Felsic Composition Research Articles

Apatite Geochemical Perspectives on the Maturation of Continental Arc Crust via Mush‐Facilitated Processes During Magmatic Flare‐Up

AbstractIn volcanic arcs, magma evolves from basaltic to intermediate and felsic composition, resulting in arc crust maturation. It remains unclear whether processes involving mush during magmatic flare‐ups would enhance this evolution. This study revealed a temporal‐compositional evolution of plutonic rocks from mafic (∼94 Ma) to intermediate (∼92–88 Ma) to felsic (∼88 Ma) during a magmatic flare‐up event in the Gangdese arc, Tibet, with increasing radiogenic Sr–Nd isotope enrichment. Apatites in mafic and felsic rocks have εNd(t) values similar to their hosts, while intermediate rocks show higher values. The elemental composition of apatites in mafic and intermediate rocks is similar but differs from those in felsic rocks. Textural and compositional features indicate varying degrees of influence of mafic rock compositions by accumulation. Triangular and linear covariation relationships between apatite‐compatible (e.g., La) and ‐incompatible (e.g., Rb) elements with SiO2, respectively, for all plutonic rocks as a whole, confirm the incorporation of apatite‐rich mushes into the mixing process. These findings suggest that mafic magma crystallized into apatite‐rich mush, which was later remobilized and mixed with felsic magma to form intermediate magma. Felsic rocks represent end‐member magmas resulting from crustal anatexis and/or mafic magma differentiation. Thus, the Gangdese arc's maturation during the magmatic flare‐up progressed sequentially through mafic magma crystallization and mush formation, mush remobilization and mixing with felsic magma, and the eventual accumulation and segregation of felsic magma. This sequence of events during flare‐ups illustrates a common crustal maturation process in volcanic arcs, as also seen in the Andean Cordillera.

Geochemical Analysis of Paleocene-Eocene Sediments from Subathu Formation, Dehradun District, Uttarakhand: Implication for Provenance, Tectonic Setting and Paleoclimate

The Palaeocene-Eocene Subathu Formation in the Nilkanth region near Rishikesh, Uttarakhand provides insights into the provenance, depositional environment, and tectonic setting prior to final India-Asia continent collision. Geochemical analysis of major oxides, trace elements, and rare earth elements from shale-dominated sedimentary rocks reveals a complex provenance. Discriminant function plots indicate the derivation of sediments from quartzose sedimentary sources, with minor input from mafic igneous sources. Trace element patterns suggest contributions from felsic igneous rocks, recycled sediments, and subduction-related components. The Chemical Index of Alteration (87.2*Avg.) indicates moderate weathering in the source area under semi-arid paleoclimatic conditions. Tectonic discrimination diagrams indicate that the deposition of sediments from Subathu Formation was occurred predominantly in an active continental margin setting. Rare earth element patterns exhibit varying degrees of fractionation, implying diverse sources ranging from mafic to felsic compositions. This multi-proxy geochemical study sheds light on the evolving paleoenvironmental conditions and sediment routing systems in the western Himalayan foreland basin leading up to the terminal India-Asia collision. Keywords: Geochemistry, Subathu Formation, Provenance, Tectonic Setting, Paleoclimate, Depositional Environment, Himalaya

Volcanic red soil in the tropical mountain region: landscape, parent materials, engineering characteristics, and its use on slope stability (case study: West Lampung, Sumatra, Indonesia)

ABSTRACT The characterization of tropical mountain volcanic red soil in the tropical mountain region, West Lampung area, Sumatra, Indonesia concerning the landslide disaster has been carried out. The research methods used in this research are remote sensing methods, field geological mapping, petrographic analysis (thin section analysis), soil geotechnical analysis, soil geochemical analysis, slope stability analysis, and statistical analysis. The findings indicated that the study area’s volcanic red soil is distinguished by rolling hills with a V-shaped valley, a moderate to severe incline, and an elevation ranging from 850 to 1150 m above sea level. The soil is volcanic residual soil which resulted in wet tropics and from tuff layer and tuffaceous breccias in the felsic composition of the hydrothermal alteration environment. The soil of the study area has a reddish-brown color, and loose characteristic, and belongs to the type of very loose to solid high plasticity uniform inorganic sandy clayey silt. Based on saturated soil slope stability analysis, volcanic red soil has a stable to unstable category if forming a slope. The intrinsic factor that affects the stability of the slope is the percentage of clay content, electrical sensitivity, plasticity index, degree of saturation, and percentage of clay mineral content.

