Mafic Volcanics Research Articles

Probabilistic hazard analyses for a small island: methods for quantifying tephra fall hazard and appraising possible impacts on Ascension Island

Proximal to the source, tephra fall can cause severe disruption, and populations of small volcanically active islands can be particularly susceptible. Volcanic hazard assessments draw on data from past events generated from historical observations and the geological record. However, on small volcanic islands, many eruptive deposits are under-represented or missing due to the bulk of tephra being deposited offshore and high erosion rates from weather and landslides. Ascension Island is such an island located in the South Atlantic, with geological evidence of mafic and felsic explosive volcanism. Limited tephra preservation makes it difficult to correlate explosive eruption deposits and constrains the frequency or magnitude of past eruptions. We therefore combined knowledge from the geological record together with eruptions from the analogous São Miguel island, Azores, to probabilistically model a range of possible future explosive eruption scenarios. We simulated felsic events from a single vent in the east of the island, and, as mafic volcanism has largely occurred from monogenetic vents, we accounted for uncertainty in future vent location by using a grid of equally probable source locations within the areas of most recent eruptive activity. We investigated the hazards and some potential impacts of short-lived explosive events where tephra fall deposits could cause significant damage and our results provide probabilities of tephra fall loads from modelled events exceeding threshold values for potential damage. For basaltic events with 6–10 km plume heights, we found a 50% probability that tephra fallout across the west side of the island would impact roads and the airport during a single explosive event, and if roofs cannot be cleared, three modelled explosive phases produced tephra loads that may be sufficient to cause roof collapse (≥ 100 kg m−2). For trachytic events, our results show a 50% probability of loads of 2–12 kg m−2 for a plume height of 6 km increasing to 898–3167 kg m−2 for a plume height of 19 km. Our results can assist in raising awareness of the potential impacts of tephra fall from short-lived explosive events on small islands.

Mantle melting in regions of thick continental lithosphere: Examples from Late Cretaceous and younger volcanic rocks, Southern Rocky Mountains, Colorado (USA)

Major- and trace-element data together with Nd and Sr isotopic compositions and 40Ar/39Ar age determinations were obtained for Late Cretaceous and younger volcanic rocks from north-central Colorado, USA, in the Southern Rocky Mountains to assess the sources of mantle-derived melts in a region underlain by thick (≥150 km) continental lithosphere. Trachybasalt to trachyandesite lava flows and volcanic cobbles of the Upper Cretaceous Windy Gap Volcanic Member of the Middle Park Formation have low εNd(t) values from −3.4 to −13, 87Sr/86Sr(t) from ~0.705 to ~0.707, high large ion lithophile element/high field strength element ratios, and low Ta/Th (≤0.2) values. These characteristics are consistent with the production of mafic melts during the Late Cretaceous to early Cenozoic Laramide orogeny through flux melting of asthenosphere above shallowly subducting and dehydrating oceanic lithosphere of the Farallon plate, followed by the interaction of these melts with preexisting, low εNd(t), continental lithospheric mantle during ascent. This scenario requires that asthenospheric melting occurred beneath continental lithosphere as thick as 200 km, in accordance with mantle xenoliths entrained in localized Devonian-age kimberlites. Such depths are consistent with the abundances of heavy rare earth elements (Yb, Sc) in the Laramide volcanic rocks, which require parental melts derived from garnet-bearing mantle source rocks. New 40Ar/39Ar ages from the Rabbit Ears and Elkhead Mountains volcanic fields confirm that mafic magmatism was reestablished in this region ca. 28 Ma after a hiatus of over 30 m.y. and that the locus of volcanism migrated to the west through time. These rocks have εNd(t) and 87Sr/86Sr(t) values equivalent to their older counterparts (−3.5 to −13 and 0.7038–0.7060, respectively), but they have higher average chondrite-normalized La/Yb values (~22 vs. ~10), and, for the Rabbit Ears volcanic field, higher and more variable Ta/Th values (0.29–0.43). The latter are general characteristics of all other post– 40 Ma volcanic rocks in north-central Colorado for which literature data are available. Transitions from low to intermediate Ta/Th mafic volcanism occurred diachronously across southwest North America and are interpreted to have been a consequence of melting of continental lithospheric mantle previously metasomatized by aqueous fluids derived from the underthrusted Farallon plate. Melting occurred as remnants of the Farallon plate were removed and the continental lithospheric mantle was conductively heated by upwelling asthenosphere. A similar model can be applied to post–40 Ma magmatism in north-central Colorado, with periodic, east to west, removal of stranded remnants of the Farallon plate from the base of the continental lithospheric mantle accounting for the production, and western migration, of volcanism. The estimated depth of the lithosphere-asthenosphere boundary in north-central Colorado (~150 km) indicates that the lithosphere remains too thick to allow widespread melting of upwelling asthenosphere even after lithospheric thinning in the Cenozoic. The preservation of thick continental lithospheric mantle may account for the absence of oceanic-island basalt–like basaltic volcanism (high Ta/Th values of ~1 and εNd[t] > 0), in contrast to areas of southwest North America that experienced larger-magnitude extension and lithosphere thinning, where oceanic-island basalt–like late Cenozoic basalts are common.

