New isotope–geochemical and mineralogical data on the ultramafic xenoliths in the volcanic rocks of the Kamchatka–Koryak region: Two types of mantle protolith in the modern island-arc system

The paper presents new isotope–geochemical and mineralogical data on mantle xenoliths of the “island-arc” (Avacha, Shiveluch, and Kharchinsky volcanoes) and “within-plate” (Valovayam River, Cape Navarin, and Bakening Volcano areas) types. In terms of paragenesis and mineral composition, the “islandarc” xenoliths correspond to the olivine–plagioclase depth facies, while the “within-plate” xenoliths came from spinel lherzolite to wehrlite facies, which is transitional to the olivine–plagioclase equilibrium. The majority of the “within-plate” xenoliths are enriched in high-field-strength elements (Ti, Nb, Hf, Zr, Yb). The “island-arc” xenoliths in general are depleted in REE, while the “within-plate” xenoliths are enriched in all REE. The former have low Pb isotope ratios, being in isotope equilibrium with lower crustal basites, while most of the latter group are enriched in radiogenic Pb. The island-arc xenoliths are of magmatic origin and were derived from the sublithospheric crust–mantle mixture, while the “within-plate” xenoliths reflect the composition of the asthenospheric mantle source. The primary appearance of the xenoliths is obliterated by secondary recrystallization and metasomatic reworking.     

Peculiarities of Subduction in the Junction of the Kuril–Kamchatka and Aleutian Island Arcs

Doklady Earth Sciences - The peculiarities of the junction between the Kuril–Kamchatka and Aleutian island arcs are studied using different techniques and experimental data. The models of...     

The Contemporary Seismic Deficit in the Kuril–Kamchatka Subduction Zone

Doklady Earth Sciences - Based on an analysis of seismicity for the period of 1900–2018, the seismic potential of the Kuril–Kamchatka subduction zone is estimated, and the geometry of...     

Dynamics of seismogenerating structures in the frontal zone of the Kolyma–Omolon superterrane

To develop a model for the dynamics of seismogenerating structures in the frontal zone of the Kolyma–Omolon superterrane (Chersky seismotectonic zone), the following aspects are analyzed: structural–tectonic position, deep structure parameters, active faults, and fields of tectonic stresses as revealed from solutions of focal mechanisms of strong earthquakes and kinematic types of Late Cenozoic fold deformations and faults. It is found that a certain dynamic setting under transpressional conditions takes place and it was caused by the interaction between structures of the Eurasian, North American, and Okhotsk lithospheric plates within regional segments of the Chersky zone (Yana–Indigirka and Indigirka–Kolyma). These conditions are possible if the Kolyma–Omolon block located in the frontal zone of the North American Plate was an indenter. Due to this, some terranes of different geodynamic origin underwent horizontal shortening, under which particular blocks of segments were pushed out laterally along the orogenic belt, on a system of conjugated strike-slip faults of different directions and hierarchical series, in the northwest and southeast directions, respectively, to form the main seismogenerating reverse-fault and thrust structures with the maximum seismic potential (M ≥ 6.5).     

Experiments on melt–rock reaction in the shallow mantle wedge

This experimental study simulates the interaction of hotter, deeper hydrous mantle melts with shallower, cooler depleted mantle, a process that is expected to occur in the upper part of the mantle wedge. Hydrous reaction experiments (~6 wt% H₂O in the melt) were conducted on three different ratios of a 1.6 GPa mantle melt and an overlying 1.2 GPa harzburgite from 1060 to 1260 °C. Reaction coefficients were calculated for each experiment to determine the effect of temperature and starting bulk composition on final melt compositions and crystallizing assemblages. The experiments used to construct the melt–wall rock model closely approached equilibrium and experienced <5% Fe loss or gain. Experiments that experienced higher extents of Fe loss were used to critically evaluate the practice of “correcting” for Fe loss by adding iron. At low ratios of melt/mantle (20:80 and 5:95), the crystallizing assemblages are dunites, harzburgites, and lherzolites (as a function of temperature). When the ratio of deeper melt to overlying mantle is 70:30, the crystallizing assemblage is a wehrlite. This shows that wehrlites, which are observed in ophiolites and mantle xenoliths, can be formed by large amounts of deeper melt fluxing though the mantle wedge during ascent. In all cases, orthopyroxene dissolves in the melt, and olivine crystallizes along with pyroxenes and spinel. The amount of reaction between deeper melts and overlying mantle, simulated here by the three starting compositions, imposes a strong influence on final melt compositions, particularly in terms of depletion. At the lowest melt/mantle ratios, the resulting melt is an extremely depleted Al-poor, high-Si andesite. As the fraction of melt to mantle increases, final melts resemble primitive basaltic andesites found in arcs globally. An important element ratio in mantle lherzolite composition, the Ca/Al ratio, can be significantly elevated through shallow mantle melt–wall rock reaction. Wall rock temperature is a key variable; over a span of <80 °C, reaction with deeper melt creates the entire range of mantle lithologies from a depleted dunite to a harzburgite to a refertilized lherzolite. Together, the experimental phase equilibria, melt compositions, and reaction coefficients provide a framework for understanding how melt–wall rock reaction occurs in the natural system during melt ascent in the mantle wedge.     

