Rhyolitic Tuff Research Articles

Provenance and tectonic implications of the Carboniferous sediments in the Bainaimiao arc belt, northern margin of the North China Craton: evidence from detrital zircon U–Pb–Hf isotopes and trace elements

Sedimentological and zircon U–Pb–Hf isotopic analyses of the Carboniferous strata in the Bainaimiao arc belt (BAB) provide provenance information for the northern margin of the North China Craton (NCC). The late Carboniferous strata in the eastern BAB were dominantly shallow marine carbonate-clastic deposits. Detrital zircon data show that: (1) sandstone from lower Sidaozhangpeng Formation displays two major age peaks of 369 Ma and 2522 Ma, with eHf (t) values of − 25.9 to − 20.4 and 2.2–6.4, respectively; (2) sandstone from upper Sidaozhangpeng Formation shows a significant age peak at 384 Ma, with eHf (t) values of − 25.3 to − 14.5. Determination of detrital zircon parent rock types based on trace element proxies suggests that the protolith rock types consist of felsic (73%) and mafic rocks (22%), with minor alkaline rocks (5%), implying a continental arc setting. The U–Pb–Hf dating of the Carboniferous strata in the western and eastern BAB shows different provenances: the Carboniferous sediments in the western BAB were derived from early Paleozoic magmatic rocks, micro-block (Hunshandake Block) and the NCC basement; the provenances of the Carboniferous strata from the eastern BAB were mainly local Devonian magmatic rocks, with subordinate input of the Neoarchean NCC basement rocks. The early Paleozoic zircon grains recorded by the early Devonian rhyolitic tuff and late Carboniferous sandstone indicate that the early Paleozoic arc magmatic activities also occurred in the eastern BAB.

The unique speciation of iron in calc-alkaline obsidians

Obsidians found in silicic domes and rhyolitic tuffs are an ideal system to understand the formation conditions of calc-alkaline felsic magmas. The Optical Absorption and Electron Paramagnetic Resonance (EPR) spectroscopic data presented in this study demonstrate that Fe-speciation in obsidians is different from that observed in other geological glasses (e.g., basaltic glasses) or in synthetic Fe-bearing silicate glasses. The presence of unusual symmetric Fe2+ octahedral sites explains the characteristic black to brown color of obsidian glasses, much distinct from that of synthetic Fe-bearing glasses. The Fe3+ sites partly occur in superparamagnetic domains, as evidenced by EPR data. The dramatic intensification of the optical absorption in the visible range indicates the presence of Fe-rich domains with Fe2+-Fe3+ and/or Fe2+-Ti4+ Inter-Valence Charge Transfer (IVCT) processes. These data are consistent with the presence of Fe-rich, mixed valence clusters, located outside the glass structure. The largest of these domains contribute to the black color of obsidians because they cause a background absorption extending throughout the visible spectrum as observed in magnetite. This unusual Fe-speciation reflects the last events of the magmatic history during emplacement and cooling, indicating that obsidians cannot be considered as a quenched witness of the initial magma.

Deep Geological Structures Associated With Terrestrial Volcanic Hydrothermal Metallogenic System: Evidence From Geophysical Survey in Taohemu Superlarge Silver‐Polymetallic Deposit, Inner Mongolia

AbstractTaohemu deposit is a superlarge silver‐polymetallic deposit discovered in the Great Xing'an Range metallogenic belt of China in recent years. The deposit occurs in the terrestrial volcanic rock sequences of Late Jurassic age. The ore bodies consist mainly of rhyolitic tuffs, brecciated tuffs, and volcanic breccia bearing Ag‐Pb‐Zn mineralization, and are largely conformable with the surrounding layered volcanic and volcaniclastic rocks with a gradual transitional relationship manifesting as strata‐bounded features in general. The mineralization system appears to be generally controlled by a paleo‐volcanic chamber. It is suggested that the late volcanic hydrothermal fluids flowed within the permeable layers of rhyolitic pyroclastics consisting mainly of volcanic breccias and breccia‐bearing tuffs. This resulted in the formation of superlarge Ag‐Pb‐Zn polymetallic ore deposit metasomatism and fracture filling. The majority of the deposit occurs as shallowly dipping concealed ore bodies, as defined by sparsely distributed drill holes. Because of its short discovery history, new mineralization style and low degree of comprehensive research, the boundaries of most ore bodies have not been effectively delineated along vertical and lateral extensions. At present, it is crucial to strengthen the comprehensive studies on the deposit, conduct more detailed geological exploration, and establish the metallogenic and prospecting models. For this reason, we have designed and implemented long sections of magnetotelluric (MT) and controlled‐source audio magnetotelluric (CSAMT) sounding across the Taohemu ore field and obtained a large number of geological structures and valuable underground exploration results that provided reliable geophysical support for promoting deep resource exploration.

