International Journal of Earth Sciences最新文献

The Aqishan-Caixiashan polymetallic ore cluster, located in the eastern Tianshan Orogenic Belt, is part of the southern Central Asian Orogenic Belt (CAOB) and experienced extensive tectono-magmatic events with polymetallic mineralization during the Carboniferous. However, the Carboniferous tectonic affinity and magmatic evolution of the ore cluster are still controversial, which limits further understanding of the tectonic evolution and regional metallogeny. With the aim of solving these problems, we have carried out geochronological and geochemical studies on Carboniferous intrusive rocks and volcanic rocks from the ore cluster. The samples of the Carboniferous magmatic rocks are enriched in LREEs and LILEs but depleted in HFSEs, which attest to the I-type affinity and are similar to those formed in a continental arc. The early Carboniferous granodiorites and granites (350–343 Ma) in the Central Tianshan Block (CTB), with high Mg# values of 37–66, low Y values of 2.3–17.2 ppm, and high Sr/Y ratios (34–85), show characteristics of adakites that are formed by partial melting of delaminated oceanic crust and/or lower crust. Additionally, they have a wide range of εHf(t) values (5.4–15.9), high εNd (t) values (1.78–6.31) and low I_Sr values (0.705–0.706), indicating a mixed source of depleted mantle and lower crust for its origin. The Late Carboniferous volcanic rocks (314–306 Ma) in the Aqishan-Yamansu Tectonic Belt (AYTB) north of the CTB have obviously negative Nb–Ta-Ti anomalies, as well as high Y contents (12–20 ppm) and low Sr/Y ratios (5.6–29.9) compared to primitive mantle, which has a relatively flat chondrite-normalized REE pattern, resembling arc-related magmas. Additionaly, they have positive values of εHf(t) values (8.7–14.7), high εNd(t) values (3.04–4.45) and low I_Sr ratios (0.704–0.707), showing a depleted mantle source, which is weakly affected by the contamination of ancient crustal material. Combining this study and previous works, we suggest that the continuous southwards subduction of the North Tianshan Ocean during the Carboniferous generated the continental arc and forearc basin in the northern part of the CTB. The expansion of the Aqishan-Yamansu forearc basin formed intrusions and volcanic formations along the arc belt. The early Carboniferous intermediate-felsic mantle-derived adakitic magmas in the CTB are probably related to regional Zn-Pb enrichment and mineralization, and the Late Carboniferous intrusions and volcanic formations in the AYTB are related to large-scale Fe-Cu-Zn-Pb-Ag mineralization.

Graphical Abstract

Crustal structure beneath the major tectonic domains comprising northeast India is investigated using receiver function analyses of teleseismic earthquakes recorded at 19 new broadband seismic stations, combined with Moho depth and Poisson’s ratio information from 14 previous studies. Velocity structure inverted from receiver functions are combined with results from H–k stacking and validated using forward modeling of gravity data. The Shillong Plateau in the western Brahmaputra River Valley, later representing the Himalayan foredeep, is inferred to have a thin (33–35 km) felsic crust. A dome-shaped, 5-km-thick high-velocity layer in the upper crust likely accounts for positive Bouguer anomaly over the plateau. The Upper Assam basin in the eastern Brahmaputra River Valley is marked by pronounced gravity low, high Poisson’s ratio, and a pair of low- and high-velocity layers in the crust. This narrow segment demarcates the crust of oceanic affinity extending from the Indo-Burmese Wedge. Further to the south of Dauki fault, under the ~ 10-km-thick wedge, a two-layered crust comprising 5–15-km-thick delta sediments overlying a ~ 20-km-thick basement of oceanic affinity extends from Bay of Bengal to the Churachandrapur-Mao Fault, where the deep basement subducts upon collision with the Burmese micro-plate. East of the collision boundary, the overriding Burmese micro-plate having a thin crust of ~ 30 km, including ~ 15-km-thick low-velocity sediments, is imaged beneath Central Myanmar Basin. The findings shed new light on the tectonic reactivation of the different domains of northeast India upon collision with the Himalaya to the north and Burmese micro-plate to the east.

