Fluid‐induced breakdown of monazite in medium‐grade metasedimentary rocks of the Pontremoli basement (Northern Apennines,Italy)

The last (decompression) stages of the metamorphic evolution can modify monazite microstructure and composition, making it difficult to link monazite dates with pressure and temperature conditions. Monazite and its breakdown products under fluid‐present conditions were studied in micaschist recovered from the cuttings of the Pontremoli1 well, Tuscany. Coronitic microstructures around monazite consist of concentric zones of apatite + Th‐silicate, allanite and epidote. The chemistry and microstructure of the monazite grains, which preserve a wide range of chemical dates ranging from Upper Carboniferous to Tertiary times, suggest that this mineral underwent a fluid‐mediated coupled dissolution–reprecipitation and crystallization processes. Consideration of the chemical zoning (major and selected trace elements) in garnet, its inclusion mineralogy (including xenotime), monazite breakdown products and phase diagram modelling allow the reaction history among accessory minerals to be linked with the reconstructed P–T evolution. The partial dissolution and replacement by rare earth element‐accessory minerals (apatite–allanite–epidote) occurred during a fluid‐present decompression at 510 ± 35 °C. These conditions represent the last stage of a metamorphic history consisting of a thermal metamorphic peak at 575 °C and 7 kbar, followed by the peak pressure stage occurring at 520 °C and 8 kbar. An anticlockwise P–T path or two clockwise P–T loops can fit the above P–T constraints. The former path may be related to a context of late Variscan strike‐slip‐dominated exhumation with minor Tertiary (Alpine‐related) reworking and fluid infiltration, while the latter requires an Oligocene–Miocene fluid‐present tectono‐metamorphic overprint on the Variscan paragenesis.     

Fluid inclusions in granulite-facies metapelites of the Hercynian ancient lower crust of the Serre,Calabria, Southern Italy

Investigations of fluid inclusions in granulitefacies metapelites of southern Calabria enable characterization of the fluid composition of these lower crustal rocks, and constrain the petrologically deduced retrograde P-T path characterized by isothermal uplift prior to isobaric cooling in middle crustal levels. Fluid inclusions in cordierite, garnet and sillimanite have a CO₂-rich composition. Inclusions in cordierite rarely contain minor amounts of N₂ and H₂O, and in garnets some CO₂–CH₄–N₂ inclusions have been analyzed by Raman microprobe. Quartz reveals the most complex fluid melusion compositions (1) CO₂-rich, (2) CO₂–CH₄–N₂, (3) CH₄–N₂, (4) H₂O–MgCl₂–CaCl₂–NaCl, (5) H₂O–NaCl and (6) H₂O–CO₂. The earliest fluid inclusions after peak metamorphism are rich in CO₂ with minor amounts of N₂ and H₂O. An early CO₂–(H₂O–N₂) fluid composition has been confirmed by detection of CO₂, H₂O and N₂ in the channels of the cordierite structure. Most of the early CO₂-rich fluid inclusions were modified during the uplift from the lower to the middle crustal level, resulting in a density decrease with CO₂ still dominant. The subsequent isobaric cooling led to further modifications of the fluid inclusions. High-density inclusions around implosion textures or scattered amongst lower-density ones must have formed during this cooling episode. Aqueous inclusions in quartz are mostly formed late and are consistent with trapping during retrograde rehydration.This project has been supported by the DFG as contribution to the special program Continental Lower Crust     

Partial melting of common metasedimentary rocks: A mass balance approach

Materials balance calculations are presented to document the progressive bulk chemical changes in metagreywacke and metapelite with separation of increments of granite of minimum melt composition. During partial melting, enrichment of Fe relative to Mg in granite and strong absorption of water leave residual rocks with increasing proportions of such Mg-rich phases as cordierite and progressively dehydrated. Extraction of most granitic compositions from metapelites results in increasing the Al value of residua, while extraction of an alkali granite melt from metagreywacke is necessary to change the Al values in their residua (from peraluminous to sub- or metaluminous). Under essentially constant temperature conditions, complex isograds may develop locally reflecting the bulk chemical changes produced by melt removal. Natural amphibolite/granulite facies rocks from three locations in Canada are compared with the calulated trends.     

A seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite-facies rocks of Calabria (Southern Italy)

Pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses, metamonzodiorites, as well as felsic and mafic granulites at the base of the section of the Hercynian lower crust exposed in Calabria (Southern Italy). The crustal section is tectonically superposed on lower grade units. Ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses, whereas sparse fault-related pseudotachylytes and thin mylonite/ultramylonite bands occur in granulite-facies rocks. The presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high-temperature conditions preceded the formation of pseudotachylytes. We have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust. Different protoliths were selected to test how lithology controls pseudotachylyte composition and textures. In migmatites and felsic granulites, euhedral or cauliflower-shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition. This indicates that pseudotachylytes originated at deep crustal conditions (>0.75 GPa). In mafic protoliths, quenched needle-to-feather-shaped high-alumina orthopyroxene occurs in contact with newly crystallized plagioclase. The pyroxene crystallizes in garnet-free and garnet-bearing veins. The simultaneous growth of orthopyroxene and plagioclase as well as almandine, suggests lower crustal origin, with pressures in excess of 0.85 GPa. The existence of melts of different composition in the same vein indicates the stepwise, non-equilibrium conditions of frictional melting. Melt formed and intruded into pre-existing anisotropies. In mafic granulites, brittle faulting is localized in a previously formed thin high-temperature mylonite bands. migmatitic gneisses are deformed into ultramylonite domains characterized by s-c fabric. Small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation. Microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high-strain rate, leading to seismic events and melting.     

Multi-stage Melting in the Lower Crust of the Serre (Southern Italy)

The lower-crustal section exposed in the Serre, southern Italy,consists mainly of Al-rich metasediments, which underwent granulite-faciesmetamorphism, partial melting and melt extraction. The paperconsiders the formation of melts in metapelites and metagreywackes.Leucosomes and host rocks have been studied to investigate themelting process. Biotite-rich and biotite-free melanosomes withscarce felsic components are present; the biotite-rich typesare widespread in the upper part of the section and the twotypes may occur side by side in the lower part. Na-rich andK-rich leucosomes including residual phases are interspersedwithin the metasediments; on the whole they do not show geochemicalsignatures suggestive of magmatic fractionation. Leucotonalitictypes prevail among the sampled leucosomes, which generallyare rare earth element (REE) depleted with positive Eu anomalieswhereas the host rocks are REE enriched with overall negativeEu anomalies. Melanosomes and migmatites show restitic chemistries.The precursor metagreywackes underwent depletion in Na₂O andenrichment in K₂O. The precursor metapelites document generaldepletion in Na₂O and they may be enriched or depleted in K₂O.All the characteristics of the migmatites and of their componentsreflect a two-stage melting: (1) H₂O-present melting, involvingmainly plagioclase, and (2) dehydration melting of micas. Allthe metasediments underwent H₂O-present melting, forming mostlysodic melts which, owing to their removal from the source asfast as they formed, did not accumulate in such proportionsas to allow migration and mostly remained within the lower-crustalmetasediments; metapelites also underwent variable dehydrationmelting, depending on chemical features and physical conditions,forming larger volumes of mobile granitic melts, most of whichmigrated far from the source. Extractions of 57–66 vol.% of total melts (sodic + potassic) from the most residual metapeliticmelanosomes and of about 27–44 vol. % of potassic meltsfrom metapelitic migmatites have been calculated. Higher volumesof the extracted melts have been calculated for the metapelitesof the lower part of the section; the most depleted metagreywackesunderwent melt extraction of about 9–13 vol. %. The two-stagemelting occurred during the prograde metamorphism and continuedduring the isothermal decompression. KEY WORDS: Calabria; lower crust; multi-stage melting     

Amphibolitization of calc-silicate metasedimentary rocks

Diopside granofels layers are associated with other metasedimentary rocks and gneisses near McMurdo Sound, Antarctica. Disrupted diopside granofels layers form tectonic inclusions which may have reaction rims of amphibolite. This amphibolite may either be a typical hornblende-plagioclase amphiholite or a hornblende-quartz amphibolite according to the initial composition of the diopside granofels. Chemical analyses of the reaction rims show that some of the amphibolites have major element compositions near tholeiitic basalt but that a sedimentary origin could be recognized for others that are high in silica. During amphibolitization, the diopside granofels loses Ca and gains Fe and Mg by mutual exchange of material with the surrounding rock over short distances. The metasomatic amphibolites fall on the igneous differentiation trend in a Niggli mg-c plot. At least small volumes of metasedimentary rock can attain the composition of basalt by amphibolitization in a metasomatic reaction rim.     