Economic geology and genesis of kaolin resources in the East African Rift system: the case of Alemtena kaolin deposit, Ethiopia

ABSTRACT The Alemtena kaolin deposit is located within the main Ethiopian rift. New geological, mineralogical, geochemical and physical test data provide evidence of the genesis and potential use of the kaolin. These data indicate that the deposit formed as a result of the intensive weathering of felsic volcanic rocks including rhyolitic tuffs, pumice, and rhyolite. The deposit contains elevated concentrations of Cr + Nb and Ce + Y+La and low concentration of Ba + Sr suggesting the deposit has a supergene origin. The presence of only kaolinite and quartz, and absence of high temperature minerals like pyrophyllite, dickite and pyrite, low bulk density (<2 g/cm3) and high crystallinity (HI > 0.9) strengthen the supergene origin. The presence of comparable grain size distribution, mainly white colour, high Al2O3 and kaolinite, relatively low concentrations of Fe2O3, TiO2 and alkali elements make the kaolin favourable for various industrial applications including paper, filler, ceramics, pharmaceuticals, and agricultural industries. Reserve estimation from section logging and pitting gave nearly 85 million tons of reserve.

Geochronology and geochemistry of late Paleozoic volcanic rocks from eastern Inner Mongolia, NE China: Implications for igneous petrogenesis, tectonic setting, and geodynamic evolution of the south-eastern Central Asian Orogenic Belt

Here we report new petrography, geochemistry, UPb zircon geochronology, and Hf isotope results for volcanic rocks of the Dashizhai Formation in the Dashizhai area, Keerqin Right Wing Front Banner, Inner Mongolia. The volcanic rocks have mafic to felsic bulk-rock compositions and include basalt, rhyolite, basaltic andesite, andesite, dacite, and rhyolitic tuff. Two dacite and rhyolitic tuff samples yield laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) 206Pb/238U zircon ages of 332 ± 1 Ma and 300 ± 1 Ma, respectively, suggesting that the timing of felsic volcanism ranged from the early to late Carboniferous. Mafic volcanic rock samples are enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE; La/Nb = 1.58–4.84), and are depleted in high-field-strength elements (HFSE; Th/Yb ratios <2.2) and heavy rare earth elements [HREE; (Tb/Yb)N ratios 1.27–1.91]. Based on these trace element characteristics, we propose that the mafic volcanic rocks represent fractionated, high-degree partial melts that were sourced from the subduction-modified and melt-depleted mantle wedge. The felsic volcanic rocks yield LILE- and LREE-rich compositions and arc-like HFSE- and HREE-poor signatures. Magmatic zircons recovered from the dated rhyolitic tuff and dacite samples yield positive εHf(t) values (+1.21 to +14.85) and two-stage Hf model ages of 1236 and 391 Ma, suggesting that arc-like felsic volcanism was sourced from, and/or interacted with, the older juvenile lower-crust underlying the Dashizhai Formation. Therefore, the mafic to felsic volcanic rocks from the Dashizhai area likely formed in a Carboniferous magmatic arc at a continental margin. Based on the isotopic and geochemical signatures of the Carboniferous–Permian volcanic rocks in the Dashizhai area, we preliminarily divide central and eastern Inner Mongolia into three zones: (i) North Zone, which formed at the magmatically active continental margin; (ii) Middle Zone, which formed in a magmatic arc peripheral to the continental marginal; and (iii) South Zone, which formed in an oceanic island arc. The age and geochemical signature of the Dashizhai Formation points to active, northward subduction during the early Carboniferous to the early Permian and suggests that the Paleo-Asian Ocean closed after the early Permian.

