Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada

The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO₂ < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO₂ < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO₂ > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H₂O-rich to CO₂-rich. The calcite in the retrograde veins has δ¹⁸O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ¹³C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO₂-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO₂ ≈ 1.13 to 1.05 g/cm³). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO₂-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO₂-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO₂-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996     

Metamorphism at the northwest contact of the Stanhope Pluton,Quebec Appalachians: Mineral equilibria in interbedded pelite and calc-schist

The interbedding of pelite and calc-schist in part of the contact aureole of a Devonian (biotite K-Ar age 346±4 Ma) granite pluton that straddles the Quebec-Vermont border offers an opportunity to compare metamorphic conditions prevailing in both rock types, and to test for internal consistency among several different geothermometers and geobarometers.Microprobe analyses and recent thermodynamic data are used with simple activity-composition models to estimate P-T-X _fluid conditions near the sillimanite zone. Calculations yield the following results: P=2 to 3 kb, and T =400° to 600° C in the inner one kilometre of the aureole; was around 0.8. Estimates are based on the calc-silicate isobaric invariant assemblages tremolite-K-feldspar-plagioclase-clinozoisite-phlogopite-calcite-quartz and diopside-tremolite-plagiolase-clinozoisite-calcite-quartz, and the paragonite-andalusite-sillimanite-albite-quartz equilibrium. The solid-solid reaction phlogopite+2 diopside + 4 quartz=tremolite+K-feldspar in the calc-schist (combined with the andalusite-sillimanite equilibrium), and phase relations in the granite yield apparently inconsistent results.The implied 6 to 10 km of cover at the time of intrusion may have been provided by subsequently eroded thrust sheets.     

The major‐ and trace‐element whole‐rock fingerprints of Egyptian basalts and the provenance of Egyptian artefacts

Discrimination diagrams have been developed that source Egyptian basaltic artefacts using whole‐rock major element geochemistry. These include K₂O versus SiO₂, TiO₂ and P₂O₅ against MgO/Fe₂O₃^t (total Fe as Fe₂O₃), and a discriminant analysis diagram using SiO₂, Fe₂O₃^t, CaO, and MnO. A complementary set of diagrams uses easily obtained trace element data (Nb/Y versus Zr/Nb; Zr [ppm] versus Rb/Sr; TiO₂ [wt % volatile free] versus V; and Cr [ppm] versus Zr/Y) to determine the bedrock sources. These diagrams have been applied to seven First Dynasty basalt vessels (Abydos), two Fourth Dynasty basalt paving stones (Khufu's funerary temple, Giza), and two Fifth Dynasty paving stones (Sahure's complex, Abu Sir). They show that the bedrock source for all the artefacts was the Haddadin flow in northern Egypt. Multidimensional scaling and cluster analysis applied to the whole‐rock data (major elements and trace elements together) and previously published mineral fingerprinting studies confirm these results. Comparing mineral versus whole‐rock fingerprinting techniques, a major advantage of the former is the small sample size required (0.001 g compared to ≥ 0.1 g). Analytical costs are similar for both methods assuming that a comparison (bedrock) database can be assembled from the literature. For most archaeological problems, a whole‐rock bedrock database is more likely to exist than a mineral database, and whole‐rock analyses on artefacts will generally be easier to obtain than mineral analyses. Whole‐rock fingerprinting may be more sensitive than mineral‐based fingerprinting. Thus, if sample quantity is not an issue, whole‐rock analysis may have a slight cost, convenience, and technical advantage over mineral‐based methods. Our results also emphasize that the Egyptians cherished their Haddadin basalt flow and used it extensively and exclusively for manufacturing basalt vessels and paving stones for at least 600 years (∼3150 B.C. to 2500 B.C., approximate ages of the vessels and Abu Sir paving stones, respectively). © 2001 John Wiley & Sons, Inc.     