A revised stratigraphic model for the ∼ 1910–1835 Ma Tanami Group, the Northern Territory, Australia: Implications for exploration targeting

The Paleoproterozoic Tanami Group of the Granites—Tanami Orogen hosts a known endowment of > 20 Moz of gold mineralization across numerous deposits. Throughout the region, host lithologies impart a first-order control on the style, grade and endowment of mineralization. However, the genetic relationship and regional distribution of three regionally correlative formations (the Dead Bullock, Mount Charles and Stubbins formations) remains enigmatic. Here, we combine lithostratigraphic analysis, lithogeochemistry, detrital zircon geochronology and geophysical observations to propose a new evolutionary model for the Tanami Group. We find that the Tanami Group was deposited in a continental back-arc basin setting and developed in four stages. Package A marked the onset of the rifting of the continental Archean basement. Sedimentary rocks within Package A are dominantly felsic in composition and display a unimodal detrital zircon age population at ∼ 2530 Ma. Package B marks the onset of mafic volcanism; consequently, volcaniclastic and sedimentary rocks within Package B display a distinct mafic geochemical signature. Detritus shed from an Archean basement persisted throughout this period, and a unimodal detrital age component at ∼ 2530 Ma persists. Package C marked the waning stages of voluminous mafic volcanism across the Tanami Basin. Sedimentary volcaniclastic rocks within Package C display a mixed mafic and felsic composition. Detrital zircon age spectra record multiple peaks at ∼ 2500, 3000 and 3350 Ma, which may reflect erosion of older Archean basement domains or changes in paleo-drainage. Package D marked a significant change in the source of detritus entering the basin from dominantly Archean to Paleoproterozoic, with dominant detrital age peaks at ∼ 1860 Ma. Despite significant lithofacies variations, the presented model highlights multiple correlations between the Dead Bullock and Mount Charles formations. The Mount Charles Formation likely represents a volcanic source-proximal, high-energy depocenter. In contrast, the Dead Bullock Formation was deposited in a lower energy setting, distal to volcanic centers. These observations are important to mineral exploration. Notably, the Mount Charles Formation is distributed over a significantly greater area than previously thought, and the under-represented Hangingwall and Footwall sequences of this formation are interpreted to host significant gold mineralization at the Oberon and Groundrush deposits.

Cretaceous Limestone Dolomitization and Argument of Its Mg2+ Sources: Exampling the Bekhme and Qamchuqa Formations in the Zagros Collision Belt

This study unveils new considerations regarding the dolomitization of the Bekhme and Qamchuqa formations (BQFs), two reefal Cretaceous units in the Iraqi Zagros collisional belt. Previous studies proposed the dolomitization of these formations due to the ascent of Mg-rich hydrothermal solutions through deep faults but lacked substantial supporting evidence. This paper present new considerations suggesting that the dolomitization of the BQFs resulted from the Cretaceous environmental conditions and basin paleogeographic setting of the platform on which these formations were deposited. To justify this consideration, seven proofs presented. First is the occurrence of dolomite as vast layers extending tens of kilometers laterally and bounded between limestone beds, which resemble a strata-bound occurrence. Second is the absence of vertical selective dolomitization such as, hydrothermal mineralization, vertical dolomite dykes, hydrothermal vents, tubes, or zones. Third is the barrenness of most of the fault surfaces and their surrounding rocks with selective dolomitization. Fourth is the sharp boundary between the dolomite and limestone layers, indicating a sudden environmental change from lagoon to reef or forereef. Fifth is the record of dolomitization in the sediments reworked by bioturbation, evident in thalassinoides burrows. Sixth is the succession of BQFs underlain by limestone and green marl devoid of dolomitization and hydrothermal upward movement. Seventh is the contradictory notion of hydrothermal dolomitization among the most recent study published in the Science journal which atributed dolomitization to normal temperate on earth surface. In the present study, the source of the Mg is argued and its presence is attributed to the warmth of the Arabian platform (plate) near the equator during the Cretaceous period. Another source is the connection of the Arabian platform with the Neo-Tethys Sea, which was the locus of Mg-rich mafic volcanism in the form of arcs and mid-oceanic ridge topography, which supplied a voluminous amount of Mg to the Arabian carbonate platform through ocean currents.

Evolution of a large Quaternary monogenetic field; the multifaceted volcanism of the Serdán-oriental basin, México

