Crustal Section Research Articles

Structural, lithostratigraphic and thermal features of a Permian lower crust from the Western Italian Alps (Valpelline Series, Valle d’Aosta)

AbstractThe Valpelline Series (Dent-Blanche Tectonic System, Western Italian Alps) is a sector of lower continental crust, which consists of Permian migmatitic metapelite with different mineral assemblages (i.e., garnet-, cordierite- and orthopyroxene-bearing), minor amphibolite and marble, intruded by aplite and pegmatite. Widespread melt production in metapelite and locally in amphibolite occurred during the development of the regional foliation. The P–T conditions during migmatisation, estimated using conventional geothermobarometers, range between 800–900 °C and 0.5–0.8 GPa, with a difference of up to ∼50 °C between cordierite- and orthopyroxene-bearing migmatites, the latter reaching higher temperatures. The Valpelline Series shows rock types, metamorphic assemblages, P–T conditions and published ages of high-temperature regional metamorphism like the archetypal lower crust section of the Ivrea-Verbano Zone in the Southern Western Alps. The latter likely represents an external portion of the same extending lower crust, at the onset of the Tethyan rifting due to lithospheric extension and asthenospheric rising.

Subcretion of altered oceanic crust beneath the SW São Francisco Craton, Brazil – A stable isotope study on diamonds and their inclusions

The presence of diamonds of lherzolitic, eclogitic and websteritic paragenesis in proximal alluvial deposits on the southwestern edge of the São Francisco Craton documents the incorporation of subducted oceanic crust and associated metasomatism through slab dehydration fluids affecting the local SCLM. To better constrain the subduction-association of diamond substrates and metasomatic events, we conducted a combined study of the δ13C-δ15N-[N] characteristics of 81 diamonds and the δ18O values of four of their eclogitic garnet inclusions. Diamond carbon isotope compositions range from −25.5 to +0.5‰, with 13C-depleted diamonds (≤ −7‰) being exclusively of eclogitic/websteritic paragenesis while the 13C-enriched (≥ −2‰) tail of the distribution is related to diamonds with lherzolitic inclusions. Nitrogen isotope values range from −14.2 to +25.5‰, with about half of the values being positive. A general absence of coherent trends in δ13C-δ15N-[N] across growth zones implies that diamond formation did not occur under fluid-limited conditions. Instead, the observed heterogeneity in carbon and nitrogen isotope compositions reflects contributions of distinct source reservoirs hosted in both altered oceanic crust and Earth's mantle. Nitrogen contents peak around a δ15N value of −3.5‰, indicating that more N-rich fluids, presumably representing a primitive endmember composition, have a mantle-like δ15N signature. While positive and negative δ15N values occur equally near the δ13C mantle value (−5 ± 2‰), 13C-depleted diamonds have nitrogen isotope compositions skewed towards positive values.13C depletion and 15N enrichment is a signature of biogenic carbonates/organic matter and low-T clays in uppermost, basaltic sections of oceanic crust that experienced low-temperature seawater alteration prior to subduction. Correspondingly, the oxygen isotope compositions of eclogitic garnet inclusions fall in a restricted range between +5.5‰ to +7.0‰. For three of the four samples, the stable isotope signatures of inclusions and host diamonds display perfect agreement, with the intensity of seawater alteration signatures, in the form of garnet inclusion 18O enrichment and host diamond 13C depletion and 15N enrichment, increasing together. For the fourth sample, the δ18O signature of the garnet inclusion (+5.5‰) and δ13C-δ15N signatures of the diamond host (−25 and + 19‰, respectively) are decoupled. While the mantle-like δ18O signature indicates a diamond substrate derived from deeper levels in oceanic crust (e.g., deep sheeted dikes), the diamond-forming fluids must have originated from sources that originally resided near the sea water interface. A viable mode of mixing such disparate isotopic signatures is the interaction of diamond-forming fluids derived from shallow oceanic crust altered at low-temperatures with eclogitized substrates originally formed in deeper levels of oceanic crust. This process likely occurred during diamond formation in a tectonic subduction mélange, which juxtaposes deeper and shallower levels of oceanic crust. In the lithospheric mantle above the subcreted oceanic slab, the elevated carbon isotope and highly variable nitrogen isotope compositions of diamonds formed in lherzolitic substrates likely relate to devolatilization and/or melting of principally oceanic sediments containing marine carbonates and clays and subsequent mixing with mantle-derived volatile components. In combination, our diamond and inclusion stable isotope data provide insight into multiple processes that promote diamond formation both inside subducted slabs accreted to the São Francisco cratonic keel and in adjacent subcontinental lithospheric mantle.

