High-pressure metamorphism in metabasites from the Betic Cordilleras (S.E. Spain) and its evolution during the Alpine orogeny

The Nevado-Filábride complex is the lowest tectonic unit of the Betic Zone sensu stricto (ss) of the Betic Cordilleras (S.E. Spain). The upper series of this complex consists of a metamorphosed sequence intruded by basic and ultrabasic igneous rocks. High-pressure metamorphism in the eclogite and blueschist facies is recorded in the metabasites, but this was partially obliterated by further successive metamorphic stages in the almandine-amphibolite and greenschist facies.Coronitic and granoblastic eclogites appear side by side in the large stocks of basic rocks. The coronitic eclogites originate from coarse-to medium-grained olivine gabbros, and the granoblastic eclogites from fine-grained basic rocks (dolerites and porphyritic basaltic rocks). Higher chemical mobility and rate of diffusion, as well as the availability of fluids during the eclogite facies metamorphism, are responsible for the greater degree of recrystallization found in the granoblastic eclogites. The availability of fluids during this metamorphic stage was controlled by the difference in the hydration of the protolith and by variable proximity to surrounding water-rich metasediments.The minerals in the eclogites are chemically homogeneous, suggesting that they are almost completely equilibrated, even in the coronitic eclogites. The estimated equilibrium P-T conditions were found to be the same (approximately 550° C at 12 kbar pressure) in both coronitic and granoblastic eclogites, and it has, therefore, been deduced that the coronitic eclogites do not represent the first and lower-grade step of a prograde metamorphism in which the granoblastic eclogites are the higher-grade step.No relationship was found between shearing and eclogite crystallization. Nevertheless, a first fabric/foliation developed in the later blueschist facies stage, and syntectonic growth of the minerals was detected in glaucophane-bearing rocks.The further metamorphic evolution of the metabasites from high-to intermediate-pressure conditions is documented by the formation of minerals belonging to albiteepidote and almandine-amphibolite facies assemblages. The application of the amphibole zonation model, in order to deduce the P-T path, does not give realistic values.High-pressure metamorphism is related to an early subduction event in the Betic Cordilleras, with a later more-or-less isothermal uplift to shallower levels.     

Eclogites in the Makuti gneisses of Zimbabwe: implications for the tectonic evolution of the Zambezi Belt in southern Africa

The gneisses of the Makuti Group in north-west Zimbabwe are characterized by complex geometries that resulted from intense non-coaxial deformation in a crustal scale high-strain zone that accommodated extensional deformation along the axis of the Zambezi Belt at c. 800 Ma. Within low-strain domains in the Makuti gneisses, undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages, which record a part of the metamorphic history that predates Pan-African events. Eclogitic rocks can be subdivided into: (1) corona-textured metagabbros that preserve igneous textures, and (2) garnet–omphacite rocks in which primary textures are destroyed. The lenses of eclogitic rocks are enveloped in a mantle of garnet–clinopyroxene–hornblende gneiss, which is a common rock type in the Makuti gneisses. The eclogites preserve multi-staged, domainal, symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression–heating. In the metagabbros, the original clinopyroxene, plagioclase and olivine domains acted separately during the peak of metamorphism, with plagioclase being replaced by garnet and kyanite, and olivine being replaced by orthopyroxene and possibly omphacite. The peak assemblage was overprinted by: (1) the multi-mineralic corona assemblage pargasite–orthopyroxene–spinel–plagioclase replacing garnet–kyanite–clinopyroxene (possibly at c. 19 kbar, 760±25 °C); (2) orthopyroxene–pargasite–plagioclase–scapolite coronas replacing orthopyroxene (15±1.5 kbar, 750±50 °C); and (3) moats of orthopyroxene–plagioclase replacing garnet (10±1 kbar, 760±50 °C). The garnet–omphacite rocks record similar peak conditions (15±1.1 kbar, 760±60 °C). Garnet–clinopyroxene–hornblende–plagioclase gneisses envelop the eclogites and record matrix conditions of 11±1.5 kbar at 730±50 °C using assemblages that are oriented in the regional fabric. These rocks are characterized by decompression-heating textures, reflecting temperature increases during exhumation of the Makuti gneisses. The eclogite facies rocks formed during a collisional event prior to 850 Ma. Their formation could be related to a suture zone that developed along the axis of the Zambezi Belt during the formation of Rodinia (between 1400 and 850 Ma). The main deformation-metamorphism in the Makuti gneisses occurred around 800 Ma and involved extension and exhumation of the high-P rocks (break-up of Rodinia), which experienced a high-T metamorphic overprint. Around 550–500 Ma, a collisional event associated with the formation of Gondwana resulted in renewed burial and metamorphic recrystallization of the Makuti gneisses.     

