Timing of metamorphism,melting and exhumation of the Leo Pargil dome,northwest India

The Leo Pargil dome, northwest India, is a 30 km‐wide, northeast‐trending structure that is cored by gneiss and mantled by amphibolite facies metamorphic rocks that are intruded by a leucogranite injection complex. Oppositely dipping, normal‐sense shear zones that accommodated orogen‐parallel extension within a convergent orogen bound the dome. The broadly distributed Leo Pargil shear zone defines the southwest flank of the dome and separates the dome from the metasedimentary and sedimentary rocks in the hanging wall to the west and south. Thermobarometry and in‐situ U–Th–Pb monazite geochronology were conducted on metamorphic rocks from within the dome and in the hanging wall. These data were combined with U–Th–Pb monazite geochronology of leucogranites from the injection complex to evaluate the relationship between metamorphism, crustal melting, and the onset of exhumation. Rocks within the dome and in the hanging wall contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached ～530–630 °C and ～7–8 kbar, ending by c. 30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top‐down‐to‐the‐west shearing during near‐isothermal decompression of the footwall rocks to ～4 kbar by 23 Ma during an exhumation rate of 1.3 mm year^?1. Monazite growth accompanied Barrovian metamorphism and decompression. The leucogranite injection complex within the dome initiated at 23 Ma and continued to 18 Ma. These data show that orogen‐parallel extension in this part of the Himalaya occurred earlier than previously documented (>16 Ma). Contemporaneous onset of near‐isothermal decompression, top‐down‐to‐the‐west shearing, and injection of the decompression‐driven leucogranite complex suggests that early crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favoured orogen‐parallel extension.     

Interrelated P-T-t-d Paths in the Variscan Erzgebirge Dome (Saxony,Germany): Constraints on the Rapid Exhumation of High-Pressure Rocks from the Root Zone of a Collisional Orogen

The Erzgebirge dome consists of several superimposed composite tectonometamorphic units of medium- to high-grade metamorphic rocks from different crustal depths. These exhibit high pressure-high temperature and even ultrahigh-pressure imprints inherited from the root zone of a Variscan orogen and were exhumed almost immediately after attainment of maximum pressures at ～341 Ma. At present, the entire stack of tectonometamorphic units lies underneath an upper-crustal sequence of Paleozoic metasediments and tectonic slivers of pre-Carboniferous metamorphic rocks.

Shear zones active at different times and at different depths are preserved, mainly recording two successive stages of the exhumation history between 340 and 330 Ma. Tectonic transport during exhumation was remarkably constant in an E-W direction, swinging to NW-SE in the eastern part of the Erzgebirge parallel to a ductile transtensional zone (Elbe zone) that was concomitantly active. The various tectonometamorphic units have characteristically correlated, convergent P-T-t-d paths (both “cooling during decompression” and “heating during decompression”) that can be deduced from the dominant quartzofeldspathic rocks. These paths indicate successive exhumation of hotter rocks from increasingly deeper structural positions and juxtaposition against cooler rocks in higher positions, concomitant with the excision of intermediate crustal levels. We interpret this type of successive vertical telescoping of the metamorphic profile to be the result of extension of the thickened tectonometamorphic stack.

Extensional unroofing in the middle and upper crust was contemporaneous with and outlasted underthrusting and hence prograde metamorphism and deformation at deeper levels of the tectonometamorphic pile. Underthrusting is documented by a major inversion of the maximum pressure conditions in the lowermost units. However, structures related to compressional stacking now generally occur only as relics transposed by extensional deformation at lower pressure, or are restricted to rare small slivers with preserved prograde structures. Sedimentation of Lower Dinantian turbidites occurred along the flanks of the Erzgebirge dome during the exhumation process.

The extrusion of high-pressure rocks is interpreted to have been driven mainly by a major regional buoyancy instability caused by the delamination of the lithospheric mantle underneath the neighboring Bohemian Massif, which represented overthickened crust at least from the Devonian to the early Visean. Major controlling factors were boundary forces exerted by the thickened crustal bulge on the neighboring thin crustal segments in the north and east, effecting lateral extension of this orogenic wedge and extrusion-i.e., convective upward flow of gravitationally unstable crustal material.     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

