Early Stages Of Exhumation Research Articles

東中国ドンハイ水晶の年代：黒雲母のAr-Ar 年代による制約

The paper presents the newest age information of the rock crystals (clear quartz crystals) in Donghai County of eastern China, and discusses the evolution of the rock crystal-bearing quartz veins with respect to the exhumation history of the Su-Lu ultra-high pressure (UHP) metamorphic belt. Biotite samples were collected from the alteration zone of the quartz veins and Ar-Ar method was used to date the biotite in an attempt to decipher the age of the rock crystals based on the relation between the biotite and the quartz veins. The dating yields biotite Ar-Ar plateau age, isochron age, and inverse-isochron age of 239.8±2.6 Ma, 241.0±2.6 Ma, and 241.1±2.7 Ma respectively. The ages are close to the peak metamorphism age (at 240-245 Ma) of the Su-Lu UHP belt, indicating that the rock crystals of the quartz veins were crystallized in the early stage of exhumation of the Su-Lu UHP metamorphic belt.

Cretaceous isochron ages from K–Ar and 40Ar / 39Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene ( X Jd = 0.48). Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial 40Ar / 36Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic 40Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.

Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

AbstractAt the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west‐oriented compression and vertical extrusion of the high‐grade rocks into higher crustal levels. The high‐pressure mineral assemblage Grt‐Ky‐Kfs‐Pl‐Qtz‐Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high‐grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite‐bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr−1 and 7–9.4 °C Myr−1, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high‐grade rocks with a cold upper‐crustal lid.

Pressure dependence and anisotropy of P-wave velocities in ultrahigh-pressure metamorphic rocks from the Dabie–Sulu orogenic belt (China): Implications for seismic properties of subducted slabs and origin of mantle reflections

The compressional wave velocities (Vp), pressure derivatives (Vp′) and anisotropy ( A) of three types of eclogites and country rocks from the Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt, China, have been measured under confining pressures up to 800 MPa. Type-1 eclogites, which are coarse-grained and subjected to almost no retrograde metamorphism, experienced recovery-accommodated dislocation creep at peak metamorphic conditions (in the diamond stability field). Type-2 eclogites are fine-grained reworked Type-1 materials that experienced recrystallization-accommodated dislocation creep under quartz/coesite boundary conditions during the early stage of exhumation. Type-3 eclogites are retrogressed samples that were overprinted by significant amphibolite facies metamorphism during a late stage of exhumation within the crust. Type-1 eclogites are richer in Al 2O 3 and MgO but poorer in SiO 2 and Na 2O than Type-2 and Type-3 eclogites. Anisotropy of Type-1 and Type-2 eclogites is generally low (<4%) because volumetrically important garnet is elastically quasi-isotropic, while Type-3 eclogites can exhibit high anisotropy (>10%) due to the presence of strongly anisotropic retrograde minerals such as amphibole, plagioclase and mica. The transition of the pressure dependence of velocity from the poroelastic to elastic regimes occurs at a critical pressure ( P c), which depends mainly on the density and distribution of microcracks and in turn on the exhumation history of rocks. The Vp–pressure relationship can be expressed by Vp= a(ln P) 2+ bln P+ c ( P≤ P c) and Vp= V 0+ DP ( P≥ P c), where P is the confining pressure, a and b are constants describing the closure of microcracks below P c, c is the velocity when P is equal to one (MPa), V 0 is the projected velocity of a crack-free sample at room pressure, and D is the intrinsic pressure derivative above P c. When data are curve-fit, pressure derivatives and anisotropy as functions of pressure are determined. The average Vp of the eclogites in the linear regime is 8.42+1.41×10 −4 P for Type-1, 7.80+1.58×10 −4 P for Type-2, and 7.33+2.04×10 −4 P for Type-3, where Vp is in km/s and P in MPa. The decrease in V 0 and increase in D from Type-1 to Type-3 eclogites are attributed to a decrease in garnet content and an increase in retrograde minerals. The NE–SW trending, NW-dipping, slab-like high Vp anomaly (8.72 km/s at a depth of 71 km) which extends from the Moho to at least 110 km beneath the Dabie–Sulu region, can be interpreted as the remnant of a subducted slab which is dominated by Type-1 eclogites and has frozen in the upper mantle since about 200–220 Ma. Such relic crustal materials, subducted and preserved as eclogite layers intercalated with felsic gneiss, garnet–jadeite quartzite, marble and serpentinized peridotite, could be responsible for regionally observed seismic reflectors in the upper mantle.

