Felsic Research Articles

Tracing the Sources of Paleoproterozoic Metasediments in the Comoé Basin, Côte d’Ivoire, West Africa : Geochemistry Implication

Petrology and geochemistry have been used jointly to study the metasediments of the Comoé Basin. The main objective is to identify the sources of the metasediments. The entire Comoé metasedimentary basin is dominated by greywackes and shales. The mineralogy of the greywackes consists of biotite, plagioclase, quartz, muscovite, chlorite, ilmenite and other oxides. The shales contain andalusite, biotite, muscovite, quartz and chlorite. Overall, the geochemistry of the major elements shows an enrichment of Fe2O3, MgO, Al2O3, K2O and TiO2 in the metapelitic rocks. These oxides are controlled by the distribution of phyllosilicates. Observation of diagrams of rare earth elements has shown that the source of these elements in these metasediments for these elements is largely controlled by the TTG. On the other hand, a large quantity of felsic material, in particular felsic rocks, contributed to the composition of the sediments. Mafic rocks contributed to a lesser extent.

The influences of sedimentary environments on carbon accumulation in lacustrine deposits: Evidences from elemental geochemistry

The carbon dynamics of lacustrine deposits play an important role in controlling the atmospheric carbon cycle. However, the influences of various sedimentary factors on carbon accumulation in lacustrine deposits are rarely known. In this study, the petrological and geochemistry analyses of organic-rich (average total organic content = 24.22 wt%) lacustrine deposits and organic-poor (average total organic content = 2.48 wt%) lacustrine deposits buried under a peatland (the Zhibian peatland) in Northeast Asia are used to explore the above issues. Results suggest that the concentrations of major oxides, trace elements and rare earth elements in the organic-poor lacustrine deposits are higher than those in the organic-rich lacustrine deposits. Furthermore, these lacustrine deposits were sourced from felsic rocks. The tectonic background was continental island arc and active continental margin. Additionally, the sedimentary models of lacustrine deposits buried under peat sequence were also concluded. Organic-poor lacustrine deposits were developed in a deep-water area, that was freshwater and oxygenic condition. This type of lacustrine deposit was influenced by the frequent input of coarse-grained terrigenous debris and low productivity. In contrast, the organic-rich lacustrine deposits were developed in a shallow-water area, that was saline and anaerobic condition. This type of lacustrine deposit was influenced by a rare input of fine-grained terrigenous debris and high productivity. These findings should assist future studies on the carbon dynamics of buried lacustrine deposits. This study would provide a representative example of carbon accumulation processes and mechanisms in buried paleolake system in Northeast China.

Discovery, geological setting and mineralization of the Shuvuutai molybdenum deposit, central-southern Mongolia

We document the newly discovered Shuvuutai Molybdenum (Mo) porphyry deposit located in the central-east Mongolian volcanic/-plutonic belt of the southern Altaids. The deposit is characterized by quartz vein- and veinlet-disseminated-type Mo ore bodies mainly hosted in a Late Permian-Early Triassic granitic intrusion. In this study, we provide a detailed study that integrates detailed geology with petrographic, mineralogic, geochemical, and geochronologic date to determine the tectonic setting of the deposit. Fluid inclusion analysis is used to reconstruct the fluid evolution history of the Shuvuutai Mo mineralization system and to understand the origin of the deposit. The ore bodies occur mainly as veins, lenses, and pods in positions from inner intrusions through contact zones to the host rocks distal to causative intrusions. The host rocks are variable in lithology, including granites, porphyries, volcanic breccias and tuffs, and sedimentary rocks. Outward from ore bodies to host rocks, the wall rock alteration is zoned from potassic (K-feldspar-quartz-mica), through phyllic (quartz-sericite-chlorite-epidote), to propylitic or argillic alterations. Two samples from porphyry stock intrusions at Shuvuutai were selected for U–Pb zircon geochronology and trace element composition studies, including the ore-hosting monzodiorite porphyry and quartz syenite porphyry. Our new ages are consistent with, or slightly postdate published zircon U-Pb ages of early phase of monzodiorite porphyry (268–232 Ma) and the main phase of quartz syenite porphyry (251.6 ± 1.7 Ma). Hydrothermal mineralization generally includes four stages, from early to late, represented by (1) potassic feldspar-quartz veins or veinlets, (2) quartz-molybdenite stockworks, (3) quartz-sulfide stockworks, and (4) quartz ± carbonate veins or veinlets. The ore-forming fluids were initially magmatic in origin and show high-temperature and high-salinity, containing daughter mineral and CO2-bearing fluid inclusions; and eventually evolved to low-temperature, low-pressure, low-salinity and CO2-poor meteoric water. S isotopic compositions of sulfides from the Shuvuutai Mo porphyry deposit indicate that the source of the ore-forming material was derived from the felsic magma. The Central-Eastern Mongolian volcanic-plutonic belt continues from Mongolia into northeast China, where similar Mo deposits are not yet recognized, suggesting that a new exploration strategy, based on the characteristics and history of the Shuvuutai discovery, is needed for the Chinese segment of the Altaids.