Unravelling the hydration history of an inverted passive continental margin using epidote U[sbnd]Pb geochronology and Pb–Sr–O–H isotope geochemistry

Inverted passive continental margins permit to comprehensively characterize fluid circulation in the continental crust in orogens from rifting to tectonic inversion. Since granitic rocks have low primary water contents and dominate the continental crust, their hydration exerts important control on metamorphism and deformation of the continental crust during orogeny. It is therefore of great interest to determine whether the granitic continental crust is hydrated during rifting phases or after tectonic inversion. Because fluid circulation may occur at both stages, a comprehensive picture of hydration of the continental crust in inverted passive continental margins can be obtained by the combination of different geochronometers and geochemical methods.In this study, we combine UPb geochronology and Pb–Sr–O–H isotope geochemistry of epidote in hydrothermal veins in the Albula Pass area (eastern Swiss Alps) as powerful tools to trace hydration events in the continental crust in orogens. The Albula Pass area is part of the inverted Adriatic passive continental margin, which was extensively affected by seawater infiltration during continental rifting in Jurassic times. Epidote geochronology elucidates the fluid circulation history in this rifted crustal section by revealing two hydration events that are not recorded by other datable minerals: (1) 85.2 ± 9.7 Ma and (2) 59.9 ± 2.7 Ma, showing that fluid circulation in the Adriatic passive continental margin continued after Late Cretaceous tectonic inversion. Epidote Pb–Sr–O–H isotope geochemistry characterizes pathways and fluid sources of the epidote forming fluids, which differ between the Late Cretaceous and the Paleocene fluid circulation events. The Late Cretaceous epidote-forming fluids were produced by compaction of sedimentary units beneath the Err nappe and release of modified seawater (i.e., formation/connate water) from marine sediments, while Paleocene veining was mainly mediated by syn-kinematically infiltrated meteoric water. The present geochronological and isotope data argue for multi-stage hydration of the continental crust in the inverted Adriatic passive continental margin. This work promotes the importance of the interplay of newly introduced and recycled fluid components in the hydrothermal alteration of the continental crust, and it highlights epidote as a powerful hygrochronometer and isotope tracer.

Мониторинговые электротомографические исследования зоны Иссык-Атинского разлома (Северный Тянь-Шань)

In 2023, monitoring geophysical studies were carried out using the electrical resistivity tomography (ERT) method along three profiles that cross the strike zone of the Issyk-Ata fault in the transverse direction. A detailed study was carried out of the observed variations in apparent electrical resistivity, which characterize the change in the electrical properties of the upper part of the Earth's crust section with a 40-minute discretization in time. Using the analysis of difference pseudo-sections of apparent resistivity, constructed based on the results of ERT monitoring, the most informative pseudo-depth interval from the point of view of geoelectric activity was determined for all three monitoring profiles. Accordingly, for a pseudo-depth of 24 m on each monitoring profile, an assessment was made of the geodynamic activity of these sections of the Issyk-Ata fault based on an analysis of the magnitude of two parameters: the amplitude of apparent resistivity variations and the correlation coefficient of electrical resistivity variations with lunar-solar solid tides. High values of these parameters, in our opinion, determine the location of modern active fault zones.

Construction of an Early Cambrian intra-oceanic arc within the West Junggar, NW China: Magmatic records from proto- to mature-arc

The Junggar orogenic collage in Central Asia is characterized by the accretion of multiple Paleozoic intra-oceanic arc terranes developed within the Junggar Ocean, yet magmatic records relating to the construction of these ancient oceanic arcs are largely disrupted. Here, we report new petrological and geochemical data of mafic-ultramafic rocks documented from the Early Cambrian Saleinuohai ophiolitic mélange in southern West Junggar. The mafic rocks include dolerites and basalts, which show geochemical features (e.g., lower TiO2 and Zr relative to MORB, depleted LREE, and high εNd(t)) resembling those of forearc basalts from the Izu-Bonin-Mariana arc. Thus, they may represent the upper section of newly-accreted oceanic crust formed during subduction initiation, upon which a nascent arc is soon built. In contrast, the ultramafic rocks include dunite, wehrlite, and clinopyroxenite, which display cumulate textures with a crystallization order of olivine-clinopyroxene-plagioclase. Similar cumulate sequences are commonly observed in island-arc environments, presumably crystallized under a relatively hydrous and high-pressure condition. The calculated equilibrium melts for these cumulates further resemble those adakitic rocks previously reported from the Saleinuohai ophiolitic mélange. Thus, they may represent the basal section of mature island arc crust that formed during intracrustal fractionation. Our data, together with previous results, provide a complete magmatic record of the development of an Early Cambrian intra-oceanic arc within the Junggar Ocean.

