Petrogenesis of andesitic lavas from Mangatepopo valley and Upper Tama lake, Tongariro volcanic centre, New Zealand

Abstract

Three geochemically and isotopically distinct andesitic magma types are recognised from lavas erupted from five discrete vents in the Mangatepopo Valley-Upper Tama Lake region of Tongariro Volcanic Centre, New Zealand. Two of these are plagioclase-phyric and fall broadly within the TYPE 1 category of Graham and Hackett (1987). They are distinguished by markedly different LILE contents and Sr isotopic ratios reflecting different degrees of crustal contamination of similar basaltic parents. A third type is pyroxene-olivine-phyric (TYPE 5) and was probably generated from a more primitive parental magma by fractional crystallisation without crustal contamination. This study highlights the subtle but important differences in andesite composition that can occur with geographically small volcanic centres and illustrates the complex nature of andesite genesis in continental arc settings.