The intermontane Serdán-Oriental basin (SOB), located in the Trans-Mexican Volcanic Belt, records a highly contrasting volcanic field with nearly 166 monogenetic volcanoes, including cones, maars, tuff rings, domes, and lava flows, with basaltic to rhyolitic compositions and from effusive to explosive, and magmatic to phreatomagmatic eruptive styles. Even when this volcanic field has been the focus of several volcanological studies, an evolutive model of its volcanism and its implications has not been reported. This is a geological study of SOB volcanism based on the spatio-temporal evolution and characteristics of its volcanoes. For this, new and published data is integrated, including chronostratigraphic, geochemical, and petrological observations, which were further processed and analyzed by GIS tools. The resulting model indicates that volcanism has been continuous from the middle Pleistocene to the Holocene. Based on parameters such as erupted magma chemistry and volcanic landforms, it can be classified into the following volcanic groups: Group 1 is mafic in composition, forming cones and lava flows during the middle Pleistocene (490 to 190 ka). Group 2 comprises predominantly andesitic volcanism characterized by cones and dome clusters erupted during the middle Pleistocene (250 to 200 ka), partially coeval with Group 1. Group 3 is characterized by rhyolitic volcanism associated with the emplacement of domes in the central part of the SOB during the middle to upper Pleistocene (220 to 24.5 ka). Group 4 includes a predominantly basaltic to andesitic volcanism with minor rhyolitic composition, with a characteristic occurrence of phreatomagmatic and magmatic activity, forming maars and tuff rings, scoria cones, and lava flows during the middle Holocene (12.2 to 6.25 ka). The youngest Group occurs in the upper Holocene (<7.5 ka), and it is marked by mafic volcanism in the form of cones and lava flows, as well as by the emplacement of the voluminous Las Derrumbadas rhyolitic twin domes. Overall, the geochemical data shows a bimodal compositional character with a gap of dacitic magmas, compatible with a general scenario dominated by felsic melt extraction from highly crystalline magma reservoirs. Eruptive rate calculations indicate a general increase with time, which goes from <0.18 km3/ky in the first three groups to 1 and 2.9 km3/ky in groups 4 and 5, respectively. Lastly, density maps and lineaments of the eruptive vents, together with crater elongations, allowed the identification of the spatial trends of the SOB volcanism and a comparison with the existing regional major tectonic structures characterizing the local basement. These findings are relevant to understanding the eruptive nature of this active volcanic field and its highly contrasting and multifaceted volcanism.

Origin of Early Cretaceous mafic volcanic rocks from the Erlian Basin west of the Great Xing’an Range of North China: Implications for the tectono-magmatic evolution of East Asia

Early Cretaceous intraplate volcanic rocks are widespread in NE Asia, but their origin remains controversial. This work presents zircon U-Pb ages, whole-rock element and Sr-Nd isotope data for mafic volcanic rocks from the Erlian Basin, a wide rift basin in NE Asia. There were two episodes of Early Cretaceous mafic volcanism in the Erlian Basin, and the eruptions show contrasting geochemical compositions. The early mafic volcanic rocks, with U-Pb ages of ca. 140−135 Ma, show slightly depleted Sr-Nd isotope compositions (ISr(t) = 0.7042−0.7052; εNd(t) = +0.82 to +3.0) and arc-like trace-element compositions, which are derived from subduction-related fluid/melt metasomatized lithosphere mantle. The late mafic volcanic rocks (dated at ca. 125 Ma) have enriched Sr-Nd isotopes (ISr(t) = 0.7055−0.7077; εNd(t) = −0.50 to −2.67) and oceanic-island basalt (OIB)-like trace-element compositions, revealing the metasomatism of melts from crustal materials and asthenosphere mantle. The two types of mafic volcanic rocks may record the interactions of the mantle and melts from the subducted paleo-Pacific oceanic slab at different depths. The landward-then-oceanward migration pattern of the Mesozoic volcanism from NE Asia can be explained by the flat subduction and subsequent slab roll-back of the Paleo-Pacific Ocean, consistent with migration patterns from the North China Craton and South China Block, implying similar Jurassic−Cretaceous subduction evolution along the entire East Asia margin. Some Late Jurassic to Early Cretaceous dates from east Mongolia and the southern margin of the Erlian Basin diverge from this trajectory. In combination with previous studies, we suggest that the Early Cretaceous pervasive intraplate volcanism in the Erlian Basin and adjacent areas of NE Asia mainly resulted from the slab roll-back of the Paleo-Pacific Ocean with a combined effect from the post-collision extension of the Mongol-Okhotsk orogen.

Geodynamic Controls on Basaltic Volcanism in the Arabian Peninsula: Evolution of Harrat Uwayrid, Saudi Arabia

AbstractBasaltic lavas from Harrat Uwayrid, Saudi Arabia, record the evolving magmatic and tectonic context of the Arabian Peninsula from at least the mid‐Miocene to the present day. New 40Ar/39Ar ages spanning from the mid to late Miocene reveal that mid‐Miocene mafic volcanism formed a large, subalkaline volcanic plateau parallel to Red Sea rifts. Subsequent volumetrically subordinate late Miocene‐Quaternary alkaline volcanism erupted monogenetic cinder cones roughly orthogonal to the earlier volcanic field. The source region for all samples was affected by both fluid and silicate metasomatism; inferred mantle mineral assemblages include amphibole for mid‐Miocene lavas and phlogopite for late Miocene‐Quaternary samples. Calculated melting depths become shallower with time across the Miocene volcanic episode (∼20–15 Ma) but become deeper in the late Miocene to Quaternary (∼10–0 Ma), indicating melting pressures and temperatures significantly higher than those recorded in Miocene lavas despite progressive lithospheric thinning. We offer a two‐stage model for the formation of Harrat Uwayrid: (a) Early‐ and mid‐Miocene rifting associated with the Red Sea opening facilitated adiabatic melting of uppermost mantle lithosphere to form the early volcanic plateau and (b) Plate motion changes in the mid‐ and late‐Miocene initiated the Dead Sea Fault and destabilized a dense pyroxenitic lower lithosphere leading to foundering or lithospheric drip beneath Harrat Uwayrid that allowed deep lithospheric melting and formed the young volatile‐rich eruptives.

