Shallow Magma Chamber Research Articles

Moment tensor inversion of microearthquakes along the Santorini-Amorgos zone: Tensile faulting and emerging volcanism in an extensional setting

The Santorini-Amorgos zone is an area rich in microseismicity at the center of the Hellenic volcanic arc. The microseismicity of the zone is distributed along the Santorini-Amorgos ridge and Kolumbo submarine volcano. In this study, we utilized crustal events that were recorded by temporary networks during September 2002 to July 2004 and October 2005 to March 2007, and also by the permanent network from 2011 to 2020. These events were inverted for their moment tensors by using P-wave polarities as well as SV/P and SH/P amplitude ratios, yielding 74 well-constrained moment tensor solutions. Most of these moment tensors have significant CLVD and isotropic components that are positively correlated to each other (R2 = 0.68). Tensile faulting due to high pore pressure is considered as the most likely cause of the observed non-DC components. The positive and negative non-DC components observed in Kolumbo may be generated by the opening and closing of cracks beneath the shallow (6–7 km) magma chamber due to a steady migration of magmatic fluids from the deep reservoir into the chamber. In Anydros, most of the microearthquakes have positive non-DC components associated with the opening of cracks. It is possible that the extensional deformation and high pore fluid pressure in the area opens subvertical cracks that become pathways for upward migrating fluids. The upward migration of magmatic fluids in an extensional regime such as the Santorini-Amorgos zone can also be viewed as an indication of emerging volcanic activity in this area.

On the uncertainty of geophysical data interpretation in volcanic areas through a case study: Ciomadul Volcano

This study mainly deals with the consequences of inadequate data coverage on the interpretation of geophysical information. The results of the joint gravity and geomagnetic survey of the Ciomadul Volcano are discussed in connection with conclusions of previously performed magneto-telluric (MT) investigations.In the first step, inversion of both the potential fields and MT data apparently suggested the existence of a shallow (i.e., <5 km) magma chamber containing residual melt, or alternatively, highly hydrothermally altered rocks. However, strong arguments exist against both of these hypotheses such as lack of strong geothermal manifestations at the surface, or the too young age of the volcano, inconsistent with the assumed advanced alteration stage.During the next step, a brief study on the limitations of the inversion approach revealed significant shortcomings generated by uneven data distribution. Based on these conclusions, the gravity inversion results were reconsidered by substituting the initially assumed single magma chamber with a hypothetical swarm of narrow volcano conduits, each with size below the Nyquist threshold of the gravity survey as determined by an observations coverage density of less than one station/km2.The new 3D density-based model was obtained by forward modelling, unveiling the structure of the shallow part of the volcanic feeder system that gradually developed and branched bottom-to-top.Given the shortcomings of the inversion due to inappropriate data coverage, it was speculated that a former solution of MT inversion might have been biased also by data scarcity. That leads to the integration of local effects (caused by narrow, but highly conductive fluid-filled conduits) into a single electrical resistivity anomaly that is improperly interpreted.

Magmatic evolution of Low-Ti lavas in the southern Paraná-Etendeka Large Igneous Province

The southern section of the Paraná-Etendeka large igneous province exposes a sequence of heterogeneous Low-Ti lava packages that range compositionally from olivine tholeiites to rhyolites. The recent advances in the volcanic stratigraphy provide a high-resolution framework that constrains the spatial and temporal evolution of the lava sequence. The stratigraphy is formed by Low-Ti lavas, separated in four volcanic units, namely: Torres, Vale do Sol, Palmas and Esmeralda. Our data shows that individual volcanic units can be separated chemically and represent distinct batches of magma. The composition of erupted lavas is mostly controlled by a complex interplay between source composition and crystallization (±assimilation) within the crust. The magmatic plumbing system evolved gradually from the onset, towards the climax and waning phase of magmatism. LIP onset is composed by more primitive (MgO > 5%) pahoehoe basalt lava flows of the Torres Formation. These low-volume eruptions were fed by a poorly connected magmatic plumbing system focused within the lower crust. Geochemical evidence suggests that the parental basalts of Torres Formation were derived from picritic primary magmas that have evolved by fractional crystallization and assimilation of enriched lithospheric materials. The main phase of magmatism is formed by thick (>20 m), tabular, crystal-poor lavas with evolved composition, including the basaltic andesite of the Vale do Sol Formation and silicic rocks of the Palmas Formation. These large volume eruptions were formed by extensive fractional crystallization (+assimilation) of precursor basaltic magmas and directly fed by shallow magma chambers within the upper crust. The waning phase of magmatism, recorded in the Esmeralda Formation basalts-basaltic andesite, is marked by low-volume Hawaiian-like eruptions of compound lava flows at the surface and the retraction of the magmatic plumbing system at depth. These lavas also mark a shift towards more primitive (asthenospheric) compositions and the shallowing of mantle melting, reflecting the progressive extension of lithosphere during magmatism. For the Low-Ti lava sequence of the Paraná-Etendeka, the surface expression and composition of lava fields are directly controlled by crystallization (+assimilation) processes, and these petrological processes can account for broad geochemical variations within large igneous provinces.

