Evolution of Caledonian deformation fabrics under eclogite and amphibolite facies at Vårdalsneset, Western Gneiss Region, Norway

The Vårdalsneset eclogite situated in the Western Gneiss Region, SW Norway, is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation. The eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite and rutile. The rocks record a five‐stage evolution connected to Caledonian burial and subsequent exhumation. (1) A prograde evolution through amphibolite facies (T =490±63 °C) is inferred from garnet cores with amphibole inclusions and bell‐shaped Mn profile. (2) Formation of L>S‐tectonite eclogite (T =680±20 °C, P=16±2 kbar) related to the subduction of continental crust during the Caledonian orogeny. Lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L>S fabric was coaxial. (3) Formation of sub‐horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90°. Disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T =690±20 °C, P=15±1.5 kbar) with top‐W kinematics may indicate, however, that non‐coaxial deformation was also active at eclogite facies conditions. (4) Development of a widespread regional amphibolite facies foliation (T =564±44 °C, P<10.3–8.1 kbar), quartz veins and development of conjugate shear zones indicate that coaxial vertical shortening and sub‐horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions. (5) Amphibolite facies mylonites mainly formed under non‐coaxial top‐W movement are related to large‐scale movement on the extensional detachments active during the late‐orogenic extension of the Caledonides. The structural and metamorphic evolution of the Vårdalsneset eclogite and related areas support the exhumation model, including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust.     

Discrete ultrahigh-pressure domains in the Western Gneiss Region, Norway: implications for formation and exhumation

New eclogite localities and new ⁴⁰Ar/³⁹Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh‐pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km²; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 × 50 × 5 km. ⁴⁰Ar/³⁹Ar mica and K‐feldspar ages show that this outcrop pattern is the result of gentle regional‐scale folding younger than 380 Ma, and possibly 335 Ma. The UHP and intervening high‐pressure (HP) domains are composed of eclogite‐bearing orthogneiss basement overlain by eclogite‐bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc‐silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15‐cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near‐isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12–17 kbar and c. 750 °C during exhumation from mantle depths, followed by a low‐pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750 °C. New ⁴⁰Ar/³⁹Ar hornblende ages of 402 Ma document that this decompression from eclogite‐facies conditions at 410–405 Ma to mid‐crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20–30 km. ⁴⁰Ar/³⁹Ar muscovite ages of 397–380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30 °C Myr^?1), perhaps because of continued tectonic unroofing.     

Discovery of coesite–eclogite from the Nordøyane UHP domain,Western Gneiss Region,Norway: field relations,metamorphic history,and tectonic significance

Ultrahigh‐pressure (UHP) rocks from the Western Gneiss Region (WGR) of Norway record subduction of Baltican continental crust during the Silurian to Devonian Scandian continental collision. Here, we report a new coesite locality from the island of Harøya in the Nordøyane UHP domain, the most northerly yet documented in the WGR, and reconstruct the P–T history of the host eclogite. The coesite–eclogite lies within migmatitic orthogneiss, interpreted as Baltica basement, that underwent multiple stages of deformation and partial melting during exhumation. Two stages of metamorphism have been deduced from petrography and mineral chemistry. The early (M1) assemblage comprises garnet (Pyr_38–41Alm_35–37Grs_23–26Spss₁) and omphacite (Na_0.35–0.40Ca_0.57–0.60Fe²⁺_0.08–0.10Mg_0.53Fe³⁺_0.01Al^VI_0.40–0.42)₂(Al^IV_0.03–0.06Si_1.94–1.97)₂O₆, with subordinate phengite, kyanite, rutile, coesite and apatite, all present as inclusions in garnet. The later (M2) assemblage comprises retrograde rims on garnet (Pyr_38–40Alm_40–44Grs_16–21Spss₁), diopside rims on omphacite (Na_0.04–0.06Ca_0.88–0.91Fe²⁺_0.09–0.13Mg_0.81–83Fe³⁺_0.08Al^VI_0.03)₂(Al^IV_0.07–0.08Si_1.92–1.93)₂O₆, plagioclase, biotite, pargasite, orthopyroxene and ilmenite. Metamorphic P–T conditions estimated using thermocalc are ～3 GPa and 760 °C for M1, consistent with the presence of coesite, and ～1 GPa and 813 °C for M2, consistent with possible phengite dehydration melting during decompression. Comparison with other WGR eclogites containing the same assemblage shows a broad similarity in peak (M1) P–T conditions, confirming suggestions that large portions of the WGR were buried to depths of ～100 km during Scandian subduction. Field relations suggest that exhumation, accompanied by widespread partial melting, involved an early phase of top‐northwest shearing, followed by subhorizontal sinistral shearing along northwest‐dipping foliations, related to regional transtension. The present results add to the growing body of data on the distribution, maximum P–T conditions, and exhumation paths of WGR coesite–eclogites and their host rocks that is required to constrain quantitative models for the formation and exhumation of UHP metamorphic rocks during the Scandian collision.     

