Felsic Volcanics Research Articles

Large-volume Permian felsic volcanism in the Tisza Mega-unit (East-Central Europe): Evidence from mineralogy, petrology, geochemistry, and geochronology

Permian felsic volcanic rocks are widespread in the Tisza Mega-unit (Carpathian–Pannonian region), covering a relatively large area from southern Transdanubia (Hungary) to the Apuseni Mountains (Romania). The present occurrence and structural position of these Permian volcanic rocks are the result of a complex tectonic evolution (including Alpine nappe stacking and Neogene extension); meanwhile, multiple hydrothermal effects and deformation modified their microtexture and chemical composition. In this study, the same emphasis was given to Permian felsic volcanism as well as the subsequent fluid- and/or deformation-related processes in the Tisza Mega-unit to extend our knowledge in regional issues (e.g., geochronology, stratigraphy, and lithologic correlations) and to investigate if the variably altered volcanic rocks can be used in petrogenetic studies.The zircon U-Pb dating yields ages of ∼270.4–262.9 Ma for the Apuseni samples and ∼268.4–260.2 Ma for those of the Pannonian Basin. Whole-rock geochemistry, considering immobile trace elements, since most of the major element compositions were severely modified by alteration processes, indicates that the studied rocks are dacite to rhyolite. They belong to a voluminous silicic volcanism during the Middle Permian, most probably related to rifts.Petrography, whole-rock geochemistry, and X-ray powder diffraction mineralogy revealed that the predominant alteration of the studied rocks is hydrothermal sericitization, locally superimposed by mylonitization in greenschist to subgreenschist conditions during Alpine orogeny. The K-Ar dating of the separated illite fractions confirms that the ductile deformation acted during the Turonian nappe stacking, as the main Alpine deformation phase in the study area.On a regional scale, the Permian magmatism in the Tisza Mega-unit looks significantly younger than the similar magmatic activity of the stable Europe (∼300–290 Ma).

Biostratigraphic and structural research in the Guedelhinha–Lançadoiras–Algaré sector in the context of the geology of the Neves–Corvo mine region, Iberian Pyrite Belt

Based on drill hole sampling and sedimentary rock dating by palynology, the present research focuses on the palynostratigraphic events established in the key geological section of Guedelhinha–Lançadoiras–Algaré located in the Portuguese Neves–Corvo mine region, Iberian Pyrite Belt. The age data allow detailing the lithostratigraphic sequence and further understanding the complex structural setting, representing an important contribution to the geological knowledge of this sector located to the immediate WNW of the Neves–Corvo VMS deposit. The combination of the studied events allows the reconstruction of the Devonian-Carboniferous sedimentation and paleoenvironments along the Iberian Pyrite Belt. Several stratigraphic hiatuses identified in the Neves–Corvo region by the palynological record are confirmed, mainly occurring from the mid Frasnian to mid Famennian, from the early and mid Strunian, and from the early to late Tournaisian, which were mainly coincident with the worldwide extinction events, in particular during Frasnian–Famennian and Late Devonian times. Extensional tectonics and related gravitational faults, local uplift mechanisms and intense volcanic activity could also explain the lack of palynological data and sedimentary hiatus. In this research, the importance of the late Strunian times in the Iberian Pyrite Belt (Miospore Biozone LN of the Neves Formation) is highlighted, confirming the contemporaneity of felsic volcanism, hydrothermalism, sulphide mineralization precipitation and black shale anoxic sedimentation.

Zirconium and neodymium isotopes record intensive felsic volcanism in South China region during the Permian-Triassic boundary crisis

