Volcanic Rock Types Research Articles

Petrography and Lithofacies of the Siwalik Group in the Core of Hazara-Kashmir Syntaxis: Implications for Middle Stage Himalayan Orogeny and Paleoclimatic Conditions

The present field and petrographic investigations of the Tortonian to Gelasian Siwalik Group in the core of the Hazara-Kashmir Syntaxis have been carried out to comprehend the middle stage Himalayan orogeny that resulted from the collision of Indian and Asian plates. The Chinji, Nagri, Dhok Pathan, and Soan Formations of the Siwalik Group were deposited by river meandering flood plains, braided rivers, and alluvial fan systems, respectively. The Siwalik Group is classified into seven major facies and many minor facies based on sedimentological properties. According to the petrographic analysis, the Siwalik Group sandstone is classified as litharenite and feldspathic litharenite petrofacies. The sandstone of the Siwalik Group is texturally mature, but compositionally it is immature. The data shown on the tectonic discrimination diagrams point to a recycled orogen provenance field for the Siwalik sandstone. In addition to quartz and feldspar, the sandstone includes clasts of volcanic, metamorphic, and sedimentary rock types. The igneous and metamorphic rock clasts were derived from the Lesser and Higher Himalayas. The sedimentary lithic fragments, on the other hand, are derived from both the earlier molasse and pre-molasse rocks. The presence of lithic fragments of the earlier molasse sandstone in the Siwalik sandstone indicates that the Siwalik Group sandstones were deposited during the Middle Stage of the Himalayan orogeny. The paleoclimatic conditions were semi-arid to semi-humid during the Siwalik Group’s deposition. The presence of clay minerals in the shale reveals the intense chemical weathering processes that occurred during their deposition on the flood plains of the river meandering system.

The first evidence of microplastic presence in pumice stone along the coast of Thailand: A preliminary study

In February and March 2022, a large amount of pumice stone appeared along the shoreline of Thailand. Pumice is a type of extrusive volcanic rock, and since there are no volcanoes in the Gulf of Thailand, an interesting question was where the pumice stones originated from. Another question was whether the pumice could be a vehicle for microplastics (MPs) which could then journey across the ocean until reaching the coast of Thailand. A preliminary study was begun, which randomly collected samples from seven beaches in five provinces along the coast of Thailand. Grayish-green pumice stones are tiny, porous, and lightweight, ranging from 0.3 to 5.0 cm in size. The examination found 5.7–12.6 MP items per pumice stone. Most of the MP particles observed were less than 1 mm in length. From Fourier transform infrared spectroscopy (FTIR) analysis, the MPs were characterized as polystyrene, polypropylene, poly (ethylene terephthalate) (PET), rayon, and nylon. The MP could have entered the holes in pumice stones while floating on the water surface over long periods. From the seasonal flow patterns, it was revealed that pumice from the South China Sea was more likely to have floated with surface currents into the Gulf of Thailand

A Preliminary Volcanological Study of North Eastern Kaba Volcano, Bengkulu Province, Indonesia

Mount Kaba is one of the active volcanoes located in the eastern part of Bengkulu Province. This research was conducted in Gunung Kaba Complex because the distribution location of the last eruption product is close to some monogenetic volcano and residential area. This is interesting to study considering because can be a potential threat of disaster. The results of this study are expected to support providing an overview of the potential threat of volcanic disasters in the Mount Kaba Complex. The research methods used include geomorphological, stratigraphic, petrographic, and geochemical analyses. The geomorphological unit of Mount Kaba consists of 6 units. The types of volcanic rocks in the research area are dominated by lava and pyroclastic rocks. Based on the petrographic analysis of 10 samples, the names of the rocks in the research area are basalt scoria, basalt, and andesite. Based on the geochemical analysis of 8 samples, it is known that silica content in the rocks ranging from 44.40925 - 63.2993 %. Based on the AFM diagram, all samples are included in the tholeiitic series. Based on the TAS (Total Alkali Silica) diagram, it is known that the igneous rocks of the study area consist of andesite, basalt, tephrite, and trachyandesite.

