Geological Journal最新文献

The ductile shear deformation of Precambrian basement rocks in Wuyishan provides a crucial perspective on intraplate orogeny in the South China Block (SCB). This study focuses on the Longquan-Badu ductile shear zone in southeastern Zhejiang, employing field observations, thin section analysis, quartz electron backscatter diffraction (EBSD), zircon U–Pb dating and ⁴⁰Ar/³⁹Ar geochronology. Two distinct phases of deformation, referred to as D1 and D2, have been identified. D1 is primarily characterized by a WNW–ESE striking foliation within a NE-plunging lineation, indicating top-to-SSW shearing. The paragneiss within the Badu complex that experienced D1 deformation has been dated to 247–239 Ma through zircon U–Pb analysis, corresponding to the prevalent high-pressure metamorphic age in the region. This correlation suggests that the D1 deformation event coincided with crustal thickening during the Early Triassic. D2 deformation exhibits folds, foliation, S-C fabrics and mylonitic microstructures and is mainly characterized by striking NNE–SSW with steeply dip, demonstrating a dominant dextral strike–slip component. Quartz c-axis orientations in mylonitic rocks indicate deformation temperatures between 350°C and 550°C with asymmetric girdle patterns suggesting simultaneous basal and prism slip. The plateau ages of muscovite from mylonitic rocks obtained through ⁴⁰Ar/³⁹Ar dating are approximately ~228 Ma implying that the D2 deformation occurred under retrograde amphibolite to greenschist facies metamorphic conditions during Middle Triassic. Collectively these data along with regional geological evidence signify two distinct intracontinental orogenic processes occurring in eastern SCB. Considering Early Mesozoic tectonothermal events in Cathaysia Block, it can be inferred that intraplate orogeny in Wuyishan resulted from plate-margin collisions between SCB and peripheral plates following scissors closure of Palaeo-Tethys from east to west.

The present investigation breaks new ground by examining the Raniganj sediments in the Kendudihi section of the Ib-River Coalfield, Odisha, India. The study identifies a megaflora consisting of 25 species of Glossopteris, Vertebraria indica, stem casts and equisetaceous stems. The microflora is predominantly composed of Striatopodocarpites spp., with a secondary presence of Densipollenites spp. The lithological signatures, including off-white fine-grained sandstone with thin bands of silty shale and grey shale, indicate that these sediments belong to the Late Permian succession of the Lower Gondwana sequence, specifically the Raniganj Formation. The macrofloral assemblage found in the lowermost grey shale can be attributed to the Wordian–Capitanian age, while the microfloral assemblage in the upper silty shale resembles that of the Wuchiapingian-Changhsingian age. Well-preserved palynomorphs and megafossils, along with the abundant occurrence of lath-shaped translucent phytoclasts in the grey and silty shale of the Raniganj sediments exposed in the Kendudihi section, explicitly suggest that the sediments were deposited in proximal, low-energy swampy settings. Additionally, the moderate occurrence of charcoal (20%), along with the existence of degraded organic matter (DOM: 7.6%) and amorphous organic matter (AOM: 16.4%), indicates that the sediments might have been deposited in oxic–dysoxic conditions. The palynological and megafloral studies reveal a warm-temperate climate with low humidity and intermittent spells of hot and cold seasons, associated with abundant rainfall. The occurrence of phosphorite in the form of nodules and thin lenses, as well as biogenic structures at the juncture of the Raniganj and Barren Measures formations, suggests a marine incursion in the area during the deposition of the late Barren Measures and early Raniganj sediments. X-ray diffraction (XRD) analysis identified fluorapatite (Ca₅(PO₄)₃F) as the predominant phosphatic mineral phase in the phosphatic nodule, siltstone and claystone. The Post-Archean Australian Shale composition, normalized rare earth element (REE) patterns of samples from this area, reveals slight positive La (average La anomaly: 1.02) and Gd (average Gd anomaly: 1.05) anomalies and heavy REE enrichment compared to light REE, explicitly indicating a marine environment.

