Pseudosection modelling for a selected eclogite body from the Koralpe (Hohl), Eastern Alps

In the Kor-, Saualpe and Pohorje regions of the Eastern Alps eclogite bodies occur within metapelitic gneisses. The bodies are between 1 meter and several hundreds of meters in size and some of them were defined by Haüy (1822) as the type locality for the rock type “eclogite”. A growing body of petrological work has documented the metamorphic evolution of the metapelites surrounding the eclogites. However, few phase diagrams have been constructed for the eclogite bodies themselves. Here we use recently available activity models for amphiboles to present new thermodynamic pseudosections for the Hohl locality of the Koralpe eclogites. We show that this eclogite reached peak conditions in a narrow PT field obliquely oriented in PT space between 16.5 and 20.5 kbar and 620°C to 720°C and that its metamorphic evolution was likely to have occurred under water saturated conditions. We conclude that eclogite and the surrounding metapelites have certainly undergone the same metamorphic peak in Eo-alpine time. Comparison of our results with different PT estimates on the eclogite from Pohorje, suggest that a PT gradient from Koralpe to Pohorje is likely.     

Refining the timing of eclogite metamorphism: a geochemical,petrological, Sm-Nd and U-Pb case study from the Pohorje Mountains,Slovenia (Eastern Alps)

High-pressure metamorphism in the Pohorje Mountains of Slovenia (Austroalpine unit, Eastern Alps) affected N-MORB type metabasic and metapelitic lithologies. Thermodynamic calculations and equilibrium phase diagrams of kyanite–phengite-bearing eclogites reveal PT conditions of >2.1 GPa at T<750°C, but within the stability field of quartz. Metapelitic eclogite country rocks contain the assemblage garnet + phengite + kyanite + quartz, for which calculated peak pressure conditions are in good agreement with results obtained from eclogite samples. The eclogites contain a single population of spherical zircon with a low Th/U ratio. Combined constraints on the age of metamorphism come from U/Pb zircon as well as garnet–whole rock and mineral–mineral Sm-Nd analyses from eclogites. A coherent cluster of single zircon analyses yields a ²⁰⁶Pb/²³⁸U age of 90.7±1.0 Ma that is in good agreement with results from Sm-Nd garnet–whole rock regression of 90.7±3.9 and 90.1±2.0 Ma (εNd: +8) for two eclogite samples. The agreement between U-Pb and Sm-Nd age data strongly suggests an age of approximately 90 Ma for the pressure peak of the eclogites in the Pohorje Mountains. The presence of garnet, omphacite and quartz inclusions in unfractured zircon indicates high-pressure rather than ultrahigh pressure conditions. The analysed metapelite sample yields a Sm-Nd garnet–whole rock scatterchron age of 97±15 Ma. These data probably support a single P-T loop for mafic and pelitic lithologies of the Pohorje area and a late Cretaceous high-pressure event that affected the entire easternmost Austroalpine basement including the Koralpe and Saualpe eclogite type locality in the course of the complex collision of the Apulian microplate and Europe.     

Timing of high-pressure metamorphism and exhumation of the eclogite type-locality (Kupplerbrunn–Prickler Halt, Saualpe, south-eastern Austria): constraints from correlations of the Sm–Nd, Lu–Hf, U–Pb and Rb–Sr isotopic systems

