Chemical balance of the Earth's crust

The chemical composition and lithologic proportions of average sedimentary and igneous rocks are analyzed by a linear least squares regression. The best set of internally consistent data for sediments in geosynclines, continental platforms, deep ocean basin and the average igneous and sedimentary rock are shown below. These results are based on published estimates of the chemical composition of igneous and sedimentary rocks and the relative volume of sediments on the continents and in the deep ocean basins.

$\text{SiO}{}_2\text{A1}{}_2\text{O}{}_3\text{Fe(total)}\text{MgO}\text{CaO}\text{Na}{}_2\text{O}\text{K}{}_2\text{O}\text{CO}{}_2\text{Geosynclines}\text{55.2}\text{12.0}\text{3.6}\text{3.2}\text{8.3}\text{1.0}\text{2.4}\text{8.6}\text{Platforms}\text{49.5}\text{10.7}\text{3.2}\text{3.3}\text{12.3}\text{0.9}\text{2.2}\text{10.8}\text{Av. sed.}\text{55.9}\text{13.8}\text{4.4}\text{3.6}\text{5.8}\text{1.2}\text{2.6}\text{5.2}\text{Av. ig.}\text{59.0}\text{15.0}\text{5.0}\text{3.6}\text{7.0}\text{3.4}\text{3.2}$

The lithologic proportions are: geosyncline—24% sandstone, 58% shale, and 16% carbonate; platform—23% sandstone, 49% shale and 26% carbonate; average sedimentary rock—13% sandstone, 33% shale, 12% carbonate, and 41% pelagic; average igneous rock is approximately 2/3 granodiorite and 1/3 tholeiite. These data indicate a chemical balance of major elements in the Earth's upper crust during the last 1.5 b.y.     

俄罗斯-蒙古地学断面地壳模型的地质-地球物理资料综合研究

地学断面是指地壳的垂直剖面,主要通过对地质和地球物理资料的综合分析来揭示构造带的性质及其空间关系。横断面的研究所采用的数据基本包括100 km宽区域地质图、上地壳的地质剖面图、重磁图(沿横断面的重磁剖面图)以及地壳的地震波速度、密度和其他地球物理属性的剖面图。这些数据被用于构建综合的数据剖面图(结果图),以展示各种地球动力学条件下(裂谷、海洋、碰撞带、造山盆地、大陆地台和岩浆弧,包括安第斯岛弧、活动大陆边缘、海沟、弧前和弧后盆地)的特定的岩石组构。本项目的研究目标是根据研究区现存的地质和地球物理数据的综合解释,统一图例,建立研究区深部剖面,以确定地体的空间关系及其在板块构造方面的地球动力学性质。 前人已分别对东西伯利亚南部和蒙古境内的多个地体进行了构造划分,并对它们的地球动力学性质和时空关系进行了分析。研究结果显示该系列地体为早古生代、中晚古生代和晚古生代—早中生代的岛弧和微大陆。此外,研究还识别出了中—晚古生代和晚古生代—早中生代安第斯型活动大陆边缘、晚古生代—早中生代被动大陆边缘和早白垩世裂谷。与岛弧和安第斯型活动大陆边缘相关的岩体被推覆至相邻大陆和微陆块上,部分推覆宽度可达150 km。目前已开展泥盆纪到晚侏罗世时期蒙古-鄂霍次克海地区的古地球动力学重建。 “非地槽”型花岗岩类岩浆作用在板块构造方面找到了直接且合理的解释,其中泥盆纪—石炭纪和二叠纪—三叠纪岩浆作用区域对应于安第斯型活动大陆边缘,中—晚侏罗世岩浆作用则与西伯利亚/蒙古-中国大陆板块碰撞有关。碰撞岩浆作用中亚碱性(地幔)元素的存在及其所在的构造区域在很大程度可以说明蒙古-鄂霍次克海闭合后,巨厚大陆岩石圈下曾经发生过持续的大洋裂谷活动(地幔热点)。在早白垩世时期,大陆裂谷活动影响到了同一时期正在发生的大陆汇聚作用。 西伯利亚南部边界大部分具有安第斯型活动大陆边缘性质,这也是蒙古—鄂霍次克缝合线沿线蛇绿岩数量较少的原因。因为当汇聚大陆一个具有安第斯类型的活动边缘,而另一个具有被动边缘时,前者的大陆地壳会最终逆冲到后者之上,并因此破坏掉先前出露的蛇绿杂岩体。部分被破坏的蛇绿岩块是俯冲带保留下来的海山残余,其可能成为增生-俯冲楔体的混沌复合体的一部分。然而,由于快速俯冲作用,这种楔形体在晚二叠世—早侏罗世的积累并不是西伯利亚活动边缘的典型特征。 沿地学断面综合的地质和地球物理资料分析表明,亚洲大陆是在显生宙时期由部分前寒武纪微陆块构造拼贴而成的。前寒武纪地块间存在不同宽度的已变形且剥蚀强烈的显生宙火山弧,它们也被归类为特定地体。     

