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The scarcity of historical rhyolite eruptions means that volcanological and petrological studies of past eruptions are a key tool for assessing the potential for future hazardous activity and improving interpretations of unrest signals. For the last 18 ky, the Laguna del Maule (LdM) volcanic complex in Chile has erupted primarily rhyolites but with differing magma compositions, eruption styles and eruptive volumes. Rapid surface uplift and episodic seismic activity at LdM over the last two decades has emphasized the need to understand both the recent evolution of the magmatic system and the most likely future eruption scenarios. Using mineral composition, geothermobarometry and MELTS modeling, we assess the influence of the magma storage and ascent conditions on the magnitude and styles of three LdM rhyolites. Magmas of the first and largest Plinian-ignimbrite eruption (LdM: rdm ; >17 km3 DRE) form a distinct mineral assemblage with An28–60 plagioclase, amphibole and quartz derived from a magma plumbing system over a large pressure range (90–350 MPa). We suggest that the rdm eruption was triggered by magma recharge and overpressure within a ~ 90 MPa magma chamber of high-silica (>76.5 wt.% SiO2) rhyolitic melt. The rdm eruption appears to have reset the LdM storage conditions, such that subsequent rhyolite eruptions have been smaller (<4 km3 DRE), involved less evolved melt (<75.8 wt.% SiO2) and produced (sub)Plinian-effusive eruption sequences. Of these, we have studied the earliest (Los Espejos: rle) and the most recent (Las Nieblas: rln). Both have mineral assemblages with An20–34 plagioclase and biotite formed in water-saturated crystal-poor rhyolite batches over limited pressure ranges (ΔP ~ 50 MPa) within a rhyodacitic mush reservoir. We suggest that rle and rln were triggered by injection of water-rich magma and volatile exsolution; the transition to effusive behavior occurred because of changes in magma temperature (rle) or magma decompression rate (rln). Similarities in the limited pressure range and high volatile content inferred for the current low-density magma body with the petrologically determined source conditions of magmas that fed past subplinian-effusive eruptions suggest that the next LdM eruption will be similar. [ABSTRACT FROM AUTHOR]

Osorno volcano (41°06'S, 72°20'W) is a composite stratovolcano of the Central Southern Volcanic Zone of the Chilean Andes. It is the southernmost member of a NE–SW trending alignment of volcanic edifices including La Picada and Puntiagudo volcanoes and the Cordón Cenizos chain. According to contemporary descriptions recorded by Charles Darwin in 1835, two eruptive events occurred: the first during January–February, and the second during November–December 1835 and January 1836. The volcano erupted basaltic andesite lavas and tephra fall deposits (52.4 to 52.9 SiO2 wt. %), which contain phenocrysts of olivine, plagioclase, clinopyroxene, and spinel. The compositions of these phenocryst phases, together with those of olivine-hosted melt inclusions, allowed us to constrain intensive parameters for the pre-eruptive magmas. These varied from 1060°C to 1140°C, with an oxygen fugacity buffer of ~ΔQFM +1.1, dissolved water concentrations of up to 5.6 wt. % (average of ~4.2 wt. %) and maximum pressures equivalent to ~7-km depth. Textural relations, such as crystal accumulations and clots, zoning in crystals and other indications of disequilibrium, lead us to infer the involvement of a crystal mush, rich in individual crystals and clots of crystals, which underwent a degree of disaggregation and entrainment into the transiting magma prior to eruption. Comparison of trace element abundances, including rare earth elements, fluid-mobile elements, and relatively fluid-immobile elements, combined with 87Sr/86Sr and 143Nd/144Nd isotope ratios, allows us to consider variations in slab-derived fluid input and the minor role of crustal contamination on the Osorno eruptive products and those from neighboring volcanic systems. Our results suggest both a greater contribution from slab-derived fluid and a higher degree of partial melting in the systems supplying stratovolcanoes (Osorno, Calbuco, and La Picada) relative to those supplying small eruptive centers built over the major regional Liquiñe-Ofqui Fault Zone. [ABSTRACT FROM AUTHOR]

Hybrid dacite magmas from Taápaca volcano in the Central Andean Volcanic Zone (18°S, northern Chile) contain sanidine crystals of unusual size (1–12 cm) and abundant mafic enclaves of variable composition throughout the entire eruptive history (1·5 Ma to recent) of the volcano. They are rich in mineral inclusions and strongly zoned in Ba with distinct growth bands separated by resorption interfaces. Resorption is followed by a sudden increase in Ba with compositional contrasts up to 2·3 wt% BaO. We argue that resorption and the sharp jumps in Ba concentration reflect distinct heating and melting events, suggesting that different growth zones formed at different temperatures. Amphibole–plagioclase thermobarometry based on mineral inclusions gives variable temperatures of ∼720–820 °C at shallow pressures (0·1–0·3 GPa) for individual growth zones. Using these temperatures for diffusion modelling, Ba profiles from X-ray scanning profiles and grey-scale gradients based on accumulated back-scattered electron images across these interfaces allow us to estimate crystal residence and reactivation times prior to eruption. This temperature control allowed the application of a 'non-isothermal' diffusion algorithm to obtain diffusion times for individual diffusive boundaries that range from 0·4 to 490 kyr and add up to total residence times of 9–499 kyr for different crystals from different stages of eruption. A combination of temperatures, pressure, diffusion times and R-MELTS modelling of the parent rhyodacite suggests storage conditions for the Taápaca reservoir at near eutectic composition at shallow depth (4–10 km). Temperatures never fell below the magma solidus but frequently cycled between 720 °C and 820 °C (i.e. between eruptible and non-eruptible state with crystallinity circling around ∼40–50 vol%) for tens to hundreds of thousands of years. We define this as 'long-term transitional temperature cycling' or LTTC storage. Frequent recharge events of basaltic andesite magma, as represented by abundant mafic enclaves, orchestrated the temperature cycling, resulted in multiple heating events that caused frequent resorptions and interrupted crystal growth, and kept the reservoir thermally 'alive'. Recharge events became more frequent only ∼3–11 kyr before the eventual eruption that carried a particular set of sanidine megacrysts to the surface. Thus, after many earlier recharge events that did not result in eruption, a final event involved mixing at a critical recharge rate to mobilize, entrain, and erupt a particular set of megacrysts from the resident rhyodacite in a hybrid dacite host. This process, happening not more than a few centuries before an eruption, has been repeated at similar time-scales at different stratigraphic stages throughout the 1·5 Myr history of Taápaca volcano. The observed mineral zonation patterns and size of sanidine crystals from the resident magma reservoir below Taápaca volcano are identical to those observed in the megacrysts from granite intrusions that also show typical age ranges of zircon crystallization that are comparable with the residence times extracted here from Ba zonation. Taápaca sanidines thus may represent an erupted equivalent and provide 'smoking gun' evidence of temperature cycling during the formation of such K-feldspar megacrysts in granites. [ABSTRACT FROM AUTHOR]

