Acid Volcanic Rocks Research Articles

Soil hydraulic properties, mineralogical alteration and pore formation in Regosols from southern Brazil

ABSTRACT Regosols (Neossolos) are soils with use limitations mainly related to effective depth, abundant presence of rock and saprolite fragments, and frequently high slope gradients; besides that, they represent a new agricultural frontier for grain production in southern Brazil. This study evaluated soil saturated hydraulic conductivity (K sat ) and water retention and availability in Regosols and saprolites derived from volcanic rocks in southern Brazil and the relationship of these variables with porosity in saprolithic horizons characterized by mineralogical weathering. The study was carried out on eight profiles derived from basic and acidic volcanic rocks of the Serra Geral Formation. We evaluated soil morphology, granulometry, porosity, bulk density (BD), K sat , water retention, electronic/ optical microscopy and chemical composition of parent materials, and soil mineralogy. Soil K sat ranged from 0.0 to 6.40 cm h -1 in the evaluated horizons, without significant difference between the A and Cr horizons. Seven soil profiles showed BD equal to or less than 1.28 Mg m -3 for the Cr samples. Total porosity in the Cr horizons was above 0.5 m 3 m -3 and not significantly different from A horizons. In five of the eight soil profiles, one or more Cr horizons presented greater available water than A horizons. Electronic and optical microscopy evidenced abundant cracks and mineralogical weathering in the rock samples. X-rays diffraction data also indicated advanced degree of weathering of Cr horizons, evidencing abundant formation of pores in the saprolite. and justifying the high-water retention in Regosols profiles in southern Brazil.

Total and Extractable Micronutrients in Tropical Acid Soils of Lampung, Indonesia

ABSTRACT The study aimed to determine the total and available amounts of iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) in the humid tropical acid soils of Lampung, Indonesia, and to identify the key factors affecting them. Topsoil samples were taken from 10 agricultural fields and 6 natural fields in 16 sub-districts of Lampung, Indonesia. The average total contents of Fe (26.36 g kg−1), Cu (8.97 mg kg−1), and Zn (53.24 mg kg−1) are generally below the world-soil average contents and those in other acid soils, which may be due to eluviation of Fe, Cu, and Zn into subsoil induced by high rainfall and the low natural contents of especially Cu and Zn in the parent rocks (acid volcanic rocks and sandstone). In contrast, the average total Mn content (1.20 g kg−1) is 2 to 8 times higher than that commonly found in acid soils and the world-soil average content, indicating that the parent materials may contain high Mn-bearing minerals. The total Fe, Mn, and Zn are positively correlated to the clay content, exchangeable-Ca and -Mg, and exchangeable-Mg, respectively, whereas the total Cu has no correlation to any soil properties. The amounts of available Fe, Mn, Cu, and Zn account for on average 0.38%, 6.20%, 21.50%, and 4.40% of the total contents, respectively. Around 6%, 12.5%, and 56% of the soil samples exhibit Mn, Cu, and Zn deficiency, respectively, and 25% face Fe toxicity. Total Fe and Mn are the main factors determining available Fe and Mn, respectively, whereas exchangeable-Mg and organic-C content mainly control available Cu and Zn, respectively.

Origin of Grain-Coating Chlorite and Implications for Reservoir Quality in the Triassic Deep-Buried Volcaniclastic Sandstones, Central Junggar Basin, Northwestern China.

The occurrence and genesis of grain-coating chlorite were investigated in order to evaluate the impact of grain-coating chlorite on preserving porosity in the deep-buried Triassic Karamay volcaniclastic sandstones based on thin sections, scanning electron microscopy, and an electron probe. Grain-coating chlorite was formed during the eogenesis, originating from the precursor of smectite through the solid-state transformation (SST) mechanism. The hydration and dissolution of unstable, intermediately basic volcanic rock fragments provided essential Fe2+ and Mg2+ ions for the formation of grain-coating chlorite. Due to relatively high stability and low susceptibility to dissolution, acidic volcanic rock fragments could not promote chlorite formation but resulted in authigenic quartz and clays as pore-filling cements. This process would destroy reservoir properties. Under high hydraulic conditions, medium- to coarse-grained sandstone experienced saltation transport, creating significant velocity differentials and pressure differentials on grain surfaces. Subsequently, clay grains adhere to the surfaces, forming grain-coating chlorite during diagenesis with good continuity. In contrast, pebbly sandstone undergoes rolling transport, resulting in smaller velocity differentials on grain surfaces. This makes relatively ineffective clay adsorption and leads to discontinuous grain-coating chlorite in subsequent stages. Under weak hydraulic conditions, grains and clay particles in fine-grained sandstone undergo suspended transport, lacking mutual movement and velocity differentials. Clay particles cannot effectively cover particles but instead fill the pores between them. Therefore, continuous grain-coating chlorite is more commonly developed in the medium- to coarse-grained sandstones and is crucial for inhibiting quartz cementation with a coverage rate exceeding 80%. Inadequate coatings fail to inhibit quartz cementation effectively, while excessive coatings may block pore throats. Optimal protection of primary porosity could occur only when grain-coating chlorite is moderately developed.

