Transcrustal Magmatic System Research Articles

Deformation at the open-vent Erebus volcano, Antarctica, from more than 20 years of GNSS observations

Erebus volcano on Ross Island in Antarctica is an iconic open-vent volcano that has hosted a convecting anorthoclase-rich phonolite lava lake for at least 50 years. Recent magnetotelluric observations have imaged a conduit system originating at least 60km below Erebus. This terminates in a seismically-imaged shallow magma reservoir about 500m NW of the crater with an upper surface at about 500m depth. A narrow and inclined terminal conduit system connects the shallow reservoir to the lava lake. Bomb-ejecting Strombolian eruptions from the lava lake and sporadically active adjacent vents are common. Larger eruptive activity with locally substantially elevated hazard has also been observed, including exceptionally energetic Strombolian activity in 1984 and an Inner Crater phreatic explosion in 1993. Despite sustained degassing and frequent eruptions, geochemical data show the composition of the lava has remained stable for the last 17ka, consistent with the long-lived transcrustal magmatic system underlying the lake.Global Navigation Satellite System (GNSS) data collection on Ross Island began in the late 1990s with campaign observations. In the early 2000s, additional benchmarks were added closer to the summit of Erebus and continuous GNSS (cGNSS) sites were co-located with a seismic network. Today, seven cGNSS sites operate on the summit plateau and flanks of the volcano, and a network of eight campaign benchmarks has been surveyed episodically.We present the first comprehensive geodetic data analysis and modeling results integrating these more than two decades of data collected at Erebus. We resolve long-term subsidence of Ross Island, which a simple viscoelastic model links to the long-term growth of Erebus over the last 20ka. The data also show multi-year cycles of inflation and deflation in the summit area, consistent with activity in the shallow summit magmatic system. These small amplitude (several mm/yr) transient dynamics suggest multi-year pulses of pressurization and depressurization, or geometric changes within the shallow magmatic system that we can reproduce with analytical source models. The most recent inflation pulse lasted from November 2020 until March 2022 when several stations moved radially away from the shallow magmatic system and upwards at 10–15mm/yr. Based on prior cycles, this deformation may result in increased eruptive activity suggesting that continued and enhanced surveillance of Erebus is warranted. These observations contrast with the long-term general stability of the lava lake, but reflect Inner Crater dynamics, which can include changes in lava lake elevation and associated topographic changes of over 20m on multi-year time scales. Our results emphasize the value of long-term and campaign-aided high-accuracy GNSS observations at open-vent volcanoes. This is especially true for volcanoes like Erebus which are remote and may only be accessible for a few months a year, and that deform at amplitudes and periods that may be difficult to resolve with satellite-based radar.

The long-lived transcrustal magmatic systems of Southeast China in response to paleo-Pacific plate subduction, recorded by the Cretaceous volcanic sequences in southeastern Zhejiang Province

Processes taking place in subduction zones are highly debated. The Cretaceous volcanic rocks are voluminously distributed along the coastal area of southeastern (SE) China. To elucidate their petrogenesis and relationship with subduction, we use new zircon U-Pb ages and Hf-O isotopes for the representative Cretaceous volcanic sequences in Zhejiang Province, China. According to stratigraphic investigations, zircon U-Pb ages, and Hf-O isotopes, these volcanic rocks can be divided into different groups corresponding to three stages of volcanic activity: the early (Stage 1, 136−129 Ma), middle (Stage 2, 125−115 Ma), and late (Stage 3, 110−94 Ma) stages. Diverse zircon populations (including antecrysts, autocrysts, and xenocrysts) are recognized. Hf-O isotopes of autocrysts suggest different protoliths for the identified three stages of volcanic activity. Xenocrysts show obvious different compositions from autocrysts. Antecrysts share similar compositions with autocrysts, which favor similarities in the magmas from which they were generated. Our observations (very small age intervals between antecrysts and autocrysts, sources for volcanic rocks throughout the entire crust, and volcanic magmatism with long-term and discontinuous characteristics), were inconsistent with the traditional melt-dominated magma chamber model. Consequently, we propose that those volcanic rocks were derived from long-lived transcrustal magmatic systems (TCMS), dominated by crystal mush, instead of melt-dominated magma chambers, maintained and recharged by a discontinuous contribution of contemporaneous underplated mantle-derived magmas, triggered by paleo-Pacific plate subduction. We suggest the different stages of volcanic activity and corresponding long-lived TCMS were produced by the change of Pacific plate motion beneath SE China during the Cretaceous period.