IBM-type forearc magmatism in the Qilian Orogen records evolution from a continental to an intra-oceanic arc system in the Proto-Tethyan Ocean

Forearc magmatism in IBM-type intra-oceanic arc systems is crucial for understanding the processes of subduction initiation and subsequent evolution of the arc system. Here we report a forearc volcanic sequence exposed in the Danghe-Nanshan, at the westernmost end of the South Qilian Accretionary Belt of the Qilian Orogen, NW China. Based on petrography and bulk-rock composition, three types of volcanic rocks (boninite, high-Mg basaltic andesite and andesite) and arc-related felsic plutons are identified. These rocks show correlating trends of trace elements and isotopes. The high-Cr spinels and clustered Ti/V ratios indicate that the boninite and high-Mg basaltic andesite magmas were derived from highly refractory peridotite sources in a forearc setting, whereas the andesite and felsic plutons were formed by fractionation of clinopyroxene and plagioclase from primary magma. Notably, slab-derived components have limited involvement in magma generation. Variation of fluid mobile/immobile element contents and Sr-Nd isotopic mixing calculations suggest less than 1% incorporation of subducted slab-derived fluids into the boninite source, whereas ∼1–3% fluids and ∼3–7% partial melts from the slab contributed to the high-Mg basaltic andesite and andesite, respectively. Therefore, an abnormally high mantle potential temperature (>1429 °C) was required for boninite production because there were little aqueous fluids involved. Zircon U-Pb dating reveals these forearc magmas formed at ∼450 Ma. Combined with the regional tectonics and published data in this region, we suggest that these magmas record a tectonic process of arc system from active continental margin to newly formed intra-oceanic subduction zone during trench retreat of the Proto-Tethys Ocean.

Geochemistry of mafic-ultramafic rocks of the 3.33 Ga Kromberg type-section, Barberton greenstone belt, South Africa: Implications for early Earth geodynamic processes

In conjunction with major, trace and rare earth element data, this study presents the first whole-rock LuHf isotope analyses of mafic-ultramafic rocks of the c. 3.3 Ga Kromberg type-section in the Barberton greenstone belt of South Africa. Three compositionally distinct volcanic rock types are identified namely Group 1 metabasalts, Group 2 metabasalts and komatiitic metabasalts. Group 1 metabasalts are FeMg tholeiitic with compositions that plot close to the Primitive Mantle composition, have isochron intercept εHf(t) value of 0.01 ± 0.4 and Eoarchean Hf model ages. Group 2 metabasalts have very high Al2O3/TiO2 ratios (c. 50), small negative Nb anomalies, LREE-depleted slightly concave up REE patterns, and negative εHf(t) values. The komatiitic basalts variably record small negative Nb anomalies in a primitive mantle normalized plot. Group 1 metabasalts and cumulate peridotites (wehrlites) yield a LuHf isochron age of 3.340 ± 0.190 Ga (95% confidence level). The highly variable major and trace element geochemistry of the rocks in the sequence implies highly dynamic and variable mantle melting environments. The Group 2 metabasalts have geochemical characteristics similar to that of boninites and are interpreted to have formed from a shallow, refractory mantle source. Group 2 metabasalts are argued to have formed in a proto-plate tectonic back-arc basin involving crustal contamination, whereas Group 1 metabasalts likely formed from within a deep mantle plume.

Mapping of Landslide Potential Zone with Frequency Ratio Method and Mitigation Efforts in Banjarnegara Regency, Central Java, Indonesia

Indonesia is a country with a very high potential for disaster. This is supported by the existence of Indonesia which is surrounded by three world plates, namely the Eurasian plate, the Indian-Australian plate and the Pacific plate. One of natural disaster which occur in Indonesia is landslide. This situation has similarities with the research area located in the Banjarnegara Regency, Central Java. Aim of this research is to provide information on the level of influence of the class and conditioning factors of landslides, and a picture of areas that have vulnerability to landslides and as an effort to provide information to related parties. This research was conducted by geological mapping and remote sensing using DEM (Digital Elevation Model) and landsat-8 as field support data and visualization of the condition of the research area. The results of this study found that the results of mapping with frequency ratio method, the percentage of low insecurity is 66.83%, moderate insecurity is 17.94%, high insecurity is 1.87%, and very high insecurity is 0.02%. Areas with very high land movement potential are in karangkobar and pandanarum districts with a total area of 0.094589 km². The area has a very high potential because it has litology with volcanic rock types, very low vegetation and the presence of straightness. Therefore, various mitigations are needed both structural and non-structural.