Tectonics of the Scandian orogeny and the Western Gneiss Region in southern Norway

Two crust-forming events dominate the Precambrian history of the Western Gneiss Region (WGR) at about 1800–1600 Ma and 1550–1400 Ma. The influence of the Sveconorwegian orogeny (1200–900 Ma) is restricted to the region south of Moldefjord-Romsdalen. A series of anorthosites and related intrusives are present, possibly derived from the now-lost western margin of the Baltic craton that may have been emplaced in the WGR as an allochthonous unit before the Ordovician.The Caledonian development is split into two orogenic phases, the Finnmarkian (Cambrian — Early Ordovician) and the Scandian (Late Ordovician/Early Silurian — Devonian). The lower tectonic units west of the Trondheim Trough may be Finnmarkian nappes ; they were part of the lower plate during the Scandian continental collision. The Blåhö nappe is correlated with dismembered eclogite bodies along the coast. A regional change of nappe transport direction from 090 to 135 marks the initiation of an orogen-parallel sinistral shear component around 425 Ma. The change caused the development of a complex sinistral strike-slip system in the Trondheim region consisting of the Möre-Tröndelag Fault Zone and the Gränse contact. The latter cut the crust underneath the already emplaced Trondheim Nappe Complex, thus triggering the intrusion of the Fongen-Hyllingen igneous complex, and initiating subsidence of the Trondheim Trough, and was subsequently turned from a strike-slip zone into an extensional fault. Minor southward transport of the Trondheim Nappe Complex rejuvenated some thrusts between the Lower and the Middle Allochthon. A seismic reflector underneath the WGR is interpreted to be a blind thrust which subcrops into the Faltungsgraben. During Middle Devonian orogenic collapse, detachment faulting brought higher units, now eroded elsewhere, down to the present outcrop level, such as the Bergen and Dalsfjord nappe and the Old Red basins.     

Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides

The Western Gneiss Region (WGR) marks the outcrop of a composite terrane consisting of variably re-worked Proterozoic basement and parautochthonous or autochthonous cover units. The WGR exhibits a gross structural, petrographic and thermobarometric zonation from southeast to northwest, reflecting an increasing intensity of Scandian (late Palaeozoic) metamorphic and structural imprint. Scandian-aged eclogites have been widely (though for kinetic reasons not invariably) stabilised in metabasic rocks but have suffered varying degrees of retrogression during exhumation. In the region between the Jostedal mountains and Nordfjord, eclogites commonly have distinctively prograde-zoned garnets with amphibolite or epidote–amphibolite facies solid inclusion suites and lack any evidence for stability of coesite (high pressure [HP] eclogites). In the south of this area, in Sunnfjord, eclogites locally contain glaucophane as an inclusion or matrix phase. North of Nordfjord, eclogites mostly lack prograde zoning and evidence for coesite, either as relics or replacive polycrystalline quartz, is present in both eclogites (ultrahigh pressure [UHP] eclogites) and, rarely, gneisses. Coesite or polycrystalline quartz after coesite has now been found in eight new localities, including one close to a microdiamond-bearing gneiss. These new discoveries suggest that, by a conservative estimate, the UHP terrane in the WGR covers a coastal strip of about 5000 km² between outer Nordfjord and Moldefjord. A “mixed HP/UHP zone” containing both HP and UHP eclogites is confirmed by our observations, and is extended a further 40 km east along Nordfjord. Thermobarometry on phengite-bearing eclogites has been used to quantify the regional distribution of pressure (P) and temperature (T) across the WGR. Overall, a scenario emerges where P and T progressively increase from 500°C and 16 kbar in Sunnfjord to >800°C and 32 kbar in outer Moldefjord, respectively, in line with the distribution of eclogite petrographic features. Results are usually consistent with the silica polymorph present or inferred. The P–T conditions define a linear array in the P–T plane with a slope of roughly 5°C/km, with averages for petrographic groups lying along the trend according to their geographic distribution from SE to NW, hence defining a clear field gradient. This P–T gradient might be used to support the frequently postulated model for northwesterly subduction of the WGC as a coherent body. However, the WGC is clearly a composite edifice built from several tectonic units. Furthermore, the mixed HP/UHP zone seems to mark a step in the regional P gradient, indicating a possible tectonic break and tectonic juxtaposition of the HP and UHP units. Lack of other clear evidence for a tectonic break in the mixed zone dictates caution in this interpretation, and we cannot discount the possibility that the mixed zone is, at least, partly a result of kinetic factors operating near the HP–UHP transition. Overall, if the WGC has been subducted during the Scandian orogeny, it has retained its general down-slab pattern of P and T in spite of any disruption during exhumation. Garnetiferous peridotites derived from subcontinental lithospheric mantle may be restricted to the UHP terrane and appear to decorate basement-cover contacts in many cases. P–T conditions calculated from previously published data for both relict (Proterozoic lithospheric mantle?) porphyroclast assemblages and Scandian (subduction-related?) neoblastic assemblages do not define such a clear field gradient, but probably record a combination of their pre-orogenic P–T record with Scandian re-working during and after subduction entrainment. A crude linear array in the P–T plane defined by peridotite samples may be, in part, an artifact of errors in the geobarometric methods. A spatial association of mantle-derived peridotites with the UHP terrane and with basement-cover contacts is consistent with a hypothesis for entrainment of at least some of them as “foreign” fragments into a crustal UHP terrane during subduction of the Baltic continental margin to depths of >100 km, and encourages a more mobilistic view of the assembly of the WGC from its component lithotectonic elements.     

Deformation-enhanced metamorphic reactions and the rheology of high-pressure shear zones, Western Gneiss Region, Norway

Microstructural and petrological analysis of samples with increasing strain in high‐pressure (HP) shear zones from the Haram garnet corona gabbro give insights into the deformation mechanisms of minerals, rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions. Scanning electron microscopy with electron backscattering diffraction (SEM–EBSD), compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization. Plagioclase with a shape‐preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the (0 0 1)[1 0 0] slip system. In omphacite, crystallographic preferred orientations indicate slip on (1 0 0)[0 0 1] and (1 1 0)[0 0 1] during deformation. Fine‐grained garnet deformed by diffusion creep and grain‐boundary sliding. Ilmenite deformed by dislocation glide on the basal and, at higher strains, prism planes in the a direction. Relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500–650 °C and 0.5–1.4 GPa and continued during prograde metamorphism to ultra‐high‐pressure (UHP) conditions. Both products and reactants show evidence of syn‐ and post‐kinematic growth indicating that prograde reactions continued after strain was partitioned away. The restriction of post‐kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn‐kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away, demonstrating that deformation played an important role in enhancing metamorphic reactions. Two important consequences of deformation observed in these shear zones are: (i) the homogenization of chemical composition gradients occurred by mixing and grain‐boundary migration and (ii) composition changes in zoned metamorphic garnet by lengthening diffusion distances. The application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100–150 MPa and lower limits for strain rates of 10^?12 to 10^?13 s^?1 as deformation conditions for the shear zones in the Haram gabbro that were produced during subduction of the Baltica craton and resulted in the production of HP and UHP metamorphic rocks.     