Crustal silica content of East China: A seismological perspective and its significance

East China has experienced multiple periods of tectonic movements, which have contributed to the composition and rheological properties of its present crust. Estimating the composition of the crust is crucial for understanding the tectonic processes. Based on the teleseismic receiver functions with data from the National Seismic Network of China, we applied the H-κ-c method to obtain the crustal bulk VP/VS ratio and to constrain the SiO2 content in the crust. We estimated the SiO2 content to range from 50.89 wt% to 73.51 wt%, with an average value of 65.87 wt%, indicating a predominant felsic composition of the East China's crust. Our study suggests that the North-South Gravity Lineament (NSGL) is an approximate delimitator of the felsic and mafic crust in East China, hinting at a widespread deficiency of mafic lower crust in the east of the NSGL. The mafic crust is extensively distributed in the Taihang orogenic region (TSR) and WuLingshan gravity gradient belts (WLG), particularly in the Datong volcanic area, which manifests the mantle materials intraplating. The scatteredly distributed mafic crust at the east of the NSGL is mainly concentrated in the southeast coast of China and in the intersection region of the Tanlu Fault (TLF) and Sulu region. In Sulu region, the TLF may primarily provide a channel for the thermal intrusion from the underlying mantle lithosphere, which has increased the mafic content. The Pacific/Philippine Sea plate subduction has triggered a significant amount of crust-mantle material exchange below southeast China that resulted in a high degree of mafic crustal composition.

Dynamic evolution of the transcrustal plumbing system in Large Igneous Provinces: Geochemical and microstructural insights from glomerocrysts and melt inclusions

Abstract The nature of the magma plumbing system of Large Igneous Provinces is still poorly understood. Among these exceptional magmatic events from Earth’s past, the end-Triassic Central Atlantic Magmatic Province (CAMP) and the end-Cretaceous Deccan Traps (Deccan) coincided in time with two of the most catastrophic biotic crises during the Phanerozoic. In order to constrain the architecture of their magma plumbing system, glomerocrysts containing abundant bubble-bearing melt inclusions from basaltic lava flows of both CAMP and Deccan were investigated via in situ geochemical and microstructural analyses. The analysed glomerocrysts, dominated by augitic clinopyroxene crystals, represent fragments of a crystal mush entrained by basaltic magmas before eruption. The analysed melt inclusions, consisting of an intermediate to felsic composition glass and CO2-bearing bubbles, represent relics of interstitial melts and fluids within a porous crystal framework forming the crystal mush. The different volume proportions between bubbles and whole inclusions reveal that melt entrapment occurred after volatile exsolution. The minimum observed bubble/inclusion fraction indicates that the CO2 concentration in CAMP and Deccan melts was at least 0.3 wt.%, consistent with a maximum entrapment pressure of about 0.5 GPa at CO2–H2O fluid-saturated conditions. The MgO-rich composition of host clinopyroxene crystals and whole rocks is in contrast with the SiO2-rich composition of (trachy-) andesitic to rhyolitic glass of melt inclusions, pointing to disequilibrium conditions. Thermodynamic and geochemical modelling shows that fractional crystallization alone cannot explain the evolved composition of glass in melt inclusions starting from their whole rock composition. On one side, the oxygen isotope composition of clinopyroxene crystals in glomerocrysts ranges from + 3.9 (± 0.3) to + 5.8 (± 0.3) ‰ and their sample-averaged oxygen isotope composition spans from + 4.4 (N = 10) to + 5.6 (N = 10) ‰, implying that glomerocrysts crystallized from mafic melts with normal (i.e., mantle-like) to slightly low δ18O values. On the other side, the oxygen isotope composition of glass in melt inclusions ranges from + 5.5 (± 0.4) to + 22.1 (± 0.4) ‰, implying that melt inclusions entrapped intermediate to felsic melts with normal (i.e., mantle-like) to extremely high δ18O values, typical of (meta-) sedimentary rocks. Some melt inclusions are compatible with fractionation from the same mafic melts that crystallized their host mineral phase, but most melt inclusions are compatible with variable degrees of crustal assimilation and partial mixing, potentially followed by minor post-entrapment isotope re-equilibration. In the CAMP, where sedimentary basins are abundant, (meta-) pelites and occasionally granitoids were the most likely assimilants. On the contrary, in the Deccan, where sedimentary basins are rare, granitoids and metapelites were the most likely assimilants. Oxygen isotope compositions of glass in melt inclusions, spanning from mantle-like to crust-dominated signatures, suggest that the CO2 within their coexisting bubbles likely derived partly from the mantle and partly from assimilated crustal materials. The investigated glomerocrysts and their bubble-bearing melt inclusions are relics of a multiphase (i.e., solid + liquid + gas phases) crystal mush revealing a dynamic evolution for the magma plumbing system of both CAMP and Deccan, where crystals, silicate melts and exsolved fluids coexisted and interacted through most of the transcrustal section.