A Large Magma Reservoir Beneath the Tengchong Volcano Revealed by Ambient Noise Adjoint Tomography

AbstractThe Tengchong volcano (TCV) is a large active volcano system in the southeastern Tibetan Plateau. It is characterized by large‐volume magmatic gas emission, active hydrothermal circulation, and intense volcanic and earthquake activities, posing a threat of near future eruptions. However, there is still no available model of the magmatic plumbing system beneath the volcano system, limiting the quantitative assessments of the eruption hazards. Here, we present a high‐resolution 3D model of the TCV constructed using ambient noise adjoint tomography. Our 3D model reveals a large basaltic magma reservoir with a volume of ∼7,000 km3 at depths of 20–35 km, which has a melt fraction of ∼2%–4.5%. Our results suggest that the deep crustal magma reservoir is fed by partial melting in the uppermost mantle and is recharging the shallow magma chambers beneath the TCV. Our results are key to understanding the volcanic activities and assessing future eruption hazards.

Geodetic monitoring of the Changbaishan volcano activity and its relationship with earthquakes, 1999–2017

The major earthquakes often trigger unrest of surrounding volcanic magma chambers. In recent years, three major earthquakes occurred around the Changbaishan volcano, but it was unclear whether these earthquakes triggered the unrest of the magma chamber. Based on geodetic data, we analyzed the volcanic activity according to the Global Position System (GPS) and leveling sites time-series and re-estimated the location and volume change of magma chamber from 2002 to 2005. Meanwhile, we calculated the dilatational strain variations of the deep magma chamber resulting from coseismic deformations caused by the 1999 Mw7.0 and 2002 Mw7.3 Wangqing deep earthquakes and the 2011 Mw 9.0 Tohoku-oki earthquake. Our results show: (1) Changbaishan has experienced four stages of unrest since 1999, and the biggest unrest of the shallow magma chamber occurred from 2002 to 2005; (2) the parameters of the shallow magma chamber simulated by the Mogi and Point Compound Dislocation Model (pDCM) show that the magma chamber is located in the northern part of the crater, with a depth of approximately 7 km. The volume of the magma chamber increased by 25–28 × 106 m from 2002 to 2005; (3) the strain variation beneath the Changbaishan volcano corresponding to the 1999 Wangqing earthquake was small. The 2002 Wangqing earthquake produced an expansion strain of about 4.4 nano-strain on the magma chamber at a depth of 550 km, and probably promoted the unrest of the Changbaishan volcano. The 2011 Tohoku earthquake induced the expansion of the shallow magma chamber and compression of the deep magma chamber. Although this event promoted shallow magma unrest, it inhibited deep magma unrest. This may explain why the Changbaishan did not show obvious unrest after the 2011 Tohoku earthquake. Therefore, more attention should be paid to earthquakes that can promote deep magma unrest in the Changbaishan volcano.

Chlorine and sulfur evolution in magmatic rocks: A record from amphibole and apatite in the Tonglvshan Cu-Fe (Au) skarn deposit in Hubei Province, south China

