as well as averages of 1.7% Cu and 0.46% Zn; the maximum may exceed 11% Cu. Sentinel‐2, ASTER, and Landsat‐8 dataset in conjunction with a portable TerraSpec Halo mineral identifier device (ASD) were employed in detecting and mapping these mineralized alteration zones within the Darhib‐Abu Jurdi metavolcanics. Petrography, mineralogy, and geochemistry of these deposits and host volcanics enabled to put constraints on their petrogenesis and evolution as a submarine hydrothermal vent system. Mineralogical characteristics of pervasive chloritization stringer zone, hypogene primary sulfide ore zone, and supergene oxidized zone are addressed and discussed. Talc deposits were formed at Darhib‐Abu Jurdi area as the main gangue phase via (a) direct precipitation on seafloor by interaction of silica‐saturated hydrothermal fluids with Mg‐rich seawater, (b) precipitation with actinolite/tremolite at shallow depth beneath the sulfide ore lenses due to penetration of Mg‐rich seawater into hydrothermal conduit, and (c) replacement of pre‐existing tremolite, actinolite, chlorite, and septechlorite.

Perspective of gold-bearing deposits in the ferruginous-siliceous formations of Egypt and Kursk Magnetic Anomaly

Banded ferruginous-siliceous formations (FSF) are confined to the Precambrian basement of the Arabian-Nubian Shield within the central part of the Eastern Desert of Egypt. Gold mineralization is spatially associated with banded ferruginous quartzite, representing one of the most ancient manifestations of gold ore. The Precambrian rocks are combined into deposits complex of the Neoproterozoic Pan-African megacycle of the territory development. Banded iron-silicate rocks occur in sedimentary-volcanogenic rocks that were formed in the subduction trough zones, and are represented by metamorphosed ferruginous quartzites, jaspilites and schists. FSF show tectonic dislocations, shear cracks, and fracture cracks made by gold-quartz-sulphide mineralization. Promising for the identification of gold-bearing mineralization in the rocks of the FSF can be iron-silicate deposits with the occurrence of epigenetic hydrothermal activity as a result of activation of the submarine volcanism of the tholeiitic type.

Remote Sensing and GIS Contribution to a Natural Hazard Database in Western Saudi Arabia

The most frequent disasters in Western Saudi Arabia are flash floods, earthquakes and volcanism, especially submarine volcanism potentially causing tsunamis in the Red Sea and submarine mass movements, dust storms and droughts. As the consequences and effects of the climate change are expected to have an increasing impact on the intensity and occurrence of geohazards as flash floods, length of drought periods, or dust storms, the systematic, continuous monitoring of these hazards and affected areas using satellite data and integration of the results into a geographic information systems (GIS) database is an important issue for hazard preparedness and risk assessment. Visual interpretation and digital image processing of optical aerial and satellite images, as well as of radar images, combined with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Shuttle Radar Topographic Mission (SRTM) and Advanced Land Observing Satellite (ALOS) PALSAR DEM data are used in this study for the mapping and inventory of areas prone to geohazards, such as flash floods or tsunami flooding. Causal or critical environmental factors influencing the disposition to be affected by hazards can be analyzed interactively in a GIS database. How remote sensing and GIS methods can contribute to the detection and continuously, standardized monitoring of geohazards in Western Saudi Arabia as part of a natural hazard geodatabase is demonstrated by several examples, such as the detection of areas prone to hydrological hazards, such as flash floods causing flooding of roads and settlements, the outlining of coastal areas of the Red Sea prone to tsunami flooding and storm surge, the mapping of traces of recent volcanic activity, and of fault/fracture zones and structural features, especially of ring structures.

Extrusion dynamics of deepwater volcanoes revealed by 3-D seismic data

Abstract. Submarine volcanism accounts for ca. 75 % of the Earth's volcanic activity. Yet difficulties with imaging their exteriors and interiors mean that the extrusion dynamics and erupted volumes of deepwater volcanoes remain poorly understood. Here, we use high-resolution 3-D seismic reflection data to examine the external and internal geometry and extrusion dynamics of two late Miocene–Quaternary deepwater (> 2 km emplacement depth) volcanoes buried beneath 55–330 m of sedimentary strata in the South China Sea. The volcanoes have crater-like bases, which truncate underlying strata and suggest extrusion was initially explosive, and erupted lava flows that feed lobate lava fans. The lava flows are > 9 km long and contain lava tubes that have rugged basal contacts defined by ∼90±23 m high erosional ramps. We suggest the lava flows eroded down into and were emplaced within wet, unconsolidated, near-seafloor sediments. Extrusion dynamics were likely controlled by low magma viscosities as a result of increased dissolved H2O due to high hydrostatic pressure and soft, near-seabed sediments, which are collectively characteristic of deepwater environments. We calculate that long-runout lava flows account for 50 %–97 % of the total erupted volume, with a surprisingly minor component (∼3 %–50 %) being preserved in the main volcanic edifice. Accurate estimates of erupted volumes therefore require knowledge of volcano and lava basal surface morphology. We conclude that 3-D seismic reflection data are a powerful tool for constraining the geometry, volumes, and extrusion dynamics of ancient or active deepwater volcanoes and lava flows.

