Hot Mafic Magma Research Articles

3D model of the El Hornito pluton, Sierras Pampeanas of Argentina: Evidence of hybridization in the re-used feeder channels for emplacement in the ductile-brittle level

A comprehensive structural, petrological, and geophysical study of the 385 ± 2 Ma Devonian El Hornito Pluton (EHP) in the Sierra Grande de San Luis, Argentina, is presented. We establish that the EHP results from the intrusion of three main magma batches. The three-dimensional shape of the pluton was modeled using stochastic litho-constrained gravity data inversion and was complemented with a field geology survey. Results suggest a shallow-rooted, irregularly shaped pluton with a mean depth of 1.5 km, 16 km long and 4 km deep with a volume of 217 km³. Two areas of negative residual Bouguer anomaly reveal the existence of two root zones interpreted to indicate feeder channels. One of the feeders does not have a surface expression, but the other coincides with the central part of the pluton. A hot mafic magma intruded into and rejuvenated a crystallizing granite mush and the thermal perturbations triggered local convection with the development of an ascending magma plume developing mixing/mingling structures. Pluton emplacement was controlled by a shear zone at the boundary between two major metamorphic units. Emplacement is evidenced by the location of the two root zones along the interpreted contacts of the metamorphic zones, and the shear zone along this basement anisotropy provided the space into which magma batches were emplaced. Internal and external structures indicate that space for pluton emplacement was generated by a combination of tectonism with ballooning and stoping assisted by floor depression, which were the most effective processes during the final emplacement of the magma. This was possible due to the contrasting rheological responses of the host rocks.

Petrogenesis of S‐type Ladakh granite and mafic microgranular enclaves in the southern margin of Ladakh batholith: An evidence of crust–mantle interaction during the collision between Indian and Eurasian plates

AbstractThe previous studies revealed the I‐type Ladakh magmatism in the Andean‐type southern margin of the Ladakh batholith (LB) was related to the subduction of the Neotethyan Ocean and India‐Eurasia collision. However, LB's S‐type granitic magmatism and associated mafic microgranular enclaves (MMEs) are poorly constrained. Here, we present the new data for S‐type Ladakh granite (LG) and associated monzodiorite MMEs in the Andean‐type orogeny in the southern margin of the Eurasian plate. The low SiO2 (47.4–53.9 wt%), high K2O (1.56–3.21 wt%), Mg# (52–65), continental‐arc tracer patterns, and slightly depleted to evolved Sr‐Nd isotopic composition ((87Sr/86Sr)i = 0.7047–0.7166; ℇNd (t = 50 Ma) = (+1.40 to −8.92)) for MME suggest that they were derived from the phlogopite‐bearing deep lithospheric mantle‐source at a depth of 5.4–10.5 km depth with 810–870°C, 1.4–2.8 kbar, and enriched by sediment‐melts addition into the mantle‐wedge from subducting Neotethyan Oceanic slab. The mantle‐derived ascending hot mafic magma mixing with felsic magma of the ancient northern Indian margin‐derived, generates monzodiorite MME by assimilation and magma mixing processes. Plagioclase, amphibole, and biotite chemistry support the magma mixing processes. LG are characterized by high SiO2 (63.4–75.0 wt%), K2O (3.93–5.67 wt%), CaO/Na2O ratio of >0.3, differentiation index (90.27–97.46), normative corundum (1.0–2.8), A/CNK values (1.00–1.18), hypersthene (0.7–5.7), and low Al2O3, MgO, TiO2, Fe2O3. They also exhibit peraluminous, variable tracer elemental abundances, variable (87Sr/86Sr)i ratios (0.6967–0.7191), and high whole rock ℇNd (t = 50 Ma) values of −4.15 to −11.92) and ancient two‐stage Nd model age of 1160 and 1858 Ma. These features suggest that S‐type Ladakh granites were derived from the melting of ancient metagreywacke‐dominated metasedimentary rocks of the northern Indian margin by a large amount of mafic magma underplating after subducted Neotethyan slab‐rollback. The formation of LG and MMEs related to the Andean‐type orogeny in the southern margin of the Eurasian plate.

Mineralogical Constraints on Magma Recharge and Mixing of the Post-Collisional Potassic Volcanic Rocks in Dahongliutan, NW Tibetan Plateau

Post-collisional potassic magmatic rocks are widely distributed in the northwestern Tibetan Plateau, yet their magmatic processes remain poorly understood. Here, we present a comprehensive analysis of the whole-rock major and trace elements, as well as the mineral textures and chemistry of the Dahongliutan volcanic rocks in the NW Tibetan Plateau, aiming to reveal the magmatic processes prior to eruption and speculate on the triggering mechanism. The results show that the Dahongliutan volcanic rocks are potassic trachyandesites, which undergo polybaric crystallization during magma ascension. The phenocrysts in these potassic rocks exhibit various textural and compositional zoning styles. The green cores of green-core clinopyroxenes show textural (e.g., resorption texture) and chemical (Fe-rich) disequilibrium with the host rock compositions, suggesting that they may be antecrysts and crystallized from early batches of more evolved magmas. Additionally, alkali feldspar phenocrysts also display disequilibrium characteristics (e.g., overgrowth rim and sieve texture), indicating hot mafic magma recharge and mixing in the magma plumbing system. Therefore, we conclude that the disequilibrium textural and compositional features of green-core clinopyroxene and alkali feldspar phenocrysts provide evidence of magma recharge and mixing prior to eruption. Furthermore, it is likely that the eruption of the Dahongliutan volcano was triggered by magma recharge.

