Baryt und ankerit aus Ammoniten des Berner Juras (Schweiz)

Barite has been found for the first time in the Swiss Jura (Hirni, Passwang). Small, white crystals of barite with yellow ankerite grow on calcite in different chambers of a Bajocian ammonite. Chemical data (by microprobe analysis) and lattice parameters (refined by leastsquares from powder patterns) are given for the two minerals. There is a distinct correlation between the cell dimensions and the chemical composition for both of them. The genesis is discussed.     

Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)

The Mont Collon mafic complex is one of the best preserved examples of the Early Permian magmatism in the Central Alps, related to the intra-continental collapse of the Variscan belt. It mostly consists (> 95 vol.%) of ol + hy-normative plagioclase-wehrlites, olivine- and cpx-gabbros with cumulitic structures, crosscut by acid dikes. Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A well-preserved cumulative sequence is exposed in the Dents de Bertol area (center of intrusion). P–T calculations indicate that this layered magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100 °C. The Mont Collon cumulitic rocks record little magmatic differentiation, as illustrated by the restricted range of clinopyroxene mg-number (Mg#_cpx = 83–89). Whole-rock incompatible trace-element contents (e.g. Nb, Zr, Ba) vary largely and without correlation with major-element composition. These features are characteristic of an in-situ crystallization process with variable amounts of interstitial liquid L trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distribution in the latter is homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in-situ crystallization equation successfully duplicated the REE concentrations in cumulitic minerals of all rock facies of the intrusion. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 20%, relative to the least evolved facies of the intrusion. L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. However, the in-situ crystallization model reaches its limitations with rock containing high modal content of REE-bearing minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of anorthositic composition, locally crosscutting the layered lithologies, evidence that the Mont Collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions. The proposed model is not able to resolve these complex open systems, but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams. Absence of significant differentiation with recurrent lithologies in the cumulitic pile of Dents de Bertol points to an efficiently convective magma chamber, with possible periodic replenishment.     

Brittle-ductile Transition Zone in the Northern Bergell Alps

Along the northern contact of Bergell granite the style of deformation changes from ductile mylonites at high metamorphic grade (amphibolite facies) in the west (Val Bondasca) to brittle faulting at low grade (greenschist facies) in the east (Maloja). This transition is documented with large scale structural observations, petrofabric data and microstructure which suggest that faulting in the Margneta-Maloja-Casaccia region is related to emplacement of the Bergell granite, and not extending beyond Isola, rather than to a large-scale strike slip fault (Engadine Line).

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Zusammenfassung Entlang des nördlichen Kontaktes des Bergeller Granits ändert sich der Deformationsstil von plastisch verformten Myloniten (Amphibolitfazies) im Westen (Val Bondasca) zum spröden Bruch (Grünschieferfazies) im Osten (Maloja). Der Übergang wird mit großstrukturellen Beobachtungen, Gefügedaten und MikroStrukturen belegt die andeuten, daß Versetzungen in der Margneta-Maloja-Casaccia-Region mit der Platznahme des Bergeller Granits zusammenhängen und sich nur bis Isola erstrecken. Sie werden deshalb nicht mit großräumigen Transversalverschiebungen assoziiert (Engadiner Linie).

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Résumé Le long du contact septentrional du granite de Bergell, le style de la déformation change, depuis des mylonites ductiles à haut degré métamorphique dans l'Ouest (Val Bondasca) (facies amphibolite) jusqu'à une fracturation à degré de métamorphisme peu élevé (facies schiste vert) dans l'Est (Maloja). La transition est apportée par des observations de structures macroscopiques, de textures et de microstructures, qui suggèrent que les déplacements dans la région sont liés à l'emplacement du granite de Begell et finissent vers Isola. Ils ne sont donc pas associés à un système de failles transversales de grande extension (ligne de l'Engadine).

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Microstructures in carbonates from the Alnø and Fen carbonatites

The nature of carbonates and related second phase minerals in carbonatite samples from the Alnø and Fen regions has been studied by optical and transmission electron microscopy, aided by X-ray diffraction and electron microprobe analysis.Calcite, dolomite and ankerite have been found to exhibit two phase microstructures. Coherent ribbons of calcitic material occur in dolomite and ankerite, while the calcite grains commonly contain a fine dispersion of plate-like and sometimes rod-like precipitates. Calcite and dolomite frequently occur in contact with the same crystallographic lattice orientation, the small lattice mismatch being indicated by moiré patterns at the grain boundaries and occasional misfit dislocations.The two-phase structure of calcite in dolomite reported for the first time in this paper is thought to be a high temperature analogue of the modulated microstructures reported recently in calcian sedimentary dolomites (Wenk et al. 1983).     

Development of preferred orientation in plane strain deformed limestone: Experiment and theory

Carbonate rocks deform preferentially by twin gliding on e={01ˉ18} and slip on r ={10ˉ14} and f={02ˉ21}. In polycrystalline aggregates strong textures develop. We report on experimentally produced textures in triaxial plane strain geometry with orthorhombic symmetry at 200° C and 400° C. Pole figure of the experimentally deformed specimens are compared quantitatively with theoretical simulations based on the Taylor theory using both slip and mechanical twinning as mechanisms. Agreement at low and high temperature is satisfactory and documents that models developed for f.c.c. metals can be applied to low symmetry minerals provided that deformation mechanisms are known and that mechanical twinning is properly accounted for. Comparison with experimental results indicates that strain was nearly homogeneous at the conditions considered and the same may apply to many geological textures. Three texture types are described which are differentiated mainly by the relative importance of e twinning.     

Polymorphism of wollastonite

Different superstructures of wollastonite from the regional metamorphic Lepontine zone (Southern Alps) and other localities (e.g., Sierra Nevada, California) have been found. Besides well ordered normal 1T, 2M (2T ?), 4T-wollastonite, all disordered intermediate states exist. The various polymorphs were studied with the precession method. The superstructure is observed in reflections hkl k=odd. 4T-wollastonite is described with a ₀=31.659 Å and is shown to be a common polymorph. The superstructure seems to be independent of metamorphic grade (in the well-zoned Lepontine area it does not follow any mineralogical isograds) and occurs only in strained rocks with well developed lineation and strong preferred orientation. Thus the formation of the superstructure is explained as a deformation effect. Contrary to temperature and pressure, both stress and strain show large local variations and inhomogeneities. Experimental deformation of a 2T-wollastonite at 4 kb and 700° C produced complete disorder. It is suggested that consecutive annealing of strained disordered wollastonite causes periodic stacking sequences of [SiO₃]-chains along the a^*-axis. It appears from these studies that superstructures in chainsilicates are more common than presently known and that they might be useful petrogenetic indicators for the deformation and cooling history of a rock.