International Journal of Earth Sciences最新文献

The dismembered ophiolites in the Wadi Al-Barramiya area, Central Eastern Desert of Egypt, are one of a series of Neoproterozoic ophiolites present in the Arabian-Nubian Shield. Here we present new fieldwork, whole-rock geochemical data, and mineral chemistry of metapyroxenite and metagabbro associated with the Al-Barramiya ophiolite in order to constrain its nature and tectonic setting; in particular whether the ophiolite was formed in a subduction or non-subduction setting. The rocks selected were obtained from the mantle section (serpentinized peridotite), an ultramafic (pyroxenite) and the crustal section (metagabbro). The serpentinized peridotite is altered to talc carbonate and listvenite, and associated with magnesite. Pyroxenite occurs as irregular coarse-grained lenses of websterite and olivine websterite within the serpentinite and occurs adjacent to the ophiolitic metagabbro. The metagabbros form scattered allochthonous masses of various sizes that are distributed across the area. The ophiolitic rocks are metamorphosed from greenschist to lower amphibolite facies. Locally, fresh relicts of olivine and Cr-spinel can be found in the serpentinite, whereas pyroxenite has fresh relicts of olivine, clinopyroxene and Cr-spinel. Cr-spinel in the metapyroxenite is zoned, with Al₂O₃, Cr₂O₃ and MgO decreasing and FeO_t increasing from cores to rims, reflecting the effects of metamorphism that selectively removed the now-depleted components. The metagabbros are characterized by enrichment in large-ion lithophile elements (LILE) over high field strength elements (HFSE) and are tholeiitic with a calc-alkaline affinity. The high Cr# (0.63–0.75) of fresh Cr-spinel relicts in the metapyroxenite, together with their low TiO₂ contents (0.04–0.24 wt%), indicate that this rock is similar to highly refractory ultramafic rocks that evolved in a fore-arc setting. This is supported by the high forsterite content (Fo = 0.91–0.93) of fresh olivine and high Mg# (0.93–0.95) of fresh clinopyroxene. Clinopyroxenes in the metapyroxenite and metagabbro have the chemical characteristics of boninite, confirming the fore-arc setting.

Graphical Abstract

Since the 1970s, it has been widely accepted that the Nördlinger Ries and the Steinheim impact structures represent a crater doublet formed by the simultaneous impact of a binary asteroid in the Middle Miocene. From a biostratigraphic point of view, however, the lowermost crater-lake sediments deposited in the drainless morphological depressions differ in age by ~ 0.5 to 1 Myr. Recent work additionally questioned the double-impact theory due to the occurrence of two vertically separated seismite horizons in North Alpine Foreland Basin deposits, interpreted to result from two different impact-induced seismic events. A continuous double-layer ejecta blanket originally surrounded the Ries crater within a minimum distance of 45 km from its center. Distal Ries ejecta consist of sedimentary and shocked basement rock fragments of the Ries area. The Steinheim crater is located 41 km WSW of the Ries crater and filled by a ‘basin breccia’ that consist of Middle and Upper Jurassic rock fragments. Most parts of the breccia and overlying crater-lake deposits are preserved. If both craters formed simultaneously, Ries ejecta would have reached the Steinheim area and should be incorporated in the Steinheim breccia or intercalated between the basin breccia and crater-lake deposits. However, no sedimentary or basement rock fragments derived from the Ries crater have ever been found in outcrops or drillings into the Steinheim crater. We conclude the Steinheim impact crater did not exist at the time of the Ries impact and the Steinheim asteroid rather impacted into the outer continuous distal Ries ejecta blanket some 0.5 to 1 Myr after the Ries impact.

Graphical abstract

Geological map of the Ries crater with the present distribution of its ejecta blanket and the geographical position of the Steinheim crater ~41 km WSW of the Ries crater. The Ries ejecta blanket consists of the more proximal type of impact breccia (Bunte Breccia) and the more distal type of impact breccia (Bunte Trümmermassen)