The Cambrian Atlas – Ossa–Morena – North Armorican Rift, West Gondwana: along- and off-axis stratigraphic and volcano-tectonic patterns

Abstract The Cambrian Atlas – Ossa–Morena – North Armorican Rift extended along West Gondwana from the end of the Pan-African and Cadomian orogenies until the diachronous beginning of drift conditions related to the opening of the Rheic Ocean. The along-axis rift cross-cut the western parts of the Anti-Atlas, High Atlas and Coastal Meseta, which were linked to the Ossa–Morena Zone and the North Armorican Domain, whereas several joint tectonic branches connected with off-axis rift transects of the Central Iberian, West Asturian–Leonese and Cantabrian zones (Iberian Massif), the Central and South Armorican domains, the Occitan Domain, the Pyrenees, and southern Sardinia. The pre-rift unconformity, post-dating the orogenic collapse, is characterized by initial (half-)graben development and subsequent infill, with slope-related breccias and conglomerates controlled by the denudation of surrounding uplands. Synrift pulses show regional extension and are distinctly identifiable on the top of rift shoulders, recording episodes of carbonate production due to their association with karst and hydrothermal processes. The break-up unconformity ranges from volcanic-free angular discordances and paraconformities to generalized uplift and denudation of subaerially exposed areas, associated with the onset of granite-dominant large igneous provinces (LIPs). The Furongian–Tremadocian (Toledanian) and Ordovician (Sardic) phases have been interpreted as due to: (i) Andean-type subduction magmatism reaching the crust in an arc–back-arc setting; (ii) post-collisional decompression melting without significant mantle involvement; and (iii) partial melting of the lower continental crust affected by the underplating of hot mafic magmas linked to superplumes.

The mechanism responsible for the geochemical diversity of the Putuoshan volcanic-plutonic complex and its implications for the late Mesozoic crustal evolution of the Cathaysia Block, Southeast China

Essential mechanisms for the geochemical diversity observed in a silicic igneous rock association, which is of paramount importance for understanding the differentiation and growth of continental crust, remain highly controversial. Here the issues are explored by comparing the Late Cretaceous intrusions (minor-phase miarolitic alkali-feldspar granite and main-phase alkali-feldspar granite) and coeval volcanic rocks (rhyolite and rhyodacite) within the Putuoshan (PTS) volcanic-plutonic complex in the coastal area of southeast (SE) China. Zircon UPb ages indicate that the rhyolite, rhyodacite, minor-phase granite and the previously studied main-phase granite are nearly contemporaneous (ca. 98–94 Ma), but the coexisting dacites are obviously earlier at ca. 160 Ma. The dacites show adakitic geochemical characteristics and extremely enriched NdHf isotopic compositions with Paleoproterozoic model ages, suggesting their derivation from remelting of Paleoproterozoic basement rocks in a thickened lower crust. In contrast, the coexisting Late Cretaceous volcanic and intrusive rocks have comparable and relatively depleted NdHf isotopic compositions. Our data, combined with previous studies suggest that parental magmas of these Late Cretaceous magmatic rocks were derived from partial melting of juvenile crust with varying degrees of involvement of mantle-derived melts, and the observed compositional variation are mainly controlled by crystal-melt segregation and rejuvenation of crystal mush facilitated by recharge of hot mafic magmas in the shallow magma reservoir. The Jurassic and Late Cretaceous magmatic rocks within the PTS complex were generated in an advancing subduction compressive regime and a retreating subduction extensional regime of the paleo-Pacific plate, respectively. The gradually increasing tectonic extension during the Cretaceous due to slab rollback and the induced underplating of depleted mantle-derived magmas not only gave rise to the continental rejuvenation of SE China but also are the main driving mechanism of this Late Cretaceous long-lived transcrustal magma system.