Multi‐Stage Magmatism During Slab Exhumation Drives the Geochemical Evolution of Continental Crust: Insights From Paleozoic Granitoids in South Altyn, Western China

AbstractThe present continental crust is characterized by a felsic upper crust and a mafic lower crust, resulting from significant geochemical differentiation over geological time. While various processes have been proposed to explain this differentiation, subduction zones remain pivotal regions for understanding the compositional evolution of continental crust. This study focuses on the South Altyn (SA) continental subduction‐collision belt in western China, a unique setting that experienced ultra‐deep (>300 km) continental subduction followed by multi‐stage exhumation. We present a comprehensive study of four granitoid suites from Tatelekebulake (TTLK) area in SA: biotite granite (BG), monzogranite (MG), K‐feldspar granite (KG), and leucogranite (LG). Comprehensive studies on petrology, geochemistry and zircon U‐Pb dating show that these granitoids formed at 494, 451, 414, and 418 Ma, respectively, and originated from protoliths with affinity to the subducted continental crust in SA. Phase equilibrium modeling suggests that BG formed at ∼800°C and 0.6 GPa, while the MG, KG, and LG formed by differentiation crystallization of the BG magma under progressively decreasing temperature and pressure conditions (750°C, 0.5 GPa; 740–700°C, 0.2 GPa; and 700–640°C, 0.1 GPa, respectively). These results, combined with previous studies, allow us to reconstruct the tectonic processes of continental exhumation and subsequent orogenic collapse in SA during the Early Paleozoic. Importantly, our findings reveal that magmatism derived from partial melting of subducted continental crust can promote the geochemical evolution of continental crust toward more felsic compositions, even in the absence of significant crustal growth or mantle‐derived magmatism. This study provides a valuable case for understanding the compositional evolution of continental crust in deep subduction zones and challenges conventional models that rely heavily on arc magmatism for crustal differentiation. Moreover, our results contribute to a broader understanding of crustal evolution processes in collisional orogens worldwide and highlight the importance of recycling and differentiation of subducted continental material in shaping crustal compositions.

Geochronology and Origin of Quaternary Dacites from the Daliuchong Volcano in the Tengchong Volcanic Field (TVF), SE Tibetan Plateau

Quaternary volcanoes from the southeastern Tibetan Plateau occur at the Tengchong volcanic field (TVF). The Daliuchong volcano is the largest volcano in the TVF, which has the most felsic compositions with explosive eruptions. The eruption history and origin of the Daliuchong volcano are a matter of debate. In the present paper, we report the groundmass K-Ar ages, whole-rock Sr-Nd-Pb-Hf isotopes, zircon U-Pb ages, and Hf-O isotopic compositions for the Daliuchong volcano to constrain its eruption history and petrogenesis. The groundmass K-Ar ages and zircon U-Pb ages indicate mid-Pleistocene (0.6 Ma to 0.3 Ma) eruptions. The presence of zircon phenocrysts with enriched mantle-like O-Hf isotopes (δ18O < 6‰, and εHf about −2) suggests the involvement of mantle-derived basaltic magmas. The whole-rock Pb isotope compositions and Sr-Nd isotope modeling reveal the involvement of magma from the lower crust. The zircon xenocrysts reveal previously unrecognized 20-Ma magmatic activity at the TVF and contamination of late Cretaceous (66–80 Ma) S-type granites during the formation of the Daliuchong dacites. The dacite magma at Daliuchong was formed by mixing of the mantle-derived magma and lower-crust-derived magma and subsequently contaminated by upper crustal materials, including late Cretaceous S-type granitic rocks.

