A re-evaluation of eclogite facies metamorphism in SW Japan: proposal for an eclogite nappe

Known eclogite occurrences in the Sanbagawa metamorphic belt of SW Japan are dominantly in metagabbro bodies which have complex polyphase metamorphic histories. These bodies are generally described as tectonic blocks and their relationship to the Sanbagawa metamorphism is unclear. New findings of foliated eclogite in the Seba and Kotsu areas show that eclogite facies metamorphism is much more widespread than generally thought. Evidence that the foliated eclogite units originated as lavas or sediments implies that these units can be treated as a high-grade part of the subduction-related Sanbagawa metamorphism. Although separated by an along-strike distance of 80 km, the Seba and Kotsu eclogites have very similar garnet and omphacite compositions, suggesting that they were formed under similar metamorphic conditions. However, differences in the associated retrograde assemblages (epidote–amphibolite in the Seba unit and epidote–blueschist in the Kotsu unit) suggest contrasting P – T  paths. In both units, the eclogite rocks occupy the highest structural level of the Sanbagawa belt and overlie rocks metamorphosed at lower pressure. The lower boundary to the eclogite units is therefore a major tectonic discontinuity locally decorated with lenses of exotic material. These features can help trace the boundary into other areas. The previously known outcrops of eclogite show enough similarities with the newly found areas to suggest that all the eclogite facies rocks in the Sanbagawa belt constitute a single nappe that lies at the highest structural levels of the orogen.     

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.     

Thermobaric structure of the Kokchetav ultrahigh-pressure–high-pressure massif deduced from a north–south transect in the Kulet and Saldat–Kol regions, northern Kazakhstan

Abstract To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh‐pressure and high‐pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO₂ (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high‐pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low‐pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.     

NORTH QAIDAM ULTRAHIGH PRESSURE METAMORPHIC (UHPM) BELT ON THE NORTHEASTERN QINGHAI-TIBET PLATEAU AND ITS EASTWARD EXTENSION

NORTH QAIDAM ULTRAHIGH PRESSURE METAMORPHIC (UHPM) BELT ON THE NORTHEASTERN QINGHAI-TIBET PLATEAU AND ITS EASTWARD EXTENSION1　YangJS ,XuZQ ,LiHB ,etal.DiscoveryofeclogiteatnorthernmarginofQaidambasin ,NWChina[J] .ChineseScienceBulletin,1998,4 3:1755～ 176 0 . 2　ZhangJX ,ZhangZM ,XuZQ ,etal.TheagesofU PbandSm NdforeclogitefromthewesternsegmentofAltynTaghtectonicbelt—theevidencesforexistenceofCaledonianorogenicroot[J] .ChineseScienc…     

鲁南苏北榴辉岩的特征及其成因

鲁南苏北榴辉岩按产状分为两类:一类是呈条带状、透镜状或肠状鳌合赋存于胶南混杂岩系中,另一类是呈透镜状赋存于石榴石橄榄岩中,以前者分布最为广泛,后者仅见于日照及东海地区。胶南混杂岩系中的榴辉岩由橄榄拉斑玄武岩深成变质作用形成,石榴石橄榄岩中的榴辉岩为地幔来源,它们共同经历了早期2.8 GPa,800℃的高温高压平衡结晶及后期600—700℃,1.1—1.3 GPa的退变质作用改造。Sm-Nd等时年龄为222±6Ma,~(143)Nd/~(144)Nd初始比值为0.511 36±3,推测本区榴辉岩形成于早三叠世华南陆块与华北陆块的碰撞事件。     

大别山超高压变质岩带氦同位素组成和某些微量元素分布

报导了大别山超高压变质岩带不同岩岩类型及六安－蔡河地学剖面中氦同位素和某些微量元素的分配分布特征，并讨论了它们的影响因素及其地质意义。     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

Ordovician eclogites from the Chinese Beishan: implications for the tectonic evolution of the southern Altaids

Numerous lenses of eclogite occur in a belt of augen orthogneisses in the Gubaoquan area in the southern Beishan orogen, an eastern extension of the Tianshan orogen. With detailed petrological data and phase relations, modelled in the system NCFMASHTO with thermocalc , a quantitative P–T path was estimated and defined a clockwise P–T path that showed a near isothermal decompression from eclogite facies (>15.5 kbar, 700–800 °C, omphacite + garnet) to high‐pressure granulite facies (12–14 kbar, 700–750 °C, clinopyroxene + sodic plagioclase symplectitic intergrowths around omphacite), low‐pressure granulite facies (8–9.5 kbar, ～700 °C, orthopyroxene + clinopyroxene + plagioclase symplectites and coronas surrounding garnet) and amphibolite facies (5–7 kbar, 600–700 °C, hornblende + plagioclase symplectites). The major and trace elements and Sm–Nd isotopic data suggest that most of the Beishan eclogite samples had a protolith of oceanic crust with geochemical characteristics of an enriched or normal mid‐ocean ridge basalt. The U–Pb dating of the Beishan eclogites indicates an Ordovician age of c. 467 Ma for the eclogite facies metamorphism. An ³⁹Ar/⁴⁰Ar age of c. 430 Ma for biotite from the augen gneiss corresponds to the time of retrograde metamorphism. The combined data from geological setting, bulk composition, clockwise P–T path and geochronology support a model in which the Beishan eclogites started as oceanic crust in the Palaeoasian Ocean, which was subducted to eclogite depths in the Ordovician and exhumed in the Silurian. The eclogite‐bearing gneiss belt marks the position of a high‐pressure Ordovician suture zone, and the calculated clockwise P–T path defines the progression from subduction to exhumation.     

Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle

Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km^?1), and this cold subduction brought a large amount of H₂O to the deep mantle in the Early Palaeozoic times.