Sm–Nd, Lu–Hf, Rb–Sr and SIMS U–Pb data are presented for meta‐gabbroic eclogites from the eclogite type‐locality ( Haüy, 1822 ) Kupplerbrunn–Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg‐rich eclogites derived from early gabbroic cumulates are kyanite‐ and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite ± kyanite. Calculated P–T conditions at the final stages of high‐pressure metamorphism are 2.2 ± 0.2 GPa at 630–740 °C. Kyanite‐rich eclogites did not yield geologically meaningful Sm–Nd ages due to incomplete Nd isotope equilibration, whereas Sm–Nd multifraction garnet–omphacite regression for a low‐Mg eclogite from Kupplerbrunn yields an age of 91.1 ± 1.3 Ma. The Sm–Nd age of 94.1 ± 0.8 Ma obtained from the Fe‐rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U–Pb SIMS age of 88.4 ± 8.1 Ma. Lu–Hf isotope analysis of a kyanite–eclogite from Kupplerbrunn yields 88.4 ± 4.7 Ma for the garnet–omphacite pair. Two low‐Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm–Nd age of 90.6 ± 1.0 Ma. A low‐Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet–whole‐rock Lu–Hf age of 93.3 ± 2.8 Ma. These new age data and published Sm–Nd ages of metasedimentary host rocks constrain the final stages of the eo‐Alpine high‐pressure event in the Saualpe–Pohorje part of the south‐easternmost Austroalpine nappe system suggesting that garnet growth in the high‐pressure assemblages started at c. 95–94 Ma and ceased at c. 90–88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high‐pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo‐Triassic Meliata back‐arc basin.     

Garnet Sm–Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P–T constraints on the thermal and tectonic history

Garnets from recrystallized, staurolite- and kyanite-bearing mica schists from the central Saualpe basement, representing the host rocks of the type-locality eclogites, give concordant Sm–Nd garnet–whole-rock isochron ages between 88.5±1.7 and 90.9±0.7 Ma. The millimetre-sized, mostly inclusion-free grains show fairly homogeneous element profiles with pyrope contents of 25–27%. Narrow rims with an increase in Fe and Mn and a decrease in Mg document minor local re-equilibration during cooling. According to phengite geothermobarometry, peak metamorphic conditions at 90 Ma were close to 20  kbar and 680  °C and similar to those recorded by the eclogites. The garnet rims record about 575  °C/7  kbar for the final stages of metamorphism. A phengitic garnet–mica schist, sampled at the immediate contact with the Gertrusk eclogite, gave a garnet–whole-rock Sm–Nd age of 94.0±2.7 Ma.
Garnet porphyroclasts separated from a pegmatite–mylonite of the Koralpe plattengneiss near Stainz are unzoned and show spessartine contents of 15%. Composition and Sm–Nd ages of close to 260 Ma point to a magmatic origin for these garnets.
The garnet data from the Saualpe document an intense Alpine metamorphism for this part of the Austroalpine basement. The mica schists recrystallized during decompression and rapid exhumation, at the final stages of and immediately following a high- P event. The Koralpe data show that high Alpine temperatures did not reopen the Sm–Nd isotope system, implying a closure temperature in excess of c . 600  °C for this isotopic system in garnet.     

P-T estimates and timing of the sapphirine-bearing metamorphic overprint in kyanite eclogites from Central Rhodope, northern Greece

Sapphirine-bearing symplectites that replace kyanite in eclogites from the Greek Rhodope Massif have previously been attributed to a high-pressure granulite-facies metamorphic event that overprinted the eclogitic peak metamorphic assemblage. The eclogitic mineralogy consisted of garnet, omphacitic pyroxene, rutile and kyanite and is largely replaced by low-pressure minerals. Omphacite was initially replaced by symplectites of diopside and plagioclase that were subsequently replaced by symplectites of amphibole and plagioclase. Garnet reacted during decompression to form a corona of plagioclase, amphibole and magnetite. Rutile was partly transformed to ilmenite and kyanite decomposed to produce a high-variance mineral assemblage of symplectitic spinel, sapphirine, plagioclase and corundum. The presence of quartz and corundum in the kyanite eclogites is evidence for the absence of bulk equilibrium and obviates a conventional analysis of phase equilibria based on the bulk-rock composition. To circumvent this difficulty we systematically explored the pressure-temperature-composition (P-T-X) space of a thermodynamic model for the symplectites in order to establish the pressure-temperature (P-T) conditions at which the symplectites were formed after kyanite. This analysis combined with conventional thermometry indicates that the symplectites were formed at amphibolite-facies conditions. The resulting upper-pressure limit (～0.7 GPa) of the sapphirine-producing metamorphic overprint is roughly half the former estimate for the lower pressure limit of the symplectite forming metamorphic event. Temperature was constrained (T ～ 720°C) using garnet-amphibole mineral thermometry. The P-T conditions inferred here are consistent with thermobarometry from other lithologies in the Rhodope Massif, which show no evidence of granulite-facies metamorphism. Regional geological arguments and ion-probe (SHRIMP) zircon dating place the post-eclogite-facies metamorphic evolution in Eocene times.     

