Petrology of late cretaceous alkaline volcanic rocks from the chatham Islands, New Zealand

Abstract

Late Cretaceous intraplate alkaline volcanic rocks on the Chatham Islands comprise a basalt-trachyte suite on the south of the two main islands (Chatham and Pitt) and isolated basaltic flows on northern Chatham Island. On southern Chatham and Pitt Islands, thinning of the flow sequence, flow dips, and lithologic similarities indicate eruption of basaltic flows from a common vent system between the two islands in present-day Pitt Strait. Trachytic rocks are dominantly pyroclastics with rare flows, erupted from centres unrelated to earlier volcanism. Variations in modal mineralogy and mineral and whole-rock chemistry of these rocks is explained by low-pressure fractionation, supported by least-squares and Rayleigh fractionation modelling, showing the dominance of clino-pyroxene then plagioclase. For relatively unfractionated rocks on northern Chatham Island and southern Chatham/Pitt Islands, incompatible element ratios (e.g., La/Ce, Zr/Nb) show some variability indicating source heterogeneity, although limited 87Sr/86Sr values (0.7029\2–0.703) indicate source isotopic homogeneity. Three rocks with 100Mg/(Mg+Fe2+) > 60 and Ni > 150 ppm have been used in conjunction with a theoretical garnet peridotite source to model non-modal batch partial melting. Estimated degrees of partial melting range from 6 to 15%; the source is enriched in La (5–1l× chondrite) and Yb (1–3× chondrite). Relative to average mantle, Rb, Sr, and Zr are enriched 2-3×, Ba 3–6×, and Nb 6–9×. © Crown copyright 1985.