喜马拉雅造山带的高压超高压变质作用与印度-亚洲大陆碰撞

喜马拉雅造山带是印度与亚洲大陆碰撞作用的产物,正在进行造山作用,是研究板块构造的天然实验室.高压和超高压变质岩分布在喜马拉雅造山带的核部.这些变质岩具有不同的形成条件、形成时间和形成过程,为印度与亚洲碰撞带的几何学、运动学和动力学提供了重要的限定.含柯石英的超高压变质岩产出在喜马拉雅造山带的西段,它们形成在古新世与始新世之间(53～46Ma),为印度大陆西北边缘高角度超深俯冲作用的产物,并经历了快速俯冲与快速折返过程.在约5 Myr内,超高压变质岩从＞100km的地幔深度折返到了中地壳深度,且仅仅叠加角闪岩相退变质作用.高压榴辉岩产出在喜马拉雅造山带中段,形成时间约为45Ma,为印度大陆低角度深俯冲作用的产物,经历了至少20Myr的长期折返过程,叠加麻粒岩相退变质作用和部分熔融.高压麻粒岩产出在喜马拉雅造山带的东端,是印度大陆东北缘近平俯冲作用的产物,峰期变质作用时间约为35Ma,经历了约20Myr的长期折返过程,叠加了麻粒岩相和角闪岩相退变质作用,并伴随有多期部分熔融.因此,喜马拉雅造山带的变质作用具有明显的时间与空间变化,显示出大陆深俯冲与折返过程的差异性,以及大陆碰撞造山带形成机制的多样性.     

喜马拉雅造山带的部分熔融与淡色花岗岩成因机制

喜马拉雅造山带核部由高级变质岩和淡色花岗岩组成,是研究大陆碰撞造山带部分熔融与花岗岩成因的天然实验室.基于最新研究成果,探讨了喜马拉雅造山带核部变质作用的条件、类型以及P-T轨迹、部分熔融的方式与程度及熔体成分以及变质作用与部分熔融的时间和持续过程.相关证据表明,造山带核部经历了高压麻粒岩相至榴辉岩相变质作用,具有以增温增压进变质和近等温降压退变质为特征的顺时针型P-T轨迹.这些高压变质岩石发生了长期持续的高温变质与部分熔融.在泥质岩石的进变质过程中白云母和黑云母脱水熔融可以形成不同成分的熔体.同时,总结了淡色花岗岩的形成时间、地球化学特征和源区熔融方式,结果表明碰撞造山过程中加厚下地壳的脱水熔融形成了喜马拉雅造山带的淡色花岗岩.      

A Comparison about Metamorphism among the Oldest-Rock Units from Orogenic Belts of Dabie, Eastern Qinling and Eastern Kunlun of Central Mountain Ranges, China

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Textural relations,P-T path,polymetamorphism and also geodynamic significance of metamorphic rocks of the Aligudarz-Khonsar region,Sanandaj-Sirjan zone,Iran

The metamorphic rocks of the Aligudarz-Khonsar region can be divided into nine groups: slate, phyllite, sericite schist, biotite-muscovite schist, garnet schist, garnet-staurolite schist, staurolite schist, mylonitic granite, and marble. In this metamorphic region, four phases of metamorphism can be identified (dynamothermal, thermal, dynamic and retrograde metamorphism) and there are three deformation phases (D1, D2 and D3). Paleozoic pelagic shales experienced prograde metamorphism and polymetamorphism from the greenschist to amphibolite facies along the kyanite geotherm. The metapelites show prograde dynamothermal metamorphism from the greenschist to amphibolite facies. Maximum degree of dynamothermal metamorphism is seen in the Nughan bridge area. Also development of the mylonitic granites in the Nughan bridge area shows that dynamic metamorphism in this area was more intense than in other parts of the AligudarzKhonsar metapelitic zone. The chemical zoning of garnets shows three stages of growth and syn-tectonic formation. With ongoing metamorphism, staurolite appeared, and the rocks reached amphibolite facies, but the degree of metamorphism did not increase past the kyanite zone. Thus, metamorphism of the pelitic sediments occurred at the greenschist to amphibolite facies (kyanite zone). Thermodynamic studies of these rocks indicate that the metapelites in the Aligudarz-Khonsar region formed at 490–550°C and 0.47–5.6 kbar.     

