Geosystems and Geoenvironment最新文献

Neoproterozoic breakup of Rodinia resulted in the formation of several oceanic realms between dispersing cratons, which were later consumed during the assembly of Gondwana. In its western portion, the interior orogenic belts of Gondwana formed during the Brasiliano-Pan African Orogeny in the late Neoproterozoic-early Cambrian. Available geophysical, structural and petrological data suggest that the complex network of shear zones that once connected the Borborema province (NE Brazil), Tuareg shield (Hoggar) and Central African domain (NW Africa) likely represent ancient sutures that mark collisional episodes between Archean-Paleoproterozoic paleocontinents such as Amazonian-West African and São Francisco-Congo. Mafic, ultramafic and sedimentary sequences associated with this set of structures respresent dismembered ophiolite slices interpreted as oceanic remnants (sensu lato) that were emplaced during the late stages of the Gondwana assembly. For instance, the composite Transbrasiliano-Khandi-In-Tedeini-Silet shear system crosscuts rock assemblages preserving a complex history of oceanic-crust-transition development (Novo Oriente complex) in association with primitive to evolved magmatic arcs and UHP rocks both in the Borborema province and NW Africa. In the central Borborema province, preserved ophiolitic slices are strongly overprinted by ductile and brittle deformation events, but partially preserved MORB-like amphibolites are akin to subduction-related-types that crystallized in early- and late Neoproterozoic times docked via terrane accretion and dispersed by strike-slip shear zones. In the southern Borborema province, an example of a Neoproterozoic ophiolitic assemblage is the Monte Orebe complex, that encompasses T-MORB mafic rocks, ultramafic lenses, and exhalative sedimentary rocks akin to early to late stages of oceanic basin spreading, emplaced during convergent plate motions between the Pernambuco-Alagoas superterrane and the São Francisco craton. Correlative units are found in Cameroon, including the strongly hydrotermalized ultramafic rocks of the Lomié and Boumnyebel complexes, that are structurally controlled by top-to-the-south verging nappes found in the N-NW margin of the Congo craton. In all scenarios, the ophiolitic complexes are related to intra-oceanic and continental magmatic arcs as well as to geophysical signatures comparable to Phanerozoic suture zones. Although strongly dismembered, scrapped off Neoproterozoic oceanic crust partially preserved within the major belts of western Gondwana demonstrate the role of accretion-collisional orogenesis during its assembly.

The Nilüfer Unit of the Karakaya Complex (northern Turkey) preserves the remnants of the voluminous Triassic Tethyan magmatism. In the Imrahor area (Ankara), the Nilüfer Unit is characterized by a megablock consisting of hydrothermally altered basalts, gabbros, and subordinate wehrlitic cumulates. These lithologies display marked positive Nb anomalies with depleted Th/La ratios and varying degrees of LREE enrichment. They can be subdivided into two groups based on Nb/Nb*, which are not related to each other via fractional crystallization or extent of partial melting. With their strong Nb-kick and broad La/Sm range, the Imrahor lithologies are more akin to FOZO (Focus Zone)- or C (common component)-type melts than EM (Enriched Mantle) and HIMU (high μ). The Zr-Nb systematics reveals that melt mixing was an essential process in the petrogenesis of these mafic rocks. Based on geochemical modeling, the trace element systematics of the İmrahor lithologies can be explained by melting metasomatized oceanic lithospheric mantle (OLM) infiltrated by very low-degree melt fractions. The metasomatized OLM may represent a recycled, plume-related material that has generated Nb-enriched melts during the Triassic Tethyan magmatism.

