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Rb-Sr and40Ar/39Ar age constraints on the Variscan metamorphism recorded by Ordovician acidic metavolcanic rocks in the Eastern Southalpine basement

Età Rb-Sr e40Ar/39Ar del metamorfismo varisico registrate da metavulcaniti ordoviciane del basamento sudalpino orientale

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Abstract

New Rb-Sr and40Ar/39Ar ages are presented for some acidic metavolcanic rocks coming from two different outcrops of the Eastern Southalpine basement (Val Sarentino and Levico): these ages refer to the timing of the two stages of the Variscan metamorphism, which affected the Southalpine basement. Whole-rock Rb-Sr ages of 345±9 and 353±56 Ma are consistent with Rb-Sr ages obtained for similar lithologies in the neighbouring basement outcrop of Val di Funes; together, these ages define a synchronous metamorphic stage during the Tournaisian (first Variscan stage).40Ar/39Ar ages on coarsegrained, post-kinematic phengites give values of 325–326 Ma for Val Sarentino and of 329–330 Ma for Levico samples. They probably represent cooling ages of the retrograde path of the second Variscan stage, during which the thermal climax of the whole Variscan metamorphic event occurred. The Levico ages are slightly older than Val Sarentino, possibly because of the closeness of the peak temperature and the assumed closure temperature range of K-Ar system in white mica. Taking into account the Levico and Val Sarentino cooling ages, the thermal peak in the Eastern Southalpine domain should be older than 331 Ma. Therefore, the two Variscan metamorphic stages in the Eastern Southalpine basement are probably closer in time than previously believed, being separated by less than 20 Ma.

Riassunto

In questo lavoro vengono presentate nuove datazioni radiometriche Rb-Sr e40Ar/39Ar di alcuni campioni di metavulcaniti acide provenienti da due zone del basamento sudalpino orientale (Val Sarentino e Levico); questi dati rappresentano dei vincoli temporali per l’evento metamorfico varisico, sviluppatosi in due stadi distinti, che ha interessato il basamento attualmente affiorante. Le età Rb-Sr su roccia totale di 345±9 e 353±56 Ma sono in accordo con età radiometriche Rb-Sr ottenute in un affioramento adiacente di basamento, sito in Val di Funes, definendo pertanto un evento metamorfico sincrono nel Tournaisiano (primo stadio varisico). Le età radiometriche40Ar/39Ar determinate su phengiti post-cinematiche a grana grossa danno valori di 325–326 Ma per i campioni della Val Sarentino e di 329–330 Ma per i campioni di Levico. L’interpretazione più verosimile per queste età è che esse rappresentino età di raffreddamento per entrambe le aree considerate, e che si possano riferire alla fase retrograda del secondo stadio metamorfico, durante il quale si registrò il picco termico dell’intero evento varisico. Le età ottenute per i campioni di Levico sono più antiche di quelle dei campioni della Val Sarentino, verosimilmente a causa della piccola differenza di temperatura fra il picco termico in questa zona e la temperatura di chiusura del sistema K-Ar nella mica chiara. In considerazione di ciò, il picco termico nel Sudalpino orientale deve essere più antico di 331 Ma, sia per la zona di Levico che per quella di Val Sarentino. Pertanto, i due stadi metamorfici varisici nel basamento sudalpino orientale sembrano essere più vicini nel tempo di quanto affermato in precedenza, dal momento che l’intervallo temporale che li separa dovrebbe essere inferiore ai 20 Ma.

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    Sandro Meli

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Nella seduta del 23 aprile 2004.

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Meli, S. Rb-Sr and40Ar/39Ar age constraints on the Variscan metamorphism recorded by Ordovician acidic metavolcanic rocks in the Eastern Southalpine basement. Rend. Fis. Acc. Lincei 15, 205–223 (2004). https://doi.org/10.1007/BF02904461

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