Volcán Quizapu, Chile, is an under-monitored volcano that was the site of two historical eruptions: an effusive eruption in 1846–1847 and a Plinian eruption in 1932, both of which discharged ∼5 km3 (dense rock equivalent) of lava and/or tephra. The majority of material erupted in both cases is trachydacite, nearly identical for each event. We present H2O-saturated, phase equilibrium experiments on this end-member dacite magma, using a pumice sample from the 1932 eruption as the main starting material. At an oxygen fugacity (f O2) of ∼NNO + 0·2 (where NNO is the nickel–nickel oxide buffer), the phase assemblage of An25–30 plagioclase + amphibole + orthopyroxene, without biotite, is stable at 865 ± 10 °C and 110 ± 20 MPa H2O pressure (P H2O), corresponding to ∼4 km depth. At these conditions, experiments also reproduce the quenched glass composition of the starting pumice. At slightly higher P H2O and below 860 °C, biotite joins the equilibrium assemblage. Because biotite is not part of the observed Quizapu phase assemblage, its presence places an upper limit on P H2O. At the determined storage P H2O of ∼110 MPa, H2O undersaturation of the magma with X H 2 O fluid = 0·87 would align P total to mineral-based geobarometry estimates of ∼130 MPa. However, X H 2 O fluid < 1 is not required to reproduce the Quizapu dacite phase assemblage and compositions. A second suite of experiments at lower f O2 shows that the stability fields of the hydrous silicates (amphibole and biotite) are significantly restricted at NNO – 2 relative to NNO + 0·2. Additional observations of Quizapu lava and pumice samples support the existing hypothesis that rapid pre-eruptive heating drove the effusive 1846–1847 eruption, with important refinements. We demonstrate that microlites in the end-member dacite lavas are consistent with in situ crystallization (during ascent), rather than transfer from an andesite. In one end-member dacite lava, newly identified reverse zoning in orthopyroxene and incipient destabilization of amphibole are consistent with small degrees of heating. Our work articulates a clear direction for future Quizapu studies, which are warranted given the active nature of the Cerro Azul–Descabezado Grande volcanic axis. [ABSTRACT FROM AUTHOR]

Where and how arc magmas are generated and differentiated are still debated and these questions are investigated in the context of part of the Andean arc (Chilean Southern Volcanic Zone) where the continental crust is thin. Results are presented for the La Picada stratovolcano (41°S) that belongs to the Central Southern Volcanic Zone (CSVZ) (38°S–41·5°S, Chile) which results from the subduction of the Nazca plate beneath the western margin of the South American continent. Forty-seven representative samples collected from different units of the volcano define a differentiation trend from basalt to basaltic andesite and dacite (50·9 to 65·6 wt % SiO2). This trend straddles the tholeiitic and calc-alkaline fields and displays a conspicuous compositional Daly Gap between 57·0 and 62·7 wt % SiO2. Interstitial, mostly dacitic, glass pockets extend the trend to 76·0 wt % SiO2. Mineral compositions and geochemical data indicate that differentiation from the basaltic parent magmas to the dacites occurred in the upper crust (∼0·2 GPa) with no sign of an intermediate fractionation stage in the lower crust. However, we have currently no precise constraint on the depth of differentiation from the primary magmas to the basaltic parent magmas. Stalling of the basaltic parent magmas in the upper crust could have been controlled by the occurrence of a major crustal discontinuity, by vapor saturation that induced volatile exsolution resulting in an increase of melt viscosity, or by both processes acting concomitantly. The observed Daly Gap thus results from upper crustal magmatic processes. Samples from both sides of the Daly Gap show contrasting textures: basalts and basaltic andesites, found as lavas, are rich in macrocrysts, whereas dacites, only observed in crosscutting dykes, are very poor in macrocrysts. Moreover, modelling of the fractional crystallization process indicates a total fractionation of 43% to reach the most evolved basaltic andesites. The Daly Gap is thus interpreted as resulting from critical crystallinity that was reached in the basaltic andesites within the main storage region, precluding eruption of more evolved lavas. Some interstitial dacitic melt was extracted from the crystal mush and emplaced as dykes, possibly connected to small dacitic domes, now eroded away. In addition to the overall differentiation trend, the basalts to basaltic andesites display variable MgO, Cr and Ni contents at a given SiO2. Crystal accumulation and high pressure fractionation fail to predict this geochemical variability which is interpreted as resulting from variable extents of fractional crystallization. Geothermobarometry using recalculated primary magmas indicates last equilibration at about 1·3–1·5 GPa and at a temperature higher than the anhydrous peridotite solidus, pointing to a potential role of decompression melting. However, because the basalts are enriched in slab components and H2O compared to N-MORB, wet melting is highly likely. [ABSTRACT FROM AUTHOR]

The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36°S, is among the most active latest Pleistocene-Holocene rhyolitic centers globally and has been inflating at a rate of >20 cm a-1 since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, ~20 km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic precursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The 238U/230Th ratios of the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied frontal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope compositions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New 40Ar/39Ar and 36Cl ages reveal a wide footprint of silicic volcanism during the early post-glacial (25-19 ka) and Holocene (c. 8-2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element compositions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of near- to sub-solidus silicic cumulates. A long-term rate of mass addition--extrapolated from surface deformation accumulated over the past decade--is comparable with those that have produced moderate- to large-volume caldera-forming eruptions elsewhere. [ABSTRACT FROM AUTHOR]

On 1 January 2008 Llaima volcano, a basaltic andesite stratocone in southern Chile, entered a phase of violent Strombolian eruption. Llaima, like many passively degassing systems, has experienced prolonged (decades-long) periods of persistent summit degassing from its open vent. The rapid transition from long-lived passive degassing to violent explosive eruption occurred with limited precursory monitoring signals. This study is motivated by the desire to understand what occurs in these systems when that switch takes place. To this end, we study the products of the 2008 violent Strombolian eruption of Llaima volcano. We present new textural analyses of scoria and geochemical data for five whole-rock samples, troctolite glomerocrysts with and without Cr-spinel, and 182 olivine-hosted melt inclusions from tephra samples. Two populations of scoria ('brown' and 'black') are distinguished by their variable crystallinity and vesicularity, but are geochemically indistinguishable. Black scoria contains abundant microlites with tabular to acicular morphologies and convolute vesicles up to 1⋅75 mm in effective diameter. The brown scoria tends to have fewer, acicular microlites, abundant matrix glass, and round vesicles with a narrower size distribution, constrained to < 0⋅4 mm in diameter. Overall, the textures of the black and brown scoria provide evidence for a textural maturation process in which shallow system magma becomes more crystal rich and probably rheologically stiffer as a result of prolonged passive degassing. The Cr-spinel-bearing and Cr-spinel-free troctolite glomerocrysts have plagioclase and olivine compositions of An65-92 and Fo81, respectively. The Cr# in the Cr-spinel ranges from 26 to 37, consistent with magma originating from the deeper plumbing system. Whole-rock compositions for the tephra average 51 wt % SiO2, 18 wt % Al2O3, and ~6 wt % MgO. The major element compositions of olivine-hosted melt inclusions range from 49 to 56 wt % SiO2 and 3⋅72 to 7⋅76 wt % MgO; there is no distinct compositional difference between olivine-hosted melt inclusions sourced from the different scoria. Melt inclusion volatile contents range from below detection to 2⋅95 wt % H2O and 1973 ppm CO2 (though not in the same melt inclusion). H2O and CO2 concentrations are consistent with open-system degassing and, when compared with differentiation indices (e.g. K2O), indicate coupled degassing and crystallization throughout the system. The majority of melt inclusions define a single liquid line of descent indicative of plagioclase and olivine fractionation. Entrapment pressures range from 8 to 342 MPa and fall into two groups: 8-100 MPa (300 m to ~4 km depth) and >100 MPa (4-14 km depth), revealing that this eruption tapped a deep plumbing system. We propose here that passive degassing at Llaima is maintained by periodic, small-batch magma injections. Consequently, owing to extensive degassing the upper plumbing system magma crystallized and increased in viscosity. Before the 2008 eruption, some volatiles sourced from the repeatedly injected magmas exsolved from the inferred crystal mush and ascending from deeply sourced degassing magmas, and gradually accumulated within the crystal mush and beneath the stiffened conduit magma. Our results support a model in which eruption triggering occurred when magma injection remobilized the mush and, importantly, unlocked the accumulated gases, which ascended rapidly and generated the observed violent Strombolian explosive activity. Our proposed model contrasts with those models for explosive mafic volcanism that require rapid magma ascent under closed-system degassing conditions. Importantly, our proposed mechanism provides a means for systems with dominantly open-system degassing behavior to switch from passive degassing to explosive eruptions. [ABSTRACT FROM AUTHOR]

