Volcanic Sequence Research Articles

Geological setting of gold-silver mineralization in the La India mining district, Nicaragua

La India gold mining district covers a 50 km² area of fault-fill gold-silver mineralized quartz-adularia veins on the western margin of a Tertiary volcanic arc in western Nicaragua. Historic mining records and modern mineral exploration data, which to-date has defined a mineral endowment of over 2.3 Moz gold, provide a wealth of geological information. This paper draws on these observations and data to describe and classify the gold-silver mineralisation at La India, identify the geological controls, and interpret the timing of mineralisation within the regional tectonic setting. The district-scale gold-silver mineralisation at La India occurs in two adjacent geological settings with distinct mineralization characteristics and exploration potential: (1) an upland area of strongly faulted felsic to andesitic volcanics where the historic mine workings are located, and (2) an adjacent downthrown graben, the Sebaco Graben, where a thick sequence of andesite is preserved overlying the felsic volcanic sequence. Gold mineralisation is classified as rift margin-type low-sulphidation epithermal gold-silver fault and fracture-fill vein mineralization. In the historic mining area erosion has exposed the top of the high-grade epithermal zone. Minimal erosion in the Sebaco Graben means that the epithermal system is fully preserved at depth, with localised hydrothermal sinter outcrops, sporadic low-grade mineralised veins and a phreatic breccia pipe exposed at surface. Apart from the one phreatic breccia, the gold-silver mineralisation occurs in quartz veins and breccias that filled brittle faults and associated fractures and fissures which developed in an extensional tectonic setting. The structures containing the gold-silver mineralisation were formed as normal and trans-tensional faults with orientations consistent with southwest-directed extension: (1) a predominant northwest to north-northwest set parallel to the subducting plate; (2) secondary but locally extensive east-west, and (3) tertiary shorter and narrower northeast and north-striking veins. A district-scale north-northwest orientated through-going structure linking the major gold-silver deposits in the historic mining area is interpreted as a deep crustal conduit for the gold-silver bearing hydrothermal fluids. Other, as yet unidentified basement feeder structures may have fed mineralised corridors in the east and west of the district. The gold-silver mineralisation is best developed where structures pass through competent felsic volcanics and welded tuffs in the historic mine area, and also in the overlying andesite flows in the Sebaco Graben. Gold-silver mineralisation is less well developed where the structure passes through less competent unwelded tuffs and volcanic agglomerates. Gold-silver mineralisation is interpreted as occurring shortly before or at 8-10 Ma at the end of a long period of slab-rollback induced extension and arc volcanism. Post-mineralisation block faulting split the La India district into the upthrown, and subsequently eroded historic mine area where epithermal mineralisation is largely exposed at surface, and the well-preserved downthrown blocks such as the Sebaco graben where much of the gold-silver mineralisation is still hidden several hundred metres below surface.

Catalytic oxidation of propane over nanorod-like TiO2 supported Ru catalysts: Structure-activity dependence and mechanistic insights

Nanorod-like titanium dioxide (TiO2-Rod) supported ruthenium catalysts with variable metal loadings (designated as x-Ru/TiO2-Rod, x = 0.1, 0.5, 1.0 and 1.5 wt.%) were prepared for the catalytic oxidation of propane. The unique nanorod-like structure and high specific surface area of TiO2-Rod support was favorable for the surface distribution of ruthenium nanoparticles, constructing highly active redox sites for propane oxidation. Light-off experimental results indicated that the x-Ru/TiO2-Rod catalysts exhibited diverse catalytic activities for propane oxidation, giving a volcanic sequence of 0.1-Ru/TiO2-Rod < 0.5-Ru/TiO2-Rod < 1.5-Ru/TiO2-Rod < 1.0-Ru/TiO2-Rod with the increasing ruthenium content. Additionally, the optimal 1.0-Ru/TiO2-Rod delivered adequate catalytic stability and CO2/water resistant ability under complex industrial conditions. Characterization results revealed that the metallic Ru0 and the surface lattice oxygen species from the Ru-TiO2 interface were demonstrated to be the catalytic active components, which played an essential role in the reaction. In situ DRIFTs analysis verified the formation of acetate, acetone and formate species as the predominant organic intermediates in propane oxidation, which may result from two plausible reaction pathways depending on the activation of terminal methyl group (–CH3) and the central methylene (CH2) group in propane molecule. This work provides fundamental guidance for the development of Ru-based catalysts and the reaction mechanism understanding for light alkanes oxidation.

