Basaltic Andesite Research Articles

Plagioclase-hosted Crystallized Melt Inclusions within Hypabyssal Volcanic Rocks of the Torud-Ahmad Abad Magmatic Belt, Iran: Analysis of Origin and Fractionation Processes

Investigating crystal-rich clots hosted in phenocrysts and phenoclasts within Eocene subvolcanic rocks in Torud-Ahmad Abad, south-southeast of Shahrood (northern part of central Iran zone). These crystal-rich clots and clusters (alias stone inclusions, nanogranitoids, microcrystal clots) are interpreted as crystallized melt inclusions (cMIs) within phenocrysts and phenoclasts, highlighting plagioclase-hosted inclusions. Least-altered hypabyssal igneous rocks are trachy-andesitic, basaltic andesitic, and dacitic porphyries. These porphyries have porphyritic, glomeroporphyritic, granular, and trachytic textures with variable-sized phenocrysts of plagioclase (albite-labradorite), green hornblende (magnesio-hastingsite), and clinopyroxene (diopside-augite), with minor biotite, and Fe-Ti oxides; large plagioclase phenocrysts, exhibiting clear normal oscillatory zoning, were consistently utilized as plagioclase-hosted inclusions due to their abundance in rocks. MIs exhibit complete post-entrapment crystallization (PEC), generally with a slightly finer grain size than the igneous groundmass, i.e. no preserved glassy MI were observed in these phenocrysts, only cMIs. These variably sized, cryptocrystalline to microcrystalline clots in various phenocrysts seem to also represent primary igneous assemblages, manifested as clusters of microphenocrysts; these are referred to as crystal clots. SEM-EDS analyses determined the composition of crystal-rich clots in various plagioclase phenocrysts forming inclusions. The major-element composition of these crystal-rich clots in plagioclase are basalt, basaltic andesite, andesite, trachy-andesite, and trachyte that seem to be melt trapped during plagioclase phenocryst growth; these trapped interface melts then form microphenocryst assemblages that are preserved in phenocrysts, which are trapped when some process interferes with the growth of a phenocryst. These cMIs exhibit compositional variations from their bulk host rock, resulting from entrapment during magma mixing during plagioclase growth.

The Neogene–Quaternary volcanism of the Carpathian–Pannonian region: from initial plate tectonic models to quantitative petrogenetic explanations

A wide range of volcanic rocks formed in the Carpathian–Pannonian region during the last 20 Myr, closely associated with the tectonic evolution of the area: (1) high-SiO 2 rhyolites and rhyodacites; (2) calc-alkaline-type volcanic suites (basalt–andesite–dacite–rhyolite and adakitic rocks); (3) potassic–ultrapotassic volcanic rocks (shoshonites, latites, leucitites and lamproites); and (4) alkali sodic volcanic rocks (basalts, trachyandesites and trachytes). During the 18.1–14.4 Ma silicic ignimbrite flare-up volcanism, explosive eruptions produced large volume of tephras, which covered extended areas of Europe. Calc-alkaline-type volcanism took place in syn-extensional and post-collisional settings. There is no classic volcanic arc in the Carpathian–Pannonian Region, and so the term ‘back-arc basin’ should be used for the Pannonian Basin only in a structural geology sense. Alkali basalt volcanism occurred mostly in the peripheral areas of the Pannonian Basin primarily owing to edge-driven convective upwelling of heterogeneous asthenospheric mantle. The youngest eruptions took place 100–30 ka ago, mostly in the geodynamically still active southeast Carpathians. After 50 years of intense research, there are still many open questions to be solved. Interdisciplinary understanding of the results of different disciplines is crucial to better constrain the evolution of this tectonically complex area, to reveal its geoenergy and to assess potential natural hazards.

Unusual mineralization in basaltic andesite of submarine volcano Esmeralda (Mariana Island Arc)

The results of studies of a basaltic andesite sample complicated by a mineralized crack and voids, with a crack and gas voids filled with secondary mineralization dredged on the Esmeralda underwater volcano, are presented. A detailed comparative study of the mineral composition of the substance lining the crack, the space around the crack, and the part of basaltic andesite unaffected by secondary changes made it possible for the first time for the underwater Esmeralda volcano to establish the presence of an association of minerals that is not characteristic of unaltered volcanic rocks. In the intracrack space and adjacent zones of basaltic andesite, wide ranges of plagioclase composition were determined, isomorphism in the Fe-Ca-pyroxene series was studied, REE oxides, hydroxides and fluorohydroxides were studied, and variability in the composition of minerals in the magnetite-hematite series was shown. It is assumed that tectonic movements led to the emergence of permeable zones in the previously formed basaltic andesites through which new portions of the melt leaked. In a limited space, high fluid gas saturation, temperature and pressure made it possible to extract metal compounds from the melt and host rocks.

