Reconnaissance thermochronology of southern Zealandia

We report new fission-track, (U–Th)/He and ⁴⁰Ar/³⁹Ar ages for minerals in surface, dredge and borehole cuttings samples from the southern South Island, Campbell Plateau and Chatham Rise regions of the Zealandia continent. Our results indicate that much of southern Zealandia underwent rapid cooling in the interval 105–80 Ma, probably during widespread intracontinental rifting before Zealandia split from Gondwana. The 108 ± 11 Ma rapid exhumation of a zircon He retention zone in the Chatham Schist, however, may record cessation of long-lived subduction at the Gondwana margin owing to Hikurangi Plateau collision and/or the onset of the extensional rifting episode. Locally, basement rocks have been appreciably heated, and thermochronological systems reset, in response to deepening of the Great South Basin and by local Miocene intraplate volcanism. An apatite fission-track age of 179 ± 17 Ma from a dredge sample of Precambrian granite at the southern tip of the Campbell Plateau indicates that it is probably ice-rafted, thus <em>in situ</em> Precambrian crust in Zealandia is still unconfirmed. In contrast, a zircon (U–Th)/He age of 99 ± 9 Ma from a Precambrian metarhyolite from Iselin Bank at the conjugate Antarctic continental margin plausibly indicates an <em>in situ</em> position.

本研究报道了西兰大陆（Zealandia）南岛南部、坎贝尔高原（Campbell Plateau）及查塔姆海隆（Chatham Rise）区域的地表、拖网及钻孔岩屑样品中矿物的全新裂变径迹（fission-track）、（铀-钍）/氦[(U–Th)/He]与氩-氩（⁴⁰Ar/³⁹Ar）热年代学年龄。研究结果表明，西兰大陆南部绝大部分区域在105~80 Ma期间经历了快速冷却，该过程大概率发生于西兰大陆从冈瓦纳大陆（Gondwana）裂解前的广泛陆内裂谷作用时期。然而，查塔姆片岩中锆石氦保留带在108±11 Ma时的快速剥露，可能记录了因希库朗伊高原（Hikurangi Plateau）碰撞导致的冈瓦纳大陆边缘长期俯冲作用终止，或是伸展裂谷事件的启动。局部区域内，受大南盆地（Great South Basin）加深及中新世板内局部火山活动影响，基底岩石经历了显著加热，热年代学体系被重置。从坎贝尔高原南端拖网采样获得的前寒武纪花岗岩样品的磷灰石裂变径迹年龄为179±17 Ma，表明该样品大概率经冰筏搬运而来，因此西兰大陆内的原位（in situ）前寒武纪地壳仍未得到证实。与之相反，采自共轭南极大陆边缘艾瑟林浅滩（Iselin Bank）的前寒武纪变流纹岩样品所测得的锆石（U–Th)/He年龄为99±9 Ma，该结果有力佐证了其原位产出的可能性。