Journal of Asian Earth Sciences最新文献

Interstitial microstructures in layered intrusions can provide crucial insights into the formation, transport, evolution, and solidification processes of magma. The symplectites commonly observed in the Shangzhuang layered intrusions in Beijing, China, can be classified into three types based on their occurrence and related primocryst minerals. Regardless of variation in types, the vermicular symplectites consistently exhibit a volume ratio of magnetite to orthopyroxene at approximately 1:4. Orthopyroxene in the symplectites shows no geochemical difference from those primocrysts. Inter-cumulus hornblende formed during the late stage after the formation of symplectites so that the symplectites are always enclosed by hornblende. By utilizing the hornblende geothermobarometer, we have also constrained that the symplectites have been formed within a relatively wide temperature range of 1040–915 °C through a reaction between the interstitial immiscible Fe-rich melt and the primocryst olivine and orthopyroxene. The reaction can be simplified as Ol/Opx (primocryst) + Fe-rich melt → Opx (symplectite) + Mt (symplectite). Besides, based on the mass balance and reaction results, the Si/O ratio of Fe-rich melt is estimated to be 1:3. The study of these symplectites contributes to refining the processes of reactive melt flow in mafic layered intrusions during late-stage magmatic crystallization.

Few constructional features of coastal geomorphology have been investigated at the northernmost extremity of the Gulf of Thailand (GoT), with a view to establishing the position (height) of local relative sea level (RSL) during the marine regression following the regional mid-Holocene highstand (MHH) that occurred at approximately 6.5 ka BP. Here, the work investigates a 2 m thick exposure of marine gravels on the coast of Ko Khang Khao islet in the eastern Bay of Bangkok. At an elevation of 3.3–5.3 m above modern sea level, the sequence is interpreted to represent a Holocene raised beach. The unlithified sediments comprise rounded quartz and mylonite pebbles and cobbles, oriented predominantly NE–SW, supported by fossiliferous sands that are rich in marine shells, coral fragments and occasional terrestrial gastropods. The juxtaposition of the marine and non-marine gastropoda of contemporaneous ages makes a compelling story for a coastal storm deposit, thrown up either by a winter monsoon storm, or by a palaeotyphoon that managed to penetrate the upper Gulf. Overlapping results of C14 and OSL age-dating of shell material and mineral sands suggest the raised (storm) beach formed between 3.5 and 4.0 ka BP, i.e. ∼ 2.5–3.0 ka after the MHH peak, at a height of ∼ 1.3–3.3 m above the local RSL position at that time (according to glacial isostatic adjustment modelling). Given the otherwise paucity of data from the upper GoT, the Ko Khang Khao raised beach provides new information that expands our current understanding of geographical variations in RSL across Southeast Asia during the Late Holocene.

On a regional scale, rare-metal pegmatite groups (pegmatites of common origin) display a zonal distribution with different chemical compositions. Whether the regional distribution of such pegmatites dike in a convergent zone is related to dehydration melting of different micas remains unclear. In recent years, the Altyn Tagh Orogen (ATO) in the northwest China has gained attention due to economically significant rare-metal mineralization. Our study focuses on the geochronology of columbite-group minerals (CGMs) and geochemistry of tourmaline from the Be-mineralized pegmatites in the Tugeman and Ayage Be deposits. CGM dating shows that these Be-mineralized pegmatites formed between 485 and 484 Ma. Subsequent hydrothermal activity, occurring around 453–450 Ma, is related to the development of a ductile shear zone. Evidence from tourmaline mineralogy and major elements reveals that the evolution of the Be-mineralized pegmatite is influenced by crystallization differentiation and subsequent fluid exsolution. Tourmalines from these pegmatites exhibit B isotope compositions ranging from –12.2 ‰ to –14.8 ‰, indicating a crustal metasediment source. Based on these findings, we propose a model for the rare-metal mineralization system in the Tugeman area: the sequential formation of Be to Li-mineralized pegmatites likely results from dehydration melting of muscovite and biotite during prolonged subduction and collision between the South Altyn Subduction–Collision Belt (SAB) and Central Altyn Block (CAB).

The northern Bozhong depression of the Bohai Bay Basin in eastern China is an important area for oil and gas exploration in buried hills, which has complex structural styles and evolutionary processes. Based on 3D seismic and drilling data, the features of the Mesozoic structural deformation of the northern Bozhong depression are analyzed, and the tectonic evolution and dynamic mechanisms are also discussed. The complex Mesozoic structural styles of the northern Bozhong depression are composed of compression, strike-slip, and extension structures, which mainly trend in the near E-W, NW, NE, and NNE directions. Triggered by the collision and subduction between the North China, South China and Izanagi plates and associated stress fields of different periods, natures and directions, the northern Bozhong depression has experienced five stages of deformation at the end of the Middle Triassic, the Late Triassic, the Early-Middle Jurassic, the Late Jurassic-Early Cretaceous and the Late Cretaceous. Cyclical tectonic evolution has controlled the development of structural styles of different strikes and natures and the spatial distribution of strata in the northern Bozhong depression, providing an important foundation for hydrocarbon accumulation in buried hills.