Tale of the Kulet eclogite from the Kokchetav Massive,Kazakhstan: Initial tectonic setting and transition from amphibolite to eclogite

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine‐grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse‐grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four‐stage metamorphic evolution: (I) pre‐eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre‐eclogite stage or coronal eclogite, and so did not identify the four‐stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE‐enriched patterns [(La/Lu)_N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO₂ (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO₂ < 0.9 wt% and FeO < 12.11 wt%) with one exception. Most Kulet eclogites plot in the MORB and IAB fields in the 2Nb–Zr/4–Y and TiO₂–FeO/MgO diagrams, although displacement from the MORB–OIB array indicates some degree of crustal involvement. All available data suggest that the protoliths of the Kulet eclogites were formed at a passive continent marginal basin setting. A schematic model involving subduction to 180–200 km at 537–527 Ma, followed by slab breakoff at 526–507 Ma, exhumation and recrystallization at crustal depths is applied to explain the four‐stage evolution of the Kulet eclogite.     

High-pressure metamorphism in Antarctica from the Proterozoic to the Cenozoic: A review and geodynamic implications

The survey of high-P metamorphic rocks in Antarctica can help clarify the geodynamic evolution of the continent by pointing out palaeo-suture zones and constraining the age of subduction and collision events. There are eclogite-facies rocks along the eastern margin of the ‘Mawson block’ (e.g., in the Nimrod Glacier region and George V Land). Some of these have been long forgotten (George V Land; Eyre Peninsula in Australia). Stillwell (1918) described rocks from George V Land containing glaucophane, lawsonite, garnet coronas and symplectites possibly after omphacite. These high-P rocks were apparently involved in the Nimrod-Kimban orogenic cycle and therefore provide a record of convergence along the eastern margin of the Mawson block at ~ 1700 Ma; they could represent one of the oldest blueschist-facies imprint. Many terranes in East Antarctica underwent a tectonometamorphic evolution during the Grenvillian (1300–900 Ma) and/or the Pan-African (600–500 Ma) orogenies, corresponding to the amalgamation of Rodinia and Gondwana, respectively. High-P relicts have been described or are suspected to occur in these terranes. Garnet-bearing coronitic metagabbros, in some cases possibly containing omphacite, are common in Dronning Maud Land and the Rayner Complex. They formed under high-P granulite-facies or eclogite-facies conditions and recall similar metabasites from the Grenville mobile belt of Canada. Note that some reconstructions of the Rodinia supercontinent consider these two Antarctic regions as an extension of the Grenvillian belt of Canada. Other eclogite-facies metamorphic rocks and ophiolites (Shackleton Range and possibly Sverdrupfjella) belong to the Pan-African mobile belt extending from Tanzania to East Antarctica. Since the Cambrian, the terranes of West Antarctica have been accreted along the palaeo-Pacific margin of Gondwana/Antarctica during several subduction-accretion orogenies. The ultrahigh-P metamorphic rocks of Northern Victoria Land formed through the accretion of an arc-backarc system during the Cambrian-Ordovician Ross orogeny; eclogites of the same orogeny also exist in Tasmania and Australia. Lastly, on the western edge of the Antarctic Peninsula, the Mesozoic–Cenozoic Andean orogeny generated a subduction-accretionary complex containing blueschist-facies rocks.     

