Problem in the Diagnostics and Petrogenesis of Ignimbrites

摘要

Correct diagnostics of rocks requires the 3D study of the structure of their basis, since a splinter-like basis structure may occur only in a single observed section of a specimen of a rock that has a more complexstructure of the plane-parallel or linear type. Ignimbrites differ from lavas with a similar silica content primarilyin the presence of fiamme, a smooth outline of the phenocrysts, and their constant cataclasis; they differ frompyroclastic lavas by having broken-down phenocrysts enclosed in an undisturbed groundmass with smooth andundulating outlines of the volcanic particles and by the presence of fiamme showing a regular pattern of evolv-ing composition. The study of the morphological and material features of all components of ignimbrites in theirrelationships with the silica content of the melt and the attitude of the ignimbrite geologic bodies they composecorroborates the hypothesis that ignimbrites are generated by the complex evolution of a gas-saturated melt ofthe emulsified type rather than by baking (welding) of ash-fall particles.