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.     

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.     

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.     

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.     

印度与欧亚两大陆块碰撞时间的厘定：来自锆石SHRIMP U Pb年龄的证据

通麦地区片麻岩中锆石SHRIMP 定年结果表明： ① 变质锆石和变质复合锆石的新壳UPb年龄为42 Ma,属于中始新世,相当于陆陆碰撞的峰期时间,而印度和欧亚两个大陆开始碰撞的时间略早于42 Ma; ② 变质复合锆石中的老核(继承锆石),有2 个点UPb年龄为201 Ma,相当于早侏罗世早期,表明通麦片麻岩是沉积岩变质的,其沉积岩的时代不会早于三叠纪; ③ 文章最后对锆石成因类型及意义、片麻岩的时代和印度与欧亚两大陆碰撞的时间等问题进行了讨论。     

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.     

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.     

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     

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.     

大别山高压-超高压片麻岩锆石的拉曼光谱学研究

对大别山高压-超高压片麻岩的锆石进行了激光拉曼探针、阴极发光(CL)、U-Pb年龄和U、Th等微量元素的测定分析.研究结果表明:锆石拉曼光谱1008cm-1峰的半高宽普遍小于20cm-1,指示锆石结晶程度较好到中等程度的蜕晶化;早期继承锆石在三叠纪高压-超高压变质作用中经历了退火和重结晶作用,所受的放射性通量是从200Ma的早侏罗纪开始积累的,表明高压-超高压岩石在这一时间已出露地表;北大别白垩纪岩浆热事件对高压-超高压岩石的影响很小;大多数锆石数据都落在天然锆石的放射性破坏趋势(RDT)中,表明锆石在高压、超高压过程中基本发生了完全的重结晶.     

A geothermobarometric investigation of garnet peridotites in the Western Gneiss Region of Norway

In the Western Gneiss Region of Norway are found numerous peridotite lenses which have been extensively recrystallized under amphibolite fades conditions during the Caledonian Orogeny. However, evidence for an earlier Caledonian high-pressure metamorphism has been recorded by abundant eclogite and granulite relicts within gneiss and by the presence of at least ten garnet perioditite bodies preserved within chlorite peridotites. Two garnet-bearing ultramafic assemblages have been recognized: olivine-orthopyroxene-clinopyroxene-garnet and olivine-ortho-pyroxene-pargasitic-amphibole-garnet.Except for olivine, minerals in the garnet peridotites are compositionally zoned, with relatively uniform cores and compositional gradients generally confined to the outer 200 micrometers, or less, of grains. The most common zoning patterns at grain margins are an increase in Fe/Mg in garnet, an increase in Al₂O₃ in orthopyroxene, and a decrease in Na₂O and Al₂O₃ in clinopyroxene, although there are exceptions to these patterns at two localities. These zoning patterns have developed mainly in response to cooling and decompression of the garnet peridotites.Application of geothermometers and barometers to the garnet peridotites has yielded temperatures of 770–860° C and pressures of 30–43 kb for cores of grains and consistently lower temperatures and pressures for rims, except for peridotites on Oterøy, where there is an apparent temperature increase from cores to rims.The petrologic and geothermobarometric evidence for most of the investigated garnet peridotites is compatible with their tectonic emplacement from the upper mantle into thickened continental crust during Caledonian collision of the Baltic and Greenland plates.     

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     

Applications of near-infrared FT-Raman spectroscopy in metamict and annealed zircon: oxidation state of U ions

We report a near-infrared Fourier-transform (FT) Raman spectroscopic method to characterize the electronic transitions of U ions and the alpha-decay damage in natural zircon. The application is demonstrated by analyzing metamict and annealed zircons from Sri Lanka. The data from crystalline zircon reveal a relatively sharp spectral feature appearing near 2733 cm^–1 in Stokes spectra with a laser excitation of 1064 nm. The feature is assigned as signals related to the previously reported U⁵⁺ absorption near 6668 cm^–1. With increasing self-irradiation dose, the feature shows a systematic decrease in intensity, accompanied by a gradual development of a broad feature between 3000 and 3400 cm^–1. On heating for 1 h, the U⁵⁺ feature shows an increase in intensity starting near ~700 K for partially metamict zircon, whereas for highly damaged zircon the first recovery of the feature takes place near 1000 K, accompanied by a decrease in the radiation-induced broad band. The changes observed in the present study reflect the variations of local environments of U ions in natural zircon during metamictization and thermal annealing.     