Nature and Origin of a Massive Sulfide Occurrence in the Karrat Group: Evidence for Paleoproterozoic VMS Mineralization in Central West Greenland

Abstract Mafic volcanic rocks of the Kangilleq Formation of the Paleoproterozoic Karrat Group host volcanogenic massive sulfide (VMS) mineralization in the area of central Kangiusap Kuua, central West Greenland. The mafic volcanic rocks display evidence of subaqueous, effusive eruption and redeposition by mass debris flows generated along fault scarps on the sea floor. A zone of semiconformable quartz alteration and disconformable chlorite alteration within hydrothermal breccias and mafic tuff breccias near the top of the volcanic sequence is interpreted to reflect a synvolcanic hydrothermal system. Conformable, massive to semimassive, and discordant, stringer-style sulfide mineralization is hosted within the quartz- and chlorite-altered volcanic rocks. The massive to semimassive sulfide mineralization is ~10 m thick and crops out along strike for ~2,000 m. The stringer zone is ≤10 m thick with individual sulfide stringers ranging in width from 5 to 90 cm. All sulfide zones are dominated by coarse pyrrhotite and pyrite, with trace amounts of sphalerite and chalcopyrite. The pillow lavas are subalkaline with geochemical characteristics typical of modern transitional to tholeiitic mid-ocean ridge or back-arc basin basalt. Trace element and Nd isotope data suggest that these lavas erupted in an epicratonic, back-arc basin. Characteristics of the host rocks indicate a period of localized rifting, volcanism, and VMS formation during genesis of the Karrat Group, which is dominated by siliciclastic rocks.

Early development of the Patagonian fold and thrust belt and the onset of foreland sedimentation in the Austral-Magallanes basin

Retroarc basins register the onset of compressional exhumation during the Cretaceous in the Andes, evidenced by sequences showing sedimentary provenance from the orogen. We present U–Pb detrital zircon geochronology from the Austral-Magallanes basin in the Southern Patagonian Andes (∼47–48°SL), where the foreland stage began in the Early Cretaceous. The littoral sandstones from basal foreland sequences of the Río Belgrano Formation yield ∼123 Ma maximum depositional ages, confirming previous studies that the basin infill is early Aptian. The detrital zircon ages showed two main clusters of 129-115 Ma and 151 -140 Ma, that are derived from the coetaneously exhumed Andean arc and earlier Jurassic magmatic sources. The zircons which overlap with ages of Upper Jurassic Andean synrift volcanics, confirms orogenic exhumation coeval with sedimentation. A sequence of retroarc volcanism dated between 119 and 97 Ma overlies the basal foreland littoral-continental deposits, and it is followed by a Late Cretaceous-Paleocene hiatus. Further analysis of detrital zircons ages distribution and geological considerations, indicates that the studied northern depocenter was a source region of sedimentary supply after its Cenomanian exhumation.

The bimodal Fii-A2-type and calc-alkaline volcanic sequence of the Aljustrel brownfield region, Iberian Pyrite Belt, SW Iberian Massif

The Iberian Pyrite Belt (IPB) is a late Devonian – Early Carboniferous world-class polymetallic VMS province that includes significant Cu-(Sn)-Pb-Zn-(Ag) deposits of massive sulphides and feeder zones. The Aljustrel brownfield region contains one of the highest concentrations of ore in the IPB in 6 known deposits (Gavião, São João, Moinho, Algares, Estação and Feitais). To delve into the petrogenesis of the Aljustrel early Carboniferous (∼355 Ma) felsic-dominated bimodal volcanism, new whole-rock trace elements and SmNd isotopes, and UPb in zircon were obtained.Based on Ga/Al and Y/Nb ratios, it is shown that Aljustrel felsic magmatism has the geochemical features of A2-type melts, typical of post-collisional and back-arc settings. UPb in zircon for a juvenile felsic volcanic rock point to antecrysts ages spanning from 387.9 to 366.6 Ma and a maximum emplacement age of 354.3 ± 2.6 Ma. These long-lasting melting events, present in both juvenile (ƐNdi = +1.79) and evolved felsic rocks (ƐNdi = −5.07), imply heterogeneous sources dominated by zircon-bearing igneous rocks. The SmNd model ages are in accordance with previous LuHf model ages in zircon, reinforcing that the isotopic variability is related to the same petrogenetic process.Subordinated Aljustrel mafic rocks, coeval with the abundant felsic volcanism, show orogenic signatures, namely Nb-Ta-Ti negative anomalies and calc-alkaline affinities, whereas SmNd isotopic data (ƐNdi = +1.54 to +5.48) points to variable to no contamination with crustal material. These geochemical results suggest derivation from an enriched mantle source modified by subduction metasomatism. In addition, the mafic rocks did not provide zircons for geochronological analysis, with the exception of one sample, in which a Concordia age of 402.1 ± 15.5 Ma was obtained from a single grain.The combined geochemical signatures of mafic and felsic volcanic rocks suggest asthenospheric rise, but this solely does not explain the abundance of zircon antecrysts in the felsic rocks. Therefore, a geodynamic model that includes a continuous evolution from Devonian to Carboniferous times is inferred. This more complex and broader geodynamic model for the Iberian Pyrite Belt in which successive metal remobilization occurred after successive melting events, fits the present geochemical data and is more likely to explain why the Iberian Pyrite Belt is a unique metallogenetic province.

