The clinopyroxene/plagioclase symplectite in retrograde eclogites: A potential geothermobarometer

The retrograde P-T-t path of intermediate and high-temperature eclogites (T 500 ± 50°C) is characterized by the transformation of omphacite into a symplectitic intergrowth of clinopyroxene and plagioclase lamellae. Often, this symplectite shows a two stage microstructural evolution, i.e. a coarse lamellar width symplectite, »S1«, followed by a thinner lamellar width symplectite, »S2«. Each symplectite may also evolve independently into a final globular microstructure, »SG«.According to metallurgical concepts, the phase transition »omphacite clinopyroxene + plagioclase« is classified as a discontinuous precipitation reaction. In discontinuous precipitation reactions, the lamellar spacing (L) depends on temperature (T) according to the growth law log (L) = A-B/T, in which A and B are constants. Therefore thin symplectites characterize low temperatures, where-as coarse symplectites characterize higher temperatures.In order of specify the thermal evolution of continental collision zones, the reliability of the discontinuous precipitation growth law has been tested on eclogites showing contrasted retrograde P-T-t histories in different geological environments, i.e. Alpine, Caledonian and Variscan Belts. Indeed, our investigations reveal that the clinopyroxene/plagioclase symplectite can be used as a potential geothermobarometer in eclogites recording temperatures above 500 ± 50°C. The temperature is obtained by the lamellar spacing and pressure by the Jadeite content of the symplectite clinopyroxene in the presence of plagioclase and quartz.

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Zusammenfassung Der retrograde P-T-t Pfad von intermediären und hochtemperierten Eklogiten (T 500 ±50°C) ist charakterisiert durch die Umformung von Omphazit in symplektitische Verwachsungen von Klinopyroxen und Plagioklas-Lamellen. Häufig zeigt dieser Symplektit eine zweiphasige mikrostrukturelle Entwicklung, das heißt, ein Symplektit mit breiten Lamellen »S1« wird gefolgt von einem Symplektit mit dünneren Lamellen »2«. Jeder Symplektittyp kann sich schließlich selbständig zu einer kugelförmigen MikroStruktur »SG« entwikkeln.Auf Grund von metallurgischen Vorstellungen wir die Phasenumwandlung Omphazit zu Klinopyroxen + Plagioklas als diskontinuierliche Ausfällungsreaktion klassifiziert. In diskontinuierlichen Ausfällungsreaktionen hängt der Lamellenabstand (L) von der Temperatur (T) nach dem Wachstumsgesetz log (L) = A-B/T ab, in dem A und B Konstanten sind. Daher zeigen dünne Symplektite niedrige Temperaturen an, während breite Symplektite höhere Temperaturen anzeigen.Um die thermische Entwicklung von kontinentalen Kollisionszonen zu spezifizieren mußte die Zuververlässigkeit des diskontinuierlichen Ausfällungswachstumsgesetz an Eklogiten getestet werden, die unterschiedlich retrograde P-T-t Geschichten aus verschieden Umgebungen zeigen; das heißt aus alpinen, kaledonischen und variszischen Gebirgszügen. Tatsächlich offenbaren unsere Bemühungen, daß der Klinopyroxen/Plagioklas Symplektit als potentielles Geothermobarometer in Eklogiten benutzt werden kann, die Temperaturen über 500°C ± 50°C erfahren haben. Die Temperatur kann man aus dem Lamellenabstand erhalten; den Druck durch den Jadeitgehalt des symplektitischen Pyroxens, in Anwesenheit von Plagioklas und Quarz.

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Résumé La déstabilisation de l'omphacite, en une symplectite à alternance de lamelles de clinopyroxène et de plagioclase, caractérise l'initiation du trajet rétrograde P-T-t pour les éclogites de température 500 ± 50°C. Cette symplectite présente parfois une évolution microstructurale depuis une symplectite grossière à alternance de lamelles larges, »S1«, relayée par une symplectite à alternance de lamelles fines, »S2«. Chaque symplectite peut aussi évoluer indépendemment en une microstructure globulaire tardive, »Sg«.En accord avec les concepts établis par les métallurgistes, la transformation »omphacite clinopyroxène + plagioclase« est une précipitation discontinue. Pour ce type de transformation, l'espace interlamellaire (L) dépend de la température (T) selon la loi de croissance Log (L) =A-B/T (*), où A et B sont des constantes. Ainsi, une symplectite fine (de type S2) caractérise des températures relativement plus basses qu'une symplectite grossière (de type S1).Afin de préciser l'évolution thermique des zones de collision continentale, nous avons testé la loi de croissance (*) sur des éclogites échantillonnées dans divers contextes géologiques (chaînes alpine, calédonide et varisque), présentant des trajets rétrogrades P-T-t contrastés. Les résultats montrent que la symplectite à clinopyroxène/plagioclase représente un géothermobaromètre potentiel de la rétromorphose des éclogites: la taille de l'espace interlamellaire dépend de la température et la teneur en jadéite des lamelles de clinopyroxène, en présence de quartz et à l'équilibre avec le plagioclase, dépend de la pression.