Effect of metamorphic reactions on thermal evolution in collisional orogens

The effects of metamorphic reactions on the thermal structure of a collisional overthrust setting are examined via forward numerical modelling. The 2D model is used to explore feedbacks between the thermal structure and exhumation history of a collisional terrane and the metamorphic reaction progress. The results for average values of crustal and mantle heat production in a model with metapelitic crust composition predict a 25–40 °C decrease in metamorphic peak temperatures due to dehydration reactions; the maximum difference between the P–T–t paths of reacting and non‐reacting rocks is 35–45 °C. The timing of the thermal peak is delayed by 2–4 Myr, whereas pressure at peak temperature conditions is decreased by more than 0.2 GPa. The changes in temperature and pressure caused by reaction may lead to considerable differences in prograde reaction pathways; the consumption of heat during dehydration may produce greenschist facies mineral assemblages in rocks that would have otherwise attained amphibolite facies conditions in the absence of reaction enthalpy. The above effects, although significant, are produced by relatively limited metamorphic reaction which liberates only half of the water available for dehydration over the lifetime of the prograde metamorphism. The limited reaction is due to the lack of heat in a model with the average thermal structure and relatively fast erosion, a common outcome in the numerical modelling of Barrovian metamorphism. This problem is typically resolved by invoking additional heat sources, such as high radiogenic heat production, elevated mantle heating or magmatism. Several models are tested that incorporate additional radiogenic heat sources; the elevated heating rates lead to stronger reaction and correspondingly larger thermal effects of metamorphism. The drop in peak temperatures may exceed 45 °C, the maximum temperature differences between the reacting and non‐reacting P–T–t paths may reach 60 °C, and pressure at peak temperature conditions is decreased by more than 0.2 GPa. Field observations suggest that devolatilization of metacarbonate rocks can also exert controls on metamorphic temperatures. Enthalpies were calculated for the reaction progress recorded by metacarbonate rocks in Vermont, and were used in models that include a layer of mixed metapelite–metacarbonate composition. A model with the average thermal structure and erosion rate of 1 mm year^?1 can provide only half of the heat required to drive decarbonation reactions in a 10 km thick mid‐crustal layer containing 50 wt% of metacarbonate rock. Models with elevated heating rates, on the other hand, facilitated intensive devolatilization of the metacarbonate‐bearing layer. The reactions resulted in considerable changes in the model P–T–t paths and ～60 °C drop in metamorphic peak temperatures. Our results suggest that metamorphic reactions can play an important role in the thermal evolution of collisional settings and are likely to noticeably affect metamorphic P–T–t paths, peak metamorphic conditions and crustal geotherms. Decarbonation reactions in metacarbonate rocks may lead to even larger effects than those observed for metapelitic rocks. Endothermic effects of prograde reactions may be especially important in collisional settings containing additional heat sources and thus may pose further challenges for the ‘missing heat’ problem of Barrovian metamorphism.     

The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps

Thermal models for Barrovian metamorphism driven by doubling the thickness of the radiogenic crust typically meet difficulty in accounting for the observed peak metamorphic temperature conditions. This difficulty suggests that there is an additional component in the thermal budget of many collisional orogens. Theoretical and geological considerations suggest that viscous heating is a cumulative process that may explain the heat deficit in collision orogens. The results of 2D numerical modelling of continental collision involving subduction of the lithospheric mantle demonstrate that geologically plausible stresses and strain rates may result in orogen‐scale viscous heat production of 0.1 to >1 μW m^?3, which is comparable to or even exceeds bulk radiogenic heat production within the crust. Thermally induced buoyancy is responsible for crustal upwelling in large domes with metamorphic temperatures up to 200 °C higher than regional background temperatures. Heat is mostly generated within the uppermost mantle, because of large stresses in the highly viscous rocks deforming there. This thermal energy may be transferred to the overlying crust either in the form of enhanced heat flow, or through magmatism that brings heat into the crust advectively. The amplitude of orogenic heating varies with time, with both the amplitude and time‐span depending strongly on the coupling between heat production, viscosity and collision strain rate. It is argued that geologically relevant figures are applicable to metamorphic domes such as the Lepontine Dome in the Central Alps. We conclude that deformation‐generated viscous dissipation is an important heat source during collisional orogeny and that high metamorphic temperatures as in Barrovian type metamorphism are inherent to deforming crustal regions.     

