Acta Geologica Sinica ‐ English Edition最新文献

Geodynamic processes following the Indo–Eurasian plate collision remain a key research focus, and the Jinshajiang–Red River tectonic zone (JRTZ), situated along this collision boundary, provides critical insights into post-collision tectonic evolution. In this study, we identify a lithological assemblage in the JRTZ, including amphibolite, granite gneiss, and migmatite. These rocks exhibit contrasting geochemical signatures, reflecting multiple source regions: asthenospheric mantle, lithospheric mantle, mafic lower and upper crust. Specifically, amphibolite (28.5 Ma) formed through the partial melting of OIB-like mantle source, whereas S-type granite gneiss (28.2 Ma) originated from the dehydration melting of metamorphosed sedimentary rocks. Amphibole monzonite (28.9 Ma) records the mixing of ancient crustal material with mantle-derived components, while migmatite (37.9 Ma) resulted from deep melting processes of metasedimentary rocks under shear conditions. We propose that the ongoing Indo–Eurasian convergence progressively thickened the crust, ultimately driving large-scale lithospheric delamination between the Eocene and Oligocene. This delamination triggered asthenospheric upwelling, which provided the thermal input required for widespread melting. This lithospheric delamination event started around 38–37 Ma and lasted at least until 28 Ma.

Better understanding of shoshonitic rocks is vital to unravel the formation process and spatial distribution of their associated ore deposits. Here, we conducted analyses on the shoshonitic granodiorite and its mafic microgranular enclaves (MMEs) from the Chadi Cu-Pb-Zn polymetallic deposit (South China), with the aim to investigate their petrogenesis and tectonic setting. Zircon U-Pb age of the MMEs (165.0 ± 1.2 Ma) is coeval with that of the host granodiorite (164.8 ± 0.63 Ma). The Chadi granitoids are enriched in large ion lithophile elements and light rare earth elements, but depleted in high-field-strength elements. The granodiorite displays low (⁸⁷Sr/⁸⁶Sr)_i (0.7069–0.7072), and negative ε_Nd(t) (–5.8 to –5.5) and zircon ε_Hf(t) (–3.6 to –0.4) values. These isotopic characteristics of the granodiorite and MMEs indicate the mixing of a mafic magma (formed from the subduction-related, metasomatically-enriched lithospheric mantle) and a felsic magma (formed from the partial melting of crustal materials), which is closely related to the Paleo-Pacific subduction. The Chadi granodioritic magma has likely low oxygen fugacity (<ΔFMQ + 1), low whole-rock Sr/Y ratio (mostly < 30), and low S (0.04 ± 0.02 wt%) and Cl (0.23 ± 0.04 wt%) contents, suggesting that the potential of forming large-scale Cu mineralization is low.

Weijia Guyot, located in the western Pacific Ocean, has become a research focus due to its abundant cobalt-rich ferromanganese (Fe-Mn) crusts. While most studies on Fe-Mn crusts on seamounts have focused on the exposed variety, less attention has been paid to potential buried crusts. This study presents a preliminary geochemical and chronological study of buried Fe-Mn crusts at Weijia Guyot. The findings suggest that these buried crusts began to form around 57.5 Ma and ceased growing at approximately 46.3 Ma. Following the formation of Weijia Guyot through volcanic eruption, it did not experience continuous and steady subsidence to its current depth. Instead, an exhumation process took place from deep to shallow depths between 46.3 and 11.6 Ma. This process brought the Fe-Mn crusts into shallow water environments, halting their growth. During this time, Weijia Guyot was located near the equatorial Pacific Ocean and was exposed to an extended period of phosphatization. This exposure led to a depletion of key metallogenic elements, such as Co, Ni and Cu, within the Fe-Mn crusts, while P₂O₅ and CaO levels increased significantly. Since the Middle Miocene, the crusts have been progressively buried by pelagic sediments.

