Search

Previously unrecognized mafic and felsic plutonic rocks, formerly interpreted as an anatectic part of the amphibolite-eclogite Marianske Lazně Complex (MLC, Bohemian Massif) were examined together with a possible upper-crustal equivalent represented by the Cista pluton intruding continental crust farther east. These plutonic rocks were studied by whole-rock geochemistry (major and trace elements, Sr-Nd isotopes) and zircon U-Pb geochronology. The MLC magmatic rocks range from pyroxene-amphibole gabbro to trondhjemite and oligoclasite. The Cista pluton consists of porphyritic granitoids. The calc-alkaline nature of these rocks, relative enrichment of fluid-mobile elements (including large-ion lithophile elements), strong fractionation of light rare earth elements over heavy rare earth elements and depletion of high field strength elements are evidence for active margin magmatism. U-Pb zircon dating indicates that the magmatism was Mid-Devonian in both the MLC (ca. 385 Ma) and Cista pluton (ca. 390 up to ca. 365 Ma). Both magmatic units show inherited cores probably linked to recycling of crustal material. Sr-Nd isotopic compositions indicate that the gabbro formed from a depleted MORB mantle source composition with a small contribution of a subduction and crustal component. A three-stage geodynamic model is proposed: 1) ca. 390–380 Ma recycling of a high-pressure accretionary wedge and migration of arc melts into more distal parts of the upper continental plate, 2) ca. 380–370 Ma compression and thickening of the accretionary wedge-arc edifice, and 3) ca. 370–365 Ma development of a giant east-dipping detachment that reworked the whole edifice in the west and also involved emplacement of the supracrustal Cista pluton along antithetic west-dipping shear zone in the east. The studied magmatic rocks record a Mid-Devonian magmatic arc related to the formation of a subduction wedge stacked beneath the upper plate that was followed by formation of a second and independent Carboniferous continental arc further east.

The NW Iberian Allochthon and the Tepla-Barrandian and Moldanubian zones represent the internal parts of the Variscan belt in their respective domains. A correlation based on the lithological association, protolith ages, metamorphic evolution, detrital zircon age spectra and tectonic setting is attempted between the NW Iberian Massif and SE Bohemian Massif in order to check whether they could have formed part of the same allochthonous stack. The Galicia-Tras-os-Montes Zone of the Iberian Massif and the internal zones of the Bohemian Massif include from bottom to top a Parautochthon and Lower Allochthon representing the outer edge of the northern Gondwana margin, an oceanic Middle Allochthon with Cambro-Ordovician and Early Devonian ophiolites and an Upper Allochthon interpreted as a peri-Gondwanan terrane. Early Variscan, subduction-related high-pressure metamorphism characterizes many of the allochthonous units, with ages younging from the structurally upper to the lower units from 400–385 Ma to 370–360 Ma, respectively. High- and ultrahigh-pressure metamorphism occurred also in the Saxothuringian Autochthon at 360–340 Ma, but not in the NW Iberian Autochthon. The different behavior of the Autochthon in the Iberian and Bohemian massifs accounts for their distinct evolutions from 360 Ma onward. We conclude that the Upper Allochthon was a unique peri-Gondwanan terrane, whereas the Middle Allochthon represents units of the same peri-Gondwanan ocean, opened at the Cambro-Ordovician boundary, and having recorded localized renewed activity in the Silurian–Early Devonian. No other oceans separated the Lower Allochthon, Parautochthon and Autochthon.

The non-cylindrical character of the Variscan Belt is established via a description of its most salient geological characteristics and correlations among the main and relatively minor massifs from Bohemia to Morocco on the basis of a bibliographical synthesis. Using the firmly established tectonostratigraphic domains, the continuity of Variscan zones is discussed, as well as the western termination of the one that occupies most of the central part of the belt. This zone is characterized by its allochthony, and breaks the continuity of the zonation, that is, the cylindrism of the belt. A proposal is made to call this zone the Mid-Variscan Allochthon and its wedge shape in map view is explained performing a step by step restoration of late Variscan deformation. Once the geometry of the belt is interpreted by a succession of regionally significant deformation events, a kinematic reconstruction is proposed. It is based on recent developments on the knowledge of Paleozoic plate tectonics and intended to show that the deformation model is compatible with current paleogeographical developments.

International audience; A multidisciplinary approach integrating potential field analysis with geological and geochemical data provides new insights into the understanding of the crustal structure and evolution of the Mongolian collage. Magnetic and gravity data demonstrate the inconsistency between the geologically defined terranes and the geophysical domains in the southwestern part of the Mongolian collage. The combination of potential field analysis and modelling with whole rock geochemistry and isotopic mapping of Carboniferous–Permian granitoids indicates the presence of a homogeneous lower crust composed of a felsic to intermediate juvenile material beneath geophysically heterogeneous upper crust. This feature is interpreted as a result of a trench-directed lower crustal emplacement of an arc type crust underplating deformed Paleozoic oceanic crust. The potential field data also confirmed the occurrence of two orthogonal late Devonian and Permian–Triassic deformation upper crustal fabrics at the scale of the southwestern Mongolian collage. The prominent magnetic highs correspond to the tectono-metamorphic domains and magmatic provinces. The gravity anomalies highlight a periodicity of the signal correlating with alternating Permian–Triassic high and low strain zones, forming a zone of major deformation wrapping around the hinge of Mongolian orocline. The geometry and kinematics of dextral and sinistral transpressive faults are explained to result from the reactivation of Permian–Triassic deformation zones in the Cenozoic stress field.