Peculiar Mg–Ca–Si metasomatism along a shear zone within the mantle wedge: inference from fine-grained xenoliths from Avacha volcano,Kamchatka

We found extremely high-Mg# (=Mg/(Mg + total Fe) atomic ratio) ultramafic rocks in Avacha peridotite suite. All the high-Mg# rocks have higher modal amounts of clinopyroxene than ordinary Avacha peridotite xenoliths, and their lithology is characteristically heterogeneous, varying from clinopyroxenite through olivine websterite to pyroxene-bearing dunite. The Mg# of minerals is up to 0.99, 0.98 and 0.97 in clinopyroxene, orthopyroxene and olivine, respectively, decreasing progressively toward contact with dunitic part, if any. The petrographical feature of pyroxenes in the high-Mg# pyroxenite indicates their metasomatic origin, and high LREE/HREE ratio of the metasomatic clinopyroxene implies that the pyroxenites are the products of reaction between dunitic peridotites and high-Ca, silicate-rich fluids. The lithological variation of the Avacha high-Mg# pyroxenites from clinopyroxenite to olivine websterite resulted from various degrees of fluid-rock reaction coupled with fractional crystallization of the high-Ca fluids, which started by precipitation of high-Mg# clinopyroxene. Such fluids were possibly generated originally at a highly reduced serpentinized peridotite layer above the subducting slab. The fluids can reach the uppermost mantle along a shear zone as a conduit composed of fine-grained peridotite that developed after continent-ward asthenospheric retreats from the mantle wedge beneath the volcanic front. The fluids are incorporated in mantle partial melts when the magmatism is activated by expansion of asthenosphere to mantle wedge beneath the volcanic front.     

The Wenquan ultramafic rocks in the Central East Kunlun Fault zone,Qinghai-Tibet Plateau—crustal relics of the Paleo-Tethys ocean

Mineralogy and Petrology - The Wenquan ultramafic rocks, located in the East Kunlun Orogenic belt in the northeastern part of the Qinghai-Tibet Plateau, consist of dunite, wehrlite,...     

Hydration and dehydration in the lower margin of a cold mantle wedge: implications for crust–mantle interactions and petrogeneses of arc magmas

Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high X_Mg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (?1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas.     

40Ar–39Ar dating of volcanic rocks of the Shyok suture zone in north–west trans-Himalaya: Implications for the post-collision evolution of the Shyok suture zone

⁴⁰Ar–³⁹Ar geochronological studies carried out on the Khardung volcanics of Ladakh, India and our earlier Ar–Ar results from the volcanics of the Shyok suture along with the available geological and geochemical data provide good constraints for post-collision evolution of the Shyok suture zone. Whole-rock samples from the Shyok volcanics yielded disturbed age-spectra and we have demonstrated earlier that the youngest tectonic event in the Shyok suture zone responsible for the thermal disturbance of these samples is Karakoram fault activation at ～14 Ma. Contrastingly whole-rock samples from the Khardung volcanics, which are in tectonic contact with these Shyok volcanics, and are exposed in the form of thick rhyolitic and ignimbritic flows, yielded undisturbed age-spectra and good plateau-ages. The whole-rock plateau-ages of two rhyolite samples are 52.8 ± 0.9 and 56.4 ± 0.4 Ma. We interpret these ages to be the time and duration of emplacement of these volcanics over thickened margin of the continental crust, which appears to be coeval with the initiation of the collision between the Indian and Asian plate. The lesser extent of post-emplacement isotopic re-equilibration in these samples unlike the Shyok volcanics indicate that these samples were present in different tectonic settings, away from the Karakoram fault, at the time of deformation in the Shyok suture zone. We propose that the two volcanic belts of contrasting nature were brought together in juxtaposition by the Karakoram strike slip faulting at ～14 Ma.     