Conditions for formation and preservation of andesite-hosted mafic enclaves during the 2018 Lower East Rift Zone eruption of Kīlauea

Andesites erupted at Kīlauea in 2018 in the Lower East Rift Zone for the first time in the known geological record. The evolved lavas erupted at Fissure 17 of the 2018 eruption, ranging from andesites to basaltic andesites, contain abundant mafic enclaves both in the lava flows and the ejecta, which are unusual at Kīlauea and in Hawai'i in general. Textural observations indicate that the enclaves originate from incomplete mixing of two magmas rather than the incorporation of cold basaltic wall rock. We suggest, on the basis of bulk and mineral compositions, that the source of the mafic enclaves is the early 2018 evolved basalt magma (phase 1b) that erupted concomitantly at adjacent fissures, which mixed with the andesite to produce the range of basaltic andesite compositions observed at Fissure 17. The coexistence of homogenized basaltic andesites and mafic enclaves within the same magma require a mixing mechanism resulting in both complete homogenization and preservation of enclaves. We propose that the range of mixing and mingling processes may be explained by spatial and temporal variability in the mixing percentages of the phase 1b basalt and the andesite within the andesite magma chamber. Field observations, chemical compositions, and 2D thermal conduction models suggest that enclaves are preserved where the basalt contribution to mixing is less than roughly 40 %, as a result of microlite crystallization leading to rigidification of the enclave magma. Above this threshold, the mixed magmas became largely homogenized. The scarcity of mafic enclaves at Kīlauea and in the Hawai'i igneous record is likely explained by mixing between magmas that lack sufficient compositional and rheological contrasts to preserve them.

Geochemistry of Mafic Rocks From the Nagrota–Kathindi Section, Himachal Pradesh, Northwestern Himalaya: A Probable Example of Plume–Lithosphere Interaction

ABSTRACTThe Northwest Himalayan region has a record of several phases of mafic magmatic activity spanning from Precambrian to Cenozoic in a dynamic tectonic setting. Here, we studied detailed petrography and new whole‐rock geochemistry of mafic volcanic and dykes from the Nagrota–Kathindi Section (NKS), Himachal region of the NW Himalaya, to understand the petrogenesis and possible tectonic setting. Both rock types have comparable mineralogical compositions (clinopyroxene + plagioclase + actinolite‐tremolite + chlorite + iron oxides ± hornblende ± epidote ± quartz ± carbonates) overprinted by greenschist to lower amphibolite facies metamorphism. The mafic volcanic and dykes of NKS exhibit subalkaline basalts to basaltic andesites and a typical tholeiite compositional character. The chondrite‐normalized rare earth element pattern exhibits similar LREE‐enrichment and strong HREE‐fractionation, whereas primitive mantle‐normalized multi‐element patterns show pronounced LILE‐enrichment of Rb, Ba, Th, LREE, and HFSE depletion of Nb, K, P, and Ti. The Zr–Y–Nb–Th relationships indicate that both rock types were derived from the plume source, whereas low Nb/La (< 1), similar high large‐ion lithophile element concentrations, and pronounced negative Nb, Zr, P, and Ti anomalies suggest that components other than mantle plume must have been involved in the generation and evolution of both rock types, that is, most likely plume and subcontinental lithosphere mantle (SCLM) interaction. The genesis of parent magma for the NKS volcanic and dykes was derived by 4%–6% and 10%–20% partial melting from the spinel + garnet lherzolite stability field. The majority of the studied samples correspond to spinel + garnet peridotite melting on (Gd/Yb)N versus CaO/Al2O3 diagram, thereby corroborating residual garnet in the mantle restite. All the basalts and dykes from the NK section did erupt/intrude in an intracontinental rift setting based on geochemical discrimination. The key petro‐tectonic processes attributed to the formation of these rocks are as follows: (i) the melting of the ascending plume by adiabatic decompression; (ii) the partial melting of this plume–SCLM source in the melting regime, which produces basaltic magma with a tholeiitic composition; and (iii) the release of heat that provides the thermal condition for melting of SCLM and interaction between upwelling mantle plume and subduction metasomatized SCLM.