In volcanic belts, magma influx into magma chambers generates excess pressure, amplifying the initial stress field to cause crustal deformation with significant ground displacements, which manifests in topographic relief. Quantifying such volcano-driven ground surface displacements is a fundamental requirement to embark on a criticality analysis of volcanotectonic events and associated hazard monitoring strategies. This study theoretically examines the underlying dynamics of surface displacements in a volcanic plumbing system comprising multiple magma chambers. The classical Mogi equation is extended to derive a set of analytical solutions to evaluate surface displacements as a function of separations between two off-axis chambers, measured along both horizontal and vertical directions. The resulting surface displacement plots, from the analytical solutions are compared with those calculated from a set of finite element (FE) model simulations run with the same parameters considered for the analytical formulations. Both the analytical and FE results suggest that horizontal (S_h) and vertical (S_v) separations of magma chambers largely control the vertical (U_z) and lateral (U_r) ground-displacement components. Spatially varying U_z attains its peak value at a specific location above the chambers, but increasing horizontal separation (S_h ~ 10 km) transforms the single-peak U_z pattern to a weakly developed double-peak U_z pattern, which eventually give way to two prominent high-amplitude peaks above the chambers when S_h ~ 25 km. Similarly, a large vertical separation (S_v ~ 6 km) yields double peaks in the U_z profile, which merge to form a single peak for small S_v (~ 1.5 km). The FE model results are used to map the stress fields around the two magma chambers to show that inter-chamber mechanical interaction can influence the deformation behaviour around the chambers, depending on S_h and S_v magnitudes. Finally, the model estimates are evaluated using available reports on the naturally occurring volcanoes: Teide volcano (Tenerife, Spain) and Long Valley Caldera (USA).

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Understanding the origin of Late Jurassic volcanism in southern Patagonia is crucial for unraveling the early Andean orogenic evolution. However, radiometric dating is not connected to stratigraphic analysis along the South Patagonian Andes, which obscures the real duration of the Late Jurassic magmatic activity. In this contribution, we present the results of a volcanic stratigraphy analysis, complemented by structural and petrographic data, on a thick succession of acidic volcanogenic rocks in the Laguna Verde district of southern Chile located along the south shore of General Carrera-Buenos Aires Lake. Through the recognition of igneous stratigraphy, we strategically sampled representative volcanogenic rocks that cover the entire duration of eruptive activity. By doing so, the new U–Pb zircon magmatic ages, combined with a compilation of U–Pb crystallization ages from the South Patagonian Andes, allows us to constrain the volcanic activity in the study area to a period of 8 My (~ 155–146 Ma, V3 stage) and 11 My considering age inherent errors. The field recognition of normal faults and the syn-kinematic emplacement of sub-volcanic bodies, which are inferred to conform to a ring-fault system, along with the presence of a thick succession of ignimbrites, suggest that the syn-extensional volcanic emplacement occurred in a caldera volcanic environment. This setting was responsible for the short-lived, voluminous eruptions. Furthermore, the high Th/U zircon ratios identified for the ~ 155–150 Ma period indicate the climax of extensional tectonics. The integration of these data supports the hypothesis that retreating-mode subduction played a major role in producing ignimbrite flare-ups.

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Reliable reconstructions of tephrostratigraphy and emplacement mechanisms of Early to Middle Miocene volcaniclastic deposits across the Alpine-Mediterranean region may yield important clues as to the nature, spread, volume, magnitude and frequency of large silicic eruptions of the Carpathian-Pannonian Region. Here we report on a sequence of Middle Miocene volcaniclastic deposits intercalated with lacustrine strata from the Livno-Tomislavgrad Basin, part of the Dinaride Lake System. We applied a multi-proxy approach to elucidate their source, decipher their emplacement mechanism, and evaluate their basin-scale and regional correlativity. New high-precision zircon geochronology (~ 14.32 Ma) reveals their simultaneity with numerous volcaniclastic deposits (and their alteration products) widely spread across the Alpine-Mediterranean region. Additionally, their correlativity is confirmed at the scale of the Livno-Tomislavgrad Basin, based on similar lithostratigraphy, mineralogy and volcanic glass geochemistry. Newly obtained zircon Hf isotope data imply that Livno-Tomislavgrad Basin distal volcaniclastic deposits and ~ 14.36 Ma Harsány ignimbrite of the Carpathian-Pannonian Region had shared a parental eruptive center. However, different volcanic glass geochemistry, bolstered by the high-precision geochronology, suggests distinct eruption events, implying more frequent explosive volcanism of the Carpathian-Pannonian Region during Middle Miocene than previously recognized. We suggest that the ~ 14.32 Ma fine fallout tephra, deposited in the distal basin in the Dinarides (> 400 km from the source), was likely re-mobilized by water-driven hillside erosion from the extensive paleo-relief, and further transported via land-derived gravity flows. Upon entering the lake, the gravity flows likely transformed into subaqueous sediment density flows. These deposited ~ 1 to 7-m-thick overall graded volcaniclastic turbidites, thinning away from the presumed source of tephra re-mobilization. Although over-thickened, the ~ 14.32 Ma Livno-Tomislavgrad Basin volcaniclastic deposits can still serve as a reliable tephro- and tectono-stratigraphic markers due to their rapid mode of accumulation.