Devonian bimodal volcanic rocks from the northeastern margin of the North China Block: Implications for post‐collisional extension and orogen‐craton boundary

Devonian is a critical period for reversal of arc polarity and transition of continental margins from passive to active settings in the northern margin of the North China Block (NCB). However, the evolution processes during this period are still poorly constrained due to limited distributions of Devonian magmatism and sedimentary records in the NCB. In this paper, we present new zircon U–Pb geochronological, whole‐rock geochemical and Sr–Nd–Hf isotopic data of some recently identified Devonian volcanic rocks from the Jiefangyingzi area in the northeastern margin of the NCB. The Jiefangyingzi volcanic rocks display a bimodal composition and consist predominantly of amphibolite (meta‐basaltic andesite) and meta‐rhyolite and rhyolitic tuff with minor meta‐andesite. New SHRIMP and LA‐ICP‐MS zircon U–Pb dating results show that the volcanic rocks erupted during the early‐middle Devonian period (397–389 Ma). Geochemical and Sr–Nd–Hf isotopic data indicate that the mafic‐intermediate components were mainly derived from an enriched lithospheric mantle source, and the parental magma experienced fractional crystallization during magma ascent. The felsic rocks were mainly originated by partial melting of ancient lower continental crust, with mixing of some mantle‐derived mafic magmas. The occurrence of Devonian bimodal volcanic rocks, combined with the Devonian alkaline magmatic belt, suggesting that the northern margin of the NCB was a post‐collisional extension environment during the early‐middle Devonian period. In addition, the Sr–Nd–Hf isotopic results of the Devonian felsic volcanic rocks suggest that the Jiefangyingzi area is located in the North China Craton (NCC) rather than the Bainaimiao arc belt, and the orogen‐craton boundary between the northern NCC and Central Asian Orogenic Belt (CAOB) is located in some areas north to the Jiefangyingzi area.

Multi-stage crustal melting from Late Permian back-arc extension through Middle Triassic continental collision to Late Triassic post-collisional extension in the East Kunlun Orogen

The East Kunlun Orogen is an important part of the East Tethys region and has received significant attention with regards to the evolution of the Tethys Ocean. This contribution presents geochronological, whole-rock major and trace element geochemical, and Sr-Nd-Hf isotopic data of magmatic rocks within the Kengdenongshe polymetallic deposit in the eastern part of the East Kunlun Orogen. Here, we report zircon UPb ages of ca. 257 Ma and ca. 211 Ma for granite porphyry intrusions, and ca. 240 Ma for the rhyolitic tuff. These rocks are characterized by high SiO2, variable Al2O3 and K2O, low Na2O, MgO and CaO contents, and high A/CNK ratios, which is typical of S-type granitic rocks. They exhibit large-ion lithophile element enrichment, depletion of high field strength elements, have low (La/Yb)N ratios, and negative Eu anomalies. They also display variable (87Sr/86Sr)i ratios (0.709981 to 0.720907), negative εNd (t) values (−8.7 to −5.5), and a wide (enriched) zircon εHf (t) range (−10.1 to −0.8). The geochemical and isotope data indicate magma derivation through dehydration melting of heterogeneous crustal sources including clay-poor meta-sedimentary rocks and amphibolite, which are both parts of the East Kunlun Orogen basement. These results provide evidence for the evolution of the Paleo-Tethys Ocean in the East Kunlun Orogen including Late Permian (266–255 Ma) back-arc extension, Late Permian to Middle Triassic (255–240 Ma) subduction, Middle Triassic (240–225 Ma) continental collision, and Late Triassic (< 225 Ma) post-collisional extension. This study further suggests that the 257 Ma and 211 Ma granite porphyries are related to the asthenosphere upwelling in a back-arc basin and post-collision extensional setting, respectively. The 240 Ma rhyolitic tuff is linked to anatexis associated with crustal thickening during continental collision.

Geology, geochemistry, fluid inclusions and O–H stable isotope constraints on genesis of the Lake Siah Fe-oxide ± apatite deposit, NE Bafq, Central Iran

