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The conditions under which halogens partition in favor of an exsolved fluid relative to the coexisting melt are key for understanding many magmatic processes, including volcanic degassing, evolution of crustal melt bodies, and ore formation. We report new F, Cl, and Br fluid/melt partition coefficients for intermediate to silicic melts, for which F and Br data are particularly lacking; and for varying CO2-H2O contents to assess the effects of changing fluid composition (XH2O) on Br fluid/melt partitioning for the first time. The experiments were conducted at pressures 50–120 MPa, temperatures 800–1100 °C, and volatile compositions [molar XH2O = H2O/(H2O +CO2)] of 0.55 to 1, with redox conditions around the Nickel-Nickel Oxygen buffer (fO2 ≈ NNO). Experiments were not doped with Cl, Br, or F and were conducted on natural crystal-bearing volcanic products at conditions close to their respective pre-eruptive state. The experiments therefore provide realistic constraints on halogen partitioning at naturally occurring, brine-undersaturated conditions. Measurements of Br, Cl, and F were made by Secondary Ion Mass Spectrometry (SIMS) on 13 experimental glass products spanning andesite to rhyolitic compositions, together with their natural starting materials from Kelud volcano, Indonesia, and Quizapu volcano, Chile. Fluid compositions were constrained by mass balance. Average bulk halogen fluid/melt partition coefficients and standard deviations are: DClfluid/melt = 3.4 (±3.7 1 s.d.), DFfluid/melt = 1.7 (±1.7), and DBrfluid/melt = 7.1 (±6.4) for the Kelud starting material (bulk basaltic andesite), and DClfluid/melt = 11.1 (±3.5), DFfluid/melt = 0.8 (±0.8), and DBrfluid/melt = 31.3 (±20.9) for Quizapu starting material (bulk dacite). The large range in average partition coefficients is a product of changing XH2O, pressure and temperature. In agreement with studies on synthetic melts, our data show an exponential increase of halogen Dfluid/melt with increasing ionic radius, with partitioning behavior controlled by melt composition according to the nature of the complexes forming in the melt (e.g., SiF4, NaCl, KBr). The fundamental chemistry of the different halogens (differing ionic size and electronegativities) controls the way in which partitioning responds to changes in melt composition and other variables. Experimental results confirm that more Cl partitions into the fluid at higher bulk Cl contents, higher melt Na, higher fluid XH2O ratios, and lower temperatures. Bromine shows similar behavior, though it seems to be more sensitive to temperature and less sensitive to Na content and XH2O. In contrast, F partitioning into the fluid increases as the melt silica content decreases (from 72 to 56 wt% SiO2), which we attribute to the lower abundance of Si available to form F complexes in the melt. These new data provide more insights into the conditions and processes that control halogen degassing from magmas and may help to inform the collection and interpretation of melt inclusions and volcano gas data.

The late Permian to Early Triassic volcano-sedimentary succession associated with the Emeishan Large Igneous Province (LIP) is well preserved in the Binchuan area, Southwest China. This paper presents coupled zircon U-Pb age, trace element, and Hf isotope analyses from the succession, together with whole-rock geochemical compositions, to reveal the origin and evolution of volcanic rocks of the LIP. Zircons from the matrix and a rhyolite clast in the Lower Triassic conglomerate bed yield identical U-Pb ages of ca. 260 Ma and have geochemical affinities to those crystallized from within-plate magmas. These features, combined with the dominance of rhyolite clasts from this horizon, imply a source related to Emeishan silicic volcanic rocks. Zircons from silicic ignimbrite and rhyolite in the upper volcanic succession of the LIP display high eHf(t) values (+4.2 to +12.9), low Th/Nb and U/Yb ratios, and similar eNd(t) values (−0.17 to +0.47) to high-Ti basalts. These chemical and isotopic characteristics are consistent with the eroded silicic volcanic rocks, indicating that the late-stage silicic volcanic rocks were generated by fractionation of high-Ti basaltic magmas without significant crustal contamination. Although the late Permian zircons from the lower and upper low-Ti basalt successions all show arc-like geochemical characteristics with high Th/Nb and U/Yb ratios, the former has much lower eHf(t) values (−11.7 to −3.6) than the latter (+4.4 to +11.6), and the zircons from the upper low-Ti basalt succession show positive eHf(t) values similar to the studied rhyolites. Samples from the lower low-Ti basalt succession have relatively higher eNd(t) values (−2.66 to +3.38) than those from the upper low-Ti basalt succession (−4.48 to −0.82). These geochemical features indicate the early-stage low-Ti basalts may be mainly derived from a previously enriched subcontinental mantle lithosphere, whereas the late-stage low-Ti basalts may be generated from the increasing involvement of asthenospheric mantle source with crustal contamination.

As ascending magmas undergo cooling and crystallization, water and fluid-mobile elements (e.g., Li, B, C, F, S, Cl) become increasingly enriched in the residual melt until fluid saturation is reached. The consequential exsolution of a fluid phase dominated by H2O (magmatic volatile phase or MVP) is predicted to occur early in the evolution of long-lived crystal-rich “mushy” magma reservoirs and can be simulated by tracking the chemical and physical evolution of these reservoirs in thermomechanical numerical models. Pegmatites are commonly interpreted as the products of crystallization of late-stage volatile-rich liquids sourced from granitic igneous bodies. However, little is known about the timing and mechanism of extraction of pegmatitic liquids from their source. In this study, we review findings from thermomechanical models on the physical and chemical evolution of melt and MVP in near-solidus magma reservoirs and apply these to textural and chemical observations from pegmatites. As an example, we use a three-phase compaction model of a section of a mushy reservoir and couple this to fluid-melt and mineral-melt partition coefficients of volatile trace elements (Li, Cl, S, F, B). We track various physical parameters of melt, crystals, and MVP, such as volume fractions, densities, velocities, as well as the content in the volatile trace elements mentioned above. The results suggest that typical pegmatite-like compositions (i.e., enriched in incompatible elements) require high crystallinities (>70–75 vol% crystals) in the magma reservoir, at which MVP is efficiently trapped in the crystal network. Fluid-mobile trace elements can become enriched beyond contents expected from closed-system equilibrium crystallization by transport of MVP from more-evolved mush domains. From a thermomechanical perspective, these observations indicate that, rather than from melt, pegmatites may more likely be generated from pressurized, solute-rich MVP with high concentrations of dissolved silicate melt and fluid-mobile elements. Hydraulic fracturing provides a mechanism for the extraction and emplacement of such pegmatite-generating liquids in and around the main parental near-solidus mush as pockets, dikes, and small intrusive bodies. This thermomechanical framework for the extraction of MVP from mushes and associated formation of pegmatites integrates both igneous and hydrothermal realms into the concept of transcrustal magmatic distillation columns.