Barium isotopes reveal the role of deep magmatic fluids in magmatic-hydrothermal evolution and tin enrichment in granites

Although deep magmatic fluids are important for the shallow magmatic-hydrothermal evolution and transport/enrichment of ore-forming elements in transcrustal magmatic system, it is still difficult to identify the influences of such fluids by using conventional geochemical indicators. Here we report Ba isotope compositions for granites from the Jurassic Qitianling batholith in the Nanling Range, South China that hosts several large tin deposits. This composite batholith consists of three stages of granites with an age range of ∼15 Ma and sharp contacts between each other, which suggest an underlying long-lived crystal mush-dominated transcrustal magmatic system. The small variation of δ138/134Ba in the first-stage less evolved granites (−0.24–0.37‰) indicates that fractional crystallization in deep crystal mush of this system does not cause significant Ba isotope fractionation. In contrary, the latter-two stage highly differentiated granites show more variable and overall lower δ138/134Ba (−1.79–0.14‰), which cannot be explained by K-feldspar-controlled fractional crystallization in shallow crystal mush. Instead, the distinctively low Ba contents (<100 μg/g) and magmatic-hydrothermal evolution features of the latter-two stage granites suggest that their low δ138/134Ba are due to modification by magmatic fluids. Further modeling demonstrates that exsolved fluids from deep crystal mush can explain the light Ba isotope compositions of the latter-two stage granites. Because these ascending deep magmatic fluids can efficiently scavenge Sn and other fluid-mobile elements then transport to shallow level, we propose that these fluids could provide critical materials for the Sn-polymetallic mineralization.

Seismic tremor reveals active trans-crustal magmatic system beneath Kamchatka volcanoes.

The occurrence and the style of volcanic eruptions are largely controlled by the ways in which magma is stored and transported from the mantle to the surface through the crust. Nevertheless, our understanding of the deep roots of volcano-magmatic systems remains very limited. Here, we use the sources of seismovolcanic tremor to delineate the active part of the magmatic system beneath the Klyuchevskoy Volcanic Group in Kamchatka, Russia. The tremor sources are distributed in a wide spatial region over the whole range of crustal depths connecting different volcanoes of the group. The tremor activity is characterized by rapid vertical and lateral migrations explained by fast pressure transients and dynamic permeability. Our results support the conceptual model of extended and highly dynamic trans-crustal magmatic systems.

Determining the State of Activity of Transcrustal Magmatic Systems and Their Volcanoes

Polygenetic volcanoes and calderas produce eruptions of a wide variety of magnitudes, chemistries, and recurrence times. Understanding the interplay between long- and short-term and deep and shallow processes associated with accumulation and transfer of eruptible magma is essential for assessing the potential for future eruptions to occur and estimating their magnitude, which remains one of the foremost challenges in the Earth sciences. We review literature and use existing data for emblematic volcanic systems to identify the essential data sets required to define the state of activity of volcanoes and their plumbing systems. We explore global eruptive records in combination with heat flux and other geological and geophysical data to determine the evolutionary stage of plumbing systems. We define a Volcanic Activity Index applicable to any volcano that provides an estimate of the potential of a system to erupt in the future, which is especially important for long-quiescent volcanoes. ▪ Magmatic plumbing systems that feed volcanic activity extend across Earth's crust and are long-lived at depth and ephemeral in their shallowest portions. ▪ We revise and update the definitions of active, quiescent, and extinct volcanoes based on physical proxies for the architecture, longevity, amount, and distribution of eruptible magma in the crust. ▪ We propose a Volcanic Activity Index, which provides a relative measure of the state of activity of a volcano with respect to all other volcanoes in the world. ▪ New imaging and monitoring strategies are required to improve our ability to detect lower and middle crust magmatic processes and forecast eruptions and their potential size.