Correlation between strontium and oxygen isotopic compositions of rocks from the Adamello massif (Northern Italy)

Rock samples from the lithologic units forming the Adamello massif were analyzed for Rb and Sr concentrations and for⁸⁷Sr/⁸⁶Sr and¹⁸O/¹⁶O ratios. The most important features are as follows:

1. the strontium and oxygen isotopes show a systematic variation from south to northwest across the massif, the older units in the south being lower both in⁸⁷Sr and¹⁸O;
2. a good positive correlation was found between strontium and oxygen isotopic compositions.

Two genetic models are advanced for explaining the results: (1) the mixing of two magmas with different¹⁸O and⁸⁷Sr contents; (2) magma formation at different depths in a crust layered with respect to both oxygen and strontium isotope composition.     

Geochemistry of the early proterozoic metasedimentary rocks of the alligator rivers region,Northern Territory,Australia

The early Proterozoic metasedimentary sequence of the Alligator Rivers Region (a part of the Pine Creek Geosyncline) in the Northern Territory, Australia, overlies an Archaean granitoid basement. Early Proterozoic sedimentary sequences, in general, record important changes in the composition of the upper continental crust about the Archaean-Proterozoic boundary. However, the geochemistry of only a few of these sequences has been documented. The geochemistry of the early Proterozoic succession in the Alligator Rivers Region is reported here and the results are interpreted in terms of differences between the stratigraphic units, their provenance—particularly in relation to crustal evolution, and their subsequent metamorphism and weathering.Clastic metasedimentary rocks throughout the Alligator Rivers Region have a remarkably uniform major and trace element geochemistry. The Kakadu Group and upper member of the Cahill Formation are relatively more enriched in SiO₂ and correspondingly more depleted in Al₂O₃ than the rest of the sequence, reflecting the greater dominance of metapsammitic assemblages. The lower member of the Cahill Formation, which hosts the major U deposits of the Alligator Rivers Region, and the metasedimentary sequence in general, exhibit no significant enrichment in U above normal background values. Rare earth element (REE) concentrations in the metasedimentary units within the Alligator Rivers Region are uniform, though in detail there are some important differences within and between formations.The composition of the early Proterozoic clastic metasediments in the Alligator Rivers Region is consistent with the composition of similar material of the same age from other areas, and supports current ideas on crustal evolution. The Alligator Rivers metasediments are enriched in Si and K, and depleted in Mg, Ca, and Na relative to the Archaean average for clastic sedimentary rocks, and their REE geochemistry resembles typical post-Archaean sedimentary rocks having a light REE enriched pattern and a distinct Eu/Eu1 depletion compared to typical Archaean sediments. However, the REE data indicate that two compositionally distinct sources are involved in the provenance of the Kakadu Group, and possibly the lower member of the Cahill Formation, where two types of REE patterns can be distinguished on their HREE concentration and Eu/Eu1 anomaly.     

Very low-temperature metamorphism in Ordovician metasedimentary rocks above and below the Sardic unconformity,SW Sardinia,Italy

International Journal of Earth Sciences - In the Iglesiente region, the Cabitza and Monte Argentu Formations are separated by an angular unconformity known as the Sardic unconformity. This is...     

Microstructures of melt inclusions in anatectic metasedimentary rocks

The occurrence of crystallized and glassy melt inclusions (MI) in high‐grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly ≤15 μm, and only rarely reaching 30 μm; they occur in clusters. In most cases inclusions are crystallized (‘nanogranites’) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K‐feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (≤8 μm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re‐homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.     