Geochemistry of Volcanic Rocks in Ponelo Island, North Gorontalo, Indonesia

Ponelo Island is located in the northern part of Sulawesi, which is still an enigma regarding the genesis of the volcanic rocks found on this island. Therefore, the objective of this study is to understand the petrogenesis and tectonic implication of these volcanic rocks. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to obtain trace and rare earth elements is the method of this study. The volcanic rocks found on Ponelo Island consist of basalt and basaltic andesite rocks with a calc-alkaline affinity. The transition data suggested a highly fractionated cause of low transition element (Ni=17-38 ppm; Cr=13-47) compared to primary magma concentration, anomalies negative of Ba, Sr, and Ti of spider diagrams, and negative anomaly of Eu (Eu/Eu*=0.88-0.99). Relationship between low concentration between Ce/Y (0.74-0.76) and La/Yb vs Sm/Yb ratio indicated ~5% spinel-lherzolite mantle source partial melting. On the other hand, incompatible element ratios, such as Ba/Nb (39.03-45.28), Ba/Th (75.52-82.67), Rb/Nb (3.93-6.22), K/Nb (1772.22-2703.45), Ba/La=13.67-14.57, Th/La (0.17-0.18), La/Nb (2.91-3.16), depleted Nb/U (6-6.74), and also lack of xenolith or enclaves indicate cryptic crustal contamination. The slab-derived fluid indicated by ratios of Rb/Y (0/019-0/05), Nb/Y (0.10-0.11), Th/Yb (0.52-0.61), and Ba/La ratio (13.29-14.57). Ponelo volcanic rocks shows typical calc-alkaline island arc tectonic setting particularly with enrichment in ion lithophile element (LILE) and light rare earth elements (LREE) along with depletion in high field strenght elements (HFSE) and heavy rare earth elemets (HREE), as shown by spider diagrams.

Growth of the Central Orogenic Belt, North China Craton through accretion of different Neoarchean arc terranes: Perspective from the Linshan complex

The onset of plate tectonics and crustal growth processes in the early Earth have been controversial scientific issues in the geoscience. The North China Craton (NCC) preserves widespread 3.8–2.5 Ga rocks, providing an ideal place to understand early continent formation and evolution. The Linshan complex located in the southern segment of the Central Orogenic Belt (COB) of the NCC, is mainly composed of TTG (tonalite-trondhjemite-granodiorite)-diorite gneisses and metamorphic volcanic-sedimentary units dominated by gabbro, basalt, basaltic andesite and biotite-plagioclase gneiss. Detailed mapping on the scale of 1:100 of a structural transect shows that the Linshan complex has mainly experienced two major deformation events including top-to-the-SE thrust faults and late NE-trending high-angle normal faults. Detailed zircon U-Pb dating shows that gabbro, basaltic andesite, and TTG gneiss mainly formed at ca. 2.52–2.50 Ga. Gabbros and basalts display enrichment of LREE and negative Nb and Ta anomalies, and basaltic andesites display mixed MORB-IAT geochemical affinities. Basalts and basaltic andesites are members of the Nb-enriched basalt series with high absolute Nb contents (>6 ppm). TTG gneisses are geochemically divided into high-pressure and low-pressure TTGs. High-pressure TTGs are characterized by high ratios of La/Ybcn (26.29–45.73) and fall into the adakitic region in the La/Ybcn-Ybcn diagram. Considering the close contact with Nb-enriched basaltic series, it is proposed that high-pressure TTGs may have formed by partial melting of a subducting oceanic slab with garnet and amphibole and/or rutile as residues. Low-pressure TTGs are characterized by low ratios of La/Ybcn and Sr/Y with marked negative Eu anomalies, indicating partial melting at shallow crustal levels. Regional tectonic relations have defined the Neoarchean Dengfeng island arc-forearc accretionary complex to the east of the Linshan complex. Thus, we propose that the gabbros-basalts-basaltic andesites in the Linshan complex mostly formed in a Neoarchean suprasubduction back-arc basin by rifting of a TTG-dominated island arc terrane. The final closure of the back-arc basin resulted in their tectonic juxtaposition forming thrust-imbricated structures. There may have been several Neoarchean “forearc-island-arc-backarc” systems in the NCC that are similar to modern accretionary tectonic orogens, indicating that plate tectonics has been in operation since at least 2.55–2.50 Ga.

Crystal mush interaction controls eruptive style during the 2018 Kīlauea fissure eruption

We use new geochemical, petrological, and rheological data to constrain the formation and emplacement of the highly compositionally unusual(andesitic basalt) Kīlauea 2018 Fissure 17 (F17) eruptive products. Despite the restricted spatial and temporal distribution, F17 samples are texturally and geochemically diverse. The western samples are enriched in SiO2 by up to 10 wt%, relative to their eastern equivalents; additionally, the western samples contain microcrystalline enclaves, absent from the homogenous eastern samples. The compositions erupted along F17 suggest interaction between the basaltic 2018 juvenile magma and a crystal mush at depth, likely a left-over from the nearby 1955 eruption. Magma mingling caused heating and local melting of remnant mush, leading to melt hybridization and volatile exsolution. Rapid water exsolution likely caused overpressurization of the reservoir underneath the western side of F17, leading to Strombolian explosions of viscous magma, in contrast to sustained Hawaiian fountaining on the eastern side. Remelting of remnant crystal mush and melt hybridization in open-conduit systems may hence be an effective mechanism in inducing volatile saturation.

