Trachytic Magma Research Articles

A Trachytic Monogenetic Volcano in the Changbaishan Volcanic Field, Xitudingzi Volcano: Petrogenesis, Magma Plumbing System and Implications for Non-basaltic Monogenetic Volcanism

Abstract The Changbaishan volcanic field is the largest Quaternary volcanic field in China and includes the most dangerous polygenetic volcano, Tianchi volcano. In addition to the Tianchi volcano, the Changbaishan volcanic field contains two other polygenetic volcanoes (Wangtian’e volcano and Namphothe volcano) and at least 150 monogenetic volcanoes. The compositions of these monogenetic volcanoes range from basalt to rhyolite. Basaltic monogenetic volcanoes are the most abundant and spread throughout the volcanic field, whereas intermediate to evolved monogenetic volcanoes are found only within the Touxi subfield. We know very little about these non-basaltic monogenetic volcanoes, especially the magmatic petrogenesis, volcanological origins and structure of the magma plumbing system beneath them. Therefore, in this study, the Xitudingzi volcano within the Touxi subfield is used as an example to study the petrogenesis of silicic magma and explore the structure of the magma system by combining new 40Ar/39Ar dating, whole-rock geochemistry, Sr-Nd isotopes, and mineral chemistry. The 40Ar/39Ar chronology results showed that the Xitudingzi volcano erupted at 1.17 Ma; this volcano is younger than the other monogenetic volcanoes in the Touxi subfield. The geochemical results indicated that the eruption products of the Xitudingzi volcano are trachytic and formed from the assimilation fractional crystallization (AFC) process of an alkaline basaltic parental magma. The results from the Rhyolite-MELTS and thermobarometry indicated that the magma plumbing system of the Xitudingzi volcano contained two magma chambers at depths of 8 km and 19 km, respectively. The trachytic magma of Xitudingzi was produced by the AFC process from the alkaline basaltic magmas in the deep magma chamber and then transported to the shallow magma chamber. Furthermore, differences in the trace element concentrations between the different units of the Xitudingzi volcano indicated that the shallow pre-eruptive magma chamber was compositionally zoned. Based on the results from our study, the upper crustal magma plumbing system of the Xitutingzi volcano existed independently of the magma plumbing system of the cone-construction stage of the Tianchi volcano, even though the two volcanoes had similar ages of activity as well as the same chemical composition of eruptive products. Based on these results and other existing studies, a relatively thick crust and the developed faults are likely the key factors for the formation of intermediate to evolved monogenetic volcanos in the Touxi volcanic field.

Chemical-memory of biotite crystals preserves the temperature and pressure conditions of the mixing process between trachytic and rhyolitic magmas in the Glória Sul stock, Sergipano orogenic System, Northeast Brazil

Biotite is an important constituent in the microgranular syenitic enclaves of the Glória Sul stock, located in the central region of the Sergipano Orogenic System, Northeast Brazil. These enclaves are centimeter-sized, rounded, and ellipsoidal, finer-grained than the surrounding granites. They are composed of microcline, orthoclase, albite-oligoclase, diopside, quartz, hornblende, titanite, apatite, zircon, epidote, magnetite, ilmenite, thorite, and pyrite. The textures present in the enclaves indicate mixing between magmas and rapid cooling of the trachyte magma that formed the microgranular enclaves (fine-grained texture, acicular apatite and zircon, blade-shaped biotite). The studied biotite exhibits a magmatic texture and is chemically classified as Fe- and Mg-biotite, particularly those with TiO2 > 2.3%. The chemical evolution of biotite is controlled by two substitutions (VI R2+ + 2 VI Al3+ ═ VI □ + 2 VI Si4+ and VI R2+ + VI Ti ═ 2 VIAl3+) reflecting a decrease in titanium and an increase in aluminum. The chemical data of biotite crystals indicate that they crystallized from I type magma, with calc-alkaline affinity, suggesting that the rocks of Gloria Sul stock were formed by the interaction between mantle- and crustal-derived magmas. Thermobarometry of magmatic biotite (enclaves and granites) provided crystallization temperatures between 900 and 800 °C and a depth of 36 to 13 km. The interaction between trachytic (enclave) and rhyolitic (granite) magmas in Glória Sul stock at occurred at about 26 km depth and at a temperature about 800 °C.

