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Notes: Zusammenfassung Das Alter der letzten Metamorphose der Sesia-Zone wurde lange Zeit für herzynisch oder älter gehalten. Als Argument dienten Grundgebirgseinschlüsse in den Trachyandesiten bis Andesiten der Sesia-Zone. Diese Vulkanite hielt man in Analogie zu den permischen Vulkaniten der Südalpen für permisch. In jüngster Zeit wurden in Tuffiten der Sesia-Vulkanite Pflanzenreste gefunden, die als oberkarbonisch beschrieben wurden. Diese Schlüsse standen in Diskrepanz zu strukturgeologischen Beobachtungen und zu den zahlreichen radiometrischen Altersbestimmungen in der Sesia-Zone und zwangen zur nochmaligen überprüfung der paläobotanischen Evidenzen. Die von uns gefundene Flora weist sehr moderne Züge auf und läßt sich mit Sicherheit ins Tertiär einstufen. Kein einziges paläozoisches Fossil wurde gefunden. Das durch die Pflanzenreste bestätigte tertiäre Alter der Vulkanite konnte mit radiometrischen K-Ar-Gesamtgesteinsaltern auf 29–33 m.y. präzisiert werden. Die letzte Hochdruck-Metamorphose in der Sesia-Zone fand nach K-Ar-Altersbestimmungen an Glimmern zwischen 60 und 90 m. y. statt. Vor ca. 38 m. y. wurden die alpeneinwärts liegenden Partien der Sesia-Zone von der lepontinischen Kristallisationsphase in Grünschieferfazies überprägt.

Notes: Zusammenfassung Im Bereich der Südgrenze des Damara-Orogens wurden Alter und Intensität der Metamorphose mit Hilfe von K/Ar-Altersbestimmungen und Illitkristallinität bestimmt. Die Untersuchungen umfaßten folgende Einheiten: 1. den südwestlichen Teil des E-W streichenden Astes des Damara-Orogens; 2. die Decken der Naukluft; 3. die Nama-Group, die sich von nördlich der Naukluft bis zum Fish-River im Süden erstreckt (einschließlich des westlichen Teils der Dwyka-Formation). Die Metamorphose der Naukluft-Decken sowie der unterlagernden Nama liegt in Bereich der höheren Anchibis unteren Epizone. Zwischen den Gesteinen der Naukluft-Decken und der südöstlich vorgelagerten gefalteten Nama besteht ein deutlicher Sprung zu niedrigerer Metamorphose. In Richtung SE nimmt der Grad der Metamorphose in der Nama kontinuierlich ab bis in den Bereich der Diagenese. In den drei oben erwähnten Einheiten wurden K/Ar-Altersbestimmungen an Hellglimmern durchgeführt. Zusätzlich wurden Muskowite aus dem Basement, das die Nama unterlagert, datiert. Die Muskowite aus dem Basement ergaben ein Alter von 1160 m. y. Bestimmungen an Hellglimmern des südlichen Damara, der Naukluft und der nördlichen Nama liegen auf zwei Isochronen mit Altern von 495 m. y. und 530 m. y. Das Alter von 530 m. y. datiert den Höhepunkt der Metamorphose und das Alter der Deformation in den synkristallin deformierten Gesteinen. Das Alter von 495 m. y. kann als Abkühlungsalter für die höher metamorphen Gesteine gedeutet werden oder zeigt eine Verjüngung durch eine zweite postkristalline Deformation in Teilen der Naukluft und Damara-Gesteine an. Dieses Alter von 495 m. y. wurde ebenfalls im Mylonit der Hauptüberschiebungsbahn der Naukluft-Decken gefunden und gibt den Zeitpunkt der Platznahme der Naukluft-Decken an.

Notes: Zusammenfassung Die während Jahren durchgeführten systematischen Untersuchungen an stratigraphisch fixierten Horizonten des unteren Miozäns haben zur Auswahl von neun datierbaren Glaukonitproben aus verschiedenen Gebieten der Tethys geführt. Nach erfolgter sorgfältiger sedimentologischer und stratigraphischer Studien wurden K-Ar-Alterswerte an diesen Glaukoniten gemessen. Die Interpretation der erhaltenen scheinbaren Alter berücksichtigt Ablagerungsbedingungen, mineralogische Qualität und derzeitige Kenntnisse über die Genese der grünen Glaukonitaggregate. Die erhaltenen Daten erlauben es, die Basis des Miozäns mit ungefähr 21–22 M. J. festzulegen und die Grenze Aquitan-Burdigal mit 18 M. J. Unsere K-Ar-Alterswerte an Glaukoniten werden von verschiedenen neueren Messungen anderer Autoren an Hochtemperaturmineralien gestützt.