The journey of the Band-e-Zeyarat ophiolite (Makran Accretionary Wedge, SE Iran) from the mid-ocean ridge to the accretionary complex: new insights from its sedimentary cover and associated basaltic dykes and sills

The Band-e-Zeyarat ophiolite (BEZO) sedimentary cover in the Makran Accretionary Prism (SE Iran) records a complex tectono-sedimentary evolution, extending from its formation at a mid-ocean ridge (MOR) setting to deformation in an accretionary prism. Stratigraphic and biostratigraphic data indicate the occurrence of a Valanginian transition zone sequence separating the volcanic sequence and pelagic sedimentary cover. The latter consists of lower Hauterivian cherty limestone passing upwards to upper Hauterivian–Barremian marl and limestone. The pelagic sedimentary cover continues with post-Barremian–Cenomanian (?) marls. Arenites are interlayered in the sedimentary cover; they are composed of rock fragments derived from volcanic-arc and continental margin settings. The BEZO sedimentary cover is intruded by dykes and sills showing enriched MOR basalt chemical affinity. Structural analysis indicates a polyphase deformation history that involved faulting and folding. Our multidisciplinary results indicate that the BEZO formed in a MOR setting and that it was subsequently overprinted by off-axis and within-plate magmatism as it spread away from this MOR. The Band-e-Zeyarat oceanic crust was incorporated into the Makran prism in the latest Late Cretaceous–Paleocene and was further deformed via strike-slip faulting along the dextral Minab-Sabzevaran fault during the Miocene–Pliocene. Supplementary material: A geological sketch map of the Band-e-Zeyarat ophiolite, photomicrographs of arenites, microphotographs of selected calcareous nannofossil taxa, a geochemical discrimination diagram, a table with semiquantitative estimation of calcareous nannofossil abundance, and a text with detailed descriptions of analytical methods, accurancy, and detection limits for whole-rock geochemical analysis are available at https://doi.org/10.6084/m9.figshare.c.6843835 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

The Colíder Paleoproterozoic felsic volcanism: New insights into stratigraphy and petrogenesis in the southern Amazonian Craton

The eastern side of Alta Floresta Mineral Province (AFMP) (União do Norte region) comprises an effusive/subvolcanic phase, explosive phase, and epiclastic rocks. The effusive/subvolcanic phase comprises massive porphyritic rhyolite with different degrees of crystallinity, porphyritic rhyolite with flow structure, and microcrystalline rhyolite. The explosive phase is characterized by welded lapilli tuff, lapilli tuff with flow structure, stratified lapilli tuff, and tuff with accretionary lapilli. Sandstones and siltstones characterize epiclastic rocks. There are porphyritic rhyolite dikes that cut the volcanic sequence. The rocks of the Colíder Group are rhyolites (>70 % SiO2) with calcic-alkaline to alkaline-calcic affinity. A rhyolitic rock is predominantly ferroan and metaluminous, while a pyroclastic rock is predominantly magnesian to ferroan and meta- to peraluminous in composition. The rocks have a high LREE content, and slight fractionation of HREE suggests they have hybrid geochemical characteristics of high potassium calc-alkaline, shoshonitic, and suggestive affinities of A-type magmas, probably produced in a post-collisional orogenic environment. These rocks have a relatively high level of Ba and Rb and a negative anomaly for Sr, P, Nb, Ta, and Ti, suggesting an intra-plate mantle source with contributions from a metasomatized mantle edge. The rocks do not have adakitic characteristics nor fertility for Cu-Au (Sr/Y < 1.9 and LaN/YbN < 38.0) as in other regions of the AFMP. The welded lapilli tuff rocks show U-Pb age of 1838 ± 17 Ma and 1817 ± 2 Ma and are the older explosive phases. The effusive/subvolcanic phase has U-Pb ages of 1800 ± 3 Ma and 1792 ± 3 Ma obtained in the massive porphyritic rhyolite. The age interval of approximately 46 Ma (1838–1792 Ma) suggests the presence of two or more volcanic cycles or distinct volcanic events. Based on the interpretation of volcanic and epiclastic deposits and remote sensing products, the paleoenvironmental reconstruction includes subaerial with effusive/subvolcanic and explosive stages in a probable eroded volcanic caldera system with epiclastic rocks from a fluvial depositional system. This volcanic scenario provides more information about the Colíder–Teles Pires volcano-plutonic event, uniquely mapped in AFMP, and offers new insights into the tectonic frame of the Amazonian craton.

