Epidote-amphibolite Facies Research Articles

Syn‐metamorphic folding in the Tauern Window, Austria dated by Th–Pb ages from individual allanite porphyroblasts

AbstractHigh‐precision 232Th–208Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn‐metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote–amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five 232Th–208Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (<1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear‐cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, 232Th–208Pb dating on microsamples offers a powerful geochronological tool.

Petrogenesis and chemogenesis of oceanic and continental orogens in Asia: Current topics, Part I

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments in the Besshi region, central Shikoku, Southwest Japan

AbstractThis report describes an investigation of the composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments, and presents discussions of the possible distribution of eclogite facies lithologies in the Besshi region, central Shikoku, southwest Japan. Garnet grains usually show chemically composite zoning with resolved inner and overgrown outer segments, respectively. The inner segment usually contains paragonite as a sodic phase inclusion and rarely contains omphacite or glaucophane, whereas the outer segment rarely includes albite but no paragonite. The inner segment often includes quartz grains that preserve high residual pressures corresponding to the eclogite facies conditions, whereas the outer segment includes quartz grains that preserve lower residual pressures corresponding to the epidote–amphibolite facies conditions. For this composite‐zoned garnet, the assemblage of sodic phase inclusions and the values of the residual pressure of quartz inclusions imply the following successive metamorphic path: prograde eclogite facies stage → decompression and hydration reaction stage → prograde epidote–amphibolite facies stage. Metasediments containing this composite‐zoned garnet with such evidence for eclogite facies conditions are more widely distributed than the eclogitic lithologies that contain the assemblage of omphacite + garnet. These results imply that the combination of (i) the chemical composite zoning of garnet, (ii) the nature of sodic phase inclusions, and (iii) the residual pressure of quartz inclusions is useful as criteria to ascertain the true distribution of the lithologies that have experienced eclogite facies metamorphism for the Sambagawa metasediments, which was difficult to recognize in earlier studies.

Geochemistry of the Osnitsk-Mikashevichy volcanoplutonic complex of the Ukrainian shield

This paper addresses the geochemistry of intrusive (Osinitsk complex) and volcanic (Klesov Group) rocks of the Osnitsk-Mikashevichy volcanoplutonic belt (OMVPB) of the Ukrainian shield, which is an active continental margin existing approximately 1980–2000 Ma ago. The Osnitsk complex comprises a wide range of rocks, from ultrabasics to granitoids, and the Klesov Group is dominated by extrusive rocks of basaltic and rhyolitic compositions metamorphosed under epidote-amphibolite facies conditions. The Sr-Nd-Hf isotopic systematics (ɛSr1990 from −4 to +10, ɛNd1990 from −0.6 to +2.3, and ɛHf1990 from 0.1 to 1.4) indicates a juvenile source for the OMVPB rocks. Geochemical data suggest independent origin of the rocks of gabbroid (SiO2 60 wt %) series. The gabbroids are subdivided into pre- and post-granite groups on the basis of the higher contents of incompatible trace elements and lower contents of compatible elements in the post-granite rocks. The geochemical characteristics of the two groups of basic rocks indicate their formation in a convergent tectonic setting. The origin of the granitoid melts is attributed to the low-degree (eutectoid) melting of basic rocks at relatively low temperatures.

Permian age of the Wudaogou Group in eastern Yanbian: detrital zircon U–Pb constraints on the closure of the Palaeo-Asian Ocean in Northeast China

The Wudaogou Group in eastern Yanbian, Northeast China, plays a key role in constraining the timing and eastward termination of the Solonker–Xra Moron River–Changchun Suture, where the Palaeo-Asian Ocean closed. The Wudaogou Group consists of schist, gneiss, amphibolite, metasedimentary, and metavolcanic rocks, all of which underwent greenschist- to epidote–amphibolite-facies regional metamorphism, with some hornfels resulting from contact metamorphism. To determine the age of deposition, the timing and grade of metamorphism, and the tectonic setting of the Wudaogou Group, we investigated the petrography and geochronology of the metamorphic rocks in this group. Zircons from the metasedimentary rocks of this group can be divided into metamorphic zircons and detrital zircons of magmatic origin. U–Pb ages of metamorphic zircons dated by LA-ICP-MS vary from 249 ± 4 to 266 ± 4 Ma, approximating the age of regional metamorphism in the eastern Yanbian area. Detrital zircons yield U–Pb ages ranging from 253 ± 5 to 818 ± 5 Ma, and indicate that the provenance of the Wudaogou Group experienced four tectonic–thermal events between 818 and 253 Ma: Neoproterozoic (ca. 818–580 Ma), Cambro–Ordovician (ca. 500–489 Ma), Devonian–Carboniferous (ca. 422–300 Ma), and middle–late Permian (ca. 269–253 Ma). The youngest detrital zircon, with a U–Pb age of 253 ± 5 Ma, defines the maximum depositional age of the Wudaogou Group. The presence of the Cambro-Ordovician and Neoproterozoic detrital zircons implies that the source of the Wudaogou Group had an affinity with Northeast China, which leads us to conclude that the Solonker–Xra Moron River–Changchun Suture extends from Wangqing to Hunchun in eastern Yanbian, and that the Palaeo-Asian Ocean may have closed at the end of the Permian or Early Triassic period.

