Breakdown Of Amphibole Research Articles

Magmatic evolution and magma chamber conditions of the Alpehué tephra from Sollipulli Volcano, Andean Southern Volcanic Zone, Chile/Argentina

The trachydacitic Alpehué tephra from Sollipulli volcano (Andean Southern Volcanic Zone), consists of ignimbrite and fallout from a Plinian eruption about 3000 years ago. It is mainly composed of (1) crystal-rich pumice and ash but also contains (2) chilled knobbly basaltic lava clasts and (3) mostly highly inflated glomerocrystic fragments with high crystal-glass ratios interpreted to represent a crystal mush zoned from basaltic to dacitic bulk compositions. Knobbly lava clasts are of three types: (a) a very phenocryst-poor basalt, (b) a basalt with large, unzoned olivine and plagioclase phenocrysts and glomerocrysts, and (c) mixtures of microcrystalline basalt with various fragments, glomerocrysts and crystals derived from a crystal mush. Clast type (4) in the tephra is banded pumices in which the three magmatic components occur variably mingled. Thermobarometry and petrographic observations, particularly presence or absence of amphibole, constrain an upper-crustal succession of a lower basaltic reservoir, a zoned basaltic to dacitic crystal mush reservoir, and a separate trachydacite magma chamber on top. All Alpehué magmatic components form a coherent liquid line of descent which supports the interpretation that the crystal mush reservoir is a gradually solidifying magma chamber, not the result of large-scale crystal-liquid segregation. The trachydacite magma may originally have formed as melt escaping from the crystal-mush reservoir but subsequently underwent a long and complex evolution recorded in large strongly zoned plagioclase phenocrysts including resorption horizons. The ascending mafic magmas collected samples from the crystal mush body and intruded the trachydacite reservoir. The phenocryst-poor basalt (a) arrived first and entrained and partially resorbed plagioclase from the host magma. The phyric basalt (b) arrived later and did not resorb entrained plagioclase before eruption. Estimated cooling times, plagioclase resorption times and ascent rates avoiding amphibole breakdown limit the duration of these pre-eruptive processes to not more than a few days.

Trans-crustal magmatic processes revealed by amphibole breakdown textures at the Quillacas monogenetic volcanic center, Bolivia

Amphibole exerts a fundamental control on arc magma petrogenesis, differentiation, and the long-term evolution of the arc crust. This study identifies two texturally distinct amphibole populations within andesitic lavas and entrained hornblendite cumulates at the Quillacas monogenetic volcanic center in the Eastern Altiplano, Bolivia. Within the hornblendites, all amphiboles are tschermakitic, large (≤800 μm) with thick, granular reaction rims (avg. 27 μm thickness). In the host andesites, tschermakites are also the dominant amphibole species but are smaller (250–400 μm) with thin, symplectic reaction rims (avg. 7–9 μm thickness). An intergrowth of symplectic and granular reaction rims is also observed in this population. Collectively, the amphibole populations within the Quillacas magmatic system also record irregular volumetric decomposition where amphibole is replaced by mineral aggregates of plagioclase, pyroxene, and oxide within the crystal. This supports a relatively slow reaction of amphibole with melt trapped in fractures and cleavages during decompression-induced degassing. Geothermobarometry suggests that the hornblendite cumulate tschermakites crystallized at P-T conditions ranging from 453 to 598 ± 12 % MPa and 928–1004 ± 22 °C. The host andesite tschermakites crystallized at P-T conditions ranging from 430 to 617 ± 12 % MPa and 928–1004 ± 22 °C. These geothermobarometric constraints correspond to depths of 16–24 km which, within this region of the Central Andean crust, also corresponds to a regionally extensive low-seismic velocity zone. The texturally distinct amphibole populations imply a multi-stage trans-crustal magmatic system likely exists beneath the Quillacas volcanic center. In this scenario, a crystal mush zone exists at upper crustal depths where cumulate amphiboles initially crystallized. Magma recharge into this mush zone initiated a reaction between hornblendite cumulates and the melt which formed the amphibole granular rims. This recharge event also transported the host andesite amphiboles that subsequently developed symplectic rims due to heating and ascent-driven decompression. This study supports the presence of amphibole-dominated mush filters in the upper crust of the Central Andean arc and advances our understanding of amphibole's role in the evolution of arc magmatic systems.

Petrographic and Geochemical Evidence for a Complex Magmatic Plumbing System beneath Bagana Volcano, Papua New Guinea

