Geological Journal最新文献

The Cuddapah Basin located in the southern part of India, is one of the largest Proterozoic basins in the world. This basin witnessed multi-stage growth that involved sedimentation, magmatism and tectonism. This region has been studied geophysically for over nine decades, nevertheless, its subsurface structural configuration, underlying crust–mantle structure and its evolutionary relationship with east Antarctica during the Columbia and Rodinia supercontinental assembly periods, remain an enigma. In the present study, we provide a 3-D crustal structural configuration of this basin based on gravity modelling along 13 east–west and 3 north–south profiles, utilising density and subsurface depth constraints from crustal seismic studies. We have delineated the presence of a 15–20 km anomalously thick, high density differentiated magmatic layer above the Moho, which varies widely from 32 km below the Eastern Ghats Belt to about 50 km below the Nellore Schist Belt. It is invariably shallower at 37–38 km north of 14°45'N, compared with 43 to 46 km south of it, indicating possible tilting of the basin from north to south. Importantly, we also notice a high order translational movement of the crustal column across the Nallamalai Basin as revealed by conspicuous change in the Moho trend, which correlates with the absence of Eastern Ghats Belt for a distance of about 400 km south of Ongole, mimicking the translational movement. We infer that the Napier complex of east Antarctica, may form the missing part of Eastern Ghats Belt of India's east coast. Similarly, a high gravity over the Nellore Schist Belt region would possibly indicate presence of remanent slab material (or magmatic material) underneath which may be related to erstwhile supra-subduction during the Rodinia supercontinental assembly period. The present study further suggests that the Iswarakuppam dome, located in northern part of the Nallamalai Basin, contains a thin veneer of Cumbum shale (5.10 km/s) followed by a high velocity (Vp: 5.70 km/s) sequences containing Bairenkonda quartzites and possibly mantle derived mafic rocks that may be correlatable with the sills of southwestern part of the Cuddapah Basin. This domal structure probably formed due to underthrusting of the western Cuddapah Basin and the collision of Nellore Schist Belt/Eastern Ghats Belt terrains after the cessation of supra-subduction below eastern Dharwar Craton at the end of the Rodinia period (around 950 Ma).

The Central Indian Tectonic Zone (CITZ) runs across peninsular India and includes Proterozoic bimodal volcanics (basalt-rhyolite), quartzite, mafic-ultramafic rocks, volcanic sediments and Banded Iron Formation (BIF). The bimodal volcanic rocks of Betul–Chhindwara belt have been subjected to upper greenschist to lower amphibolite-grade metamorphism and have well-preserved remnants of pillow structures. Total alkali vs. silica diagram clearly discriminates all the samples into subalkaline basalts and rhyolites which corresponds to their bimodal nature. Mafic volcanic sequence of Betul–Chhindwara belt is represented by high Ti and low Ti Groups. I. High Ti basalt has undergone low degree of partial melting (~5%), whereas low Ti basalt has undergone a high degree of partial melting (~20%) of the same source rock. Fe and Ca decrease with decreasing Ti indicating clinopyroxene and iron-titanium oxide fractionation in both the groups of basalt. These basalts are generally enriched in incompatible trace elements such as Rb and Ba and depleted in Nb, P and Ti, which collectively are good indicators of continental crust/lithosphere involvement in their genesis. The basalts show no Eu anomaly, which indicates little role of plagioclase during fractionation process. Positive anomalies of U–Th–Zr for the basalts indicate crustal involvement. Whole-rock Sm–Nd isochrons for the mafic volcanic rocks indicate an age of crystallisation for these volcanic rocks at about 1232 ± 37 Ma (initial ¹⁴³Nd/¹⁴⁴Nd = 0.510752 ± 0.000035, mean square weighted deviate [MSWD] = 1.20) which is much younger than the basement rocks ca. 1500 Ma. The ε _Ndt (t = 1232 Ma) vary from −5.93 to −3.1 for the mafic volcanic rocks and between −5.81 and +0.14 for felsic volcanic rocks. Depleted mantle model ages of basalts vary from 2204 to 3040 Ma, and for rhyolites, these vary from 2174 to 2863 Ma, respectively. The ε _Nd value for all the basaltic samples includes both the groups of basalts plot away from the CHUR line indicating their derivation from a depleted source and evolves to lower values, indicating longer crustal residence or more crustal contribution. Mafic magma might have been produced at the subduction zone interacted with the lower continental crust while ascending to the surface. This lowered the melting point of the continental crust which led to the production of felsic melt. Episodic mafic magma could have led to the production of rhyolite, produced at different levels of the continental crust.