In order to trace chlorine and sulfur evolution during magma evolution, detailed petrographic observation, major- and trace-element analysis of amphibole and apatite were carried out on the ore-related pluton in the Tonglvshan Cu-Fe (Au) skarn deposit, Hubei Province, South China. Several generations of apatite and amphibole are identified, from early to late including: (1) apatite in euhedral amphibole; (2) euhedral amphibole; (3) apatite in subhedral-anhedral amphibole/plagioclase; (4) subhedral-anhedral amphibole + plagioclase; (5) apatite in K-feldspar or quartz; and (6) K-feldspar + quartz. Most apatites show homogeneous textures, but a few apatite grains enclosed in amphibole and plagioclase (1 and 3) display oscillatory zoning. These zoned apatites crystallized in semi-closed environments created as growth of amphibole or plagioclase removed components of the melt. The magmatic Cl and S evolution can be divided into three stages: (I) In the deep magma chamber, at ∼ 5 km below the surface, magma underwent ∼ 39% fractional crystallization, and its Cl and S contents increased simultaneously; (II) In a shallow magma chamber at ∼ 2.5 km depth, fractional crystallization elevates Cl and S contents in the residual melt, but fluid exsolution depletes them; (III) At near-solidus stage, when fluid exsolution becomes weak, crystallization elevates Cl and S contents in the residual melt simultaneously. This study suggests that the silicate mineral (fractional) crystallization can elevate Cl content in residual melt, and fluid exsolution can efficiently remove Cl from the magma. Bulk sulfur in magma may be transported not only as dissolved sulfur, but also as sulfates like anhydrite. The sulfur content of apatite is controlled by the sulfur composition in melt from which the apatite crystallizes, but Cl in apatite may represent re-equilibration between the apatite and the last melt in contact with the apatite, because of rapid diffusion of Cl in apatite. These characteristics along with complex apatite generations imply that in-situ study is necessary when using apatite to trace volatile composition in magma.

Graben type calderas: The Bolaños case, Sierra Madre Occidental, Mexico

Graben calderas are volcano-tectonic structures that use the faults of a graben as main vents from which magma is massively erupted from a shallow magma chamber during collapse of intra-graben blocks. These eruptions are generally silicic and explosive and form large volume ignimbrite sheets without a previous Plinian major depressurization event. Graben calderas can be associated to tectonic settings ranging from pure extension to strike-slip transtension, including complete grabens, half-grabens or pull-apart grabens. The Sierra Madre Occidental (SMO) in western Mexico includes several of these graben type calderas with the corresponding large volume ignimbrite sheets that result from fissure type eruptions along the graben's border and interior faults. The Bolaños graben constitutes one of the best examples of these tectonically controlled collapse calderas and is the second largest caldera of the world after Toba caldera. It is a 90 × 25 km rectangular caldera and the vent of the 25.37 ± 0.36 Ma silicic Alacrán ignimbrite, with a minimum Dense Rock Equivalent volume of 2650 km3 (3800 km3 rock volume). Post-collapse silicic domes were emplaced just after the Alacrán ignimbrite from 25.02 ± 0.33 Ma to 23.94 ± 0.33 Ma, with a total minimum lava volume of 171 km3. Both ignimbrite and domes account for at least 2800 km3 rhyolitic magma output from Bolaños graben caldera, without considering the distal deposits, and in particular the co-ignimbrite ash-cloud deposit. We describe the characteristics of the Bolaños graben caldera focusing on its major products, the Alacrán ignimbrite and the post-collapse dome volcanism, providing the geological frame of the Bolaños graben area documented with 40Ar-39Ar ages. We finally propose a conceptual model to explain the dynamics of the Bolaños graben that can be applied to other similar volcano-tectonic depressions of the Sierra Madre Occidental and elsewhere in the world with similar geological settings.

Origin and Evolution of the Fatu Kapa Magmatic System (North-Western Lau Back-arc Basin): Insight on the Genesis of High-Silica Lavas