The Permian-Triassic mass extinction (PTME), the largest biocrisis in Earth's history, is regarded as having been triggered by volcanism, although the source and nature of this volcanic activity in the South China region remains in debate. In this regard, determining the geochemical composition of the volcanic and terrestrial source fluxes to marine depositional systems is crucial for differentiating material derived from the Siberian Traps Large Igneous Province versus regional volcanic arcs. Here, we report zirconium (Zr) isotopic compositions (δ94/90Zr) of Permian-Triassic boundary strata from a deep carbonate-ramp setting at Xiakou, South China, providing new insights regarding the provenance of volcanic ash layers and their host silicic mudstones. The δ94/90Zr composition of magmatically sourced materials is generally controlled by degree of melt crystallization and has the potential to discriminate between primitive and evolved igneous sources. At Xiakou, the pre-PTME Lower Unit contains volcanic ash layers with δ94/90Zr (+0.05 ± 0.03‰, 2SD) similar to that of upper continental crustal (UCC), recording dominantly detrital zircon sources. About 10 kyr before the mass extinction, a single ash layer (Bed 249) records a transient excursion to mantle-like values (∼0‰), followed by a protracted shift across the Permian-Triassic boundary (PTB) toward signatures characteristic of more evolved felsic sources (+0.19 ± 0.12‰) accompanied by a positive shift in Nd isotopes (εNd(t)). The high δ94/90Zr of volcanic ash layers from the Middle Unit (corresponding approximately to the interval between the PTME and PTB) indicates dominance of regional felsic volcanic sources. In this interval, volcanic ash δ94/90Zr values at Xiakou are distinctly higher than those at Meishan, suggesting that primary volcanic signatures are better preserved in deep-water facies (Xiakou) than in shallow-water facies (Meishan). In the Middle (excluding the Bed 249) and Upper (post-PTME) units, positive covariation of δ94/90Zr with Al2O3/TiO2, Zr/TiO2 and Zr/Sc ratios was the product of mixing of detrital and volcanic materials, with the latter having a dominantly felsic source. This study provides new insight into the sources of zircons in PTB sections of South China, and it demonstrates the general utility of sedimentary Zr isotopic compositions in determining magmatic provenance.

Miocene lamprophyre and felsic volcanics in the outer zone of SW Japan: arc building through multiple sources in an active convergent margin

ABSTRACT Subduction-related magmatism is a significant contributor to arc building and continental growth. In the active convergent margin of the SW Japan arc, voluminous volcanic rocks and mafic dykes formed through subduction and back-arc rifting during Miocene. In this study, we investigate a suite of felsic volcanic rocks (rhyolite and dacite) and lamprophyres from central Shikoku Island, which occur in the north-central part of the Chichibu belt and Sanbagawa belt in the Outer zone of the SW Japan arc. Whole-rock geochemistry results show that the rhyolite and dacite samples are peraluminous, belong to the high-K calc-alkaline series, show LREE and LILE enrichment, HREE and HFSE depletion that suggest arc volcanic affinity. The lamprophyre samples belong to alkali lamprophyre, show LREE and HFSE enrichments, and display OIB (oceanic island basalt)-like geochemical features. The 206Pb/238U weighted mean ages of the zircons from the rhyolite and dacite samples range from 14.39 ± 0.21 Ma to 13.92 ± 0.30 Ma, whereas the lamprophyre shows 206Pb/238U weighted mean age of 12.21 ± 0.35 Ma. Zircon Lu-Hf isotope data suggest that the rhyolite and dacite were derived from the remelting of the arc crustal rocks and that the lamprophyre was derived from depleted mantle sources and was contaminated by crustal materials. The felsic volcanic rocks (rhyolite and dacite) can be correlated to Miocene (15.5 Ma to 13.5 Ma) magmatism in the Outer Zone of the SW Japan arc. The formation of lamprophyre dykes in the SW Japan arc may be related to the subhorizontal mantle flows beneath the SW Japan arc associated with the opening of the Japan Sea back-arc basin or asthenosphere upwelling through the slab-tear of the Philippine Sea Plate beneath the SW Japan arc. Our study shows that the SW Japan Arc was built by multiple magmatism from diverse sources in an active convergent margin.

In Situ LA-ICP-MS U-Pb Geochronology, Sr-Nd-Hf Isotope and Trace Element Analysis of Volcanic Rocks from the Gacun Volcanic-Hosted Massive Sulfide Deposit in Sichuan, China