Detection of the 2012 Havre submarine eruption plume using Argo floats and its implications for ocean dynamics

More than 85% of the Earth's volcanism and geothermal heat release occurs underwater at mid-ocean ridges, intra-plate hotspots, and oceanic island arcs. However, we lack a clear understanding of how a submarine eruption interacts with the overlying water column, the structure of the submarine eruptive plume, and the transport of volcanic products in the ocean by these plumes. We describe here the detection of eruption-derived temperature increase and salinity decrease from the 2012 Havre submarine caldera eruption by an existing array of temperature/salinity profiling floats, known as Argo. The Argo floats observed the signal, significant at >3 sigma level with respect to the background ocean variability (noise), at ocean depths of ∼1750–2000 m deeper than the eruptive vent itself. In order to understand this signal, we develop a new conceptual submarine plume model which suggests that the sub-vent level temperature and salinity anomalies are caused by large-scale ocean mixing induced by the eruptive plume, rather than heating directly from the eruption. Using results from scaling analysis and a modified numerical plume-gravity current model, we show that submarine eruptions can efficiently advect water masses from the deep ocean closer to the surface and form multiple subsurface intrusions during the process. Consequently, the submarine eruptive plumes can initiate significant ocean mixing, a process with significant implications for both long- and short-term influence of submarine volcanism on ocean chemistry, dynamics, and ecology. Finally, our model confirms that a detectable anomaly would persist out to 100s of km from the eruption location, comparable to Argo's spatial sampling. Given the global ocean coverage and a decadal baseline, the Argo floats (and other similar datasets), in concert with hydrophone and seismic arrays, can provide significant new constraints on subaqueous volcanism, eruption dynamics, and the impact on Earth systems through enhanced ocean mixing and nutrient supply, e.g. during submarine Large Igneous Provinces.

Thermal Effects of Magma Intrusion on Ocean Temperatures Through Ice Ages, with Particular Reference to Interglacial Events

Recent observations are used to crudely quantify and scale the direct heating of the global ocean surface by submarine volcanism. These suggest a current submarine magma budget that is substantially greater than the prevailing consensus. The current ocean heating effect is estimated at c. 0.7W/m2. Interglacial events are geologically sudden and predictably cyclical, overtly caused by the combined effect of Milankovitch cycles. Several possible causes for interglacial warmings arising from these cycles have been proposed, and rejected as inadequate. These are summarily reviewed. It is concluded, in the absence of other capable causes, and in particular given the evidence of continued ocean warming throughout the Dryas cooling events, that ice age interglacial events are probably caused by a sustained increase in the rate of submarine volcanic heating of the oceans. The primary cause of increasing emissions is suggested as the rising levels of solid gravitational tides caused by the combined orbital forcing of all three main Milankovitch cycles. These tides are at their largest during interglacial warmings. How the dominant climate control system of oceanic feedback responds to this internal perturbation change is also discussed. In particular how the upper equilibrium temperature of an interglacial period is imposed by this control.

Gold Mineralization in Izu Peninsula, Central Japan, during Crustal Extension in Response to Double Subduction

AbstractIzu Peninsula in central Japan, the northern tip of the Izu‐Bonin arc, hosts numerous epithermal Au–Ag vein deposits of low‐sulfidation style. All have similar vein textures, mineralogy, and alteration. Geochemical data from fluid inclusions in vein quartz, the mineralogy and mineral chemistry of alteration, and stable isotope data indicate that auriferous hydrothermal activity occurred under subaerial conditions. The K–Ar ages of auriferous vein minerals are <1.5 Ma, indicating that the mineralization took place after extensive submarine volcanism for the host rocks. These observations suggest that Au–Ag mineralization was synchronous with the development of an extensional regime of the Izu block after its collision with the Honshu arc after 1.5 Ma. This collision resulted in the shifting of the Izu block far from the trench to the rear position, and the subduction of the Izu block along the Suruga trough to the west and along the Sagami trough to the east. The reararc position of the Izu block and double subduction resulted in crustal extension, upwelling of asthenospheric mantle, and tholeiitic magmatism reflected by mafic dyke swarms and subsequent monogenetic volcanic activity in the Izu peninsula. The timing of the Au mineralization in the Izu Peninsula during the beginning of lithospheric extension is similar to that of the Sado Au–Ag deposit on Sado island in the Japan Sea. Two mineralization events coincide with extensive tholeiitic mafic volcanism and injections of dyke swarms related to the back‐arc opening of the Japan Sea. The geological setting of the Au–Ag mineralization in Izu and Sado is also similar to that of the epithermal Au–Ag deposits in northern Nevada, where mineralization was contemporaneous with crustal extension and tholeiitic mafic magmatism derived from the asthenospheric mantle. This study suggests that epithermal Au mineralization at shallow crustal depths is a product of large‐scale lithospheric evolution.