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. , .. , «S 1», «S 2». «SG». , . (L) () — — log (L) = - /, . , . , , , .: , . , , / , , 500° + 50°. , — , .

     

Exsolution of garnet within clinopyroxene of mantle eclogites: major- and trace-element chemistry

Eclogite xenoliths from the mantle have experienced a wide variety of processes and P-T conditions, many of which are recorded in the mineral compositions and textures. Exsolution of garnet from clinopyroxene is one such texture, occurring in a minority of mantle eclogites. New analyses of clinopyroxene and garnet of eclogite xenoliths from kimberlites at Bellsbank (South Africa) and Obnazhënnaya (Yakutia, Russia) are presented here, and these are combined with data from the literature. Exsolution of garnet from clinopyroxene is generally lamellar, although lens-shaped garnets are also present. Major- and trace-element characteristics show a wide range of compositions and include eclogite Groups A, B, and C. Rare-earth element (REE) concentrations of garnet and pyroxene were determined by SIMS, and the REE patterns are subtly different from those in ordinary eclogites. Differences include the absence of prominent Eu anomalies in samples of this study and differences in the slopes of chondrite-normalized REE patterns. It is possible that these signatures are unique to exsolved eclogites, a result of subsolidus elemental partitioning during exsolution. Some reconstructed whole-rock compositions are aluminuous; comparison with ordinary eclogites shows only minor differences, implying a similar origin. If the immediate precursor to the exsolved eclogites was a monomineralic pyroxenite, the excess aluminium was tied up in Tschermak's molecule, although the occasional presence of kyanite exsolution lamellae is indicative of a Ca-Eskola component. Reconstructed pyroxenes from kyanite- and corundum-rich samples contain unrealistic amounts of aluminium for mantle pyroxenes. A protolith (or parental pyroxene) threshold of 24% Al₂O₃ may exist, above which (as in a plagioclase cumulate) the final assemblage is kyanite- and/or corundum-bearing.     

Strange attractors in plagioclase oscillatory zoning: petrological implications

Oscillatory zoning in plagioclase crystals from different mixed lava flows cropping out on the Island of Capraia (Italy) has been investigated. An-calibrated profiles from back-scattered-electron images are used as compositional time series of oscillatory zoning. They are analyzed by a qualitative visual method that allows to appreciate at first sight the global structure of the time series and by a quantitative method to reconstruct the attractors associated to the oscillatory patterns, and to calculate the fractal dimension of attractors. Results show that attractors have fractional dimensions, indicating that series are chaotic. In addition, it is evidenced that there is a wide variation in oscillatory zoning in crystals from the same lava flow and that plagioclase populations from the different lava flows differ in the shape of frequency histograms of attractor dimension. The development of oscillatory zoning is simulated by considering a chemically inhomogeneous magmatic mass governed by chaotic flow fields, coupled with chemical diffusion, in which plagioclase crystals grow according to the availability of nutrients in their neighborhoods. Results show that in such a dynamical system plagioclase crystals develop chaotic zoning patterns analogous to those observed in natural plagioclases. This approach allows us to explain the differences observed in plagioclase crystals from the same lava flow and the differences in the shape of frequency histograms of attractor dimensions in the four lava flows.     