The magma plumbing system of the potentially hazardous Laoguipo volcano in the Tengchong Volcanic Field, SW China

The Late Pleistocene-Holocene Laoguipo volcano in the Tengchong Volcanic Field (TVF), southwestern China, displays significant geochemical and geophysical anomalies characteristics. Here we present petrographic observations, mineral chemistry, bulk rock geochemistry, thermobarometry, and thermodynamic simulation to evaluate the crystallization conditions and pre-eruptive magmatic processes occurring within the magma plumbing system. This study reveals the existence of two magma reservoirs beneath the Laoguipo volcano. The deep magma reservoir is composed of basaltic trachyandesite (SiO2 = 54–57 wt%), which is located at 15–19 km depths with 1087–1160 °C, 1.5–2 wt% H2O content, oxygen fugacity of ΔNNO+1 (Ni-NiO buffer), melt viscosity of 101.7–102.6 Pa·s, and density of 2.5–2.6 g/cm3. The formation of the deep magma reservoir is attributed to the 31% mass fractional crystallization of primitive basalt in the TVF. The shallow magma reservoir is composed of trachyte (SiO2 = 63–64 wt%), which is located at 6–11 km depths with 780–825 °C, 5.9–6.5 wt% H2O content, oxygen fugacity of ΔNNO+1 (Ni–NiO buffer), melt viscosity of 103.9–104.8 Pa·s, and density of 2.2–2.3 g/cm3. The shallow magma reservoir formed after the basaltic trachyandesite had assimilated 19% mass of the upper crustal material and fractionated 41% mass of the crystals. This study suggests that the shallow trachyte magma reservoir is being heated by the ascending deep basaltic trachyandesite magma, resulting in crystal dissolution, remobilization of crystal mush, and magma convection, which may be the main factors responsible for the geochemical and geophysical anomalies characteristics. The Laoguipo volcano is forming a mature magma plumbing system, which is of significance for forecasting future volcanic eruptions.

Magmatic processes forming replacement textures with fluorite alignments in feldspars in an evolved trachyte from Oki‐Dogo Island, Sea of Japan

AbstractInternal microtextures of ternary alkali feldspars in sanidine trachyte from Oki‐Dogo Island were examined using an electron microprobe analyzer, a scanning electron microscope, a transmission electron microscope and cathodoluminescence instruments, to develop the understanding of volcanic processes of alkaline magmas related to feldspar crystallization. The examined trachyte is an evolved rock of the Oki‐Dogo Pliocene trachyte group. Its phenocryst feldspars are commonly associated with lamellar‐wavy‐domain textures with scales approximately from 100 nm up to several hundreds of μm that show complex and gradual variations in composition: however, anti‐rapakivi zoning textures common in other Oki‐Dogo alkaline rocks are almost completely absent in the trachyte. These textures are produced by extensive magmatic ion‐exchange replacement reactions progressively advanced in the evolved magma. Characteristic braided fluorite alignments are developed consistently with lamellar‐wavy‐domain textures in phenocryst feldspars, and similar braided alignments are also present in groundmass feldspars with complicated microtextures. Most of fluorite grains are <100 nm in diameter, and the patterns of braided fluorite alignments vary greatly in individual feldspars. The whole occurrence of the feldspar microtextures represents an extreme example of diffusion‐controlled replacement reactions, progressively advanced in the dry (relatively anhydrous) trachyte magma. The genetic processes forming fluorite alignments in feldspars are related to magma compositions, especially F and P contents, and the crystallization of F‐bearing minerals, especially of fluorapatite.

Apatite Geochemical and Nd Isotopic Insights into Trachyte Petrogenesis in the Tianchi Volcanic Area of Changbai Mountain, NE China

AbstractWe report the oxide, element geochemistry and Nd isotopic geochemical data of apatite in the middle Pleistocene medium‐ and fine‐grained trachyte in the Tianchi volcanic area (TVA) of Changbai Mountain, discussing the relationship between apatite and the composition of the whole rock. The purpose is to use the apatite geochemical data to constrain the evolutionary process of trachytic magma and the petrogenesis of trachyte in the cone‐forming period of the Tianchi volcano. Apatite (Ca5(PO4)3(OH, F, Cl)) is a common accessory mineral that occurs widely in volcanic rocks in the TVA. The apatites in the trachyte are mainly subhedral–anhedral, having the characteristics of magmatic apatite. In terms of oxide and element geochemistry, they have homogeneous Al2O3, SiO2, MgO, P2O5, K2O, CaO and heterogeneous TiO2, with high F content. They are generally enriched in Th, U and LREEs, depleted in Nb, Ta, Zr, Hf and HFSEs, showing negative Ba, Sr and Ti anomalies, similar to those of the whole‐rock host trachytes. The ratios of high (La/Yb)N, low δEu (Eu/Eu*), Sr/Y value and ΣREE content in apatite, and the F, Sr, Y, Th/U, La/Sm, and Nd/Tb with ΣREE and δEu anomalies showed a linear correlation, all of those indicating that the host magma has the characteristic of high differentiation. The apatite grains generally having 147Sm/144Nd, 143Nd/144Nd ratios and εNd(t) values of 0.1072–0.1195, 0.5123–0.5126 and –3.49 to –0.10, respectively, are similiar to those of the host rock. The Nd model ages TDM1 are 949–803 Ma in apatite. Combined with the εNd(t) value of the apatite core (–7.06 to –3.49), we conclude that the initial magma of the host trachyte was derived from the partial melting of Proterozoic crustal material and there was an assimilation of wall rocks during its evolution.