Historical mine and mineral deposit datasets are routinely used to inform quantitative mineral assessment models, but they also can contain a wealth of supplementary qualitative information that is generally underutilized. We present a workflow that uses correspondence analysis, an exploratory tool commonly applied to multivariate abundance data, to better utilize qualitative data in these historical datasets. The workflow involves extraction of qualitative information on ore mineralogy from a mineral deposit database, attaches those data to a target geological feature, and analyzes the underlying data structure with correspondence analysis and hierarchical clustering. The output of correspondence analysis is inversely weighted to the relative frequency of ore minerals, and therefore rare mineral species (i.e., those with unusually low frequencies) can disproportionately contribute to the total variance of the dataset. We present a novel technique for aggregating frequencies of rare mineral species that minimizes this effect. We apply this workflow to evaluate how ore mineral assemblages in former and active mines vary in spatial relation to silicic calderas in the southwestern United States. The most common ore mineral associations observed spatially and genetically associated to calderas include those related to polymetallic, base metal-rich systems and epithermal Au–Ag systems. Three other groups of mineralized calderas were identified, including: (1) Hg–Sb mineralized calderas in the northern Great Basin and western Nevada volcanic field; (2) calderas associated with elevated abundances of Mn oxides/hydroxides, fluorite, and Be-minerals, mostly in eastern Utah and New Mexico; and (3) calderas with numerous U ± F deposits, which are located in central Colorado, the eastern Great Basin and in northern Nevada. The latter three groups are associated with economically significant critical mineral resources, including the Li resources of the McDermitt complex and Be associated with the Spor Mountain on the margin of the Thomas caldera complex. We conclude that correspondence analysis is a promising technique that can enhance data exploration of the qualitative information held within mineral deposit datasets. Consequently, it could have numerous applications for mineral potential mapping, resource assessment projects, and characterization of mineral systems.

Voluminous silicic (rhyolitic–dacitic) eruptive units overlie mafic lavas in many continental flood basalt (CFB) provinces of the world. These silicic units comprise lavas as well as pyroclastic deposits, including lava-like ignimbrites, and many were erupted from identified calderas. Silicic volcanic units are widespread in the northern and northwestern parts of the Deccan Traps CFB province of India, particularly the Saurashtra peninsula. In the Alech Hills in Saurashtra, whose geology is poorly known, basal mafic lavas are intruded by a ring dyke of granophyre with a diameter of 30 km. Inside this ring, silicic eruptive units are exposed. The lowest and most extensive unit is a grey, feldspar-phyric, ≥ 3.8 km3 flood rhyolite lava. This is overlain successively by rheomorphic and lava-like ignimbrites, eutaxitic ignimbrites, and non-welded tuffs and breccias. Based on a combination of geological features, such as the large granophyre ring dyke, the effusive and explosive rhyolite volcanism, fault-controlled arcuate and annular ridges of the rhyolites, possible caldera-collapse mesobreccias, and widespread hydrothermal alteration, we recognize the Alech Hills as a giant (30 km-diameter, 660 km2) effusive-explosive silicic caldera. A recently published CA-ID-TIMS 206Pb/238U date of 65.765 ± 0.018 Ma (2σ) for the granophyre ring dyke suggests that the Alech Hills caldera formed contemporaneously with the K/Pg boundary mass extinction, and may have significantly contributed to it.

The main phase of silicic volcanism from the Afro-Arabian large igneous province preserves some of the largest volcanic eruptions on Earth, with six units totaling >8,600 km3 dense rock equivalent (DRE). The large volumes of rapidly emplaced individual eruptions present a case study for examining the tempo of voluminous silicic magma generation and emplacement. Here were report high-precision 206Pb/238U zircon ages and show that the largest sequentially dated eruptions occurred within 48 ± 34 kyr (29.755 ± 0.023 Ma to 29.707 ± 0.025 Ma), yielding the highest known long-term volumetric extrusive rate of silicic volcanism on Earth. While these are the largest known sequential silicic supereruptions, they did not cause major global environmental change. We also provide a robust tie-point for calibration of the geomagnetic polarity timescale by integrating 40Ar/39Ar data with our 206Pb/238U ages to yield new constraints on the duration of the C11n.1r Subchron., Quantifying the tempo of large-volume silicic magma generation and eruption is a long-standing but elusive task. Here we show that the three largest sequentially dated eruptions, totaling >4,300 km3, occurred within 48 ± 34 kyr and yield the highest known long term volumetric extrusive rate of silicic volcanism on Earth.

Vein, stockwork, skarn, and carbonate-replacement Sn deposits commonly contain little or no Cu, but examples rich in Cu are also well known in many Sn belts worldwide. The origin of Sn-Cu deposits in association with fractionated and reduced, ilmenite-series granites of mostly metasedimentary crustal parentage is enigmatic because Cu, in contrast to Sn, normally accompanies oxidized, magnetite-series intrusions ultimately of mantle origin. Decompression mantle melting in back-arc or post-collisional settings generates mafic magmas, which are commonly invoked as the triggers for partial melting of thick overlying metasedimentary crust to form the fractionated, peraluminous, ilmenite-series magmas required to generate any associated Sn mineralization. Therefore, it is reasonable to suppose that oxidized fluids exsolved from these mafic magmas can contribute Cu to the synchronous and coexisting silicic chambers, thereby adding Cu to reduced Sn-bearing fluids and any resultant Sn deposits. Copper could also be released from Cu-bearing immiscible magmatic sulfides, formed during early crystallization of the reduced silicic magmas, as a result of either the introduction of these oxidized fluids and/or magma degassing. These putative mechanisms are restricted to thick metasedimentary crust, whereas in dominantly metaigneous crust the resultant magnetite-series magmas generate Cu deposits. This fundamental crustal influence on Sn versus Cu metallogeny of back-arc or post-collisional settings is responsible for both along- and across-strike transitions from the Central Andean Sn belt to regions displaying partly coeval Cu ± Au mineralization.

The volcanic rocks of the Colider and Roosevelt formations are extensively exposed in the south-central portion of the Amazonian Craton where effusive and pyroclastic rocks have been mapped. Both units, topped by chemical sediments and oceanic facies as rhyolite and andesite lavas, rhyodacite, and porphyritic dacite, with frequent intercalations of pyroclastic and epiclastic deposits. Whole-rock geochemistry for 55 samples of rhyolitic to andesitic composition suggests the involvement of fertile mantle-derived components with E-MORB to OIB compositions. The analyzed rocks display calc-alkaline to shoshonitic affinity consistent with generation related to an active continental margin. The whole-rock Sm-Nd isotope data from selected felsic volcanic rocks of the Colider and Roosevelt formations yield negative initial eNd values between –3 and –9, indicating the predominantly crustal nature of the parental magmas with early Archean to late Paleoproterozoic (ca. 2.5–2.0 Ga) depleted mantle model ages.