Zircons of igneous rocks at the Erdenetuin-Obo porphyry Cu-Mo deposit (Mongolia): U-Pb dating and petrological implications

The Erdenetuin-Obo porphyry Cu-Mo deposit was formed at the final stage of development of magmatic activity occasionally manifested in the Late Permian-Early Triassic in the period of at least 40 Ma. Early plutonic (host) and late ore-bearing porphyry intrusive complexes were formed in that period. The plutonic complex is multiphase, while the porphyry complex is polyrhythmical and multiphase within rhythms. The obtained data on the U-Pb isotopic composition (SHRIMP II) of zircons from unaltered rocks of the ore field are discussed: gabbro, diorite, and granodiorite of the plutonic complex and granodiorite-porphyry I and II of the first and second rhythms of the ore-bearing complex, respectively. Zircons of different age levels and genotypes were identified in the course of performed investigations. Gabbro are dominated by postmagmatic (superimposed) zircons with the datings of 239–225 Ma. The age of xenogenic zircon brought out from the basement rocks is estimated at 1146 ± 11 Ma. Zircons occur as magmatic and postmagmatic (superimposed) minerals dated 252–247, 244–233 Ma in diorite and 244–242, 239–224 Ma in granodiorite. The ages of postmagmatic zircons from diorite are partially overlapped by datings of magmatic zircons from granodiorite and granodiorite-porphyry. In the porphyry complex, the datings of magmatic zircons are 240–234 and 222–220 Ma in granodiorite-porphyries I and II, respectively. There are also inherited zircons with datings coinciding with those of magmatic zircons from precursor intrusive rocks. Datings of such zircons are 249–241 and 257–231 Ma for granodiorite-porphyries I and II, respectively. As a whole, zircon datings in all studied igneous rocks forming a virtually uninterrupted range in the period of 257–220 Ma allow us to suggest the relation of the ore magmatic system to the long-living constantly active deep source occasionally delivering melt to the upper levels.     

Disequilibrium in co-existing clinopyroxene and garnet from blueschist-facies Fe-rich eclogites, Voltri Group, Italy

Abstract Clinopyroxenes and garnets from 11 blueschist-facies Fe-rich eclogite samples from the Voltri Group show a wide range of chemical compositions. Detailed analyses of single pyroxene and garnet grains show wide and scattered chemical inhomogeneity, the K_D(K_D= (Fe₂₊/Mg)^Gt/(Fe²⁺/Mg)^Cpx) ranges from 20 to 87 based on rim analyses only. The data obtained indicate that the mineral pairs never attained equilibrium under uniform P-T conditions and that the compositions of the metamorphic minerals were influenced mainly by the composition of the pre-metamorphic minerals and by topotactical reactions.     

Mineral chemistry of Monte Vulture volcanics: petrological implications

Summary Two distinct periods of volcanic activity are distinguishable at Monte Vulture volcanic complex: the first activity started with tephritic and foiditic volcanics, documented by xenoliths in the phonotrachytic (Ph-T) ignimbrites, lava blocks in a basal explosion breccia deposit beneath the Ph-T ignimbrites, sandy lenses rich in volcanic components in fluviatile conglomerates, and ended with emplacement of phonotrachytic deposits (Ph-T ignimbrites and lava domes); whereas subsequent activity produced deposits of foiditic and tephritic composition which built up the central volcano. The volcanics of the two periods do not fit a simple evolutionary model on the basis of geochemical signatures.New whole-rock and microprobe analyses of mafic minerals from volcanics of the first period are presented here, together with the published data on M. Vulture volcanics, to give a more comprehensive picture of the magma evolution.Principal features can be summarized as follows: (i) melanitic garnet occurs as a stable phase only in the volcanics of the first period of activity; (ii) olivine and diopside occur only in the cone building deposits; (iii) pyroxenes from the volcanics of the first stage crystallized under different physico-chemical conditions with respect to those occurring in volcanics of the central edifice; (iv) the analyzed lavas occurring as blocks in the explosion breccia deposit of the first stage display diversities with respect to the lavas of the central volcano, and it is likely that diverse parental magmas fed the first and the subsequent activities. During the first stage, crystal/liquid fractionation processes determined the evolution from the mafic parental magmas towards phonotrachytic compositions, as with the foidite-tephrtte sequence of the second stage (cone building deposits). Mineralogical evidence of mixing processes is present in the volcanics of the sequences of the two stages.