Eclogite-facies polyphase deformation of the Drøsdal eclogite, Western Gneiss Complex, Norway, and implications for exhumation

Exhumation of the deep crust during orogenic extension is accepted as a geological phenomenon, but structures formed during burial and the earliest stages of exhumation are often overprinted by deformation events occurring at shallower crustal levels. The Western Gneiss Complex (WGC) of Norway comprises variably retrogressed high-pressure (HP) and ultrahigh-pressure (UHP) eclogite bodies enclosed in predominantly felsic amphibolite-facies rocks. The Drøsdal body, located in the Sunnfjord area, is one of the largest and best-preserved eclogites in the WGC, and comprises over ca. 3 km 2 of exposed eclogite-facies rocks. It is an excellent example of an eclogite tectonite, and displays a wealth of structures formed during deformation at a minimum depth of 50 km and peak temperatures of ca. 800 °C. Large volumes of mylonites with the eclogite-facies assemblage garnet±clinopyroxene±quartz±amphibole±clinozoisite±phengite±kyanite±rutile are preserved. Structures associated with the eclogite-facies metamorphism include E–W-trending isoclinal folds, boudinage, hinge-parallel lineations, and meter-scale kyanite-dominated veins. The Drøsdal body was tightly folded on the kilometer scale under eclogite-facies conditions. Subsequently, the shapes of the Drøsdal body and other mafic bodies in the Sunnfjord area were modified during eclogite-to-amphibolite-facies E–W-directed stretching, to give boudin-like lenses. The timing of formation of a pervasive eclogite-facies lineation, eclogite-facies folds, and kyanite bearing veins overlapped substantially, and this portion of the deformation history was dominated by a constrictional strain field. Partitioning of deformation occurred after a substantial amount of eclogite-facies deformation had already taken place, and resulted in the relative rotation of linear features in specific zones on the scale of ∼400 m. The eclogite-facies lineations and fold hinges within the Drøsdal body are subparallel to amphibolite and greenschist-facies structures throughout the WGC. Although circumstantial, this suggests that structures belonging to each of these three metamorphic facies formed during one progressive deformation event corresponding to extensional exhumation. Our observations are consistent with models involving inhomogeneous deformation of the lower crust during early orogenic extension. We envisage that transtensional (noncoaxial) deformation dominated the extensional deformation of the WGC, but that coaxial deformation must have occurred locally.

Exhumation Paths of High-Pressure–Low-Temperature Metamorphic Rocks from the Lycian Nappes and the Menderes Massif (SW Turkey): a Multi-Equilibrium Approach

The Menderes Massif and the overlying Lycian Nappes occupy an extensive area of SW Turkey where high-pressure–low-temperature metamorphic rocks occur. Precise retrograde P–T paths reflecting the tectonic mechanisms responsible for the exhumation of these high-pressure–low-temperature rocks can be constrained with multi-equilibrium P–T estimates relying on local equilibria. Whereas a simple isothermal decompression is documented for the exhumation of high-pressure parageneses from the southern Menderes Massif, various P–T paths are observed in the overlying Karaova Formation of the Lycian Nappes. In the uppermost levels of this unit, far from the contact with the Menderes Massif, all P–T estimates depict cooling decompression paths. These high-pressure cooling paths are associated with top-to-the-NNE movements related to the Akçakaya shear zone, located at the top of the Karaova Formation. This zone of strain localization is a local intra-nappe contact that was active in the early stages of exhumation of the high-pressure rocks. In contrast, at the base of the Karaova Formation, along the contact with the Menderes Massif, P–T calculations show decompressional heating exhumation paths. These paths are associated with severe deformation characterized by top-to-the-east shearing related to a major shear zone (the Gerit shear zone) that reflects late exhumation of high-pressure parageneses under warmer conditions.

The early stages of the Alpine collision: an image derived from the upper Eocene–lower Oligocene record in the Alps–Apennines junction area