Chemistry,mineralogy and petrology of an eclogite from the type locality (Saualpe,Austria)

An eclogite and five of its coexisting minerals (omphacite, garnet, carinthine, kyanite and zoisite) from the probable type locality of eclogites (Kupplerbrunn, Saualpe, Austria) described by Haüy (1822) have been analysed. Optical and X-ray data for these minerals are also given. Comparison of the Kupplerbrunn rock with those of other eclogites from the Saualpe region indicates they all have roughly similar compositions. When plotted on an A-C-F diagram the majority of these analyses fall in the region of kyanite-bearing eclogites suggested by Tilley (1936) although the Kupplerbrunn rock is the only sample containing kyanite; the others containing zoisite. The garnet and omphacite compositions of the Kupplerbrunn rock differ markedly from those of other Saualpe eclogites, possibly due to different metamorphic conditions of their formation. Carinthine analyses are all very similar for eclogites from Saualpe. On the basis of geological, analytical and limited experimental evidence, it is postulated that the Kupplerbrunn eclogite was derived from an original gabbroic rock low in water content such that amphibole and zoisite formed from plagioclase, pyroxene and water; omphacite, garnet and kyanite formed from plagioclase and pyroxene, once all the water was used up in the form of amphibole and zoisite. These reactions are believed to have taken place at 5–8 kb pressure at around 600° C; a value close to that suggested by Lodemann (1966) from field data.     

Metamorphic evolution and geochemistry of kyanite eclogites in central Rhodope,northern Greece

In central Rhodope of northern Greece, kyanite eclogites were discovered in the area of Thermes. They are strongly overprinted and exhibit a multi-stage development of minerals and mineral assemblages formed during successive stages of the exhumation. The initial high-pressure assemblage was garnet+omphacite+kyanite+zoisite+phengite+rutile+quartz. Corundum, Fe-Mg-spinel, sapphirine and högbomite occur as products of a first, high-temperature overprint, still at high pressures, whereas various symplectites [corundum-plagioclase (pl), spinel-pl, sapphirine-pl, clinopyroxene-pl, biotite-pl, amphibole-pl] grew during subsequent stages of the exhumation. Diablastic amphibole+plagioclase formed as end-products of the amphibolitization. According to geochemical data, the protoliths of the kyanite eclogites were basalts to basaltic andesites with “volcanic arc” affinities. For the high-pressure stage of metamorphism, minimum PT conditions were around 19 kbar, 700°C, while for the initial stages of the overprint, high-pressure granulite-facies conditions prevailed (T>800°C, at P>15 kbar). The PT conditions of the amphibolite facies were 8–11 kbar, 580–690°C. The kyanite eclogites of Thermes record the highest temperatures of metamorphism within the whole of Rhodope.     

Dehydration of metapelites during high‐P metamorphism: The coupling between fluid sources and fluid sinks

