Zirkon fission track ages of for middle Pleistocene Rangitawa Tephra, New Zealand

Rangitawa Tephra is an important stratigraphic marker in mid-Pleistocene marine and terrestrial sequences in New Zealand and adjacent ocean basins. Zircon fission track ages (ZFTA) on Rangitawa Tephra from five sites in the southern North Island yield mean site ages in the range 0.34 to 0.40 Ma with a weighted mean of 0.35 + 0.04 Ma (1 sigma). On the basis of glass shard major-element chemistry, ferromagnesian mineralogy, ZFTA and similarity of paleomagnetic dates of proposed tephra correlalives in deep-sea cores, it is concluded that Rangitawa Tephra represents a major eruptive event in the Taupo Volcanic Zone most probably associated with eruption of the Whakamaru-group ignimbrites (0.35 0.39 Ma) or less likely the Paeroa Range Group Ignimbrites (0.36 -0.38 Ma). Pollen analyses from two onshore sites, together with regional loess stratigraphy, show that Rangitawa Tephra was erupted during a glacial period. The ZFTA and previously reported oxygen isotope data from DSDP Site 594 indicate that Rangitawa Tephra was erupted near the end of oxygen isotope stage 10.

兰吉塔瓦火山碎屑（Rangitawa Tephra）是新西兰及毗邻洋盆中更新世中期海相与陆相地层序列中的重要地层标志层。对北岛南部5个采样点位的兰吉塔瓦火山碎屑开展锆石裂变径迹年龄（Zircon Fission Track Ages, ZFTA）测试，获得各点位的平均年龄介于0.34~0.40 Ma，加权平均年龄为0.35±0.04 Ma（1倍标准偏差）。基于玻璃碎屑主量元素化学特征、铁镁矿物矿物组成、锆石裂变径迹年龄，以及深海岩心中已提出的对应火山碎屑的古地磁年代相似性，研究认定兰吉塔瓦火山碎屑代表陶波火山带的一次大规模喷发事件，其最可能与瓦卡马鲁组熔结凝灰岩（0.35~0.39 Ma）的喷发相关，亦存在较小概率对应帕罗阿岭组熔结凝灰岩（0.36~0.38 Ma）的喷发。对两个陆上采样点位的孢粉分析结果结合区域黄土地层序列表明，兰吉塔瓦火山碎屑形成于冰期时段。取自深海钻探计划（Deep Sea Drilling Project）594站位的锆石裂变径迹年龄与此前已发表的氧同位素数据显示，兰吉塔瓦火山碎屑的喷发发生于氧同位素10期末期附近。