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Experimentally calibrated models to recover pressures and temperatures of magmas are widely used in igneous petrology. However, large errors, especially in barometry, limit the capacity of these models to resolve the architecture of crustal igneous systems. Here, we apply machine learning to a large experimental database to calibrate new regression models that recover P–T of magmas based on melt composition plus associated phase assemblage. The method is applicable to compositions from basalt to rhyolite, pressures from 0.2 to 15 kbar, and temperatures of 675°C to 1400°C. Testing and optimisation of the model with a filter that removes estimates with standard deviation above the 50th percentile show that pressures can be recovered with root-mean-square-error (RMSE) of 1.1 to 1.3 kbar and errors on temperature estimates of 21°C. Our findings demonstrate that, given constraints on the coexisting mineral assemblage, melt chemistry is a reliable recorder of magmatic variables. This is a consequence of the relatively low thermodynamic variance of natural magma compositions despite their relatively large number of constituent oxide components. We apply our model to two contrasting cases with well-constrained geophysical information: Mount St. Helens volcano (USA), and Askja caldera in Iceland. Dacite whole-rocks from Mount St Helens erupted 1980 to 1986, inferred to represent liquids extracted from cpx–hbl–opx–plag–mt–ilm mush, yield melt extraction source pressures of 5.1 to 6.7 kbar in excellent agreement with geophysical constraints. Melt inclusions and matrix glasses record lower pressures (0.7–3.8 kbar), consistent with magma crystallisation within the upper reaches of the imaged geophysical anomaly and during ascent. Magma reservoir depth estimates for historical eruptions from Askja match the location of seismic wave speed anomalies. Vp/Vs anomalies at 5 to 10 km depth correspond to hot (~990°C) rhyolite source regions, while basaltic magmas (~1120°C) were stored at 7 to 17 km depth under the caldera. These examples illustrate how our model can link petrology and geophysics to better constrain the architecture of volcanic feeding systems. Our model (MagMaTaB) is accessible through a user-friendly web application (https://igdrasil.shinyapps.io/MagmaTaBv4/). [ABSTRACT FROM AUTHOR]

Metamorphosed dacitic porphyry dikes were first found in the western part of the Vorontsovka terrane, which is located in the Paleoproterozoic Volga–Don orogen at the margin of Archean Sarmatia and Volga–Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. These are ferrous, metaluminous calc-alkali I-type granitoids. The sodium specialization of the rocks and their low concentrations of Mg, Cr, Ni, and incompatible elements, with significant REE fractionation, the absence of Eu* anomalies, high Sr/Y ratio, remarkably high (Gd/Yb)n values (>10), and the radiogenic Nd isotopic composition indicate that the dacitic melts were derived from a juvenile mafic source. According to petrogenetic estimations, such conditions could be caused by the partial melting of depleted N-MORB basites in equilibrium with an eclogitic residue. The dacitic magmas were likely generated by the partial melting of mafic rocks at lower levels of the significantly thickened crust (>60 km) in relation to collision processes. [ABSTRACT FROM AUTHOR]

We present a reconstruction of the physicochemical conditions of melts in the Pleistocene storage and plumbing system of the Doña Juana Volcanic Complex (SW Colombia): a poorly known, potentially active polygenetic volcano of dacitic composition comprising four major edifices and showing periods of long quiescence. Compositional data for plagioclase, amphibole, pyroxene, and Fe-Ti oxides were combined with new and existing whole-rock data from representative eruptive products, allowing for the implementation of equilibrium tests and geothermobarometry calculations within an established stratigraphic, petrographic, and geochronological framework. Textural and geochemical variabilities of all mineral phases suggest the existence of a trans-crustal magmatic system feeding the Pleistocene eruptions of Doña Juana, and cyclic rejuvenation of a crystal mush following each volcano edifice collapse. The assemblage of different crystal cargos before magma recharge and final eruption is attested by (i) the coexistence of equilibrium and disequilibrium textures and variable compositions in crystals of all studied species, (ii) felsic cores in antecrysts, (iii) mafic overgrowth rims, and (iv) significantly less differentiated microcrysts relative to the composition of meso- and macrocrysts. By integrating multiple mineral-only and mineral-liquid geothermobarometers, after careful textural analyses, we estimate the intensive parameters of the mush–melt interaction zone of the plumbing system in the middle crust, providing a preliminary view of the architecture of a trans-crustal magmatic system in a complex tectonic setting at a previously understudied area of the north-Andean volcanic zone. [ABSTRACT FROM AUTHOR]

Compositional variations of amphibole stratigraphically recovered from multiple eruptions at a given volcano have a great potential to archive long-term magmatic processes in its crustal plumbing system. Calcic amphibole is a ubiquitous yet chemically and texturally diverse mineral at Mount St. Helens (MSH), where it occurs in dacites and in co-magmatic enclaves throughout the Spirit Lake stage (last ~4000 years of eruptive history). It forms three populations with distinct geochemical trends in key major and trace elements, which are subdivided into a high-Al (11–14.5 wt% Al2O3), a medium-Al (10–12.5 wt% Al2O3), and a low-Al (7.5–10 wt% Al2O3) amphibole population. The oldest investigated tephra record (Smith Creek period, 3900–3300 years BP) yields a bimodal amphibole distribution in which lower-crustal, high-Al amphibole cores (crystallized dominantly from basaltic andesite to andesite melts) and upper-crustal, low-Al amphibole rims (crystallized from rhyolitic melt) document occasional recharge of a shallow silicic mush by a more mafic melt from a lower-crustal reservoir. The sudden appearance of medium-Al amphiboles enriched in incompatible trace elements in eruptive periods younger than 2900 years BP is associated with a change in reservoir conditions toward hotter and drier magmas, which indicates recharge of the shallow silicic reservoir by basaltic melt enriched in incompatible elements. Deep-crystallizing, high-Al amphibole, however, appears mostly unaffected by such incompatible-element-enriched basaltic recharge, suggesting that these basalts bypass the lower crustal reservoir. This could be the result of the eastward offset position of the lower crustal reservoir relative to the upper crustal storage zone underneath the MSH edifice. Amphibole has proven to be a sensitive geochemical archive for uncovering storage conditions of magmas at MSH. In agreement with geophysical observations, storage and differentiation have occurred in two main zones: an upper crustal and lower crustal reservoir (the lower one being chemically less evolved). The upper crustal silicic reservoir, offset to the west of the lower crustal reservoir, has captured compositionally unusual mafic recharge (drier, hotter, and enriched in incompatible trace elements in comparison to the typical parental magmas in the region), resulting in an increased chemical diversity of amphiboles and their carrier intermediate magmas, in the last ~3000 years of MSH's volcanic record. [ABSTRACT FROM AUTHOR]

The İstanbul-Zonguldak Tectonic Unit is an Amazonia-derived continental fragment added to Baltica during the middle Paleozoic, and represents Far East Avalonia. The soft-docking time of two continental blocks, thus the consumption of the Teisseyre-Tornquist Ocean, is poorly known. This paper reports biotite-bearing dacite and pyroxene-bearing basaltic andesite and dacite dykes of Ordovician igneous crystallization ages in the İstanbul-Zonguldak Tectonic Unit (NW Turkey). They have porphyritic to spherulitic textures. U–Pb dating on igneous zircons from arc- and syncollisional-related dykes yielded Ordovician ages of ca. 484.1 ± 2.5 Ma (2σ) and 444.4 ± 3.7 to 443.0 ± 2.1 Ma (2σ). They display calc-alkaline signatures and are noteworthy with subduction components, as deduced by the presence of marked negative Nb anomalies. Biotite-bearing dykes intruded to it in an arc setting whereas pyroxene-bearing dykes emplaced into it in a syncollisional setting. Besides, Middle-Late Ordovician granites (c. 464–446 Ma) intruding the high-grade metamorphic rocks, known as rift-related intrusions in previous studies, show calc-alkaline affinities and contain subduction components, and they formed in a volcanic arc. I suggest that Early-Late Ordovician magmatism is related to the Teisseyre-Tornquist Ocean subducting under Far East Avalonia, and Late Ordovician magmatism is associated with soft-docking between two continental blocks, thus the destruction of the Teisseyre-Tornquist Ocean. Overall, its suture in Far East Avalonia overlaps with the Vardar suture in Balkans, and it can be traced from İzmir to Söğüt in Anatolia and represents the missing part in Far East Avalonia. [ABSTRACT FROM AUTHOR]

