Revisión de las colecciones de rocas riolíticas catalanas del Museu de Geologia y de la Facultat de Geologia de la Universitat de Barcelona

Abstract

A revision of the collections of catalonian rhyolitic rocks from Museu de Geologia de Barcelona and Faculty of Geology of University of Barcelona

The Museu de Geologia de Barcelona and the Faculty of Geology of the University of Barcelona own twin collections of rhyolitic volcanic rocks from the Catalonian Pyrenees, permo-carboniferous in age. Samples of these collections are currently exposed in the museum and used for didactic purposes at the university (petrography and hand sample studies). This revision corroborates that the collections have a initial common origin in the San Miguel de la Cámara collection (1934). The university’ collection consists of lavic rocks from the Gréixer-Coll de Pi sector, rhyolitic in composition, and with textural evidence (perlitic textures, skeletal phenocrysts, thermal shocked quartz, etc.) of magma-water interaction during the mise-en-place process. Despite of their age these rocks show fine preservation of the original textural features, but they seem unadequate for didactic purposes in introductory courses of petrography because of several facts (peraluminous character expressed in phenocrysts, presence of specific textures of magma-water interaction, advanced stage of devetrification and silicification, etc.). These samples seem more idoneous for advanced didactic purposes (second cycle or PhD level studies).

The Museu de Geologia contains two groups of samples: the aforementioned lavic rocks, and pyroclastic rocks from several localities of the catalonian Pyrenees. The pyroclastic rocks offer evidence of their character related to explosive volcanism (shards, pumice, crystal fragments, etc.) but they are deeply transformed and therefore inadequated for didactic purposes. Most of the thin slides are in fact too thick to allow a correct study of pyroclastic textures. The petrographic features allow us to suspect not only rhyolitic terms but also rhyodacitic and dacitic ones.

The available data (bibliography and this study) shows some inconsistences in the current interpretation of the lavic silicic rocks. We offer a synthetic tentative explanation that could be tested in future research.