Monthly Archives: September 2018

Central Iberian basement-a multiphase evolution

Congratulations to team member Jürgen von Raumer for his latest paper on the Central Iberian basement-A multiphase evolution. 

The Cambro‐Ordovician rhyodacitic to dacitic volcanics from the Central Iberian basement, currently known as Ollo‐de‐Sapo (toads eye), have been reported as a specific group of felsic porphyritic rocks with blue quartz and large phenocrysts of K‐feldspar, in a partly vitreous or fine‐grained matrix. Interpreted to form Cambro‐Ordovician volcanic domes, they are accompanied by tuffs, ignimbrites and products of reworking in a near‐surface environment.

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The coarse‐ to fine‐grained rocks exhibit rather large K‐feldspar phenocrysts, plagioclase and rounded blue quartz, representing former corroded phenocrysts. Their colouration indicates unmixing of TiO2 at around 900 °C during cooling from relatively high crystallisation temperatures, indicating their origin at hot lower crustal conditions. We propose at least a two‐step evolution (1) starting around 495 Ma in the lower crust of a collapsing cordillera, generating a phenocryst‐rich mush and adiabatic melting of the lower crustal protolith to produce the spectacular Ollo‐de‐Sapo porphyrites, before (2) magma ascent and crustal extension leading to a different thermal regime around 483 Ma. This article is protected by copyright. All rights reserved.

Ollo de Sapo Cambro-Ordovician volcanics from the Central Iberian basement-A multiphase evolution.