Geochronology and Geochemistry of the Early Paleozoic Ore‐host Volcanic Sequence in the Kalatag Area, East Tianshan, NW China: Implications for the Tectonic Evolution

AbstractThe East Tianshan contains many late Paleozoic magmatic and polymetallic deposits. Recent studies demonstrate that the early Paleozoic volcanic rocks discovered in the northern region of East Tianshan can be subdivided into the Daliugou, Hongliuxia and Kalatag formations. Here, we report zircon U‐Pb ages of volcanic rocks and quartz diorite porphyry, together with whole‐rock geochemical and Sr‐Nd isotope data of early Paleozoic volcanic rocks, in order to investigate their petrogenesis and geodynamic setting. Zircon U‐Pb analyses of the rocks suggest that the Kalatag Formation formed at ca. 438–413 Ma. Geochemical characteristics of the early Paleozoic volcanic rocks exhibit typical subduction‐related features. They have high Mg#(44–75), positiveεNd(t) values (0.25–7.88), low87Sr/86Sr ratios (0.70457 to 0.70588) and young two‐stage Hf isotope model ages (551–446 Ma), consistent with a depleted mantle origin with limited crustal contamination. The basaltic andesite and andesite have relatively high MgO contents (6.1–9.4 wt%), suggesting that they belong to high‐magnesian andesites. They were most likely derived from the partial melting of mantle peridotite caused by the addition of fluids released by subducted oceanic slab. Based on regional geology, geochemical characteristics and previous studies, we infer that the southward subduction of the Junggar Ocean slab resulted in the generation of early Paleozoic volcanic rocks and arc setting‐related metallogenic systems in the Kalatag area.

Review of bomb and ash flows: Genesis and case studies of a subset of block and ash flow deposits

Pyroclastic density currents with an abundance of cauliflower-shaped bombs are an uncommon type of deposit, called bomb and ash flow (BoAF) deposits in several papers. Although they are similar to block and ash flow (BAF) deposits (e.g., rich in juvenile blocks and breadcrusted bombs), they are often related to eruptions of mafic to intermediate magmas. In the current study, we analyze and compare historical and prehistorical BoAF-generating eruptions at Asama and Aso (Japan), Mayon (Philippines), Mt. Spurr (United States), Fuego (Guatemala), Arenal (Costa Rica), Cotopaxi and Tungurahua (Ecuador), and Láscar and Tilocálar volcanoes (Chile). Our review indicates that BoAFs show a substantial contribution of juvenile rounded material but with different rheologies and fragmentation mechanisms. This juvenile material is typically basaltic or andesitic, as it is more susceptible to form volcanic bombs with scoriaceous cauliflower textures. Thus, BoAFs could be a subset of the BAF deposits. The study and recognition of this type of deposit in volcanic sequences could be misinterpreted as a ballistic bomb deposit or even a hot bomb-rich lahar deposit, therefore, its appropriate interpretation is fundamental for volcanic hazard assessment.

Paleoproterozoic rift-related volcanism of the Karrat Group, west Greenland: New insights from lithofacies analysis and geochemistry of the Kangilleq Formation

Mafic volcanic rocks of the Paleoproterozoic Kangilleq Formation of the Karrat Group of West Greenland represent a submarine mafic volcanic sequence, erupted and deposited on the Rae craton of Greenland. The formation ranges from <100 m-thick to ∼500 m-thick, and consists of mafic volcaniclastic rocks, pillow lavas and massive basalt lavas that have undergone greenschist facies metamorphism, yet preserve primary volcanic textures such as pillows, drainage cavities, lobes, flow banding, autoclastic breccias, hyaloclastite, scoria and layering.Rocks of the Kangilleq Formation overlie the Archean Qeqertarssuaq Complex and are interleaved with and overlain by the ca. 1950 Ma Nûkavsak Formation. Based on detailed (1:200) mapping and high-precision geochemistry, rocks of the Kangilleq Formation are subdivided into the Kangiusiap and Umiammakku members. The Kangiusiap member is dominated by mafic, scoria-rich breccias, has sulfide mineralization, and is bound at the bottom and top by the Nûkavsak Formation. This member has subalkaline, transitional to tholeiitic, mid-ocean ridge basalt (MORB) to back arc basin basalt (BABB) signatures with a Nd-isotope value similar to the depleted mantle at 1.88 Ga. The Umiammakku member is dominated by pillow lavas, with subordinate amounts of massive lavas and volcaniclastic rocks, overlies the Qeqertarssuaq Complex and is overlain by, and interleaved with, the Nûkavsak Formation. It is alkaline with characteristics similar to modern ocean island basalts and Nd-isotopic values slightly lower than the depleted mantle at 1.88 Ga.The rocks of the Kangilleq Formation represent non-arc suites or suites that have only minor subduction zone or crustal input, and display geochemical signatures similar to basalts found in modern intracontinental back arc settings. Lithofacies, bedforms, internal stratigraphy, and the identification of synvolcanic faults, suggest it is the product of volcanic eruptions during incipient rifting within a back arc setting, and seafloor mass wasting events at fault scarps related to rifting. Based on the geological characteristics and its petrogenetic and tectonic history, the Kangilleq Formation is interpreted to be correlative with the Bravo Lake Formation of the Piling Group of northeastern Canada and records incipient rifting of the southern margin of the Rae craton in the Paleoproterozoic.

