Mineralogy and chemistry of a pillow lava, northland, new zealand, and its tectonic significance

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 (mg 3647) 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, Galapagos Spreading Centre, DSDP Leg 34, and Leg 65. The occurrence of a lens of more-HYG depleted ocean-floor lava within the Waipapa Group suggests a transitional tectonic setting between a midocean ridge spreading centre with very low sedimentation rates > 10 m/Ma) and an ensialic environment with much higher rates of sedimentation. A possible present-day continental margin analogue occurs in the Gulf of California, where magma extrusion and intrusion is hosted by flysch sediments and initiated from short spreading centres joined by long transform faults. © Crown copyright 1985.