Computer simulation of Pb and Sr isotope evolution of the Earth's crust and upper mantle

Isotope evolution in a differentiated (crust and upper mantle) and chemically heterogeneous Earth has been computed for a model with isotopic exchange between crust and mantle at an exponentially decreasing rate. To simulate the effects of subduction, cells of crust and mantle are selected at random, their contents mixed and then redistributed into the cells from which they came—to give new chemically heterogeneous but momentarily isotopically homogeneous systems. Daughter isotopes in each cell grow according to equations appropriate for closed chemical systems. Rock age distributions and isotopic data created by the computer calculation mimic nature. Pb isotope data changes through geologic time are illustrated to demonstrate that two-stage interpretations applied to Pb data for rocks with complex histories may be misleading. The intercept of the Pb ore growth curve and regressions fit to Pb data gives minimum values only for the duration of heterogeneous $\text{U}\text{Pb}$ systems, not the time when heterogeneous distribution first occurred. An intercept derived time of about 3 b.y., in the Pb system is shown to be quantitatively compatible with an average $\text{Rb}\text{Sr}$ age of crustal rocks of 1.5 b.y. and with a constant degree of chemical heterogeneity for all of Earth history.     

Helium concentration in the Earth's lower atmosphere

Measurements in 1981 of the helium content of the Earth's lower atmosphere have given a value of 5.222 ± 0.017 ppm by volume. This value, obtained by isotope dilution mass spectrometry, is 0.3% lower, but in essential agreement with the currently accepted value of 5.239 ± 0.004 ppm determined by Glueckauf in the 1930's. A consideration of processes that could have altered the helium concentration since the 1930's indicates that the concentration could have increased measurably due to release of helium by natural gas production. Possible net helium loss from the atmosphere is, however, not readily quantifiable.     

Crustal features of some main suture zones along the European Geotraverse

The European Geotraverse (EGT) crosses along a 4000 km profile from the North Cape to Tunisia the following main suture zones:

the Tornquist-Teisseyre zone between the Baltic Shield and the Variscan realm,

the transition zones between Rhenohercynian and Saxothuringian as well as between Saxothuringian and Moldanubian zones in the Variscan part of central Europe, and

the collision zone between the European continent and the Adriatic microplate.

Some structural aspects of these suture zones are described.     