The Lake Siah iron ± apatite deposit is situated in the Bafq Mining District (BMD), Central Iran. The iron ± apatite orebodies are hosted by a succession of rhyolite, rhyolitic tuff, trachyandesite, and andesite of Lower Cambrian age. The host rock has undergone widespread alteration and mineralization stages including Na (albite–quartz), followed by Na–Ca (albite–calcite–amphibole–magnetite ± quartz ± epidote), magnetite–apatite, K (biotite ± K-feldspar), hydrolytic (sericite–quartz), and argillic (kaolinite–montmorillonite ± dickite), respectively. Electron microprobe analyses (EPMA) from magnetite and hematite show significant variations of trace elements. Based on Ni/Cr + Mn and Ca + Al + Mn versus Ti + V diagrams, the majority of magnetite samples belong to Kiruna-type deposits. At least, three generations of fluid inclusions, including solid-bearing (L + V + S) (Type I), liquid-rich (L + V) (Type II-A), vapor-rich (L + V) (Type II-B), and liquid or vapor mono-phase (Type III), are recognized in quartz and apatite. The solid-bearing fluid inclusions in quartz completely homogenized at temperatures of 150 to > 530 °C with salinities of 30–58 wt% NaCl equiv. Liquid-rich fluid inclusions in apatite homogenized to a liquid phase at 175–210 °C, whereas the vapor-rich fluid inclusions homogenized to a vapor at 335–350 °C. Oxygen isotope analysis was carried out on quartz and magnetite. The hydrogen and oxygen isotopic compositions of quartz (δ18Ofluid values of 7.57–11.04‰) show that with progress in time the ore-forming solutions gradually evolved from a magmatic to meteoric-dominated source.

AMS dating and ancient DNA analysis of bone relics associated with St John the Baptist from Sveti Ivan (Sozopol, Bulgaria)

Excavations in 2010 on the Black Sea island of Sveti Ivan (Bulgaria) revealed the remains of a miniature marble sarcophagus containing human and animal remains, along with an inscribed tufa ossuary, beneath a 4th to 5th-century church floor. We found that the tufa ossuary had strong links with Cappadocian rhyolite tuff. The inscriptions on its surface suggest links to John the Baptist since they refer to him by name and contain a reference to his nativity date of June 24. The ancient Greek text also mentions him in the genitive case, i.e. ‘of Saint John’. Here, we review the historical evidence pertaining to the relics and conclude that they most likely reached Sveti Ivan via Constantinople during the 4–5th century CE. To shed more light on the relics, we sampled three of the remains for radiocarbon dating and ancient DNA analysis. Radiocarbon dating of one of the bones (a metacarpal) yielded a first century AD date (5–75 cal AD, 68.2% probability). Ancient DNA analysis of this bone, as well as a rib fragment and a tooth, using shotgun sequencing yielded very little human DNA, ranging between 0 and 0.9% of sequenced reads. However, the DNA did not show any of the post-mortem damage patterns that are typical for ancient DNA. We conclude that the sequences we obtained are most likely the result of modern contamination.

Study on volcanic reservoir characteristics of Yingcheng Formation in Xudong area of Xujiaweizi fault depression

Types of deep reservoir rich in Yingcheng formation of Xudong area in Xujiaweizi fault depression, the study of volcanic rocks of Yingcheng formation in the main, using the date of core, logging, oil test, from lithology, lithofacies and physical properties to study volcanic reservoir characteristics. The main lithology of volcanic reservoir in Yingcheng formation of Xudong area include rhyolite, rhyolitic tuff, rhyolitic tuff fused, rhyolitic breccia and rhyolitic tuff breccia. In accordance with lithology combination of features, volcanic facies can be divided into volcanic explosive facies, volcanic effusive facies and volcanic sedimentary facies. The main types of porosity have air vent of primary pore, remnants pores of air vent filled after, cracks, micro-cracks and feldspar corrosion hole of secondary porosity, volcanic ash corrosion hole, micro-pore produced by rhyolitic glass from glass off in spherules rhyolite, quartz crystal chip corrosion hole, and so on. Through studying the ratio of reservoir and formation for a single well, the frequency distribution of reservoir thickness, production capacity situation and pool-forming regularity, Xudong area can be divided into three reservoir development zone, so we will find out the most favorable reservoir development zone, then it can play a guiding role for the next step of the exploration deployment.

Hydrocarbon source rock potential of Miocene diatomaceous sequences in Szurdokpüspöki (Hungary) and Parisdorf/Limberg (Austria)