Geochemical Characterization and Genetic Analysis of Rocks of the Dianzhong Formation in Geda Township Area,South-Central Tibet

The study on the genetic and geochemical characteristics of volcanic rocks of the Cretaceous-Paleogene Dienzhong Formation in central and southern Tibet is of great significance for the establishment of magma evolution sequence in the working area. The volcanic rocks of Dianzhong Formation in Geda area can be roughly divided into dite, andesite and rhyolite. Zircon U-Pb isotopic dating results show that the weighted average age of Pb206/U238 of oligopleural rhyolite and Andesite samples is 65. 5±2. 3Ma (MSWD=2. 0) and 68. 7±3. 1Ma (MSWD=2. 0) , respectively. This indicates that the middle acid volcanic rocks of Dianzhong Formation erupted from Late Cretaceous to Paleocene. Geochemical analysis of rocks shows that the acidic volcanic rocks in Dianzhong Formation are a set of meta-aluminous-peraluminouscalc- alkaline andesite-dacite-rhyolite rock assemblages. Large ion lithophile elements and light rare earth elements are enriched, and high field strength elements are deficient. In the process of magmatic evolution, there is a large amount of crystallization of plagiase or a large amount of refractory residue of plagiase, and there are a lot of continental crust materials in the volcanic rocks of the Dian Zhong Formation. Comprehensive analysis shows that the volcanic rocks of Dianzhong Formation have the chemical characteristics of continental arc volcanic rocks, and may be magmatic products in the transition stage from subduction mechanism to continental collision mechanism. Zircon U-Pb dating, in situ Lu-Hf isotope and whole rock principal and trace analysis were carried out on the acidic volcanic rocks in Dianzhong Formation, Geda Area. The aim is to study the geochemical characteristics and evolutionary origin of rocks in this area. It provides the geochemical data support for the discussion on the division of tectonic evolution stages in the study area since Jurassic.

New Evidences of Early Peopling of Central Patagonia: A Discoidal Stone and the First Fishtail Projectile Point from Chubut Province (Argentina, South America)

ABSTRACT In this paper, two new artifacts, a Fishtail projectile (or Fell 1) point and a discoidal stone from central Patagonia, Argentina, are presented. These pieces, mainly associated with the initial peopling of South America, come from an area in which until now no artifacts of this type had been recovered. Both instruments, which were part of the technological equipment of Paleoamericans, are complete. The projectile point is made of light brown translucent opal, and the discoidal stone is made of rhyolite, an acidic volcanic rock. Three-dimensional models of the instruments are presented with the aim of providing a detailed and precise record of both lithic pieces. These archaeological artifacts give information on the geographical distribution of indicators of initial human presence and the use of space during the Pleistocene–Holocene transition in central Patagonia, in the south of South America.

Spatial-temporal variation, source apportionment and risk assessment of lead in surface river sediments over ∼20 years of rapid industrialisation in the Pearl River Basin, China