Re-Os Geochronology of the Hitachi Volcanogenic Massive Sulfide Deposit: The Oldest Ore Deposit in Japan

The Hitachi volcanogenic massive sulfide (VMS) deposit in the Hitachi belt is one of the largest Cu-Zn sulfide deposits in Japan. However, its primary formation age has not been precisely determined because regional and contact metamorphism have disturbed some radiometric isotope systems, e.g., K-Ar ages on wall rocks. Further metamorphism has greatly reduced the fossil record, which has limited the use of biostratigraphy. To establish the ore deposit age, we applied Re-Os isotope dating to samples from two representative sulfide deposits of the greater Hitachi VMS deposit: the Fudotaki and Fujimi deposits. Re-Os isotope data from the Fudotaki deposit yields a model 3 age of 533 ± 13 Ma, with an initial 187 Os/ 188 Os ratio of 0.35 ± 0.20. In contrast, Re-Os isotope data from the Fujimi sulfide ore did not exhibit a linear correlation, suggesting that the Re-Os isotope system there has been disturbed by regional metamorphism of higher grade than the Fudotaki deposit as well as contact metamorphism during intrusion of Cretaceous granitic rocks. Although the Fudotaki sulfide ores have been metamorphosed to lower epidote-amphibolite facies, the well-defined Re-Os isochron allowed us to interpret the timing of primary sulfide deposition on the paleoseafloor. Some orebodies of the greater Hitachi VMS deposit have been estimated to be Early Carboniferous based on the scarce fossil records and considered to be distributed in the Daioin Formation, but our results revealed that the Fudotaki deposit were formed during the Cambrian and occurs in the Upper Akazawa Formation, which strongly supports the idea that the Cambrian beds are widely distributed in the Hitachi belt and that a significant unconformity exists between the Cambrian and Early Carboniferous above the Fudotaki deposit. The Cambrian Re-Os date classifies the Hitachi VMS deposit as the oldest dated ore deposit in Japan, and in turn provides an insight to understand the geologic framework and genesis of the proto-Japanese islands and the East Asian region.

Late Paleozoic onset of subduction and exhumation at the western margin of Gondwana (Chilenia Terrane): Counterclockwise P–T paths and timing of metamorphism of deep-seated garnet–mica schist and amphibolite of Punta Sirena, Coastal Accretionary Complex, central Chile (34° S)

