Oceanic Island Arc Research Articles

Кокчетау-Иссыккульский террейн Казахстанского палеоконтинента и его палеоширотное положение в ордовике

Research subject. The Kazakhstan (Kazakh-Kyrgyz) paleocontinent attracts much research attention. In this article, we present data on the tectonic structure of the Early Paleozoic Kokchetau-Issykkul sialic terrain. Materials and methods . The geodynamic position of the Kokchetau-Issykkul terrain was determined by analysing samples of Ordovician rocks collected across 11 sites using the paleomagnetism method. Thus, we studied the paleomagnetism of red sandstones, siltstones, tuffaceous siltstones and basalts, as well as that of andesite and tuffaceous rocks in marine sediments, which affiliation to different Ordovician formations was established on the basis of trilobite, brachiopod, conodont and graptolite fauna. Results. Early Paleozoic rocks are exposed in the northern part of the studied terrain (Kokchetau district) and in the Issyk-Kul region, which covers southern Kazakhstan, northern Kyrgyzstan and western Xinjiang. The sutures of Chistopol-Terskei and Iradir-Ili ocean basins serving as the boundaries of the Kokchetau-Issykkul terrain are located in the allochthonius position. A multi-storey system of napes composed of pre-Paleozoic and Early Paleozoic rocks of the oceanic crust, volcanic oceanic island arcs and continental slope were studied near the Terskei margin of the terrain. This system of napes is covered by sediments containing graptolites of the Floian-Darriwilian age. Subduction-related volcanism near this margin of the terrain is of the Darriwilian and Sandbian age, while I-type granites are of the Darriwilian age. The over-thrusting of the Iladyr-Ili oceanic crust on the Ili border of the terrain also occurred in the middle Ordovician. Subduction-related mag-matism occurred here in the Late Darriwilian and Sandbian periods. Conclusion. The conducted studies testify to the geodynamics of the region. In the Early and Middle Ordovician, the Kokchetau-Issykkul terrain was a sialic volcanic arc separated from other sialic terrains by ocean basins. In the Middle Ordovician, the counter subduction of the oceanic crust under the Kokchetau-Issykkul island arc occurred. This process led to the closure of the Chistopol-Terskei and Iradir-Ili ocean basins in Middle Ordovician and to the formation the Kazakhstan continent by the collision of the Early Paleozoic sialic terrains and oceanic island arcs. According to the study of the paleomagnetism of Ordovician rocks in the Kokchetau-Issykkul island, the position of the middle part of this arc is determined to be at the paleogeographic latitude of 9.1 ± 5.4°.

О ПРИМЕНЕНИИ ГЕОДИНАМИЧЕСКИХ ЛИТОХИМИЧЕСКИХ ДИАГРАММ ПРИ ИЗУЧЕНИИ ТЕФРОГЕННЫХ ПЕСЧАНИКОВ

Актуальность. Диаграммы, позволяющие на основе литохимических особенностей кластолитов предполагать геодинамическую позицию седиментационных бассейнов, используются уже на протяжении многих десятилетий. Вместе с тем возможность их применения для изучения тефрогенных песчаников остается неясной, а результаты противоречивыми. Цель исследования состоит в сравнительном анализе положения на наиболее часто применяемых диаграммах точек, отвечающих составу двух групп пород – тефрогенным песчаникам и вулканитам их источника сноса. Объектами исследования являются тефрогенные граувакки улутауской свиты и вулканиты (от риолитов до базальтов) урлядинской толщи Магнитогорской мегазоны Южного Урала. Улутауская свита относится к живетскому и нижней части франского яруса. Слагающие ее граувакки являются тефрогенными турбидитами и дебритами, сформированными в глубоководной впадине, расположенной у подножия энсиматической Магнитогорской островной дуги. В современном эрозионном срезе осевой зоне островной дуги соответствуют вулканиты урлядинской толщи. Методы. В основу сопоставления положены результаты силикатного анализа граувакк улутауской свиты (202 образца) и вулканитов урлядинской толщи (202 образца). Сравнение проведено на основе диаграмм Мейнарда, Бхатии, Розера–Корша и Верма–Армстронга-Алтрина. Результаты. Анализ диаграмм показал, что более реалистичные результаты дает применение основных вулканокластических граувакк (SiO2<63 %). Исключением в данном случае является диаграмма (Fe2O3+MgO)–TiO2: большинство точек оказалось за пределами каких-либо полей. Точки, отвечающие составу кислых разностей граувакк (SiO2>63 %), попадают на многих диаграммах в поля энсиалических островных дуг, окраин андского типа и пассивных континентальных окраин. Наилучшие результаты для кислых разностей были получены при использовании диаграммы DF1–DF2 и модифицированной диаграммы М. Бхатии Al2O3/(CaO+Na2O)–K2O/Na2O. Поля граувакк на большинстве построенных диаграмм совпадают с полями вулканитов и являются по сравнению с ними более компактными, что вызвано смешанным составом песчаников и низкой эксплозивностью базальтовых расплавов. Полученные результаты во многом не согласуются с результатами аналогичных исследований. Следовательно, выводы, получаемые при построении литохимических геодинамических диаграмм, не могут считаться окончательными и должны дополняться другими методами.

