Metamorphic petrology of the Barchi–Kol metabasites, western Kokchetav ultrahigh-pressure–high-pressure massif, northern Kazakhstan

Abstract In the Barchi–Kol area, located at the westernmost part of the Kokchetav ultrahigh pressure (UHP) to high-pressure (HP) massif, northern Kazakhstan, metabasites from the epidote amphibolite (EA) facies to the coesite eclogite (CEC) facies are exposed. Based on the equilibrium mineral assemblages, the Barchi–Kol area is divided into four zones: A, B, C and D. Zone A is characterized by the assemblage: epidote + hornblende + plagioclase + quartz, with minor garnet. Zone B is characterized by the assemblage: garnet + hornblende + plagioclase + quartz + zoisite. Zone C is defined by the appearance of sodic–augite, with typical assemblage: garnet + sodic–augite + tschermakite–pargasite + quartz ± plagioclase ± epidote/clinozoisite. Zone D is characterized by the typical eclogite assemblage: garnet + omphacite + quartz + rutile, with minor phengite and zoisite. Inclusions of quartz pseudomorph after coesite were identified in several samples of zone D. Chemical compositions of rock-forming minerals of each zone were analyzed and reactions between each zone were estimated. Metamorphic P-T conditions of each zone were estimated using several geothermobarometers as 8.6 ± 0.5 kbar, 500 ± 30 °C for zone A; 11.7 ± 0.5 kbar, 700 ± 30 °C for zone B; 12–14 kbar, 700–815 °C for zone C; and 27–40 kbar, 700–825 °C for zone D.     

Overview of the geology, petrology and tectonic framework of the high-pressure–ultrahigh-pressure metamorphic belt of the Kokchetav Massif, Kazakhstan

Abstract High‐ to ultrahigh‐pressure metamorphic (HP–UHPM) rocks crop out over 150 km along an east–west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet‐like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north‐dipping, fault‐bounded orogen‐parallel units (I–IV, from base to top). Constituent metabasic rocks exhibit a systematic progression of metamorphic grades, from high‐pressure amphibolite through quartz–eclogite and coesite–eclogite to diamond–eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780–1000°C, P = 37–60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730–750°C, P = 11–14 kbar; Unit IV: T = 530°C, P = 7.5–9 kbar) and bottom (Unit I: T = 570–680°C; P = 7–13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit opposing senses of shear at the bottom (top‐to‐the‐north) and top (top‐to‐the‐south) of the pile. The orogen‐scale architecture of the massif is sandwich‐like, with the HP–UHPM nappe group juxtaposed across large‐scale subhorizontal faults, against underlying low P–T metapelites (Daulet Suite) at the base, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V). The available structural and petrologic data strongly suggest that the HP–UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjacent lower‐grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30–40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab‐breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP–UHPM rocks.     

Kyanite-anthophyllite schist and southwest extension of the Dabie Mountains ultrahigh- to high-pressure belt

Abstract Kyanite-anthophyllite schist preserves the first record of high pressure in the amphibolite-facies unit of the SW Dabie Mountains, whereas ultrahigh- and high-pressure (UHP and HP) metamorphism has been well documented by the occurrence of coesite, diamond and mafic eclogite in the SE Dabie Mountains. Textural evidence indicates that minerals of the kyanite-anthophyllite schist formed mainly in two stages: (i) garnet + kyanite + antho-phyllite + rutile formed at pressure in excess of 1.2 GPa at T < 650°C; (ii) cordierite±staurolite formed by reaction of anthophyllite + kyanite at P < 0.5 GPa, T∼530°C. Plagioclase and ilmenite replaced garnet and rutile respectively during decompression. In a still later stage, secondary biotite recrystallized, accompanied by sillimanite replacing kyanite, and spinel replacing staurolite. The P-T information suggests that the amphibolite unit in the SW Dabie Mountains is part of the Triassic collision belt between the Sino-Korean and Yangtze cratons. The P-T paths of the UHP eclogite in the eastern Dabie Mountains and the HP kyanite-anthophyllite schist in the SW Dabie Mountains show similar decompression and equivalent late stage Barrovian-style metamorphism. Emplacement of voluminous granitoid at middle crustal levels between 134–118 Ma contributed to the development of the Barrovian-type metamorphism in the Dabie Mountains.     

