Red-coloured pedosediments of the Lori Basin, Armenia

<p>In this work, the samples of red-coloured pedosediments were analyzed. They were found in two sections of the Lori Basin, Armenia - Kurtan-IV and Yagdan. Micromorphological analysis, the measurement of magnetic susceptibility, particle size distribution, CHN, determination of bulk, and biomorphic composition was made. Based on the results of previous studies, the age of the deposits overlying these sections was known. The pedosediment from the Kurtan-IV section is overlain by a mixture of sand and volcanic ash, which is dated at about 1.4 Ma (Calabrian). The pedosediment from the Yagdan section is overlain by 2-2.5 Ma basaltic lava (Gelasian). As a result of our research, it was revealed that both pedosediments were formed in conditions differ from current ones and have differences between themselves also. The pedosediment from the Yagdan section was formed during the dominance of the subtropical climate. It was revealed the presence of manganese-ferruginous and clayey films, a low index of carbonization and salinity, and a relatively high index of weathering. Pedo-sediments from the Yagdan section can be classified as Cambisols with vitric, argic, chromic qualifiers. By the time the later section Kurtan-IV was formed, the climate became cooler, which was reconstructed by the appearance of phytoliths of coniferous plants. Pedosediment from the Kurtan-IV section can be classified as Stagnic Luvisols. The later factors influencing the preservation of pedosediments and changes in their composition and properties were identified. Thus, the upper horizon of the Yagdan section sharply differs from the underlying ones in increased weathering and oxidation, and these properties are retained up to the third layer. It has increased indicators of magnetic susceptibility and the content of copper elements, cobalt, nickel, vanadium, chromium, which were brought in by basalt lava. The pedosediment from the Kurtan-IV section was overlain by lacustrine deposits, which led to a decrease in the magnetic susceptibility and an increase in the SiO<sub>2</sub> content and carbonation index. This work was supported by RFBR, grant N19-29-05024 mk.</p>

The Mandalovoo–Gurvansayhan terranes in the southern Gobi of Mongolia: new insights from the Bayankhoshuu Ruins section

The Bayankhoshuu Ruins section in southern Mongolia is characterized by strongly thrusted and folded sequences. Overall, three sections ranging from Ordovician to Carboniferous rocks were studied. Facies analysis combined with stratigraphic data provide improved lithostratigraphic descriptions of Palaeozoic successions in the Mushgai region. The overall marine sedimentary sequence is punctuated by volcanic rocks–basaltic lava of Silurian and Middle Devonian age and volcaniclastic bentonite and tuff in the Middle to Late Devonian and Mississippian suggesting an island arc setting. The Minjin Member of the Botuulkhudag Formation (Middle Devonian to Late Devonian) is primarily composed of thick basaltic and subaerial volcanic rocks with minor silicified siltstone and chert inclusions. Thicker successions of limestone occur in the Ordovician/Silurian, Early Devonian, and the Mississippian. The macrofauna is scarce, except distinct limestone horizons where different fossil groups were recognized. Microfossils, such as radiolarians and conodonts, are scarce and generally poorly preserved. However, based on the re-study of collections from earlier publications and new conodont data, a more detailed biostratigraphic record of the Khoyormod, Botuulkhudag, and Arynshand formations of the Bayankhoshuu Ruins section can be developed. For instance, the Arynshand Formation likely ranges from the late Bispathodus ultimus conodont biozone to the Scaliognathus anchoralis–Doliognathus latus conodont biozone. A tectonic breccia occurs in the early Mississippian and is overlain by a red shale of remarkable thickness at the top of this formation which points to subaerial exposure in the early Mississippian (near the Tournaisian/Visean transition). Due to strong tectonic overprint and/or facies, some unconformities/hiatuses occur. Most strata are intensively folded and faulted, ranging from centimeter to meter scale. Overall, deposition likely occurred on either the Mandalovoo or Gurvansayhan Terrane.

The effects of basaltic lava flows on the petrophysical properties and diagenesis of interbedded aeolian sandstones: an example from the Cretaceous Paraná Basin, Brazil

An analysis of the petrophysical and diagenetic effects of the emplacement of Cretaceous basaltic lava flows (Serra Geral Formation) on aeolian sandstones (Botucatu Formation) has been undertaken on core samples from the Paraná Basin, Brazil. Between 0.1 to 1 m from the contact zone, acoustic wave velocities and porosities in sandstones show a significantly wider scatter than those located >1m away from the lava contact. Higher P-wave values (av. 3759.3 ms-1) occur between 0.1 to 1 m from the lava contact in contrast to those areas > 1 m away (av. 3376.8 ms-1), whilst the average porosity is 6.5% near the contact (0.1 to 1 m), and 10.7% away from the contact (>1 m). Petrographic evaluation reveals two diagenetic pathways responsible for modification of the petrophysical properties: early hydrothermal Mg-rich authigenesis (Type 1) and early chemical dissolution (Type 2). Type 3 diagenesis occurs away from the lava-sediment contact (>1 m) with the appearance of poikilitic calcite and smectite. The sandstone samples associated with Types 1 and 2 diagenesis display a decrease in porosity and increased acoustic velocities in relation to Type 3, while Type 3 samples show little or no variation in reservoir properties. The lava-induced diagenetic effects at the sandstone-lava contacts (0.1 to 1 m) may form a baffle or seal to fluids around the margins of the sandstone bodies. Therefore, whilst diagenesis associated with lava emplacement may hinder reservoir quality around the margins, the original reservoir properties are preserved within these large sandstone bodies.Supplementary material: Petrophysical and petrographic data is available as annex files. https://doi.org/10.6084/m9.figshare.c.5244473