Construction of the Caroline Ridge uppermost basement in the West Pacific: Implications from intrabasement seismic reflectors

The construction model of the Caroline Ridge uppermost basement is still unresolved, requiring more inference from limited geophysical observational data. Here, we systematically reveal intrabasement seismic reflectors of volcanic sequences within the rifted and subsidence domains of the Caroline Ridge. Extrusive centres and three types of intrabasement reflectors, that is, relatively horizontal, ridgeward‐dipping and folded reflectors, have been identified. Extrusive centres in the rifted domain are characterized by domal shapes and produce sub‐parallel stratified intrabasement reflectors within the conduits that connect with the relatively horizontal reflectors distributed on both sides of the basement highs. Intrabasement reflectors display increasing dip angles away from the extrusive centre and present ridgeward‐dipping reflectors but not troughward‐dipping reflectors in subsidence domain 1, suggesting a brittle deformation process. Layered intrabasement reflectors are developed within subsidence domain 2 but display folded and mounded morphologies, suggesting a ductile deformation process. We propose that the Caroline Ridge formation was supported by discrete extrusive centres, and the uppermost basement construction model has experienced stages of transition from brittle deformation to ductile deformation, which can provide new clues for the early‐stage crustal evolution of global oceanic plateaus.

Experimental comparison between dynamic and static parameters for volcanic rocks in Sulcis basin (Italy)

This work compares the results of expensive and time-consuming destructive tests and cheap non-destructive ultrasonic analyses for the assessment of rock parameters (i.e. uniaxial compressive strength, Young's modulus, Poisson's ratio, etc.), with the aim to geologically characterize the site and to find correlations between the mechanical and acoustic properties of rocks for applications in the field of the monitoring of CO2 leakage. The assessment considers 60 samples, 10 per each of the six formations of the Sulcis Miocene volcanic sequence. Samples have been initially subjected to non-destructive tests for the measurement of density and compressional and shear wave velocity and then mechanically tested to determine the uniaxial strength. As it happens for other kinds of rock, a linear direct correlation has been found between uniaxial compressive strength and compressional wave velocity, as well as between dynamic and static Young's moduli and for the dynamic and static Poisson's ratios, with a correlation coefficient R2 significantly affected by the nature of the pyroclastic formation. Helpful information on the pyroclastic rocks have been obtained, collecting parameters currently not available in the literature for this kind of geological formations.

Potential for permanent CO2 sequestration in depleted volcanic reservoirs in the offshore Campos Basin, Brazil

The pre-salt oil and gas production in Brazil faces a significant challenge due to the high CO2 content in these reservoirs. Approximately 600,000 t of CO2 are reinjected monthly in the reservoirs, but the increased production of CO2 will demand alternatives for sequestration. Experiments and pilot projects have demonstrated the viability of CO2 sequestration through the mineralization method in basaltic rocks. Here, we present a study aimed at demonstrating the feasibility of using the volcanic rocks of the Cabiúnas Formation, located at the base of the pre-salt section in shallow waters of the Campos Basin, for CCS projects. We used legacy data to determine the regional characteristics and porosity distribution of the volcanic sequence to assess the feasibility of geological sequestration in this region. Our estimates demonstrated that the Cabiúnas flood basalts have a good to excellent storage capacity. The modeling of a 31km2 hypothetical reservoir with a thickness of 300 m in the upper part of the sequence above the Badejo Field revealed a storage estimate of 16–47 Mt. The technical aspects discussed in this study provide valuable insights that can help with the development of future CCS projects in the volcanic rocks of this petroleum province.