Abstract Diatomaceous sediments are often prolific hydrocarbon source rocks. In the Paratethys area, diatomaceous rocks are widespread in the Oligo-Miocene strata. Diatomites from three locations, Szurdokpüspöki (Hungary) and Limberg and Parisdorf (Austria), were selected for this study, together with core materials from rocks underlying diatomites in the Limberg area. Bulk geochemical parameters (total organic carbon [TOC], carbonate and sulphur contents and hydrogen index [HI]) were determined for a total of 44 samples in order to study their petroleum potential. Additionally, 24 samples were prepared to investigate diatom assemblages. The middle Miocene diatomite from Szurdokpüspöki (Pannonian Basin) formed in a restricted basin near a volcanic silica source. The diatom-rich succession is separated by a rhyolitic tuff into a lower non-marine and an upper marine layer. An approximately 12-m thick interval in the lower part has been investigated. It contains carbonate-rich diatomaceous rocks with a fair to good oil potential (average TOC: 1.28% wt.; HI: 178 to 723 mg HC/g TOC) in its lower part and carbonate-free sediments without oil potential in its upper part (average TOC: 0.14% wt.). The composition of the well-preserved diatom flora supports a near-shore brackish environment. The studied succession is thermally immature. If mature, the carbonate-rich part of the succession may generate about 0.25 tons of hydrocarbons per square meter. The diatomaceous Limberg Member of the lower Miocene Zellerndorf Formation reflects upwelling along the northern margin of the Alpine-Carpathian Foreland. TOC contents are very low (average TOC: 0.13% wt.) and demonstrate that the Limberg Member is a very poor source rock. The same is true for the underlying and over-lying rocks of the Zellerndorf Formation (average TOC: 0.78% wt.). Diatom preservation was found to differ considerably between the study sites. The Szurdokpüspöki section is characterised by excellent diatom preservation, while the diatom valves from Parisdorf/Limberg are highly broken. One reason for this contrast could be the different depositional environments. Volcanic input is also likely to have contributed to the excellent diatom preservation in Szurdokpüspöki. In contrast, high-energy upwelling currents and wave action may have contributed to the poor diatom preservation in Parisdorf. The hydrocarbon potential of diatomaceous rocks of Oligocene (Chert Member; Western Carpathians) and Miocene ages (Groisenbach Member, Aflenz Basin; Kozakhurian sediments, Kaliakra canyon of the western Black Sea) has been studied previously. The comparison shows that diatomaceous rocks deposited in similar depositional settings may hold largely varying petroleum potential and that the petroleum potential is mainly controlled by local factors. For example, both the Kozakhurian sediments and the Limberg Member accumulated in upwelling environments but differ greatly in source rock potential. Moreover, the petroleum potential of the Szurdokpüspöki diatomite, the Chert Member and the Groisenbach Member differs greatly, although all units are deposited in silled basins.

Geochronology, geochemistry, and Sr–Nd–Hf–O isotopes of the Zhongqiuyang rhyolitic tuff in eastern Guangdong, SE China: Constraints on petrogenesis and tectonic setting

A‐type magmas generally occur in extension‐related environments and shed new light on the tectonic setting by metallogenic and petrological studies of A‐type felsic volcanic rocks. The Zhongqiuyang porphyry Cu deposit is located in eastern Guangdong, SE China. Here, we present zircon U–Pb geochronology of rhyolitic tuffs, which yields a weighted mean206Pb/238U age of 164.7 ± 1.3 Ma, consistent with the age of the Jurassic porphyry Cu mineralization in the southeastern coastal metallogenic belt. The Zhongqiuyang rhyolitic tuffs are characterized by high FeOT/MgO ratios and low MgO, CaO, and TiO2contents and enrichments in Rb, Th, and U and depletions in Ba, Nb, Ta, Sr, P, and Ti, showing an A2‐subtype affinity. The Sr–Nd–Hf–O isotopic compositions imply that the rhyolitic tuffs were generated by partial melting of the crustal melts (~90% of the Palaeoproterozoic metaigneous rocks and 10% of the Palaeoproterozoic metasedimentary rocks) in the Cathaysia Block, with input of 10~20% mantle‐derived materials. Element variations indicate that crystal fractionation was also required for the generation of the studied samples after partial melting. In combination with previous studies, we propose that a window or a break‐off of the subducted slab triggered by upwelling asthenosphere during the Middle to Late Jurassic underneath the Cathaysia Block, and the ascending mafic magmas not only supplied heat for crustal remelting but also added mantle‐derived melts, resulting in the formation of I‐ or A‐type felsic intrusions related to Cu–Au/Cu–Mo deposits along the active continental margin of SE China.