High abundance of early Archaean grains and the age distribution of detrital zircons in a sillimanite-bearing quartzite from Mt Narryer, Western Australia

SHRIMP U–Pb analyses are reported for a detrital zircon population from a sample of sillimanite-bearing quartzite from the Narryer sedimentary succession in the Narryer Terrane of the northwestern Yilgarn Craton. The detrital zircons define two distinctive age groups, an older group from 4000 Ga to 4280 Ma and a younger group from 3750 to 3250 Ma. The abundance of older group zircons of about 12% far exceeds the abundance of about 2% reported in the first discovery of ancient zircons in a quartzite from the Narryer metasediments, and is equivalent to the abundance of >3900 Ma zircons in metaconglomerate sample W74 from the Jack Hills, confirmed by new measurements reported in this paper. Most analyses of the Narryer and the Jack Hills detrital zircon populations are discordant. The Jack Hills zircon analyses are dominated by strong recent Pb loss whereas the Narryer zircon analyses have had a more complex history and have experienced at least one Pb loss event, possibly associated with the high-grade metamorphism at ca. 2700 Ma, and a further disturbance of the U–Pb systems during relatively recent times. Although the number of analyses is limited and many of the zircon analyses are discordant, the age distributions of the older (>3900 Ma) zircons from the Narryer and Jack Hills samples are different, suggesting a complex provenance for the ancient zircons. The distribution of ages in the younger population of Mt Narryer zircons is similar to that reported for zircons from the surrounding Meeberrie gneiss, supporting previous suggestions that zircons from the gneisses or their precursors were a major contributor to the detrital zircon suite. The younger zircon population from Jack Hills sample (W74), lacks the strong age peak from 3600 to 3750 Ma present in the Narryer zircon population, and conversely the strong zircon age group at ca. 3350–3500 Ma in the Jack Hills population is only weakly represented in the Narryer zircon population. The age distributions for the Narryer and the Jack Hills zircon populations are taken as benchmarks for comparing zircon populations from quartzite occurrences elsewhere in the Yilgarn Craton.     

New constraints on the origin and age of Variscan eclogitic rocks (Ligurian Alps,Italy)

Gabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneiss Variscan basement of the Savona Crystalline Massif (Ligurian Briançonnais, Italy). Whole rock trace element patterns, low initial εNd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at ~468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392–376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 ± 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration.     

印度与欧亚两大陆块碰撞时间的厘定:来自锆石SHRIMP U-Pb年龄的证据

通麦地区片麻岩中锆石SHRIMP定年结果表明:①变质锫石和变质复合锆石的新壳U-Pb年龄为42 Ma,属于中始新世.相当于陆陆碰撞的峰期时间,而印度和欧亚两个大陆开始碰撞的时间略早于42 Ma;②变质复合锫石中的老核(继承锆石),有2个点U-Pb年龄为201 Ma,相当于早侏罗世早期,表明通麦片麻岩是沉积岩变质的,其沉积岩的时代不会早于三叠纪;③文章最后对锆石成因类型及意义、片麻岩的时代和印度与欧亚两大陆碰撞的时间等问题进行了讨论.     