Kinematic analysis of ultrahigh-pressure–high-pressure metamorphic rocks in the Chaglinka–Kulet area of the Kokchetav Massif, Kazakhstan

Abstract The central part of the Kokchetav Massif is exposed in the Chaglinka–Kulet area, northern Kazakhstan. The ultrahigh-pressure–high-pressure (UHP–HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I–IV) metamorphosed under different pressure–temperature (P–T) conditions. The coesite- and diamond-bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite-dominant Unit I on the bottom and the orthogneiss-dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP–HP metamorphic belt are affected by at least four deformational events post-dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non-coaxial ductile flow in a NW–SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top-to-the-northwest motions and those from Unit III provide top-to-the-southeast, south or south-west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear-indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large-scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike-slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north-westward extrusion of UHP Unit II.     

Metamorphic zoning and thermal structure of the Dabie ultrahigh- pressure–high-pressure terrane, central China

The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.     

Ultrahigh-pressure metamorphic records hidden in zircons from amphibolites in Sulu Terrane, eastern China

Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD‐PP1 and CCSD‐PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite‐facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite‐bearing ultrahigh‐pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631–780°C and pressures of ≥2.8 × 10³ MPa, representing the P–T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642–726°C; P ≥ 2.8 × 10³ MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites.     

Mineral parageneses and metamorphic P-T paths of ultrahigh-pressure eclogites from Kyrghyzstan Tien-Shan

Abstract Eclogites occur in three districts of the northern and southern parts of Tien-Shan. Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P-T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite. Aktyuz and Makbal eclogites have not preserved the prograde path. An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600°C, P = 12 kbar) has recorded three stages of retrograde metamorphism. Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet-omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition. Garnet from a garnetchloritoid-talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low-temperature part of coesite eclogite fades metamorphism. Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample. The prograde path from stage I to stage III has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included. The peak metamorphism of stage III took place at about 660°C at 25 kbar. The stages IV and V are retrograde. UHP eclogite facies metamorphism took place twice in Kyrghyzstan. The Aktyuz and Atbashy eclogites gave Rb-Sr mineral-isochron ages of about 750 Ma and 270 Ma, respectively. The K-Ar age of paragonite from the Makbal eclogite is about 480 Ma.     

The versatility of petrological modeling: Thermobarometry of high-pressure metabasites from the Renge and Sanbagawa belts and phase evolution during warm subduction at Nankai

Petrological modeling is a powerful technique to address different types of geological problems via phase-equilibria predictions at different pressure–temperature-composition conditions. Here, we show the versatility of this technique by (1) performing thermobarometrical calculations using phase equilibrium diagrams to explore the petrological evolution of high-pressure (HP) metabasites from the Renge and Sanbagawa belts, Japan and (2) forward-modeling the mineral–melt evolution of the subducted fresh and altered oceanic crust along the Nankai subduction zone geotherm at the Kii peninsula, Japan. In the first case, we selected three representative samples from these metamorphic belts: a glaucophane eclogite and a garnet glaucophane schist from the Renge belt (Omi area) and a quartz eclogite from the Sanbagawa belt (Besshi area). We calculated the peak metamorphic conditions at ~2.0–2.3 GPa and ~550–630 °C for the HP metabasites from the Renge belt, whereas for the quartz eclogite, the peak equilibrium conditions were calculated at 2.5–2.8 GPa and ~640–750 °C. According to our models, the quartz eclogite experienced partial melting after peak metamorphism. In terms of the petrological evolution of the subducted uppermost portion of the oceanic crust along the warm Nankai geotherm, our models show that fluid release occurs at ~20–60 km, likely promoting high pore-fluid pressure, and thus, seismicity at these depths; dehydration is controlled by chlorite breakdown. Our petrological models predict partial melting at >60 km, mainly driven by phengite and amphibole breakdown. According to our models, the melt proportion is relatively small, suggesting that slab anatexis is not an efficient mechanism for generating voluminous magmatism at these conditions. Modeled melt compositions correspond to high-SiO₂ adakites; these are similar to compositions found in the Daisen and Sambe volcanoes, in southwest Japan, suggesting that the modeled melts may serve as an analog to explain adakite petrogenesis.     

Two contrasting petrotectonic domains in the Kokchetav megamélange (north Kazakhstan): Difference in exhumation mechanisms of ultrahigh‐pressure crustal rocks,or a result of subsequent deformation?