The Gacun deposit is a typical Volcanic Hosted Massive Sulfide (VHMS) associated with Late Triassic seafloor calc-alkaline felsic volcanics. Studies of zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf isotope geochemistry of volcanic rocks from the Northern Yidun arc were undertaken in this paper. We reshaped the Gacun magmatic system activity time, defined the origin of magma evolution, and proposed a metallogenic model of the deposit. Whole-rock major element compositions of the magmatic rocks in the Northern Yidun island arc indicate that they are a complete basalt–andesite–dacite–rhyolite assemblage, showing three obvious stages of composition evolution. They are enriched in large-ion lithophile and light rare earth elements, but depleted in high field-strength and heavy rare earth elements, with weak-to-negligible Eu anomalies (obvious in rhyolite). These geochemical features indicate that the Northern Yidun island arc is a magmatic arc based on ancient continental crust. The Ganzi–Litang oceanic subduction induced mantle melting and produced calc-alkaline basaltic magma, while the MASH processes at the bottom of the crust produced andesitic magma. Part of the andesite magma erupted to form andesite lava. The remaining part was mixed with magma produced via anatexis of ancient crust (approximately 20%–40% of the ancient crustal component), forming the ore-bearing rhyolite. Zircon U-Pb age data defines Gacun magmatic–hydrothermal mineralization sequence of events: At 238 Ma, arc magmatism led to the formation of andesite in the eastern part of the deposit. At 233 Ma, in the arc zone (the western part of Gacun deposit), a large-scale bimodal magmatism formed the main ore-bearing rock series of Gacun deposit, rhyolitic volcanic rocks. At 221 Ma, volcanic eruptions tended to end and sub-volcanic intrusion occurred, forming a lava dome, which was located under the ore-bearing rhyolitic volcanic rocks. The lava dome acted as a thermal engine and promoted hydrothermal circulation. The hydrothermal activity reached a peak at 217 ± 1 Ma, and the Gacun VHMS deposit was formed.

Origin of Fe-rich clay minerals in Early Devonian volcanic rocks of the Northern Minusa basin, Eastern Siberia

The formation of clay minerals in volcanic successions follows various evolutionary trends, the understanding of which is crucial for estimating the age of volcanism and the prospecting clay deposits. Early Devonian volcanics within the Byskar Series of the Minusa Depression (Khakassia) are variably altered to a range of phyllosilicate mineral assemblages. This study highlights the origin and formational conditions of the clay minerals, including illite, glauconite, smectite (saponite) and chlorite. Detailed mineralogical and chemical investigation identifies the predominant phyllosilicates, including 1 M illite, illite-smectite (R = 3), 1 M glauconite, smectite, chlorite and kaolinite within trachyandesite, trachyte, and rhyodacite. The phyllosilicates formed by the diagenetic transformation of volcanics after eruptive phases. Evolution trends of phyllosilicates depend largely on the composition of the volcanic substrate. The smectite to illite trend developed due to the dissolution of felsic volcanics and the alteration of smectites into illites with the K+, Al3+ and Mg2+ dominating the mineral formation system. The illites evolved by the substitution of Fe2+ and Mg2+ for Al3+ in the octahedral sheets of smectites. The K+, which was released due to devitrification of felsic substances, was introduced into the interlayer sites for compensating the layer charge originated by the isomorphic substitution in tetrahedral and octahedral sheets. The smectite to glauconite transition trend corresponds to the devitrification and dissolution of alkaline intermediate-felsic volcanics. Early formed dioctahedral smectites evolved into glauconite probably with the predominance of K+ and Fe2+/3+ ions in the formation environment. The substitution of Fe2+/3+ to Al3+ and Mg2+ in the octahedral sheets of smectite contributed to the formation of a layer charge and the entry of exchangeable K+ into the interlayers of glauconite. The third trend involved the formation of smectites (saponites) or chlorites within the intermediate volcanics possibly with Mg-rich but K-poor mineral system.

Clastic-volcaniclastic Rocks in Papaghni Sub-basin: Implications for Syn-sedimentary Felsic Volcanism during the Deposition of Paleoproterozoic Vempalle Carbonate Sequence, Cuddapah Basin, India

Abstract Based on detailed field, petrographic, mineral chemistry and geochemical studies, succinct details on the clastic - volcaniclastic sequence recorded within the dolomite horizon in Paleoproterozoic Vempalle Formation, Cuddapah basin, India are presented. The clastic rocks represented by rudaceous-arenaceous sequence and the volcaniclastic rock represented by an intervening tuffaceous horizon are noticed in the Vempalle dolomite near Tummalapalle area, in southwestern part of the Cuddapah basin. The conglomerate is matrix supported with dominantly chert clasts and the arenaceous unit is represented by fine grained quartz arenite. The tuffaceous unit is finely laminated exhibiting typical; clast matrix texture characteristic of fine grained pyroclastic rocks. Zircon, apatite, rutile and monazite are the accessory phases in the tuff. Geochemically, the felsic tuff is rhyolitic, peraluminous and shows relative enrichment of LREE. The chemical composition of the feldspar falls close to the K-feldspar end member in the Or-Ab-An plot. The clastic - volcaniclastic sequence in the Vempalle carbonate sequence indicate a localised break in carbonate precipitation accompanied by syn-sedimentary felsic volcanism during the evolution of Paleoproterozoic segment in the Cuddapah basin, India.