Polymetamorphic metapelites and embedded eclogites share a complex, episodic interplay of dehydration and fluid infiltration at the eclogite type‐locality (Saualpe–Koralpe, Eastern Alps, Austria). The metapelites inherited a fluid content (i.e. mineral‐bound OH expressed in terms of mol.% H₂O) of ～6–7 mol.% H₂O from high‐T–low‐P metamorphism experienced during the Permian. At or near P_max of the subsequent Eoalpine event (～20 kbar and 680^°C), the breakdown of paragonite to Na‐rich clinopyroxene and kyanite in metapelites released a discrete pulse of hydrous fluid. Prior to the dehydration event, the rocks were largely fluid absent, allowing only limited re‐equilibration during the prograde Eoalpine evolution. Similarly, Permian‐aged gabbros have persisted metastably due to the absence of a catalyst prior to fluid‐induced re‐equilibration. The fluid triggered partial to complete eclogitization along a fluid infiltration front partially preserved in metagabbro. Near‐isothermal decompression to ～7.5–10 kbar and 670–690°C took place under fluid‐absent conditions. After decompression, a second breakdown of phengitic white mica and garnet produced muscovite, biotite, plagioclase and ～0.1–0.7 mol.% H₂O that enhanced extensive fluid‐aided re‐equilibration of the metapelites. Potential relicts of high‐P assemblages were largely obliterated and replaced by the recurrent amphibolite facies assemblage garnet+biotite+staurolite+kyanite+muscovite+plagioclase+ilmenite+quartz. The hydrous fluid originating from the metapelites infiltrated the embedded eclogites at these P–T conditions and induced the local breakdown of the peak assemblage omphacite and garnet to fine‐grained symplectites of diopside and plagioclase. Further fluid infiltration led to the formation of hornblende–quartz poikiloblasts at the expense of the symplectites. The metapelites re‐equilibrated until the growth of retrograde staurolite consumed any remaining free fluid, thereby terminating the process. Further re‐equilibration is inhibited by both the lack of a catalytic fluid and H₂O as a reactant essential for rehydration reactions. The interplay between fluid sources and fluid sinks describes a closed cycle for the rocks at the eclogite type‐locality. Final, near‐isobaric cooling is indicated by a slight increase of X_Fe in garnet rims. Post‐decompression dehydration and fluid‐aided re‐equilibration arrested by the introduction of staurolite might explain the apparently homogeneous retrogression conditions as well as the notorious absence of diagnostic high‐P assemblages in metapelites at the eclogite type‐locality.     

华北克拉通中部造山带早前寒武纪变质演化历史评述

根据变质作用程度不同,华北中部造山带早前寒武纪基底可以进一步分为高级区和花岗-绿岩带。前者变质程度可达高角闪岩相-麻粒岩相,包括太华、吕梁、阜平、恒山、怀安、宣化等杂岩,花岗-绿岩带的变质程度较低,多为绿片岩相-角闪岩相,包括登封、赞皇、五台等杂岩。已有变质演化研究表明,高级区恒山、怀安和宣化杂岩中的麻粒岩和(或)退变榴辉岩记录的峰期变质压力最高,恒山杂岩、阜平杂岩和宣化杂岩中的麻粒岩记录的峰期变质温度最高;花岗-绿岩带中的赞皇杂岩和五台杂岩出露高压斜长角闪岩和高压变泥质岩。中部带各变质杂岩中可识别出早期进变质、峰期、峰后快速降压和晚期冷却等变质阶段,拥有顺时针近等温降压型的变质作用P-T轨迹,与华北克拉通中部的俯冲碰撞有关。大量变质年代学数据显示,中部带各变质杂岩中至少记录了~1.85Ga、~1.95Ga和~2.5Ga三组变质年龄,其中,~1.85Ga的变质年龄占据了主导地位,大致与区域片麻理形成的时间一致,代表变质高峰期时代;~1.95Ga的变质年龄代表了峰期前的某个进变质片段;~2.5Ga的变质年龄则指示了更早一期的变质事件,推测与古老陆块~2.5Ga所遭受的大量幔源岩浆的侵入和底垫作用有关。然而,变质年龄与变质阶段的对应关系尚不明确。     

Sapphirine in SW Sweden: a record of Sveconorwegian (–Grenvillian) late-orogenic tectonic exhumation