Delineation of shallow volcanic structures from audio-frequency magnetotelluric data beneath Ulleung Island, East Sea (Sea of Japan)

As volcanoes are closely related to the living environment of humans, in particular via natural hazards and geothermal energy, scientific studies on volcanic edifice structures are required. Ulleung Island is a Quaternary volcanic island located in the mid-western East Sea (Sea of Japan). In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to image the substructure of the Ulleung volcanic edifice. In addition, the electrical structure was interpreted from the geothermal system perspective, geochemical compositions of volcanic rocks, and the possibility of the presence of a magma reservoir. AMT data were obtained from 25 stations and processed using the remote reference technique. Then, the three-dimensional (3-D) approach was reasonably adopted according to dimensionality analysis. Before conducting the 3-D inversion, the effects on topography and ocean were analyzed using a simplified 3-D synthetic model, because Ulleung Island is surrounded by sea and the topography is undulating. Most AMT stations on Ulleung Island are distorted by topographical and oceanic effects; in particular, oceanic effects are significant at frequencies lower than 10 Hz. The 3-D inversion was conducted with full impedance components and vertical magnetic transfer functions in a frequency range of 10,000–1 Hz. The results show that the Ulleung volcanic edifice is characterized by two layers of electrical structures as follows: near-surface resistive anomalies and an underlying conductive layer. Considering the high geothermal gradient on Ulleung Island, we suggest that the conductive layer of the volcanic edifice is due to hydrothermal alteration of basaltic rocks, with a potential heat source underneath. Based on the geochemical characteristics of the Ulleung volcanic rocks, the possibility of heat transfer from a trachytic magma reservoir within the shallow crustal is suggested. In summary, this study presents a geological interpretation of shallow volcanic structures beneath Ulleung Island and the possibility of an active magma reservoir as a potential heat source for the volcanic edifice.Graphical

Modelling the post-caldera plumbing system of Changbaishan volcano (China) from integrated geochemical, isotopic, geobarometric, and geophysical data

The Changbaishan Tianchi volcano is the largest active volcano of China and the 946 CE VEI 6 “Millennium eruption” (ME) is responsible for the formation of its summit caldera. After the ME, the volcanic activity is characterized by smaller eruptions most of which are recorded only in historical documents. Here we analyze the deposits of the ME and 1403 CE eruption outcropping in the Heishigou section, which is located east of the Chagbaishan Tianchi caldera. We present whole-rock, glass, mineral, isotopic, and geobarometric data to obtain information on the vertical extent of the caldera and post-caldera plumbing system of the volcano. Results show that the plumbing system of the 1403 CE eruption is located between 1.9 and 5.4 km depth, which is also the depth range of the shallower reservoir of the ME. Therefore, the magma reservoir of the ME was active also after the caldera collapse. The 1403 CE magma stored in the ME reservoir evolved by fractional crystallization of the residual ME trachytic magma and underwent the contamination of upper crust material. The 1403 CE event was triggered by the injection of less evolved, mafic magma from depth. The inferred depth of the shallower ME reservoir and that of the 1403 CE eruption is the same of the present-day main geophysical anomalies (low seismic velocity and low resistivity) recorded below the Changbaishan Tianchi caldera. In addition, the source of the deformations related to the uplift during unrest episodes between 2002 and 2020 suggests a refilling of the 1403 CE plumbing system. We conclude that the plumbing system of the 1403 CE, post-caldera activity at Changbaishan Tianchi volcano is the same of the ME eruption and it is still active. This system may be recharged from the arrival of less evolved magma from depth. The proposed conceptual model of the post-caldera volcanism at Changbaishan Tianchi represents a reference framework to interpret the signals of unrest and decipher the dynamics of the present-day plumbing system.

An isotopically enriched mantle component in the source of Rodrigues, Réunion volcanic hotspot