Abstract In northern Kazakhstan the WNW striking Kokchetav megamélange includes different crustal sequences with high‐pressure/ultrahigh‐pressure (HP/UHP) remnants of their 540–520 Ma subduction metamorphism. Two domains separated by the north‐east trending Chaglinka fault are distinguished. The western domain exhibits NE–SW structures within a single Kumdy–Kol megaunit of diamond‐bearing UHP metasediments and high‐temperature (HT) eclogites. The eastern domain consists of the composite Kulet megaunit with the Kulet UHP unit (coesite‐bearing metasediments, whiteschists and eclogites), the Enbek–Berlyk medium‐pressure (MP) unit (kyanite‐bearing, high‐alumina rocks with interleaved coronitic metagabbro), and ortho‐ and paragneisses with eclogites and amphibolites included. All eclogites in the eastern domain are of the relatively low temperature (LT) type. Sillimanite is common and appears after kyanite in the sheared MP unit. A regional and moderately ESE plunging linear fabric coincides with the fold‐axis of the foliation poles from the eastern domain. Whether this also reflects a regional top to the WNW transport, as inferred from the dextral strike‐slip on steeply to SSW dipping foliation, needs further study. Top to the WNW shear is shown by weakly inclined low pressure (LP) cordierite rocks that flank the eastern domain in the south. Some new ³⁹Ar/⁴⁰Ar mica cooling ages (519, 521 Ma) from the Kulet UHP micaschists reflect the same early stage evolutionary event as was previously shown for the Kumdy–Kol UHP rocks (515, 517 Ma) in the west. Similar ³⁹Ar/⁴⁰Ar ages (500, 517 Ma) are recorded by micas and amphibole that outline a top to NNW shear fabric in the non‐subducted Proterozoic basement, north of the megamélange. A 447 Ma overprint of the MP sequences is considered to reflect the strike‐slip deformation with sillimanite and the reworking of an early kyanite‐bearing tectonite. Biotites from the LP cordierite rocks yielded approximately 400 Ma ³⁹Ar/⁴⁰Ar ages. In case they reflect the WNW shear deformation, the latter is considered to be associated with a regional granite magmatism (420–460 Ma) extending south of the eastern domain. In their present different structural domains the Kulet and Kumdy–Kol UHP units display a similar early stage event. Subsequent LP deformation, which is likely to be associated with regional granite magmatism (420–460 Ma), is assumed to have obliterated any common or uniform early exhumation structure for the whole megamélange. The north‐east structured Kumdy–Kol domain is assumed to have preserved the most information about the early stage exhumation. This domain is at an angle to the regional WNW strike of the megamélange.     

Ultrahigh-pressure metamorphism and decompressional P-T paths of eclogites and country rocks from Weihai, eastern China

Abstract Altered quartz-rich and nearly quartz-free eclogitic rocks and completely retrograde quartz-rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh-P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three-stage metamorphic events including ultrahigh-pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P-T conditions for the three-stage recrystallization sequence are 840 ± 50°C, >28 kbar, about 760±50°C, 9 kbar, and ~650°C, <8 kbar respectively. Most country rock gneisses contain dominant amphibolite-facies assemblages; some garnet-bearing clinopyroxene gneisses recrystallized under granulite-facies conditions at about 740±50°C and 8.5 kbar; similar to granulite-facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale-silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14-35 kbar. Eclogitic boudins and quartzo-feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P-T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula.     

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance

The present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm^?1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω₁ > 8.5 cm^?1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.     

Kinetic modeling of the coesite–quartz transition in an elastic field and its implication for the exhumation of ultrahigh-pressure metamorphic rocks

The present paper examines a kinetic model of the coesite–quartz transition under an elastic field. This model is applied to discuss the possible exhumation path of ultrahigh-pressure (UHP) metamorphic rocks. By incorporating the model of transition kinetics into a three-shelled composite sphere model in linear elasticity, the internal stresses in coesite, quartz, and garnet shells were calculated for given external pressure ( P )–temperature ( T ) paths. The occurrence of rupture provides a constraint on the temperature and the amount of quartz inverted from coesite at the rupture for each P–T path. Comparison of calculated results and the natural occurrence of coesite inclusion from the Dora Maira Massif, containing ∼ 27% quartz at the rupture, enables us to constrain the possible exhumation path and possible transition kinetics. A steep decompression path with slow transition kinetics is most favorable, which is consistent with the estimated P–T path during exhumation for most UHP metamorphic rocks.     