Devonian ignimbrites of central Victoria: explosive magmas from multiple sources, and deep crustal structure in the Selwyn Block

The magmas that formed each Devonian, S-type, volcanic complex in central Victoria were generated through partial melting of a distinct assemblage of rock types in the crustal source region. The more silicic rhyolites were formed independently of the rhyodacitic to dacitic magmas, rather than as differentiates. Within every complex, each ignimbritic unit is also heterogeneous in its chemical and isotopic characteristics. Thus, in this part of Paleozoic Australia, magma heterogeneity occurred on at least three scales—the individual rock unit, the complex and across the region. This means that the Selwyn Block, the logical source of the ignimbrite magmas, is highly heterogeneous, probably in three dimensions. The statistically significant differences in the ages of the various complexes suggest that the heat sources for partial melting of the deep crust were themselves rather local in extent, and probably reflected numerous asynchronous intrusions of mantle-derived mafic magmas, most likely in relatively small batches. Thus, there should be no thick underplated or intraplated layer of gabbroic rock in the deep crust here. Thinner bodies of hot mafic magmas were probably emplaced in different places and at different levels, inducing local high-grade metamorphic conditions. Published radiometric dates for various volcanic units suggest that the magmatic heating of the crust occurred over a period of at least 17 Myr, although maximum intensity occurred between ca 375 and ca 371 Ma. Thus, a model of the deep crust here should involve numerous, perhaps thin metasedimentary units with highly variable ages and original clay contents, as well as sills of Late Devonian mafic rocks scattered both vertically and horizontally and intruded episodically. The dominantly meta-igneous sources of crustally derived I-type magmas generally lie at higher structural levels, where they did not experience metamorphic temperatures sufficient to cause the higher-temperature melting reactions needed to form I-type volcanic magmas. KEY POINTS Each Late Devonian, S-type, volcanic magma in central Victoria formed through melting of a distinct assemblage of crustal source rocks, most likely within the Selwyn Block. The rhyolites formed independently of the rhyodacitic to dacitic magmas, not as differentiates. The mafic magma heat sources for partial melting of the deep crust were emplaced as numerous, relatively small batches, in sheets distributed horizontally and vertically. The dominantly meta-igneous and shallower crustal sources of I-type magmas did not experience the higher temperatures required to form I-type volcanic magmas.

Multistage Neoarchean magma genesis in the Bundelkhand Craton, India: Evidence from whole-rock elemental and Nd isotopic study of mafic magmatic enclaves and granitoids

Using field, petrography and geochemistry (elemental and Nd isotopic compositions), an integrated study has been carried out on the Neoarchean lithologies comprising mafic magmatic enclaves (MMEs) and undeformed granitoids of the Bundelkhand Craton (Central India) to understand their genesis and crustal evolution. Based on the degree of mingling/mixing with the host granitoids, the MMEs are classified into three types as E-1, E-2 and E-3 types. The E-1 type MMEs represent the early fragments formed by fractional crystallization of mafic/ultramafic magma. The E-2 type MMEs formed due to restricted mingling and rapid crystallization of a hot mafic magma that interacted with felsic granitoid magma. The E-3 type MMEs show compositional similarity with its felsic host and are proposed to have formed by magma mixing process. The MMEs underwent varying degrees of isotopic equilibration with the host granitoids. The E-1 and E-2 type MMEs were resulted from depleted mantle-wedge and the E-3 type MMEs formed from a metasomatized mantle-wedge during subduction. Underplating of basaltic magma probably played a key role in the crustal formation and also provided heat for large scale melting of the crustal rocks giving rise to granitic magma. Such tectono-magmatic events also produced mafic magmas that variably interacted with the granitic magma and played a significant role in the generation of huge Bundelkhand granitoid massif during the Neoarchean.

Generation of crystal-poor rhyolites from a shallow plutonic reservoir in the Famatinian arc (Argentina)

The Ordovician rocks of Sierra Las Planchadas are an exceptionally preserved natural example of a plutonic-volcanic connection. Located in northwestern Argentina, the Las Planchadas Formation consists of a monzogranitic batholith that displays a window into the source of large subvolcanic rhyolitic rocks, cryptodomes and volcanic feeder-dykes. Mafic intrusions and felsic dykes occur cutting the plutonic rocks. Grain-scale relationships, whole-rock compositions and mineral chemistry indicate that monzogranites and rhyolites are two comagmatic members which differentiated within upper crust magma reservoirs. Monzogranites accumulated some plagioclase and trapped a large volume of melt, denoted by a high modal percentage (~64–71%) of near-solidus minerals, comprised of alkali feldspar, albite-rich plagioclase (An<30), and late crystallized quartz. The formation of felsic dykes and rhyolitic cryptodomes is the result of a late melt extraction stage facilitated by the injection of hot mafic magmas from deeper in the system. The mafic-induced thermal reactivation increased the amount of melt-filled pore stored within a highly-crystallized monzogranite reservoir. The breakdown of the low-temperature phases overpressurized the mush and opened a network of microfractures into which residual melts, depleted in feldspar-compatible trace elements (Ba, Sr, and Eu), infiltrated. The draining system involved thin interconnected vein-like channels that merged into wider sheet-like conduits. Our results suggest that channelized segregation of late-stage melts in the subvolcanic environment is a mechanism for triggering rhyolitic eruptions.

