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

New major and trace element, and Sr, Nd and Pb isotope results are presente d for selected basaltic rocks from young volcanic fields (<10 Ma) in the No rthland province of the Northland-Auckland Peninsula, and Mercury Island on the eastern side of the Coromandel Peninsula, North Island, New Zealand. T he 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 Pb-206/Pb-204, but low Pb-207/Pb-204 and Pb-208/Pb-204, and so low Delta(207)Pb/Pb-204 (4.1 +/- 2.2) and Delta(208)Pb/Pb-204 (10.7 +/- 6.7). Ti Point and Stony Patter samples (TPSB) and Mercury Island basa lts (MIB) have similar compositions, relatively high silica contents and lo w Nb/La ratios (0.23-0.58), high Sr (0.7039-0.7048) and low Nd (0.51278-0.5 1293) isotope ratios, with higher Delta(207)Pb/Pb-204 and Delta(208)Pb/Pb-2 04 (8.1 +/- 1.6 and 30.9 +/- 5.0, respectively). The rocks from Whangarei-P uhipuhi field (WHP) have intermediate compositions, in that their Sr and Nd isotope ratios are similar to KBI, but their Pb isotope ratios are more si milar to the:TPSB and MIB. Furthermore, the WHP rocks have similar large io n lithophile element (LILE) abundances to KBI but with negative Nb anomalie s which are a feature of TPSB and MIP. Although AFC processes could explain co-variations of isotope ratios in the samples from the different fields t hey are difficult to reconcile with the observed relationships between isot ope ratios, major elements and Nb/La. Thus, the major differences in the ge ochemical features of the Northland volcanic rocks are more likely to repre sent source variations in the underlying mantle. The KBI samples have isoto pe and trace element compositions similar to the Auckland basalts, and to m any ocean island basalts (OIB). In contrast, negative Nb anomalies and high Delta(207)Pb/Pb-204 and Delta(208)Pb/Pb-204, high Sr and low Nd isotope ra tios are features of subduction-related volcanism. Therefore, two very diff erent mantle enrichment processes have been identified in the mantle beneat h the Northland-Auckland peninsula, reflecting changes in the tectonic sett ing from plate convergence to intraplate over the last 30 Ma. The KBI, WHP, TPSB and MIB have similar MgO and FeOt (total iron as FeO) contents at mg# similar to 0.70 indicating that they were generated at similar depths and temperatures. However, the Auckland basalts have significantly higher FeOt and MgO contents, and they are therefore considered to have been derived fr om greater depths and higher temperatures. This constrains the spatial rela tionships between the two mantle domains with intraplate sources dominant a t depth and the subduction related enrichment processes at shallower levels in the mantle underlying the Northland Peninsula. (C) 2000 Elsevier Scienc e B.V. All rights reserved.