Sanbe volcano: Long‐term evolution of an arc magmatic system

AbstractA suite of samples was studied that represents the major explosive eruptions of Sanbe volcano, SW Japan. We demonstrate how rate of magma flux into the Trans‐Crustal Magmatic System (TCMS) presents a major control on the type and style of the subsequently forthcoming eruptions. Erupted products can be separated into two distinct groups. An older group is characterized by highly evolved, high‐K, LILE‐rich rhyolitic magmas, showing a supressed adakitic trace element signature (otherwise characteristic for the young stratovolcanoes in the SW Japan arc) with low Ca, Sr concentrations and a negative Eu anomaly. In contrast, the younger group (dominantly of andesitic—dacitic composition) displays a strong adakitic trace element signature with characteristic steep REE profiles and high Sr concentrations. An Eu anomaly is generally lacking here. The two groups are also distinct in their petrographic features, with the early group being almost aphyric showing simple log linear crystal size distributions and homogeneous, uniform mineral chemistries. In contrast, products of the younger group show complex crystal size distributions with diverse mineral compositions and abundant disequilibrium features. Our study shows that an initial high melt‐production rate allowed dehydration melting of lower crustal rocks leading to the formation of highly evolved K‐rich magmas. These magmas intruded into the shallow crust and produced two large Plinian rhyolitic, caldera forming eruptions. Subsequently the primary magma production rate decreased and the lower crust became too refractory for additional dehydration melting by these lower volume magma batches, causing the conventional adakitic magmatism to produced several additional eruptions of smaller magnitude, mainly of Sub‐Plinian or Pelean styles.

Degassing and environmental effect of intracontinental transcrustal magmatic system

岩浆作用是联系地球深部与表层碳储库的桥梁，是全球碳循环的重要一环。揭示岩浆作用与碳循环之间的关系对理解地球系统演化过程中的环境效应和气候变化有重要意义。本文综述了前人在岩浆系统研究的重要进展，介绍了陆内穿地壳岩浆系统，梳理了从幔源岩浆底侵和地壳熔融，到岩浆上升、累积、混合、分异直至喷出等物理化学过程，构建了地壳岩浆活动的统一框架。岩浆过程同时控制着挥发分行为，岩浆自身的去气作用将地球深部挥发分带到地表，而接触热变质作用可改造围岩并将其中的碳脱出。基于穿地壳岩浆系统，本文系统阐述了岩浆挥发分的来源、组成、行为，及其释放到大气中对气候与环境产生的潜在影响。尤其是针对中国华南地区分布的大量陆内中生代的侵入岩和火山岩，结合同期广泛分布的巨厚碳酸盐岩，介绍和展望了陆内岩浆系统和去气作用的联系及环境效应，提出该时期的岩浆活动及其与碳酸盐岩的反应很可能对局部乃至全球的气候造成了显著的影响，该领域有较大研究潜力。

Episodic transport of discrete magma batches beneath Aso volcano

Magma ascent, storage, and discharge in the trans-crustal magmatic system are keys to long-term volcanic output and short-term eruption dynamics. How a distinct magma batch transports from a deep reservoir(s) to a pre-eruptive storage pool with eruptible magma remains elusive. Here we show that repetitive very-long-period signals (VLPs) beneath the Aso volcano are preceded by a short-lived (~50–100 s), synchronous deformation event ~3 km apart from the VLP source. Source mechanism of a major volumetric component (~50–440 m3 per event) and a minor low-angle normal-fault component, together with petrological evidence, suggests episodic transport of discrete magma batches from an over-pressured chamber roof to a pre-eruptive storage pool near the brittle-ductile transition regime. Magma ascent velocity, decompression rate, and cumulative magma output deduced from recurrent deformation events before recent 2014 and 2016 eruptions reconcile retrospective observations of the eruption style, tephra fallouts, and plume heights, promising real-time evaluation of upcoming eruptions.