Lead (Pb) is a highly toxic element and is not essential to the human body. Lead pollution caused by human activities and a high geological background is considered a global environmental issue. According to the China Geochemical Baseline (CGB) project, the Pearl River Basin had the highest Pb content in alluvial sediments of 30 first-level basins in China. For this reason, it is of great significance to determine the temporal and spatial variations in Pb and their influencing factors in the Pearl River Basin. In this study, 956 stream sediment samples collected in the 1980 s (early stage) and 129 river sediment samples collected from 2008 to 2010 (late stage) were used to study the background value and spatial-temporal variation characteristics of Pb in river sediments in the Pearl River Basin. The Pb source apportionment and an ecological risk assessment were also carried out. The background value of Pb (36.2 mg·kg–1) in the river sediments of the Pearl River Basin was significantly higher than that in China (22.1 mg·kg–1). The parent rocks determine the Pb background in sediments and the high Pb background areas mainly comprised carbonate rocks and acid volcanic rocks. Over 20 years of rapid industrialisation, the average Pb increased from 43.3 to 68.3 mg·kg–1 in the Pearl River Basin. The BCR analysis revealed that Pb mainly existed in the reducible phase (48 % on average) and residue phase (42 % on average). The enrichment factor and geo-accumulation index indicated that the late-stage sediments experienced more Pb pollution than the early-stage sediments. However, the risk assessment code (RAC) showed that there was a low ecological risk of Pb in the late-stage sediments. The factor analysis results for the two rounds of data were significantly different. The Pb content in early-stage sediments was closely related to Al2O3 and Zr, while Pb in the late-stage sediments was mainly related to Zn, As, Sb, Au and Hg, indicating that the increase in Pb in the later samples was mainly influenced by human activities. The Pb isotope composition of the late-stage sediments confirmed that low Pb content was mainly controlled by natural sources, while high Pb content was significantly affected by anthropogenic sources. Combining the results of spatial-temporal variation, chemical speciation and source apportionment indicated that the rapid rise of Pb in late-stage sediments in certain areas could be attributed to mining and smelting activities during the process of industrialisation over 20 years. The anthropogenic exogenous Pb could be immobilised by Fe-Mn (hydro)oxides when it entered the soil, so although there was a high background the ecological risk of Pb in river sediments was low. In the future, Pb pollution control and remediation needs to be strengthened in the Pearl River Basin to avoid the outbreak of potential ecological risks linked to Pb.

The geochemical characteristics of Cretaceous volcanics in southern Hainan Island and implications for tectonic evolution in the South China Sea

The southeastern Eurasian plate, where the South China Sea (SCS) is located, lies in a complex tectonic setting between the Pacific and Tethys tectonic belts. It is widely accepted that the tectonics of the SCS area were influenced by subduction in the late Mesozoic, but there is still controversy over whether it was paleo-Pacific subduction or Tethyan subduction. Volcanic activity in the south of Hainan Island was intense during the Cretaceous, and geochemical analysis of the collected basaltic andesite, andesite and rhyolite samples in this study indicate those intermediate-acid series igneous rocks are high-K calc-alkaline or calc-alkaline. Some andesites have high MgO contents and Mg# values (2.04–5.34 wt% and 36.83–55.29; Mg# = 100× Mg2+/(Mg2+ + TFe2+). Light rare earth elements (LREEs) and large ion lithophile elements (LILEs) are enriched in all the samples, but high field strength elements (HFSEs) are depleted. The negative Eu anomalies are more obvious in the rhyolites than andesites. The geochemical characteristics of the volcanic arc igneous rocks show that the mid-Cretaceous tectonic setting of Hainan Island can be classified as an Andean active continental margin. During the mid-Cretaceous, intermediate volcanism occurred in Hainan Island and its adjacent areas. The zircon-saturation temperatures of the acid volcanic rocks in study area exhibit relatively low values (ranging from 746°C–790°C). Unlike igneous rocks forming in the coastal area east of the South China Block at the same time, no A-type granitoids with alkaline dark minerals appear in Hainan Island. During the late Mesozoic, the western SCS, where Hainan Island was located, may not have been affected by the subduction of the paleo-Pacific Plate, but rather Neotethyan subduction which dominated the Cretaceous magmatic and tectonic activities along the western margin of the SCS. This finding helps to understand the late Mesozoic tectonic evolution of the southeastern edge of the Eurasian plate.