Iron (Fe)-skarn mineralizations (Çambaşı, Dereli, Eğrikar, Karadağ, Kopuz, Sekü, Donguldere, Arnastal, Özdil, Ögene, İkizdere, Ovit Dağı, Kartiba, etc.) in the eastern part of the Pontides (NE Türkiye) are accommodated in the Pontide paleomagmatic arc and the eastern part of the Pontides Metallogenic Belt containing numerous various deposits. Fe-skarn mineralization around the Arnastal Plateau (Gümüşhane-Türkiye) is fragmentary in this area, which is covered with plants (grasses, flowers, etc.). Thus, it was attempted to describe the subsurface structures and Fe mineralization using an interpretation of the available geological and existing magnetic data in this area. Granitoid and volcano-sedimentary series (Upper Cretaceous) outcrop in the study area. These volcanosedimentary series consist of limestone, sandy limestone, marl, andesite, quartz andesite, basalt, and their equivalent pyroclastics. They include a thin layer of the red limestone (Upper Cretaceous) and olistolith recrystallized limestones (Jurassic-Lower Cretaceous). Zigana granitoid, intruding into all of these rocks, is high-K calc-alkaline and metaluminous and is classified as quartz monzonite, monzogranite, granite, and syenogranite as a result of its modal composition. Contact pyrometasomatic mineralization occurs in between the limestone and granitoids in the Arnastal Plateau. In addition to petrographic and mineralogical studies, magnetic methods were applied to an area of approximately 10 km2 to find any covered Fe deposits. Magnetic susceptibility values measured on the outcropped rocks ranged from 1 to 34 (10-7 SI) for limestone, 78 to 3750 (10-7 SI) for basalt, and 105 to 3946 (10-7 SI) for granitoids. It is considered that these ranges express the alteration (physical or chemical) of the rocks, their homogeneity, and the lack or presence of Fe minerals. The magnetic survey was conducted along a study area measuring 12,075 m long with 25 and/or 50 m station spacing. Processing of the magnetic data revealed the presence of eight buried Fe ores that could represent either massive or disseminated mineralization between the Sarıtaş and Kurtdere plateaus. Magnetite ore bodies may be present from the surface to a depth of approximately 15-25 m inside the limestone. In addition to the identification of new magnetite ore mineralization or bodies, a new geological map was designed by determining probable formation boundaries with this magnetic survey. [ABSTRACT FROM AUTHOR]

Antimilos volcano in the South Aegean Volcanic Arc, Greece, comprises an andesite–dacite suite that follows a distinct evolutionary path than the main edifice of the Milos volcanic field, despite their proximity. Petrographic and geochemical analyses reveal that basaltic andesite to low-Si dacite lavas have similar phenocryst assemblages that indicate crystallization from hot, relatively dry magmas in an upper crustal storage region. Rare antecrystic high-Mg# clinopyroxene cores with low Y, low Dy, and high Sr contents record the cryptic involvement of amphibole, a phase nominally absent from the erupted products, in the deeper parts of the plumbing system. Low temperature antecrysts with textures recording various degrees of disequilibrium suggest a protracted history of interaction between the upper crustal reservoir and deeper mafic melts, forming mobile hybrid magmas that consequently erupt as highly mingled, crystal-rich lava domes. Antimilos magmas seem to have escaped recharge filtering in the upper crust and prolonged stalling, which is the process that is probably responsible for the paucity of mafic eruptions in the rest of the Milos volcanic system. Large extensional structures offshore of Antimilos promote rapid ascent of mafic melts, inhibiting prolonged stalling and interaction with the arc crust. This model highlights the dominant role of the regional stress field in generating petrologically distinct suites in the marginal parts of some volcanic fields. [ABSTRACT FROM AUTHOR]

We would like to thank Geist et al. (Contrib Miner Petrol, 2023) for the opportunity to further discuss the arguments presented in our paper "Long or short silicic magma residence time beneath Hekla volcano, Iceland?" (Sigmarsson et al. Contrib Miner Petrol 177:13, 2022). The disagreement centers around the origin of the silicic magmas at Hekla, namely whether it is by (1) fractional crystallization and a long crustal residence time before eruption or (2) partial melting of altered basaltic crust and short transfer time to the surface. We disagree with the arguments presented by Geist et al. (Contrib Miner Petrol, 2023) against the model for the origin of dacite at Hekla from dehydration melting of amphibolite, a model that still explains most if not all results obtained so far on the Hekla magma suite. [ABSTRACT FROM AUTHOR]

Between 38.5 ka cal BP and 32.4 ka cal BP, a dacitic Volcanic Explosivity Index 5 eruption at Misti volcano emplaced the Sacarosa tephra-fall deposit. Its biotite phenocrysts, fine grain size, scarce lithics, and abundant loose crystals characterize the deposit at locations sampled. The eruption's ~ 800 °C magma rose rapidly from ~ 10 km depth, culminating in a Plinian eruption which reached a mass eruption rate of 7.7 × 106–4.1 × 107 kg/s and emplaced about 3 km3 of tephra within tens of hours. The unit comprises two layers of subequal thickness separated by a diffuse contact with the upper distinguished by being slightly coarser and less well sorted than the lower. The deposit's coarser upper layer indicates either climactic conditions or a lesser degree of fragmentation during the latter half of the eruption. Strong winds distributed the deposit southwest of Misti, where it crops out over at least 800 km2 and drapes the present site of Arequipa with up to 100 cm of tephra. The Sacarosa deposit is the first among the Cayma stage deposits, a distinctive group of felsic, biotite-bearing units, to be carefully described and its eruption characterized. Several Cayma stage deposits were emplaced by voluminous explosive eruptions similar to the Sacarosa eruption, representing a ~ 8.9–15.5 ky interval of powerful eruptions. Such an explosive eruption today would threaten Arequipa's over 1,100,000 residents, many of whom live within the Sacarosa deposit's distribution. [ABSTRACT FROM AUTHOR]

Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide. [ABSTRACT FROM AUTHOR]

The Azufre volcano (21°47′S; 68°14′W) is emplaced above the western boundary of the Altiplano Puna Magma Body. The youngest lava of the Azufre volcano (50–331 ka) was selected for detailed studies because of the relevant petrological information that could emerge to understand its reservoir as a potential heat source of the neighboring Cerro Pabellón geothermal system. The studied lava corresponds to an andesite-dacite (61–63 SiO2 wt%) with phenocrysts of plagioclase, amphibole (Group 1), biotite, pyroxenes, quartz, and olivine. The lava also contains aphanitic enclaves (58–60 SiO2 wt%), whose groundmass have the same mineralogy of lava samples groundmass consisting of amphibole microphenocryts (Group 2) and microlites of plagioclase, pyroxenes, and Fe–Ti oxides. Disequilibrium textures are commonly observed in the studied samples such as partially resorbed plagioclase phenocrysts, amphibole breakdown, and reverse zoning in pyroxene and plagioclase phenocrysts. Thermobarometry calculations indicated pressures of ~ 2 kbar for Group 1 amphiboles, temperatures of 905–1097 °C for Mg-rich pyroxene phenocrysts and Ca-rich plagioclase (An ≥ 66), and near-solidus temperatures of 712–788 °C for Group 1 amphibole-plagioclase pairs. Group 1 amphiboles also indicate crystallization from an evolved liquid (63–79 wt% of SiO2). Oxidation conditions of QFM + 0.9–2.5 log units were recorded in amphibole and Fe–Ti oxides. Rhyolite-MELTS models reproduce the composition of the high-temperature phases from a melt composition (andesitic enclave) at similar P–T-fO2. The arrival of new hot andesite magma into a crystal-rich shallow reservoir, thermally and compositionally zoned, would have triggered the studied eruption. Diffusion models in Fe–Ti oxides microlites indicated cooling temperatures of 742–866 °C during the sub-aerial emplacement of the lava. [ABSTRACT FROM AUTHOR]

Mount Elbrus, Europe's tallest and largely glaciated volcano, is made of silicic lavas and is known for Holocene eruptions, but the size and state of its magma chamber remain poorly constrained. We report high spatial resolution U–Th–Pb zircon ages, co-registered with oxygen and hafnium isotopic values, span ~ 0.6 Ma in each lava, documenting magmatic initiation that forms the current edifice. The best-fit thermochemical modeling constrains magmatic fluxes at 1.2 km3/1000 year by hot (900 °C), initially zircon-undersaturated dacite into a vertically extensive magma body since ~ 0.6 Ma, whereas a volcanic episode with eruptible magma only extends over the past 0.2 Ma, matching the age of oldest lavas. Simulations explain the total magma volume of ~ 180 km3, temporally oscillating δ18O and εHf values, and a wide range of zircon age distributions in each sample. These data provide insights into the current state (~ 200 km3 of melt in a vertically extensive system) and the potential for future activity of Elbrus calling for much-needed seismic imaging. Similar zircon records worldwide require continuous intrusive activity by magmatic accretion of silicic magmas generated at depths, and that zircon ages do not reflect eruption ages but predate them by ~ 103 to 105 years reflecting protracted dissolution–crystallization histories. [ABSTRACT FROM AUTHOR]

Previous studies of Pinatubo have shown that basaltic magma was injected into what was thought to be a homogeneous dacitic magma reservoir just before the 1991 eruptions. Mingling of the basalt and dacite resulted in a volatile-charged hybrid andesitic magma that ascended through the overlying dacite reservoir, triggering initial andesitic eruptions and the climactic eruption of dacite a few days later. Using mineral and whole-rock chemistry and isotopic data, we show that the dacitic magma was not homogeneous, but was instead heterogenous, produced from heterogeneous dacitic magma, which is approximated by at least two distinct silicic end-member magmas. Our data show that these silicic magmas were not produced by fractional crystallization of basaltic magma, such as that which formed the basalt enclaves of the 1991 and Buag period basalts (~ 1.0–0.5 ka). Instead, we argue that the 1991 dacite magmas were produced independently but simultaneously by the melting of heterogeneous crustal materials. Based on these results, we propose the following sequence of events. First, basaltic magma distinct from that of the Buag period underplated and heated the crust to form silicic melt pockets of various compositions in a crustal mush zone. During the period between 0.5 ka and 1991, these silicic melts separated from the crustal mush zone and accumulated in the crust, forming the 1991 dacitic magma reservoir. Finally, during the climactic phase of the 1991 eruption, heterogeneous dacitic pumices were erupted, having a composition range that is representative of their variable crustal origins. [ABSTRACT FROM AUTHOR]