Abstract Bagana is a persistently active stratovolcano located on Bougainville Island, Papua New Guinea. Characteristic activity consists of prolonged lava effusion over months to years, with occasional shifts to explosive vulcanian or sub Plinian eruptions that threaten surrounding communities. Satellite observations have shown that Bagana is a major SO2 emitter, particularly during eruptive intervals. Despite persistent and potentially hazardous activity, no previous geophysical, petrological, or geochemical studies have constrained the magma storage conditions and reservoir processes at Bagana. To address this knowledge gap, we present new bulk rock major, trace element, and radiogenic isotope data, plus mineral phase major element compositions, for Bagana lavas erupted in 2005 and 2012 and ash erupted in 2016. We use our new data to understand the magmatic processes controlling the typical effusive activity and provide the first estimates of magma storage conditions beneath Bagana. The basaltic andesite bulk rock compositions (56–58 wt% SiO2) of our Bagana lavas reflect accumulation of a plagioclase + clinopyroxene + amphibole + magnetite + orthopyroxene crystal cargo by andesitic-dacitic (57–66 wt% SiO2) carrier melts. Constraints from clinopyroxene and amphibole thermobarometry, amphibole hygrometry, and experimental petrology suggest that the high-An plagioclase + clinopyroxene + amphibole + magnetite assemblage crystallizes from basaltic-basaltic andesite parental magmas with >4 wt% H2O, over a temperature interval of ~1100–900°C, at pressures of ~130–570 MPa, corresponding to ~5–21 km depth. Continued crystallization in the magma storage region at ~5–21 km depth produces andesitic to dacitic residual melts, which segregate and ascend towards the surface. These ascending melts entrain a diverse crystal cargo through interaction with melt-rich and mushy magma bodies. Degassing of carrier melts during ascent results in crystallization of low-An plagioclase and the formation of amphibole breakdown rims. The radiogenic isotope and trace element compositions of Bagana lavas suggest that parental magmas feeding the system derive from an enriched mantle source modified by both slab fluids and subducted sediments. Our findings suggest that the prolonged lava effusion and persistently high gas emissions that characterise Bagana’s activity in recent decades are sustained by a steady state regime of near-continuous ascent and degassing of magmas from the crustal plumbing system. Our characterisation of the Bagana magmatic plumbing system during effusive activity provides a valuable framework for interpreting ongoing monitoring data, and for identifying any differences in magmatic processes during any future shift to explosive activity.

Dehydration‐driven deformation of eclogite: Interplay between fluid discharge and rheology

AbstractAqueous fluids released during dehydration of a subducting slab have a large effect on the rheology of the subduction interface. While high‐pressure experiments and natural‐case studies link deformation with critical dehydration reactions during eclogitization, the exact interplay between these processes remains ambiguous. To investigate fluid–rock interaction and associated deformation at high‐pressure, we studied a suite of eclogites from the Tsäkkok Lens of the Scandinavian Caledonides that record prograde metamorphism within an Early Palaeozoic cold subduction zone. Our results show that in‐situ dehydration during the blueschist to eclogite facies transition produces fluid fluxes leading to rheological weakening and densification, consequently promoting ductile‐brittle deformation. Petrographic evidence, supported by thermodynamic modelling and thermobarometry, attest to a prograde passage from lawsonite‐blueschist to peak eclogite facies of ~2.5 GPa and ~620°C. Phengite‐bearing eclogites imply interaction with an externally‐derived fluid, whereas rare phengite‐free, kyanite‐eclogites only record internally‐derived fluid production. Models predict that prograde breakdown of chlorite, lawsonite and amphibole between 500 and 610°C lead to progressive dehydration and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal elongated shapes of highly strained omphacite porphyroblasts, displaying minor yet gradual changes in misorientation towards the grain boundaries. Occasionally, these intragranular structures form subgrain cells that have similar sizes to those of neoblasts in the rock matrix. These observations point to the potential onset of dynamic recrystallization processes via dislocation creep. Moreover, the omphacite neoblasts and rutile show non‐random crystallographic preferred orientations (CPOs), which are characterized by the subparallel alignment of point‐like maxima in rutile [001] and [100] axes to those of [001] and (010) of omphacite neoblasts, respectively. Additionally, the [001] axes of these minerals are also subparallel to the weak stretching mineral lineation, and the (100) of rutile and the (010) of omphacite neoblasts are distributed in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to the foliation are dilated and sealed predominantly by omphacite. The lack of obliquity between CPO and foliation plane, as well as the systematic orientation of garnet microfracture orientations, are consistent with coaxial deformation at peak‐pressure conditions. Unlike other studies, we show that neither an external fluid source nor channelized fluid flow is needed to facilitate a ductile‐brittle deformation of eclogite in a subduction setting.

Partial Melting under Shallow-Crustal Conditions: A Study of the Pleistocene Caldera Eruption of Mendeleev Volcano, Southern Kuril Island Arc

Abstract The southern part of the Kuril Island Arc is one of the world’s most active modern volcanic zones, with widespread felsic caldera volcanism, but it has been less well studied compared with other arcs. The Mendeleev caldera-forming eruption (40 ka) on Kunashir Island, southern Kurils, is one of the most explosive Pleistocene eruptions to have occurred in this region. This study aimed to establish the origin and storage conditions of magma preceding the Pleistocene caldera eruption of Mendeleev volcano. Mineralogical and melt inclusion analyses reveal that the primary melts had felsic compositions and that the early stage of magmatic evolution involved amphibole breakdown into a two-pyroxene, plagioclase, and Fe–Ti oxide assemblage under pressure–temperature conditions of 107–314 MPa and 807–932°C. The caldera-forming products are represented by dacitic pumice composed of plagioclase + augite + hypersthene+ quartz + Fe–Ti oxides and melt with uniform low-K rhyolite composition. Pre-eruptive magma was stored in a reservoir at 77–195 MPa (3.0–7.6 km depth) and 830–890°C under H2O-saturated conditions. The mechanism of magmatic evolution implies the following two-step scenario: (1) generation of magma by the partial melting of an amphibole-bearing substrate accompanied by the formation of early Mg-rich clino- and orthopyroxene, plagioclase, Fe–Ti oxides, and peritectic rhyolitic melt; and (2) crystallization of late plagioclase and quartz directly from these partial melts. Local or regional extension during the Pleistocene, accompanied by increasing heat flow in the supra-subduction mantle, generated an active mafic intrusion into the upper crust. This process was accompanied by abundant subaerial eruptions of basaltic volcanoes and could have caused intense heating and partial melting of upper-crustal rocks. Our results indicate that the partial melting of amphibole-bearing substrates in island arcs may serve as a universal mechanism for the generation of silicic magmas during powerful caldera eruptions in young island arcs.