The upper Eocene–lower Oligocene sediments deposited in the eastern part of the Tertiary Piedmont Basin in northern Italy provide a complete record of the unroofing of the Alpine orogenic prism during the early stages of exhumation in the Ligurian sector. From late Priabonian till late Rupelian time, the sediments in the study area were derived from two different sources, one characterised by white micas with Si<6.5 pfu and Permian 40Ar/ 39Ar ages (270 Ma), and the other characterised by white micas with Si>7 pfu and Eocene–Oligocene 40Ar/ 39Ar ages (32–50 Ma). The first source is considered to be indicative of low-pressure metamorphic rocks that covered the HP rocks of the Ligurian Alps, and were completely eroded by Chattian time. From this time on, the study area started to record the first input from western Alpine sources characterised by a larger span of ages with a more frequent Eoalpine signal. Thus, sediments deposited in the eastern part of the Tertiary Piedmont Basin contain the only available evidence of rocks belonging to high crustal levels in the Alpine orogenic prism that were not affected by the Alpine overprint. These data also provide time constraints to the poorly dated first conglomerates deposited in this area. 40Ar/ 39Ar geochronology reveals a minimum age of 33±1.4 Ma for the Pianfolco Conglomerates in the type locality, and of 31.4±3.5 Ma for the Borbera Conglomerates.

Triassic polyphase deformation in the Feidong-Zhangbaling Massif (eastern China) and its place in the collision between the North China and South China blocks

The Feidong-Zhangbaling Massif is located between the Dabieshan and Sulu areas along the Tan-Lu fault in eastern China. Five tectonic-metamorphic events are distinguished there. The earliest deformation (D1) corresponds to a southward compression that occurred during subduction of the South China Block below the North China Block. Top-to-the-south shearing is coeval with Late Permian-Early Triassic blueschist facies metamorphism, and possibly with the development of south-verging recumbent folds in the Neoproterozoic-Paleozoic sedimentary cover of the South China foreland. The main ductile deformation (D2) is an extensional one, characterized by top-to-the-north shearing, coeval with the early stage of exhumation of the high-pressure rocks. A top-to-the-NW ductile shearing, and microfolds overturned to the northwest, belong to a second deformational phase of exhumation (D3) which is distinct from the main event (D2). Previous 40Ar– 39Ar mica dates ranging between 245 and 212 Ma suggest Late Permian-Early Triassic ages for the D1 to D3 events. The D4 event produced NE–SW trending folds in the sedimentary cover interpreted as gravity collapse structures. A Late Cretaceous brittle extensional event (D5) controls the opening and infill of continental half-grabens. In the study area, the Tan-Lu fault is a Cretaceous brittle normal fault. The lack of ductile deformation presented along the Tan-Lu fault suggests that it did not play a significant role in the exhumation of high-pressure metamorphic rocks.

Exhumation of alpine high-pressure rocks: insights from petrology of eclogite clasts in the Tertiary Piedmontese basin (Ligurian Alps, Italy)

The study of late and postorogenic sedimentary basins is a powerful tool to understand uplift, exhumation and erosion of an orogen. In the Ligurian Alps, high-pressure ophiolitic rocks are directly overlain by the sediments of the Tertiary Piedmontese basin (TPB); the conglomerates at the bottom of the TPB succession contain clasts of metaophiolites and metasediments which display deformation structures acquired at peak metamorphic conditions ranging from eclogite to blueschist facies. The deformation events prevalently recorded by the high-pressure rocks of the Ligurian Alps are either related to subduction and peak metamorphism, or to the late stage greenschist collision. The main structures which accomplish the early exhumation of such high-pressure terrains are only locally recorded and poorly explored. Further information on these early tectonic events can be gathered through the study of the clasts in the conglomerates. In this paper, we present a textural and petrologic outline and some thermobarometric estimates of the main types of high-pressure clasts sampled and their comparison with the high-pressure equivalents presently exposed in the Ligurian Alps. In particular, some clasts displaying a Na-amphibole+white mica+epidote+sphene blueschist foliation superimposed to an eclogitic garnet+Na-clinopyroxene+rutile tectonitic assemblage have been studied in detail. This kind of rock records a pressure–temperature ( P– T) path implying cooling during exhumation, whereas the high-pressure bedrocks are characterized by either isothermal decompression, or initial heating and subsequent cooling. We conclude that the blueschist foliation of these clasts most likely developed along shear zones and/or contacts among different slices, formed during tectonic coupling of warm, uprising eclogite units, with cooler blueschist slices during the early stages of exhumation.

