Petrogenetic implications of the 1954 Ngauruhoe lava and its xenoliths

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Abstract
The Ngauruhoe lava erupted in 1954 is an olivine-bearing basaltic andesite, and belongs to the Taupo volcanic association of basalts, basaltic andesites, pyroxene andesites, dacites, rhyolites, and ignimbrites. This association, as indicated by a variation diagram, is divided into three distinct series: (a) a main basic series, displaying continuous variation from basalts to dacites; (b) a continuous acid series of rhyolites and ignimbrites; and (c) an accumulative series of basalts and basaltic andesites. A characteristic feature of the 1954 Ngauruhoe lava is the abundance of small quartzose and feldspathic xenoliths, and intensely vitrified gneissic xenoliths of boulder size. Structural, petrographic, and chemical data indicate that all the xenoliths are derived from acid gneiss. Limited miscibility between acid syntectic melt and basic host magma is shown by gradational contact of the vitrified xenoliths. Petrochemical study of the effusive rocks of the Taupo volcanic association indicates that, with the exception of the accumulative series, the sum of the alkaluminous oxides remains constant during magmatic differentiation. This constancy is one of the characteristics distinguishing the Taupo volcanic association from the Hawaiian basalt-trachyte association. The parent magma of the main basic series has been derived from olivine basalt magma by selective assimilation of acid gneiss. Limited' fractional crystallization of this contaminated magma, controlled by the constancy of the sum of the alkaluminous oxides, produced the lavas of the basalt-dacite range, the dacites being the last differentiates. The rhyolites and ignimbrites are excluded from being derived by fractional crystallization of the contaminated basaltic magma. The accumulative basic lavas have formed from the uncontaminated olivine-basalt magma by gravitational crystal accumulation and limited fractional crystallization. The similarity in chemical composition between the vitrified xenoliths and the rhyolites and ignimbrites suggests that the acid effusive rocks of the Taupo volcanic association originated from an acid magma produced by transfusion of acid gneiss in prolonged contact with olivine basalt magma at great depth. Within the acid series fractional crystallization is also restricted by the constancy of the sum of the alkaluminous oxides.