Acta Geologica Sinica ‐ English Edition最新文献

The origin of tight reservoirs in the Yanchang Formation of the Ordos Basin and their relationship with hydrocarbon charging remain unclear. Based on petrological observations, physical property analysis, fluid inclusion system analysis and in situ U-Pb dating, the sequence of tight sandstone reservoir densification and oil charging was determined. Through petrological observations, fluid inclusion analysis and physical property analysis, it is concluded that compaction and cementation are the primary causes of reservoir densification. When the content of calcite cement is less than or equal to 7%, compaction dominates densification; otherwise, cementation becomes more significant. However, determining the exact timing of compaction densification proved challenging. Microscopic observations revealed that oil charging likely occurred either before or during the densification of the reservoir. According to in situ U-Pb dating and the porosity evolution curve, cementation densification occurred between 167.0 ± 20.0 Ma and 151.8 Ma. Temperature measurements of the aqueous inclusions indicate that oil charging occurred between 125.0 and 96.0 Ma, suggesting that densification preceded oil charging. This study provides valuable insights for the future exploration of tight oil reservoirs in the Ordos Basin.

Compared to discrete continental marginal basins, the mechanisms of hydrocarbon migration and enrichment in transform continental marginal basins are poorly understood. In this study, we conducted a comprehensive analysis of the main source rocks, reservoirs, and vertical migration pathways within the Rovuma (RB) and Tanzania (TB) basins in East Africa utilizing drilling, logging, seismic, and geochemical data. The results indicate that the enhanced preservation conditions of the Lower Jurassic source rocks in the southwest could lead to the discovery of large natural gas fields in the southern TB and RB. The primary reservoir is a deep-water turbidite sandstone. Due to topographic differences, the expanse of the turbidite sandstones in the RB is significantly larger than those in Tanzania. The main vertical migration pathways are the western boundary fault zone of the Kerimbas Graben (WBFZ) and the Seagap fault zone (SFZ). In the RB, natural gas migrates vertically along the WBFZ and preferentially accumulates in the deep-water turbidite sandstones of the footwall under the control of the fluid potential. Conversely, in the southern TB, the deep natural gas first migrates upward along the SFZ, then moves along the shallow branch faults in the sandstones on both sides of the SFZ.

The North China Craton (NCC) experienced significant lithospheric thinning of over 100 km during the Mesozoic, accompanied by extensive magmatic activity and extensional tectonics. However, the timing and mechanism of this thinning remain the subjects of debate. This study presents zircon U-Pb ages, Hf isotopic data and whole-rock elemental and Sr-Nd isotopic compositions of the Guanshui monzonites and diorites in the eastern NCC. Zircon U-Pb dating reveals that both rock types formed at ca. 130 Ma. The monzonites, characterized by high Mg^# (50.9–57.9), low Nb/U ratios (2.53–3.89) and depleted isotopic compositions, suggest derivation from asthenospheric mantle modified by slab-derived fluids. The diorites, distinguished by low SiO₂ (49.5–50.8), high Mg^# (66.7–68.5) and an EM2-type enriched mantle isotopic signature, point to a lithospheric mantle source modified by subducted sediment melts. The coexistence of monzonites and diorites suggests a transition in magma source from lithospheric to asthenospheric mantle, implying that lithospheric thinning may have commenced around 130 Ma. The destruction of the NCC was likely driven by localized, small-scale drip-style detachment processes, rather than wholesale lithospheric removal.