Chalcophile element processing beneath a continental arc stratovolcano

The chalcophile elements are important both in terms of their economic value and as potential tracers of magmatic processes at convergent margins. However, because of analytical difficulties, comprehensive datasets of chalcophile element concentrations for volcanic rocks are rare. Here, we present analyses of a near complete suite of chalcophile elements (S, Cu, Ag, Se, As, Sb, Sn, W, Mo, Pb, Bi, Tl, Zn, Ga, Co) for volcanic rock samples collected from a typical continental arc stratovolcano in southern Chile (Antuco). Enrichment in Pb, Bi, W, Tl, Sb and As relative to Parental-MORB indicates that these elements have been mobilised from the subducting slab into the sub-arc mantle wedge, in contrast to Cu and Ag. Very low Se concentrations suggest that Se, like S, was lost during co-eruptive degassing of the Antuco magmas. Previous studies on oceanic arcs have demonstrated that as higher fO2 subduction-related magmas ascend through the overlying lithosphere, magnetite fractionation may trigger sulfide fractionation during crystallisation. If such a process is extensive and has a sharp onset, this would result in a plummet in the Cu, Se and Ag contents of the residual melt. At Antuco, although a decrease in the Fe2O3(T) and TiO2 concentrations at ∼55 wt.% SiO2 (∼3 wt.% MgO) indicates magnetite fractionation, this is not associated with a corresponding drop in Cu contents. Instead, we observe a general decrease in Cu and a decrease in Cu/Ag with increasing SiO2 and decreasing MgO. Furthermore, Cu/Ag in the most primitive Antuco rocks are lower than the global MORB array, indicating that the melts were sulfide saturated at an early stage in their crustal evolution. Through modelling fractional crystallisation, we show that only a minor volume (0.5–0.6 vol.%) of fractionating sulfide is needed to produce divergent trends in Cu and Ag, as observed in the Antuco samples. Our results show that sulfide fractionation occurred from an early stage during the crustal evolution of Antuco's magmas. We infer that this was promoted by stalling in the lower crust, which for oxidised magmas at depths >20 km is within the sulfide stability field. However, elevated DyN/YbN of the Antuco magmas compared to oceanic island arc magmas provides an additional, or alternate mechanism to inducing sulfide fractionation in the lower crust prior to ascent, through initial garnet fractionation. Fractional crystallisation within this depth range meant that later magnetite fractionation had only a minor impact on the partitioning behaviour of the chalcophile elements. In contrast, arc magmas transiting thinner crust may not experience sulfide saturation until a later stage in their evolution, induced by magnetite fractionation. Our results imply that convergent margin crustal thickness, and therefore the depth range of magmatic differentiation, determines the dominant control on initial magmatic sulfide saturation and therefore the primary distribution of chalcophile elements. This implies that secondary processes are required to explain the transport and concentration of sulfides and chalcophile elements at shallower crustal levels.

Basalt Tectonic Discrimination Using Combined Machine Learning Approach

Geochemical discrimination of basaltic magmatism from different tectonic settings remains an essential part of recognizing the magma generation process within the Earth’s mantle. Discriminating among mid-ocean ridge basalt (MORB), ocean island basalt (OIB) and island arc basalt (IAB) is that matters to geologists because they are the three most concerned basalts. Being a supplement to conventional discrimination diagrams, we attempt to utilize the machine learning algorithm (MLA) for basalt tectonic discrimination. A combined MLA termed swarm optimized neural fuzzy inference system (SONFIS) was presented based on neural fuzzy inference system and particle swarm optimization. Two geochemical datasets of basalts from GEOROC and PetDB served as to test the classification performance of SONFIS. Several typical discrimination diagrams and well-established MLAs were also used for performance comparisons with SONFIS. Results indicated that the classification accuracy of SONFIS for MORB, OIB and IAB in both datasets could reach over 90%, superior to other methods. It also turns out that MLAs had certain advantages in making full use of geochemical characteristics and dealing with datasets containing missing data. Therefore, MLAs provide new research tools other than discrimination diagrams for geologists, and the MLA-based technique is worth extending to tectonic discrimination of other volcanic rocks.

REE, Y, Th, U and Mn systematics of Upper Devonian conodonts in the West Uralian Folded Zone (Southern Urals)

Research subject. This article presents the results of a study undertaken to investigate the systematics of rare-earth elements (REE), Y, Th, U and Mn in the Upper Devonian conodonts of the Western Ural region of the foreland fold of the Southern Urals (Askyn and Makarovo horizons, Kukkarauk, Ryauzyak and Lemezinsky sections).Methods. The conodonts were isolated from carbonate rocks using the conventional method of their dissolution in organic acids, mainly formic acid. Sample preparation and mass spectrometric analysis were carried out in the class 1000 and 10 000 cleanroom facilities of the Zavaritsky Institute of Geology and Geochemistry. A PerkinElmer ELAN 9000 quadrupole ICP mass-spectrometer was used to determine the concentration of the trace elements under study. Sm and Nd isotope ratios were measured from a 3% nitric acid solution by a Thermo Fischer Neptune Plus multicollector inductively coupled plasma mass-spectrometer. The long-term reproducibility and accuracy of the measurement procedure were evaluated using a standard Merck Nd solution based on the NIST Nd2O3 and yielded 143Nd/144Nd = 0.511720 ± 15 (1 SD, n = 40).Results. The established features of PAAS-normalised lanthanide distributions in the conodont bulk samples (10.4–21.8 mg), Ce-anomalies, high REE values (173–1211 ppm) and a number of other parameters indicate the leading role of late diagenetic processes in the formation of conodont REE systematics. This is also evidenced by the Y/Ho values (≈26–32) specific for the studied conodont samples.Conclusions. In general, the distribution of lanthanides in the conodonts suggests that this process was mainly controlled by a lithogenic (from 90 to more than 99%) REE source. The εNd (t) (–4.0…–2.8) values characteristic of the cono donts of the Askyn and Makarovo horizons suggest that their Nd isotopic composition was either formed under the influence of the open ocean (island arc basin), characterised by a significant share of radiogenic Nd, or due to the entry of radiogenic waters of the ocean into the shelf zone at the peak of the marine transgression that took place in the region under consideration in the Famennian.