A c. 1900 Ma Tethyan-type ophiolite in the Penokean Orogen, Pembine, Wisconsin (USA): Insights from the volcanic stratigraphy

Orogenic ophiolites are a hallmark of Phanerozoic plate tectonics, containing igneous lithologies that provide constraints on fundamental tectono-magmatic processes. The c. 1900Ma Pembine Ophiolite (Wisconsin, USA) is associated with the Penokean Orogen and represents a rare example of a proposed Paleoproterozoic ophiolite. The Penokean Orogen shares broad characteristics with Phanerozoic (<541 Ma) orogens, but the origin of the Pembine Ophiolite remains unclear, with the mafic volcanic rocks interpreted as representing either an intra-oceanic arc or continental back arc setting. To test these hypotheses, we present the results of petrography, bulk-rock geochemistry and mineral chemistry for a suite of 34 Pembine rocks, as well as U-Pb zircon geochronology for two samples. Based on trace elements established as immobile in the studied rocks, we demonstrate that mafic volcanism progressed (up-stratigraphic-section) from mid-ocean ridge-like to boninitic. The chemical evolution is identical to that observed in < 250 Ma ophiolites in the Himalayan–Alpine Orogen, which record forearc spreading during the nascent stages of subduction in the Tethys Ocean. We interpret the Pembine Ophiolite as forearc lithosphere formed during subduction initiation and obducted to the margin of the Superior Craton during the Penokean Orogeny. The processes responsible for forming (and preserving) this example of a Paleoproterozoic ophiolite may not have been dissimilar to those operating on the Phanerozoic Earth.

The Petrology and Geochemistry of REE-Enriched, Alkaline Volcanic Rocks of Ambitle Island, Feni Island Group, Papua New Guinea

Ambitle in the Feni Island Group is located within the NW trending Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain, Melanesian Arc, northeastern Papua New Guinea. The TLTF chain is renowned for its alkaline magmatism, geothermal activity, copper–gold mineralization, and world-class gold mining. Although its geochemical patterns indicate island arc signatures (i.e., high LILE and depleted HFSE), TLTF volcanism is not directly related to the older Melanesian Arc subduction system. However, it may have been influenced by source mantle metasomatism linked to the older subduction. The purpose of this study is to (1) present and interpret the petrographic, mineralogical, and geochemical data from Feni within the context of the tectonic evolution of the TLTF and (2) propose a geodynamic, petrogenetic model for the Feni volcanic rocks. The key methodologies used in this study are field mapping and sampling, petrographic analysis using the optical microscope, whole-rock geochemical analysis via XRF and ICP MS, and mineralogical analysis using an electron microprobe. The main rock types sampled in this study include feldspathoid-bearing basalt, trachybasalt, phonotephrite, trachyandesite, and trachydacite. Minerals identified include forsteritic olivine, diopside, augite, labradorite, andesine, anorthitic plagioclase, nepheline, and leucite in the primitive mafic suites, whereas the more evolved intermediate and felsic hypabyssal suites contain amphibole, albite, orthoclase, biotite, and either rare quartz or feldspathoids. Amphibole composition is primarily magnesiohastingsite with minor pargasite formed under polybaric conditions. Accessory minerals include apatite, titanite, and Ti-magnetite. We propose that limestone assimilation followed by fractional crystallization are plausible dominant processes in the geochemical evolution of the Ambitle volcanics. Clinopyroxene fractionation is dominant in the mafic volcanics whereas hornblende fractionation is a major petrologic process within the intermediate suites proven by the enrichment of LREE and depletions in MREE and HREE. Feni magmas are also highly enriched in REEs relative to neighboring arcs. This study is globally significant as alkaline magmas are important sources of Cu, Au, and REE as critical elements for green energy and modern technology.

Fate of an oceanic plate in the Neo-Tethys intra-oceanic subduction system: Evidence from elemental and Rb/Sr – Sm/Nd isotopic systematics

We report the coexistence of normal mid-ocean ridge basalt (NMORB)-type and alkaline ocean island basalt (OIB)-type mafic volcanics from the Zildat Ophiolitic Mélange (ZOM), Indus Suture Zone (ISZ), eastern Ladakh Himalaya. Trace element modeling portrays chemically heterogeneous mantle sources, wherein blueschists and sub-alkaline mafic volcanics depict almost flat REE-patterns [(La/Sm)N = 1.04–2.9] and respectively <20 % and >20 % degrees of partial melting from a depleted spinel-lherzolite mantle source. In contrast, alkaline mafic volcanics depict enriched LREE-patterns [(La/Yb)N = 35–48] with <5 % degrees of partial melting from a transitional garnet-spinel lherzolite enriched mantle source. Since the prevalent hypothesis considers seamounts and island arc rocks of the subducting oceanic plate within the Neo-Tethys Ocean to be the protolith for the contemporaneous Shergol Ophiolitic Melange (SOM) blueschists. In contrast, ZOM blueschists depict different protolith signatures. The εNd(t = 140 Ma) values for ZOM blueschists (+3.7 to +7.4) and sub-alkaline mafic volcanics (+9.2 to +9.3) are analogous to present-day Indian Ocean NMORB, while for OIB-type mafic volcanics (+3.7 to +4.3) are comparatively lower and analogous to Hawaii and Kerguelen OIB. Therefore, the observed protolith decoupling in the contemporaneous blueschists suggest the involvement of different protolith contenders, which were partially off-scraped from of the subducting plate during prolonged deep subduction followed by exhumation along the ISZ.