Chemical equilibration of the Earth's core and upper mantle

The oxygen fugacity (fO₂) of the Earth's upper mantle appears to lie somewhat above that of the iron-wüstite buffer, its fO₂ is assumed to have been similar to the present value at the time of core formation. In the upper mantle, the Fe-rich liquid protocore that would form under such conditions of fO₂ at elevated temperatures would lie predominantly in the system Fe-S-O. Distribution coefficients for Co, Cu, Ni, Ir, Au, Ir, W, Re, Mo, Ag and Ga between such liquids and basalt are known and minimum values are known for Ge. From these coefficients, upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole Earth and equilibrium between the Fe-S-O protocore and upper mantle. These calculated abundances are surprisingly close to presently known upper mantle abundances; agreements are within a factor of 5, except for Cu, W, and Mo. Therefore, siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late-stage meteoritic bombardment, and a protocore formed from the upper mantle containing S and O seems likely.As upper mantle abundances fit a local equilibrium model, then either the upper mantle has not been mixed with the rest of the mantle since core formation, or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of $\text{P}$, $\text{T}$, and fO₂. The latter possibility seems unlikely over such a $\text{P-T}$ range.     

Seismic investigations along the European Geotraverse and its surroundings in Central Europe

Since 1975 several high-resolution seismic-refraction and reflection surveys have been carried out in western Germany to investigate the structure of the Earth's crust and uppermost mantle. The investigation culminated in the seismic-refraction survey along the 825 km long central part of the European Geotraverse (EGT) in 1986. This contribution summarizes the main results of the more recent crustal investigations along and around the EGT. The internal crustal structure throughout the area of the Variscides is very complex and changes laterally considerably. Distinct crustal blocks differing in their internal structure can be assigned to geologically defined units of the Variscan and Caledonian orogeny. In spite of local deviations, in general a more or less transparent and low-velocity upper crust contrasts with a highly reflective lower crust. A subdivision of upper and lower crust by a well-defined boundary (Conrad discontinuity) is not always seen. Towards the Alps the average velocity of the lower crust is as low as 6.2 km s^?1, in contrast to the area north of the Swabian Jura where the velocities above Moho vary between 6.8 and 7.2 km s^?1. In Northern Germany, the Elbe line separates the lower crust into two regions with 6.4 km s^?1 average velocity in the south and 6.9 km s^?1 in the north. The total crustal thickness under the Variscan part of Germany is fairly constant between 28 and 30 km, except under the Rhine Graben area with 25–26 km and beneath the central part of the Rhenish Massif where an anomalous crustal thickening to 37 km is observed. Under northern Germany the Moho rises to about 26 km depth and the data indicate at least one fault-like step of 1 km before the crust thickens toward the Ringkobing-Fyn basement high. The synthesis of seismic velocity structure and petrological information from xenolith studies allows us to propose a mafic composition for the deeper levels of the crust and uppermost mantle which may be valid at least for the central part of the Variscan crust along the European Geotraverse in Central Europe.     

Crustal structure of the Southern Alps beneath the intersection with the European Geotraverse

Newly digitized and amplitude controlled record sections from the 1977 Southern Alps refraction campaign permitted a reinterpretation of the crustal structure in the area between western Lombardy and the Giudicaria fault. The resulting model exhibits considerable lateral heterogeneity: in the west, below 7.5 km of sediments of the Lombardy Basin, the crust reaches a depth of only 31 km, whereas it thickens towards the more mountainous area in the east, reaching a depth of 46 km below the Adamello Massif. Although the signal character of the corresponding reflections is somewhat erratic, the data are satisfied best by models with a low-velocity zone in the upper crust. An additional small velocity discontinuity from 6.2 to 6.4 km/s was found in the middle crust at around 20 km. Earlier interpretations, based on travel-times alone, included a layer with a velocity of about 7 km/s at this depth. This high-velocity layer was then interpreted as lower-crustal material of the Adriatic — African plate, which had been overthrust onto the European plate during the Alpine orogeny, thus explaining the uplift of the Southern Alps. However, this model of crustal doubling is questionable, because such a mid-crustal high-velocity layer is not in agreement with the amplitude data. The relatively thin crystalline part of the crust under the Lombardy Basin is interpreted, in accordance with geological evidence, as a relic of a Late Hercynian rifting event.     