Pressure–temperature evolution, protolith characteristics and tectonic setting of granulite facies rocks from Laojinchang area in the Longgang Block, China

The Archean supracrustal rock series in the Laojinchang area of Jilin Province, China provide valuable insights into the Precambrian geological evolution of the North China Craton. This study presents new results on petrography, geochemistry, mineralogy, isotope geochronology and phase equilibrium modeling of the Laojinchang granulite facies metamorphic rocks. The protolith ages of garnet two-pyroxene granulite and amphibolite from Sidaolazihe Formation, and two-pyroxene granulite from Laoniugou Formation are 2565 ± 5.9 Ma, 2512 ± 17 Ma, and 2509.2 ± 8.6 Ma, respectively, with the metamorphic age recorded by amphibolite being 2475.6 ± 9 Ma. New geochemical data indicates that TTGs (tonalite-trondhjemite-granodiorite) weathering in an island arc environment formed the protolith of argillaceous gneiss of Sidaolazihe Formation. Basaltic andesite, formed under an island arc environment, was the protolith of garnet two-pyroxene granulite of Sidaolazihe Formation; while calc-alkaline basalt was the protolith of amphibolite from the same formation and two-pyroxene granulite from Laoniugou Formation. All these Archean supracrustal rocks exposed in the Laojinchang area have recorded nearly isobaric cooling (IBC) anticlockwise P-T paths with distinct prograde, peak and post-peak stages. The peak P-T conditions reached granulite facies, suggesting a heat source from underplated magmas. A tectonic model involving an island-arc setting is proposed for the evolution of these Neoarchean supracrustal rocks in Laojinchang area.

Analysis of Mineral Types, Density, and Porosity in the Lam Teuba Formation Using Infrared Spectroscopy Method

The Lam Teuba Formation is one of the local fault areas of the Seulimuem segment located in the Grand Mosque sub-district of Aceh Besar regency. The Lam Teuba Formation has several types of rocks such as basaltic andesite lava, pyrolastic flows, pyroclastic falls, and sedimentary rocks, namely claystone, limestone sandstone, agglomerate, limestone. Rocks have physical characteristics, namely density and porosity which have different values in each rock. This difference can be caused by the minerals contained in the rock. The method used to calculate the density and porosity values used the Archimedes method, and the identification of mineral types using the infrared spectroscopy method. The results of data processing obtained the highest original density value at location 1 sample 1 with a value of 3.42 and the lowest value at location 5 sample 2. The highest dry density value was in location 1 of sample 1 with a value of 3.33 and the lowest value was in location 5 of sample 2. The highest saturation density value was in location 1 of sample 1 with a value of 3.43, the lowest value was in location 5 of sample 2 with a value of 2.07. The average porosity value was the highest with a value of 5.41 in the poor quality category in sample 4, while the lowest value was 4.62 in the very bad category in sample 1. The types of minerals found in the Lam Teuba formation are illite, montmorillonite, kaolinite minerals with a mixture of smectite which is a clay mineral group and siderite minerals which are carbonate mineral groups.

Induced subduction initiation near an ultra-slow spreading ridge: A case study from the Beila ophiolite in the north-central Tibetan Plateau

Subduction initiation of oceanic lithosphere is the key scientific problem of the plate tectonics on Earth. When, where, and how an oceanic subduction began remained obscure because of little geological records. Here, we report a newly documented ophiolitic sequence, including ultra-slow spreading ridge fragments (V1) and associated fore-arc lavas (V2) units in the Bangong-Nujiang suture zone, north-central Tibetan Plateau. The V1 units are controlled by detachment fault with peridotite, gabbro and pillow basalt. Peridotite exhibits uniform characteristics of abyssal peridotite, representing residues after low-degree melting of asthenospheric mantle. Mafic rocks display geochemical features of normal mid-ocean ridge basalt (N-MORB). These features, combined with zircon U-Pb dating, indicate that formation of the oceanic lithosphere occurred along an ultra-slow spreading ridge around 177–170 Ma in this area. The V2 units occur as a complete sequence from bottom to top of fore-arc basalt (FAB), basaltic andesite, boninite, and high-Mg arc-related rocks with formation ages of 160–150 Ma, which directly extruded and overlay on the V1 units. This sequence is identical to those of the Izu-Bonin-Mariana (IBM) fore-arc, and represent magmatic response to subduction initiation (SI). Thus, we propose that the Beila ophiolite preserved an entire process of oceanic transition from ultra-slow spreading oceanic ridge to the SI. Subduction initiation in the Beila area was induced by southward subduction transition after the closure of the northern branch of the Bangong-Nujiang Meso-Tethys Ocean and subsequent continental collision. This study provides direct geological evidence for delineating the geodynamic mechanism of an oceanic initial subduction.