EXHUMATION OF ALPINE HIGH PRESSURE ROCKS: INSIGHTS FROM PETROLOGY OF ECLOGITE CLASTS IN THE TERTIARY PIEDMONTESE BASIN (LIGURIAN ALPS, ITALY)

The study of late and post-orogenic sedimentary basins is a powerful tool to understand uplift, exhumation and erosion of an orogen. In the Ligurian Alps, high-pressure ophiolitic rocks are directly overlain by the sediments of the Tertiary Piedmontese Basin (TPB); the conglomerates at the bottom of the TPB succession contain clasts of metaophiolites and metasediments which display deformation structures acquired at peak metamorphic conditions ranging from eclogite- to blueschist- facies. The deformation events prevalently recorded by the highpressure rocks of the Ligurian Alps are either related to subduction and peak metamorphism, or to the late-stage greenschist collision. The main structures which accomplish the early exhumation of such high-pressure terrains are only locally recorded and poorly explored. Further information on these early tectonic events can be gathered through the study of the clasts in the conglomerates. In this paper we present a textural and petrologic outline and some thermobarometric estimates of the main types of high-pressure clasts sampled and their comparison with the high pressure equivalents presently exposed in the Ligurian Alps. In particular, some clasts displaying a Na-amphibole + white mica + epidote + sphene blueschist foliation superimposed to an eclogitic garnet + Na-clinopyroxene + rutile tectonitic assemblage have been studied in detail. These kind of rocks record a pressure-temperature path implying cooling during exhumation, whereas the high-pressure bedrocks are characterized by either isothermal decompression, or initial heating and subsequent cooling. We conclude that the blueschist foliation of these clasts most likely developed along shear zones and/or contacts among different slices, formed during tectonic coupling of warm, uprising eclogite units, with cooler blueschist slices during the early stages of exhumation.

Continental subduction and exhumation of UHP rocks. Structural and geochronological insights from the Dabieshan (East China)

In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics. The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons. North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the 40Ar/ 39Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands. The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the 40Ar/ 39Ar dates. A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented.

High‐ to ultrahigh‐pressure (UHP) ductile shear zones in the Sulu UHP metamorphic belt, China: implications for continental subduction and exhumation

AbstractHigh‐ to ultrahigh‐pressure (HP‐UHP) metamorphic rocks that resulted from deep continental subduction and subsequent exhumation in the Sulu orogenic belt, China, have experienced multiphase deformation and metamorphic overprint during its long journey to the mantle and return to the surface. HP‐UHP shear zones are strain‐localized weak zones on which the UHP slab is transported over long distances. HP‐UHP shear zones are well exposed along a 200‐km belt in the Sulu UHP metamorphic belt. The shear zones lie structurally below the UHP rocks and above the non‐UHP rocks, suggesting the early exhumation of the UHP rocks by thrusting. The large area distribution, HP‐UHP nature, high strain and structural association of the shear zones with the UHP rocks suggest that the shear zones are probably a regional detachment developed during the early stage of exhumation of the UHP rocks. Kinematic indicators suggest top‐to‐the N–NW motion of the UHP slab during the exhumation, which, combined with isotope signature in Mesozoic igneous rocks, leads us to the interpretation that the subduction polarity is the North China plate down to the south rather than the Yangtze plate down to the north in the Sulu region.

Ultrahigh-pressure metamorphism: tracing continental crust into the mantle

More and more evidence is being discovered in Phanerozoic collision belts of the burial of crustal rocks to previously unsuspected (and ever increasing) depths, presently on the order of 150–200 km, and of exhumation from such depths. This extends by almost one order of magnitude the depth classically ascribed to the metamorphic cycling of continental crust, and demonstrates its possible subduction. The pieces of evidence for this new, ultrahigh-pressure (UHP) metamorphism exclusively occur in the form of relics of high-pressure minerals that escaped back-transformation during decompression. The main UHP mineral indicators are the high-pressure polymorphs of silica and carbon, coesite and microdiamond, respectively; the latter often demonstrably precipitated from a metamorphic fluid and is completely unrelated to kimberlitic diamond or any shock event. Recent discoveries of pyroxene exsolutions in garnet and of coesite exsolutions in titanite suggest a precursor garnet or titanite containing six-fold coordinated silicon, therefore still higher pressures than implied by diamond stability, on the order of 6 GPa. The UHP rocks raise a formidable geological problem: that of the mechanisms responsible for their burial and, more pressingly, for their exhumation from the relevant depths. The petrological record indicates that large tracts of UHP rocks were buried to conditions of low T/P ratio, consistent with a subduction-zone context. Decompression occurred in most instances under continuous cooling, implying continuous heat loss to the footwall and hangingwall of the rising body. This rise along the subduction channel – an obvious mechanical discontinuity and weak zone – may be driven by buoyancy up to mid-crustal levels as a result of the lesser density of the acidic crustal rocks (even if completely re-equilibrated at depth) after delamination from the lower crust, in a convergent setting. Chronological studies suggest that the rates involved are typical plate velocities (1–2 cm/yr), especially during early stages of exhumation, and bear no relation to normal erosion rates. Important observations are that: (i) as a result of strain partitioning and fluid channelling, significant volumes of subducted crust may remain unreacted (i.e. metastable) even at conditions as high as 700°C and 3 GPa – with implications as to geophysical modeling; (ii) subducted continental crust shows no isotopic or geochemical evidence of interaction with mantle material. An unknown proportion of subducted continental crust must have escaped exhumation and effectively recycled into the mantle, with geochemical implications still to be explored, bearing in mind the above inefficiency of mixing. The repeated occurrence of UHP metamorphism, hence of continental subduction, through time and space since at least the late Proterozoic shows that it must be considered a common process, inherent to continental collision. Evidence of older, Precambrian UHP metamorphism is to be sought in high-pressure granulite-facies terranes.