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh-pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well-preserved phengite-bearing eclogite sample from the Nordfjord region. The sample was investigated using phase-equilibrium modelling, trace-element analyses of garnet, trace- and major-element thermobarometry and quartz-in-garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near-isobaric heating to 660–680°C. Near-isothermal decompression to 10–14 kbar is recorded in fine-grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end-member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny.     

Structural,petrological and chemical analysis of syn‐kinematic migmatites: insights from the Western Gneiss Region,Norway

Evidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P (ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H₂O‐rich fluid phase at great depth to facilitate H₂O‐present partial melting.     

Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

The Solund–Hyllestad–Lavik area affords an excellent opportunity to understand the ultrahigh‐pressure Scandian orogeny because it contains a near‐complete record of ophiolite emplacement, high‐pressure metamorphism and large‐scale extension. In this area, the Upper Allochthon was intruded by thec. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410–400 Ma. Within < 5–10 Myr, all these units were exhumed by the Nordfjord–Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near‐ultrahigh‐pressure rocks to within c. 3 km vertical distance from the low‐grade Solund Conglomerate.     

High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction.     

Transformation of Archaean Lithospheric Mantle by Refertilization: Evidence from Exposed Peridotites in the Western Gneiss Region, Norway

Orogenic peridotites occur enclosed in Proterozoic gneissesat several localities in the Western Gneiss Region (WGR) ofwestern Norway; garnet peridotites typically occur as discretezones within larger bodies of garnet-free, chromite-bearingdunite and are commonly closely associated with pyroxenitesand eclogites. The dunites of the large Almklovdalen peridotitebody have extremely depleted compositions (Mg-number 92–93·6);the garnet peridotites have lower Mg-number (90·6–91·7)and higher whole-rock Ca and Al contents. Post-depletion metasomatismof both rock types is indicated by variable enrichment in thelight rare earth elements, Th, Ba and Sr. The dunites can bemodelled as residues after very high degrees (>60%) of meltextraction at high pressure (5–7 GPa), inconsistent withthe preservation of lower degrees of melting in the garnet peridotites.The garnet peridotites are, therefore, interpreted as zonesof melt percolation, which resulted in refertilization of thedunites by a silicate melt rich in Fe, Ca, Al and Na, but notTi. Previous Re–Os dating gives Archaean model ages forthe dunites, but mixed Archaean and Proterozoic ages for thegarnet peridotites, suggesting that refertilization occurredin Proterozoic time. At least some Proterozoic lithosphere mayrepresent reworked and transformed Archaean lithospheric mantle. KEY WORDS: Archaean mantle; Proterozoic mantle; Western Gneiss Region, Norway; mantle metasomatism; garnet peridotite     

造山带挤出构造与高压-超高压岩石剥露机制:以大别山为例

造山带挤出构造阐述了被边界断裂所围限的造山带深变质块体,在造山带内部垂向和(或)侧向应力的作用下折返变形的过程。研究主要集中在挤出块体的几何形态及其内部变形样式、边界断裂特征、挤出路径以及挤出动力来源等4个方面,其研究目的主要是为了解决造山带深变质岩石折返剥露的机制问题。依据挤出块体的挤出方向与造山带主体走向之间的关系,在三维球形坐标系Lx-Ly-Lz中,将造山带挤出构造大致分为7个端员类型(Ⅰ型～Ⅶ型)。其中Lx为造山带或俯冲带的主体走向;Ly呈水平方向并与Lx相垂直;Lz垂直于Lx和Ly所构成的平面。这些基本端员类型的组合及其之间的过渡类型可以详尽地诠释大别山印支期高压-超高压岩石的挤出过程。其中榴辉岩相挤出阶段介于Ⅳ型与Ⅶ型挤出构造之间,角闪岩相挤出阶段介于Ⅱ型与Ⅵ型挤出构造之间并可能具有渠道流挤出模式,而绿片岩相挤出阶段类似于Ⅴ型挤出构造。     

