Acta Geologica Sinica ‐ English Edition最新文献

The 7 ka old Qixiangzhan lava flow (QXZ, Tianchi volcano) represents the last eruptive event before the 946 CE, caldera-forming ‘Millennium’ eruption (ME). Petrographic, whole rock, mineral composition, Sr-Nd isotopic data on QXZ show that: (a) the lava consists of two components, constituted by comenditic obsidian fragments immersed in a continuous, aphanitic component; (b) both components have the same geochemical and isotopic variations of the ME magma. The QXZ and ME comendites result from fractional crystallization and crustal assimilation processes. The temperature of the QXZ magma was about 790°C and the depth of the magma reservoir around 7 km, the same values as estimated for ME. QXZ had a viscosity of 10^5.5–10⁹ Pa s and a velocity of 3–10 km/yr. The emplacement time was 0.5–1.6 yr and the flow rate 0.48–1.50 m³/s. These values lie within the range estimated for other rhyolitic flows worldwide. The QXZ lava originated through a mixed explosive–effusive activity with the obsidian resulting from the ascent of undercooling, degassing and the fragmentation of magma along the conduit walls, whereas the aphanitic component testifies to the less undercooled and segregated flow at the center of the conduit. The QXZ lava demonstrates the extensive history of the ME magma chamber.

As one of the most serious geological disasters in deep underground engineering, rockburst has caused a large number of casualties. However, because of the complex relationship between the inducing factors and rockburst intensity, the problem of rockburst intensity prediction has not been well solved until now. In this study, we collect 292 sets of rockburst data including eight parameters, such as the maximum tangential stress of the surrounding rock σ_θ, the uniaxial compressive strength of the rock σ_c, the uniaxial tensile strength of the rock σ_t, and the strain energy storage index W_et, etc. from more than 20 underground projects as training sets and establish two new rockburst prediction models based on the kernel extreme learning machine (KELM) combined with the genetic algorithm (KELM-GA) and cross-entropy method (KELM-CEM). To further verify the effect of the two models, ten sets of rockburst data from Shuangjiangkou Hydropower Station are selected for analysis and the results show that new models are more accurate compared with five traditional empirical criteria, especially the model based on KELM-CEM which has the accuracy rate of 90%. Meanwhile, the results of 10 consecutive runs of the model based on KELM-CEM are almost the same, meaning that the model has good stability and reliability for engineering applications.

We report new SHRIMP zircon U-Pb ages, zircon Lu-Hf isotopic and whole rock geochemical data from Permian granitoids located in the Alxa area of Inner Mongolia, China. In combination with published geochronological and geochemical data, the granitoids in the region can be divided into two age groups: ca. 285 Ma and ca. 269 Ma. The granitoids of the first group are mainly composed of calc-alkaline to high-K calc-alkaline, weakly peraluminous I-type granodiorites with ɛ_Hf(t) values of -19.6 to -4.3, which demonstrates evidence of crustal reworking; the granitoids of the second group, however, mainly consist of A-type granites that are high-K calc-alkaline to shoshonite, metaluminous to weakly peraluminous, and have high 10,000 × Ga/Al ratios (2.59–3.12) and ɛ_Hf(t) values ranging from –11.3 to –2.7, all of which demonstrates a mixed crust-mantle source. We interpret the granitoids of the first group to have formed during the subduction of Central Asian oceanic crust and the second group to have formed by the asthenospheric upwelling caused by the formation of slab windows during late ocean ridge subduction.