Notes: Abstract In Normandy (France), the glauconite bearing base of the Cenomanian has a wide distribution in outcrops and boreholes. Glauconite pellets are subjected to natural weathering and fluvial transport following separation from the glauconite-bearing bedrock by streams and creeks which dissolve the carbonate fraction. From the 100 samples collected, 19 samples from the studied horizon have been selected after X ray, diffaction studies. X ray diffractron techniques show that the non weathered glauconite pellets are composed of well-ordered gilauconites. In the reworked glauconite, the alteration is manifested by a slight opening of the structure. In addition, the diffractogramm yield a fast and sensitive estimation of the K contents of the glauconites. Lowering of the K content goes hand in hand with the opening of the glauconite structure. 12 samples were selected from the autochtonous horizon (fresh outcrop and borehole) a second group of 7 samples was taken from alluvions (streams and quaternary terraces) to study the effects of natural weathering. As actually, no other glauconite-bearing horizons are found in the region, the glauconite pellets in the alluvions can only be derived from the studied bedrock. Reworking and natural weathering of the glauconites leads to a decrease of the K content of 10–15%. Argon analysis shows that comparable percentage of radiogenic argon is lost at the same time so that the apparent ages do not change (in the errors limits). These results are only valid for well-ordered glauconites and continental alteration under moderate climate with basic fresh water. In conclusion, it can be stressed that the credibility of glauconite K-Ar ages does not change after such a reworking.

Notes: Abstract Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mössbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, 40Ar/39Ar and stable isotope methods. With increasing metamorphic grade based on illite “crystallinity” data (XRD and IR) the following continuous changes are observed: (i) the 1Md→2M1 polymorph transformation is completed in the higher grade anchizone; (ii) K2O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction 〈2 μm decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age. These results suggest that the prograde evolution from 1 Md illite to 2M1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of 40Ar and 87Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the 40Ar/39Ar age spectra. Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant 40Ar/39Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian.

Notes: Abstract The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with ɛ 607 Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by “phlogopite metasomatism”. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial ɛ 293 Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic “crustal residence ages” of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.

Notes: Abstract Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The δ18O(smow) values of the quartz (after coesite) (δ18O=8.1 to 8.6‰, n=6), phengite (6.2 to 6.4‰, n=3), kyanite (6.1‰, n=2), garnet (5.5 to 5.8‰, n=9), ellenbergerite (6.3‰, n=1) and rutile (3.3 to 3.6‰, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H2O, the a(H2O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X(CO2)〉0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are ∼680 °C/30 kb at a(H2O)=1.0 and are calculated to be ∼70°C higher at a(H2O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (δ18O=11.6‰) and garnet (δ18O=8.7‰) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (δD(SMOW)=-27 to-32‰), on secondary talc and chlorite rite after pyrope (δD=-39 to -44‰) and on the surrounding biotite (δD=-64‰) and phengite (δD=-44‰) gneiss. All phases appear to be in nearequilibrium. The very high δD values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar δD, but dissimilar δ18O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.

Notes: Abstract Some 150 white K-micas from the Central Alps were analysed for their polymorph and phengite content. Pre-Alpine white K-micas and those belonging to the Meso-Alpine Lepontine Metamorphic “High” show exclusively the 2M1 polymorph. The 3T structural form, on the other hand, has been found in one third of the white K-micas formed during the Alpine regional metamorphism. In most cases this trigonal structure coexists with varying amounts of the 2M1 form. The 3T distribution pattern suggests that this polymorph originated during the Eo-Alpine high-pressure/“low temperature” metamorphism. Provided this interpretation is correct, the sporadic occurrence of this polymorph within the Meso-Alpine staurolite zone may be used as a tracer for the Eo-Alpine metamorphism. The following improved correlation between the (060, 331) reflections of 2M1 white K-micas and the RM-content (= 2Fe2O3+FeO+MgO in molar proportions), based on 24 micas from granitoid rocks, is presented: d(060, 331)= 1.498+0.082 RM. The phengite content of Alpine white K-micas belonging to the assemblage muscovite-biotite-K-feldspar-quartz was estimated from RM values or derived from chemical analyses and was found to be clearly related to metamorphic grade. Phengite-rich micas were formed during the Eo-Alpine high-P/“ low-T” metamorphism while aluminous muscovite was found within the Meso-Alpine thermal high of the Lepontine gneiss area. White K-micas from areas which underwent both the Eo-Alpine and the Meso-Alpine metamorphism display variable phengite contents. Although these micas show Tertiary Rb-Sr and K-Ar ages, the variable phengite content presumably reflects conditions during the Eo-Alpine high-P/“low-T” metamorphism. This interpretation implies that the cations occupying the interlayer positions are more easily equilibrated than those in octahedral and tetrahedral structural sites. A compilation of 3T white K-mica occurrences described in the literature is given in the appendix.