Distinct stores and the routing of water in the deep critical zone of a snow-dominated volcanic catchment

Abstract. This study combines major ion and isotope chemistry, age tracers, fracture density characterizations, and physical hydrology measurements to understand how the structure of the critical zone (CZ) influences its function, including water routing, storage, mean water residence times, and hydrologic response. In a high elevation rhyolitic tuff catchment in the Jemez River Basin Critical Zone Observatory (JRB-CZO) within the Valles Caldera National Preserve (VCNP) of northern New Mexico, a periodic precipitation pattern creates different hydrologic flow regimes during spring snowmelt, summer monsoon rain, and fall storms. Hydrometric, geochemical, and isotopic analyses of surface water and groundwater from distinct stores, most notably shallow groundwater that is likely a perched aquifer in consolidated collapse breccia and deeper groundwater in a fractured tuff aquifer system, enabled us to untangle the interactions of these groundwater stores and their contribution to streamflow across 1 complete water year (WY). Despite seasonal differences in groundwater response due to water partitioning, major ion chemistry indicates that deep groundwater from the highly fractured site is more representative of groundwater contributing to streamflow across the entire water year. Additionally, the comparison of streamflow and groundwater hydrographs indicates a hydraulic connection between the fractured welded tuff aquifer system and streamflow, while the shallow aquifer within the collapse breccia deposit does not show this same connection. Furthermore, analysis of age tracers and oxygen (δ18O) and stable hydrogen (δ2H) isotopes of water indicates that groundwater is a mix of modern and older waters recharged from snowmelt, and downhole neutron probe surveys suggest that water moves through the vadose zone both by vertical infiltration and subsurface lateral flow, depending on the lithology. We find that in complex geologic terrain like that of the JRB-CZO, differences in the CZ architecture of two hillslopes within a headwater catchment control water stores and routing through the subsurface and suggest that shallow groundwater does not contribute significantly to streams, while deep fractured aquifer systems contribute most to streamflow.

Мікробіота ризосфери Salix sp. за умов використання різних агротехнічних прийомів

The study has established the regularities of distribution of certain physiological groups of microbial cenosis in conditions of growing of fruit and energy plants. In case of willow, the numbers of ammonifiers, micromycetes, actinomycetes, cellulolytic and oligonitrophilic microorganisms were observed to reduce compared to the control. In case of apple-trees, the number of microorganisms was higher but still within the control values. Growing of willows leads to stabilization of soil solution reaction, higher concentration of ammonium nitrogen, and lower concentration of nitrate nitrogen and potassium, compared with the control. We have established decreased concentrations of humus vs. control in both cases – growing of willows and apple-trees, which figure correlates with the number of pedotrophs. Notwithstanding the lowered number of some physiological groups of microorganisms, the soil with willows was characterized by highly intensive mineralization processes, in particular those of transformation of organic matters. An open-field long-term fertilization experiment was set up during 2011 with willow (Salix triandra x Salix viminalis ’Inger’), grown as an energy crop. The brown forest soil was treated during June 2011, May 2013, and May 2016 with municipal biocompost (MBC), municipal sewage sludge compost (MSSC), willow ash (WA) and sulfuric urea (SU) and two times (2011, 2013) with rhyolite tuff (RT). Thus, the most indicative changes in the soil microbiota against the control plot were found in case of the use of municipal sewage sludge compost; rise in the number of intestinal bacteria, ammonifiers, micromycetes and actinomycetes. In case of the use of municipal biocompost rise in the levels of intestinal bacteria, microscopic fungi, actinomycetes, pedotrophs and cellulolytic bacteria were found. While calculating the mineralization/immobilization index, it was shown that the most significant deviation from the control plot was found in rhyolite tuff treated soil – a decrease by 6 times, and in case of willow ash by 2.3 times, which proves the inhibition of mineralization of the organic substances in the soil.

Generation of late Mesozoic felsic volcanic rocks in the Hailar Basin, northeastern China in response to overprinting of multiple tectonic regimes