Transcrustal magmatic systems: evidence from andesites of the southern Taupo Volcanic Zone

Studies synthesizing field work, numerical simulations, petrology, geochemistry and geophysical observations indicate that the compositional diversity of arc lavas results from evolution of mantle-derived magmas by mixing, assimilation and fractional crystallization. This evolution occurs within complexes called transcrustal magmatic systems. The mafic lower parts of such zones, called hot zones, are difficult to probe. However, a satellite vent near the stratovolcano Ruapehu in the southern Taupo Volcanic Zone (New Zealand) comprises materials that may originate from a hot zone. Magnesian andesites (Mg# 60–65 ) from the Ohakune scoria cone contain primitive olivine (Fo 85–91 ), high-Mg# clinopyroxene (Mg# 81–88 ) and orthopyroxene (Mg# 76–83 ), but lack plagioclase. Disequilibrium of Ohakune crystals and groundmass suggests that the crystal cargo of Ohakune andesites was scavenged from deeper and more primitive levels of the magmatic system. Mineral constraints on temperature and pressure indicate that the hot zone initially formed at mid- to lower-crustal pressures (3.5–7.0 ± 2.8 kbar). We interpret the mafic mineralogy and presence of disequilibrium features as evidence that these andesites and their crystal cargo are products of a hot zone in the middle to lower crust. Products of the hot zone may appear before products of the systems that form the bases of mature stratovolcanoes such as Ruapehu. Supplementary material: Details of petrography, geochemical and arc olivine data, and MELTS modelling data are available at https://doi.org/10.6084/m9.figshare.c.5494984

Evolution of deep crustal hot zones constrained by the diversity of Late Mesozoic magmatic rocks in SE China

The formation of widespread Late Mesozoic magmatic rocks (dominantly felsic rocks) in SE China needs a physical and chemical balance between crustal and mantle melting, which could be reconciled by complicated transcrustal magmatic systems in deep crustal hot zones with time. After the relative magmatic quiescence at ca. 210–180 Ma, magmatic activities were mainly developed in the interior of SE China with an areal distribution and subsequently concentrated in the present coastal area with an overall linear pattern after ca. 140 Ma except the Lower Yangtze region. The spatiotemporal change requires a unique crust-mantle geodynamic coupling with the evolution of deep crustal hot zones from intracontinental to continent marginal setting. Correspondingly, the magmatic rocks show a compositional change from bimodal distribution in Early and Middle Jurassic to unimodal and weakly bimodal pattern in Late Jurassic and Early Cretaceous. In general, most of the Late Mesozoic intermediate to felsic magmatic rocks show higher Mg# than typical rifting-related rocks at given SiO2, which requires the addition of water and possible mass transfer and mixing of underplated hydrous basaltic magma in the deep crustal hot zones. The occurrence of high-Sr/Y and La/Yb rocks and possibly thickened crust (ca. 40 km) before ca. 140 Ma, combined with the current thin crust thickness (ca. 31-33 km) as revealed by seismic data, indicates a widespread crustal thinning after ca. 140 Ma. The relatively thickened crust before ca. 140 Ma is beneficial for melting of underlying old crust that is composed of dominantly Neoproterozoic and Paleoproterozoic components, the formation of porphyritic Cu deposits, the subsequent magmatic differentiation and associated giant W, Sn, Nb, Ta and Li ore deposits. In contrast, the deep crustal hot zones progressively migrated to the present coastal area after ca. 140 Ma, driving the increasing amounts of intermediate rocks and intermediate-felsic volcanic rocks. In general, the diversity and compositional change of Late Mesozoic magmatic rocks and associated ore deposits in SE China may be controlled by the evolution of deep crustal hot zones in different tectonic settings in response to the subduction of Paleo-Pacific Plate.