Geochemistry of fine-grained sedimentary rocks: Implications for source area, paleoweathering, paleoclimate, and provenance of the Santa Maria Formation (Triassic, Paraná Basin, Brazil)

The integration of ICP-OES, XRD, chemical analysis, and scanning electron microscopy was used to investigate the source area, weathering, paleoclimate conditions, and provenance of the siltstones of the Santa Maria Formation (Middle to Upper Triassic, Southern Brazil). This unit is regionally subdivided, from base to top, into the Passo das Tropas Member (Late Anisian–Middle Ladinian) and the Alemoa Member (Middle Ladinian–Carnian). The Chemical Index of Alteration (CIA) was calculated for both members to highlight the weathering processes of primary minerals. The CIA results indicate moderate to extreme weathering of the source rocks for the siltstones. The extreme chemical alteration observed in the Passo das Tropas siltstones (CIA = 76.97–90.83) and in the mid-section of the Alemoa Member (CIA = 87.96–89.63) suggests weathering under more humid conditions, related to an increase in paleorainfall. Paleoclimate was interpreted as semi-arid to arid, with alternating wet and dry seasons. These results align with the geochemical behavior of uranium and thorium. Samples with higher CIA values show higher Th/U ratios, indicating a shift toward a more humid climate. X-ray diffraction analysis of the fine fraction (FF < 2 μm) of the samples identified mixed-layered illite-smectite (I/S) clay minerals with minor amounts of quartz. Regarding provenance, Th/U ratios coupled with Th vs. Sc and La-Th-Sc plots, suggest an upper crustal protolith for the southern Santa Maria siltstones, consisting of uplifted rocks from the crystalline basement of the Sul-Rio-Grandense shield and sedimentary rocks of the Camaquã Basin. The Th/Sc, Sc/Th, Co/Th, and La/Sc ratios, along with the contents of Al₂O₃, SiO₂, TiO₂, Co, V, Ni, and Sc, indicate a more felsic composition for the Passo das Tropas Member. Meanwhile, the source rocks of the Alemoa Member sediments demonstrate a more mafic composition in the mid-section, transitioning to a felsic composition in the uppermost succession.

Formation of a Large Cold Groundwater Mantle Helium Anomaly and High Temperature Geothermal Resources in Response to Bimodal Magmatism Near Roosevelt Hot Springs and Utah FORGE, Milford Valley, Southwest Utah

AbstractA large mantle helium anomaly and separate domains of high heat flow are the predominant manifestations of bimodal magmatic activity in the Milford valley. The mantle helium anomaly (1.9–2.6 R/Ra) covers 270 km2 and is subdivided into two separated domains: a cold shallow groundwater regime and high temperature hydrothermal activity. The zone of anomalous heat flow covers &gt;100 km2 and is also subdivided into two adjacent domains, comprising hydrothermal activity at Roosevelt Hot Springs (RHS) (3–7 W/m2) and conductive heat flow (100–180 mW/m2). While the transfer of heat and mantle helium at RHS are coupled, heat and helium transfer are decoupled in the adjacent cold groundwater regime to the west. Both the mantle helium and geothermal anomalies are attributed to recent mafic‐felsic magmatic intrusions of &gt;400 km3, however, the absence of volcanic eruptions &lt;500,000 years indicates magmas stall before rising to shallow crustal level &lt;10 km depth. Deep level magmatism produces a felsic composition melt, which is inferred to be responsible for the widespread and near uniform range of diluted mantle helium values. A thick and impermeable mass of crystalline granitic basement rock at the mid‐crustal level divides the ascent of mantle helium into separate flow paths. It may also impede the rise of buoyant magma trapping thermal energy that facilitates partial melting, slow cooling, and development of a thick thermal aureole. Partitioning of convective and conductive thermal regimes and independent flow paths supplying deeply derived helium characterize the development of a large long‐lived magma‐related geothermal system.

Submarine cores record magma evolution toward a catastrophic eruption at Kikai Caldera

Magma evolution toward a caldera-forming eruption remains uncertain in many cases owing to the lack of successive volcanic records before catastrophic eruptions. Here we take an approach to this issue by analyzing a submarine core sampled near Kikai Caldera, southern Japan, which has recorded two caldera-forming eruptions at 95 and 7.3 ka and small eruptions between them. Discovery of mafic glass fragments in the submarine deposits of the 95-ka eruption, which had not been recognized in subaerial outcrops, implies the involvement of mafic magma in felsic magma-driven caldera-forming eruption. Inter-caldera volcanic activity resumed with binary mafic and felsic magma extrusions but then shifted to eruptions predominated by felsic magmas. In the final stage preceding the 7.3-ka caldera-forming eruption, the most felsic composition did not appear in glass fragments. We suggest that this period was the phase of felsic melt accumulation to grow a magma reservoir toward the next catastrophic eruption.