In this study, the Paleozoic albite–epidote–amphibolite occurring as meter-sized intercalations within garnet–mica schist at Punta Sirena beach (Pichilemu region, central Chile) is characterized for the first time. These rocks constitute an unusual exposure of subduction-related rocks within the Paleozoic Coastal Accretionary Complex of central Chile. Whereas high pressure (HP) greenschist and cofacial metasediments are the predominant rocks forming the regional metamorphic basement, the garnet–mica schist and amphibolite yield higher P–T conditions (albite–epidote amphibolite facies) and an older metamorphic age. Combining detailed mineral chemistry and textural information, P–T calculations and Ar–Ar ages, including previously published material from the Paleozoic Accretionary Complex of central Chile, we show that the garnet–mica schist and associated amphibolite (locally retrograded to greenschist) are vestiges of the earliest subducted material now forming exotic bodies within the younger HP units of the paleo-accretionary wedge. These rocks are interpreted as having been formed during the onset of subduction at the southwestern margin of Gondwana. However, we show that the garnet–mica schist formed at a slightly greater depth (ca. 40 km) than the amphibolite (ca. 30 km) along the same hot-subduction gradient developed during the onset of subduction. Both lithotypes reached their peak-P conditions at ca. 335–330 Ma and underwent near-isobaric cooling followed by cooling and decompression (i.e., counterclockwise P–T paths). The forced return flow of the garnet–mica schist from the subduction channel started at ca. 320 Ma and triggered the exhumation of fragments of shallower accreted oceanic crust (amphibolite). Cores of phengite (garnet–mica schist) and amphibole (amphibolite) grains have similar chemical compositions in both the S1 and S2 domains, indicating rotation of these grains during the transposition of the burial-related (prograde peak-T) foliation S1 by the non-coaxial exhumation-related foliation S2. During exhumation and retrograde D2 deformation, the garnet–mica schist and amphibolite were tectonically mingled at a depth of ca. 30 km at ca. 315 Ma. We propose that the Punta Sirena unit comprises a “pseudo”-coherent sequence formed by heterogeneous lithologies that followed non-chaotic exhumation mingling, now representing the remnants of the fossil subduction channel developed at the onset of the Late Paleozoic subduction at central Chile.

Isotope composition of strontium, carbon, and oxygen in magnesian carbonates of the Onot greenstone belt

341 The Onot greenstone belt (OGB) forms a trough (paleorift) structure within the Prisayan marginal block of the Siberian Platform basement (Fig. 1). The OGP occurs on ancient tonalite–trondhjemite–gran odiorite gneiss containing zircons with a U–Pb age of ≈3.34 Ga [1] and is intruded by Early Proterozoic granite of the Shumikhin Complex (the U–Pb age of zircons is 1.87 Ga [2]). The belt is composed of metavolcanic rocks of the tholeiite–rhyolite series, metaterrigenous and biotite–garnet gneisses, amphib olite, quartzite, and magnesite and dolomite marbles [3, 4]. Rocks are metamorphosed under the condi tions of the amphibolite and epidote–amphibolite facies and have undergone intense ultrametamorphic and metasomatic transformations. The age of the pro tolith of metarhyolite from the OGB is 2.89 Ga (U– Pb age by zircon) [5]. The Rb–Sr age of ultrameta morphic granitoids from the Onot Complex is 2.64 Ga [6]. Thus, the supracrustal complex of the OGB is one of the oldest in Asia, and its age is consistent with that of typical Archean greenstone belts of the Earth.

Compositional zoning and inclusions of garnet in Sanbagawa metapelites from the Asemi-gawa route, central Shikoku, Japan

Compositional and textural characteristics of the zonal structure and inclusions of garnet in metapelites from areas of high-grade greenschist facies (garnet zone) to areas of epidote-amphibolite facies (albite-biotite and oligoclase biotite zones) located along the Asemi-gawa route were examined using EPMA and Raman spectroscopy to investigate evolution of the P-T history of Sanbagawa metamorphism. Garnet grains exhibit two types of compositional zoning patterns: bell-shaped and composite types with spessartine variation. Bell-shape zoning is characterized by a monotonous decrease in the spessartine component from the crystal core towards the margin. Almandine and pyrope components exhibit the opposite trend. The grossular content reaches a maximum at an intermediate position between the core and the rim, and then decreases towards the outermost rim. The composite-zoned garnet is divided into core and mantle parts and it exhibits discontinuous compositional variations according to the associated boundary, evidencing resorption of the core and overgrowth of the mantle during crystallization; implying a two-stage growth of garnet during the Sanbagawa metamorphism. Composite-zoned garnet is characteristically observed in metapelites from a part of the albite-biotite zone, and quartz grains included in its core retain a higher residual pressure than those in garnet from other mineral zones. Isolated paragonite crystal occurs particularly as an inclusion in the core part of this composite-zoned garnet. These data suggest that part of the Sanbagawa metamorphic rocks in the Asemi-gawa region recrystallized under higher-pressure conditions up to the eclogite facies, prior to regional metamorphism from the greenschist, to the epidote-amphibolite facies that formed the regional thermal structure of the Sanbagawa belt of central Shikoku.