We performed zircon U–Pb age dating and geochemical analyses of late Mesozoic felsic volcanic rocks in the Hailar Basin, NE China, with the aim of eclucidating their emplacement ages, origin and geodynamic significance. The volcanic rocks consist of dacites, rhyolites and rhyolitic tuffs. Laser ablation–inductively coupled plasma–mass spectrometry zircon U–Pb dating results suggest that the rocks were erupted during the Late Jurassic–Early Cretaceous (161–117 Ma). They belong to the high-K calc-alkaline series and can be divided into two groups. Group I rocks are metaluminous to weakly peraluminous, contain low concentrations of heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and have low zircon saturation temperatures (average 786 °C), all of which indicate an I-type affinity. In contrast, Group II rocks have higher HREE and HFSE concentrations and zircon saturation temperatures (average 918 °C), suggesting an A-type affinity. All the felsic volcanic rocks have positive εHf(t) values of 1.43–12.32 with two-stage model ages of 1110–401 Ma. Our data indicate that the I-type felsic volcanic rocks formed from magmas generated by partial melting of a dominantly juvenile mica-bearing K-rich basaltic lower crust, whereas the A-type felsic volcanic rocks originated from the partial melting of a dry mafic–intermediate middle–lower crust that was dehydrated but not melt depleted. Based on the present results and previous research, we propose that the Late Jurassic I- and A-type felsic volcanic rocks in the Hailar Basin were formed in a post-collisional environment related to break-off of the subducted oceanic slab of the Mongol–Okhotsk Ocean and the subsequent gravitational collapse of the orogenically-thickened crust after closure of the ocean. In contrast, the Early Cretaceous I- and A-type felsic volcanic rocks were erupted in an extensional setting related to rollback of the subducted Paleo-Pacific Plate.

Geochronology and Geochemistry of Ore-Hosting Rhyolitic Tuff in the Kengdenongshe Polymetallic Deposit in the Eastern Segment of the East Kunlun Orogen

The Kengdenongshe Au–Ag–Pb–Zn polymetallic deposit, a newly discovered large-scale polymetallic deposit in the southeastern section of the East Kunlun Orogen (EKO), contains an Au resource of 40 t, Ag resource of 690 t and Pb–Zn resource of 10.5 × 105 t. The ore-bearing rocks are mainly composed of laminar barite and rhyolitic tuff. In this study, LA-ICP-MS zircon U–Pb dating and whole rock major and trace elements analyses have been conducted on the ore-bearing rhyolitic tuff. LA-ICP-MS zircon U–Pb dating data show that these rhyolitic tuffs were emplaced at ca. 243.3 ± 1.6 Ma. The samples show similar features to those of S-type granites, including high contents of SiO2 (76.4–82.6 wt. %) and Al2O3 (11.0–12.7 wt. %) and relatively low concentrations of Na2O (0.35–2.43 wt. %) and CaO (0.095–0.124 wt. %), with high A/CNK (molar [Al2O3/(CaO + Na2O + K2O)]) (1.72–2.03) and K2O/Na2O ratios (1.41–17.1). Further, they exhibit depletion in HFSEs (High Field Strength Elements) and enrichment in LREEs (Light Rare Earth Element) with negative Eu anomalies (Eu/Eu* = 0.51–0.64). These geochemical characteristics indicate that the Kengdenongcuo rhyolitic tuff originated from the fluid-absent melting of a plagioclase-poor, clay-rich metapelitic source and experienced minor fractional crystallization. In combination with arc-type magmatism and contemporaneous syn-collision granitoids in the region, the Kengdenongcuo tuff formed in a continental collision setting, implying that the Bayan Har–Songpan Ganzi Terrane collided with the East Kunlun Terrane and the Paleo-Tethys Ocean was closed at the period of ~243 Ma. The Kengdenongcuo polymetallic deposit formed at about the same time.

High‐resolution stratigraphy of a Quaternary fluvial deposit based on magnetic susceptibility variations (Jászság Basin, Hungary)

Quaternary fluvial succession of the Jászság Basin (Hungary) was investigated, challenging the stratigraphical potential of ‘early postglacial fluvial magnetic susceptibility episodes' recognized earlier in the Körös Basin. Low field magnetic susceptibility (MS) was measured in four boreholes from the basin centre and margins, representing channel and flood‐plain environments. Statistical distributions of MS data contain significant sets of outliers, regardless of facies conditions. The downhole distribution of these outliers produces magnetic susceptibility cycles. Supported by magnetic susceptibility cycles, high‐resolution facies correlations were performed reflecting a steady palaeohydrographical situation in the area, with a trunk river to the southeast and tributaries to the northwest. SEM‐EDX data revealed that ferromagnetic grains are responsible for the outlying MS values. The heterogeneous association of magnetite indicates a catchment area with volcanic and metamorphic rocks, while the group of small (&lt;5 μm) magnetite octahedrons originated from nearby rhyolitic tuff formations. Magnetic grains were transported along channel belts, while small (&lt;2 μm) magnetic particles were floated onto flood‐plains attached to clays. Climatic control is indicated by peaks at ~100 ka frequency in spectra of MS records and was also detected in palaeosol development and in flood frequency using the spectra of measured colour (~100 ka) and logged resistivity (~100 ka, ~41 ka), respectively. The climate‐dependent MS signal traceable far into the basin in both channel and flood‐plain environments can be summoned when the Quaternary fluvial succession of the Pannonian Basin is investigated, assuming some sources of magnetite in the catchment areas. According to the concept of ‘fluvial magnetic susceptibility episodes’, the early postglacial escape and spreading of the magnetite fraction control the MS signal that can support mapping of the unconformable Quaternary base and building of high‐resolution models of aquifers.

