New U–Pb zircon ages for the Glenroy Complex and McKnee Intrusives, southeast Nelson: strengthening links with northern Fiordland and constraining the timing of metamorphism

The Glenroy Complex and McKnee Intrusives, southeast Nelson, have previously been correlated with similar rocks in northern Fiordland: the Worsley Pluton of the Western Fiordland Orthogneiss and George Sound Paragneiss, and Mistake Diorite and Darran Leucogabbro, respectively. Two samples of the Glenroy Complex (Woodham Orthogneiss) and one from the McKnee Intrusives yielded laser ablation inductively-coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon ages of 123.7 ± 1.4 and 121.9 ± 1.1 Ma, and 138.8 ± 1.3 Ma, respectively. These new ages fall within the ranges previously published for the Glenroy Complex, McKnee Intrusives and their northern Fiordland counterparts and, thus, provide further evidence for their correlation. An age of 116.3 ± 2.4 Ma was also determined for a post-metamorphic pegmatite–aplite dike, constraining metamorphism of the Woodham Orthogneiss to within 2.0–9.0 Ma following magmatic crystallisation. The time gap between crystallisation and metamorphism is similar to that of the Worsley Pluton (3.7–8.9 Ma), lending additional support for their direct correlation.

此前，纳尔逊东南部的格伦罗伊杂岩（Glenroy Complex）与麦基侵入体（McKnee Intrusives）曾被认为与菲奥德兰北部的同类岩石具有可对比性：前者对应西菲奥德兰正片麻岩（Western Fiordland Orthogneiss）中的沃斯利岩体（Worsley Pluton）与乔治湾副片麻岩（George Sound Paragneiss），后者对应米斯特克闪长岩（Mistake Diorite）与达兰浅色辉长岩（Darran Leucogabbro）。本次研究对2件采自格伦罗伊杂岩（含伍德哈姆正片麻岩（Woodham Orthogneiss））的样品及1件麦基侵入体样品开展定年分析，获得激光剥蚀电感耦合等离子体质谱法（laser ablation inductively-coupled plasma mass spectrometry，LA–ICP–MS）测试的U-Pb锆石年龄：其中格伦罗伊杂岩的两件样品年龄分别为123.7±1.4 Ma与121.9±1.1 Ma，麦基侵入体的样品年龄为138.8±1.3 Ma。这些新测年结果落在此前已发表的格伦罗伊杂岩、麦基侵入体及其菲奥德兰北部对应岩体的年龄区间内，进一步为二者的对比关系提供了佐证。研究还为1件变质后伟晶岩-细晶岩岩脉测得年龄为116.3±2.4 Ma，据此将伍德哈姆正片麻岩的变质作用时代约束在岩浆结晶后的2.0~9.0 Ma范围内。其岩浆结晶与变质作用之间的时间间隔与沃斯利岩体（3.7~8.9 Ma）的区间相近，为二者的直接对比关系提供了额外支撑。