Jadeite–garnet glaucophane schists in the Bizan area,Sambagawa metamorphic belt,eastern Shikoku,Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X_Jd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low X_Ca bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X_Ca bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.     

High-pressure eclogites in northern Jiangsu – southern Shandong province, eastern China

Abstract Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino-Korean and Yangtze cratons in central and eastern China. These eclogites usually have high-P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high-P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO₂ phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO₂ phase, and are divided into prograde eclogites and mantle-derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle-derived eclogites have high MgO/(FeO + Fe₂O₃) and Cr₂O₃ compositions in bulk rock and minerals, and consist mainly of pyrope-rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular-rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile. Pargasitic and taramitic amphiboles, calcic plagioclase (An₆₈), epidote, zoisite, K-feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near-isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high-P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near-isothermal decompression during the retrograde stage. The conditions at which symplectites re-equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle-derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.     

Evidence for multiple burial–partial exhumation cycles from the Onodani eclogites in the Sambagawa metamorphic belt,central Shikoku,Japan

Eclogites from the Onodani area in the Sambagawa metamorphic belt of central Shikoku occur as layers or lenticular bodies within basic schists. These eclogites experienced three different metamorphic episodes during multiple burial and exhumation cycles. The early prograde stage of the first metamorphic event is recorded by relict eclogite facies inclusions within garnet cores (X_Sps 0.80–0.24, X_Alm 0–0.47). These inclusions consist of relatively almandine‐rich garnet (X_Sps 0.13–0.24, X_Alm 0.36–0.45), aegirine‐augite/omphacite (X_Jd 0.08–0.28), epidote, amphiboles (e.g. actinolite, winchite, barroisite and taramite), albite, phengite, chlorite, calcite, titanite, hematite and quartz. The garnet cores also contain polyphase inclusions consisting of almandine‐rich garnet, omphacite (X_Jd 0.27–0.28), amphiboles (e.g. actinolite, winchite, barroisite, taramite and katophorite) and phengite. The peak P–T conditions of the first eclogite facies metamorphism are estimated to be 530–590 °C and 19–21 kbar succeeded by retrogression into greenschist facies. The second prograde metamorphism began at greenschist facies conditions. The peak metamorphic conditions are defined by schistosity‐forming omphacites (X_Jd ≤ 49) and garnet rims containing inclusions of barroisitic amphibole, phengite, rutile and quartz. The estimated peak metamorphic conditions are 630–680 °C and 20–22 kbar followed by a clockwise retrograde P–T path with nearly isothermal decompression to 8–12 kbar. In veins cross‐cutting the eclogite schistosity, resorbed barroisite/Mg‐katophorite occurs as inclusions in glaucophane which is zoned to barroisite, suggesting a prograde metamorphism of the third metamorphic event. The peak P–T conditions of this metamorphic event are estimated to be 540–600 °C and 6.5–8 kbar. These metamorphic conditions are correlated with those of the surrounding non‐eclogitic Sambagawa schists. The Onodani eclogites were formed by subduction of an oceanic plate, and metamorphism occurred beneath an accretionary prism. These high‐P/T type metamorphic events took place in a very short time span between 100 and 90 Ma. Plate reconstructions indicate highly oblique subduction of the Izanagi plate beneath the Eurasian continent at a high spreading rate. This probably resulted in multiple burial and exhumation movements of eclogite bodies, causing plural metamorphic events. The eclogite body was juxtaposed with non‐eclogitic Sambagawa schists at glaucophane stability field conditions. The amalgamated metamorphic sequence including the Onodani eclogites were exhumed to shallow crustal/surface levels in early Eocene times (c. 50 Ma).     