A revised stratigraphic model for the ∼ 1910–1835 Ma Tanami Group, the Northern Territory, Australia: Implications for exploration targeting

The Paleoproterozoic Tanami Group of the Granites—Tanami Orogen hosts a known endowment of > 20 Moz of gold mineralization across numerous deposits. Throughout the region, host lithologies impart a first-order control on the style, grade and endowment of mineralization. However, the genetic relationship and regional distribution of three regionally correlative formations (the Dead Bullock, Mount Charles and Stubbins formations) remains enigmatic. Here, we combine lithostratigraphic analysis, lithogeochemistry, detrital zircon geochronology and geophysical observations to propose a new evolutionary model for the Tanami Group. We find that the Tanami Group was deposited in a continental back-arc basin setting and developed in four stages. Package A marked the onset of the rifting of the continental Archean basement. Sedimentary rocks within Package A are dominantly felsic in composition and display a unimodal detrital zircon age population at ∼ 2530 Ma. Package B marks the onset of mafic volcanism; consequently, volcaniclastic and sedimentary rocks within Package B display a distinct mafic geochemical signature. Detritus shed from an Archean basement persisted throughout this period, and a unimodal detrital age component at ∼ 2530 Ma persists. Package C marked the waning stages of voluminous mafic volcanism across the Tanami Basin. Sedimentary volcaniclastic rocks within Package C display a mixed mafic and felsic composition. Detrital zircon age spectra record multiple peaks at ∼ 2500, 3000 and 3350 Ma, which may reflect erosion of older Archean basement domains or changes in paleo-drainage. Package D marked a significant change in the source of detritus entering the basin from dominantly Archean to Paleoproterozoic, with dominant detrital age peaks at ∼ 1860 Ma. Despite significant lithofacies variations, the presented model highlights multiple correlations between the Dead Bullock and Mount Charles formations. The Mount Charles Formation likely represents a volcanic source-proximal, high-energy depocenter. In contrast, the Dead Bullock Formation was deposited in a lower energy setting, distal to volcanic centers. These observations are important to mineral exploration. Notably, the Mount Charles Formation is distributed over a significantly greater area than previously thought, and the under-represented Hangingwall and Footwall sequences of this formation are interpreted to host significant gold mineralization at the Oberon and Groundrush deposits.

Volcanic activity during the Early–Middle Triassic transition in the Sichuan Basin, South China: Duration, evolution and implications

The felsic volcanogenic tuff known as ‘green bean rock’ (GBR) and its carbonates are widely dispersed across the western Yangtze Block. This study investigates the origin, tectonic setting and genesis of GBR, as well as the environmental disruptions evident in carbonates from the western edge of the Yangtze Block. The analyses include mineralogy, whole‐rock geochemistry and the isotopic composition of zircon Hf, carbon and oxygen in GBR samples from the western edge of the Yangtze Block. The geochemical profile of GBR shows enriched LREE, Th and U content, depleted levels of Nb, Ta, Sr, Ba, K, Rb and Ti, and strong‐to‐moderate negative Eu anomalies (δEu = 0.15–0.18). Zircon Hf isotopes exhibit S‐type geochemical affinities with low negative εHf(t) values (−13.3 to −5.7) and TDM2 ages of 1684–2110 Ma. This suggests that the volcanic ashes originate from the magma of an intermediate to felsic composition. X‐ray directionality data show that the most prevalent clay minerals are illite and illite/smectite. Lithium fixed in these minerals is likely to have leached from brine. Early Triassic variances in δ13C profiles are reliable indicators of environmental disturbances, pointing to cycles of devastation and restoration in marine ecosystems, interspersed with extraneous events including volcanic activity. The study posits that volcano eruptions may have prolonged biotic recovery after the end‐Permian mass extinction.