Comment on the papers by Hovland et al. (2018) “Large salt accumulations as a consequence of hydrothermal processes associated with 'Wilson cycles': A review” (part 1 and 2)

Abstract The geological model of large hydrothermal salt accumulations linked to Wilson cycles as reviewed by Hovland and Coworkers, un-adequately addressed: (1) the lack of well exposed submarine volcanogenic hydrothermal feeder zone, (2) the sequential precipitation of different hydrothermal evaporite minerals and (3) the absence of hydrothermal alteration mineralogy around the major evaporites. These aforementioned points are the main classical features to be linked to the hydrothermal model of evaporites. Experimentally, heating seawater during submarine volcanism firstly results in precipitation of anhydrite followed by halite, other evaporite minerals, serpentine, brucite, talc, metallic sulfides-oxides and related hydrothermal alterations. In this regard, Mg/Ca – Cl, Mg/Ca – SO4, Mg/Ca – Na/Cl and Mg/Ca - TDS biplots show that hydrothermal brines of riftogenic and subduction zone settings plot in a separate field than the subduction-related Andean Salars as well as seawater. This indicates that the hydrothermal serpentinization of basaltic oceanic crust and mantle peridotites removes magnesium from seawater and incorporates the Mg into serpentine, brucite and other hydrous Mg-silicates rather than the hydrothermal brines. Finally, our comment arose from a recent global classification of the most distinguished marine, non-marine and exhalative hydrothermal evaporites.

Anomalous Pacific‐Antarctic Ridge Volcanism Precedes Glacial Termination 2

AbstractWe present results from a well‐dated sediment core on the Pacific‐Antarctic Ridge (PAR) that document a ∼15 cm thick layer of basaltic ash shards that precedes the penultimate deglaciation (Termination 2). The glasses have MORB composition consistent with an axial source and their morphologies are typical of pyroclastic deposits created by submarine volcanism. The ash layer was deposited ∼7 km from the PAR axis, a distance that implies buoyant plumes lofted debris high into the water column with subsequent fallout to the core location. We infer plume rise height using grain settling velocities, the water depth at the core site, and deep ocean current speeds from ARGO floats. Rise heights of 1.5 km or less require unrealistically large current speeds to transport grains to the core site. Instead, the data are consistent with a plume rise height of at least 2 km, implying that T2 was an interval of anomalous volcanism along this segment of the PAR. The timing and duration of the ash deposit is consistent with glacial‐interglacial modulation of ridge magmatism. Volcaniclastic records from additional locations will be necessary to assess whether the PAR record is a rare find or it is representative of mid‐ocean ridge volcanism during glacial terminations.

Crustal plumbing system of post-rift magmatism in the northern margin of South China Sea: New insights from integrated seismology

The origin and evolution of magmatism in rifting margins are a fundamental geological subject, and remain the focus of intense studies. Unlike the classical magma-poor and magma-rich rifting margins, the northern margin of the South China Sea (SCS) exhibits a strong post-rift magmatism, but the evolution and mechanism of this behavior are still poorly understood. In this work we investigate the detailed structural features of the post-rift magmatism in the crust of the SCS northern margin. Our analysis is based on wide-angle and multichannel reflection seismic data, combined with previous seismological results. Our results show a wide distribution of shallow igneous sills and intrusions. These features are spatially and tectonically linked to the lower crustal high-velocity bodies (HVBs) and submarine volcanism, indicating a causative connection between the three features. Considering the existence of an obvious low-velocity branch of the Hainan plume in the northern SCS and uplift of the lithosphere-asthenosphere boundary, we propose that the HVBs reflect consolidated mafic intrusions which formed the lower crustal reservoirs feeding the overlying igneous sills and intrusions. In the SCS northern margin where abundant extensional and detachment faults can act as magmatic channels, such a dynamic process might cause post-rift volcanism. Our results suggest that the crustal magmatic system of post-rift volcanism has a multilevel upward migration mode, and the HVBs in the lower crust could be the product of post-rift magmatism, further indicating that the SCS northern margin had a magma-poor property at the rifting phase but has undergone a strong magmatic rejuvenation by the subsequent mantle plume.

