NEW TRACE ELEMENT CHEMICAL AND GEOCHRONOLOGICAL STUDIES OF THE NORTHLAND OPHIOLITE, NEW ZEALAND , REVEAL THE NEED FOR A REASSIGNMENT OF ITS ENVIRONMENT OF FORMATION AND TIMING OF GENERATION

The Northland ophiolite of New Zealand has traditionally been described as a Late Cretaceous, tholeiitic mid-ocean ridge (MOR) complex of basalt and lesser dolerite and gabbro, upon which were built younger, volumetrically minor alkalic seamounts. Results from this study suggest instead that the Northland ophiolite formed in the Mid Oligocene above a suprasubduction zone (SSZ), the formation of the tholeiitic and alkalic suites is essentially synchronous, and that a third suite of basalt, transitional in trace element chemistry to the tholeiitic basalt/dolerite/gabbro and alkalic basalt, is present. Seven whole rock 40Ar/39Ar analyses of the tholeiitic suite yield plateau dates between 25.1 +/-1.2 Ma and 29.6 +/-1.0 Ma; a single 40Ar/Ar39 analysis from the alkalic suite yields a plateau date of 25.0 +/- 0.8 Ma. Zircons liberated from a plagiogranite confer a 206Pb/238U age of 28.3 +/-0.2Ma. Trace element signatures confirm the existence of the three distinct suites, a SSZ modified tholeiitic suite, an alkalic suite that plots as within-plate alkalic basalts, and a transitional suite that plots as within-plate tholeiitic basalts. These findings warrant a new interpretation for the tectonic environment in which the Northland ophiolite formed, and the timing of its generation.