Hypothesis for the origin of convergent margin granitoids and Earth’s continental crust by thermal migration zone refining

The production of Earth’s granitoids is generally attributed to magma intrusion, fractional crystallization and assimilation but the details of how granitoid plutons form remains widely debated. In light of recent experimental results which indicate that partially molten wet andesite in a temperature gradient evolves into a granitic bulk composition at the cooler end of the gradient (in a process called thermal migration), I present a model for at least some of Earth’s granitoids forming by a top-down thermal migration zone refining process. According to this model, convergent margin igneous activity builds a thick volcanic pile which becomes a barrier to further ascent of magma, leading to magma underplating by injection of sills at the base of the pile. When magmas arrive at the location of underplating, they react and release heat and water to the overlying materials (previously intruded sills), resulting in a downward moving zone having a near-steady-state temperature gradient. This leads to compositional differentiation by wet thermal migration taking place over million year time scales; this in situ differentiation process occurs in the middle of the underplated region but not on the more rapidly cooled edges of the sills. Modeling using the IRIDIUM program shows this process can produce sequences of granitoid that are kilometer or greater in thickness; regardless of granitoid thickness, the bottom of the system maintains a near constant thickness of hornblende gabbros. The model provides a logical connection between andesitic stratovolcanoes and underlying, more silicic intrusive series plutons—both reflect ascent of andesitic composition magmas, with the implication that convergent margin magmatic systems evolve temporally from stratovolcanoes to plutons once magma ascent is inhibited and underplating begins.The model provides an alternative to the standard view that granitoids result from cooling of large bodies of magma and could help to resolve long-standing questions concerning: geophysical observations of magma chambers; the compositions of minerals in granitoids; and the development of preferred mineral orientations in granitoids. It provides a consistent model in that it explains the systematic normal compositional zoning of plutons within the context of an incremental growth process dictated by geochronology. Most importantly, the model is predictive, emphasizing the importance of examining granitoids in the vertical dimension. The hypothesis that thermal migration plays a role in granitoid formation can be tested by analysis of non-traditional stable isotope systems such as Fe, Mg and Si that should show a signature of thermal diffusion. The model predicts that the tops of overlying granitoids will have relatively heavy isotopic compositions whereas underlying hornblende gabbros will have relatively light isotopic compositions. Examination of existing iron isotope data and new silicon isotope data are consistent with the hypothesis and point to the need for more thorough testing.     

Seismic reflection and refraction studies for investigating fault zones along the Geotraverse Rhenoherzynikum

In 1973 detailed seismic crustal studies were performed across the prominent fault zone between the Hercynian fold systems of the Rhenish Slate Mountains (western part of Rhenoherzynikum) and the Saar-Nahe trough. Reflection data show a zone of strongly dipping reflectors, separated from another area with nearly horizontal layering. Data from refraction stations confirm the picture of a fault zone cutting two old crustal blocks down to the Moho. A similar but smaller survey was performed in 1975 across the western Rhine graben fault near Landau. This fault is tensionsal and could not be observed with the same certainty and up to the same depth range as the former one. Apparently, its dip near the surface is smaller than anticipated and may even assume still smaller values at intermediate crustal depths. Moreover, high temperatures in this area tend to limit the maximum depth of the fault zone, in accordance with the concept of a direct relationship between the depths of seismicity along faults and the temperature—viscosity regime. The area between the two reflection surveys was studied by refraction observations, making use of the shots of the reflection work. In general, the reflection investigations are well able to reveal the geometry and the maximum depth of fault zones and show many structural details, while the supplementing refraction work determines the overall velocity depth relation and may follow important horizons.     

The earthquake foci and deep structure of the Earth’s crust and upper mantle along the southern Sakhalin-Sea of Okhotsk-Kamchatka profile

The Kuril-Kamchatka seismofocal zone was thought to be a single plate approximately 90 km wide and dipping to a depth of 700 km at an angle of 40°–45°. This concept reflects primarily the physical differences (elastic wave velocities, density, temperature, etc.) between the seismofocal zone and the mantle hosting it. Detailed investigations show that the seismofocal zone proper is also heterogeneous with earthquake hypocenters variably concentrated and clustered within this zone, where both seismogenic and aseismic strata, as well as subvertical zones, can be identified. The latter are reflected in the structure and faults of the Earth’s crust and upper mantle.     