Cobalt is a critical and strategic metal mainly found as associated element in several types of deposits, of which skarn-type deposits are the major sources. Han-Xing type skarn iron deposit, having high grade iron ore and associated cobalt, is a typical skarn-type iron ore in China. But the complete recovery and exploitation of cobalt are restricted because of the lower grade of related cobalt and the dearth of prior research on its occurrence condition and enrichment mechanism. In this paper, pyrite from five typical ore deposits in the Han-Xing area was studied by using electron probe microanalysis (EPMA) and laser-ablation inductively-coupled-plasma mass-spectrometry (LA–ICP–MS) techniques to decipher the occurrence state and enrichment mechanism of associated cobalt in skarn-type iron deposits. The results show that Co²⁺ replaces Fe²⁺ in pyrite through isomorphism. The distribution of cobalt in pyrite from different deposits varies greatly, that is, in the Xishimen iron deposit, the cobalt content is comparatively enriched in the pyrite's core. In contrast, in other deposits, the cobalt content is concentrated in the pyrite's rims, where it can be up to 1000 times higher than in the core. The cobalt mineralization in Han-Xing area can be divided into several stages. The sulphur element of sulphide is mainly derived from evaporite, while cobalt mineralization occurred in the early stage with pyrite formation or in the late stage by metasomatism/cementation of Co-rich ore-forming fluid. The magma assimilated with the Ordovician evaporite not only promoted iron mineralization, but also became the main controlling factor for cobalt enrichment.

The Salina Cruz and Puerto Ángel beach areas in the Gulf of Tehuantepec, Mexican Pacific coast represent an important economic sector of the region. In this study, the mineralogy and geochemistry of bulk sediments, and geochronology of 400 detrital zircons recovered from the beach sediments were analysed to investigate their origin. The sediments are abundant in quartz, feldspar, ilmenite, cordierite, aragonite and anorthite. The chemical index of weathering revealed a moderate to intense weathering in the source area. The chondrite normalized REE patterns of bulk sediments are similar to the found in the Upper Continental Crust, suggesting the derivation of sediments from felsic igneous rocks. The REE patterns of zircons and the trace elemental ratios reveal a continental crust origin. Zircon U–Pb ages in the Salina Cruz beach were represented by Proterozoic (~545.1–1314.1 Ma; n = 170) and Cenozoic (~0.01–66 Ma; n = 20). The Puerto Ángel beach was abundant in Proterozoic zircon grains (~600.9–1171.4 Ma; n = 109) and followed by Mesozoic grains (~73.78–246.9 Ma; n = 40). The comparison of zircon U–Pb ages of this study with probable source rocks reveals that the Oaxaquia Terrane and Chiapas Massif Complex were the major contributors of Proterozoic zircons to the coastal areas. Similarly, the results indicate that the Cenozoic zircons were contributed by the Chiapas Massif Complex, coastal batholith and Cuicateco Terrane. The Mesozoic zircons are very few, derived from the nearby Xolapa Complex and the Chuacús Terrane.

We present significant findings of the Kausani Granite Gneiss within the Inner Lesser Himalayan Sedimentary Zone (iLHSZ) just north of the North Almora Thrust (NAT). The Kausani Granite Gneiss body lies within the quartzite of the Someshwar Formation and has a tectonized contact and a discordant relationship on the north side. Detailed seismic profiling across the body also confirms a similar result. Cathodoluminescence images of zircon from the Kausani body show no inheritance of older cores. The U–Pb ages from the zircon populations separated from the Kausani body give a crystallization age of 1866 ± 3 Ma. Along with the Upalda granite gneiss and Toneta granite gneiss near the Alakhnanda Thrust in the Garhwal Himalaya and the Dungeshwari granite gneiss near the Dailekh Thrust in Nepal, the Kausani Granite Gneiss north of NAT Kumaun Himalaya forms a major terrain boundary. These gneissic bodies mark the southernmost extent of felsic magmatism at NAT, rather than the Main Central Thrust. An about 1.8 billion years-old magmatic event in the LHSZ suggests that it is a currently active continental margin inside the ‘Greater India’ region, now situated in the Himalayan domain. However, the Pb-loss modelling of the U–Pb zircon data reveals thermal events during the Himalayan Orogeny (~45 Ma).

