Petrological Results Research Articles

New Rock Magnetism and Magnetic Fabrics Studies on the Late Triassic Volcanic Rocks from Qaidam Block, Northern Tibetan Plateau

The Qaidam Block, located at the northern Qinghai–Tibet Plateau, is a pivotal area in unraveling the closure time of the Kunlun Ocean basin which might have recorded the transformation process between the Proto-Tethys and Paleo-Tethys Ocean basins. However, the late Triassic position of the Qaidam Block remains enigmatic, largely due to the scarcity of paleomagnetic data essential for quantitatively determining its paleolatitude. The widespread presence of the Elashan formation, particularly along the southern periphery of the Qaidam block, presents good material for conducting paleomagnetic work. Nevertheless, the primary magnetic carriers preserved within the Elashan formation might be influenced by multiple tectonic thermal events, particularly those associated with collisions between southern blocks and the Qaidam Block. Here we present rock magnetism and magnetic fabrics studies to identify the content and composition of magnetic minerals within the Elashan formation. The rock magnetic and petrologic results show that the magnetic carriers in the samples from the Elashan formation are dominated by magnetite with a small amount of goethite, pyrrhotite, and hematite. The results of Anisotropy in Magnetic Susceptibility indicate that the south of the Longwalangku section might not be obviously influenced by the tectonic events. Our results also provided guidance for future paleomagnetic research, emphasizing the importance of conducting further sampling away from adjacent faults, particularly in the southern Longwalangku area.

Structure and provenance of the Cretaceous Pingshanhu Basin in the Hexi Corridor: Implications for Mesozoic tectonics in the northern Tibetan Plateau

Abstract The construction of Earth’s largest highland, the Tibetan Plateau, is generally considered to have been generated by the Cenozoic India-Asia collision. However, the extent to which high topography existed prior to the Cenozoic remains unclear. The Hexi Corridor foreland basin of the northern Tibetan Plateau is an ideal region in which to investigate this history, given its widespread exposure of Early Cretaceous sedimentary sequences. In this study, we examined the Early Cretaceous strata in the northern Hexi Corridor to understand the relationships between pre-Cenozoic sedimentation and tectonic deformation and constrain the late Mesozoic tectonic setting of the adjacent Qilian Shan and Alxa blocks bordering the northern Tibetan Plateau. Results of sandstone petrology analyses, paleocurrent observations, and U-Pb geochronology suggest that the oldest Early Cretaceous sediments deposited in the northern Hexi Corridor were sourced from the southern Alxa block during the earliest Cretaceous. By the late Early Cretaceous, Hexi Corridor sediments were sourced from both the southern Alxa block to the north and the Qilian Shan to the south. Sandstone petrologic results indicate that the northern Hexi Corridor experienced a tectonic transition from contraction to extension during the Early Cretaceous. These findings suggest that the northern Tibetan Plateau region was partially uplifted to a high elevation during the late Mesozoic before the India-Asia collision.

Metamorphic Ages and PT Conditions of Amphibolites in the Diebusige and Bayanwulashan Complexes of the Alxa Block, North China Craton

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase ± K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825–910 °C/7.2–10.8 kbar, which is similar to that (800–870 °C/7.0–10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl–pl–qz thermobarometry yielded the metamorphic PT conditions of 732–810 °C/3.0–6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 °C, 768 °C and 780 °C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800–910 °C/7.0–10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl–pl–qz thermobarometer. In addition, LA-ICP-MS U–Pb analysis on metamorphic zircons yielded weighted mean 207Pb/206Pb ages of 1901 ± 22–1817 ± 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex.