Probing the hidden magmatic evolution of El Misti volcano (Peru) with the Pb isotope composition of fumaroles

This work proposes a new method to probe the hidden magmatic evolution of quiescent Andean volcanoes from the Pb isotope composition of gases. The method is based on an assimilation-fractional crystallisation-degassing model linking the Pb isotope composition of gases with the SiO2 content of their magmatic source. The model is applied to El Misti volcano that threatens Arequipa, the second most densely populated city of Peru. Gas condensates and Pb-rich solid deposits (PbS, PbCl2, PbSO4) collected in 2018 in the bottom of El Misti crater at 260–150°C fumarole vents were used to reconstruct the mean composition of degassing magmas (60.8–61.8 wt% SiO2). These compositions are slightly more evolved than the lavas from the last AD 1440–1470 eruption, suggesting either the secular differentiation of the main magma reservoir, or the contribution of more evolved magmas to volcanic gases. On the other hand, the slight but significant difference between the instantaneous composition recorded in gas condensates and the time-integrated composition recorded in solid deposits points to the degassing of less evolved magmas over the last decades. This trend is ascribed to a recent recharge of El Misti reservoir with hot mafic magmas, in agreement with the evolution of fumarolic deposit mineralogy in the last half a century. The Pb isotope composition of gas appears to be a promising tool for probing the hidden magmatic evolution of quiescent volcanoes where assimilation-fractional crystallisation operates.

Deciphering Cryptic Multi-Stage Crystal-Melt Separation during Construction of the Tonglu Volcanic–Plutonic Complex, SE China

Abstract Revealing the origin of explosive eruptions of silica-rich magma is of paramount importance for understanding the evolution of continental crust and volcanic hazards. However, it remains controversial as to how the erupted magmas form and how they connect with plutonic realms, partly owing to the rarity and the obscurity of the ‘cumulate’ complementary to such eruptions of silica-rich magmas. Here the issues are explored by comparing the volcanic rocks (rhyodacite + rhyolite) and their associated subvolcanic intrusions (monzodiorite + monzonite + quartz monzonite) within Tonglu volcanic basin (SE, China). The Tonglu plutonic and volcanic units are consistent with each other in age (~130 Ma), space and source materials (e.g. Sr-Nd-Hf isotopes), strongly suggesting that they are cogenetic. Mineral mapping demonstrates that abundant plagioclase clusters (and chains in monzodiorite) occur in plutonic units, implying the processes of crystal gathering and/or accumulation. Rhyolite-MELTS modeling, and geochemical studies coupled with textural observations suggest that the Tonglu volcanic and plutonic rocks represent the residual melts and the complementary cumulate residues, respectively. The compositional and mineral variations in the plutonic rocks can be explained by two-stage, low-pressure crystal-melt separation of a dacitic magma. The monzodiorite represents the first-stage cumulate that was unsaturated in zircon and biotite/K-feldspar on the basis of low Zr and Ba concentrations and the occurrence of these two phases in the interstices between plagioclase and amphibole. The monzonite and quartz monzonite are the second-stage cumulates after saturation of zircon and biotite/K-feldspar as indicated by abrupt increases in Zr and Ba concentrations and zircon inclusions within euhedral biotite. Mass balance calculation and textural estimation indicate that the cumulates actually are a mixture of crystals and melt, containing ~40 vol% interstitial melt. Interstitial zircons from monzodiorite, largely crystallized from trapped melt, show contrasting trace-element trends (e.g. Ti, Zr/Hf, Eu/Eu*, Gd/Yb) to those of the other rock types (i.e. monzonite, quartz monzonite, rhyodacite and rhyolite), consistent with fractional crystallization (FC). We interpret these divergences to heating-induced partial dissolution of a basal crystal framework of monzodiorite due to recharges of hot mafic magmas, but the limited dissolution fails to rejuvenate the crystal mush. The Tonglu volcanic–plutonic system demonstrates that compositional distillation via crystal–liquid separation within the upper crust is an effective mechanism by which a potentially eruptible rhyolitic cap can be generated. Recharges of hot mafic magmas at shallow crustal levels may prolong the lifespan of granitic magma reservoirs. A combined study including texture, geochemistry, thermodynamic modeling and mass-balance calculations can help us identify the fingerprints of cumulates in felsic magma systems and thus track the processes responsible for producing large eruptions of silica-rich magmas.

