P2 - Ryhiolitic ignimbrite

Rock

Macroscopic description:

Medium-grained rock containing crystals with a mean grain size of 1-5 mm set in a grey-coloured matrix. The larger crystals exhibit a milky white colour and a fragmented shape; these crystals are identified as feldspar and quartz by the thin section examination. The sample comes from the ignimbrite deposits of the Atesina Volcanic Complex (Dolomites, Southern Alps, North Italy); see the P1 ignimbrite for further details.

Thin section description

The rock is of the same type of to the P1 ignimbrite. It is an ignimbrite sample composed of crystals of quartz (Qz), colourless in the PPL view, and altered crystals of feldspar (some plagioclase (Pl) crystals can be identified in the XPL view by the albite twinning) and biotite (Bt). The crystals are embedded in a glassy-rich matrix (see P1 sample for the glassy matrix description). One Pl crystal in the right side of the field has the inner portion replaced by epidote (Ep), recognizable in the XPL view, because of the high birefringence colours.