The northeast margin of Ordos Basin, China has a large area with ‘bleached sandstone’, a unique rock characterised by a low diagenetic degree and poor cementation degree. Long-term wind and hydraulic erosion have led to significant soil erosion, vegetation degradation and low ecological carrying capacity in this region, with huge amounts of sediments being transported to the Yellow River. Therefore, it is critical to investigate the weathering mechanism and physicochemical characteristics of bleached sandstone. In this study, the pore structure, elemental composition, mineral composition and radon exhalation characteristics of the bleached sandstone were analysed using low-temperature nitrogen adsorption instrument, inductively coupled plasma mass spectrometer, polarising microscope and environmental radon detector, respectively. Moreover, the physicochemical characteristics of the bleached sandstone were analysed to determine the correlation among them. The results demonstrated that the degree of pore development in bleached sandstone is significantly higher than that in original rock, and the content of mesopores of size from 2 to 50 nm is the highest. These pores were found to be predominantly slit pores with parallel plate structure. The mineral composition is primarily quartz, feldspar, clay minerals and muscovite, with quartz being the highest. Quartz and feldspar exhibited significant weathering, broken crystals and more impurities. A large amount of biotite was altered to form muscovite. The intergranular spaces of bleached sandstone are filled with heterobase, primarily composed of clay minerals. Additionally, clay minerals formed a ring band structure around the particle skeleton. Bleached sandstone was found to be rich in P, Ba, Mn, Sr, Zr and other elements, with Ba showing the highest proportion. The bleached sandstone is enriched in toxic elements Be, As, Cd, Sb, Hg, Tl and ²³⁸U compared with the original rock, indicating that the latter has a certain degree of adsorption. Among them, Be has the highest enrichment degree. Moreover, it is enriched with light rare earth and deficient in heavy rare earth. δEu and δCe exhibit a slight deficiency. Although the rate of radon exhalation in bleached sandstone is low, it is higher than that of the original rock. The high porosity and micropore content can promote radon exhalation. High ²³⁸U content can enhance radon generation, which exhibited a positive correlation with radon exhalation rate. The research results can provide a reference for ecological management, development and utilisation of bleached sandstone in this region.

Understanding the Late Palaeozoic accretionary processes responsible for the formation of the eastern Central Asian Orogenic Belt (CAOB) is crucial for unravelling continental growth mechanisms in this region. This study systematically investigates newly identified volcanic rocks in the northern margin of the North China Block (NCB) and Bainaimiao Arc Belt (BAB), aiming to elucidate their petrogenesis, tectonic setting and implications for the evolution of the Paleo-Asian Ocean (PAO). Zircon U–Pb dating reveals that these volcanic rocks were emplaced between 278 and 260 Ma. The intermediate-mafic volcanic rocks exhibit typical subduction-related geochemical signatures, including low TiO₂ contents, enrichment in Rb, Ba, U, K, Pb and Sr., and depletion in Nb and Ta, reflecting derivation from a subduction-modified lithospheric mantle. The acidic volcanic rocks, characterised by high SiO₂ and Al₂O₃ contents and pronounced negative Eu anomalies, are interpreted as products of partial melting of the lower continental crust with fractional crystallisation. The _εHf(t) values range from highly positive in the BAB region to lower values in the NCB, highlighting the incorporation of juvenile crustal material in the north and ancient crustal components in the south. Geochemical and isotopic evidence suggests that the volcanic rocks formed in an Andean-type continental arc during the southward subduction of the PAO beneath the northern margin of the NCB. After 260 Ma, the tectonic setting transitioned to an extensional environment, as reflected in the geochemistry of younger intraplate granites. These findings suggest that the PAO continued subduction until the Late Permian, followed by slab break-off and post-collisional extension. The crustal thickening (40–66 km) and widespread magmatism indicate significant juvenile crustal growth during the Middle to Late Permian. Combined with regional tectonic data, this study provides critical insights into the geodynamic processes driving crustal evolution in the eastern CAOB.