Transcrustal and source processes affecting the chemical characteristics of magmas in a hyperactive volcanic zone

Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand’s active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate.

The ~1.1 Ga St. Ignace Island complex, Northern Ontario, Canada: Evidence for magma mixing and crustal melting in the generation of Midcontinent Rift-related bimodal magmas and implications for regional metallogeny

Abstract The St. Ignace Island complex in Northern Ontario is a package of dominantly felsic rocks emplaced within the upper portions of the Osler Volcanic rocks of the ~1.1 Ga Midcontinent Rift System. The Osler volcanic rocks are predominantly tholeiitic basalts intercalated with rare interflow sediments and rhyolites. The St. Ignace Island complex is a ~26 km2 stock with a felsic core of quartz-feldspar-phyric rhyolites and dacites and an outer ring of anorthosite and gabbro. Textures at a variety of scales within the rocks of the complex show clear evidence of the mingling and mixing of partially crystallized mafic and felsic liquids. Two multigrain (zircon/baddeleyite) fractions from a sample of the gabbro define a Discordia line with an upper intercept date of 1107±8.9 Ma. The core of the complex consists of dacites and rhyolites with similar REE abundances with negative Nb anomalies whereas the surrounding mafic rocks are gabbros to monzogabbros that are less LREE-enriched than the felsic rocks but with similar HREE. Felsic units have a narrow range of 87Sr/86Sri (0.7032-0.7045) and 143Nd/144Ndi (0.51051-0.51057) whereas the mafic end members have similar 87Sr/86Sri (0.7040-0.7061) but more radiogenic 143Nd/144Ndi (0.51067-0.51085). Very well-preserved silicate melt inclusions (MI), many completely glassy, were observed in quartz, clinopyroxene and some plagioclase phenocrysts from the complex. These represent some of the oldest unrecrystallized silicate melt inclusions described to date. Melt inclusions within quartz from the felsic volcanics are broadly rhyolitic in composition whereas MI from plagioclase in the mafic volcanics range from basalt to basaltic andesite; these felsic and mafic melt compositions are interpreted to represent the end-member liquids in the system and bulk rock analyses affirm mixtures of the two. Concentrations of Cu and Ag (in both mafic and felsic MI), and Mo (in felsic MI), are up to an order of magnitude higher in the mafic and felsic MI than in continental crust. Bulk rock metal concentrations are also significantly lower than in the MI, suggesting that the melt inclusions may preserve pre-eruptive metal tenors that were subsequently modified by sulfide saturation, degassing, or post-solidus hydrothermal alteration. The whole rock and MI geochemistry of the St. Ignace complex are broadly similar to the Central Osler Group and, given the broad similar ages, suggests they may have been derived from a similar mantle source, but distinct from the source of rhyolites in the Black Bay Peninsula. The negative Nb anomalies and negative εNd values for the St. Ignace complex are consistent with mixing with older continental crust during ascent and emplacement. The rocks of the St. Ignace Island complex likely formed as the result of emplacement of a large mafic magma chamber at the base of the Osler volcanic pile that triggered partial melting to generate the rhyolite end members. The felsic melts ascended to shallower levels in the crust where they mixed with mafic magmas derived directly from the deeper chamber. Generally, melt inclusions in the complex have very high Cu and Ag contents, similar to those observed in arc-related and extremely oxidized early rift-related rocks and may account for the world-class volcano-sediment-hosted Cu-(Ag) deposits within the rift and the presence of small porphyry-style deposits.

Petrogenesis of Early Triassic Felsic Volcanic Rocks in the East Kunlun Orogen, Northern Tibet: Implications for the Paleo-Tethyan Tectonic and Crustal Evolution