Trace element modeling of aqueous fluid – peridotite interaction in the mantle wedge of subduction zones

Recently measured partition coefficients for Rb, Th, U, Nb, La (Ce), Pb, Sr, Sm, Zr, and Y between lherzolite assemblage minerals and H₂O-rich fluid (Ayers et al. 1997; Brenan et al. 1995a,b) are used in a two-component local equilibrium model to assess the effects of interaction between slab-derived aqueous fluids and wedge lherzolite on the trace element and isotopic composition of island arc basalts (IAB). The model includes four steps representing chemical processes, with each process represented by one equation with one adjustable parameter, in which aqueous fluid: (1) separates from eclogite in the subducted slab (Rayleigh distillation, mass fraction of fluid released F  ^fluid); (2) ascends through the mantle wedge in isolated packets, exchanging elements and isotopes with depleted lherzolite (zone refining, the rock/fluid mass ratio n); (3) mixes with depleted lherzolite (physical mixing, the mass fraction of fluid in the mixture X  ^fluid); (4) induces melting to form primitive IAB (batch melting, mass fraction of melt F ^melt). The amount of mantle lherzolite processed by the fluid in step (2) determines its isotopic and trace element signature and the relative contributions of slab and wedge to primitive IAB. Assuming an average depleted lherzolite composition and mineralogy (70% olivine, 26% orthopyroxene, 3% clinopyroxene and 1% ilmenite) and using nonlinear regression to adjust parameter values to obtain an optimal fit to the average composition of IAB (McCulloch and Gamble 1991) yields values of F  ^fluid= 0.20, n= 26, X  ^fluid= 0.17, and F  ^melt= 0.15, with r  ²= 0.995 and the average relative error in trace element concentration = 6%. The average composition of IAB can also effectively be modeled with no contribution from the slab other than H₂O (i.e., skip model step 1): n= 27, X  ^fluid= 0.21, F  ^melt= 0.17, with r  ²= 0.992. By the time the fluid reaches the IAB source, exchange with depleted wedge lherzolite reduces the ⁸⁷Sr/⁸⁶Sr ratio isotopic composition to near-mantle values and the slab contribution to <50% for all but the most incompatible elements (e.g., Pb). The IAB may retain the slab signature for elements such as B and Be that are highly incompatible and that have very low concentrations in the depleted mantle wedge. The relatively high equilibrium D  ^{mineral /   fluid} values measured by Ayers et al. (1997), Brenan et al. (1995a) and Stalder et al. (1998) suggest that large amounts of fluid (>5 wt%) must be added to lherzolite in the IAB source. Decreasing X  ^fluid below 0.05 causes model results to have unacceptably high levels of error and petrologically unreasonable values of F  ^melt. That H₂O contents of IAB are generally <6 wt% suggests that not all of the H₂O that metasomatizes the IAB source remains in the source to dissolve in the subsequently formed melt. Modeling of the compositions of specific primitive IAB from oceanic settings with low sediment input and depleted mantle wedges (Tonga, Marianas) shows a generally lower level of fluid-wedge interaction (low n), and therefore a larger slab component in primitive IAB. Received: 6 October 1997 / Accepted: 8 May 1998     

Estimation of the stress state of the Earth’s crust and the upper mantle in the area of the Southern Kuril Islands

The catastrophic Shikotan earthquake of October 4 (5), 1994, occurred in the Pacific Ocean. Its focus was located 80 km eastward of Shikotan Island. The stress state of the Earth’s crust in this area was estimated by the method of the cataclastic analysis of the whole range of the earthquake mechanisms. The performed reconstruction of the parameters of the current stress state of the Earth’s crust and the upper mantle in the area of the Southern Kuril Islands made it possible to establish that this area is characterized, on the one hand, by the presence of extensive areas of steady behavior of the stress tensor parameters and, on the other hand, by the presence of local sections of anomalously fast changes in these parameters.     

