International Journal of Earth Sciences最新文献

The Yermak Plateau (YP) north of Svalbard is a prominent bathymetric feature in the Eurasia Basin of the Arctic Ocean, forming the northwesternmost margin of the Eurasian plate. Seismic data indicate that the YP comprises continental basement; however, little is known about its geology. New petrographic, geochemical, Sr–Nd isotopic, and Ar–Ar geochronological data were obtained on rock fragments, which were previously recovered from basement highs of the northeastern and southwestern YP and are dominantly of magmatic origin. These new data combined with available literature data, and comparisons with volcanic and sedimentary rocks from onshore and offshore areas adjacent to the YP indicate that the northeastern YP and the southwestern YP are different regarding their geological evolution. The southwestern YP comprises an alkaline basaltic suite for which an Ar–Ar biotite age of 51 Ma was previously reported. The suite was formed in a continental extensional regime offshore northern Svalbard. Associated sedimentary rocks (sandstone, several limestones) show petrographic similarity with rocks of the Devonian Old Red Sandstone on Svalbard. From the northeastern YP, in contrast, we recovered mildly alkaline basaltic rocks with mid-Cretaceous Ar–Ar ages (102 ± 3 and 98 ± 3 Ma). The rocks show certain geochemical characteristics (partial enrichments of P, Ba, and Eu), which overlap with similar-aged Cretaceous basaltic rocks from northern Ellesmere Island of Canada and North Greenland. We suggest that the northeastern YP is a continental fragment derived from the North American plate, which was separated from the conjugate Morris Jesup Rise and juxtaposed to the geologically distinct southwestern YP by the propagation of the Gakkel Ridge spreading center since the early Oligocene.

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Northern Anatolia has been the subject of many studies due to its complex tectonic structure. Here we report on a paleomagnetic study sampled from upper Cretaceous to middle Miocene sedimentary and volcanic rocks at 54 sites running along the traces of the North Anatolian Fault in the Pontides, Northern Türkiye, to define the post-late Cretaceous tectonic evolution of the area. The results were evaluated together with previously published paleomagnetic data around the Kırşehir Block and the Pontides. The rotational pattern showed distinct phases in the Pontides after the late Cretaceous. A predominant late Cretaceous-middle Eocene clockwise rotation up to 38.1° ± 17.4° probably occurred due to the progressive collision between the Pontides and the Kırşehir Block from west to east. The ongoing deformation continued along the İzmir-Ankara-Erzincan Suture Belt during the middle Eocene-middle Miocene time as evidenced by the paleomagnetic rotation pattern, while no significant rotations were observed further north of the suture zone at this time period. On the other hand, middle Miocene paleomagnetic sites showed a counterclockwise rotation, reflecting the westward escape of the Anatolian microplate, while local fault-bounded vertical block rotations were apparent in places.

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Granite genesis is crucial to understanding the evolution of continental crust, yet many concerns about granite genesis remain not well answered, such as whether I-type granite contains metasedimentary components, what controls granite compositional diversity, and how granitic plutons are constructed. To explore these issues, we conducted a detailed study on the two-mica plagiogranite, tonalite, and biotite plagiogranite units of the Wujinxia composite pluton in the eastern Qilian orogen, NE Tibetan Plateau. These units comprise two-mica plagiogranite, tonalite (with diorite enclave), and biotite plagiogranite. Zircon U–Pb data reveal that three granitic units formed at ~ 487 Ma, ~ 464 Ma, and ~ 430 Ma, respectively. Magmatic and xenocrystic garnet were identified from the tonalite and biotite plagiogranite, respectively. The two-mica plagiogranite, tonalite, and biotite plagiogranite all belong to low-K series rocks (K₂O/Na₂O = 0.10–0.26), and were derived from deep crustal sources mainly consisting of juvenile mafic rocks, with involvement of minor metasedimentary rocks in the magma sources of the two-mica plagiogranite and tonalite. The diorite enclave within the tonalite was probably derived from an enriched mantle-derived basaltic magma. Mineral compositions, thermobarometric calculations, and whole-rock geochemical data indicate that the low-K intrusive units of the Wujinxia composite pluton resulted from multiple magmatic systems at different depths. The results suggest that I-type granites can contain metasedimentary components by partial melting of a mixed crustal source, and high-Mn content helps the preservation of high-Ca garnet within such rocks. For a composite pluton spanning a large compositional variation, its compositional diversity is jointly controlled by magma source composition, melting condition and thermal evolution of individual magma pulses, and the resulted assembly style during pluton construction.