Geochronological and geochemical studies, including platinum group element (PGE) analyses, were undertaken on samples from the El Abra-Pajonal igneous complex, northern Chile, to investigate the magmatic evolution of the suite. Special attention was paid to identifying the onset of sulfide saturation and to documenting how it influenced the geochemistry of the chalcophile elements and the formation of the El Abra porphyry Cu deposit. The PGE have extreme sulfide melt-silicate melt partition coefficients, making them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. In arc-related intermediate to felsic magmatic systems, which have the potential to produce porphyry deposits, the timing and extent of sulfide saturation relative to orefluid saturation may control the capacity of these systems to produce economic mineralization and, if they do, whether the deposits are Cu-only or Cu-Au. This study incorporates the first comprehensive analysis of PGE in a felsic magmatic suite associated with an economic porphyry system. The suite comprises a series of quartz monzodiorite to granite intrusions with U-Pb zircon ages between 43 and 35 Ma. Their petrography and major element chemistry, including increasing Sr/Y ratios and rare earth element patterns, suggest that crystal fractionation and crustal assimilation were the key magmatic processes governing the evolution of the El Abra-Pajonal suite. Plagioclase fractionation dominated the oldest intrusions, and their associated granites and aplites. Following the injection of a more primitive, wetter, mafic magma at 41-40 Ma, plagioclase fractionation became suppressed and amphibole became the dominant fractionating phase, leading to the formation of the El Abra porphyry intrusion and Cu deposit. Abundances of Pt and Pd in felsic rocks from the El Abra-Pajonal intrusive complex drop rapidly in samples with MgO values below 25wt %, following sulfide saturation of the magmas, which occurred slightly before ore-fluid saturation and formation of the Cu deposit. Modeling suggests that the amount of sulfide formed was very small, enough to strip the PGE and Au from the magma but not Cu, because of the lower partition coefficient of Cu relative to the precious metals, which explains why the mineralization at El Abra is a Cu-only porphyry deposit, rather than a Cu-Au deposit. [ABSTRACT FROM AUTHOR]

The shallow crustal Torres del Paine Intrusive Complex in southern Patagonia offers an opportunity to understand the chemical evolution and timing of crystallization processes in shallow plutonic rocks. It is characterized by hornblende-gabbros, gabbronorites, monzodiorites and granitic plutonic rocks. The exceptional exposure of the intrusion permits the identification of two structurally and petrographically different zones. Layered gabbronorite, olivine-bearing pyroxene–hornblende gabbronorite and monzodiorite forming vertical sheets and stocks in the west are referred to here as the feeder zone. These mafic rocks are in vertical contact with younger granitic rocks on their eastern border. The eastern part is a laccolith complex. It is characterized by three major units (I, II, III) of granitic rocks of over 1000 m vertical thickness; these are underlain in places by a sequence of hornblende-gabbro sills intermingled with evolved monzodiorite granite. Chilled, crenulated margins as well as flame structures between gabbroic rocks and monzodiorites suggest that the mafic sill complex remained partially molten during most of its construction. Bulk-rock major and trace element data indicate that the Paine mafic rocks follow a high-K calc-alkaline to shoshonitic differentiation trend. The parental magmas were basaltic trachyandesite liquids, with variable H2O and alkali contents. The majority of the feeder zone gabbronorites have high Al2O3 contents and positive Eu and Sr anomalies, consistent with accumulation of plagioclase and efficient extraction of intercumulus melt. The mafic sill complex largely lacks these cumulate signatures. Comparisons of the intercumulus groundmass in the hornblende-gabbros with intra-sill dioritic stocks and pods reveal similar rare earth element patterns and trace element ratios indicating incomplete extraction of evolved interstitial liquids. The Sr, Nd and Pb isotopic compositions of the mafic and granitic rocks exhibit ranges of 87Sr/86Sr of 0·704–0·708, ɛNd +3·8 to −1·2, 206Pb/204Pb 18·61–18·77, 207Pb/204Pb 15·67–15·67 and 208Pb/204Pb 38·56–38·77. Crystal fractionation and assimilation–fractional crystallization modelling, combined with high-precision U–Pb dating of zircons, indicates that the western feeder zone gabbronorites are linked to the uppermost Paine granite (granite I), whereas the mafic sill complex is younger and not directly related to the voluminous granite units II and III. These results are interpreted to indicate that crystal–liquid separation is facilitated in subvertical, dynamic feeder systems whereas subhorizontal sill complexes are inefficient in separating large volumes of mafic cumulates and complementary felsic rocks. [ABSTRACT FROM PUBLISHER]

Tephra and lava pairs from two summit eruptions (ad 2008 and 1957) and a flank fissure eruption (∼ ad 1850) are compared in terms of textures, phenocryst contents, and mineral zoning patterns to shed light on processes responsible for the shifts in eruption style during typical eruptive episodes at Volcán Llaima (Andean Southern Volcanic Zone, Chile). The mineralogy and whole-rock compositions of tephra and lavas are similar within eruptive episodes, suggesting a common magma reservoir for Strombolian paroxysms and lava effusion. The zoning profiles and textures of plagioclase record successive and discrete intrusions of volatile-rich mafic magma accompanied by mixing of these recharge magmas with the resident basaltic-andesitic crystal mushes that are commonly present at shallow levels in the Llaima system. Each recharge event destabilizes the plagioclase in equilibrium with the resident crystal mush melt and stabilizes relatively An-rich plagioclase, as is recorded by the numerous resorption zones. Lavas typically have ∼15–20 vol. % more phenocrysts than the tephra. Differences in plagioclase and olivine textures and zoning, combined with different phenocryst contents, indicate that a greater volume fraction of recharge magma is present in the explosively erupted magma than in subsequent effusively erupted magma. We propose that Strombolian paroxysms at Volcán Llaima are triggered by interactions with large volume fractions of recharge magma, which decrease the bulk viscosity and increase the volatile contents of the erupted magmas, leading to the conditions required for the fragmentation of basaltic-andesite. Lava effusion ensues from reduced interactions with the recharge magma, after it has partially degassed and crystallized, thereby impeding rapid ascent. This process could be operating at other steady-state basaltic volcanoes, wherein shallow reservoirs are periodically refilled by fresh, volatile-rich magmas. [ABSTRACT FROM PUBLISHER]