Basaltic rocks from ophiolitic mélanges provide information on geodynamic setting, origin, and later tectonometamorphic conditions. This paper resolves the P–T conditions and timing of high-pressure metamorphism in an accretionary wedge that formed during the Late Jurassic closure of the Neotethyan Meliata Basin. Blueschist-facies metabasites of the Meliatic Bôrka Nappe and the Albian conglomerate pebbles of the Fatric Klape Nappe contain rare assemblages of garnet in association with glaucophane, phengite, rutile, and epidote (±albite). Here, we compare a Lu–Hf garnet age from the Meliatic Bôrka Nappe in the southern margin of the Inner Western Carpathians (IWC) with a garnet age from inferred Meliatic blueschists of the Klape Nappe overlying the IWC northern margin. The Hačava type locality of the Bôrka Nappe hosts calc-alkaline type metabasite (∼VAB-C; εNd₍₂₄₅₎ = +0.9) embedded within Middle to Upper Triassic marbles of the Neotethyan Meliata Basin northern continental margin. The investigated Klape Nappe island arc tholeiite (∼VAB-T; εNd₍₂₄₀₎ = +5.9) blueschist pebble, and associated deepwater metasedimentary rock pebbles were found in a conglomerate layer of unmetamorphosed Albian flysch. The garnet ages of 153.95±0.69 Ma and 152.1±1.5 Ma correspond to closure of the Meliata Basin during southward intraoceanic and continental margin subduction. This was followed by the exhumation of HP blocks in serpentinite mélange and the formation of an accretionary wedge with included anchimetamorphosed Jurassic flysch. The P–T conditions of the blueschists were constrained by Perple_X modelling to be 520°C and 1.55 GPa for the Bôrka Nappe, and 490–510°C and 1.68–1.72 GPa for a pebble from the Klape Nappe conglomerate. The similarity of garnet dates and metamorphic conditions between the two samples suggest that the blueschists formed during the Late Jurassic Meliatic subduction. The north-vergent Meliatic nappes are the inferred source of the Albian flysch conglomerates deposited in the foreland Fatric Basin. This material was subsequently transported by the Fatric Klape Nappe to the IWC orogenic front during the Turonian, following the closure of the Fatric Basin.

The study area is composed of mainly basement rocks, with problems of low yield of groundwater in most of the boreholes drilled. Therefore, this study was carried out to evaluate the hydraulic parameters of the basement aquifers using the vertical electrical sounding (VES), constant rate pumping test, and borehole logs. The mean value of aquifer resistivity and thickness are 408.4 Ωm and 20.2 m. The depth ranges from 20.9-88.0 m with a mean value of 55.5 m; showing appreciable groundwater potential mostly within the southern part. The value of transmissivity and hydraulic conductivity from the resistivity data ranges from 2.058-1761.033 m²/day and 0.205-138.664 m/day with a mean value of 152.251 m²/day and 8.841 m/day, respectively, indicating porous and permeable zones within the southern and northwestern part of the study area. The value of longitudinal conductance and transverse unit resistance ranges from 0.0034-4.2333 m-hom and 38.1-146439 Ωm² with a mean value of 0.3250 m-hom and 9704.1 Ωm²; indicating poor to moderate protective capacity in almost the entire area except for the small portion within the southern part. The mean value of fracture contrast and the reflection coefficient are 0.743 and -0.333, showing that the southern part has more water-filled fractures. Conclusively, this study reveals, that the aquifer depth ranges from 60-90 m, the transmissivity and hydraulic conductivity demonstrate high porosity and permeability within the southern parts of the study area, and serve as a baseline for groundwater resource management policy in Nigeria.

