Geochemistry of Central North Island Greywackes and Genesis of Silicic Magmas

Abstract

This thesis investigates the role of the sedimentary basement in the genesis of silicic volcanic rocks of the Taupo Volcanic Zone (T.V.Z.). The petrography of the Mesozoic sediments surrounding the zone show that the basement has two provenances; Western Basement and Eastern Basement. The Western Basement, derived from a largely andesitic terrain has a higher plagioclase/ quartz ratio and higher ferromagnesian mineral content than the Eastern Basement, which is derived from a mainly granitic and high grade metamorphic terrain. The plagioclase compositions of the Western Basement rocks show a bimodal distribution with the majority having a composition of An 40-50 while a smaller group have An contents 0-10. Eastern Basement plaqioclase compositions all lie in the range An 0-10. Both basement groups have developed a sparse metamorphic assemblage consisting of calcite and quartz veins as well as isolated grains of prehnite and epidote. Samples from the Kawerau geothermal field are hydrothermally altered to produce local development of wairakite and sphene as well as increased amounts of calcite and quartz. Sulphides (mainly pyrite, with some sphalerite) have grown in some samples and the crystallinity of the phyllosilicates has also increased. The major and trace element chemistry of the basement further supports the separate provenances of the two basement types. The Western Basement has the general composition of a high-SiO2 andesite although with a small granitic input. The average SiO2 content is ≈ 63%. The Eastern Basement is granodioritic in composition with an average SiO2 content of ≈ 72%. Both basement groups have Na2O/K2O > 1. Only Na2O, CaO, Rb and Ba have possibly been mobilized during sedimentation and metamorphism. The effects of hydro-thermal alteration on basement chemistry is negligible. Data available for the rhyolitic rocks of T.V.Z. show that these rocks have a phenocryst assemblage dominated by plagioclase with plagioclase/quartz > 1. K-feldspar is rare in the rhyolites of T.V.Z. The ferromagnesian mineral assemblages can be divided into several groups which correlate with eruption temperatures: cummingtonite + hornblende + hypersthene hornblende + hypersthene biotite + hornblende + cummingtonite + hypersthene hypersthene + augite olivine + hypersthene + augite The rhyolitic rocks typically have Na2O/K2O > 1 and do not show marked enrichment in incompatible trace elements (Rb, Ba, Cs) or incompatible element ratios. Eruption temperatures of the rhyolites are low ranging from 725-900°C with most having eruption temperatures < 800°C. There is little difference in the chemistry of the rhyolitic lavas and the associated pyroclastic deposits except that Ba and Rb are enriched in the pyroclastics. Nearly all the rhyolites equilibrated under conditions of PH2O ≈ PTOTAL prior to eruption. Hydrothermal experiments on natural greywackes and argillites from the basement at 3 Kb total pressure under water-saturated conditions show that rhyolitic melts are formed between 650 and 850°C in equilibrium with an amphibolitic restite assemblage. It is however noted that at lower water pressure a granulitic assemblage would be expected with a reduction in the amount of melt formed. Trace element modelling using published data for the rhyolites as well as new R.E.E. determinations favours an origin by partial melting. The trace element characteristics of the rhyolites could also be produced by fractionation of andesite for some elements but this produces unfavourable Cr/V and K/Rb ratios. Basalt fractionation would not reproduce the trace element characteristics of the rhyolites. Similar trace element modelling techniques are used to discuss the origin of the dacitic rocks occurring in T.V.Z. and these models show that the dacites fall into two groups. One is derived by fractional crystallization of andesite while the other is produced by higher degrees of partial melting of basement rocks. For both types there is evidence of contamination. The fractionation derivatives have been contaminated by crustal rocks while the dacites formed by crustal fusion have undergone contamination by basic magma. This basic magma may have initiated melting of the crust. Oxygen isotope data confirms the provenances of the two basement groups distinguished, with the Western Basement having whole rock δ18O values from 7-10 ‰ and the Eastern Basement have δ18O from 10-14 ‰. δ18O values of Tertiary-Recent sediments from Hawkes Bay are high (≈ 20 ‰) indicating that they were not involved in magma production to any large extent. The igneous rocks fall into two distinct groups. Basalts and andesites fall on a normal igneous trend with δ18O ranging from 5.5 ‰ for basalts to ≈ 7.5 ‰ for andesites. Values do however indicate that some crustal contamination has occurred. The rhyolites fall on a totally different negative trend when plotted against SiO2. This distinction further supports a separate origin for the rhyolites and the more basic igneous rocks of T.V.Z.. A conceptual model of rhyolite genesis based on this data together with available geophysical data suggests that blocks of Western Basement may be subsiding into a plastic mantle. The crust-mantle interface is estimated to be of a depth of 20 km under T.V.Z. because of a diapiric uprise of mantle material into the zone. This probably results from pressure release due to the opening of the Taupo "back-arc basin" within the last 1 m.y..