Paleogeography and high-precision geochronology of the Neoarchean Fortescue Group, Pilbara, Western Australia

While rates of Phanerozoic plate movements and magnetic field reversals have been well studied, little is known about such phenomena on early Earth. The ca. 2.8–2.7 Ga Fortescue Group on the Pilbara craton in Western Australia has been recognized as a well-preserved sequence of Archean rift volcanics thought to derive from a flood basalt province, and may have been moving rapidly across the globe at two different intervals in its depositional history. We present the results of a magnetostratigraphic study integrated with high-precision U-Pb ID-TIMS geochronology aiming to quantify rates of cratonic motion and provide a continuous time series for changes in Pilbara paleogeography during these two rapid intervals, at ∼2.77 and 2.72 Ga. We provide six new or updated high-quality paleomagnetic poles for inclusion in databases tracking Precambrian cratonic motion. During the craton’s largest geographic displacement at ∼2.77 Ga, we resolve a minimum drift rate of 23 ± 20 cm/a if there was substantial rotation of the Pilbara craton along with translational motion, and a more rapid minimum estimate of 64 ± 23 cm/a if the motion was dominated by translation; these estimates exceed both Mesoarchean and most modern rates of plate motion. We provide a new high-precision U-Pb zircon age of 2721.23 ± 0.88/0.88/6.9 Ma for the Tumbiana Formation stromatolite colony, which developed as the Pilbara craton drifted from 51.5 ± 7.0° to 32.1 ± 5.7° paleolatitude. Although the Fortescue Group has been considered an early prototype of large igneous provinces, it was emplaced over a longer duration than its Phanerozoic counterparts and does not fit at least one definition of a large igneous province (LIP). But as a potential prototype of LIP magmatism, the Fortescue succession chronicles eruptive dynamics, rapid paleogeographic changes, and a series of robustly determined magnetic field reversals during the Neoarchean.

Subaqueous felsic volcanic sequence and its contribution to the ancient alkaline lacustrine deposits in the Mahu Sag, Junggar Basin, NW China

Subaqueous eruptions alter the chemical conditions of the lake by intensifying the interaction between hot volcanic rocks and water, resulting in the exchange of alkalis. The lower part of the Fengcheng Formation in the Mahu Sag, NW China, has abundant rhyolitic welded tuff and related pyroclastic deposits, which were deposited under ancient alkaline lacustrine deposits of the Fengcheng Formation. Despite this, the effect of the eruption process on the formation of the alkali lake remains unclear. To gain a better understanding of the eruptive behaviour and emplacement processes, detailed drilling core descriptions, microscopic observations, and interpretations of textural characteristics of rhyolitic welded tuff units and adjacent volcano‐sedimentary associations have been conducted. The rhyolitic welded tuff can be categorized into seven lithofacies units based on the degree of welding, devitrification characteristics, pore development, and filling characteristics. The age of eruption was determined through zircon U–Pb dating of the rhyolitic welded tuff, revealing it to be approximately 300 Ma, thereby providing constraints on the deposition time of alkaline lacustrine deposits. Mudstones that encase the welded tuff unit and quenched structures such as perlitic fractures, quench cracks of spherulite and lithophysae, and chalcedony spherules indicate that the welded tuff unit was formed through subaqueous eruption. Alkali fractionation (sodium and potassium) occurs in the spherulite and lithophysae zone and at the top condensation boundary, demonstrating that the subaqueous eruption process significantly enhanced devitrification and hydration, causing sodium to escape from the welded tuff and released into the lake water as sodium ions. The rhyolitic welded tuff provides evidence of the evolution of a subaqueous felsic eruption where the water‐rock reaction played a fundamental role in alkalizing the lake by providing the lake with sodium ions in Mahu Sag.

Compositional and geothermobarometric analysis of the Upper Miocene tholeiitic volcanic products in the northern Andes at 5-6° N latitude: The Combia Volcanic Province