Forearc magmatism in the Pannonian basin recorded by metasomatised skarnoid xenoliths in Pliocene basalt (Novohrad-Gemer Volcanic Field, Slovakia)

Anorthite-diopside-spinel-forsterite xenoliths occur in basaltic lava flow in northern part of the Pannonian basin in the Novohrad-Gemer Volcanic Field (NGVF). Several lines of geochemical evidence suggest that the typical skarn assemblage originated by multiple carbonatitic and alkaline silicate metasomatism of pristine basalt to picrobasalt protolith rather than by the modification of crustal carbonates around shallow magma chambers. Interaction with mantle-derived magnesite or dolomite carbonatite melt triggered the crystallization of neoformed forsterite at the expense of primary clinopyroxene. The alkaline silicate metasomatism resulted in the replacement of primary diopside and augite by neoformed aegirine and aegirine-augite accompanied by amphiboles (richterite, katophorite, arfvedsonite). Weakly metasomatised Group 1 xenoliths retained their fore-arc magmatic geochemical signatures: flat REE patterns, low concentrations of high-field strength elements (Ti, P, Zr) and Ni compared to MORB, low ratios of highly-to-less incompatible elements (Ti/V, Zr/Y) relative to MORB, low Na2O, low total REE, and supra-subduction affinity of primary clinopyroxenes, being thus analogues to fore-arc basalts (FAB). In contrast, strongly metasomatised Group 2 xenoliths exhibit LREE-enriched patterns sub-parallel with those of host trachybasalts. Some Group 1 xenoliths also exhibit large positive Eu anomalies and strongly depleted signatures diagnostic of low-pressure plagioclase-pyroxene cumulates. One xenolith with complementary negative Eu, Sr and positive Rb anomalies has been identified as high-Mg basaltic andesite melt residuum. Sr and Nd isotopes plot along the OIB-MORB array. The Group 1 converges at initial 87Sr/86Sr and 143Nd/144Nd ratios of 0.7038–0.7041 and 0.5127–0.5128, respectively, thus recording a 140–190 Ma old mantle source. Thermobarometry data on metasomatically unmodified pyroxenes have been arranged along decompression cooling trend, propagating from ∼1230 °C and ∼ 1 GPa and ending at ∼1160 °C around atmospheric pressure. The maximum pressure indicates the melt generation zone ∼35 km deep, reflecting thin lithospheric mantle. The FABs identified for the first time in the AlCaPa megaunit corroborate the affinity of the NGVF lithospheric segment to Dinarides.

Arc building and maturation of the Lombok Island, East Sunda Arc

Intra-oceanic subduction zones are major sites of crust–mantle material exchange and crustal growth; however, the processes responsible for the magmatic evolution and transformation of nascent arcs into mature oceanic arcs with thicker crust are debated. We present mineral chemistry, zircon UPb age and Hf isotopic data, and whole-rock chemical and Sr–Nd–Hf–Pb isotopic data for Cenozoic volcanic rocks on Lombok in the East Sunda Arc, Indonesia, to investigate their petrogenesis and arc maturation. SIMS zircon UPb analyses of lavas from southern Lombok yield an early Oligocene age of ∼30 Ma. The lavas can be divided into three groups based on their ages and geochemical characteristics: group I (late Eocene) and II (∼30 Ma) lavas from southern Lombok, and group III lavas (∼3–0 Ma) from northern Lombok. The lavas are characterized by a progressive enrichment in K2O and incompatible element contents, and NdHf isotopic compositions with time. Group I lavas are tholeiitic basalts with low K2O contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and NdHf isotopic compositions (εNd = +7.4; εHf = +14.5), enrichment in large-ion lithophile elements (e.g., Rb, U, and K), and typical forearc basalt (FAB) geochemical characteristics. Group II lavas are low-K basaltic andesites to dacites that yield flat chondrite-normalized REE patterns [(La/Sm)N = 0.9–1.4], uniform initial 87Sr/86Sr ratios (0.7038–0.7042), and positive εNd(t) (+5.9 to +6.8) and εHf(t) (+13.9 to +14.4) values. Group III lavas are typically medium- to high-K calc-alkaline basalt to andesite and are enriched in light REEs [(La/Sm)N = 1.6–3.3] with uniform initial 87Sr/86Sr ratios (0.7038–0.7042) and slightly low εNd(t) (+3.9 to +5.4) and εHf(t) (+10.5 to +12.5) values. Progressive enrichment in incompatible element contents and NdHf isotopic compositions over time, from FAB to high-K calc-alkaline lavas, reflect the maturation of the East Sunda Arc. Sr–Nd–Pb–Hf isotopic mixing models, along with trace element ratios (e.g., Ba/Th, La/Yb, Th/Yb), suggest that increasing amounts of sediments were incorporated into the mantle sources of the magmas from group I (∼0%) to group II (<0.5%) and group III (0.5%–1.0%) lavas. Partial melting models show that the formation of group I and II lavas can be reproduced by high-degree (∼10%) partial melting of spinel peridotite, whereas the group III lavas were formed by low-degree (∼5%) partial melting of spinel peridotite. We propose that the evolution of Lombok arc magmas was controlled mainly by the fluxes of subducted slab material, along with decreasing degrees of partial melting as a result of crustal thickening over time.