Fluorine-rich mafic lower crust in the southern Rocky Mountains: The role of pre-enrichment in generating fluorine-rich silicic magmas and porphyry Mo deposits

AbstractFluorine-rich granites and rhyolites occur throughout the southern Rocky Mountains, but the origin of F-enrichment has remained unclear. We test if F-enrichment could be inherited from ancient mafic lower crust by: (1) measuring amphibole compositions, including F and Cl contents, of lower crustal mafic granulite xenoliths from northern Colorado to determine if they are unusually enriched in halogens; (2) analyzing whole-rock elemental and Sr, Nd, and Pb isotopic compositions for upper crustal Cretaceous to Oligocene igneous rocks in Colorado to evaluate their sources; and (3) comparing batch melting models of mafic lower crustal source rocks to melt F and Cl abundances derived from biotite data from the F-rich silicic Never Summer batholith. This approach allows us to better determine if the mafic lower crust was pre-enriched in F, if it is concentrated enough to generate F-rich anatectic melts, and if geochemical data support an ancient lower crustal origin for the F-rich rocks in the southern Rocky Mountains.Electron microprobe analyses of amphibole in lower crustal mafic granulite xenoliths show they contain 0.56–1.38 wt% F and 0.45–0.73 wt% Cl. Titanium in calcium amphibole thermometry indicates that the amphiboles equilibrated at high to ultrahigh temperature conditions (805 to 940 °C), and semiquantitative amphibole thermobarometry indicates the amphiboles equilibrated at 0.5 to 1.0 GPa prior to entrainment in magmas during the Devonian. Mass balance calculations, based on these new measurements, indicate parts of the mafic lower crust in Colorado are at least 3.5 times more enriched in F than average mafic lower crust. Intrusions coeval with the Laramide Orogeny (75 to 38 Ma) pre-date F-rich magmatism in Colorado and have Sr and Nd isotopic compositions consistent with mafic lower crust ± mantle sources, but many of these intrusions contain elevated Sr/Y ratios (>40) that suggest amphibole was a stable phase during magma generation. The F-rich igneous rocks from the Never Summer igneous complex and Colorado Mineral Belt also have Sr and Nd isotopic compositions that overlap with the lower crustal mafic granulite xenoliths, but they have lower Sr/Y, higher Nb and Y abundances, and distinctly less radiogenic 206Pb/204Pbi compositions than preceding Laramide magmatism. Batch melt modeling indicates low-degree partial melts derived from rocks similar to the mafic lower crustal xenoliths we analyzed can yield silicic melts with >2000 ppm F, similar to estimated F melt concentrations for silicic melts that are interpreted to be parental to evolved leucogranites.We suggest that F-rich silicic melts in the southern Rocky Mountains were sourced from garnet-free mafic lower crust, and that fluid-absent breakdown of amphibole in ultrahigh temperature metamorphic rocks was a key process in their generation. Based on the composition of high-F amphibole measured from lower crustal xenoliths, the temperature of amphibole breakdown and melt generation for these F-enriched source rocks is likely >100 °C higher than similar lower crust with low or average F abundances. As such, these source rocks only melted during periods of unusually high heat flow into the lower crust, such as during an influx of mantle-derived magmas related to rifting or the post-Laramide ignimbrite flare-up in the region. These data have direct implications for the genesis of porphyry Mo mineralization, because they indicate that pre-enrichment of F in the deep crust could be a necessary condition for later anatexis and generation of F-rich magmas.

Experimental Investigation of Trachydacite Magma Storage Prior to the 1257 Eruption of Mt Samalas

Abstract The caldera-forming Samalas eruption of the Samalas–Rinjani volcanic complex on Lombok, Indonesia, in 1257 CE ranks as one of the most explosive and sulphur-rich eruptions of the past thousand years. Along with other significant events (e.g. Tambora 1815, El Chichón 1982), Samalas 1257 forms a class of powerful eruptions of magmas of intermediate alkaline composition, which remain relatively understudied. This study aims to better understand the pre-eruptive magma reservoir conditions and volatile storage capacities of intermediate alkaline systems, using the 1257 Samalas eruption as a case study. Systematic partial-equilibrium experiments were run to constrain potential magma reservoir conditions (25–200 MPa and 850–1000°C) by examining changes in the chemistry and textures of experimental glass and mineral products. Natural trachydacite pumice from the 1257 Samalas eruption was used as the experimental charge for a series of water-saturated experiments run in cold-seal pressure vessels, buffered at an oxygen fugacity ƒO2 of NNO/NNO + 1 log units. Assessing the match between natural and experimental glass compositions and mineral phases and textures reveals that pre-eruptive magma reservoir conditions were between 875°C and 930°C and ca. 100–150 MPa (4.5 ± 1km depth). Breakdown of experimental amphibole at pressures below 75 MPa, and plagioclase instability at 950°C/100 MPa, provide the strongest constraints on phase stabilities that are consistent with the magma storage region. In the observed temperature and pressure range, the natural compositions of plagioclase, amphibole and orthopyroxene are replicated in experimentally precipitated rims. Clinopyroxene and high anorthite plagioclase (An>64) in the natural mineral assemblage could not be replicated in the experiments, implying that these compositions most likely did not grow in the shallow pre-eruptive reservoir of Mt Samalas. The pre-eruptive storage depths of trachydacite magma are significantly shallower than the postulated magma storage at the currently active basaltic-andesite magmatic system at Rinjani, and thus monitoring magma storage depth may be a useful indicator of changing magma composition in the future. Our findings show that highly explosive VEI 7 eruptions of intermediate alkaline magma can be fed from a relatively limited range of storage pressures (100–150 MPa), suggesting accumulation of magma in one place, rather than the rapid extraction of magma from a vertically extensive, transcrustal magma system.