The felsic volcanic rocks in orogenic belts are vital probes to understand the tectonic evolution and continental crust growth. Here, we present a comprehensive study on the zircon U–Pb geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of Early felsic volcanic rocks from the Hongshuichuan Formation, East Kunlun Orogen, Northern Tibet, aiming to explore their petrogenesis and implications for the Paleo-Tethyan orogeny and crustal evolution. The studied felsic volcanics comprise rhyolite porphyry and rhyolite, exhibiting coeval zircon U–Pb ages of ca. 247–251 Ma. Rhyolite porphyries show metaluminous to peraluminous nature (A/CNK = 0.88–1.24) with high SiO2 contents (72.1–78.9 wt%) and moderate Mg# values (22–40), and they display enrichment of LREE with (La/Yb)N ratios of 6.02–17.9 and depletion of high field strength elements. In comparison, the rhyolites are strongly peraluminous (A/CNK = 1.09–1.74) with high SiO2 contents (71.7–74.3 wt%) and high Mg# values (43–52) and are also enriched in LREE ((La/Yb)N of 6.65–18.4) and depleted in HFSE (e.g., Nb, Ta, Ti). Combining with their different zircon Lu-Hf isotopes, i.e., enriched isotopes for the rhyolite porphyries (εHf(t) = −7.3 to −3.8) and depleted Hf isotopes for the rhyolites (ɛHf = −0.6 to +3.0), we interpret that the studied rhyolite porphyries and rhyolites were derived by partial melting of Mesoproterozoic metagreywacke sources followed by plagioclase-dominated fractional crystallization, but the latter shows the significant contribution of crust–mantle magma mixing. The mixed mantle-derived magma comes from an enriched lithospheric mantle source that had been metasomatized by subduction-related fluids. Combining with other geological evidence, we propose that the studied Early Triassic felsic volcanic rocks were formed in a subduction arc setting, and the reworking of ancient continental crust with crust–mantle magma mixing is the major mechanism of crustal evolution in the East Kunlun Paleo-Tethyan orogenic belt.

Cogenetic Origin of Magmatic Enclaves in Peralkaline Felsic Volcanic Rocks from the Sanshui Basin, South China

Centimeter-scale magmatic enclaves are abundant in peralkaline felsic volcanic rocks in the Sanshui Basin. Their lithology is mainly syenite and syenitic porphyry, and they mainly comprise alkali feldspar and amphibole, which is similar to the mineral assemblage of the host trachyte and comendite. The SiO2 content in the syenitic enclaves is ~63 wt%, which is similar to that of the host trachyte but lower than that of the comendite. Thermobarometric calculations showed that the syenitic enclaves crystallized at similar temperature and pressure conditions as their host trachyte. The results of mass-balance modeling and MCS modeling indicate that the syenitic enclaves likely experienced an approximately 74% fractional crystallization from the basaltic parental magma. Combined with the similar mineral assemblages and geochemical characteristics of the host trachyte, we think that the enclaves resulted from the in situ crystallization of trachytic magma in the shallow crust and that they had a cogenetic origin with their host volcanic rocks, which means that they were likely to derived from the identical magma chamber which was formed from different batches of magma mixing/mingling. The recharge and mixing of basaltic magma triggered the eruption of trachytic magma eruption. The syenitic crust may have been disaggregated by the ascending trachytic magma and brought to the surface as syenitic enclaves. The syenitic enclaves in volcanic rocks provide unique information on the magmatism of the shallow crust as evidence of magma mixing/mingling.

Detrital zircon geochronology from the Nueva Segovia Schist, Nicaragua: evidence for the tectonic evolution of the Chortis Block?

Abstract The Chortis Block of Central America is a cratonic-type peri-Gondwanan terrane and is commonly included in Neoproterozoic–Paleozoic palaeogeographical reconstructions. At present, most research has focused on the Mesozoic evolution of the Chortis Block, however, its earlier history remains poorly constrained. As a result, there is considerable debate surrounding the internal complexities of the Chortis Block and its tectonothermal evolution has not been well established by geochronological and geochemical data. New field investigations from the Nueva Segovia Schist (Northern Nicaragua), considered one of the oldest exposed parts of the Chortis Block, reveal it is composed primarily of deformed sequences of greenschist facies marine clastic and chemical sediments in conformable contact with felsic volcanics. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from two samples taken from the Nueva Segovia Schist reveal a youngest age peak of c. 250 Ma with other significant peaks at c. 500 Ma, c. 1.0 Ga and c. 1.2 Ga. Taken together with field observations, these data suggest the Nueva Segovia Schist was likely deposited between c. 250 and 110 Ma proximal to Amazonia during the Late Paleozoic, and they support a Precambrian age for the basement of the East Chortis Terrane. Taken together the data support a Pangaean position of the Chortis Block, adjacent to Amazonia inboard of Oaxaquia.