Partial transformation of gabbro to coesite-bearing eclogite from Yangkou, the Sulu terrane, eastern China

An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region of China consists of blocks of eclogite facies metagabbro, metagranitoid, ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss. A gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra-high-pressures was identified in a single 30 m block; metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins. The metagabbro contains an igneous assemblage of Pl+Aug+Opx+Qtz+Bt+Ilm/Ti-Mag; it shows relict magmatic textures and reaction coronas. Fine-grained garnet developed along boundaries between plagioclase and other phases; primary plagioclase broke down to Ab+Ky+Ms+Zo±Grt±Amp. Augite is rimmed by sodic augite or omphacite, whereas orthopyroxene is rimmed by a corona of Cum±Act and Omp+Qtz layers or only Omp+Qtz. In transitional rocks, augite and orthopyroxene are totally replaced by omphacite, and the lower-pressure assemblage Ab+Ky+Phn+Zo+Grt coexists with domains of Omp (Jd_70–73)+Ky±Phn in pseudomorphs after plagioclase. Both massive and weakly deformed coesite-bearing eclogites contain Omp+Ky+Grt+Phn+Coe/Qtz+Rt, and preserve a faint gabbroic texture. Coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz; some intergranular coesite and quartz pseudomorphs after coesite also occur. Assemblages of the coronal stage, transitional and UHP peak occurred at about 540±50 °C at c. 13 kbar, 600–800 °C at ≥15–25 kbar and 800–850 °C at >30 kbar, respectively. Garnet from the coronal- through the transitional- to the eclogite-stage rocks show a decrease in almandine and an increase in grossular±pyrope components; garnet in low-grade rocks contains higher MnO and lower pyrope components. The growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex; the application of garnet zoning to estimate P–T should be carried out with caution. Some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries; these quartz aggregates, as well as other primary and low-P phases, persisted metastably at UHP conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde P–T evolution.     

Petrology and P–T evolution of garnet peridotites from central Sulawesi, Indonesia

Alpine‐type orogenic garnet‐bearing peridotites, associated with quartzo‐feldspathic gneisses of a 140–115 Ma high‐pressure/ultra‐high‐pressure metamorphic (HP‐UHPM) terrane, occur in two regions of the Indonesian island of Sulawesi. Both exposures are located within NW–SE‐trending strike–slip fault zones. Garnet lherzolite occurs as <10 m wide fault slices juxtaposed against Miocene granite in the left‐lateral Palu‐Koro (P‐K) fault valley, and as 10–30 m wide, fault‐bounded outcrops juxtaposed against gabbros and peridotites of the East Sulawesi ophiolite within the right‐lateral Ampana fault in the Bongka river (BR) valley. Six evolutionary stages of recrystallization can be recognized in the peridotites from both localities. Stage I, the precursor spinel lherzolite assemblage, is characterized by Ol+Cpx+Opx±Prg‐Amp ± Spl±Rt±Phl, as inclusions within garnet cores. Stage II, the main garnet lherzolite assemblage, consists of coarse‐grained Ol+Opx+Cpx+Grt; whereas finer‐grained, neoblastic Ol+Opx+Grt+Cpx±Spl±Prg‐Amp±Phl constitutes stage III. Stages IV and V are manifest as kelyphites of fibrous Opx+Cpx+Spl in inner coronas, and Opx+Spl+Prg‐Amp±Ep in outer coronas around garnet, respectively. The final (greenschist facies) retrogressive stage VI is accompanied by recrystallization of Serp+Chl±Mag±Tr±Ni sulphides±Tlc±Cal. P–T conditions of the hydrated precursor spinel lherzolite stage I were probably about 750 °C at 15–20 kbar. P–T determinations of the peak stage IIc (from core compositions) display considerable variation for samples derived from different outcrops, with clustering at 26–38 kbar, 1025–1210 °C (P‐K & BR); 19–21 kbar, 1070–1090 °C (P‐K), and 40–48 kbar, 1205–1290 °C (BR). Stage IIr (derived from rim compositions) generally records decompression of around 4–12 kbar accompanied by cooling of 50–240 °C from the IIc peak stage. Stage III, which post‐dates a phase of ductile deformation, yielded 22±2 kbar at 750±25 °C (P‐K) and 16±2 kbar at 730±40 °C (BR). The granulite–amphibolite–greenschist decompression sequence reflects uplift to upper crustal levels from conditions of 647–862 °C at P=15 kbar (stage IV), through 580–635 °C at P=10–12 kbar (stage V) to 350–400 °C at P=4–7 kbar (stage VI), respectively, and is identical to the sequence recorded in associated granulite, gneiss and eclogite. Sulawesi garnet peridotites are interpreted to represent minor components of the extensive HP‐UHP (peak P >28 kbar, peak T of c. 760 °C) metamorphic basement terrane, which was recrystallized and uplifted in a N‐dipping continental collision zone at the southern Sundaland margin in the mid‐Cretaceous. The low‐T , low‐P and metasomatized spinel lherzolite precursor to the garnet lherzolite probably represents mantle wedge rocks that were dragged down parallel to the slab–wedge interface in a subduction/collision zone by induced corner flow. Ductile tectonic incorporation into the underthrust continental crust from various depths along the interface probably occurred during the exhumation stage, and the garnet peridotites were subsequently uplifted within the HP‐UHPM nappe, suffering a similar decompression history to that experienced by the regional schists and gneisses. Final exhumation from upper crustal levels was clearly facilitated by entrainment in Neogene granitic plutons, and/or Oligocene trans‐tension in deep‐seated strike–slip fault zones.     