Chemical Differentiation by Mineralogical Buffering in Crustal Hot Zones

Abstract Chemical diversity in convergent margin magmas is a product of igneous differentiation in crustal hot zones, vertically extensive regions characterised by a low-volume (<20%) mobile melt phase dispersed in a crystal-rich mush. Chemical reaction between buoyant, percolating melts and the surrounding mush leads to chemical buffering by the local mineral assemblage. Where this assemblage has low thermodynamic variance (e.g. six mineral phases plus melt and H2O-CO2 fluid) the resultant multiply saturated melts will show limited chemical variability. Plutonic xenoliths from many volcanic arcs, as well as exhumed arc crustal sections, testify to the ubiquity of low-variance, broadly gabbroic, mineral assemblages. Here I use the concept of multiple saturation to explore the chemical consequences of percolative reactive melt flow in crustal hot zones using data from published experimental studies on a wide variety of different starting materials. I show that the common, low-variance hornblende gabbronorite assemblage clinopyroxene-hornblende-orthopyroxene-magnetite-plagioclase-ilmenite (CHOMPI) coexists with fluid-saturated melt over a wide range of pressure (1–10 kb) temperature (800–1050 °C) and fluid composition (molar fraction H2O, XH2O, of 1.0 to 0.3). The CHOMPI stability field is bounded by the following: the appearance of garnet at high pressure, the hydrous haplogranite granite liquidus at low temperature, and amphibole breakdown at high temperature and low pressure. CHOMPI melts cover a wide compositional range (54–74 wt% SiO2; 4.4–0.1 wt% MgO) that can be parameterised in terms of five independent variables: pressure, temperature, fO2, molar CO2/H2O in the fluid and melt K2O content. The compositional diversity and broad stability field of CHOMPI-saturated melts make them extremely common in the rock record. Melt composition parameterisations can be inverted to recover pressure (±1.3 kb), temperature (±16 °C) and fluid molar CO2/H2O (±0.43) of CHOMPI-saturated melts. If a natural magma composition can be shown to lie on or close to the CHOMPI saturation surface then the conditions under which that melt was last in equilibrium with this mineral assemblage can be established. I apply this method of magma source thermobarometry and hygrometry to the most recent eruptions from 15 Cascades arc volcanic centres. Calculated pressures range from 1.3 to 5.8 kb (5–21 km depth) with significant along-arc variation. Temperatures correlate with pressure and match independent estimates of eruption temperatures from mineral thermometry with the exception of two eruptions where significant (≤10°C) cooling occurred during pre-eruptive magma storage. Fluid XH2O is in the range 0.47–0.92 and inversely correlates with pressure. Mineralogical buffering of melt chemistry in hot zones is proposed as an important mechanism of chemical differentiation in volcanic arcs. Mineralogical buffering can operate at the low-melt fractions observed in geophysical surveys of arc crust, providing an alternative to traditional concepts of assimilation-fractional crystallisation and liquid lines of descent that operate most effectively in melt-rich systems.

Strengths and limitations of in situ U–Pb titanite petrochronology in polymetamorphic rocks: An example from western Maine, USA

AbstractTitanite is a potentially powerful U–Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, which may lead to inaccurate and geologically ambiguous U–Pb dates if a proper correction is not or cannot be applied. Here, we present laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS)‐derived titanite U–Pb dates and trace element concentrations for two banded calcsilicate gneisses from south‐central Maine, USA (SSP18‐1A and SSP18‐1B). Single spot common Pb‐corrected dates range from 400 to 280 Ma with ±12–20 Ma propagated 2SE. Titanite grains in sample SSP18‐1B exhibit regular core‐to‐rim variations in texture, composition, and date. We identify four titanite populations: (1) 397 ± 5 Ma (95% CL) low Y + HREE cores and mottled grains, (2) 370 ± 7 Ma high Y + REE mantles and cores, (3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and (4) 295 ± 6 Ma LREE‐depleted rims. We interpret the increase in titanite Y + HREE between ca. 397 and ca. 370 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr‐in‐titanite temperatures (803 ± 36°C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 370 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (>80 vol%), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite dates the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well‐defined date–composition–texture relationships observed for titanite from SSP18‐1B, titanite grains from sample SSP18‐1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed dates spanning 380–330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U–Pb data along with chemical and textural data to screen for inherited radiogenic Pb.