Characteristics of Carboniferous Volcanic Reservoirs in Beisantai Oilfield, Junggar Basin

In recent years, petroleum exploration in the Carboniferous volcanic rock reservoirs in the Junggar Basin has been the focus of important petroleum energy development in western China. The lithologic identification of volcanic rock reservoirs seriously restricts the accuracy of reservoir prediction and affects the success rate of oil exploration. Different types of volcanic rocks have different petrological characteristics and mineral assemblages, especially affected by the depositional environment. The volcanic rocks in different regions have their own uniqueness. This paper takes the Carboniferous volcanic reservoirs in Xiquan block, Beisantai Oilfield, Junggar Basin as the research target. Through a large number of core observations, casting slices, scanning electron microscopy, and X-ray diffraction methods, the Carboniferous volcanic rocks are analyzed. The petrology, pore characteristics, physical properties, and diagenetic evolution history of the reservoir are analyzed. The study shows that the volcanic facies in the Xiquan block can be divided into explosive facies, overflow facies, and volcanic sedimentary facies, among which the explosive facies is subdivided into empty subfacies (volcanic breccia-breccia tuff combination) and thermal base wave subfacies (tuff). The lithology of the reservoir is pyroclastic rock and volcanic lava, belonging to medium-porous and ultralow permeability reservoirs, and the storage space can be divided into three types: primary pores, secondary pores, and fractures. The lithology of key exploration is breccia tuff, followed by breccia tuff and volcanic breccia.

Quantitative characterization of fracture-pore distribution and effects on production capacity of weathered volcanic crust reservoirs: Insights from volcanic gas reservoirs of the Dixi area, Junggar Basin, Western China

Igneous hydrocarbon reservoirs worldwide are of significant importance to the petroleum industry for oil and gas exploration. The Carboniferous weathered volcanic crust reservoirs of the Dinan salient in the northern Junggar Basin are major exploration targets of PetroChina. The present study analyzes the geological, well logging, and production data to formulate development and distribution methods for quantitative characterization of weathered volcanic crust fractures and examine their fracture-pore distributions. The effects of the fracture-pore distribution on the productivity of various lithologies of weathered volcanic crust reservoirs were investigated. Various volcanic lithofacies demonstrate the lithological differences in their physical properties, fracture development, and distributions due to the conspicuous variations in weathering resistance among different volcanic rock types. The present study revealed the level of physical properties of volcanic rocks, which are ranked as follows: volcanic breccia > mafic lava > tuff > felsic intrusive rock. However, the degree of fracture development ranked differently from most developed to least developed as follows: felsic intrusive rock > mafic lava > volcanic breccia > tuff. The fracture distribution was primarily controlled by lithology, rock stratum thickness, degree of weathering and leaching, along with tectonism. Compared to other rock types, felsic intrusive rock and mafic lava were more prone to fracturing owing to different petrophysical properties. Additionally, fractures typically developed in single lithological bodies with a thickness of less than 15 m and were characterized by the “single-section” developing mode. For lithological bodies of thickness greater than 15 m, fracture development was more limited, primarily concentrated at the top and bottom of the lithological body and characterized by the “bi-section” or “tri-section” developing modes. These differences can be quantitatively characterized by the fracture development thickness, fracture layer density, and fracture development thickness ratio. Moreover, the influence of the degree of fracture development on the productivity of mafic lava and felsic intrusive gas reservoirs was higher than that of the porosity; however, volcanic breccia and tuff exhibited the opposite effect. The production capacity of weathered volcanic crust reservoirs is affected by the following factors in the order of degree of fracture development > porosity > lithology > thickness of a single lithological body > distance from the top surface of the weathered crust. These quantitative characterization methods can provide valuable technical support for optimizing favorable exploration targets.