The Qinghai–Tibet Plateau possesses the thickest continental crust on Earth, yet the timing of its formation remains debated. In this study, we conducted zircon U-Pb isotopic dating, geochemical and Sr-Nd-Pb-Hf isotopic analyses on the Xuexiumaer biotite quartz monzonite porphyry (BQMP) sampled from the Lake Dajia area in southern Gangdese. This study aims to estimate the paleo-crustal thickness beneath this region during the early India-Asia collision stage using whole-rock Sr/Y and (La/Yb)_N ratios as proxies. Results reveal that the Xuexiumaer BQMP was formed at ~51 Ma in a collisional tectonic setting following Neo-Tethyan slab breakoff, and is an I-type granitoid derived primarily from partial melting of juvenile mafic lower crust with subordinate ancient crustal input. The estimated paleo-crustal thickness in the Lake Dajia area at ~51 Ma is less than 40 km. This indicates that although the Qinghai–Tibet Plateau had already undergone significant crustal thickening and attained an exceptionally thick crust (>50 km) prior to the India–Asia collision as demonstrated by previous studies, some regions still maintained a crust only slightly thicker than the average continental crust (~35 km) at the initial collision stage. This limited crustal thickening likely resulted from underplating of subduction-related mafic magma at the mantle-crust boundary.

Multiple instances of the Late Cretaceous granodiorites within the Anglonggangri region of the northwestern Lhasa Block were identified, their petrogenesis were explored and mineralization potential were assessed. The zircon U-Pb dating of the Anglonggangri granodiorites revealed ages of 82.8 and 80.8 Ma. Granodiorite samples have SiO₂ contents of 64.36–68.33 wt%, with high Sr/Y (55–95) and A/CNK ratios (0.99–1.06). Zircon ε_Hf(t) values range from –0.3 to +16.2. Two granodiorite samples have (⁸⁷Sr/⁸⁶Sr)_i values of 0.7034 and 0.7043 and positive ε_Nd(t) values of 3.51 and 3.83. These geochemical properties indicate that they are adakitic rocks formed by partial melting of the juvenile thickened lower crust, slightly contaminated with material from the mantle due to the small-scale delamination of the lower crust. The zircons in the granodiorites have moderate Ce/Nd (2.5–43), logfO₂ (–20.0 to –9.6), and ΔFMQ (–1.28 to +4.00) values; low (Ce/Nd)/Y (0.001–0.049) ratios; and high Dy/Yb (0.17–1.16) ratios, which indicate that these granodiorites exhibit moderate oxygen fugacity and lower magma water content than the Miocene Gangdese porphyry copper deposits associated with high-Sr/Y granites. Their ability to create porphyry-type copper deposits could have been hampered by their low magma water content and moderate oxygen fugacity.

The Suzhou granitic pluton is the first identified Nb-Ta-rich granite in China. To reveal the genetic link between the sequence of magmatic and hydrothermal evolution and Nb-Ta mineralization in different intrusive phases of the Suzhou granite, whole-rock geochemistry, geochemistry and U-Th-Pb dating of monazite was analyzed. The unique geochemical characteristics show that the Suzhou pluton can be discriminated as an A-type granite. LA-ICP-MS U-Th-Pb dating of monazite in both the medium- and coarse-grained biotite granite (MBG) and the fine-grained biotite granite (FBG) indicates that the granite formed between 124 and 127 Ma. Based on geochemical characteristics and mineral textures, the MBG (Mnz-Ia) and FBG (Mnz-Ib) monazites are classified as magmatic monazites; another monazite (Mnz-II) from the MBG formed during a magmatic-hydrothermal transitional stage. Nb-Ta in the Suzhou pluton gradually concentrated during fractional crystallization and alteration of Ti-rich minerals and biotite. Ultimately, with the involvement of F-Li-rich fluid, Nb-Ta mineralization occurred during the magmatic–hydrothermal transition. The Suzhou pluton is considered part of a 600-km- and NE–SW-trending Nb-rich A-type granite belt together with other Early Cretaceous A-type granites in the Jiangnan Orogen that offers prospects of a new target for Nb-Ta prospecting.