Melt breccias in parautochthonous basement rocks of the Morokweng impact structure, South Africa – Evidence for a friction origin

Archaean granitoid gneisses and younger metadolerite and dolerite intrusions in a drill core from the late-Jurassic Morokweng impact structure, host mm- to m-wide aphanitic dykes that are spatially and geometrically associated with cataclasite-bearing reverse-slip faults. The dykes contain lithic and mineral clasts derived from their wallrocks. Petrographic and BSEM analysis indicates that the original dyke matrices are completely altered to a smectite-zeolite-magnetite paragenesis. However, a melt precursor is argued based on evidence for low-viscosity flow, high dyke aspect ratios and intrusive relationships into both the wallrocks and cataclasite, and the sharply contrasting matrix hydrothermal mineral grain size and textures relative to those found in the cataclasites. A subset of cm-wide pink-orange monomictic dykes have a felsic composition closely resembling their granitic wallrocks. More voluminous, polymictic, red dykes show compositions intermediate between the mafic and felsic wallrock endmembers in the drill core, which both occur in close proximity to the dykes. These hybrid compositions are interpreted to have developed either through mixing of mafic and felsic melts along faulted contacts between the doleritic and gneissic rocks, or by mechanical mixing, with or without assimilation, of a dolerite-derived melt with incohesive granitoid-derived cataclasite within the fault zones. The structural, petrographic and geochemical evidence support the dykes originating by friction melting (pseudotachylite, s.s.) caused by high-strain-rate faulting of the crater basement during the excavation and/or modification stages of the Morokweng impact, rather than by contamination of impact melt intrusions. Based on their spatial context relative to the impact melt sheet and the moderately high shock levels of their host rocks, these pseudotachylite dykes are interpreted as part of the parautochthonous peak ring of the Morokweng impact structure.

Tonian juvenile arc crust provenance recorded in the Paraná Basin, Western Gondwana proto-Andean margin

A provenance study through petrography, whole-rock geochemistry, and zircon U-Pb-Th and Lu-Hf systematics suggests that the lowermost strata of the Paraná Basin (460–380 Ma) record a recycled orogen provenance with mixing trend between mature polycyclic quartzose detritus and mafic/intermediate contributions. Sources with felsic compositions are indicated by the Lower Paraná Group, as it is suggested by the relative enrichment of Zr/Y, Th/Sc, and Y/Sr ratios. Meanwhile, the Iapó Formation received additional contributions from Archean mafic/intermediate plagioclase-rich sources (Eu* = 0.81). The Tonian juvenile (900–800 Ma) detrital zircon population from the Pre-Carboniferous strata with εHf(t) ranging from +3 to +11, in additional, late-Tonian population with an important crustal component (εHf(t) ranging from −12 to −8), provide evidence that the Ribeira Fold Belt constitutes the primary sedimentary source for the lowermost strata. Juvenile early-Tonian (950–900 Ma) and middle-Tonian (810–750 Ma) detrital zircon population suggests an additional provenance derived from the São Gabriel Domain, located at the southern border of the basin. Late-Ediacaran to early-Cambrian detrital zircon population suggest sources from the Eastern Pampean Range whereas a restricted detrital zircon population younger than 500 Ma with εHf(t) signatures ranging from 3 to −32, is consistent with a provenance from the Famatinian magmatic arc and associated rocks. In contrast to the detrital zircon population from the Pre-Carboniferous strata, the Itararé Group (∼310 to 298 Ma) reveals juvenile Ordovician sources (εHf(t) ranging from −2 to +2) and a restricted population encompassing both evolved and juvenile sources with a “Grenvillian” signature.

Provenance history, depositional conditions andtectonic settings during late Cenomanian – early Turonian time in the Gongola Sub-Basin of the Upper Benue Trough Nigeria: Evidence from major and trace elements geochemistry of the Kanawa shales from the Pindiga Formation

Newly analysed major and trace element compositions of the Kanawa shales deposited in the Pindiga Formation reveal the provenance history and depositional conditions during the late Cenomanian - early Turonian time in the Gongola Sub-Basin of the Upper Benue Trough. We conclude that the source of the siliciclastic detritus in the Kanawa shales of the Pindiga Formation are igneous rocks of predominantly intermediate chemistry with a temporal trend towards more felsic compositions. The source rocks presumably underwent intense chemical weathering with CIA, PIA and CIW values up to 89.51, 97.30, and 97.56, respectively, which led to enhanced shale deposition in the Gongola Sub-basin. Obtained relatively high Fe/Mn and Zr/Rb ratios in the Kanawa shales suggest deposition under relatively strong hydrodynamic conditions in presumably shallow water. Additionally, integrated bivariate plots of Ga/Rb vs. K2O/Al2O3, SiO2 vs. Al2O3+K2O + Na2O with the ratios of Sr/Ba, Rb/K, MgO/Al2O3x100, V/Cr, V/(V + Ni) V/(V + Cr), P/Ti and Ba/Ti indicate deposition under a warm, humid climate in dominantly anoxic and brackish water conditions under relatively low primary productivity conditions. This interpretation aligns with the global climate and redox conditions during the late Cenomanian – early Turonian times. The tectonic setting bivariate diagrams of K2O/Na2O vs. SiO2 and SiO2/Al2O3 vs. K2O/Na2O, as well as the high and low quartz multidimensional diagrams, reveal a rift basin in a passive margin depositional environment, confirming earlier works on the evolution of the Benue Trough.

