Geology and geochronology of the Sub-Antarctic Snares Islands/Tini Heke, New Zealand

The first comprehensive geological map, a summary of lithologies and new radiogenic isotope data (U–Pb, Rb–Sr) are presented for crystalline rocks of the Sub-Antarctic Snares Islands/Tini Heke, 150 km south of Stewart Island. The main lithology is Snares Granite (c. 109 Ma from U–Pb dating of zircon), which intrudes Broughton Granodiorite (c. 114 Ma from U–Pb zircon) on Broughton Island. Rafts of schist within Snares Granite are common on the outlying Western Chain islets, and rare on North East and Broughton islands. Zircon grains extracted from one schistose raft on Broughton Island are prismatic and yield an essentially unimodal age population of c. 116 Ma that is within error of the granodiorite. These properties suggest that the dated raft represents a meta-igneous rock despite its mica-rich nature. Some schistose rocks on the Western Chain contain coarse relict plagioclase phenocrysts and appear to have an igneous protolith. No conclusive metasedimentary rocks have been identified, although sillimanite-bearing mica-rich schist occurs on Rua. Deformation of the crystalline rocks occurred after Snares Granite intrusion and before cooling below muscovite K–Ar closure at 400 ± 50 °C at 95 Ma. This period overlaps the age of extensional ductile shear zones on Stewart Island. The discovery of several basaltic dykes, which cut across fabrics and are unmetamorphosed, indicates that volcanic rocks are associated with all Sub-Antarctic island groups. The larger of the islands are overlain by peat, which on North East Island also contains gravel deposits.

本数据集收录了位于斯图尔特岛以南150千米的亚南极斯奈尔斯群岛/蒂尼赫基（Sub-Antarctic Snares Islands/Tini Heke）结晶岩的首幅综合地质图、岩性概要与全新放射同位素测试数据（U–Pb、Rb–Sr）。区域内主要岩性为斯奈尔斯花岗岩，其锆石（zircon）U–Pb定年结果约为109 Ma，侵入了布劳顿岛上的布劳顿花岗闪长岩，后者的锆石U–Pb定年结果约为114 Ma。斯奈尔斯花岗岩体内的片岩岩块在偏远的西链小岛中较为常见，而在东北岛与布劳顿岛上则较为稀少。从布劳顿岛的一块片岩岩块中提取的锆石呈柱状，其年龄分布基本为单峰态，约为116 Ma，误差范围内与花岗闪长岩的年龄一致。这一特征表明，尽管该岩块富云母，但其本质上属于变火成岩。西链的部分片岩含有粗大的残留斜长石斑晶，推测其原岩为火成岩。尽管鲁阿岛（Rua）产出含矽线石的富云母片岩，但目前尚未发现确凿的变沉积岩。结晶岩的变形作用发生于斯奈尔斯花岗岩侵入之后，且早于95 Ma时白云母K–Ar封闭温度（400 ± 50 °C）对应的冷却阶段。该变形时期与斯图尔特岛的伸展韧性剪切带形成时期重合。本次研究发现多条切割岩石组构且未发生变质的玄武岩岩脉，表明火山岩与所有亚南极岛群均存在成因关联。面积较大的岛屿被泥炭覆盖，其中东北岛的泥炭中还含有砾石沉积。