The Combia Volcanic Province (~11-5 Ma), is a volcaniclastic sequence located in northwest Colombia between the Central and Western cordilleras at 5-6° N latitude. Its source is associated with the volcanic activity of the magmatic arc produced by the subduction of the Nazca plate under the South American plate in the northern Andes. The distribution, composition, and chronostratigraphy of the province’s deposits are well-known, with the volcanic sequences characterized as compositionally bimodal. The older rocks (ca. 11-9 Ma) display tholeiitic affinity, whereas the younger (ca. 9-5 Ma) are mostly calc-alkaline, with some adakite-like signature recognized. While the magmatic system for the calc-alkaline magmas has been previously extensively studied, the processes that occurred during the magma stagnation and ascent are unknown for the tholeiitic magmas. This work bridges this gap by the study of tholeiitic lava flows outcropping at the center of the province, through petrography, mineral chemistry, whole-rock analysis, and geothermobarometry calculations of the crystallization conditions. Texturally, the rocks are porphyritic with plagioclase (An50-90) and clinopyroxene (augite and pigeonite) phenocrysts and microphenocrysts, embedded in a glassy and microcrystalline groundmass. Compositionally, the rocks vary from basaltic andesite to andesite (52.8-57.8 wt% SiO2), with relative enrichments of LILE to HFSE and REE to chondrite. Crystallization conditions, based on several plagioclase-melt and pyroxene-melt geothermobarometers, were estimated at T=1,095-1,153 °C and P=0.22-0.60 GPa for the plagioclase, T=1,046-1,131 °C and P=0.09-0.21 GPa for the augite, and T=867-1,039 °C and P=0.40-0.60 GPa for the pigeonite. These results suggest a relatively rapid magma ascent for the tholeiitic products as well as an evolution mostly through fractional crystallization. The LILE elements enrichment, the negative trend in the FeOt, TiO2 and CaO versus SiO2 content, together with some disequilibrium textures, are also evidence of crustal contamination and magma recharge. Thus it is proposed that the Combia Volcanic Province started as a simple magmatic system, where the tholeiitic products were generated by a relatively rapid magma ascent. Then, a more complex magmatic system linked to long-term magma stagnation, allowed melt evolution to form calc-alkaline magmas as previously defined.

Magma Composition and Formation Conditions of Alkaline-Salic Rocks of the Early Mesozoic Adaatsag Bimodal Association of the Kharkhorin Rift Zone, Central Asia: Data from the Study of Melt Inclusions in Minerals

The composition and formation conditions of magmas of comendites of the Early Mesozoic Adaatsag volcanic association are determined using electron and ion microprobe analysis of melt inclusions, and mechanisms favorable for the accumulation of trace and rare earth elements are identified. The melt and fluid inclusions are found in quartz of comendites sampled in various parts of the section of a volcanic sequence. The melt inclusions consist of glass, a gas bubble, and daughter minerals (fluorite, polylithionite, and K-feldspar). The composition of a gas phase in melt inclusions was studied using Raman spectroscopy. The dominant gas phase components include water and hydrogen. The fluid inclusions are aqueous fluids probably corresponding to KF. The thermometric studies of melt inclusions and the analysis of glasses of homogenized melt inclusions in quartz phenocrysts of comendites showed that magmas crystallized from water-saturated rare metal melts with high Li, Zr, F, Rb, Nb, Y, and Th contents at a temperature of 880–930°C and a pressure of 1000 bar at a depth of ~3.5 km accompanied by degassing. The comparison of our data of the study of melt inclusions in phenocrysts of alkaline salic rocks of the coeval Adaatsag, Dzarta-Khuduk, and Sant volcanic associations, which occur within the Kharkhorin Rift Zone, revealed common principles of magma evolution. This allowed us to propose a similar mechanism of their formation, involving the accumulation of many rare and rare-earth elements, as well as volatile components (F, H2O) in the process of crystallization differentiation. Subsequently, a salt melt rich in Li, F and water could be separated from such comendite magmas. The detection of fluoride aqueous inclusions in quartz allows us to suppose the further evolution of the salt melt leading to the appearance of a concentrated aqueous fluid and the possible participation of the latter in metasomatic processes.

Cenozoic clockwise rotation of the northeastern Tibetan Plateau: Paleomagnetic evidence from volcanic sequences in the West Qinling Mountain

Cenozoic clockwise rotation of the northeastern Tibetan Plateau: Paleomagnetic evidence from volcanic sequences in the West Qinling Mountain

Late Paleozoic alkaline granitoids of the southwestern and Northern Mongolia: U–Pb ID TIMS zircon dating, petrogenesis and implications for post-accretion and anorogenic activity of the Central Asian Orogenic Belt