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Modern cold-water corals (CWCs) occur in a wide range of water depths, with Desmophyllum pertusum being one of the most common species. Pleistocene, Holocene, and modern coral mound formation by living CWC reefs have previously been described in the Porcupine Seabight from water depths greater than 700 m in the vicinity of the transitional zone between the Eastern North Atlantic Water and Mediterranean Outflow Water. Here we document occurrence of fossil corals retrieved from two cores at 370 m depth in the Macnas Mounds, a relatively shallow occurrence for mounds on the Irish shelf-edge. Both cores feature D. pertusum restricted to the upper two metres, immediately overlying an erosive surface and a coeval major down-core change in grain size from sand to mud. Radiocarbon dating of coral specimens indicates the CWC mounds initiated 7.82 Cal ky BP. Our study unequivocally documents the existence of Holocene shelf-edge coral mounds in the eastern Porcupine Seabight and highlights the possibility of other occurrences of CWCs in similar settings elsewhere in the northeast Atlantic. Given that no living CWCs were encountered in the study area, we suggest that the area previously experienced more favourable conditions for CWC mound initiation and development along the shelf-edge margin, possibly due to differing conditions in the European Slope Current which flows northward along the continental slope from south of the Porcupine Bank to the Faroe-Shetland Channel.

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To better understand the geodynamic evolution of northeastern China during the Late Mesozoic, we analyzed zircon U–Pb geochronological, Lu–Hf isotopic, and geochemical data for Early Cretaceous volcanic rocks from the southeastern margin of the Songliao Basin. Newly identified A-type rhyolite and trachyandesite yielded zircon ²⁰⁶Pb/²³⁸U ages of ca. 123 Ma and 117 Ma, respectively. The rhyolites are high in SiO₂ (72.24–78.89 wt%) and total alkali (K₂O + Na₂O = 8.81–10.03 wt%), and low in MgO (0.10–0.26 wt%), CaO (0.32–0.36 wt%), Ni (0.08–2.69 ppm), and Cr (0.39–4.87 ppm) concentrations, with negative Nb, Ta, and Sr anomalies. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs); the calculated Zr saturation temperatures are high (828–915 °C). The A-type rhyolites possess variable zircon ε_Hf(t) values ranging from + 5.69 to + 10.49. Petrogenetic analysis leads us to propose that the A-type rhyolites were probably formed by partial melting of a Neoproterozoic–Early Paleozoic juvenile lower crust. The trachyandesites have Nb/Ta (14.9–17.25), Zr/Hf (35.04–42.75), Rb/Sr (0.25–0.40), and Lu/Yb (0.14–0.15) ratios that are similar to those of mantle-derived magma, indicating a mantle source. They have ε_Hf(t) values of + 4.71 to + 7.29 and show enrichment in LILEs and LREEs, and weak depletion in HFSEs, suggesting that the parent magma originated from partial melting of a depleted lithospheric mantle, and was subsequently metasomatized by subduction-related fluids, followed by extensive fractional crystallization during the magma evolution. Combined with the temporal and spatial distribution of Late Mesozoic igneous rocks from the southeastern margin of the Songliao Basin, we propose that Early Cretaceous volcanic rocks formed in an extensional tectonic setting that was closely related to rollback of the Paleo-Pacific (Izanagi) oceanic slab.