大陆中部地壳应变局部化与应变弱化

大陆岩石圈流变学研究是构造地质学学科发展的必然,也是发展板块构造理论、探索大陆板块内部变形与动力学演化的核心问题。大陆中部地壳是大陆岩石圈中一个具有特殊性的圈层,其主要成分以花岗质岩石为代表,位于岩石脆-韧性转变域。在中部地壳层次上,岩石既具有脆性变形特点,又具有韧性变形属性,而且常常表现出多种流变强度。研究成果显示,中部地壳岩石流变具有许多特殊性:1)应变局部化是中部地壳流动最为典型表现形式;2)存在大陆地壳多震层:多震与强震,显示出中部地壳既弱又强的流变学属性;3)液/岩反应强烈,流体相直接影响着岩石的流变性;4)在许多地区存在有地球物理异常体(低速高导体)。大陆中部地壳应变局部化是板块相互作用过程中地壳层次上应变积累与集中的重要表现。在宏观尺度、中小型尺度和微观尺度上都有着重要的构造特点。地壳岩石的应变弱化,是诱发应变局部化的主要机制。多种形式的水致弱化(包括液压致裂、反应弱化、水解弱化等)与结构弱化(包括细粒化、晶格取向、成分分带性等)对于应变局部化具有重要的贡献。大陆地壳岩石流变学、中部地壳弱化与应变局部化研究,是未来岩石圈流变学研究的重要方向。     

Crustal stacking and extension recorded by tectonic fabrics of the SE margin of the Tauern Window, Austria

Structural and metamorphic analyses show that Alpine deformation in the Austroalpine-Pennine contact zone around the margin of the SE Tauern Window can be divided into two main stages: (i) early crustal thickening associated with prograde metamorphism; and (ii) a younger history of ductile flow that added to cumulative displacement of the upper units to the NW quadrant but was associated with substantial subvertical attenuation of the contact zone, and most probably of the overriding Austroalpine plate as well. During the history of this region strain localization progressively shifted down section. Radiometric ages constrain the early deformation to be older than 75 Ma. The onset of contact-zone attenuation and upper-plate extension was after this date but before 35 Ma (before major involvement of European basement in the collisional orogen), and associated with both retrograde metamorphism and a degree of non-coaxiality less than simple shear. Estimates of thinning in the contact zone and on a regional scale are in good agreement and indicate vertical attenuation of approximately 40%. These results suggest that pre-collisional tectonic thinning of the Austroalpine domain may be more widespread and significant than generally recognized.     

A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion.     

Deformation of a pervasively molten middle crust: insights from the neoproterozoic Ribeira‐Araçuaí orogen (SE Brazil)

Pervasive melting of the middle crust, as inferred in Tibet and the Altiplano, probably influences the deformation of the lithosphere. To constrain strain distribution in a pervasively molten crust, we analysed the deformation in an eroded analogue of these orogens. The Ribeira‐Araçuaí orogen (SE Brazil) comprises a stack of allochthons containing large volumes of anatectic and magmatic rocks. The upper allochton (∼300 km long, 50–100 km wide and >10 km thick) involves peraluminous diatexites and leucogranites resulting from partial melting of the middle crust. It overlies another allochthon containing huge early‐ to syn‐collisional plutons intruding metasediments. Both anatexites and magmatic intrusions display a pervasive strain‐induced magmatic fabric. Homogeneous strain distribution suggests inefficient localization. U–Pb ages of ∼575 Ma imply that anatexite melting was synchronous to the early‐ to syn‐collisional magmatism. Similarity in ages magmatic and solid‐state fabrics indicates that intrusions and anatexites deformed coherently with solid‐state rocks while still molten, in response to a combination of gravity‐driven and collision‐driven deformation.     

3D model of deep structure of the Early Precambrian crust in the East European Craton and paleogeodynamic implications

The integral 3D model of the deep structure of the Early Precambrian crust in the East European Craton is based on interpretation of the 1-EU, 4B, and TATSEIS seismic CDP profiles in Russia and the adjacent territory of Finland (FIRE project). The geological interpretation of seismic images of the crust is carried out in combination with consideration of geological and geophysical data on the structure of the Fennoscandian Shield and the basement of the East European platform. The model displays tectonically delaminated crust with a predominance of low-angle boundaries between the main tectonic units and the complex structure of the crust-mantle interface, allowing correlation of the deep structure of the Archean Kola, Karelian, and Kursk granite-greenstone terrane with the Volgo-Uralia granulite-gneiss terrane, as well as the Paleoproterozoic intracontinental collision orogens (the Lapland-Mid-Russia-South Baltia orogen and the East Voronezh and Ryazan-Saratov orogens) with the Svecofennian accretionary orogen. The lower crustal “layer” at the base of the Paleoproterozoic orogens and Archean cratons was formed in the Early Paleoproterozoic as a result of underplating and intraplating by mantle-plume mafic magmas and granulite-facies metamorphism. The increase in the thickness of this “layer” was related to hummocking of the lower crustal sheets along with reverse and thrust faulting in the upper crust. The middle crust was distinguished by lower rigidity and affected by ductile deformation. The crust of the Svecofennian Orogen is composed of tectonic sheets plunging to the northeast and consisting of island-arc, backarc, and other types of rocks. These sheets are traced in seismic sections to the crust-mantle interface.     