Neoarchean-Paleoproterozoic magmatic arc evolution in the Wutai-Hengshan-Fuping area, North China Craton: New perspectives from zircon U–Pb ages and Hf isotopic data

The Wutai granite-greenstone belt, which formed in the Neoarchean-Paleoproterozoic, with the adjoining high-grade metamorphic Hengshan-Fuping region, record important transitions in crustal and tectonic evolution, although these evolutionary processes have still been in dispute. In this study, new U–Pb and Lu–Hf LA-ICP-MS zircon analyses were obtained from eight metamorphosed magmatic rocks in the Wutai-Hengshan-Fuping area in North China Craton (NCC). The results reveal that most of Wutai granitoids yield emplacement ages of 2.56–2.52 Ga, with corresponding εHf(t2) of +4.7 to +8.3. The tonalitic gneiss from Dongshandi intrusion, however, gives a new age of 2482 Ma and εHf(t2) of +2.5 to +4.6, which are similar with Yixingzhai Pluton marked by ages of 2501–2499 Ma and εHf(t2) of +3.2 to +4.4. Also, ∼2.7 Ga and ∼2.5 Ga continental crust generation are further revealed. But ∼2.5 Ga crustal growth is more complicated due to the involvement of old crustal components, which can be identified by variable εHf(t2) values ranging from −1.8 to +7.5. Thus, based on the distinctions of Hf isotopes, compositional and spatial features, two consecutive groups of 2.56–2.52 Ga and 2.52–2.48 Ga are recognized and a two-step tectonic evolution model for the Wutai-Hengshan-Fuping area is proposed. During 2.56–2.52 Ga, intra-oceanic subduction led to the formation of Wutai oceanic island arc, accompanied mainly by the development of Wutai granitoids (εHf(t2) of +4.7 to +8.3) and coeval volcanics (predominant arc-tholeiitic chemical affinity), corresponding to the juvenile crustal growth. Between 2.52 Ga and 2.48 Ga, with intervening oceanic crust between intra-oceanic arc and continent consumed, the Wutai intra-oceanic arc then collided and was obducted onto the western edge of the Hengshan-Fuping continental block. Large amounts of TTG gneisses produced in both the Wutai and Hengshan-Fuping (εHf(t) of −1.8 to +7.5), along with calc-alkaline volcanism in Wutai, possibly in response to the thickening through arc accretion and commencing the continental arc. This proposed two-step tectonic evolution model, combined with a late Neoarchean (2.6–2.5 Ga) intra-oceanic arc system along the northwestern of the Eastern Block (EB), probably provides a better explanation for the fundamental geodynamic mechanism for continental arc development at the Proterozoic dawn.

Геологическое строение и этапы тектонической эволюции палеозоид Казахстана

Research subject. In this paper, we set out to investigate main paleostructures in Kazakhstan. In particular, we attempt to clarify or revise their existing geodynamic structures and to show key stages in their development on the basis of new data on the age of some stratigraphic units and rocks of the ophiolitic association. Materials and methods . The work was based on the materials of long-standing geological research performed by the authors, including government-funded geomapping, tectonic zoning and paleoreconstruction, as well as the revision of literature data. Results . It is shown that the main sedimentary, volcanogenic, volcanogenic-sedimentary and intrusive complexes were originated in the western part of the Central Asian fold belt (Paleozoic Kazakhstan) in place of various structures differing in their geodynamic environment and tectonic history throughout the Proterozoic and entire Paleozoic periods. The Epigrenville microcontinent disintegrated into individual massifs has been described, along with rift depressions, basins with oceanic crust, island arcs and island arc systems, the Epicaledonian continent, continental depressions, regional volcano-plutonic belts and marginal basins . The tectonic zonality of Kazakhstan has been presented in the form of diagrams. The mapping of the Precambrian Ulutau massif has been conducted. The distribution of the Devonian complexes and the Late Paleozoic complexes of Kazakhstan is schematically represented. Conclusions . We have refined and partially revised the existing concepts and tectonic consecution on the territory of Kazakhstan. On the basis of paleoreconstructions, key stages in the development of main paleostructures have been established; features of the geological evolution of Kazakhstan paleozoids and their relationship with tectonic processes in the Central Asian fold belt have been determined. It is shown that the modern structure of Kazakhstan paleozoids was evolving over a long period and in many stages for almost 900 million years.