The hydrocarbon activity in Pengyang area, situated in the southwestern Ordos Basin, is notably prominent. Investigation on the migration laws of hydrocarbons is imperative for comprehending the involvement in uranium mineralization. Based on the analysis of spatial distribution of hydrocarbon containing fluid and hydrocarbon generation conditions of sandstone in the Luohe Formation, the organic geochemical characteristics including hydrocarbon components, carbon isotopes and biomarker compounds were analysed. The research results indicate that: (1) hydrocarbon fluid activities in the Luohe Formation are predominantly observed in layers exhibiting higher uranium mineralization. The mudstone of the Luohe Formation had low organic matter content and low thermal maturity, which was not conducive to hydrocarbon generation. (2) Hydrocarbon-containing fluid in the sandstone of Luohe Formation not only contained reducing gases such as methane and hydrogen but also chloroform asphalt components. The carbon isotopes of hydrocarbon in sandstone inform Luohe Formation resemble oil and gas in the Mesozoic. The biomarker parameter inferred that the parent rock of hydrocarbons in the Luohe Formation was formed under reducing and freshwater conditions, and hydrocarbon generation occurred at the mature stage. As above mentioned, a comparison was carried out between the affinity of hydrocarbon-containing fluid in the Luohe Formation and different layers of hydrocarbon source rocks. The migration behaviour of hydrocarbon-containing fluid in the Pengyang area has been summarized, and the involvement of hydrocarbon-containing fluid in uranium mineralization has been discussed. The main concepts are as follows: the sedimentary environment and thermal evolution conditions of hydrocarbons in the sandstone of Luohe Formation resemble those of the primary hydrocarbon source rocks in the Yanchang Formation. The main hydrocarbon charging events in the Luohe Formation occurred before the Late Cretaceous period, which is primarily related to two hydrocarbon generation events from 130 to 100 Ma in the Yanchang Formation and fault conduits connecting the Triassic to the Cretaceous Strata. The hydrocarbon-containing fluid released from Yanchang Formation migrating to the Luohe Formation provides reducing conditions for the precipitation of uranium in oxygen-bearing water bodies.

The Colatina Fracture Zone (CFZ) defines a distinct NNW–SSE-oriented linear zone of fractures and brittle faults that represents an inherited weak zone in the current crustal structure of the (Pre)Cambrian Araçuaí Orogen. In the Early Cretaceous, the CFZ was reactivated during rifting of West Gondwana and subsequent opening of the South Atlantic Ocean, as evidenced by the emplacement of dykes along its structural network and the development of major depocentres of the Campos Basin in the offshore segments of the CFZ. Previous thermochronological studies have demonstrated that the CFZ was also rejuvenated during the drift phase of the South Atlantic. However, a number of questions regarding differential surface uplift and basement exhumation between the CFZ and its surrounding areas, such as the Doce River Valley (DRV), are still unresolved. In this study, we aim to investigate the CFZ as a distinctive structure in the tectonic rejuvenation of the passive margin of south-east Brazil. Samples from the CFZ and the DRV were collected for apatite fission-track (AFT) analyses. In the DRV, samples yield AFT central ages from 87 to 97 Ma with mean track lengths (MTL) from 12.6 to 13.3 μm. In contrast, in the CFZ, AFT central ages from 70 to 83 Ma with MTL values from 13.2 and 13.4 μm are obtained. The correlation between AFT age and elevation suggests that the tectonic development of these regions was markedly different and uncoupled. The thermal history models from the AFT data further constrain this differential evolution. On the one hand, thermal history modelling for the DRV indicates a slower and protracted cooling since the incipient Atlantic rifting in the Early Cretaceous. On the other hand, the models for CFZ reveal a rapid cooling phase between the Late Cretaceous to the Palaeocene. In the DRV, the observed basement cooling was most probably triggered by erosion of the uplifted rift shoulder generated by Gondwana break-up. The more recent, Late Cretaceous–Palaeocene rock cooling, localized in the CFZ, was synchronous with a major phase of the Andean orogeny. This suggests that reactivations and erosional exhumation of the CFZ basement could be a consequence of far-field propagation of intraplate compressional stress. The higher susceptibility of the CFZ to reactivating over its surroundings shows that structural inheritance is a key factor in the differential tectonic evolution of passive margins. Further research on the Late Cretaceous–Palaeocene reactivation in the CFZ's offshore extension may be crucial for the exploitation of hydrocarbons in the Campos and Espírito Santos basins.

