Trace elements, variation in alkalis, and the ratio Sr⁸⁷/Sr⁸⁶in selected rocks from the Taupo Volcanic Zone

Abstract

The volcanic rocks of the Taupo Zone are mostly acid tuffs and flows, with less abundant andesites, minor dacites, and rare basalts. A preliminary examination has been made of distribution of Cr, V, Sr, Rb, Cs, Li, Ba, and Zr in selected rocks from the Taupo Zone. The data support the hypothesis that the Taupo Zone volcanics belong to a continuous chemical sequence, non-porphyritic aluminous basalt-andesite--dacite--calc-alkaline rhyolite, with accumulative olivine and pyroxene andesites. Low total alkalis, low Zr, high K/Rb ratios and high Ba contents distinguish the acid rocks of the Taupo Zone from those in other New Zealand Tertiary provinces. Low concentrations of K, Rb, Cs, and Zr, high K/Rb ratios and low Rb/Sr ratios, compared with values for the average continental crust and average granite, suggest a subsialic source for the Taupo volcanics. Sr"'/Sr 8• ratios of O· 7047 for basalt and o· 7066 for rhyolite, together with the overall chemistry, indicate that the acid rocks are not the fusion products of ancient sial or reworked material derived from this and suggest a common source for both the acid and basic rocks in the Taupo Zone.