A bimodal source for the generation of tonalitic to granitic magmas in a non-subduction-related magmatic belt: An example from the Sierra Chica of Córdoba, Argentina

A notable consensus prevails about the hybrid nature of intermediate calc-alkaline magmatism. Accordingly, petrogenetic models envisage the addition of young mantle material and cortical recycling in those geological settings where intermediate magmas are dominant. Most exhaustively studied examples are: 1) magmatic arcs associated with subduction zone settings where oceanic crust and sediments are introduced into the mantle wedge, and 2) deep crustal sections are pervaded by hot, H2O-rich mantle-derived liquids. Nevertheless, back-arc or foreland terranes of several hundred kilometers in width, frequently characterized by high heat flux, crustal melting, recycling and melt fractionation, are crucial areas to evaluate the generation of intermediate magmatism. The Sierra Chica of Córdoba (Argentina) provides an excellent example to study these processes. The high-grade crustal segment shows metamorphosed pelitic and basaltic protoliths that were part of an old sedimentary succession. We evaluate the evolution of this bimodal source during two consecutive early Paleozoic orogenic periods when it was established as the source area of intermediate to silicic magmatism far from the contemporary magmatic arcs. Field relations and new geochemical and geochronological data revealed the occurrence of two magmatic events during Cambrian and early Ordovician periods. Cambrian migmatization and partial melting (∼528 to 505 Ma) is represented by irregular-shaped monzogranites and tonalites, while early Ordovician magmatism (∼480 Ma) is characterized by dyke-shaped pegmatites and tonalite-trondhjemites. The preservation of two contrasting igneous lineages suggests a genetic linkage between metapelitic and amphibolitic migmatites and between granitic and tonalitic-trondhjemitic partial melts. Coeval with this magmatism, Famatinian tonalitic-trondhjemitic to monzogranitic magmas intruded upper crustal levels in Sierra Chica and other parts of Sierras of Córdoba. Geochemical signatures point to a bimodal (mafic/felsic) source for this intermediate to silica-rich magmatism. The evolution of partial melts studied in the Sierra Chica is depicted on the basis of geochemical projections with fractionation and hybridization models, in order to establish the primary liquids of this Famatinian magmatic belt. Trends defined by Famatinian tonalites, trondhjemites and granodiorites suggest that primary Ca-rich melts (tonalites and trondhjemites of this study) evolved through fractionation of plagioclase (30–50 vol%) compatible with the preservation of inferred co-magmatic Pl-cumulates within these plutonic bodies. Peraluminous granites and monzogranites can be explained by a simple restite unmixing process involving alkali-rich liquids generated in the metapelitic part of the source. The addition (hybridization) of up to 30 vol% of granitic melts to the tonalite-trondhjemite end-member may account for the composition of other Famatinian granodioritic to monzogranitic intrusive rocks from the plutonic belt. The crustal segments of the Sierra Chica have undergone metamorphism and partial melting in out-of-arc settings: the Cambrian fore-arc and the early Ordovician back-arc. The voluminous magmatic belt rooted in the paired metapelitic-amphibolic source area is involved in the origin of intermediate to silicic Famatinian magmatism without significant mass transfer from the mantle and crustal growth, instead it is dominated by crustal melting, recycling and fractionation.

Variations in the Composition of MORB Chilled Glasses from the Mid-Atlantic Ridge, 12°–31° N: Reflection of Composition Evolution of Parent Melts and the Influence of the Hydrothermal Component

The geochemical peculiarities of sample collection of MORB chilled glasses obtained in six areas of the axial zone of the Mid-Atlantic Ridge (MAR), 12°–31° N have been studied. The results of this study provided information on the composition of the parent melts for these glasses and made it possible to assess probable geochemical effects reflecting the interaction of magmatic melts with hydrothermal systems of the MAR axial zone or with altered oceanic crust (AOC). It is shown that basalts of the E-MORB family which includes most of samples are localized mainly in the “cold” segments of the MAR in the crustal section of which serpentinites predominate. On the other hand, samples with depleted signature (N-MORB) belong to segments of the MAR in which serpentinite outcrops are either absent or play a subordinate role. The E-MORB chilled glasses from “cold” segments of the MAR show signs of contamination of basaltic melts with components borrowed either from the host serpentinites or from aqueous-saline fluids circulating in hydrothermal systems located in serpentinites (“serpentinite hosted”). Judging by the data obtained on the nature of variations in the content of Cl, U, and Sr in the studied chilled glasses belonging to the N-MORB family, there are no signs of intracrustal contamination. It is assumed that relics of the ancient continental lithosphere preserved under axial zone of the MAR and involved in the process of partial melting of the shallow mantle took part in the formation of E-MORB parental melts in some segments of the MAR.