The Mascarene Islands in the western Indian Ocean, encompassing La Réunion, Mauritius, and Rodrigues, are the recent (<10 Ma) surface expressions of the Réunion hotspot. Ocean island basalts (OIB) from these islands exhibit a remarkably homogeneous long-lived radiogenic isotopic composition, coinciding with the convergence field of many global OIB trends in the mantle array. Réunion plume-related OIB therefore provide one of Earth’s most pristine representations of this “focal zone” component, which may have a primordial heritage. Besides this signature, Mascarene lavas have been suggested to retain contributions from sources with distinct compositions, including: (1) Archaean-aged zircons assimilated from continental crust within the oceanic lithosphere by trachytic magmas from Mauritius; (2) more deeply recycled continental crust components preserved by elevated 87Sr/86Sr and 208Pb/206Pb in lavas from the Piton des Neiges volcano of Réunion; and (3) an isotopically depleted mantle component resulting from interaction with Central Indian Ridge material.In this study we use Sr-Nd-Pb isotope systematics, along with major and trace element compositions of basaltic lavas from all three Mascarene Islands to investigate the relationship of their sources to well-characterized mantle endmembers. Among the Mascarene Islands, Rodrigues lavas are the most enriched in highly incompatible elements, likely reflecting shallower and lower degrees of partial melting than Réunion or Mauritius. Combined Sr-Nd-Pb isotope compositions indicate that lavas from the Older Series of Mauritius resemble those from Réunion, whereas lavas from the Younger and Intermediate Series, together with Rodrigues, are consistent with contributions from an isotopically depleted component. In addition, the Pb isotopic compositions of Rodrigues samples require an additional contribution from a component with a long-term enrichment in its Th/U ratio. Based on isotope mixing models, direct assimilation of continental crust embedded within the oceanic lithosphere is unlikely to account for the Pb isotopic variation of Rodrigues. A metasomatized mantle component, previously envisaged as a “fossil” Réunion plume near the Central Indian Ridge, is partially able to reproduce the trace element signature, but not the observed Sr-Nd-Pb isotopic compositions of Rodrigues. Instead, small proportions (<5%) of an EM1-like component provide a preferred endmember. The composition and origin of this component, which is exclusively reflected within Rodrigues lavas, may constitute a new geochemical feature of the Mascarene Islands and is consistent with geodynamical predictions that small-scale enriched mantle domains may be widespread in the source regions of hotspot volcanoes.

Historical explosive activity of Mount Melbourne Volcanic Field (Antarctica) revealed by englacial tephra deposits

Five tephra layers named BRH1 to 5 were sampled in an ice cliff located on the north-eastern flank of Mount Melbourne (northern Victoria Land, Antarctica). The texture, componentry, mineralogy, and major and trace element compositions of glass shards have been used to characterize these layers. These properties suggest that they are primary fall deposits produced from discrete eruptions that experienced varying degrees of magma/water interaction. The major and trace element glass shard analyses on single glass shards indicate that Mount Melbourne Volcanic Field is the source of these tephra layers and the geochemical diversity highlights that the eruptions were fed by compositionally diverse melts that are interpreted to be from a complex magma system with a mafic melt remobilizing more evolved trachy-andesitic to trachytic magma pockets. Geochemical compositions, along with textural and mineralogical data, have allowed correlations between two of the englacial tephra and distal cryptotephra from Mount Melbourne, recovered within a marine sediment core in the Edisto Inlet (~ 280 km northeast of Mount Melbourne), and constrain the age of these englacial tephra layers to between the third and the fourth century CE. This work provides new evidence of the intense historical explosive activity of the Mount Melbourne Volcanic Field and better constrains the rates of volcanism in northern Victoria Land. These data grant new clues on the eruptive dynamics and tephra dispersal, and considerably expand the geochemical (major and trace elements) dataset available for the Mount Melbourne Volcanic Field. In the future, this will facilitate the precise identification of tephra layers from this volcanic source and will help define the temporal and spatial correlation between Antarctic records using tephra layers. Finally, this work also yields new valuable time-stratigraphic marker horizons for future dating, synchronization, and correlations of different palaeoenvironmental and palaeoclimatic records across large regions of Antarctica.

Explosive volcanism of Piton des Neiges (Reunion Island) and excess age dispersion in sanidine: Insights into magma chamber processes in a hotspot setting

Explosive eruptions of trachytic to trachyandesitic composition produced thick and widespread pyroclastic deposits during the recent volcanic activity of Piton des Neiges (PdN), the largest volcano of La Réunion Island (France). However, without precise geochronology for these eruptions, the future behavior of the dormant volcano is difficult to assess. New high-resolution single crystal 40Ar/39Ar dates on alkali feldspar, together with petrochemical data on juvenile clasts, indicate that instead of multiple and small eruptions, PdN produced a single large-volume event (“Dalle Soudée” eruption) at 198.8 ± 2.5 kyrs in which its entire crustal plumbing system was reactivated by mafic injections in the silicic reservoir. After 60 kyrs of quiescence, PdN activity resumed with several small silicic eruptions. From a dome-forming trachytic event (Belouve eruption) represented by some of the most recent block-and-ash flow deposits near the present-day summit we report in-situ 238U/230Th disequilibrium dates on pristine zircons. Sanidine and zircon crystalized at ca. 875 °C in the trachytic magma. For this event, the corresponding 238U/230Th isochron age of the zircons overlaps with the youngest 40Ar/39Ar dates on single sanidine crystals and yields an age of 42.7 ± 3.8 kyr. The variability in trace elements of zircon argues for a prolonged crystallization during this phase prior to eruption. Similarly, the 40Ar/39Ar sanidine dates show a significant spread in both eruptions, covering ca. 80 kyrs, uncommon for a method with a predicted low closure temperature. Such a pattern of age distribution argues for the presence of inherited argon in sanidines and for long residence times in the magma reservoir. The recent eruptive history of Piton des Neiges, involving evolved magma compositions, long-living silicic reservoirs and millennia-long quiescence in volcanic activity, suggests that future potentially hazardous eruptions from this volcano cannot be excluded.