FEATURES OF VOLCANIC ROCKS AS MATERIALS FOR POLYMERIC COPOSITES

Fillers or reinforcing components are an integral part of composite materials. A classification of composites according to structural features and fineness has been adopted. Ideas about the effect of fillers on the physical, mechanical and operational characteristics of composite materials and products have been defined and are constantly being deepened. In this regard, the use of volcanic rocks as fillers is noteworthy. The prospects for the use of certain types of volcanic rocks as disperse fillers of polymer composites are justified by their significant distribution and accumulation of fine fractions of screenings as waste during their extraction and processing. At the same time, the achievement of the effectiveness of such raw materials requires a deepening of ideas about the composition and physico-chemical properties, in the direction of which this work has been done. Physical and chemical studies in terms of evaluating the suitability of volcanic rocks (perlite, zeolite, andesite) as ingredients of polymer composite materials made it possible to obtain quantitative data on their chemical and mineralogical composition, hydrophilic-lyophilic balance and energy state of their surface. The interpretation of the obtained results concerning the potential interaction of the studied materials with polymeric binders is given taking into account the above features of their composition, structure and quantitative IR spectroscopy data in terms of structure-forming bonds Si–O–Si, S –O–Al, hydroxyl groups and adsorbed water. These features indicate the possibility of choosing volcanic rocks as fillers for polymer composites with the required lyophilic-lyophobic balance and energy state of their surface and providing an unlimited level of interaction with polymer binders as a factor in shaping the structure and properties of composites.

PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS ON TECTONIC SETTINGS OF THE BIRIMIAN SUPERGROUP VOLCANIC ROCKS, EVIDENCE FROM NEW DROBO ENVIRONS SOUTH OF JAMAN DISTRICT IN THE BONO REGION OF GHANA

The petrographic and geochemical studies of Birimian Supergroup meta-volcanic rocks in the New Drobo environs in Ghana help to decipher the tectonic settings at the study area. Twenty thin sections were prepared with rock samples from the field at the KNUST Geological Engineering Laboratory and petrographic microscope used to determine the different types of minerals in the samples and also the rock type. Whole rock geochemical analysis was done, using both X-Ray Fluorescence and Inductively Coupled Plasma Mass Spectrometer methods at Australian Laboratory Services in Canada. The main volcanic rock types at the study area comprise of basalt, andesite, dacite and rhyo-dacite with porphyritic mineralogical composition such as plagioclase feldspar, augite, olivine, hornblende, biotite, quartz, rutile, chlorite. The average concentrations of Zr < 150 ppm, TiO2 < 1.5 % and P2O5 < 0.25 % and the ratios of Nb/Y < 1.2 and Y/Nb > 1 reveal the magma type in the study area to be continental tholeiitic basalt in nature. The mafic volcanic basalt is from a plate margin tectonic setting and thus of normal MORB and volcanic arc basalts. Also, the low basaltic values of the ratios of Ti/Y and Nb/Y confirm the tectonic setting of the area to be of plate margin. Zr is highly incompatible with respect to Ti and renders the ratio of Zr/TiO2 to be influenced by partial melting and causes heterogeneity in the mantle. The heterogeneity in the source of the mantle can be confirmed with the high value of the ratio of Zr/Y.

The millstone trade from the most exploited Italian volcanic areas: an overview from the phoenicians to the roman period