This study systematically investigates the concentration of ²²²Rn in geothermal fluids and the distribution of geothermal radon mineral water in Shandong Province, with the aim of elucidating formation mechanisms and influencing factors. The findings indicate that the overall abundance of ²²²Rn in geothermal fluids across the region is relatively low. Geothermal radon mineral water is primarily located within banded thermal reservoirs associated with bedrock fracture structures in the Ludong and Luxi uplift geothermal zones. The study reveals that the ionic composition, radioactivity intensity, and extent of water-rock interactions exert only effects in the concentration of ²²²Rn in geothermal fluids. The formation of geothermal radon mineral water is predominantly governed by “fracture-controlled” mechanisms, with thermal reservoir lithology, fracture tectonics, and seismic activity serving as key determinants. Additionally, the enrichment of ²²²Rn in geothermal fluids is influenced by factors such as geothermal fluid temperature, depth of occurrence, cap rock thickness, and alteration processes. The genetic mechanisms of geothermal radon mineral water can be categorized into two types: ‘native’ and ‘composite’. These findings provide critical insights into the exploration and development of geothermal radon mineral water resources in Shandong and similar regions.

The Jinchuan magmatic Ni-Cu-PGE deposit is the largest single Ni sulfide deposit in the world. It consists primarily of orebody-24 in segment I, and orebody-1 and orebody-2 in segment II. The contents of platinum-group elements (PGE) in these orebodies decrease significantly from west to east across the deposit. However, the PGE characteristics of platinum-group minerals (PGM) and alloys, as well as their roles during mineralization in different orebodies, remain unclear. In this study, PGM and alloy occurrences in orebody-24 and orebody-2 had been observed by scanning electron microscopy (SEM) and spherical-aberration corrected scanning transmission electron microscope (Cs-STEM). The PGE contents were analyzed by SEM–EDS for submicron-scale PGM and alloys. The results show that olivine in lherzolite mainly hosts PGM of PtTe and PdBi₂, whereas pyroxene contains small amounts of PdBiTe. Pyrrhotite predominantly hosts (Ir-Rh-Pt)AsS, Pd(BiTe), and other PGM, as well as PtSn and PtOs alloys. Pentlandite mainly encloses Pd(BiTe), PdBi/PdBi₂ and other PGM, as well as PtSn and PtFe alloys. Chalcopyrite primarily encloses PdBi/PdBi₂, and other PGM, along with PtSn and IrOs. Distinct distribution patterns of PGE in PGM and alloys had been observed between different orebodies. Orebody-24 contains more (Ir-Rh-Pt)AsS minerals and PtFe/PtSn alloy grains, whereas orebody-2 has a higher proportion of Pt- and Pd-bearing PGM. The presence of euhedral alloys in silicate minerals from orebody-24 suggests that its parent magma had a higher PGE content before sulfide saturation than that of orebody-2. More than 90% of PGM and alloys in both orebody-24 and orebody-2 contain Pt and Pd, emphasizing their contributions to the elevated Pt and Pd concentrations. The different PGE distributions of PGM and alloys in the two orebodies suggest that thermodynamic conditions (fO₂ and fS₂) and semimetals, especially As, play critical roles in controlling PGE behavior and occurrence.

The Litang fault zone is an important seismogenic structure along the southeastern margin of the Tibetan Plateau. It caused the M7¼ earthquake in Litang in 1948 AD. The fault zone intersects the Sichuan–Tibet transportation corridor and poses a serious risk to its safe operation. This study, utilizing high-resolution remote sensing interpretation, field geological verification, UAV photogrammetry, UAV LiDAR, paleoearthquake trench excavation, and AMS ¹⁴C and OSL dating methods, reveals the geometric structure, slip rates, paleoearthquake sequence, and earthquake rupture segmentation of the Litang fault zone; analyzes the rupture distribution range of the 1729 AD Litang earthquake and estimates its magnitude. The study indicates that the Litang fault zone is a relatively immature strike-slip fault, which has developed as a new active fault zone within the Northwestern Sichuan sub-block during the southeastward material migration of the southeastern margin of the Tibetan Plateau. This reflects a transformation in the deformation model of the Northwestern Sichuan sub-block crust from the ‘Rigid Block’ model to the ‘Continuous Deformation’ model.