How does a monzogranite turn into a trachydacitic extrusion mantled by basinal volcaniclastics and peperites? The case of South-Ouessant, Armorican Variscides (France)

Abstract

How many volcanic bodies are being confused with plutonic ones worldwide? The purpose of this study is not to provide an answer to this question, but rather to illustrate this issue through an example from the French Armorican Variscides. It concerns a magmatic body cross-cutting highly strained terranes in the Ouessant Island, regarded for decades as a granitoid (monzogranite) on the basis of both its coarse-grained texture and its mineralogy. However, the volcanic origin of the metamorphosed series flanking this foliated body is here recognized by pillow lavas, deposit layers, and fiamme-bearing volcaniclastics, all emplaced onto the soft-substrate floor of a fault-bounded basin. Among other things, similarities between feldspar megacrysts/porphyrocrysts in both the volcaniclastics and the adjoining (formerly) monzogranitic massive body lead us to reinterpret the latter as a trachydacitic extrusion. In our model, the corresponding viscous lava progressively flowed in the basin, recovering the earlier volcanic formations and inducing load effects on the underlying soft sediments, along with compaction of the previously deposited pumices, to produce fiamme. The interpretation of the South-Ouessant area as a Visean transtensional volcano-sedimentary basin provides a new perspective on the distribution of the Variscan pull-apart basins in the Armorican Massif.