Evidence for a lava lake on Mt. Michael volcano, Saunders Island (South Sandwich Islands) from Landsat, Sentinel-2 and ASTER satellite imagery

Mt. Michael is an active stratovolcano on Saunders Island in the South Sandwich Islands; a remote, oceanic island arc in the southern Atlantic Ocean, bordering the Southern Ocean. The arc contains the only active volcanoes in the South Georgia and South Sandwich Islands British Overseas Territory, yet little is known of their activity. Despite lava lakes being extremely rare with only a few global examples, previous analyses of satellite AVHRR imagery of Mt. Michael in the 1990s showed persistent thermal anomalies not associated with magma overflowing the crater. This suggested the existence of a lava lake inside Mt. Michael's crater. However, their study relied on 1 km resolution imagery, and there have been no long-term investigations to determine if this is a persistent feature.In-situ observations of Mt. Michael are extremely difficult given the location. We used Landsat, Sentinel-2 and ASTER satellite data to monitor activity and detect thermal anomalies within the crater. Persistent volcanic plumes and eruptions were identified throughout the thirty-year period studied. On all 15 occasions that a suitable satellite image was available a thermal anomaly within the crater was detected in the high transmissivity shortwave infrared bands (SWIR) centred on 1.65 μm and 2.2 μm. Pixel-integrated surface temperatures over the crater floor yield temperatures of 284–419 °C; comparable to lava lakes elsewhere. Unmixing these temperatures using the dual-band method reveals a molten lava component at basaltic temperatures (989–1279 °C) among a cooler crust. This is the first evidence for a lava lake within Mt. Michael from data that can resolve the crater floor, and the first evidence for magmatic temperatures. As thermal anomalies were detected in all images showing the crater, we suggest that the lava lake is a common and persistent feature.

Provenance and magmatic-tectonic setting of Campanian-aged volcaniclastic sandstones of the Kannaviou Formation in western Cyprus: Evidence for a South-Neotethyan continental margin volcanic arc

Regionally developed late Cretaceous subduction-related magmatism in the eastern Mediterranean records progressive closure of the Southern Neotethys. In western Cyprus, the late Cretaceous (c. 90 Ma) Troodos ophiolite is depositionally overlain by volcaniclastic sandstones (up to 750 m thick) that are dominated by redeposited pyroclastic fallout, interbedded with both non-calcareous and calcareous radiolarian-bearing mudstones. The sands were mainly deposited by channelised mass-flow processes and to a lesser extent by turbidity currents in a deep-water forearc basin. The succession was folded and locally thrust-deformed related to latest Cretaceous emplacement of adjacent Mesozoic continental margin and oceanic units (Mamonia Complex). SIMS U-Pb analysis of euhedral to subhedral magmatic zircon crystals yielded a weighted mean 206Pb/238U age of 80.1 ± 1.1 Ma (Campanian). Sandstone petrography including compositional framework (Q12F21L67) suggests sediment supply from a volcanic arc. Subordinate continentally-derived detritus suggests a continental margin arc setting for the mafic to felsic fallout ash. SIMS analysis of little-altered volcanic glass indicates high Th/Nb and Th/La ratios that suggest the involvement of continental crust and/or subducted terrigenous sediments in magma genesis. Whole-rock chondrite-normalised REE patterns have comparable trends to modern and ancient forearc basin volcaniclastic sands and sandstones (e.g., northwest Pacific region). Trace element chemistry, although scattered towards the continental island arc fields on some geochemical diagrams (e.g., Th-Sc-Zr), mainly suggests an oceanic island arc source for the Kannaviou Formation sandstones, with variable enrichments in V-Cr-Ni-Sc, depletion in Nb-Ta, and relatively low trace element ratios (e.g., La/Co, Th/Co, La/Sc, Th/Sc). An explanation for this apparent discrepancy is that the volcaniclastic sandstones were derived from a relatively primitive arc constructed on previously depleted, rifted (thinned) Neotethyan continental crust. The inferred continental margin arc developed during early-stage northward subduction of the Southern Neotethys beneath a Tauride microcontinent to the north. Large volumes of volcanic ash were derived from continental margin arc volcanism, probably in the vicinity of the Kyrenia Range in northern Cyprus.

The Tamrau Block of NW New Guinea records late Miocene–Pliocene collision at the northern tip of the Australian Plate

The Tamrau Block is a geological terrane located in the Bird’s Head Peninsula of NW New Guinea. Relatively little is known about this terrane and how it relates to other parts of western New Guinea. The geological history and provenance of this terrane is difficult to resolve because it is found at the boundary between the Australian, Philippine Sea, Caroline, and Pacific plates. This segment of the plate boundary records episodes of deformation, magmatism, and exhumation associated with the interaction of these three tectonic plates. For instance, the southern edge of the Tamrau Block is bounded by a major strike-slip fault zone (the Sorong Fault Zone), previous studies imply that the terrane may have been translated some distance along this fault zone during the Cenozoic. We seek to understand the geological history and provenance of the Tamrau Block. To do this, we report new field observations made during a three-month field campaign in a poorly exposed region, together with new petrographic and geochronology data. U–Pb detrital zircon age spectra were obtained from five sedimentary and metamorphic sequences and the results were combined with existing biostratigraphic age data to reassess the stratigraphy and tectonic history of the region. The oldest rocks in the Tamrau Block (the Tamrau Formation) represent Jurassic–Cretaceous passive margin sediments. These were deformed and metamorphosed in at least three distinct events – the first phase of which involved amphibolite facies conditions (and produced the first reported instance of metamorphic kyanite from the Bird’s Head), possibly in the Oligo–Miocene. These Jurassic–Cretaceous rocks are unconformably overlain by the Ajai Limestone. Both units are cross-cut and overlain by intrusives and eruptives of the middle Miocene Moon Volcanics. The heat associated with these intrusives baked the Tamrau Formation and the overlying Ajai Limestone, resulting in andalusite growth in the Tamrau Formation and hornfelsing of the limestones. Deposition of the overlying Koor Formation occurred from the middle–late Miocene, and was partially contemporaneous with volcanism. An episode of crustal shortening occurred after the deposition of the Koor Formation with the development of asymmetric folds and steeply-inclined reverse faults. This is marked by a late Miocene–Pliocene break in deposition between the Koor Formation and undeformed Opmorai Formation (∼10.5–4.5 Ma) and records the collision of part of an oceanic island arc (Tosem Block) to the Tamrau Block, an event also recognised in other parts of New Guinea. A comparison of the U–Pb detrital zircon age spectra from sedimentary rocks within the Tamrau Block and those reported in other studies indicate that there is no relationship between the rocks of the Tamrau and Kemum blocks (which are in faulted contact). Instead, the U–Pb detrital age data share similarities with rocks from the Lengguru Fold and Thrust Belt and Weyland Overthrust in the south-east. We therefore propose that the Tamrau Block was transported westwards ∼300 km to its current position along the Sorong Fault Zone after the late Miocene–Pliocene collision between the Tosem Block and the northern margin of the Australian Plate.