Metamorphic P–T conditions, in‐situ LA–ICP–MS biotite–muscovite Rb–Sr geochronology and tectonic implications of two‐mica–quartz schist from the Kuanping Group in the North Qinling Orogenic Belt

The Kuanping Group is an important lithotectonic unit in the North Qinling Tectonic Belt, which has experienced metamorphism from greenschist to high amphibolite facies and is of great significance in defining the orogenic process of the Qinling Orogenic Belt. This study presents the results of petrology, geothermobarometry, phase equilibrium modelling, in‐situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) biotite–muscovite Rb–Sr analysis, for the two‐mica–quartz schist of the Kuanping Group in the Hongtuling and Heihe areas. These results are employed to better understand their metamorphic P–T conditions and ages, as well as geological implications. Garnet‐bearing two‐mica–quartz schist in the Hongtuling area records a clockwise P–T trajectory on the basis of phase equilibrium modelling, with peak P–T conditions of ~600°C and ~0.85 GPa. In addition, the Ti‐in‐biotite thermometry yielded metamorphic temperature conditions of ~578°C–580°C and ~590°C–596°C for the garnet‐bearing two‐mica–quartz schist from the Hongtuling area and the two‐mica–quartz schist from the Heihe area, respectively. These results indicate that metamorphic temperature conditions in both areas are higher than the closure temperature of the Rb–Sr isotopic system of muscovite and biotite. In‐situ LA–ICP–MS biotite and muscovite Rb–Sr analyses yielded three groups of isochron age, i.e., ca. 410 Ma, ca. 360 Ma and ca. 320–300 Ma. On the basis of previously published data and the data in this study, the Kuanping Group underwent ca. 440 Ma regional metamorphism in the NQB, retrograde cooling at ca. 410–360 Ma due to exhumation and ca. 320–300 Ma tectonothermal overprinting.

Paleoproterozoic U Mineralization in Huayangchuan Deposit, Xiaoqinling Area: Evidence from the U–Rich Granitic Pegmatite

The Huayangchuan uranium deposit, located in the west of the Xiaoqinling belt on the southern margin of the North China Craton, is a large U–Nb–Pb deposit accompanied with rare–earth elements. The Huayangchuan uranium deposit, discovered in the 1950s, has long been known as a carbonatite–type uranium deposit. Recently, new geological work has found uranium mineralization in many granitic pegmatite veins in the Huayangchuan deposit and adjacent areas. Here, we report a systematic investigation of the petrography, whole–rock geochemistry, zircon U–Pb ages, and in situ Lu–Hf isotopic characteristics of newly discovered U–rich granitic pegmatite veins in the west of Huayangchuan deposit. The petrological results showed that the lithology of the samples is granite pegmatite. The U–Pb ages of zircon were 1826.3 ± 7.9 and 1829 ± 11 Ma. Microscopically, the paragenetic characteristics of zircon, betafite, and uraninite exist in the intergranular fissures of K–feldspar and quartz, reflecting metallogenic phenomena in the rock formation process. Almost all whole–rock samples were rich in SiO2 (64.37−70.69 wt.%), total alkalis (K2O + Na2O = 8.50–10.30 wt.%), and Al2O3 (12.20–14.41 wt.%) but poor in TiO2 (0.23–0.73 wt.%), MgO (0.38–0.90 wt.%), CaO (1.23–2.22 wt.%), P2O5 (0.14–0.83 wt.%), and MnO (0.04–0.57 wt.%). Additionally, they showed enrichment of LILEs (such as Rb, Ba, Th, U, and K), depletion of HFSEs (such as Ta, P, Ti, and Hf), and no alkaline dark minerals, and the characteristics are intraplate A1–type granite. The A1–type granite displayed low zircon εHf(t) values (−19.42–−15.02) with zircon two–stage Hf model aged 3.10–2.76 Ga, indicating that the U–rich granitic pegmatite was derived predominantly from partial melting of the ancient continental crust (such as the early Taihua group formed in Archean–Neoarchean). Combined with the above results and regional geological data, the U–rich granitic pegmatite discovered in the Huayangchuan deposit was formed in a post–collisional regime after the Luliang movement in the late Paleoproterozoic. This study suggests that future uranium prospecting work in this area should focus on late Paleoproterozoic U–rich granitic pegmatites.