The geomagnetic field of the Ukrainian Carpathians and a 3D magnetic model of the Transcarpathian Depression

The article presents the results of a qualitative and quantitative analysis of the geomagnetic field of the Ukrainian Carpathians and 3D magnetic modeling along the PANCAKE geotransect and Transcarpathian Depression, as well as a comparison of the obtained results with fault-block tectonics, deep structure, and distribution of a number of types of endogenous ore deposits and hydrocarbon accumulations.
 It is shown that the overthrust northeastern part of the Carpathian arc lies on the magnetic crust, and the southwestern one — on its non-magnetic lower part, and between the Rava-Rus’ka and Krakovets and Pre-Carpathian faults, the Earth’s crust is magnetized throughout.
 For the territory of the Transcarpathian Depression, the regional and local components of the geomagnetic field were identified for the first time, geological and geophysical characteristics were provided, and their tectonic interpretation was proposed.
 A three-dimensional magnetic model of the Transcarpathian Depression was created taking into account the magnetization of rocks according to measurements. A detailed magnetic model of the upper part of the crustal section of the Transcarpathian Depression was developed. The connection of magnetic sources with fault zones was analyzed; the results were compared with the deep structure and distribution of ore deposits and hydrocarbon accumulations.
 The regional source with magnetism І=1.0 A/m is located within the Mukachevo Depression and is located at a depth of 6.0 km to 13 km. The local component of the geomagnetic field reflects the magnetic inhomogeneity of the Earth’s crust in the upper 3—4 km of the section and mainly reflects the volcano-tectonic structures and dike formations of the Vyhorlat-Gutyn Ridge, the Chop-Berehove Uplift, and the Velika Dobron’ Uplift. The maximum depths of magnetic sources (up to 3.5 km) and their magnetization (1.22 A/m) are characteristic of the structures of the Vyhorlat-Gutynsky Ridge, intermediate values of depths (2.0—3.0 km) and magnetization (up to 0.93 A/m) belong to the Chop-Berehove Uplift, and the minimum depths (up to 1.1 km) and magnetization (up to 0.7 A/m) are characteristic of the Velika Dobron’ Uplift.
 It is shown that the gas fields of Transcarpathia correspond to local positive magnetic anomalies and are localized above a deep magnetic source. Within the Berehove Uplift, positive anomalies and magnetic sources indicate andesiteporphyrite shafts and andesite domes of Sarmatian age associated with gold, gold-polymetallic and silver mineralization. A zone of antimony mineralization is associated with the Pannonian-Pontic volcanic structures along the southern foot of the Vyhorlat-Gutyn Ridge, and deposits and occurrences of bismuth and mercury correlate well with intrusive formations of the Dacian-Romanian age.

OH incorporation and retention in eclogite-facies garnets from the Zermatt–Saas area (Switzerland) and their contribution to the deep water cycle

Abstract. The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens include grossular–andradite–uvarovite solid solutions in serpentinite and rodingite and almandine–grossular–pyrope–spessartine solid solutions in eclogite, mafic fels and meta-sediment. All rocks experienced the same peak metamorphic conditions corresponding to a burial depth of ∼ 80 km (∼ 540 ∘C, 2.3 GPa), allowing determination of the OH content in garnet as a function of rock type. The capacity for OH incorporation into garnet strongly depends on its composition. Andradite-rich (400–5000 µg g−1 H2O) and grossular-rich garnet (200–1800 µg g−1 H2O) contain at least 1 order of magnitude more H2O than almandine-rich garnet (< 120 µg g−1 H2O). Microscale analyses using FTIR and EPMA profiles and maps reveal the preservation of OH zoning throughout the metamorphic history of the samples. The OH content correlates strongly with Mn, Ca and Ti zoning and produces distinct absorption bands that are characteristic of multiple nano-scale OH environments. The use of 2D diffusion modelling suggests that H diffusion rates in these rocks is as low as log(D[m2 s−1]) = −24.5 at 540 ∘C. Data were collected for the main garnet-bearing rock types of the Zermatt–Saas area allowing a mass balance model of H2O to be calculated. The result shows that ∼ 3360 kg H2O km−1 (section of oceanic crust) yr−1 could be transported by garnet in the subducting slab beyond 80 km depth and contributed to the deep-Earth water cycle during the Eocene subduction of the Piemonte–Liguria Ocean.

Determination of geothermal temperature and temperature gradient via measurements in the production well sump

Data on the geothermal temperature and the temperature gradient of the earth's crust section are necessary for solving a number of fundamental and applied problems. However, the currently used methods for determining the geothermal temperature based on the temperature measured in the wellbore have known disadvantages and can only be used in a small number of wells in the field under study. The article proposes a new method for determining the geothermal temperature and the temperature gradient from the measured temperature distribution in production wells with a long enough sump. It is shown that every other well out of more than four hundred considered wells of oil fields in the Republic of Bashkortostan (Russia) has an appropriate sump. The influence of heat transfer from the working formation, metal casing and natural thermal convection in the borehole fluid on the temperature field in the sump of the production well is studied analytically and numerically. An algorithm for finding an area with undisturbed temperature distribution in the sump of the wellbore is described. The requirements for the choice of a well and corresponding sump section are given, as well as the procedure for the subsequent processing of thermograms, which makes it possible to determine the geothermal temperature and the temperature gradients in the sump. The efficiency of the proposed technique is demonstrated by two cases of field data examples.