Determination of the early Paleozoic accretionary complex in Southwestern Yunnan, China: Implications for the tectonic evolution of the Proto-Tethys Ocean

Accretionary complex study provides important knowledge on the subduction and the geodynamic processes of the oceanic plate, which represents the ancient ocean basin extinction location. Nevertheless, there exist many disputes on the age, material source, and tectonic attribute of the Lancang group, located in Southwest Yunnan, China. In this paper, the LA-ICP-MS detrital zircon U-Pb chronology of nine metamorphic rocks in the Lancang group was carried out. The U-Pb ages of the three detrital zircons mainly range from 590–550 Ma, 980–910 Ma, and 1150–1490 Ma, with the youngest detrital zircons having a peak age of about 560 Ma. The U-Pb ages of the six detrital zircons mainly range from 440–460 Ma and 980–910 Ma, and the youngest detrital zircon has a peak age of about 445 Ma. In the Lancang group, metamorphic acidic volcanic rock, basic volcanic rocks, intermediate-acid intrusive rocks, and high-pressure metamorphic rocks are exposed in the form of tectonic lens in schist, rendering typical melange structural characteristics of “block + matrix”. Considering regional deformation and chronology, material composition characteristics, and the previous data, this study thinks the Lancang group may be an early Paleozoic tectonic accretionary complex formed by the eastward subduction of the Changning-Menglian Proto-Tethys Ocean, which provides an important constraint for the Tethys evolution.

Investigation of hot spring gas components and soil gas fluxes in Arxan Holocene volcanic field, Inner Mongolia, NE China

The latest research results show that there is a unified magma system and heating channel beneath the Arxan volcanic field, indicating a potential risk of eruption. The Arxan volcanic field features multiple gas emission sites (e.g., Jinjianggou hot springs and Yinjianggou hot springs) and exhibits strong hydrothermal activity. In this study, measurements of the hot spring gas composition and soil CO2 flux in the Arxan Holocene volcanic field were conducted, and the results were combined with previous research results to analyze the degassing characteristics of this region. The results show that the volcanic gases in the Arxan volcanic field are composed of 0.07%–1.09% CO2, 0.33–12 ppm CH4, 1.57–53 ppm H2, 800–30,241 ppm He, and 1.14%–1.86% Ar. The He content in this area is notably higher than that in other dormant volcanoes in China. This difference is possibly caused by U–Th decay in the Mesozoic granodiorite and acidic volcanic rocks in the study area, which can produce substantial radiogenic He. The soil gas concentrations near the Jinjianggou and Yinjianggou hot springs are higher than those of two Holocene volcanoes. The peak CO2 concentration in the soil near the Jinjianggou hot spring can reach 35,161 ppm. The single-site soil microseepage CO2 flux in the Arxan volcanic field is 4.66–107.18 g m−2 d−1, and the estimated annual CO2 emission flux from the volcanic field to the atmosphere is 0.63 × 105 t, which also demonstrates that soil CO2 flux of Arxan volcano is comparable to the soil CO2 emission level of the Iwojima volcano.

The Geology, Petrography, and Geochemistry of Egyptian Dokhan Volcanics: A Potential Source for Construction Aggregate

The present paper focuses on the geology, petrography, and geochemistry of the well-known Dokhan volcanics encountered in the northern part of the Eastern Desert of Egypt. The basalts, andesites, rhyolites, and agglomerates exposed at the Makhar Seal (flood plain) as well as Wadi Abu Zoghot, Wadi El-Ghafiryia, Wadi Al-Radah Luman, Wadi Al-Ushsh, Wadi Umm Sidrah, and Gabal Ghuwayrib are herein examined as sources of coarse aggregate in concrete mixes. A representative total of 28 samples—collected from different Dokhan volcanics—was studied in terms of field geology, petrography, and geochemistry wherein a variety of experiments related to construction material validation apply. The petrographic examination revealed that the studied Dokhan volcanics consist of basic, intermediate, and acidic volcanic igneous rocks. These rocks are represented through basalts, andesites, imperial porphyry, dacites, rhyodacites, rhyolites, and their pyroclastics. Furthermore, the applied geochemical analysis indicated that the studied Dokhan volcanics are alkaline to sub-alkaline, calc-alkaline and classified as basalts, basaltic andesites, andesites, trachyandesites, trachydacites, trachytes, and rhyolites, indicating an initial potential as aggregate for concrete mixes. Finally, the results obtained from incorporating Dokhan volcanics as aggregates in concrete mixes demonstrated a significant improvement in regard to the properties of the comprising concrete mixes. Herein, a higher compressive strength was witnessed after 28 days for Dokhan volcanic concrete, when compared to concrete comprising dolomite aggregate, amounting to an average increase that exceeded 36%.