Igneous rocks related to porphyry Cu‐Au mineralization at the Dizon mine, Philippines

AbstractThe Dizon Au‐rich porphyry Cu deposit, 0.67 Mt Cu and 174 t Au, is hosted by diorite and andesite porphyry intrusions, dated at ~2.5 Ma. Amphibole and Fe‐Ti oxides in relatively unaltered rocks were used to evaluate the magma conditions of intrusions. Parental magma for diorite porphyry was ~950°C at a depth of ~15 km, whereas the parental magmas for the andesite porphyry had lower temperatures, 760–820°C at a depth of ~5 km. The deposit formed at the locus of multiple intrusions, with evidence for injections of hot mafic magmas, including destabilization texture of plagioclase phenocrysts. Parental magmas at Dizon were oxidized, above FMQ + 2.0, and water‐rich, &gt;5 wt%, comparable to conditions of many large porphyry deposits elsewhere in the world. The occurrence of thick opacitic bands of amphibole in the diorite porphyry at Dizon reflects the release of aqueous fluids from the magma; such magmatic fluids were likely responsible for magmatic hydrothermal mineralization of the Dizon porphyry deposit. Subduction of the Scarborough Seamount caused a compressive regime in the overlying plate, which likely contributed to favourable tectonic conditions for mineralization.

A-type granite in the Boein-Miandasht Complex: Evidence for a Late Jurassic extensional regime in the Sanandaj-Sirjan Zone, western Iran

The Boein-Miandasht Complex (BMC) in the central part of the Sanandaj-Sirjan Zone, western Iran, is composed of metamorphic rocks, and gabbro, gabbro-diorite, and granite intrusions, in which co-magmatic mingling structures are commonly found. Zircon U-Pb ages demonstrate that crystallization of the granitic rocks occurred during the Late Jurassic (158 ± 5 Ma to 156 ± 4 Ma). Whole-rock granite geochemistry indicates low abundances of MgO (0.43–1.16 wt%), CaO (0.92–2.34 wt%), P2O5 (0.12–0.20 wt%) and Sr (72.2–254 ppm), high contents of SiO2 (66.9–72.0 wt%), alkalis (Na2O + K2O = 8.04–8.94 wt%), Rb (153–213 ppm), high field-strength elements (HFSE), rare earth elements (REE), and high FeOt/MgO ratios. The rocks are calc-alkalic to alkali-calcic and metaluminous to slightly peraluminous, typical characteristics of ferroan A-type granites. Chondrite-normalized REE diagrams display slight enrichment of light REE, flat HREE patterns and significant negative Eu anomalies. The primitive mantle-normalized multi-element variation diagrams show negative Eu, Nb, Sr, P and Ti anomalies. Both REE and other incompatible elements are consistent with A-type granite affinities for these rocks. The thinning of the continental crust and injection of hot mafic magmas within the crust were probably responsible for the partial melting of the lower crust to produce A-type granitic magmas in the region. This process is consistent with an extensional tectonic regime in the central part of the Sanandaj-Sirjan Zone during the Late Jurassic.

Tectonic significance of the late Eocene (Bartonian) calc-alkaline granitoid body in the Marivan area, Zagros suture zone, northwest Iran

ABSTRACT The Marivan granitoid body of the northern Sanandaj–Sirjan Zone (N-SaSZ) of northwest Iran was emplaced into Cretaceous sediments, and is associated with minor amounts of iron skarn-type mineralization along the contact between the intrusion and surrounding country rocks. The granitoids within the intrusion include (hornblende-rich) diorite, granodiorite, and granite phases, and zircon U–Pb dating of four samples from this intrusion indicate they were emplaced during the late Eocene (Bartonian; 37.7 ± 1.0 Ma). These plutonic rocks can be classified into two main types, granitic and dioritic rocks, according to petrography and whole-rock major element geochemistry. Both the granitic and dioritic rocks are high-K, and some of the granitic rocks have an I-type affinity and peraluminous to metaluminous compositions. Both the granitic and dioritic rocks have εNd(t) values that cluster around zero (–3.3 to +3.0) and low 87Sr/86Sr(i) values (0.7040–0.7076), suggesting derivation from a mafic juvenile crustal or depleted mantle source, which was possibly mafic calc-alkaline rocks or amphibolites within the lower crust. The new zircon U–Pb ages and petrological and geochemical data presented in this study suggest that the Palaeocene–early Eocene collision of the Arabian and Iranian plates caused the ascent and emplacement of hot mafic magma through a slab window into the crust. This increased the geothermal gradient within the root zone of the developing magmatic system at the base of the continental crust, causing the partial melting of lower crust in this region. This generated mafic to intermediate magmas that were either directly emplaced into the upper crust or mixed with varying amounts of upper crustal material prior to emplacement, which formed the diorite to leucogranite range of compositions within the Marivan granitoid intrusion.

