DETERMINATION OF COMPACTION DURING WELDING OF ANDESITIC IGNIMBRITE, GORELY VOLCANO, KAMCHATKA PENINSULA, FAR EAST RUSSIA

Fiamme shape and plagioclase phenocryst alignment were quantitatively analyzed to characterize the welding process and assess the post welding flow in a welded andesitic tuff from Gorely volcano, Kamchatka Peninsula, Far East Russia. Marked reduction or elimination of porosity and the appearance of black lenses referred to as fiamme characterizes densely welded ignimbrites. Fiamme within a rock sample from Gorely volcano were used as a passive marker to estimate amount of relative compaction, and type of strain. Plagioclase crystals in the andesite were measured with respect to length and orientation of their long axes. Analysis of this data was used to measure textural anisotropy and relative compaction direction. The estimated minimum compaction of the fiamme is between 33% and 42%, as determined through mean ellipse measurements. Axis measurements of fiamme on three oblique faces cut perpendicular to the horizontal foliation are consistent with each other and show an average major axis of 0.79 cm and an average minor axis of 0.31cm. The major and minor axes on a face cut parallel to the horizontal foliation (face 4) were equal at 0.08 cm. Measurements indicate a “Pancake” shape of fiamme, suggesting coaxial (pure flattening) strain. Qualitative petrographic observation, however, revealed non-coaxial (simple) shear indicators such as boudinage and asymmetric grain tail complexes on the margins of fiamme and lithic clasts in thin section that suggest a component of post-welding flow in the ignimbrite. Link vector distribution of plagioclase crystals revealed an orientation of 19 degrees above horizontal and an estimated direction of relative compaction of 70.8 +/- 8.9 degrees to the predominant horizontal foliation defined by the elongate fiamme. The estimated amount of relative compaction was 49.3%. This data is consistent with the results of orientation and minimum compaction for the fiamme. Both the mean ellipse data and the link vector data are constant with respect to the amount of compaction during welding. However, the shape of fiamme indicates pure flattening, while the petrographic analysis suggests a component of flow contributed to the post-emplacement deformation.