Periodic Mixing of Magmas Recorded by Oscillatory Zoning of the Clinopyroxene Macrocrysts from an Ultrapotassic Lamprophyre Dyke

Abstract Ultrapotassic rocks are volumetrically minor, but widely distributed in different geological settings. Extensive studies have concerned mantle melting processes that generated these rocks. However, crustal processes that they may have involved are poorly known. In this paper, we describe complex oscillatory zoning patterns of clinopyroxene (Cpx) macrocrysts from an ultrapotassic lamprophyre dyke in the Kyrgyz North Tianshan orogen. These macrocrysts commonly have a corroded or patchy-zoned core surrounded by a mantle with distinct oscillatory zoning, which is, in turn, surrounded by a euhedral rim. The oscillatory zoning of the mantle is composed of alternating coarse and fine layers with a clear resorption surface, or closely packed layers with a straight or wavy boundary in back-scattered electron images. High-amplitude oscillation of Mg#, Ti, Al, Cr and Sr across the layers of the mantle is attributed to magma mixing. Low-amplitude, high-frequency oscillation of Mg# across the closely packed layers was probably developed as a result of kinetic effects or crystal movement under thermal and chemical gradients. In addition, cryptic sector zoning of some macrocrysts clearly shows a Si- and Mg-rich hourglass sector and an Al- and Ti-rich prism sector. The sector zoning indicates crystallization of these macrocrysts under low degrees of undercooling, and the presence of concentric Cr-rich and Cr-poor layers within the same grain indicates that the growth process was disrupted by multiple magma recharging events. The cores of the macrocrysts have Mg# with three distinctive ranges: &amp;lt;84–90 (Core I), 74–84 (Core II) and 60–70 (Core III). The mantles have Mg# ranging from 64 to 90 without a distinct gap. The rims have a narrow range of Mg# from 76 to 80. The cores and mantles with high Mg# (≥85) have variable La/Yb from 1·8 to 5·0 and Dy/Yb from 2·3 to 4·6. The macrocrysts overall have variable 87Sr/86Sr from 0·7072 to 0·7084. Highly variable trace elements and 87Sr/86Sr within a single grain indicate that both primary and evolved magmas with different compositions were periodically recharged into the crustal magma reservoirs. Modelling results reveal that the melts in equilibrium with the Cpx macrocrysts may have been derived from the magma reservoirs at three different depths equivalent to crystallization pressures of ∼5·4, ∼3·3 and ∼1·6 kbar, respectively, making up a transcrustal magmatic system. The Cpx-laden melts in deep magma reservoirs may have been frequently transported to shallower reservoirs. Magma mixing in the shallower reservoirs led to heterogeneous magmas with different cooling rates and chemical compositions. Early crystallized Cpx crystals were overprinted with diverse zoning patterns during overgrowth and accumulation. Thus, the complex zoning patterns and compositions of the Cpx macrocrysts have important implications for a transcrustal magmatic system in the formation of ultrapotassic rocks.

Tectonic constraints on a magmatic plumbing system: The Quetrupillán Volcanic Complex (39°30′ S, 71°43′ W), Southern Andes, Chile

The Quetrupillán Volcanic Complex is a composite system, active since the Pleistocene. We combine petrological and geochemical data from its erupted products with structural and geometrical constraints applied to the overall system and region. We conclude that a basaltic source melt is hindered on its ascent through the crust due to a compressional tectonic regime, influenced further by the structural control imposed by the Liquiñe-Ofqui Fault Zone. The stalled melt evolves by fractional crystallisation within the crust and undergoes a degree of crustal contamination, resulting in a network of trachytic melt pockets within a transcrustal magmatic system. Eruptions sourced from these pockets have generated numerous lava flows with trachytic compositions, which have occurred from the summit and flanks of Quetrupillán during the Holocene. Occasionally, some of the basaltic source melt has reached shallow levels within the plumbing system with minor interaction with the trachytic melt, resulting in the eruption of lavas with basalt, basaltic andesite and trachyandesite compositions. We propose a conceptual model for the magmatic system of Quetrupillán, in which the Liquiñe-Ofqui Fault Zone plays an important role in exerting a structural control on the crust on which Quetrupillán resides, influencing magma residence times and pathways to the surface.

