A NEW PARADIGM FOR ANDESITIC TEPHRA CORRELATION

Tephra beds can be widely dispersed from their source volcano, with emplacement occurring in a very short time (i.e. hours to weeks). Tephra beds are therefore essentially isochronous geologic marker horizons and are valuable for correlation and geochronology.

The vast majority of studies for which tephras have been used as chronostratigraphic marker beds have relied on the correlation of rhyolitic tephras. These tephras are typically well-preserved and readily distinguished by their ferromagnesian mineral assemblages and major-element glass chemistries. EMP and LA-ICPMS analysis of glass shards are now proven and routine techniques for identifying volcanic source and discriminating eruptives from the same centre.

Andesitic tephras are now also proving to be important marker beds within Quaternary sedimentary sequences, and there is thus a growing need to correlate distal andesitic tephras to known eruptives at source. Analysis of andesitic tephras from two arc volcanoes, Mounts Ruapehu (NZ) and Rainier (USA), however, shows that discriminating andesitic eruptives is problematic due to their petrographic complexity, restricted mineral assemblages, and similarity in major-element glass and phenocryst geochemistry.

Andesitic eruptions, typified by smaller melt volumes and more complex processes, produce tephras that show only small-scale variations. Understanding this variation in near source eruptives is necessary to elucidating distal tephra records. We are, therefore, cognizant of the need to approach the correlation of these tephras from a different perspective to that used for rhyolitic tephras.

Our approach is to geochemically 'fingerprint' the magmatic system (to identify characteristic geochemical behaviour) and use this to determine inherent geochemical variability within the eruptive sequence. In essence, this allows the determination of the 'correlation potential' of various formations and member units at source, and thus identification of geochemical parameters useful to correlating distal tephras.

In hindsight this would appear a logical approach, but it is one that has emerged only as a result of the establishment of a geochemical data base for andesitic tephras (that demonstrates the difficulties in discriminating tephras and that permits modeling) and stratigraphers working more closely with geochemists.