Lavas were widely used in antiquity to produce millstones. This is mainly due to their superior properties for grinding cereals and availability when compared with other rock-types. In the past four decades, several studies have been published about lava millstones discovered in subaerial and submarine archaeological sites of the Central-Western Mediterranean. Although the morphological evidence of old quarries is rarely present, all these studies were aimed at recognizing provenance and manufacturing areas of the volcanic raw material. Typologies of grinding tools coexisted in different periods, even if some technological developments marked transitions between cultures. The main chronology is: Archaic saddle quern, Greek hopper-rubber (Olynthian), small to medium size rotary device (Morgantina type) and large hourglass rotary millstone (Pompeian style). Potential volcanic sources are widespread throughout the entire Mediterranean region, but two main Italian quarrying areas of volcanic rocks for the manufacture of millstones from the Phoenician to the Roman period were pointed out. These are the Latium-Umbria border in Central Italy, and Sicily (Eastern Sicily and Sicilian Channel) in Southern Italy. In detail, analysis of the lava lithotypes shows that grinding tools were mainly constructed of: (i) a leucite phonolite of the so called “Orvieto quarries” between the localities of Sugano and Buonviaggio in the Roman Volcanic Province (High-K alkaline series); (ii) hawaiites and mugearites (Na-alkaline series) from Etna volcano; (iii) basalts (Tholeiitic/Transitional series) of the Hyblaean Mountains and (iv) basalts (Na-alkaline series) from Pantelleria Island (Sicilian Channel). Although some lava millstones from other volcanic regions are recorded, the above four Italian volcanic rock types represent the most exploited in antiquity. A comparison between volcanic millstones and outcropping lavas already exists, from literature data, through thin section modal mineralogy and conventional igneous petrology (i.e., TAS classification, magmatic affinities, and major-trace elements signature). Therefore, on this basis we propose a set of discriminating geochemical parameters (major-trace elements and element ratios diagrams) useful for a quick assessment tool to possibly evaluate one of these four exploited volcanic areas of Italy matching millstones. A sketch of volcanic millstone trade networks and commercial routes in antiquity throughout the Central-Western Mediterranean has been also reported and overviewed on the basis of the literature data.

Interaction of magma and wet, unconsolidated sediments: a case study from Ilindenski Kamak, Chelopech Formation, Central Srednogorie

The Ilindenski Kamak is situated along the Ilindenska river valley, northwest of Chelopech village, and is composed of different types of volcanic and volcaniclastic rocks, which are interbedded by sandstones and mudstones, all of Late Cretaceous age. The architecture of the pile of extrusive andesitic rocks is interpreted as a subaqueous cryptodome or sill. The presented research deals with a variety of volcanic facies: 1, coherent andesitic domain; 2, quench fragmented domain (closely packed peperites); 3, globular peperites developed at the contact of wet, unconsolidated sediments.

Insights into the pore structure and pore development pattern of subaqueous volcanic rocks in the Santanghu Basin, western China

Pore structure plays an important role in controlling the oil migration and accumulation mechanism of subaqueous volcanic rocks (SVRs). The Santanghu Basin in western China is a unique region for providing insight into the pore structure characteristics of SVRs; however, little attention has been given to them. Thin-section analysis, scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), pressure-controlled porosimetry, and nuclear magnetic resonance (NMR) experiments are performed on SVRs in the Santanghu Basin to reveal their pore structure characteristics and pore development patterns. The results show that the SVRs are dominated by ink bottle-shaped pores and have the characteristics of rather low matrix permeabilities (the Swanson parameters are mainly <4.0%/MPa), fine pore throats (rapex mainly <0.5 μm) and high capillary pressures (P50 > 14.0 MPa). An effective method, namely, integrating NMR and LTNA experiments, is proposed to estimate the full-range pore size distribution (PSD) of the SVRs. In the full-range PSD, three peaks are identified at pore sizes of <1 μm, 1–10 μm and >10 μm, mainly corresponding to intercrystalline pores (IPs), dissolved pores (DPs), and residual vesicles (RVs), respectively. The storage space of SVRs is dominated by IPs, while the contents of DPs and RVs are the key to SVRs becoming effective reservoirs. The content of RVs is mainly controlled by the depth of the water body during deposition and the distance from the volcanic strata boundary. The higher the content of K and Na in the original composition and the closer the distance to the source rock is, the higher the proportion of DPs. In accordance with the original composition and the depth of the water body during deposition, the SVRs are divided into four types. Moreover, we propose the pore development patterns of the four types of volcanic rocks. RVs mainly exist in type I and type II SVRs, while DPs are important storage spaces in type I and type III SVRs. Finally, type IV SVRs only develop IPs and a few nanoscale DPs.