The Honghai volcanogenic massive sulfide (VMS) deposit occurs in the Kalatage inlier of the Dananhu-Haerlik arc, Eastern Tianshan, in the southern Altaids. The deformed deposit, hosted in a suite of early Paleozoic basic to intermediate volcanic, volcaniclastic, and sedimentary rocks, consists of lenticular massive to semimassive, and sulfide-silicate stockwork orebodies. The orebodies are characterized by zoning of metallogenic elements and sulfide minerals outward from a central stockwork zone as follows: (Fe +/- Cu), (Fe + Cu + Zn + Au + Ag; Cu > Zn), (Fe + Zn + Cu + Au + Ag; Zn > Cu), and (Fe +/- [Cu/Zn]). A typical VMS hydrothermal alteration zone that formed around the foot of the massive orebodies is several times larger than the lenticular massive orebody. The hydrothermally altered rocks contain sericite, chlorite, and epidote zones from the core outward. Re-Os and argon dating of pyrite and sericite shows that the Honghai VMS deposit formed in the Early Silurian (436 +/- 2 Ma) and was deformed in the Early Devonian (410 +/- 4 Ma). This is the first documented early Paleozoic VMS deposit in the Eastern Tianshan. Our multidisciplinary data indicate that the Honghai Cu-Zn deposit formed in an immature/nascent island arc, where early tholeiitic lavas evolved into transitional basic-andesitic volcanic rocks and calc-alkaline intrusions with relatively high epsilon(Nd)(t) values (+6.2-+8.4) and low (Sr-87/Sr-86)(i) ratios (0.70412-0.70541). The Sr-Nd-Pb-S isotopic and geochemical data suggest that the ore-forming melts started in mantle-derived volcanic rocks and deep subvolcanic intrusions, and some alteration was influenced by seawater. The deposit formed in a high-level subvolcanic intrusion driven by sub-seafloor CO2-rich, NaCl-CaCl2-H2O hydrothermal fluids (temperatures: 99-377 degrees C, salinities: 6%-18% NaCl equivalent) circulating in active extensional faults in a submarine caldera. Extension of the immature (nascent) intra-oceanic arc was one of the important factors that controlled the development and preservation of the VMS deposit. Future work will benefit from this improved understanding of the VMS deposits formed in immature (nascent) intra-oceanic arcs, especially in early Paleozoic island arcs in the Eastern Tianshan in the southern Altaids.

International audience; The high‐pressure Heilongjiang Complex belongs to a Jurassic accretionary complex in Yilan area, NE China. The complex is located in the junction of three orogenic systems in the NE Asia: the E‐W‐striking Central Asian Orogenic Belt (CAOB), the N‐S‐striking Paleo‐Pacific system, and the ENE‐WSW‐striking Mongol‐Okhotsk system. Thus, a detailed tectono‐metamorphic analysis coupled with accurate radiometric dating of the Heilongjiang Complex provided in this study represent important evidence for understanding the complex interactions between the three orogenic systems in the Late‐Palaeozoic to Mesozoic. New structural and microstructural data combined with the strain analysis highlight the presence of three distinct deformation events affecting the high‐pressure Heilongjiang Complex. The first deformation event D1 is related to development of blueschist facies prolate to plane strain fabrics that were subsequently reworked by the blueschist‐amphibolite facies event D2 that transposed all previous structures by the main west‐dipping schistosity. This planar fabric was folded by upright F3 folds and heterogeneously reworked by greenschist facies E‐W‐striking vertical cleavage during large scale N‐S shortening. Each deformation event was related to specific strain fabrics and metamorphic conditions highlighting dynamic evolution of the subduction complex during burial, exhumation, and final shortening. The first D1 event is related to westward directed prograde constrictional flow and progressive dismembering of competent mafic, ultramafic layers, and quartzites in highly stretched meta‐sedimentary matrix. This mélange was folded by recumbent detachment folds resulting in emplacement of the exhumed unit progressively reaching amphibolite facies conditions atop of W‐WSW‐dipping channel. This event is associated with dominant flattening implying increasing mechanical coupling between the colliding Songliao and Jiamusi blocks. Subsequently, the accreted subduction complex was reworked during N‐S‐directed horizontal shortening. This event was associated with an important post‐buckle flattening of fold limbs and rotation of competent blocks with inherited D1 and D2 strain ellipsoids along F3 fold axes. Our new 40Ar‐39Ar plateau ages on muscovite and phengite (173–177 Ma), together with previous radiometric data, are interpreted as reflecting exhumation of subduction channel during the blueschist‐amphibolite facies event while the N‐S shortening (D3) probably operated in Late Jurassic as indicated by some previously published cooling ages (~140–160 Ma). The westward‐oriented subduction and exhumation fabrics of the high‐pressure Heilongjiang accretionary complex followed by N‐S‐directed upright folding are interpreted to reflect transition from E‐W Pacific subduction regime to N‐S‐directed shortening related to the closure of the Mongol‐Okhotsk Ocean.

International audience; We present a new analogue model of oroclinal buckling, which is based on recent advances in our geological understanding of the Mongolian orocline in the Central Asian Orogenic Belt. The model simulates an amplification of a preexisting, gently arcuated accretionary system. In the analogue models, we established three neighboring subdomains that represent contrasting lithospheric domains separated by a steep “subduction” interface. The shortening of the model is parallel to the pseudo-linear interface between these subdomains and produces two different deformation patterns in the amplified interlimb areas of the orocline. We used analogue modelling techniques to investigate both the flow of a ductile lithosphere and the brittle deformation of an upper crust that is associated with the development of an orocline. The modelling results show that: (1) two types of regional-scale folds develop both of which are upright. The largest are steeply plunging oroclines and the smaller have sub-horizontal axes; (2) the overall deformation distribution displays significant thickening/thinning and exhumation of the lower ductile layers with variations of upward transfer of crustal material; (3) the hinge regions of the two adjacent oroclines studied consist of different materials, one contains oceanic and the other felsic lower crustal rocks. It is found that the geometry of the lithosphere beneath the two regions is very different; (4) the geometry of the orocline is non-cylindrical and exhibits an increasing plunge of fold axes when traced from the upper-crust to deeper lithospheric layers. The resulting model is used to geophysically constrain deep crustal structures and surface deformation patterns in Mongolia.

Coupled age and trace element mapping in monazite grains using the Laser-Ablation Split-Stream ICP-MS technique was carried out for a sequence of samples from (U)HP crustal section in the Eger Crystalline Complex interpreted as a Variscan continental subduction wedge. The sampled section comprises banded orthogneiss with monazite protolith ages of 486 ± 3[10] Ma. The orthogneiss was heterogeneously transformed to migmatite, HP felsic granofels and U(HP) fine-grained Ky-Kfs granulite during coupled deformation and percolation of silicate melts in a subduction channel. Age of the (U)HP metamorphism and related melt percolation in the granulite is dated by Th-rich and HREE-poor monazite cores at 353.6 ± 2.7[8] Ma, that are in equilibrium with HREE-rich garnets. Th-rich cores of monazites in the migmatite, granofels and granulite that lack the (U)HP signatures show younger ages of 347.8 ± 3.4[8] Ma, 345.8 ± 2.4[8] Ma and 339.3 ± 1.3[7] Ma, respectively. The monazite rims with lower Th and higher HREE contents than the monazite cores are likely a result of (re)crystallization in equilibrium with melt that partly dissolved the HREE-rich prograde garnet during decompression. This (re)crystallization is constrained by ages of 344.2 ± 1.6[8] Ma in (U)HP granulites and 338–340 ± [8] Ma in migmatites, granofelses and granulites without the (U)HP signature. The groups of ages from 353 Ma to 338 Ma, are significantly different when only their in-run uncertainties (7–10 Ma) are compared and indicate a 15 ± 3 Ma period of monazite (re)crystallization from burial/peak to exhumation.