Abstract The South Eastern Futuna Volcanic Zone (SEFVZ; 50 km × 80 km) is located in the NW of the Lau back-arc basin at the northern boundary of the Futuna and YAO microplates. Fatu Kapa is an area (∼15 km × 10 km) of diffuse volcanic activity at the center of the SEFVZ and at the intersection of two volcanic axes with the Futuna fracture zone. At this place, the volcanic, tectonic, and hydrothermal activities are enhanced. The most differentiated volcanic series of the whole Futuna region are found in the Fatu Kapa area. Based on major and trace element data, four groups of volcanic rocks are distinguished in the Fatu Kapa area. The main groups form a well-defined transitional suite from basalts to andesitic and dacitic lavas that display enrichments in Zr, Cl and heavy rare earth elements. The most mafic lavas of this group are extracted from an Indian-type depleted mantle mixed with a component with trace element and isotopic characteristics similar to the nearby Samoa mantle plume. There is no evidence of subduction input. Petrological modelling shows that the mafic melts of the main group (Group 1A) evolve towards trachyandesites through fractional crystallization coupled with assimilation of 10 % of a Cl-rich and Th-poor melt generated by low-degree (≤2 %) partial melting of a hydrothermally altered basaltic crust. Rocks of Groups 1B(a) and 1B(b) were then generated by mixing of a mafic melt from Group 1A with a melt obtained by partial melting of andesitic and hydrothermally altered wallrock in shallow magma chambers. Extremely high Cl concentrations can be explained by assimilation of interstitial K- and Cl-rich brines in the pore water of the hydrothermally altered volcanic rocks before melting of these rocks. This is in good agreement with unusual extension of the Fatu Kapa hydrothermal area and indicates that, in the north of the Lau basin, the microplate boundaries are privileged zones to generate a complex felsic magmatism within areas of widespread hydrothermal alteration. Our study confirms the importance of assimilation and fractional crystallization and mixing processes in generating high-silica lavas in back-arc environments decoupled from subduction.

Geology and history of the Malea Planum region: A new view of Mars’ oldest large volcanic province

Geology and history of the Malea Planum region: A new view of Mars’ oldest large volcanic province

Petrologic evolution of boninite lavas from the IBM Fore-arc, IODP Expedition 352: Evidence for open-system processes during early subduction zone magmatism

Abstract Boninite samples from several intervals within Hole U1439C, recovered during IODP Expedition 352, show highly variable mineral chemistries that imply complex crystallization histories. Small pyroxene grains show oscillatory zoning with cores and zones ranging from pigeonite to augite. Late crystallizing augite has highly variable Al2O3 contents (1.9–13.7 wt%) and Ca-Tschermak component contents (3–13 mol%), which reflect disequilibrium conditions. Large, euhedral, low-Ca pyroxene (i.e., enstatite/clinoenstatite) crystals exhibit complex sector and oscillatory zoning patterns. Cr-rich spinel is found as inclusions both in olivine and low-Ca pyroxene. Early crystallized olivine phenocrysts have embayed and reacted margins, and some early crystallized olivines exhibit zoning. A few olivines have multiple zones, with both normal and reverse zoning between Fo86–92. Olivine xenocrysts also have embayed and reacted margins; however, xenocrysts do not exhibit chemical zoning patterns and have consistent Fo88 compositions. Disequilibrium crystallization of pyroxene rims reflects rapid cooling during an eruption. Sector zoning in low-Ca pyroxenes is the result of crystallization during periods of moderate undercooling between mixing events. Oscillatory, normal, and reverse zoning in olivine and pyroxene appears to have formed in response to multi-stage magma mingling or mixing processes, which introduced additional Ca, Fe, Ti, and Al into parental boninitic melts. The presence of olivine xenocrysts and orthopyroxene indicate equilibrium at 2–4 kbar (Whattam et al. 2020) indicates that boninite magma mixing events likely occurred within shallow magma chambers containing olivine-rich cumulate piles. Large mixing events probably destabilized the magma chamber, resulting in devolatilization and eruption. In contrast, small mixing events lacked the energy to destabilize the chamber, resulting in repeated compositional oscillations in minerals affected by multiple small mixing events.

The Temporal Variation of Magma Plumbing System of the Kattadake Pyroclastics in the Zao Volcano, Northeastern Japan

The geologic and petrologic study of the Kattadake pyroclastics (around 10 ka) from the Zao volcano (NE Japan) revealed the structure of the magma plumbing system and the mixing behavior of the shallow chamber. The Kattadake pyroclastic succession is divided into lower and upper parts by a remarkable discontinuity. All rocks belong to medium-K, calc-alkaline rock series and correspond to ol-cpx-opx basaltic-andesite to andesite with 20–28 vol% phenocrystic modal percentage. All rocks were formed by mixing between andesitic magma and near aphyric basalt. The petrologic features of andesites of lower and upper parts are similar, 59–61 wt% SiO2, having low-An plagioclase and low-Mg pyroxenes, with pre-eruptive conditions corresponding to 960–980 °C, 1.9–3.5 kb, and 1.9–3.4 wt% H2O. However, the basalts were ca. 49.4 wt% SiO2 with Fo~84 olivine in the lower part and 51.8 wt% SiO2 with Fo~81 olivine and high-An plagioclase the in upper one. The percentage of basaltic magma in the mixing process was lower, but the temperature of the basalt was higher in the lower part than the upper one. This means that the shallow magma chamber was reactivated more efficiently by the hotter basalts and that the mixed magma with a 70–80% of melt fraction was formed by a smaller percentage of the basaltic magma.