Fluid evolution and exhumation history of ultrahigh-pressure rocks at Lanshantou, Sulu terrane, Eastern China

Fluid inclusions in coesite-bearing eclogites and eclogite-facies quartz veins at Lanshantou in the Sulu ultrahigh-pressure metamorphic (UHPM) terrane, Eastern China, have preserved remnants of peak metamorphic and exhumation fluids. Three types of fluids have been distinguished: (1) mixed H 2O CO 2 ± N2 fluid, (2) hypersaline aqueous fluid and (3) low salinity H 2O fluid. Mixed H 2O CO 2 ± N2 fluids were introduced during or just after peak metamorphism. The hypersaline fluids might be products of dehydration melting during early stage of exhumation. Low salinity H 2O inclusions can be related to the latest evolutionary stage of the high-pressure veins during exhumation.

Coesite eclogite facies metamorphic tectonite: Evidence for ultrahigh pressure deformation

An ultrahigh pressure ductile shear melange crops out on the beach of Yangkou Bay near Qingdao City. The melange is composed of weakly deformed blocks in a highly ductilly flow mylonites. Ultrahigh pressure metamorphic (UHPM) tectonite includes strongly deformed eclogite and mylonitized eclogite. Coesite occurs in the tectonite as both interstitial mineral and inclusions in garnet and omphacite, indicating that the deformation took place in the stability field of coesite (800–850°C, >30 GPa) in the upper mantle. Coesite is rounded or short prismatic grains with undulatory extinction, and often fractured, suggesting brittle deformation. Garnet is also characterized by brittle fractures and sometimes necked and slightly elongated. Omphacite is elongated, with long axis preferred orientation. Undulatory extinction, subgrains and dynamically recrystallized grains suggest plastic flow of omphacite. Ultrahigh pressure metamorphic tectonite was probably formed in the ductile shear zone during the early stage of exhumation of the ultrahigh pressure metamorphic rocks. Its kinematic indicators point to the transport direction of the UHPM slab during the early stage of exhumation.

Oxygen isotope composition of granulites from dabieshan in eastern China and its implications for geodynamics of yangtze plate subduction

The oxygen isotope composition of whole-rock and mineral separates was measured for granulite rocks from Dabieshan. According to their whole-rock δ18O values relative to the normal mantle δ18O values of 5.7±0.5‰, two groups are classified: (1) mafic granulite which exhibits lower δ18O values of 3.5 to 4.7‰, and (2) felsic granulite which shows higher δ18O values of 7.6 to 7.8‰. Consistent isotope temperatures of 800 to 900 °C are obtained for mineral pairs containing such refractory minerals as pyroxene, garnet, hornblende and iron oxides, suggesting the achievement and preservation of oxygen isotope equilibrium at the conditions of the peak granulite-facies metamorphism. This not only points to a rapid cooling and ascent for the granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the 18O-depletion in the mafic granulite. It is evident that the granulite rocks acquired the low δ18O values before the granulite-facies metamorphism by interaction with a certain 18O-depleted surface fluid. The surface fluid is assumed to exchange oxygen isotopes with the granulite protoliths prior to plate subduction. Fluid-absent metamorphism is suggested for the formation of the granulites on local scales. It is likely that the granulites together with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassic metamorphism but experienced differential two-stage uplifts prior to amphibolite-facies retrogression.

