Geochemistry of late Cenozoic basaltic volcanism in Northland and Coromandel, New Zealand: implications for mantle enrichment processes

Abstract

New major and trace element, and Sr, Nd and Pb isotope results are presented for selected basaltic rocks from young volcanic fields (<10 Ma) in the Northland province of the Northland–Auckland Peninsula, and Mercury Island on the eastern side of the Coromandel Peninsula, North Island, New Zealand. The rocks are mainly alkali basalts and tholeiites which are different from those in the adjacent Auckland volcanic field. Samples from Kaikohe–Bay of Islands (KBI) tend to have low silica contents and high Nb/La ratios (1.05–1.62), low Sr (0.7027–0.7030) and high Nd (0.51299–0.51304) isotope ratios and relatively high 206 Pb / 204 Pb , but low 207 Pb / 204 Pb and 208 Pb / 204 Pb , and so low Δ 207 Pb / 204 Pb (4.1±2.2) and Δ 208 Pb / 204 Pb (10.7±6.7). Ti Point and Stony Batter samples (TPSB) and Mercury Island basalts (MIB) have similar compositions, relatively high silica contents and low Nb/La ratios (0.23–0.58), high Sr (0.7039–0.7048) and low Nd (0.51278–0.51293) isotope ratios, with higher Δ 207 Pb / 204 Pb and Δ 208 Pb / 204 Pb (8.1±1.6 and 30.9±5.0, respectively). The rocks from Whangarei–Puhipuhi field (WHP) have intermediate compositions, in that their Sr and Nd isotope ratios are similar to KBI, but their Pb isotope ratios are more similar to the TPSB and MIB. Furthermore, the WHP rocks have similar large ion lithophile element (LILE) abundances to KBI but with negative Nb anomalies which are a feature of TPSB and MIB. Although AFC processes could explain co-variations of isotope ratios in the samples from the different fields they are difficult to reconcile with the observed relationships between isotope ratios, major elements and Nb/La. Thus, the major differences in the geochemical features of the Northland volcanic rocks are more likely to represent source variations in the underlying mantle. The KBI samples have isotope and trace element compositions similar to the Auckland basalts, and to many ocean island basalts (OIB). In contrast, negative Nb anomalies and high Δ 207 Pb / 204 Pb and Δ 208 Pb / 204 Pb , high Sr and low Nd isotope ratios are features of subduction-related volcanism. Therefore, two very different mantle enrichment processes have been identified in the mantle beneath the Northland–Auckland peninsula, reflecting changes in the tectonic setting from plate convergence to intraplate over the last 30 Ma. The KBI, WHP, TPSB and MIB have similar MgO and FeO t (total iron as FeO) contents at mg# ∼0.70 indicating that they were generated at similar depths and temperatures. However, the Auckland basalts have significantly higher FeO t and MgO contents, and they are therefore considered to have been derived from greater depths and higher temperatures. This constrains the spatial relationships between the two mantle domains with intraplate sources dominant at depth and the subduction related enrichment processes at shallower levels in the mantle underlying the Northland Peninsula.