The Palaeocene-Eocene (66–41 Ma) hosts the most intense stage of extensive magmatism in the Gangdese belt of southern Tibet. However, the magma sources and tectonic setting of this magmatic belt have been a matter of discussion for decades. In this study, we conducted zircon geochronological and geochemical studies on Eocene gabbroic rocks and granitic rocks from the Jiacha area in the eastern Gangdese belt. Laser ablation-ICP-MS zircon U–Pb dating suggests almost synchronous crystallization ages of 50–47 Ma for both rock types. Geochemically, the gabbroic rocks have low total rare earth elements and high positive zircon eHf(t) values (from +8.4 to +15.0), indicating that the parental magma was derived from an isotopically depleted mantle. The granitic rocks are magnesian calc-alkalic to calcic granites that are characterized by variable zircon eHf(t) values (from +3.0 to +10.8), indicating that the magmas were generated by differentiation of a mantle-derived magma with variable contributions from ancient crustal materials. Furthermore, an enlarged database of 66–41 Ma plutonic rocks and coexisting Linzizong volcanic rocks reveals an evolutionary trend from mafic to silicic rocks. The zircon Hf and Nd isotopic compositions become more evolved with increasing whole-rock SiO2. The whole-rock and isotope geochemistry of the 66–41 Ma plutonic rocks and the Linzizong volcanic rocks indicate an origin and evolution of these rocks in an active continental margin setting. Therefore, we argue that the Gangdese belt was associated with the subduction of the Neo-Tethyan Oceanic plate throughout the Early Eocene, thereby implying that the initial India-Asia collision occurred after ~ 41 Ma.

Structural hydroxyl content of volcanic quartz phenocrysts was investigated with unpolarized Fourier-transform infrared spectroscopy. The phenocrysts originated from five pyroclastic fallout deposits from the Bükk Foreland Volcanic Area (BFVA), Hungary, and two from the AD 1314 Kaharoa eruption (KH eruption), Okataina Volcanic Complex (Taupo Volcanic Zone), New Zealand. All investigated quartz populations contain structural hydroxyl content in a narrow range with an average of 9.3 (±1.7) wt ppm. The earlier correlated horizons in the BFVA had the same average structural hydroxyl content (within uncertainty). Thus, it can be concluded that the structural hydroxyl content does not depend on the geographical distance of outcrops of the same units or the temperature or type of the covering deposit. The rare outlier values and similar structural hydroxyl contents show that the fallout horizons cooled fast enough to retain their original structural hydroxyl content. The similarity of the structural hydroxyl contents may be the result of similar P, T, and x (most importantly H2O and the availability of other monovalent cations) conditions in the magmatic plumbing system just before eruption. Therefore, we envisage common physical–chemical conditions, which set the structural hydroxyl content in the quartz phenocrysts and, consequently, the water content of the host magma (∼ 5.5 wt %–7 wt % H2O) in a relatively narrow range close to water saturation.

Many silicic plutonic rocks of the Late Permian Emeishan large igneous province (ELIP) are spatially and temporally related to oxide ore-bearing layered mafic–ultramafic intrusions. Their association suggests that there may be a petrogenetic link between the two rock types where the layered intrusion represents the cumulate portion of the magma system and the silicic rocks are the residual liquid portion. An alkali feldspar syenite collected in close spatial association with the Hongge-layered mafic–ultramafic intrusion yielded a weighted-mean zircon U–Pb age of 261.5 ± 2.4 Ma with a corresponding weighted-mean eHf(t) value of + 6.5 ± 0.5. The Hongge syenite is alkalic, metaluminous, and ferroan and somewhat similar to other A-type granitoids of the ELIP but, it is composed of ~ 90 vol% alkali feldspar. The rocks have initial 87Sr/86Sr ratios of 0.70570 and 0.70515 and eNd(t) values of − 0.3 and + 1.4. Rhyolite-MELTS and isotopic modeling indicate that the Hongge syenite is likely produced by crystal accumulation of a residual silicic liquid derived from a mafic parental magma that experienced localized crustal contamination during emplacement. Moreover, the age, location, and composition of the Hongge syenite suggests that it is the uppermost portion of the Hongge-layered intrusion and is representative of the residual silicic liquid of the mafic magmas that form the plagioclase-rich cumulate gabbros of the upper zone.

The Masjed Daghi porphyry-epithermal Cu-Au-Mo deposit in the northern Arabian-Eurasian collision zone of the Alborz Magmatic Assemblage, NW Iran, is hosted by an early Miocene quartz monzodiorite to diorite intrusion that intruded Eocene volcanic rocks. Potassic, phyllic, argillic, and propylitic alterations associated with four stages of porphyry mineralization (I to IV) are distinguished. Late high-sulfidation epithermal veins of mainly quartz or quartz-barite are enclosed in concentric zones of advanced argillic, argillic, silicic, and propylitic alterations. Poly-phase brine inclusions from the stage ΙΙ porphyry mineralization have homogenization temperatures (Th) between 305 and 600 oC, with salinity from 30.2 to 73.9 wt% NaCl equivalent. Brines inclusions of stages ΙΙΙ and ΙV have Th from 192 to 466 oC and salinity from 20.6 to 59.2 wt% NaCl equivalent. These brine inclusions were trapped with vapor-rich inclusions, which have Th from 122 to 318 oC and low-moderate salinity of 0.3 to 22.3 wt% NaCl equivalent. Fluid inclusions from quartz and sphalerite in epithermal veins yielded Th ranges of 123–298 °C and 121–218 °C, and salinity ranges of 1.9–12.8 and 1.9–11.2 wt% NaCl equivalent, respectively. The δ34S values of sulfide minerals from stages ΙΙ and ΙΙΙ porphyry mineralization vary from + 0.9 to + 2.3‰, whereas the δ34S values of sulfides from the late epithermal veins range from + 1.2 to -1.1‰. These characteristics are consistent with a similar magmatic source for both the fluids of porphyry mineralization and subsequent high-sulfidation epithermal veins. The Masjed Daghi deposit that represents a telescoped porphyry-epithermal system of copper–gold mineralization in the center and peripherals of the early Miocene intrusive stocks shows both similarities and differences to other Tethyan deposits in the Alpine-Himalayan orogenic belt.

First data on oxygen isotopic composition in phenocrysts in volcanic rocks from Okhotsk–Chukotka volcanic belt (106–78 Ma, North-Eastern Russia) together with Sr and Nd isotopic data gives additional geochemical evidence on belt segmentation. Calculated δ18Omelt in calc-alkaline magmas of belt vary from normal-δ18O (from +5.2 to +6.9‰) consistent with mantle-like signatures of radiogenic isotopes. Low-δ18O (from +4.7 to +4.8‰) silicic magmas were found in Western–Okhotsk segment of the belt interpreted as assimilation of hydrothermally altered rocks from pre-caldera rifted basement. The highest δ18O values were obtained for Chukotka segment of the belt reaching +10.8‰ and these are correlated with elevated 87Sr/86Sr due to assimilation of relatively mature high-δ18O continental crust. Comparison with Kamchatka volcanic belt is performed.