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Die Mineralchemie der Monte Vulture Vulkanite: Petrologische Konsequenzen

Zusammenfassung Zwei Stadien vulkanischer Aktivität sind im Monte Vulture Komplex zu unterscheiden. Das erste Stadium ist durch tephritische und foiditische Vulkanite, die als Xenolithe in den phonotrachytischen (Ph-T) Ignimbriten erhalten sind, durch Lava Blöcke in einer basalen Explosionsbrekkzie im Liegenden der Ph-T Ignimbrite und durch Sandlinsen in fluviatilen Konglomeraten, die reich an vulkanogenen Komponenten sind, gekennzeichnet. Es endet mit der Eruption der Phontrachyte (Ph-T Ignimbrite und Lava Kuppen). Während des zweiten Stadiums wurden vulkanogene Ablagerungen foiditischer und tephritischer Zusammensetzung, die heute den Zentralteil des Vulkans aufbauen, eruptiert. Die Geochemie der Vulkanite läßt sich nicht mit Hilfe eines einfachen Bildungsmodelles erklären. Neue Gesamtgesteins- und Mikrosondenanalysen von Vulkaniten des Erststadiums und deren mafischer Minerale werden gemeinsam mit bereits publizierten Daten über die Vulkanite des Monte Vulture präsentiert, um ein zusammenfassendes Bild seiner Magmenentwicklung zu geben.Grundsätzlich kann folgendes festgehalten werden: (i) Granat tritt als stabile Phase nur in den Vulkaniten des ersten Stadiums auf. (ii) Olivin und Diopsid sind auf die Vulkanschlote beschränkt. (iii) Die Pyroxene der Vulkanite des Frühstadiums kristallisierten unter anderem physikalisch-chemischen Bedingungen, als die im Zentralteil des Vulkans. (iv) Die untersuchten Laven, die als Blöcke in der basalen Explosionsbrekkzie auftreten, unterscheiden sich ebenfalls von denen des zentralen Teiles. Wahrscheinlich sind sie von unterschiedlichen Ausgangsmagmen abzuleiten. Durch Fraktionierung entstanden während des ersten Stadiums aus einem ursprünglich mafischen ein phonotrachytisches Magma und während des zweiten die foiditisch-tephritische Abfolge der Vulkanschlote. Hinweise auf Mischungsprozesse liegen vor.

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With 6 Figures     

Diffusion-controlled olivine corona textures in granitic rocks from Lofoten, Norway: calculation of Onsager diffusion coefficients, thermodynamic modelling and petrological implications

Reactions occurring during cooling of charnockitic intrusives on the Lofoten Islands produce characteristic diffusion-controlled textures around fayalite and Fe–Ti oxides. Thermobarometry indicates the corona textures formed at 780–840 °C and pressures of 4–10 kbar, whereas the magmatic assemblage of the charnockite (clinopyroxene–olivine–quartz) crystallized at about 850–870 °C and 4 kbar. The succession olivine|orthopyroxene+magnetite|orthopyroxene+garnet and olivine|orthopyroxene+magnetite|amphibole developed where olivine reacted with adjacent plagioclase or K-feldspar, but the modes and the thicknesses of the corona textures vary according to the feldspar type, indicating that the primary magmatic ternary feldspar was already exsolved into albitic plagioclase and alkali feldspar when the corona formation began. Simultaneously, in other parts of the rock, primary magmatic clinopyroxene reacted to amphibole and Fe–Ti oxides reacted to orthopyroxene+garnet coronas or to amphibole. Textures demonstrate significant Al diffusion in the rocks under granulite facies conditions and they suggest that no pervasive fluid influx occurred and that amphibole formation was dependant on a local source of H₂O probably related to water-release during the last stages of magmatism. Calculation of the net reaction by accounting for all observed reactions at different sites in the rock indicates that the system can be regarded as balanced on a hand-specimen scale with respect to all elements except for Na and H₂O. The larger variety of textures developed in rocks of granitic bulk composition provide more constraints than textures from gabbroic compositions, and permitted calculation of a set of relative diffusion coefficients which also reproduce textures in the gabbroic and anorthositic rocks from the Lofoten Islands. The following set of relative diffusion coefficients (L_i/L_Fe) reproduces the observed textures in the Lofoten rocks: Si=0.82, Mg=0.59, Mn=0.05, Na=0.38, K=0.39, Al=0.05 and Ca=0.07.     