The classical concept of collisional orogens suggests that mountain belts form as a crustal wedge between the downgoing and overriding plates. However, this orogenic style is not compatible with the presence of (ultra-)high pressure crustal and mantle rocks far from the plate interface in the Bohemian Massif of Central Europe. Here we use a comparison between geological observations and thermo-mechanical numerical models to explain their formation. We suggest that continental crust was first deeply subducted, then flowed laterally underneath the lithosphere and eventually rose in the form of large partially molten trans-lithospheric diapirs. We further show that trans-lithospheric diapirism produces a specific rock association of (ultra-)high pressure crustal and mantle rocks and ultra-potassic magmas that alternates with the less metamorphosed rocks of the upper plate. Similar rock associations have been described in other convergent zones, both modern and ancient. We speculate that trans-lithospheric diapirism could be a common process., Communications Earth & Environment, 2 (1), ISSN:2662-4435

The Barrovian inverted metamorphism of the Svratka dome developed within two nappes derived from the Brunia continent that was thrust beneath the Moldanubian orogenic root. The metamorphism increases from biotite–chlorite zone in the basement to very closely spaced staurolite, kyanite and sillimanite zones at the top of the nappe pile. The sequence of mineral growth, chemical zoning of garnet, and pseudosection modelling indicate prograde paths from 4.5 kbar/510 °C to 5.5 kbar/540 °C in the garnet zone, from 6 kbar/530 °C to 7 kbar/600 °C in the staurolite zone, and from 3.5 kbar/510 °C to 8.5 kbar/650 °C in the kyanite zone. The age of monazite inclusions in garnet and staurolite is interpreted to reflect prograde metamorphism at 338 ± 7 Ma and 336 ± 7 Ma, respectively. An older matrix monazite crystal is interpreted as dating prograde crystallization at 345 ± 7 Ma, whereas a younger monazite group records recrystallization at/or down to 334 ± 7 Ma. While these petrological and geochronological data are consistent with data from an inverted metamorphic sequence of the southern Thaya dome, the spacing and distribution of metamorphic zones, nappe thicknesses, and late structures are different in the two domes. An antiformal stack of imbricated basement sheets and the extreme attenuation of metamorphic isograds at the top of the nappe pile in the Svratka dome are explained by a relatively cold overthrusting Moldanubian domain, formed mainly of middle orogenic crust. The homogeneous thickening of the hinterland-dipping basement duplexes and the regular spacing of metamorphic isograds in the Thaya dome are explained by a hot overriding Moldanubian domain, which in this region has a high proportion of exhumed lower orogenic crust and formed a hot mid-crustal channel.

The high-pressure Heilongjiang Complex records a long and complicated evolution from the late Paleozoic to Mesozoic in NE China, which is still subject to controversy. In this paper we present a new detailed study including structural, geochemical and geochronological analyses (Ar–Ar dating on metamorphic minerals, U-Pb SIMS dating on zircons). Two main periods of oceanward accretionary growth are identified between 305 Ma and 195 Ma and around 112–101 Ma. These periods of accretionary growth were associated with the emplacement of voluminous magmatism, slab roll-back with continental back-arc extension, and growth of a forearc accretionary prism. Between these periods of accretionary growth, a major phase of regional subduction erosion took place between 195 Ma and 142 Ma at the eastern edge of the Songliao Block. The main effects of this subduction erosion phase were that regional Jurassic-early Cretaceous extrusion overlapped with Mesozoic accretion and erosion in NE China, which disturbed the initial orogenic architecture. With a review of relevant zircon ages of this accretionary orogen, we unravel the complicated geodynamic history of accretion, subduction erosion and tectonic extrusion that shaped NE China from the late Paleozoic to Mesozoic.

This study uses gravimetric data integrated with recent seismic data published on south Madagascar to investigate geometry of crust–mantle interface. The regional tectonic framework of Madagascar is characterised by anastomosing network of up to 15-km-wide, 600-km-long and north-oriented high-strain zones, which originated during Neoproterozoic convergence. The studied Bouguer anomalies obtained from the International Gravimetric Bureau were high-pass filtered to emphasise short-wavelength gravimetric variations (shorter than 200 km). The Pan-African high-strain zones coincide with the positive gravimetric anomalies suggesting a link with deep seated high-density material. Considering the present-day thickness of the crust (35 km) and its seismic velocity record, the gravimetric anomalies can be visualised as narrow vertical tabular bodies located at the base of the Moho. Modelling further confirmed that such narrow vertical bodies could be stable over geologic time scale since these structures are relatively small (10 to 30 km wide). The vertical tabular bodies possibly reflect material transfer such as vertical motion of sub-crustal weak and possibly partially molten mantle along vertical deformation zones. It is proposed that these structures were initiated by folding of weak mantle–crust interface characterised by low-viscosity contrast between weak mantle and stronger granulitized lower crust during bulk pure shear-dominated horizontal shortening. It is proposed that the cuspate-lobate “mullion-type” geometry mimics rheological inversions of mafic and felsic rocks and shape of folds of variable scale observed in southern Madagascar. The formation of such mega-mullion structures is possibly an expression of “creme brulee” rheological model, where the deformation of the lithosphere is governed by stronger granulitic lower crust and weaker partially molten and/or hydrated mantle.

High-pressure granitic orthogneiss of the south-eastern Orlica-Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P-T path deduced from the thermodynamic modelling indicates decompression from ~15−16 kbar and ~650-740 ºC to ~6 kbar and ~640 ºC. Melt was already present at the P-T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6-8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse.

The Kutna 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.

International audience

A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust.