A long-lived mafic magma reservoir: Zircon evidence from a hornblende peridotite in the Hida Belt, Japan

Abstract Zircon geochronology has contributed to our understanding of the longevity of transcrustal magmatic systems; however, most studies focus on zircon records from felsic rocks due to the restricted occurrence of zircon in mafic-ultramafic rocks. We present U–Pb age, geochemical, and Hf–O isotope data for zircons from a hornblendite peridotite in the Hida Belt, Japan, that offers a unique opportunity to investigate the lifetime of a long-lived mafic plumbing system in an arc setting. We found two zircon U–Pb age clusters: an incompatible element-rich cluster at 196 Ma and an incompatible element-poor cluster at 186 Ma. Their homogeneous isotopic signatures (δ18O = 7.7‰ ± 0.8‰, εHf = 10.3‰ ± 1.7‰) indicate the same magma source despite the 10 m.y. age gap. These two clusters are explained by different zircon formation mechanisms that differ depending on whether or not zircon saturation requires differentiated melt with high SiO2. The enriched older zircons formed by local zircon saturation at the mafic melt-olivine interface, whereas the younger depleted zircons precipitated from the last drop of interstitial felsic melt co-existing with hornblendes. Our finding substantiates the longevity of mafic systems at lower crusts, which sustain transcrustal magma systems and crustal evolution.

Global radiant flux from active volcanoes: the 2000–2019 MIROVA database

Since 2000, the Moderate Resolution Imaging Spectroradiometer (MODIS) has acquired infrared images of the Earth’s surface daily. These data have made it possible to measure the thermal energy radiated by the world’s most famous volcanoes and also to discover and track eruptions in remote and poorly monitored regions. In this work, we present the database of Volcanic Radiative Power (VRP, in W) time series, recorded by the MIROVA (Middle Infrared Observation of Volcanic Activity) system over 2 decades of MODIS observations (2000–2019) at 111 active volcanoes. The database reveals that globally, the number of thermally active volcanoes each year varies between 60 and 80, almost equally partitioned between volcanoes with a basic (50%) and intermediate (45%) composition, while only 5% is represented by volcanoes erupting acidic lavas. Within the investigated period, the global-scale heat flux was almost stationary, and occasionally punctuated by peaks associated with the largest effusive eruptions (e.g., Bardarbunga and Kilauea). The Volcanic Radiative Energy (VRE, in J) emitted by basic volcanoes (∼1.8 × 1018 J) in 20 years constitutes 91% of the total, while intermediates and acids contribute only 8% (∼1.8 × 1017 J) and 1% (∼1.7 × 1016 J), respectively. A comparison with the volume of lava erupted effusively by the same volcanoes reveals that this difference is attributed to the lower efficiency in radiating thermal energy of increasingly acidic (viscous) lava bodies. Each compositional group is associated with a specific relationship between VRE and erupted volume which characterises most of the effusive volcanoes. On the other hand, some open-vent volcanoes reveal that much more heat is released than that theoretically radiated by the erupted lava. This imbalance (hereby called excess radiation) is attributed to an additional heat source, likely associated with an underlying convective magma column and/or to outgassing through a permeable conduit. We are convinced that the database presented in this work will be useful to support new emerging studies on global-scale volcanism and will contribute to a better understanding of each volcanic system.

Alternating Subplinian and phreatomagmatic phases during the construction of a phonolitic maar-diatreme volcano (Caldera del Rey, Tenerife, Canary Islands)

The Early Pleistocene, well exposed, Caldera del Rey maar-diatreme volcano, Tenerife, Canary Islands was constructed during a ∼ VEI 4 phonolitic eruption that involved two cycles of magmatic-to-phreatomagmatic activity and resulted in two overlapping craters aligned NE-SW. Magmatic phases fed unsteady Subplinian eruption columns that reached 8–12 km altitude and dispersed tephra to the west and southwest of the volcano and shed pyroclastic density currents. Phreatomagmatic phases, driven by explosive interactions between magma and groundwater, constructed an extensive tephra ring via deposition from ballistic curtains, pyroclastic density currents, and tephra fall. Near-optimal-scaled depth phreatomagmatic explosions (strong and/or shallow) excavated a substantial diatreme beneath the north crater and constructed a substantial tephra ring. This abruptly transitioned to deeper-than-optimal scaled depth explosions (weak and/or deep) that erupted mostly fine ash which was dispersed by dilute pyroclastic density currents and fallout and filled the south crater. At distances of >4 km from the volcano, over a metre of ash and pumice accumulated during the phreatomagmatic phases. The Caldera del Rey volcano provides an instructive study on how interaction between ascending felsic magma and groundwater can modify Subplinian eruptions.