Influence of the diagenetic oxidation and allochthonous organic matter input on geochemical and petrological results – Insights from the Upper/Middle Triassic strata, Polish Basin

Set of 24 Lower Keuperian (Upper Ladinian) and Grabfeld Formation (Lower Carnian) grey/black claystones core samples from the central part of the Polish Basin was examined by geochemical (Rock-Eval and Gas Chromatography-Mass Spectrometry) and petrological analyses. The Rock-Eval and petrological analyses revealed that the analysed organic matter is mostly mature/overmature (1.0–2.5%Ro). A combination of biomarker maturity indicators and petrological analyses suggest that autochthonous organic matter did not reach 0.9%Ro, and the higher mature organic matter particles have an allochthonous origin. Most samples contain significant input of allochthonous organic matter, which influenced the results of some geochemical indicators describing the origin of organic matter and palaeoenvironmental conditions.Investigated strata contain mostly mixed planktonic-bacterial and terrigenous organic matter. A higher contribution of terrigenous organic matter was observed within the Grabfeld Fm., where geochemical traces of fluvial-deltaic palaeoenvironment was recorded. Deposition of both strata occurred mostly under anoxic bottom waters in brackish or normal-saline marine conditions. However, an elevated salinity level was observed within Grabfeld Fm. sample from Byczyna 1 borehole.All samples contain phenylnaphthalenes (PhN), phenylphenanthrenes (PhP), and aromatic ketones, widely used as diagenetic oxidation indicators. The juxtaposition of two ratios based on phenanthrene (P), methylphenanthrenes (MP), and PhP was proposed to define the degree of organic matter diagenetic oxidation. Combined results of ƩMP/(ƩMP + ƩPhP) and P/(P + ƩPhP) revealed that Grabfeld Fm. contains more oxidised organic matter than Lower Keuperian strata. Moreover, the distribution of the PhN isomers – 1-PhN and 2-PhN – is controlled mainly by maturity level only in the least oxidised samples, where P/(P + ƩPhP) values range from 0.8 to 1.0. In other samples, the PhN distribution is controlled by the diagenetic oxidation degree. For this reason, the distribution of 1-PhN versus 2-PhN should not be used as a maturity indicator within more oxidised samples.

Metamorphic P–T evolution of the pelitic granulites from the Helanshan in the North China Craton revealed by phase equilibrium modeling and garnet trace elements: Implications for Paleoproterozoic collisional orogenesis

Granulite-facies metamorphic rocks are widely exposed in Paleoproterozoic orogens, and their prograde metamorphic evolution is critical for a better understanding of the Paleoproterozoic collisional orogenic and geodynamic processes. In this paper, we present the results of petrology, trace element geochemistry of garnet, P–T conditions calculated by phase equilibrium modeling and garnet trace element geothermobarometry, for the pelitic granulites of the Helanshan Complex in the western Khondalite Belt of the North China Craton (NCC). These results are used for determination of their metamorphic evolution and tectonic implications. Petrographic observations show that the metamorphic evolution of the pelitic granulites can be divided into four stages (M1–M4), and these metamorphic stages are recorded by the mineral inclusions and reaction textures of garnets. Compared with the nearly flat major-element zoning, the zoning of yttrium (Y) and heavy rare earth elements (HREEs) in the garnet cores and mantles show bell-shaped distributions as well as inner-rim troughs and increases on the outer-rims. Different garnets have the same REE patterns in the same textural domain. The bell-shaped distributions of trace elements (Y and HREEs) in the garnets were controlled mainly by Rayleigh fractionation processes under subsolidus conditions, whereas the increases in these elements in the garnet rims were related to the breakdown of major and accessory phases under suprasolidus conditions. By combination of phase equilibrium modeling and garnet trace element geothermobarometry, a complete clockwise P–T path is constructed for the pelitic granulites. This P–T path is characterized by the heating as pressure rising during evolution from early metamorphic stage (M1: 4.2 ± 0.3 kbar/524 ± 17 °C) to peak metamorphic stage (M2-1; 9.8–10.6 kbar /794–815 °C), which is followed by a isothermal decompression (ITD) to the condition of 5.8–6.5 kbar/808–833 °C and later near isobaric cooling (IBC) to subsolidus. As a result, the pelitic granulites revealed that the formation of the Khondalite Belt experienced initially crustal thickening and post collisional extensional uplift. Integrated with previously reported results, the prograde geothermal gradient of the Helanshan pelitic granulites (14 ± 1 °C/km) is similar to that the pelitic schists in the Himalayan Orogen (13 ± 2 °C/km), which suggests that the Paleoproterozoic and present-day orogens have similar geothermal gradient during crustal thickening.