Graphical Abstract

Abstract

Portezuelo de Pajas Blancas' primary metasomatic deposits host sapphires that differ in size, colour, and colour distribution through different lithological units. The discovery of sapphire-bearing secondary aeolian placer deposits enabled a detailed analysis of sapphire types to determine the causes of this heterogeneity. Representative single sapphires from the primary and secondary deposits were analysed using various techniques, including SEM-CL, OM-CL, Micro-XRF, Raman spectroscopy, and polarised transmitted light multi-focus optical microscopy. Based on the results, seven sapphire types were identified according to their colours and colour distribution. Fe and Ti concentrations mainly contribute to the colour and micro-textures occurrence. The primary growth textures include cores, progressive, oscillatory, and diffusion zoning, whereas the secondary alteration features correspond to micro-brecciation, re-arrangement, and overgrowth. Inclusions such as euhedral andalusite, anhedral anhydrite, and anhedral rutile were identified in the sapphires. The formation of Portezuelo de Pajas Blancas sapphires can be divided into five genetic stages: (i) core growth development; (ii) chemical imbalance due to open system behaviour that produces progressive and oscillatory zoning; (iii) low-temperature deformations that produce micro-brecciation; (iv) re-arrangement of pre-formed sapphire fragments and subsequent sapphire overgrowth; (v) diffusion zoning that produce Fe- and Ti-enriched outer rims. In conclusion, the Portezuelo de Pajas Blancas' sapphires were formed through at least five stages of oxidised contact metasomatism at varying temperatures and low pressures, where a heterogeneous forming condition predominates during sapphire formation.

Graphical Abstract

The environmental consequences of mine flooding in the Saar hard coal district, post-mining re-use concepts in the course of the energy transition, and the potential of coalbed methane production require an understanding of subsurface rock properties on the microscale. In this study, mineralogy, microtexture, microstructure, porosity, permeability, and geochemistry of an Upper Carboniferous (Stephanian A–B) drill core recovered in the Saar–Nahe basin are quantified. Based on these data, the diagenetic history and reservoir quality are analyzed regarding mine flooding and coalbed methane potential. The feldspar-poor and igneous rock fragment-free siliciclastic rock succession shows multiple fining upward sequences deposited in a fluvial environment during the pre-volcanic syn-rift phase of the Variscan intramontane Saar–Nahe basin. Intercalated small-scale coarsening upward sequences are related to the floodplain where near-surface soft-sediment deformation and paleosol formation took place. Porosity (< 7%) of the tight siliciclastic rocks is mainly controlled by an interplay of authigenic microporous kaolinite, dissolution porosity, and quartz cement, whereas permeability (< 0.05 mD) shows no systematic variation with petrography. During burial, quartz cements preserved porosity by stabilizing the granular framework against mechanical compaction, while phyllosilicates were ductilely deformed reducing reservoir quality. Relative phyllosilicates and quartz contents and mean grain size are reliably inferred from SiO₂/Al₂O₃ ratios (1.8–28.8), Ba (0.0108–0.0653 wt%), Rb (0.0024–0.0181 wt%), and Sr (0.0013–0.0086 wt%) concentrations measured with a portable x-ray fluorescence analyzer. Regarding coalbed methane production and mine flooding, sealing of cleats and heterogeneous subsurface rock properties due to dynamically changing depositional settings during the Late Carboniferous need to be considered.

Graphical abstract

The Western Xihu Basin (WXB), part of the East China Sea Shelf Basin (ECSSB), demonstrates distinct structural differences between different sub-areas. Understanding the origin and mechanism of these differences is critical for unravelling the formation and evolution of the Western Xihu Basin and the ECSSB. Based on high-resolution 2D and 3D seismic data, we investigate the structural characteristics and evolution of the hinged margin and discuss the underlying formation mechanisms. The results suggest that, while controlled by NNE-, NE- and NW-striking major faults, there are distinct differences in the fault geometry, margin structure, fault displacement rate, and margin evolution in different basin areas. In contrast to the conventional division scheme which divides the WXB into three general sub-areas, our results suggest that the WXB should be divided into seven sub-areas with different tectonic structures and stress histories. The evolution of the WXB can be divided into three stages: (1) the synrift stage in the Palaeocene and early Eocene, when NNE-, NE- and NW-striking major faults were highly active and controlled the initial formation of the hinged margin structure; (2) late synrift stage in the late Eocene, when the faulting activity diminished, and the control of faults on the margin evolution decreased; (3) postrift stage from the Oligocene onwards, when active faulting ceased and sedimentation and differential basement subsidence became the main factors controlling basin evolution. The formation and reactivation of NW-striking faults under influence of the Izanagi-Pacific ridge subduction during the Mesozoic provided the basis for the differential evolution of the WXB in Cenozoic.