A possible lower crustal flow channel in the Middle Urals based on reflection seismic data

The Europrobe Seismic Reflection Profiling in the Urals Experiments (ESRU) reflection seismic data from the Middle Urals images c. 10‐km thick band of strong, subhorizontal lower crustal reflectivity and a thinning of the crust that is associated with the East Uralian Zone, a broad strike‐slip fault system containing high‐grade metamorphic rocks and syn‐orogenic to post‐orogenic granitoids. The lower crustal reflectivity consists of discontinuous to continuous, high‐amplitude reflections. Reflections are subparallel to slightly oblique and have a layered to oblate appearance. Geometrical relationships indicate that the reflectivity post‐dates fault activity, suggesting that late‐orogenic processes modified the lower crust. The surface geology indicates that the conditions for lower crustal flow were met in the East Uralian Zone. We suggest that the lower crustal reflectivity imaged by the ESRU data is related to a flow channel that developed at the base of the crust in the interior of the orogen.     

Footwall dip of a core complex detachment fault: thermobarometric constraints from the northern Snake Range (Basin and Range,USA)

Low‐angle detachment faults are common features in areas of large‐scale continental extension and are typically associated with metamorphic core complexes, where they separate upper plate brittle extension from lower plate ductile stretching and metamorphism. In many core complexes, the footwall rocks have been exhumed from middle to lower crustal depths, leading to considerable debate about the relationship between hangingwall and footwall rocks, and the role that detachment faults play in footwall exhumation. Here, garnet–biotite thermometry and garnet–muscovite–biotite–plagioclase barometry results are presented, together with garnet and zircon geochronology data, from seven locations within metapelitic rocks in the footwall of the northern Snake Range décollement (NSRD). These locations lie both parallel and normal to the direction of footwall transport to constrain the pre‐exhumation geometry of the footwall. To determine P–T gradients precisely within the footwall, the ΔPT method of Worley & Powell (2000) has been employed, which minimizes the contribution of systematic uncertainties to thermobarometric calculations. The results show that footwall rocks reached pressures of 6–8 kbar and temperatures of 500–650 °C, equivalent to burial depths of 23–30 km. Burial depth remains constant in the WNW–ESE direction of footwall transport, but increases from south to north. The lack of a burial gradient in the direction of footwall transport implies that the footwall rocks, which today define a sub‐horizontal datum in the direction of fault transport, also defined a sub‐horizontal datum at depth in Late Cretaceous time. This suggests that the footwall was not tilted about the normal to the fault transport direction during exhumation, and hence that the NSRD did not form as a low‐angle normal fault cutting down through the lower crust. Instead, the following evolution for the northern Snake Range footwall is proposed. (i) Mesozoic contraction caused substantial crustal thickening by duplication and folding of the miogeoclinal sequence, accompanied by upper greenschist to amphibolite facies metamorphism. (ii) About half of the total exhumation was accomplished by roughly coaxial stretching and thinning in Late Cretaceous to Early Tertiary time, accompanied by retrogression and mylonitic deformation. (iii) The footwall rocks were then ‘captured’ from the middle crust along a moderately dipping NSRD that soled into the middle crust with a rolling‐hinge geometry at both upper and lower terminations.     