Tectonics of Volcanogenic Massive Sulphide (VMS) Deposits at Flores Back Arc Basin: A Review

The bathymetry, petrology, marine magnetic, and seismic-SBP data have identified the northwest-southeast direction submarine ridge that shows hydrothermal activity. This activity occurred through Mount Baruna Komba, Abang Komba, and Ibu Komba. The volcanic rocks are andesite basaltic lava flows, tuff, and pumice. The andesite basaltic lava shows porphyritic, intergranular, intersertal to glomeroporphyritic textures. The rock composes anhedral minerals of k-feldspar, plagioclase, and pyroxene. These minerals present in small-sized, short prismatic dispersed in very fine groundmass minerals or glasses. Most of the volcanic rocks have experienced various degrees of alteration. The k-feldspar and plagioclase are most dominantly transformed into sericite, clay mineral, carbonate, epidote and oxide mineral, opaque mineral, and secondary plagioclase through the albitization process, while pyroxene replaced by chlorite. Other minerals are biotite and quartz, and base metals are present Cu, Zn, Ag, As, Pb, and gold. Mineralization categorizes as the phyllic zone, sub-prophylithic zone, and phyllic-potassic zone that formed at a temperature range of 250-400oC. The submarine hydrothermal alteration in the Komba Ridge is associated with a volcanogenic sulphide deposit controlled by crust thinning due to the crust rifts in the back-arc tectonic setting.

Volcanic Processes and Magmatic Evolution of Tianchi Volcano, Changbaishan

The Changbaishan volcanic field located on the Gaima (Gaema, Gaiman) Plateau witnessed plateau-forming eruptions along with the uplift of the Gaima Plateau. The Tianchi basaltic lava shield volcano was formed at the main peak of Changbaishan, with cone construction eruptions that formed a huge and steep trachytic composite cone on the gentle lava shield. At the peak of the Millennium Eruption (ME), height of the eruption column (HB) reached 25 km and the bulk volume of tephra was about 120 km3. The ME eventually formed Tianchi caldera, after which several eruptions occurred, albeit of a much smaller scale.The magmas involved in the shield-forming eruptions are characterized by both alkalic series trachybasalt and basaltic trachyandesite and subalkalic tholeiite and basaltic andesite. In the cone-construction and ignimbrite-forming eruption stages, the magma is completely composed of alkalic series trachyte and comendite. The largest negative Eu anomalies observed in ME magmas indicate that plagioclase was strongly crystallized and differentiated.

The Geochemistry of Magnetite and Apatite from the El Laco Iron Oxide-Apatite Deposit, Chile: Implications for Ore Genesis

The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models—liquid immiscibility and complete metasomatic replacement of andesitic lava flows—attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, and Extensión Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with original crystallization of titanomagnetite from silicate melt or high-temperature magmatic-hydrothermal fluid. The systematic decrease of trace element concentrations in magnetite from intermediate to shallow depths is consistent with progressive growth of magnetite from a cooling magmatic-hydrothermal fluid. Apatite grains from surface outcrops are F rich (typically >3 wt %) and have compositions that overlap igneous and magmatic-hydrothermal apatite. Magnetite and fluorapatite grains contain mineral inclusions (e.g., monazite and thorite) that evince syn- or postmineralization metasomatic alteration. Magnetite grains commonly meet at triple junctions, which preserve evidence for reequilibration of the ore minerals with hydrothermal fluid during or after mineralization. The data presented here are consistent with genesis of the El Laco orebodies via shallow emplacement and eruption of magnetite-bearing magmatic-hydrothermal fluid suspensions that were mobilized by decompression-induced collapse of the volcanic edifice. The ore-forming magnetite-fluid suspension would have rheological properties similar to basaltic lava flows, which explains the textures and presence of cavities and gas escape tubes in surface outcrops.

Expanding the toolbox for dating basaltic lava sequences: 40Ar–39Ar dating of silicic volcanic glass from interbeds

40Ar–39Ar dating of glass shards from silicic tuffs of the Ellensburg Formation (NW, USA) interbedding basaltic lavas yielded accurate, precise, reproducible plateau and isochron ages that are within error at the 2σ level. The age-spectra have flat plateaus and the inverse isochrons have atmospheric 40Ar/36Ar at the 2σ level. Ages of 12.00 ± 0.24, 11.37 ± 0.15, 10.67 ± 0.21 and 10.70 ± 0.18 Ma are consistent with the stratigraphy of four of the dated layers; the age of 10.77 ± 0.18 Ma for a fifth layer is at odds with the stratigraphy. This discrepancy arises due to the effect of glass alteration that induced K- and Ar-loss. There is no evidence of excess 40Ar or 39Ar recoil. The new ages indirectly constrain the timing of eruption of the lavas above and below the ash beds. This demonstrates that volcanic glass from interbeds can be used as an additional tool for indirectly dating basaltic lava sequences, which is independent of the lavas and complementary to other materials. Considering the numerous studies in which volcanic glass failed to provide reliable 40Ar–39Ar ages, additional and supportive constraints are still needed to assess the validity of the ages from glass shards.Supplementary material: A1 – sample location, stratigraphic position and depositional environments; A2 – electron microprobe information with calibration and sample data; A3 – Ar isotopic data of blanks and samples, data reduction software, J values, criteria for age calculation and images of age spectra and inverse isochrons are available at https://doi.org/10.6084/m9.figshare.c.5077705