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.

Electrical Structure of the Lithosphere From Rio de la Plata Craton to Paraná Basin: Amalgamation of Cratonic and Refertilized Lithospheres in SW Gondwanaland

AbstractWe conducted a magnetotelluric (MT) study from Paleoproterozoic Rio de la Plata Craton, in Uruguay, toward Paleozoic‐Mesozoic Paraná Basin, in Brazil. The 850‐km‐long MT transect comprises 35 evenly spaced broadband electromagnetic soundings sites. In the Paraná Basin, 11 additional long‐period measurements were acquired to extend the maximum depth of investigation. All data were inverted using two‐ and three‐dimensional approaches obtaining the electrical resistivity structure from the surface down to 200 km. The Rio de la Plata Craton is >200‐km thick and resistive (~2,000 Ωm). Its northern limit is electrically defined by a lithosphere scale lateral transition and lower crust conductive anomalies (1–10 Ωm) interpreted as a Paleoproterozoic suture at the southern edge of Rivera‐Taquarembó Block. The latter is characterized by an approximately 100‐km thick and moderate resistive (>500 Ωm) upper mantle. The Ibaré shear zone is another suture where an ocean‐ocean subduction generated the 120‐km thick and resistive (>1,000 Ωm) São Gabriel juvenile arc. Proceeding northward, a 70‐ to 80‐km thick, 150‐km wide, and inclined resistive zone is imaged. This zone could be remnant of an oceanic lithosphere or island arcs accreted at the southern border of Paraná Basin. The MT transect terminates within the southern Paraná Basin where a 150‐ to 200‐km‐thick less resistive lithosphere (<1,000 Ωm) may indicate refertilization processes during plate subduction and ocean closure in Neoproterozoic‐Cambrian time. Our MT data support a tectonic model of NNE–SSW convergence for this segment of SW Gondwanaland.

The Calc-Alkaline Hidden Bay and Kagalaska Plutons and the Construction of the Central Aleutian Oceanic Arc Crust

Abstract Calc-alkaline plutons are the major crustal building blocks of continental margins, but are rarely exposed in oceanic island arcs. Two of the best examples are the ∼10 km wide Hidden Bay and Kagalaska plutons that intrude Eocene mafic volcanic–sedimentary rocks on Adak and Kagalaska islands in the central Aleutian arc. Twenty new Ar/Ar and U/Pb ages, coupled with published ages, show that the Hidden Bay pluton was intruded in multiple stages from ∼34·6 to 30·9 Ma, whereas the Kagalaska pluton was intruded at ∼14 Ma. The plutons largely consist of medium- to high-K2O hornblende-bearing cumulate diorite (53–55 wt % SiO2) and hornblende–biotite granodiorite (57–64 wt %), with lesser amounts of gabbro (50–52 wt % SiO2), leucogranodiorite (67–69 wt % SiO2) and aplite (76–77 wt % SiO2) that can generally be linked to each other by crystal fractionation. The compositions of these plutons are generally similar to those of continental plutons, except for more oceanic-like large ion lithophile element and isotopic signatures (87Sr/86Sr = 0·703–0·7033; ɛNd = 9·4–7·7) that reflect oceanic- rather than continental-type crustal contaminants. Chemical similarities between the Hidden Bay homogeneous gabbros and high-Al basalts in Adak Pleistocene-Holocene volcanoes indicate little temporal evolution in the general character of the mantle-derived basalts. Rather than a unique arc setting and distinctive magmas, formation of the Aleutian calc-alkaline plutons seems to require a sufficient crustal thickness (∼37 km) and a high enough water content to stabilize pargasitic hornblende amphibole in a relatively closed magma system that favors increasing K, Ti and H2O at the end of a magmatic cycle. This termination of magmatism coincides with a northward migration of the magmatic front that is inferred to be associated with fore-arc subduction erosion. In accord with Adak region crustal architecture based on seismic data, crystallization models for the plutons suggest that mantle-generated hydrous arc basalts fractionated olivine and clinopyroxene in the lower crust to form high-Al basaltic composition magmas that rose into the mid-crust, where gabbro and diorite crystallized to form the magmas that buoyantly rose into the upper crust and crystallized to form the volumetrically dominant granodiorite (58–63 wt % SiO2). The most important temporal changes in chemistry can be explained by fore-arc crust incorporated into the mantle wedge by fore-arc subduction erosion creating ‘adakitic’ signatures at times of northward arc migration and a change to a more continental subducted sediment component at the time of Plio-Pleistocene glaciation.