Crystal zoning as well as temperature and pressure estimates from phenocryst phase equilibria are used to constrain the architecture of the intermediate-sized magmatic system (some tens of km3) of Volcán Quizapu, Chile, and to document the textural and compositional effects of magma mixing. In contrast to most arc magma systems, where multiple episodes of open-system behavior obscure the evidence of major magma chamber events (e.g. melt extraction, magma mixing), the Quizapu magma system shows limited petrographic complexity in two large historical eruptions (1846–1847 and 1932) that have contrasting eruptive styles. Quizapu magmas and peripheral mafic magmas exhibit a simple binary mixing relationship. At the mafic end, basaltic andesite to andesite recharge magmas complement the record from peripheral cones and show the same limited range of compositions. The silicic end-member composition is almost identical in both eruptions of Quizapu. The effusive 1846–1847 eruption records significant mixing between the mafic and silicic end-members, resulting in hybridized andesites and mingled dacites. These two compositionally simple eruptions at Volcán Quizapu present a rare opportunity to isolate particular aspects of magma evolution—formation of homogeneous dacite magma and late-stage magma mixing—from other magma chamber processes. Crystal zoning, trace element compositions, and crystal-size distributions provide evidence for spatial separation of the mafic and silicic magmas. Dacite-derived plagioclase phenocrysts (i.e. An25–40) show a narrow range in composition and limited zonation, suggesting growth from a compositionally restricted melt. Dacite-derived amphibole phenocrysts show similar restricted compositions and furthermore constrain, together with more mafic amphibole phenocrysts, the architecture of the magmatic system at Volcán Quizapu to be compositionally and thermally zoned, in which an andesitic mush is overlain by a homogeneous dacitic magma that is the source for most of the 1846–1847 and 1932 erupted magmas. Dacite formation is best explained by mineral–melt separation (crystal fractionation) from an andesitic mush, which is inferred to have thermally and compositionally buffered the dacite magma thereby keeping it at relatively low crystallinity (<30 vol. %). The dominant cause of compositional diversity is melt separation. Back-mixing of mush (i.e. crystals with signatures of growth both in the andesitic mush and in the dacite magma) into the overlying dacite magma is rarely observed. Recharge events that increase crystal and magma diversity in the dacite magma are limited to an episode of mafic recharge and mixing just prior to the 1846–1847 eruption, where evidence for magma mixing is present on all scales. Chamber-wide mixing was incomplete (mixing efficiency of ∼0·53–0·85) as flow lobes vary significantly in composition along the proposed mixing array. Estimates of viscosity variations during the course of magma mixing suggest that mixing dynamics and the degree of magma interaction on all scales were established at the beginning of the recharge event. [ABSTRACT FROM PUBLISHER]

El Teniente, the world’s largest Cu deposit, is hosted in Late Miocene and Pliocene plutons that intrude the older Teniente Volcanic Complex (or Farellones Fm; 14·2–6·5 Ma). The Late Miocene and Pliocene plutonic host rocks of the deposit include, sequentially, the relatively large (>50 km3) Teniente Mafic Complex laccolith (8·9 ± 1·4 Ma), the smaller (∼30 km3) Sewell equigranular tonalite complex (7·05 ± 0·14 Ma) and associated andesitic sills (8·2 ± 0·5 to 6·6 ± 0·4 Ma), small dacitic porphyry stocks (<1 km3; 6·09 ± 0·18 Ma), the unusual Cu- and S-rich ‘Porphyry A’ anhydrite-bearing granitoid stock (<1 km3; 5·67 ± 0·19 Ma), the Teniente Dacite Porphyry dike (<1 km3; 5·28 ± 0·10 Ma), minor latite dikes (4·82 ± 0·09 Ma), and finally a small dacite intrusion (4·58 ± 0·10 Ma). These plutonic rocks are all isotopically similar to each other (87Sr/86Sr = 0·7039–0·7042; ɛNd =+2·5 to +3·5) and also to the Teniente Volcanic Complex extrusive rocks, but distinct from both older Late Oligocene to Early Miocene volcanic rocks (87Sr/86Sr = 0·7033–0·7039; ɛNd = +3·8 to + 6·2) and younger Pliocene post-mineralization mafic dikes and lavas (87Sr/86Sr = 0·7041–0·7049; ɛNd = +1·1 to −1·1). Multiple Cu-mineralized magmatic–hydrothermal breccia pipes were emplaced into these plutonic rocks during the same time period as the felsic porphyry intrusions, between at least 6·31 ± 0·03 and 4·42 ± 0·02 Ma. These mineralized breccia pipes, which formed by exsolution of magmatic fluids from cooling plutons, have their roots below the deepest level of mining and exploration drilling and were derived from the same magma chamber as the felsic porphyries, >4 km below the paleosurface. To produce the ∼100 × 106 tonnes of Cu in the deposit requires a batholith-size (>600 km3) amount of magma with ∼100 ppm Cu. We suggest that both the mineralized magmatic–hydrothermal breccias and the progressively smaller volumes of more fractionated, but isotopically equivalent, Late Miocene and Pliocene felsic plutonic rocks that host the deposit were derived from the roof of a large, long-lived, thermally and chemically stratified, open-system magma chamber, or magmatic plumbing system, recharged from below by mantle-derived magmas. Only when this system fully solidified did post-mineralization mafic olivine-hornblende-lamprophyre dikes (3·85 ± 0·18 to 2·9 ± 0·6 Ma) pass through the system from the mantle to the surface. The significant progressive temporal isotopic evolution, to higher 87Sr/86Sr (from 0·7033 to 0·7049) and lower ɛNd (from +6·2 to −1·1), that occurred between the Late Oligocene and Pliocene in the vicinity of El Teniente for mafic mantle-derived magmas, and by implication their sub-arc mantle-source region, was due in part to increased mantle-source region contamination by subducted crust tectonically eroded off the continental margin. The post-mineralization olivine-hornblende-lamprophyres also imply extensive hydration of the mantle below this portion of the Andean arc by the Pliocene, which may have played a role in producing oxidized volatile-rich magmas and mineralization at El Teniente. [ABSTRACT FROM AUTHOR]