Oceanic intraplate explosive eruptions fed directly from the mantle

Constraining the volatile content of magmas is critical to our understanding of eruptive processes and their deep Earth cycling essential to planetary habitability [R. Dasgupta, M. M. Hirschmann, Earth Planet. Sci. Lett. 298, 1 (2010)]. Yet, much of the work thus far on magmatic volatiles has been dedicated to understanding their cycling through subduction zones. Further, studies of intraplate mafic volcanism have disproportionately focused on Hawaii [P. E. Wieser et al., Geochem. Geophys. Geosyst. 22, e2020GC009364 (2021)], making assessments of the overall role of intraplate volcanoes in the global volatile cycles a challenge. Additionally, while mafic volcanoes are the most common landform on Earth and the Solar System [C. A. Wood, J. Volcanol. Geotherm. Res. 7, 387-413 (1980)], they tend to be overlooked in favor of silicic volcanoes when it comes to their potential for explosivity. Here, we report primitive (olivine-hosted, with host Magnesium number - Mg# 78 to 88%) melt inclusion (MI) data from Fogo volcano, Cabo Verde, that suggest that oceanic intraplate silica-undersaturated explosive eruptions sample volatile-rich sources. Primitive MI (melt Mg# 70 to 71%) data suggest that these melts are oxidized (NiNiO to NiNiO+1) and very high in volatiles (up to 2 wt% CO2, 2.8 wt% H2O, 6,000 ppm S, 1,900 ppm F, and 1,100 ppm Cl) making Fogo a global endmember. Storage depths calculated from these high volatile contents also imply that magma storage at Fogo occurs at mantle depths (~20 to 30 km) and that these eruptions are fed from the mantle. Our results suggest that oceanic intraplate mafic eruptions are sustained from the mantle by high volatile concentrations inherited from their source and that deep CO2 exsolution (here up to ~800 MPa) drives their ascent and explosivity.

The ~1.1 Ga St. Ignace Island complex, Northern Ontario, Canada: Evidence for magma mixing and crustal melting in the generation of Midcontinent Rift-related bimodal magmas and implications for regional metallogeny

Abstract The St. Ignace Island complex in Northern Ontario is a package of dominantly felsic rocks emplaced within the upper portions of the Osler Volcanic rocks of the ~1.1 Ga Midcontinent Rift System. The Osler volcanic rocks are predominantly tholeiitic basalts intercalated with rare interflow sediments and rhyolites. The St. Ignace Island complex is a ~26 km2 stock with a felsic core of quartz-feldspar-phyric rhyolites and dacites and an outer ring of anorthosite and gabbro. Textures at a variety of scales within the rocks of the complex show clear evidence of the mingling and mixing of partially crystallized mafic and felsic liquids. Two multigrain (zircon/baddeleyite) fractions from a sample of the gabbro define a Discordia line with an upper intercept date of 1107±8.9 Ma. The core of the complex consists of dacites and rhyolites with similar REE abundances with negative Nb anomalies whereas the surrounding mafic rocks are gabbros to monzogabbros that are less LREE-enriched than the felsic rocks but with similar HREE. Felsic units have a narrow range of 87Sr/86Sri (0.7032-0.7045) and 143Nd/144Ndi (0.51051-0.51057) whereas the mafic end members have similar 87Sr/86Sri (0.7040-0.7061) but more radiogenic 143Nd/144Ndi (0.51067-0.51085). Very well-preserved silicate melt inclusions (MI), many completely glassy, were observed in quartz, clinopyroxene and some plagioclase phenocrysts from the complex. These represent some of the oldest unrecrystallized silicate melt inclusions described to date. Melt inclusions within quartz from the felsic volcanics are broadly rhyolitic in composition whereas MI from plagioclase in the mafic volcanics range from basalt to basaltic andesite; these felsic and mafic melt compositions are interpreted to represent the end-member liquids in the system and bulk rock analyses affirm mixtures of the two. Concentrations of Cu and Ag (in both mafic and felsic MI), and Mo (in felsic MI), are up to an order of magnitude higher in the mafic and felsic MI than in continental crust. Bulk rock metal concentrations are also significantly lower than in the MI, suggesting that the melt inclusions may preserve pre-eruptive metal tenors that were subsequently modified by sulfide saturation, degassing, or post-solidus hydrothermal alteration. The whole rock and MI geochemistry of the St. Ignace complex are broadly similar to the Central Osler Group and, given the broad similar ages, suggests they may have been derived from a similar mantle source, but distinct from the source of rhyolites in the Black Bay Peninsula. The negative Nb anomalies and negative εNd values for the St. Ignace complex are consistent with mixing with older continental crust during ascent and emplacement. The rocks of the St. Ignace Island complex likely formed as the result of emplacement of a large mafic magma chamber at the base of the Osler volcanic pile that triggered partial melting to generate the rhyolite end members. The felsic melts ascended to shallower levels in the crust where they mixed with mafic magmas derived directly from the deeper chamber. Generally, melt inclusions in the complex have very high Cu and Ag contents, similar to those observed in arc-related and extremely oxidized early rift-related rocks and may account for the world-class volcano-sediment-hosted Cu-(Ag) deposits within the rift and the presence of small porphyry-style deposits.