In tectonic history of many orogenic belts, alkaline granitoids with A-type affinity mainly reveal lithosphere extension events. However, precise dating of these rocks is often hampered by several problems either due to absence of accessory zircon or due to high U and Th contents disturbing the U–Pb isotope system. This study reports results of chemical abrasion isotope dilution–thermal ionization mass spectrometry (CA ID TIMS) U–Pb dating of zircon from five plutons of aegirine- and/or alkaline amphibole-bearing alkali-feldspar granites and quartz syenites from the Trans-Altai Gobi (SW Mongolia) and Northern Mongolia in the Central Asian Orogenic Belt and reveals their close petrogenesis in different tectonic position. In the SW Mongolia the earliest Carboniferous alkaline granitoids (ca. 358–346 Ma) were formed during the post-accretion extensional event suggesting the Late Devonian accretion of the Trans-Alai Gobi–Eastern Junggar oceanic island arc to the Mongolian–Chinese Altai peri-Siberian continental margin. Alkaline granitoids of the Northern Mongolia were emplaced in the Early Permian during two independent geographically overlapping anorogenic near-coeval igneous pulses. The earliest pulse (ca. 285 Ma) related to evolution of the western half of the Northern Mongolia–Western Transbaikalia volcano-plutonic belt of alkaline granitoids with the bimodal volcanic sequences, is ascribed to the most significant Early Permian magmatic event throughout the Central Asian Orogenic Belt. The latest pluton (276 ± 1 Ma) identifies the initial emplacement stage of the Middle Permian–Early Triassic Khangai granitoid batholith.Alkaline granitoids from the Trans-Altai Gobi and Northern Mongolia possess similar ferroan, mainly peralkaline, A2-type granite geochemical characteristics, but differ in Nd isotopic signatures. For the Trans-Altai Gobi plutons εNd(t) = +6.7; for Northern Mongolian plutons εNd(t) varies between –1.7 to + 0.6. These values are consistent with the Nd isotopic evolution of the continental crust of the hosting terranes, ‘Hercynian’ isotope province for the Trans-Altai Gobi and ‘Caledonian’ province with a pre-Ediacaran microcontinent crust for the Northern Mongolia. Thereby studied alkaline granitoids were formed manly though anatexis of the host terrane crust during post-accretion and anorogenic extension events in different parts of the Central Asian Orogenic Belt.

Rock Magnetism and Magnetic Fabric Study of the Icelandite and Rhyodacite Long Volcanic Sequence at Mauna Kuwale, Wai’anae Volcano, Oahu, Hawaii, USA

In order to understand further the emplacement (i.e., volcanic growth) of 22 Icelandite and 3 Rhyodacite cooling units in one of the long volcanic sequences known as Mauna Kuwale of the Wai’anae volcano (ca. 3.3 Ma), Oahu Hawaii we have conducted appropriate rock magnetic experiments described below as well as anisotropy of magnetic susceptibility (AMS) studies of such 25 units. We have undertaken rock magnetic investigations such as continuous and partial thermo-magnetic cycles of low field magnetic susceptibility versus temperature dependence, (k-T) curves experiments. We classified the k-T heating-cooling dependence of susceptibility in three groups A, B and C. Type A: yielded two components of titano-magnetite with a predominat Ti rich phase and occasionally a relevant magnetite component phase. Type B: samples are characterized by Ti poor magnetites. Magnetite dominates as the main magnetic carrier. Type C: k-T curves show one single phase of titanomagnetite, and Ti poor magnetite. The coercivity or remanence, determined by back field magnetization is always &lt;60 mT, which suggest the predominance of magnetic components of low coercivity, like magnetite. Usually, two coercivity components are identified in the specimens. In addition we also conducted magnetic granulometry analyses on 27 specimens to determine the domain state of the flows. The ratio of hysteresis parameters (Mr/Mrs versus Hcr/Hc) show that overall samples fall in the Pseudo-Single Domain (PSD) region with high values of Mr/Mrs and very low values of Hcr/Hc. Only two samples from cooling units 17 and specially 22 show a Single Domain (SD) magnetic behavior and a sample from one unit approaches the SD-MD mixture region. We measured the magnetic susceptibility of all cooling units and we found out that in all analyzed units the magnetic susceptibility is low 13.7 ± 8.8 (10−3 SI). Magnetic anisotropy/magnetic fabric is used as a tool in rock fabric analyses to investigate the preferred orientation of magnetic minerals in rocks. Magnetic anisotropy is low on all (measured) flows from the Icelandite cooling units from 1 to 17 (mean P’ = 1.010), but becomes noticeably distinct and high in rhyodacite cooling units 23, 24 and 25 (mean P’ = 1.074). Four units show a magnetic fabric with k3 axes vertical to sub-vertical which may be denoted as normal for the horizontal to sub horizontal units. Two Icelandite cooling units display oblate shapes and two other cooling units triaxial shapes. K1 axes are horizontal but point in different directions, i.e., NE and NW. Remaining cooling units show different magnetic fabric. Units 17, 23, 24 and 25, despite important variations in anisotropy (low for units 25 and high for units 23 and 24) and shape of ellipsoid (oblate in cooling unit 23, prolate in 24 and triaxial in 25) the k3 axes show the same orientation, SW to SSW dipping around 45° and a very steady magnetic lineation azimuth NW nearly horizontal to sub horizontal. The magnetic mineralogy and magnetic fabric indicate that both the Icelandite and Rhyodacite cooling units the magmatic evolution during the shield stage of the entire Wai’anae volcano and that such growth was not affected by tectonic deformation.