Abstract
Rare outcrops of a glassy rock very similar to an obsidian occur among the Permian volcanics of the Lugano region, about 15 km North of Varese (Italy). Such a rock is known as «vitrofiro di Gràntola» (the «vitrophyric dacite» of Stille and Buletti, 1987, 262 Ma in age). XRF analyses for major and some trace elements performed on four selected samples indicate a rhyodacitic chemistry with a calcalkaline affinity for these rocks. Microprobe analyses of Mg-Fe and Fe-Mg-orthopyroxene, Fe-augite, Fe-pargasite, Fe-edenite, Fe-olivine, plagioclase, sanidine and allanite are given as a support for the pre-eruption differentiation history.
Riassunto
Circa 15 km a Nord di Varese (Italia), si osservano limitati affioramenti di una roccia dall’aspetto vetroso, molto simile a quello di un’ossidiana, facente parte delle vulcaniti permiane del Luganese. Tale roccia, conosciuta come «vitrofiro di Gràntola», corrisponde allavitrophyric dacite datata 262 Ma da Stille e Buletti (1987). La composizione (determinata con XRF) in element! maggiori, minori ed in traccia di 4 campioni opportunamente selezionati, dimostra il loro carattere riodacitico con affinità calcalcalina. La composizione, determinata alla microsonda elettronica, dei minerali femici (orto-e clinopirosseni, anfiboli, olivine), sialici (plagioclasi e sanidino) e di un’allanite ha fornito un contribute) alla comprensione del processo evolutivo precedente alla messa in posto.
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Nella seduta del 10 novembre 1990.
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Bocchio, R., Crespi, R., Capitani, L.D. et al. Caratteri chimici e mineralogici del «vitrofiro di Gràntola» (Varese). Rend. Fis. Acc. Lincei 2, 39–49 (1991). https://doi.org/10.1007/BF03010411
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DOI: https://doi.org/10.1007/BF03010411