Geochronology, Mineralogy, and Geochemistry of the Tonsteins from the Permo–Carboniferous Benxi Formation, Ordos Basin, North China Craton

AbstractTonstein layers are found worldwide in the Permo–Carboniferous coal‐bearing strata. This study investigates the geochronology, mineralogy, and geochemistry of four tonstein samples from the Permo–Carboniferous Benxi Formation, Ordos Basin, North China Craton (NCC). The typical features of the studied tonsteins include thin beds, lateral continuity, angular quartz grains, and euhedral zircons with similar U‐Pb ages, indicating a significant pyroclastic origin. In addition, the tonstein samples have low TiO2/Al2O3 ratios (<0.02) and rare earth elements and yttrium (REY) concentrations with obvious negative Eu anomalies, indicating that the tonsteins have a felsic magma origin. Moreover, compared with the mean composition of clay shale, the studied tonsteins are characterized by high concentrations of the elements Nb and Ta, which may affect the concentration of the corresponding elements in surrounding coal seams. The zircon U‐Pb ages of the tonsteins (293.9–298.8 Ma) provide a precise chronological framework on the Benxi Formation in the Ordos Basin, constraining the Gzhelian–Aselian stages. The tonsteins were probably sourced from arc volcanism along the western margin of the NCC during the early Permian, implying that the Alxa Terrane had not amalgamated with the NCC at that time.

Tying Shock Features to Impact Conditions: The Significance of Shear Deformation During Impact Cratering

AbstractImpact cratering is associated with extreme physical conditions with temperatures and pressures far exceeding conditions otherwise prevailing at the surfaces of terrestrial planets. As a consequence, shock‐metamorphosed rocks contain unique deformation features such as planar deformation features in quartz, high‐pressure mineral polymorphs and melted rock. While the physical conditions of formation for impact‐induced melting following the highest pressure and temperature conditions is relatively well understood, aspects of the formation of melt‐veins in otherwise seemingly relatively low shock material has been the topic of discussion. In a new study, Hamann et al. (2023, https://doi.org/10.1029/2023JE007742) are able to largely reproduce the current classification of progressive shock metamorphism of felsic rocks using a modern experimental set up that eliminates multiple shock wave reflections at sample containers and excavation and ejection of target material. Importantly, however, they find that shear deformation results in the formation of melt veins at pressures as low as 6 GPa. The authors recover stishovite in melt veins formed at low‐moderate (<18 GPa) shock pressure, lower than most previous studies. These results have bearing on our understanding of the conditions of progressive shock metamorphism at terrestrial impact structures. However, since the results are similar to data obtained from experiments on basaltic rocks, the results also have broader implications for understanding the shock histories of meteorite parent bodies. Hamann et al. show the importance of experimental impact cratering for bridging the gap between observations in shocked rocks from terrestrial impact structures, in meteorites, and in returned samples, and their formational conditions.

Translithospheric magmatic plumbing system of a late Early Cretaceous intraplate volcano in NE China: Insights from geochemistry and phenocryst composition