Timing of Transition from Proto- to Paleo-Tethys: Evidence from the Early Devonian Bimodal Volcanics in the North Qaidam Tectonic Belt, Northern Tibetan Plateau

The transition from the Proto- to the Paleo-Tethys is still a controversial issue. This study reports a new petrology, zircon U–Pb geochronology, and whole-rock geochemistry of volcanic rocks from the Maoniushan Formation in the Nankeke area, northern Qaidam (NQ) of the Tibetan Plateau, to provide new evidence for the transition from the Proto- to the Paleo-Tethys oceans. The volcanic suite consists mainly of rhyolitic crystal lithic tuff lavas and minor basalts. Zircon U–Pb data indicate that the bimodal volcanic rocks were formed during the Early Devonian (ca. 410–409 Ma). Geochemically, the basalts have low contents of SiO2 (48.92 wt.%–51.19 wt.%) and relatively high contents of MgO (8.94 wt.%–9.99 wt.%), TiO2 (1.05 wt.%–1.29 wt.%), K2O (2.35 wt.%–4.17 wt.%), and K2O/Na2O ratios (1.04–2.56), showing the characteristics of calc-alkaline basalts. Their rare earth element (REE) patterns and trace element spider diagrams are characterized by enrichments in LREEs (LREE/HREE = 18.31–21.34) and large ion lithophile elements (LILEs; Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; Nb, Ta, P, and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.82–0.86), which are similar to Etendeka continental flood basalts (CFB). These features suggest that the basalts were most likely derived from low degree (1%–5%) partial melting of the asthenospheric mantle, contaminated by small volumes of continental crust. In contrast, the felsic volcanics have high SiO2 (68.41 wt.%–77.12 wt.%), variable Al2O3 (9.56 wt.%–12.62 wt.%), low MgO, and A/CNK ratios mostly between 1.08 and 1.15, defining their peraluminous and medium-K calc-alkaline signatures. Their trace element signatures show enrichments of LREEs and LILEs (e.g., Rb, Th, U, K, and Pb), depletion of HFSEs (e.g., Nb, Ti, Ta, and P), and negative Eu anomalies (Eu/Eu* = 0.22–0.66). These features suggest that the felsic volcanics were derived from partial melting of the middle crust, without interaction with mantle melts. Considering all the previous data and geochemical features, the Maoniushan Formation volcanic rocks in NQ formed in a post-collisional extensional setting associated with asthenospheric mantle upwelling and delamination in the Early Devonian. Together with the regional data, this study proposed that the Proto-Tethys Ocean had closed and evolved to the continental subduction/collision orogeny stage during the Middle to Late Ordovician, evolved to the post-collisional extensional stage in the Early Devonian, and finally formed the Zongwulong Ocean (branches of the Paleo-Tethys Ocean) in the Late Carboniferous, forming the tectonic framework of the Paleo-Tethys Archipelagic Ocean in the northern margin of the Tibetan Plateau.

Main stages of the Kostomuksha greenstone belt Banded Iron Formation genesis, Karelian Craton: based on U-Th-Pb dating of zircon

We studied zircon from banded iron formations (BIFs) in the Kostomuksha greenstone belt (KGSB), Karelian Craton, for the first time. Here, BIFs are associated with Mesoarchean basalt-komatiites – BIF-1 and felsic volcanics – BIF-2, as well as with Neoarchean graywackes and felsic volcanics – BIF-3. All the BIFs contain accessory zircon. Analysis of its isotopic age shows the presence of 1-4 age clusters in each sample. There are single grains of detrital zircon in most of the studied BIFs. The grains are of commensurable or older age than the BIF enclosing rocks. The ages of detrital zircons differ among BIF association: 2.84 Ga in BIF-1, no detrital zincon in BIF-2, and 2.98–2.75 Ga in BIF-3. Most of the zircon in the BIFs is of metamorphic origin. All BIF groups contain 2.75–2.72 Ga zircon, while BIF-3 also has 2.65–2.64 Ga zircon associated with felsic magmatism. Rims and grains of 1.89–1.85 Ga zircon from BIF-1 and BIF-3, indicative of local tectono-thermal processes simultaneous with the formation of the Svecofennian Orogen, are reported here for the first time.