Preface to thematic issue: Petrotectonic characteristics of the Kokchetav Massif, northern Kazakhstan

Preface to thematic issue: Petrotectonic characteristics of the Kokchetav Massif, northern Kazakhstan

Transformation of Fe–Ti gabbro to coronite, eclogite and amphibolite in the Baie du Nord segment, Manicouagan Imbricate Zone, eastern Grenville Province

Fe–Ti gabbros from the Baie du Nord Segment of the Manicouagan Imbricate Zone, metamorphosed under high P–T conditions during the Grenvillian orogeny, have been the focus of a detailed micropetrological study. Textures and mineral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas, pseudomorphs after igneous phases (transitional) and true, granoblastic eclogites. The transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine. The deformed margins of coronitic Fe–Ti gabbros are transformed to amphibolite and contain clinopyroxene‐bearing leucosomes with garnet poikiloblasts that are indicative of high‐P–T dehydration melting. Interpretation of garnet zoning and thermobarometry suggest that the highest P–T conditions are recorded by coronas around xenocrysts (c. 720–800 °C at 14–17 kbar) and garnet–clinopyroxene cores in granoblastic assemblages (c. 740–820 °C at 13–17 kbar) in the eclogitic samples. Re‐equilibration during the early stages of exhumation at high‐T conditions (&gt;700 °C) affected all samples, and is evidenced by the widespread development of pargasite‐bearing plagioclase collars in the coronitic and transitional metagabbros and by widespread re‐equilibration of the eclogites giving lower P–T estimates at grain boundaries. However, the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressure (depth) from the coronites (11–13 kbar) to the eclogites (13–17 kbar). The P–T conditions recorded by these rocks define a metamorphic field gradient which suggests high heat flow through the lower crust during the Grenvillian orogeny.

Low differential stress during high-pressure metamorphism: The microstructural record of a metapelite from the Eclogite Zone, Tauern Window, Eastern Alps

Constraints on the state of stress at deep levels of convergent plate boundaries can be obtained from the microstructural record of high-pressure metamorphic rocks, provided that this record has remained unchanged during exhumation. The inclusion patterns in poikiloblastic garnet of a metapelite from the Eclogite Zone of the Tauern Window, Eastern Alps, reveal a foam microstructure of quartz controlled by grain boundary free energy. The garnet blasts show concentric chemical zonation. Thermobarometry based on correlation between compositional domains in garnet, chloritoid and phengite coexisting with kyanite, rutile and quartz shows that garnet growth commenced before peak P-T conditions close to 25 kbar and 600°C had been reached and continued during the early stage of decompression. This implies that the microstructure of quartz, controlled by grain boundary free energy, persisted continuously during the latest stage of burial to ca. 80 km depth and the early stage of exhumation, involving detachment from the downgoing slab and release into the subduction channel. The foam structure of the overgrown quartz indicates that the differential stress was too low to drive dislocation creep during that period, when the rock crossed the interface between the plates, with a high differential stress resulting in ‘tectonic overpressure’ or significant shear heating being ruled out. Outside the protective garnet the interfacial free energy controlled quartz microstructure has been destroyed completely due to dynamic recrystallization during deformation by dislocation creep. This happened while the crustal slice was exhumed to a depth of about 45 km. The preservation of this synkinematic quartz microstructure indicates a tectonic scenario where the final cooling was rapid and no marked stress peak was associated with the brittle-plastic transition.

Late orogenic extension of the Betic Cordillera and the Alboran Domain: A lithospheric view

The Betic Cordillera of southern Spain provides a clear example of a collisional orogen that has undergone large‐scale extensional collapse while convergent motion of the bounding plates continued. Extension was accommodated by coeval shortening in thin‐skinned fold and thrust belts around the periphery of the system, and much of the region has now subsided to form a large marine basin. The thermal and deformational record of these processes is preserved in rocks from the upper mantle, crystalline crust, and sedimentary cover. Upper mantle peridotites record evidence for exhumation in several stages from asthenospheric depths to the surface. Early stages of exhumation probably occurred during Mesozoic rifting. Cooling at midlithospheric depths reflects continental convergence, and subsequent heating indicates loss of most of the underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse. Crustal rocks in the internal zone of the Betic Cordillera were metamorphosed down to 50 km depth and are now exposed beneath major low‐angle normal detachment zones that separate them from heavily faulted low‐grade rocks above. Cooling ages of associated mylonites indicate that these detachments were active during the early to middle Miocene. Fault‐bounded intramontane basins, developed during the early to middle Miocene, contain coarse continental sediments heavily affected by normal fault systems, followed by a less deformed late Miocene marine succession. All of these phenomena can be explained by convective removal of the lithospheric root beneath a Paleogene collisional orogen, leading to large‐scale extension followed by thermal subsidence of the center of the system.