Mount Elbrus, Europe's tallest and largely glaciated volcano, is made of silicic lavas and is known for Holocene eruptions, but the size and state of its magma chamber remain poorly constrained. We report high spatial resolution U–Th–Pb zircon ages, co-registered with oxygen and hafnium isotopic values, span ~ 0.6 Ma in each lava, documenting magmatic initiation that forms the current edifice. The best-fit thermochemical modeling constrains magmatic fluxes at 1.2 km3/1000 year by hot (900 °C), initially zircon-undersaturated dacite into a vertically extensive magma body since ~ 0.6 Ma, whereas a volcanic episode with eruptible magma only extends over the past 0.2 Ma, matching the age of oldest lavas. Simulations explain the total magma volume of ~ 180 km3, temporally oscillating δ18O and εHf values, and a wide range of zircon age distributions in each sample. These data provide insights into the current state (~ 200 km3 of melt in a vertically extensive system) and the potential for future activity of Elbrus calling for much-needed seismic imaging. Similar zircon records worldwide require continuous intrusive activity by magmatic accretion of silicic magmas generated at depths, and that zircon ages do not reflect eruption ages but predate them by ~ 103 to 105 years reflecting protracted dissolution–crystallization histories. [ABSTRACT FROM AUTHOR]

The subduction of the Bangong–Nujiang Ocean is important in the geological evolution of the Tibetan Plateau. In this paper, we report new zircon U-Pb age and Lu-Hf isotopic data and whole-rock elemental and Sr-Nd-Pb isotopic data for Early Cretaceous dacites from the Rena-Co area (RCA) in the southern Qiangtang Terrane (QT), central Tibet and use these data to better understand the tectonic evolution of the Bangong–Nujiang suture. LA–ICP-MS dating of zircons yields ages of 109.5 ± 0.6 Ma to 109.6 ± 0.8 Ma for the dacites from the RCA. Geochemically, these dacites are medium-K calc-alkaline and show high SiO2 contents of 64.79–70.37 wt.%, high Sr contents of 517–598 ppm and low Y contents of 8.45–10.7 ppm, similar to those of typical adakites. Additionally, all the rocks are strongly enriched in light rare earth elements and some large ion lithophile elements (e.g. Rb, U, K and Cs) but significantly depleted in high-field-strength elements (e.g. Nb, Ta and Ti), consistent with the geochemical characteristics of arc-type magmas formed in the subduction zone. Moreover, these adakite-like dacites show whole-rock initial (87Sr/86Sr)i ratios of 0.705119 to 0.705491, (206Pb/204Pb)i ratios of 18.489 to 18.508, (207Pb/204Pb)i ratios of 15.591 to 15.612, (208Pb/204Pb)i ratios of 38.599 to 38.686, ϵ Nd (t) values of −0.28 to +1.25 and single-stage Nd model ages of 642 to 818 Ma, as well as significantly positive zircon ϵ Hf(t) values of 3.9–13.1, with young Hf-depleted mantle ages of 331 to 923 Ma. These geochemical and isotopic data indicate that they are most likely derived from the juvenile thickened mafic lower continental crust, which contains partial melts of metasomatized peridotite and subduction-related fluids in the magma source region. Based on previous studies and our new data, we propose that the RCA adakite-like dacites are most likely a result of the northwards subduction of the Bangong–Nujiang Ocean lithosphere beneath the southern QT during the Early Cretaceous and that a slab rollback model could explain the formation of the RCA adakite-like dacites. [ABSTRACT FROM AUTHOR]

Garnet's stability in arc magmas and its influences on their differentiation were explored experimentally in a typical basalt, andesite, and dacite at conditions of 0.9–1.67 GPa, 800–1300 °C, with 2–9 wt.% added H2O, and with oxygen fugacity buffered near Re + O2 = ReO2 (~ Ni-NiO + 1.7 log10 bars). Garnet did not grow at 0.9 GPa in any of the compositions, even with garnet seeds added to facilitate nucleation. At 1.0–1.2 GPa, garnet grew as thin rims (< 5 µm) on introduced garnet seeds coexisting with dacitic to rhyodacitic liquids at temperatures ≤ 1000 °C. At 1.3 GPa, garnet grew readily with no seeds from 900 to 1100 °C coexisting with liquids ranging from peraluminous basaltic andesite to rhyodacite, and at 1.46 GPa, garnet was stable as hot as 1150 °C in metaluminous basaltic liquid. Garnet grew as a liquidus phase only in the dacite, a composition similar to the average upper continental crust. Inverse experiments on the dacite determined a liquidus multiple-saturation point with garnet, plagioclase, orthopyroxene, calcic clinopyroxene, and amphibole at 975 °C, 1.46 GPa, with 7 wt.% dissolved H2O. Such dacitic and more evolved melts can be products of peritectic reactions that with decreasing temperature consume garnet, calcic clinopyroxene, and melt components, producing amphibole and less abundant but more evolved melts. For this reason, experiments on product melts need not produce reactant minerals, accounting for some disparities in published experimental results on the apparent stability of garnet in intermediate-to-evolved arc magmas. Results on more mafic compositions are more reliable guides and show that liquids of arc dacitic composition, and more evolved compositions, would coexist stably with garnet only in the deepest portions of continental-margin arc crust with average thickness and density (~ 43 km, ~ 1.2 GPa) or in the underlying shallow mantle. Metaluminous arc basaltic, basaltic andesitic, and many andesitic liquids would not coexist stably with garnet at pressures ranging from the crust to at least the midpoint of the mantle wedge, but results in the literature allow that some andesitic liquids with higher Fe/Mg than common in arcs may also saturate with garnet in the deeper portions of average-thickness continental arc crust. A persistent issue, however, is that, at pressures of the lower continental crust or shallow mantle (0.7–1.67 GPa), arc basalts may crystallize or differentiate within a regime that includes a clinopyroxene-dominated high-T interval (1250–1150 °C) with lesser orthopyroxene. This crystallizing assemblage drives coexisting liquids to become peraluminous at 53–60 wt.% SiO2 (normalized anhydrous), whereas arc igneous suites mainly attain peraluminous compositions at 65–70 wt.% SiO2. Thus, simple, progressive crystallization-differentiation of appropriately hydrous, oxidized basalts near the base of continental arc crust does not generate, or does not act alone to produce, the dominantly metaluminous arc andesites. Scarcity of natural peraluminous andesites and basaltic andesites, and of correlative intrusions, despite their demonstrated production by deep basalt differentiation, may result from mixing accompanying crystallization-differentiation. Cumulates produced by arc basalts at deep crustal and upper mantle pressures have densities equal to or exceeding those of upper mantle peridotite until coexisting liquid compositions reach or exceed that of silicic andesite or dacite, after which cumulates become buoyant relative to the mantle. Deep differentiation may therefore be efficient until that point, with cumulates being lost to the mantle and melts evolving rapidly to silicic andesite through dacite compositions. This process results in both an intermediate overall composition for the buoyant crust, and deep-crustal dacitic through rhyolitic melts that can mix with deep basalts thereby producing metaluminous basaltic andesites and andesites. [ABSTRACT FROM AUTHOR]

To enrich understanding of how fluid flows through the natural filled jointed rock, the permeability experiments of the natural filled jointed dacite rock under repeated loading and unloading conditions are carried out. Experimental results show that the response of permeability of filled jointed dacite rock to confining pressure under repeated loading and unloading conditions is considered as porous media. Permeability and gas pressure are positively correlated under repeated loading and unloading conditions. Relationship between permeability and gas pressure shows that the higher confining pressure, the smaller permeability increasing rate. The calculated equivalent crack apertures are very small at the order of magnitude about 10−8 m. The power law is found to well describe the relationship between equivalent crack aperture and effective stress of natural filled jointed dacite rock. With the parameters determined based on experiment results, an empirical expression is proposed. [ABSTRACT FROM AUTHOR]

New geochemical data, including Sr–Nd–Pb isotopes for whole-rock and groundmass samples, are reported for edifice-forming eruptives at Tongariro volcano, New Zealand, which span its ~ 350 ka to late Holocene history. Tongariro eruptives are medium-K basaltic-andesites to dacites (53.0–66.2 wt% SiO2) that evolved via assimilation-fractional crystallisation (AFC) processes partly or mostly in the uppermost 15 km of the crust. When ordered chronologically using a high-resolution 40Ar/39Ar-dated eruptive stratigraphy, the compositional data show systematic 10–130 kyr cycles. Mafic replenishment events inferred from MgO values occurred at ~ 230, ~ 151, ~ 88 and ~ 56 ka and in the late Holocene, with high-MgO flank vents erupting at ~ 160, ~ 117, ~ 35 and ~ 17.5 ka. Cycles in Sm/Nd, 87Sr/86Sr, 143Nd/144Nd and Pb isotopic ratios, which are decoupled from MgO, K2O and Rb/Sr cycles, indicate periods of prolonged crustal residence of magmas from ~ 230 to ~ 100 ka and ~ 95 to ~ 30 ka. AFC modelling shows that intermediate and silicic melt compositions, with r-values between 0.1 and 1, are needed to reproduce Tongariro compositional arrays. AFC models also indicate that ~ 20% of the average Tongariro magma comprises assimilated (meta)sedimentary basement material. Locally, Tongariro and adjacent Ruapehu volcanoes attain their most crust-like 87Sr/86Sr and 143Nd/144Nd compositions at ~ 100 and ~ 30 ka, paralleling with zircon model-age crystallisation modes at the rhyolitic Taupō volcano ~ 50 km to the NNE. These coincidences suggest that the timing and tempo of magma assembly processes at all three volcanoes were contemporaneous and may have been tectonically influenced since at least 200 ka. [ABSTRACT FROM AUTHOR]