Boundaries of mantle–lithosphere domains in the Bohemian Massif as extinct exhumation channels for high-pressure rocks

Five domains (microplates) have been recognized by seismic anisotropy in the mantle lithosphere of the Bohemian Massif. The mantle domains correspond to major crustal units and each of the domains bears a consistent fossil olivine fabric formed before their Variscan assembly. The present-day mantle fabric indicates that this process consisted of at least three oceanic subductions, each followed by an underthrusting of the continental lithosphere. The seismic anisotropy does not detect remnants of the oceanic subductions, but it can trace boundaries of the preserved continental domains subsequently underthrust along the paths of previous oceanic subductions. The most robust continent–continent collision was followed by westward underthrusting of the Brunovistulian mantle lithosphere, still detectable by seismic anisotropy more than 100 km beneath the Moldanubian mantle lithosphere. Major occurrences of the high-pressure/ultra high-pressure (HP–UHP) rocks follow the ENE and NNE oriented sutures and boundaries of the mantle–lithosphere domains mapped from three-dimensional modeling of body-wave anisotropy. The HP–UHP rocks are products of oceanic subductions and the following underthrusting of the continental crust and mantle lithosphere exhumed along the mantle boundaries. The close relation of the mantle sutures and occurrences of the HP–UHP rocks near the paleosubductions testifies for models interpreting the granulite–garnet peridotite association by oceanic/continental subduction/underthrusting followed by the exhumation of deep-seated rocks. Our findings support the bivergent subduction model of tectonic development of the central part of the Bohemian Massif. The inferences from seismic anisotropy image the Bohemian Massif as a mosaic of microplates with a rigid mantle lithosphere preserving a fossil olivine fabric. The collisional mantle boundaries, blurred by tectonometamorphic processes in easily deformed overlying crust, served as major exhumation channels of the HP–UHP rocks.     

Geochemical features of Viséan rocks in the north of Kolyvan'-Tomsk fold zone

Viséan rocks of the Kolyvan' - Tomsk fold zone are divided into two main groups. The third comprises the residuum. On the basis of statistical processing of many spectral analyses, the rocks of different groups are correlated by their average content of minor elements.--Author.     

Variations in the Pb isotope composition in polyformational magmatic rocks of the Ketkap–Yuna igneous province of the Aldan Shield: Evidence for mantle–crust interaction

The Pb isotope composition of polyformational Mesozoic igneous rocks of the Ketkap–Yuna igneous province (KYIP) and lower crustal metamorphic rocks of the Batomga granite–greenstone area (the complex of the KYIP basement) of the Aldan Shield was studied for the first time. Based on the data obtained, several types of material sources participating in petrogenetic processes were distinguished. The mantle source identified as PREMA is registered in most of the igneous formations and predominates in mafic alkaline rocks. According to the isotope characteristics, the upper crustal source corresponds to a source of the “Orogen” type by the model of “plumbotectonics” or to the average composition of the continental crust by the Stacey–Kramers model. The lower crust is the third material source; however, the type of lower crustal protolith involved in the igneous process is still not defined, which makes difficult to estimate its role in the petrogenetic processes.     

Tectonic significance of Triassic mafic rocks in the June Complex,Sanandaj–Sirjan zone,Iran

This study concentrates on the petrological and geochemical investigation of mafic rocks embedded within the voluminous Triassic June Complex of the central Sanandaj–Sirjan zone (Iran), which are crucial to reconstruct the geodynamics of the Neotethyan passive margin. The Triassic mafic rocks are alkaline to sub-alkaline basalts, containing 43.36–49.09 wt% SiO₂, 5.19–20.61 wt% MgO and 0.66–4.59 wt% total alkalis. Based on MgO concentrations, the mafic rocks fall into two groups: cumulates (Mg# = 51.61–58.94) and isotropic basaltic liquids (Mg# = 24.54–42.66). In all samples, the chondrite-normalized REE patterns show enrichment of light REEs with variable (La/Yb)_N ratios ranging from 2.48 to 9.00, which confirm their amalgamated OIB-like and E-MORB-like signatures. Enrichment in large-ion lithophile elements and depletion in high field strength elements (HFSE) relative to the primitive mantle further support this interpretation. No samples point to crustal contamination, all having undergone fractionation of olivine + clinopyroxene + plagioclase. Nevertheless, elemental data suggest that the substantial variations in (La/Sm)_PM and Zr/Nb ratios can be explained by variable degrees of partial melting rather than fractional crystallization from a common parental magma. The high (Nb/Yb)_PM ratio in the alkaline mafic rocks points to the mixing of magmas from enriched and depleted mantle sources. Abundant OIB alkaline basalts and rare E-MORB appear to be linked to the drifting stage on the northern passive margin of the Neotethys Ocean.     