The statherian anorogenic magmatism in the Paramirim Aulacogen, são Francisco-Congo paleoplate: New data, synthesis and regional correlations

Fragmentation of continental masses is usually associated with anorogenic magmatism, and in this context, major silicic magmatic events may be associated with Large Igneous Provinces (LIPs). The Paramirim Aulacogen evolves between 1.77 and 0.67 Ga, filling anorogenic siliciclastic and acidic volcanic rocks with subordinate carbonates. A rhyolite from the Algodão Formation, the oldest unit of the Aulacogen, was dated in this work using the U–Pb method (LA-ICPMS), finding a crystallization age of 1777±1 Ma, with a model age (TDM) of 3.10 Ga and εNd (t) equals −10.4. The anorogenic plutonic magmatism is represented by the Lagoa Real Intrusive Suite, with over 5000 km2. New geochemical and geochronological data are presented, and the integration with the literature allowed the interpretation of four plutonic magmatism cycles: (i) Cycle 1: represented by Lagoa do Barro granitoids (1762 ± 9 Ma) and the metavolcanic rocks of the Algodão Formation (1775 ± 77–1777±1 Ma); (ii) Cycle 2: with the São Timóteo (1741 ± 4–1746 ± 5 Ma) and Caetité (1743 ± 5 Ma) granitoid and syenitoid, in addition to charnockitic rocks (1742±7 Ma); (iii) Cycle 3: is related to phonotephrite, tephriphonolite and phonolite. Trachyte, trachyandesite and trachydacite of São Simão Formation (1731 ± 5 Ma) and tuff layer of Ouricuri do Ouro Formation (1722 ± 13 Ma), as well as rhyolites and dacites that ocurr in the basement of Paramirim Aulacogen. The Caldeirão granitoid and syenitoid (1724 ± 5 Ma) is associated with this cycle; and (iv) Cycle 4: with the Suçuarana intusive rocks (1686 ± 6 Ma) and pyroclastic rocks of trachyandesitic composition. The Sm–Nd model ages (TDM) obtained in this work and the literature varies between 3.1 and 2.2 Ga, with values of εNd (t) between −15.8 and −8.6. The formation of the Statherian magmas possibly occurred by melting the enriched mafic mantle, followed by underplating in the continental crust and assimilation of Archean crustal rocks, with fractionation of hornblende, biotite, apatite, and Fe–Ti oxides during magma evolution. The Lagoa Real Intrusive Suite, and their volcanic correlates, represent part of a Statherian Silicic Large Igneous Province (SLIP) developed in a crust of the Archean-Rhyacian-Orosirian age of the São Francisco-Congo Paleoplate. This magmatism marks the attempt at the breakup of the Columbia supercontinent, with correlates in other regions of Brazil and the world.

Major, trace and rare earth element geochemistry of the Permian Lucaogou oil shales, eastern Junggar Basin, NW China: implications for weathering, provenance and tectonic setting

The Permian Lucaogou oil shales in the eastern Junggar Basin have long been regarded as important source rocks and reservoirs for tight oil exploration. The oil shales can also be used to assess the provenance and tectonic setting of the East Junggar region using major, trace and rare earth element geochemistry. The low chemical index of alteration ratios (37.36–64.18, 45.54 on average), low plagioclase index of alteration ratios (26.99–62.95, 42.99 on average), and high index of compositional variability ratios (0.64–1.66, 1.10 on average) suggest that the Permian Lucaogou oil shales mostly consist of immature sediments that have experienced a weak weathering intensity. The samples have low TiO2/Al2O3 and TiO2/Zr ratios that indicate a felsic origin with minor intermediate igneous rocks. The high Th/Sc and Zr/Sc ratios suggest a felsic origin without sediment recycling. The samples show fractionated light rare earth elements and relatively flat heavy rare earth elements patterns with weak negative Eu anomalies, which implies that the main provenance is lower Carboniferous intermediate–acid volcanic rocks. In addition, multiple major- and trace-element‐based discrimination diagrams show that the parent rock of clastic sediments formed mainly in a continental island arc and active continental margin environment, consistent with previous studies on the tectonic background of the East Junggar region during the late early Carboniferous. Key Points Immature sediments are present in the Permian Lucaogou oil shales and have a weak weathering intensity. The provenance of the clastic sediments of the Permian Lucaogou oil shales was predominantly early Carboniferous intermediate–acid volcanic rocks. The clastic sediments of the Permian Lucaogou oil shales were developed in a continental island arc and active continental margin environments. The Permian Lucaogou oil shales provide crucial information for tectonic setting and evolution of the northern Xinjiang during the Carboniferous–Permian.