A systematic study of early Paleozoic S-type granites in Pinghe enhances our understanding of the tectonic evolution of proto-Tethys and provides a foundation for exploring rare metal deposits in the region. The Pinghe granites consist of monzogranite and leucogranite. Zircon U-Pb dating shows that the emplacement ages of the monzogranite and leucogranite are 502.0 Ma and 500.9 Ma, respectively. All samples have high SiO₂ content and a weakly to strongly peraluminous character (A/CNK = 1.08–1.23), consistent with S-type granites. The monzogranite has relatively high CaO, Sr, Ba, and CaO/Na₂O ratios but lower Rb. In contrast, the leucogranite has lower CaO, Sr, Ba, and CaO/Na₂O ratios but higher Rb. The similar ε_Nd(t) values (–9.3 to –8.4) and Pb isotopic compositions ((²⁰⁶Pb/²⁰⁴Pb)_t = 18.03–19.36, (²⁰⁷Pb/²⁰⁴Pb)_t = 15.66–15.76, (²⁰⁸Pb/²⁰⁴Pb)_t = 37.97–38.55) suggest that the monzogranite formed through partial melting of crustal greywacke, while the leucogranite originated from partial melting of crustal pelite. Regional geological studies suggest that these S-type granites in Pinghe were emplaced in an active continental margin setting, associated with the westward subduction of the proto-Tethys oceanic slab. The geochemical characteristics of leucogranite are consistent with those of tungsten-tin-related granites, indicating significant metallogenic potential for W and Sn deposits.

The lower Yarlung Tsangpo River basin of the Qinghai–Tibet Plateau frequently experiences geo-hazardous occurrences such as landslides, ice/rock avalanches and debris flows, causing loss of human lives and damage to infrastructure. However, a comprehensive inventory map of geohazards is lacking for this region, due to the extreme challenges of the geomorphological and environmental conditions (i.e., steep terrain, dense vegetation cover, and the presence of ice and snow). To this end, we propose a novel approach for mapping active geohazards in complex mountainous regions through InSAR phase gradient measurements based on a deep learning algorithm, which is then applied to the lower Yarlung Tsangpo River basin for the first time, in order to prepare an inventory map of active geohazards using ascending and descending Sentinel-1 SAR images acquired between March 2017 and July 2023. First, the InSAR phase gradient stacking method was introduced to estimate ground deformation, which offers significant advantages in minimizing the influence of InSAR decorrelation and effectively suppressing topographic residuals and atmospheric delays. InSAR phase gradient rates effectively retrieve patterns of localized ground deformation associated with geohazard activity. Then, a DeepLabv3 deep learning model was established and trained with phase gradient rate maps of manually labeled geohazards, in order to achieve the automatic identification of active geohazards. Our results show that there are 277 active geohazards within the lower Yarlung Tsangpo River basin, encompassing an area of ~25600 km². The DeepLabv3 model achieved good precision, recall rate and F1 scores at 92, 86 and 90%, respectively. The distribution of detected geohazards is closely correlated with the topographic factors, faults and river system. Compared to the results derived from Small Baseline Subset InSAR (SBAS-InSAR) and optical images, the proposed approach can obtain high density pixels of InSAR measurement in low-coherence scenarios, thus enabling high-accuracy mapping of active geohazards in complex mountainous areas.

The Xingyuan large fluorite deposit in Fengning, Hebei Province, China, is a significant deposit in the North Hebei–West Liaoning fluorite mineralization belt. The ore bodies are structurally controlled and occur in fault zones near granitic porphyry veins. Previous studies have focused on the geology and ore-controlling factors, whereas the general features of the hydrothermal systems that contributed to the formation of the deposit remain unclear. This study investigated the nature, origin, and evolution of mineralizing fluids in the Fengning deposit, based on fluid inclusion and H-O isotope data. The fluid inclusions in fluorite are mostly H₂O-rich, gas–liquid, two-phase inclusions, along with a few three-phase inclusions containing halite daughter crystals and CO₂ gas. The ore-forming fluid was variable in homogenization temperature (108–388°C), salinity (0.2–47.4 wt% NaCl equivalent), and density (0.58–1.11 g/cm³), which indicate it was a H₂O-NaCl-CO₂ system of moderate–low temperature, low salinity, and low density. Fluorite H-O isotopes (δD_V-SMOW = –123.5‰ to –111.8‰; δ¹⁸O_V-SMOW = –10.3‰ to –6.5‰), temperature data, and fluid compositions indicate the mineralizing fluid was initially dominated by magmatic waters, but then experienced a large influx of meteoric waters. The fluid temperature and salinity decreased and the density increased from the early to late stages of mineralization. The main mechanisms of fluorite precipitation were water–rock reactions and fluid cooling. The Xingyuan fluorite deposit is a post-magmatic hydrothermal deposit.