The magmatic plumbing systems of intraplate volcanoes are key to understanding the structure of intraplate continental crust. A late Early Cretaceous volcano in NE China provides an opportunity to reconstruct the magmatic plumbing system of an intraplate paleo-volcano. ArAr and zircon SIMS UPb dating results indicate that olivine basalts formed at 102.81 ± 0.17 Ma and hornblende andesitic rocks formed at 101.1 ± 1.1 Ma–102.84 ± 1.2 Ma. Compared with coeval olivine basalts, the hornblende andesitic rocks have lower rare earth element (REE) abundance and parallel REE patterns, indicating that the basalts and andesitic rocks have independent origins. Olivine phenocrysts only occur in the basalts and are often zoned, with cores of Fo78–86 and rims of Fo62–68, the crystalline temperatures of olivine cores are 1204–1234 °C. The plagioclases from the basalt groundmass have An = 62–63. There are two types of plagioclase phenocryst in the andesitic rocks. One has An = 62–50, the other has An = 27–34.The former is normally zoned crystals (An = 62–50 from the core to rim) with high-An rims (60–63), similar in composition to plagioclase in the groundmass of the coeval basalt. They formed at temperatures of 1140–1162 °C and pressures of 4.7–7.0 kbar, implying that they were crystallized from basaltic melt with H2O contents of 0.60–0.82 wt%. The latter is low-An phenocrysts (27–34) with high-An (∼44) rims, suggesting that they were crystallized from felsic melt and that the injection of mafic magma had happened. The orthopyroxene phenocrysts only occur in andesitic rocks and have Mg# of 80–82, and are estimated to have crystallized at 6.7–8.5 kbar, and 1206–1222 °C, implying that they were crystallized from basaltic melt. The hornblende phenocrysts in andesitic rocks are pargasite, tschermakite, magnesio-hornblende, and magnesio-hastingsite and formed at 2.4–3.5 kbar and 883–957 °C. The estimated pressure and temperature of orthopyroxene and plagioclase (An>55) phenocrysts indicate that mafic magma chambers was located at depths of over 31.0 km, whereas those of hornblende phenocrysts show that felsic magma chamber was located at 8.9–12.8 km and has H2O content of 5.8–7.3 wt%. Using these data, we construct a new model of translithospheric magmatic plumbing system for intraplate volcanoes. Mantle-derived mafic magma chamber was emplaced at the crust–mantle boundary and causes partial melting of the lower crust, which in turn produces felsic magma that would be stored in a magma chamber in the upper crust. The mafic magma chamber at the crust-mantle boundary does not crystallize in-situ, as mafic magma could occasionally rise and be repeatedly injected and mix into the felsic magma chamber in the upper crust.

Zircon trace-element and isotopes (U–Pb, Lu–Hf, δ 18 O) response to fluid-deficient metamorphism of a subducted continental terrane (North Muya, Eastern Siberia)

Abstract The orogenic continental crust of accretionary and collisional belts worldwide is dominated by felsic and metasedimentary rocks, which show variable responses to high-grade metamorphism. Transformation of felsic rocks is commonly limited as compared to that of the enclosed mafic rocks (including eclogites sensu stricto ), which is widely attributed to availability of H 2 O–CO 2 fluids, kinetically controlled growth of high-grade assemblages, and their preferential preservation in metabasites more competent to rehydration. We report on the results of studies of the geochemical behaviour of zircon (trace-element, U–Pb, Lu–Hf and δ 18 O) in three felsic samples (two metagranitoids and one paragneiss), which are spatially (geographically and at the outcrop-scale) juxtaposed with mafic eclogites within the North Muya block (Neoproterozoic Baikalides, northern Central Asian Orogenic Belt). The data imply that metagranitoids and metasediments within the buried continental lithosphere might follow a single subduction-related P – T – t trend, whereas contrasting degrees of mineralogical and zircon transformation were governed by mineral buffer reactions in the absence of external fluids. The latter was significant only in an H 2 O-enriched protolith of metasediments. The formation of 18 O-depleted zircon recrystallization rims together with Mn enrichment of garnet rims indicate a distinct metamorphic stage without or with minor localized fluid infiltration, most likely, related to peak temperature conditions during collision. Supplementary material: The data on the mineral composition, trace-element and U–Pb–Lu–Hf–O isotopic composition of zircon are available at https://doi.org/10.6084/m9.figshare.c.6794043

Zircon and the role of magmatic petrogenesis in the formation of felsic-hosted volcanogenic massive sulfide (VMS) deposits: a case study from the mid-Paleozoic Yukon-Tanana terrane, northern Canadian Cordillera

Magmatism is a critical component in sustaining hydrothermal convection and metal transport during the formation of volcanogenic massive sulfide (VMS) deposits. Previous studies of magmatic petrogenesis in VMS systems have demonstrated that ore-related volcanic rocks have distinct whole-rock geochemical and isotopic signatures (i.e., high HFSE, REE, Th, εHf-Nd, zircon saturation T) relative to barren volcanic rocks, which supports models of elevated crustal heat flow during periods of ore deposition; however, the petrologic characteristics and intrinsic parameters (e.g., T, fO2) related to these magmatic events in VMS districts remain poorly understood. Arc–back-arc assemblages from the mid-Paleozoic Yukon-Tanana terrane are well-characterized and include the Finlayson Lake VMS district, which is host to several felsic-hosted deposits (e.g., Kudz Ze Kayah, GP4F, Wolverine) that were generated in a peri-Laurentian continental back-arc tectonic setting. In this study, zircon from back-arc and coeval arc rocks in the Yukon-Tanana terrane was used as a proxy for primary magma formation conditions that generated VMS-proximal and VMS-distal stratigraphy. Our results indicate that zircon grains in VMS-proximal environments have unique textural, geochemical, and isotopic characteristics (e.g., low-aspect ratios, greater abundance of zircon-phosphate intergrowths, Th/U > 1, Zr/Hf > 80, Tzrc > 780 °C, εHfi > –7) that are clearly distinguished from zircon in VMS-distal rocks in both the back-arc and arc settings (Th/U < 1, Zr/Hf < 80, Tzrc < 780 °C, εHfi < –7). These signatures correlate to VMS-proximal magmas that were hotter, less fractionated, and contained greater juvenile melt contributions compared to VMS-distal magmas and reflect a series of high-flux magmatic events that directly correspond to the early tectonic development of Yukon-Tanana terrane. Moreover, this study underscores the importance of mineral-scale petrology, geochemistry, and geochronology in defining the primary magmatic conditions that generated VMS-related felsic rocks and highlights the utility of zircon as a prospectivity tool in both grassroots and brownfields VMS exploration.