Wet calc-alkaline magmatic fractionation in the middle−upper crustal sections of the continental arc: Insights from the Neoproterozoic Nanba intrusive complex, western Yangtze Block, South China

Elucidating the petrological and geochemical characteristics of continental arc crustal fragments can provide unique insights into the magmatic evolutional path and physicochemical conditions (e.g., P−T−H2O) of continental magmatic arcs. This contribution provides a comprehensive set of petrological, mineralogical, geochronological, and geochemical data for the Neoproterozoic Nanba intrusive complex of the western Yangtze Block, South China, which features well-exposed middle−upper crustal sections of the Neoproterozoic continental magmatic arc, to evaluate their magmatic source and fractionation process. The Neoproterozoic Nanba intrusive complex, composed of cumulate gabbros, hornblende gabbros, gabbro-diorites, diorites, granodiorites, K-feldspar granites, and monzogranites, was emplaced at ca. 796−790 Ma. The different lithologies of the Nanba intrusive complex display relatively homogeneous Sr-Nd-Hf isotopes, which indicates a cogenetic evolutional process. The predominantly depleted isotopic characteristics, together with high values of Ba, Rb/Y, and Ba/La, demonstrate that these rocks mainly originated from a subduction fluid−metasomatized mantle source. Petrological and geochemical characteristics, as well as hornblende thermobarometric data, collectively support that the ca. 796−790 Ma Nanba intrusive complex, following the calc-alkaline trend, was dominated by the wet cogenetic fractionation of hornblende accompanied by plagioclase and accessory minerals beneath the middle−upper crustal levels (2.4−4.7 kbar). The cumulate gabbros represent the residue of accumulated hornblende + plagioclase + Fe-Ti oxides from fractionated basaltic melts. The high-SiO2 K-feldspar granites and monzogranites display complementary trace elements, which represent residual silicic cumulates and extracted interstitial liquids in the shallower crystal mush reservoir, respectively. In combination with previous studies, we propose that the middle−upper crustal sections of the Neoproterozoic continental arc of the western Yangtze Block underwent hornblende-dominated fractionation that was followed by significant crystal-liquid separation, which led to the genesis of high-SiO2 melts at the shallower crustal level. The cogenetic evolution played a significant role in molding the petrological and geochemical diversity of Neoproterozoic igneous rocks and the differentiation process of continental arc crust in the western Yangtze Block.

Internal structure and kinematics of the Zvenyhorod-Brats’k fault zone of the Ukrainian shield according to geophysical data

Complex geomagnetic and tectonophysical studies were performed in order to characterize in detail the heterogeneity of the Earth’s crust of the Zvenyhorod-Bratskfault zone and to clarify its internal structure and kinematics. Based on the results of magnetometric studies, maps of the different wavelength components of magnetic anomalies were obtained, the sources of which are associated with different layers of the crust; the magnetization of the upper, middle and lower parts of the crust was estimated; 2D and 3D magnetic models of individual structures and areas were developed; geological and tectonic interpretations were performed.
 The Zvenyhorod-Bratskfault zone is clearly manifested in the regional component of the geomagnetic field, separating its maxima and minima zones, and is also reflected by linear anomalies and bands of the short-wavelength component of the upper part of the crustal section. At the same time, it separates areas of the Earth’s crust with fundamentally different types of sources and their magnetization values: in the west, linear sources of the upper and middle crust of the GSZ are characterized by significant magnetization (up to 3—6 A/m) in contrast to the oval-shaped sources of the Ingul block with magnetization up to 1 A/m m.
 Tectonophysical measurements of structural and textural elements of rocks were carried out at four sites along the rivers Velyka Vys’, Kilten’, Suhyi Tashlyk and Mertvovid. Processing and interpretation of field measurements of structural textural elements of rocks were done by the structural-paragenetic method of tectonophysics for III-IV depth levels using the Stereonet program.
 It was established that the Zvenyhorod-Bratskfault zone was formed ≤2.45 billion years ago and consists of rectilinear shear zones related to different stages of fault formation. However, the main factor in the formation of the fault zone is the Pervomaisk stage, during which the L-shears with a strike azimuth of 345° and R-shears with a strike azimuth of 357° were formed. The presence of Archean phases of shear in it can testify to the earlier formation of the Zvenyhorod-Bratskfault zone relative to the Pervomaysk one.
 The Proterozoic cycle of activation (~2.1—1.70 billion years ago) contributed to the formation of deposits and ore manifestations of minerals within the Zvenyhorod-Bratskfaults zone.

