Acta Geologica Sinica ‐ English Edition最新文献

The Tarim Craton is an ancient Precambrian continental block, and detailed knowledge of its thermo-tectonic history is crucial for understanding the early history of continental evolution. Abundant layered mafic rocks, which have commonly been regarded as basalts, occur within the Ediacaran Sugetbrak Formation (Fm.) in the Aksu region of the northwestern Tarim Craton. Clear intrusive features have now been discovered, including mafic rocks truncating Ediacaran sedimentary layers, exhibiting an intrusion-baked margin where they interact with both the overlying and bottom wall rocks, and displaying a fine-grained transition zone from their interior to their margins. The new findings demonstrate that these mafic rocks within the Aksu Ediacaran strata were not erupted basalts but instead are intrusive diabase dykes. Therefore, these mafic rocks cannot be used to constrain the timing of the Sugetbrak Fm. in the Aksu area, nor as marker layers for regional stratigraphic correlation. Furthermore, the Ediacaran thermo-tectonic evolution in this region, deduced from the assumption that the mafic rocks are lavas, needs to be revised.

The Upper Cretaceous Qingshankou Formation black shales, deposited in the late Turonian (LTB shales), are the main source rocks of the Songliao Basin. The origins of organic matter enrichment of the shales is a contentious subject fuelling many ongoing debates. This study investigates the genesis of the organic matter-rich shale by using molecular geochemistry. The LTB shales can be divided into three sections. The Section I shales were deposited in saline, stratified and anoxic water conditions, which are related to seawater incursion events. At least three episodic and periodic seawater incursion events were recognized during Section I shale deposition. The Section II shales deposited in brackish to fresh and deep lake-level conditions with high primary productivity, which are related to lake-level transgression. The Section III shales were deposited under fresh and slightly oxidized water conditions, which are related to lake-level regression. Two organic matter enrichment models for the LTB shales are identified, that is, the seawater incursion model and the maximum lake-level transgression sedimentation model, which act on different shale sections, both playing significant roles in the enrichment of organic matter.

The Jenkyns Event, more widely known as the Toarcian Oceanic Anoxic Event (T-OAE), is marked by globally distributed negative carbon-isotope excursions, widespread oxygen depletion, and large-scale organic carbon burial, which indicate major climate/environmental perturbations in Earth's surface systems during the Early Jurassic. Although extensive research has been conducted in European continental settings, particularly in the western peri-Tethys regions, the impacts of this event beyond Europe remains largely unexplored. Here, a multiapproach study including investigations into the spore-pollen assemblages, pyrite framboids, clay minerals, total organic carbon (TOC) levels, and organic carbon isotope (δ¹³C_org) levels in a lacustrine borehole section (MED1) from the Yin'gen–Ejinaqi Basin, North China, provides evidence of the occurrence of the Jenkyns Event and its extensive sedimentary responses in the eastern Tethys terrestrial systems. Two distinct spore-pollen assemblages have been identified in MED1 (drilling depth: 982.4 m to 1267.5 m), with the Cycadopites–Protopinus–Osmundacidites assemblage in the lower part (1267.5 m to 1132.9 m) indicating a middle Early Jurassic age and the Classopollis assemblage in the upper part (1132.9 m to 985.7 m) suggesting a Toarcian age. Framboidal pyrite data suggest more anoxic conditions during the deposition of black mudstone and shale intercalations in the lower part of the Classopollis assemblage (1132.9 m to 1066.9 m), which combined with organic carbon enrichment and negative δ¹³C_org excursions, are considered the paleoenvironmental response to the Jenkyns Event in the study area. Furthermore, the evolution of vegetation groups changed from plant groups characterized by bisaccate and cycad pollen, as well as fern spores, to vegetation groups represented by Cheirolepidiaceae pollen across the Jenkyns Event, as evidenced by spore-pollen data, together with the clay mineral assemblage change characterized by a notable increase in illite at the expense of kaolinite, suggests that while a subtropical-temperate climate persisted, a change toward warmer and drier conditions most likely occurred in the early Toarcian in the study area. In contrast to the humidification evidenced in many coastal settings, this aridification trend in the Yin'gen–Ejinaqi Basin aligns with the conditions in many inland areas. It is hypothesized that the underlying cause of these divergent changes may be linked to certain patterns of spatially variable water availability on land, potentially driven by extremified hydrological conditions.