Petrogenesis of the Miocene Morro del Cobre Cu (Mo)-porphyry deposit in the Frontal Cordillera over the Pampean flat-slab segment, Argentina

The Morro del Cobre porphyry (Cu-Mo-Au) deposit, genetically linked to the upper Miocene magmatism (~9–10 Ma), is located at the eastern margin of the Andean Frontal Cordillera of San Juan, Argentina, over the Pampean flat-slab segment (28–33°S) of the Central Andes. The petrogenetic analysis of these rocks reveals an arc affinity and an adakite-like signature with the source magmas evolving under thickened crust conditions which resulted in a high pressure (amphibole ± garnet) residual mineralogy. These mineralizing magmas, that evolved towards shallower magma chambers (<10 km) have hydrous and oxidant character, typical of fertile suites worldwide. The emplacement of Morro del Cobre coincides with the passage of the aseismic Juan Fernandez Ridge as many other Miocene productive porphyry deposits in this segment of the Andes.

Voluminous Paleogene volcanism in the southern Mesa Central, Mexico: Unravelling the fissure-fed origin of rhyolitic ignimbrites of the Villa Garcia-Loreto Volcanic Complex

The southern portion of the Mesa Central (MC) physiographic province is covered by numerous Oligocene to Miocene volcanic successions that are part of the Sierra Madre Occidental silicic large igneous province. The MC merges with the Sierra Madre Occidental and the southern Basin and Range extensional province. The voluminous Villa Garcia-Loreto Volcanic Complex (VGLVC) is located in the southern portion of the MC and consists of predominantly rhyolitic lava flows, and voluminous products of explosive rhyolitic volcanism, mainly as lava-like to rheomorphic ignimbrite sheets. The volcanism in the area appears in two events, first, at 60.4 ± 0.3 Ma (K–Ar whole-rock age) with the fissure-fed eruption of an andesitic lava and second, with the fissure-fed eruption of two densely-welded, crystal-poor ignimbrites at 31.59 ± 0.14 Ma and 28.66 ± 0.07 Ma (40Ar–39Ar sanidine ages). Chondrite-normalized rare earth element patterns and primitive mantle normalized multi-element patterns show enrichment in light rare earth elements (LREE) and high field strength elements (HFSE). Geochemical analysis and mineral chemistry show that the andesitic magmas were generated by partial melting of the uppermost mantle in a subduction-related setting and ascended directly through the brittle crust and erupted along N-S trending faults. Rhyolitic magmas were generated by partial melting of the lower crust in an intra-plate and extensional setting, associated with an intense phase of the southern Basin and Range extension. During the ascent and storage in a shallow magma chamber fractional crystallization of mainly feldspars and pyroxene was the major process taking place in the evolution of the magma. Geophysical results suggest that the volcanic rocks of the VGLVC were originated by fissure-fed eruptions associated with the Loreto Graben fault. The major Basin and Range faults locally reach a depth of 7 km and trending in NNW-SSE direction. The combined results show that the Basin and Range extension favored the formation, ascent, and eruption of rhyolitic and andesitic magmas in the southern MC.

The 2002–2005 Changbaishan Volcanic Unrest Triggered by the 2002 M 7.2 Wangqing Deep Focus Earthquake

One of the most active intraplate volcanoes in East Asia, Changbaishan volcano experienced unrest from July 2002 to July 2005. On 2002/06/28, the M 7.2 Wangqing deep-focus earthquake occurred ∼290 km northeast of Changbaishan volcano. While some studies have suggested a possible triggering relationship, the physical mechanism of such distant interaction is still not well understood. Using a template matching technique, which cross-correlates waveform of known events with continuous data, we perform systematic detection of microseismic events recorded by station CBS near Changbaishan volcano from July 1999 to July 2007. The detected earthquakes can be further categorized into three different types: volcano-tectonic (VT) events, long-period (LP) events and harmonic-spectra (HS) events. We detect 3763 VT events between July 2002 and July 2007. The intense VT earthquake swarm during the period from July 2002 to July 2005, along with recurring LPs and HSs and other geodetic/geochemical evidence, suggest magma movement during unrest. Compared with the hand-picked catalogue, the catalogue obtained by template matching technique reveals a delayed-triggering relationship between Wangqing deep-focus earthquake and unrest. The small magnitudes of the VT events and the limited numbers of LP and HS events suggest that the Wangqing mainshock likely triggered bubble excitation in the mid-crust magma system, resulting in overpressure and a small magma injection into the shallow magma chamber at a depth of ∼5 km, leading to the 3-years unrest.