Zircon stability grids in crustal partial melts: implications for zircon inheritance

Zircon inheritance is a common phenomenon in igneous rocks, although more frequent in granitoids. Zircons inherited from granite magmas mostly come from the source, not from wall rocks or xenoliths. Consequently, they can provide invaluable information about the source materials, melting temperature, and melt segregation conditions. Miller et al. (Geology 31:529–532, 2003) divided granite rocks according to their zircon saturation temperature (TZr) into “hot” (TZr > 800 °C, with little or no inherited zircon) and “cold” (TZr < 800 °C; with abundant inherited zircon). Nevertheless, we have found that coeval and neighboring two-mica granites with TZr < 750 °C, presumably derived from similar sources, may have a radically different inheritance, from about 95 to near 0%. This paper aims to understand the reasons for these differences, in particular, and the survival of source zircons in granitoids, in general. To this end, we modeled the relationships between source composition, temperature, pressure, water content, zircon solubility, and melt fraction, on one hand; and melt production and zircon solution rates, on the other hand. Our results foresee that zircon survival during crustal melting is more probable if the source is a fertile peraluminous metasedimentary rock than if it is a metaluminous source with similar SiO2. Elevated zircon inheritance is characteristic of mid-crustal S-type, water-rich granite magmas generated within 4.5 and 6 kbar. Moderate or no inheritance is characteristic of water-poor granite magmas, because their sources require higher temperatures to produce the same melt fraction. Fast melt extraction does not cause perceptible effects on our models, because melt generation is slower than zircon dissolution, except in the case of crustal underplating by hot mafic magmas. We propose to refine the “hot” and “cold” classification by splitting the “cold” granites (TZr < 800 °C) into two categories, “dry” with little inheritance and “wet” with a very high zircon inheritance. Wet granites require a source water-fluxed from outside. They are characteristic of mid-crustal anatectic complexes with highly fertile gneisses alternating with unfertile mica-rich metapelites. We suggest that the extra water should come in most cases from dehydration reactions in the unfertile metasedimentary rocks beneath the crustal section undergoing anatexis.

Comparative geochemical study on Furongian–earliest Ordovician (Toledanian) and Ordovician (Sardic) felsic magmatic events in south-western Europe: underplating of hot mafic magmas linked to the opening of the Rheic Ocean

Abstract. A geochemical comparison of early Palaeozoic felsic magmatic episodes throughout the south-western European margin of Gondwana is made and includes (i) Furongian–Early Ordovician (Toledanian) activities recorded in the Central Iberian and Galicia–Trás-os-Montes zones of the Iberian Massif, and (ii) Early–Late Ordovician (Sardic) activities in the Eastern Pyrenees, Occitan Domain (Albigeois, Montagne Noire and Mouthoumet massifs) and Sardinia. Both phases are related to uplift and denudation of an inherited palaeorelief, and stratigraphically preserved as distinct angular discordances and paraconformities involving gaps of up to 22 million years. The geochemical features of the predominantly felsic Toledanian and Sardic activities point to a predominance of magmatic byproducts derived from the melting of metasedimentary rocks, rich in SiO2 and K2O and with a peraluminous character. Zr ∕ TiO2, Zr ∕ Nb, Nb ∕ Y and Zr vs. Ga ∕ Al ratios, and rare-earth element (REE) and εNd(t) values suggest the contemporaneity, for both phases, of two geochemical scenarios characterized by arc and extensional features evolving to distinct extensional and rifting conditions associated with the final outpouring of mafic tholeiite-dominant lava flows. The Toledanian and Sardic magmatic phases are linked to neither metamorphism nor penetrative deformation; on the contrary, their unconformities are associated with foliation-free open folds subsequently affected by the Variscan deformation. The geochemical and structural framework precludes subduction-generated melts reaching the crust in a magmatic arc-to-back-arc setting and favours partial melting of sediments and/or granitoids in the lower continental crust triggered by the underplating of hot mafic magmas related to the opening of the Rheic Ocean.

Magmatic stoping during the caldera-forming Pomici di Base eruption (Somma-Vesuvius, Italy) as a fuel of eruption explosivity

Magma-carbonate interactions and the subsequent CO2 release can occur before and during an eruption, critically affecting magma storage and ascent processes. However, the mechanisms and timescales of those interactions are unclear, particularly during the fast magma withdrawal that feeds high-intensity eruptions. In order to better understand magma‑carbonate interactions, we selected the caldera-forming Pomici di Base plinian eruption, the oldest (22 ka) and largest (> 4.4 km3) explosive event in the history of the Somma-Vesuvius volcanic system, as case study. During this event the emission of trachytic and latitic-shoshonitic (~25% and ~75% of the erupted magma volume respectively) magmas generated a long-lasting plinian column, hypothetically driven by CO2 liberation during magma‑carbonate interaction. In this study, we reconstruct in detail the evolution of the plumbing system during the Pomici di Base eruption combining geochemical (major/minor elements and radiogenic/stable isotopes) analyses of juvenile products with thermodynamic and kinetic calculations. Our results demonstrate that magmatic stoping (i.e., the formation and transport of host-rock pieces into a magma body) during caldera collapse evolution can promote rapid magma assimilation of carbonate blocks and CO2-rich fluids resulting from destabilization of the carbonate bedrock, thus triggering CO2 release and acting as a fuel for the eruption explosivity, especially when residual hot mafic magmas are involved. Our findings suggest that the accurate knowledge of these processes and their influence on eruptive dynamics are critical for improving the hazard assessments of volcanoes with plumbing systems located in carbonate bedrocks.

