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The youngest basic oceanic magmatism in the Alps (Late Cretaceous; Chiavenna unit, Central Alps): geochronological constraints and geodynamic significance

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Abstract

Cathodoluminescence-controlled radiometric dating (U–Pb SHRIMP) was carried out on zircon domains from metabasic rocks of the Chiavenna unit, a major mafic/ultramafic-bearing unit in the Central Alps. Co-magmatic zircon domains from amphibolites near Chiavenna and Prata areas yielded weighted mean 206Pb/238U ages at 93.0±2.0 and 93.9±1.8 Ma, respectively, interpreted as the age of crystallization of the magmatic protoliths. These ages fit well with the time of late spreading in the Valais Ocean, as suggested by previous paleogeographic reconstructions. Inherited zircon grains and/or core domains (Permo-Triassic, Carboniferous, Proterozoic) are abundant, indicating proximity of the Chiavenna unit to thinned continental crust. This is in line with the origin of this unit from subcontinental mantle sources, as suggested previously on petrological and structural grounds. Metamorphic zircon domains from one amphibolite near Chiavenna yielded a weighted mean 206Pb/238U age at 37.1±0.9 Ma, identical to the 38.5±0.9 Ma SHRIMP age of an amphibolitized eclogite of the Antrona ophiolites (Valais domain, Western Alps). Precise metamorphic ages were difficult to obtain from the composite (poly)metamorphic rim domains of the Prata amphibolite. This is attributed to the location of the Prata area close to the granulite-facies Gruf unit (metamorphosed at ca. 33 Ma) and to the 24–25 Ma old Novate granite, where metamorphic/fluid events probably caused multiple resetting to various degrees. The ca. 93 Ma old magmatism, identified for the first time in the Chiavenna unit, is the youngest basic oceanic magmatism reported in the Alps. The 37.1±0.9 Ma old metamorphism in the Chiavenna unit, attributed to the Valais domain, confirms the model suggesting stepwise younging of metamorphic ages from the south (Adriatic plate) to the north (European plate). It is older than metamorphism in the European margin (ca. 35–31 Ma) lying to the north of the Valais domain and younger than that in the Piemont–Ligurian Ocean (ca. 44–45 Ma) lying to the south of the Valais domain.

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Acknowledgements

We very much appreciate the help of M. Hamilton and R.A. Stern during various stages of the SHRIMP work. R. Huber is thanked for her help in part of the field work and for discussions. W. Wittwer is cordially thanked for zircon separation. Numerous fruitful discussions with N. Froitzheim, Bonn, as well as his help with the geochemical analyses are greatly appreciated. Constructive comments by V. Trommsdorff, Zürich, on an early draft of the manuscript, as well as by W. Frisch, Tübingen and A. Möller, Potsdam, who reviewed this paper, are gratefully acknowledged. Many thanks also to W. Schreyer, Bochum, for the editorial support. This study was supported by a grant of the Swiss National Science Foundation (20-52662.99).

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  1. Institute of Isotope Geology and Mineral Resources, Swiss Federal Institute of Technology (ETH), Sonneggstrasse 5, 8092 Zurich, Switzerland

    Anthi Liati & Dieter Gebauer

  2. Research School of Earth Sciences, The Australian National University, Mills Road , Canberra, ACT 0200, Australia

    C. Mark Fanning

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Liati, A., Gebauer, D. & Fanning, C.M. The youngest basic oceanic magmatism in the Alps (Late Cretaceous; Chiavenna unit, Central Alps): geochronological constraints and geodynamic significance. Contrib Mineral Petrol 146, 144–158 (2003). https://doi.org/10.1007/s00410-003-0485-7

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