Aragonite and magnesite in eclogites from the Jæren nappe,SW Norway: disequilibrium in the system CaCO3–MgCO3 and petrological implications

Eclogites from the Jæren nappe in the Caledonian orogenic belt of SW Norway contain aragonite, magnesite and dolomite in quartz‐rich layers. The carbonates comprise composite grains that occur interstitially between phases of the eclogite facies assemblage: garnet + omphacite + zoisite + clinozoisite + quartz + apatite + rutile ± dolomite ± kyanite ± phengite. Pressure and temperature conditions for the main eclogite stage are estimated to be 2.3–2.8 GPa and 585–655 °C. Published ultrahigh pressure (UHP) experiments on CaO‐, MgO‐ and CO₂‐bearing systems have shown that equilibrium assemblages of aragonite and magnesite form as a result of dolomite breakdown at pressures >5 GPa. As a result, recognition of magnesite and aragonite in eclogite facies rocks has been used as an indicator for UHP conditions. However, petrological testing showed that the samples studied here have not experienced such conditions. Aragonite and magnesite show disequilibrium textures that indicate replacement of magnesite by aragonite. This process is inferred to have occurred via a coupled dissolution–precipitation reaction. The formation of aragonite is constrained to eclogite facies conditions, which implies that the studied rocks have experienced metasomatic, reactive fluid flow during their residence at high pressure (HP) conditions. During decompression, the bimineralic carbonate aggregates were overgrown by rims of dolomite, which partially reacted with aragonite to form Mg‐calcite. The well‐preserved carbonate assemblages and textures observed in the studied samples provide a detailed record of the reaction series that affected the rocks during and after their residence at P–T conditions near the coesite stability field. Recognition of the HP mechanism of magnesite replacement by aragonite provides new insight into metasomatic processes that occur in subduction zones and illustrates how fluids facilitate HP carbonate reactions that do not occur in dry systems at otherwise identical physiochemical conditions. This study documents that caution is warranted in interpreting aragonite‐magnesite associations in eclogite facies rocks as evidence for UHP metamorphic conditions.     

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.     

Recrystallisation of oscillatory zoned zircon: some geochronological and petrological implications

Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U–Pb age. Recrystallisation provides a mechanism for resetting zircon U–Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integraded radiation damage further enhanced by high concentrations of trace-element contaminants. The U–Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.Dedicated to Borwin Grauert on the occasion of his sixtieth birthday     

Production of 3He in rocks by reactions induced by particles of the nuclear-active and muon components of cosmic rays: Geological and petrological implications

The paper presents data on the production of the ³He nuclide in rocks under the effect of cosmicray particles. The origin of the nuclide in the ground in neutronand proton-induced spallation reactions, reactions induced by high-energy muons, and negative muon capture reactions is analyzed. The cross sections of reactions producing ³He and ³H are calculated by means of numerical simulations with the GEANT4 simulation toolkit. The production rate of the ³He nuclide in the ground is evaluated for the average level of solar activity at high geomagnetic latitudes and at sea level. It is proved that the production of ³He in nearsurface ground layers by spallation reactions induced by cosmic-ray protons may be approximately 20% of the total production rate of cosmogenic ³He. At depths of 10–50 m.w.e., the accumulation of ³He is significantly contributed by reactions induced by cosmic-ray muons. Data presented in the paper make it possible to calculate the accumulation rate of ³He in a rock depending on depth that is necessary for the evaluation of the exposure time of the magmatic or metamorphic complex on the Earth’s surface (³He dating).     

南阿尔金地区榴辉岩特征及意义

野外地质调查工作表明,阿尔金地区西起且末河到茫崖长达300km范围内的"阿尔金山岩群"中多处出露榴辉岩透镜体。榴辉岩在产状和岩石学特征上可以分为两种:第1种是中细粒结构,块状构造,退化变质较弱,围岩多与片麻状花岗岩或花岗闪长岩关系密切;第2种是细粒结构,片麻状构造,具有透入性片麻理,有明显退化变质现象,围岩多与大理岩、黑云变粒岩、石英片岩等关系密切。在分布上,榴辉岩限于古元古代"阿尔金山岩群"中,含有榴辉岩的阿尔金构造杂岩带是柴达木盆地北缘碰撞杂岩带的西延部分。     

The assemblage diaspore+quartz in metamorphic rocks: a petrological, experimental and thermodynamic study

