Exhumation of high-pressure rocks of the Kokchetav massif: facts and models

The exhumation of ultrahigh-pressure (UHP) metamorphic units from depths more than 100-120 km is one of the most intriguing questions in modern petrology and geodynamics. We use the diamondiferous Kumdy-Kol domain in the Kokchetav Massif to show that exhumation models should take into consideration initially high uplift velocities (from 20 down to 6 cm/year) and the absence of the deformation of UHP assemblages. The high rate of exhumation are indicated by ion microprobe (SHRIMP) dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds.Diamondiferous rocks in the Kumdy-Kol domain occur as steeply dipping (60°-80°) thin slices (few hundred metres) within granite-gneiss. Using geological, petrological and isotopic-geochemical data, we show that partial melting of diamondiferous metamorphic rocks occurred; a very important factor which has not been taken into account in previous models.Deformation of diamondiferous rocks at Kumdy-Kol is insignificant; diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation. All these facts could be explained by partial melting of metapelites and granitic rocks in the Kumdy-Kol domain. The presence of melt is responsible for an essential reduction of viscosity and a density difference (Δρ) between crustal rocks and mantle material and reduced friction between the upwelling crustal block, the subducting and overriding plates. Besides Δρ, the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle.We construct different models for the three stages of exhumation: a model similar to “corner flow” for the first superfast exhumation stage, an intermediate stage of extension (most important from structural point of view) and a very low rate of exhumation in final diapir+erosional uplift.     

Structural and Morphological Features of the Participation of Microorganisms in the Formation of Nb–REE–Rich Ores of the Tomtor Field (Russia)

Doklady Earth Sciences - Data indicating the important role of microorganisms in the redistribution of REEs in the weathering crust and the decisive role in the concentration of REEs during the...     

Distinctive petrological, geochemical, and geodynamic features of subduction-related magmatism

Geological-petrological and geochemical data on subduction-related magmatism (including the volumes and compositions of the corresponding magmatic series) are compared to the results of experiments and numerical simulation. The subduction zone is subdivided into five depth sectors and volcanic zones I, II, and III: 1 is the accretionary wedge that controls the geodynamic stability of subduction; 2 is the sector of dehydration and fluid filtration; 3 is the zone of eclogitization and initial partial melting in the slab above which boninite volcanic zone I is formed during early stages; 4 is the main zone of melting of the sedimentary-basite layer and the development of volcanic zone II with the predominance of andesites; and 5 is the zone of higher degree melting, above which volcanic zone III (basaltic andesite and alkali basalt) is formed. The criterion of volcanism intensity, which was obtained within the scope of the melting model, is proportional to the subduction velocity and the thickness of the melting zone, and the distance between the groups of volcanics along the subduction zone is 75–100 km, at a thickness of the melting zone of 15–20 km. The calculated isotherm of 600°C, which controls the stability of serpentine and chlorite, is not identified at depth above 150 km, and this is confirmed by the composition and P-T conditions of the high-pressure rocks (containing diamond and coesite), which were brought from depths of 150–200 km in subduction zones. Seismic sections constructed with regard for the amplitude characteristics of seismic waves show two melting zones (“wet” melting at a depth of 100–200 km and “dry” melting at a depth of 150–200 km) and a complicated thermal structure of the suprasthenospheric wedge, which can include slant magma conduits. The mineralogical and geochemical features of arc magmatic series are formed at a decisive role of an H₂O-CO₂ fluid and an elevated oxidation potential. The predominant buffer minerals are as follows: garnet in the slab melting zone; magnetite, Ca-pyroxene, and amphibole in intermediate magmatic chambers; and amphibole, protoenstatite-bronzite (in place of olivine), and Cr-spinel (in place of magnetite) for boninite series generated in a “hot” asthenospheric wedge at interaction with fluids or water-rich melts. Actively disputable problems are the interactions scale of melts and fluids generated in a subduction zone with a “hot” mantle wedge, the possibility of transporting water-rich minerals deep into the mantle (to depths greater than 150 km), and the evolution of the scale at which young continental crust is generated by subduction melts.     

Plate reconstructions in the Arctic region based on joint analysis of gravity,magnetic, and seismic anomalies

Based on the analysis of various geophysical data, namely, free-air gravity anomalies, magnetic anomalies, upper mantle seismic tomography images, and topography/bathymetry maps, we single out the major structural elements in the Circum Arctic and present the reconstruction of their locations during the past 200 million years. The configuration of the magnetic field patterns allows revealing an isometric block, which covers the Alpha–Mendeleev Ridges and surrounding areas. This block of presumably continental origin is the remnant part of the Arctida Plate, which was the major tectonic element in the Arctic region in Mesozoic time. We believe that the subduction along the Anyui suture in the time period from 200 to 120 Ma caused rotation of the Arctida Plate, which, in turn, led to the simultaneous closure of the South Anyui Ocean and opening of the Canadian Basin. The rotation of this plate is responsible for extension processes in West Siberia and the northward displacement of Novaya Zemlya relative to the Urals–Taimyr orogenic belt. The cratonic-type North American, Greenland, and European Plates were united before 130 Ma. At the later stages, first Greenland was detached from North America, which resulted in the Baffin Sea, and then Greenland was separated from the European Plate, which led to the opening of the northern segment of the Atlantic Ocean. The Cenozoic stage of opening of the Eurasian Basin and North Atlantic Ocean is unambiguously reconstructed based on linear magnetic anomalies. The counter-clockwise rotation of North America by an angle of ~ 15° with respect to Eurasia and the right lateral displacement to 200–250 km ensure an almost perfect fit of the contours of the deep water basin in the North Atlantic and Arctic Oceans.     