In the Sveconorwegian granulite region of SW Sweden, sapphirine occurs in reaction coronas in Mg- and Al-rich kyanite eclogites which form parts of mafic complexes. Aluminous to peraluminous sapphirine forms symplectitic intergrowths with plagioclase±corundum±spinel after kyanite. Kyanite and omphacite were the main reactants in the formation of sapphirine. The sapphirine formed during decompression from the eclogite facies ( P >15  kbar) through the high- to medium-pressure granulite and upper amphibolite facies at c. 750  °C. Preserved growth zoning in garnet, frozen-in reaction textures, and chemical disequilibrium suggest a rapid tectonic exhumation. Ductile deformation in the surrounding gneisses and parts of the mafic complex is characterized by foliation development, WNW–ESE stretching and dynamic recrystallization under granulite to upper amphibolite facies conditions, simultaneous with the sapphirine formation. This decompression, high-grade re-equilibration and associated deformation took place during the exhumation of the Sveconorwegian eclogites, bracketed between 969±14 and 956±7  Ma. Probable tectonic causes are late-orogenic gravitational collapse and/or plate divergence following the Sveconorwegian–Grenvillian continent–continent collision. There are no indications of metastability of aluminous and peraluminous sapphirine in the decompressed kyanite eclogites; sapphirine is stable in amphibole-poor and amphibolitized varieties, including rocks that have undergone dynamic recrystallization. Close similarities between rocks from different parts of the world with respect to reaction textures suggests that sapphirine+plagioclase-forming reactions are a universal feature in high-temperature decompressed kyanite eclogites.     

Mechanisms of exsolution in omphacites from high temperature,type B,eclogites

The microstructural development during the transformation of omphacite into pyroxene-plagioclase symplectites has been studied in some eclogites from the Seve nappe, Central Scandinavian Caledonides. The omphacite transformation can be described as a discontinuous precipitation reaction that occurred in two clearly defined stages to produce a coarser type A, followed by a finer, type B symplectite. Each type has its distinctive chemistry. The combination of microstructural and chemical characteristics of the transformation is used to reconstruct the early stages of the cooling history of the eclogites. In addition, based on a classification of phase transformations according to growth processes, the continuous exsolution reaction reported in high-temperature omphacites is combined with the discontinuous reaction in a time-temperature transformation (TTT) diagram to produce a more unified view of the exsolution in the omphacites.     

P–T evolution and reaction textures in retrogressed eclogites from Svetlik, the Moldanubian Zone (Czech Republic)

Summary Lenses and pods of mafic rocks from the Monotonous Unit near Svetlik are characterized by eclogite facies mineral assemblages; however some inclusion patterns (oriented quartz rods in clinopyroxene and cuboids of disordered graphite in garnet) that are usually known from ultra-high pressure rocks were also observed in one sample. Conventional thermobarometry yielded maximum P–T conditions of 2.0–2.5 GPa and 750 °C. Decompression and heating at amphibolite and granulite facies conditions resulted in the formation of at least five distinct types of symplectites. These include symplectitic intergrowth of ilmenite and clinopyroxene after titanite, described here for the first time from the Moldanubian Zone. In addition, symplectites of plagioclase and biotite with accessory amounts of spinel after tabular pseudomorphs (after phengite?) are also reported here. Mass balance relations indicate that symplectites of diopside + plagioclase after omphacite and plagioclase + spinel (sapphirine) after kyanite + garnet, formed by nearly isochemical reactions. All other symplectite-forming reactions were allochemical and were accelerated by the presence of fluid in the primary phases. Preserved zoning pattern in garnet with high compositional gradient in some samples suggests that the rocks were affected briefly by granulite facies overprint.     