Continental crust delamination in a retreating subduction zone: A case study in the southern Alxa (Inner Mongolia, China), Central Asian Orogenic Belt

Carboniferous-Permian igneous rocks in the southern Alxa (Inner Mongolia, China) of the southern Central Asian Orogenic Belt exhibit an oceanward migration trend, which provides an ideal area to reconstruct the process of crust-mantle material exchange within a retreating subduction zone. This study presents new geochronological and geochemical data for the Shouji plutons from the southern Alxa to address crust-mantle material exchange process. All these diorite, granodiorite, and K-feldspar granite samples from the Shouji batholith exhibit Early Permian zircon U-Pb ages (ca. 282−270 Ma). These diorite and granodiorite are characterized by high Sr contents as well as low Y, and Yb concentrations, coupled with distinctively high Sr/Y values. They exhibit fertile Nd-Hf isotopic compositions with negative whole-rock εNd(t) values, and zircon εHf(t) values, similar to those of continental crust-related adakites, which might originate from partial melting of lower crust, and followed by melt-mantle interactions. These K-feldspar granite samples display peraluminous and high-K calc-alkaline compositions, and negative zircon εHf(t) and whole-rock εNd(t) values, and originated from partial melting of Paleoproterozoic igneous rocks, with additions of mantle-derived melts in their sources, in low-temperature, low-pressure conditions. Previously published zircon U-Pb age data show that numerous mafic igneous rocks were mostly generated during the Early Permian, marking the onset of mafic volcanism in the southern Alxa. Moreover, compiled detrital zircon U-Pb age data for Late Carboniferous−Permian sedimentary succession imply a regional topographic uplift during the Early Permian. Both of these two lines of evidence suggest an Early Permian lower crustal delamination in the southern Alxa. These Shouji adakitic plutons were most likely to be generated during the lower crustal delamination. Furthermore, Sr-Nd isotope data of Carboniferous-Permian igneous rocks show that the continental crust was recycled back into the mantle during Early Permian within the southern Alxa by means of subduction erosion and lower crustal delamination.

Tectonic Implication of the Permo-Triassic Khao Yai Mafic Volcanic Rocks in Nakhon Nayok Province, Thailand: Evidence from Geochemistry and Rare Earth Elements (REEs)

The Permo-Triassic Khao Yai Volcanics in Nakhon Nayok Province, Thailand, is a part of the Loei-Phetchabun-Nakhon Nayok Volcanic Belt. The purpose of this study is to clarify the geochemistry and REEs characteristic of Khao Yai Volcanics that is useful for identifying the paleo-environment or tectonic setting eruption in this area. The least-altered, mafic volcanic rocks from the Permo-Triassic Khao Yai volcanics, are seriate-textured to porphyritic, with variable amounts of phenocrysts/microphenocrysts. The mineral compositions include plagioclase, clinopyroxene, orthopyroxene, olivine, minor Fe-Ti oxide mineral, amphibole, apatite, biotite/phlogopite, monazite/zircon, glassy, and quartz. The geochemical characteristic of the Khao Yai Volcanics informed that the rocks had the same parental magma with different degrees of crystal fractionation. Most of the mafic volcanic rocks are subalkalic andesite on the basis of their Zr/TiO2 and Nb/Y ratios and calc-alkalic on diagrams Ti-Zr, Ti-Zr-Y, Hf-Th-Ta and Y-La-Nb. The least-altered mafic volcanic rocks have (La/Sm)cn and (Sm/Yb)cn ranging from 2.41 to 2.71 and 1.86 to 2.22, respectively. The studied calc-alkalic andesite are analogous to the Quaternary calc-alkalic dacite and andesite from Maca Volcano, Patagonian Andes and the Middle Eocene andesite from Shimokoh cauldron, SW Japan in terms of chondrite-normalized REEs and N-MORB normalized patterns. Accordingly, the studied Khao Yai volcanics have been developed in an active continental margin, formed by eastward underthrusting Paleo-Tethys, the leading edge of Shan-Thai, beneath Indochina in the Late Permian to Early Triassic. HIGHLIGHTS New tectonic model of the Permo-Triassic Khao Yai mafic volcanic rocks in Nakhon Nayok Province, Thailand The geochemistry and rare earth elements (REEs) analysis by XRF and ICP-MS The REEs analog to modern volcanic rock The classification of pre-Jurassic volcanic belt in Thailand GRAPHICAL ABSTRACT

Early Paleozoic Cenerian (Sardic) geodynamic relationships of peripheral eastern north Gondwana affinities: revisiting the Ordovician of the Getic/Kučaj nappe (eastern Serbia)