Petrology of an intrusion-related high-grade migmatite: implications for partial melting of metasedimentary rocks and leucosome-forming processes

Intrusion-related migmatites comprise a substantial part of the high-grade part of the southern Damara orogen, Namibia which is dominated by Al-rich metasedimentary rocks and various granites. Migmatites consist of melanosomes with biotite+sillimanite+garnet+cordierite+hercynite and leucosomes are garnet- and cordierite-bearing. Metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies (5–6 kbar at 730–750 °C). Field evidence, petrographic observations, chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites. The intruding melts were significantly modified by magma mixing with in situ partial melts, accumulation of mainly feldspar and contamination with garnet from the wall rocks. However, it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith. The occurrence of LILE-, HFSE- and LREE-enriched and -depleted residues within the leucosomes implies that both quartzo-feldspathic and pelitic rocks were subjected to partial melting. Isotope ratios of the leucosomes are rather constant (¹⁴³Nd/¹⁴⁴Nd (500 Ma): 0.511718–0.511754, ε Nd (500 Ma): ?3.54 to ?5.11) and Sr (⁸⁷Sr/⁸⁶Sr (500 Ma): 0.714119–0.714686), the metasedimentary units have rather constant Nd isotope ratios (¹⁴³Nd/¹⁴⁴Nd (500 Ma): 0.511622–0.511789, ε Nd (500 Ma): ?3.70 to ?6.93) but variable Sr isotope ratios Sr (⁸⁷Sr/⁸⁶Sr (500 Ma): 0.713527–0.722268). The most restitic melanosome MEL 4 has a Sr isotopic composition of ⁸⁷Sr/⁸⁶Sr (500 Ma): 0.729380. Oxygen isotopes do not mirror the proposed contamination process, due to the equally high δ¹⁸O contents of metasediments and crustal melts. However, the most LILE-depleted residue MEL 4 shows the lowest δ¹⁸O value (<10). Mass balance calculations suggest high degrees of partial melting (20–40%). It is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites. High degrees of partial melting can be reached as long as the available H₂O, derived from the crystallization of the intruding granites, is efficiently recycled within the rock volume. Due to the limited amounts of in situ melting, it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies. The high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas. The Davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures, and illustrates some of the reactions that may modify leucosome compositions. The area provides constraints on melting processes operating in high-grade metasedimentary rocks.     

Isotopic equilibrium/disequilibrium in granites, metasedimentary rocks and migmatites (Damara orogen, Namibia)—a consequence of polymetamorphism and melting

The overwhelming part of the continental crust in the high-grade part of the Damara orogen of Namibia consists of S-type granites, metasedimentary rocks and migmatites. At Oetmoed (central Damara orogen) two different S-type granites occur. Their negative ε_Nd values (− 3.3 to − 5.9), moderately high initial ⁸⁷Sr/⁸⁶Sr ratios (0.714–0.731), moderately high ²⁰⁶Pb/²⁰⁴Pb (18.21–18.70) and ²⁰⁸Pb/²⁰⁴Pb (37.74–37.89) isotope ratios suggest that they originated by melting of mainly mid-Proterozoic metasedimentary material. Metasedimentary country rocks have initial ε_Nd of − 4.2 to − 5.6, initial ⁸⁷Sr/⁸⁶Sr of 0.718–0.725, ²⁰⁶Pb/²⁰⁴Pb ratios of 18.32–18.69 and ²⁰⁸Pb/²⁰⁴Pb ratios of 37.91–38.45 compatible with their variation in Rb/Sr, U/Pb and Th/Pb ratios. Some migmatites and residual metasedimentary xenoliths tend to have more variable ε_Nd values (initial ε_Nd: − 4.2 to − 7.1), initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr: 0.708–0.735) and less radiogenic ²⁰⁶Pb/²⁰⁴Pb (18.22–18.53) and ²⁰⁸Pb/²⁰⁴Pb (37.78–38.10) isotope compositions than the metasedimentary rocks. On a Rb–Sr isochron plot the metasedimentary rocks and various migmatites plot on a straight line that corresponds to an age of c. 550 Ma which is interpreted to indicate major fractionation of the Rb–Sr system at that time. However, initial ⁸⁷Sr/⁸⁶Sr ratios of the melanosomes of the stromatic migmatites (calculated for their U–Pb monazite and Sm–Nd garnet ages of c. 510 Ma) are more radiogenic (⁸⁷Sr/⁸⁶Sr: 0.725) than those obtained on their corresponding leucosomes (⁸⁷Sr/⁸⁶Sr: 0.718) implying disequilibrium conditions during migmatization that have not lead to complete homogenization of the Rb–Sr system. However, the leucosomes have similar Nd isotope characteristics than the inferred residues (melanosomes) indicating the robustness of the Sm–Nd isotope system during high-grade metamorphism and melting. On a Rb–Sr isochron plot residual metasedimentary xenoliths show residual slopes of c. 66 Ma (calculated for an U–Pb monazite age of 470 Ma) again indicating major fractionation of Rb/Sr at c. 540 Ma. However, at 540 Ma, these xenoliths have unradiogenic Sr isotope compositions of c. 0.7052, indicating depleted metasedimentary sources at depth. Based on the distinct Pb isotope composition of the metasedimentary rocks and S-type granites, metasedimentary rocks similar to the country rocks are unlikely sources for the S-type granites. Moreover, a combination of Sr, Nd, Pb and O isotopes favours a three-component mixing model (metasedimentary rocks, altered volcanogenic material, meta-igneous crust) that may explain the isotopic variabilty of the granites. The mid-crustal origin of the different types of granite emphasises the importance of recycling and reprocessing of pre-existing differentiated material and precludes a direct mantle contribution during the petrogenesis of the orogenic granites in the central Damara orogen of Namibia.     