A new insight into the eastern extension of the Proto-Tethyan margin of Gondwana by Early Paleozoic volcanic rocks in South China

South China is located in a key area of East Gondwana. The volcanic rocks in the unambiguous tectonic setting of Hainan Island were systematically studied based on detailed field, petrological, geochemical, and UPb zircon geochronological data. The petrology and geochemistry reveal three types of volcanic rocks, i.e., andesites, N-MORB-type metabasalts, and E-MORB-type metabasalts. Their eruption ages are well defined between 446 and 430 Ma. The andesite is characterized by negative Nb, Ta, and Eu anomalies and positive Th and U anomalies. The E-MORB-type metabasalt is characterized by Rb, Ba and Sr depletions, slight Nb and Ta enrichments, and negative Eu anomalies. The N-MORB-type metabasalt has more depleted trace element compositions and is generally enriched in Rb, Ba, and Sr; depleted in Nb and Ta; and it does not exhibit significant Eu anomalies. The whole rock SrNd isotopic analyses show that the andesites, N-MORB-type metabasalts, and E-MORB-type metabasalts have εNd(t) values of −0.2 to −3.6, +3.7 to +6.8, and + 5.5 to +6.7, respectively. These results suggest that their formation is closely related to the subduction of Proto-Tethyan oceanic crust. These detailed descriptions of the Early Paleozoic volcanic rocks on Hainan Island indicate that South China was likely the eastern extension of the Proto-Tethyan margin and was located on the easternmost periphery of East Gondwana.

Geochronology and Geochemistry of Late Paleozoic Volcanic Rocks and Their Relationship With Iron and Molybdenum Deposits in Xilekuduk Area, Northern Margin of Junggar

A large number of intermediate basic volcanic rocks and porphyry Cu-Mo deposits as well as volcanic-hosted magnetite deposit have been recently discovered in the Xilekuduk area. However, no reports concerning petrogenesis and age or their relationship with mineralization have been published to date. The purpose of this study was to make up for the absence of previous studies on Devonian volcanic activities in the area and to confirm the relationship between two stages of volcanic activities and mineralization so as to provide important theoretical basis for mineral exploration. Based on research results of zircon U-Pb geochronology and element geochemistry of volcanic rocks in the area, the ages of dacite, andesite, and stomatal andesite are considered as 375.2 ± 2.9 Ma, 386.5 ± 3.0 Ma, and 317.9 ± 2.9 Ma, respectively, corresponding to the Middle Devonian and Late Carboniferous Period. The Devonian volcanic rocks belong to the high-K calc-alkaline series and island arc volcanic rocks, which are enriched in LREE, strongly enriched in large ion lithophile elements Th, Rb, Ba, and K and relatively depleted in high-field strength elements (HFSEs) Nb, Ta, and Ti. The Carboniferous volcanic rocks are enriched in LREE, as well as the large ion lithophile elements Th, Rb, Ba, and K are strongly enriched, while depleted in the HFSEs Nb, Ta, and Ti; moreover, the contents of TiO2and V are 0.94–0.97% and 178–183×10–6, which are higher than those of island arc basalts. According to mineralogical typomorphic characteristics and geochemical analysis, magnetite mineralization is divided into two phases. The early stratiform magnetite ore MT1 has magmatic characteristics, forming a volcanic rock type magnetite deposit related to Devonian volcanic eruption and sedimentation (375–386 Ma). The magnetite MT2 in the magnetite-quartz vein is considered as hydrothermal genesis, which is a metal mineral in the early metallogenic stage of Carboniferous (317.1 ± 2.9 Ma) volcanic eruption and subvolcanism, and may be related to porphyry molybdenum mineralization. Therefore, the volcanism and Fe-Cu-Mo mineralization in this area is characterized by multistage superimposed mineralization.