Deciphering Degassing and Source Effects in Cl Isotopes in Melt Inclusions: The Possible Role of Amphibole in the Magma Source of Stromboli (Aeolian Island Arc)

Chlorine isotopes have emerged as a new geochemical tool over the past 15 years. Most of the data consist of bulk rock data, with a minority carried out in situ on melt inclusions using secondary ion mass spectrometry. More data are necessary to understand the relationship between δ37Cl measured in melt inclusions and that in bulk rocks from the same volcanic center. Here we have analyzed a suite of melt inclusions entrapped in olivine Fo63-85, as well as some from clinopyroxene crystals, from a single hand-sample from the Vancori unit of Stromboli, Aeolian Islands. The 27 selected melt inclusions have major element compositions ranging from high potassium alkali basalt to evolved shoshonite. Their δ37Cl vary from −2.6 ± 0.1‰ to +1.2 ± 0.2‰, a far larger range than for Stromboli bulk rocks. In this dataset, the δ37Cl variation in melt inclusions is not related to Cl degassing, or to fractional crystallization. Instead, correlations between δ37Cl and S/Cl, K2O and trace element ratios suggest mixing of two Cl endmembers with distinct δ37Cl signatures. A first endmember is characterized by high potassium alkali basalt compositions, high Ba/La (∼28), high S/Cl, and high δ37Cl (>1‰), confirming the influence in the mantle source of an aqueous fluid and providing a new constraint on its composition: that it derives from the breakdown of amphibole. The second endmember has a more evolved composition, high La/Yb, low S/Cl, and low δ37Cl (<−2‰), identifying the influence of a solute-rich component derived from subducted sediments. The δ37Cl data thus help refine the two sources initially identified from bulk rock studies and δ37Cl proves to be a potential tracer for amphibole.

Building a continental arc section: Constraints from Paleozoic granulite-facies metamorphism, anatexis, and magmatism in the northern margin of the Qilian Block, northern Tibet Plateau

AbstractContinental arcs in active continental margins (especially deep-seated arc magmatism, anatexis, and metamorphism) can be extremely significant in evaluating continent building processes. In this contribution, a Paleozoic continental arc section is constructed based on coeval granulite-facies metamorphism, anatexis, and magmatism on the northern margin of the Qilian Block, which record two significant episodes of continental crust growth. The deeper layer of the lower crust mainly consists of medium-high pressure mafic and felsic granulites, with apparent peak pressure-temperature conditions of 11–13 kbar and 800–950 °C, corresponding to crustal depths of ~35–45 km. The high-pressure mafic granulite and local garnet-cumulate represent mafic residues via dehydration melting involving breakdown of amphibole with anatectic garnet growth. Zircon U-Pb geochronology indicates that these high-grade metamorphic rocks experienced peak granulite-facies metamorphism at ca. 450 Ma. In the upper layer of the lower crust, the most abundant rocks are preexisting garnet-bearing metasedimentary rocks, orthogneiss, and local garnet amphibolite, which experienced medium-pressure amphibolite-facies to granulite-facies metamorphism at depths of 20–30 km at ca. 450 Ma. These metasedimentary rocks and orthogneiss have also experienced partial melting involving mica and rare amphibole at 457–453 Ma. The shallow to mid-crust is primarily composed of diorite-granodiorite batholiths and volcanic cover with multiple origin, which were intruded during 500–450 Ma, recording long-term crustal growth and differentiation episode. As a whole, two episodes of continental crust growth were depicted in the continental arc section on the northern margin of the Qilian Block, including: (a) the first episode is documented in a lithological assemblage composing of coeval mafic-intermediate intrusive and volcanic rocks derived from partial melting of modified lithospheric mantle and subducted oceanic crust during southward subduction of the North Qilian Ocean at 500–480 Ma; (b) the second episode is recorded in mafic rocks derived from partial melting of modified lithospheric mantle during transition from oceanic subduction to initial collision at 460–450 Ma.

Deciphering magma storage and ascent processes of Taranaki, New Zealand, from the complexity of amphibole breakdown textures

Lava flows that constitute the current edifice of Taranaki volcano, New Zealand, contain a complex crystal cargo including plagioclase, pyroxene, amphibole, oxides, and rarely olivine. Amphibole textures and mineral chemistry indicate that crystals are pargasites entrained at various depths by ascending magmas. The crystals record a complex growth history within a range of temperature and pressure conditions (c. 950–1010 °C at mid- to deep crustal levels). Most amphiboles show the development of distinct reaction rims where the mineral is in contact with the ambient melt. Texturally, the rims are identified as detached, symplectic, granular or coarse. True rim thicknesses vary little (±20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from <5 to >50 μm. Reaction rim formation on Taranaki amphiboles is attributed to degassing during magma ascent as well as small increases in temperature. Associated with the formation of rims, the amphiboles also show two types of volumetric decomposition, identified as irregular and aligned. The former is indicative of slow reaction of amphibole with melt entrapped in fractures and cleavages during decompression-induced degassing, while the other is indicative of heating-induced breakdown of amphibole triggered by uptake into hot magmas prior to the onset of eruption. The combined evidence indicates that despite the complex and variable growth history of the remobilized crystal cargo, such crystals may be useful in constraining volcanic pre- and syn-eruptive processes and, when appropriate experimental data become available, their rates, which may be of the order of days to weeks.