Construction of a trans-crustal magma system: Building the Bear Valley Intrusive Suite, southern Sierra Nevada, California

The Bear Valley Intrusive Suite (BVIS) in the Southernmost Sierra Nevada exposes a trans-crustal magmatic system that spans emplacement pressures from 3-10 kbars, and was emplaced at all crustal levels between 100.1-101.5 Ma. As such, it represents an unparalleled snapshot of magmatic processes within continental arc crust. In this study we present new field observations combined with whole rock geochemistry that show a fundamental dichotomy within the BVIS. The lower crust of the BVIS is dominantly composed of mafic cumulates that preserve originally shallow to horizontal magmatic fabrics, while the middle and upper crust is dominantly composed of voluminous homogeneous tonalites with steep fabrics. Using a stochastic model of melt fractionation and extraction, we show that these observations strongly constrain the P-T paths along which BVIS magmas must have been emplaced: to create the observed abrupt transition from mafic lower crust to felsic middle and upper crust, evolving melts must cool nearly isobarically in the lower crust before being rapidly emplaced in the upper crust along near-isothermal paths. Our modeling results show that the BVIS magmas must have cooled below 900°C near 7 kbars depth.These modeling results additionally require that the BVIS was emplaced into an unusually warm lower crust. The Sierra Nevada Batholith is typically characterized by felsic crust with low seismic velocities between 6.0-6.5 km/s to at least 30-35 km depth, significantly deeper than the observed transition at ∼28 km depth in the BVIS to mafic cumulates with calculated velocities >7.0 km/s. Given this observation, we conclude that the bulk of the Sierra Nevada Arc magmas must have stalled, cooled and differentiated at greater depths than the BVIS magmas.

Large Silicic Eruptions, Episodic Recharge, and the Transcrustal Magmatic System

AbstractLarge silicic eruptions result from rapid evacuation of large, upper crustal reservoirs of silicic magmas. These silicic magmas are thought to be replenished by melt extracted from underlying crystal‐rich source mushes, but the timescales and mechanisms of such melt extraction are unclear. Geochemical observations suggest that the replenishing melt is often more primitive than the eruptives and must thus cool and crystallize to generate the highly silicic magmas that eventually lead to large eruptions. Motivated by these observations, we use thermal models to explore the replenishment conditions capable of building an eruptible silicic reservoir to generate a large eruption. Results show that the minimum melt replenishment rate required for a silicic reservoir to start growing increases with the effective thermal diffusivity of the overlying crust and decreases with the depth of the reservoir. For an eruptible reservoir at 6 km depth to grow, the initial replenishment rate needs to be greater than 2 × 10−9 m/s. High replenishment rates are required to provide enough advected heat to counterbalance rapid heat loss and consequent freezing that would prevent the eruptible reservoir from growing. However, these high initial replenishment rates must then subside over time for the eruptible reservoir to cool, crystallize, and evolve to highly silicic melts. Thermal histories of some natural systems suggest assembly of large eruptible reservoirs in &lt;20 kyr. The rapid replenishment followed by its decay suggests that replenishment was triggered by a pronounced but ephemeral increase in the porosity and permeability of the underlying crystal‐rich source mush, allowing for rapid melt expulsion. We speculate that this perturbation may be driven by the sudden incursion of deep‐seated, hotter magmas into the base of the crystal‐rich source mush.

Cretaceous volcanic-plutonic magmatism in SE China and a genetic model

Intensive and extensive Cretaceous volcanic-plutonic magmatism occurred in the coastal area of SE China, which is an important part of circle Pacific continental margin associated with copper and gold metallogenesis. Representative calderas are studied in detail, which show “coeval”, “cogenetic” and “complementary” geochemical and petrological signatures for the eruptive and intrusive units. Contemporary abyssal plutonic associations of gabbro-diorite-granite are revisited for their micropetrographical and geochemical fingerprints, which show distinct compaction-driven fractionation of plagioclase and amphibole for the gabbroic rocks. Microtextures and trace elements are also analyzed for underplating/intraplating gabbroic rocks in the lower crust, which were carried out by Cenozoic basalt as deep-seated xenoliths, to test accumulation and fractionation of the mantle-derived magma. A dataset of geochemistry for the Cretaceous volcanic-plutonic rocks is compiled from previous and this studies. Comparisons with magmatism of typical extensional continental arc (northern Cascades in North America), compressional continental arc (Andes in South America) and rifted arc (Taupo in New Zealand) show that the Cretaceous magmatism in the coastal area of SE China is more approximate to the rifted arc one and dominated by felsic magmatism. A series of numerical modeling using major and trace elements and isotopes, as well as petrographic evidence reveal that a combination of fractional crystallization of mantle-derived magma, partial melting of pre-existed mid-lower crust and magma mixing played an important role in the compositional variations of the Cretaceous volcanic-plutonic rocks in SE China.By integrating petrology, geochemistry and geophysical data, we propose a trans-crustal magma system model for the Cretaceous volcanic-plutonic magmatism in SE China. In this model, significant fractionation combining distinct contribution of crustal materials can interpret the dominant felsic component feature. Therefore, the Cretaceous volcanic-plutonic rocks in SE China recorded the evolution of a continental arc built on a complex trans-crustal magmatic system.