The timing of stabilisation and the exhumation rate for ultra-high pressure rocks in the Western Gneiss Region of Norway

New petrographic evidence and a review of the latest radiometric age data are taken to indicate that formation of the ultra‐high pressure (UHP) eclogites within the Western Gneiss Region of Norway probably occurred within the 400–410 Ma time frame. Thus, this event took place significantly later than the previous, widely accepted age of c. 425 Ma for the timing of the high pressure metamorphism in this part of the Scandinavian Caledonides. Garnet growth under UHP (coesite‐stable) conditions is recognised as a discrete, younger event following on from earlier garnet formed under firstly amphibolite facies then quartz‐stable, eclogite facies conditions. Currently, the best constrained and most precise age, specifically for UHP mineral growth, is the 402 ± 2 Ma U–Pb age for metamorphic zircon (some of which retain coesite inclusions) from the Hareidland eclogite. Exhumation must have followed shortly thereafter and, based on synoptic pressure–temperature and depth–time curves, must have been very fast. Our data and those of others indicate an initial fast exhumation to about 35 km depth by about 395 Ma at a mean rate of about 10 mm a^?1. This rapid exhumation rate may have been driven by the appreciable residual buoyancy of the deeply subducted continental crustal slab due to incomplete eclogitization of the dominant Proterozoic orthogneisses during the short‐lived UHP event. Subsequent exhumation to 8–10 km depth by about 375 Ma occurred at a much slower mean rate of about 1.3 mm a^?1 with the late‐stage extensional collapse of the Caledonian orogen playing an increasingly important role, especially in the final unroofing of the Western Gneiss Region with some remarkably preserved UHP rocks.     

The strain‐dependent spatial evolution of garnet in a high‐P ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X‐ray microtomography study

Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono‐metamorphic processes, however, they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron‐based X‐ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite–garnet eclogite in four dimensions, where the 4^th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain. The microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. The microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X‐ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low‐strain domain, garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow, and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet shows a near‐random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo‐nucleation and overgrowth. Microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies, becoming more almandine‐rich. We interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization, and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming.     

Fast exhumation of the ultrahigh-pressure Alpe Arami garnet peridotite (Central Alps, Switzerland): constraints from geospeedometry and thermal modelling

Previous studies suggest that the metamorphic evolution of the ultrahigh‐pressure garnet peridotite from Alpe Arami was characterized by rapid subduction to a depth of c. 180 km with partial chemical equilibration at c. 5.9 Gpa/1180 °C and an initial stage of near‐isothermal decompression followed by enhanced cooling. In this study, average cooling rates were constrained by diffusion modelling on retrograde Fe–Mg zonation profiles across garnet porphyroclasts. Considering the effects of temperature, pressure and garnet bulk composition on the Fe–Mg interdiffusion coefficient, cooling rates of 380–1600 °C Myr^?1 for the interval from 1180 to 800 °C were obtained. Similar or even higher average cooling rates resulted from thermal modelling, whereby the characteristics of the calculated temperature‐time path depend on the shape and size of the hot peridotite body and the boundary conditions of the cooling process. The very high cooling rates obtained from both geospeedometry and thermal modelling imply extremely fast exhumation rates of c. 15 mm yr^?1 or more. These results agree with the range of exhumation rates (16–50 mm yr^?1) deduced from geochronological results. It is suggested that the Alpe Arami peridotite passively returned towards the surface as part of a buoyant sliver, caused as a consequence of slab breakoff.     