Cogenetic Origin of Magmatic Enclaves in Peralkaline Felsic Volcanic Rocks from the Sanshui Basin, South China

Centimeter-scale magmatic enclaves are abundant in peralkaline felsic volcanic rocks in the Sanshui Basin. Their lithology is mainly syenite and syenitic porphyry, and they mainly comprise alkali feldspar and amphibole, which is similar to the mineral assemblage of the host trachyte and comendite. The SiO2 content in the syenitic enclaves is ~63 wt%, which is similar to that of the host trachyte but lower than that of the comendite. Thermobarometric calculations showed that the syenitic enclaves crystallized at similar temperature and pressure conditions as their host trachyte. The results of mass-balance modeling and MCS modeling indicate that the syenitic enclaves likely experienced an approximately 74% fractional crystallization from the basaltic parental magma. Combined with the similar mineral assemblages and geochemical characteristics of the host trachyte, we think that the enclaves resulted from the in situ crystallization of trachytic magma in the shallow crust and that they had a cogenetic origin with their host volcanic rocks, which means that they were likely to derived from the identical magma chamber which was formed from different batches of magma mixing/mingling. The recharge and mixing of basaltic magma triggered the eruption of trachytic magma eruption. The syenitic crust may have been disaggregated by the ascending trachytic magma and brought to the surface as syenitic enclaves. The syenitic enclaves in volcanic rocks provide unique information on the magmatism of the shallow crust as evidence of magma mixing/mingling.

Petrogenesis of Late Campanian Alkaline Igneous Rocks in Eastern Anatolia: Magmatism related to a subduction transform edge propagator (STEP) fault?

ABSTRACT In eastern Anatolia, the Divriği-Hekimhan Magmatic Province (DHMP) includes ~77–69 Ma alkaline rock units which are located to the NW of the Baskil Arc of ~85–74 Ma. The magmatic rocks are composed of nepheline (Ne)–to quartz (Q)–normative alkaline basaltic to trachytic/syenitic units. Among them, the basaltic rocks are composed of plagioclase (Or2–11Ab32–51An39–64) + clinopyroxene (Wo47–51En35–42Fs9–16) + Fe-Ti oxide ± alkali feldspar (Or57–98Ab2–42An1) ± biotite ± olivine. Their 87Sr/86Sr(I) ratios and ɛNd(I) values vary in the ranges of 0.70591–0.70871 and−3.2–1.6, respectively. The subvolcanic trachytic rocks are composed of perthitic alkali feldspar phenocryst in a matrix of feldspar (Or45–61Ab38–54An0–2), biotite, and Fe-Ti oxides. The trachytic volcanic rocks are made up of feldspar (Or38–63Ab34–59An1–4) in a fine-grained matrix. Their 87Sr/86Sr(I) ratios and ɛNd(I) values vary in the ranges of 0.70532–0.70952 and−3.2–0.7, respectively. The syenitic rocks in the region contain both quartz- and nepheline-sodalite-bearing syenites. Geochemical features reveal that the Ne-normative basaltic magmas have undergone mafic mineral fractionation coupled with crustal contamination to produce the Q-normative derivatives. Enhanced differentiation of the Ne- and Q-normative fractionated magmas via feldspar-dominated fractionation created the silica-undersaturated and -oversaturated trachytic magmas, respectively. During the feldspar-dominated differentiation, the re-melting of accumulated alkali feldspars in the magma chamber likely gave rise to the formation of trachytic rocks with alkali feldspar-like whole rock compositions. The final products of the Ne-normative magmas are represented by the phonolites and foid-syenites with silica-undersaturated eutectic compositions. A geochemical evaluation of the basaltic rocks revealed that the alkaline magmatism mainly originated from a shallow asthenospheric mantle source which had previously been metasomatized by oceanic to continental subduction. We suggest that the DHMP was formed in response to STEP fault-controlled rolling back of the northward subducting slab of the Baskil Arc, which created a localized gap for asthenospheric upwelling.