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The Nigerian Younger Granite (NaYG) province is renowned for its world-class anorogenic alkaline ring granite complexes and has been recognized as a potential host for abundant Nb–Sn–W–Mo mineralization. In this study, a comprehensive investigation was conducted on the southernmost complex of the province, known as the Afu granite complex, encompassing petrography, zircon U–Pb dating and Lu–Hf isotopes, and whole-rock geochemical analyses. The dominant rocks within the Afu complex consist of biotite granite and albite biotite granite. Both rock units exhibit typical characteristics of A₁-type granite, including elevated levels of alkali oxides (Na₂O + K₂O = 8.01–8.52 wt%) and high field strength elements (Zr + Nb + Ce + Y = 495–950 ppm), as well as high FeO^T/(FeO^T + MgO) and Ga/Al ratios, similar to other younger granites found in Nigeria. Bulk rock Sm–Nd and zircon Lu–Hf isotopic data (εNd(t) = −2.6 to 0.9; εHf(t) = −7.2 to −0.5) indicate a predominantly crustal material-dominated magma source, with some contribution from mantle-derived magma. The linear correlations observed between Rb/Sr and Nb/Ta ratios as well as Sr contents from biotite granite to albite biotite granite suggest the significant role played by magma differentiation. The elevated bulk Nb contents observed in all rocks, along with the presence of Sn–W-bearing quartz veins, contribute to the rare metal mineralization of the Afu complex and indicate a combination of magmatic and hydrothermal processes. Additionally, zircon U–Pb dating results from the Afu granites reveal their relatively young ages (149–148 Ma) compared to other NaYGs (263–151 Ma), suggesting the cessation of anorogenic alkaline magmatism within the NaYG province.

The Posidonia Shale in the basement of the North Alpine Foreland Basin of southwestern Germany represents an important archive for environmental changes during the Toarcian oceanic anoxic event and the associated carbon isotope excursion (T-CIE). It is also an important hydrocarbon source rock. In the Salem borehole, the Posidonia Shale is ~ 10 m thick. The lower 7.5 m (1763.5–1756.0 m) of the Posidonian Shale and the uppermost part of the underlying Amaltheenton Formation were cored and studied using a total of 62 samples. Rock–Eval, palynological, maceral, biomarker and carbon isotope data were collected to assess variations in environmental conditions and to quantify the source rock potential. In contrast to most other Toarcian sections in southwest Germany, TOC contents are high in sediments deposited during the T-CIE, but reach a peak in post-CIE sediments. Biomarker ratios suggest that this reflects strong oxygen-depletion during the T-CIE (elegantulum to lower elegans subzones), but also during the falciferum Subzone, which is also reflected by a prolonged dinoflagellate cyst blackout. While sediments of the tenuicostatum Zone to the elegans Subzone are thinner than in neighbouring sections (e.g., Dotternhausen), sediments of the falciferum Subzone are unusually thick, suggesting that increased subsidence might have contributed to anoxia. The T-CIE interval is very thin (0.75 m). δ¹³C values of n-alkanes show that the maximum negative isotope shift predates the strongest basin restriction during the T-CIE and that the carbon isotope shift is recorded earlier for aquatic than for terrigenous organisms. In Salem, the Posidonia Shale is thermally mature and highly oil-prone. The residual source petroleum potential is about 0.8 tHC/m².

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Borehole image data and a 1D-stress model built on open hole logs, leak-off tests (LOT) and image logs are used to evaluate the potential of seismicity caused by fault triggering during geothermal heat production in the city of Vienna. Data were derived from a 4220 m deep geothermal exploration well that investigated the geothermal potential of fractured carbonates below the Miocene fill of the Vienna Basin. The well penetrated several normal faults of the Aderklaa Fault System (AFS) that offset Pleistocene terraces at the surface and hence are regarded as active. Stress-induced borehole failures and 1D geomechanical modeling proves that the potential reservoirs are in a normal fault stress regime with S_v > S_Hmax > S_hmin. While stress magnitudes in the upper part of the well (down to about 2000 m) are significantly below the magnitudes that would trigger the rupture of critically oriented faults including the AFS, stresses in the lower part of the drilled section in the pre-Neogene basement (below about 3300 m) are not. Data evidence a rotation of S_Hmax for about 45° at a fault of the AFS at 3694 m to fault-parallel below the fault suggesting that the fault is active. Critical or near-critical stressing of the fault is corroborated by the stress magnitudes calculated from the 1D geomechanical model. The safety case to exclude unintended triggering of seismic fault slip by developing geothermal reservoirs in close vicinity to one of the branch faults of the AFS may therefore be difficult or impossible to make.