Along-dip variations of subduction fluids: The 30–80 km depth traverse of the Schistes Lustrés complex (Queyras-Monviso, W. Alps)

The Queyras-Monviso traverse of the Schistes Lustrés complex, is a stack of underplated nappes of oceanic rocks subducted from blueschist- to eclogite-facies conditions.This study reports on salinities and gas contents of primary fluid inclusions trapped in high pressure veins from metasediments and metamafic rocks (445 fluid inclusions in 22 samples). These data provide snapshots of the compositions of fluids present at peak burial conditions, varying from ~30 to 80 km depth and illustrate the evolution of fluid composition with burial along a cold subduction zone.Fluid inclusions trapped in lawsonite- and carpholite-bearing veins in metasediments contain moderately saline aqueous fluids (average salinity of 4.6 wt% NaCl eq.) with subordinate amounts of CO2 and CH4 in the vapor phase. The observed salinity decrease with increasing grade is interpreted as reflecting progressive dilution of initial seawater-like pore fluid by low-salinity fluids released locally by dehydration reactions. Less-frequently measured higher salinities in the uppermost metasedimentary-dominated tectonic unit suggest brine infiltration from embedded blocks of margin units containing evaporites. CO2 and CH4 (and scarce potential hydrocarbons) appear to be locally released from fluid interaction with carbonates and carbonaceous matter-rich pelitic horizons, respectively.Fluid inclusions in high pressure omphacite veins in metagabbros record higher salinities (mean salinity about 17 wt% NaCl eq.) with small amounts of N2 in eclogitic veins only, and a variety of minerals (calcite, white mica, salts) indicative of complex chemical systems. These high salinities are interpreted as partly inherited from seafloor high-temperature hydrothermal alteration of gabbros, resulting in phase separation and brine formation. Progressive breakdown of hydrothermal Cl-rich amphibole to glaucophane (blueschist-facies) and then omphacite (eclogite-facies) and release of brines trapped in fluid inclusions could account for the high salinity fluids. Therefore, in metagabbros, fluid inclusions record progressive release of Cl in the fluid phase with increasing grade.Fluid signatures of metasediments and metamafics appear characteristic of each rock type. Local fluid signatures and redox conditions were preserved within units, possibly due to restricted and transient fluid circulation (at the hm-scale at most).Fluid inclusions in Alpine metasediments show salinities and gas contents comparable with other subducted fragments of oceanic lithosphere worldwide, whereas fluid salinities of Alpine metagabbros are higher than salinities recorded elsewhere, either due to (1) higher-temperature hydrothermal alteration and brine formation in Alpine metagabbros (compared to metavolcanics) or (2) more restricted infiltration by sediment-derived fluids compared to block-in-mélange subduction complexes.

No chemical change during high-T dehydration and re-hydration reactions: Constraints from Erzgebirge HP and UHP eclogite

Meta-basaltic eclogite occurs in the Erzgebirge in three high-pressure (HP) units (Unit 1, 2, and 3) as numerous lenses within high-grade felsic rocks. Units 2 and 3 are common HP units with quartz eclogite, and Unit 1 is an ultra-high pressure (UHP) unit with coesite eclogite, garnetite (metarodingite) and garnet peridotite. The conditions of peak metamorphism increase from Unit 3 (600–650 °C, 20–22 kbar), to Unit 2 (670–730 °C, 24–26 kbar) and Unit 1 (840–920 °C, ≥30 kbar), correlating with a decreasing abundance of hydrous minerals (from >10 vol% in Unit 3 to 0 vol% in Unit 1). In all units, the dominating eclogite type is dark-colored with a composition typical of MORB. A subordinate light type is chemically more variable and has higher contents of Mg, Al, Ca, Cr, Ni, and large ion lithophile elements as well as lower Fe, Zr, Y, and rare earth element concentrations than dark eclogite. Light eclogite formation is interpreted by plagioclase accumulation in a MOR magma chamber.No compositional difference is visible between well-preserved eclogite and samples with variable degrees of post-eclogitic overprint. In addition, dark eclogite from all units is compositionally indistinguishable, implying that prograde dehydration reactions did not modify major and trace element concentrations. This conclusion applies to all reactions in the temperature interval between c. 600 °C (peak T in Unit 3) and c. 900 °C (peak T in Unit 1), including prograde breakdown of calcic amphibole, zoisite, paragonite, and phengite. Together with previous studies on dehydration reactions in blueschist and eclogite at <600 °C (Spandler et al., 2003; Spandler et al., 2004), the present results imply that de-volatilization of the basaltic portion of subducting slabs plays only a minor, if any, role for the enrichment of fluid-mobile elements in the mantle wedge. We infer that most, if not all, of the H2O produced by dehydration reactions between 600 and 900 °C is preserved in eclogite due to the pressure-enhanced capability of garnet and omphacite to incorporate structural water. Incorporation of H2O, produced during the final dehydration stage (c. 800 ± 50 °C, 25–30 kbar), in nominally anhydrous minerals may explain why partial melting obviously did not occur in eclogite.