Geometry and Rheology of Plumes: Common Features in the Probabilistic Gravity Models

By means of the gravity models reflecting a rheological states of geological environments 3D distribution of density contrast in the heads of six plumes (Yellowstone, Emeishan, Indigiro-Kolyma, Sea of Okhotsk, Indigiro-Kolyma, and Maya-Selemdzha) up to the depth of 200 km are studied and compared with all geological-geophysical data. According to the obtained data, astenospheric parts of plumes have mushroom-like shape, and astenosperic magmas spread under the lithosphere bottom and more rare ‒ under the crust bottom. At the distance of 250‒300 km from central trunks of plumes they are narrowed to diameter of 200–300 km at a depth of 100‒120 km. In heads of the majority of plumes astenospheric magmas merge with the subcrustal viscous layer and approach the Earth’s surface to 40‒50 km. In the majority of the considered plumes their lithospheric and crustal fragments are curved towards the Earth’s surface, In the upper crust layers upwards are sometimes complicated by local downwards (Yellowstone and May-Selemdzha plumes) that is explained by sagging of the dome roofs over the magmatic chambers into subcrustal viscous layer and in asthenosphere. Plumes are often accompanied by zones of the lithosphere stretching (rifts) therefore in the lower lithospheric and crustal sections of plumes linear zones of the lowered viscosity are mapped. The structural position of considered plumes is controlled by borders of lithospheric plates and large segments of the second rank. Identical geometry and rheology of plumes created at different times (Triassic‒Neogene) in the regions which are far removed from each other (the North East Russia, North West of the USA, Southern China, Sea of Okhotsk) demonstrate universality of the tectonic situations promoting penetration of mantle streams into upper layers of the Earth. The main of them are the lithosphere stretching zones, in particular ‒ sites of crossing of multidirectional fractures of a lithosphere and crust.

Geoelectrical model of the Earth’s crust along the Shu-Sarysu geotraverse according to magnetotelluric soundings

Purpose. To study and assess the possibility of effective application of electrical prospecting by magnetotelluric sounding (MT-sounding) and to determine the range of geological problems to be solved in the geoelectric conditions of the Shu-Sarysu geotraverse. Methodology. The issues of methodology of modern technologies of field observations, peculiarities of processing and interpretation of MT-sounding data are considered, which showed that modern instrumental and methodical electrical survey technologies of MT-sounding allow obtaining results of field measurements of increased reliability, due to their accuracy, productivity, mobility, noise immunity, level of automation. To identify the main features of the geoelectric structure of the Earth’s crust and the upper mantle, the modern technique of interpreting MT data allows for 1, 2D inversion of the effective curves of apparent resistivity and impedance phase. Findings. Based on MT-sounding data, a geoelectric section of the Shu-Sarysu geotraverse was constructed, characterizing the position and morphology of five geoelectric boundaries in the crustal section in the 0–40 km depth interval. The identified five geoelectric horizons show good coincidence with the conventional seismic horizons and confidently correlate with the geological references established by GIS data. Originality. On the basis of MT-sounding studies, the geological interpretation of the structure of suprasalt and subsalt structural-formation complexes is presented; the geological efficiency of MT-sounding electrical prospecting in revealing the complex structures in the conditions of the Shu-Sarysu geotraverse is shown. On the basis of model constructions of the geoelectric section, new data on the deep geological structure of the Shu-Sarysu geotraverse were obtained: new data on the structure of Mesozoic terrigenous-sedimentary rock complexes and the Paleozoic part of the section with the allocation of the supposed faults; on the basis of MT-sounding results, recommendations for further detailed geological exploration work were given. Practical value. A comprehensive interpretation is recommended of the obtained MT-sounding data together with the results of geological and geophysical research on the regional profile in order to establish search criteria for the discovery of mineral deposits and the prospects of gas content in the area.

Tectono-metamorphic evolution of a post-variscan mid-crustal shear zone in relation to the Tethyan rifting (Ivrea-Verbano Zone, Southern Alps)

The Ivrea-Verbano Zone, in the Italian Southern Alps, samples a complete section of middle to lower continental crust, which records the transition between the Variscan and the Alpine Tethys rift-related tectonics. We present a structural-metamorphic characterization of one of the most important structures, the Forno-Rosarolo shear zone which drove the post-Variscan tectonic evolution of the Ivrea-Verbano Zone. The Forno-Rosarolo shear zone is a NE-SW-oriented, subvertical shear zone, mainly made of metapelites, amphibolites, calc-silicates and granulites involved in anastomosed proto- to ultra-mylonite layers enveloping weakly deformed lenses. Mylonitic deformation developed on an already folded and sheared metamorphic (Variscan) complex but predates brittle fracturing and faulting, locally associated with pseudotachylites.In present day orientation, the kinematic indicators point to a sinistral sense of shear. Removing the Alpine ∼90° tilt of the IVZ crustal section, this kinematic results in a former extensional shear zone. Investigations on the kinematic of the flow in the mylonites revealed a non-coaxial deformation with a major component of pure shear acting together with simple shear. Metamorphic conditions of the wall rocks vary from upper amphibolite (SE, footwall) to granulite facies (NW, hanging wall). Estimations from the mineral assemblages within the mylonites point to amphibolite facies conditions during deformation (∼650 °C and ∼5.5 kbar).In absence of precise geochronological constraints, we discuss the role of the Forno-Rosarolo shear zone within the Tethyan rifting system in comparison with recent reconstructions. Kinematic data and metamorphic conditions allow to constrain the development of the Forno-Rosarolo shear zone mylonitic deformation during the intermediate phase of the Tethyan rift known as “thinning mode”. This stage was characterized by general shear conditions (pure shear between 70% and 50%) suggesting a phase of transition from a symmetric to an asymmetric configuration of rift.