Diachronous slab-driven Early Cretaceous crustal extension and pulsed magmatic response in southeastern China: Insight from A-type granite and related rocks

The Late Mesozoic magmatic belt in southeastern (SE) China, stretching over 1300 km, resulted from southwestward subduction of the paleo-Pacific plate. Our study focuses on the origin of Early Cretaceous syenite and A-type granite in SE China, utilizing zircon U-Pb and Hf isotopic analyses, bulk-rock compositions, and Nd-Hf isotopic compositions. Findings indicate that ∼ 142 Ma silica-unsaturated syenites in the southwest form through small degrees of partial melting of the metasomatized lithospheric mantle, while ∼ 142 Ma A-type granites originate from partial melting of juvenile felsic rocks. The ∼ 130 Ma A-type granites in the northeast result from mixing of ancient crustal-derived magma and mantle-derived magma. The southwest and northeast regions display distinct magmatic histories, with flare-ups and lulls occurring at different times. These variations suggest diverse extensional and compressional histories. A spatially migrating extensional regime is observed from the southwest (∼147 Ma) to the northeast (∼120 Ma) of SE China. Magmatic flare-ups in the southwest involve reworking juvenile crust, while in the northeast, they modify ancient crust, likely related to paleo-Pacific plate subduction and northeastward slab rollback. Slab subduction rate correlates strongly with observed geological events, indicating diachronous crustal extension, magmatic flare-ups, and lulls during ca. 147–110 Ma due to northeastward slab rollback and changing subduction rate.

Zircons in NYF-type pegmatites in the Emeishan large igneous province, SW China: A record of Nb and REE mineralization processes

The metallogenic mechanism of Nb and rare earth elements (REEs) during the magmatic-hydrothermal evolution of alkaline rocks remains ambiguous. Zircon is a common accessory mineral occurring in both barren aplite and the Nb-Y-F mineralized pegmatite (NFY-type) in Miyi County of the Pan-Xi area, the central zone of the Permian Emeishan large igneous province (ELIP) in SW China. Here, we present in situ geochemical analyses on these zircons to reveal the Nb-REE mineralization processes during magmatic-hydrothermal evolution, as well as the prospecting potential of plume-related NYF-type pegmatites. Four types of zircons are identified in the Miyi aplite and pegmatite. Zrn-I with well-developed oscillatory zoning (OZ) represents magmatic zircons in the aplite. Zrn-II and Zrn-III are magmatic zircons in the pegmatite. Zrn-II is closely associated with albite and is restricted to the center of an individual crystal, indicating early crystallization. There are abundant nanoscale britholite and thorite inclusions in Zrn-II, indicating that the primary melt is enriched in Ca, Y, REEs, Nb, Th, and F. Zrn-III presents as the rim of Zrn-II, with a sharp boundary between them. Zrn-IV is a metamict zircon formed through hydrothermal alteration after its structure was destroyed due to high Th and/or U contents. In situ zircon U-Pb dating of both Zrn-I and Zrn-III yields concordant ages of ca. 258 Ma, coeval with the felsic magmatism in the ELIP. For positive εHf(t) values of Zrn-I and Zrn-III (1.8–5.8 and 1.5–9.1, respectively), the Miyi pegmatite is also plume-related and might be derived from magma resembling A-type granite in the ELIP. Both Zrn-I and Zrn-III show negative Eu anomalies, and there is a decrease in Sc contents from Zrn-I to Zrn-III, indicating that the Miyi pegmatite formed from a highly evolved magma. Furthermore, the synchronous increases in Na, Nb, Y and total REEs from Zrn-I to Zrn-III indicate that excess Na in the melt plays an essential role in the further enrichment of Nb, Y, and REEs. In addition, a dramatic decrease in Nb, Y and total REE contents and LREE (light REE)/HREE (heavy REE) ratios from Zrn-III to Zrn-IV illustrates that Na- and F-rich hydrothermal fluids can leach and facilitate the precipitation of these elements, particularly LREEs. Therefore, there are two-stage Nb and REE mineralization processes related to magmatic-hydrothermal evolution in the Miyi pegmatite, which may be a general model for the mineralization of plume-related NFY-type pegmatites.