The Underwater Esmeralda Volcano (Mariana Island Arch) and some Features of the Composition of its Composition Rocks

A generalization of the available original data and literature data on the geological and geophysical knowledge of the underwater volcano Esmeralda, located in the Mariana Island Arc, has been carried out. As a result of studying the rocks dredged during the 4th and 5th cruises of the R/V Vulkanolog at the present level, new data were obtained on the silicate and rare-element composition of the rock samples that make up this underwater volcano. It has been established that the studied volcanic edifice is composed of five types of rocks: basalts, basaltic andesites, dacites, gabbro, and basanites. For the first time, samples of dacite and basanite have been discovered, indicating that the petrochemical diversity of the underwater volcano Esmeralda is wider than previously thought. All dredged rocks are characterized by a slightly increased content of incoherent elements LILE and HFSE. The studies carried out made it possible to attribute the main part of the dredged rocks to the association of island-arc ferruginous tholeiites (IAB, IAT) and only the composition of a single sample of alkaline basalt (basanite) falls into the field of alkaline basalts of oceanic islands (OIB, OIA). The increased content of iron in plagioclase phenocrysts confirms that the rocks belong to the high-iron tholeiite association.

Petrology and geochemistry of Mount Lurus lavas: evidence for alkaline to subalkaline transition in the rear arc of Java, Sunda Arc

Abstract Mount Lurus, a small Quaternary volcanic center in the rear of Java Island, Sunda arc, has erupted both alkaline and subalkaline lavas in a short span of active period. Furthermore, this small volcanic edifice, together with its neighboring alkaline volcanic center of Ringgit-Beser, corresponds to a shallower slab depth (∼160 km) than Muria in Central Java (270 km). We report the mineralogical and geochemical characteristics of Lurus lavas and compare them to the Ringgit-Beser lavas. All Lurus lavas show typical characteristics of island arcs with elevated Ba, Sr, Pb, and K and negative anomalies of Nb and Ti. The older Lurus edifice (Old Lurus) produces alkaline lavas with high alkalinity, which are trachyandesite and phonotephrite in composition. The younger Lurus (Young Lurus) generates calc-alkaline lavas with basaltic andesite composition. Both groups have plagioclase, clinopyroxene, olivine, and opaque minerals as phenocrysts phase. Leucite is found in one sample of Old Lurus with the lowest SiO2 content (48.92 wt%) and highest alkali content (5.38 wt%). Old Lurus lavas tend to show higher contents of incompatible elements (Pb, Ba, Nd, and Rb) than Young Lurus. The mineralogical and compositional differences between these lavas suggest that these magmas are sourced from at least three magma chambers of distinct composition: highly potassic magma chamber (comparable to the evolved-K series of Ringgit-Beser) and hydrous-potassic magma which fed Old Lurus edifice; and calc-alkaline magma chamber which fed Young Lurus edifice.

Insights into the late Archean evolution of the Carajás Province: Geochemical and geochronological constraints from the Santana do Araguaia complex, Amazonian craton, Brazil