Heat transfer between a thermochemical plume channel and the surrounding mantle in the presence of horizontal mantle flow

Heat and mass transfer processes in the conduit of a thermochemical plume located beneath an oceanic plate far from a mid-ocean ridge (MOR) proceed under conditions of horizontal convective flows penetrating the plume conduit. In the region of a mantle flow approaching the plume conduit (in the frontal part of the conduit), the mantle material heats and melts. The melt moves through the plume conduit at the average velocity of flow v and is crystallized on the opposite side of the conduit (in the frontal part of the conduit). The heat and the chemical dope transferred by the conduit to the mantle flow are carried away by crystallized mantle material at the velocity v.The local coefficients of heat transfer at the boundary of the plume conduit are theoretically determined and the balance of heat fluxes through the side of the plume conduit per linear meter of the conduit height. The total heat generation rate, transmitted by the Hawaiian plume into the upper and lower mantle, is evaluated. With the use of regular patterns of heat transfer in the lower mantle, which is modeled on the horizontal layer, heated from below and cooled from above, the diameter of the plume source, the kinematic viscosity of the melt in the plume conduit, and the velocity of horizontal lower-mantle flows are evaluated and the dependences of the temperature drop, viscosity and Rayleigh number for the lower mantle on the diameter of the plume source are presented.     

Two contrasting petrotectonic domains in the Kokchetav megamélange (north Kazakhstan): Difference in exhumation mechanisms of ultrahigh‐pressure crustal rocks,or a result of subsequent deformation?

Abstract In northern Kazakhstan the WNW striking Kokchetav megamélange includes different crustal sequences with high‐pressure/ultrahigh‐pressure (HP/UHP) remnants of their 540–520 Ma subduction metamorphism. Two domains separated by the north‐east trending Chaglinka fault are distinguished. The western domain exhibits NE–SW structures within a single Kumdy–Kol megaunit of diamond‐bearing UHP metasediments and high‐temperature (HT) eclogites. The eastern domain consists of the composite Kulet megaunit with the Kulet UHP unit (coesite‐bearing metasediments, whiteschists and eclogites), the Enbek–Berlyk medium‐pressure (MP) unit (kyanite‐bearing, high‐alumina rocks with interleaved coronitic metagabbro), and ortho‐ and paragneisses with eclogites and amphibolites included. All eclogites in the eastern domain are of the relatively low temperature (LT) type. Sillimanite is common and appears after kyanite in the sheared MP unit. A regional and moderately ESE plunging linear fabric coincides with the fold‐axis of the foliation poles from the eastern domain. Whether this also reflects a regional top to the WNW transport, as inferred from the dextral strike‐slip on steeply to SSW dipping foliation, needs further study. Top to the WNW shear is shown by weakly inclined low pressure (LP) cordierite rocks that flank the eastern domain in the south. Some new ³⁹Ar/⁴⁰Ar mica cooling ages (519, 521 Ma) from the Kulet UHP micaschists reflect the same early stage evolutionary event as was previously shown for the Kumdy–Kol UHP rocks (515, 517 Ma) in the west. Similar ³⁹Ar/⁴⁰Ar ages (500, 517 Ma) are recorded by micas and amphibole that outline a top to NNW shear fabric in the non‐subducted Proterozoic basement, north of the megamélange. A 447 Ma overprint of the MP sequences is considered to reflect the strike‐slip deformation with sillimanite and the reworking of an early kyanite‐bearing tectonite. Biotites from the LP cordierite rocks yielded approximately 400 Ma ³⁹Ar/⁴⁰Ar ages. In case they reflect the WNW shear deformation, the latter is considered to be associated with a regional granite magmatism (420–460 Ma) extending south of the eastern domain. In their present different structural domains the Kulet and Kumdy–Kol UHP units display a similar early stage event. Subsequent LP deformation, which is likely to be associated with regional granite magmatism (420–460 Ma), is assumed to have obliterated any common or uniform early exhumation structure for the whole megamélange. The north‐east structured Kumdy–Kol domain is assumed to have preserved the most information about the early stage exhumation. This domain is at an angle to the regional WNW strike of the megamélange.     

Structure of the upper mantle in the Circum-Arctic region from regional seismic tomography

We present a new three-dimensional model of P-velocity anomalies in the upper mantle beneath the Circum-Arctic region based on tomographic inversion of global data from the catalogues of the International Seismological Center (ISC, 2007). We used travel times of seismic waves from events located in the study area which were recorded by the worldwide network, as well as data from remote events registered by stations in the study region. The obtained mantle seismic anomalies clearly correlate with the main lithosphere structures in the Circum-Arctic region. High-velocity anomalies down to 250–300 km depth correspond to Precambrian thick lithosphere plates, such as the East European Platform with the adjacent shelf areas, Siberian Plate, Canadian Shield, and Greenland. It should be noted that lithosphere beneath the central part of Greenland appears to be strongly thinned, which can be explained by the effect of the Iceland plume which passed under Greenland 50–60 million years ago. Beneath Chukotka, Yakutia, and Alaska we observe low-velocity anomalies which represent weak and relatively thin actively deformed lithosphere. Some of these low-velocity areas coincide with manifestations of Cenozoic volcanism. A high-velocity anomaly at 500–700 km depth beneath Chukotka may be a relic of the subduction zone which occurred here about 100 million years ago. In the oceanic areas, the tomography results are strongly inhomogeneous. Beneath the North Atlantic, we observe very strong low-velocity anomalies which indicate an important role of the Iceland plume and active rifting in the opening of the oceanic basin. On the contrary, beneath the central part of the Arctic Ocean, no significant anomalies are observed, which implies a passive character of rifting.