Mineralogy and chemistry of a pillow lava, Northland, New Zealand, and its tectonic significance

Abstract

Abstract The mineralogy and chemistry of a pillow lava situated within a predominantly terrigenous sedimentary sequence (Waipapa Group; ?Permian) on Motukawanui Island, Cavalli Islands, indicate that the lava varies from Q saturated to oversaturated tholeiite with high total iron (mg3647) and TiO2 Cr and Ni are moderate to low in concentration. Although the extrusive rocks have undergone variable low-grade alteration (prehnite-pumpellyite metamorphic facies), minor and trace element ratios of the freshest sample indicate depletion in the more-hygromagmatophile (more-HYG) elements (K, Ba, La, Ce) relative to the hygromagmatophile (HYG) elements (Sr, Nd, P, Zr, the heavy rare earth elements (HREE), Ti, and V). Elemental ratios obtained are La/Yb ≃ I, (Ba/La)N < 1, Nb/Zr < 0.08, Ba/Zr ≃ 0.27, Y/Zr ≃ 0.3, Ti/V ≃ 25-28, and Ti/Zr ≃ 87. Chemical comparisons may be made with the “fractionated” Group 1 mid-ocean ridge basalts (MORBs) or Fe-Ti basalts, which have been described from the Juan de Fuca Ridge, Galapag...