Dating the activity of complex faults and fracture systems is crucial for creating reliable geological models for various tectonics and subsurface engineering applications. This study presents a comprehensive study integrating U–Pb fracture cement dating, trace elements, and fluid inclusion temperature analysis with seismic analysis of faults, fault diagenesis, and burial history studies to better constrain faulting and fracture activities in an intra-cratonic strike-slip fault system in the northern Tarim Basin. Seismic profiles indicate at least three distinct phases of fault activity corresponding to the Middle Ordovician, Permian, and Paleogene periods. Fracture cementation and crosscutting relationships corroborate the identification of three fracturing stages. U–Pb dating of fractured cement has widely detected Middle Ordovician and Early Permian age intervals. Fluid inclusion homogenization temperatures from the fractured cements, ranging across < 50 °C, 70–130 °C, and 150–180 °C, correspond to three episodes of rapid subsidence during the Ordovician, Permian, and Neogene, respectively. These results suggest three phases of fault/fracture reactivation in the Middle Ordovician (prior to 470 Ma), Early Permian (prior to 295 Ma), and Cenozoic. The fault/fracture reactivation in the Ordovician is closely related to the regional tectonic transition from extension to compression, while fault and fracture reactivation in the Early Permian may be related to hydrothermal activity associated with large-scale igneous province and oil emplacement. Fault/fracture activity in the Cenozoic may be related to a reduction in subsidence, gradual reduction of geothermal gradients, and massive oil emplacement. This research underscores the significance of integrating geochemical and subsurface datasets for accurately determining the timing of faulting and fracturing in sedimentary basins.

The breakup of eastern Gondwana during the Early Cretaceous has been well-documented. However, the relationship between Cretaceous mafic magma and the Kerguelen mantle plume has not been well-constrained. It is also unknown if the Kerguelen mantle plume in the Tethyan Himalaya played a key role in triggering the breakup of eastern Gondwana. The petrogenesis and geodynamic processes of the coeval magmatism in the Tethyan Himalaya might provide value insights into this issue. In this study, we conducted petrological, geochronological, and geochemical investigations on a recently identified diabase from the Yamdrok Lake. The diabase contains zircon grains with a U-Pb age of 138 ± 8 Ma, suggesting the emplacement at the Early Cretaceous. Geochemically, the studied samples display OIB-like trace elemental features that are enriched in light rare earth elements (LREEs) but no significant depletion in high field strength elements (HFSEs), such as Nb, Ta, and Ti. Furthermore, they have relatively uniform Sr-Nd-Pb-Hf-O isotopic compositions with initial ⁸⁷Sr/⁸⁶Sr ratios varying from 0.70563 to 0.70623, ε_Nd(t) and ε_Hf(t) values range from 0.6 to 0.8 and 1.0–2.4, respectively, whole-rock δ¹⁸O values of 6.42 ‰ to 7.50 ‰, and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.995 ∼ 39.285, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.673 ∼ 15.698, and (²⁰⁶Pb/²⁰⁴Pb)_i = 18.689 ∼ 18.914, similar to an EMII-type enriched mantle. And recycled shallow continental crustal materials into the OIB-like mantle source might account for the EMII-type mantle features observed in this study. Considering the Kerguelen mantle plume locating beneath the triple junction of Australia, Antarctica, and Greater India, we propose that the mafic rocks in this study might be genetically related to the early activity or even initiation of the Kerguelen plume during the Early Cretaceous. Our study further indicates that the Kerguelen mantle plume contains EMI- and EMII-type isotopic compositions, which are characterized by geochemical complexity and variation.

The Loei and Truong Son fold belts are well known as being the most geologically important and highly mineralized magmatic arc-related terranes in mainland southeast Asia. Numerous studies have examined the geology, geochemistry, and geochronology but only a few metallogenic and detailed deposit characterizations have been undertaken. This study of zircon geochemical analyses uses laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to provide U-Pb ages and magmatic fertility of felsic to intermediate volcanic and plutonic rocks from ten prospects in the Loei and Truong Son fold belts, in the northwest Laos region. The geochemical and geochronological analyses suggest at least three episodes of magmatism occurred: Silurian-Devonian (434–411 Ma), Early to Middle Permian (299–277 Ma), and Late Permian to Middle Triassic (253–243 Ma). The variable ranges of ɛHf values (−12 to + 12) suggest that mantle-derived and crustal contamination signatures are related to the history of subduction and arc magmatism in this region. Key trace elements and ratios associated with oxygen fugacity of magmas (e.g., Eu/Eu*, Dy/Yb, Ce/Nd, ΔFMQ) imply that the Phu Kham and the west of Vientiane along the Mekong River are likely to be fertile for porphyry copper deposits. In contrast, the Pha Gnai and other suites are less fertile. Using zircon as a fertility indication can be a valuable tool to distinguish between fertile and barren magmatic suites in this area.