Petrology of the diamond-grade eclogite in the Kokchetav Massif, northern Kazakhstan

Abstract The Kokchetav Massif of northern Kazakhstan is unique because of the abundant occurrence of microdiamond inclusions in garnet, zircon and clinopyroxene of metasediments. In order to determine precise pressure–temperature (P–T) conditions, we have systematically investigated mineral inclusions and the compositions of major silicates in Ti–clinohumite–garnet peridotite and diamond-grade eclogite from Kumdy–Kol. It was found that garnet peridotites from Kumdy–Kol contain assemblages of garnet, olivine, Ti–clinohumite and ilmenite. The garnet contains inclusions that are indicative of both ultrahigh pressure (UHP) and retrograde conditions. Inclusions of hydrous phases such as chlorite, amphibole and zoisite were formed at the post-UHP stage. The study also found that eclogite from Kumdy–Kol contains albite–augite symplectites after omphacitic pyroxene. The core of pyroxene (sodic augite) contains high K₂O (up to 1wt%; average 0.24wt%). Phengite is included in the core. Applying the K₂O-in-augite geobarometry, which is based on recent experiments, and the garnet–clinopyroxene (Grt–Cpx) geothermometer for peak metamorphism, the eclogites yield P–T estimates of > 6 GPa and > 1000 °C, and the diamond-grade eclogites yield lower temperature estimates at 900–1000 °C and 5 GPa.     

Petrology and retrograde P-T path for eclogites of the Maksyutov Complex, Southern Ural Mountains, Russia

Abstract The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M_1-3) and two stages of deformation (S₁ and S₂) were observed in thin sections: M₁) garnet (Alm_55-60, Prp_22-28, Grs_16-20) + omphacite (Jd_46-56) + phengite (Si ≅ 3.5) + rutile; M₂) garnet + glaucophane ± lawsonite + white mica; and M₃) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.     

The chemical Th-U-total Pb isochron ages of Jiaodong and Jiaonan metamorphic rocks in the Shandong Peninsula, eastern China

Abstract The chemical Th-U-total Pb isochron method (CHIME) was applied to determine the age of monazite and thorite in five gneisses and zircon in an ultra high-pressure (UHP) phengite schist from the Su-Lu region, eastern China. The CHIME ages and isotopic ages reported in the literature show that gneisses in the Su-Lu region are divided into middle Proterozoic (1500–1720 Ma) and Mesozoic (100–250 Ma) groups. The Proterozoic group includes paragneiss and orthogneiss of the amphibolite-granulite facies, and their protolith age is late Archean-early Proterozoic. The Mesozoic group is mainly composed of orthogneiss of the greenschist-epidote amphibolite facies, and the protolith age is Middle Paleozoic-Early Proterozoic. The Proterozoic and Mesozoic gneisses occupy northern and southern areas of the Su-Lu region, respectively, which are divided by a major Wulian-Qingdao-Yantai fault. Ultra high-pressure metamorphic rocks occur as blocks in the Mesozoic gneisses, and form a UHP complex.
The UHP phengite schist in the Mesozoic orthogneiss contains detrital zircons with late Proterozoic CHIME age ( ca 860 Ma). Age of the UHP metamorphism is late Proterozoic or younger, and protolith age of the UHP metamorphic rocks is probably different from that of the surrounding Mesozoic gneisses.     

Duplex of metamorphic units including ultramafic rocks in the central region of Tokunoshima,Southwest Japan

Geological observations in the central part of Tokunoshima in the Amami Islands, Southwest Japan, reveal that discrete layers of serpentinite, dioritic gneiss, and amphibolite are intercalated into pelitic schist and these rock bodies form a northwest‐dipping tectonic stack. A subhorizontal psammitic schist layer overlies them. These rocks underwent ductile deformation that is denoted by penetrative foliation and mineral lineation. Microstructures of the sheared metamorphic rocks and serpentinite indicate top‐to‐the‐east, ‐southeast or ‐south (hanging‐wall up) displacements. The en echelon array of rock bodies is interpreted as a duplex with the psammitic schist layer on its top and the pelitic schist layer on its bottom. It is inferred that the serpentinite‐bearing duplex was formed due to the tectonic erosion and the subsequent accretionary growth operated in a Cretaceous or older subduction zone. Tokunoshima has been considered to belong to the Shimanto Belt. However, regional low‐pressure and high‐temperature type amphibolite‐facies metamorphism and related ductile deformation have not been recognized in the other areas of the Shimanto Belt. There is no metamorphic rock occurrence comparable to that of Tokunoshima in the neighboring islands. The metamorphic rocks in Tokunoshima can be correlated to any of low‐pressure/temperature type metamorphic regions in Kyushu.     