The Shanan sag in the central–western Bohai Bay Basin hosts high-quality Paleogene source rocks within the Shahejie Formation's third member (E₂s₃). Despite hydrocarbon indications in Cenozoic strata, no commercial accumulations have been discovered. An integrated approach combining geochemical analysis, fluid inclusion thermometry, apatite fission-track (AFT) thermochronology, and basin modeling was employed to unravel the paleogeothermal regime and hydrocarbon generation history of E₂s₃ source rocks. AFT data from the Shahejie Formation's second member (E₂s₂) reveal a tectonothermal event at 25 Ma that accelerated E₂s₃ maturation. Outside three sub-sag depocenters, current E₂s₂ reservoir temperatures remain below the 25 Ma paleo-geothermal maxima despite subsequent Neogene burial. Hydrocarbon-bearing brine inclusions in E₂s₂ reservoirs exhibit peak homogenization temperatures (Th) at 25 Ma, with minimal high-temperature signals, indicating that E₂s₃ hydrocarbon generation peaked during the Paleogene thermal event, with limited late-stage accumulation. The regional effects of the Dongying Movement necessitate thick Neogene sedimentation to compensate for the 25 Ma paleo-geothermal anomaly. Our findings emphasize targeting Neogene depocenters in petroleum exploration to mitigate the inhibitory effects of high paleo-heat flow on late hydrocarbon generation, thereby enhancing current accumulation potential.

Geochemical surveys are essential for understanding the spatial distribution of ore-forming elements. However, these surveys often involve compositional data, the weight concentrations, which do not meet the requirements of statistical methods due to the closure effect. In this study, we applied an integrated approach combining compositional data, multifractal, and multivariate statistical analyses to identify the nonlinear complexity of the spatial distributions of elemental concentrations in the Er'renshan ore field. Initially, the raw concentrations were transformed into log-ratios following the principles of composition data theory to alleviate the impact of the closure effect. Multifractal analysis was then conducted to characterise the nonlinear complexity of the concentration distributions. Furthermore, principal component analysis (PCA) and factor analysis (FA) were applied to identify spurious correlations and the potential factors controlling the distribution patterns. The results demonstrate that: a) the raw data are biased, while the log-ratio data are unbiased and more reliable; b) the spatial distributions of elemental concentrations exhibit nonlinear complexity; and c) the elemental distribution in the study area is largely controlled by structural factors.

The Disuga Cu deposit, located in the eastern porphyry belt of the Zhongdian arc, southwest China, provides a window into magmatic–hydrothermal processes controlling porphyry Cu mineralization. Based on zircon U-Pb geochronology, hydrothermal mineral chemistry, short-wave infrared spectroscopy, and mass balance modeling, this study investigated the alteration zonation and element mobility in the Disuga Cu deposit. Zircon U-Pb ages of the ore-hosting quartz dioritic porphyries (222.4 ± 3.1 and 219.3 ± 2.4 Ma) are similar to those of Late Triassic subduction-related magmatism. High zircon-crystallization temperatures (727 ± 26°C) and elevated oxygen fugacity (ΔFMQ + 2.0) confirm these porphyries were favorable for mineralization. Hydrothermal sericite (Si = 6.49 atoms per formula unit [apfu]; Al^VI = 3.39 apfu) and chlorite (Fe/(Fe + Mg) = 0.59–0.63) compositions indicate an acidic reduced fluid. Three distinct hydrothermal stages were identified: (1) phyllic alteration (370°C); (2) propylitic alteration (315°C); and (3) low-temperature hydrothermal alteration (242°C). Mass balance calculations show that the Cu migration rate (155.6%/114.4%) in the propylitic/phyllic alteration zones was higher than that of Mo (14.3%; limited to the propylitic alteration zone). The alteration mineralization assemblages indicate the occurrence of deep potassic alteration zones and porphyry Cu-(Mo) mineralization in the Disuga area.

The Precambrian Homrit–Waggat granite is a post-orogenic batholithic intrusion located in the northern region of the Nubian Shield, characterized by a typical annular morphology and significant secondary alteration. This study aims to elucidate the processes that have shaped the intrusion in both macroscopic and microscopic perspectives, employing a combination of field observation and petrographic analysis alongside major and trace element compositions of minerals. Within the central region of the pluton, biotite and amphibole are observed sporadically, while the predominant crystallization of anhydrous oligoclase in the outer regions has led to a progressive increase in volatile components within the residual melt, ultimately resulting in a volatile-saturated aluminosilicate melt. The exsolved fluids subsequently interacted with the previously crystallized mineral assemblage, producing metasomatic overprinting. As the cooling and crystallization continued, the water pressure within the magma chamber gradually escalated until it equaled or surpassed the confining pressure, leading to the formation of fractures and veins filled with minerals that crystallized from the residual aqueous fluids. The ongoing degassing and expulsion of aqueous fluids from the magma chamber's interior ultimately contributed to the collapse of the chamber's roof, resulting in the annular ring-dike morphology observed in the Homrit Waggat pluton.