International audience

International audience

International audience; The timing and mechanism of formation of the Tibet Plateau remain elusive, and even the present-day structure of the Tibetan lithosphere is hardly resolved, due to conflicting interpretations of the geophysical data. We show here that significant advances in our understanding of this orogeny could be achieved through a better assessment of the composition and rheological properties of the deepest parts of the Tibetan lithosphere, leading in particular to a reinterpretation of the global tomographic cross sections. We report mantle phlogopite xenocrysts and carbonate-bearing ultramafic cumulates preserved in Eocene potassic rocks from the Eastern Qiangtang terrane, which provide evidence that the lithospheric mantle in Central Tibet was enriched in H 2 O and CO 2 prior to the India-Asia collision. Rheological calculations and numerical modeling suggest that (1) such metasomatized mantle would have been significantly weaker than a normal mantle but buoyant enough to prevent its sinking into the deep mantle; (2) the slow seismic anomalies beneath Central Tibet may image a weakened lithosphere of normal thickness rather than a lithosphere thinned and heated by the convective removal of its lower part; and (3) melting of such soft and fusible metasomatized mantle would have been possible during intracontinental subduction, supporting a subduction origin for the studied Eocene potassic magmatism. These results demonstrate that the inheritance a soft and buoyant precollisional Tibetan lithosphere may have conditioned the growth and the present-day structure of the Tibet Plateau. Plain Language Summary The Tibetan Plateau is the largest relief on the Earth's surface, but also one of the least understood geodynamic phenomena, due to the lack of constrain on its growth evolution and mechanism. Here we have discovered, enclosed in Eocene (35-Ma-old) lavas from Central Tibet, hydrous and carbonated minerals derived from the underlying upper mantle, which reveal an unsuspected abundance of water and CO 2 in this upper mantle. We show that, consequently, the upper mantle beneath Central Tibet has been anomalously weak since before the collision of India with Asia. It was previously thought to have disappeared, foundered in a process called "delamination," but we demonstrate here that this was an artifact: Most of the tibetan mantle is still there, but it is very weak and therefore does not appear on the seismic images. This implies that models expecting the Tibet Plateau to be a result of a delam-ination process are probably wrong; instead, we propose a model in which the weak upper mantle beneath our study area was easily indented by the strong adjacent continental block; this model manages to explain both Eocene magmatism and uplift in our study area.

A clear insight of the Indo-Burma amalgamation is required for an understanding of the complex processes of the Indo-Asian collision. Where and when the northeastern part of the Indian subcontinent amalgamated with the Burma plate has not yet been well defined. In order to better understand the tectonic affiliations and evolution of the Indo-Burma Ranges (IBR), we report new detrital zircon U-Pb and Hf isotope data of nine sandstone/siltstone samples (late Miocene–Pliocene) from the Chittagong-Tripura Fold Belt, Bangladesh, which is in the Neogene belt of the IBR. Our results show that the zircon ages of all nine samples have three peaks at

New geochemical and geochronological data are used to characterize the geodynamic setting of metasediments, felsic orthogneisses, and eclogite and amphibolite lenses forming the Beishan complex, NW China, at the southern part of the Central Asian Orogenic Belt. The metasediments correspond compositionally to immature greywackes receiving detritus from a heterogeneous source involving a magmatic arc and a Precambrian continental crust. Metagranitoids, represented by felsic orthogneisses, show both composition of greywacke-derived granitic melt with incompatible trace element patterns similar to the host metasediments. The eclogite lenses are characterized by high Nb contents (5.34–27.3 ppm), high (Nb/La)N (>1), and low Zr/Nb ratios (

This paper examines the rheological behaviour of power law viscous materials arising from mixtures of deformation mechanisms and/or different mineral assemblages. The mixing relation is based on classical thermodynamics mixing relations where the enthalpy of the mixture is the sum of the molar volume proportions of the individual molar enthalpies; this is equivalent to the 1887 Arrhenius mixing relation for Newtonian viscosities. We compare this mixing relation to viscosities obtained from finite element simulations of deforming mixed mineral phases and from experimental results. The mixing relation helps understanding of apparent anomalies in experimental results for calcite and quartz. This then enables the construction of flow laws and of deformation mechanism maps for mixtures of deformation mechanisms and of mineral assemblages over a range of grain size distributions. The rheological implications for changes in the topology (such as anisotropy) of mineral arrangements, for mixtures of grain sizes, for the interpretation of experimental results and for geodynamic modelling are discussed.

Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical and zircon U–Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mts. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U–Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian–Ordovician and culminating at ca. 500 Ma. We propose that the Ordovician succession forms an ‘Altai sedimentary wedge’, the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (early Cambrian) to the development of the ‘Ikh-Mongol Magmatic Arc System’ and the giant ‘Altai sedimentary wedge’ above a north-dipping subduction zone (Late Cambrian–Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro–Ordovician geodynamic evolution of the Mongolian collage system.

A polyphase tectono-metamorphic evolution of the Mongolian tract of the Central Asian Orogenic Belt (CAOB) is inferred from a structural, petrological and geochronological study in the Hovd Zone at the NE periphery of the Altai Domain. Four tectonic events are recorded here. A relict sub-vertical fabric (S1) is associated with the growth of garnet cores and staurolite, typical of MP–MT Barrovian-type metamorphism (M1). This fabric was isoclinally folded and overprinted by sub-horizontal foliation (S2) associated with growth of garnet rims, sillimanite and cordierite, typical of LP–HT metamorphism (M2). The P–T path inferred from petrology and thermodynamic modelling shows that the peak conditions reached 590–610 °C and c. 5.8 kbar (M1) and were followed by decompression to 4–5 kbar associated with heating to 630–690 °C (M2). Subsequently, the studied region was strongly reworked by NW–SE trending upright folds (F3) locally associated with development of muscovite- and chlorite-bearing cleavage (S3). The region is weakly affected by post-metamorphic D4 overprint. Monazite in-situ electron-microprobe dating was complemented by LA–ICP-MS U–Pb dating, revealing three distinct texturally defined age groups at c. 455, 435 and 385 Ma. The ages at c. 455 Ma are related with monazite included in garnet cores, the age group at c. 435 Ma with monazite in garnet rims and in S2 foliation, while the ages of c. 385 Ma come mostly from monazite in chlorite–muscovite-bearing bands. Based on the monazite textural position and REE patterns, these ages are correlated with the tectono-metamorphic events D1–D3, reflecting short-lived Late Ordovician shortening, prolonged Middle Silurian–Early Devonian extension and Late Devonian–Early Carboniferous compression, respectively. Alternation of shortening and extension periods is interpreted as being caused by tectonic switching, i.e. repeated alternation of advancing and retreating subduction regimes. The latest Permian event resulted from the collision of the Mongolian orogenic collage with the Tarim and North China cratons in the south and caused rotation of previous fabrics to the current configuration.

International audience; Magma transfer between lower and upper continental crust is a fundamental process linking the dominantly mafic composition of the lower crust with the more felsic composition of the upper crust. We explore the mechanisms of mafic magma ascent and emplacement in the middle crust by studying a mid-crustal gabbroic to dioritic magmatic system (Sondalo complex, Eastern Central Alps, N Italy). We characterize the structure and Anisotropy of Magnetic Susceptibility (AMS) fabric of concentric gabbroic to dioritic intrusions. The significance of AMS fabrics is discussed using Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Crystallographic Preferred Orientation (CPO) data acquired on different test sites. The magmatic and magnetic fabrics of the pluton were acquired during its emplacement in the Permian and were not subsequently tilted: the fabrics are essentially vertical, indicating vertical magma transfer through the crust with a two-phase intrusion history. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggest that the first magma ascent phase occurred along pathways subparallel to the vertical fabric of the host metasedimentary rocks. (2) The second magma ascent phase was controlled by a change in the rheology of the host-rock and the mafic magma. Heat dissipation to the contact aureole induced partial melting, thereby lowering the mechanical strength of the host-rocks, whereas the viscosity of the mafic magma increased due to cooling and associated fractional crystallization. This caused an en-masse rise of the pluton resulting in the formation of a structural aureole, i.e. a vertical foliation in the contact aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase accounts for the P−T evolution recorded by metasedimentary rocks in the contact aureole of the pluton.