The magma evolution of Ngebel Volcano, both temporally and spatially, is represented by the characteristics of its lava. Ngebel Volcano, located in East Java, is a Quaternary andesitic stratovolcano. This volcano is part of the Wilis Volcanic Complex. The volcanism stage of Ngebel Volcanic Complex can be divided into the Jeding with andesitic basalt (SiO2 49 - 59%), pyroxene andesite Kemlandingan (SiO2 49 - 59%), Manyutan with hornblende andesite (SiO2 49 - 59%), and Ngebel with dacite (SiO2: 49 - 59%). The variation of major elements combined with petrographic features such as plagioclase, pyroxene, hornblende, quartz, and opaque minerals from basaltic andesite to dacite is interesting. The minerals show that the magma differentiation process of Ngebel Volcanic Complex is the results of fractional crystallization of magma. The purpose of this study is to determine the evolution of magma from volcanic rocks of which stratigraphic positions have been determined. The analytical methodology used is petrographic and geochemical analysis. Detailed temporal evolution shows that magma from the Ngebel Volcanic Complex underwent a differentiation process that changed the magma composition from mafic to more felsic. [ABSTRACT FROM AUTHOR]

The Late Paleozoic and Mesozoic tectonic framework of the Nanpanjiang Basin has much been disputed. Herein, the middle-acid volcanic rock, dacite, exposed to the Zhesang gold district, southeast Yunnan Province, has been analyzed. The results show that the dacite belongs to a calc-alkaline series, SiO2 contents range from 62.79 to 76.66 wt%. Zircon SHRIMP U–Pb dating of dacite demonstrates that they were formed in the Early Triassic (247.8 ± 1.7 Ma, MSWD = 1.2). All samples exhibit enrichment in LILE (e.g. Rb, K, Th, and U), and depletion in HFSE (e.g. Nb, Ta, and Ti), which has the geochemical affinity of I-type granite. La–La/Sm and La–La/Yb discrimination diagrams show that the partial melting, mainly of the mafic lower crust, of rocks, plays a major role in the formation process. The dacite has low initial 87Sr/86Sr ratios (0.706954 to 0.708589) and negative εNd(t) values (− 11.77 to − 10.88). Zircons in dacite have εHf(t) values of − 16.2 to − 8.3, and the two-stage Hf model ages are 1799–2301 Ma, mostly concentrated between 1800 and 1900 Ma, indicating that the magma source area is the reconstructed ancient lower crust mixed with some mantle materials, and crystal fractionation process underwent in the late stage of magma migration. This study reveals that the arc-volcanic rocks of the Early Triassic in the southern margin of the Nanpanjiang Basin were formed by the subduction of the Late Paleozoic ocean basin within the border region between China and Vietnam. [ABSTRACT FROM AUTHOR]

U–Th–Pb LA-ICP-MS was used for dating detrital zircon extracted from the cement of Neoarchean and Paleoproterozoic polymict conglomerates of the Karelian Province, Fennoscandian Shield. The chemical composition of terrigenous and volcanogenic rocks of the Gimolskaya Formation is presented for the first time. The data obtained showed that the detrital zircon from the cement of conglomerates of different ages yields similar results of about 2750 Ma. The clasts of the gneiss tonalite and plagioporphyry dominate in the Neoarchean conglomerates of the Sukkosero area. They are identical in chemical composition with the Neoarchean (2.78–2.73 Ga) TTG, granodiorite, and metadacite of the Central Karelian domain. These rocks are distinguished by an increased content of Ba (900–1200 ppm) and Sr (600–700 ppm), and the ratios Sr/Y = 34–90 and (La/Yb)n = 17–30. The chemical composition of the cement (values of CIA < 55 and negative values of the function DF(x)) from –1.0 to –0.4 is similar to the composition of clasts, which means the cement is composed of finer clasts of the same rocks. The Paleoproterozoic conglomerates of the Vottomuks area contain zircon from felsic rocks of the Neoarchean basement. The chemical composition of the cement of the Paleoproterozoic conglomerates and the predominance of basaltic andesites of Sumian are evidence of the post-Sumian(<2.45 Ga) age of these rocks. The absence of the Paleoproterozoic zircon in cement could have resulted from its absence in the Paleoproterozoic source rocks. The age of Neoarchean conglomerates from the Gimolskaya Formation is less than 2.75 Ga. [ABSTRACT FROM AUTHOR]

The Early Cretaceous geology of Northwestern (NW) Peru was characterized by emplacement of voluminous felsic volcanic rocks (e.g. the Oyotún Fm.), which have important implications for the understanding of Early Cretaceous crust-mantle interactions in the Central Andes. We present U-Pb-Hf isotopic compositions and whole-rock and trace elemental compositions of a series of Early Cretaceous volcanic rocks from the Western Cordillera and the Eastern Cordillera of the Central Andes. The volcanic rocks outcrop sporadically with limited thickness and uniform volcanic lithofacies in the western region, while the eastern region has distributions of larger scale volcanic rocks with more complex volcanic lithofacies. LA-ICP-MS zircon U-Pb dating defines the age of the Oyotún Fm. range from 138.8 Ma to 131.0 Ma in the Western Cordillera, and from 139.7 Ma to 132.3 Ma in the Eastern Cordillera. In situ Hf isotope analysis on dated zircon yield an εHf(t) range of +3.37 to +8.42 and a TDM2(Hf) range from 983 Ma to 650 Ma with a peak at ca. 840 Ma. The Early Cretaceous felsic magmas are predominantly dacites and rhyolites with minor components of basalt and andesite, and show relatively homogeneous Hf isotopic compositions of zircon in single samples. Both the εHf(t) of zircons and the magma temperature increase from the bottom to the top of the Early Cretaceous volcanic rocks in each studied area. All these facts imply an increasing contribution of contemporaneous underplated hot and dry mantle-derived magmas with time, both as a heat source inducing crustal melting and a source of material (melt) that variably mixed with the ancient continental crustal materials. [ABSTRACT FROM AUTHOR]

In order to properly protect and restore cultural assets, it is important to correctly identify the natural materials used in historical buildings and determine the origin of these materials and the quarries they were taken from. This study therefore examines the rampart building stones of Ankara Castle, which is one of the most important historical buildings in the city, and determines the locations of the quarries used in its construction. In the study, samples were taken from the castle, rampart walls and the rocks in the castle's foundation. The samples were then compared mineralogically, petrographically and geochemically with other samples taken from quarries where the stones may possibly have originated from. Ankara Castle, which is located in the Altındağ district of Ankara, is comprised of an inner and an outer castle. The castle was basically built on vertically articulated andesite rock and has the same composition as the rampart stones. Brick and limestone were also used in certain parts of the rampart walls. The rampart stones are mainly composed of pink porphyroafanitic textured andesites, but also contain locally crystallized and biosparitic limestone. Andesites are seen to have a hyaloplitic porphyritic texture when examined under the microscope, and consist mainly of plagioclase, amphibole, biotite, pyroxene, ± quartz and opaque minerals. Quartz and plagioclase crystals are embayed and the plagioclase was also zoned. Kaolinization, chloritization and opacification type alterations were detected in the andesites. These rocks, which are composed of quartz andesite, contain high levels of K-calcalkaline. In order to determine the origins of the rocks used in the construction of the rampart walls, andesite samples were compared with other samples taken from different locations around the castle and Ankara. It was determined that the rampart andesite samples most resemble those taken from seven old quarries on Yenidogan Hill. [ABSTRACT FROM AUTHOR]

To obtain time-dependent responses of rock in excavation engineering projects, a series of creep tests of dacite are conducted through stepwise unloading of confining pressure. Based on the test results, time-dependent behaviour and unloading failure mechanism of dacite are studied in terms of creep rate, failure mode, long-term strength, and time-dependent damage. According to the creep rate, creep deformation can be divided into decelerating creep stage, steady creep stage, and accelerating creep stage during the last unloading failure process. Time-dependent deformation exhibits an anisotropic characteristic. Local shear bands are found in failed samples, which is markedly different from tensile failure in a short-term unloading test. Time-dependent damage of dacite under unloading condition is the result of combined effects of cracks. Creep failure of dacite under unloading condition caused by concentrating in a local zone and coalescing to form one or more macroscopic shear bands. This study gives an insight to understand the mechanism of unloading creep failure of rock and provides an experimental foundation for developing a time-dependent constitutive model under unloading condition. [ABSTRACT FROM AUTHOR]

