Silicic Magma Chambers Research Articles

Comment on , Lithos 456-457, 107325, Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

An article (Gennaro et al., 2023, 456–457, 107325) recently published in Lithos proposes a model for the transcrustal magmatic plumbing system at the Marsili Seamount, a massive and still active volcanic centre at the Southern Tyrrhenian back-arc spreading ridge. Their model is based on a combination of geophysical and petrochemical data, the former confined to the axial zone of the Marsili volcano while the latter mostly refer to literature data dealing with samples collected from various sectors of the volcano. This comment questions the rationality of the proposed model, documenting that it is poorly constrained and, at places, incorrect.

Reply to the comment by on , Lithos 456–457, 107325, Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

Reply to the comment by on , Lithos 456–457, 107325, Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

Apatite Textures, Elemental and Isotopic Compositions Unmask the Homogenizing Process in Silicic Magma Chambers

AbstractSilicic magmas are the most viscous of all magmas, however some granitic plutons display remarkably homogeneous compositions, which contradicts the hypothesis that mechanical mixing is the main homogenizing process in the magma chamber. Thus much remains controversial about the mechanisms responsible for the homogeneities of silicic plutons. Here, we present textural observations, elemental mapping, and in situ elemental and Nd‐O isotopic data of apatites from the compositionally homogeneous Late Permian Yuanyang A‐type granitic pluton (SW Yunnan, South China). Apatite grains display oscillatory chemical zonation and resorption‐precipitation texture, suggesting incremental growth dominated by co‐genetic magma batches injection. The intra‐/inter‐grain core to rim elemental and Nd‐O isotopic variations imply crystal transfer and crystallization from different melt domains within the crystal mush. We propose that rejuvenation events associated with hotter cogenetic intermediate magma batch injection has induced crystal mush reactivation and convective stirring in silicic magma chambers, thereby homogenizing the entire reservoir.

Mafic magma recharge triggered eruption of a long-lived shallow silicic magma reservoir beneath a submarine volcano from the southwestern Okinawa Trough

Abundant submarine volcanoes have been identified in the southwestern Okinawa Trough, yet the duration of shallow magma reservoirs and processes driving their eruptions remains unclear. Here, the texture and in situ element compositions of orthopyroxene and magnetite in the dacite and mafic magmatic enclaves (MMEs) from the FDV-1 submarine volcano were investigated to decipher the duration of pre-eruption shallow silicic magma storage and eruption triggering processes. Both unzoned and zoned orthopyroxene phenocrysts were observed in the host dacite. The rims of zoned and unzoned orthopyroxene have similar compositions with Mg# from 49 to 55, Cr = 2.31–9.26 μg/g, V = 54.10–137 μg/g, and Ni = 16.44–21.19 μg/g, and they are in equilibrium with the rhyolitic matrix, which is consistent with growth in the shallow silicic magma reservoir. In contrast, the high Mg# cores (Mg# = 63–79) in the zoned orthopyroxenes are in equilibrium with basalt, with significant enrichment in Cr (1092–1737 μg/g), Ni (106–181 μg/g), and V (183–295 μg/g), which indicates a recycled (antecryst) origin by mafic magma recharge and magma mixing. Furthermore, the timescales from FeMg interdiffusion in orthopyroxene phenocrysts suggest that the magma mixing is approximately 600 years before eruption. Correspondingly, the shallow silicic magma reservoir beneath the submarine volcano is longevity. The presence of bubble and V-depleted magnetite aggregates in the MMEs suggests that volatile-saturated and oxidized mafic magma recharges into the shallow silicic magma chamber. Moreover, the distinct contents of relatively fast-diffusing Ni in magnetite between the MMEs and host dacite suggest that the mafic magma injection into the silicic magma reservoir and formation of the MMEs must have occurred over very short timescales (< 24 h) before the eruption to inhibit complete re-equilibration. Therefore, this last mafic magma recharge induces destabilization of the long-lived shallow silicic magma reservoir and leads to the dacitic magma eruption.

Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

The plumbing system of volcanoes in crustal convergent settings consists of vertically superimposed, sill-like reservoirs. The shallower reservoirs are generally filled by less dense and SiO2-richer magmas. The plumbing systems of spreading ridges are supposed to be simpler and characterized by prevailing sub-parallel vertical dykes. Here, we present the results of thermo-barometric determinations from minerals and modelling of potential field data at the Marsili spreading ridge in the Southern Tyrrhenian Sea back-arc oceanic basin. The Marsili plumbing system consists of sill-like, large SiO2-rich magma reservoirs located at the Moho depth and sub-vertical basaltic dykes crossing the whole oceanic crust. The formation of deep-seated silicic reservoirs is associated with a decrease in the back-arc spreading rate. The geometry of the plumbing system of spreading ridges may be not dissimilar from that of volcanoes at convergent margins.

Using Apatite to Track Volatile Evolution in the Shallow Magma Chamber below the Yonaguni Knoll IV Hydrothermal Field in the Southwestern Okinawa Trough

The Yonaguni Knoll IV is an active seafloor hydrothermal system associated with submarine silicic volcanism located in the “cross back-arc volcanic trail” (CBVT) in the southwestern Okinawa Trough. However, the behavior of volatiles during magmatic differentiation in the shallow silicic magma chamber is unclear. Here, the volatile contents of apatite inclusions trapped in different phenocrysts (orthopyroxene and amphibole) and microphenocrysts in the rhyolite from the Yonaguni Knoll IV hydrothermal field were analyzed by using electron microprobe analysis, which aims to track the behavior of volatiles in the shallow magma chamber. Notably, the ‘texturally constrained’ apatites showed a decreasing trend of XCl/XOH and XF/XCl ratios. Based on the geochemical analyses in combination with thermodynamic modeling, we found that the studied apatites were consistent with the mode of volatile-undersaturated crystallization. Therefore, volatiles were not saturated in the early stage of magmatic differentiation in the shallow rhyolitic magma chamber, and consequently, the metal elements were retained in the rhyolitic melt and partitioned into crystalline magmatic sulfides. Additionally, previous studies suggested that the shallow rhyolitic magma chamber was long-lived and periodically replenished by mafic magma. The injection of volatile-rich and oxidized subduction-related mafic magmas can supply abundant volatiles and dissolve magmatic sulfide in the shallow magma chamber. These processes are important for the later-stage of volatile exsolution, while the forming metal-rich magmatic fluids contribute to the overlying Yonaguni Knoll IV hydrothermal system.

Periodicity of Karoo rift zone magmatism inferred from zircon ages of silicic rocks: Implications for the origin and environmental impact of the large igneous province

New U-Pb ages for zircons constrain the duration of silicic magmatism and timing of coeval mafic magmatism across the main rift zone of the Karoo large igneous province in Mozambique. Our 190 ± 2 Ma, 188.4 ± 0.9 Ma, 181.7 ± 1.0 Ma, 180 ± 3 Ma, 178 ± 2 Ma and 172 ± 2 Ma ages support periodicity of Karoo magmatism previously inferred from 40Ar/39Ar age data. The ∼ 190–188 Ma ages confirm early onset of magmatism and the ∼ 182–178 Ma ages correlate the bimodal volcanic successions of the Lower Zambezi and the Movene Formation with widespread silicic magmatism across the rift zone. The ∼ 172 Ma age corresponds to waning magmatic activity. The age range and Hf isotopic compositions of zircons indicate up to ∼9 Ma lifespan for the Jurassic silicic magma chambers and suggest that the ∼2700–400 Ma xenocrysts represent crustal sources of the host rocks. The available chronological data indicate that the ∼183 Ma main phase magmatism was largely confined within the main Karoo and Kalahari basins and that the preceding and subsequent phases were mainly associated with the Karoo rift zone. Judging from geochemical literature, different kinds of magmas were erupted during the successive magmatic phases. We calculate from published geochemical data that the mafic main phase magmas were relatively poor in CO2 and SO2 and the lava stacking patterns point to low eruption rates, which suggests that degassing of sedimentary wall-rocks of intrusions probably triggered the coeval Pliensbachian-Toarcian extinction. In contrast, the mafic late phase magmas were rich in CO2 and SO2 and at least some of the lavas indicate high eruption rates. We propose that efficient degassing from widespread mafic magmatism and explosive eruption of over 30,000 km3 of silicic magmas in the Karoo rift zone linked the ∼182–178 Ma late phase magmatism with contemporaneous global biosphere crises.