Episodic crustal growth and reworking of the Yudongzi terrane, South China: Constraints from the Archean TTGs and potassic granites and Paleoproterozoic amphibolites

The Yudongzi terrane is featured by well-preserved lithologic outcrops of basement rocks, including the ~2.7 Ga tonalite-trondhjemite-granodiorite (TTGs), ~2.5 Ga potassic granites, Neoarchean supracrustal rocks and ~1.85 Ga amphibolites. The TTG rocks contain ~2.8 Ga inherited/xenocrystic zircons, which indicates the existence of ancient rocks in the magma source. Most zircon grains in the TTG rocks have chondrite-like Hf isotopic compositions, indicative of derivation of magmas from the Paleo-Mesoarchean (3.3–3.0 Ga) crustal sources, consistent with their whole-rock Nd model ages (3.3–2.9 Ga). Zircon oxygen isotopic compositions (5.4 to 6.1‰) are mainly in equilibrium with the mantle, indicating insignificant surficial processes for the source rocks. Thus, the Archean TTGs likely originated from melting of an overthickened oceanic island arc where shallow subduction occurred. The compositional diversity of the TTG rocks can be ascribed to varying residual minerals after melting of mafic precursors at different crustal depths. The subsequent intracrustal melting of the TTG-dominated proto-crusts formed more potassic granites at ~2.5 Ga, probably associated with dense residue delamination and concomitant mantle upwelling. Detrital zircon age pattern and geochemical features of the meta-sediments from the Yudongzi Group demonstrate that the adjacent meta-volcanic components are the major sedimentary source, and thus they were proximal erosion and deposition products of an arc system within a proto-subduction zone. Protolith Hf isotope signature in metamorphic zircons of the amphibolites provides evidence for the existence of early Archean basaltic proto-arc crusts which were metamorphosed at ~1.85 Ga. Collectively, the above-stated Neoarchean to Paleoproterozoic tectonothermal events imply significant crustal growth and reworking, which were coincident with global-scale continental lithospheric development and stabilization during assembly and dispersal of supercontinents Kenorland and Columbia. Similar tectonostratigraphic, paleomagnetic and geochronologic records support a Neoarchean-Paleoproterozoic connection of the Yangtze Block with the circum-N Laurentia and southern Siberia. Furthermore, the late Archean geochemical transition from sodic TTG to potassic crust record, as a sign of cratonic maturation, is collectively related to the kinematic adjustment of supercontinent process and thermal evolution of the Earth system, therefore sheding lights on subduction tectonic operation and varied crustal growth mechanism.

Heterogeneous Oceanic Arc Volcanic Rocks in the South Qilian Accretionary Belt (Qilian Orogen, NW China)

Primitive arc magmas in oceanic island arcs are probes of sub-arc magmatic processes and are crucial for understanding oceanic subduction. We report data for an Early Paleozoic oceanic arc volcanic complex in the Lajishan–Yongjing terrane, South Qilian Accretionary Belt (SQAB), Qilian Orogen, including zircon U–Pb dating and Hf–O isotopes, mineral and whole-rock geochemistry, and Sr–Nd isotope compositions. New zircon ages of ∼455–440 Ma constrain the timing of arc volcanism and the subduction of the Qilian Ocean. Based on petrography and bulk-rock composition, five lithological types have been identified: ankaramite; high-Mg basaltic andesite; high-Al andesite; boninite; sanukite. The volcanic sequence thus is one of the few island arcs where three types of near-primitive arc rocks including boninite, ankaramite and sanukite have been simultaneously produced. All these rocks have variably enriched Sr–Nd isotopic compositions, positive to slight negative zircon εHf(t) values and elevated zircon δ18O values. Boninites, ankaramites and sanukites are interpreted as contemporary near-primitive melts generated from different sources and conditions within an island arc setting. Boninites are characterized by low Ti and REE concentrations and high Cr# chrome spinel, and are interpreted as melts of refractory, Cpx-poor, spinel lherzolite or harzburgite at >25% partial melting. Anomalous zircon δ18O values of 6·57–7·61‰ and Sr–Nd mixing calculations suggest less than 2% incorporation of subducted oceanic sediments into the mantle source of the magmas. The ankaramites are characterized by low SiO2 and high MgO (Mg#), Cr, Ni and La/Yb ratios, and have similar isotopic ratios to tectonically adjacent ocean island basalt (OIB) lavas. The ankaramite lavas are likely to have derived from mantle sources similar to those of OIB; that is, pyroxenite-bearing garnet peridotite enriched in incompatible elements. High-Mg basaltic andesites and high-Al andesites may be derived from parental ankaramite magmas. Sr–Nd–Hf isotopic mixing modeling constrains the amount of silicic melt to ∼1–4% for ankaramite magma. Sanukites are of andesitic–dacitic composition with high Mg#, Cr and Ni, and enriched large ion lithophile elements and high La/Yb ratios. They are interpreted as having been generated by reaction of mantle peridotite with a silicic melt, itself derived from subducted sediments. Enriched Sr–Nd–Hf isotopic compositions constrain the amount of silicic melt to ∼10–15% for sanukite. Large compositional variations among the volcanic rocks from the same arc reflect heterogeneous mantle sources and variable degrees of mantle metasomatism by sediment-derived hydrous fluids or silicic melts, accompanied by secondary assimilation–fractional crystallization processes during magma ascent to the surface. The generation of the island arc volcanic sequence in the Lajishan–Yongjing Terrane is a response to the collision between the Lajishan–Yongjing Oceanic Plateau (recorded by the Lajishan–Yongjing Ophiolite) and the pre-existing trench–continental margin. Evolution from a continental margin in the North Qilian Accretionary Belt to an oceanic island arc in the SQAB records subduction advance and retreat in the history of the Qilian Ocean.