Regional tectonic synthesis suggests that a segment of the bipartite eastern Gondwana-type Carpathian-Balkan nappe-stacked Getic/Kučaj/Supragetic basement (eastern Serbia) experienced Cambro-Ordovician Cenerian (Sardic) accretionary-type deformation. The Ordovician basement of the Alpine Getic/Kučaj nappe exposes an earlier-mapped shallow-marine transgressive-type Fe-silicate-rich ironstone sequence. The Ordovician ironstone is used as second-order evidence of a hitherto untraceable tectonically-driven unconformity. Early Paleozoic compression is consistent with the controversial latest Cambrian to intra-Ordovician Cenerian (Sardic) interval, documented by (i) a 488 Ma metamorphic event and available detrital zircon data (Serbo-Macedonian gneissic unit), (ii) a deformed Lower Ordovician Getic/Kučaj brachiopod assembly, and (iii) an intra-Ordovician unconformity dividing the Supragetic basement/”Vlasina complex”. The data further imply that mafic gabbro-dominating sills, cropping out in the northern Getic/Kučaj unit, are consistent with Ordovician back-arc activity. The Getic/Kučaj gabbro is Ordovician in age, piercing a Neoproterozoic–Cambrian (Lower Ordovician) Supragetic/”Vlasina complex”, overlain by a transgressive Silurian–Devonian sedimentary sequence. The emergence of Ordovician mafic intrusions reflects submarine volcanism, while deep-water redox conditions were capable of a sustained supply of Fe (similar to Sardinia). In terms of tectono-palaeogeographic reconstructions, the origin of Ordovician shortening and mafic volcanism is often challenged. The latter is broadly analogous with the embryonic eastern Rheic Ocean, corresponding additionally to the Armorican spur and related intra-continental magmatism.

Evidence of rare earth elements origin in acid mine drainage from the Iberian Pyrite Belt (SW Spain)

Acid mine drainage (AMD) is a worldwide pollution problem of watersheds. In addition to toxic metal(oid)s and acidity, many elements of economic interest are released into the environment, which make AMD a potential strategic secondary source of these elements such as rare earth elements (REE). Despite the importance of these metals, their origin in AMD is still uncertain. Recent hypotheses suggest preferential leaching of REE-enriched minerals as a possible source. Leaching tests with H2SO4 have been developed to simulate the interaction under AMD formation conditions with sulfide bodies and host rocks from two representative mining areas in the Iberian Pyrite Belt: the Perrrunal and Poderosa mines (SW of Spain). The REE patterns and Ce and Eu anomalies of the rock leachates have confirmed the geochemical relationship between the AMD and certain country rocks (felsic and mafic volcanics, and shales). A detailed chemical and mineralogical study has confirmed the existence of a diversity of minerals with high concentrations of REE. Thus, the minerals with the highest REE contents are also those with the fast dissolution kinetics under acid conditions: REE phosphates (monazite and xenotime type) and carbonates (parisite type). Finally, petrographic evidence of the selective leaching of these minerals clearly supports these minerals as the main source of REE in the AMD.

Geological evolution of the Proterozoic Betul belt (∼2.16–0.95 Ga) of the Central Indian tectonic Zone: Its linkage to the assembly and dispersal of Columbia and Rodinia supercontinents

The Proterozoic Betul Belt is an important supracrustal unit that records the protracted history of crustal accretion and growth in the Central Indian Tectonic Zone (CITZ). Here, we present results from systematic geological mapping, petrological, geochemical (including Sm-Nd isotope) studies, and U-Th-Pb geochronology on rocks from the Betul belt, with the objective of reconstructing its Proterozoic evolution and its link with past supercontinents. The Betul belt comprises ca. 2167 ± 11 Ma granite gneisses, ca. 2051 ± 80 Ma mafic volcanics (pillowed lava), 1715 ± 10 Ma rhyolite, and older granitoids (1671 ± 29 Ma) that have undergone multi-stage deformation and amphibolite-facies regional metamorphism. The bulk rock geochemistry of gneisses, pillow lava, and mafic intrusions are characterized by arc-like signatures, whereas the rhyolite and granites are anorogenic and back-arc related. The whole-rock geochemical and Sr-Nd isotopic compositions of pillow lavas and mafic intrusions suggest the involvement of recycled crustal material in their source, possibly in the form of associated sediments. The Betul granite gneisses were emplaced at 2167 Ma in a collisional setting by melting crustal protoliths having an affinity with the Bundelkhand craton and may constitute the basement for the supracrustal sequence. The supracrustal units, i.e., mafic volcanics and rhyolites, occur as a bimodal suite formed in the arc and back-arc environment with penecontemporaneous sedimentation. Bulk rock geochemistry and age data indicate two phases of A-type granite magmatism at 1674 Ma and 1079 Ma, with post-collisional geochemical traits forming within an anorogenic and back-arc setting, respectively. Younger post-orogenic granites (Navegaon) were emplaced in a back-arc setting at ca 1079–954 Ma, synchronous with a collisional event in the Sausar belt. Spot ages from monazite in Hbl gabbro yield a weighted mean of 1026 ± 97 Ma (MSWD = 0.15), reflecting Grenville-age regional metamorphism in the CITZ. The geological events recorded in the rocks of the Betul belt between ca 2167 Ga and 954 Ma correlate well with events that affected Laurentia, Baltica, Australia, and East Antarctica within the frameworks of the Columbia (ca. 2.1–1.7 Ga) and Rodinia (ca.1.2–0.9 Ga) supercontinents.