Age, genesis, and geological significance of Permian–Triassic boundary volcanic tuffs in the northeastern margin of South China

The Permian–Triassic boundary (PTB) volcanic tuffs are widely distributed in the East Palaeo‐Tethys region, but their age and origin have been controversial. The Permian–Triassic deep‐water sediments in the northeastern margin of South China are favourable research objects, and we report new results from the PTB deep basin sediments in the Susong area, Anhui Province, in this paper. Two volcanic tuffs are found in the Zuoshan PTB in the Zuoshan section, and Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U–Pb zircon dating indicates that the upper and lower tuffs are 251.3 ± 1.7 Ma and 252.3 ± 2.1 Ma, respectively. The PTB age is 251.6 Ma according to a numerical interpolation method of the average deposition rate of the PTB. Fusulinids and non‐fusulinid foraminifers in the limestone overlying the lower tuff define the rock as end‐Permian. Zircon trace elements show that both tuffs are sourced from felsic volcanism in continental arc background, and their source magmas were derived from the continental crust. Zircons of PTB age from the upper tuff on the bottom of the Triassic have negative εHf(t) value from −8.02 to −3.75, and the Palaeo‐Mesoproterozoic two‐stage model ages of 1502–1780 Ma, showing that their source rocks were only derived from the remelting of the Palaeo‐Mesoproterozoic continental crust. The zircons from the lower tuff on the top of the Permian show two types, including negative εHf(t) values from −6.48 to −1.25 and Palaeo‐Mesoproterozoic two‐stage model ages of 1350–1679 Ma, and positive εHf(t) values from +3.98 to +10.15, and Neoproterozoic two‐stage model ages of 629–1023 Ma, indicating that their sources were derived from the reworking of the Palaeo‐Mesoproterozoic continental crust and early Neoproterozoic depleted mantle joined in juvenile crust growth in the northern margin of the South China Block at the beginning of the collision between the South China and North China blocks. The initial collision started no later than 252.3 Ma and developed significantly from east to west.

The role of long-lived arc volcanism in the formation of the VMS deposits: A case study of the volcanic-sedimentary sequence of Kangbutiebao formation associated with VMS deposits, Altai Mountains

Questions about the role of volcanism in the formation of VMS deposits and the origin of metals in pre-ore volcanic rocks in accretionary orogen generally puzzle scholars. The volcanic-sedimentary sequence of Lower Devonian Kangbutiebao Formation (KBTB Fm) associated with VMS deposits in the Altai Mountains is studied to attempt to solve these issues. The KBTB Fm is predominantly composed of felsic volcanic rocks with subordinate sedimentary rocks. Zircon U-Pb dating of rhyolite and rhyodacite suggests that felsic volcanism occurred in the Early Devonian (400.1 ± 0.8 Ma to 395.3 ± 0.9 Ma). Partial melting of juvenile crust under the environment of lower temperature (662 to 728 °C) and pressure (<∼15 km) resulted in volcanic rocks with arc-like trace elements and depleted Hf isotopes. Both compositional heterogeneity of source rocks and contrasting melting temperatures are responsible for the variety of petrochemistry. Sedimentary rocks exhibit low CIA values (51–66) and varied ICV values (0.76–1.78), implying that the provenance was affected by weak weathering. As suggested by detrital zircon geochronology, detrital materials from sedimentary rocks are dominated by pyroclastic materials that erupted from the volcano, together with minor eroded detrital materials from adjacent intrusions. It is proposed that the KBTB Fm was formed under continental rifted arc or back-arc condition, and local extensional setting was induced by slab-rollback of the Paleo-Asian Ocean, which produced bimodal volcanic rocks and VMS deposits. Large volumes of sequences with low metallic abundance were produced and protracted hydrothermal circulation was sustained under the condition of long-lived arc volcanism, leading to most VMS deposits with low to moderate grade, and rarely, high grade. All these parameters are grouped together (i.e., abundant but low contents of metals, long-lived heat source, and syn-volcanic faults), leading to most VMS deposits hosted by the KBTB Fm in the southern Chinese Altai.

Role of Depleted-MORB Mantle in the Genesis of Basalts from the Neoarchean Eastern Felsic Volcanic Terrane of the Sandur Greenstone belt, Dharwar Craton, India

Abstract The mafic volcanic rocks from the eastern felsic volcanic terrane (EFVT) of the Neoarchean Sandur greenstone belt of Western Dharwar Craton (WDC) are studied to evaluate their geochemical characteristics and tectonic implications. Characterised as sub-alkaline, tholeiitic basalts, they display moderate SiO2 (49.82-53.74 wt.%), Al2O3 (9.08-15.91 wt.%), TiO2 (0.25-0.66 wt.%) and total alkalies (Na2O+K2O= 0.60-3.97 wt.%) with Mg# ranging from 37-55 at low Cr (7-33 ppm), Ni (8-24 ppm) and V (252-612 ppm). Their rare earth and trace element patterns are coherent exhibiting moderate fractionation (La/Yb=0.16-1.09), slight negative Eu anomalies indicating limited plagioclase fractionation (Eu/Eu*= 0.69-0.96), negative Nb, Ta, Ti and Zr-Hf anomalies revealing their arc signatures. The Nb-Th and Zr-Y relationships of these basalts suggest their derivation from the partial melting of depleted mantle in an intra-oceanic arc setting. Trace element modelling of these basalts indicates their genesis by ~15% partial melting of depleted MORB-mantle and depletion of the source is attributed to previous melting events. These basalts are geochemically similar with the depleted arc and forearc basalts of the Phanerozoic Tonga and Izu-Bonin-Mariana arcs emphasising the operation of analogous tectonic processes during the Archean era.