A counter-clockwise P–T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.     

Quartz and orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic P–T path of Belomorian eclogites

The Shirokaya Salma eclogite‐bearing complex is located in the Archean–Palaeoproterozoic Belomorian Province (Russia). Its eclogites and eclogitic rocks show multiple clinopyroxene breakdown textures, characterized by quartz–amphibole, orthopyroxene and plagioclase lamellae. Representative samples, a fresh eclogite, two partly retrograded eclogites, and a strongly retrograded eclogitic rock, were collected for this study. Two distinct mineral assemblages—(1) omphacite+garnet+quartz+rutile±amphibole and (2) clinopyroxene+garnet+amphibole+plagioclase+quartz+rutile+ilmenite±orthopyroxene—are described. Based on phase equilibria modelling, these assemblages correspond to the eclogite and granulite facies metamorphism that occurred at 16–18 kbar, 750–800°C and 11–15 kbar, 820–850°C, respectively. The quartz–amphibole lamellae in clinopyroxene formed during retrogression with water ingress, but do not imply UHP metamorphism. The superfine orthopyroxene lamellae developed due to breakdown of an antecedent clinopyroxene (omphacite) during retrogression that was triggered by decompression from the peak of metamorphism, while the coarser orthopyroxene grains and rods formed afterwards. The P–T path reconstructed for the Shirokaya Salma eclogites is comparable to that of the adjacent 1.9 Ga Uzkaya Salma eclogite (Belomorian Province), and those of several other Palaeoproterozoic high‐grade metamorphic terranes worldwide, facts allowing us to debate the exact timing of eclogite facies metamorphism in the Belomorian Province.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

Two-stage decompression in garnet-bearing mafic granulites from Sostrene Island, Prydz Bay, East Antarctica

Mafic garnet-bearing granulites from Sostrene Island, 150 km southwest of Davis Station on the coast of Prydz Bay, East Antarctica, exhibit two-stage symplectic coronas on garnet, formed after peak metamorphic conditions (M1). An outer corona of Opx (Mg₆₆) + Pl (An_94–97) + minor Hbl mantles a finer-grained inner corona of Opx (Mg₆₇) + Pl (An_95–96) + Spl (Mg₃₆). Both symplectites contain minor ilmenite–magnetite intergrowths. The finer-grained symplectite also occurs along a fracture cleavage in the garnet. The outer corona originated during a second metamorphic event (M2) via the reaction Grt + Cpx (Hbl) + SiO₂= Opx + Pl (1), whereas the inner corona formed later in response to decompression and minor deformation, resulting in the fracture cleavage in the garnet, according to the reaction Grt = Opx + Pl + Spl (2). The grossular content of the garent (X_Grs= 0.168) is almost exactly that which is required for the stoichiometric breakdown by reaction (2) (calculated X_Grs= 0.167). The mafic rocks are silica undersaturated, and the SiO₂ for reaction (1) was most probably derived externally from the surrounding felsic gneisses. Preferred P–T estimates for M1 based on garnet core (Prp₄₀Alm₄₂Grs₁₇Sps₁)–matrix Opx–Cpx–Hbl pairs are c. 10 kbar at 980°C. The fine-grained symplectite formed post-peak M2 at c. 7 kbar and 850°C. The enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar, which compare with conditions of c. 6 kbar and 775°C in the nearby Bolingen Islands. These lower P–T estimates are considered to be representative of the widespread 1100-Ma metamorphic event recognized in outcrops along the Prydz Bay coast. The high-P, high-T estimates derived from the garnet relics provide evidence for an earlier, possibly Archaean, high-grade metamorphic event.     