Geochemistry, age, and petrogenesis of rocks from the Garevka metamorphic complex, Yenisey Ridge

The mineralogical, petrological, geochemical and geochronological data on the Garevka metamorphic complex (GMC) of the Yenisey Ridge were used to evaluate the age, nature, and provenance of their protoliths. The evolution of the GMC occurred in two stages with different ages, thermodynamic regimes, and metamorphic field gradients. The final emplacement of granitoids was marked by high-pressure (HP) amphibolite facies regional metamorphism (970 Ma). At the second stage, these rocks experienced Late Riphean (900–870 Ma) retrograde epidote-amphibolite facies metamorphism accompanied by the formation of blastomylonitic complexes within narrow zones of brittle-ductile deformation. The metamorphism of migmatites (850 Ma) is coeval with the collisional medium-pressure metamorphism of the kyanite-sillimanite type. The GMC is different from the other rock complexes of the Yenisey Ridge in the presence of rapakivi-type granites. The geochemistry of these rocks, which is characterized by stronger enrichment in K2O, FeO, Y, Th, U, Zr, Hf, Nb, Ta, and REE relative to the other mineral assemblages of the GMC, is typical of anorogenic (A-type) within-plate granites. Among other distinctive features of these rocks are the strong iron enrichment of the melanocratic minerals, the presence of ilmenite as the sole Fe-Ti oxide, and crystallization from higher temperature (T = 825°C vs. T = 750°C) water-poor magmas under reducing conditions below the FMQ buffer. Significant variations in the geochemical and petrological characteristics of the GMC rocks suggest that they could not be derived from a single source. The main volume of the high-K rocks varying in composition from A-type to S-type granites was generated by melting of mixed mantlecrustal sources. The products of melting of the Late Archean-Early Proterozoic infracrustal gneisses of the Siberian Craton could be a possible source for the least oxidized rocks.

High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 1. Petrography and mineralogy

Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al2O3. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.

New age constraints for the geodynamic evolution of the Sistan Suture Zone, eastern Iran

The Sistan Suture Zone (SSZ) in eastern Iran extends as a N–S trending belt over more than 700km along the border area between Iran and Afghanistan. The SSZ formed as a result of eastward-directed subduction of a Neotethyan ocean basin beneath the Afghan block and includes a tectonic mélange consisting of disrupted meta-ophiolitic rocks within a low-grade matrix of ultramafic, mafic and pelitic schists. Some mélange blocks were affected by eclogite-, blueschist- and/or epidote amphibolite-facies P–T conditions. Understanding of the petrological and geochronological record of these rocks plays a key role in unravelling the geodynamic evolution of the SSZ. The main aim of the present study was to assess the geological significance of previously published 40Ar/39Ar ages (c. 116–139Ma) which have not provided robust age constraints for geodynamic reconstructions on a regional scale. For this purpose, samples were collected within a NNW–SSE trending belt spanning a distance of c. 120km that exposes the major occurrences of high-pressure/low temperature rocks and epidote amphibolites.Multi-point RbSr mineral isochrons indicate a regional consistent pattern with ages ranging between c. 83 and c. 87Ma for eight samples representing different metamorphic grade and widely separated locations (weighted mean=85.7±0.8Ma). Additional 40Ar/39Ar dating for five of these samples yielded in most cases ages that are identical to the RbSr results within analytical uncertainty. For one sample the 40Ar/39Ar age of 81.3±1.3Ma is about 3myr younger than the corresponding RbSr age. A still younger RbSr age of 78.9±0.5Ma was determined for a retrograde epidote-biotite assemblage in an overprinted domain of an eclogite. Ion probe UPb zircon ages for two eclogites and two meta-acidic rocks from the mélange yielded weighted mean 206Pb/238U ages of 87.3±1.4Ma, 86.1±1.5Ma, 88.7±1.4Ma and 87.2±1.2Ma, respectively. The results of this study do not support previous interpretations suggesting >125Ma high-pressure/low-temperature metamorphism and amphibolite-facies overprinting, but instead document the importance of Late Cretaceous subduction zone processes for the geodynamic evolution of the SSZ.