Stratovolcanoes are commonly characterised by cyclic eruptive activity marked by transitions between dome-forming, Vulcanian, Subplinian and Plinian eruptions. Guagua Pichincha volcano (Ecuador) has been a location of such cyclicity for the past ~ 2000 years, with Plinian eruptions in the first and tenth centuries AD (Anno Domini/after Christ), and CE (Common Era) 1660, which were separated by dome-forming to Subplinian eruptions, such as the recent 1999–2001 eruption. These cycles are therefore a prominent example of effusive-explosive transitions at varying timescales. Here, we investigate the reasons for such shifts in activity by focusing on degassing and outgassing processes within the conduit. We have coupled a petrophysical and textural analysis of dacites from the CE 1660 Plinian eruption and the 1999–2001 dome-forming/Vulcanian eruption, with different percolation models in order to better understand the role of degassing on eruptive style. We demonstrate that the transition from dome-forming to Plinian activity is correlated with differences in phenocryst content and consequently in bulk viscosity. A lower initial phenocryst content and viscosity is inferred for the Plinian case, which promotes faster ascent, closed-system degassing, fragmentation and explosive activity. In contrast, dome-forming phases are promoted by a higher magma viscosity due to higher phenocryst content, with slower ascent enhancing gas escape and microlite crystallization, decreasing explosivity and yielding effusive activity. Resumen: Los estratovolcanes se caracterizan comúnmente por presentar actividad eruptiva cíclica, marcada por transiciones entre erupciones formadoras de domos y erupciones de tipo Vulcanianas, Subplinianas y Plinianas. El volcán Guagua Pichincha (Ecuador) ha dado lugar a tal ciclicidad durante los últimos ~ 2000 años, con erupciones Plinianas tanto en los siglos Primero y Décimo, como en el año 1660, las cuales estuvieron intercaladas por erupciones formadoras de domos y de tipo Subplinianas, tal como ocurrió durante la erupción reciente de 1999–2001. Estos ciclos son, por lo tanto, ejemplos destacados de transiciones eruptivas de tipo efusiva-explosiva a escalas de tiempo variadas. En este trabajo, investigamos las razones de tales cambios de actividad enfocándonos en procesos de exsolución y pérdida de gases del magma en el conducto (desgasificación en sistemas cerrado y abierto). Hemos acoplado análisis petrofísicos y texturales tanto de dacitas de la erupción Pliniana de 1660, como de la erupción formadora de domos/Vulcaniana de 1999–2001, junto con diferentes modelos de percolación, para así comprender mejor el rol de la exsolución de volátiles en el estilo eruptivo. Demostramos que la transición desde una actividad efusiva formadora de domos a una Pliniana está correlacionada con diferencias en el contenido de fenocristales y, subsecuentemente, con la viscosidad total del magma. Un contenido inicial menor de fenocristales y una menor viscosidad se infiere para el caso Pliniano, lo que promueve un ascenso más rápido, desgasificación en sistema cerrado, fragmentación y finalmente actividad explosiva. Por el contrario, las fases formadoras de domos son promovidas por una viscosidad mayor debido a un contenido mayor de fenocristales, con ascenso más lento promoviendo a su vez el escape de gases y la cristalización de microlitos, disminuyendo la explosividad y produciendo actividad efusiva. [ABSTRACT FROM AUTHOR]

Partial melting during high-temperature to ultrahigh-temperature (UHT) metamor-phism facilitates crustal differentiation, element transfer, and the evolution of topography in orogens, however the mechanisms that drive heating of Earth's crust remain controversial. We provide new evidence from ca. 2.3 Ma dacites in the Tibetan Plateau, representing the youngest known UHT metamorphic event. Our results show that these dacites were mainly generated by fluid-absent melting of metasedimentary rocks and minor mafic rocks at peak temperatures of 1100-1150 °C and pressures of 0.8-0.9 GPa. The dacites represent mixtures of UHT melts and granulite residues and are geochemically similar to A-type granites with extremely high heat-production values (5.33-5.99 µW m-3). Compared with the geological and geophysical observations, numerical modeling indicates that the key factor determining the thermal evolution of Tibet is the thickness of the radioactive layer. Orogens dominated by rocks of felsic composition, like Tibet, could easily reach UHT conditions within a short period of time (20-40 m.y.) after crustal thickening by radioactive heating, without the need for an additional tectonic mechanism. [ABSTRACT FROM AUTHOR]

Late'iki (previously known as Metis Shoal) is a highly active volcano in the Tofua arc with at least four temporary island-building eruptions and one submarine eruption in the last 55 years. The most recent eruption, commencing in October 2019, resulted in lava effusion and subsequent phreatic explosions, the construction of a short-lived island that was quickly eroded by wave action and possibly further phreatic activity that continued into January 2020. The two-pyroxene dacite from the 2019 eruption is similar to the 1967/8 eruptions suggesting the magma is residual from earlier eruptions and has not undergone further differentiation in the last 50 years. New observations of the 2019 eruption site confirm the lava-dominant character of the volcano summit but a thin veneer of wave-reworked, finely fragmented lava material remains that is interpreted to have been produced by phreatic explosions from hot rock-water interactions during the effusive eruption. A notable absence of quench-fragmented hyaloclastite breccias suggests that non-explosive quench fragmentation processes were minimal at these shallow depths or that hyaloclastite debris has resedimented to greater depths beyond our summit survey area. [ABSTRACT FROM AUTHOR]

Nb-rich trachytes from Virginia, shoshonite from Colorado and high-K dacite from the Bolivian Andes contain diverse suites of native metals and alloys. Virginia trachytes are differentiates of mafic within-plate magmas derived from oceanic island basalt (OIB)-type mantle sources in the post-orogenic extensional regime of the Appalachian mobile belt. Colorado shoshonite and Bolivian dacite were produced via crystal fractionation of Fe–Mg silicates from trachybasalt and basalt magma, respectively, derived from subduction-related mantle modified by either crustal contamination (both regions are underlain by thick, mature continental-type crust) or recycling of marine sediment. Metal assemblages include refractory metals (Pt with minor Fe and Cr, native W, Co–W and Fe–W alloys), Cu–Ag–Au alloys, chalcophile metals (Cu–Sn alloy, native Cu and Zn) and associated sulfides (galena). Textural evidence (metals occur as inclusions in phenocrysts, high temperatures of their formation, etc.) and compositional features of metallic phases suggest their magmatic origin. Metal-rich magmas (both mafic and differentiated) transport metals in convergent and within-plate tectonic settings and deliver ore metals to the oxidized porphyry and epithermal environments in the upper crust. [ABSTRACT FROM AUTHOR]

The Pauzhetka Caldera (27 × 18 km) was formed in the South Kamchatka during the Golygin Ignimbrite eruption (420–440 ka), the largest known eruption in the region in the past 1 Myr. The eruption was preceded by the 3 Ma-old mafic and intermediate volcanism. After the caldera-forming eruption, a variety of products, from basalt to rhyolite, were ejected within the caldera. For understanding the origin of voluminous silicic magmatism in thin mafic South Kamchatka crust, we used geochemical and isotope data. Our research has characterized the major and trace element composition of Golygin ignimbrite, intra-caldera hydrothermally altered deposits, pre-caldera (Mt. Orlinoe Krylo, Mt. Klyuchevskaya) and post-caldera (Kambalny Ridge, Chernye Skaly) eruptive centers. The Sr–Nd isotope composition of the Golygin ignimbrite and some eruptive post-caldera products was investigated. The isotope variations indicate that parental magmas for all rocks of the Pauzhetka area were obtained from a weakly evolved source derived through fluid-assisted melting of a subducted slab. Geochemical data support that the formation of most magmas of the Pauzhetka caldera was mainly controlled by fractional crystallization in the lower to middle crust. MELTS-modelling agrees with geochemical data. The fractional crystallization of Kambalny basalt with 2 wt % H2O at 6 kbar provides the best fit to the observed composition of the Golygin dacite. [ABSTRACT FROM AUTHOR]

When the modern-like geomorphology and climate pattern of the Himalaya-Tibetan Plateau were established still remains unclear. In this study, we apply paired stable isotope compositions of carbonate (d18Oc) and leaf waxderived n-alkanes (d2Halk) from the upper Gazhacun Formation in the Namling Basin to reconstruct the middle Miocene elevation and climate of southern Tibet. Depositional age of the upper Gazhacun Formation has been precisely constrained to between 15.5 Ma and 15.1 Ma by zircon U-Pb ages of dacite interlayers. Paired carbonate derived d18Ow values (-17.9 ± 1.3‰ to -18.3 ± 1.3‰) and leaf-wax derived d2Hw values (-131.5 ± 20‰ to -145.7 ± 20‰) plot on or very close to the global meteoric water line suggesting that these samples experienced little evaporation enrichment and isotopic alternation. Based on these two independent proxies, paleoelevation estimates for the Namling Basin are consistently between 4.6+0.7/-0.8 km and 5.2+0.7/-0.8 km, supporting a high elevation for southern Tibet in the mid-Miocene. Integrated with published paleoelevation estimates for the Himalaya, central and northern Tibet in literature, a near-present elevation across the whole Himalaya-Tibetan Plateau has already been established since the middle Miocene (ca. 15 Ma). Besides, stable isotopic values across the Himalaya-Tibetan Plateau show that the d2Hw values gradually increase northward from the Himalaya to northeastern Tibet, quite similar to that of the present day. This pattern suggests that during the middle Miocene, the Himalaya-Gangdese system may have blocked southerly monsoonal moisture from reaching northern Tibet. Westerlies or local recycling of moisture might be the dominant moisture sources across northern Tibet, with enriched d18Ow and d2Hw precipitation values that could lead to erroneous paleoelevation estimates over central and northern Tibet. [ABSTRACT FROM AUTHOR]