Petrogenesis of Late Cretaceous volcanic-plutonic complex from Xiaoxiong caldera in East Zhejiang

破火山内出露的火山岩与浅成侵入岩为硅质岩浆演化研究提供了一个重要窗口，从而备受关注。小雄破火山内的火山-侵入杂岩是中国东南沿海晚白垩世岩浆活动的典型代表，包括小雄组火山岩（K₂x）与两类侵入岩（花岗斑岩、正长斑岩）。本文以小雄火山-侵入杂岩为研究对象，开展了系统的锆石U-Pb年代学、岩石学和地球化学研究，旨在深入探讨破火山内火山岩与侵入岩之间的成因联系和岩浆演化过程。系统的LA-ICP-MS 锆石U-Pb年代学研究表明，小雄组火山岩形成于98~88Ma，并具有多期次喷发的特点，可分为下段、中段和上段，年龄分别为98~96Ma（K₂x¹）、95~92Ma（K₂x²）、~ 88Ma（K₂x³）。小雄花岗斑岩形成年龄为90Ma；正长斑岩形成稍晚，约88Ma。与下段流纹质玻屑凝灰岩的Nd-Hf同位素组成[ε_Nd（t）=-8.3~-7.2， ε_Hf（t）=-11.8~-7.2]相比，中段流纹岩要更为亏损[ε_Nd（t）=-5.84~-5.32， ε_Hf（t）=-10.1~-0.5]。研究表明，小雄组流纹质火山岩的母岩浆可能起源于发生在深部岩浆房中渐进的壳幔相互作用，中段流纹岩的源区混入了更多的亏损幔源组分。中段流纹岩与花岗斑岩具有相似的Nd-Hf同位素组成，以及 互补的微量元素地球化学特征，由发生在浅部岩浆房的分离结晶作用和堆晶作用所制约。值得注意的是，正长斑岩与花岗斑岩并不存在直接的成因演化关系，两者应是不同的起源。不同的正长斑岩岩株具有高度一致的结晶年龄、微量元素特征以及Nd-Hf同位素组成，以上特征均表明小雄破火山内的正长斑岩具有相同的起源。正长斑岩母岩浆起源于富集岩石圈地幔的部分熔融，岩浆源区混入了来自亏损的软流圈地幔组分，其地球化学成分变化主要受普通辉石+磷灰石+钛铁矿的分离结晶所控制。;The association of volcanic and shallow plutonic rocks in calderas may provide important clues to the geochemical evolution of silicic magma systems, and thus it has received a lot of attention. The Xiaoxiong caldera, mainly composed of volcanic rocks of the Xiaoxiong Formation (K₂x) and two types of plutonic rocks (granite-porphyry and syenite-porphyry), is a typical product of Late Cretaceous magmatism in SE China. In this paper, we conducted petrological and geochemical studies, as well as zircon dating and trace element analysis for the coexisting volcanic and plutonic rocks from the Xiaoxiong caldera, aiming to constrain the magmatic origin and evolution processes and evaluate the petrogenetic relationship between the volcanism and plutonism. LA-ICP-MS zircon U-Pb dating results revealed that the Xiaoxiong Formation was formed in 98~88Ma with characteristics of multi-staged eruptions. The ages of the Ⅰ-, Ⅱ-, Ⅲ-cycle volcanic rocks are constrained at 98~96Ma (K₂x¹), 95~92Ma (K₂x²) and ~88Ma (K₂x³), respectively. Meanwhile, zircon U-Pb dating provides crystallization ages of ca. 90Ma for granite-porphyry and ca. 88Ma for syenite-porphyry. However, in terms of Nd-Hf isotopic compositions, the rhyolites from the middle section [ε_Nd(t)=-8.3~-7.2, ε_Hf(t)=-11.8~-7.2] record increasingly depleted characteristics to the rhyolitic vitric tuffs from the lower section [ε_Nd(t)=-5.84~-5.32, ε_Hf(t)=-10.1~-0.5], implying a progressive crust-mantle interaction, and more juvenile, Nd-Hf isotopically depleted materials were involved into the source of these rhyolites. The rhyolites from the middle section and the granite-porphyry show cogenetic and complementary geochemical signatures. Therefore, we suggest that they could be closely linked by fractional crystallization and crystal accumulation in a shallow magma chamber, i.e., these rhyolites are formed by the extraction of interstitial melt from crystal mush, while the granite-porphyry represents the residual crystal mush. It is worth to note that there is no directly evolutional relationship between granite-porphyry and syenite-porphyry, and they are more likely to have different magmatic origins. In addition, the syenite-porphyries of different stocks in the Xiaoxiong caldera have a highly consistent crystallization ages, trace element characteristics and Nd-Hf isotope compositions, which indicates that they have the same magmatic origins. The syenite-porphyry was generated probably by pyroxene+apatite+ilmenite -dominated fractional crystallization from basaltic magma that was produced by magma mixing between melts derived from depleted asthenosphere and subduction-related enriched lithospheric mantle.