Structure of the Silicic Magma Chamber and Mechanism of Magma Mixing Estimated by Two Types of Mafic Inclusions: Petrological Study of the Kurodake in the Taisetsu Volcanic Group, Central Hokkaido, Japan

Structure of the Silicic Magma Chamber and Mechanism of Magma Mixing Estimated by Two Types of Mafic Inclusions: Petrological Study of the Kurodake in the Taisetsu Volcanic Group, Central Hokkaido, Japan

Element and Sr isotope zoning in plagioclase in the dacites from the southwestern Okinawa Trough: Insights into magma mixing processes and time scales

The processes and time scales of magma mixing in shallow silicic magma chambers prior to eruptions in the Okinawa Trough (OT) are still poorly understood. Here, we employed in situ major-trace elements and Sr isotopic analyses, coupled with textural investigations, on plagioclase phenocrysts in the mafic magmatic enclaves (MMEs) and their host dacites from the southwestern OT to address this issue. The plagioclase phenocrysts in the MMEs have high An% contents (82–88) and Sr/Ba ratios (7.2–17.2) and relatively unradiogenic 87Sr/86Sr ratios (0.70536–0.70595), suggesting that they crystallized in the chilled basaltic andesitic magma. In contrast, plagioclase phenocrysts in the host dacites show a wide composition range, with An% contents of An47 to An93, Sr/Ba ratios of 3.4–18.0, and 87Sr/86Sr ratios of 0.70535–0.70791. The plagioclase phenocrysts are zoned, with high-An% cores characterized by resorption textures and mantled by euhedral low-An% rims. The transition between cores and rims is very abrupt, with a decrease of 30 to 40 mol% An. This shift coincides with large and abrupt changes in Sr/Ba ratios. The plagioclase phenocryst rims have low Sr/Ba values (3.4–5.1) and 87Sr/86Sr ratios (0.70632–0.70675) that are similar to those of the whole-rock dacites, suggesting that the plagioclase rims grew from the host dacitic magma. In contrast, Sr isotopes in the plagioclase cores are either radiogenic (0.70724–0.70791) or unradiogenic (0.70535–0.70595) compared with rims, combined with glomerocryst textures, consistent with their derivation from distinct deep mafic crystal mush zones, and were then entrained by ascending melts and transported rapidly to the shallow silicic magma chamber and underwent decompression during ascent, leading to plagioclase core resorption. Mg diffusion calculations indicate that incorporation of the calcic cores into the shallow silicic magma chamber experienced a long pre-eruptive storage (~600 years). The distinct Sr isotopes of the plagioclase, particularly the different compositions of titanomagnetite in the MMEs and host dacites, reflect the mafic magma injection into the silicic magma chamber and formation of the MMEs must have occurred over very short time scales before eruption to inhibit complete re-equilibration, which likely triggered the dacitic magma to finally erupt. Hence, the silicic magma in the southwestern OT contains a crystal cargo with a complex, open-system crystallization history and that magma mixing/mingling is an important process for controlling the chemical and textural diversity of these silicic magmas as well as a likely eruption trigger.

Geochemistry and petrogenesis of the post-collisional high-K calc-alkaline magmatic rocks in Tengchong, SE Tibet

Post-collisional high-K calc-alkaline (HKCA) magmatic rocks (8 Ma to present) are extensively distributed in the Tengchong volcanic field (TVF), SE Tibet. Here, an integrated study of whole-rock major-trace elements, Sr-Nd-Pb-O isotopes and zircon U-Pb-Hf-O isotopes, together with mineral texture and geochemistry, are presented for these volcanic suites. The HKCA magmatic rocks exhibit high concentrations of incompatible trace elements with negative Nb-Ta-Ti anomalies and high (87Sr/86Sr)i (0.705–0.710), low (143Nd/144Nd)i (0.5120–0.5126) with variable δ18O (5.7–9.3‰). They may be divided into four groups from early to late stages: Groups I and III are basaltic-trachybasaltic-trachyandesitic assemblages (MgO = 3.3–9.2 wt.%); Groups II and IV mainly consist of trachyandesites and dacites (MgO = 0.6–5.1 wt.%). Incompatible trace element and Sr-Nd isotope compositions of the mafic magmas (MgO = 6.0–9.2 wt.%) from Groups I and III are postulated to reflect fluid enrichment of a mantle source by silicate and carbonatitic metasomatism as a consequence of eastward underthrusting of the Indian continental slab following the India-Asia collision. The petrogenesis of differentiated magmas (Groups II and IV) is interpreted in terms of control by crustal magma chamber AFC processes. The results of P-T estimation and EC-AFC modelling suggest at least two magma storage regions: a deep one (17–28 km) in which the trachyandesitic magmas were produced by AFC of basaltic magmas and a shallow one (9–14 km) fed by trachyandesitic magmas which gave rise to dacitic magmas via AFC. Phenocryst zoning textures also record mafic replenishment into silicic magma chambers.