The Madre de Dios Metamorphic Complex (MDMC) in southern Chile is a fossil frontal accretionary prism, which is mainly composed of metapsammopelitic rocks, intercalations of oceanic rocks (greenstone and metachert) and platform carbonate. We concentrated on the metabasite to decipher the metamorphic evolution. This rock type contains assemblages of the pumpellyite–actinolite facies: pumpellyite ± actinolite–chlorite ± grandite ± phengite ± epidote–albite–quartz–titanite ± K-feldspar ± calcite. The metamorphic phases mainly grew by prograde hydration reactions during various episodes of restricted fluid influx. Fundamental phase relations of the pumpellyite–actinolite facies and adjacent facies were reproduced by pseudosections calculated for the system K2O–Na2O–CaO–FeO–O2–MgO–Al2O3–TiO2–SiO2–H2O–CO2 at 200–400°C and 1–9 kbar. The calculated stability fields of the metamorphic assemblages as realized in the MDMC metabasite indicate highest metamorphic conditions restricted to 290–310°C, 4–6 kbar for the MDMC, presumably as a result of the main fluid influx at these conditions. Nevertheless, earlier local equilibria are still preserved as a result of strongly kinetically controlled mineral reactions and a lack of recrystallization and compositional homogenization at thin-section scale. Hence, thermodynamic calculations of local multivariant mineral equilibria using the entire compositional variation of minerals in the MDMC show that the prograde PT path evolved from 4 ± 1 kbar, 200–220°C to 5 ± 1 kbar, 290–330°C. The prograde PT path reflects nearly horizontal particle paths after reaching the maximum depth typical for frontal accretionary prisms. Long residence at maximum depth resulted in thermal re-equilibration. 40Ar/39Ar spot ages were measured by in situ UV laser ablation of local phengite concentrations in a deformed metapelite at 233·2 ± 1·8 Ma and in an undeformed metabasite at 200·8 ± 2·4 Ma. Whereas the first age represents an age of accretion, the latter age can be attributed to mineral growth either during a younger stage of accretion or during a retrograde stage. 40Ar/39Ar isotopic analyses of two further metabasite samples reflect a prominent resetting of ages at 152·0 ± 2·2 Ma and white mica growth during external fluid access triggered by either a local intrusion or a late Jurassic extensional episode. [ABSTRACT FROM PUBLISHER]

A multi-method geochronological approach is applied to unravel the dynamics of a paired metamorphic belt in the Coastal Cordillera of central Chile. This is represented by high-pressure–low-temperature rocks of an accretionary prism (Western Series), and a low-pressure–high-temperature overprint in the retro-wedge with less deformed metagreywackes (Eastern Series) intruded by magmas of the coeval arc. A pervasive transposition foliation formed in metagreywackes and interlayered oceanic crust of the Western Series during basal accretion near metamorphic peak conditions (∼350–400°C, 7–11 kbar) at 292–319 Ma (40Ar/39Ar phengite plateau ages). 40Ar/39Ar UV laser ablation ages of phengite record strain-free grain growth and recrystallization with a duration of 31–41 Myr during a pressure release of 3–4 kbar. During early accretion the main intrusion in the arc occurred at 305 Ma (Pb–Pb evaporation; zircon) and the Eastern Series was overprinted by a short high-temperature metamorphism at 3 kbar, 296–301 Ma (40Ar/39Ar muscovite plateau ages). Fission-track ages of zircon (206–232 Ma) and of apatite (80–113 Ma) are similar in both series, indicating synchronous cooling during distinct periods of exhumation. Early exhumation (period I) during continuing basal accretion proceeded with mean rates of ≥0·19–0·56 mm/yr, suggesting that erosion in a tectonically active area was an important unroofing mechanism. At the same time mean rates were 0·03–0·05 mm/yr in the Eastern Series, where crustal thickening was minor. A shallow granite intruded into the Western Series at 224 Ma, at the end of basal accretion activity, when exhumation rates decreased to 0·04–0·06 mm/yr in both series during period II (∼100–225 Ma). Major extension, basin formation and local bimodal dyke intrusion at 138 Ma were accompanied by mean cooling rates of ∼1–2°C/Myr. Accelerated cooling of 3–5°C/Myr at ∼80–113 Ma suggests a mid-Cretaceous convergence event (period III). After 80 Ma cooling rates decreased to 1–2°C/Myr (period IV). The pressure–temperature–deformation–time information for subduction, basal accretion and exhumation in the accretionary wedge of central Chile illustrates that these processes reflect a continuous cyclic mass flow that lasted nearly 100 Myr, while the retro-wedge remained stable. After the cessation of accretion activity a similarly long period of retreat of the subducting slab occurred; this ended with renewed convergence and shortening of the continental margin. [ABSTRACT FROM AUTHOR]

In the Chilean Coastal Cordillera, two units, the Western and Eastern Series, constitute coeval parts of a Late Palaeozoic paired metamorphic belt dominated by siliciclastic metasediments. The Western Series also contains rocks from the upper oceanic crust and represents an accretionary prism. Omnipresent high-pressure conditions are reflected by Na–Ca-amphibole and phengite in greenschists. Peak PT conditions of 7·0–9·3 kbar, 380–420°C point to a metamorphic gradient of 11–16°C/km. Three unique occurrences of blueschist yield deviating conditions of 9·5–10·7 kbar, 350–385°C and are interpreted as relics from the lowermost part of the basal accretion zone preserving the original gradient of 9–11°C/km along the subducting slab. Pervasive ductile deformation related to basal accretion occurred near peak PT conditions. Deformation and PT evolution of the metapsammopelitic rocks is similar to that of the metabasites. However, a rare garnet mica-schist yields peak PT conditions of 9·6–14·7 kbar, 390–440°C reflecting a retrograde stage after cooling from a high-temperature garnet-forming stage. It is considered to be an exhumed relic from the earliest siliciclastic rocks subducted below a still hot mantle wedge. A retrograde overprint of all rock types occurred at ∼300–380°C. Continuous reactions caused crystal growth and recrystallization with abundant free water mostly under strain-free conditions. They record a pressure release of 3–4 kbar without erasing peak metamorphic mineral compositions. The Eastern Series lacks metabasite intercalations and represents a less deformed retro-wedge area. In the study area it was entirely overprinted at a uniform depth at 3 ± 0·5 kbar with temperatures progressively rising from 400°C to 720°C towards the coeval Late Palaeozoic magmatic arc batholith. The interrelated pattern of PT data permits a conceptual reconstruction of the fossil convergent margin suggesting a flat subduction angle of ∼25° with continuous basal accretion at a depth of 25–40 km and a short main intrusion pulse in the magmatic arc. The latter was accompanied by the formation of a thermal dome in the retro-wedge area, which remained stable relative to the vertically growing accretionary prism characterized by cyclic mass flow. [ABSTRACT FROM AUTHOR]

