Mid-Pleistocene silicic tephra beds in the Auckland region, New Zealand: Their correlation and origins based on the trace element analyses of single glass shards

Abstract

The trace element composition of individual glass shards from twelve distinct mid-Pleistocene silicic tephra beds from the Auckland region, New Zealand, has been determined by laser ablation (LA) ICP-MS analysis. This is the first application of such data to tephra correlation studies in New Zealand. These analyses support earlier major-element based correlations between three surface sections (Oruarangi and Okariha at Port Waikato, and St Kentigern's at east Auckland) and a long core (Patiki Road, west Auckland). The correlations provide a higher resolution record of Taupo Volcanic Zone (TVZ) activity than is preserved in more proximal deposits, where burial and erosion obscure much of the volcanic record. A combination of major and trace element analysis enables the correlation of three units exposed in the Auckland region with their proximal equivalents (ignimbrites) from the TVZ, namely deposits associated with (1) the Ongatiti ignimbrite (1.21±0.04 Ma), (2) an un-named, post-Ahuroa/pre-Unit E ignimbrite (sample UT1274) from the Rocky Hill section, King Country and (3) the Unit E ignimbrite (∼1 Ma). Combined major and trace element data show a marked evolution in magma chemistry, with the large Ongatiti and Unit E ignimbrite eruptions occurring after a change in composition caused by replenishment/reactivation of the magma chamber. Changes in magma chemistry between these deposits resulted from a combination of assimilation of metasedimentary country rocks and periods of fractional crystallisation of a plagioclase-dominated assemblage. Trace element data and the wider dispersal of the silicic tephras in the Auckland region suggests that they are all sourced from the Mangakino Volcanic Centre (MVC) in the TVZ. This study illustrates for the first time in a New Zealand context the potential for correlating tephra deposits using single glass shard trace element chemistry.