Heterogeneity in kimberlite structure and composition as a reflection of its petrogenesis peculiarities

New data on the geological structure and petrochemical characteristics of the first kimberlite pipe in the Syuldyukar kimberlite field and its position in the Ygyatta diamond-bearing area are presented. The petrographic identification of the petrochemical varieties of kimberlites in this area was performed for the first time. The compositions of the rockforming and accessory minerals of kimberlites were studied using a modern hardware complex. The results showed that the directed change in the structural and material characteristics of kimberlites from peripheral to central parts in extended vertical bodies of the Syuldyukar tube type is expressed in the replacement of calcium kimberlites by magnesium kimberlites. Mg-kimberlites are characterized by the largest crystals of diamond, garnets, and ilmenites, high concentrations of magnesium, and a number of other components. All of these changes in kimberlites are caused by flow differentiation. This information allowed us to apply the obtained petrological results to the kimberlites of the Syuldyukar tube as search criteria for forecasting and identifying new kimberlite bodies in the Ygyatta diamond-bearing area.

Classification and Provenance on Geochemical Lithogenes: A Case Study on Rock–Soil–Sediment System in Wanquan Area of Zhangjiakou, North China

Geochemical lithogenes have been successfully applied as an innovative concept in the field of composition classification and source traceability of geological materials recently. This paper introduces the background of the development of geochemical genes and the construction and application of LG01 and LG03 lithogenes. Based on LG01 and LG03, the LG_CR classification and provenance are applied and verified on a weathering profile, ten gully sedimentary profiles and regional stream sediments in the Wanquan area of Zhangjiakou city, Hebei province, China. The geochemical lithology of the weathering profile shows a gradual variation from basic-like in the bottom rock to acidic-like at the upper soils compositionally with heterogeneity. The classification results on 10 sedimentary gully profiles (each with five samples) indicate that soils at the bottom of the gully system are dominated with 11 types of LG_CR materials, while the top materials are made up of 21 types, reflecting the mixing of the upstream soils. The results of stream sediments from a regional geochemical survey with a scale of 1:200,000 in this area illustrate that the classification results of LG_CR on stream sediments are basically consistent with the petrological results derived from regional geological mapping. Therefore, LG_CR can be used not only as an effective tool for classification and traceability of geological materials but also has great potential in lithological mapping in petrological-overburdened areas.

Rock Magnetic Results from the Early Ordovician Limestone Rocks in the Northern Qaidam Block, Tibetan Plateau

The early Ordovician location of the Qaidam Block remains unclear, mainly due to the sparse paleomagnetic data to quantitatively determine the coeval position. The early Ordovician limestone is widespread in the northern Qaidam Block providing an ideal window to obtain a paleomagnetic result. The results derived from the limestone are, however, frequently constrained by the hydrothermal fluid activities after the strata formed and complex magnetic minerals in the samples. Therefore, systematic petrological and rock magnetic studies were conducted to identify the content and composition of magnetic minerals in the limestone samples from the early Ordovician Duoquanshan Formation in the northern Qaidam Block. The rock magnetic and petrologic results show that the magnetic carriers in the limestone samples are dominated by magnetite with a small amount of pyrrhotite, goethite, and hematite. These results are consistent with the characteristics of stepwise demagnetization. Combined with the results published previously, the secondary remanence component carried by pyrrhotite might be produced by thermal fluid or magma active by the multiple Tethys evolution.