To the south of the Carajás Province, Amazonian Craton, 2.85–1.88 Ga old rock units underlie the Santana do Araguaia Domain (SAD), bounded by the Iriri-Xingu and Rio Maria domains and the Tocantins Province. The SAD differs from the Rio Maria Domain, as indicated by the scarcity of TTGs and the dominance of both high-K to weakly sodic granites, sodic granodiorites, and monzodiorite/quartz monzodiorite with high-Ba + Sr contents. Therefore, the SAD is a particular case to assess tectonic settings in the Archean, continental crust generation and the timing/nature of the Meso-Neoarchean transition from TTG-type to calc-alkaline high-K and sodic granitoid magmatism to the south of the Rio Maria Domain. The magmatic evolution of the SAD started at 2.85 and extended up to ∼1.88 Ga with a significant contribution from the pre-existing crust, differing from that of the Rio Maria Domain (3.0–2.86 Ga). The Santana do Araguaia Complex (SAC) comprises deformed to non-deformed rocks divided into (i) tonalite (TTG with high Al2O3 and medium La/Yb ratio), (ii) sodic granodiorite group (K2O/Na2O < 1) with Ba- and Sr-rich varieties, which can be divided into type 1 displaying high Sr/Y and La/Yb ratios, and type 2 with low Sr/Y and low to intermediate La/Yb, both resembling the sanukitoids low-Ti, (iii) hybrid granite group (high-K calc-alkaline), composed of two sub-types of high-Ba + Sr monzogranites, one of high Sr/Y that shares many features with the CA2-type of Archean calc-alkaline granites and HSG, and a second type, with low Sr/Y ratio and akin chemistry with the CA1 and normal granite (NG). Group (iv) is of biotite or low Ba–Sr granites classified as syenogranites CA1/NG strongly fractionated calc-alkaline rocks. Group (v) consists of rocks with high Ba–Sr but with moderate Mg# (mean = 44.8), low contents of transition elements, and moderate K2O/Na2O ratios (mean = 0.72); the group is divided into monzodiorite with high Sr/Y, La/Yb ratios, and low Y, and quartz monzodiorite with low Sr/Y, moderate La/Yb ratio and Y values, both with sanukitoids high-Ti characteristics. The rocks that constitute the studied complex show arc-like geochemical signatures, suggesting emplacement during the end of phase B (collisional) and the start of phase c (thermal relaxation/colappse) of stage 2 in the two-stage tectonic-magmatic model (subduction/collision-post-collision). In the Carajás Province the first stage of the model is marked by the genesis of TTG (3.0–2.94 Ga) –juvenile magmatism of the Rio Maria Domain, and the second stage, collisional phase, between 2.89 and 2.85 Ga with sanukitoid rocks, hybrid and biotite granites as well as significant occurrences of high Ba + Sr rocks and post-collisional granitic suites (∼2.74 Ga). However, the elemental whole-rock geochemistry and geochronology datasets show that the Santana do Araguaia Complex (SAC) is marked by high-K and Ba + Sr rocks such as abundant sanukitoid suites, hybrid and biotite granites, but with very scarce TTGs. The ages between 2.85 and 2.81 Ga obtained from SAC suggest the final phase “a” and beginning of phase “b” of the second evolutionary stage, collisional phase, and slab-breakoff/post-collisional, probably the terminal tectonic-thermal episode of Archean accretion and differentiation in the Amazonian Craton. The SAC rocks' contrasting Sr/Y and La/Y ratios cannot be explained exclusively by variation in the melting pressure (melts of mafic crust at >50 km) in a thickened continental crust. Multiple crustal petrogenetic processes or heterogeneous sources can produce the heterogeneous geochemical signatures that characterize the rocks of the complex. The admitted sources of these Archean granitoid magmas include basaltic andesite, sanukitoids, and metagreywackes, subordinate TTGs, at low to moderate pressures melting as they suggest low values of (Dy/Yb)n below 1.9 and relatively high-Sr contents. The mesostructure data indicate that the studied rocks were affected by high-temperature deformation (>600 °C) with partial melting.

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

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

Proterozoic mafic dyke swarms of Bundelkhand Craton, North India: A connection to Columbia supercontinent

The present paper addresses petrography, geochemistry and Ar‐Ar geochronology of a significant number of mafic dykes from the Paleo‐ to Neoarchean Bundelkhand Craton in central India. The majority of the dykes are NW‐SE oriented (with a few NE‐SW and ENE‐WSW) with tholeiitic, sub‐alkaline and basalt to basaltic andesite composition. The trace element geochemistry of these dykes indicates an island arc setting during emplacement. The Ar‐Ar mineral dating (plagioclase) of three representative dykes reveals an emplacement age between 1.53 and 1.46 Ga. This finding and earlier reports (2.1–1.73 Ga) point to sustained mafic magmatism throughout the Bundelkhand Craton in a preferred structural orientation between 2.1 and 1.46 Ga. Mafic magmatism was episodic and can be linked to the perpetual subduction accretion processes between the central Indian Archean continents during the development of the Columbia supercontinent. The mafic dykes were emplaced at 45° to the maximum compression direction (E‐W), that is, along the line of no finite longitudinal strain. This time equivalent widespread NW‐SE and NE‐SW trending mafic dyke system is also relatable along the adjacent continents (Singhbhum, Bastar) and thus opened up a new paradigm for the dyke's emplacement across the Indian cratons.

