The Tangihua Complex, New Zealand : Implications for Cretaceous-Oligocene convergent margin processes in the SW Pacific from comparison with the Poya terrane, New Caledonia

Abstract

New major oxide, trace element and isotopic analyses of basaltic lavas from the Tangihua Complex, Northland, New Zealand, have led to the development of a new model explaining the generation of the Tangihua Complex ophiolite and a new interpretation of the tectonic setting of New Zealand during the Cretaceous. New 40 Ar/39 Ar dates fiom the Tangihua complex confirm their age of formation is most likley ± l00Ma and they have suffered a major episode of alteration at ± 30Ma, which most likely corresponds to the age of emplacement of the Northland Allochthon. The Tangihua Complex basalts are relatively homogeneous, differentiated (Mg#<45) and dominantly tholeiitic with lesser calc-alkaline and minor alkaline affinities. The primary phenocryst assemblage is dominated by plagioclase (An66-87) with lesser amounts of clinopyroxene (En31-52 Fs10-33 Wo28-45; Mg# = 61), orthopyroxene and magnetite whereas olivine, biotite and hornblende are rare. Unusually, the lavas reveal a continuum between arc and back-arc chemistries, with a depletion of Nb and HFSE suggesting derivation from a depleted mantle source. Geochemical modelling shows the back-arc lavas have undergone = 35% fractional crystallisation of a source depleted by = 2-3% and the arc lavas have undergone = 30% fractional crystallisation of a source depleted by less than 5%. High LILE/HFSE and LILE/LREE ratios suggest LILE enrichment by a slab-related aqueous fluid and possibly small amounts of a slab-derived silicic melt. The pervasive, low-temperature alteration assemblage can be divided into three main phases based primarily on temperature but also influenced by the water/rock ratios: an initial phase of Na-rich zeolite precipitation, followed by a transitional cooling phase, characterised by K-, Na- and Ca-rich zeolites, and finally, at <50°C, a period of K- and Ca-dominated alteration. The alteration patterns in the Tangihua complex suggests that little or no tectonic activity occurred between formation and obduction, enabling a classic seafloor alteration sequence to develop. The trace element contents and 87Sr/86Sr and 143Nd/144Nd ratios of the Tangihua complex are typical of arc and back-arc systems found within the SW Pacific. The Poya terrane ophiolite of New Caledonia is the geographically closest ophiolite complex of a similar age to the Tangihua Complex but despite many similarities between the systems, close inspection of the alteration and geochemistry show that the systems are unrelated. The combined geochemistry and tectonic constraints suggest that the Tangihua complex formed either in a transitional zone between an arc and a back-arc setting, or in a zone of migration from arc to back-arc volcanism. TIre end of the Late Cretaceous, and the break-up of the eastern and southern Gondwana margin, saw the approach and incipient collision of the spreading ridge between the Phoenix and Pacific plates. Initially subducted, the small, buoyant. Phoenix plate was captured by the Pacific plate, which led to the initiation of rifting between the newly formed Phoenix-Pacific plate and West Antarctica. Stalling of the Phoenix-Pacific ridge and associated subduction system, in combination with the initiation of rifting in the Tasman Sea, resulted in the partial subduction and dehydration of portions of the Phoenix Plate, which then reacted with the previously depleted mantle wedge. Remnants of the ensuing volcanism include the obducted Tangihua Complex of Northland, New Zealand.