大别山榴辉岩带片麻岩的锆石拉曼光谱研究

我们对来自大别山榴辉岩带11个片麻岩样品的锆石进行了拉曼光谱研究,这些研究集中在1008和356cm^-1峰和它们的半高宽上。依据阴极发光（CL）图像特征,锆石可分为三种结构类型：（1）核边结构,由一个老的岩浆锆石核和一个变质边构成。也可由两期变质锆石构成;（2）CL不均匀强反差发光的锆石,这些锆石的一些部位明显发亮,这些发亮的部分是在早期岩浆锆石发育而来的;（3）CL总体是均匀的或弱反差发光锆石。在CL图像的基础上,我们用拉曼扫描的方法分析了这3类不同结构的锆石。1008和356cm^-1峰显示了相同或相似的图像特征,而它们的半高宽图像有相同或相似的图像特征。峰和半高宽图像可以有一定差异。核边结构的和CL不均匀强反差发光的锆石的CL结构也反映在拉曼光谱的图像上,半高宽的图像更相似于锆石的CL图像。CL总体是均匀的或弱反差发光锆石的拉曼光谱图像通常也是相对均匀的。11个样品锆石的1008cm^-1峰位值和它的半高宽落在放射破坏趋势上,我们认为早期继承的锆石在超高压变质过程中发生了完全退火和重结晶。一粒锆石拉曼光谱图像的结构反映的是不同的部分蜕晶化程度的差异,由于有相同的蜕晶化历史,这些部分的蜕晶化程度与这些部分的U和Th含量应该是正相关的。     

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.     

Raman confirmation of microdiamond in the Svartberget Fe‐Ti type garnet peridotite,Western Gneiss Region,Western Norway

Ultra‐high pressure metamorphic rocks have been found worldwide. The volume and areal extent of an exhumed UHPM domain are important for understanding the geodynamic mechanisms responsible for the high pressure and relatively medium temperature conditions needed for their creation. We report here Raman microspectroscopical data that prove the existence of microdiamonds at the Svartberget Fe‐Ti type peridotite locality in the Western Gneiss Region of Norway. Raman microscopy of two carbon microinclusions belonging to polyphase inclusion assemblages included in garnets from a garnet‐phlogopite websterite vein yielded a sharp, narrow, intense peak at 1332 cm^?1, characteristic of diamond. The diamond is associated with polyphase solid inclusions possibly originating from supercritical, dense, H‐C‐N‐O‐F‐P‐S‐Cl fluids. Lithological, textural and geochronological evidence points towards a Caledonian origin of the trapped fluid and subsequent diamond formation.     

大别-苏鲁造山带高级变质岩中锆石微区U，Th和Pb化学组成特征统计

对大别-苏鲁造山带27个高级变质岩(以榴辉岩和片麻岩为主)中锆石进行了337次微区离子探针U.Th和Pb测定和U-Pb年龄研究。根据测定点年龄的不同，可以把变质岩中锆石分成三种不同成因区域，一是变质岩原岩(继承)锆石区域，它遭受了不同程度的变质重结晶作用，其^206Pb/^238U年龄值大于277Ma；二是三叠纪高压.超高压变质阶段形成的变质锆石区域，其年龄限定为273-184Ma；三是碰撞造山变质作用后形成的锆石区域，其年龄小于148Ma。对三种类型锆石中U，Th和Pb特征分析发现：片麻岩锆石中U的含量是榴辉岩类锆石中U的丰度的两倍，片麻岩类和榴辉岩类锆石中Th的丰度大致相近。各类岩石变质阶段形成的变质锆石和原岩锆石区域相比，U含量有升有降，而Th含量则大幅下降，下降幅度一般在2-10倍，高者可达20-30倍以上。除石榴石橄榄岩外，各类岩石变质锆石中区域的Th/U比都小于0.1，这是判断高压-超高压变质成因锆石的极重要化学指标。大别山超高压变质阶段后形成的锆石的Th/U比多数类似于其原岩锆石，少数接近变质锆石，说明白垩纪后期热扰动对变质岩中已存在的锆石的影响如同岩浆结晶过程。各类岩石中锆石Pb的含量变化比较复杂，Pb-U正相关性表明锆石中普通铅含量很低，绝大部分为放射成因铅。