Petrogenesis of Zeiatit gabbroic rocks in the Southern Eastern Desert of Egypt: Discrimination of arc-related Neoproterozoic gabbros

The area of Gabal Zeiatit, South Eastern Desert, Egypt represents one of a series of ophiolitic-island arc complexes found within the Arabian–Nubian Shield (ANS). The present study introduces new major, trace, and rare earth element analyses and mineral composition data of gabbroic rocks encountered in this area with the goal of constraining their geotectonic setting. The investigated area consists of two types of gabbroic rocks. The Zeiatit ophiolitic metagabbros (ZMG) are always associated with serpentinites of ophiolitic affinity and were subjected to metamorphism of greenschist to lower amphibolite facies and exhibit tholeiitic affinity and MORB-like characters. Serpentinites and metagabbros are tectonically thrusted over a thick pile of metavolcanic rocks. The Zeiatit intrusive gabbros (ZIG) are related to Egyptian Alaskan-type mafic-ultramafic complexes and intrude the studied ophiolitic rocks and island arc rocks. Geochemical data reveal that both studied gabbroic types originated in sub-arc setting. Mineral chemistry data for the two types of the studied gabbroic rocks indicate that they possess different characteristics. The NiO (<0.15 wt%) and Fo (<83 mol.%) contents of olivine from the investigated ZIG are lower than those of the Egyptian ophiolites. The clinopyroxene composition of the ZIG show arc related cumulate. Geochemically, the ZMG exhibit MORB-like and arc-like signatures as they display Ti/V ratios ranging from 14.52 to 21.09. Such phenomenon of hybrid mixture between island arc tholeiite (IAT) and MORB is generally acknowledged being unique to forearc tectonic regime. Moreover, depleted LREE and HFSE and relatively high CaO in the studied ZMG suggest derivation from Cpx-rich mantle at forearc tectonic setting. The ZIG have Nb/Yb (0.28–1.04) and Th/Yb (0.1–0.42) which are typical for oceanic island arcs. The ZIG are highly depleted in Nb, Ta, and Zr, but enriched in Ba, Pb, Sr, U and Th suggest their derivation from a metasomatized lithospheric mantle source that modified by hydrous fluids derived from the subducting slab in a subduction zone. ZIG have Th/Ta (0.69–2.05) and Ta/Hf (0.16–0.28) indicating that they were mainly derived from partial melting of a depleted mantle source, being mixed with crustal material. The present geochemical and mineralogical studies revealed that, the investigated ZMG are fragment of oceanic lithosphere probably developed in a supra-subduction zone environment and originated by 10–30% partial melting of a spinel lherzolite mantle in a sub-arc (forearc) setting at shallow depth which consistent with the most recent conclusions of the Egyptian ophiolitic metagabbros. However, the investigated ZIG are akin to the Alaskan-type rocks.

Precambrian crustal evolution of the central Jiangnan Orogen (South China): Evidence from detrital zircon U-Pb ages and Hf isotopic compositions of Neoproterozoic metasedimentary rocks

The Neoproterozoic amalgamation and timing of collision between the Yangtze and Cathaysia sub-blocks along the Jiangnan Orogen in South China remain disputed. With a view to constrain the crustal evolution in this major orogen, here we present new results from petrology, geochemistry, zircon U-Pb chronology and Lu-Hf isotopes, on a suite of meta-sedimentary rocks from the Shuangqiaoshan Group and tuffaceous rock from the Liantuo Formation in the Jiuling terrane of central Jiangnan Orogen. Magmatic zircon grains in meta-sedimentary and sedimentary samples from the bottom to top part of the lower and upper Shuangqiaoshan groups constraints the deposition during 863–820 Ma and 797–780 Ma, respectively. The unconformities were formed during 820–797 Ma, reflecting the final amalgamation of Yangtze with Cathaysia sub-blocks along the central Jiangnan Orogen. Our data also identify four major events at 1000–800, 1500–1900, 1900–2200, 2300–2600 Ma. The Neoproterozoic detrital zircons in the lower Shuangqiaoshan Group possess variable εHf(t) values of −44.0 to +13.4. Combined with the geochemical features, these data suggest that the lower Shuangqiaoshan Group was deposited in a back-arc basin with a mixed provenance of granitic and felsic volcanic components, whereas the upper Shuangqiaoshan Group was formed in a rift basin with more granitic or felsic components in the source region. Integrating the results presented in this study with those from previous works, a five-stage crustal evolution model is proposed for the Neoproterozoic Jiangnan Orogen involving (1) intra-oceanic subduction beneath the Huaiyu oceanic island arc (1000–880 Ma), (2) collision between Huaiyu arc and Yangtze Sub-block (880–860 Ma), (3) back-arc extension beneath the Yangtze Sub-block (860–830 Ma), (4) collision between Cathaysia and Yangtze sub-blocks (830–800) and (5) intracontinental rifting throughout South China Block (800–750 Ma).