The petrogenesis and genetic link to polymetallic mineralization of the granites in the Dayishan complex, southern Hunan province remain debated. Here, we present an integrated study on the petrology, zircon U-Pb ages and whole-rock geochemistry for this complex. Our findings indicate that the southern complex consists of (amphibole-bearing) biotite granites and muscovite granites emplaced at 153–151 Ma, and the central and northern complex consists of two-mica granites and tourmaline-bearing muscovite granites, respectively with the former emplaced at 164 Ma and the latter at 150 Ma. The (amphibole-bearing) biotite granites have SiO₂ contents of 68.0–73.8 wt% and are enriched in alkalis and rare earth elements and depleted in Sr and Ba. They display Zr + Y + Ce + Nb > 350 ppm and 10000 × Ga/Al > 2.6 along with high zircon saturation temperatures (821–883°C). The two-mica granites and (tourmaline-bearing) muscovite granites have high SiO₂ (74.4–77.3 wt%) and low Ga/Al, Zr + Nb + Ce + Y, K/Rb, Zr/Hf, and Nb/Ta along with low zircon saturation temperatures (709–817°C). Geochemical characteristics suggest that the (amphibole-bearing) biotite granites are A-type granites generated through shallow dehydration melting of early Paleozoic granitoids, and that the two-mica granites and (tourmaline-bearing) muscovite granites are fractionated A-type granites produced through fractionation crystallization from the (amphibole-bearing) biotite granites accompanied by fluid fractionation.

Deciphering high-pressure granulite-facies metamorphism and anatexis within a collisional orogeny can provide crucial constraints on geodynamic evolution and melt activity during subduction and exhumation. Combining petrographic observations, mineral chemistry, REE in Grt-Cpx thermobarometry, and previous work, at least four stages are suggested for the metamorphic evolution of the mafic granulites in the South Altun, including the protolith stage, the high-pressure granulite-facies stage (909–1037°C and 17.3–30 kbar), medium-pressure granulite-facies overprint (9.1–11.9 kbar and 753–816°C), and subsequent late amphibolite-greenschist-facies metamorphism. Zircon U-Pb dating shows that the mafic granulites underwent high-pressure granulite-facies metamorphism at 497.2 ± 3.7 Ma, while the leucosome formed at 498.2 ± 2.9 Ma. Thus, the leucosomes from the host mafic granulite may have been formed at the high-pressure granulite-facies metamorphic event. The characteristics of zircon morphology, mineral inclusions, low Th/U values, HREE enrichment, and negative Eu anomalies indicate that these zircons from the leucosome were formed from the metamorphic melts. The characteristics of whole-rock major and trace elements as well as Hf isotopic features of zircons between the leucosomes and the host mafic granulite indicate that the melt may have been generated by the partial melting of the host mafic granulite.

The temporal and spatial evolution of the Ailao Shan–Red River (ASRR) fault zone, which serves as an important accommodation zone for the extrusion and escape of the Southeastern Tibetan Plateau, is crucial for analyzing the uplift and growth of the plateau. Based on the petrology and apatite fission track analysis, the tectonic history and active pattern of the ASRR fault zone since the middle Miocene are determined in this study. The ASRR fault zone exhibits 12–8 Ma and 8–4 Ma rapid cooling phases since the middle Miocene. The 12–8 Ma and 8–4 Ma cooling may imply that the dextral movement of the ASRR fault zone presents a migration trend from northwest to southeast, accompanied by the weakening of the activity intensity, which is directly related to deformation processes, including extrusion boundary migration and active tectonic movements of the southeastern Tibetan Plateau, since the middle–late Miocene.

This study focuses on tuffaceous clastic rocks of the Lower Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin. It aims to explore the influence of sedimentation and parent rock on this kind of reservoir development. The results show that the tuffaceous components formed by the denudation of ultramafic and mafic rocks can transform into chlorite coating or hematite, while those from intermediate rock denudation can be dissolved or transformed into illite. Sedimentary facies and lithofacies are essential in controlling the evolutionary result of tuffaceous components. Matrix-supported medium conglomerate and grain-supported medium-fine conglomerate that developed in the fan delta plain, with a closed original geochemical systems, have been in the oxidizing environment for a long time. The tuffaceous matrices mainly transforms into hematite or illite. These minerals occupy the primary pores and are difficult to dissolve by felsic fluids, which inhibits the development of high-quality reservoirs. The grain-supported sandy fine conglomerate developed in the fan delta front was in the underwater reductive environment with an open original geochemical system. The tuffaceous matrices not only can transform into chlorite coating to strengthen the particle's compaction resistance, but also can be fully dissolved, which promotes the formation of high-quality reservoirs.