Two Stages of Sapphirine Formation During Prograde and Retrograde Metamorphism in the Palaeoproterozoic Lewisian Complex in South Harris, NW Scotland

Two types of sapphirine occurrences were found in the Lewisiancomplex in South Harris, NW Scotland: (1) inclusions withinporphyroblasts; (2) symplectic grains together with secondarycordierite, plagioclase and orthopyroxene. The presence of sapphirineinclusions implies that sapphirine was stable at the early stageof ultra-high-temperature metamorphism, whereas symplectic sapphirinegrains were formed during decompressional retrograde metamorphism.The sapphirine occurrences and compositions of associated mineralsdepend on the host rock composition. Sapphirine inclusions occuronly in rocks with high bulk-X_Mg, and sapphirine is never presentas porphyroblastic grains because of its breakdown in responseto pressure increase. Sapphirine symplectites can be seen inthe relatively low bulk-X_Mg rocks, and the texture suggestslocal equilibrium in the Mg–Al-rich domain that is formedby metamorphic segregation of the Mg–Al-rich mineralsin response to partial melting. The various sapphirine occurrencesobserved in South Harris were controlled by not only protolithcomposition but also local, mineral-scale, composition in acontinuous metamorphic history. KEY WORDS: sapphirine; Lewisian complex; high-Mg garnet; partial melting     

Petrogenesis of sapphirine-bearing metatroctolites from the Buck Creek ultramafic body,southern Appalachians

The Buck Creek ultramafic body, North Carolina, includes aluminous lenses that have been described as troctolites. These lenses preserve mineral assemblages which record several different stages of metamorphism. The first stage is characterized by anhydrous reactions between olivine and plagioclase to produce coronas of orthopyroxene+ clinopyroxene/spinel symplectite. Thermo barometric results indicate minimum pressures of c. 6 kbar and c. 800 ^oC. Sapphirine replaces spinel in some clinopyroxene symplectites, and occurs as anhedral grains within amphibole, observations which in combination suggest peak metamorphic conditions of c. 9-10 kbar and c. 850 ^oC. Sapphirine-bearing hydrous assemblages formed at the expense of the coronas, indicating a second metamorphic episode involving deeper burial, deformation and hydration. Schistose rocks from the margins of the lenses are composed of anorthite+amphibole+margarite+corundum, and probably record a later, lower P-T event. Whole rock analyses for the Buck Creek lenses suggest an accumulate protolith of magnesian olivine and calcic plagioclase. Trace element data for the troctolites are consistent with data for adjacent amphibolites in suggesting that the Buck Creek mafic and ultramafic cumulates crystallized from magmas derived from a mantle source similar to that which produces modern intraplate or rift-related basalts. We propose that the Buck Creek ultramafics represent basal cumulates(± uppermost mantle) from ocean crust formed in a marginal basin in the latest Precambrian. Subduction-induced burial to at least 18 km under dry conditions induced corona formation. Collisional events of the Taconic orogeny thrust the Buck Creek rocks into the orogenic pile to at least 30 km depth and hydrated them along zones of weakness, locally producing P-T -P_H2O conditions appropriate for formation of sapphirine and hydrated assemblages, but still preserving some dry symplectites.     

Silica-rich Melts in Quartz Xenoliths from Vulcano Island and their Bearing on Processes of Crustal Anatexis and Crust-Magma Interaction beneath the Aeolian Arc, Southern Italy

Quartz-rich xenoliths in lavas and pyroclastic rocks from VulcanoIsland, part of the Aeolian arc, Italy, contain silicic meltinclusions with high SiO₂ (73–80 wt %) and K₂O (3–6wt %) contents. Two types of inclusions can be distinguishedbased on their time of entrapment and incompatible trace element(ITE) concentrations. One type (late, ITE-enriched inclusions)has trace element characteristics that resemble those of themetamorphic rocks of the Calabro-Peloritano basement of theadjacent mainland. Other inclusions (early, ITE-depleted) havevariable Ba, Rb, Sr and Cs, and low Nb, Zr and rare earth element(REE) contents. Their REE patterns are unfractionated, witha marked positive Eu anomaly. Geochemical modelling suggeststhat the ITE-depleted inclusions cannot be derived from equilibriummelting of Calabro-Peloritano metamorphic rocks. ITE-enrichedinclusions can be modelled by large degrees (>80%) of meltingof basement gneisses and schists, leaving a quartz-rich residuerepresented by the quartz-rich xenoliths. Glass inclusions inquartz-rich xenoliths represent potential contaminants of Aeolianarc magmas. Interaction between calc-alkaline magmas and crustalanatectic melts with a composition similar to the analysed inclusionsmay generate significant enrichment in potassium in the magmas.However, ITE contents of the melt inclusions are comparablewith or lower than those of Vulcano calc-alkaline and potassicrocks. This precludes the possibility that potassic magmas inthe Aeolian arc may originate from calc-alkaline parents throughdifferent degrees of incorporation of crustal melts. KEY WORDS: melt inclusions; crustal anatexis; magma assimilation; xenoliths; Vulcano Island     