Graphical Abstract

The Sibela Mountains of the island of Bacan in eastern Indonesia contain one of the Earth’s youngest metamorphic complexes that is now exposed at elevations up to 2000 m. New mica ⁴⁰Ar/³⁹Ar and apatite (U–Th–Sm)/He data from metamorphic and igneous rocks indicate that these rocks were rapidly exhumed in the Pleistocene (c. 0.7 Ma). Exhumation of the metamorphosed Permo-Triassic basement (c. 249–257 Ma) was accompanied by metamorphism (recorded by schists) as well as partial melting (recorded by c. 1.4 Ma granitic dykes). These processes must have occurred at extremely high cooling and exhumation rates. The rapid exhumation on land was associated with significant subsidence in adjacent basins offshore that reach depths up to 2.4 km. Neogene metamorphic core complexes and other metamorphic complexes are well-known from eastern Indonesia, but they usually record much higher exhumation rates than those reported from older classic metamorphic core complexes found in other parts of the world and require a different formation mechanism. Unlike classic metamorphic core complexes that are characterized by low-angle detachment faults, the Bacan metamorphic rocks were exhumed on steep bounding normal faults forming a rectilinear block pattern. We suggest such complexes are termed metamorphic block complexes (MBC). The Bacan MBC is exceptionally young and like the other east Indonesian complexes was rapidly exhumed during subduction rollback. A flexure formed during arc-arc collision as the Sangihe forearc loaded the Halmahera forearc which reactivated steeply-dipping faults in a rectilinear chocolate block pattern.

Graphical abstract

The western Argentinian sector of Gondwana has been the focus of several recent studies related to the Famatinian orogeny; however, the geologic history of arc activity in hinterland areas remains poorly understood. We present new data from the Monte Guazú Complex that reveal arc-related magmatism in the Sierras de Córdoba, which we consider part of the Famatinian hinterland. Igneous rocks comprise a diorite unit which includes an amphibole quartz-gabbro/diorite and a tonalite unit comprising amphibole- and biotite-bearing tonalites to minor granodiorites. Both units constitute a medium-K calc-alkaline series ranging in composition from metaluminous to moderately peraluminous. Trace-element signatures show Ti and Nb depletion and strong incompatible element enrichments (large-ion lithophile elements, Pb, Th, U, and light-rare earth elements) relative to normal mid-ocean ridge basalts, suggesting that they formed in an arc setting. U–Pb zircon geochronology constrains magmatism to 455–498 Ma, while weighted mean ages of 474–489 Ma are mostly synchronous with the Famatinian arc beginning. We demonstrate that fractional crystallization of mantle wedge-derived melts controlled the early magmatic evolution, while country rock assimilation and anatectic melt mixing were prevalent in evolved rock members. These results spatially extend the Late Cambrian–Late Ordovician Famatinian retro-arc to the southern Sierras de Córdoba. Our findings show that magmatism involved coeval anatexis of host rocks, conversely to previous geodynamic models. Our data have fundamental implications for Paleozoic tectonic and magmatic processes operating along the western Argentinian sector of Gondwana, demonstrating the importance of crustal reworking and the addition of mantle material in the Famatinian arc inboard sector.

Graphical Abstract

Jurassic sedimentary sequences suitable for nuclear waste storage in northern Germany consist of organic-lean claystone and were uplifted to < 100 m depth in the Hils Syncline area (southern Lower Saxony Basin). This Hils Syncline, showcasing a northwestward increase in thermal maturity, facilitates the study of shale petrophysical properties influenced by burial history. This study introduces a 3D-thermally calibrated numerical model of the Hils Syncline area to analyze its geodynamic evolution and maturity variations. It provides new vitrinite reflectance and sonic velocity data for modeling calibration and erosion estimation. The Hils Syncline area has undergone continuous subsidence, interrupted by a Cretaceous uplift documented by an erosional unconformity. During the latest Early Cretaceous, Jurassic rocks underwent maximum burial reaching up to several thousand meters depth and temperatures up to 160 °C in the northwest. The Late Cretaceous inversion caused stronger erosion towards the northwest removing up to 3300 m of sediment compared to about 1300 m in the south, according to vitrinite reflectance-based estimations. Numerical modeling results along the study area indicate decreasing porosity and permeability northwestward with increasing thermal maturity. Porosity and vertical permeability decreased to 5–14% and 2.8 × 10^–23 to 1.5 × 10^–19 m² [1 mD = 10⁻¹⁵ m²], respectively, while vertical thermal conductivity increased to 1.30–2.12 (W/m/K). These trends of porosity/permeability and thermal conductivity with burial align with sonic velocity and published experimental porosity data, except for the thermally most mature region (Haddessen). This anomaly is tentatively attributed here to localized overpressure generation in the Posidonia Shale during maximum burial, affecting both the underlying Pliensbachian and overlying Doggerian units.