Genesis of Pyroxenite Veins in the Zedang Ophiolite, Southern Tibetan Plateau

AbstractUnderstanding the nature of parental melts for pyroxenite veins in supra‐subduction zone (SSZ) ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones. The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro, diabase, and basalt. Veins of two pyroxenite varieties cut the southern part of the Zedang massif. These pyroxenite rocks have different geochemical characteristics, where the first variety (type‐I) has relatively higher contents of SiO2 (51.82–53.08 wt%), MgO (20.08–23.23 wt%), and ΣPGE (3.42–13.97 ppb), and lower Al2O3 (1.59–2.28 wt%) and ΣREE (1.63–2.94 ppm). The second pyroxenite variety (type‐II) is characterized by SiO2 (45.44–49.61 wt%), MgO (16.68–19.78 wt%), Al2O3 (4.24–8.77 wt%), ΣPGE (14.46–322.06 ppb), and ΣREE (5.82–7.44 ppm). Pyroxenite type‐I shows N‐MORB‐like chondrite‐normalized REE patterns. Zircon U‐Pb ages of pyroxenite type‐I (194 ± 10 Ma), associated ophiolitic gabbro (135.3 ± 2.0 Ma), and plagiogranite (124.2 ± 2.3 Ma) evidently imply episodic evolution of the Zedang ophiolites. The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction‐initiated hydrous melting of metasomatized sub‐arc mantle, later overprinted by sub‐slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times. The geochemical variations in pyroxenite vein composition, coupled with age differences amongst the other ophiolite units, may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.

Paleo‐Permeability Structure of the Crustal Section of the Samail Ophiolite Based on Automated Detection of Veins in X‐Ray CT Core Images From the Oman Drilling Project

AbstractTo assess the paleo‐permeability structure of oceanic crust, we used 3‐D X‐ray Computed Tomography (XCT) images to quantify the distribution and geometry of mineral veins in core samples from Oman Drilling Project Holes GT1A, GT2A, and GT3A, which correspond to the upper to lower crustal sections of the Samail ophiolite. We developed a new method that automatically detects veins in the XCT core images based on iterative adaption of the two‐step Hough transform combined with multiscale Hessian filtering for identifying an elongate structure. Application of the developed method allowed us to identify the geometry and Computed Tomography number of more than 1500 veins with millimeter‐scale apertures in core sections with a total length of ∼1,200 m. High‐CT (HCT) veins in the drilled cores can be related to relatively high‐temperature fluid circulation near the mid‐ocean ridge, whereas Low‐CT (LCT) veins can be related to subsequent low‐temperature fluid circulation. Applying fracture fluid‐flow models to the geometric information for the detected veins, we found that the HCT and LCT vein systems both yielded bulk permeability of 10−13–10−9 m2 for each hole. This indicates that millimeter‐wide fractures can control crustal‐scale permeability, even in the lower oceanic crust. However, these vein systems show different depth dependencies and anisotropies of permeability, possibly reflecting the different spatial variations of high‐ and low‐temperature fluid circulation in oceanic crust.

Continentalization of an intraoceanic arc as exemplified by the Jurassic Bonanza arc of Vancouver Island, Canada

We present new U-Pb zircon ages, bulk rock chemistry, and field observations for volcanic and plutonic rocks of the Jurassic Bonanza arc of Vancouver Island, Canada, a proposed southern extension of the Talkeetna arc crustal section in Alaska. Rather than a synchronous and contiguous arc crustal section, the new field and geochronological data reveal three distinct periods in the arc development, wherein the oldest, “prima” (205−195 Ma), was built upon by the youngest, “dopo” (<180 Ma), with both preserving their volcanic and plutonic sections of the upper crust. In contrast, the middle “mezza” period of arc construction (195−180 Ma) has no volcanic expression and is represented almost exclusively as plutons in the middle crust emplaced below the brittle-ductile transition, and exhumed mainly along the western and southern coast of Vancouver Island. The new data and structural interpretation refute earlier models suggesting that the deeper, more mafic rocks of the middle crust (West Coast Complex) are either restite or the crystal cumulates that form the more silicic plutonic rocks (Island Plutonic Suite) in the upper crust. Both the middle and upper plutonic components are remarkably similar to average continental crust. The growth and development of the Bonanza arc in an oceanic realm—but on a thick, pre-existing marine plateau—promoted a “continentalization” of its bulk composition within ∼10 m.y., similar to what is proposed to have occurred in Costa Rica over the same timescale.