Plate tectonics,flood basalts and the evolution of Earth’s oceans

The chemical composition of the bulk silicate Earth (BSE) indicates that the present‐day thermal budget of Earth is likely to be characterized by a significant excess of surface heat loss over internal heat generation, indicating an important role of secular cooling in Earth’s history. When combined with petrological constraints on the degree of secular cooling, this thermal budget places a tight constraint on permissible heat‐flow scaling for mantle convection, along with implications for the operation of plate tectonics on Earth, the history of mantle plumes and flood basalt magmatism, and the origin and evolution of Earth’s oceans. In the presence of plate tectonics, hotter mantle may have convected more slowly because it generates thicker dehydrated lithosphere, which could slow down subduction. The intervals of globally synchronous orogenies are consistent with the predicted variation of plate velocity for the last 3.6 Gyr. Hotter mantle also produces thicker, buoyant basaltic crust, and the subductability of oceanic lithosphere is a critical factor regarding the emergence of plate tectonics before the Proterozoic. Moreover, sluggish convection in the past is equivalent to reduced secular cooling, thus suggesting a more minor role of mantle plumes in the early Earth. Finally, deeper ocean basins are possible with slower plate motion in the past, and Earth’s oceans in the Archean is suggested to have had about twice as much water as today, and the mantle may have started as dry and have been gradually hydrated by subduction. The global water cycle may thus be dominated by regassing, rather than degassing, pointing towards the impact origin of Earth’s oceans, which is shown to be supported by the revised composition of the BSE.     

An introduction to the European Geotraverse Project: First results and present plans

The European Geotraverse (EGT) is an international, multidisciplinary project focused on a north-south orientated lithospheric profile, 4000 km long and of varying width, extending from northernmost Scandinavia to North Africa. This profile consists of three interlinking Segments (Northern, Central, and Southern) comprising a continuous succession of tectonic provinces ranging from the oldest Precambrian areas of the Baltic Shield to the currently active area of the Western Mediterranean. The broad aim of the EGT Project is to obtain a better three-dimensional picture of the structure, state, and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics. All of the 12 major projects that constitute the EGT “Joint Programme” have now been initiated, and several of these projects are nearing completion.Along the Northern Segment, data from “The Fennoscandian Long-Range Project” (FENNOLORA), a 2000 km long seismic profile across the Precambrian Baltic Shield, show that except beneath southern Sweden, the Shield is characterized by a high-velocity, 40–50 km thick crust—including a 5–10 km thick crust-mantle transition zone. An alternating series of 4–6 high- and low-velocity zones is present in the subcrustal lithosphere, the base of which increases in depth from ca. 110 km to ca. 230 km from south to north beneath the Shield. The top of the mantle transition zone lies at a depth of about 450 km. The second major project along this Segment, the EUGENO-S (European Geotraverse Northern Segment—Southern Part) project, is a multidisciplinary study of the Fennoscandian Border Zone, and was largely completed in 1984 with the realization of a large-scale seismic experiment. Preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the Danish Basin. Field work for a third major project, a multidisciplinary transect of the Archaean and Early Proterozoic terrains in the northernmost part of the Shield (the “Polar Profile”), was carried out in 1985.A series of deep seismic reflection lines has so far been realized in the area of the Central Segment in the context of German national programmes. First interpretations of the seismic data from a 260 km long profile across the two main intra-Variscan (Hercynian) lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied, and distinct changes in reflectivity between the main Variscan tectonic zones. In 1986, the entire Segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies.A series of seismic experiments (termed EGT-S) have been carried out across the Southern Segment (in 1982, 1983, and 1985). Interpretation of data from the 1982 and 1983 experiments have led to several interesting results, including:

• 1.(1) the suggestion that two “crust-mantle”-like interfaces exist beneath the Po Basin (at depths of about 35 and 50 km) and adjacent tectonic units, these interfaces marking a deep contact zone between the Adriatic and European plates,
• 2.(2) in the area between Genoa and Corsica, the Ligurian Sea is underlain by a greatly thinned, distinctly layered section of continental crust, and
• 3.(3) Corsica and Sardinia are underlain by bowl-shaped, “typically” Variscan continental crusts.