Highly differentiated trachytic magma linked with rare metal mineralization: A case study from the Shuanghekou Nb deposit, South Qinling

Highly differentiated trachytic magma linked with rare metal mineralization: A case study from the Shuanghekou Nb deposit, South Qinling

Two million years of evolution of the southern central andes retroarc, Payenia Volcanic Province, Argentina, from the study of the Cerro Nevado Volcanic Complex

New K–Ar ages, major and trace element analyses, and Sr and Nd isotopic data for the Cerro Nevado Volcanic Complex (CNVC) coupled with those previously published allow us to better document the hot thermal anomaly which affected the upper mantle beneath the Andean back-arc Payenia Volcanic Province and generated alkaline magmatism. These data, along with previously published ones from this and neighbor volcanic centers, suggest that this anomaly stagnated there, during at least the last 2 million years. Geochemical features from the CNVC basaltic magmas indicate that they were sourced within an OIB-type mantle with an EMI signature, similar to that underlying the neighboring Auca Mahuida Shield Volcano to the south, and attributed to the rise of a mantle thermal anomaly. We identify at least five events of partial melting and seven eruptive phases from 1.88 to 0.75 Ma. Continental crust contamination in the San Rafael Block crustal basement occurred coupled with fractional crystallization processes, probably in magma chambers and across several cycles, producing the trachybasaltic, trachyandesitic and trachytic magmas of the Cerro Nevado volcanic cone (1.34–1.22 Ma).

Petrogenesis of submarine volcanic rocks dredged from Dokdo, Ulleungdo, and the neighboring seamounts in the East Sea: Constraints from mineral chemistry, geochemistry, and 40Ar/39Ar ages

Several seamounts, including the Dokdo (DD), Ulleungdo (UD) Anyongbok (AYB), Simheungtack (SHT), and Isabu, were formed from intraplate volcanism in the southwest of the East Sea back-arc basin during the Quaternary. In the present study, mineralogy and geochemistry data generated using submarine volcanic rocks collected from the DD, UD, AYB, and SHT locations were utilized to investigate characteristics of the source, evolution, and storage conditions of the associated magmas. Analyses of representative samples from the DD, UD, and AYB using the 40 Ar/ 39 Ar method yielded plateau ages of 1.566–2.658, 0.533–0.714, and 1.736 Ma, respectively. The dredged samples consist of trachybasalt, phonotephrite, trachyandesite, and trachyte and have oceanic island basalt (OIB)-like characteristics. According to numerical modeling using trace element data, the primary magma linked to the formation of these samples can originate from ≤5% partial melting of an enriched garnet–peridotite facies metasomatized by melt derived from a subducted slab. The composition of major oxides, from the trachybasalts to trachytes, and mass balance calculations suggest that the trachyandesites and trachytes were formed via fractional crystallization from a basaltic melt. These diverse rock compositions probably highlight pulses of magma that separated from the main reservoir in the shallow crust. Results from mineral–melt thermometry indicate that the olivine and plagioclase in these basaltic rocks crystallized at temperatures of 1213 ± 56 °C and 1163 ± 23 °C, respectively. Thermobarometric calculations using clinopyroxene–melt pairs indicate that the Quaternary volcanism was associated with two magma storage regions that correspondingly involved pressures of 791 ± 253 MPa (1098 ± 34 °C) and 187 ± 123 MPa (938 ± 77 °C). The primary magma was likely present in a reservoir beneath the Moho (17–32 km), and it evolved by fractional crystallization to produce rocks, such as trachybasalts and phonotephrites. Crystallization and differentiation of the basaltic magma that was transported to crustal reservoirs explain the evolved compositions. The trachytic magma generated were present at depths of 2–11 km, and crystallization produced a paragenetic assemblage comprising clinopyroxene, amphibole, plagioclase, biotite, apatite, and titanomagnetite. • Quaternary intraplate volcanisms occurred in the East Sea back-arc basin. • New 40 Ar/ 39 Ar ages of submarine volcanic rocks were obtained. • Basaltic rocks were originated from an enriched garnet-bearing mantle source. • Trachytes were formed by fractional crystallization from basaltic magma. • The Quaternary volcanism had at least two magma storage regions.

