MAGMAS OF THE KARIOI VOLCANIC COMPLEX, NEW ZEALAND: A WINDOW INTO MAGMA GENERATION IN THE MANTLE

Karioi, a Pliocene volcanic complex of the North Island, New Zealand ("NINZ"), comprises (oldest to youngest): i) alkalic basalts and tuffs flanking the Karioi edifice; ii) porphyritic calc-alkalic basalts erupted from monogenetic vents at the base of the Karioi edifice, iii) fissure-erupted shield-building basalts making up the bulk of the edifice, iv) basic and intermediate lavas, tuffs, and lahars of the Karioi composite cone perched on the shield, and v) young alkalic basalts erupted from vents near the main edifice.

Except for lavas of unit ii, these rocks do not exhibit typical geochemical traits of convergent-margin calc-alkalic rocks. They are alkalic and transitional basalts of intra-plate character, similar to young intra-plate basaltic rocks of NINZ north of Karioi. There is no evidence of crustal contamination of the magmas now represented by these rocks, and both geochemical traits and stratigraphic relations of these rocks suggest that origins of those magmas were somehow linked, despite variations in petrologic and petrographic character of the rocks.

Isotopic ratios of Sr, Nd, and Pb were determined for 20 of these rocks. On a Sr-Nd diagram, points for Karioi rocks form an array very similar to that for rocks from the Azores, and superimpose on points for rocks from the North Fiji back-arc basin with relatively radiogenic Sr. On a 208/204 vs 206/204 Pb diagram, points for Karioi rocks lie well above the NHRL, between extensions of arrays for Pacific and Indian Ocean MORB but closer to the I-MORB array. The Karioi array resembles that for the Lau back-arc basin but with more-radiogenic Pb. Following Zhang et al. (1999), we have plotted 87/86Sr vs 206/204Pb. The Karioi array is narrow and nearly vertical, with marked variation in 87/86Sr but hardly any variation in 206/204Pb, closely similar to the array for 55-14 Ma rocks of New South Wales ("NSW"; ibid.). Note that the dispersion of the Karioi array on that diagram is nearly as great as that for the NSW rocks, representing numerous volcanic complexes in seven discrete fields. Much the same kinds of mantle source rocks for formation of magmas were present beneath NSW 55-14 Ma and beneath the Karioi complex about 2 Ma ago. Diverse models of mixing of mantle components P-MORB, I-MORB, EM2, and HIMU could explain isotopic traits of NSW and Karioi rocks. These models cannot be convincingly tested at present.