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.     

P–T evolution and tectonic significance of lawsonite-bearing schists from the eastern segment of the southwestern Tianshan,China

Robust quantification of pressure (P)–temperature (T) paths for subduction-related HP/UHP metamorphic rocks is fundamental in recognizing spatial changes in both the depth of detachment from the down-going plate and the thermal evolution of convergent margin sutures in orogenic belts. Although the Chinese southwestern (SW) Tianshan is a well-known example of an accretionary metamorphic belt in which HP/UHP metabasites occur in voluminous host metasedimentary schists, information about the P–T evolution of these rocks in the eastern segment is limited, precluding a full understanding of the development of the belt as a whole. In this study at Kekesu in the eastern segment of the SW Tianshan, we use microstructural evidence and phase equilibrium modelling to quantify the peak and retrograde P–T conditions from two lawsonite-bearing micaschists and an enclosed garnet–epidote blueschist; for two of the samples we also constrain the late prograde P–T path. In the two micaschist samples, relics of prograde lawsonite are preserved in quartz inclusions in garnet, whereas in the metabasite, polymineralic aggregates included in garnet are interpreted as pseudomorphs after lawsonite. For garnet micaschist TK21, which is mainly composed of garnet, phengite/paragonite, albite, chlorite, quartz and relict lawsonite, with accessary rutile, titanite and ilmenite, the maximum P–T conditions for the peak stage are 18.0–19.0 kbar at 480–485°C. During initial exhumation, the retrograde P–T path passed through metamorphic conditions of 15.0–17.0 kbar at 460–500°C. For garnet–glaucophane micaschist TK33, which is mainly composed of garnet, glaucophane, phengite/paragonite, albite, chlorite, quartz, relict lawsonite and minor epidote, with accessary titanite, apatite, ilmenite and zircon, the maximum P conditions for the peak stage are >24.0 kbar at 400–500°C. During exhumation, the P–T path passed through metamorphic conditions of 17.5–18.5 kbar at 485–495°C and 14.0–17.5 kbar at 460–500°C. For garnet–epidote blueschist TK37, which is mainly composed of garnet, glaucophane, epidote, phengite, chlorite, albite and quartz, with accessary titanite, apatite, ilmenite, zircon and calcite, the prograde evolution passed through metamorphic conditions of ~20.0 kbar at ~445°C to P_max conditions of ~21.5 kbar at 450–460°C and T_max conditions of 19.5–21.0 kbar at 490–520°C. During exhumation, the rock passed through metamorphic conditions of 17.5–19.0 kbar at 475–500°C, before recording P–T conditions of <17.5 kbar at <500°C. These results demonstrate that maximum recorded pressures for individual samples vary by as much as 6 kbar in the eastern segment of the SW Tianshan, which may suggest exhumation from different depths in the subduction channel. Furthermore, the three samples record similar P–T paths from ~17.0 to 15.0 kbar, which suggests they were juxtaposed at a similar depth along the subduction interface. We compare our new results with published information from eclogites in the same area before considering the wider implications of these data for the orogenic development of the belt as a whole.     