The cliff embankments are covered as sediments from the floodplain run on both sides of the river. Various sediments transportation and depositional mechanism are contained in these floodplain deposits. For better understanding of depositional history of the floodplain sediments cliff embankments along the Sai River at the villages of Chaukhada and Khatihara in the districts of Pratapgarh and Jaunpur, respectively, were analysed using a laser particle size analyser. The common facies observed in the studied sections are very silty sandy mud, very silty sand, slightly silty sand and very silty slightly sandy mud accordingly. Indurated calcretes are also observed in the very silty sandy mud layer in the Chaukhada section. The Chaukhada section has mean size value ranging from 2.80 phi to 4.98 phi while Khatihara section has these values ranging from 3.80 phi to 4.27 phi, implying coarse silt to very fine sand size range for both the cliff section. The sediments in both of the investigated cliff sections are poorly to very badly sorted, as seen by the sorting (standard deviation) values for the Chaukhada section, which are ranging from 1.12 phi to 2.22 phi, and the Khatihara section, ranging from 1.30 phi to 1.84 phi. The sediment samples displayed the skewness values ranging from 0.23 to 0.34 for the Chaukhada section and 0.09 to 0.32 for the Khatihara section, indicating that the sediments are very finely skewed. The kurtosis value ranging between 1.24 to 1.73 for the Chaukhada section and 1.22 to 1.72 for the Khatihara sections shows that the sediments are leptokurtic to very leptokurtic in nature. Bivariate plots of textural parameters have been also utilized to figure out the depositional environment. The C–M plot indicates that the graded to uniform suspension was the primary mechanism of transportation for all the sediments.

Turbidity currents are important ocean dynamic processes that influence sediment transport, ocean engineering, and marine environments. As a turbidity current evolves, its macroscopic and mesoscopic properties are controlled by ambient fluid mechanisms, including entrainment and mixing. These ambient fluid mechanisms are, in turn, influenced by changes in the multiscale properties of the turbidity current. These two aspects are often studied separately, and the feedback relationship between them is not fully understood. To reveal the feedback relationship and understand the control of ambient fluid on turbidity current evolution, this review summarizes and discusses the multiscale properties, ambient fluid mechanisms, feedback relationship, and the influencing factors from the relevant researches in the past 20 years. It is believed that ambient fluid entrainment and mixing change the multiscale properties of a turbidity current by affecting the overall dilution of the current and behavior of sediment particles. Changes in velocity, density, and stratification of the turbidity current enhance or suppress ambient fluid entrainment and mixing. Factors influencing the pattern and extent of the feedback interaction include the initial conditions of turbidity currents and their runout environments. The concept of ambient fluid – turbidity current – marine geological system is proposed. Under this system, the natural marine geological environment is considered in detail when studying turbidity current. Establishing this system necessitates improvements in research methods; particularly, improving the two-dimensional resolution of in-situ observations, developing new physics and numerical turbidity current generation methods, and using the Euler-Lagrange model to analyze interactions between particles and fluids, and between particles. Furthermore, complex external hydrodynamic fields as well as seabed geological fields could be considered in this system; this includes internal waves, bottom currents, typhoon-induced waves and currents, and canyon topography.

The early Palaeozoic palaeogeographic position of the North China Craton (NCC), including with respect to Gondwana, remains subject to debate. Detrital zircon chronology of lower Palaeozoic strata in the NCC should give insights into this scientific problem. Previous studies have reported zircon U–Pb ages for lower Palaeozoic clastic rocks from the western, southern, and eastern margins of the NCC. However, data remain limited for western Liaoning Province, northeastern China, in which representative Cambrian–Ordovician strata are well exposed. Data from this region should provide valuable constraints on the early Palaeozoic palaeo-location of the NCC, including with respect to Gondwana. Here we report laser-ablation–inductively coupled plasma–mass spectrometry detrital zircon ages from siltstone–mudstone of the Cambrian Mantou Formation in the Xingcheng area of Liaoning Province. Our new age determinations reveal two major source areas for the Mantou Formation: (1) the northeastern margin of East Gondwana, which yields zircons with ages of 1300–1000 and 600–510 Ma; and (2) the palaeo-highlands of the western NCC, which yields zircons of ca. 2500 and ca. 1800 Ma. Combining these new results with previously published data from lower Palaeozoic strata in the NCC, age spectra for the NCC exhibit dominant age peaks at ca. 2.5, ca. 1.9, ca. 1.0, and ca. 0.5 Ga, similar to those for Cambrian–Ordovician strata in northern India, western Australia, and Antarctica. Therefore, the NCC is interpreted to have been located close to East Gondwana during the early Palaeozoic. Integrating evidence from palaeobiogeography, palaeomagnetism, and detrital zircon geochronology, the NCC is inferred to have been situated near the boundary between northern India and western Australia during the early Cambrian–Middle Ordovician.