Nephelinites in eastern China originating from the mantle transition zone

Nephelinites, a highly alkaline volcanic rock type, have widely been thought to be formed in a shallow lithosphere previously impregnated with phlogopite and/or amphibole. Here we argue that nephelinites start melting at much greater depths than previously thought. Taking four Cenozoic nephelinitic volcanoes in eastern China as specific examples, we show that nephelinites have exceptionally low K/U and Ba/Th ratios, similar to most nephelinites worldwide, but their Nd-Sr-Hf isotopic compositions are inconsistent with a local lithospheric origin. They also have low, marine‑carbonate-like δ26Mg. We explain these features by partial melting of recycled, carbonate-bearing sediments in the mantle transition zone (MTZ; 410–660 km), where the high-pressure mineral liebermannite (KAlSi3O8, previously known as K-hollandite) is stable as a residual phase, preferentially retaining K, Ba, Rb and Pb relative to Th, U and light REE, thus releasing liquids with the characteristic trace-element patterns. Group-I kimberlites and oceanic HIMU basalts globally share the same peculiar geochemical anomalies characterizing the nephelinites, suggesting they may all have their roots in the MTZ.The nephelinites show correlations for Mg-Sr isotopes, 143Nd/144Nd-MgO, and δ26Mg-Ba/Th ratios, suggesting binary mixing between melts. We therefore invoke a two-stage process to generate these nephelinites. In the first stage, an initial carbonated silicate melt is formed by melting of carbonate-bearing sediments in the MTZ. Subsequently, during the ascent, the initial melt reacts with the surrounding peridotite, thereby generating a reacted melt, which ultimately mixes with the initial, unreacted carbonated silicate melt. Accordingly, Cenozoic nephelinites in eastern China can be regarded as evolved melts from the MTZ.

Late Jurassic–Early Cretaceous irregular slab rollback of paleo-Pacific plate beneath southeastern China: Insights from the petrogenesis of volcanic rocks of Moshishan Group in Dazhou volcanic basin, Gan-Hang Belt

The geodynamic model of the paleo-Pacific plate subduction beneath southeastern China during the Late Mesozoic, though generally accepted, remains controversial with respect to the exact plate subduction model. The Moshishan Group volcanic strata are well developed in the Dazhou volcanic basin in the Gan-Hang Belt (GHB). In this study, detailed investigations on geochronology, elemental geochemistry, and isotopic compositions of these volcanic rocks were performed to constrain their formation ages and petrogenesis as well as the tectonomagmatic evolution of the GHB. LA-ICP-MS zircon UPb dating results show that the volcanic eruptions in the Dazhou volcanic basin began approximately 150.9 ± 2.7 Ma to 128.8 ± 1.9 Ma and lasted for ~20 Myr from the Late Jurassic to Early Cretaceous. These volcanic rocks show high Ga/Al ratio, high K2O + Na2O and Zr + Nb + Ce + Y contents, and high formation temperature (~812–1022 °C), indicating A-type granite affinity. Such types of A-type volcanic rocks formed in an extensional tectonic environment, likely corresponding to a back-arc extension due to the slab rollback of the paleo-Pacific plate. The εNd(t) values of the volcanic rocks of Moshishan Group vary from −11.60 to −5.41, and the zircon εHf(t) values vary from −18.30 to −3.84, indicating that these volcanic rocks were mainly derived from the partial melting of para-metamorphic rocks. Different proportions of mantle-derived materials were also involved in their sources, such as little or no addition of mantle-derived material in the early Dashuang Formation. However, a significantly increased proportion of mantle-derived material addition was found in the later Gaowu Formation, Xishantou Formation, and Jiuliping Formation. We found that the volcanic rocks in the southwestern GHB were formed during the Early Cretaceous (137–132 Ma), while those in the northeastern GHB formed during the Late Jurassic to Early Cretaceous (150–120 Ma). Jurassic calc-alkaline granites (181–151 Ma) that formed in a continental arc environment related to the paleo-Pacific plate subduction are also distributed along the GHB. This indicates that the shift from an arc environment dominated by the subduction of the paleo-Pacific plate to a back-arc extensional environment characterized by slab rollback began at ~150 Ma in the GHB. We propose an irregular slab rollback model beneath southeastern China to interpret the distribution pattern of the volcanic basins in the GHB. The slab rollback in the GHB did not initiate simultaneously, or with homogeneous velocity, throughout the region. Slab rollback in the southwestern part of the GHB occurred between 137 and 132 Ma, whereas in the northeastern part, rollback occurred between 150 and 120 Ma. In addition, the mantle-crust interaction that produced A-type granitic magmas along the northeastern part of the GHB gradually intensified from the early to late stage between ~150 and ~ 120 Ma.