Coal facies of the Middle Permian Baruunnaran deposit, South Mongolia

The Baruunnaran coal deposit is located in the northeastern part of the South Gobi Basin, southern Mongolia, and hosted in the middle Permian Tavantolgoi Formation. In this paper we present indices of coal facies determined from 34 coal samples obtained from three seams in the lower and upper part of the formation (III, IXG and X), by studying their organic petrography and the geochemistry of the coal ash. The results of coal petrography revealed that seams III, IXG and X are composed of 46.9-80.9 vol.% vitrinite, 11.6-47.5 vol.% inertinite and 1.2-18.2 vol.% liptinite. In samples from seams III and X the average content of mineral matter is low at 11 and 13.4 vol.%, respectively, and 6.3 vol.% in seam IXG, and consists of clay, silica, pyrite, carbonate, and other minerals. The inorganic content mostly occurs as fillings of cell cavities, cracks, and fissures of vitrinite and inertinite macerals. The vitrinite random reflectance values range from 0.81-1.07%. The Gelification Index and Tissue Preservation Index suggest the peats accumulated in wet forest swamp environments with high tree density. The majority of the seams accumulated in mildly oxic to anoxic conditions with good tissue preservation. The peat mire water ranged from weakly to strongly acidic. Further, it was determined by Al2O3/TiO2 ratios that the clastic sediments were probably sourced from volcanic basement characterized by intermediate felsic composition.

Composition and Sources of Clastic Material of Terrigenous Rocks of the Khabarovsk Accretionary Complex (Sikhote-Alin)

Abstract ––The results of comprehensive studies of terrigenous rocks of the Khabarovsk Sikhote-Alin accretionary complex are presented. It is established that the fragments of Jurassic and Permian–Triassic sandstones are dominated by poorly rounded and poorly separated material mainly from local provenance areas. The detrital part of the rocks is mainly represented by quartz, in a smaller amount by feldspar and rock fragments. Sandstones are characterized by high silica content, moderate alumina content, low concentrations of femic elements and calcium, moderate alkali content with significant variations in the K/Na ratio. Both Jurassic and Permian–Triassic rocks are typically characterized by reduced contents of LILE, REE, to a lesser extent HFSE and negative values of the ɛNd(T) parameter – compared to PAAS. The model Nd age of Jurassic sandstones varies from 1.36 to 1.71 Ga, Permian–Triassic – from 1.14 to 1.35 Ga. Most of the detrital zircon population is of late Paleozoic–early Mesozoic age, approximately 25% are older (pre-Paleoproterozoic). The studied sandstones are mainly rocks of the first cycle of weathering (petrogenic), formed during the erosion of igneous rocks of felsic composition. The synthesis of the obtained data suggests that the main source of the cluster material for the Mesozoic sedimentary rocks was the geological formations of the northern part of the Bureya–Khanka superterrane (Bureya and Malokhingan blocks), as well as, possibly, the eastern part of the Mongol–Okhotsk belt. The Khabarovsk terrane has not drifted significantly along the Tan Lu stike-slip system and is an “autochthonous” block in the present-day structure of Sikhote-Alin.

Major, Volatile, Ore, and Trace Elements in Magmatic Melts in the Earth’s Dominant Geodynamic Environments. I. Mean Concentrations