The Daihai Cenozoic intraplate basalts are distributed in the western North China Craton (NCC), which is a part of the Cenozoic volcanic province in eastern China, and they are all alkaline basalts. The fine-scale determination of their mantle source region properties, partial melting mechanisms, and petrogenesis can provide crucial information for exploring lithospheric destruction and thinning in the western NCC. ⁴⁰Ar-³⁹Ar dating of potassium feldspar grains from the Daihai alkaline basalts yielded plateau ages of 18.22 ± 1.84 Ma and 26.86 ± 0.72 Ma, indicating that the Daihai basalts underwent multiple eruptive cycles during the Late Oligocene-Middle Miocene. These basalts exhibit ocean island basalt (OIB)-like geochemical features and were subjected to negligible crustal contamination. Moreover, basaltic magmas underwent intense fractional crystallization of olivine and clinopyroxene. The geochemical differences in the Daihai basalts were controlled by partial melting. The Daihai basaltic magmas were composed of at least two types (type I-enriched mantle and prevalent mantle) of mantle end-members that partially melted and then mixed, and the lithology of the mantle source region was predominantly peridotite. Under enriched mantle conditions, the mixing of garnet lherzolite partial melts (<1%) and spinel lherzolite partial melts (2-5%) can reasonably explain the elemental variations characteristic of the Daihai basalts. Most importantly, the melting depth and lithospheric thickness of the Daihai basalts were < 70 km or even close to 50 km, implying that the western NCC underwent lithospheric destruction and thinning during the Cenozoic. However, these effects were spatially heterogeneous. The most plausible genetic mechanism for the generation of the Daihai basalts was the coupled effects of subduction of the Pacific slab and subduction–collision of the Indo–Eurasian Plate since the Oligocene. The mantle flows generated by these two events convected, blocked and triggered upwelling mantle flows at the eastern margin of the Ordos Block. The upwelling mantle flows resulted in frequent magmatism in the region.

The westward subduction of the Paleo-Pacific plate had a significant effect on the evolution of the eastern margin of the Eurasian continent during the Mesozoic. However, records for the initial subduction of the Paleo-Pacific plate from mafic rocks are still lacking. In this paper, we deal with Middle Jurassic mafic rocks and coeval granites in the eastern Hebei district to constrain the impact of the subduction on the igneous event. Our new U-Pb ages of zircon grains confirm the emplacement of these rocks during the Middle Jurassic. These mafic dikes are composed of appinites and lamprophyres with high MgO (7.8–12.2 wt%), Cr (>500 ppm) abundances relative to high-Ti mafic rocks. They display large ion lithophile element enrichment, high field strength element depletion, and enriched isotopic components (ε_Hf (t) = -9.6 – –22.7), indicating arc-like geochemical signatures. These mafic dikes were likely formed from mafic magmas derived from subduction-related metasomatized subcontinental lithospheric mantle sources under the spinel- garnet transition zone. The coeval Shijiatai (SJT) granitic pluton, which is suggested as the appinite-granite suite, is high-silica granites with low Mg# (<30) values. The SJT granitic magmas were likely sourced from the chemically modified lower crust caused by underplating of mafic magmas derived from hydrated lithospheric mantle. More importantly, the new identification of Jurassic, rather than Triassic water-rich appinitic rocks in eastern China strongly points to the subduction of the Paleo-Pacific plate. Accordingly, our study provides new geochemical evidence for westward subduction of the Paleo-Pacific plate.

The Wubaduolai deposit is a newly discovered porphyry copper deposit in the Zhunuo-Beimulang ore-concentrated area of the Gangdese belt. The monzogranite porphyry is the causative intrusion in the deposit with a zircon U-Pb age of ∼16 Ma. Monzogranites have high Sr (332–673 ppm) and Sr/Y ratios (35–69), typical of adakite affinity. They have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7083–0.7093 and ε_Nd(t) values between −9.2 and −7.1. The zircon ε_Hf(t) values range from −6.45 to −2.09 and zircon δ¹⁸O values have a narrow range of 7.4 to 8.1 ‰ for the porphyry. These geochemical characteristics indicate that the parental magma was derived from juvenile Tibetan lower crust, with contributions from Indian plate-released fluids and metasomatized mantle-derived mafic magmas. The porphyry shows high values of log w(Fe₂O₃/FeO) (>0) and V/Sc (mostly > 12) in whole rock and high values of △FMQ (4.13–5.79), 10000 × (δEu)/Y(>4), and low Dy/Yb (<0.3) in zircons, suggesting that the magma was oxidized and hydrous. The oxidized and hydrous parental magma is consistent with those evaluated for the Zhunuo and Beimulang deposits. The three-component mixing model result shows that compared with the ore-related porphyries in the Zhunuo and Beimulang deposits, the Wubaduolai intrusion has higher proportion of Indian plate-released fluids and mafic magmas and lower proportion of juvenile Tibetan lower crust. Therefore, we suggest that although the causative intrusions have similar oxygen fugacity and water content, the source has more Indian continental crust and mantle-derived materials and fewer juvenile Tibetan lower crust materials at Wubaduolai, implying that the Wubaduolai monzogranite porphyry has lower potential for mineralization.