Graphical Abstract

Schematic illustration of the formation of studied ~14.32 Ma volcaniclastic successions of the SE Livno-Tomislavgrad Basin, Dinaride Lake System

Sedimentary provenance analysis has rarely been applied to continental rifts, because source-rock terrains are assumed to be uniform and supply systems stable and closely connected to the formation of graben shoulders. Here, we test the suitability of multi-proxy sedimentary provenance analysis to unravel the evolution of a segment of the East African Rift System from its sedimentary record, where an extreme relief has been created and a continent-wide drainage system has been affected. Multi-proxy techniques include petrographic-mineralogical analysis (framework, heavy mineral spectra, zircon morphology, and colour), as well as single-grain garnet geochemistry, Zr-in-rutile thermometry, and geochronological methods (detrital zircon U–Pb ages). Published data are compiled and complemented by new U–Pb detrital zircon data of rift sediments. Because a comprehensive database of basement source-rock compositions from East Africa is missing, published modern river sand compositions have been added to provide endmember source fingerprints. The selected Albertine Rift is surrounded by four main source terrains within the East African Basement, which cannot be separated clearly by individual provenance indicators. Most useful were heavy mineral spectra, garnet composition, and with limitation Zr-in-rutile thermometry showing different metamorphic overprint. U–Pb detrital zircon spectra assisted well in detecting juvenile magmatism of different orogenic events, but repeated inheritance in younger units prevented a strict allocation. Petrographic maturity turned out to be more dependent on multi-cycle detritus as on climate shift. Applying endmember composition to rift sediments, three evolutionary stages of the Albertine Rift could be identified which coincide with basin-scale unconformities. Moreover, we could prove a large, southwest trending drainage network in East Africa during the Miocene, which was destructed step-wise by ongoing rifting. Extreme uplift of the Rwenzori Mountains up to 5109 m could be documented by a specific set of provenance indicators justified by modern river sand. Three main lessons can be drawn from this study: (1) basement units can be only reliably characterized by a combination of provenance-sensitive parameters, and here, the frequently used U–Pb detrital zircon ages are not sufficient; (2) it is possible to define subunits, e.g., individual fault-blocks within the rift; and (3) chemical weathering is unproblematic when applying a multi-parameter approach even in a tropical setting.

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The use of the HVSR (Horizontal-to-Vertical Spectral Ratio) method on single-station microtremor measurements is well documented in small alluvial plains for bedrock mapping. In large sedimentary basins, like the Po Plain, its application is still debated. To shed some light on this issue, we investigated two seismogenic structures buried below the Po Plain Quaternary deposits: the Mirandola and Casaglia anticlines. We acquired and analysed a dense distribution of HVSR data covering the two areas and mapped the frequency and amplitude values of the observed resonance peaks. The top of both anticlines is highlighted by high amplitude peaks picturing E-W elongated sectors with high-impedance contrast, where Quaternary deposits are reduced in thickness to about 60–130 m and directly overlay the Pliocene (Mirandola) and Miocene (Casaglia) marine units. In Mirandola, the high-amplitude peaks also correspond to higher resonance frequencies, while in Casaglia, the distribution of resonance frequencies is relatively uniform suggesting a flatter crestal region and the lateral continuity of the resonance surface. The combination of peak frequency and amplitude information on a dense grid of measurement points is thus confirmed to be useful for identifying and mapping buried geological structures such as structural highs. Further modelling is being carried out to estimate the depth of the surface responsible for the observed resonances, through calibration with borehole information.

