P1 - rhyolitic ignimbrite

Coarse-crystal rich rock with a compact, massive aspect. The rock contains milky white quartz and feldspar crystals (with a fragmented shape) set in a red-coloured matrix. The sample comes from the Ora Formation, a well-exposed Permian ignimbrite succession of the Atesina Volcanic Complex (Dolomites, North Italy). The Atesina Volcanic Complex (280-270 Ma) represents the largest igneous rock (intrusive and volcanic) association of Permian age in the Southern Alps; the igneous rocks have a calc-alkaline affinity and display a wide array of chemical compositions (from basic to acid terms).

Thin section description:

The crystal-rick character of this rock sample is clearly perceived in thin section. The crystals recognised in the PPL view are: quartz (Qz), colourless; feldspar (plagioclase: Pl; alkali feldspar: Afs), with a dusty aspect due to alteration to an aggregate of fine-grained white mica (sericite); biotite (Bt), partially or totally black due to oxidation (OH anf F replaced by O) and at places showing local alteration (i.e., chloritization, sericite). The crystals are embedded in a matrix that is black in the XPL due to a high proportion of glass (glass is isotropic and hence black in the XPL view as the microscope stage is rotated); the matrix also contains widespread tiny laths of crystals. This rock sample is an ignimbrite, formed by the compaction and solidification of the hot magmatic fragments (welding compaction) explosively erupted from a volcano; in this case, the original shape of the glass shards is not seen because of the high degree of welding compaction of the rock (Willcok et al., 2015 – J Volc Geoth Res, v. 306, 17-40).

thin section PPL	thin section XPL