Geochemistry of the Pepom tephra deposits: The most recent intracaldera volcanism of Sete Cidades volcano, São Miguel, Azores

Many volcanoes have the ability to impact human infrastructure with explosive activity and understanding the processes of magma generation and conditions of storage in these systems remains a priority. The Sete Cidades volcano on the island of São Miguel, Azores archipelago is exactly such a volcano lying only 12 km from the island's capital city of Ponta Delgada and has been repeatedly active during the Holocene. Sete Cidades is the westernmost central volcano of São Miguel and its most recent explosive volcanism produced the Pepom series of 17 trachytic pumice deposits that were erupted from vents within the caldera. These variably magmatic and phreatomagmatic eruptions involved trachytic magmas that represent the end point of a magmatic evolution dominated by crystal fractionation processes. The magmas contain a mineral assemblage dominated by feldspar, with biotite, clinopyroxene, FeTi oxides, amphibole, and trace apatite. Mineral thermometry and hygrometry reveal that the Pepom magmas were stored at temperatures of 789 ± 23 °C to 894 ± 20 °C under volatile-rich conditions (mostly >5 wt% water). Intercalated within the trachytic pyroclastic succession are mafic products that were erupted from vents on the flanks of the volcano suggesting that the evolved, shallow magmatic system may be acting as a barrier to ascending mafic magmas. The Pepom tephra deposits represent trachytic magmas that show limited evidence for the involvement of feldspar-dominated cumulates as has been observed in other settings whereas the preceding, caldera-forming, Santa Bárbara eruption suggests such involvement. Despite the predominant role of fractionation in petrogenesis, the Pepom tephras formed from mildly low-δ18O magmas (4.1 to 5.6‰ for trachyte) and as such require the addition of hydrothermally altered material. The cause of the low-δ18O values remains unknown, but low-δ18O, hydrothermally altered syenitic clasts found within deposits from the nearby Fogo volcano may be potentially analogues.

Eruptive history of Mason Spur, a Miocene—Pleistocene polygenetic volcanic complex in southern Victoria Land, West Antarctic Rift System, Antarctica

Mason Spur is a deeply eroded Middle Miocene to Pleistocene (c. 13 to 0.37 Ma) volcanic complex in southern Victoria Land, within the West Antarctic Rift System (WARS). The oldest rocks include a large volume of trachyte ignimbrites that provided abundant volcanic detritus recovered in McMurdo Sound drill cores. The ignimbrites together with early-formed intrusions were strongly deformed during a substantial caldera collapse at c. 13 Ma. Intense erosion modified the volcanic landscape, creating a paleo-relief of several hundred metres. Deep ravines were cut and filled by deposits of multiple lahars probably linked to gravitational collapses of trachyte dome(s). Small-volume trachytic magmas were also erupted, forming lavas and at least one tuff cone. The youngest trachytic activity comprises a lava dome and related block-and-ash-flow deposits, erupted at 6 Ma. Basanite erupted throughout the history of the complex and eruptions younger than 12 Ma are almost exclusively basanite, forming scoria cones, water-cooled lavas, and tuff cones. Three peripheral outcrops are composed of basanitic ‘a‘ā lava-fed deltas, probably erupted from vents on neighbouring volcanoes at Mount Discovery and Mount Morning. Abundant ignimbrite deposits at Mason Spur differentiate this volcanic complex from others in the WARS. Eruptions were triggered by rift extension initially, yielding the voluminous trachytes sourced from a magma chamber on the margin of the WARS. Later mafic eruptions were associated with deep crustal faults related to residual intraplate deformation. These results add important details to the eruptive history of the intracontinental WARS.

Zircon geochronological and geochemical insights into pluton building and volcanic-hypabyssal-plutonic connections: Oki-Dōzen, Sea of Japan—A complex intraplate alkaline volcano

Abstract The relationship between plutonic and volcanic components of magmatic plumbing systems continues to be a question of intense debate. The Oki-Dōzen Islands, Sea of Japan, preserve outcrops of temporally associated plutonic, hypabyssal, and volcanic rocks. Post-intrusion uplift juxtaposed Miocene syenites in inferred faulted contact with volcanic trachytes that are cut by rhyolite hypabyssal dikes. This provides a window deep into the timing and origins of magma storage architecture and dynamics. Zircon is ubiquitous in all samples; our aim is to determine what its age and composition can reveal about the plutonic-volcanic connection. Here we show magma source characteristics are recorded in zircon Hf isotopes; source composition and assimilation of heterogeneous hydrothermally altered crust in zircon O isotopes; and extensive fractional crystallization in zircon trace elements. Combined with new UTh-Pb SHRIMP zircon ages, 6.4–5.7 Ma, compositional data show pluton formation was by protracted amalgamation of discrete magma pulses. The rhyolite dike preserves an evolved fraction segregated from these discrete magmas. Synchronous with plutonism was a volcanic eruption of trachyte magma derived from the same source, which may have stalled at a relatively shallow depth prior to eruption. Stalling occurred at least above the amphibole stability zone because amphibole-compatible Sc and Ti were not depleted in the trachyte melt resulting in elevated values of these in volcanic compared to plutonic zircon. Identifying smaller episodic magma pulses in a larger magmatic complex places constraints on potential magma fluxes and eruptible volumes. High-flux, large volume, plume-related ocean island magmatic systems may have extensive vertically distributed multi-stage magmatic reservoirs and subduction-related systems transcrustal magma reservoirs. By contrast, Oki-Dōzen was a low-flux system with incremental pluton growth and small- to moderate-scale eruptions.