Sedimentary mechanisms of a modern banded iron formation on Milos Island, Greece

Abstract. An early Quaternary shallow submarine hydrothermal iron formation (IF) in the Cape Vani sedimentary basin (CVSB) on Milos Island, Greece, displays banded rhythmicity similar to Precambrian banded iron formation (BIF). Field-wide stratigraphic and biogeochemical reconstructions show two temporal and spatially isolated iron deposits in the CVSB with distinct sedimentological character. Petrographic screening suggests the presence of a photoferrotrophic-like microfossil-rich IF (MFIF), accumulated on a basement consisting of andesites in a ∼ 150 m wide basin in the SW margin of the basin. A banded nonfossiliferous IF (NFIF) sits on top of the Mn-rich sandstones at the transition to the renowned Mn-rich formation, capping the NFIF unit. Geochemical data relate the origin of the NFIF to periodic submarine volcanism and water column oxidation of released Fe(II) in conditions predominated by anoxia, similar to the MFIF. Raman spectroscopy pairs hematite-rich grains in the NFIF with relics of a carbonaceous material carrying an average δ13Corg signature of ∼ −25‰. A similar δ13Corg signature in the MFIF could not be directly coupled to hematite by mineralogy. The NFIF, which postdates large-scale Mn deposition in the CVSB, is composed primarily of amorphous Si (opal-SiO2 ⋅ nH2O) while crystalline quartz (SiO2) predominates the MFIF. An intricate interaction between tectonic processes, changing redox, biological activity, and abiotic Si precipitation are proposed to have collectively formed the unmetamorphosed BIF-type deposits in a shallow submarine volcanic center. Despite the differences in Precambrian ocean–atmosphere chemistry and the present geologic time, these formation mechanisms coincide with those believed to have formed Algoma-type BIFs proximal to active seafloor volcanic centers.

Three‐Dimensional Seismic Imaging of Ancient Submarine Lava Flows: An Example From the Southern Australian Margin

AbstractSubmarine lava flows are the most common surficial igneous rock on the Earth. However, they are inherently more difficult to study than their subaerial counterparts due to their inaccessibility. In this study, we use newly acquired 3‐D (three‐dimensional) seismic reflection data to document the distribution and morphology of 26 ancient, buried lava flows within the middle Eocene‐aged Bight Basin Igneous Complex, offshore southern Australia. Many of these lava flows are associated with volcanoes that vary from 60 to 625 m in height and 0.3 to 10 km in diameter. Well data and seismic‐stratigraphic relationships suggest that the lava flows and volcanoes were emplaced offshore in water depths of <300 m. The lava flows range from 0.5 to 34 km in length and 1 to 15 km in width and are typified by tabular and dendritic forms. This morphological variation may result from differing lava effusion rates and/or the volumes of lava erupted. We demonstrate that: (1) the dendritic flows contain complex lava distribution systems and kipukas, features never‐before observed from seismic data; and (2) the distribution and morphology of the lava flows was strongly controlled by the emplacement of magmatic intrusion‐induced forced folds. This suggests that magmatic intrusions may play an important role in controlling the distribution of lava flows elsewhere. Our study highlights the usefulness of seismic data in studying the manifestation of submarine volcanism, and provides quantitative data on the extent and distribution of an ancient submarine volcanic province along the southern Australian margin.

New Frontiers and Technologies in Submarine Volcanism Research

AGU Chapman Conference on Submarine Volcanism: New Approaches and Research Frontier; Hobart, Tasmania, Australia, 29 January to 3 February 2017

Submarine basaltic fountain eruptions in a back-arc basin during the opening of the Japan Sea

Basaltic rock generated during the middle Miocene opening of the Japan Sea, is widely distributed on the back-arc side of the Japanese archipelago. Few studies have investigated on submarine volcanism related to opening of the Japan Sea. The present study aimed to reconstruct details of the subaqueous volcanism that formed the back-arc basin basalts (BABB) during this event, and to discuss the relationship between volcanism and the tectonics of back-arc opening, using facies analyses based on field investigation. The study area of the southern Dewa Hills contains well-exposed basalt related to the opening of the Japan Sea. Five types of basaltic rock facies are recognized: (1) coherent basalt, (2) massive platy basalt, (3) jigsaw-fit monomictic basaltic breccia, (4) massive or stratified coarse monomictic basaltic breccia with fluidal clasts, and (5) massive or stratified fine monomictic basaltic breccia. The basaltic rocks are mainly hyaloclastite. Based on facies distributions, we infer that volcanism occurred along fissures developed mainly at the center of the study area. Given that the rocks contain many fluidal clasts, submarine lava fountaining is inferred to have been the dominant eruption style. The basaltic rocks are interpreted as the products of back-arc volcanism that occurred by tensional stress related to opening of the Japan Sea, which drove strong tectonic subsidence and active lava fountain volcanism.