The Jiangnan Orogenic Belt in South China formed as a result of the amalgamation between the Yangtze Block and the Cathaysian Block and records the geological information of collision. However, the timing of the amalgamation of two blocks remains controversial. Here, structural deformation and zircon U-Pb geochronological results from Precambrian metamorphosed sedimentary-volcanic rock units at the eastern domain of the Jiangnan orogenic belt in southern Anhui Province are presented. The pre-Nanhua rocks in the study area were widely involved in the metamorphism and deformation to with NW or SE-dipping foliation, tight folds and local development of strike-slip ductile shear zones, which is induced by collision of Yangtze Block and the Cathaysia Block. In this study, a mylonitic granodiorite sample intruding in the Xikou Group, a mylonitic dacite sample from Jingtan Formation and two deformed quartz veins samples in the Shexian region were dated by LA-ICP-MS zircon U-Pb dating method. Zircons from the mylonitic granodiorite and dacite yield the weighted mean ages of 811.2 ± 7.5 Ma (MSWD = 0.51, n = 30) and 779 ± 7.4 Ma (MSWD = 0.42, n = 29), respectively, interpreted as the crystallization ages of the original rocks, whereas the zircons from deformed quartz veins yield the weighted mean ages of 779 ± 14 Ma (MSWD = 2.6, n = 22) and 769 ± 16 Ma (MSWD = 2.4, n = 16), respectively, representing the crystallization age of the quartz veins. Combined with previous data, our results show that the metamorphosis and deformation of Pre-Nanhua rocks took place during the period of 820–770 Ma. Therefore, we consider that the Yangtze Block and the Cathaysian Block the eastern Jiangnan Orogenic Belt don't amalgamate until 770 Ma. [ABSTRACT FROM AUTHOR]

The thermal histories of upper crustal magma reservoirs place important constraints on the formation, evolution, and remobilization of crustal magmas, yet quantifying thermal histories remains challenging. We report new in situ plagioclase trace-element data, Sr in plagioclase diffusion timescales, and plagioclase 238U-230Th-226Ra disequilibria data from Mount Saint Helens (MSH) 1980 cryptodome and 2004–2005 dacite domes to evaluate the thermal storage conditions and compositional diversity of recent MSH magmas. This approach allows us to more directly link the thermal (and therefore physical) conditions within the MSH magma reservoir to timescales of storage, thereby constraining the fraction of time 1980–2005 MSH magmas have spent in a mobile state. Plagioclase trace-element data and U-series characteristics reveal a compositionally heterogenous magmatic system beneath MSH and also require multi-stage plagioclase growth histories. The data also show that 2004–2005 dacites contain a different plagioclase population relative to the 1980 cryptodome dacite, comprised of either a compositionally (and possibly temporally) distinct plagioclase component or composed of the same plagioclase components but in significantly different proportions. Discordant plagioclase 238U-230Th and 230Th-226Ra apparent ages require a mixture of young (likely eruption related) and old (>20–40 ka) plagioclase crystals in both 1980 and 2004–2005 dacites. The low (230Th)/(232Th) in all measured 1980 and 2004–2005 plagioclase requires a significant fraction (>10–30%) of the plagioclase to be old (>10s kyr). Maximum modeled Sr diffusion timescales at 750°C range from decades to centuries for both eruptions, with a maximum of ~600 years found in 1980 cryptodome plagioclase. However, partially equilibrated Sr in the innermost exposed parts of some crystals found in both 1980 and 2004–2005 plagioclase indicate that a fraction (>20%) of plagioclase may have experienced a total of >10 kyrs at temperatures ≥750°C. Coupling Sr diffusion timescales with U-series measurements indicates that significant fraction (at least ~40%) of modeled MSH plagioclase spent <5% of their storage time at temperatures ≥750°C and thus in an easily mobilized rheological state. In contrast, the fraction of partially equilibrated plagioclase possibly spent >25–50% of their storage time at hotter conditions. Our data combined with data from other arc magmatic systems (e.g. Mt. Hood) imply that the process of remobilizing magma in arc systems toward successive eruptions requires thermally rejuvenating largely crystalline material, as opposed to sequential tapping of a persistent liquid-dominated magma body, even if successive eruptions are spaced closely in time (decades). In addition, rejuvenation events responsible for successive eruptions may sample spatially localized, but potentially overlapping, portions of the broader magma reservoir. [ABSTRACT FROM AUTHOR]

We explore crystal growth and magma recharge during the formation of intermediate lavas using bulk rock compositions and zoning patterns and textural variation in plagioclase feldspars from Hasandağ volcano in south-central Turkey. Hasandağ intermediate lavas formed primarily through fractionation of the observed mineral phases, and also show abundant evidence for magma mixing and thermochemical disequilibrium. Sparse basaltic andesites originated through mixing of observed mafic and felsic lavas rather than fractionation. Plagioclase phenocrysts from all lava types have uniform core (An~39) and variable rim (An39–64) compositions, suggesting plagioclase feldspars from a common magma reservoir are present in all erupted lavas. An-rich crystal rims are regularly enriched in Mg, Fe and Ti, indicating mafic recharge and magma mixing. Mixing timescales determined by diffusion modeling of Mg profiles across core-rim boundaries suggest recharge to eruption occurs on the order of up to ~ 45 days. Thermobarometric calculations constrain shallow crustal storage of dacite to 1.5–2 kbar (~ 5 km) and 800–890 °C, with deeper reservoirs for more mafic magmas (up to 35–40 km for basalts). Abundant disequilibrium textures (sieve-textured zones, oscillatory zoning, resorbed and patchy-zoned cores) indicate mobilization of a homogeneous crystal-rich dacitic reservoir by injection of mafic magma. We suggest that incomplete and dynamic physical mixing at shallow crustal levels results in distinct crystal morphologies, some of which record punctuated ascent and storage, while others are erupted rapidly after the influx of new magma. [ABSTRACT FROM AUTHOR]

The pebbles of volcanic rocks and volcanic products in the gravels of the Cotmeana Piedmont are of rhyolites, dacites, trachytes, andesites, basalts, lavas, ignimbrites, volcanic breccias and tuffs. The rhyolites and dacites pebbles are relatively uniformly distributed throughout the Piedmont area. Andesite pebbles occur more frequently in the north-eastern area. All the other petrographic types are more common in the western half of the piedmont, some types appearing only in this region (breccias and tuffs). The source area of the gravels in the Cotmeana Piedmont corresponds to the hydrographic basins of the Olt and Argeș rivers. The Olt River brought eruptive clastic material from proximal (Cindrel Mountains and Getic Subcarpathians) and distal source areas (Perșani and Baraolt Mountains, north-eastern Făgăraș Mountains and southeastern Transylvanian Basin). The clastic material imput of the Argeș River comes only from the proximal areas (southern slope of the Făgăraș Mountains and Getic Subcarpathians). There are some unclear aspects regarding the provenance of some pebbles in the eastern half of the piedmont, having as possible explanation the presence of a primitive course of the Olt River inside the piedmont area. [ABSTRACT FROM AUTHOR]

In this study, we report the petrography and mineralogy of a brecciated eucrite Northwest Africa (NWA) 8021, which shows a locally layered texture with one layer containing graphic clasts and Si,Ti‐rich regions. The graphic clasts contain rod‐like grains of silica phases, augite, K‐feldspar, and Ca‐phosphate minerals included in anorthite. Some of the clasts contain relatively coarse grains of quartz, K‐feldspar, and augite, which are chemically different from the rod‐like phases, indicating different origins. All of the augite grains in the graphic clasts have rare earth element (REE) concentrations higher than those in typical eucrites. The bulk Na2O+K2O contents of the graphic clasts are higher than typical eucrites. All of these chemical features indicate that the graphic clasts were probably derived from an evolved parent rock. Low‐degree partial melting of the eucritic crust (<10%) is required to generate a melt equilibrated with the REE‐rich rod‐like pyroxene from the graphic clasts. The Si,Ti‐rich regions contain high abundances of silica phases (~52 vol%) and ilmenite (~9 vol%), probably derived from an evolved Si,Ti‐rich rock (dacite). The evolved components observed in NWA 8021 are different from other evolved components observed in howardites and indicate more diverse evolution in the eucrite parent body than previously thought. [ABSTRACT FROM AUTHOR]

We examine the connected history of dacite-dominant volcanic rocks of the Tschicoma Formation, erupted between 5.5 and 2 Ma from the Jemez Mountains volcanic field, western USA. Zircon samples from two separate eruptions have continuous SHRIMP U–Pb age spectra spanning 0.84–1.08 Myr duration (3.12–3.96 Ma and 3.50–4.58 Ma, respectively), following an episode of zircon crystallization 0.28–0.50 Myr earlier (at 4.46 Ma and 4.86 Ma, respectively). Zircon chemical variations, as well as ubiquitous resorption textures that commonly show large core-rim age differences (up to 720–740 kyr), suggest that they grew in separate melt lenses. Zircons were likely stored at near-solidus or even sub-solidus conditions after crystallization, but may have been reactivated in response to at least four major magma recharge events every 300–400 kyr and smaller events in between. A cycle of zircon dissolution (from heating), recrystallization (during cooling), and storage repeated in different locations in the Tschicoma mush system throughout its lifespan; each recharge-induced heating stage may last for several hundred to more than a thousand years based on calculations of zircon dissolution. We envisage the melt lenses to be distributed in a crystal mush zone, coalescing into a single magma batch as magma recharge occurs shortly before eruption. Once active, increasing magma supply rates may trigger large-scale partial melting of the pre-existing mush and caldera-forming eruptions. [ABSTRACT FROM AUTHOR]

Late'iki (previously known as Metis Shoal) is a highly active volcano in the Tofua arc with at least four temporary island-building eruptions and one submarine eruption in the last 55 years. The most recent eruption, commencing in October 2019, resulted in lava effusion and subsequent phreatic explosions, the construction of a short-lived island that was quickly eroded by wave action and possibly further phreatic activity that continued into January 2020. The two-pyroxene dacite from the 2019 eruption is similar to the 1967/8 eruptions suggesting the magma is residual from earlier eruptions and has not undergone further differentiation in the last 50 years. New observations of the 2019 eruption site confirm the lava-dominant character of the volcano summit but a thin veneer of wave-reworked, finely fragmented lava material remains that is interpreted to have been produced by phreatic explosions from hot rock-water interactions during the effusive eruption. A notable absence of quench-fragmented hyaloclastite breccias suggests that non-explosive quench fragmentation processes were minimal at these shallow depths or that hyaloclastite debris has resedimented to greater depths beyond our summit survey area. [ABSTRACT FROM AUTHOR]