Microscopic observations of sites and forms of ettringite in the microstructure of deteriorated concrete

The determination of delayed ettringite formation (DEF) in hardened concrete relying simply on the identification of ettringite by electron microscopy or powder X-ray diffractometry can be imperfect because of the high risk of missing other possible deterioration phenomena. The presence of ettringite can be easily biased as an indication of DEF while the actual cause of deterioration is ASR. This paper identifies the deterioration causes and presents different ettringite formation factors based on the petrological observation results. The experiments conditions including depth of carbonation, mix proportions of concrete, curing temperature and others were considered. The deterioration of the samples seem to be correlated to ASR, except for the precast concrete product which presented DEF. In order to determine the deterioration causes and demonstrate the importance of petrological approach, different observations using the same methods were carried out on a concrete specimen blended with fly ash showing some cracks.

Precursory Signal Detected for the 2018 Sierra Negra Volcanic Eruption, Galápagos, Using Seismic Ambient Noise

AbstractThe changes in physical properties leading up to a volcanic eruption provide clues to processes occurring within volcanoes and may reveal premonitory signals. The Sierra Negra shield volcano, located in the Galápagos Islands, erupted on 26 June 2018 after months of continued inflation and escalating earthquake activity. We applied ambient noise interferometry to five months of continuous seismic data from 12 broadband stations to calculate crustal shear‐wave velocity changes before and during the eruption. Using the Moving Window Cross‐Spectral technique and a pre‐eruption stack of ambient seismic data as reference, we found a −0.27% decrease in velocity 17 days before the eruption in station‐pairs that pass beneath the caldera's north‐eastern sector. Sensitivity kernels suggest that the velocity changes of this precursory signal are greatest at depths corresponding to the shallow sill (∼2 km) beneath the wide caldera. Our results, considered in light of geodetic, seismicity, and petrological results, suggest that this velocity decrease is in part caused by dilatation from a ML 4.8 earthquake, and degassing after a possible magma intrusion below or at the base of the sill. The precursory velocity decrease within the sill region offers an important tool for forecasting future eruptions at Sierra Negra.

Ultrahigh temperature metamorphism and isobaric cooling of Neoarchean ultramafic‐mafic granulites in the southern granulite terrain, India: Phase equilibrium modelling and SHRIMP zircon U–Pb dating

AbstractThis study presents the results of petrology, SHRIMP zircon U–Pb analysis, conventional Fe–Mg geothermobarometry, REE (rare earth element)‐based geothermobarometry, and phase equilibrium modelling, for newly discovered ultramafic‐mafic granulites (garnet pyroxenite with minor plagioclase and garnet‐bearing granulite) in the Namakkal Block of the southern granulite terrain (SGT), India. Results from this study shed light on the growth, evolution, and thermal state of lower continental crust during the Neoarchean, as a cross‐section of the Archean lower continental crust is well‐exposed in the SGT. SHRIMP zircon U–Pb dating of a garnet pyroxenite sample (18ID‐41) yielded a weighted mean 207Pb/206Pb age of 2531 ± 6 Ma, which is the same within uncertainty as one weighted mean SHRIMP metamorphic zircon 207Pb/206Pb age of 2519 ± 9 Ma from mafic granulite sample (18ID‐43). In addition, zircon U–Pb analyses also yielded an apparent 207Pb/206Pb age of 2474 ± 3 Ma (1σ) for the garnet pyroxenite, and a weighted mean 207Pb/206Pb age of 2489 ± 8 Ma for the mafic granulite. Thus, the timing of metamorphism of the ultramafic‐mafic granulites was constrained to be ca. 2530–2470 Ma. The peak metamorphic phase assemblage of the garnet pyroxenite is grt–cpx–pl–ilm–liq, which was modelled to be stable at 1058–1172°C/7.8–11.6 kbar, whereas the retrograde mineral assemblage of grt–cpx–hbl–pl–qz–ilm–liq occurred at 877–888°C/9.9–11 kbar. The mafic granulite was demonstrated to record similar peak and retrograde metamorphic P–T conditions to those of the garnet pyroxenite but have a relatively small P–T range of the peak mineral assemblage. The ultrahigh temperature metamorphism of these ultramafic‐mafic granulites was further confirmed by the temperature results (1057–1087°C) from the REE‐in‐two‐pyroxene thermometer. As a result, a retrograde metamorphic P–T path characterized by near‐isobaric cooling from ~1080°C at ~10 kbar to ~900°C at ~10 kbar was constructed for these rocks, which is consistent with an extremely high thermal state that persisted for ca. 60 Ma in the lower crust.