A newly discovered 2030–2010 Ma magmatic suite records the dawn of Proterozoic extension on the southern margin of the Yilgarn Craton

Uranium–lead zircon geochronology, zircon lutetium–hafnium isotope analyses and whole-rock geochemistry are used to investigate concealed basement geology and the location of Archean–Paleoproterozoic terrane boundaries in the northeastern Albany–Fraser Orogen (AFO). We identify a new major component of the AFO, and the first compelling evidence for ca. 2030–2010 Ma A-type felsic magmatism in western Australia. Prior to this study, the oldest rocks in the AFO that formed by extension and reworking of the southern Yilgarn Craton margin had protolith emplacement ages of ca. 1810–1800 Ma; our results extend this record by ca. 200 Myr and provide new insights into the crustal and tectonic evolution of the AFO. This phase of magmatism marks the start of a long-lived and widespread Paleoproterozoic extensional regime that prevailed across the West Australian Craton (WAC) for ∼ 400 Myr. Extension in the WAC was coeval with localised magmatism, volcanism and volcano-sedimentary basin formation in the North and South Australian cratons. Globally, our findings also correlate with the onset of prolonged extension and the generation of a vast tract of oceanic crust around the periphery of Nuna. We posit a petrogenetic link between ca. 2030–2010 Ma felsic rocks in the AFO and age–isotope-equivalent ultramafic–mafic rocks in the eastern Yilgarn Craton that are located along-strike and are interpreted to reflect a possible far-field and late-stage magmatic expression of the ca. 2060–2055 Ma Bushveld superplume.

Insights from distribution and fractionation of the rare earth elements into As enrichment in the Singe Tsangpo River Basin

The geogenic As enrichment occurs extensively in the major river basin from the Tibetan Plateau, while the knowledge involved with the underlying mechanisms is far from completion. The present study utilized the geochemical behaviors of rare earth elements (REE) to study the hydrogeochemical evolution and As enrichment in the Singe Tsnagpo River basin, a typical As-rich river basin in the Tibetan Plateau. The river water was characterized by significant positive Eu anomalies and slight negative Ce anomalies, indicating the hydrogeochemical control of oxidative weathering of sourcing rocks and the contribution of felsic rocks. The PHREEQC modeling results suggested that the carbonate weathering contributed to the complexation of REE in the river water, where REE(CO3)+ and REE(CO3)2− were the predominant complex species. Besides, the reversing scenarios of HREE/LREE enrichment in the river water/sediments suggested a critical control of iron (hydr)oxides on the REE fractionation due to the preferential adsorption of LREE compared with HREE. Interestingly, the variations in Y/Ni and Cr/V ratios from the river sediments suggested a different contribution of sourcing rock weathering along the river flow path, where ultramafic rock showed a substantial contribution to the river sediments in the lower reaches and granite source is predominant in the upper reaches. It was also notable that the concurrent enrichment of REE and As in the river waters showed a response to the substantial enhancement of chemical weathering in the upper reaches of the STR basin, which was evidenced by the corresponding increases in the electrical conductivity and the δ18O values in the river waters. The present study thus provides new insights into utilizing REE as environmental tracers for studying hydrogeochemical evolution and As enrichment in the STR basin, which could also apply to similar alpine arid and cold river basins.

Lithium in felsic magmas: a volcanological perspective

Volcanic eruptions are unpredictable phenomena that pose a challenge to crisis management, owing to the fact that contrasted eruptive styles (explosive versus effusive) exhibited at the surface depend on unobservable deep processes occurring in the reservoir and the volcanic conduit. Constricting the behaviour of magma during ascent, and the degassing in particular, allows for a clearer understanding of the relationships between petrological and volcano monitoring signals, and hence a better description of the volcanic hazard. To this aim, lithium (Li) has been used to track magmatic and post-eruptive processes, as a geospeedometer for processes operating on short time scales due to its high mobility in silicate melts and crystals. Yet, the accurate use of Li to assess syn- and post-eruptive processes still lack complete dataset. We propose a review of our current knowledge on Li behavior, with an emphasis on felsic (andesitic to rhyolitic) magmas whose explosive behavior during volcanic eruptions is still poorly understood. We present current knowledge regarding the Li concentration and isotopic compositions, intracrystalline diffusion, and crystal-melt-fluid partition coefficients discovered in felsic magmas and primary crystals. We describe difficulties in interpreting Li data to investigate the differentiation, degassing, ascent rate, volatile fluxing, and cooling of magmas. Finally, we suggest future directions for expanding our understanding of Li behavior.