Architecture of a Super-sized Magma Chamber and Remobilization of its Basal Cumulate (Peach Spring Tuff, USA)

Abstract Using a combination of petrological and geochemical approaches, we investigate processes prior to and during eruption of the Miocene supereruption of the Peach Spring Tuff (PST; Arizona–California–Nevada), including those leading to assembly and destruction of its reservoir(s). We compare the dominant high-silica rhyolite outflow of the PST with the sparsely exposed but distinctive crystal-rich trachyte capping unit, which matches intracaldera trachyte in composition, texture, and phenocryst content. The details of the diverse glass chemistry in fiamme and pumice in the capping unit, coupled with glass compositions in the rhyolite outflow and phase chemistry in general, illuminate critical aspects of chamber geometry, conditions, and processes at the onset of the supereruption. Our results are consistent with a relatively simple single-chamber reservoir for the PST where the crystal-poor, high-silica rhyolite portion directly overlies a mushy, cumulate base. Rhyolite-MELTS phase-equilibria and amphibole geobarometers indicate that the high-silica rhyolite was extracted from its cumulate mush at a depth of ∼9·5–11 km (∼260–300 MPa) and subsequently stored and crystallized at ∼7·0–8·5 km (190–230 MPa). Three types of glass are distinguishable in PST pumice: trachyte (Trg; ∼68 wt% SiO2), low-silica rhyolite (LSRg; ∼72), and high-silica rhyolite (HSRg; ∼76·5). As many as three discrete, complexly mingled glasses are present in single trachyte fiamme. Trace element concentration profiles in sanidine and plagioclase phenocrysts from both the trachyte and HSR support growth from multiple distinct melts (Trg, LSRg, and HSRg). Glasses in trachyte fiamme have zircon saturation temperatures ≥100 °C higher than HSR glasses (850–920 vs ∼770 °C) and compositions indicating dissolution of cumulate phases: very high Zr and Zr/Hf (zircon), REE (chevkinite and titanite), Ba and Sr (feldspars), and P (apatite). Dominant processes of crystal accumulation in the formation of a mushy base, followed by efficient melt extraction, led to the formation of the voluminous high-silica rhyolite melt-rich body overlying a residual cumulate of trachytic composition. This was followed by heating, partial dissolution, and remobilization of the basal cumulate. This history is reflected in the contrasts that are evident in the PST (elemental compositions of pumice, phenocrysts, and glasses; crystal-fraction; temperatures). Reheating was presumably a result of injection of hot mafic magma, but isotopic uniformity of trachyte and rhyolite indicates minimal chemical interaction with this magma. Variability in dissolution textures in phenocrysts in the trachyte, revealed by resorbed and embayed shapes, and the large range of glass trace element concentrations, together with variable temperatures recorded in glasses by zircon and apatite saturation thermometry, suggest that heat transfer from the hotter rejuvenating magma was unevenly distributed. The late-stage heating event probably contributed to the onset of eruption, providing the thermal energy necessary to reduce the crystal fraction within the cumulate below the mechanical lock point. We estimate ∼50 % of the original cumulate phenocrysts dissolved before eruption, using Rhyolite-MELTS and trace element modeling. Sharp contacts with micron-scale compositional gradients between contrasting glass types in individual trachyte fiamme suggest that juxtaposition of contrasting magmas from different parts of the reservoir occurred during eruption.

Petrology, geochemistry, and zircon U–Pb isotopes of Xintian Complex in Yanbian area, Northeast China: Evidence for magma mixing and geodynamics processes

Mixing/mingling of magmas is an important mode of magmatic evolution, crust–mantle interaction, and compositional exchanges, and the mafic microgranular enclaves (MMEs) in the host granites are commonly generated by the injection of hot mafic magmas into felsic reservoir at the crustal level. However, the exhaustion of most mafic melts causes the absence of the critical mantle marks after homogenization. The Xintian Complex in Yanbian area is composed of the gabbro diorite and granitic rocks enclosing the abundant MMEs, which likely provides the significant, but rare mantle end‐member information. On the basis of geochronology study, both MMEs (126.3 ± 1.2 Ma) and granodiorite (126.0 ± 2.0 Ma) were formed in the Early Cretaceous, which is coincident with the earlier emplacement of gabbro diorite (128.5 ± 1.0 Ma) within the errors. Geochemically, all of these distinguished phases have arc affinities related to subduction of oceanic slabs. Nevertheless, the dominant granodiorite rocks, typical high‐K calc‐alkaline I‐type granites, have the source of partial melting of mafic lower crust modified by subducted sediment melts, with additions of mafic melts as indicated by high Mg# values (48–56) and special textures (acicular apatite and re‐equilibrium plagioclase) of minerals in the hosted MMEs. In contrast, gabbro diorites are typical of calc‐alkaline affinities with deficit of the incompatible Zr and Hf, which were formed by re‐melting with low degree (3–10%) of mantle wedges metasomatized by subduction fluids with the crystallization of dominant pyroxene minerals. However, the MMEs is characterized by higher SiO2 (55.71–56.02 wt%), Cr (546–569 ppm), Ni (83.7–92.8 ppm), MgO (6.03–6.63 wt%), and heavy rare earth element (HREE) (546–569 ppm) with weak enrichment in Zr and Hf and lower (La/Yb)N values (17.98–20.93) compared to gabbro diorite. Therefore, the MMEs and host granodiorite are the products of heterogeneous mixing of weakly evolved mafic melts and subsolid felsic melts in the shallow storages, and this crust–mantle interaction is under a back‐arc extensional setting related to rollback of subducting palaeo‐Pacific slab.

