Factors controlling the internal facies architecture of maar-diatreme volcanoes

Abstract

Most if not all kimberlite pipes show a multitude of facies types, which imply that the pipes were emplaced under an episodic re-occurrence of eruptive phases, often with inter- mittent phases of volcanic quiescence. The majority of these facies can be related to either the fragmentation behaviour of the magma during emplacement or changing conditions dur- ing sedimentation of volcaniclastic deposits, as well as their alteration and compaction after deposition. An additional factor controlling pipe-facies architecture is the degree of mobility of the loci of explosions in the explosion chambers of the root zone or root zones at the base of the maar-diatreme volcano. In a growing pipe, the root zone moves downward and, with that movement, the overlying diatreme enlarges both in size and diameter. However, during the life span of the volcano, the explosion chamber can also move upward, backintothelower diatreme,where renewedexplosionsresult inthedestructionofolderdepositsandtheirstructures.Nextto vertical shifts of explosion chambers, the loci of explosions can also move laterally along the feeder dyke or dyke swarm. This mobility of explosion chambers results in a highly com- plex facies architecture in which a pipe can be composed of several separate root zones that are overlain by an amalgam- ated, crosscutting diatreme and maar crater with several lobes. Pipe complexity is amplified by periodic changes of the fragmentation behaviour and explosivity of kimberlite mag- ma. Recent mapping and logging results of Canadian and African kimberlite pipes suggest that kimberlite magma frag- mentation ranges from highly explosive with abundant entrained country rock fragments to weakly explosive spatter-like production with scarce xenoliths. On occasions, spatter may even reconstitute and form a texturally coherent deposit on the crater floor. In addition, ascending kimberlite magma can pass the loci of earlier fragmentation events in the root zone and intrudes as coherent hypabyssal kimberlite dykes in high pipe levels or forms extrusive lava lakes or flows on the crater floor or the syneruptive land surface, respectively. This highly variable emplacement behaviour is typical for basaltic maar-diatreme volcanoes and since similar deposits can also be found in kimberlites, it can be concluded thatalsothevolcanologicalprocessesleadingtothesedeposits are similar to the ones observed in basaltic pipes.