Fragmentation is a common feature of rockfall that exerts a strong control on the trajectories of the generated blocks, the impact energies, and the runout. In this paper, we present a set of four real-scale rockfall tests aimed at studying the fragmentation of the rocky blocks, from the global design of the field procedure to the data analysis and the main results. A total of 124 limestone, dacite, or granite blocks ranging between 0.2 and 5 m3 were dropped from different heights (8.5 to 23.6 m) onto four slopes with different shapes (single or double bench) and slope angles (42º to 71º). The characteristics of the blocks, in particular the size, surface texture and joint condition, were measured before the drops. The trajectories of the blocks and both the initial and the impact velocities were tracked and recorded by means of three high-speed video cameras. A total of 200 block-to-ground impacts have been studied. On average, 40% of the blocks broke upon impact on the slope or on the ground, making it necessary to measure the fragments. The initial and final sizes of the blocks/fragments were measured by hand with tape, though photogrammetric techniques (UAV and terrestrial) were also used for comparison purposes. The information gathered during the field tests provides a deep insight into the fragmentation processes. On the one hand, the high-resolution slow-motion videos help to describe when and how the block breakage takes place and the spatial distribution of the pieces. On the other hand, it is possible to compute the block trajectories, the velocities, and the energy losses using videogrammetry. The results include, for instance, a block average fragmentation of 54% and 14% for the limestone and granitoids, respectively; the systematic inventory of the size fragments, which may be used for fitting the power law distributions; and after each breakage, the total angle of aperture occupied by the fragments has been measured, with values in the range 25º–145º. To figure out the different behavior of the blocks in terms of breakage/no breakage, each block-to-ground impact has been characterized with a set of parameters describing the energy level, the robustness of the substrate, and the configuration of the block contact at the impact point, among others. All these terms are combined in a function F, which is used to adjust the field data. The adjustment has been carried out, first, for the whole 200 events and later for a subset of them. The procedure and the results are described in the paper. Although the discrimination capability of F is moderately satisfactory, it is very sensitive to the test site and setup. It must be highlighted that these field tests are a unique source of data to adjust the parameters of the numerical simulation models in use for rockfall studies and risk mitigation, especially when fragmentation during the propagation is considered. [ABSTRACT FROM AUTHOR]

Permo-Carboniferous felsic magmatic rocks are widely exposed on the northern half of the south-eastern Central Asian Orogenic Belt. However, their origin and spatiotemporal distribution are unknown. Herein, zircon U-Pb-Hf isotopic data, major and trace element compositions of the whole-rock, and Sr-Nd isotopic data derived for felsic volcanic rocks and granites of the Chaganaobao and Baiyinaobao areas in central Inner Mongolia are combined with the data from previous studies. The integration of these results reveals that the Uliastai Continental Margin, Erenhot-Hegenshan Ophiolite Belt, and Northern Orogenic Belt have coeval magmatic lull or trough at c. 307 Ma, which overlap with the closure timing of the Hegenshan Ocean. The Latest Carboniferous−Early Permian (c. 303−295 Ma) felsic volcanic rocks in the Chaganaobao area of the Uliastai Continental Margin comprise dacite, trachydacite, and rhyolite, while demonstrating positive whole-rock εNd(t) (+2.9 to +4.7) and zircon εHf(t) values (+7.65 to +13.78). Early Permian granites of the Baiyinaobao area located within the Northern Orogenic Belt show positive εHf(t) values (+ 2.70 to +11.33) and low TFe2O3 + MgO + TiO2 contents. The geochemical composition reveals that those felsic magmatic rocks mainly originate from the partial melting of juvenile crustal components with subordinate input of mantle-derived melts. Those c. 305−275 Ma felsic magmatic rocks from the north of the south-eastern Central Asian Orogenic Belt exhibit affinities of post-collision type granites in geochemical nature. Therefore, the flare-up of the Early Permian magmatism surrounding the Erenhot-Hegenshan Ophiolite Belt could be related to the extension after the final closure of the Hegenshan Ocean. [ABSTRACT FROM AUTHOR]

New trace-element, radiogenic Sr-Nd-Pb isotopic and geochronological data from Middle-Late Cretaceous Zagros ophiolites of Iran give new insights into the tectono-magmatic history of these supra-subduction zone (SSZ)-type ophiolites. The distribution of Middle-Late Cretaceous SSZ-type ophiolites in Iran comprises two parallel belts: (1) the outer Zagros ophiolitic belt and (2) the inner Zagros ophiolitic belt. These Middle-Late Cretaceous ophiolites were generated by seafloor spreading in what became the fore-arc and back-arc during the subduction initiation event and now define a ~3000-km-long belt from Cyprus to Turkey, Syria, Iran, the UAE, and Oman. The Zagros ophiolites contain complete (if disrupted) mantle and crustal sequences. Mantle sequences from both outer-belt and inner-belt ophiolites are dominated by dunites, harzburgites, and lherzolites with minor chromitite lenses. Peridotites are also intruded by gabbros and a variety of mafic to minor felsic (plagiogranite and dacite) dikes. Crustal rocks comprise ultramafic-mafic cumulates as well as isotropic gabbros, sheeted dike complexes, pillowed and massive lavas, and felsic rocks. Our new zircon U-Pb ages indicate that the outer-belt and inner-belt ophiolites formed near coevally during the Middle-Late Cretaceous; 100-96 Ma for the outer belt and 105-94 Ma for the inner belt. Both incompatible-element ratios and isotopic data confirm that depleted mantle and variable contributions of subduction components were involved in the genesis of outer-belt and inner-belt rocks. Our data for the outer belt and inner belt along with those from better-studied ophiolites in Cyprus, Turkey, the UAE, and Oman lead to the conclusion that a broad, ~3000-km-long swath of fore-arc lithosphere was created during Middle-Late Cretaceous time. [ABSTRACT FROM AUTHOR]

The Sanjiang Indosinian orogen, located in the eastern part of the Paleo-Tethys tectonic domain, is a critical region to study the Paleo-Tethyan Ocean evolution. Middle Permian–Late Triassic magmatic rocks are widespread in the Deqin–Weixi–Madeng area of southwestern (SW) Sanjiang Indosinian orogen, yet their petrogenesis and tectonic setting remain disputed. In this study, LA-ICP-MS zircon U-Pb age and Hf isotopes, and whole-rock elemental and Sr-Nd isotope geochemistry of Madeng dacite were studied. The Madeng dacite was dated at ca. 241.7 and 243.4 Ma. The samples had high Al2O3 (12.91 to 14.39 wt.%) but low MgO (0.62 to 1.76 wt.%) contents, and were alkali-rich (Na2O + K2O = 6.97 to 8.66 wt.%) with A/CNK > 1.1, strongly resembling peraluminous S-type granites. The rocks were enriched in Rb, K, Th, U and LREE, but depleted in Ba, Sr, Nb, Ta, P and Ti, and showed obvious negative Eu anomalies, suggesting fractionation of Ti-bearing minerals (e.g., rutile and ilmenite) and plagioclase. The dacite had an initial 87Sr/86Sr value of 0.705698 to 0.710277, and negative εNd(t) (−11.28 to −10.64) and εHf(t) (−13.99 to −8.60), indicating a continental meta-sedimentary source. Their average Nb/Ta (12.24) and Th/U (4.65) were also consistent with continental crust. According to the lithological assemblage and geochemical features, we propose that the Deqin–Weixi–Madeng area intermediate-felsic magmatism was generated in a subduction-related tectonic setting. [ABSTRACT FROM AUTHOR]

In Central Asia, the Carboniferous is a crucial period in the formation of the Tianshan Belt and associated bending of the Kazakhstan tectonic collage. In order to reveal Carboniferous magmatic events of the region and their tectonic implications, we conducted field investigations, zircon U–Pb dating, whole-rock geochemical and Sr–Nd isotopic studies on the Early Carboniferous Dahalajunshan Formation and Late Carboniferous Yishijilike Formation volcanic rocks of the Wusun Mountain Range (southern Yili-Central Tianshan Block). Volcanic rocks of the Dahalajunshan Formation consist of calc-alkaline basalt, andesite and dacite, yielding new zircon U–Pb ages of ~ 350 Ma. They have positive whole-rock εNd(t) values (+ 0.5 to + 1.6). In contrast, the Yishijilike Formation volcanic rocks dominantly comprise alkaline and calc-alkaline bimodal suites that erupted at ~ 337 Ma to 313 Ma and have higher whole-rock εNd(t) values (+ 2.3 to + 4.3). These two episodes of Carboniferous magmatism were correlated with partial melting of depleted mantle that metasomatized by slab-derived fluids. The late Carboniferous Wusun Mountain magmatic belt shows characteristics of a back-arc system that evolved due to trench retreat relative to the southern margin of the Yili-Central Tianshan Block. This mechanism induced an extensional regime with gradually depleting magma sources. The asymmetric retreat of the paleo-subduction zones of the South Tianshan Ocean and Junggar Ocean relative to the Yili-Central Tianshan Block was hence a vital driving force for the bending of the Kazakhstan Orocline. [ABSTRACT FROM AUTHOR]