Rock-ice avalanches have frequently occurred in the Eastern Himalayan Syntaxis region due to climate change and active tectonic movements. These events commonly trigger catastrophic geohazard chains, including debris flows, river blockages, and floods. This study focuses on the Zelongnong Basin, analyzing the geomorphic and dynamic characteristics of high-altitude disasters. The basin exhibits typical vertical zonation, with disaster sources initiating at elevations exceeding 4000 m and runout distances reaching up to 10 km. The disaster chain movement involves complex dynamic effects, including impact disintegration, soil-rock mixture arching, dynamic erosion, and debris deposition, enhancing understanding of the flow behavior and dynamic characteristics of rock-ice avalanches. The presence of ice significantly increases mobility due to lubrication and frictional melting. In the disaster event of September 10, 2020, the maximum flow velocity and thickness reached 40 m/s and 43 m, respectively. Furthermore, continuous deformation of the Zelongnong glacier moraine was observed, with maximum cumulative deformations of 44.68 m in the distance direction and 25.96 m in the azimuth direction from March 25, 2022, to August 25, 2022. In the future, the risk of rock-ice avalanches in the Eastern Himalayan Syntaxis region will remain extremely high, necessitating a focus on early warning and risk mitigation strategies for such basin disasters.

The Ziyoutun Cu-Au district is located in the Jizhong–Yanbian Metallogenic Belt and possesses excellent prospects. However, the thick regolith and complex tectonic settings present challenges in terms of detecting and decomposition of weak geochemical anomalies. To address this challenge, we initially conducted a comprehensive analysis of 1:10,000-scale soil geochemical data. This analysis included multivariate statistical techniques, such as correlation analysis, R-mode cluster analysis, Q–Q plots and factor analysis. Subsequently, we decomposed the geochemical anomalies, identifying weak anomalies using spectrum-area modeling and local singularity analysis. The results indicate that the assemblage of Au-Cu-Bi-As-Sb represents the mineralization at Ziyoutun. In comparison to conventional methods, spectrum-area modeling and local singularity analysis outperform in terms of identification of anomalies. Ultimately, we considered four specific target areas (AP01, AP02, AP03 and AP04) for future exploration, based on geochemical anomalies and favorable geological factors. Within AP01 and AP02, the geochemical anomalies suggest potential mineralization at depth, whereas in AP03 and AP04 the surface anomalies require additional geological investigation. Consequently, we recommend conducting drilling, following more extensive surface fieldwork, at the first two targets and verifying surface anomalies in the last two targets. We anticipate these findings will significantly enhance future exploration in Ziyoutun.

Hydrous Cr-bearing uvarovite garnets are rare in natural occurrences and belong to the ugrandite series and exist in binary solid solutions with grossular and andradite garnets. Here, we report the occurrence of hydrous uvarovite garnet having Cr₂O₃ upto 19.66 wt% and CaO of 32.12–35.14 wt% in the serpentinized mantle peridotites of Naga Hills Ophiolite (NHO), India. They occur in association with low-Cr diopsides. They are enriched in LILE (Ba, Sr), LREEs, with fractionating LREE-MREE [avg. (La/Sm)_N = 2.16] with flat MREE/HREE patterns [avg. (Sm/Yb)_N = 0.95]. Raman spectra indicate the presence of hydroxyl (OH^–) peaks from 3500 to 3700 cm^–1. Relative abundances in fluid mobile elements and their close association with clinopyroxenes are suggestive of the formation of uvarovite garnets through low temperature metasomatic alteration of low-Cr diopsides by hydrothermal slab fluids. The high LREE concentration and absence of Eu anomaly in the garnet further attest to alkaline nature of the transporting slab dehydrated fluid rather the involvement of low-pH solution. The chemical characteristics of the hydroxyl bearing uvarovite hosted by the mantle peridotite of NHO deviate from the classical features of uvarovite garnet, and their origin is attributed to the fluid-induced metasomatism of the sub arc mantle wedge in a suprasubduction zone regime.