Zircon U–Pb, Hf, and O isotopic constraints on the tectonic affinity of the basement of the Himalayan orogenic belt: Insights from metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India

Zircon U–Pb geochronology and Hf–O isotope geochemistry of metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India, provide new insights into the tectonic affinity of the basement of the Himalayan orogenic belt. Detrital zircon U–Pb ages from metasedimentary rocks in both the Lesser Himalayan Sequence (LHS) and the overlying Main Central Thrust (MCT) zone yielded dominant Paleoproterozoic ages with the youngest age peak of 1840 Ma. Along with that, the variable ɛHf (t) values (−14.56 to 8.44) indicate that these metasedimentary rocks could have been derived from the juvenile Paleoproterozoic orthogneiss basement in the Himalayas and/or the ancient Indian craton in the south. In contrast, the metasedimentary rocks in the Ediacaran Harsil Formation are dominated by the ca. 1200–800 Ma zircons with the youngest age peak of ca. 630 Ma and variable ɛHf (t) values (−16.57 to 13.32), which belong to the underlying High Himalayan Crystalline Sequence (HHCS), rather than the overlying Tethyan Himalayan Sequence. Orthogneisses in the MCT zone record ca. 1.87–1.85 Ga felsic magmatism associated with the Nuna supercontinent. The Bhagirathi leucogranites intruding the Harsil Formation were formed at ca. 496–490 Ma. These Paleoproterozoic orthogneisses and early Paleozoic leucogranites show only positive ɛHf (t) values (0 to +15) correlatable with a juvenile mantle input. However, the zircon Hf model ages older than the U–Pb ages and the oxygen isotope values (δ18O: 7.74–8.91 ‰), significantly higher than zircons crystallized from mantle-derived magma, indicate large degrees of melting and contamination of crust materials in both the granitoids. In combination with the pre-existing geochronological data, the current study indicates that the Garhwal HHCS is characterized by bimodal Neoproterozoic zircons at ca. 1100–900 Ma and ca. 860–800 Ma with wide εHf (t) values which are similar to those in the outer LHS and the Cathaysia Block (including the Jiangnan Orogen) of the South China Craton. These observations suggest that a close relationship existed between North India and South China during the Neoproterozoic since Tonian. The provenance of the Neoproterozoic HHCS is possibly a mixed source from both southern Gondwana terranes (e.g., east Antarctica) and northern South China.

The genetic relationship between coeval Ediacaran mafic and granitoid plutons in the Antigonish Highlands, Nova Scotia

Abstract Granitic plutons dominated by felsic-intermediate compositions are commonly spatially and temporally associated with mafic intrusions; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is controversial. To better understand this relationship, we present new lithogeochemical and isotopic data from coeval late Neoproterozoic plutonic rocks in the Antigonish Highlands of Nova Scotia where the regional context is well documented. The predominantly mafic Greendale Complex contains lamprophyre, appinites and leucocratic dykes. The appinites are remarkably variable in their textures, and consist of hornblende pegmatites, hornblende cumulates, porphyritic hornblende gabbro and coarse-grained, equigranular hornblende gabbro. Geochemical data show enrichment in large-ion lithophile elements and depletion in the high field strength elements suggesting an arc setting. ε Nd (607) values from the Greendale Complex range from +3.2 to +5.0 and are on average slightly more juvenile than coeval granitic rocks which have petrological characteristics typical of continental arc magmas. Hydrous mafic magmas were likely contaminated by subducted sediments and their ascent was facilitated by lithospheric-scale faults. Felsic magmas were derived by anatexis of heterogeneous Avalonian crust that oscillated between fluid-saturated to fluid-deficient (dehydration) melting, consistent with the evolution from arc to intra-arc rift environment.

Pore Characteristics, Oil Contents and Factors Influencing Laminated Shale in the First Member of the Qingshankou Formation in the Gulong Sag, Northern Songliao Basin