The high-velocity layer in the lower crust is widely distributed in the northern continental margin of the South China Sea. A detailed anatomy of the high-velocity layer is crucial for understanding the continental rifting and crustal thinning. Based on three seismic reflection profiles across the Pearl River Mouth Basin (PRMB) and the global free-air gravity anomaly data in this study, by the gravity modelling we construct the crustal structure along three seismic reflection profiles across the depression and uplift zones. The free-air gravity anomaly data within the uplift and depression zones indicates the distinct zonation, and the high and low values of free-air gravity anomalies in the basin show the northeastward trend. Based on the gravity modelling along the three seismic profiles, the crustal thickness is of 12–23 km beneath the basin and thickness of the crust is gradually thinning from the continental shelf to the continental slope. The high-velocity layer extends eastward to the Dongsha Uplift and terminates westward beneath the Baiyun Sag. The average thickness of the high-velocity layer is 4–6 km and the maximum thickness is about 8 km at the Dongsha Uplift. The thickness variation of the high-velocity layer suggests the continuous distribution of mantle underplating and intense magma activity. According to the isolated distribution and the continuous distribution of the high-velocity layer found by previous studies in the northern South China Sea, we use gravity modelling to test which models of the high-velocity layer within the PRMB are reasonable. According to testing models and preferred models of the crustal structure by gravity modelling, we propose that the continuous distribution pattern of the high-velocity layer is the best model to interpret the crustal thinning characteristics and the gravity anomaly responses of the high-velocity layer in the basin. We propose that the high-velocity layer in the Pearl River Mouth Basin was formed by the long-term underplating of the high-temperature melted mantle.

Mantle plumes related to Large Igneous Provinces have been linked to continental break-up and validated by the outpouring of mafic-ultramafic magmas that range from continental flood basalt magmatism to submarine plateau volcanism. This study presents a new set of geochemical and mineralogical data on mafic magmatic rocks from the Sylhet Trap of the Shillong Plateau, northeast India. The investigated mafic rocks (basalt and dolerite) are predominantly sub-alkaline-tholeiitic, composed of bytownite+labradorite and diopside+augite, with ophitic to sub-ophitic and glomeroporphyritic textures, the dark interstitial region of much finer grains consisting of opaque minerals and devitrified glass. The mafic rocks of Sylhet Trap show light rare earth elements enrichment with (La/Yb)_N ratio (1.92–2.86) and (La/Sm)_N ratio (1.11–1.40), an almost flat pattern of heavy rare earth elements along with mild europium anomalies (Eu/Eu*= 0.94–1.11). Trace element characteristics suggest their affinity towards enriched mid-oceanic ridge basalt and generated from low degree of partial melting of spinel source with minor involvement of crustal contamination. The similarity in geochemical characteristics of the investigated mafic rocks with the magmatism of Rajmahal Traps, eastern Peninsular India, Abor Volcanics, eastern Himalaya, along with Bunbury Basalt of western Australia and Cona Mafic exposed in southeastern Tibet, suggests their genetic linkage with mantle plume activities. Thus, we argue that the magmatic event of the Sylhet Trap is related to the Kerguelen mantle plume activity that played a significant role in the fragmentation of eastern Gondwana during the Lower Cretaceous period, giving rise to Greater India, Antarctica and northwest Australia.

Trillions of cubic meters of gas reserve have been found in the Sinian Dengying carbonate reservoirs with normal pressure in the central Sichuan Basin, while no industrial gas reservoir have been detected in the Sinian Dengying reservoir with normal pressure in the eastern Sichuan Basin. The pore fluid pressure of gas reservoir is usually closely related to total gas content. To investigate the pore fluid pressure evolution and its implication for gas reserve preservation in the Sinian Dengying reservoir of the central and eastern Sichuan Basin, we conducted a comprehensive analysis including fluid inclusion petrography, microthermometry and Raman spectroscopy. The timings of gas inclusions captured in the central and eastern Sichuan Basin occurred from 175 to 92 Ma and 191 to 183 Ma, respectively. The presence of two-phase vapour-solid bitumen inclusions with similar phase proportions in a single fluid inclusion assemblage of fluorite provides direct evidence of in situ oil cracking to gas. The widespread solid bitumen from the Sinian Dengying reservoir in the central Sichuan Basin indicates the existence of massive oil cracking, which results in the formation of overpressure in the reservoir. Pore fluid pressure evolution of the Sinian Dengying reservoir of the central Sichuan Basin experiences normal pressure stage (200–155 Ma), overpressure development stage (155–90 Ma) and overpressure release stage (90–0 Ma). The maximum pore fluid pressure and its corresponding pressure coefficient of the Sinian Dengying reservoir of the central Sichuan Basin are approximately 141.4 MPa and 1.95, respectively. The overpressure development stage reflects the processes of oil cracking and gas accumulation, and the overpressure release stage reflects the dissipation of some natural gas in the Sinian Dengying reservoir of the central Sichuan Basin. The pore fluid pressure of the Sinian Dengying reservoir in the eastern Sichuan Basin has maintained at normal pressure since 200 Ma, indicating that the gas reservoir was small during the oil cracking stage and natural gas completely leaked due to tectonic uplift.