Graphical Abstract

The Neoarchean Qingyuan greenstone belt (QGB) is located at the northeastern margin of the North China Craton (NCC) and has experienced amphibolite- to granulite-facies metamorphism. The supracrustal rock assemblage is composed of biotite-hornblende (Bt-Hb) monzonitic gneiss, garnet-biotite-hornblende (Grt-Bt-Hb) monzonitic gneiss, Grt-Bt-Hb plagioclase gneiss, amphibolite and orthopyroxene-biotite-hornblende (Opx-Bt-Hb) plagioclase gneiss. This study involves thorough field and petrographic observations, whole-rock geochemistry and LA-ICP-MS zircon U‒Pb ages of these rocks to constrain the Neoarchean geodynamic setting of the QGB. Zircon U‒Pb dating indicates that these metavolcanic rocks formed during 2.57‒2.52 Ga and experienced subsequent regional metamorphism at 2.52‒2.47 Ga. Lithological and geochemical characteristics show that the QGB protoliths were tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites. The andesite-dacite-rhyolite assemblage exhibits high SiO₂ and Mg# values, low Yb and Y contents, strongly fractionated chondrite-normalized REE patterns and depletion in Nb, Ta, Ti and P, resembling Phanerozoic adakites. These geochemical characteristics suggest an origin from the partial melting of a subducted slab interacting with mantle wedge material. The tholeiitic to calc-alkaline basalts exhibit slightly LREE-enriched chondrite-normalized REE patterns and negative Nb and Ti anomalies, like island arc basalts. These basalts might have originated by partial melting of a mantle source affected by metasomatism from subduction-derived fluids and melts. The lithological associations and geochemical characteristics imply that the Qingyuan greenstone belt developed in a continental margin arc setting.

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In this study, we use Broad Ion Beam polishing and Scanning Electron Microscopy (BIB-SEM) to characterize the microstructure of selected core samples of immature Upper Cretaceous carbonate-rich oil shales from Jordan and to link the observations to porosity and compositional and geochemical data. The aim of this study is to understand the distribution of pore space, primary organic matter, and organic sulfur on a sub-micron scale, particularly in carbonate- and silicate-dominated layers. The thermal maturity of these marine carbonate mudstone samples of pelagic origin was found to be influenced by the elevated sulfur contents in these Type II-S kerogen source rocks. This was confirmed through both organic geochemistry and BIB-SEM observations, which revealed high sulfur content. Porosity in the carbonate mudstone exists within foraminifera, and aggregates of microfossil fragments. Initially, these voids provided significant inter- and intra-particle porosity which were later filled by organic matter during diagenesis. This ‘mobile’ organic matter is interpreted as microscopic bitumen, which exists as a solid or highly viscous fluid at surface conditions. It is likely a residue of low-temperature (“early”) bitumen generation. By examining the samples before and after dichloromethane (DCM) extraction and subsequent BIB-SEM analyses, we observed that the specimens contained a significant amount of soluble organic matter (SOM), mostly present in the micropores associated with calcite. The microscopic solid bitumen is observed to remain stable even under various conditions, such as in vacuum oven conditions of 105 °C (24 h), or exposure to ultra-high vacuum, broad ion beam (heat > 70 °C) and an electron beam of 15 keV. This suggests that the solid bitumen acts as a solid at elevated temperatures and confining pressures (85 °C and 250 MPa), and its presence can lead to the buildup of significant fluid overpressures. Our observations indicate that the pores associated with calcite provide high storage capacity in the shales during the early stages of hydrocarbon generation. In contrast, it suggests that siliciclastic-rich samples are more prone to hydrofracturing as the (early) generated hydrocarbons cannot be expelled easily. These findings highlight the complex distribution and behavior of pore space, organic matter, and sulfur in shales, shedding light on their potential for hydrocarbon generation and storage.

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The organic geochemistry and coal petrology of Palaeogene coals from northeast Greenland (Thyra Ø Island and Kronprins Christian Land) and central Spitsbergen (Longyearbyen and Grumantbyen) were studied using Rock–Eval and gas chromatography–mass spectrometry, as well as microphotometry and maceral group analyses. Bulk data and biomarker distributions of the coals demonstrate a low coal rank for both, but a lower coalification degree of coals from Greenland (0.49–0.55% VRr) compared to those from Svalbard (0.68–0.75% VRr). Maceral group analyses revealed relatively similar distributions with a strong predominance of vitrinite. The generally high abundance of hopanoids (hopanes/hopenes and hopanoic acids) implies a strong bacterial reworking of the organic matter, whereas sulphur occurrences indicate a marine influence after organic matter deposition. A great variety of higher plant biomarkers was detected in all coals. Distinctive compounds recorded in the coals are aliphatic and aromatic diterpenoids as well as partly hydrogenated picenes, suggesting strong input of conifers and angiosperms. Pristane/phytane ratios indicate that the organic matter in the ancient swamps was deposited in an oxic, fluvio-deltaic setting at both sites. This study provides a detailed geochemical investigation of understudied coals from northeast Greenland. Moreover, it enhances our understanding of probably interrelated Palaeogene depositional settings from Greenland and Spitsbergen in terms of their palaeoecology, primary input into coal swamps, and individual thermal history.

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