Petrogenesis and tectonic setting of TTG rocks from the Katoro area, Sukumaland Greenstone Belt, NW Tanzania: Evidence from zircon U-Pb geochronology, Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd isotopes

The northern part of the Tanzania Craton is mainly comprised of several low-grade metamorphic granite-greenstone belts that are exposed on the east and south shores of Lake Victoria. Due to their gold-endowment, these greenstone belts have been the main focus of research and were interpreted to form in various tectonic settings involving back-arc basin, island arc or continental arc, and oceanic plateau. However, the TTG gneiss, as the dominant component of Archean crust, has received far less attention from the scrutiny of modern analytical methods than its counterpart, the mafic volcanics of the greenstone belts. Here, we present a comprehensive geochemical dataset including zircon U-Pb age, zircon Lu-Hf isotope results, as well as bulk-rock geochemical compositions and Sm-Nd isotopic analyses for TTG suites from the Katoro area in northwestern part of Tanzania. Zircon U-Pb dating for three TTG samples yielded 207Pb/206Pb ages of 2688 ± 19 Ma, 2717 ± 19 Ma and 2695 ± 15 Ma, respectively, and the results fall within the peak crustal growth period in northern part of the Tanzania Craton. The Katoro TTGs have relatively high contents of SiO2 (70.77–74.69 %) and Na2O (4.74–5.16 %), high ratios of Sr/Y (59.49–167.25) and (La/Yb)N (17.27–50.72), but low MgO (0.24–0.70 %) and K2O (1.71–3.26 %) contents, low Nb/Ta ratios (8.50–16.60), and most of them belong to the trondhjemitic series rocks. They are characterized by strong enrichment of LILEs and LREEs with slightly positive Eu anomalies (Eu/Eu*=1.05–1.38), but depleted in Nb-Ta and Ti, as well as heavy REEs and Y. They have positive εHf(t) values (average εHf(t) = +1.90 ± 0.50 or + 2.07 ± 0.33) and εNd(t) values (+1.19 to + 2.40) but lower than the contemporaneous depleted mantle, which is inconsistent with a subducting slab signature, and it is proposed that the Katoro TTGs most likely originated from partial melting of rutile-free, garnet- and amphibole-bearing lower mafic crust of an oceanic plateau. The results presented here suggest that the tectonic evolution in this part of Tanzania Craton needs to invoke a scenario in which crustal growth caused by subduction-related modification of an oceanic plateau.

Constraining the crustal composition of the continental U.S. using seismic observables

The composition of the crust is one of the most uncertain and controversial components of continental estimates due to (1) limited direct access and (2) inconsistent indirect assessments. Here we show that by combining high-resolution shear velocity (Vs) models with newly measured with newly measured ratio of compressional wave velocity (Vp) and Vs, or Vp/Vs ratio, for the crystalline crust, a 3-D composition (SiO2 wt%) model of the continental crust can be derived with uncertainty estimates. Comparing the model with local xenolith data shows consistency at mid and lower crustal depths. The spatial patterns in bulk SiO2 content correlate with major geological provinces, including the footprints of Cenozoic and Mesozoic mafic volcanism in the western U.S., and offer new insight into the composition and evolution of the continental U.S.

Petrogenesis and tectonic implications of Cenozoic mafic volcanic rocks in the Kahak area of central Urumieh–Dokhtar magmatic arc, Iran

• Mafic volcanic rocks in the Kahak area were erupted at ∼ 60–55 Ma and ∼ 21 Ma. • The Kahak basaltic rocks are divided into two groups of low and high Nb/Ta ratios. • The petrogenesis of the Kahak basaltic rocks involved slab dehydration and melting. • Slab rollback and mantle metasomatism occurred during the Paleocene–Eocene and Miocene . Cenozoic volcanic rocks are widely distributed in the Urumieh–Dokhtar magmatic arc (UDMA), Iran, but their petrogenesis and tectonic implications remain enigmatic. The volcanic rocks in the Kahak area of the central UDMA are mostly basalt and basaltic andesite. They show enrichment in light rare earth elements and large ion lithophile elements and depletion in the high-field-strength elements (Nb, Ta, and Ti), corroborating a subduction-related origin. U–Pb ages of zircon from these volcanic rocks demonstrate two episodes of magmatism during the Paleocene–Eocene (∼60–55 Ma) and Miocene (∼21.9–21.4 Ma). These rocks have 87 Sr/ 86 Sri values from 0.70485 to 0.70577, ɛ Nd (t) values from +2.0 to +5.0, and zircon Hf isotopes ε Hf (t) values from +8.3 to +10.2. These features with isotopic modeling support derivation from partial melting of an asthenospheric mantle wedge in an extensional intracontinental-arc basin. This mantle source was previously metasomatized by subducted slab components of the Neotethys Ocean. The mafic melts underwent fractional crystallization and minor crustal contamination during magma evolution. The Paleocene–Eocene basaltic rocks have subchondritic Nb/Ta ratios, indicating that the magma source was affected by fluids with low-Nb/Ta ratios, which were released during shallow metamorphism of a subducted slab. However, the Miocene basaltic rocks have unusual superchondritic Nb/Ta ratios, indicating slab melt involvement. Geochemical data and field relationship indicate that mafic volcanism in the Kahak area occurred in an extensional setting associated with the Paleogene shallowing of the northeastward-subducting Neotethyan oceanic lithosphere followed by Late Paleogene and Neogene slab rollback and eventual partial melting of a deeply subducted oceanic slab.