Only three localities of mantle xenoliths are known from all of East Antarctica, occurring at the Jetty Peninsula (Lambert–Amery Rift), Vestfold Hills and Gaussberg volcano. The latter two are spinel-facies peridotites, whereas the Jetty Peninsula rocks also include garnet-spinel lherzolites; all come from Indo-Antarctica. The mantle xenoliths of Jetty Peninsula and Vestfold Hills contain abundant geochemical and mineralogical evidence for multiple enrichment events that are attributed to infiltration of melts and their fluid products. Many of these episodes are spatially related to precursory activity along major trans-lithospheric structures that eventually led to the separation of India from Antarctica. Mantle rocks also occur at Schirmacher Oasis (Dronning Maud Land) and Haskard Highlands (Shackleton Ranges) as blocks tectonically emplaced in high-grade crustal rocks. These show varying degrees of alteration due to reaction with silicic crustal rocks or hydrous fluids: none correspond to unchanged mantle compositions. Geophysical surveys are our only information on the mantle lithosphere beneath the inland ice, and these can be used to infer the locations of thicker lithosphere probably related to cratons by southward extrapolation of coastal geological correlations. Intense local modification of the mantle lithosphere by melt infiltration and fluid movements may influence the large-scale images derived from geophysical data, and may be incorrectly interpreted as homogeneous compositions.

We present new age constraints for igneous rocks and ore-metasomatic formations of the gold deposits in the Akzhal-Boko-Ashalin ore zone. In terms of their ore formation, these deposits correspond mainly to the orogenic type, which generally reflects specific metallogeny of the West Kalba gold-bearing belt in East Kazakhstan. Gold-quartz veins and mineralized zones of the gold-sulphide formation are confined to fractures feathering regional NW-striking and sublatitudinal faults. Their common features include the following: gold-bearing veinlet-disseminated pyrite-arsenopyrite ores that are localized in carbonaceous-sandy-schist and turbidite strata of different ages; structural-tectonic control of mineralization, numerous dikes of medium-basic compositions in ore-control zones; and the presence of post-orogenic heterochronous granite-granodiorite rocks, although their relation to gold-ore mineralization is not obvious. Igneous rocks of the study area have similar ages in a narrow range from 309.1±4.1 to 298.7±3.2 Ma, which is generally consistent with the previously determined age of granitoid massifs of gold-ore fields in East Kazakhstan. A younger age (292.9±1.3 to 296.7±1.6 Ma) is estimated for felsic rocks of the dyke complex. For the ore mineralization, the 40Ar/39Ar dating of sericite from near-ore metasomatites yields two age intervals, 300.4±3.4 Ma and 279.8±4.3 Ma. A gap between of the ages of the ore mineralization and the igneous rocks is almost 20 Ma, which may indicate that the processes of ore formation in the ore field continued in an impulse-like pattern for at least 20 Ma. Nevertheless, this confirms a relationship between the hydrothermal activity in the study area and the formation and evolution of silicic igneous rocks of the given age interval, which belong to the Kunush complex, according to previous studies. This interpretation is supported by reconstructed tectonic paleostress fields, showing that directions of the main normal stress axes changed during the ore mineralization stage, which is why the ore bodies significantly differ in their orientations. The above-mentioned data are the first age constraints for the study area. Additional age determinations are needed to further improve understanding of the chronology of ore-forming processes. Actually, all the features characterizing the gold mineralization of the Akzhal, Ashalin and Dauba ore fields, including the data on lithology, stratigraphy, structural tectonics, magmatism, isotope geochronology, mineralogy and geochemistry, can be used as criteria when searching for similar ore fields in East Kazakhstan.

We present whole-rock geochemistry, LA-ICP-MS zircon U–Pb ages and Hf isotopic compositions of subsurface lithologies in the northwestern corner of Mesa Central, Mexico. Zircon U–Pb ages identify five magmatic events: (1) ca. 162 Ma Nazas volcano-sedimentary province; (2) 88–61 Ma early stage Laramide age magmatism; (3) 57–48 Ma late stage Laramide age rhyodacitic/dacitic magmatism; (4) 46–41 Ma transition period andesitic magmatism; and (5) 32–28 Ma silicic volcanism of the Sierra Madre Occidental silicic large igneous province. A ca. 162 Ma zircon U–Pb crystallization age obtained for a dacitic flow interbedded within the Guanacevi conglomerate yielded generally negative eHf (t) composition varying from −3.3 to +0.8 suggesting magmas derived from crustal source(s). The new geochronological data, lithological features, and stratigraphic position indicate deposition of the conglomerate was synchronous with intermittent volcanism of the Nazas volcano-sedimentary province. Late stage Laramide age (ca. 51 Ma) rhyodacites/dacites yielded largely positive and heterogeneous eHf (t) compositions ranging from −1.6 to +6.1 indicating the dominant role of mantle-derived sources. These ca. 51 Ma rhyodacites/dacites have radiogenic Hf isotopic compositions comparable to those of Late Cretaceous – Early Tertiary Laramide plutons of the southern granite domain suggesting that they share similar subcontinental mantle sources. The 46–41 Ma andesite samples yielded dominantly negative and heterogeneous eHf (t) compositions varying from −4.8 to +4.0. Zircon U–Pb ages, and geochemical and Hf isotopic compositions indicate a rapid shift (~ 2 Ma) from predominantly mantle-derived magmas during the 57–48 Ma late stage Laramide age magmatism to more crustal dominated sources during the 46–41 Ma transition period magmatism. Therefore, we propose the development of a tear in the subducting slab underneath central Durango. The proposed tear in the subducting slab would have facilitated asthenospheric upwelling, providing the necessary heat to induce crustal melting during the 46–41 Ma transition period magmatism.

In the northernmost segment of the Arabian–Nubian Shield, a post-collisional high-K calc-alkaline volcanic sequence is exposed along Wadi Abu Ma’amel, Eastern Desert of the Nubian Shield. It comprises a series of intermediate to silicic volcanics and associated pyroclastics that include the Imperial Porphyry and calc-alkaline volcanics typical of the Dokhan Volcanics. The Imperial Porphyry occurs as subvolcanic sill-like intrusions forming the young member of the Dokhan Volcanics. The volcanic sequence extruded through synorogenic granite and was intruded by post-collisional granite, which also caused thermal contact metamorphism. The red and purple colors of the Imperial Porphyry reflect hydrothermal alterations, which resulted in the formation of dispersed flakes of hematite, epidote, and piemontite. The entire high-K calc-alkaline volcanic sequence, ranging from andesite through dacite and rhyodacite, exhibits post-collisional geochemical characteristics. Most samples of the Imperial Porphyry and some of the typical Dokhan Volcanics have characteristics of adakitic rocks, including high Sr (694–889 ppm), low Y (10.6–18.8 ppm), high Sr/Y (41.1–83.8), (La/Yb)n (8.6–15.6), and low (Yb)n (5.4–9.0). The mostly calc-alkaline character and other traits of the studied volcanics that were previously interpreted to indicate arc magmatism reflect, instead, remelting of earlier (pre-collisional) arc-related material. The formation of Wadi Abu Ma9amel volcanics resulted from upwelling of hot asthenospheric material during thinning of the previously thickened lithosphere as a consequence of lithospheric delamination. The parental magma was generated by partial melting of mafic lower crust that mixed with upper-crust-derived magma. It evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe–Ti oxides in the more-evolved dacitic and rhyodacitic rocks.