Data from our original database, which includes more than 2 600 000 analyses for 75 elements of mineral-hosted melt inclusions and quench glasses in volcanic rocks, are generalized to calculate the mean concentrations of major, volatile, ore, and trace elements in magmatic melts from the following dominant geodynamic environments: (I) spreading zones of oceanic plates (mid-oceanic ridges), (II) environments affected by mantle plumes in oceanic plates (oceanic islands and lava plateaus), (III, IV) environments related to subduction processes (III is zones of arc magmatism on the oceanic crust, and IV is zones of magmatism in active continental margins in which magma-generating processes involve the continental crust), (V) environments of continental rifts and areas with continental hotspots, and (VI) environments of backarc spreading. A histogram of SiO2 distribution in natural magmatic melts shows a bimodal distribution: one of the maxima falls onto SiO2 concentrations of 50–52 wt % and the other onto 72–76 wt %. The most widely spread melts contain 62–66 wt % SiO2. Mean temperatures and pressures are calculated for each of the environments. The normalized multielemental patterns presented for environments I through VI show the ratios of the mean concentrations of elements in magmatic melts of mafic, intermediate, and felsic composition to the concentrations in the primitive mantle. Mean ratios of incompatible, trace, and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, etc.) are evaluated for the melts of each of the environments. The variations in these ratios are calculated, and it is demonstrated that the ratios of incompatible elements are mostly statistically significantly different in the different environments. The differences are particularly significant between the ratios of the most differently incompatible elements (e.g., Nb/Yb) and some ratios involving volatile components (e.g., K2O/H2O).

Geochemical evolution of the REE-enriched Cave Peak porphyry Mo-deposit, Trans-Pecos Texas, USA

The Cave Peak deposit is a rift-related, breccia-hosted, fluorine-rich porphyry molybdenum deposit that is enriched in Nb, REE, and other critical minerals. Cave Peak is the easternmost member of a northwest-trending group of Paleogene intrusions in the Diablo Plateau along with the Marble Canyon stock, an unmineralized, petrogenetically related, compositionally zoned mafic alkaline pluton. The Cave Peak intrusions show within-plate geochemical affinity and is the product of a highly differentiated magma series. Curvilinear trends in Harker variation and trace element diagrams record magma differentiation from mafic to intermediate (44–69 wt% SiO2) to felsic (70–76 wt% SiO2) compositions at Marble Canyon and Cave Peak, respectively. Quartz syenite and monzonite from the Marble Canyon stock yielded zircon UPb ages of 36.2 ± 0.15 Ma and 36.1 ± 0.09 Ma, respectively. The youngest major intrusion at Cave Peak, an alkali feldspar granite porphyry, has a zircon UPb age of 34.8 ± 0.4 Ma, supporting that these intrusions represent a magma differentiation trend. The δ98Mo of Cave Peak molybdenites range between −0.39 ± 0.06 ‰ and + 0.52 ± 0.06 ‰ with a total Mo isotope range of 0.91 ‰. Cave Peak samples predominantly fractionated towards heavier δ98Mo. The Cave Peak data are distinctly heavier than data obtained from the nearby subduction-related Red Hills porphyry Cu deposit which has δ98Mo values of −0.25‰ to −0.51‰.

Refining Boron Isotopic Measurements of Silicate Samples by Multi‐Collector‐Inductively Coupled Plasma‐Mass Spectrometry (MC‐ICP‐MS)

Solution MC‐ICP‐MS is an established technique for high precision boron isotope measurement results (δ11BSRM 951) in carbonates, yet its application to silicate rocks has been limited. Impediments include volatilisation during silicate dissolution and contamination during chemical purification. To address this, we present a low‐blank sample preparation procedure that couples hydrofluoric acid‐digestion and low‐temperature evaporation (mannitol‐free), to an established MC‐ICP‐MS measurement procedure following chemical purification using B‐specific Amberlite IRA 743 resin. We obtain accurate δ11BSRM 951 values (intermediate precision ±0.2‰) for boric acid (BAM ERM‐AE121 19.65 ± 0.14‰) and carbonate (NIST RM 8301 (Coral) 24.24 ± 0.11‰) reference materials. For silicate reference materials covering mafic to felsic compositions we obtain δ11BSRM 951 with intermediate precision &lt; ±0.6‰ (2s), namely JB‐2 6.9 ± 0.4‰; IAEA‐B‐5 ‐6.0 ± 0.6‰; IAEA‐B‐6 ‐3.9 ± 0.5‰ (2s). Furthermore, splits of these same reference materials were processed by an alternative fusion and purification procedure. We find excellent agreement between δ11BSRM 951 measurement results by MC‐ICP‐MS of the reference materials using both sample processing techniques. These measurement results show that our sample processing and MC‐ICP‐MS methods provide consistent δ11BSRM 951 values for low B‐mass fraction samples. We present new data from Mid Ocean Ridge Basalt (MORB) glass, documenting a range in δ11BSRM 951 from ‐5.6 ± 0.3‰ to ‐8.8 ± 0.5‰ (2s), implying some upper mantle δ11BSRM 951 heterogeneity.