Geochronology of high-pressure mafic granulite dykes in SW Sweden: tracking the P–T–t path of metamorphism using Hf isotopes in zircon and baddeleyite

Although the U–Pb zircon chronometer has been widely used for dating metamorphism in moderate‐ to high‐grade rocks, it is generally difficult to link the U–Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back‐projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U–Pb, Sm–Nd and Lu–Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ～5 km east of the NNE‐trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic, granulite facies assemblage. U–Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of c. 1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high‐pressure metamorphism at ～15 kbar and ～740 °C. The growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. The identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite‐to‐zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage. The estimated P–T conditions are ～10 kbar and ～700 °C. Analyses of small (～30 μm), clear and round zircon in this sample yield a Concordia U–Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu‐Hf and Sm‐Nd mineral isochron ages of 1027 ± 9 and 1022 ± 34 Ma, respectively. This type of secondary zircon plots at the lower end of the Lu‐Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.     

Mineral Corona Formation During High-P Retrogression of Granulitic Rocks, Ungava Orogen, Canada

Crystalline basement exposed in tectonic windows within theUngava Orogen records a polycyclic Archean granulite-faciesto Paleoproterozoic amphibolite-facies history. Amphibolite-faciesassemblages comprise garnet coronas around plagioclase, clinopyroxeneor cummingtonite coronas on orthopyroxene, hornblende coronason clinopyroxeneorthopyroxene, sodic rims on calcic plagioclase,and/or titanite coronas on ilmenite. Petrographic observationsand model reactions suggest that growth of coronitic garnetis closely associated with amphibolitization of twopyroxenegneisses. Calcic plagioclase constitutes a key reactant in allgarnet-producing reactions and possibly acted as a rate-controllingphase. Multi-equilibrium thermobarometric calculations showgood convergence of possible equilibria in the amphibolite-faciesrocks, indicating that coronitic textures need not imply completechemical disequilibrium. P—T determinations for the amphibolite-faciesgneisses beneath the thrust belt of Ungava Orogen are in therange 7.7–9.8 kbar and 585–723C. These values areconsistent with prograde determinations from pelitic schistswithin the thrust belt. Estimates of water activity clusterinto two populations. High a_H2O values are obtained for highlystrained basement rocks adjacent to the thrust belt whereaslow a_H2O values are derived for orthogneiss samples which showno thrust-related fabrics and are distal to the thrust belt. KEY WORDS: corona; high-P retrogression; multi-equilibrium thermo-barometry ^*Corresponding author. Telephone: (613) 995-4935. Fax: (613) 995-9273. Internet: mstonge{at}cc2smtp.emr.ca     

Amphibole evolution in Variscan eclogite-amphibolites from the Savona crystalline massif (Western Ligurian Alps, Italy): Controls on the decompressional P-T-t path

The Variscan metamorphic evolution of eclogite-amphibolites from the Savona Crystalline Massif is marked by the occurence of calcic amphiboles. Microtextural relations together with coupled electron microprobe and X-ray investigations on amphiboles pointed out a complex decompressional evolution.

A prograde stage of the evolution in the eclogite facies (temperature conditions estimated as ca. 620°C) is recorded by compositional zoning and inclusions in garnet. The eclogitic event (pressure conditions 12kbar) produced assemblages of garnet, omphacite, Ca-amphibole, zoisite, quartz and rutile. Composition and zoning of matrix eclogitic amphiboles reveal that the subsequent evolution in the eclogite facies contemplates a late re-equilibration under lower conditions of temperature (ca. 540°C) and pressure. The eclogitic amphiboles are pargasitic to edenitic hornblendes, and are marked by rather high Na contents at the M4 site (up to 0.5 apfu).

The forming of coarse-grained diopside-plagioclase symplectites and of amphibole-plagioclase coronas represent the early results of the destabilization of omphacites and garnets in the amphibolite facies. Pressure conditions of this stage have been estimated of ca. 10 kbar, while temperature conditions are poorly constrained. However, the increase in edenite (NaAlSi₋₁) at the outermost rims of the matrix eclogitic amphiboles and the very high values of edenite and tschermack (Al₂Mg₋₁Si₋₁) components in the coronitic amphiboles (subsilicic pargasites) suggest that temperature increase accompanied the early decompression to the amphibolite facies.

Subsequent retrogression in the low-grade amphibolite facies resulted in the complete breakdown of omphacite, producing finer-grained symplectites. This stage is also recorded by the amphibole growth (magnesio-hornblendes to edenitic hornblendes with negligible Na amounts at the M4 site) at the expense of the clinopyroxene symplectites and as outward rims around the coronitic amphiboles.

