Genesis of crystal-rich volcaniclastic facies in the Permian red beds of the Central Pyrenees (NE Spain)

Abstract

The Permo-Carboniferous volcanism of the Central Pyrenees developed in subaerial conditions, and is mainly represented by the products of explosive eruptions of calc-alkaline silicic magmas occasionally associated with calderaforming events. The sedimentological record of the Permo-Carboniferous basins of the Central Pyrenees is characterized by the abundance of crystal-rich volcaniclastic deposits reflecting a high crystal content in the erupting magmas (18–40 vol.%). Sedimentation during the Early Permian was dominated by massflows, stream floods and braided channels, organised in poorly developed alluvial fans at basin margins. A wide spectrum of crystal-rich deposits can be identified in the Permian terrigenous red beds. The grouping of these deposits into facies and facies associations, allows the recognition of several volcaniclastic marker horizons extending through the Central Pyrenees. These can be used to establish stratigraphical correlations between the different Permo-Carboniferous basins. The study of these crystal-rich volcaniclastic deposits has revealed that the Lower Permian crystal-rich voleaniclastic deposits of the Central Pyrenees originated from pyroclastic and reworking processes that sometimes acted simultaneously. Pyroclastic deposits derived from high intensity explosive magmatic eruptions of rhyodacitic and rhyolitic crystal-rich magmas. These eruptions generated high eruption columns that occasionally collapsed, giving rise to the formation of widespread pyroclastic flows and associated pyroclastic surges. The entrance of the eruption columns into the atmosphere had a significant effect on the local weather, provoking sporadic storms that led to reworking of pyroclasts by rainfall run-off during eruption. Different crystal concentration processes acted in the eruption columns and in the subsequent pyroclastic flows and overriding ash clouds, producing the accumulation of a high volume of crystals in the resulting deposits. Reworking and redeposition of pyroclastic deposits by fluvial processes further enhanced crystal concentration.