U–Pb and fission‐track dating of a submarine pyroclastic rock from southwest Japan

AbstractThe Okinoshima Formation crops out on Okinoshima Island and comprises a thick sequence (> 200 m) of pyroclastic rocks and alternating beds of sandstone and mudstone. Because Okinoshima Island is located between Honshu and Tsushima Island, the Okinoshima Formation potentially provides an important record of volcanism during the opening of the Japan Sea in northwest Kyushu, as well as a record of the formation of the present Genkai Sea region. In consideration of the lack of previous geochronological work, dating (fission‐track and U–Pb) of igneous zircons extracted from the Okinoshima Formation were undertaken and studied the clay mineral alteration in the pyroclastic material in order to reveal its thermal history. These data are used to constrain the age of the Okinoshima Formation and the present Genkai Sea region. Our results show that no thermal event has reset the fission‐track age after deposition of the pyroclastic rocks, and that the Okinoshima Formation was deposited at 16.2 Ma. The present Genkai Sea region is a deep‐sea basin, and its formation at 16.2 Ma was accompanied by submarine volcanism and rapid subsidence that marked the climactic stage of Japan Sea formation. After 16 Ma, the tectonic setting of the present Genkai Sea region changed from one of extension (related to the formation of the Japan Sea) to one of compression, with uplift occurring under the influence of the clockwise rotation of southwest Japan. Consequently, after 16 Ma the present Genkai Sea region became isolated from the forming processes of the Japan Sea.

The possible synglaciogenic Ediacaran hematitic banded iron salt formation (BISF) at Hormuz Island, southern Iran: Implications for a new style of exhalative hydrothermal iron-salt system

The Ediacaran BISF at Hormuz Island is a newly identified glaciogenic iron-salt deposit in the Tethyan margin of Gondwana. The BISF was formed by synchronous riftogenic A-type submarine felsic volcanism and evaporate deposition. The mineralization occurs in a proximal felsic tuff cone and jaspilitic distal zones and contains 1 million tonne of hematite-rich ore with an average grade of 58% Fe. The ore structure shows cyclicity of macrobandings, mesobandings and microbandings of anhydrite, halite, hematite and chert, which marks a new record in BIFs geohistory. The alteration minerals in the proximal and distal zones are actinolite, ripidolite, epidote, sericite, tourmaline, clinochlore, anhydrite and clay minerals. The occurrence of metamorphosed polygenetic bullet-shape dropstones in BISF attests that there was probably a continuous process of ice melting, episodic submarine volcanism and exhalative hydrothermal banded iron salt formation during the Late Ediacaran time. The non-metamorphosed Neoproterozoic stratigraphy, the presence of genus Collenia, U-Pb dating (558±7Ma) and the marked negative δ13C excursion in cap carbonates are representative of Late Ediacaran glaciation, which has been identified worldwide. The REE+Y display light REE enrichment, unusually strong Tb-Tm anomaly, a weak positive Y anomaly, but no distinguished Eu and Ce anomalies, reflecting the glaciogenic nature of the BISF. The contents of Zr, Hf, Nb, Ta, Th, La, Ce and Y in BISF, dropstones, halite and cap carbonates are similar to those of the Neoproterozoic glaciogenic BIFs. Also, the Ni/Fe, P/Fe ratios and Fe/Ti – Al/Al+Fe+Mn+Ca+Na+K diagram suggest an exhalative hydrothermal Ediacaran-type BISF. The absence of brecciated magnetite in the ore association and the low contents of copper (9–493ppm) and gold (<5–8ppb) are not in favor of the IOCG – Kiruna-type iron oxide ores. The co-paragenesis of hematite with several alteration minerals, in particular actinolite, tourmaline and anhydrite, indicates that the exhalative hydrothermal fluids were generated by the interaction of seawater with the felsic rocks and sediments at about 200–500°C. The interaction of seawater with felsic magma and sediments led to the formation of Mg-rich alteration minerals, leaching Si, Fe, Mn and other elements and forming the potential ore fluids. It is highlighted that the A-type alkaline submarine felsic volcanism could be considered as an exploration target for BISF.