Fig. 3
Thin-section micrographs of dropstones. a, b Sample 1, Rosso Ammonitico Lombardo Formation, Toarcian, south of the village of Arzo (Ticino). The rhyolite exhibits a porphyritic texture and a microfelsitic matrix composed of quartz and feldspar. Phenocrysts are composed of (1) clear subhedral quartz (qtz), locally displaying embayment; (2) euhedral alkalifeldspar (Ak-fsp) with Karlsbad twinning, partly zoned and strongly sericitized; (3) minor idiomorphic plagioclase (plg) is characterized by polysynthetic twinning, partly to strongly sericitized. Average anorthite-content is around An25. Minor biotite (bio) has completely been transformed to fine-grained aggregates of sericite and opacs (Fe-oxides). c, d Sample 3, Molino Member of Moltrasio Formation, Pliensbachian, Val Molinaccio, west of Bédero north of Varese. Subhedral quartz (qtz) and sanidine (san) crystals are embedded in a devitrified felsitic matrix with fluidal texture and some volcanic lithic clasts (lit); plagioclase is a very subordinate phenocryst phase, whereas variably altered biotite is ubiquitous. e, f, Rosso Ammonitico Lombardo Formation (Toarcian), Breggia River (Ticino); The crystal-rich rhyolite fragment displays a porphyritic texture and a microfelsitic matrix composed of quartz and feldspar. Phenocrysts are: (1) clear subidiomorphic quartz (qtz) crystals with embayments and melt inclusions; (2) polysynthetically twinned plagioclase (An25) partly replaced by saussurite and sericite (plg), many crystals are broken; (3) alkalifeldspar with Karlsbad twins, some perthitic exolutions, but still exhibiting sanidine optics (nearly uni-axial, negative, Ak-fsp); and (4) biotite completely replaced by chlorite (chl) and Fe-oxides. a, c, e Nicols parallel; b, d, f Nicols crossed