Frequently, amphibolites are completely recrystallized and do not retain any relic of high pressure assemblages. Temperature conditions for this late amphibolitic event have been estimated of ca. 510°C, while pressure conditions are inferred in the range 4–7 kbar.

Such a complicated decompressional evolution is indicative of complex uplift tectonics after a subduction event.     

Proterozoic anticlockwise P–T path of the Lewisian Complex of South Harris, Outer Hebrides, NW Scotland

The Leverburgh Belt and South Harris Igneous Complex in South Harris (northwest Scotland) experienced high-pressure granulite facies metamorphism during the Palaeoproterozoic. The metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic, quartzofeldspathic and mafic gneisses, especially in pelitic gneisses from the Leverburgh Belt: (1) some coarse-grained garnet in the pelitic gneiss includes biotite and quartz in the inner core, sillimanite in the outer core, and is overgrown by kyanite at the rims; (2) garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims; (3) the Al^VI/Al^IV ratio of clinopyroxene from mafic gneiss increases from core to rim; (4) retrograde reaction coronas of cordierite and hercynite+cordierite are formed between garnet and kyanite, and orthopyroxene+cordierite and orthopyroxene+plagioclase reaction coronas develop between garnet and quartz; (5) a P–T path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet+sillimanite and garnet+sillimanite+hercynite with increasing temperature; and (6) in sheared and foliated rocks, hydrous minerals such as biotite, muscovite and hornblende form a foliation, modifying pre-existing textures. The inferred metamorphic history of the Leverburgh Belt is divided into four stages, as follows: (M1) prograde metamorphism with increasing temperature; (M2) prograde metamorphism with increasing pressure; (M3) retrograde decompressional metamorphism with decreasing pressure and temperature; and (M4) retrograde metamorphism accompanied by shearing. Peak P–T conditions of the M2 stage are 800±30 °C, 13–14 kbar. Pressure increasing from M1 to M2 suggests thrusting of continental crust over the South Harris belt during continent–continent collision. The inferred P–T path and tectonic history of the South Harris belt are different from those of the Lewisian of the mainland.     

Reconstruction of the prograde PT history of high-P migmatitic paragneisses via melt-reintegration approach and thermodynamic modelling (Allochthonous Complexes,NW Iberian Massif)

The Upper Units of the allochthonous complexes of the NW Iberian Massif constitute a terrane with continental affinity. They represent the vestiges of a Cambrian magmatic arc developed in the periphery of Gondwana (West African Craton) which was involved in the Devonian Variscan collision, undergoing high-P, high-T metamorphism. This includes ultramafic rocks, high-P mafic rocks (eclogites and granulites) and high-P migmatitic paragneisses. The latter rocks show an extensive migmatization with the leucosomes oriented parallel to the regional foliation. The migmatitic paragneisses are composed of garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and Ti-hematite. Thermodynamic modelling using the measured bulk composition in the NCKFMASTHO system indicates metamorphic peak conditions of ~15 kbar and ~800 to 835°C, followed by a significant cooling. The prograde evolution is assessed by means of a melt-reintegration approach, using the composition of the garnet and its inclusions. An appropriate composition of liquid is added to the measured bulk composition to emulate the pre-melting bulk composition. Modelling of this melt-reintegrated composition allows to identify a colder high-P episode below ~500°C. Zircon crystals extracted from the leucosomes show overgrowths crystallized from the partial melt at c. 389 Ma (U–Pb system). The P–T–t path proposed reveals a subduction of the peri-Gondwanan arc-derived section down to mantle depths. An isobaric heating stage occurred as a result of residence at great depths and/or inception of a transient oceanic basin at c. 395 Ma. The ensuing near-isothermal exhumation occurred due to the extension related to the inception of the basin, reaching the thermal peak shortly before c. 389 Ma. Subsequent cooling is related to the underthrusting of colder oceanic and transitional crust below the HP-HT Upper Units.