Tectonometamorphic Evolution of Jutulsessen, Gjelsvikfjella, cDML, East Antarctica

The Jutulsessen area, can provide a vital clue to the supercontinent assembly of Gondwana Land as it is situated within the Circum East Antarctic Mobile Belt just east of the Penksockett rift marking the divide between the central Dronning Maud Land from the Western Dronning Maud Land. This landmass is dominated by migmatitic quartzo-feldspathic rocks intruded by syn to post-tectonic granites. The work highlights the data from western part cDML area with a view to arrive at a more comprehensive model for the cDML and subsequently to the super continent assembly. Granitic and migmatitic gneisses comprising of amphibolitic and biotite rich enclaves. The gneisses show variations from quartzo-felspathic gneiss to amphibolitic gneiss. The area has witnessed complex geological history involving at different deformational episodes with concomitant metamorphism. The pervasive dominant foliation trends NW-SE with shallow to medium dips towards SW. In the Stabben area, a nonfoliated intrusive syenite-gabbro pluton limits the gneissic exposures. Compositionally, the orthogneisses plot in the monzogranitegranodiorite field where as the mafic dykes/enclaves plot in the basalt-andesite-rhyodacite field. The bulk geochemical characteristics suggest significant crustal contamination. Garnet-biotite Fe-Mg exchange thermometry gives peak metamorphic temperature of 483° C for the gneisses and 628° C for the dioritic enclave within gneisses. A peak metamorphic grade of upper amphibolite to granulite facies is deduced from the mineral assemblages. Widespread anatexis has led to extensive occurrence of migmatites in the area. Recent geochronological studies assign an age of 1170 Ma to 970 Ma for the migmatites/gneisses and an emplacement age of 501 Ma for the Stabben gabbro and syenite. The discriminant plots of the Jutulsessen rocks indicate diverse origin ranging from pre-plate collision to post-collision orogenic tectonic setting. The mafic enclaves/dykes show ocean island arc to MORB affinities. Voluminous addition of juvenile crust during the Pan-African orogeny strongly overprints earlier structures.

Geochemical attributes of late Neoproterozoic Salt Range Formation, Pakistan: constraints on provenance, paleoclimate, depositional and tectonic settings

Detailed geochemical analyses of clastic fraction of the oldest exposed Salt Range Formation in the famous salt producing region (Salt and Trans-Indus Ranges) Pakistan was carried out in order to understand provenance, paleoclimate, depositional and tectonic settings. Sandstone and shale samples of Salt Range Formation were analysed for major, trace and Rare Earth Elements and various geochemical models and ratios were used to interpret. Geochemical analyses of both sandstone and shale fraction mostly display similar results and interpretations. The samples are classified as arkose, subarkose, litharenites and were sourced from felsic and/or intermediate igneous rocks. The sedimentation of the detritus was occurred in fluvial to restricted shelf conditions under arid to semi-arid and oxygenated climate conditions. The tectonic settings of the sediments were mainly Oceanic and Continental Island arcs as well as Active Continental margin.

Co-seismic ionospheric GPS-TEC disturbances from different source characteristic earthquakes in the Himalaya and the adjoining regions

Co-seismic ionospheric disturbances (CIDs) due to different source characteristic earthquakes are investigated through GPS-derived vertical total electron content (VTEC). We investigated VTEC changes related with Moderate, Strong, Major and Great earthquakes occurred at different tectonic settings in both Himalaya and non-Himalayan regions. The VTEC and its anomalies are computed using GPS data obtained from the local network of GPS stations in Himalaya with the IGS stations surrounding the epicentre region. Irrespective of the source characteristics, significant CIDs are observed during Major and Great earthquakes, namely; (i) the Mw 7.8, 25th April 2015 Gorkha earthquake, (ii) the Mw 7.6, 8th October 2005 Kashmir earthquake, and (iii) the Mw 8.6, 28th March 2005 Nias–Simeulue earthquake.The VTEC anomalies due to the Gorkha event are observed 21.15 min after the earthquake origin time and continued till 22.78 min with the amplitude range from −0.530 to 0.517 (±0.11) TECU. CIDs associated with the Kashmir earthquake are noticed only at two GPS sites (NADI and PAN2) roughly after 23 min of the earthquake occurrence. But, it continued around 14 min with the amplitude range from −0.12 to 0.177 TECU (±0.02 TECU = 1σ). However, for the case of Ocean-Island Arc Nias–Simeulue earthquake, anomalous CIDs were observed only at GPS site NTUS, where the disturbances started around 25.58 min later and continued around 17.92 min with amplitude range from −0.077 to 0.058 TECU (±0.02 TECU = 1σ). We inferred that the CIDs originated because of earthquakes in Himalaya have relatively larger VTEC magnitudes (>15 TECU) that propagates faster in reaching and registering in the ionospheric layers compared with the non-Himalayan earthquakes. Secondly, different lithospheric-ionospheric coupling mechanism is operating in transferring the seismic energy, which originated from the Continent-Continent and the Tsunami genic Ocean-Island Arc collision zone, to the ionosphere.