Mid-Cretaceous granitic magmatism during the transition from subduction toextension in southern New Zealand: a chemical and tectonic synthesis

Regional geochronological studies indicate that mid-Cretaceous plutonism (t he Hohonu Suite at similar to 110 Ma) in the Hohonu Batholith, Western Prov ince of New Zealand, occurred during a period of rapid tectonic change in t he SW Pacific portion of Gondwana, The 30-40 m.y. preceding Hohonu Suite ma gmatism were dominated by the subduction-related plutonism of the Median Te ctonic Zone volcanic are. Between 125-118 Ma there was a major collisional event, inferred to be the result of collision between the Median Tectonic Z one and the Western Province. This collision resulted in melting of the Med ian Tectonic Zone are underplate and generation of a distinctive suite of a lkali-calcic granitoids, termed the Separation Point Suite. At similar to 1 10 Ma there was another pulse of magmatism, restricted to the Buller terran e of the Western Province, and including the Hohonu Suite granitoids. This was followed almost immediately by extension, culminating in the opening of the Tasman Sea some 30 m.y. later. The Hohonu Suite granitoids overlap tem porally with the last vestiges of collisional Separation Point magmas and t he onset of crustal extension in the Western Province, and thus represent m agmatism in a post-collisional setting. Hohonu Suite magmas are typically c alc-alkaline, but retain a chemical signature which suggests that the earli er Separation Point Suite magmas and/or sources were involved in Hohonu Sui te petrogenesis. A model is proposed in which rapid isothermal uplift, resu lting from the post-collisional collapse of continental crust previously th ickened during the Median Tectonic Zone collision, caused melting of lower continental crust to generate the Hohonu Suite granitoids. In this example, granitoid composition is a consequence of the composition of the source ro cks and the conditions present during melting, and no geochemical signature indicative of the tectonic setting during magmatism is present. (C) 1998 E lsevier Science B.V. All rights reserved.