Rapid response petrology for the opening eruptive phase of the 2021 Cumbre Vieja eruption, La Palma, Canary Islands

How and why magmatic systems reactivate and evolve is a critical question for monitoring and hazard mitigation efforts during initial response and ongoing volcanic crisis management. Here we report the first integrated petrological results and interpretation provided to monitoring authorities during the ongoing eruption of Cumbre Vieja, La Palma, Canary Islands, Spain. The first eruptive products comprised simultaneous Strombolian fountain-fed lava flows and tephra fall from near-continuous eruption plumes. From combined field, petrographic and geochemical analyses conducted in the 10 days following sample collection, we infer low percentage mantle melts with a variably equilibrated multimineralic crystal-cargo and compositional fractionation by winnowing during eruptive processes. Hence ‘rapid response’ petrology can untangle complex magmatic and volcanic processes for this eruption, which combined with further study and methodological improvement can increasingly assist in active decision making.

COMPARATIVE STUDIES OF SOME OF THE ROCKS IN THE SEKONDIAN SERIES – IMPLICATIONS FOR PETRO-MECHANICAL STRENGTH OF THE ROCKS

This study petrologically and mechanically assesses and compares five of the seven stratigraphic units of the Sekondi Group comprising Elmina sandstone from Central region; Ajoa, Takoradi, Takoradi Harbour and Essipong shales from the Western region in Ghana. All the studied shales were detrital clastic sedimentary rocks observed to have angular, near rounded and elongated crystal habits which are randomly distributed within fine-grained clay minerals as cementing matrix. These characters were clearly observed in the Ajua shales than the others. Unlike the shales, no chlorite, organic materials or foliations were observed in the Elmina sandstone. Also, the most consisted minerals in the shales were the feldspars (K- and plagioclase) and quartz, whereby the K-feldspar dominated the other crystals in the sandstone. The finer texture of the shales may be inferred that the shales have undergone longer times and distances of transportation process. The UCS test carried out on the Elmina sandstone reveals it to be a weak rock with a strength value of 37.3 MPa whilst the Schmidt Hammer test carried out on the four shale rock samples define the shales to be delaminated with the average rebound value of zero (0) each. Both results confirm the megascopic and microscopic petrological results, since both revealed the occurrences of laminated sections within the rocks, and the fissile property of all the shales. It also proves megascopic observation of Elmina sandstone being the hardest of the rocks observed, although the sandstone is still relatively weak from the rock hardness classification.

