The sandstone memory of the evolution of a Triassic volcanic arc in the Southern Alps, Italy

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Abstract
The erosion of a southern volcanic belt provided the bulk of the fine‐grained and moderately sorted siliciclastic detritus in the whole deltaic to lagoonal Carnian (Upper Triassic) sequence of the Bergamasc Alps. Volcanic input began in the Early Carnian (Calcare Metallifero Bergamasco) and became prominent in Early‐Middle (Val Sabbia Sandstone) and Late Carnian times (S. Giovanni Bianco Fm.), after an intervening period of diminished supply (Gorno Fm.).The mineralogical and chemical composition of the sandstones compares closely with that of siliciclastics derived from modern, Pacific‐type magmatic arcs and it testifies to an evolution of the source rocks from andesites and dacites during the deposition of the Val Sabbia Sandstone, towards rhyodacitic ignimbrites for the S. Giovanni Bianco Fm. The marked reduction in feldspars at the top of the sequence contrasts with the classical trend from Undissected to Dissected Magmatic Arc provenance and is ascribed to the progressive deepening of erosion into Middle Triassic felsic volcanics. An increase in undulatory and polycrystalline quartz grains in the Late Carnian shows that the erosion did reach deeper into the crystalline roots of the arc, although non‐volcanic detritus never exceeded 10%.After a terminal pyroclastic event in Middle‐Late Carnian times, suggested by the sudden appearance of pumiceous vitric clasts at the top of the Val Sabbia Sandstone, the mid‐Triassic orogenic magmatism ended and was followed by the tensional stage which led to the birth of Neotethys.