The upper pressure limit of pyrophyllite is given by the equilibria (i) pyrophyllite=diaspore+quartz and (ii) pyrophyllite=diaspore+coesite. High- P experimental investigations carried out to locate equilibrium (i) yield brackets between 497 °C/24.8  kbar and 535 °C/25.1  kbar, and between 500 °C/23  kbar and 540 °C/23  kbar. Equilibrium (ii) was bracketed at 550 °C between 26.0 and 28.3  kbar. In the experimental P–T  range, equilibria (i) and (ii) are metastable with respect to kyanite. A stable P–T  grid is calculated using thermodynamic data derived under consideration of the present experimental results. According to these data, the lower pressure limit of the assemblage diaspore+quartz according to equilibrium (i) range from about 12  kbar/300 °C to 20  kbar/430 °C (in the presence of pure water). The upper stability of diaspore+quartz is limited by the reaction diaspore+quartz=kyanite+H₂O at about 450 °C (nearly independent of pressure) and, to higher pressure, by the quartz=coesite transition. Equilibrium (ii) is metastable over the whole P–T  range.
Natural occurrences600.S of the diaspore–quartz assemblage in metamorphic rocks in Sulawesi, New Caledonia, Amorgos and the Vanoise are characterized by minerals indicative of high- P such as ferro-magnesiocarpholite, glaucophane, sodic pyroxene and lawsonite. The metamorphic P–T  conditions of these rocks are estimated to be in the range 300–400 °C, >8  kbar. These data are compatible with the derived P–T  stability field of the diaspore+quartz assemblage. We conclude that, in metamorphic rocks, diaspore+quartz is, as ferrocarpholite, an indicator for unusual low- T  /very high- P settings.     

Chloritoid-bearing rocks associated with blueschists and eclogites, northern New Caledonia

Abstract Chloritoid-bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the Tertiary high-pressure metamorphic belt of northern New Caledonia. The typical assemblage of chloritoid-bearing rocks in the epidote zone is quartzchlorite-muscovite-garnet-chloritoid. In the omphacite zone, epidote is an additional member of the chloritoid-bearing assemblage. Paragonite is rare, plagioclase was not detected, and rutile and ilmenite are the Fe-Ti oxide phases. Chloritoid-glaucophane is not a common assemblage. Chloritoid-bearing rocks have relatively low (Ca+K+Na)/Al ratios and the chloritoids are relatively Mg-rich with Mg/ (Mg+Fe) up to about 0.4. A comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560° C in the omphacite zone and a minimum temperature limit near 450° C at 10 kbar. An empirical garnet-chlorite Fe-Mg exchange thermometer does not yield consistent results for the higher-grade rocks, suggesting T s ranging from 390 to 535° C in the omphacite zone and 420–465° C in the epidote zone. The distribution coefficient K _D = (Fe/Mg)_ctd/(Fe/Mg)_chl for chloritoid and chlorite ranges from 3.9 to 6.4, values which are lower than those (=10) from lower greenschist facies rocks, but are near those of upper greenschist facies and albite-epidote amphibolite facies.     

High-pressure blueschists and eclogites in Hong'an: a framework for addressing the evolution of high- and ultrahigh-pressure rocks in central China

The Hong'an region in the Qinling–Dabie collisional zone in eastern China hosts a series of metamorphic rocks exposing a south-to-north distribution from blueschist/blueschist–greenschist, amphibolite, eclogite (kyanite free) and kyanite–eclogite to coesite–eclogite facies rocks that represent progressively deeper levels of the Mesozoic subduction–collision complex. The Hong'an area is interesting for three reasons: (1) it escaped the thermal and structural overprint imparted on much of the Dabie Mountains during Early Cretaceous intrusion of voluminous granites and granodiorites; (2) the high-pressure (HP) Hong'an eclogites are widely distributed, often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist/blueschist–greenschist rocks exposed to the south; (3) the blueschist/blueschist–greenschist facies rocks are generally better exposed than their equivalents in the southeastern Dabie Mountains and offer some opportunity for simultaneous structural and metamorphic analysis. The Hong'an area HP rocks offer perhaps the closest approximation to a preserved snapshot of Mesozoic pressure–temperature (P–T) conditions attending early subduction–exhumation in the region, and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh-pressure (UHP) rocks. The purpose of this contribution is twofold: (1) to document previously unpublished metamorphic and structural data characterizing these HP sequences and their relative continuity in Hong'an; (2) to incorporate these data with recent geochronologic, structural and paleomagnetic information in the context of protracted, late Paleozoic through Mesozoic subduction, collision and exhumation. Metamorphism and exhumation of some of the southern Hong'an HP sequences appear to have occurred concomitant with oceanic subduction immediately to the west, and thus may have preceded widespread continental subduction/collision. Moreover, all of the HP–UHP sequences in the region were exhumed before the end of collision between the Sino-Korean and Yangtze cratons at ca. 160 Ma. Exhumation of HP–UHP rocks both before and during continental plate collision is neither novel for central China nor for other HP–UHP zones, but is important to take into account when reconstructing the evolution of such orogens.     

Distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks

A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe²⁺ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Al^iv to balance the effect of increasing (Fe³⁺+Ti⁴⁺+Al^vi+Cr³⁺)_M1.The Fe²⁺/Fe³⁺ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe²⁺ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe²⁺ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.     

Late Paleozoic volcanic rocks of the Intra-Sudetic Basin, Bohemian Massif: petrological and geochemical characteristics

Late Paleozoic volcanic rocks in the Intra-Sudetic Basin of the Bohemian Massif in the Czech Republic can be subdivided into two series: (I) a minor bimodal trachyandesite-rhyolite series of Upper Carboniferous age with initial ⁸⁷Sr/⁸⁶Sr of ca. 0.710 and ε_Nd values of −6.1 also characteristic of volcanics of the near Krkonoše Piedmont Basin (0.707 and −6.0, Ulrych et al., 2003) and (II) a major differentiated basaltic trachyandesite-trachyandesite-trachyte-rhyolite series of Lower Permian age with lower initial ⁸⁷Sr/⁸⁶Sr of ca. 0.705-0.708 and ε_Nd values ranging from −2.7 to −3.4/−4.1/. The newly recognized volcanic rocks of trachytic composition indicate that the rocks were formed by magmatic differentiation of similar parental melts rather than constituting a bimodal mafic-felsic sequence from different sources. Both series are generally of subalkaline affinity and calc-alkaline character with some tholeiitic tint (FeO/MgO vs. SiO₂, presence of orthopyroxene). The magmatic activity occurred in cycles in a layered chamber, each starting primarily with felsic volcanics and ending with mafic ones. The mafic rocks represent mantle-melt(s) overprinted by crust during assimilation-fractional crystallization. The Sr-Nd isotopic data confirm a significant crustal component in the volcanic rocks that may have been inherited from the upper mantle source and/or from assimilation of older crust during magmatic underplating and shallow-level melt fractionation.     

General relations in the trace-element composition of zircons from eclogites with implications for the age of eclogites in the belomorian mobile belt

Analysis of currently available data (877 individual high-precision zircon analyses) on the composition of zircons from eclogite complexes worldwide reveals general relations in the zircon composition: an anomalous decrease in the Th concentration (no higher than 3 ppm on average) and the Th/U ratio (0.33 on average), a significant decrease in the concentrations of all REE (to 22 ppm) and particularly LREE (<2 ppm), and relatively low concentrations of Y (34 ppm), U (100 ppm), and P (41 ppm) at an elevated Hf concentration (11 400 ppm on average). The REE patterns of eclogitic zircons are noted for pronounced flat HREE patterns, poorly pronounced (if any) negative Eu anomalies, strongly reduced positive Ce anomalies (Ce/Ce* = 11 on average), and U-shaped configurations of LREE patterns up to the development of negative Nd anomalies. The relations detected in the distribution of trace elements and REE in eclogitic zircons are of universal nature and occur irrespective of the rock type (metabasites, metaultrabasites, or gneisses) and the metamorphic pressure (eclogites of high and ultrahigh pressure). The application of the aforementioned criteria makes it possible to reliable distinguish eclogitic zircons from those of magmatic or metamorphic genesis (not related to high-pressure metamorphism). Eclogites in the Belomorian Mobile Belt (in the Salma and Gridino areas) were determined to contain zircons in metagabbro eclogites; the cores of these zircons have an age of 2.8?C2.9 Ga and are of magmatic genesis, whereas their outer metamorphic zones have an age of 1.9 Ga and a trace-element composition typical of eclogitic zircons. Hence, the Belomorian Mobile Belt was affected only by single (Svecofennian, at ??1.9 Ga) episode of eclogite metamorphism of Archean rocks.     

Thallium in basalts and related rocks

The average K, Rb, T1 abundances in 26 Mid-Atlantic Ridge basalts are 2,660 p.p.m., 6.6 p.p.m., 205 p.p.b.; abundances in 19 Hawaiian basalts and other rocks are 12,430 p.p.m., 32 p.p.m., 84 p.p.b.; 20 continental rocks (mostly basic) average 12,580 p.p.m., 56 p.p.m., 305 p.p.b. Abundances were obtained also for 8 meteorites.