Zircon xenocryst evidence for crustal recycling at the Mid-Atlantic Ridge

The presence of zircon xenocrysts in Mid Atlantic Ridge (MAR) rocks poses essential questions concerning the path they have followed from their sources, how they have survived transport through the mantle, and why they are much more abundant than in any other ocean. Sixteen out of twenty-five seafloor samples collected between S 21° to N 42° contained xenocrysts, here defined as those grains that coexist with a significantly younger population or are significantly older than the band of the Atlantic seafloor age from which they were collected. The xenocrysts are of two types, young and old. Hf and O isotopes and trace elements indicate that the younger crystals (3.1 Ma to 25.6 Ma) are oceanic, derived from previous MAR rocks, whereas the older crystals (190 Ma to 3.2 Ga) are continental, derived from crust slivers located at the MAR or immediate vicinity. The high ∂18O (~7‰ to 11‰) of many older xenocrysts indicates ultrafast recycling through the mantle; otherwise, the fast diffusivity of oxygen would have driven the xenocrysts ∂18O to that of their mantle hosts, ∂18O ≈ 5.3‰. The incorporation of xenocrysts into MAR magmas mainly occurs after ridge jumps, when hot mafic magma pervades a new crustal section scavenging xenoliths, or locally melts felsic wall-rocks and mixes with the resulting zircon-laden low-T magmas. In either case, the fusible component of the xenocrysts source contaminates the upwelling mafic magma. When the source was oceanic, the resulting chemical and isotopic signatures of the contaminated magma would remain MORB-like, albeit enriched in incompatible elements. When the source was continental, the resulting magma would reflect a perceptible continental influence. Crustal contamination at the ridge explains the abundance of zircon xenocrysts and the distinctive chemistry of the MAR magmas compared with other MORBs. These features are ultimately related to the Atlantic slow-spreading because it promotes ridge jumping and, hence, active crustal recycling.

Evidence of mingling between contrasting magmas in the Ribeirão do Óleo Pluton, Coastal Terrane and the tectonic implications on the Ribeira Belt, Brazil

Gneiss–migmatitic rocks and granites between the Cubatao and Itariri shear zones characterize the Mongagua Complex-Coastal Terrane. Three major units are defined: (1) the Itariri Suite (IS) composed of monzogranites, granodiorites, tonalites, and locally gneiss–migmatitic rocks; (2) the Areado pluton composed of monzogranite and locally tonalite; and (3) the Ribeirao do Oleo Pluton mainly composed of monzogranites, which is the subject of this article. Combined field observations, petrography, Sr–Nd–Hf isotopes, U–Pb zircon data, and preliminary geochemical studies are presented for the Ribeirao do Oleo Pluton (ROP), intrusive in granite-gneisses of the IS. It is a plutonic association of weakly peraluminous, medium-to-high-K biotite monzogranites that surround microgranular intermediate enclaves, which are products of magma mingling. Different from the most examples of magma mingling described in the literature, the ROP mingling did not result from the intrusion of hot mafic magma into colder host felsic magma. Conversely, the emplacement of the monzogranite started most likely during the medium-to-final stage of crystallization of the quartz microdiorite. A detailed study of LA-ICP-MS U–Pb zircon ages, including zircon morphology and internal structure, was useful to determine more precisely the emplacement sequence and the relation between the units of the ROP. Intermediate and acid facies have Paleoproterozoic and Neoproterozoic inheritances, moreover a mixture of xenocrysts, antecrysts, and autocrysts, indicative of a complex evolution of the ROP. Several magma crystallization pulses are recorded at 655–624 Ma, 627–601 Ma, and 613–586 Ma. The negative eHf(t) ranging from − 17.26 to − 12.51 in monzogranites, eNd(t) values of − 10.66 to − 7.83 at t = 600 Ma in both facies, 87Sr/86Sr of ca. 0.708 for the monzogranite and 0.706 for the quartz microdiorite indicate a strong crustal contribution in the genesis of intermediate and acid magmas of this pluton.