The versatility of petrological modeling: Thermobarometry of high‐pressure metabasites from the Renge and Sanbagawa belts and phase evolution during warm subduction at Nankai

AbstractPetrological modeling is a powerful technique to address different types of geological problems via phase‐equilibria predictions at different pressure–temperature‐composition conditions. Here, we show the versatility of this technique by (1) performing thermobarometrical calculations using phase equilibrium diagrams to explore the petrological evolution of high‐pressure (HP) metabasites from the Renge and Sanbagawa belts, Japan and (2) forward‐modeling the mineral–melt evolution of the subducted fresh and altered oceanic crust along the Nankai subduction zone geotherm at the Kii peninsula, Japan. In the first case, we selected three representative samples from these metamorphic belts: a glaucophane eclogite and a garnet glaucophane schist from the Renge belt (Omi area) and a quartz eclogite from the Sanbagawa belt (Besshi area). We calculated the peak metamorphic conditions at ~2.0–2.3 GPa and ~550–630 °C for the HP metabasites from the Renge belt, whereas for the quartz eclogite, the peak equilibrium conditions were calculated at 2.5–2.8 GPa and ~640–750 °C. According to our models, the quartz eclogite experienced partial melting after peak metamorphism. In terms of the petrological evolution of the subducted uppermost portion of the oceanic crust along the warm Nankai geotherm, our models show that fluid release occurs at ~20–60 km, likely promoting high pore‐fluid pressure, and thus, seismicity at these depths; dehydration is controlled by chlorite breakdown. Our petrological models predict partial melting at &gt;60 km, mainly driven by phengite and amphibole breakdown. According to our models, the melt proportion is relatively small, suggesting that slab anatexis is not an efficient mechanism for generating voluminous magmatism at these conditions. Modeled melt compositions correspond to high‐SiO2 adakites; these are similar to compositions found in the Daisen and Sambe volcanoes, in southwest Japan, suggesting that the modeled melts may serve as an analog to explain adakite petrogenesis.

Subduction-related metasomatism in the lithospheric mantle beneath the Calatrava volcanic field (central Spain): constraints from lherzolite xenoliths of the Cerro Gordo volcano

ABSTRACT Two types of lherzolite xenoliths appear in the Cerro Gordo maar from the Cenozoic Calatrava volcanic field. Hydrous lherzolites (group-1) show a major proportion of metasomatic amphibole (>5 modal%), whereas group-2 lherzolites have only accessory amounts or an absence of amphibole. Group-1 lherzolites show a higher LREE-Ba-Sr-Th content in whole-rock composition compared to group-2 lherzolites. The most depleted group-2 lherzolite xenoliths indicate a low degree of partial melting (<5%) of the mantle source, similarly to other Calatrava xenolith suites. Trace element content combined with Sr-Nd isotopic ratios suggests a protolith mantle source with the characteristics of a depleted MORB Mantle (DMM type). This mantle protolith was later refertilized by subduction-related metasomatism, causing the formation of modal amphibole and obliterating the original MORB-like chemical features in the most transformed xenoliths (group-1 lherzolites). The entrainment of these mantle fragments in the volcanic alkaline magma promoted amphibole breakdown reactions during its transport, generating silica-rich glasses with the associated unusual formation of secondary orthopyroxene. The suprasubduction character of the protolithic mantle combined with previously published Re-Os model ages in nearby lherzolite xenoliths suggests that the sampled lherzolites could represent the old Cadomian mantle wedge of the Central Iberian zone.

Partial melting due to breakdown of phengite and amphibole in retrogressed eclogite of deep Precambrian crust: An example from the Algonquin terrane, western Grenville Province, Canada

Characterizing partial melting of eclogite in Precambrian orogens is critical to understanding the petrological processes that operate in the deep orogenic crust. Retrogressed eclogite from the Algonquin terrane in the western Grenville Province, Canada, was taken as an example to study such processes. The studied samples contained the assemblage of garnet, omphacite, amphibole, phengite, biotite, and rutile at peak pressure. Various exhumation- and melt-related textures formed show evidence for a complex post-peak pressure history. Rutile is only found in the core of garnet, whereas ilmenite-bearing assemblages predominate in the garnet rim and the matrix. Symplectite of diopside + plagioclase + amphibole replaced former omphacite. Symplectite of amphibole and plagioclase formed as a corona around garnet. Polymineralic inclusions of plagioclase + K-feldspar ± ilmenite ± biotite ± amphibole ± apatite ± sulfides in garnet crystallized from melt, which was captured during garnet growth and reacted with the host. Cusps of plagioclase into the boundaries of other phases (e.g. garnet and clinopyroxene) mimic the original melt-solid interface. Neoblasts of garnet with euhedral crystal faces were formed as a peritectic phase during eclogite melting. Leucocratic veins without external connection, present in the sampled mafic pod at outcrop scale, are interpreted as crystallized pockets of melt which derived internally. Pseudosection modeling of the studied samples along with compositional isopleths of chemically zoned garnet yielded an exhumation path from 1.7 ± 0.1 GPa, 680 ± 50 °C to 1.3 ± 0.1 GPa, 920 ± 50 °C. Along this path, the modal content of melt increased in response to the breakdown of phengite and amphibole along with involvement of omphacite and rutile. The melt crystallized during cooling and equilibrated at 0.6–0.8 GPa, 710–750 °C, estimated using empirical thermobarometry. This study shows that metabasite from Proterozoic collisional orogens can be partially melted on the exhumation and heating path through phengite- and amphibole-dehydration melting.