Volca´n San Pedro in the Andean Southern Volcanic Zone (SVZ) Chile, comprises Holocene basaltic to dacitic lavas with trace element and strontium isotope ratios more variable than those of most Pleistocene lavas of the underlying Tatara–San Pedro complex. Older Holocene activity built a composite cone of basaltic andesitic and silicic andesitic lavas with trace element ratios distinct from those of younger lavas. Collapse of the ancestral volcano triggered the Younger Holocene eruptive phase including a sequence of lava flows zoned from high-K calc-alkaline hornblende–biotite dacite to two-pyroxene andesite. Notably, hornblende–phlogopite gabbroic xenoliths in the dacitic lava have relatively low 87Sr/86Sr ratios identical to their host, whereas abundant quenched basaltic inclusions are more radiogenic than any silicic lava. The latest volcanism rebuilt the modern 3621 m high summit cone from basaltic andesite that is also more radiogenic than the dacitic lavas. We propose the following model for the zoned magma: (1) generation of hornblende–biotite dacite by dehydration partial melting of phlogopite-bearing rock similar to the gabbroic xenoliths; (2) forceful intrusion of basaltic magma into the dacite, producing quenched basaltic inclusions and dispersion of olivine and plagioclase xenocrysts throughout the dacite; (3) cooling and crystallization–differentiation of the basalt to basaltic andesite; (4) mixing of the basaltic andesite with dacite to form a small volume of two-pyroxene hybrid andesite. The modern volcano comprises basaltic andesite that developed independently from the zoned magma reservoir. Evolution of dacitic and andesitic magma during the Holocene and over the past 350 kyr reflects the intrusion of multiple mafic magmas that on occasion partially melted or assimilated hydrous gabbro within the shallow crust. The chemical and isotopic zoning of Holocene magma at Volca´n San Pedro is paralleled by that of historically erupted magma at neighboring Volca´n Quizapu. Consequently, the role of young, unradiogenic hydrous gabbro in generating dacite and contaminating basalt may be underappreciated in the SVZ. [ABSTRACT FROM PUBLISHER]

Two groups of gabbroic xenoliths (I and II) containing large proportions of late-crystallized hornblende (up to 50 vol. %) and Na-rich phlogopite (up to 15 vol. %), were brought to the surface by a late Holocene eruption of Volcán San Pedro, the youngest edifice of the Tatara–San Pedro Volcanic Complex (36°S, Chilean Andes). Group I are inferred to be fragments of partially solidified Holocene plutons because they contain residual interstitial glass, whereas exsolution and deformation textures in Group II indicate that they are fragments of pre-Quaternary plutonic basement. On the basis of textural relations plus the mineral and whole-rock compositions of both groups of xenoliths, we suggest that hornblende and phlogopite with high mg-numbers and Cr contents have formed by reactions between refractory cumulus minerals (olivine, Cr-spinel, pyroxenes or plagioclase) and evolved melts ± aqueous fluids that migrated through partly solidified crystalline frameworks. Thus, the hydrous minerals are not early-crystallized phases in the basaltic magmas from which the cumulus minerals precipitated. The high proportions of hornblende in many subduction-related gabbroic plutons and xenolith suites compared with its paucity in basaltic or basaltic andesitic lavas may be partially explained by multistage plutonic crystallization histories involving reaction and migration of evolved melt ± aqueous fluids that either could have originated within the cumulus pile of the mafic intrusion or were derived externally, from broadly contemporaneous felsic magmas. [ABSTRACT FROM PUBLISHER]

Neogene plateau lavas in Patagonia, southern Argentina, east of the volcanic gap between the Southern and Austral Volcanic Zones at 46·5° and 49·5°S are linked with asthenospheric slab window processes associated with the collision of a Chile Ridge segment with the Chile Trench at 12 Ma. The strong ocean-island basalt (OIB)-like geochemical signatures (La/Ta <20; Ba/La <20; 87Sr/86Sr = 0·7035–0·7046; 143Nd/144Nd = 0·51290–0·51261; 206Pb/204Pb = 18·3–18·8; 207Pb/204Pb = 15·57–15·65; 208Pb/204Pb = 38·4–38·7) of these Patagonian slab window lavas contrast with the mid-ocean ridge basalt (MORB)-like, depleted mantle signatures of slab window lavas elsewhere in the Cordillera (e.g. Antarctic Peninsula; Baja California). The Patagonian lavas can be divided into a voluminous ∼12–5 Ma, tholeiitic main-plateau sequence (48–55% SiO2; 4–5% Na2O + K2O) and a less voluminous ∼7–2 Ma alkaline post-plateau sequence (43–49% SiO2; 5–8% Na2O + K2O). Moderately high FeOT (9–11%), and low heavy rare earth element (HREE), Y, and Sc concentrations in all lavas are consistent with melt generation just below the garnet–spinel transition at a depth of ∼70 km. The main-plateau lavas from the western back-arc can be modeled by ∼10–15% partial melting of an OIB-like asthenospheric mantle source with additions from slab fluid–melt components coupled with crustal contamination (AFC). A three-stage petrogenetic model is envisaged: (1) decompression melting and source region contamination of an OIB-like subslab asthenospheric source by slab melts of the trailing edge of the subducted Nazca Plate; (2) minor contamination of slab window melts with arc components ‘stored’ in the supraslab mantle wedge and/or basal continental lithosphere; (3) further modification by addition of crustal components during magma ascent. The main-plateau lavas from the eastern back-arc can be modeled by ∼7% partial melting of the same asthenospheric source as the influence of arc components diminishes and the intensity of mantle upwelling into the slab window decreases. All post-plateau lavas can be modeled as 1–4% partial melts of the pristine OIB-like asthenospheric source in the widening slab window. [ABSTRACT FROM PUBLISHER]

Voluminous rhyolitic volcanism along the palaeo-Pacific margin of Gondwana was marked by three principal episodes of magmatism. The first of these ( V1) is essentially coincident with the main episode of Karoo–Ferrar magmatism at ∼184 Ma. A younger ( V2) episode occurred at ∼168 Ma, and a third episode ( V3) occurred in the interval 157–153 Ma. We evaluate the origin of V1 and V2 rhyolites from the Antarctic Peninsula using major and trace element and isotopic (Sr, Nd, O) data. An isotopically uniform (87Sr/86Sri ∼0·707; εNdi ∼ −3) andesite–dacite magma was generated as a result of anatexis of ‘Grenvillian age’ hydrous mafic lower crust, linked to earlier, arc-related underplating. The lower-crustal partial melts would have mixed with fractionated components of the mafic underplate, followed by subsequent storage and homogenization. Early Jurassic ( V1) rocks of the southern Antarctic Peninsula are interpreted as melts of upper-crustal paragneiss, which have mixed with the isotopically uniform magma in upper-crustal magma chambers. The V2 rhyolites are the result of assimilation–fractional crystallization of the isotopically uniform magma. This occurred in upper-crustal magma chambers involving assimilants with similar isotopic composition to that of the magma. A continental margin setting was crucial in developing hydrous, readily fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the Discovery–Shona–Bouvet group of plumes, thought to be responsible for the Karoo magmatic province. The progression (old to young) of volcanism from NE to SW in Patagonia and south to north in the Antarctic Peninsula is consistent with migration away from the mantle plumes towards the proto-Pacific margin of Gondwana during rifting and break-up. [ABSTRACT FROM PUBLISHER]