Omphacite breakdown reactions and relation to eclogite exhumation rates

Clinopyroxene + plagioclase (±Hbl ± Qtz) symplectites after omphacite are widely cited as evidence for prior eclogite-facies or high-pressure (HP) metamorphism. Precursor omphacite compositions of retrograde eclogites, used for reconstructing retrograde P–T paths, are commonly estimated by reintegrating symplectite phases with the assumption that the symplectite-forming reactions were isochemical. Comparisons of broadbeam symplectite compositions to adjacent unreacted pyroxene from various symplectites after clinopyroxene from the Appalachian Blue Ridge (ABR) and Western Gneiss Region (WGR) suggest that the symplectite forming reactions are largely isochemical. Endmember calculations based on reintegrated symplectite compositions from the ABR and WGR suggest that a minor Ca-Eskola (CaEs) component (X_CaEs = 0.04–0.15) was present in precursor HP clinopyroxene. WGR symplectites consist of fine-grained (∼1 μm-scale), vermicular intergrowths of Pl + Cpx II ± Hbl that occur at grain boundaries or internally. ABR symplectites contain coarser (∼10 μm-scale) planar lamellae and rods of Pl + Cpx II + Qtz + Hbl within clinopyroxene cores. The contrasting textures correlate with decompression and cooling rate, and degree of overstepping of the retrograde reaction (lamellar: slow, erosionally controlled exhumation with slow/low overstepping; fine-grained, grainboundary symplectite: rapid, tectonic exhumation with rapid/high overstepping). Variations in X_CaEs, X_jd, and X_CaTs of precursor HP omphacite are related to the symplectic mineral assemblages that result from decompression. Quartz-normative symplectities indicate quartz-producing retrograde reactions (e.g., breakdown of precursor CaEs); quartz-free symplectities (e.g., diopside + plagioclase after omphacite) indicate quartz-consuming reactions (jd, CaTs breakdown) outpaced quartz-producing reactions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Contrasting Eoalpine P-T evolutions in the southern Koralpe,Eastern Alps

Summary The Koralpe crystalline complex and the Plankogel unit represent two lithologically distinct units within the Koralpe region of the southeastemmost Austroalpine crystalline basement. The Eoalpine P-T evolution of these two units is derived from new petrographical data. The Plankogel unit and the Koralpe crystalline complex show markedly different P-T evolutions during the early stages of the Eoalpine event. The rocks of the Koralpe crystalline complex experienced eclogite facies conditions with minimum pressures in the range of 15–16 kbar and temperatures in excess of 700°C. At the same time the Plankogel unit resided in a shallower environment at pressures of 10–11 kbar and temperatures of less than 600°C. The tectonic emplacement of the Plankogel unit into its present position on top of the Koralpe crystalline complex took place after the eclogite facies event in a relatively shallow crustal level. After their juxtaposition the Koralpe crystalline complex and the Plankogel unit were affected by a common amphibolite facies metamorphic overprint. The distinctly different P-T evolution during the early stages of the Eoalpine event and a common history at later stages imply that major tectonic processes were operative in this part of the Austroalpine crystalline basement during the Cretaceous. Such processes may have involved subduction of oceanic and continental lithosphere which may have lead to significant crustal shortening within the Austroalpine basement.