C–O–H fluid-melt-rock interaction in graphitic granulites and problems of quantifying carbon budget in the lower continental crust

Estimates on the geological carbon cycle are subject to large uncertainties that can be reduced by thorough observation of rocks. In this contribution, we focus specifically on C–O–H fluid-melt-rock interactions in graphitic metapelitic granulites and on their bearing to the carbon budget of granulitic roots of continents. We provide robust microstructural and thermometric constraints on the coexistence of anatectic silicate melts and C–O–H fluids up to near ultrahigh temperature conditions in the archetypal crustal section of Ivrea-Verbano Zone (IVZ, Italian Alps). Fluid inclusions in garnet are investigated before and after high-temperature experiments, and contain considerable proportions of CO2, CH4, N2, but lower H2O than predicted for graphitic systems at granulite facies. When comparing and contrasting the melt compositions obtained by Perple_X and rhyolite-MELTS with natural melts from IVZ, a much better match is obtained by the former, questioning the choice of rhyolite-MELTS for modelling melting equilibria of metasedimentary rocks and for quantifying carbon budget of the lower crust. Overall, data show that assuming only a limited extent of fluid-melt immiscibility in the deep crust contradicts the evidence from natural rocks and prompts to an incomplete view of actual carbon behavior and carbonic fluids. The available experimental dataset on CO2 solubility in felsic melts cannot be used to interpret the volatile budget of melt inclusions in graphitic migmatites and granulites, as most solubility experiments were conducted under carbonate-saturated (i.e. highly oxidizing) conditions which maximize CO2 content of melt, compared to graphitic (i.e. more reducing) protoliths. As a consequence, thermodynamic models still cannot account for all the complexities related with interactions among H2O–CO2–CH4 ternary fluids, H2O- and CO2-bearing anatectic melts and graphite-bearing residues in graphitic metapelites. Targeted experimental studies are therefore crucial to boost substantial computational efforts, before any precise estimates on carbon budget and fluxes in the lower anatectic crust can be made.

Geochemical and radiometric data for mafic rocks from the Guleman Ophiolite (SE, Turkey): New insights on the geodynamic evolution of the southern Neo-Tethyan ocean

The Guleman Ophiolite is located in the southeast of Turkey and mainly consists of harzburgites and dunites hosting economically viable chromitite bodies, and lesser amounts of cumulate and isotropic gabbro, diabase dykes, pyroxenite and locally plagiogranite. This study reports the combined whole-rock geochemical and radiogenic isotope (Sr and Nd) data for gabbro and diabase dykes, and in-situ zircon U-Pb geochronological data for plagiogranite and gabbro from the Guleman Ophiolite. Two different geochemical groups were determined based on immobile minor and trace elements, and radiogenic isotope characteristics of gabbro and diabase dykes investigated here. The first group is represented by boninites and displays highly depleted elemental patterns compared to NMORB, and spoon-shaped rare earth element profiles. Contrarily, the second group resembles back-arc basin basalts and is characterized by NMORB-like rare earth element patterns associated with negative Nb anomalies. The boninitic samples are characterized by less radiogenic 143Nd/144Nd(i) (0.51229 to 0.51267; ƐNd = −4.8 to +2.7) and more radiogenic 87Sr/86Sr(i) (0.70549 to 0.70758) isotope compositions compared to the back-arc basin basalt-like samples (143Nd/144Nd(i) = 0.51289 to 0.51296; ƐNd = +7.0 to +8.4; 87Sr/86Sr(i) = 0.70496 to 0.70629), indicating derivation from distinct and/or heterogeneous mantle source regions within a supra-subduction zone. The LA–ICP–MS zircon U-Pb analyses yielded concordant late Cretaceous ages for gabbro (82 ± 2.9 Ma) and plagiogranite (79.9 ± 0.8 Ma) samples. The overall geochemical, isotopic and geochronological data presented here suggest that the Guleman Ophiolite consists of forearc and initial back-arc spreading sections of oceanic crust, which may have been formed in an intra-oceanic subduction zone within the Berit Ocean (northern branch of the southern Neo-Tethyan ocean). Moreover, the generation of continental arc magmatism (i.e. Baskil and Esence Granitoids) was well-documented on the active margin of the Malatya-Keban Metamorphics to the north. Therefore the closure of this oceanic basin should have resulted in multiple coeval northward subduction events during the late Cretaceous.