大别山超高压变质岩的冷却史及折返机制

大别山超高压变质岩及其围岩 T-t 冷却曲线显示了超高压变质岩的冷却史从800℃到300℃经历了三个阶段:两次快速冷却(226±3Ma 到219±7Ma 期间从800℃到500℃的第一次快速冷却,180～170Ma 期间从450℃到300℃的第二次快速冷却)和介于二者之间的等温过程。这一具有两次快速冷却的 T-t 曲线已被近年来获得的高精度金红石 U-Pb 年龄(218±1.2Ma)(Li et al.,2003),高压变质和退变质独居石 Th-Pb 年龄(Ayers et al.,2002),和强面理化榴辉岩二次多硅白云母的Rh-Sr 年龄(182.7±3.6Ma)(Li et al.,2001)所证实。超高压变质岩的二次快速冷却事件反映了二次快速抬升过程。在东秦岭及苏鲁地体东端发育的同碰撞花岗岩 U-Ph 年龄为225～205Ma,与超高压变质岩第一次快速冷却时代吻合。考虑到同碰撞花岗岩与俯冲板片断离的成因联系,这种时代耦合关系表明俯冲板片断离可能是超高压变质岩第一次快速抬升和冷却的重要机制之一。大别山 Pb 同位素填图揭示出南大别带超高压变质岩具有高放射成因 Pb 特征,因而源于俯冲的上地壳;而北大别带超高压变质岩具有低放射成因 Pb 特征,源于俯冲长英质下地壳。这表明在陆壳俯冲过程中上、下地壳之间可发生挤离(detachment)或脱耦(decoupling)。已有实验证明脱耦的上地壳在俯冲过程中可沿挤离面逆冲抬升(Chemenda et al.,1995)。同理,由于俯冲镁铁质下地壳在大别山没有出露,可以推测俯冲长英质下地壳和镁铁质下地壳之间也最终发生了挤离或脱耦。大陆岩石圈在不同深度存在若干低粘度带(Meissner and Mooney,1998)是上述俯冲陆壳分层脱耦现象发生的依据。因此,俯冲上地壳及部分长英质下地壳的第一次快速抬升折返是俯冲过程中大陆地壳内部分层脱耦和俯冲板片断离的综合结果。上述过程只能使已脱耦的上地壳及部分长英质下地壳抬升折返,而未与俯冲岩石圈脱耦的下地壳在板片断离后仍可继续俯冲。俯冲板片断离后,两大陆块在晚三叠世和早-中侏罗世继续汇聚,导致华南陆块下地壳继续俯冲,及已经脱耦并折返至中上地壳的超高压岩片向北仰冲。这一仰冲可能是导致超高压变质岩第二次快速抬升的重要机制。强面理化榴辉岩二次多硅白云母的 Rb-Sr 年龄(182.7±3.6Ma)可能记录了这一超高压岩片仰冲事件发生的时代。惠兰山基性麻粒岩年代学研究揭示了罗田穹隆在早白垩世的快速抬升,与此同时大别山发生了大规模岩浆事件。山体快速抬升与大规模岩浆事件的耦合关系指示了大别造山带早白垩世的去根作用,或岩石圈拆离事件。伴随这一山体快速抬升,大别山超高压变质岩开始大面积出露地表。     

Evolution of metamorphic volatiles during exhumation of microdiamond-bearing granulites in the Western Gneiss Region, Norway

Fluid inclusions in garnet, kyanite and quartz from microdiamond-bearing granulites in the Western Gneiss Region, Norway, document a conspicuous fluid evolution as the rocks were exhumed following Caledonian high- and ultrahigh-pressure (HP–UHP) metamorphism. The most important of the various fluid mixtures and daughter minerals in these rocks are: (N₂ + CO₂ + magnesian calcite), (N₂ + CO₂ + CH₄ + graphite + magnesian calcite), (N₂ + CH₄), (N₂ + CH₄ + H₂O), (CO₂) and (H₂O + NaCl + CaCl₂ + nahcolite). Rutile also occurs in the N₂ + CO₂ inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions. Volatiles composed of N₂ + CO₂ + magnesian calcite characterise the ambient metamorphic environment between HP–UHP (peak) and early retrograde metamorphism. During progressive decompression, the mole fraction of N₂ increased in the fluid mixtures; as amphibolite-facies conditions were reached, CH₄ and later, H₂O, appeared in the fluids, concomitant with the disappearance of CO₂ and magnesian calcite. Graphite is ubiquitous in the host lithologies and fluid inclusions. Thermodynamic modelling of the metamorphic volatiles in a graphite-buffered C-O-H system demonstrates that the observed metamorphic volatile evolution was attainable only if the f _O2 increased from c. −3.5 (±0.3) to −0.8 (±0.3) log units relative to the FMQ oxygen buffer. External introduction of oxidising aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the f _O2. The oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental sediments previously subducted during an extended period of continental collision in conjunction with the Caledonian orogeny. Received: 15 December 1997 / Accepted: 25 May 1998     