International audience; The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.

Grapevines are highly sensitive to environmental conditions, with variability in weather and climate (particularly temperature) having a significant influence on wine quality, quantity and style. Improved knowledge of spatial and temporal variations in climate and their impact on grapevine response allows better decision-making to help maintain a sustainable wine industry in the context of medium to long term climate change. This paper describes recent research into the application of mesoscale weather and climate models that aims to improve our understanding of climate variability at high spatial (1 km and less) and temporal (hourly) resolution within vineyard regions of varying terrain complexity. The Weather Research and Forecasting (WRF) model has been used to simulate the weather and climate in the complex terrain of the Marlborough region of New Zealand. The performance of the WRF model in reproducing the temperature variability across vineyard regions is assessed through comparison with automatic weather stations. Coupling the atmospheric model with bioclimatic indices and phenological models (e.g. Huglin, cool nights, Grapevine Flowering Véraison model) also provides useful insights into grapevine response to spatial variability of climate during the growing season, as well as assessment of spatial variability in the optimal climate conditions for specific grape varieties.

Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide.

1 Czech Geological Survey, Klarov 3, 118 21 Prague 1, Czech Republic; karel.schulmann@geology.cz 2 Ecole et Observatoire des Sciences de la Terre, Universite de Strasbourg, UMR 7516 of CNRS, 1 rue Blessig, 67084 Strasbourg Cedex, France 3 Mongolian University of Science and Technology, Baga Toiruu 34, Ulaanbaatar, Mongolia 4 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China * Corresponding author

Hf values of +7.4 to +11.5. The latter suggest an exclusively juvenile, depleted mantle-derived magma source. According to calculated P–T conditions, we propose three main stages in the Sagsai Pluton evolution. The gabbro started to grow at >1000 °C and 0.8–0.9 GPa. The ensuing magma mixing with crustal melts and final crystallization of gabbro occurred at similar P–T conditions (740–830 °C and 0.5–0.6 GPa) as partial melting of migmatite xenoliths in the Sagsai Pluton. The P–T conditions of the Sagsai Pluton final emplacement calculated from quartz syenite (c. 600 to 670 °C and 0.4–0.5 GPa) are consistent with the mineral assemblage in the contact aureole (cordierite–biotite and biotite–chlorite schists). The magmatic evolution of Sagsai Pluton confirms a model of deep lithosphere thermal anomaly associated with heterogeneous melting of crust in a post-subduction extensional regime, perhaps shortly after the slab break-off.

Highly deformed banded phengite-biotite metagranite from the Snieznik dome in the Bohemian Massif has been modified locally to have stromatic, schlieren or nebulitic textures typical of migmatites. This occurred mostly along subvertical deformation zones at eclogite-facies conditions, at a scale of several centimetres to several metres, mostly parallel to the foliation. The transition from banded to migmatite types of orthogneiss is marked by an increase in the amount of phases interstitial along grain boundaries in the dynamically recrystallized monomineralic feldspar and quartz aggregates, and by increasing consumption of recrystallized K-feldspar grains by fine-grained plagioclase and quartz, as well as myrmekite (intergrowth of Pl􏰄Qz). The new minerals are in textural equilibrium with phengite. The myrmekite, quartz and feldspars can be coarse-grained (grain size 􏰅0􏰀5cm). These features are considered to be the result of grain-scale melt infiltration that caused dissolution-reprecipitation along grain boundaries in the presence of phengite. The infiltration was pervasive at the grain scale, but localized at hand-specimen to outcrop scales. All the rock types have the same mineral assemblage of Grt􏰄+Ph+􏰄Bt+􏰄Ttn+􏰄Kfs+􏰄Pl+􏰄Qz+Rt+Ilm; they have similar garnet, phengite and biotite compositions, and based on mineral equilibria modelling we infer equilibration at a pressure of 1􏰀5-1􏰀7GPa and a temperature of 690-740􏰂C. Because the rocks are inferred to be H2O-undersaturated and above the temperature conditions of the wet solidus, infiltration must have involved a hydrous melt, as opposed to an H2O fluid. Stability of melt-bearing mineral assemblages and mineral compositions are almost independent of the melt proportion in the system, thus explaining the identical assemblage and mineral compositions observed in all the migmatite types. This precludes the estimation of the amount of melt infiltrated. Migmatite textures, however, suggest that variable degrees of melt-rock interaction occurred, being low in the banded migmatite types and higher in the nebulitic and schlieren types. Retrograde equilibration was largely restricted to retrograde zoning in phengite, garnet and plagioclase, and crystallization of biotite around phengite and garnet, presumably in a continuous reaction consuming melt. This may have occurred down to 􏰁0􏰀7-1􏰀0 GPa. According to Sr-Nd isotope data, the infiltrating melt is probably derived from similar rocks, structurally beneath the observed ones. The infiltration may have facilitated exhumation of a 2 km wide structural domain from 􏰁1􏰀7 to 􏰁0􏰀7 GPa, within which are the subvertical deformation zones along which the infiltration occurred.