La Pacana is one of the largest known calderas on Earth, and is the source of at least two major ignimbrite eruptions with a combined volume of some 2700 km3. These ignimbrites have strongly contrasting compositions, raising the question of whether they are genetically related. The Toconao ignimbrite is crystal poor, and contains rhyolitic (76–77 wt % SiO2) tube pumices. The overlying Atana ignimbrite is a homogeneous tuff whose pumice is dacitic (66–70 wt % SiO2), dense (40–60% vesicularity) and crystal rich (30–40 % crystals). Phase equilibria indicate that the Atana magma equilibrated at temperatures of 770–790°C with melt water contents of 3·1–4·4 wt %. The pre-eruptive Toconao magma was cooler (730–750°C) and its melt more water rich (6·3–6·8 wt % H2O). A pressure of 200 MPa is inferred from mineral barometry for the Atana magma chamber. Isotope compositions are variable but overlapping for both units (87Sr/86Sri 0·7094–0·7131; 143Nd/144Nd 0·51222–0·51230) and are consistent with a dominantly crustal origin. Glass analyses from Atana pumices are similar in composition to those in Toconao tube pumices, demonstrating that the Toconao magma could represent a differentiated melt of the Atana magma. Fractional crystallization modelling suggests that the Toconao magma can be produced by 30% crystallization of the observed Atana mineral phases. Toconao melt characteristics and intensive parameters are consistent with a volatile oversaturation-driven eruption. However, the low H2O content, high viscosity and high crystal content of the Atana magma imply an external eruption trigger. [ABSTRACT FROM PUBLISHER]

Spinel-facies mantle xenoliths occur in a diatreme cutting through the Neogene Southern Patagonia Plateau at Gobernador Gregores (Santa Cruz Province, Argentina). This plateau is in a back-arc position with respect to the Chile trench. Xenoliths differ in their whole-rock composition from other South America occurrences, having higher CaO/Al2O3 ratios and, in some samples, TiO2 enrichment, whereas the Na2O/Al2O3 variation range is similar. Three assemblages can be distinguished. Assemblage 1, in anhydrous protogranular lherzolites and harzburgites, contains clinopyroxene with a depleted major and trace element composition, indicating pre-metasomatic depletion processes. This assemblage fully recrystallized to Assemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing) during a metasomatic episode. This causes clinopyroxene to acquire geochemical characteristics often attributed to carbonate-melt metasomatism. Noticeably, amphibole is markedly enriched in Nb (up to 298 ppm), especially when depleted in Ti. A further event, related to decompression during xenolith uplift to the surface, induces closed-system (perhaps with the exception of CO2 addition) disequilibrium melting of Assemblage 2, dominantly of amphibole. It is found in pockets (where amphibole is a residual phase) consisting of Na–Si-rich glass and carbonate (Mg-rich calcite) drops, and in veins originating from the pockets (Assemblage 3). Euhedral olivine, clinopyroxene and spinel crystallize only in the silicate glass. So do new, euhedral apatite crystals when glass is in contact with previous Assemblage 2 apatite. Textural evidence and comparison with experimental work suggest that silicate glass and carbonates are the result of unmixing of a former homogeneous melt. Because of the different flow rates of carbonate and silicate melt, the xenoliths become enriched in carbonate, which is found in the veins during their migration. Thus, the high CaO/Al2O3 ratio of whole rocks provides inconclusive evidence of carbonatite metasomatism. This factor, and other minor deviations from the expected results of carbonatite metasomatism, lead us to hypothesize an aqueous, Cl-rich fluid, possibly slab derived, as an alternative agent. Amphibole, resulting from reactive porous flow of this agent in the mantle, could fully explain the observed geochemical features, as indicated by estimates of its partition coefficients. [ABSTRACT FROM PUBLISHER]

New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ar data are presented for the Jurassic silicic volcanic rocks and related granitoids of Patagonia and the Antarctic Peninsula. U–Pb is the only reliable method for dating crystallization in these rocks; Rb–Sr is prone to hydrothermal resetting and Ar–Ar is additionally affected by initial excess 40Ar. Volcanism spanned more than 30 My, but three episodes are defined on the basis of peak activity: V1 (188–178 Ma), V2 (172–162 Ma) and V3 (157–153 Ma). The first essentially coincides with the Karoo–Ferrar mafic magmatism of South Africa, Antarctica and Tasmania. The silicic products of V1 are lower-crustal melts that have incorporated upper-crustal material. The geochemistry of V2 and V3 ignimbrites is more characteristic of destructive plate margins, but the presence of inherited zircon still points to a crustal source. The pattern of volcanism corresponds in space and in time to migration away from the Karoo mantle plume towards the proto-Pacific margin of Gondwana during rifting and break-up. The heat required to initiate bulk crustal fusion may have been supplied by the spreading plume-head, but thinning of the crust during continental dispersion would also have facilitated anatexis. [ABSTRACT FROM PUBLISHER]

The Chonos Metamorphic Complex forms part of a belt of low-grade metamorphic rocks in the Chilean Coastal Cordillera that are interpreted as Palaeozoic–Mesozoic accretionary complexes. It comprises metapsammopelitic schists, metabasites and meta-ironstones occurring in two contrasting units. Special attention during microprobe study of key samples was given to the chemical zonation of minerals. Subsequently, conventional geothermobarometry and that using thermodynamic calculations were applied. The Eastern belt comprises rocks that are metamorphosed to pumpellyite–actinolite facies conditions and show a low degree of deformation with well-preserved sedimentary and igneous structures. Maximum P–T conditions were around 5·5 kbar and 250–280°C. The rocks of the Western belt are characterized by a transition between greenschist and albite–epidote–amphibolite facies metamorphism and show a penetrative tectonic transposition foliation S2 formed close to the pressure maximum. Maximum P–T conditions vary around 8–10 kbar and 380–500°C overstepping the stilpnomelane + phengite stability. High pressures in this belt are confirmed by regionally distributed phengites with high Si contents up to 3·5 Si per formula unit. Regional distribution of maximum temperatures is reflected by the composition of actinolitic hornblendes within the metabasites. In a garnet-bearing metabasite of the Western belt, oscillatory growth zoning of garnet was observed. The composition of corresponding mineral inclusions suggests that a prograde P–T path during garnet growth evolved from 7·5 kbar and 375°C to about 9·4 kbar and 500°C. Late garnet grew synkinematically with penetrative deformation. The retrograde P–T path in the rocks of the Western belt is constrained by the composition of mainly late strain-free minerals and involves slight cooling during decompression. Both belts are part of a subduction system. The apparent P–T gap between the belts is due to their juxtaposition during exhumation. The Eastern belt constitutes the transition towards the backstop system of the accretionary prism that is represented by the Western belt, whereas the absence of very low grade rocks west of the Western belt is attributed to tectonic erosion, which was possibly caused by subduction of a ridge. [ABSTRACT FROM PUBLISHER]