Nature of the Late Ordovician-Early Silurian Xiaohe section, Hunan-Hubei area, South China: implications for the Kwangsian Orogeny

ABSTRACT The intraplate Kwangsian Orogeny is a key orogenic event in South China in the mid-Paleozoic. We re-examined the evidence for the Yichang Uplift, an inferred geographic feature during the Kwangsian Orogeny, to evaluate its timing and nature. Field, sedimentological, mineralogical and geochronological data were collected from the Late Ordovician-Early Silurian Xiaohe section, Hunan-Hubei area. Results suggest that the Xiaohe section is composed of the Late Ordovician Wufeng Formation black shale in the lower part and the Early Silurian Longmaxi Formation black shale in the upper part. We found that the clay layers interbedded in the Wufeng Formation are altered rhyolitic tuffs instead of parts of a subaerial wreathing crust. LA-ICP-MS U-Pb dating of zircons in the top tuff layer of the Wufeng Formation yielded an age of 447.0 + 1.4/- 2.2 Ma, consistent with biostratigraphic data, providing a radiometric constraint for the sedimentary break existed between the Wufeng and Longmaxi formations and confirming the absence of the Hirnantian (latest Ordovician) Guanyinqiao Formation in the study area. Our data support that the Yichang Uplift was a submarine highland possibly initiated by the reactivation of the inherited Jianshi-Enshi Fault in the Hunan-Hubei area during the Kwangsian Orogeny.

Kaolinite Occurrence of Kachchh, Gujarat: A Product from Rhyolitic Tuff by Hydrothermal Alteration

ABSTRACT Kaolinite associated with Mesozoic sediments of Kachchh and alkaline igneous rocks are not well studied in view of its genesis. The petrography, EPMA and DTA studies support that the studied kaolinite samples formed by mature transformation from rhyolitic tuff by hydrothermal alteration in epithermal condition. Such postulation indicates that there was intermittent acid volcanic episode along with alkaline basic igneous magmatism in the Cretaceous of Kachchh basin, western part of India.

Characterisation of fibrous ferrierite in the rhyolitic tuffs at Lovelock, Nevada, USA

AbstractFerrierite is the name for a series of zeolite-group of minerals which includes three species with the same ferrierite framework (FER) crystal structure but different extra-framework cations. Recent studies have shown that ferrierite can exhibit a fibrous-asbestiform crystal habit and may possess the same properties as carcinogenic fibrous erionite. Characterisation of the ferrierite in and around a mine location will be helpful in assessing the potential for toxic outcomes of exposure in the mine and any local population.The zeolite-rich tuff deposit of Lovelock, Nevada, USA is the largest occurrence of diagenetic ferrierite-Mg. A previous survey reported that ferrierite hosted in these rocks displays a fibrous morphology. However, these observations concerned a limited number of samples and until now there has been little evidence of widespread occurrence of fibrous ferrierite in the Lovelock deposit.The main goal of this study was to perform a mineralogical and morphometric characterisation of the tuff deposit at Lovelock and evaluate the distribution of fibrous ferrierite in the outcrop. For this purpose, a multi-analytical approach including powder X-ray diffraction, scanning and transmission microscopies, micro-Raman spectroscopy, thermal analyses, and surface-area determination was applied.The results prove fibrous ferrierite is widespread and intermixed with mordenite and orthoclase, although there are variations in the spatial distribution in the bedrock. The crystal habit of the ferrierite ranges from prismatic to asbestiform (elongated, thin and slightly flexible) and fibres are aggregated in bundles. According to the WHO counting criteria, most of the ferrierite fibres can be classified as breathable. While waiting for confirmatory in vitro and in vivo tests to assess the actual toxicity/pathogenicity potential of this mineral fibre, it is recommended to adopt a precautionary approach for mining operations in this area to reduce the risk of exposure.