The present study aimed to evaluate the ability of a mixture of gabbro powder and dacite powder to promote plant growth, provide nutrients and alter soil chemical characteristics. Two experiments were conducted under different field conditions, in which black oat and beans were added with increasing doses (0 to 10 t ha-1) of the powder rock blend. The grain yield (beans), dry matter production (oats) and the nutritional status of the crops were evaluated. At the end of the experiments, soil samples were collected and subjected to chemical analysis. The application of the rocks resulted in an increase in bean productivity corresponding to 440 kg ha-1 at the dose of 7 t ha-1 in relation to the control. The application of the blend also resulted in an overall improvement in most commonly fertility parameters used, although this improvement was more evident only for phosphorus, copper, and zinc nutrients. In addition, the application of the rock mixture resulted in an increase in the foliar contents of potassium, zinc, copper, and manganese nutrients. Therefore, the material obtained by the mixture of gabbro and dacite, in the proportion of mixture tested, can be considered a multi-nutrient source with evident agronomic viability. [ABSTRACT FROM AUTHOR]

This study presents petrographic and geochemical characterization of 46 pre-impact rocks and 32 impactites containing and/or representing impact melt rock from the peak ring of the Chicxulub impact structure (Yucatán, Mexico). The aims were both to investigate the components that potentially contributed to the impact melt (i.e., the pre-impact lithologies) and to better elucidate impact melt rock emplacement at Chicxulub. The impactites presented here are subdivided into two sample groups: the lower impact melt rock--bearing unit, which intrudes the peak ring at different intervals, and the upper impact melt rock unit, which overlies the peak ring. The geochemical characterization of five identified pre-impact lithologies (i.e., granitoid, dolerite, dacite, felsite, and limestone) was able to constrain the bulk geochemical composition of both impactite units. These pre-impact lithologies thus likely represent the main constituent lithologies that were involved in the formation of impact melt rock. In general, the composition of both impactite units can be explained by mixing of the primarily felsic and mafic lithologies, but with varying degrees of carbonate dilution. It is assumed that the two units were initially part of the same impact-produced melt, but discrete processes separated them during crater formation. The lower impact melt rock--bearing unit is interpreted to represent impact melt rock injected into the crystalline basement during the compression/excavation stage of cratering. These impact melt rock layers acted as delamination surfaces within the crystalline basement, accommodating its displacement during peak ring formation. This movement strongly comminuted the impact melt rock layers present in the peak ring structure. The composition of the upper impact melt rock unit was contingent on the entrainment of carbonate components and is interpreted to have stayed at the surface during crater development. Its formation was not finalized until the modification stage, when carbonate material would have reentered the crater. [ABSTRACT FROM AUTHOR]

Our understanding of magmatism in the New Zealand North Island subduction system is limited by sparse knowledge of the structure and composition of the deep arc crust, particularly in the northern Taupo Volcanic Zone (NTVZ), where there are multiple major tectono-magmatic transitions impacting the crust. Herein, we use detailed textural and micro-chemical analyses from multiple crystalline phases and melt inclusions to probe the magmatic processes occurring in the lower crust that ultimately control the early evolution of magmas beneath Pūtauaki (Mt. Edgecumbe) in the NTVZ. Here, two-pyroxene-plagioclase glomerocrysts in dacite lavas have textures, compositions, and phase associations consistent with a lower crustal cognate cumulate origin. Importantly, the glomerocrysts record a multi-stage evolution in the lower crust. Elevated 87Sr/86Sr isotope ratios (0.7058–0.7062) in high-An (up to An92) plagioclase, along with relatively low Mg/Fe ratios in olivine and pyroxene within the glomerocrysts, record the deep assimilation of isotopically enriched lower crust and the significant fractionation of Mg-rich phases, near the base of the crust. The evolved melts appear to have then ascended into the lower crust and subsequently crystallized the gabbroic mineral assemblage preserved in the xenoliths. Two pyroxene thermobarometry indicates that crystallization/equilibration occurred at temperature between 926 and 1001 °C and pressure between 4 and 7 kbars, consistent with crustal depths of 15–26 km. Melt inclusions hosted within pyroxenes in glomerocrysts are rhyodacitic to rhyolitic (68–74 wt% SiO2) indicating that fractionation of the glomerocryst assemblage resulted in the production of melts. We propose that these represent melts that ascend into the upper crust and fractionate to produce rhyolites that erupt from the large calderas of the central TVZ (CTVZ). Our findings also indicate that crustal architecture and magma pressure–temperature–time pathways beneath the NTVZ are similar to magmatic systems in the STVZ, but distinct from those beneath the large rhyolitic systems in the central TVZ. These differences may arise from variations in mantle flux and rates of extension along the arc. [ABSTRACT FROM AUTHOR]

Primitive arc magmas provide our closest glimpse of the original mantle-derived magmas that produce the more ubiquitous andesites and dacites found in subduction zones and that ultimately construct Earth's continental crust. This study examines the crustal storage and ascent history of the Mt. Shasta primitive magnesian andesite (PMA), a demonstrated parent magma for the voluminous mixed andesites erupted at Mt. Shasta. Our petrographic and geochemical observations of the PMA identify a mid-crustal magma mixing event recorded in multiple populations of reversely zoned clinopyroxene and orthopyroxene phenocrysts. Thermobarometric calculations conducted as part of this study and prior phase equilibrium experiments (Grove et al., Contrib Miner Petrol 145:515–533, 2003; Krawczynski et al., Contrib Miner Petrol 164:317–339, 2012) suggest the PMA experienced storage, mixing, and subsequent crystallization at ~ 500 MPa and ~ 975 °C. Modeling of Fe–Mg interdiffusion between the rims and cores of the reversely zoned pyroxenes suggests this mixing event and the resulting crystal rim growth occurred less than 10 years prior to eruption ( 2.9 - 2.2 + 6.4 ). Ascent from 500 MPa (~ 15 km) during the calculated diffusion timescales suggests minimum crustal transit rates of ~ 170 MPa (~ 5 km)/year and cooling rates of ~ 5–7 °C/km, consistent with conductive cooling models. This ascent rate is slower than the handful of previously documented trans-crustal magmatic ascent rates and significantly slower than syn-eruptive decompression rates. If this behavior is representative, ~ the 10% mafic magmas erupted as part of the modern Mt. Shasta edifice fluxed through the crust within decades. Coupled with a review of the U–Th–Ra residence times for Shasta andesites to dacites, we suggest that crustal magma flux and assembly beneath modern Mt. Shasta occurred in discrete pulses that occupy a minority of the 700 k.y. period of edifice construction. The results of this study thus constrain the pre-eruptive history and ascent characteristics of a hydrous primitive arc magmas in the upper crust between their shallowest storage region in the mid-crust and volatile exsolution and provide constraints on crustal magma flux beneath continental arc volcanoes. Should future earthquake swarms indicative of magma movement in the middle to upper crust occur beneath Shasta, the results presented here also provide the first estimates of the possible magma ascent rates and the time intervals that could accompany related magma ascent to eruption at Mt. Shasta. [ABSTRACT FROM AUTHOR]

• δ26Mg variation in lavas from the EPR is resolved within 0.03‰ uncertainty (tSE). • Mg isotopes can be significantly fractionated during fractional crystallization. • Δ26Mg Ol-melt , Δ26Mg Cpx-melt , and Δ26Mg Ti − Mgt-melt are estimated. • δ26Mg deviation should be considered when tracing mantle source with evolved MORBs. The magnesium (Mg) isotope composition of oceanic basalts has provided useful constraints on the evolution of the upper mantle and crust-mantle interactions. However, the behavior of Mg isotopes during oceanic magma differentiation is still unclear because of the small range in Mg isotope values in typical normal mid-ocean ridge basalts (N-MORB). Here, we present high-precision Mg isotope data on a well-characterized suite of mid-ocean ridge (MOR) lavas from the 9–10°N segment of the East Pacific Rise. These samples range from relatively primitive basalt to evolved dacite with MgO contents decreasing from 8.62 to 0.80 wt.%, and display a resolvable variation in δ26Mg from -0.27 to -0.17‰. The less-evolved samples (MgO > 7.0 wt.%) that have experienced olivine and plagioclase fractional crystallization have δ26Mg values ranging from -0.23 to -0.17‰ that are negatively correlated with MgO content. Samples containing MgO of 3.5–7.0 wt.%, that have experienced significant crystallization of clinopyroxene, together with plagioclase, show a limited variation of δ26Mg (-0.20 to -0.18‰). For those highly evolved samples (MgO < 3.5 wt.%) saturated with Fe–Ti oxides, the δ26Mg vary from -0.27 to -0.20‰ and are positively correlated with MgO content. The variation of δ26Mg in the MOR lavas is consistent with three stages of magma differentiation, beginning with fractional crystallization of olivine and plagioclase followed by increasing amounts of clinopyroxene and finally joined by Fe-Ti oxides. Quantitative modeling of the Mg isotopic variation in EPR samples, shows that the variation of δ26Mg in MOR lavas is primarily a consequence of fractional crystallization of the Mg-bearing minerals when Δ26Mg Ol-melt ∼ -0.10‰, Δ26Mg Cpx-melt ∼ 0.00‰, and Δ26Mg Ti − Mgt-melt ∼ 0.20‰. This study provides evidence that shallow level crystal fractionation can produce significant and predicable variations in Mg isotopic compositions of MOR lavas and that this process should be carefully considered when using evolved lavas to trace mantle source compositions. [ABSTRACT FROM AUTHOR]