Fluid-inclusion petrography in calcite stalagmites: Implications for entrapment processes

ABSTRACT Fluids trapped in speleothems have an enormous potential in frontier fields of paleoclimate and paleohydrological research. This potential is, however, hampered by diverse scientific and technical limitations, among which the lack of a systematic methodology for genetically characterizing fluid inclusions is a major one, as these can have different origins, and thus, the trapped fluid (usually water), different meanings. In this work, we propose a systematic petrological classification of fluid inclusions, based on: 1) the temporal relation between fluid inclusions and the host calcite, 2) the spatial relation between fluid inclusions and the “crystallites” and crystals aggregates, and 3) the phases (water, air) trapped inside fluid inclusions. The first criterion allows dividing fluid inclusions in two main categories: primary and secondary, whose identification is critical in any research based on trapped fluids. The other two criteria allow the definition of eight types of primary and four types of secondary fluid inclusions. Primary fluid inclusions contain the drip water that fed stalagmites at the time of crystal growth, and can be intercrystalline, i.e., located between adjacent crystallites, or intracrystalline, i.e., with the fluid trapped within crystallites. We differentiate six main types among the intercrystalline fluid inclusions (elongate, thorn-shaped, down-arrow, interbranch, macro-elongate, and bucket) and other two among intracrystalline inclusions (pyriform and boudin). In primary inclusions, water is the main phase, while gas is much less abundant. The presence of gas could be related to slow drip rates or degassing in the cave, but also to later leakage due to changes in temperature and humidity often occurring during inadequate handling of speleothem samples. Secondary fluid inclusions were clearly related to younger water inlet through stratigraphic disruptions or unconformities. They are formed after water infiltration, but sealed before the renewed crystal growth. We differentiate four main types of secondary inclusions: interconnected, rounded, triangular, and vertical fluid inclusions. The identification of primary and secondary fluid inclusions in speleothems is a key for interpretation in paleoclimate studies. Integration of petrological results allow establishment of three different genetic scenarios for the formation of fluid inclusions, whose identification can be relevant because of their predictive character.

An Almahata Sitta EL3 fragment: implications for the complex thermal history of enstatite chondrites

Almahata Sitta is a polymict breccia, consisting of many kinds of clasts. Here we present our mineralogical and petrological results on an EL3 fragment, MS-177 from Almahata Sitta. This fragment shows a typical type 3 chondritic texture, consisting of well-defined chondrules, isolated silicate minerals, and opaque nodules. Most chondrules are enstatite-rich with some having olivine. Although these components are typical of EL3 chondrites, the mineral abundances and compositions are different from the other EL3s. Diopside is unusually abundant in MS-177. On the other hand, perryite and daubreelite were not found. The major pyroxene is orthoenstatite, and the silica phase is quartz. Fe–Ni metal has relatively high P contents. Troilite is enriched in Cr and Mn. Keilite and buseckite are present in MS-177. From the mineralogy and texture, MS-177 experienced a high-temperature event under subsolidus conditions. Shock-induced heating for a short duration might explain this high-temperature event. We suggest that other E3 chondrites also experienced heating events under such subsolidus conditions on their parent bodies. On the other hand, the high abundance of diopside cannot be explained by a secondary thermal event and may have been a primary feature of MS-177, formed before accretion to the parent body.

Identifying deep recycled carbonates through Miocene basalts in the Maguan area, SE Tibetan Plateau

Cenozoic mantle-derived magmas are widespread on the Tibetan Plateau, and provide evidence for the evolution of deep mantle and its influence on the Plateau development. Miocene basalts in the Maguan area on the southeastern Plateau have high MgO (9.13–13.10 wt%) and Mg# (0.60–0.70) with high Ce/Pb (10.6–32.5) and Nb/U (43.7–52.9) ratios, similar to those of oceanic basalts. Distinct from Eocene-Oligocene mantle-derived potassic magmas in Western Yunnan and Cenozoic basic volcanic rocks in Tengchong, these Maguan basalts are characterized by high large ion lithophile elements (LILEs) concentrations, positive anomaly in high field strength elements (HFSEs) and depleted SrNd isotopes, consistent with the melt of an asthenospheric mantle origin. The high Ce/Pb and Nb/Y (1.80–2.68) ratios together with low Ba/Y and Ba/Th ratios indicate a significant input of slab-derived melt into the asthenospheric source. Besides, Hf/Hf* and Ti/Ti* ratios are significantly lower than those of ocean island basalt (OIB), which are proportional to the lighter δ26Mg (-0.6‰ to -0.4‰) values. Based on the results of experimental petrology, the SrMg isotope mixing model suggests that the asthenospheric mantle beneath the Maguan area had undergone the significant metasomatism of recycled carbonates prior to the late Miocene. The above petrological and geochemical understanding, together with the geophysical data, allows us to propose that the mantle metasomatism is most probably associated with the Neo-Tethys seafloor subduction, which is further testified by the decoupling between depleted SrNd isotopes and elevated LILE concentrations.