The Demeni-Mocidade domain (DMD) comprises a large area of granitoid rocks, located on the border between the states of Roraima and Amazonas, Brazil, within the inner part of the Ventuari-Tapajos Province of the Amazonian Craton, where large amounts of granitoid rocks, formed between 2.0-1.8 Ga, are predominant. This study examines six granite samples collected from the Mocidade and Demeni mountains, which are related in time to the Uatuma Silicic Large Igneous Province (SLIP), which, in turn, covers several tectonostratigraphic domains whose evolution is associated with an intercontinental setting, allowing a common association with I- and A-type granitoid rocks formed within the same 1.88 to 1.87 Ga time interval. These rocks are monzogranites and correspond petrographically to holocrystalline anisotropic lithotypes with fine to medium grain sizes. Their textural relationships, including some lithotypes containing phenocrysts with oscillatory zoning and resorbed rims, indicatethat crystallization occurred at subvolcanic or hypabyssal depths. Moreover, the association between plutonic and subvolcanic rocks in the same suite shows variations in the crustal development of the magmatic chambers. The granitoids of the DMD share a common geochemical signature with those of the Agua Branca Suite, which occurs within the Uatuma and Trombetas-Erepecuru domains of the Guiana Shield, suggesting that their crystallization occurred in similar magmatic chambers. Six U-Pb zircon ages show that most zircon crystals are concordant and the few ones which are discordant are well aligned along Discordia straight lines descending to zero. The calculated Concordia ages, covering the 1884 to 1877 Ma interval, agree within experimental error, indicating a probably similar crystallizationage. Such apparent age values, close to 1900 Ma for the DMD, made it possible for the domain of the Uatuma SLIP to spread to the West, and the area of the Ventuari-Tapajos Province could extend towards the Amazonas State of Venezuela. Finally, the coexistence between I- and A-type granitoids in the DMD, with ages within the range 1.88-1.87 Ga, stimulates a discussion, and there are three possibilities: (1) The calc-alkaline magmatism can be associated with late (post-collisional) processes related to subduction. (2) The granitoid rocks are formed in an intracontinental setting under more stable (post-orogenic) tectonic conditions. (3) The granitoid rocks are predominantly formed by A-type and alkaline magmatism in intraplate settings.

It is generally hypothesized that high-silica (SiO2 > 75 wt%) granite (HSG) originates from crystal fractionation in the shallow crust. Yet, identifying the complementary cumulate residue of HSG within plutons remains difficult. In this work, we examine the genetic links between the porphyritic monzogranite and HSG (including porphyritic granite, monzogranite, and alkali feldspar granite) from the coastal area of southeastern China using detailed zircon U-Pb ages, trace elements, Hf-O isotopes, and whole-rock geochemistry and Nd-Hf isotopic compositions. Zircon U-Pb ages indicate that the porphyritic monzogranite and HSG are coeval (ca. 96–99 Ma). The HSG and porphyritic monzogranite have similar formation ages within analytic error, identical mineral assemblages, similar Nd-Hf isotopic compositions, and consistent variations in their zircon compositions (i.e., Eu/Eu*, Zr/Hf, and Sm/Yb), which suggests that their parental magma came from a common silicic magma reservoir and that the lithological differences are the result of melt extraction processes. The porphyritic monzogranite has relatively high SiO2 (70.0–73.4 wt%), Ba (718–1070 ppm), and Sr (493–657 ppm) contents, low K2O and Rb concentrations and low Rb/Sr ratios (0.1–0.2), and it displays weak Eu anomalies (Eu/Eu* = 0.57–0.90). Together with the petrographic features of the porphyritic monzogranite, these geochemical variations indicate that the porphyritic monzongranite is the residual silicic cumulate of the crystal mush column. The HSG (SiO2 = 75.0–78.4) has variable Rb/Sr ratios (2–490) and very low Sr (1–109 ppm) and Ba (9–323 ppm) contents. Zircon from the HSG and porphyritic monzogranite overlap in Eu/Eu*, Zr/Hf, and Sm/Yb ratios and Hf contents; however, some zircon from the HSG show very low Eu/Eu* (

Massive CO2 and SO2 degassed from large igneous provinces (LIPs) are thought to cause ocean acidification and calcification crisis with a significant loss of calcified marine biota. The ocean acidification caused by the Emeishan LIP event has been proposed as one of the important factors that triggered the end-Guadalupian (Middle Permian) crisis, although the driving mechanism remains unclear. Here we represent a detailed field investigation combined with LA-ICP-MS, SIMS and CA-ID-TIMS zircon geochronology of tuffs from four volcanic sections in the eastern part of the Emeishan LIP. Our new data combined with previous results from this LIP confirm that: (i) the submarine volcanism occurred between ~260.7 Ma and ~ 257.9 Ma, with most basaltic eruptions at ~260.1 Ma to 259.5 Ma; and (ii) explosive tuff eruptions occurred during ~257.9 Ma to ~256.9 Ma in the subaerial environment. We propose a model involving SO2 release from the submarine basaltic eruptions in the Emeishan LIP as the trigger of the end-Guadalupian ocean acidification, due to the high solubility of SO2 in seawater that could produce strongly acidic conditions. The late explosive silicic volcanism in the subaerial setting during Early to Middle Wuchiapingian (Late Permian) might have been a potential strong driver of the global Wuchiapingian cooling event.

The end-Permian crisis was the most severe extinction event in the Phanerozoic. In Southwestern Gondwana, this crisis was related to the development of the Choiyoi Silicic Large Igneous Province. I...

U–Pb zircon dating of felsic volcanic rocks from the Konachan Complex (Algan Mountains, northwestern Koryak Highland) confirmed their Late Miocene age (34.6 ± 0.5–38.8 ± 0.4 Ma, Priabonian). It is shown that these rocks are represented by moderate-potassium peraluminous (ASI = 1.23–1.30), moderate- and high-Mg varieties, frequently with high Cr and Ni contents. The Sr, Y, and Yb contents are similar to those of adakites. The high 143Nd/144Nd and low 87Sr/86Sr ratios in the rocks of the complex in combination with their trace and rare-earth distribution patterns suggest that metabasites weakly contaminated by sedimentary material were one source of silicic melt. The magmatic rocks of the Konachan Complex were formed during accretion of the terranes of the Olyutorka–Eastern Kamchatka island-arc system; the compression locked the upwelling front of continental asthenosphere, thus initiating crustal anatexis and silicic magmatism.