Amphibole perspective to unravel the rhyolite as a potential fertile source for the Yonaguni Knoll IV hydrothermal system in the southwestern Okinawa Trough

Yonaguni Knoll IV is an active seafloor hydrothermal system containing massive Zn–Pb sulfides located in the southwestern Okinawa Trough. Previous research has suggested that the metal elements Zn and Pb were dominantly derived from the local volcanic rocks via water–rock interactions. However, it is not known whether or why the volcanic rocks are rich in these metals. Here, we performed a detailed in situ analysis of the major and trace elements in amphiboles in the rhyolite from the hydrothermal field to decipher the physical‐chemical conditions of the silicic magma, including temperature (T), pressure (P), oxygen fugacity (fO2), volatile contents (Cl, H2O) and metal contents (Pb, Zn), and investigate the behavior of the metals (Zn and Pb) during melt evolution. Our results show that the amphiboles are euhedral calcic magnesio‐hornblende and are in equilibrium with the hydrous and oxidized host rhyolitic melt characterized by high H2Omelt (~5.8–6.5 wt.%), fO2 (NNO +0.1 to +0.6) and SiO2melt (~71.73–72.62 wt.%) values at 761–797°C and 102–137 MPa, corresponding to an approximate depth of 3.9–5.2 km. In addition, the rare earth element (REE) patterns of the melt reconstructed from the amphiboles are analogous to those of cold‐wet‐oxidized rhyolites. Thus, the amphiboles crystallized in a cold‐wet‐oxidized shallow silicic magma chamber. The high oxygen fugacity is inferred to have favored sulfur and metal enrichment in the melt. The most abundant metal in the amphiboles is Zn (237–294 ppm), and the concentrations of Pb are much lower (0.86–3.32 ppm), suggesting that Zn preferentially entered the amphibole and that Pb was retained in the melt. The Cl content of the amphiboles ranges from 0.11 to 0.25 wt.%, and accordingly, the Clmelt content estimated from the amphiboles varies from 0.15 to 0.28 wt.%. The low (Cl/H2O)melt ratios, which are &lt;0.05 (i.e., ~0.02–0.04), suggest that the melt exsolved an H2O‐rich vapor rather than a hydrosaline fluid with insufficient Cl to transport metals (e.g., Pb and Zn) into the vapor phase during degassing both in the magma chamber and during ascent. The lack of exsolved hydrosaline fluid and the high oxidation conditions resulted in the metal elements Pb and Zn remaining dissolved in the rhyolitic melt and/or partitioning into crystalline minerals upon eruption. Thus, the large volumes of rhyolitic magma that intruded into the upper crust of the Yonaguni Knoll IV hydrothermal field are a potential fertile source of these elements via leaching by heated seawater.

Multiple timescale constraints for high-flux magma chamber assembly prior to the Late Bronze Age eruption of Santorini (Greece)