Hisingerite in Trachydacite from Tarim: Implications for Voluminous Felsic Rocks in Transitional Large Igneous Province

Unlike the typical large igneous provinces (LIPs) that are dominated by mafic-ultramafic rocks, the Tarim large igneous province (TLIP) is characterized by a high proportion of felsic rocks, based on which the TLIP is classified as a transitional LIP. In this study, we focus on the trachydacite from the TLIP in which we report the characteristics of hisingerite employing a variety of techniques such as EMPA, LA-ICPMS, CCD single crystal diffraction, and bulk-rock oxygen isotopes. The hisingerite in this rock is associated with plagioclase, amphibole, apatite and ilmenite. These minerals occur as aggregates of fine curled fibers in micron-scale and display heavy rare earth elements (HREE) enriched signature with significant negative Eu anomalies. In the primitive mantle-normalized trace element spider diagrams, they show pronounced Th and U spikes and Nb, Zr, Hf troughs. Petrological observation and mineralogical study reveal a closely genetic relationship between hisingerite and amphibole, indicating that the hisingerite might have been derived from the breakdown of amphibole during the magma ascent. The formation of hisingerite requires excess water from the surrounding melts, suggesting a hydrous parental magma. The hisingerite and amphibole assign a hydrous crustal source for the rock, and extensive crustal melting accounts for the voluminous felsic rocks in the TLIP.

The genesis of felsic magmatism during the closure of the Northeastern Paleo-Tethys Ocean: Evidence from the Heri batholith in West Qinling, China

To better understand the origin of voluminous silicic rocks in a convergent continental margin, we conducted an integrated study in which we have obtained geochronological, mineralogical, and isotopic (including whole-rock Sr–Nd–Pb, in-situ zircon Hf) data of the Heri batholith in West Qinling on the NE Tibetan Plateau. The batholith is composed of metaluminous to weakly peraluminous granodiorites (235–233 Ma) and porphyritic granodiorites (230–223 Ma) with an I-type affinity. Both lithologies share similar major, trace elemental and SrNd isotopic compositions. Detailed elemental data demonstrate that these granodioritic rocks underwent fractional crystallization of hornblende and apatite, with plagioclase (i.e. sieve-textured plagioclase cores) accumulation to some extent. Except for porphyritic granodiorites, the Pb isotopes for other analyzed samples are characterized by high radiogenicity and uniformity ((206Pb/204Pb)t: 17.263–18.472, (206Pb/204Pb)t: 15.571–15.591, and (206Pb/204Pb)t: 38.032–38.304), together with limited variations in initial Sr ((87Sr/86Sr)t: 0.707251–0.708103) and Nd (εNd(t) = −7.1 to −6.3) isotopes with two-stage model ages (TDM2) of 1.58–1.52 Ga. These factors collectively point to a derivation from the Mesoproterozoic basement rocks at the lower crustal level, or a comprehensive mixing of different-age components that generated an average crustal residence age. The SrNd isotopic compositions of the porphyritic granodiorites are strikingly similar to those of granodiorites. Compared with the experimental melt compositions of amphibolites, the Heri granitoids are probably derived from an amphibolitic source under fluid-absent conditions due to the incongruent breakdown of amphibole and biotite. Based on the temporal–spatial distribution of granitic intrusions in West Qinling and the regional tectonic evolution, our interpretation is that the Heri batholith was formed during the initial collision between the North China Craton (NCC) and the South China Craton (SCC), which was accompanied by the closure of the Paleotethyan Ocean. Considering both previously published data and our new data, we propose that the Heri granitoids were mainly generated by the partial melting of lower crustal amphibolites, with minor mantle-derived melts.

Adakite Volcanism at Continental Margin and Associated Problems. Part II. Adakites from the Sea of Okhotsk, Kamchatka, and Bering Sea Regions: Typification and Genesis

Based on a compilation of the vast literature, we typify the peculiar mantle-derived adakitic volcanic rocks in the Sea of Okhotsk, Kamchatka, and Bering Sea regions. The low-silica (LSA) and high-silica (HSA) types are distinguished in each region. The prevailing adakite type corresponds to the bimodal basalt–andesite series in the Kamchatka region and the basanite–dacite series in the Bering Sea region. In the Kamchatka region, the formation of LSA was related to the fractionation and melting of pyroxene, while the origin of HSA was related to the fractionation, melting, and breakdown of amphibole. Alkaline metasomatism plays a subordinate role in these processes. In the Bering Sea region, the formation of both types of adakites was mainly controlled by amphibole, with the assistance of alkaline metasomatism. The isotope–geochemical compositions of the adakites and within-plate volcanic rocks shows that both these rock types can be equally regarded as indicators of plume petrogenesis and geodynamics. Based on a comparison of the compositions of the adakites and glasses derived by xenolith melting, a two-stage genetic model is proposed: formation of black pyroxenites through melting of mantle lherzolites and generation of the entire rock complex owing to the repeated melting of the same lherzolites and newly formed pyroxenites. The formation of adakitic and within-plate signatures in the volcanic products beginning from the Eocene indicates the transition of the spacious territory of the NE continental margin from subduction-related compression to a setting of rift extension.