Petrology and geochemistry of quaternary basalts from northland, New Zealand

Abstract

The Northland volcanic fields lie between 330 and 400 km northwest of the Taupo volcanic zone. In contrast to the voluminous rhyolitic and andesitic volcanism found there, volcanism in Northland over the last 2.3 m.y. has produced two volcanic fields containing alkalic, transitional, and tholeiitic basalt. Petrographic variation is small and augite-, olivine-, and ophitic basalts are recognised. Variation in major element chemistry is limited but all of the lavas are distinctly aluminous (15.5–18.6% Al 2O 3) and many of the tholeiitic basalts have TiO 2 around 1%. Older lavas tend to be ophitic and are predominantly alkalic, while younger ones are porphyritic and tholeiitic. This division is reflected in the incompatible elements, which are enriched in alkalic basalts. Tholeiitic basalts have K/Rb ratios of around 300; values as high as 2100 are found in some alkalic types. While some tholeiitic lavas are apparently genetically related, the alkalic lavas have been derived from individual magma batches, each the result of a unique partial melting/fractionation process. The petrological and geochemical patterns can be related to progressive melting of ascending mantle diapirs. The occurrence of minor alkalic and peralkalic rhyolites in the northern field is thought to be the result of partial melting of the base of the crust and rhyolite intrusions may be the heat source for a geothermal system found in the northernmost field.