Biostratigraphic and petrological characteristics of Cretaceous\u2013Paleogene sediments in the eastern Cuu Long delta

The basic structure of reservoir models is strongly affected by the stratigraphic interpretation and the properties of reservoir rock. The estimation of the volume of hydrocarbon accumulation will depend on the accuracy of predicting and distributing reservoir quality. Biostratigraphic and sedimentary petrologic results provide geological information to clarify the stratigraphy and properties of sedimentary rocks on the eastern margin of the Cuu Long delta plain trough. On the basis of stratigraphic correlation of the wells and studying the structures of the area, the deposition, thickness of sediments and the ability to correlate with prospect rocks on the southeastern Vietnam continental shelf were determined. The results have identified the basement rock of wells TC-1 and TC-2, which are Cretaceous metasandstones and were deposited in freshwater fluvial environments with high-energy conditions. The upper Paleogene sediments were overlaid directly on the Cretaceous basement rock in the TC-2 well but were completely absent in the TC-1 well. The lithologic composition is mainly feldspathic litharenite sandstone, which is formed in freshwater fluvial and freshwater lacustrine environments. This study provides new data on stratigraphic column of the eastern Cuu Long delta plain. The data show that the age of the sedimentary basement rock is Cretaceous which has not been determined in previous studies. The findings of this study can help for better understanding of the geological development history and the completely stratigraphic column of the Cuu Long delta to correlate with objects that are hydrocarbon accumulation in Cuu Long sedimentary basin in order to improve efficiency in petroleum exploration activities.

Hydrothermal Origin of Early Permian Chert Nodules in the Central North China Craton Linked to Northern Margin Cratonic Activation

AbstractSedimentary chert phases from the Archean to the present are widely used to trace sedimentary environments and tectonic settings. Recently, chert nodules occurring within carbonates have been the subject of possible hydrothermal or biogenic origin, in lieu of a diagenetic origin. However, chert nodules from a vast cratonic basin represent extremely rich silica accumulations but less noted is how they respond to submarine hydrothermal activity (and/or surface siliceous organism productivity). The links between the cratonic‐type chert depositions and environmental changes regarding cratonic evolution need to be revisited at a large temporal‐spatial scale. The chert nodules are widespread throughout the Lower Permian Taiyuan Formation in the North China Craton (NCC). Several Taiyuan chert‐rich successions across the NCC have been selected to study possible links between chert deposition and cratonic evolution in scenario of partial cratonic activation of the northern NCC margin during the Late Paleozoic. Based on stratigraphic correlation, the chert nodules are ubiquitously, evenly distributed throughout the Taiyuan Formation at a large craton‐basin scale from the northern to southern interior NCC. Petrological results, elemental abundances, together with silicon and oxygen isotopic compositions of chert samples infer significant hydrothermal contributions for the silica accumulations. Therefore, the cratonic‐scale chert depositions of hydrothermal origin infer a giant and remote silica reservoir, linking to the large igneous province and magmatism in the NCC northern margins. The Taiyuan chert nodules could be unique marine sedimentary archives recording the Late Paleozoic NCC partial activation, which also generated continental records of igneous rocks, bauxites and tuffs. The strong tectonics of the northern margin, south‐dipping topography and northward transgression of the Early Permian NCC facilitated the chert deposition on the shallow marine platform in the cratonic interior.