Photo-interprétation à différentes échelles et relevés de terrain: exemples de corrélations structurales dans le Bassin Méditerranéen et sur le socle Ouest africain

Résumé L'interprétation structurale des linéaments, décelés sur clichés photographiques pris par satellite, est discutée sur les bases suivantes: d'une part en pratiquant au minimum un changement d'échelle (plus grande); d'autre part en utilisant les relevés de terrain. Des exemples pris dans le Bassin Méditerranéen sédimentaire et sur le socle cristallophyllien de l'Afrique de l'Ouest permettent de définir une méthodologie commune. Ainsi en région sédimentaire, partant de prises de vue par exemple au 1/2 500 000, les échelles au 1/15 000 et au 1/50 000 se révèlent être des compléments précieux en photo-interprétation. De la même façon en zone de socle, les échelles au 1/20 000 et au 1/50 000 sont complémentaires du 1/200 000. De plus grâce aux donnés de terrain, il est possible de qualifier l'information photogéologique et d'en exprimer sa nature. Enfin les problèmes de corrélations entre rhegmatisme de socle et fracturation de couverture sont abordés de façon préliminaire: la similitude directionnelle observée devrait orienter les futures recherches en ce domaine.

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The structural interpretation of disclosed lineaments on satellite's pictures ist discused on the following basis: first, with a minimum scale change (making it bigger); on the other hand using field plotting. The examples took from the Mediterranean sedimentary basin and from the western Africa crystalographic shield allow us to define a common methodology. Thus, in sedimentary area, through 1/2 500 000 scale aerial photographys, the 1/15 000 and 1/50 000 scales would be real valuable complements in photo-interpretation. Same way as, in shield zone, the 1/20 000 and 1/50 000 scales would be complements of the 1/200 000 one. More than that, because of field plotting utilisation is possible to quality photogeological information and even to explain its nature. In short correlation problems are exposed in a preliminary approach; the observed directional similitude could orientate the future research.

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Zusammenfassung Die strukturelle Deutung von Lineamenten, die aus Satellitenphotos abzulesen sind, wird nach folgenden Voraussetzungen erörtert: zum einen wird zumindest eine Vergrößerung des Maßstabes vorgenommen, zum anderen werden Geländebefunde herangezogen. An Hand von Beispielen aus dem sedimentären Mittelmeerbecken sowie dem Kristallo-phyllitischen Sockel Westafrikas ist es möglich, eine gemeinsame Methodik zu definieren. So erweisen sich im sedimentären Gebiet die Maßstäbe 115 000 und 150 000 als wertvolle Ergänzungen zur Photointerpretation zum Beispiel einer Aufnahme von 12 500 000. Entsprechend ergänzen im Bereich des Sockels die Maßstäbe 120 000 und 150 000 die Aufnahme von 1200 000. Darüber hinaus ist es mit Hilfe von Geländedaten möglich, die photogeologische Information zu qualifizieren und zu beinhalten. Schließlich werden die Probleme der Korrelation zwischen Bruchbildung im Sockel und Brücken im Deckgebirge angesprochen: Die beobachtete Annäherung in der Ausrichtung soll zukünftige Untersuchungen auf diesem Gebiet orientieren.

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, , : , no- , , — . . , 1: 15.000 1:50.000 12.500.000. 1: 200.000 1: 20.000 1: 50.000. . . .

     

Selenium isotopes trace the source and redox processes in the black shale-hosted Se-rich deposits in China

We analyzed the Se isotopic composition of black shales and related kerogen and sulfide fractions from the Zunyi Ni-Mo-Se deposit, the La’erma Se-Au deposit and the Yutangba Se deposit in southern China to constrain metal sources and accumulation processes, both subjects of disagreement in the scientific community. Se at the Zunyi Ni-Mo-Se polymetallic deposit displayed a restricted range of δ⁸²Se values (−1.6‰ to 2.4‰ with a mean of 0.6‰) suggesting a major hydrothermal origin where aqueous Se was probably transported as H₂Se, along with H₂S, and precipitated directly as selenides or in sulfides. Se at the La’erma Se-Au deposit covers a larger range in δ⁸²Se values (−3.8‰ to 5.4‰ with a mean of 0.3‰), suggesting Se redistribution following redox transformations, leading to kinetic isotopic fractionation. The largest Se isotopic variation so far in natural terrestrial samples was found in the Yutangba Se deposit, with δ⁸²Se values varying from −12.77‰ to 4.93‰. On the basis of variations in Se isotopes in the deposit, along with other geological and geochemical evidence, the “redox model” (supergene alteration) explains the occurrence of native Se in the deposit. Overall, hydrothermal systems may be a potentially important Se source to form economic deposits in comparison to seawater sources. Significantly, our study indicates that either secondary hydrothermal or supergene alteration is a key factor in Se enrichment in black shales. Redistribution of Se, and probably other redox-sensitive metals like Mo, Cr and V, leads to isotopic fractionation which may be used to fingerprint such alteration/precipitation processes.     