Stage II of Lascar Volcano, Chile, involved development of an andesite to dacite volcanic complex and associated hypabyssal porphyry intrusions above the main magma chamber. The system culminated in development of a large zoned magma chamber that erupted in the large-magnitude (8 km3) Soncor explosive eruption at 26·5 ka, forming a Plinian pumice deposit and ignimbrite. Vent-derived lithic clasts in the Soncor deposits sample the pre-existing Stage II complex. The Piedras Grandes hornblende andesite unit represents a pre-Soncor dome complex. The andesite consistsof a heterogeneous phenocryst assemblage of plagioclase-amphibole-orthopyroxene-oxides and minor biotite, clinopyroxene, quartz, apatite, anhydrite and olivine together with commingled basaltic andesite inclusions and streaks. Temperature estimates from zoned orthopyroxenes and Fe-Ti oxides and disequilibrium between phenocrysts indicate an origin by remobilization and remelting of an igneous protolith by influx of hydrous mafic magmas so that the andesite is a mixture of partial melt, restite crystals, mafic components and phenocrysts. More silicic Stage II rocks are also interpreted as partial melts with entrained restite. The zoned Soncor chamber contained dacite (67 wt % SiO2) to silicic andesite (61 wt % SiO2) crystal-rich magmas with an assemblage of plagioclase-orthopyroxene-clinopyroxene-oxides with minor biotite, amphibole, apatite, zircon, anhydrite, pyrrhotite and olivine. Hornblende-rich mafic andesite pumice from late flow units in the ignimbrite provides evidence for influxes of hydrous mafic magmas at the base of the chamber at sufficient depths to stabilize amphibole. The hydrous mafic magmas are interpreted to have evolved in the lower crust by high-pressure fractionation with some lower-crustal contamination. The Soncor zoned magma chamber developed in the upper crust at about 6 km depth as a result of repeated influx of hydrous mafic magmas. The magmas in the chamber evolved by open-system fractionation with magma mixing being important in the less evolved magmas. Repeated influxes of hydrous mafic magmas resulted in convective stirring and addition of heat, volatiles and mafic components to the chamber. This produced complex histories of individual crystals and heterogeneous character of phenocrysts in individual samples. Halogen contents of amphibole, biotite, apatite and glass inclusions, S contents of glass inclusions, stabilization of anhydrite in the silicic magmas, and mass balance calculations imply major transfer of volatile components from the hydrous mafic magmas into the interior of the zoned chamber in the form of a co-magmatic fluid phase. [ABSTRACT FROM PUBLISHER]

Upper Palaeozoic silicic magmatism is widespread in the Central Andes, but its origin is poorly constrained. We investigated whole-rock chemical and isotopic composition of Upper Palaeozoic granitoids and their Early Palaeozoic high-grade country rocks in the Chilean Coastal Cordillera and Precordillera at ~22°S, in comparison with an Upper Cretaceous granitoid. The age of the Late Palaeozoic granitoids from a Rb-Sr isochron of ~300 Ma is consistent with K-Ar cooling ages of hornblende and biotite. Similar major and trace element patterns as well as Nd and Pb isotopic composition of Upper Palaeozoic granitoids and gneisses point to a source of the granitoids that is similar to the gneisses at outcrop. Sr isotope ratios of the Upper Palaeozoic granitoids are less radiogenic than those in many of the gneisses. We propose a stratification of the Early Palaeozoic crust with a Rb-deficient granulitic mid-lower crust, resulting in less radiogenic Sr compared with the upper crust, based on the interpretation of the P-T-t history and isotopic composition of the Lower Palaeozoic metamorphic basement and of the isotopic composition of the Late Palaeozoic granitoids and younger magmatic rocks. Nd isotopic composition is identical in lower and upper crust and in crustal melts from the Late Palaeozoic to Recent. The Cretaceous granitoid evolved from partial melts of a mantle-derived source with considerable contamination by the old crustal component. The crust that formed in the Early Palaeozoic is the major source of material for the Cenozoic tectonic thickening of the Andean crust. [ABSTRACT FROM PUBLISHER]

Heterogeneous andesitic and dacitic lavas on Cordón El Guadal bear on the general problem of how magmas of differing compositions and physical properties interact in shallow reservoirs beneath continental arc volcanoes. Some of the lavas contain an exceptionally large proportion (<40%) of undercooled basaltic andesitic magma in various states of disaggregation. Under-cooled mafic magma occurs in the silicic lavas as large (<40 cm) basaltic andesitic magmatic inclusions, as millimeter-sized crystal-clots of Mg-rich olivine phenocrysts plus adhering Carich plagioclase microphenocrysts (An50–70), and as uniformly distributed, isolated phenocrysts and microphenocrysts. Compositions and textures of plagioclase phenocrysts indicate that inclusion-forming magmas are hybrids formed by mixing basaltic and dacitic melts, whereas textural features and compositions of groundmass phases indicate that the andesitic and dacitic lavas are largely mechanical mixtures of dacitic magma and crystallized basaltic andesitic magma. This latter observation is significant because it indicates that mechanical blending of undercooled mafic magma and partially crystallized silicic magma is a possible mechanism for producing the common porphyritic texture of many calc-alkaline volcanic rocks. The style of mafic-silicic magma interaction at Cordon El Guadal was strongly dependent upon the relative proportions of the endmembers. Equally important in the Guadal system, however, was the manner in which the contrasting magmas were juxtaposed. Textural evidence preserved in the plagioclase phenocrysts indicates that the transition from liquid-liquid to solid-liquid mixing was not continuous, but was partitioned into periods of magma chamber recharge and eruption, respectively. Evidently, during periods of recharge, basaltic magmas rapidly entrained small amounts of dacitic magma along the margins of a turbulent injection fountain. Conversely, during periods of eruption, dacitic magma gradually incorporated small parcels of basaltic andesitic magma. Thus, the coupled physical-chemical transition from mixed inclusions to commingled lavas is presumably not coincidental. More likely, it probably provides a partial record of the dynamic processes occurring in shallow magma chambers beneath continental are volcanoes. [ABSTRACT FROM AUTHOR]

Prahuaniyeu, located on the NW margin of the Somoncura Large Igneous Province in northern Patagonia, is one of two known localities in Argentina where mantle-derived garnet- and spinel-bearing peridotites occur associated with alkali basalts; the other locality is the Pali Aike volcanic field of southern Patagonia. Most of the Prahuaniyeu garnet-bearing peridotites are fertile in terms of their Al2O3 and CaO contents, whereas the spinel-bearing peridotites cover a wide range from fertile to depleted compositions. Whole-rock light rare earth element (LREE) enrichment in the garnet-bearing peridotites and partly in the spinel-peridotites is consistent with intergranular percolation of the host basalt melt, as hydrous phases are not present and the clinopyroxenes and garnets are not enriched in LREE. Lack of slab-derived component(s) in the metasomatic products rules out the participation of a subducted slab in the generation of these basalts. In situ clinopyroxene analyses suggest that a group of spinel-peridotites experienced cryptic metasomatism by carbonatitic melts. Non-metasomatized garnet- and spinel-peridotites have experienced fractional melting ranging from 1 to 3% and from 5 to 12%, respectively. The Prahuaniyeu xenoliths lie on an elevated geotherm (high temperatures at low pressure) implying convective heat transport. The two most fertile samples, which indicate apparent internal ‘ages’ between c. 10 and 30 Ma for the sub-Prahuaniyeu lithospheric mantle, suggest resetting of the Sm–Nd isotopic system under a high-temperature regime and most probably reflect closure of the system following this ‘high-T event’, which can be related to extensive magmatic activity within the Somoncura province, starting in the Eocene and finishing in the Miocene. [ABSTRACT FROM AUTHOR]