Peperitic textures of Ordovician dacitic synsedimentary intrusions in Argentina’s Puna Highland: clues to emplacement conditions

Abstract

Synsedimentary crystal-rich dacitic sills, laccoliths and cryptodomes emplaced in early Ordovician submarine thin-bedded silt/sandstone sequences show a variety of magma–water interaction features. Megapeperites occurring along the igneous contact contain angular dacitic megaclasts that are dispersed in dominantly structureless host sediment. They result mainly from quench fragmentation of magma and dynamic stressing of cooling magma bodies on contact with wet sediments, combined in some instances with fluidization of the sediments or minor explosive expansion of water incorporated by magma breaching the sediment–water interface or stopping of water-saturated roof sediments by shallowing intrusions. Globular peperites, having bulbous to globular dacitic clasts immersed in the host sediment with evidence of fluidization along the contact zone, are also recognized at the top and base of sills. In this case, a vapor film might have formed at the magma–sediment interface; insulating the magma from direct contact with the sediment and suppressing quench fragmentation and steam explosions. The diversity of peperite textures observed in the same intrusive and sedimentary rocks appears to be mainly the result of different levels of emplacement of the synsedimentary dacitic intrusions. Scarcity of peperites related to mafic sills is attributed to their having been emplaced late during the evolution of the basin.