In situ LA-ICP-MS zircon U-Pb age of ultrahigh-pressure eclogites in the Yukahe area,northern Qaidam Basin

Zircon grains were selected from two types of ultrahigh-pressure (UHP) eclogites, coarse-grained phengite eclogite and fine-grained massive eclogite, in the Yukahe area, the western part of the North Qaidam UHP metamorphic belt. Most zircon grains show typical metamorphic origin with residual cores in some irregular grains and sector, planar or misty internal textures on the cathodoluminescence (CL) images. The contents of REE and HREE of the core parts of grains range from 173 to 1680 μg/g and 170 to 1634 μg/g, respectively, in phengite eclogite, and from 37 to 2640 μg/g and 25.7 to 1824 μg/g, respectively, in massive eclogite. The core parts exhibit HREE-enriched patterns, representing the residual zircons of protolith of the Yukahe eclogite. The contents of REE and HREE of the rim parts and the grains free of residual cores are much lower than those for the core parts. They vary from 13.1 to 89.5 μg/g and 12.5 to 85.7 μg/g, respectively, in phengite eclogite, and from 9.92 to 45.8 μg/g and 9.18 to 43.8 μg/g, respectively, in massive eclogite. Negative Eu anomalies and Th/U ratios decrease from core to rim. Positive Eu anomalies are shown in some grains. These indicate that the presence of garnet and the absence of plagioclase in the peak metamorphic mineral assemblage, and the zircons formed under eclogite facies conditions. LA-ICP-MS zircon U-Pb age data indicate that phengite eclogite and massive eclogite have similar metamorphic age of 436±3Ma and 431±4Ma in the early Paleozoic and magmatic protolith age of 783–793 Ma and 748–759 Ma in the Neo-proterozoic. The weighted mean age of the metamorphic ages (434±2 Ma) may represent the UHP metamorphic age of the Yukahe eclogites. The metamorphic age is well consistent with their direct country rocks of gneisses (431±3 Ma and 432±19 Ma) and coesite-bearing pelitic schist in the Yematan UHP eclogite section (423–440 Ma). These age data together with field observation and lithology, allow us to conclude that the Yukahe eclogites were Neo-proterozoic igneous rocks and may have experienced subduction and UHP metamorphism with continental crust at deep mantle during the early Paleozoic, therefore the metamorphic age of 434±2 Ma of the Yukahe eclogites probably represents the continental deep subduction time in this area.

     

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.     

Geological and mineralogical study of eclogite and glaucophane schists in the Naga Hills Ophiolite,Northeast India

A variety of low‐ to high‐pressure metamorphic assemblages occur in the metabasic rocks and metachert in the Upper Cretaceous–Eocene ophiolite belt of the central part of the Naga Hills, an area in the northern sector of the Indo–Myanmar Ranges in the Indo–Eurasian collision zone. The ophiolite suite includes peridotite tectonite containing garnet lherzolite xenoliths, layered ultramafic–mafic cumulates, metabasic rocks, basaltic lava, volcaniclastics, plagiogranite, and pelagic sediments emplaced as dismembered and imbricated bodies at thrust contacts between moderately metamorphosed accretionary rocks/basement (Nimi Formation/Naga Metamorphics) and marine sediments (Disang Flysch). It is overlain by coarse clastic Paleogene sediments of ophiolite‐derived rocks (Jopi/Phokphur Formation). The metabasic rocks, including high‐grade barroisite/glaucophane‐bearing epidote eclogite and glaucophane schist, and low‐grade greenschist and prehnite–clinochlore schist, are associated with lava flows and ultramafic cumulates at the western thrust contact. Chemically, the metabasites show a low‐K tholeiitic affinity that favors derivation from a depleted mantle source as in the case of mid‐ocean ridge basalt. Thermobarometry indicates peak P–T conditions of about 20 kb and 525°C. Retrogression related to uplift is marked by replacement of barroisite and omphacite by glaucophane followed by secondary actinolite, albite, and chlorite formation. A metabasic lens with an eclogite core surrounded by successive layers of glaucophane schist and greenschist provides field evidence of retrogression and uplift. Presence of S‐C mylonite in garnet lherzolite and ‘mica fish’ in glaucophane schist indicates ductile deformation in the shear zone along which the ophiolite was emplaced.     

An introduction to ultrahigh-pressure metamorphism

Abstract Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).