Paragonite in marbles from the Tauern Window, Austria: Compositional and thermobaric controls

Paragonite coexists with phengite, chlorite, zoisite, tremolite and quartz in dolomitic marbles from near Döllach in the Sonnblick area of the SE Tauern Window, Austria, prompting an investigation into the petrogenetic significance of paragonite in carbonate rocks.Thermodynamic calculations on pure-end-member reactions indicate that the assemblages paragonite+CaCO3+quartz and paragonite+dolomite+quartz are stable over wide ranges of X(CO2) and have P–T stability fields almost as wide as that of paragonite+quartz. P–T pseudosection calculations spanning 0.35–1.4GPa and 400–590°C in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–H2O–CO2 yield wide fields for paragonite-bearing assemblages in the Döllach paragonite-marbles at low X(CO2). The bulk compositions of these rocks are characterised by high values of molar Na/(Na+K) and Al/(Na+K) and these parameters are influential in stabilising paragonite-bearing assemblages in marbles. A large proportion of published limestone and dolostone whole-rock compositions would be capable of supporting paragonite at P–T conditions within the greenschist, blueschist or epidote–amphibolite facies. Paragonite is probably much more common in low- and medium-grade marbles than the rarity of reports of its occurrence suggest.Apart from zoisite, the hydrous silicates in the Döllach marbles are characterised by modest fluorine contents, with XF values of coexisting minerals decreasing in the order Tr>Phe>Pa≈Chl.Calcite–dolomite thermometry and pseudosection calculations indicate peak-metamorphic conditions for the Döllach marbles of T=510±20°C, P>0.77GPa and X(CO2)<0.065. The P–T data agree with previous estimates for the area.

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies metamorphism in the Sanbagawa belt

Quantitative analyses of garnet-bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high-grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn-tectonic prograde-zoned garnet and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P)-temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H2O) give consistent peak-P estimates of 1.3-1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ∼ 0.7 GPa, which is as high as the values reported in well-characterized eclogite samples in central Shikoku. The inferred peak-P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ∼ 0.8 GPa, 580 °C suggest decompression to the epidote-amphibolite facies (M2). The M2 minerals locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re-equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.

Metamorphic pressure-temperature evolution of garnet-chloritoid schists from the Lake Zone, SW Mongolia

Garnet-chloritoid schists occur in the Chandman district of the Lake Zone, SW Mongolia, which is located within the central part of the Central Asian Orogenic Belt. The Chandman district is composed of Neoproterozoic ophiolites of the Khantaishir Formation, eclogite-bearing orthogneisses and micaschists of the Alag Khadny metamorphic complex, marbles of the Maykhan Tsakhir Formation, and the basement block of the Zamtyn Nuruu Formation. Garnet-chloritoid schists occur as lenses or layers within marbles of the Maykhan Tsakhir Formation, which lie in contact with eclogite bodies. They consist mainly of garnet, chloritoid, phengitic muscovite, chlorite and quartz, with minor amounts of kyanite, rutile, ilmenite, zircon and carbonaceous matter. The texture and mineral chemistry of the garnet-chloritoid schists record prepeak and peak stages of high-pressure intermediate type metamorphism. The pre-peak stage of epidote-amphibolite facies conditions (T = 500-510 °C and P = 7-8 kbar) is deduced from cores of garnet containing mineral inclusions of chloritoid, Fe-rich chlorite, phengitic muscovite, and quartz. The peak mineral assemblage of the garnet-chloritoid schists defined by garnet rims containing chloritoid, phengitic muscovite, Mg-rich chlorite, kyanite and quartz as inclusions indicates T = 560-590 °C and P = 10-11 kbar, falling in transitional conditions between the epidote-amphibolite and the eclogites facies. The pressure conditions of the garnet-chloritoid schists are distinctly lower than those of the eclogites (T = 590-610 °C and P = 20-25 kbar), whereas temperature conditions are similar. The eclogites are considered to have formed by subduction of oceanic crust. In contrast, the metamorphism of the garnet-chloritoid schists was probably related to collisional tectonic event in the Central Asian Orogenic Belt.

Tectonometamorphic evolution of the Garevka polymetamorphic complex (Yenisei Ridge)