The general structure of the Chinese Altai has been traditionally regarded as being formed by five tectono-stratigraphic ‘terranes’ bounded by large-scale faults. However, numerous detrital zircon studies of the Paleozoic volcano-sedimentary sequences shown that the variably metamorphosed Cambro-Ordovician sequence, known as the Habahe Group, is present at least in four ‘terranes’. It structurally represents deepest rocks unconformably covered by Devonian and Carboniferous sedimentary and volcanic rocks. Calc-alkaline, mostly Devonian, granitoids that intruded all the terranes revealed their syn-subduction related setting. Geochemistry and isotope features of the syn-subduction granitoids have shown that they originated mainly from the melting of youthful sediments derived from an eroded Ordovician arc further north. In contrast, Permian alkaline granitoids, mostly located in the southern part of the Chinese Altai, reflect a post-subduction intraplate setting. The metamorphic evolution of the metasedimentary sequences shows an early MP-MT Barrovian event, followed by two Buchan events: LP-HT mid-Devonian (ca. 400–380 Ma) and UHT-HT Permian (ca. 300–270 Ma) cycles. The Barrovian metamorphism is linked to the formation of a regional sub-horizontal possibly Early Devonian fabric and the burial of the Cambro-Ordovician sequence. The Middle Devonian Buchan type event is related to intrusions of the syn-subduction granitoids during an extensional setting and followed by Late Devonian-Early Carboniferous NE-SW trending upright folding and crustal scale doming during a general NW-SE shortening, responsible for the exhumation of the hot lower crust. The last Permian deformation formed NW-SE trending upright folds and vertical zones of deformation related to the extrusion of migmatites, anatectic granitoids and granulite rocks, and to the intrusions of gabbros and granites along the southern border of the Chinese Altai. Finally, the Permo-Triassic cooling and thrust systems affected the whole mountain range from ca. 265 to 230 Ma. In conclusion, the Chinese Altai represents different crustal levels of the lower, middle and upper orogenic crust of a single Cambro-Ordovician accretionary wedge, heterogeneously affected by the Devonian polyphase metamorphism and deformation followed by the Permian tectono-thermal reworking event related to the collision with the Junggar arc. It is the interference of Devonian and Permian upright folding events that formed vertical boundaries surrounding the variously exhumed and eroded crustal segments. Consequently, these crustal segments should not be regarded as individual suspect terranes.

International audience

International audience

1 Czech Geological Survey, Klarov 3, 118 21 Prague 1, Czech Republic; pavel.hanzl@geology.cz 2 Ecole et Observatoire des Sciences de la Terre, Universite de Strasbourg, UMR 7516 of CNRS, 1 rue Blessig, 67084 Strasbourg Cedex, France 3 Institute of Petrology and Structural Geology, Charles University in Prague, Faculty of Science, Albertov 6, 128 43 Prague 2, Czech Republic 4 Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar 15160, Mongolia 5 School of Geology and Mining Engineering, MUST, P.O. Box 654, Ulaanbaatar, Mongolia 6 State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China * Corresponding author

Sedimentological and detrital zircon provenance study of the Lower Palaeozoic Zuunnuruu, Tsetseg and Sagsai sedimentary formations was carried out in the eastern part of the Hovd Zone in western Mongolia. Sedimentological analysis has revealed two distinct and consecutive types of sedimentary environments. The Lower Ordovician–earliest Silurian sediments had dominantly volcano-sedimentary character, interpreted as reflecting deposition in a proximal part of a Pacific-type accretionary wedge. The upper, mainly Devonian part of the profile has generally siliciclastic flysch-like nature and indicates platform-type depositional setting related to the syn-extensional thinning of the accretionary wedge-system. Detrital zircon age populations of all the three studied formations uniformly show a dominant Neoproterozoic–Ordovician age group at ca. 560–460 Ma, a broad Neo- to Mesoproterozoic peak at ca. 1050–720 Ma, several minor Meso- to Palaeoproterozoic age clusters at ca. 1.4, 1.9 and 2.4 Ga and ca. 400–360 Ma peaks in the youngest Devonian formation. The early Palaeozoic part of the age spectra is interpreted as detritus mainly derived from the magmatic rocks of the Cambrian–Ordovician Ikh-Mongol Arc System within the nearby Lake Zone and the youngest ages from the neighbouring Devonian granites. The other sources were identified as more distal Tonian magmatic-arc complexes, Rodinia break-up-related volcanic rocks and basement of the Precambrian Zavkhan and Baidrag continental blocks even further east. The maximum sedimentary ages, determined by the youngest detrital zircons, shift the end of deposition of the Sagsai Formation at least to the latest Devonian. Nearly identical detrital zircon age spectra from Lower Palaeozoic sequences of the Hovd Zone and other parts of the Altai belt support an existence of a single giant accretionary complex developed along the entire outer margin of the Ikh-Mongol Arc System. The change in the sedimentary style suggests the latest Ordovician–earliest Silurian termination of magmatic-arc activity in the western Lake Zone, marking the onset of late Silurian–Devonian crustal extensional period in the Altai accretionary system.

New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate relationship between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome-like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5−4 kbar and 530−570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N-S compression. The re-heating of the massif follows late massive E-W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)-Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma-rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe. This article is protected by copyright. All rights reserved.

The European Variscan orogeny can be compared to the Tibetan-Himalayan system for three main reasons: (1) The Variscan belt originated through progressive amalgamation of Gondwanan blocks that were subsequently squeezed between the Laurussia and Gondwana continents. Similarly, the Tibetan-Himalayan orogen results from amalgamated Gondwanan blocks squeezed between Asia and India. (2) The duration of the collisional period and the scale of the two orogens are comparable. (3) In both cases the collisional process resulted in formation of a thick crustal root and long lasting high-pressure granulite facies metamorphism. Recent petrological data allow a more detailed comparison pointing to similarities also in the midcrustal re-equilibration of the granulites and their association with specific (ultra)potassic magmatic rocks. In both orogens, the origin of the granulites was attributed to relamination and thermal maturation of lower crustal allochthon below upper plate crust. Subsequent evolution was explained by midcrustal flow eventually leading to extrusion of the high-grade rocks. We propose that the lower and middle crustal processes in hot orogens are connected by gravity overturns. Such laterally forced gravity-driven exchanges of material in the orogenic root were already documented in the Variscides, but the recent data from Tibet and Himalaya show that this process may have occurred also elsewhere. Using numerical models, we demonstrate that the exchange of the lower and middle crust can be efficient even for a minor density inversion and various characteristics of the crustal layers. The modeled pressure-temperature paths are compatible with two-stage metamorphism documented in Tibet and Himalaya.

The relative strengths of constituent minerals in a deforming poly-mineralic rock depend on the grain size distributions of the various phases, the operating deformation mechanisms in those phases, and the topology of the microstructure. It is observed that more than one deformation mechanism operates, and the resulting constitutive relations (especially for calcite) are extremely variable. To explore such relations, deformation mechanism maps for each mineral are required together with mixing relations for multiple deformation mechanisms. We construct reference deformation mechanism maps for calcite and alkali-feldspars that are based on theoretical expressions for constitutive relations, together with experimental or molecular dynamic estimates of constitutive parameters. A thermodynamically based mixing relation is employed that enables bulk constitutive relations to be established for mixtures of mineral phases and deformation mechanisms with a range of grain sizes. This provides a basis for comparison and understanding of experimental results. We map out fields on deformation mechanism maps where switches in the relative strengths of calcite and quartz and of feldspar and quartz occur. We introduce the concept of geometrically necessary deformation mechanisms that minimise the energy of the system.