The role of structural inheritance in continental break-up and exhumation of Alpine Tethyan mantle(Canavese Zone,Western Alps)

The Canavese Zone(CZ)in the Western Alps represents the remnant of the distal passive margin of the Adria microplate,which was stretched and thinned during the Jurassic opening of the Alpine Tethys.Through detailed geological mapping,stratigraphic and structural analyses,we document that the continental break-up of Pangea and tectonic dismemberment of the Adria distal margin,up to mantle rocks exhumation and oceanization,did not simply result from the syn-rift Jurassic extension but was strongly favored by older structu ral inheritances(the Proto-Canavese Shear Zone),which controlled earlier lithospheric weakness.Our findings allowed to redefine in detail(i)the tectono-stratigraphic setting of the Variscan metamorphic basement and the Late Carbonife rous to Early Cretaceous CZ succession,(ii)the role played by inherited Late Carboniferous to Early Triassic structures and(iii)the significance of the CZ in the geodynamic evolution of the Alpine Tethys.The large amount of extensional displacement and crustal thinning occurred during different pulses of Late Carbonife rous-Early Triassic strike-slip tectonics is wellconsistent with the role played by long-lived regional-scale wrench faults(e.g.,the East-Variscan Shear Zone),suggesting a re-discussion of models of mantle exhumation driven by low-angle detachment faults as unique efficient mechanism in stretching and thinning continental crust.     

The formation of foliated (garnet-bearing) granites in the Tongbai-Dabie orogenic belt: partial melting of subducted continental crust during exhumation

Foliated (garnet-bearing) (FGB) granites are associated closely with and are usually the major wall rocks of the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Tongbai-Dabie region, the mid segment of the Qinling-Dabie-Sulu orogenic belt in central China. These granites appear either as small plutons or as veins, which commonly intrude into or surround the HP and UHP metamorphic eclogites or gneisses. The veins of FGB granites usually penetrate into the retrograded eclogites or gneisses along the foliations. Condensation rims can occasionally be found along the margins of granite veins. These granites are rich in Si and alkali with high Ga/Al ratios, and depleted in Ca, Mg, Al, Ti, Sc, V, Ni, Co, Cr and Sr, which are similar to A-type granites. In a chondrite normalized diagram, the samples are light rare earth elements enriched with different extent of negative Eu anomaly. Moreover, Rb, Nb, Ta, Sr, P and Ti show different degrees of negative anomalies, whereas Ba, K, La, Zr and Hf show positive anomalies in the primitive mantle normalized diagram. Negative anomalies of Eu and Sr indicate strong influence of plagioclase. In conventional discrimination diagrams, these FGB granites belong to the A-type granite, with geochemical characteristics affinitive to post-collisional granites. The εNd (230 Ma) values (−15.80 to −2.52) and T _DM values (1.02–2.07 Ga) suggest that magma for the FGB granites were derived from a heterogeneous crustal source. Therefore, the FGB granites may provide clues for deciphering the formation of post-collisional granites. It is proposed that the magma of the FGB granites both in the HP and UHP units was formed in an extensional tectonic setting slightly post-dating the HP and UHP metamorphism, most likely as a result of decompressional partial melting of UHP retrograded eclogites during exhumation.