---

Unterschiedliche Eoalpine P-T Entwicklungen in der Südlichen Koralpe, Ostalpen

Zusammenfassung Das Koralpenkristallin und die Plankogelserie stellen zwei unterschiedliche lithologische Einheiten in der südlichen Koralpe des ostalpinen Kristallins dar. Die P-T Entwicklung dieser beiden Einheiten während der Eoalpinen Metamorphose wurde anhand neuer petrographischer Daten abgeleitet. Das Koralpenkristallin und die Plankogelserie zeigen deutlich unterschiedliche P-T Entwicklungen in einem frühen Stadium der Eoalpinen Metamorphose. Die Gesteine des Koralpenkristallins waren eklogitfaziellen Bedingungen mit Mindestdrucken im Bereich von 15 bis 16 kbar und Temperaturen von über 700°C ausgesetzt. Die Plankogelserie verweilte zur gleichen Zeit in einem relativ seichten Niveau bei Drucken von 10 bis 11 kbar und Temperaturen unterhalb 600°C. Die Platznahme der Plankogelserie in ihrer heutigen Position im tektonisch Hangenden des Koralpenkristallins erfolgte nach dem eklogitfaziellen Ereignis in einem relativ seichten Krustenniveau. Nach ihrer Vereinigung erfuhren die beiden Einheiten eine gemeinsame amphibolitfazielle Überprägung. Die markant unterschiedlichen P-T Entwicklungen in einem frühen Stadium der eoalpinen Orogenese und die gemeinsame Entwicklung in einem späteren Stadium können als Hinweis auf eine umfangreiche tektonische Aktivität in diesem Teil des ostalpinen Grundgebirges in kretazischer Zeit gewertet werden. Diese Tektonik bestand eventuell in einer Subduktion von ozeanischer und kontinentaler Lithosphäre, und kann zu einer signifikanten Krustenverkürzung im Ostalpinen Kristallin geführt haben.

---

---

With 7 Figures     

High-pressure relics in meta-sediments intercalated with the Weissenstein eclogite,Münchberg gneiss complex,Bavaria

The mineral assemblages in the eclogites and meta-sediments of the Münchberg gneiss complex suggest minimum pressures of about 15 kbar, and temperatures of 600±30° C for the eclogite-facies metamorphism. Both rock types underwent a subsequent amphibolite-facies metamorphism at almost the same temperature range. In the Weissenstein borehole the eclogites and meta-sediments show an intimate interlayering on a centimetre scale. Contacts between the two rock types are often sharp and untectonized suggesting in-situ metamorphism of the eclogites. The following features indicate that the gneisses were subjected to eclogite-facies metamorphism:

1. Phengite relics and phengite inclusions in garnet contain up to 3.45 Si per formula unit while amphibolite facies phengites have considerably lower Si-contents (3.0–3.25 p.f.u.).
2. Omphacite relics occur in the form of Na-augite (6–9 mol% jadeite)-oligoclase symplectites.
3. Garnets with up to 40 mol% of the pyrope component occur.

The geological and mineral-chemical data indicate that large volumes of crustal material have been buried to depths possibly exceeding 70 km. The preservation of primary eclogitic textures as well as symplectitic textures in the presence of a fluid phase, are indicative of a rapid decompression. This would suggest a tectonic uplift (e.g. underthrusting) as is also indicated by the virtually constant temperature range during the uplift.     

Constraining peak P–T conditions in UHP eclogites: calculated phase equilibria in kyanite‐ and phengite‐bearing eclogite of the Tromsø Nappe,Norway

Kyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (X_Grs = 0.25–0.29), but lower pyrope‐content (X_Prp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa & 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (X_Ca^Grt = 0.18–0.21) and symplectite clinopyroxene (X_Na^Cpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (X_Mg = 0.86–0.87) and spinel (X_Mg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out‐of‐sequence thrust during the Scandian lateral transport of nappes.     

Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Canadian Shield

A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year^?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.