Influence of ferric iron on phase equilibria in greenschist facies assemblages: the hematite‐rich metasedimentary rocks from the Monti Pisani (Northern Apennines)

We have investigated the effects of different Fe₂O₃ bulk contents on the calculated phase equilibria of low‐T/intermediate‐P metasedimentary rocks. Thermodynamic modelling within the MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (MnNKFMASHTO) chemical system of chloritoid‐bearing hematite‐rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe₂O₃ is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P–XFe₂O₃ diagrams, obtaining equilibration conditions of 475 °C and 9–10 kbar related to a post‐compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock‐forming silicates that often yield important thermobaric information. In Fe₂O₃‐rich compositions, garnet‐ and carpholite‐in curves shift towards higher temperatures with respect to the Fe₂O₃‐free systems. The presence of a ferric‐iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P–T–XFe₂O₃ phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.     

Parageneses of garnet in granulite-facies rocks,Lofoten-Vesteraalen,Norway

Garnet occurs widely as a secondary mineral in the granulite-facies rocks of the Lofoten-Vesteraalen islands of North Norway. The garnet-forming reactions may be inferred from the resulting textures and are interpreted as being retrograde reactions. Microprobe analyses show that garnets with high proportions of CaO and MgO occur in coronas around olivine grains in anorthosites, whereas the most iron-rich garnets formed by reaction between plagioclase and the very iron-rich pyroxenes of some monzonitic (mangeritic) rocks. Garnets ranging in composition between these extremes formed by reactions involving biotite, plagioclase and magnetite. Textural features indicate that these reactions have been induced by oxidation of the biotite; the liberated water has converted the pyroxenes to amphibole. The net effect of the reactions is to transform the granulite-facies assemblages to amphibolite-facies assemblages.The secondary garnet is nearly ubiquitous in the granulites of the outer Lofoten islands, which may show no other signs of retrogression. On Langöy in Vesteraalen garnet occurs only within a zone of strongly retrograded gneisses. Differences in bulk composition do not satisfactorily explain the distribution of secondary garnet. This distribution, and the observed oxidation textures, imply widespread penetration of the Lofoten granulites by a transient oxidizing fluid. On Langöy these fluids apparently affected only the narrow retrograded zone. The model suggests that the 1700 to 1800 m.y. Rb/Sr date obtained by Heier and Compston (1969) for the Lofoten granulites represents the time of oxidation, whereas the 2800 m.y. date for the Langöy granulites represents the original granulite-facies metamorphism.Publication number 3 in the Norwegian Geotraverse Project.     