Baaran and Baytag “terranes” form important part of the Trans-Altai Zone of the Central Asian Orogenic Belt in the SW Mongolia. The Trans-Altai Zone represents an oceanic domain built by Ordovician to Devonian ophiolites covered by Devonian–Carboniferous oceanic sediments. This oceanic assemblage was intruded by a spectrum of Carboniferous volcanic rocks ranging from basalts to rhyolites (325.6 ± 1.4 Ma to 351.0 ± 5.9 Ma; LA-ICP-MS U–Pb dating on zircon) that display similar whole-rock trace-element and Sr–Nd isotopic signatures regardless their silica contents and the “terrane” they belong to. They are normal-K calc-alkaline rocks whose NMORB-normalized multielement patterns show an arc-like enrichment in LILE (Large Ion Lithophile Elements) and deep negative TNT (Ta, Nb, Ti) anomalies. Based on the relatively high Nb/Yb and Th/Yb ratios, the arc was probably formed on immature continental rather than typical oceanic crust. Low (87Sr/86Sr)i (0.7043–0.7047), high e Nd 350 (+5.2 to = +5.8) and the observed in-situ Hf isotopic variation in dated zircons ( e Hf t = +7.4 to +13) are all attributed to a partial melting of primitive and youthful (Neoproterozoic–Ordovician) arc-type lower continental crust (metamorphosed intermediate–acid igneous rocks and/or arc-derived greywackes), augmented with, and most likely facilitated by, contemporaneous arc-related basaltic intrusions. A distinct possibility remains the previously formulated model assuming that the Carboniferous magmatism in Gobi-Altai and Trans-Altai zones sampled the juvenile Neoproterozoic to Cambrian arc crust relaminated under the Ordovician–Devonian oceanic crust. In any case, the studied arc association, together with lithological sequence of ophiolites, can be closely correlated with the Dulate arc in eastern Junggar. It is proposed that the two arc sequences form a single belt, c. 500 km long, that represents one of the main known arc structures in the Central Asian Orogenic Belt.

Structural analysis and U–Pb geochronological study on zircons from the southern Chinese Altai (the Kalasu area, SE of the Altai city) show that the Cambro-Ordovician accretionary wedge (ca. 520–492 Ma) underwent four major geological events: 1) emplacement of Early Devonian magmas (ca. 410–400) associated with formation of a volcano-sedimentary cover, 2) major Middle Devonian (ca. 390–374 Ma) tectono-metamorphic event, 3) Late Devonian-Early Carboniferous folding without apparent metamorphism, and 4) a regional folding with localized Early Permian high- to ultrahigh-temperature reworking (ca. 300–280 Ma). The Early Devonian magmatism is characterized by emplacement of mafic rocks and granitoids in the centre of the NE-SW profile, coevally with granitoid magmatism and rhyolite volcanism in the southwest and northeast, respectively. The whole volcano-sedimentary and magmatic edifice was transposed by sub-horizontal metamorphic fabric associated with variable metamorphic degrees in different areas ranging from greenschist facies in the northeast (mu + bi±g) to amphibolite facies in the southwest (st + g ± sill) and granulite facies in the centre (g + sill+kfs). This metamorphic architecture, distribution of magmatism and character of metamorphic zircon populations allow to correlate these areas with upper, middle and lower orogenic crust that developed during important vertical shortening and horizontal flow in Middle Devonian. Subsequently, the whole edifice was affected by regional NE-SW trending upright (possibly Late Devonian-Early Carboniferous) folding. Finally, Early Permian shortening produced NW-SE trending regional upright folds in the southwest and northeast and a crustal-scale vertical, tabular deformation zone in the centre. The Permian deformation is accompanied by granulite facies (kfs + cd + sill+g) metamorphism and anatexis reworking the Devonian lower orogenic crust, with extensive resetting and growth of new zircons and with intrusions of Permian granites and gabbros. This study suggests that the Early Permian event was related to massive perturbation of thermal structure of the mantle lithosphere due to the collision of the Junggar arc with the Chinese Altai terrane.

Lu-Hf and Sm-Nd garnet-whole rock geochronology combined with petrographic observations, minero-chemical variations, thermodynamic modelling and structural data was used to constrain the P–T–t–d evolution of eclogites from the Marianske Lazně Complex (Bohemian Massif). Boudins of mostly isotropic eclogite with relict steep eclogite-facies fabric are affected by steep migmatitic foliation, which is followed on a regional scale by the development of almost pervasive, predominantly SE-dipping, extensional foliation. The structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite. A least retrogressed sample of eclogite shows clusters of fine-grained inclusion-poor garnet, omphacite relicts surrounded by a fine-grained clinopyroxene-plagioclase symplectite with minor amphibole, biotite-plagioclase intergrowths after white mica, kyanite with plagioclase-spinel coronas and accessory rutile. Rare potassic white mica occurs as inclusions in omphacite. A more retrogressed eclogite, with no omphacite or kyanite relicts, contains inclusion-poor garnet surrounded by amphibole-plagioclase corona in a matrix dominated by plagioclase-amphibole symplectite with minor clinopyroxene. In places, the symplectite is overgrown by coarse-grained amphibole. Peak P–T conditions, inferred from combined conventional thermobarometry and phase-equilibria modelling and based on inclusions of white mica (up to 3.33 Si p.f.u.), matrix omphacite (Jd33–36) and garnet core (Alm33–38Prp38–42Grs22–25Sps1) compositions are ~25 kbar at 650–750 °C. A HT overprint occurred at ~14–18 kbar and >800 °C based on coexisting clinopyroxene (Jd18–24), plagioclase (An18–35), and amphibole (Na(B) Lu-Hf and Sm-Nd garnet geochronology has been applied to both samples, an older age (c. 390 Ma) obtained by the Lu-Hf method is interpreted as the timing of HP metamorphism, while c. 15 Ma younger ages were obtained by the Sm-Nd method. As temperatures for the HT overprint exceed the empirically and experimentally determined closure temperature of the Sm-Nd system the Sm-Nd ages are interpreted to date cooling following the HT overprint. Combined together, contrasting eclogite and amphibolite-facies migmatite fabrics, the mineral textures, calculated P–T conditions, and distinct Lu-Hf and Sm-Nd ages, provide a complete P–T–t–D path characterised by rapid (~15 Ma) transition from HP subduction, crustal thickening to extensional HT shearing. This unconventional exhumation path does not fit to models of monocyclic exhumation in a subduction channel proposed for the Munchberg and ZEV Devonian HP units to the west. In contrast, the post-peak extensional low angle shearing shortly after subduction and collision resembles more the geodynamic model typical for the Iberian subduction system both in time scales and sequence of tectonic events.