The mafic sills and unmetamorphosed shales and siltstones of Tadpatri Formation in Cuddapah Supergroup are associated with thin flows of quartz keratophyre and an occasional albitite dyke. The textural characteristics of these quartz keratophyre essentially establish them as igneous extrusive rocks. A detailed study of texture and chemistry, coupled with comparison of available records in geological literature, suggests that these rocks were derived through very small degree of partial melting of metasomatized mantle at pressures more than plagioclase stability field and less than jadeite stability field. High alkali content and associated volatiles led to significant decrease in viscosity and emplacement of these lavas in an extensional tectonic set-up.

Rocks from subvolcanic silicic magmatism (SSM) are exposed intermittently over a wide area along the Sonora coast, NW Mexico. This region is characterized by syn-rift volcanism related to the devel...

Micro-organisms have long been implicated in the construction of stromatolites. Yet, establishing a microbial role in modern stromatolite growth via molecular analysis is not always straightforward because DNA in stromatolites can have multiple origins. For example, the genomic material could represent the microbes responsible for the construction of the stromatolite (i.e., "builders"), microbes that inhabited the structure after it was built (i.e., "tenants"), or microbes/organic matter that were passively incorporated after construction from the water column or later diagenetic fluids (i.e., "squatters"). Disentangling the role of micro-organisms in stromatolite construction, already difficult in modern systems, becomes more difficult as organic signatures degrade, and their context is obscured. To evaluate our ability to accurately decipher the role of micro-organisms in stromatolite formation in geologically recent settings, 16/18S SSU rRNA gene sequences were analyzed from three systems where the context of growth was well understood: (a) an actively growing stromatolite from a silicic hot spring in Yellowstone National Park, Wyoming, where the construction of the structure is controlled by cyanobacteria; (b) a mixed carbonate and silica precipitate from Little Hot Creek, a hot spring in the Long Valley Caldera of California that has both abiogenic and biogenic components to accretion; and (c) a near-modern lacustrine carbonate stromatolite from Walker Lake, Nevada that is likely abiogenic. In all cases, the largest percentage of recovered DNA sequences, especially when focused on the deeper portions of the structures, belonged to either the tenant or squatter communities, not the actual builders. Once removed from their environmental context, correct interpretation of biology's role in stromatolite morphogenesis was difficult. Because high-throughput genomic analysis may easily lead to incorrect assumptions even in these modern and near-modern structures, caution must be exercised when interpreting micro-organismal involvement in the construction of accretionary structures throughout the rock record.

Reworked detrital sediments were discovered in the syn-rift to post-rift Cretaceous to Quaternary (Pleistocene) detrital deposits of the Douala coastal sub-basin in the eastern margin of the South Atlantic Ocean at West Cameroon. This contribution presents the geochemical, mineralogical, and sedimentological data to constrain the origin of these reworked sediments. The investigated samples are characterized by the high values of Plagioclase Index of Alteration (PIA, 97.83–99.90), Chemical Index of Alteration (CIA, 71.9–99.08), and Chemical Index of Weathering (CIW, 98.67–99.90). Data gleaned from these indices suggest that the reworked sediments and their potential source rocks experienced severe weathering. The A-CN-K plot along with the mineralogical indicates a probably post-depositional K-enrichment. The high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (15.82–79.13) suggest that the reworked were mainly derived from felsic igneous rocks. This interpretation is further confirmed by the Zr versus TiO2, TiO2/Al2O3, Th/Co versus La/Sc, and La/Th versus Th/Yb plots, which indicate felsic igneous and silicic source rocks. Meanwhile, the high variability of LREE/HREE ratios (15.82–79.13) and the positive Eu anomalies (1.10–1.32) of studied samples could indicate a little contribution of mafic source rocks. Analysis of the distribution of the heavy minerals (e.g. tourmaline, aluminum silicate, and rutile) revealed that these felsic and silicic source rocks are mainly granite and gneiss, probably belonging to the Nyong Group and the Western part of the Neoproterozoic Yaounde Group. The morphoscopic analyses of the host rocks (very angular to angular shapes) have revealed that the reworked sediments have proximal to subproximal onshore origins, probably from former sedimentary deposits, as is indicated by variation of their color. The tectonic discriminant diagram indicates the Oceanic Island Arc and Active Continental Margin which are inconsistent with the geological history of the Douala coastal sub-basin and could be explained by the recycling effect experienced by studied sediments which have been deposited in the coastal plain to the fluvial environment in the Low System Track context.

Silicic magmatism is a feature of all continental LIP events, and where volumetrically significant, occurs as high‐frequency (~1,000–10,000 yr recurrence intervals), large‐magnitude (>M8) explosive supereruptions producing vast ignimbrite sheets. Silicic supereruptions inherently have the eruptive mechanism to deliver aerosols and ash to the stratosphere for global dispersal, and thus overcome eruptive barriers that exist for flood basalts built up by long‐lived, low effusion and low vigor fountains that lack height and persistent stratospheric penetration. The historical record demonstrates the climate forcing capabilities of silicic supereruptions, which during LIP events, were likely associated with large CO2, SO2, halogen, and Hg emissions, and through tephra deposition, could cause iron fertilization in the world's oceans, thereby kick‐starting phytoplanktonic biological pumps to significantly draw down atmospheric CO2. What may be important, therefore, for LIP events to cause the most environmental impact and trigger a mass extinction, is the combined effect of closely spaced basaltic and silicic, or effusive and explosive, eruptions that work in tandem to overload the troposphere and stratosphere with volcanic aerosols producing rapid decadal‐scale, extreme fluctuations in pH driven by acid rain, S‐, or iron fertilization‐driven temperature chills, and toxic UV radiation bursts. These effects could be repeated within as little as a few hundred years of each other particularly during hyperactive LIP pulses.

Although it is widely accepted that large silicic calderas are associated with voluminous synvolcanic intrusive complexes at depth, geological evidence for caldera-forming eruptions preserved in plutonic rocks has largely been elusive. Here, we document a rare example of such evidence in the Cadillac Mountain intrusive complex, Maine (USA), where erosion has revealed a remarkable marginal “shatter zone” that records evidence for a major caldera-forming eruption. This shatter zone, up to >1 km wide, is bounded by a steep ring fault at its outer margin, which grades inward into Cadillac Mountain granite. Its outer margins are characterized by intensely brecciated and deformed country rock injected by felsite veins, reflecting explosive fragmentation associated with eruptive decompression. This marginal facies grades inward to a chaotic mélange of variably rounded and remelted country rock blocks in granitic matrix, reflecting debris eroded from ring fault conduit walls and milled in an eruptive jet before collapsing onto crystal mush. Further inward, blocks up to 80 m in size were stoped from the collapsing chamber roof and settled onto strong mush. Textural and chemical variations in the shatter zone matrix reveal syneruptive ascent of distinct silicic and more mafic magma from depth, which was likely drawn through the highly permeable shatter zone toward areas of low pressure beneath active vents. The Cadillac Mountain shatter zone provides clear evidence for a major eruption preserved in the plutonic record and supports the origin of some granites as the cumulate roots of large silicic volcanic systems.