Accumulative Landforms in Valleys with Gas-Hydrothermal Occurrences (from the Example of Watercourses of Some Volcanic Massifs in the Kuril—Kamchatka Region)

Doklady Earth Sciences - The sulfur isotope anomalies ∆33S and ∆36S known today in ancient rocks exhibit common geochemical patterns reflected in the existence of a correlation like...     

Recycling of sediment into the mantle source of K-rich mafic rocks: Sr–Nd–Hf–O isotopic evidence from the Fushui complex in the Qinling orogen

Potassium (K)-rich mafic rocks are viewed as being derived from partial melting of an enriched mantle source, but it is controversial about which processes cause the mantle enrichment. The Fushui intrusive complex is the largest early Paleozoic K-rich intrusive complex in the eastern Qinling orogen. Therefore, detailed studies on the Fushui complex can contribute not only to understanding of the petrogenesis of K-rich mafic rocks, but also to unraveling the Paleozoic evolution of the Qinling orogen. In this study, we provide an integrated investigation of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock major and trace elements, as well as Sr–Nd–Hf isotopes, for the Fushui intrusive complex. In situ zircon secondary ion mass spectrometry (SIMS) / laser ablation induction coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating reveals that different rock types of the Fushui complex have identical formation ages of 488–484 Ma. The Fushui complex belongs to the shoshonitic series, and is characterized by extreme large ion lithophile element (LILE, e.g., Ba, U, Th and Sr) and Pb enrichment and depletion of high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P and Ti). It shows high initial Sr isotopic ratios (0.7100–0.7151), negative whole-rock ε _Nd(t) (?3.97 to ?5.68) and negative to slight positive whole-rock (?2.24 to 2.38) and zircon (?2.85 to 0.34) ε _Hf(t) values, as well as high zircon δ¹⁸O values (6.86 ± 0.13 ‰). The Hf–Nd isotopic systems are decoupled with positive Δε _Hf values (3.85–5.37). These geochemical features indicate that the mantle source has incorporated subducted zircon–barren oceanic sediments. A simple two-end-members mixing model constrains the amount of subducted sediments in the Fushui mantle source to 5–8 %. The Fushui complex originated from 1 to 6 % equilibrium melting of a phlogopite-bearing garnet lherzolite by non-modal melting. As shoshonitic magmas have been discovered in modern nascent arcs, we suggest that the generation of the Fushui complex was induced by the subduction of the Paleotethyan Ocean, when it jumped from the northern to the southern boundary of the North Qinling microcontinent.     

Fault-related rocks: deciphering the structural–metamorphic evolution of an accretionary wedge in a collisional belt,NE Sicily

The Alpine chain exposed in the Western Mediterranean area represents a front several kilometres in width, dismembered by more recent tectonics and by opening of the Tyrrhenian Basin. In most exposures of this mountain belt, relics of older metamorphic rocks occur. The deformational sequence of events may be revealed by the recognition of metamorphic records associated with different structures. Within a tract of the Alpine front cropping out in the Peloritani Mountains (NE Sicily), we distinguished two metamorphic complexes characterized by different tectonometamorphic histories. Their present tectonic juxtaposition is a cataclastic thrust linked to the recent Africa-verging Sicilian–Maghrebian fold-and-thrust belt. The Lower Complex is characterized by Hercynian metamorphism (P > 0.2 GPa and T ≈ 350°C) exclusively. It essentially consists of very low-grade metapelites and metavolcanic rocks overlain by an unmetamorphosed sedimentary cover. The Upper Complex, comprising different tectonic slices, consists of medium- to high-grade Hercynian metamorphic rocks (P?=?0.3–0.8 GPa and T up to 650°C) with Alpine metamorphic overprint (T > 250°C) affecting also the Mesozoic–Cenozoic cover. Lithotypes, structures, and inferred P–T conditions of investigated rocks suggest the existence of an Alpine accretionary wedge during the Cretaceous deformational collision. Within the Upper Complex, a polyphase Palaeogene mylonitic horizon involving rocks belonging to different tectonic slices fully preserves the tectonometamorphic evolution. For this reason, we focused our attention on these sheared rocks in order to reconstruct the entire tectonic history of this geologically complex area. Our new basic model allows the complex structure of the nappe-pile edifice of the Peloritani Mountains to be simplified, casting new light on the tectonic evolution of this key sector of the southern Calabrian-Peloritani Orogen.