This study presents petrography, mineral chemistry, whole-rock major and trace elemental systematics for Recent pyroclastic deposits from volcanic edifices of Baïgom and Petpenoun, located in the Noun Plain graben along the Cameroon Volcanic Line (CVL), Central Africa, to better understanding their petrogenesis and geodynamic environment. The samples are mainly ash, lapilli and scoria, belonging to two different magmatic series: an alkaline series and a sub-alkaline series. Petrographic and mineralogical evidence in alkaline samples show a common crystallizing mineral assemblage of olivine, clinopyroxene and/or plagioclase, scattered in a fine-grained to glassy matrix. Clinopyroxene occurs in variable sizes, and includes megacrysts, which show either little or strong zonation, and thus represent different processes. The large, unzoned crystals provide clear evidence for prolonged magma chamber processes involving high and near-constant melt/Clinopyroxene ratios. The megacryst abundances are interpreted as crystal accumulation. The thermobarometry results in alkaline samples provide temperatures of 1188–1267 °C and pressure estimates in the range of 10.3–23.8 kbar. All the Recent pyroclastic deposits display high (La/Yb)_N (11.20–20.35) and (Tb/Yb)_N ratios (1.93–2.64), suggesting that their parental magmas originated from partial melting in the garnet-bearing mantle zone, at a depth > 80 km. The near-constant Ce/Y (3.14–3.76) and the higher Zr/Nb (5.85–10.71) ratios in sub-alkaline lava samples adequately fit with derivation of the magmas from a melt column extending across the garnet-spinel transition zone. Our results suggest two distinct origins for Recent pyroclastic samples from volcanic edifices of Baïgom and Petpenoun and imply an underlying heterogeneous mantle source beneath the CVL.

Charnockites (orthopyroxene-bearing granitoids) constitute one of the major components in many Precambrian high-grade metamorphic terranes of the world. They are broadly classified into two types based on the nature of occurrence as massive charnockite that forms large (magmatic) batholiths, and incipient charnockite which occurs as patches, veins and lenses of mesoscopic scale developed through local dehydration reactions within felsic (both igneous and sedimentary) protoliths. Here we present petrological data on representative charnockites of different ages and tectonic settings from various parts of the world and evaluate their formation conditions based on phase equilibrium modeling in the system NCKFMASHTO. Massive charnockites ranging from Neoarchean to Paleoproterozoic age from the Salem and Nagercoil Blocks (southern India), Napier Complex (Antarctica), and the Limpopo Complex (Zimbabwe) record P-T conditions that are broadly equivalent to the regional P-T conditions of high-grade metamorphism recorded in these terranes, ranging from normal granultes to ultrahigh-temperature metamorphic rocks. Typical unmetamorphosed magmatic charnockite in a late Paleoproterozoic post-collisional extensional setting and carrying K-feldspar phenocrysts from the North China Craton yield a wide P-T stability range. Incipient charnockite from southern India records P-T conditions that are slightly lower than the peak P-T regimes estimated for the region. Regardless of the discrepancies in P-T conditions estimated from the phase equilibria approach, the data confirm that these rocks, in all the cases considered in this study, were generated under low aH₂O conditions. The available fluid inclusion data from both massive and incipient charnockites confirm that the water activity was buffered to low levels, possibly through the presence of CO₂-rich fluids, to stabilize the index mineral orthopyroxene in these rocks. Charnockites formed in different plate tectonic settings; in most cases as arc magmas within subduction-collision settings and some cases, post-collisional extension during different periods in Earth history, and serve as potential archives of melt and fluid processes in the continental crust.