Geochemistry of Archean metasedimentary rocks from West Greenland

Archean metasedimentary rocks occur as components of the Isua supracrustals, Akilia association and Malene supracrustals of southern West Greenland. Primary structures in these rocks have been destroyed by metamorphism and deformation. Their chemistry and mineralogy is consistent with a sedimentary origin, but other possible parents (e.g. acid volcanics, altered pyroclastic rocks) cannot be excluded for some of them. There is little difference in the composition of metasedimentary rocks from the early Archean Isua supracrustals and probable correlative Akilia association. Both have a wide range in rare earth element (REE) patterns with $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ ranging from 0.61?5.8. The REE pattern of one Akilia sample, with low $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$, compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources. Other samples with very steep REE patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the Isua supracrustals. Samples with intermediate REE patterns are best explained by mixing of basaltic and felsic end members. Metasedimentary rocks from the Malene supracrustals can be divided into low silica (≤55% SiO₂) and high silica (>77% SiO₂) varieties. These rocks also show much variation in $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ (0.46?14.0) and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks. The wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation. Accordingly, these data do not rigorously test models of early Archean crustal composition and evolution. The systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early Archean crust.     

Multiple intrusions in the Cima d'Asta pluton (Northern Italy)

The systematic modal analysis of four sectors of the Cima d'Asta pluton (North Italy) has shown the existence of multiple adjacent intrusions.The main data supporting this interpretation are reported. Some problems concerning the mechanism of adjacent intrusions and the plutonic magmatic evolution are also discussed in a preliminary way.This paper is published with the help of a grant from the Consiglio Nazionale delle Ricerche, Italia.     

Subsolidus segregation of quartz in metamorphic differentiation of metasedimentary rocks

Thermodynamic modeling of the SiO₂–TiO₂–Al₂O₃–Fe₂O₃–MnO–MgO–CaO–Na₂O–K₂O–P₂O₅–H₂O (STAFMMCNKPOH) system at 600°C, 5 kbar has been applied to investigate dissolution and re-precipitation of quartz. Comparing silica molality in the STAFMMCNKPOH and SiO₂–H₂O systems, there is seen to be no effect of mineral assemblage on quartz solubility. From quantitatively estimated water/rock ratio required to dissolve quartz completely, one can deduce that the segregation of quartz appears to be due to diffusive transport of silica in inner pore fluid rather than to advective transport (in fluid flow).     

西昆仑东段首次发现前寒武纪麻粒岩相变质岩

在塔里木南缘铁克里克构造带,出露了大面积的前寒武纪变质岩系,前人将其时代划归于古元古代(新疆地质矿产局,1991)。我们对这一地区前寒武系研究表明,它主要由三部分组成:变质双峰式火山岩组合(以布琼一带出露最完     

Eclogites in the Northern Dora-Maira Nappe (Western Alps,Italy)

Summary Eclogitised metabasics of early-Alpine age are described from the northern Dora-Maira nappe (internal Pennidic, western Italian Alps), a tectonic element which has become widely known for the coesite-bearing assemblages discovered in its southern part. The P-T conditions inferred in this paper for the eclogitic peak (P = 9–13 kbar, T = 500 ± 50 °C) are much lower than those proposed in the southern Dora-Maira by Chopin (1987) for the coesite-bearing unit. Consequently, only the latter underwent a peculiar early-Alpine evolution at extremely high-pressures and temperatures, while the northern Dora-Maira nappe was eclogitised at P-T conditions comparable to those of the other internal Pennidic units. The post-eclogitic path proposed here for the northern Dora-Maira occurred at isothermal conditions or at slightly increasing temperatures.

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Les éclogites dans la nappe Dora-Maira septentrionale (Alpes Occidentales, Italie)

Resumé Metabasites éclogitiques d'âge éoalpine sont décrites dans la nappe Dora-Maira septentrionale (Pennique interne, Alpes occidentales italiennes), une unité connue pour la découverte de parageneses à coesite dans le secteur meridional. Les conditions P-T de la recrystallization éclogitique proposées dans ce travail (P = 9–13 kbar, T = 500 ± 50 °C) sont plus basses que les conditions suggerées parChopin (1987) pour les roches de l'unité à coesite. D'autre part ces éstimations des pressions et témperatures sont comparables à celles des autres nappes Penniques internes. La trajectoire post-éclogitique proposée içi pour le Dora-Maira septentrional est caracterisée par une diminution de pression à témperatures plus ou moins constantes (ou légèrment croissantes).