In intracratnoic basins, the effect of strike-slip faults on sedimentary microfacies is generally underestimated due to their small scale. Based on the integration of core, well logs, and three-dimensional seismic data, this study presents a comprehensive analysis of the Permian carbonate platform and strike-slip faults in the southwestern Kaijiang-Liangping trough of the Sichuan Basin. The relationship between strike-slip faults and Permian carbonate microfacies is investigated. The results reveals the existence of a NW-trending strike-slip fault zone along the platform margin, exhibiting clear segmentation. The western side of the study area exhibits a rimmed platform margin characterized by type I reefs, which corresponds to the presence of a large-scale strike-slip fault zone. In contrast, the eastern side is characterized by a no-rimmed and weak rimmed platform margin, accompanied by type II reefs, which align with smaller strike-slip fault zones. It was found that the strike-slip fault had some effects on the platform and reef-shoal complex of the Permain Changxing Formation. First, the platform was divided by strike-slip fault into three segments to show rimmed, week rimmed and no-rimmed platform. Second, reef-shoal complex devolped along the faulted high position in the strike-slip fault zone, and separated by faulted depression. Third, strike-slip faults can offset or migrated the reef-shoal complex and platform margin. Additionally, the thickness of the platform margin varies across strike-slip fault zone, which is related to the activity of strike-slip faults. The strike-slip faults affect the microfacies by controlling the pre-depositional paleotopography. This case suggests that the strike-slip faults play a crucial role in the diversity and distribution of carbonate microfacies in the intracratonic basin.

Magmatic periodicity is recognized in continental arcs worldwide, but the mechanism responsible for punctuated arc magmatism is controversial. Continental arcs in the Trans-Himalayan orogenic system display episodic magmatism and the most voluminous flare-up in this system was in early Eocene during the transition from subduction to collision. The close association of the flare-up with collision is intriguing. Our study employs zircon Lu-Hf and bulk rock Sr-Nd isotopes, along with mineral geochemistry, to track the melt sources of the Nymo intrusive complex and the role of mantle magma during the early Eocene flare-up of the Gangdese arc, Tibet. The Nymo intrusive complex is composed of gabbronorite, diorite, quartz diorite, and granodiorite which define an arc-related calc-alkaline suite. Zircon U-Pb ages reveal that the complex was emplaced between ~50–47 Ma. Zircon Hf isotopes yield ε_Hf(t) values of 8.2–13.1, while whole-rock Sr and Nd isotopes yield ε_Nd(t) values of 2.7–6.5 indicative of magmatism dominated by melting of a juvenile mantle source with only minor crustal assimilation (~15%–25%) as indicated by assimilation and fractional crystallization modeling. Together with published data, the early Eocene magmatic flare-up was likely triggered by slab breakoff of subducted oceanic lithosphere at depths shallower than the overriding plate. The early Eocene magmatic flare-up may have contributed to crustal thickening of the Gangdese arc. This study provides important insights into the magmatic flare-up and its significant role in the generation of large batholiths during the transition from subduction to collision.

Mississippi Valley-type (MVT) Zn-Pb deposits predominantly form within both orogenic forelands and fold-and-thrust belts, yet the mineralization process within the latter tectonic setting remains inadequately understood. This study, through a comprehensive review of MVT deposits across global fold-and-thrust belts, introduces a novel model elucidating the mineralization process in the context of tectonic belt evolution. It is demonstrated that during the stage I, regional compression is introduced by early stages of plate convergence, causing the folding and thrusting and creating structural or lithological traps such as evaporite diapirs and unconformity-related carbonate dissolution-collapse structures. Thereafter, in stage II, hydrocarbons begin to migrate and accumulate within these traps, where reduced sulfur is generated through thermochemical or bacterial sulfate reduction concurrent with or preceding Zn-Pb mineralization. In the subsequent stage III, as plate convergence persists, the regional stress transitions from compression to transpression or extension. Under these conditions, steeply-dipping extensional faults are generated, facilitating the ascent of metalliferous brines into early-formed structural or lithological traps. Precipitation of Zn and Pb sulfides occurs through the mixing of Zn-Pb-transporting fluids with pre-existing reduced sulfur or by interaction with hydrocarbons.