Chemo-stratigraphy, petrology and U-Pb geochronology of South-eastern part of the Yuntdağ volcano (Karakılıçlı – Manisa) in Western Anatolia

There are a number of volcanic eruption centres that form large caldera structures, and possibly feeding from relatively large magma chambers during the Miocene period in Western Anatolia. One of them is Yuntdağ volcano, and its products have formed a thick volcanic succession covering large areas. In this study, an area where the volcanic sequence is seen well in the south-eastern part of the volcano was selected. Geochemistry and geochronology analyses were carried out along a line between Kalpakkaya and Çamlık Hills in order to better understand the petrological evolution of melting, assimilation, storage and homogenisation processes in the magma chamber (and/or chambers) feeding the volcano within a limited time period (~4 Ma). Samples acquired from the bottom, middle and upper portions of these sections display Early-Middle Miocene ages of 17.64 ± 0.20, 17.22 ± 0.15, 16.16 ± 0.17 and 16.36 ± 0.13, 15.79 ± 0.71 and 13.61 ± 0.20 Ma, respectively. Although, felsic crystal melts, magma mixing and multiple mafic inputs were the dominant processes, fractional crystallisation was less efficient. Also, geochemistry data also indicate that the studied volcanic rocks generated by the melting of the lithospheric mantle, by slab retreat and roll-back of the Hellenic – Cyprus slab, and evolved in shallow magma chambers during the Early – Middle Miocene.

Extreme Heterogeneity in Mid‐Ocean Ridge Mantle Revealed in Lavas From the 8°20′N Near‐Axis Seamount Chain

AbstractLavas that have erupted at near‐axis seamounts provide windows into mid‐ocean ridge mantle heterogeneity and melting systematics which are not easily observed on‐axis at fast‐spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near‐axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near‐ridge mantle using a ∼200 km‐long chain at ∼8°20′N. High‐resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near‐axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on &lt;1 km scales, and reveal a continuum of depleted, normal, and enriched basalts spanning the full range of ridge and seamount compositions in the northeast Pacific. There is no systematic compositional variability along the chain. Modeling suggests that depleted mid‐ocean ridge basalt (DMORB) lavas are produced by ∼5%–15% melting of a depleted mid‐ocean ridge (MOR) mantle. Normal mid‐ocean ridge basalts (NMORB) form from 5% to 15% melting of a slightly enriched MOR mantle. Enriched mid‐ocean ridge basalts (EMORB) range from &lt;1% melting of 10% enriched mantle to &gt;15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub‐ridge mantle is much more heterogeneous than is commonly observed on‐axis and heterogeneity exists over small spatial scales.