The 1985 phase of seismic surveys focused on crustal structure beneath Tunisia and the adjacent seas.In addition, two off-traverse projects are being realized. First, a wide-aperture network of autonomously recording seismic stations (“NARS”), installed along the line Gothenburg-Málaga between 1982 and 1984, is already yielding high-quality data on the upper 600–700 km of the mantle. Second, an investigation of lithospheric seismic anisotropy in the area of the Iberian Peninsula is being organized for 1987–1988.Finally, of great importance are the systematic compilation of existing data and, where needed in critical regions, collection of new geophysical and geological data presently being carried out for the entire area encompassed by the EGT. It is expected that these compilations will be completed by 1987, at about the same time that full results from the main large-scale seismic experiments become available, enabling the construction of an integrated lithospheric cross-section along the EGT, requiring a final phase of intensive multidisciplinary collaboration.     

CaAl ratio and composition of the Earth's upper mantle

Undifferentiated meteorites (chondrites) have the same relative abundances of refractory lithophile elements (Ca, Al, Ti, Sc, REE, etc.), despite variable absolute concentrations. The reasonable assumption of chondritic ratios among refractory elements in the bulk Earth is used to constrain the chemical composition of the upper mantle in the following way: Correlations of the compatible refractory elements Ca, Al, Ti, Sc and Yb with MgO are worldwide very similar in suites of spinel-lherzolite xenoliths from basaltic rocks. Such suites represent upper mantle material depleted to differing degrees by extraction of partial melts. From these refractory elements vs. MgO correlations, ratios of pairs of refractory elements were calculated at various MgO contents. Chondritic $\text{Al}\text{Ti}$ and $\text{Sc}\text{Ti}$ ratios were only obtained for MgO contents below 36%. A chrondritic $\text{Sc}\text{Yb}$ ratio requires an MgO content above 35%. We therefore accept 35.5% as the most reasonable MgO content of undepleted upper mantle. This MgO content is slightly below the spinel-lherzolite with the lowest measured MgO content (36.22%). The corresponding Al₂O₃ content of 4.75% is higher than in previous estimates of upper mantle composition. The concentrations of other elements were obtained from similar correlations at a MgO content of 35.5%. The resulting present upper mantle composition is enriched in refractory elements by a factor of 1.49 relative to Si and Cl and by a factor of 1.12 for Mg relative to Si and Cl. These enrichments are in the same range as those for the Vigarano type carbonaceous chondrites. The Mg/Mg + Fe ratio of 89 is slightly lower than previous estimates.The $\text{Ca}\text{Al}$ ratio in spinel lherzolite suites is, however, uniformly higher worldwide than the chondritic ratio by about 15%. Orogenic peridotites as well as komatiites appear to have similar non-chondritic $\text{Ca}\text{Al}$ ratios. It is therefore suggested that this non-chondritic $\text{Ca}\text{Al}$ ratio is a characteristic of the upper mantle, possibly since the Archean. A minor fractionation of about 4% of garnet in an early, global melting event (deep magma ocean?) is presented as the most likely cause for the high $\text{Ca}\text{Al}\text{-}\text{ratio}$. In this case the addition of 4% of such a garnet component to the undepleted present upper mantle would be required to obtain the composition of the primordial upper mantle. The $\text{Ca}\text{Al}\text{-}\text{ratio}$ of this primordial mantle would be 15% higher than that of the undepleted present upper mantle, resulting in an enrichment of refractory elements of 1.70 ($\text{Al}\text{Si}$ relative to Cl) for the primordial upper mantle.     