Mineralogical Evidence of Pre-caldera Magma Petrogenesis in the Jemez Mountains Volcanic Field, New Mexico, USA

Abstract The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO2 group, and a high-Al, high-temperature, low-fO2 group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.

Bolivian hornblendite cumulates: Insights into the depths of Central Andean arc magmatic systems

The eruptive products of continental arc volcanoes provide wide-ranging insights into the processes governing storage, differentiation, and final eruption (or emplacement) of magmas from trans-crustal magmatic systems. The important role and fractionation of amphibole in continental arc magmatic systems is well documented even if amphibole is generally absent as a prevalent phenocrystic phase in erupted, differentiated lavas. This study presents a comprehensive textural and geochemical investigation of rare plagioclase-bearing hornblendite cumulates entrained in c. 1.4 Ma (trachy-)andesites at the Quillacas monogenetic center on the Bolivian Altiplano, Central Andes. MicroXRF mapping and imaging via SEM-EDS clearly reveals their mesocumulate, idiomorphic nature characterized by cumulus hornblende (≥90%) with intercumulus sodic plagioclase (~8%), minor Fe-Ti oxides and accessory apatites. The trace element compositions of cumulus amphiboles are consistent with their crystallization from a silicate melt with (La/Yb)N < 5 and low (Pb/Ce)N at <0.6. Amphibole TiO2 contents imply crystallization between 700 and 764 °C consistent with depths in the arc crust of 50–55 km as constrained by the Central Andean geotherm. Bulk hornblendite isotopic signatures (Sr-Nd-Pb) correspond to compositions associated with the Central Andean mid-lower arc crust consistent with the presence of hornblendite cumulate piles at this depth. Intercumulus (titano)magnetite exhibits oxy-exsolution ilmenite lamellae, the formation of which is associated with oxygen fugacity (ƒO2) increase during magmatic cooling and differentiation. The ubiquitous presence of gabbroic dehydration (reaction) rims (20–40 μm in width) on the amphiboles is interpreted to be the result of a decrease in ƒO2 during ascent. The hornblendite mesocumulates from this study validate the extensive “cryptic” fractionation of amphibole at arcs and its important role on the petrogenesis of (continental) arc magmas.

Magma recharging beneath the Weishan volcano of the intraplate Wudalianchi volcanic field, northeast China, implied from 3-D magnetotelluric imaging

AbstractThe last volcanic eruptions at the intraplate Wudalianchi volcanic field in northeast China were ∼300 yr ago. Recent ambient noise tomography (ANT) imaged a potential magma chamber beneath one of its volcanoes, the Weishan volcano, which last erupted at ca. 50 ka. To image the spatial distribution of the magmatic system and estimate the melt fractions beneath the Weishan volcano, we use a dense magnetotelluric (MT) network (average site spacing of ∼1 km) around the Weishan cone to image a three-dimensional (3-D) resistivity structure beneath the volcano. For the first time, 3-D MT inversion illuminates the high-resolution spatial distribution of a very low-resistivity body of ∼0.3–3 Ω·m at depth of ∼2–15 km beneath the Weishan volcano. From the 3-D resistivity model, it can be deduced there exists a magma chamber in the upper and middle crust. From both low-velocity anomalies from ANT and low-resistivity anomalies from MT imaging, melt fractions of magma reservoirs are reliably estimated to be &amp;gt;∼15%. From the morphology of magma reservoirs and the shallow magma chamber, the Weishan volcano can be best described by the model of transcrustal magmatic system. Considering the significant melt fractions and active earthquakes and tremors occurring around magma reservoirs, the Weishan volcano is likely in an active stage with magma recharging. Therefore, it needs more active monitoring for better forecasting of its potential future eruptions.