Analysis of rock structure stability in coal mines

In this paper, the theory of limit point instability is used to analyse the stability of rock structures in coal mines. A general method of analysing stability of rock structures is put forward and a uniform instability condition of rock structures is set up. Some instability phenomena, such as rock bursts in circular roadways, pillars and long walls, and the outburst of coal and gas from circular roadways, are discussed analytically. At a later stage, the critical point of rock structure instability is determined. The influence of relative parameters (such as the mechanical properties of rock, coal, and the geometric sizes) on the stability of the rock structures is carefully analysed. Copyright © 2005 John Wiley & Sons, Ltd.     

Hydrogen isotopic composition of water from fossil micrometeorites in howardites

We have measured the hydrogen isotopic composition (D/H ratios) of the water from 13 carbonaceous chondritic microclasts (CCMs, size <1 mm) trapped in two howardites (Kapoeta and Yamato-793497) early in the evolution of Solar System. The division into tochilinite-rich; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich CCM types is corroborated by the hydrogen isotopic compositions. Both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite-rich CCMs represent CM2 chondritic matter. In contrast, there is no good match between the isotopic and mineralogical properties of the magnetite-rich CCMs and the known groups of carbonaceous chondrites, suggesting that magnetite-rich CCMs represent a new kind of chondritic matter, not yet sampled in meteorite collections. This demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete. The study of (micro)clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids.The average hydrogen isotopic composition of water belonging to CCMs, D/H = (152.0 ± 4.8) × 10⁻⁶ (1σ_m), is similar to that of Antarctic micrometeorites (AMMs), D/H = (161.2 ± 3.8) × 10⁻⁶ (1σ_m). The similarity, in terms of mineralogy and hydrogen isotopic composition, between CCMs and AMMs demonstrates that the composition of the micrometeorites has not been modified over the whole history of the Solar System. It indicates that the composition of the micrometeorite flux onto Earth has been, and is, dominated by a mixture of CM2-like; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich carbonaceous chondritic matter exemplified by CCMs found in howardites. Because CCMs have not suffered atmospheric entry, they provide an abundant source of pristine micrometeorites.The average D/H ratio of the whole population of CCMs is identical within errors to that of the Earth (149 ± 3 × 10⁻⁶). The match between the CCMs D/H ratio and that of the Earth is especially remarkable because 1) three different populations of CCMs are needed to make the D/H ratio of the Earth; 2) there is no single carbonaceous chondrite group for which a similar match exists. This observation suggests that CCMs population might be representative of the late veneer agent(s) that delivered water to the Earth.     

Early Permian extensional shearing of an Ordovician granite: The Saint-Eutrope “C/S-like” orthogneiss (Montagne Noire,French Massif Central)

Dating the magmatic events in the Montagne Noire gneiss dome is a key point to arbitrate between the different interpretations of the Late Carboniferous–Early Permian tectonics in this southern part of the Variscan belt. The Saint-Eutrope orthogneiss crops out along the northern flank of the dome. We show that the protolith of this orthogneiss is an Ordovician granite dated at 455 ± 2 Ma (LA-ICP-MS U-Pb dating on zircon). This age is identical to that previously obtained on the augen orthogneiss of the southern flank, strongly suggesting that both orthogneiss occurrences have the same Ordovician protolith. The Saint-Eutrope orthogneiss experienced intense shearing along the Espinouse extensional detachment at ca. 295 Ma (LA-ICP-MS U-Pb-Th on monazite), an age close to that determined previously on mica by the ³⁹Ar-⁴⁰Ar method and contemporaneous with the emplacement age of the syntectonic Montalet granite farther to the west. This normal sense shearing reworked previous fabrics related to Variscan thrusting that can be still observed in the augen orthogneiss of the southern flank, and is responsible for the spectacular “C/S-like” pattern of the Saint-Eutrope orthogneiss. This work also shows that care is needed when dealing with C/S-type structures, since they can develop not only in syntectonic intrusions, but also in orthogneisses affected by an intense secondary deformation, at decreasing temperature.