The Petrogenesis of Mid-Cretaceous Continental Intraplate Volcanism in Marlborough, New Zealand, during the Break-up of Gondwana
The Lookout Volcanics are the remnants of an extensive sheet of mid-Cretaceous (ca. 96 Ma) continental intraplate volcanic rocks erupted just prior to the rifting of New Zealand from Gondwana. Preserved in a fault angle depression bounded by the Awatere Fault located in Marlborough, South Island, New Zealand, the volcanic rocks cover an area of ca. 50 km2 with exposed thicknesses up to 1000 m. On the basis of stratigraphic evidence the dominantly terrestrial lavas flows are inferred to have erupted from dykes of a coeval radial dyke swarm. A detailed sampling of the lava flows of the Lookout Volcanics has been undertaken with a ca. 700 m composite stratigraphic section being constructed, largely based on a continuous sequence of lava flows outcropping in Middlehurst Stream. New Rb-Sr age constraints for the Lookout Volcanics (97.6 plus or minus 3.4 Ma) and Blue Mountain Igneous Complex (97.1 plus or minus 0.7 Ma) are consistent with previous radiometric dates of plutonic complexes in the Central Marlborough Igneous Complex, and suggest a rapid accumulation of volcanic material from ca. 98-96 Ma during the initial extension of proto-New Zealand. The predominantly mafic and alkaline samples include basalt, picrobasalt, basanite, trachybasalt and basaltic trachyandesite rock types. No samples represent primary magmas with all samples having undergone fractionation (or accumulation) of olivine plus clinopyroxene plus or minus plagioclase plus or minus Fe-Ti oxides. Initial Sr-Nd-Hf-Pb isotopic variations (87Sr/86Sr = 0.7030-0.7039; 143Nd/144Nd = 0.51272-0.51264; 176Hf/177Hf = 0.28283-0.28278; 206Pb/204Pb = 20.32-18.82) reflect mixing between melts of a HIMUlike mantle component with up to 25-30% of an Early Cretaceous upper crustal component. Oxygen isotope ratios determined by laser fluorination analysis from 6 lava flows yielded delta 18O = 4.7-5.0 per thousand for olivine, 4.8-5.4 per thousand in clinopyroxene cores, 3.9-5.5 per thousand in clinopyroxene rims. Average olivine (4.8 per thousand) and clinopyroxene core (5.1 per thousand) values are 0.4-0.5 per thousand lower than those of average mantle peridotite but comparable to those of HIMU OIB, and are consistent with New Zealand intraplate magmas being generated by a low delta 18O mantle. However, oxygen isotopic disequilibrium between clinopyroxene cores and rims (Delta 18O = -1.4 to +0.3) records the overprinting of this signature by crustal processes. Negative disequilibrium between clinopyroxene rims and cores in primitive samples suggests these phenocrysts grew in a shallow crustal magma chamber with an active meteoric water system. The effects of crustal assimilation can also be observed with clinopyroxene phenocrysts from the most evolved sample exhibiting coupled elevated delta 18O and 87Sr/86Sr. Variations in incompatible trace element ratios are consistent with the Lookout Volcanics being the small degree (2-5%) partial melts of an amphibole-bearing garnet pyroxenite. Furthermore, the elevated NiO contents of olivine phenocrysts are consistent with melting of a pyroxenitic mantle source. The presence of residual amphibole constrains melting to the hydrous subcontinental lithospheric mantle. The Lookout Volcanics and coeval plutonic complexes are the oldest occurrences of HIMU magmatism in Zealandia. This source was generated by small degree silicate melts from recycled oceanic lithosphere that metasomatised the base of the subcontinental lithospheric mantle beneath East Gondwana over 200 Ma ago.