Sub-surface magma movement inferred from low-frequency seismic events in the off-Nicobar region, Andaman Sea

Monitoring volcanic activity along the submarine volcanoes that are usually induced by subsurface magmatism is a challenge. We present fresh set of Ocean Bottom Seismometer (OBS) data that shows geophysical evidence indicative of subsurface magmatism along the submarine volcanoes in the off Nicobar region, Andaman Sea. In this region, we observed for the first time, hybrid very long-period earthquakes documented by passive OBS experiment. These events were initiated by high-frequency (5–10 Hz) with a clear onset of P-phase followed by low-frequency (0.01–0.5 Hz) oscillations in the range of 300–600 s with a prominent high-frequency (10–40 Hz) hydro-acoustic phase. A total of 141 high-frequency events were detected on 21st and 22nd March 2014 out of which 71 were of low-frequency oscillations. These events are distributed in the northwest–southeast direction along the submarine volcanic arc and Seulimeum strand of Great Sumatra fault. Off Nicobar region has been witnessing frequent earthquake swarms since 26th December 2004 tsunamigenic Sumatra earthquake. These swarms occurred in January 2005, March and October 2014, November 2015 and March 2019. The occurrence of low-frequency earthquakes and prominent hydro-acoustic phase are suggestive of sub-surface tectonic and magmatic influence. We propose that upward movement of magma pulses from deeper magma reservoir to the shallow magma chamber activated the strike-slip movement of sliver faults and induced earthquake swarms in the off Nicobar region.

Numerical Modeling Of Pressure Source Of Sinabung Volcano Based On GPS Data In 2011-2012 Using Particle Swarm Optimization (PS0)

Mogi Model with particle swarm optimization (PSO) scheme have been applied to the local GPS data of Sinabung Volcano during 2011 to 2012 to receive subsurface parameters as pressure sources in terms of misfit and inversion model parameter. The size of displacement was inverted by PSO. From the inversion concluded that the position pressure source showing shallow magma pockets at a depth between ±1.3 km volume change around +0.95x106 m3 . It indicates the presence of a huge magma supply and continuous into shallow magma chamber up to the surface of Sinabung Volcano.

The Kirganik alkalic porphyry Cu-Au prospect in Kamchatka, Eastern Russia: A shoshonite-related, silica-undersaturated system in a Late Cretaceous island arc setting

The Kirganik prospect is a silica-undersaturated, shoshonite-related, alkalic, porphyry Cu-Au system developed within the Late Cretaceous island arc of Central Kamchatka. It is associated with a potassic volcano-plutonic suite of rocks that range from monzogabbro- and monzodiorite- to monzonite- and syenite-porphyry, followed by a second, compositionally overlapping magmatic cycle of trachytic syenite-porphyry, and possibly other, more differentiated monzonitic phases. The rocks exhibit a subduction-related, island arc rock affinity, with the magma generation involving low degree (~1–3 vol%) partial melting of metasomatized, K-rich lithospheric mantle, followed by amphibole fractionation in a deep, probably lower crustal, magma chamber, with subsequent fractionation of accessory minerals, clinopyroxene and possibly biotite in shallower magma chambers. An early calc-potassic alteration assemblage of K feldspar-pyroxene-apatite-magnetite is closely related to small syenite intrusions. This stage is overprinted by more extensive potassic alteration zones of similarly quartz-free biotite-K feldspar-magnetite, some of which contain abundant Cu-sulfides (mostly bornite and chalcopyrite, with minor chalcocite), minor native gold and PGE minerals. Subsequent sodic-potassic to calcic-sodic-potassic alteration comprising albite-magnetite-calcite-epidote, and propylitic assemblages of chlorite-magnetite-epidote ± pyrite are much more weakly developed. These hydrothermal alteration assemblages form lens-shaped, replacement-style zones, rather than stockworks of well-defined veinlets/stringers, particularly quartz-sulfide veinlets, which are absent. Apatite from calc-potassic and potassic alteration assemblages contains low salinity (5–10 wt% NaCl-eq.) liquid-gaseous fluid inclusions indicating a homogenous, low salinity, supercritical aqueous fluid exsolved directly from a cooling and degassing pluton, with no boiling and unmixing into hypersaline and gaseous phases. Under inferred high temperatures of calc-potassic and potassic alteration (450–550 to 400–420 °C) and significant (2.1–2.2 to 1.5–1.8 kbar) pressure, this suggests a deep (>5 km?), possibly root-level of a larger, shallower, since eroded, porphyry Cu-Au system. The current geological setting and distribution of alteration and mineralization can be explained by a sharp, up to 90°, post-mineral tilting of the initial narrow, subvertical, 'finger-like' intrusions and related mineralized system, followed by deep exhumation. As a consequence, a significant part (>1.5 km) of the deep porphyry system is currently exposed at surface.