Deutung diskordanter Zirkonalter der Silvrettadecke und des Gotthardmassivs (Schweizer Alpen)

U-Pb isotopic analyses are reported for zircons of nine igneous rocks and high grade metamorphic paragneisses from the Silvretta nappe and the Gotthard massif, Switzerland.Following Silver and Deutsch (1961) the zircon populations isolated from six of these rocks were devided into series of size fractions with varying uranium content. An extensive Rb-Sr investigation has been made on some of these rock samples and their minerals (Arnold and Jäger, 1965: Gotthard massif, Grauert, 1966: Silvretta nappe). Rb-Sr measurements on biotites and mineral isochrons of individual gneiss samples yield ages which indicate the end of the Alpine and the Hercynian (=Variscan) metamorphism. Other measurements give mixed ages between these events. The U-Pb investigation of zircons, however, yield criteria for an even older event, 400 to 500 m.y. ago.The isotopic relationship indicates a strong episodic disturbance of the U-Pb systems in the paragneiss zircons with high losses of radiogenic lead. In a Concordia diagram the best fit lines to the data points of the paragneiss zircons from the Gotthard massif as well as from the Silvretta nappe trend both toward the data points of the orthogneiss zircons and intersect the Concordia at approximately 440 m.y. Rb-Sr analyses of the granitic orthogneisses from the Silvretta nappe (Flüela granite gneisses) gave a well defined isochron of 428 m.y. (=1.47×10^–11y^–1). This value agrees rather well with the age given by the intersection of the best fit lines to the paragneiss zircons with the Concordia.Considering the geological and petrographical observations we interpret this disturbance of the U-Pb systems in the paragneiss zircons as the result of an influence of regional high grade metamorphism and anatectic melting together with the formation of igneous rocks during the Caledonian petrogenesis.In the Silvretta the uranium rich zircons from the Flüela granite gneisses reflect a disturbance and lead loss probably during the Hercynian metamorphism. In the Gotthardmassif the zircons from a quartz diorite and an ultrabasic inclusion in this diorite, however, show no or only a slight influence of the later Hercynian and Alpine metamorphisms. The apparent ages of the zircons in the ultrabasic inclusion are nearly concordant (Pb²⁰⁶/U²³⁸: 448 m.y., Pb²⁰⁷/ U²³⁵: 452 m.y., Pb²⁰⁷/Pb²⁰⁶: 472 m.y.). There is in all orthogneiss zircons some evidence of older inhereted U-Pb components besides new zircon growth.If we assume an episodic model for the lead loss and a disturbance 450 m.y. ago the minimum primary age of the detrital zircon components in the paragneisses would be 1500 m.y.

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Der eine von uns (B.G.) möchte den Herren Prof. Dr. M. Grünenfelder und Prof. Dr. P. Signer vom Labor für Geochronologie des Instituts für Kristallographie und Petrographie der Eidgenössischen Technischen Hochschule Zürich seinen Dank für ihr Interesse und die großzügige Unterstützung bei der Durchführung dieser Arbeit aussprechen. Herr Prof. Dr. M. Grünenfelder und Herr Dr. R. Pidgeon haben ihn in die Arbeitsweise der U-Pb-Isotopenanalyse eingeführt. Herr Dr. V. Köppel sowie alle oben genannten Herren haben durch ihre wertvollen Ratschläge und kritischen Diskussionen diese Arbeit wesentlich gefördert. Ihnen und auch allen anderen Angehörigen des Labors für Geochronologie, die zum Gelingen der Arbeit beigetragen haben, sei herzlichst gedankt.

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Herr Dr. L. Rybach hat in dankenswerter Weise von vier Zirkonfraktionen die Urankonzentration durch Gamma-Spektrometrie bestimmt.

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Wir beide möchten Frau Prof. Dr. E. Jäger und Herrn Prof. Dr. E. Niggli vom Mineralogisch-Petrographischen Institut der Universität Bern für ihr wohlwollendes Interesse und die kritische Durchsicht des Manuskriptes danken.