Pleistocene to recent evolution of Mocho-Choshuenco volcano during growth and retreat of the Patagonian Ice Sheet

Mocho-Choshuenco volcano (39.9°S, 72.0°W) produced ∼75 explosive eruptions following retreat of the &amp;gt;1.5-km-thick Patagonian Ice Sheet associated with the local Last Glacial Maximum (LGM, from 35 to 18 ka). Here, we extend this record of volcanic evolution to include pre- and syn-LGM lavas that erupted during the Pleistocene. We establish a long-term chronology of magmatic and volcanic evolution and evaluate the relationship between volcanism and loading/unloading of the Patagonian Ice Sheet via twenty-four 40Ar/39Ar and two 3He age determinations integrated with stratigraphy and whole-rock compositions of lava flows and glass compositions of tephra. Our findings reveal that the edifice is much younger than previously thought and preserves 106 km3 of eruptive products, of which 50% were emplaced immediately following the end of the penultimate glaciation and 20% after the end of the LGM. A period of volcanic inactivity between 37 and 26 ka, when glaciers expanded, was followed by the eruption of incompatible element-rich basaltic andesites. Several of these syn-LGM lavas dated between 26 and 16 ka, which crop out at 1500−1700 m above sea level, show ice contact features that are consistent with emplacement against a 1400- to 1600-m-thick Patagonian Ice Sheet. Small volume dacitic eruptions and two explosive rhyolitic eruptions dominate the volcanic output from 18 to 8 ka, when the Patagonian Ice Sheet began to retreat rapidly. We hypothesize that increased lithostatic loading as the Patagonian Ice Sheet grew prohibited dike propagation, thus stalling the ascent of magma, promoting growth of at least three discrete magma reservoirs, and enhancing minor crustal assimilation to generate incompatible element-rich basaltic andesitic to dacitic magmas that erupted between 26 and 17 ka. From an adjacent reservoir, incompatible element-poor dacites erupted from 17 to 12 ka. These lava flows were followed by the caldera-forming eruption at 11.5 ka of 5.3 km3 of rhyolite from a deeper reservoir atop which a silicic melt lens had formed and expanded. Subsequent eruptions of oxidized dacitic magmas from the Choshuenco cone from 11.5 to 8 ka were followed by andesitic to dacitic eruptions at the more southerly Mocho cone, as well as small flank vent eruptions of basaltic andesite at 2.5 and 0.5 ka. This complex history reflects a multi-reservoir plumbing system beneath Mocho-Choshuenco, which is characterized by depths of magma storage, oxidation states, and trace element compositions that vary over short periods of time (&amp;lt;2 k.y.).

Tectonic evolution of the South Pamir Orogen: Insights from the Permian to cretaceous magmatism

The Pamir orogen originated through the amalgamation of various arc terranes. Despite undergoing numerous studies, the subduction polarity and time regarding the accretion processes remain inadequately constrained. To address these issues, we carried out a detailed study of zircon and apatite U-Pb ages, zircon Hf isotopes, and whole-rock geochemistry for the Permian to Cretaceous gabbro, volcanic rocks and granitoids from the Southern Pamir and the Rushan-Pshart suture. Three stages of magmatism are identified, including 279–266 Ma gabbro and andesite to dacite, 209–208 Ma granitoids, and 108–103 Ma gabbro and granitoids, respectively. The Permian gabbro and basaltic andesite to dacite in Pshart Range are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but are depleted in Ta, Nb, and Ti, suggesting formation from an enriched mantle source in a subduction zone. The Late Triassic I-type granitoids are typical arc calc-alkaline magmatic rocks with εHf(t) values varying from −8.7 to −3.5, which mainly sourced from the partial melting of mixed crustal sources predominately consisting mafic rocks with minor metasedimentary rocks. The Early Cretaceous Pshart granites are I-type granites with relatively low εHf(t) values ranging from −14.6 to −2.9, suggesting that they were crust-derived rocks. The Early Cretaceous Bazardara granites are identified as A2-type granites with the εHf(t) values ranging from −18.3 to +5.3, and were derived from mixing of mantle-derived and crust-derived magmas, or interaction between mantle melts and old crustal rocks. The early Cretaceous gabbro exhibits -MORB-like geochemical characteristics with flat REE pattern (La/YbN = 4.96), moderate enrichment of LILEs and weak depletion of Nb, Ta and Ti (Nb/LaPM = 0.94). Thus, the early Cretaceous A2-type granite, E-MORB-like gabbro and volcanic rocks were formed in an extensional setting. Our new data indicate that the southward subduction of the Rushan-Pshart oceanic slab started in the Early Permian and continued to the Late Triassic. Moreover, combining the previous data, we conclude that the Early Cretaceous granites and gabbro in the Southern Pamir formed in an extensional environment due to the southward rollback after northward near-flat subduction of the Neo-Tethyan oceanic lithosphere.