Petrogenesis of the volcanic rocks of the Central North Island, New Zealand, as indicated by a study of [formula omitted] ratios, and Sr, Rb, K, U and Th abundances

Abstract

The data cover the Pleistocene to Recent basaltic, andesitic, and especially rhyolitic (including lavas, ignimbrites, and pumice deposits) volcanic rocks of the Taupo Volcanic Zone, and adjacent areas. Analytical data are presented for approximately one hundred volcanic samples, including forty-four new Sr isotope determinations. In addition, limited data are presented on the Triassic-Jurassic eugeosynclinal and miogeosynclinal greywacke-argillite sequences which flank the western and eastern margins of the volcanic zone. The following points are shown by the data: 1. (a) The rhyolitic extrusives have relatively low concentrations of Rb (average 108 ppm), Sr (125 ppm), K (2.69%), U (2.53 ppm), and Th (11.3 ppm), when compared with recent estimates of average “granite” concentrations, suggesting that the rhyolitic magmas have no history involving strong fractional crystallization processes; 2. (b) a complete continuity exists, for the elements studied, through the basaltandesite-dacite-rhyolite compositional range; 3. (c) the initial (i.e. present day) Sr isotopic ratios of the andesitic, dacitic, and rhyolitic extrusives are identical, and exhibit a small but similar range of variation (0.7046–0.7067). This variation is possibly real. The initial Sr isotopic ratios of the basalts are identical within experimental error (0.7041–0.7044), being consistently lower than the Sr isotopic ratios of the intermediate and acidic volcanics; 4. (d) only minor differences exist between the rhyolitic eruptive types in their element abundances. For example, the ignimbrites have slightly higher average Rb, K and Sr abundances than the rhyolite lavas, while the pumices have lower average Rb, K, U, and Th, and higher Sr than the rhyolitic ignimbrites and lavas; 5. (e) the element ratios R Kb , U K , Th K , and Th U exhibit a greater range of variation in the basaltic and andesitic volcanic rocks compared to the rhyolitic rocks; 6. (f) the Triassic-Jurassic eugeosynclinal greywacke-argillite sediments (referred to as the “marginal facies” of the New Zealand geosyncline) have present day Sr isotopic ratios which are closely similar to those of the rhyolitic and andesitic volcanics. The Sr isotopic ratios of these sediments vary according to lithology. The overall composition of the sediments is andesitic to dacitic. It is suggested that the rhyolitic magmas were derived by widespread partial fusion of these eugeosynclinal greywacke-argillite sediments, although this suggestion is based on a number of assumptions concerning the lateral and vertical extent of the sediments. It is shown that such an origin is consistent with the main chemical and mineralogical characteristics shown by the rhyolitic volcanic rocks. The data presented for the andesites are interpreted to be inconsistent with their derivation by fractional crystallisation from the outcropping basalts, or by direct mantle derivation involving a single stage process. The data are believed to be most consistent with crustal hybridisation of basalt, or of a primary andesitic magma (as the data cannot be interpreted to be inconsistent with the existence of a third primary magma type). It is concluded, in the light of the data presented, that the Mesozoic eugeosynclinal greywacke-argillite sediments would constitute the most logical choice of crustal material involved in this process.