The anorogenic late Ediacaran granite-rhyolite porphyries of Gabal Abu Durba, Sinai: termination of magmatism in the Neoproterozoic Arabian-Nubian Shield crust

ABSTRACT Anorogenic magmatism in the northern Arabian-Nubian Shield occurred during a long-lasting crustal extension event (<580 Ma), which succeeded the formation of the Pan-African orogenic belt in NE Africa and Arabia. Late Neoproterozoic anorogenic felsic volcanic-subvolcanic rocks, along with post-collisional granitoid suites, are exposed in the Gabal Abu Durba mountain range along the eastern flank of the Gulf of Suez in Sinai. These rocks include granite and rhyolite porphyries, metaluminous to weakly peralkaline with distinct potassic alkaline and ferroan affinities. Major and trace element characteristics such as K2O/MgO >16, total alkalis >8.5 wt.%, (K2O + Na2O)/CaO >10, agpaitic index (NK/A) >0.85 and Zr (>250 ppm), Nb (>20 ppm), Y (>80 ppm), Zn (>100 ppm) and Ce (>100 ppm), and 10,000 × Ga/Al >2.6 and [Zr + Nb + Y + Ce] >350 ppm are suggestive of an A-type granite genesis. Zircon U-Pb dating of two representative samples returned crystallization ages of 569 ± 2.6 Ma and 561.7 ± 3.2 Ma for granite and rhyolite porphyries, respectively. The melting temperatures estimated at 997–1020°C are consistent with high-temperature liquidus conditions. Fractional crystallization, coupled with less significant crustal assimilation, was likely the main mechanism of formation of these rocks from a common primitive asthenospheric mantle-derived trachytic magma in an anorogenic intra-plate setting. The A-type alkali granite and rhyolite porphyries of Gabal Abu Durba are manifestations of mantle upwelling and lower crust underplating in the late Ediacaran anorogenic stage. These felsic subvolcanic intrusions (≈ 569–562 Ma) along with their volcanic counterparts constitute a new unmatched rock unit that most probably represent the last chapter of the anorogenic magmatism in the Neoproterozoic Arabian-Nubian Shield crust before the beginning of Phanerozoic era.

Rare metal enrichment of the Tianbao trachytic complex, North Daba Mountains (South Qinling): Insights from textures and geochemistry of trachytes and Nb-REE minerals

South Qinling hosts abundant alkaline igneous rocks characterized by rare metal mineralization, which, however, is not well understood, particularly for those extrusive alkaline magmatic systems. This study focuses on the Tianbao trachyte Nb(-REE) deposit located in the North Daba Mountains (South Qinling) and investigates textures and geochemistry of ore-bearing trachytes and Nb-REE minerals to reveal relative contributions of magmatic and hydrothermal processes to rare metal enrichment. Low K2O/Na2O ratios (0.26–1.64), depletions of Ba, Sr, and enrichments of Nb, Ta, Zr, Hf in the Tianbao trachytic complex resemble geochemical signatures of rifting-related alkaline igneous rocks, highlighting an intraplate rifting origin. Positive εNd(t) values (3.1 – 3.6) and constrained La/Yb (21.2 – 35.8) and Nb/Zr (3.7 – 4.8) ratios indicate that they might have originated from mixing of the asthenospheric mantle and metasomatized lithospheric mantle and formed due to protracted magmatic differentiation of coeval mafic magmas. Niobium and REEs in the Tianbao trachytic complex primarily originated from an enriched mantle source, and then upgraded to initial enrichment due to intense differentiation of the trachytic magmas. Niobium and REEs in the trachytic magmas were firstly incorporated into magmatic mineral phases (e.g., biotite, niobian titanite, niobian ilmenite and apatite) under relatively reduced (below the Ni-NiO oxygen fugacity buffer) conditions. With the differentiation of trachytic magmas, niobium, REEs and anion ligands (F-, CO32– and PO43-) got saturated in the late-stage alkaline melts and then entered the exsolved fluids. Ca-rich components should have been leached from the magmatic minerals during the hydrothermal stage and reacted with these ligands to develop Ca-rich hydrothermal mineral phases, including fluorite and calcite veins, allanite and apatite overgrowth under relatively oxidized (between the Ni-NiO and hematite-magnetite oxygen fugacity buffer) conditions. The remaining Nb and REEs finally precipitated as the economic minerals (e.g., allanite-(Ce), loparite-(Ce) and niobian Y-rich zircon) along fractures. Mass balance calculations indicate that these hydrothermal economic mineral phases account for the majority of Nb-REE reserve in the trachytic complex, and thus, the hydrothermal process might have played the key role in leading to Nb and REE concentrating to ore grade.