Products of weathering usually clay minerals, are commonly characterized through mineralogical, chemical and geochemical analysis, emphasizing their implications in industrial applications, initiation of landslides and their impact on surface water geochemistry. In this study the vertical variations in textural characteristics and clay mineral type within weathered profiles in the Sabga area were investigated. Two exposed weathered profiles were logged from bed rock to topsoil and each horizon was sampled separately. Granulometric analysis on the samples indicated mixtures of clay, silt and sand size particles in all horizons. Smectite and kaolinite were identified by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The two logs show that the lower horizons display whitish colors with fine laminations, middle horizons are brownish and the upper horizons display dark brown colors. Graphic mean values for both sections gave values that range between 2.02 - 3.7f with an average standard deviation value of 1.9f, indicating that the grains are poorly sorted sand, silt and clay. SEM micrographs show laths of smectite in the lower horizons, flakes of kaolinite and laths of smectites in the middle horizons while the upper horizons show flakes of kaolinite with microlites of quartz + feldspars. XRD patterns show broad basal reflections for kaolinite at 2θ 36.8° (3.15 ?), smectite at 2θ 33.5° (3.34 ?), both contaminated with quartz at 2θ 36.5° (3.32 ?) typical for these minerals.

The break-up of Gondwana during the Early–Middle Jurassic was associated with flood basalt volcanism in southern Africa and Antarctica (Karoo–Ferrar provinces), and formed one of the most extensive episodes of continental magmatism of the Phanerozoic. Contemporaneous felsic magmatism along the proto-Pacific margin of Gondwana has been referred to as a silicic large igneous province, and is exposed extensively in Patagonian South America, the Antarctic Peninsula and elsewhere in West Antarctica. Jurassic-age silicic volcanism in Patagonia is defined as the Chon Aike province and forms one of the most voluminous silicic provinces globally. The Chon Aike province is predominantly pyroclastic in origin, and is characterized by crystal tuffs and ignimbrite units of rhyolite composition. Silicic volcanic rocks of the once contiguous Antarctic Peninsula form a southward extension of the Chon Aike province and are also dominated by silicic ignimbrite units, with a total thickness exceeding 1 km. The ignimbrites include high-grade rheomorphic ignimbrites, as well as unwelded, lithic-rich ignimbrites. Rhyolite lava flows, air-fall horizons, debris-flow deposits and epiclastic deposits are volumetrically minor, occurring as interbedded units within the ignimbrite succession.

We present a detailed review of the petrological and geochemical aspects of rhyolite and associated silicic volcanic rocks (up to 20 ​vol% of all rocks) reported to date from twelve well known Phanerozoic continental mafic Large Igneous Provinces (LIPs). These typically spread over ≤104 ​km2 (rarely 105 ​km2 for Paraná-Etendeka) area and comprise ≤104 ​km3 of extrusive silicic rocks, erupted either during or after the main basaltic eruption within 900 ​°C. Rhyolites formed by significant crustal contribution are characterized by strong negative Nb-Ta anomalies, absence of clinopyroxene phenocrysts, and are likely to have a magma temperature

The Taupō Volcanic Zone (TVZ) is characterised by a negative residual gravity anomaly that correlates with a zone of normal faulting, subsidence and voluminous silicic volcanism from c. 2 Ma. A ste...

The genesis and timing of molybdenum (Mo) mineralization in the Mada Ring Complex, north-central Nigeria, have been constrained using a combination of whole-rock elemental geochemistry, Nd-Sr isotopes, zircon U-Pb-Hf isotopes, and molybdenite Re-Os geochronological data. The Mada Ring Complex is one of fifty-three within-plate alkaline ring complexes with economically significant Sn and Nb production from largely alluvial placer deposits in north-central Nigeria. Molybdenum mineralization in the ring complex is dominantly disseminated with minor stockwork veins hosted within biotite granite. Integrated zircon U-Pb and molybdenite Re-Os geochronology constrain initial magma emplacement to between 152 and 151 (± 1) Ma and the Mo-mineralization event to ≤ 150 (± 1) Ma. The granites associated with Mo mineralization are highly silicic and display elevated whole-rock concentrations of Nb, Zr, Y, and REE (except Eu), as well as high 87Rb/86Sr ratios, but low concentrations of Sr, Ba, CaO, MgO, and TiO2, consistent with highly differentiated A-type granites. They yield moderately negative whole-rock eNd(t) (− 5.87 to − 5.47) and zircon eHf(t) (− 4.97 to − 10.06), suggesting that their parental magmas were largely derived from the lower crust with a contribution from the upper mantle. Magmatic oxygen fugacity (ƒO2) of the A-type granites (log FMQ = +1.16), constrained from zircon compositions, is lower than values reported from typical within-plate porphyry molybdenum deposits (log FMQ = +2 to + 3). However, compared to regional Sn ± Nb-mineralized A-type granites, the Mada A-type granites have higher ƒO2 and Ce4+/Ce3+, but are less differentiated.

Diamonds containing fluid inclusions provide invaluable samples of upper mantle fluids, the study of which illuminates not only diamond formation but also the long-term evolution of the subcratonic, lithospheric mantle. The very large range of inclusion compositions worldwide has been interpreted to represent four end-member fluids: saline (rich in Na + K + Cl); silicic (rich in Si + Al); and carbonatitic (rich in Ca + Mg + Fe, with low-Mg and high-Mg end members). However, the sources and evolution of these fluids and the processes involved in diamond formation are still unclear. We used an unusual study of diamonds from the Panda kimberlite (Ekati Mine, Northwest Territories, Canada) in which both mineral and fluid inclusions in the diamonds were analyzed (Tomlinson et al., 2006) to develop models of the saline, silicic, and low-Mg carbonatitic fluids present in the Panda fluid inclusions. The models used aqueous speciation and solubility calculations to link the solid and fluid inclusion chemistry with model upper mantle rock types. We used the extended Deep Earth Water model to calculate equilibrium constants previously calibrated with experimental rock solubilities referring to upper mantle temperatures and pressures ( Huang and Sverjensky, 2019 ). Our results at 950 °C and 4.5 GPa suggest that the saline fluid could originate from peridotite, the silicic fluid from eclogite, and the low-Mg carbonatitic fluid from carbonated dunite. The fluid models were then used to predict the irreversible, chemical mass transfer when the carbonatitic fluid infiltrated a harzburgite containing a saline fluid. Simultaneous reduction of formate and bicarbonate in the carbonatitic fluid and oxidation of aqueous hydrocarbons from the peridotitic fluid during mixing and reaction with harzburgite resulted in the formation of diamond, olivine, garnet, and clinopyroxene, and increases in the logf O 2 and pH . Olivine was predicted to become more Fe-rich and garnet more Ca and Fe-rich with reaction progress, in agreement with reported temporal trends (core-to-rim) in the Panda mineral inclusions. The fluid at the site of diamond formation became more saline with reaction progress and the predicted aqueous phase concentrations of all elements changed consistent with trends in Panda fluid inclusions. In contrast, a prediction for a saline fluid infiltrating a harzburgite containing a carbonatitic fluid resulted in trends of the silicate minerals and the salinity with reaction progress that were in the opposite direction to data from the Panda diamonds. Overall, our study strongly supports the notion that fluids from subducting slabs could mix and precipitate diamonds containing carbon from both oxidized and reduced sources, while adding Ca and Fe to the sub-lithospheric cratonic mantle through metasomatic reactions.