Permian Radiolarians and Spicules from Conglomerate of the Lower Jurassic Kuruma Group in Itoigawa, Niigata Prefecture, Central Japan

Conglomerate of the Lower Jurassic Kuruma Group contains microfossil-bearing clasts. In the present study, these microfossils were investigated via the hydrofluoric acid (HF)-etched surfaces of the clasts and residues produced during acid treatment. As a result, Permian (mainly Guadalupian–Lopingian) radiolarians were obtained from mudstone and chert clasts; spicules of indeterminate age were obtained from the chert clasts. The mudstone clasts were derivable from the Akiyoshi, Ultra-Tamba, Maizuru, and/or Hida-Gaien belts or equivalent units, while the spicule-dominated chert clasts likely originated from the Akiyoshi belt or equivalent units. Except for tuff and acidic volcanic rocks, the clast components of the Kuruma Group in this study are similar to the component rocks of the Akiyoshi belt.

Geochemical Studies of Rare Earth Elements (REE) in Ion Adsorption Clays (IAC) in Gua Musang, Kelantan

Rare earth element (REE) become the ‘critical metals’ for green technology development that have been rapidly expanded worldwide in these days. REE is mainly originated from granitic rocks. REE in ion adsorption clay (IAC) is the product from weathering of granite. IAC are believed to store high concentration of heavy rare earth element (HREE) and light rare earth element (LREE). Gua Musang is selected for this study because it is located on the three longitudinal belts that composed of acid volcanic igneous rocks from Main Range, Senting and Boundary Range Granites. In this study, the characteristics of ion adsorption clays and REE distribution in Gua Musang have been studied by mineralogy and geochemical analyses. Rocks and soil samples were collected closed to the granite bodies and its surrounding to represent its weathering products. Polarised optical microscopy was used for petrography and mineralogy studies. From fieldwork observation, Gua Musang lithologies composed of carbonate facies, argillaceous facies and pyroclastic facies. X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD) and Inductive Coupled Plasma Microspectrometry (ICP-MS) were used accordingly to characterise the composition of major and trace elements in IAC samples. REE value in Pulai are the highest concentration as iron nodule have been found in the sampling area. Sample from Boundary Range granite also reported store high concentration of REEs in this study.

ПОЗДНЕКАМЕННОУГОЛЬНЫЕ РИОЛИТЫ ПРИАМУРСКОГО ФРАГМЕНТА НОРА-СУХОТИНСКОГО ТЕРРЕЙНА: ГЕОХИМИЯ И ГЕОХРОНОЛОГИЯ

The publication presents the first results of geochemical, isotope-geochemical (Sm-Nd) and geochronological (U-Pb, LA-ICP-MS) studies on acid volcanic rocks collected in the Bogdanikha River basin of the Amur fragment of the Nora-Sukhotino terrane in the northeast of the South Mongoli–Khingan orogenic belt. In terms of the content of rock-forming components, the studied volcanic rocks correspond to high-silica and high-alumina rhyolites. Elevated contents of alkalis, Ga, Zr, Nb and Y, reduced concentrations of Ba, Sr, Ti, Eu, and mantle values of εNd(t) = +3.0…+3.6 enable the rhyolites from the Bogdanikha river basin to be classified as A2-type rhyolites. The concordant age of the youngest zircon population from the rhyolite, according to geochronological (U-Pb, LA-ICP-MS) studies, is 301  4 Ma which corresponds to the Late Carboniferous. Taking into account the geochemical features of the studied rhyolites and the existing models for the formation of the South Mongolian–Khingan orogenic belt, it is most likely that they formed in a collisional setting as a result of slab detachment.