Exhumation of a Variscan orogenic complex: insights into the composite granulitic–amphibolitic metamorphic basement of south-east Corsica (France)

A structural, petrological and geochronological (U‐Th‐Pb of zircon and monazite) study reveals that the lower crust sequences of the Variscan high‐grade basement cropping out between Solenzara and Porto Vecchio, south‐east Corsica (France) have been tectonically juxtaposed along with middle crustal rocks during the extrusion of the orogenic root of the Variscan chain. We propose that a system of high‐temperature, orogen‐parallel shear zones that developed under a transpressive dextral tectonic regime caused the exhumation of the entire sequence. This tectonic complex is thus made up of rocks having undergone different P–T conditions (eclogite‐?, high‐pressure granulite facies and amphibolite facies) at different times, reflecting the progressive foreland migration of the orogenic front. The Solenzara granulites were derived from burial of continental crust to high‐pressure (1.8–1.4 GPa) and high‐ to ultrahigh‐temperature conditions (900–1000 °C) during the Variscan convergence: U–Pb ELA‐ICPMS zircon dating constrained the timing of this metamorphism at c. 360 Ma. The gneisses cropping out at Porto Vecchio are middle crustal‐level rocks that reached their peak temperature conditions (700–750 °C at <1.0 GPa) at c. 340 Ma. The diachronism of the metamorphic events, the foliation patterns and their geometry suggest that the granulites were exhumed to middle crustal levels through channel flow tectonics under continuous compression. The amphibolite facies gneisses of Porto Vecchio and the granulites of Solenzara were accreted through the development of a major dextral mylonitic zone forming under amphibolite facies conditions: in situ monazite isotope dating (ELA‐ICPMS) revealed that this deformation occurred at c. 320 Ma and was accompanied by the emplacement of syntectonic high‐K melts. A final HTLP static overprint, constrained at 312–308 Ma by monazite U‐Th‐Pb isotope dating, is related to the emplacement of the igneous products of the Sardinia‐Corsica batholith and marks the transition from the Variscan orogenic event to the Permian extension.     

Metamorphic history of a syn‐convergent orogen‐parallel detachment: The South Tibetan detachment system,Bhutan Himalaya

The South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal‐sense displacement on an orogen‐parallel shear zone during lithospheric contraction. Here, in situ monazite U(–Th)–Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure–temperature–time (P–T–t) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile/brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite‐bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite‐bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal‐sense (top‐to‐the‐north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P–T–t paths are compatible with modelled P–T–t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid‐crustal material.     

Olivine‐bearing symplectites in fractured garnet from eclogite,Moldanubian Zone (Bohemian Massif) – a short‐lived,granulite facies event

Recent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.     

Contrasting metamorphic evolution of metasedimentary rocks from the Çine and Selimiye nappes in the Anatolide belt, western Turkey

Abstract P–T conditions, mineral isograds, the relation of the latter to foliation planes and kinematic indicators are used to elucidate the tectonic nature and evolution of a shear zone in an orogen exhumed from mid‐crustal depths in western Turkey. Furthermore, we discuss whether simple monometamorphic fabrics of rock units from different nappes result from one single orogeny or are related to different orogenies. Metasedimentary rocks from the Çine and Selimiye nappes at the southern rim of the Anatolide belt of western Turkey record different metamorphic evolutions. The Eocene Selimiye shear zone separates both nappes. Metasedimentary rocks from the Çine nappe underneath the Selimiye shear zone record maximum P–T conditions of about 7 kbar and >550 °C. Metasedimentary rocks from the overlying Selimiye nappe have maximum P–T conditions of 4 kbar and c. 525 °C near the base of the nappe. Kinematic indicators in both nappes are related to movement on the Selimiye shear zone and consistently show a top‐S shear sense. Metamorphic grade in the Selimiye nappe decreases structurally upwards as indicated by mineral isograds defining the garnet‐chlorite zone at the base, the chloritoid‐biotite zone and the biotite‐chlorite zone at the top of the nappe. The mineral isograds in the Selimiye nappe run parallel to the regional S_R foliation, parallel the Selimiye shear zone and indicate that the Selimiye shear zone formed during this prograde greenschist to lower amphibolite facies metamorphic event but remained active after the peak of metamorphism. ⁴⁰Ar/³⁹Ar mica ages and the tectonometamorphic relationship with the Eocene Cyclades–Menderes thrust, which occurs above the Selimiye nappe in the study area, suggests an Eocene age of metamorphism in the Selimiye nappe. Metasedimentary rocks of the Çine nappe 20–30 km north of the Selimiye shear zone record maximum P–T conditions of 8–11 kbar and 600–650 °C. An age of about 550 Ma is indicated for amphibolite facies metamorphism and associated top‐N shear in the orthogneiss of the Çine nappe. Our study shows that simple monophase tectonometamorphic fabrics do not always indicate a simple orogenic development of a nappe stack. Preservation in some areas and complete overprinting of those fabrics in other areas apparently occur very heterogeneously.