To clarify the reservoir characteristics of laminated shale, the occurrence mechanism of shale oil and its influencing factors in the Gulong Sag, northern Songliao Basin, are studied to better guide the exploration and development of shale oil there. First, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) are used to characterize the pore types, pore geneses and factors influencing the pore volume in the study area. Second, the organic matter of the samples is extracted with a mixture of dichloromethane and methanol. Total organic carbon (TOC), nitrogen adsorption and Rock-Eval tests are performed on the samples before and after extraction to reveal the pore size distribution after extraction. The factors influencing free and adsorbed shale oil and the lower limit of pore size are discussed in detail. The results show that interparticle pores (interP pores), intraparticle pores (intraP pores), organic matter pores (OM pores) and microfractures can be found in the laminated shale (Q1) in the Gulong Sag, Songliao Basin, and that the interP pores and intercrystalline pores in clay minerals are the main pores. The FE-SEM results show that the diameters of interP pores vary from several hundred nanometers to several microns, and their morphologies are mainly triangular, strip-shaped or irregular. The morphology of the intercrystalline pores in the clay minerals is generally irregular, depending on the crystal type and arrangement of clay minerals. According to the characteristics of the nitrogen adsorption and desorption curves, the pore morphologies are mainly slit-shaped pores, parallel-plate-shaped pores and ink-bottle-shaped pores. The pore size distribution is mostly bimodal, and the pore volume contribution is the greatest in the pore size range of 10~20 nm. Before and after extraction, the overall characteristics of the pore size distribution change only slightly, but the number of micropores increases significantly. Different minerals have different degrees of influence on the proportions of micropores, mesopores and macropores. Quartz mainly inhibits the formation of micropores, while the overall effect on mesopores and macropores is positive depending on the diagenetic period. Feldspar has a strong positive correlation with the micropore and mesopore proportions but is not highly correlated with the macropore proportions. The influence of the carbonate mineral content on the pore volume is not obvious because of its complex composition. The TOC content and vitrinite reflectance (Ro) are the two most important factors controlling free oil and adsorbed oil, and the contents of mineral components, such as felsic minerals, carbonate minerals and clay minerals, have no obvious correlation with shale oil content. With increasing pore volume, the contents of free oil and adsorbed oil increase, but the proportion of adsorbed oil decreases gradually. The correlation between the specific surface area and adsorbed oil content is poor. At normal temperatures and pressures, the lower limit of the pore diameters that can contain free oil is 4 nm, and the lower limit of the pore diameters that can contain movable oil is 10 nm.

Barium isotope fractionation during granitic magmatism and potential of δ138/134Ba for distinguishing magmatic-hydrothermal transition in granitic magma systems

The geochemistry of Ba is commonly used to study igneous processes and fluid-melt interaction in subduction zone and granitic environments. Recent advances in analytical techniques have also indicated that Ba isotope geochemistry has the potential to significantly augment these studies. Understanding the Ba isotope fractionation mechanisms is thus critical if this potential is to be fully realized. Hence, we have used the first-principles calculations based on the density functional theory (DFT) and long-time ab initio molecular dynamics (AIMD) simulation to calculate Ba isotope fractionation during felsic magma evolution. In fluids, the reduced partition function ratio (β factor) of aqueous Ba2+ is highly variable, depending on the coordination number (n) of H2O in the first hydration shell. At ambient surface conditions on the Earth, n = 8 best represents the aqueous Ba2+ species in natural fluid, and the temperature-dependent equilibrium Ba isotope fractionation between barite/witherite and fluids can be described by the relationships: 103lnαwitherite-fluid = −0.00009×(106/T2)2 + 0.0603×(106/T2) + 0.0003 and 103lnαbarite-fluid = −0.0001×(106/T2)2 + 0.0648×(106/T2) + 0.0004, which is consistent with previous theoretical and experimental studies. In hydrothermal fluids, the hydration number of Ba2+ varies from 4 to 6 at high P-T, and the cumulative average of 103lnβ is derived to be 0.0798 ± 0.005‰ in 923.15 K and 0.2 GPa. In silicate minerals, the enrichment of heavy Ba isotopes decreases in the sequence of muscovite > microcline ≈ celsian ≈ sanbornite > barylite > phlogopite. Variations in Ba concentrations and the degree of Al-Si disordering only induce limited Ba isotope fractionation, which would be hard to distinguish analytically. At high temperatures (> 600 °C), the melt phase is enriched in heavy Ba isotopes relative to any coexisting aqueous fluid and crystallizing minerals. During the early stages of felsic magma differentiation involving plagioclase crystallization as the main mineral, the δ138/134Ba and concentration in the residual melt remain close to their initial values, but any fluid exsolved from the melt will have light δ138/134Ba values. The high Ba distribution coefficient of K-feldspar means that the resultant K-feldspar granite inherits > 90% of the Ba contained in the initial magma reservoir. Hence, the δ138/134Ba of K-feldspar is close to that of the initial melt. During the final stage of felsic magma evolution, mixing of exsolved fluids from deeper reservoirs may significantly reduce the δ138/134Ba of the now Ba-depleted melt, and can account for the low δ138/134Ba values measured in highly differentiated leucogranites and pegmatites.