ALBERT

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 40Ar/39Ar incremental-release analyses were carried out on whole-rock and constituent white mica (illite)-rich size fractions (0.63–1 to 6.3–20 μm) within two very-low grade, penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade (northeastern Rheinisches Schiefergebirge, Federal Republic of Germany). One sample from the upper anchizone displays internally concordant 40Ar/39Ar spectra with plateau ages ranging between ca. 316 and 325 Ma. These are similar to conventional K-Ar ages determined for the whole-rock and size fractions. Together the isotopic results suggest that cleavage formed at ca. 320 Ma during a concomitant very-low grade metamorphism. This is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the Westphalian. A metatuff sample from the middle anchizone records more internally discordant 40Ar/39Ar age spectra with total-gas ages ranging from 366 to 372 Ma. These are ca. 35–45 Ma older than corresponding conventional K-Ar ages, indicating marked recoil-loss of 39Ar occurred during irradiation. Transmission electron microscopy reveals that white mica grains within size fractions from the upper anchizone sample have clearly defined, straight edges whereas those within the middle anchizone samples are embayed and diffuse. This results in an increase in surface/volume ratio and therefore greater susceptibility for recoil-loss of 39Ar in the middle anchizone sample. Grain-edge morphology appears to be a major factor in determining the extent of recoil-loss of 39Ar during 40Ar/39Ar analysis of fine-grained size fractions.

Notes: Zusammenfassung Mit Hilfe von Luftaufnahmen aus 11 000 m Höhe unter Anwendung des Standard-Me\bild-Verfahrens wurde eine Grundkarte der Sudbury-McGregor-Bucht in Ontario, Kanada, hergestellt. Unter Berücksichtigung geologischer Arbeiten ergaben sich daraus neue Gesichtspunkte für Lithologie und Tektonik des mittleren PrÄkambriums in diesem Gebiet.

Notes: Zusammenfassung Die Amphibolitkonzentrate aus vier Proben unterschiedlich regressiver Eklogite aus der Tsäkkok-Linse des Seve-Deckenkomplexes spiegeln diskordante40Ar/39Ar Altersspektren wieder. Diese deuten an, daß eine Argonkontamination von außen stattgefunden hat.36Ar/40Ar gegen39Ar/40Ar Isotopen sind für zwei Proben genau bestimmt und liefern Alter von 464,7 ± 1,3 Mio. und 463,2 ± 6,3 Mio. Jahre. Inverse Ordinatenabschnitte sind sehr viel größer als 295 und bestätigen erhebliche Argon-Fremdkomponenten. Zwei Hellglimmerkonzentrate aus metasedimentären Einheiten der Tsäkkok-Linse zeigen intern konkordante40Ar/39Ar Altersspektren, welche Plateaualter von 448,2 ± 1,6 Mio. und 468,4 ± 0,9 Mio. Jahre besitzen. Ein Amphibolkonzentrat aus einem metamorphen basischen Gang innerhalb der Sarek-Linse des Seve-Deckenkomplexes liefert ein Isotopenkorrelationsalter von 463,8 ±12,3 Mio. Jahre, welches eine postmetamorphe thermale Entwicklung andeutet, die ähnlich der Tsäkkok-Linse ist. Die Amphibol- und Hellglimmeralter werden dahingehend interpretiert, daß sie die postmetamorphe thermale Abkühlung durch entsprechende Temperaturen für das intrakristalline Zurückhalten des Argons datieren, die im Anschluß an die Amphibolitfaziesregression der Eklogitfaziesvergesellschaftung folgte. Der Seve-Deckenkomplex scheint aus einer distalen Fazies spätprotero- bis frühpalaeozoischen Miogeosynklinale von Baltika hervorgegangen zu sein. Die jetzigen40Ar/39Ar Ergebnisse liefern eine Aufzeichnung von frühpaleozoischen (Vor-Mittelordovizium) tektono-thermalen Aktivitäten, und sie sind daher vergleichbar mit früheren Vermutungen von einer bedeutenden frühkaledonischen Orogenese innerhalb der skandinavischen Kaledoniden.

Notes: Abstract New petrologic and 40Ar/39Ar geochronologic data constrain conditions of Alpine metamorphism along the northwestern border of the Tauern Window. The P-T estimations based on phengite barometry were determined for samples from units of the Lower Austroalpine nappe complex exposed above the Southpenninic interior of the Tauern Window, and from upper parts of the Southpenninic “Bündner Schiefer” sequence. Results suggest that both Mesozoic metasedimentary nappe units (Reckner and Hippold Nappes) and an ophiolitic nappe (Reckner Complex) of the Lower Austroalpine nappe complex have been metamorphosed at pressures between 8 and 10.5 kbar and temperatures around 350 °C. The structurally highest Lower Austroalpine unit (Quartzphyllite Nappe) was not affected by high-pressure metamorphism and records maximum P-T conditions of approximately 4 kbar and 400 °C. Highest parts of the structurally underlying Southpenninic Bündner Schiefer sequence were metamorphosed at intermediate pressures (6–7 kbar). Temperatures increased in all structural units during decompression. Whole-rock 40Ar/39Ar plateau ages of silicic phyllites and cherts with abundant high-Si phengites record ages around 50 Ma in the Reckner Nappe, and 44–37 Ma in the Hippold Nappe and Southpenninic Bündner Schiefer sequence. These ages are interpreted to date closely the high-pressure metamorphism. The Lower Austroalpine-Southpenninic border area in the NW Tauern Window appears to have evolved along an indented, fragmented active continental margin where the Reckner Complex represents one of the oldest sections of the Southpenninic (Piemontais) Oceanic tract that was originally situated close to, or even within, the Lower Austroalpine continent. During closure of the Piemontais Ocean, the resultant subduction zone did not entrain components of the Reckner Complex or its cover sequences (Reckner and Hippold Nappes): therefore “Eoalpine” high-pressure metamorphism did not occur. Sequences exposed within the study area were subducted to relatively shallow depths during the last stage of consumption of oceanic crust and immediately prior to final continental collision.

Notes: Abstract Generally synmetamorphic granitic stocks intrude high-grade, Silurian-lowermost Devonian metasedimentary rocks near Augusta, Maine. Rb-Sr whole-rock isochrons (8 points each) define mutually overlapping crystallization ages of 394±8 m.y. (Togus quartz monzonite), 387±11 m.y. (Hallowell quartz monzonite), and 381±14 m.y. (Three Mile Pond biotite granodiorite), thereby providing a narrow chronologic bracket for Acadian tectonothermal activity in the area. Igneous hornblende, muscovite, and biotite display internally concordant 40Ar/39Ar age spectra with plateau dates of 350 m.y. (hornblende) and 300-265 m.y. (muscovite and biotite), with an overall southwestward younging trend. The mineral dates are similar to those recorded in adjacent portions of the regional metamorphic terrain and suggest a prolonged postmagmatic cooling which closely followed the diachronous northeast-southwest post-Acadian cooling of the country rocks. No evidence for a distinct Permian thermal overprint of older isotopic systems has been observed.