The Margaree pluton extends for >40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U–Pb (zircon) ages of 363 ± 1.6, 364.8 ± 1.6, and 365.5 ± 3.3 Ma, respectively, consistent with field and petrological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm–Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen.

This paper describes the geology and eruptive history of a Quaternary silicic volcanic complex at Shiribetsu, Hokkaido, Japan, with a focus on volcanic landforms produced by silicic explosive eruptions that were not associated with caldera collapse. The Shiribetsu volcanic complex comprises a dacitic pyroclastic plateau and an overlying dacitic central dome complex. The pyroclastic plateau is 22 × 25 km in size

Interaction between amphibole and H2O–HCl fluid was experimentally studied in an internally heated pressure vessel (IHPV) at 650–800°C and 5–7 kbar. The stability of the anorthite–amphibole association was approved to be constrained at T = 650–800°C, (P = 5–7 kbar) within a range of HCl fugacity (fHCl) 50 100 µm) corundum and quartz crystals. Based on experimental data, it is demonstrated that interaction between metabasite with acidified fluid resulted in Ca, Fe, and no so much Mg removal from the metabasite (as inferred for rocks cropping out on Kii Island in the White Sea). The HCl fugacity in the metasomatizing fluid was approximately 50–200 bar at T = 650 – 800°C, P = 5–7 kbar. Rock depletion in bases led to that the residual matrix enriched in Al2O3 and SiO2, and this was favorable for the origin of corundum, on the one hand, and highly silicic rocks, like quartzite, on the other.

Degassing dynamics play a crucial role in controlling the explosivity of magma at erupting volcanoes. Degassing of magmatic water typically involves bubble nucleation and growth, which drive magma ascent. Crystals suspended in magma may influence both nucleation and growth of bubbles. Micron- to centimeter-sized crystals can cause heterogeneous bubble nucleation and facilitate bubble coalescence. Nanometer-scale crystalline phases, so-called “nanolites”, are an underreported phenomenon in erupting magma and could exert a primary control on the eruptive style of silicic volcanoes. Yet the influence of nanolites on degassing processes remains wholly uninvestigated. In order to test the influence of nanolites on bubble nucleation and growth dynamics, we use an experimental approach to document how nanolites can increase the bubble number density and affect growth kinetics in a degassing nanolite-bearing silicic magma. We then examine a compilation of these values from natural volcanic rocks from explosive eruptions leading to the inference that some very high naturally occurring bubble number densities could be associated with the presence of magmatic nanolites. Finally, using a numerical magma ascent model, we show that for reasonable starting conditions for silicic eruptions, an increase in the resulting bubble number density associated with nanolites could push an eruption that would otherwise be effusive into the conditions required for explosive behavior.

We present results of lithogeochemical, diatomic and palynological studies of sediments from the Tunka-13 well that was drilled in the southeastern part of dry Tunka basin in the Baikal rift zone. At the base of the section, there is an eroded basaltic flow of 16–15 Ma. From lithogeochemical signatures, we identify nine sedimentary units. The seven lower ones (interval 7.2–86.5 m) belong to the Tankhoi formation, the eighth (interval 2.7–6.6 m) to the Anosov formation, the ninth (interval

The age of the Gow Lake impact structure (Saskatchewan, Canada) is poorly constrained, with previous estimates ranging from 100 to 250 Ma. Using a combination of step-heating and UV laser in situ 40Ar/39Ar analyses we have sought to understand the 40Ar/39Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for 40Ar/39Ar geochronology due to its small size (∼5 km diameter), the silicic composition of the target rock, and the large difference in age between the impact event and the target rock (∼1.2 Ga). These factors can serve to inhibit argon mobility in impact melts, leading to retention of ‘extraneous’ 40Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous 40Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ 40Ar/39Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of 40Ar inherited from silica-rich melts, data still had to be corrected for extraneous 40Ar by using isotope correlation plots to define the initial trapped 40Ar/36Ar components. Our inverse isochron age of 196.8 ± 9.6/9.9 Ma (2σ, analytical/external precision) demonstrates that the Gow Lake event occurred within uncertainty of the Triassic-Jurassic boundary, but there is no evidence that it was part of an impact cluster.

Silicic magmas span a wide range of eruptive styles between explosive and effusive, and transitions between these styles are commonplace. Yet the triggers of switches in eruptive style remain poorly understood. Eruptions are mostly driven by degassing of magmatic water and their eruption style - effusive or explosive - is likely governed by the efficiency of outgassing as well as magma ascent rate. Microlites and phenocrysts are often purported to promote heterogeneous bubble nucleation and outgassing, both key variables in the degassing dynamics that become crucial in controlling the eruptive style. Here, in order to shed light on the role of nature, size and abundance of crystals on degassing of silicic magma, we experimentally investigate (heating-induced) vesiculation in a multiphase (microlite- and phenocryst-bearing), low-water content, and bubble-free, natural rhyolite. The experiments were conducted at magmatic temperature (∼900-1100 °C) and atmospheric pressure in an optical dilatometer. Our results indicate that microlites exert a large influence on bubble nucleation while the effect of phenocrysts is subordinate. Amongst the microlite phases, bubbles nucleate more easily on Fe-Ti oxides than other mineral phases. Bubble coalescence and connectivity are, in contrast, enhanced by the phenocrysts, more than microlites, in low-crystallinity magma. Comparing the bubble textures of the post-experimental samples with those produced in a phenocryst-free and microlite-poor silicic magma, we observe that the phenocryst- and microlite-bearing magma exhibits significantly more bubble coalescence and connectivity, as well as higher bubble number densities. These findings help to constrain the roles that pre- and syn-eruptive crystalline phases may play in degassing processes during ascent of silicic magma.