The rhyodacitic magma discharged during the 30–80 km3 DRE (dense rock equivalent) Late Bronze Age (LBA; also called ‘Minoan’) eruption of Santorini caldera is known from previous studies to have had a complex history of polybaric ascent and storage prior to eruption. We refine the timescales of these processes by modelling Mg–Fe diffusion profiles in orthopyroxene and clinopyroxene crystals. The data are integrated with previously published information on the LBA eruption (phase equilibria studies, melt inclusion volatile barometry, Mg-in-plagioclase diffusion chronometry), as well as new plagioclase crystal size distributions and the established pre-LBA history of the volcano, to reconstruct the events that led up to the assembly and discharge of the LBA magma chamber. Orthopyroxene, clinopyroxene and plagioclase crystals in the rhyodacite have compositionally distinct rims, overgrowing relict, probably source-derived, more magnesian (or calcic) cores, and record one or more crystallization (plag ≫ opx > cpx) events during the few centuries to years prior to eruption. The crystallization event(s) can be explained by the rapid transfer of rhyodacitic melt from a dioritic/gabbroic region of the subcaldera pluton (mostly in the 8–12 km depth range), followed by injection, cooling and mixing in a large melt lens at 4–6 km depth (the pre-eruptive magma chamber). Since crystals from all eruptive phases yield similar timescales, the melt transfer event(s), the last of which took place less than 2 years before the eruption, must have involved most of the magma that subsequently erupted. The data are consistent with a model in which prolonged generation, storage and segregation of silicic melts were followed by gravitational instability in the subcaldera pluton, causing the rapid interconnection and amalgamation of melt-rich domains. The melts then drained to the top of the pluton, at fluxes of up to 0.1–1 km3 year− 1, where steep vertical gradients of density and rheology probably caused them to inject laterally, forming a short-lived holding chamber prior to eruption. This interpretation is consistent with growing evidence that some large silicic magma chambers are transient features on geological timescales. A similar process preceded at least one earlier caldera-forming eruption on Santorini, suggesting that it may be a general feature of this rift-hosted magmatic system.

Application and reliability of calcic amphibole thermobarometry as inferred from calc-alkaline products of active geothermal areas in the Andes

Igneous thermobarometry may contribute to locate the upper-crust heat source/s representing one of the main factors required to establish the potential of high enthalpy geothermal areas through models of the heat flux. To test this possibility, we applied amphibole thermobarometry to some of the youngest lavas and pyroclastics (andesite to rhyolite) of Apacheta and La Torta (Chile) and Chachimbiro (Ecuador) areas representing medium to high enthalpy geothermal systems located in two different crustal contexts along the Andean active continental margins of South America. We used the robust application of Ridolfi and Renzulli (2012) allowing to estimate the intrinsic physico-chemical parameters with reasonably low uncertainties (T ± 24 °C, P ± 12%, fO2 ± 0.4 log units) and the composition of the melt in equilibrium with Mg-rich calcic amphibole in a wide range of conditions, up to 1130 °C and 2.2 GPa. In addition, analyses of the intra-crystalline compositional and textural variations allowed us to propose a method to distinguish between chemically homogeneous and heterogeneous amphiboles, avoiding some incorrect applications of the Ridolfi and Renzulli (2012)'s equations and thus unreliable results.Thermobarometric constraints indicate a polybaric crystallization of magmas below the Chilean geothermal areas of Apacheta and La Torta. In the last 150 ka, the feeding systems of these areas mainly consisted of shallow silicic magma chambers at depths of 4–8 km (740–840 °C), giving rise to dacite domes (tortas). Amphiboles in the rhyolite lava flows, erupted earlier (Pliocene-Pleistocene) during the activity of the nearby Apacheta-Aguilucho Volcanic Complex, show a similar T-depth range (780–810 °C, 4–6 km). At deeper levels (10–20 km), the depths estimated using amphiboles from the magmatic enclaves (in the dacite domes) and from the andesite lavas are approximately aligned along the upper morphologic margins of the well geophysically constrained Altiplano-Puna Magma Body (APMB; a partially melted zone in the upper crust where the S-waves velocities are <2.9 km/s), with estimated temperatures of 850–1010 °C. Heat source contributions for the investigated Chilean geothermal areas should therefore be represented by both the shallower magma bodies and the huge APMB.The Holocene products of the Ecuadorian Chachimbiro geothermal area indicate amphibole crystallization from 7 to 17 km depth at temperature conditions in the range of 800–910 °C. The occurrence of two separated regions of magma storage at approximately 230 ± 30 MPa and 380 ± 60 MPa is discussed. They should be taken into account as heat sources for this Ecuadorian geothermal area.