Origin and migration of fluoride in the area of the Aluto Volcanic Complex (Main Ethiopian Rift)

Fluoride-enriched ground and surface waters represent a major health risk for the local population in many areas along the East African Rift. The present study investigates the origin of fluoride and the reason for its accumulation in the rift waters, following two hypotheses: (i) fluid-rock-interactions release fluoride from minerals into the water and (ii) magmatic-derived fluoride-containing liquids and gases migrate along permeable fault zones until they dissolve in ground- and surface water or be released to the atmosphere.Rock-, gas, water-, soil-, and plant samples were collected from the area within and close by the Aluto Volcanic Complex, which is part of the Main Ethiopian Rift. Most analyzed waters showed fluoride concentrations above the drinking water limit (>1.5 mg/L) with the highest values in hot springs (up to 70 mg/L) and the geothermal well (76 mg/L). In the solid phase, a high fluoride content was found in acid volcanic rocks (ignimbrite: 4391 ppm; rhyolite: 3248 ppm) as well as in pumice (up to 1955 ppm). The fluoride content of soil samples collected within the volcanic complex varied between 82 and 1036 ppm, whereas former lake sediments from outside the Aluto Volcanic Complex showed higher fluoride contents ranging from 674 to 8171 ppm.Identified fluoride-rich minerals are various amphiboles (about 3 wt.-% F−), fluor-apatite (about 3 wt % F−), minerals of the fluor-caphite group (about 5 wt.-% F−), parisite (up to 9 wt.-% F−), and fluorite (CaF2). Elevated fluoride concentrations were also measured in some gas samples from fumaroles (up to 50 ppm) and in plant samples collected next to the fumaroles (up to 65 ppm). Leaching experiments of solid samples with deionized water demonstrated that fluoride can easily be mobilized from pumice and sediments but hardly from consolidated rocks. This fluoride release increased with temperature (up to 150 °C) and correlated roughly with dissolved silica indicating the binding of some fluoride to the amorphous or weakly crystalline silica fraction.Based on these results it was concluded that fluoride migrates via different pathways through the environment: At the depth of the magma chamber during magmatic differentiation fluoride enriches initially in the magmatic melt and accumulates in some late-crystallizing minerals of igneous rocks such as fluorite or mica. Upon volcanic eruption fluoride is predominantly incorporated in the glass (ignimbrite) and ash phase (pumice). On the surface, these rocks are exposed to weathering and fluoride leaches partly out into the aqueous phase. Soft and porous rocks such as pumice release fluoride first whereas hard extrusive/igneous rocks are less prone to weathering and retain the fluoride. Pumice and (lake) sediments might act both, as source and as sink for fluoride in the area. Although some fluoride might drain from the surface into the deeper subsoil, we conclude that magmatic fluids (liquid and gas) contribute more substantially to the overall fluoride ground water content because (i) of the much higher fluoride content in deep geothermal waters and hot springs as compared to surface-near waters; (ii) active geothermal surface manifestations located along fault zones indicate that fluids migrate from deep magmatic intrusions (as gas and liquid) towards the surface, where fluoride dissolves in groundwater; (iii) and the good correlation between bicarbonate (deriving from dissolution of magmatic CO2) and fluoride content in all analyzed water samples.

Geochemistry of the Neoproterozoic Volcanic Rocks of the Nakora Area of Malani Igneous Suite, Barmer District, Western Rajasthan, India

The Neoproterozoic rocks of the Nakora Ring Complex (NRC) consist of three phases (Extrusive, Intrusive and Dyke) that are based on the detailed geological mapping (contact relationship, mode of occurrence, position of xenolith, flows, dykes and veins) and their stratigraphic position. NRC consists mainly of acid volcanic rocks besides minor amount of basic rocks with intermediate calc-alkaline to tholeiitic affinities and occurs in the form of ring structures. The Nakora basaltic rocks show LREE enriched nature and they have consistent negative Nb, Ta, Sr and Zr anomalies. The HREE pattern is showing parallel arrangement with HREE pattern of other basic rocks. The Nakora acid volcanic rocks exhibit high LREE enrichment than the HREE with negative Ba, Sr, Eu and Ti anomalies in primitive mantle normalized multi-element diagrams. All the samples show negative Ba, Sr, Eu and Ti anomalies. The diminution in amounts of Sr and Eu is apparently related to the fractionation of feldspars or their retention in the refractory minerals resistant to melting in the lower crust. As compared to trachytes, the rhyolites show high SiO2, high Al2O3, low total alkalis, low total iron, low TiO2, high CaO and high MgO. The petro-mineralogical and geo-chemical data specifies that the NRC rocks are generated from a co-magmatic source through a co-genetic process in a rift tectonic context. Petrogenetic modeling indicates that both the basic rocks and acid volcanic rocks of Nakora may have been derived from rocks akin to Bhilwara mafic metavolcanic/mixed Nakora gabbros and Siwana rhyolite/banded gneiss from Kolar Schist Belt by different degrees of partial melting respectively.

