Emplacement dynamics of phonolite magma into maar-diatreme structures — Correlation of field, thermal modeling and AMS analogue modeling data

Emplacement mode and original shape and dimensions of a well exposed phonolite body in the České středohoří Mountains (Czech Republic) were reconstructed using combined techniques of structural analysis of magmatic fabrics and columnar jointing together with analogue and thermal mathematical modeling of cooling for different shapes of experimental bodies. Phreatomagmatic rocks in the vicinity of some phonolite stocks in the area of interest suggest that the phonolite bodies were likely emplaced into maar-diatremes. Our modeling revealed that intrusion of magma into phreatomagmatic maar-diatreme craters can result in cryptodomes, extrusive domes, lava lakes or branched intrusions. The fabric and columnar jointing pattern of the selected phonolite body reveals best fit with an asymmetric extrusive dome emplaced into the maar crater. The scaling analysis and thermal modeling also suggest that the phonolite extrusion could have formed within 6–66 days and cooled to the background temperature after 10,000 years. Combined analogue and thermal modeling also revealed that the phonolite extrusions into maar-diatreme craters are marked by upper tier (collonade) of vertical columns and lower tier of curved and outward flaring columns. Both tiers in the phonolite extrusions are divided by a subhorizontal suture.

Research Highlights
► Complex structural analysis of magmatic fabrics and cooling joint pattern on a selected phonolite stock revealed that it represents a remnant of an extrusive dome emplaced into a maar-diatreme crater.
► Analogue modeling results suggest that intrusions of crystal-rich magma into maar-diatremes can result in cryptodomes, extrusive domes, lava lakes or branched intrusions depending on the yield strength of the magma, degree of initial magma overpressure and the degree of heterogeneity of magma in its source.
► Columnar joints that form during cooling of the extrusions define two tiers of columns divided by a subhorizontal suture. Thick vertical columns of the upper collonades in contrast to thinner curved columns of the lower collonades are explained by slower cooling of the top surface of the extrusions due to presence of an insulating layer of vesicular pumice.
► Microscopic cavities form due to rapid lateral diffluence of the crystal-rich phonolite lava at relatively high flow stresses during lava extrusion. The cavities form around large phenocrysts, where local flow can not be fully accommodated by slip along individual microphenocrysts. Lenticular cavities can be distorted into sigmoidal shapes during resumed ductile flow.