Deposits, petrology and mechanism of the 2010–2013 eruption of Kizimen volcano in Kamchatka, Russia

Kizimen volcano in Kamchatka is well known as a source of highly heterogeneous poorly mingled magmas ranging from dacites to basaltic andesites. In 2010–2013, the volcano produced its first historical magmatic eruption with the deposition of 0.27 km3 of block and ash pyroclastic flows accompanied by slow extrusion of a 200-m-thick, highly viscous (1010–1011 Pa s) block lava flow with a volume of 0.3 km3. The total volume of erupted magma comprised approximately 0.4 km3 DRE. We provide description of the eruption chronology, as well as the lithology and petrology of eruptive products. The erupted material is represented by banded dacite and high-silica andesite. The dacitic magma was formed during a long dormancy after the previous magmatic eruption several hundred years ago with mineral compositions indicating average pre-eruptive temperatures of ~ 810 °C, fO2 of 0.9–1.6 log units above the nickel–nickel oxide (NNO) buffer and shallow crustal storage conditions at ~ 123 MPa. The silica-rich andesite represents a hybrid magma, which shows signs of recent thermal and compositional disequilibrium. We suggest that the hybrid magma started to form in 1963 when a swarm of deep earthquakes indicated an input of mafic magma from depth into the 6–11-km-deep silicic magma chamber. It took the following 46 years until the magma filling the chamber reached an eruptible state. Poor mingling of the two melts is attributed to its unusually high viscosity that could be associated with the pre-eruptive long-term leakage of volatiles from the chamber through a regional tectonic fault. Our investigations have shown that shallow magma chambers of dormant volcanoes demonstrating strong persistent fumarolic activity can contain highly viscous, degassed magma of evolved composition. Reactivation of such magma chambers by injection of basic magma takes a long time (several decades). Thus, eruption forecasts at such volcanoes should include a possibility of long time lag between a swarm of deep earthquakes (indicating the recharge of basic magma from depth) and the following swarm of shallow earthquakes (indicating final ascent of the hybrid magma towards the surface). Due to the high viscosity of the magma, the shallow swarm can last for more than a year. The forthcoming eruption can be of moderate to low explosivity and include extrusion of viscous lava flows and domes composed of poorly mingled magmas of contrasting compositions.

Magma Emplacement Rates and Porphyry Copper Deposits: Thermal Modeling of the Yerington Batholith, Nevada

Abstract Many porphyry copper deposits are associated with granitoid plutons. Porphyry copper deposit genesis is commonly attributed to degassing of pluton-forming intermediate to silicic magma chambers during slow cooling and crystallization. We use numerical simulations of thermal evolution during pluton growth to investigate the links between pluton construction, magma accumulation and solidification, volatile release, and porphyry copper deposit formation. The Jurassic Yerington batholith, Nevada, serves as a case study because of its exceptional exposure, revealing the geometry of three main intrusions. The last intrusion, the Luhr Hill granite, is associated with economic porphyry copper deposits localized over cupolas where dikes and fluid flow were focused. Our simulations for the conceptual model linking porphyry copper deposits with the presence of large, highly molten magma chambers show that the Luhr Hill granite must have been emplaced at a vertical thickening rate of several cm/yr or more. This magma emplacement rate is much higher than the time-averaged formation rates of other batholiths reported in the literature. Such low rates, although common, do not lead to magma accumulation and might be one of the reasons why many granitoid plutons are barren. Based on our results, we formulate the new testable hypothesis of a link between porphyry copper deposit formation and the emplacement time scale of the associated magma intrusion.

B20 珪長質マグマ溜まりにおける結晶沈降と再平衡化の微量元素組成変化モデル(噴火のダイナミクス (1), 日本火山学会2005年秋季大会)

B20 珪長質マグマ溜まりにおける結晶沈降と再平衡化の微量元素組成変化モデル(噴火のダイナミクス (1), 日本火山学会2005年秋季大会)

P65 斑晶とメルト包有物の組成および組織から推定する富士山宝永噴火の珪長質マグマ溜まりでのマグマプロセス(ポスターセッション)

P65 斑晶とメルト包有物の組成および組織から推定する富士山宝永噴火の珪長質マグマ溜まりでのマグマプロセス(ポスターセッション)

Rapid assembly and crystallization of a fossil large-volume silicic magma chamber