Age and origin of tuffites from the Hekou Formation in the Napo basin, Southwest Guangxi and its tectonic implications

桂西南那坡盆地位于八布-Song Hien构造带，发育早-中三叠世岩浆岩和巨厚海相碎屑岩系，是研究华南地块南缘古特提斯造山过程的关键地区。本文对那坡盆地中三叠统河口组中-上部的两套沉凝灰岩开展了详细的岩石学、地球化学和锆石U-Pb-Hf同位素分析研究。两件沉凝灰岩样品LA-ICP-MS锆石U-Pb定年结果分别为240.6±1.0Ma和236.3±0.8Ma，指示河口组碎屑岩沉积时代为中三叠世拉丁期，部分为晚三叠世卡尼期早期。全岩地球化学结果显示，该沉凝灰岩属于亚碱性英安质，富集轻稀土（LREEs）和大离子亲石元素（LILEs）、Rb、Th和U，亏损高场强元素（HFSEs），具有显著的Nb、Ta和Ti负异常。沉凝灰岩锆石具有较低的ε_Hf（t）值（-18.6~-6.3）和较老的地壳模式年龄（t_DM^C=1664~2434Ma），结合锆石微量元素特征认为其形成于俯冲环境。整体上，那坡盆地河口组沉凝灰岩表现出俯冲作用形成的岛弧火山岩地球化学特征，推断其来源于桂西南凭祥地区的中-晚三叠世酸性火山岩。结合已有区域地质调查成果，认为华南地块南缘八布-Cao Bang古特提斯分支洋盆于晚二叠世至晚三叠世早期向北俯冲，随后自西向东呈剪刀式逐渐闭合。华南地块与北越地块完全碰撞拼合的时间发生在晚三叠世中晚期。;The Napo basin, Southwest Guangxi, locates in the Babu-Song Hien tectonic belt and shows widespread Early-Middle Triassic magmatic and massive marine clastic rocks, which plays a key role in studying the evolution of the Paleo-Tethys orogeny along the southern margin of South China block. In this paper, we have carefully analyzed the petrology, geochemistry and zircon U-Pb-Hf isotope for two sets of tuffites in the middle-upper part of the Middle Triassic Hekou Formation, Napo basin. LA-ICP-MS zircon U-Pb dating results of the two tuffite samples yield the ages at 240.6±1.0Ma and 236.3±0.8Ma, respectively, indicating that the clastic rocks of Hekou Formation should be deposited during the Middle Triassic Ladinian stage, some of which may extend to the early Carnian of Late Triassic. Whole-rock geochemical results indicate that the tuffites are subalkaline dacitic, enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), Rb, Th and U, depleted in high field strength elements (HFSEs), and have significant negative anomalies of Nb, Ta and Ti. The tuffite zircons have lower ε_Hf(t) values (-18.6~-6.3) and older crustal model ages (t_DM^C=1664~2434Ma). Combined with trace elements, we suggest these zircons may have been formed in subduction-related setting. In summary, the tuffite samples from the Hekou Formation in Napo basin display the geochemical characteristics of island-arc volcanic rock, which are inferred to be derived from the Middle-Late Triassic acidic volcanic rocks in Pingxiang area, Southwest Guangxi. Given the existing geological survey results, it is concluded that the Babu-Cao Bang Paleo-Tethys branch ocean along the southern margin of the South China block should subduct northward during the Late Permian-early Late Triassic, and subsequently closed with scissor-style from west to east. The collision between the South China block and North Vietnam terrane should occur in the middle to late stage of Late Triassic.

Soybean crops cultivated with dacite rock by-product: A proof of a cleaner technology to soil remineralization

Soybean crops cultivated with dacite rock by-product: A proof of a cleaner technology to soil remineralization

Upper Pliocene Acid Volcanic Rocks in the Upper Chegem and Lower Chegem Volcanic Highlands, North Caucasus

Upper Pliocene Acid Volcanic Rocks in the Upper Chegem and Lower Chegem Volcanic Highlands, North Caucasus