The formation of Mesozoic natural gas in the Pengyang area of southwestern Ordos Basin is discussed, from the perspective of microbial community characteristics, in order to clarify the relationship between the origin of natural gas and its associated indigenous microbial community. The types and diversity of indigenous microbial communities associated with the oil reservoir were studied by means of collecting reservoir formation water samples from exploration wells. The indigenous microbial communities in the Chang 8 member of the Yanchang Formation were primarily distributed within Proteobacteria and Firmicutes, including the specific species and genera of Methylobacter, Pseudomonas, Haibacter, Toxobacillus, Acinetobacter and Adura actinomyces. The results of diversity analysis shows that the number of common genes was 5448, while the number of unique genes and information was less. This reflects the fact that the strata in the study area are relatively closed and not invaded by external water sources, which leads to the development of biological community diversity. In conjunction with the analysis of geochemical characteristics of oil and gas reservoirs in this area, this indicates that the study area possesses the necessary geological conditions for microbial degradation. It is the first time that the species and diversity of the indigenous microbial community in the Ordos Basin have been analyzed, showing that microbial degradation is the main cause of natural gas formation here, changes the characteristics of crude oil in this area and provides first-hand information on the impact of indigenous microorganisms on the reservoir.

The Southwest Tianshan is the suture zone between the Central Tianshan and the Tarim Craton. To better illustrate the subduction polarity of the Southwest Tianshan Ocean, a systematic detrital zircon LA-ICP-MS U-Pb geochronology of sedimentary rocks of the Devonian Apadaerkang Formation and the Carboniferous Akeqiayi Group of the Biedieli area in the Wushi region is the focus. Detrital zircon ages indicate that the youngest grains of the targeted quartz sandstones are Late Devonian and Early Carboniferous in age, respectively. These ages also have relatively similar age spectra, indicating a similar tectonic setting and source in the Biedieli area during the Devonian to Carboniferous. The main spectrum peaks occur at 2360, 1960, 810, 640 and 440 Ma, with a principal age peak in the early Paleozoic (474–430 Ma), and the Central Tianshan Terrane is indicated as the main source. The detrital zircon geochronology and sedimentary characteristics of the Biedieli rocks indicate that the Paleozoic sedimentary rocks in the northern Wushi region were formed at an active continental margin, associated mainly with the early Paleozoic subduction of the South Tianshan Ocean.

Black soil is essential for maintaining regional food security and promoting global agricultural production. Understanding the weathering process of parent material and the accumulation of organic carbon is crucial to comprehending the developmental history and future trends of black soil, especially against the background of large-scale global cultivation and climate change. Although the importance of black soil formation and evolution cannot be ignored, the relevant research is still very scarce. In this study, a typical eight-meter-deep soil core was collected from the Keshan area of the Songnen Plain, Northeast China, where surface black soil developed on paleo-sediments. Using ¹⁴C dating, the formation age of the black soil was determined. Based on the characteristics of the geochemical composition, grain size and the magnetic susceptibility of the sediments, it was demonstrated that the black soil and its parent material originated from reworked loess. Furthermore, the mass transfer coefficient (τ) of some elements was determined, in order to explore the soil weathering process. By calculating the transported amount of alkaline and alkaline-earth elements, the weathering rate of parent material to black soil was found to be weak, at 0.16 kEq·ha^–1·year^–1. Combining the results of dating and carbon density in the different layers of black soil, the accumulation rate of organic carbon was determined as follows: rapidly increasing in the initial period of 13.2–2.2 ka, reaching its maximum average value of 34.0 g·cm^–2·a^–1 at 2.2–0.8 ka, then showing a decreasing trend with an average value of -77.5 g·cm^–2·a^–1. Compared with regional climate change, Keshan black soil has developed under a colder and wetter climate during the Holocene. Predictably, ongoing global warming may lead to the degradation of black soils in the Songnen Plain, as well as in other regions. Our results will enrich geological knowledge of black soil formation and future evolutionary trends.