Petrogenesis and Origins of Mid-Cretaceous Continental Intraplate Volcanism in Marlborough, New Zealand: Implications for the Long-lived HIMU Magmatic Mega-province of the SW Pacific

Continental intraplate basalts with a HIMU-like mantle signature in the SW Pacific (e.g. New Zealand, Australia and Antarctica) have been erupted over an area of similar to 25 million km(2) sporadically over the last c. 100 Myr. This long-lived HIMU signature has recently been ascribed to both melting in the subcontinental lithospheric mantle and the asthenospheric mantle. The Lookout Volcanics represent the oldest samples of this HIMU magmatic mega-province and are remnants of extensive mid-Cretaceous (98 Ma) intraplate volcanism preserved in South Island, New Zealand, which occurred prior to the rifting of Zealandia from Gondwana. We have carried out a detailed petrographic, chemical and isotopic study of the Lookout Volcanics based on sampling of a 700 m composite stratigraphic section that comprises predominantly mafic alkaline rocks. Initial Sr-Nd-Hf-Pb isotopic variations (Sr-87/Sr-86 = 0 center dot 7030-0 center dot 7038; Nd-143/Nd-144 = 0 center dot 51272-0 center dot 51264; Hf-176/Hf-177 = 0 center dot 28283-0 center dot 28278; Pb-206/Pb-204 = 20 center dot 3-18 center dot 8) are explained by mixing between melts of a HIMU-like mantle source and up to 22% of Early Cretaceous upper crust. Oxygen isotope data for six lava flows yielded delta O-18(olivine) = 4 center dot 7-5 center dot 0 parts per thousand, delta O-18(cpx cores) = 4 center dot 8-5 center dot 4 parts per thousand and delta O-18(cpx rims) = 3 center dot 9-5 center dot 5 parts per thousand. Average olivine and clinopyroxene core values are in oxygen isotopic equilibrium and comparable with data for phenocrysts from HIMU ocean island basalts. Oxygen isotopic disequilibrium between clinopyroxene cores and rims records phenocryst growth in a shallow magma chamber interacting with an active meteoric water system. The scavenging of such phenocrysts suggests that volcanic phenocrysts with low delta O-18 cannot always be used as a mantle fingerprint of recycled crustal components with low delta O-18. Clinopyroxene and plagioclase phenocrysts from the most evolved sample have elevated delta O-18 and Sr-87/Sr-86 consistent with crustal contamination. Variations in incompatible trace element ratios (e.g. La/Yb-N = 12 center dot 5-20 center dot 2; Dy/Yb-N = 1 center dot 90-2 center dot 20) are consistent with small degrees (c. 2-4%) of partial melting of an amphibole-bearing garnet pyroxenite or peridotite mantle source. Moreover, the elevated NiO contents of olivine phenocrysts from the Lookout Volcanics may also be consistent with melt generation from a pyroxenitic mantle source. However, given the documented effect of increasing pressure causing a decrease in the melt-olivine partition coefficient for Ni, the elevated Ni in the olivine phenocrysts may simply reflect high Ni contents in the primary magmas of the Lookout Volcanics as a result of deep, garnet-facies, mantle melting. The pervasive HIMU mantle source throughout Zealandia was most probably produced along the East Gondwana subduction margin by cooling of subduction-modified upper mantle and its incorporation into the subcontinental lithospheric mantle that accompanied the crustal formation of Zealandia.