Graphical abstract

3D numerical model of the Hils Syncline and surrounding area revealing that a northwestward increase in maximum burial resulted in higher temperatures and varying maturity levels. While most locations align well with calibration data (i.e. measured vitrinite reflectance and porosity), discrepancies arise in the Haddessen/Bensen area. The mismatch between porosity, vitrinite reflectance, and sonic velocity response indicates local overpressure in the northernmost region mainly during the Cretaceous. It was likely caused by gas generation in the Posidonia Shale affecting nearby Lower and Middle Jurassic units.

Hydrocarbon enrichment in faulted basins is often controlled by the activity of faults with some degree of sealing capacity. However, the rules that control the migration and accumulation of hydrocarbons in reservoirs dominated by faults are poorly understood. The Liuzhuang fault in the Bohai Bay Basin is selected for hydrocarbon migration and accumulation research. Interpretation of seismic and log data, tests of rock mechanics, and quantitative fluorescence analysis are used to quantitatively evaluate the migration of hydrocarbons. The results reveal that the Liuzhuang fault has been a long-term active fault since the Cenozoic with gradually reduced fault activity from the NE to the SW. At least four fault-bounded traps with various degrees of trap filling in the lower segment of the first member of Shahejie Formation (Es₁^L) were identified in the hanging wall of this fault. This differences in the degree of filling are related to the continuity of the smear structure that formed from the Es₁^m cap rock in the brittle‒ductile transition stage. The development degree of the smear structure, which is quantitatively evaluated by the shale smear factor (SSF) and shale gouge ratio (SGR), directly affected the fault sealing capacity. Therefore, three cases of hydrocarbon migration and accumulation, i.e., continuous smearing along the fault and complete fault sealing with SSF values of < 3, reduced continuity of the smear structure and partial fault sealing with SSF values in the range of 3–5, and discontinuous smearing and ineffective fault sealing with SSF values of > 5, were defined. The new results have implications for further exploration in faulted basins, including the Bohai Bay Basin.

Graphical abstract

The Sanjiang Tethyan Orogen plays a pivotal role in elucidating the geodynamic processes related to the evolution of the Paleo-Tethys Ocean. Research on the Gicha Complex within this orogen is imperative for comprehending the intricate evolution of the region. This study presents new findings from zircon U‒Pb dating, Hf isotope analysis, and whole-rock elemental analyses of gabbro, olivine gabbro, and the recently discovered mylonitic rhyolite and diabase in the Gicha Complex. The zircon U‒Pb dating yields ages of 703 ± 3 Ma for the diabase, 296 ± 2 Ma for the gabbro, and 231 ± 1 and 228 ± 2 Ma for the mylonitic rhyolite, demonstrating that these rocks record the extended evolution from the breakup of Rodinia to the closure of the Paleo-Tethys Ocean. The Neoproterozoic diabase samples exhibit shoshonitic and arc-like geochemical properties, with positive zircon ε_Hf(t) values ranging from 1.1 to 3.7 and high Th/Yb values ranging from 1.34 to 3.07, suggesting that they were generated by partial melting of enriched subduction-modified lithospheric mantle in an active continental margin. The early Permian gabbro samples are part of the tholeiitic series and exhibit low SiO₂ and K₂O contents; depletions in Nb, Ta and Ti; and enrichment in Pb, indicating derivation from a combination of depleted mantle peridotite and subduction-related components. The whole-rock geochemical features suggest that these rocks formed in a back-arc setting featuring a small, mature ocean basin. The mylonitic rhyolite samples exhibit weakly peraluminous characteristics (A/CNK = 0.98–1.12) with high-K calc-alkaline geochemical features. The samples are depleted in Ba, Nb, Ta, and Ti and enriched in Th, U, Pb, and LREEs and exhibit negative to positive zircon ε_Hf(t) values ranging from − 0.5 to + 5.9, indicating that they originated from partial melting of Meso-Neoproterozoic juvenile lower crust. Geochronological, petrological, and geochemical analyses indicate that the subduction of oceanic crust occurred along the western margin of the Yangtze Block until the late Neoproterozoic (~ 703 Ma).

Graphical abstract