The North Yellow Sea Basin in the Eastern North China Craton preserves the Upper Jurassic–Lower Cretaceous, and the sedimentary processes of the basin provide clear tectonic transformation records. The Upper Jurassic–Lower Cretaceous detrital mode analysis of sandstone and Lower Cretaceous detrital zircon U–Pb chronology was used to trace the sedimentary provenance and discuss how the sedimentary process records tectonic transformation. The detrital mode analysis of the sandstone revealed significant compositional changes over time. There were more metamorphic lithics and less feldspar in the Upper Jurassic sedimentary rocks, and fewer metamorphic lithics and more feldspar in the Lower Cretaceous sedimentary rocks. These changes in sandstone composition suggest that the source area of the basin changed. Detrital zircon U–Pb geochronology indicated that the sources of Early Cretaceous sediments were no longer from the Sulu orogenic belt but originated from the Liaodong Peninsula and the Korean Peninsula. The change in the sediment source direction from the Upper Jurassic to the Lower Cretaceous in the North Yellow Sea Basin is a good record of compressional orogenesis during the Late Jurassic and transitioning to strike-slip and extensional processes in the Early Cretaceous.

A suite of organic-carbon-rich mudstones has been recently identified in the lower Carboniferous Zaduo Group in the eastern North Qiangtang Basin. Fossils from this group suggest an early Carboniferous depositional age. However, an unconformity recognised between the lower clastic rock unit and the upper carbonate unit of the Zaduo Group indicates a sedimentary hiatus. Here, we report detrital zircon U–Pb geochronological and sporopollen fossil data from the lower clastic rock unit of the Zaduo Group to examine its age. All the eight samples contain Triassic zircons and give a maximum depositional age of Late Triassic, which could also be indicated by the sporopollen results. This age revision has been roughly checked over a spatial span of 300 km in the eastern North Qiangtang Basin. The detrital zircon U–Pb age patterns highly resemble those from the Triassic Bagong Formation in the Qiangtang Basin and Upper Triassic flysch deposits in the Songpan-Garze Terrane. Considering the similarities in organic-carbon content, detrital zircon age patterns, rock assemblages, and spatial distributions between the lower clastic rock unit of the Zaduo Group and the Bagong Formation, it is quite possible that the lower unit of the Zaduo Group is an extension of the Bagong Formation. Moreover, the mudstones of Bagong Formation are the most important source rocks in the Qiangtang Basin, which requires more details and caution to verify the depositional age of the Zaduo Group. Further, the age revision highlights the need for caution to reevaluate the distribution of source rocks in the Qiangtang Basin.

The central Qinzhou-Hangzhou metallogenic belt in south China hosts large to supra-large hard-rock type lithium ore deposits. This contribution presents an integrated study of zircon U–Pb geochronology, whole-rock and in situ Lu-Hf isotopic geochemistry for the Dagang muscovite-granite type Lithium ore deposit. LA-ICP-MS zircon U–Pb dating of five muscovite-granitic samples yielded weighted average ²⁰⁶Pb/²³⁸U ages of 145 ± 3, 143 ± 1, 143 ± 1, 144 ± 3 and 144 ± 6 Ma, respectively, which are interpreted as the intrusion and mineralization ages of the granite. The Dagang granites have high SiO₂, Na₂O + K₂O and Al₂O₃ contents that are similar to those of strongly peraluminous granites. All the samples are depleted in Ba, Nb and Ti elements, but enriched Rb, Ta, Pb and U elements, as well as they have large negative ε_Hf(t) values of −11 ± 1.3, −8.1 ± 0.6, −6.7 ± 1.5, −8.6 ± 0.9 and −10.3 ± 2.3, which are indicative of dominantly Mesoproterozoic continental crustal metasedimentary in origin. There is no obvious correlation between SiO₂ and Li, whereas SiO₂/Al₂O₃ show negative correlation with Li, in combination with positive correlations of LOI, Rb, Cs and Li, as well as consensual genetic views of high-U zircons, it is proposed that lithium enrichment of the muscovite-granite type lithium ore deposit occurred late stage of crystallisation differentiation of silicic magmatic system during Al-rich mineral precipitation, and are superimposed of hydrothermal alteration.