The Garevka metamorphic complex (GMC), located at the junction of the Central Angara and Isakovka terranes (western part of the Transangarian Yenisei Ridge), was studied in terms of its tectonometamorphic evolution and geodynamic processes in the Neoproterozoic history of the region. Geological, structural, geochronological, and petrological data permitted the recognition of two stages in the GMC evolution, which differ in thermodynamic regimes and metamorphic field gradients. These stages were related to crustal contraction and extension within the Yenisei regional shear zone, a large lineament structure in the region. Stage 1 was marked by the formation of metamorphic complexes in the middle to upper amphibolite facies moderate-pressure regional metamorphic settings at ∼960 Ma, P = 7.7–8.6 kbar, and T = 582–631 °C. This suggests subsidence of the area to the middle continental crust with dT/dH = 20–25 °C/km. During stage 2, the rocks experienced Late Riphean (∼880 Ma, SHRIMP II U–Pb and 40Ar–39Ar dating) dynamic metamorphism under epidote-amphibolite facies conditions (P = 3.9–4.9 kbar; T = 461–547 °C), indicating a metamorphic field gradient of dT/dH no greater than 10 °C/km, with the formation of blastomylonites in narrow zones of ductile and brittle deformations. In these zones, high-grade GMC blocks were exhumed to the upper continental crust and underwent low-temperature metamorphism. Comparison of the structural, geologic, and other evolutionary features (nearly identical age constraints in view of exhumation rate, similar PT-paths, and different types of metamorphism associated with different geodynamic settings, etc.) of the Garevka and Teya complexes suggests that they constitute a single polymetamorphic complex.

Zr-in-rutile thermometry in eclogite and vein from southwestern Tianshan, China

Trace element LA–ICPMS analyses of rutile from eclogites and veins from southwestern Tianshan, China, are carried out for testing the validity of Zr-in-rutile thermometry in low-T/UHP eclogites. Mineral inclusions in rutile and time-resolved mass spectrometer data are carefully examined to identify and reject contaminated data. The Zr contents vary between individual grains from the same sample; rutile inclusions in garnet, amphibole, and paragonite have a narrower range of Zr contents than matrix rutiles that always show the highest Zr contents, which reflects the progressive rutile growth in the eclogites. Average Zr concentrations in rutile from all six investigated eclogite samples statistically belong to one well-defined population (34±5ppm; 33±11ppm; 26±7ppm; 36±17ppm; 22±6ppm; 40±17ppm), while average Zr concentration in rutile from the three veins belong to a second well-defined population (55±7ppm; 66±6ppm; 55±5ppm). The peak temperature of eclogite can be estimated as 535–570°C at a pressure of 2.7GPa inverted from the discovery of well preserved coesite in an increasing number of rocks from southwestern Tianshan. The mineral assemblage of the veins suggests their retrograde formation in the epidote–amphibolite facies; the growth temperature of vein rutile is 530–540°C at ∼1.0GPa. A robust P–T path constructed by Zr-in-rutile thermometry in eclogites and veins indicates that the rocks underwent a rapid uplift involving an early isothermal decompression from eclogite to epidote–amphibolite facies. Zr in rutile from eclogites and veins can serve as a pressure indicator in study area that suggests the pressure has a significant effect to the Zr-in-rutile thermometry.

Interaction of deformation and metamorphism during subduction and exhumation of hydrated oceanic mantle: Insights from the Western Alps

AbstractPetrological investigations supported by multi‐scale structural analysis of eclogitized serpentinite in the Zermatt–Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P–T–d–t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta‐rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote‐amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti‐clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti‐clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti‐clinohumite is associated with veins that are older than D2 and pre‐date D3. Veins that post‐date D3 are characterized by amphibole + chlorite or by serpentine. P–T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures &gt;2.5 ± 0.3 GPa at temperatures slightly higher than 600 °C. The late epidote–amphibolite facies re‐equilibration associated with D3 and D4 developed during late syn‐exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt–Saas Zone registered different P–T peak conditions and underwent different exhumation paths. In addition, the inferred P–T–d–t path suggests that the Valtournanche serpentinites re‐equilibrated close to the UHP conditions registered by the Cignana meta‐cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small‐size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.