The rates at which large volumes of eruptible, silicic (>65 wt% SiO2) magma (magma chambers) are assembled, as well as their longevity in the upper crust, remain controversial. This controversy is due, in part, to a missing record of granitoid plutonic complexes that represent large fossil upper crustal magma chambers. We present new geologic mapping and high-precision U-Pb zircon geochronology from the Eocene Golden Horn batholith in Washington State, USA. These data reveal that the batholith was constructed as a series of sills over 739 ± 34 k.y. Topographic relief of >2 km permits volume estimates for 4 of the sills, the largest of which, a >424 km3 rapakivi granite, was emplaced over 26 ± 25 k.y. at a rate of ∼0.0125 km3/yr. This rate exceeds those needed to build large, silicic magma chambers in thermal models, and we suggest that that this unit may represent the first fossil magma chamber of this type recognized in the geologic record.

Mafic replenishments into floored silicic magma chambers

Commingling between contemporaneous mafic and felsic magmas is now widely recognized in a broad range of intrusions and intrusive complexes. These interactions are important features for two main of reasons: (1) the rapidly chilled margins of mafic magma against silicic magma commonly preserve the compositions of mafic liquids, and (2) because the mafic magma solidifies rapidly, the resulting (final) configurations of mafic and felsic magmas can provide insights into physical processes and changing viscosity contrasts and rheologies of magmas and felsic crystal mush during crystallization of the mafic magma. Mingling of contrasted magmas was first recognized in the 1950s. Wider recognition of interactions between mafic and silicic liquids led to concepts of “net-veining” in the 1960s, “intramagmatic flows” (chilled basaltic layers separated by felsic cumulates) in the 1970s, and in the 1990s to “mafic-silicic layered intrusions” (MASLI), which could be as much as a few kilometers thick and more than 100 km2 in area. It was quickly appreciated that these MASLI preserved stratigraphic records of mafic replenishments into silicic magma chambers floored by felsic crystal mush. Volcanic studies had anticipated the occurrence of this last type of intrusion on the grounds that extensive ponding of basaltic magmas beneath silicic chambers was seen to be essential to keep large silicic systems like Yellowstone active for millions of years. This paper looks at the history of changing perceptions and interpretations of magma mingling and whether or not “sill complexes” are distinct from mafic-silicic layered intrusions. The stratigraphy of mafic-silicic layered intrusions records changing magmatic compositions, events, and processes in a temporal framework comparable to that provided by coeval volcanic rocks. As a result, careful study of MASLI has great potential for linking plutonic and volcanic processes and events.

Evolution and Eruption Processes of a Highly Porphyritic Silicic Magma System: Petrology of the Historical Eruptive Stage of Hokkaido-Komagatake Volcano, Japan

After about 5000 years of dormancy, the Hokkaido-Komagatake volcano started its historical eruptive stage with a plinian eruption in AD 1640. During this eruption, a zoned magma chamber was formed with upper mafic and lower silicic layers. This study focuses on the temporal evolution and eruption processes of the magma chamber based on petrological investigations of subsequent eruptions in AD 1694, 1856, 1929 and 1942. In all the eruptive products the majority of the clasts are strongly porphyritic white pumice (silicic andesite), which was derived from the silicic end-member magma. These are associated with small amounts of mafic ejecta (mafic to intermediate andesite). Based on density calculations for the magmas, the layered structure of the zoned magma chamber is likely to have been stable throughout the historical eruptive stage. In each eruption since AD 1694, relatively mafic magma erupted initially followed by more silicic magma. In addition, although there is petrological evidence for a mafic magma injection into a resident silicic magma chamber prior to the AD 1640 eruption, all eruptions from AD 1694 onwards appear to have taken place without new mafic injections. This suggests that since the AD 1694 eruption magmas have been erupted sequentially from the upper mafic part of the magma chamber. The composition of scoria, the most mafic ejecta, changed from relatively mafic magma in the AD 1694 eruption to gray pumice in later eruptions, which represent a hybrid magma. This temporal change is interpreted to indicate that mafic magma mixed gradually with more silicic magmas during repeated eruptions. In addition, the volume of the mafic and hybrid magmas decreased because of repeated eruptions without new mafic magma injections, so that the AD 1942 eruption was dominated by the eruption of silicic magma. We consider the present chamber to be filled with highly porphyritic, viscous silicic magma.