Petrologic Reconstruction of the Tieshan Magma Plumbing System: Implications for the Genesis of Magmatic-Hydrothermal Ore Deposits within Originally Water-Poor Magmatic Systems

Abstract Most genetic models for magmatic-hydrothermal ore deposits are based on the prerequisite that the parental magmas associated with mineralization are enriched in water (&amp;gt; ∼4 wt %). However, it has been recognized that a number of magmatic-hydrothermal ore deposits also formed within tectono-magmatic settings that produce initially water-poor magmas such as Climax-type porphyry deposits. Here, we present a detailed reconstruction of the Tieshan magma plumbing system related to skarn-porphyry Cu–Fe–Au mineralization in the Edong district, in which primitive magmas typically show water-poor features. Applications of multiple thermodynamic calibrations on various magmatic units from the Tieshan and Tonglushan deposits provide a wealth of information regarding the structure and evolution of the transcrustal magmatic system. Petrographic observations and clinopyroxene-liquid thermobarometry calculations indicate that the Tieshan magmatic-hydrothermal system was fed by a deep crustal magma reservoir. An accurate picture of the evolution of H2O within the magma plumbing system is presented using the plagioclase-liquid hygrometer in combination with the amphibole hygrometer. Three critical stages during the evolution of water within the plumbing system have been recognized, associated with H2O contents of 0·8–1·7 wt %, 2·1–2·8 wt % and 3·2–4·6 wt %, respectively. The first enrichment of water in the magmas can be attributed to the separation and transfer of evolved melts from the deep magma reservoir to the shallow crust. Continuous cooling and solidification of the shallow magma body gave rise to the second enrichment of H2O in residual melts, leading to magmas that were fertile for the formation of ore deposits. The detailed chemical evolution of the magma plumbing system was investigated using mineral trace element compositions in combination with the partition coefficients predicted by the lattice strain model. The earliest equilibrium melts are characterized by high Sr contents (the average = 658 ± 64 ppm), suggesting that high Sr/Y signatures were likely derived from their magma sources or fractionation at deeper levels in initially water-poor environments. Variations of some particular geochemical fingerprints in equilibrium melts such as, Dy/Dy* and Eu/Eu*, also provide fundamental information on the evolution of the magma plumbing system. Our study confirms the critical role of a deep crustal magma reservoir on the formation of magmatic-hydrothermal ore deposits. The fertility of magmas with respect to ore deposit formation was enhanced by the extraction and transfer of evolved magmas from the deep reservoir to shallower levels, particularly due to the enrichment of magmatic water contents. In addition, the presence of a deep magma reservoir also sustains the incremental growth of shallow magma chambers, which provide ore-forming fluids.

Aseismic mid-crustal magma reservoir at Cleveland Volcano imaged through novel receiver function analyses

Processes related to eruptions at arc volcanoes are linked by structures that transect the entire crust. Imaging the mid- to lower-crustal portions (here, ~5–15 km and >15 km respectively) of these magmatic systems where intermediate storage may occur has been a longstanding challenge. Tomography, local seismic source studies, geodetic, and geochemical constraints, are typically most sensitive to shallow (<5 km) storage and/or have insufficient resolution at these depths. Geophysical methods are even further limited at frequently-erupting volcanoes where well-developed trans-crustal magmatic systems are likely to exist, due to a lack of deep seismicity. Here we show direct evidence for mid-crustal magma storage beneath the frequently erupting Cleveland volcano, Alaska, using a novel application of seismic receiver functions. We use P-s scattered waves from the Moho as virtual sources to investigate S-wave velocities between the Moho and the surface. Our forward modeling approach allows us to provide direct constraints on the geometry of low velocity regions beneath volcanoes despite having a comparatively sparse seismic network. Our results show clear evidence of mid-crustal magma storage beneath the depths of located volcanic seismicity. Future work using similar approaches will enable an unprecedented comparative examination of magmatic systems beneath sparsely instrumented volcanoes globally.