Slow subduction and buoyant exhumation of the Sanbagawa eclogite

The Western Iratsu body of the Sanbagawa belt (SW Japan) represents a <2km thick garnet–epidote amphibolite slice with minor intercalated occurrences of eclogite. Equilibrium phase assemblage diagrams (pseudosections) for eclogite and garnet–epidote amphibolite from this body can be successfully used to reconstruct the pressure (P)–temperature (T) path and the evolution of the associated mineral assemblages. Microstructural observations combined with calculated pseudosections reveal two distinct stages of subduction-related metamorphism referred to as M1 and M2. P–T conditions of M1 evolved from 0.9GPa, 570°C to 1.2GPa, 660°C, implying relatively high thermal gradients possibly developed shortly after the initiation of subduction. The available age data and the characteristics of the P–T path suggest that the Western Iratsu body was detached from the slab at 1.2GPa (~40km depth) and the subsequent subduction occurred at a very slow rate due to weak mechanical coupling to the downgoing slab. The prograde metamorphism associated with the second subduction phase (M2) reached the eclogite facies (~1.8GPa, 510–560°C), but mafic rock compositions recrystallized into either eclogite or garnet–epidote amphibolite assemblages, depending on the bulk rock (CaO+Na2O)/(FeO+MgO) ratio. The thermal peak of M2 (~620°C at 1.6GPa) was attained during the early phase of exhumation. Further decompression and re-equilibration (M3) took place in the epidote-amphibolite facies. The M2 to M3 P–T evolution is associated with a progressive increase in thermal gradient (T/P ratio), which can be explained by thermal modeling as the result of the inflow of a very young hot section of the slab. The bulk density of the whole Western Iratsu body at the point of onset of exhumation—the return point—is estimated to be ~3180kgm−3, which is less than the density of mantle, implying that buoyant rise is a viable mechanism for its initial exhumation. Buoyant rise is in accordance with the widespread early-exhumation related deformation that has a dip-parallel stretching lineation. Exhumation of the buoyant discrete mafic slice along the decoupled slab–mantle interface could have been triggered by the heating that lead to fluid liberation and a resultant reduction of the rock strength.

Rutile inclusions in quartz crystals record decreasing temperature and pressure during the exhumation of the Su-Lu UHP metamorphic belt in Donghai, East China

Donghai County in the Jiangsu Province of East China is known for its large-scale production of high-quality quartz crystals. The quartz crystals are hosted within the Su-Lu ultrahigh-pressure (UHP) metamorphic belt. They form in quartz veins hosted by eclogite and gneiss, or along contacts between eclogite and gneiss and are mined as placer deposits in the Quaternary sediments. Backscattered electron imaging of the rutile in rutile-bearing euhedral quartz crystals from this region reveal three compositionally and texturally distinct generations of rutile, encapsulated within the host quartz crystal. The earliest generation of rutile (R1) is brightest in BSE, reflecting enrichment in Fe2O3 (0.75–2.08 wt%), Nb2O5 (0.37–0.93 wt%), WO3 (0.48–2.99 wt%), and ZrO2 (0.005–0.105 wt%), relative to R2 and R3. R2 rutile overgrows R1 rutile and contains 0.66–1.11 wt% Fe2O3, 0.45–0.74 wt% Nb2O5, 0.09–0.20 wt% WO3, and <0.001–0.013 wt% ZrO2. R3 rutile, which is the only rutile generation in contact with the host quartz crystals is darkest in BSE and always overgrows R1 and rounded aggregates of R2. R3 rutile contains 0.40–0.68 wt% Fe2O3, 0.13–0.37 wt% Nb2O5, 0.03–0.12 wt% WO3, and <0.001 wt% ZrO2. Oxygen isotopes decrease progressively from 0.6 to −1.5‰ in R1 rutile to 0.1‰ in R2 rutile to −5.1 to −0.3‰ in R3 rutile, consistent with decreasing temperature of rutile crystallization. Combined application of the Zr in rutile and Ti in quartz thermobarometers indicate that these rutile generations precipitated at progressively lower pressures and temperatures, consistent with previously determined retrograde metamorphic conditions of the Su-Lu UHP belt. R1 grew after peak metamorphism during retrograde eclogite facies metamorphism at temperatures between 720 and 800 °C and pressures between 21.7 and 26.6 kbar. During subsequent retrograde metamorphism, R1 was fractured and partially brecciated and overgrown by R2 during epidote-amphibolite facies retrograde metamorphism in the range of 400 to 500 °C and 5 kbar. Zirconium concentrations of <14 ppm in R3 rutile and Ti concentrations of <1 ppm in the co-precipitated host quartz suggest that they both grew at low temperatures of <300° and pressures <2 kbars. Oscillatory growth zones in quartz revealed by cathodoluminescence suggest that no subsequent metamorphism or hydrothermal activity occurred after the formation of the host euhedral quartz crystals and is consistent with a low temperature of quartz crystallization. The random distribution of rutile crystals in the quartz and the lack of evidence for rutile transport by hydrothermal fluids indicate that the host quartz formed by a dissolution-replacement process, whereby the original host mineral, likely omphacite or garnet, was replaced by quartz, but the rutile was not significantly replaced or dissolved due to the low solubility of Ti in low-temperature hydrothermal fluids.