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The Nagaland–Manipur Hill Ophiolite (NMHO) is an NNE-SSW trending linear ophiolite zone exposed in the northeastern states of Nagaland and Manipur in India. Basaltic rocks of NMHO are geochemically divisible into two broad groups in the Zr/Ti versus Nb/Y classification diagram. Samples with TiO₂ < 2 wt% and Zr = 38–118 ppm plot within the basalt field, whereas samples with TiO₂ > 2 wt% plot within the alkali basalt field. The latter can be subdivided into the alkali basalt group-1 (AB-1) with Zr = ~ 200 ppm and the alkali basalt group-2 (AB-2) with Zr = ~ 400 ppm. In a normal mid-ocean ridge basalt (N-MORB) normalized trace element pattern, basalt displays a near-horizontal trend from Lu to Pr at rock/N-MORB = ~ 1 and then increases slightly from Pr to Rb, whereas alkali basalt is relatively more enriched than basalt. In the chondrite-normalized rare earth element (REE) pattern, basalt displays near-horizontal trends with (La/Yb)_CN ranging between 0.78 and 1.89. On the other hand, alkali basalt displays parallel and steadily increasing enrichment trends from Lu to La [(La/Yb)_CN = 11.07–17.61], with a slight drop at Eu and Sm. Mantle melting models suggest: (1) partial melting of N-MORB-like sources at degrees of melting (F) = 4–8% for basalts; and (2) partial melting of ocean island basalt (OIB)-like sources at F = 7.5–17.8% for alkali basalt. Occurrences of basalts with N-MORB-like and and alkali basalt with OIB-like signatures within the NMHO complex suggest their origin from distinct magma batches with direct or indirect involvement of the Kerguelen plume.

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Chondrite normalised pattern for elements Nb, Zr and REE showing estimated compositions of source S1 and residue R1 for alkali basalt (estimated using DMI modeling) and source S2 for basalt (estimated using NBM modeling) in this study. DMM and OIB (Workman and Hart 2005) values are also shown for comparison. Average abyssal peridotite value is from Niu (2004) database.

Bali island is part of the Sunda volcanic arc with an active volcano at its center. The Buyan-Bratan caldera, the Batur volcano, and the Agung volcano are active volcanoes aligned from the west to the east of the island of Bali. The activity and age of volcanoes progress eastward. Therefore, the physical properties and conditions of each body of magma beneath the volcanoes on the island of Bali requires investigation. Ground magnetic measurements were carried out throughout the island and integrated with the Bouguer anomaly map. Hand samples were taken that represented each lithology of Bali Island. Furthermore, magnetic susceptibility and density measurements were carried out, which were used as initial data for generating magnetic and gravity models. Apart from that, geological maps and previous geological and geophysical research in the Bali Island area were also used as supporting data to create magnetic and gravity models. Magnetic and gravity modeling shows a carbonate rock formation in Bali Island's southern volcanic area, surrounded by a graben up to 6 km deep and covered by volcanic deposits. Furthermore, magnetic and gravity models indicate an intrusive rock between Agung and Batur volcanoes as well as shallow and deep magma chambers beneath the Buyan-Bratan Caldera, Agung, and Batur volcanoes. The magnetic susceptibility and density differences from the magnetic and gravity models in this study are one way to trace the eastward shift in volcanic activity on Bali Island. These findings will contribute to better understanding the volcanic processes and the migration of volcanic centers, which can help volcanic and earthquake hazard mitigation.

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