The coal-forming environment during mass extinction in the latest permian: Evidence from geochemistry of rare Earth elements

The C1 coal of Latest Permian during mass extinction in eastern Yunnan was studied to reveal the terrestrial paleoenvironment and influence of geological events on coal-formation during mass extinction. An analysis of Rare Earth Elements (REEs) was conducted on the C1 coal from the Yantang Mine of Xuanwei, eastern Yunnan Province, which was deposited during the latest Permian. A total of 24 samples from coals, partings, roofs and floors from the C1 coal were taken from the fresh face in the underground mine. The results of the REEs analysis indicated that the total REE content (∑REE) in the C1 coal varies from 23.99 μg/g to 267.94 μg/g, averaged 122.69 μg/g. The C1 coal is enriched in light REE (LREE) relative to heavy REE (HREE), signifying the fractionation between LREE and HREE. Most samples of the coal seam C1 are depleted in Eu in various degrees and slightly depleted in Ce, especially two partings in sub-seams B1 and B3 which show the significant negative Eu anomalies. The geochemical characteristics of REE reveal that the C1 coal was deposited in a weak oxidation environment; the sedimentary environment was turbulent during the middle-later stage of coal-forming process; the C1 coal was affected by the basalt clastic materials from the Khangdian Oldland and acidic synsedimentary volcanic ash in the coal-forming period. The geochemical characteristics of Tonsteins in C1 coal are similar to those of marine Permian-Triassic boundary (PTB) volcanic ash layers in South China, which are both derived from the felsic volcanism caused by the closure of the Paleo-Tethys at the southwestern margin of the South China. Furthermore, the C1 coal was also affected by the eruption of Siberian large igneous province (SLIP) in the early stage.

Large-Volume Permian Felsic Volcanism in the Tisza Mega-Unit (East-Central Europe): Evidence from Mineralogy, Petrology, Geochemistry, And Geochronology

Large-Volume Permian Felsic Volcanism in the Tisza Mega-Unit (East-Central Europe): Evidence from Mineralogy, Petrology, Geochemistry, And Geochronology

Late Tonian explosive volcanism and hyaloclastites in northern Paraguay Belt, Central Brazil: A record of Rodinia break-up in western Gondwana

Explosive volcanism and oceanic crust generation are key aspects to understanding the break-up history of the Rodinia. Its impact on the extreme glaciations of the Cryogenian was marked by ocean chemistry fluctuations in these times, which are still poorly known and preserved in western Gondwana. Here we describe in detail and provide a series of U-Pb ages for the Nova Xavantina Metavolcanosedimentary Sequence, where extrusive rocks, such as hyaloclastite and metabasalts, as well as mafic and ultramafic intrusive rocks and ignimbrites, are associated with diamictites, limestones, and iron formations in the Paraguay belt, Central Brazil. Two styles of pyroclastic eruptions were identified in the studied area. Basic-ultrabasic Surtseyan eruption was enhanced by magma-water interaction and is represented by hyaloclastites and incorporated bombs of scoria. Felsic Plinian eruption, in its turn, was formed by violent explosions in a subaerial environment and is represented by widely exposed acidic tuffs and ignimbrites. Shallow subaqueous environments, such as lakes, salt lakes, and possibly proto-oceans, developed in this precursor basin concomitant with bimodal volcanism. High resolution U-Pb geochronology results indicate they were formed in at least three magmatic events, starting with the formation of ca. 745 Ma seafloor metabasalts and felsic volcanics dated at ca. 735 Ma, and intrusive gabbros at ca. 714 Ma. Alkaline ultramafic rocks of ca. 577 Ma intrude the region and may be associated with similar rocks that crop out regionally as the Planalto da Serra Alkaline Complex. Our results from Nova Xavantina volcanism explain the main Neoproterozoic source until now unrecognized and previous recorded in detrital zircons from the Paraguay belt sediment provenance.