Automatic classification of volcanic rocks from thin section images using transfer learning networks

In this study, efficient deep transfer learning models are proposed to classify six types of volcanic rocks, and this paper has a novelty in classifying volcanic rock types for the first time using thin section images. Convolutional neural network-based DenseNet121 and ResNet50 networks, which are transfer learning methods, are used to extract the features from thin section images of rocks, and the classification process is carried out with a single-layer fully connected neural network. The proposed models are trained and tested on 1200 thin section images using four different optimizers (Adadelta, ADAM, RMSprop, SGD). AUC, accuracy, precision, recall and f1-score are used as performance metrics. Proposed models are run 10 times for each optimizer. DenseNet121 classifies volcanic rock types using RMSprop with an average accuracy of 99.50% and a maximum of 100.00%, and ResNet50 classifies using ADAM with an average accuracy of 98.80% and a maximum of 99.72%. Thus, the applied deep transfer learning is promising in geosciences and can be used to identify rock types quickly and accurately.

Paleoarchean variole-bearing metabasalts from the East Pilbara Terrane formed by hydrous fluid phase exsolution and implications for Archean greenstone belt magmatic processes

Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original volcanic textures and relict magmatic minerals, which are invaluable for inferring volcanic processes and by extension magma genesis. A feature observed in many Archean greenstone belts but rare in modern volcanic rocks is variolitic texture with millimetre- to centimetre-sized leucocratic globular structures (varioles), in mafic lavas. Models proposed to explain this texture include rapid undercooling, magma mingling and liquid immiscibility. Here, we investigate samples of Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt of the Pilbara Supergroup, Western Australia. The samples contain relict magmatic clinopyroxene and spinel and preserve sharp contacts between varioles and host. No plagioclase is observed in the samples. The clinopyroxene mineral composition is indistinguishable between in the varioles and host. This is despite the variole material having lower MgO and Fe2O3(total) and higher SiO2 and Al2O3 concentrations. However, the variole and host materials have indistinguishable TiO2, Cr2O3, NiO2 and Sm-Nd isotope systematics. These observations are consistent with an immiscibility reaction from a parental melt to a hydrous fluid phase and conjugate mafic liquid, where minerals that formed before the reaction become incorporated into both endmembers where they continued to grow by diffusive exchange between the hydrous fluid and silicate melt. Two lines of evidence indicate that hydrous flux exsolution occurred due to H2O saturation in the precursor magmas, including: (1) the lack of plagioclase, for which crystallisation is supressed at high H2O content; and (2) Nb/Th < 7, which indicates a hydrated mantle source. The variole-bearing pillow lavas are part of the 10-Myr-duration Coongan Subgroup volcanic cycle and occur interspersed with tholeiite basalts and minor komatiites of the Mount Ada Basalt and mafic to felsic volcanic rocks of the Duffer Formation. These distinctly different volcanic rock types, which have εNdi −0.3 ± 0.4, ~1ε unit lower than contemporary tholeiites, indicate that there was variation in magma genesis, which included hydrous fluxed melting likely genetically related to delamination, during the 10 Myr formation of the Coongan Subgroup volcanic cycle, as part of the Paleoarchean East Pilbara Terrane formation.

Effect of high salinity and temperature on water–volcanic rock interaction

In order to understand the processes occurring in natural hydrothermal systems, it was carried out a series of water–volcanic rock interaction studies in the laboratory and an intermediate volcanic rock samples from geothermal production wells in Tuzla geothermal field (TGF) in western Turkey. A high-pressure autoclave was used to conduct water–rock interaction experiments under similar conditions of the field. Rainwater and seawater were treated with volcanic rocks at 140 °C (reservoir temperature) and 4.5 bar pressure. The change in the ionic content of the resulting fluids was examined in terms of the type of volcanic rocks and mineral saturation index. The results indicate that talc and diopside minerals in geothermal systems may cause scaling at high temperatures depending on the geothermal fluid and pH.