Kinetics of dissolution of mechanically comminuted rock-forming oxides and silicates—II. Deformation and dissolution of oxides and silicates in the laboratory and at the Earth's surface

Comparison of the patterns of fracture under tensile stress, indentation, and scratching of periclase. quartz, and corundum indicates that the properties relevant to dissolution of rock-forming oxides and of rock-forming non-layer silicates should be changed by mechanical comminution in essentially the same way as those of quartz. The changes are accomplished by brittle fracture under the tensile component of the stress field, which does not generate subsurface damage, and by microplastic behavior under local stresses with high net compressive and shear components, which does.Mechanical comminution should therefore affect the apparent rates of dissolution (rates calculated with respect to the initial interface area) of rock-forming oxides and of rock-forming non-layer silicates in essentially the same way in which it affects the apparent rate of dissolution of quartz. This is supported by the available evidence on the effect of dry grinding on the kinetics of dissolution of feldspars, pyroxenes, and olivines in aqueous solutions.Different effects of mechanical comminution on solubilities and dissolution rate constants can be related to certain measured or calculated properties of the considered minerals. Notably, the effect of grain size on the dissolution rate constant can be rigorously related to the Kelvin effect.The available evidence on the mechanical comminution at the bases of dry-based glaciers in highgradient segments of streams, in certain high-energy coastal and epeiric environments, and in sandy deserts indicates that such mechanical comminution should significantly affect the simultaneous or subsequent dissolution of the comminuted material.     

Exhumation of subducted continental crust along the arc region

The Kutná Hora crystalline complex (KHCC) in the Bohemian Massif is a HP/HT complex adjacent to the magmatic arc. It is dominated by migmatite, orthogneiss and granulite with bodies of eclogite and peridotite. The KHCC migmatite consists of K-feldspar, plagioclase, quartz, phengite, biotite, garnet and kyanite. Melting conditions were estimated at 780 °C and >16 kbar and inferred melt volume varies between 1 and 4 vol%. Peak temperature is 865 °C at 18–19 kbar followed by decompression in the presence of melt to 12–13 kbar and 770–800 °C. U-Pb monazite geochronology reveals a spread of ages between 550 Ma and 330 Ma. REE patterns show low Yb/Gd for 550–500 Ma, high Yb/Gd for ages at ~480 Ma, and decreasing Yb/Gd towards ~340 Ma. First monazite in equilibrium with garnet constrains the HP metamorphism to ~350 Ma, which is followed by recrystallization of monazite down to 325 Ma. U-Pb zircon geochronology displays an age range from ~670 Ma to ~430 Ma. The broad age range records a span of protolith crystallization and/or old metamorphism. The presence of HP ky + mu migmatite, their P–T path, protolith zircon and monazite metamorphic ages and whole-rock geochemistry are similar to HP migmatites in the Eger crystalline complex (ECC) in the Saxothuringian domain further in the west. We propose the following geodynamic scenario for subduction-relamination-exhumation mechanism: (i) subduction of the Saxothuringian continental lithosphere at 360 Ma related to early stage of trans-lithospheric diapirism triggered by arc-related magma weakening; (ii) large-scale emplacement of relaminant into the upper plate lithosphere at 350–340 Ma; and (iii) return flow of the relaminant along the subduction interface (the ECC) and emplacement of relaminant in the upper–middle crust in the rear part of the arc system (the KHCC) at 340–330 Ma.     

Velocity structure of the crust and upper mantle in the Baikal rift zone from the long-term observations of broad-band seismic stations

Using inversion of SV receiver functions, defined for various directions at each of the three broad-band stations located in the Baikal rift zone, detailed S velocity models of the crust and upper mantle down to 260 km have been obtained. These models reflect peculiarities of the velocity structure beneath Baikal depressions and mountains.