The Khantaishir Magmatic Complex (KMC) (south–central Mongolia) exposes a section of a magmatic system consisting of deep crustal, ultramafic cumulates (coarse-grained Amp gabbros and hornblendites; c. 0.35–0.5 GPa) to shallower crustal levels dominated by Amp–Bt tonalites ( c. 0.1–0.2 GPa). The magmatic rocks were emplaced during most of the Cambrian ( c. 538–495 Ma) and are mostly geochemically primitive (Mg# ~ 50), Na-rich and metaluminous. The (normal-) calc-alkaline signature and characteristic trace-element enrichment in hydrous-fluid mobile large-ion lithophile elements (LILE) relative to high-field strength elements (HFSE) suggest an origin within a magmatic arc. Multiple intrusions of basic magma derived from a subduction-modified depleted mantle developed by fractional crystallization and/or accumulation of (Ol, Cpx) Amp + Bt, later joined by Pl. Magma mixing with, or without, exchange of xenocrysts between compositionally dissimilar melt batches was also important. Over time, partial melting of older, lower crustal metabasic rocks became increasingly significant, again with a strong subduction signature. The lack of zircon inheritance in the magmatic products and rather high zircon e Hf t values (all >+3, but for most samples > +8) as well as whole-rock Sr–Nd isotopic compositions imply that the arc was not founded on mature continental crust. It was probably located at the margin of the Baydrag microcontinent, dominated by accreted metabasic rocks of an older (early Tonian?) island arc covered by a thin layer of subordinate metasediments containing detrital zircons with Tonian and ill-defined Palaeoproterozoic ages. The KMC represents a small vestige of an extensive Cambrian–Ordovician subduction system (termed here the Ikh-Mongol Arc System), bordering the western margin of a chain of Precambrian microcontinents (Tuva-Mongolia, Zabkhan and Baydrag) that, together with accreted Neoproterozoic marginal basins (the Lake Zone), defines the external part of the Mongolian orocline.

International audience

International audience; Assessment of sediment redistribution by end-Ordovician ice sheets is crucial for the reconstruction of Lower Paleozoic source-to-sink patterns. Focusing on the ice-distal, deepwater Tazekka depocenter (Moroccan Meseta), we performed a provenance study that combined whole-rock geochemistry, petrography and insights from high-resolution detrital zircon ages. The results show that the glacigenic sediments are compositionally — mineralogically and geochemically — more mature than preglacial strata. This observation points to a preferential cannibalization of the “great Lower Paleozoic quartz-rich sandstone sheet”, with a limited input of first-cycle, far-travelled clastic sediments. Differentiation of glacial units is not straightforward, yet the glaciation acme is typified by a highly mature sedimentary source and an age spectrum lacking Mesoproterozoic zircon grains, both features strongly indicating derivation from the Cambrian–Lower Ordovician cover of the Tuareg Shield. More regional sources are expressed during the earlier glaciation stages, during which lowstand remobilizations unrelated to subglacial erosion are also suspected. Subordinate but notable late Tonian (∼0.8 Ga) and latest Stenian to early Tonian (∼1 Ga) zircon populations are also evidenced in Morocco, which may have implications for future paleogeographic reconstructions.

International audience

Paleomagnetic investigations of Early Permian and Triassic magmatic rocks from Catalan Coastal Ranges provide three magnetic directions which are interpreted as primary magnetizations and partly as overprints. The succession of magnetic directions is interpreted in terms of a succession of large-scale clockwise rotations, which bring the whole assembly of Pyrenees, Catalonia, Corsica-Sardinia and Maures-Esterel massifs from the southeast of the Massif Central to the west during Permian. This major movement is associated with clockwise rotation of northern limb of the Iberian orocline during Latest Carboniferous and Early Permian (~ 305–290 Ma). Subsequently, Permian large-scale dextral transtensional–extensional shearing operated along Aquitanian Shear Belt between the Massif Central in the northeast and the Iberian Variscan massifs in the southwest. The latest phase of the rotation in the Late Permian-Early Triassic is associated with alkaline magmatism probably linked to Neo-Tethys opening activity in Western Europe.

The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the “Khantaishir” arc (520–495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

Potassium–argon method on muscovite and biotite and chemical U–Th–Pb method on monazite have been used to date various tectonic and thermal processes affecting the Silesian orogenic wedge at the eastern Variscan front (NE Bohemian Massif). This wedge is composed of three structural units showing an increasing Barrovian metamorphic gradient from the east to the west and LP–HT reworking related to voluminous granite intrusion of the Žulova Pluton in the central part. Three groups of ages are reported: 1) Mississippian (340–320 Ma) K–Ar muscovite ages from the western kyanite zone and easternmost biotite–chlorite zone, ~320 Ma U–Th–Pb ages of monazite inclusions in syn-burial S1fabric preserved in garnets of the kyanite zone, 2) Pennsylvanian–Early Permian (~310–290 Ma) K–Ar ages on muscovite and biotite and matrix monazite from the sillimanite and staurolite zones of the central part of the wedge, 3) Early to Mid-Permian K–Ar muscovite and biotite and U–Th–Pb matrix monazite ages (~280–260 Ma) from the southern part of the area, adjacent to the Sudetic fault system. The sequence of obtained ages is interpreted as reflecting the Mississippian formation of the Silesian orogenic wedge that was followed by crustal-scale detachment related to Pennsylvanian–Early Permian intrusion of a voluminous Žulova Pluton accompanied by important fluid flow. Finally, the southern part of the studied domain was probably reworked by Permian fault system associated with renewed fluid activity.

New and published paleomagnetic measurements from Trans Altai and South Gobi zones in south Mongolia document large tectonic motions in between Late Carboniferous and Triassic. Magnetic inclinations confirm equatorial position of south Mongolian terranes in Late Carboniferous–Permian times. The evolution of magnetic declinations indicates 90° anticlockwise rotation in between latest Carboniferous and Early Triassic of all studied tectonic units around the Eulerian pole located close to axis of Mongolian orocline. The anticlockwise rotation continues in Triassic being accompanied by a major drift to the north. The structural and published geochronological data suggest Carboniferous E–W shortening of the whole region resulting in N–S trend of all continental and oceanic geological units followed by orthogonal N–S shortening during Late Permian to Early Jurassic. Both paleomagnetic and geological data converge in a tectonic model of oroclinal bending of Mongolian ribbon continent, westerly back arc oceanic domain and Mongol–Okhotsk subduction zone to the east. The oroclinal bending model is consistent with the coincidence of the Eulerian pole of rotation with the structural axis of Mongolian orocline. In addition, the Mesozoic collisional tectonics is reflected by late remagnetizations due to formation of wide deformation fronts and hydrothermal activity.