U–Th–Pb monazite dating and the timing of arc–continent collision in East Timor

The Timor Orogen represents one of the youngest arc–continent collisions exposed on the Earth. It has the potential to provide some of the key parameters about how this style of orogeny evolves. However, the metamorphic age of the highest-grade rocks formed in the collision remains controversial. Using U–Th–Pb dating of monazite from amphibolite-grade sillimanite and garnet-bearing schists we show the peak metamorphism occurred at 5.5–4.7 Ma. The young age of the monazite and the presence of significant amounts of common Pb required the development of a new protocol to simultaneously account for the ²³⁰Th disequilibrium and the ²⁰⁷Pb common Pb correction. The new estimate of metamorphic age is consistent with the estimates based on plate reconstructions for the initiation of arc–continent collision in East Timor. The metamorphic event is a result of this collision.

帝汶造山带（Timor Orogen）是地球上出露的最年轻的弧-陆碰撞带之一，可为该类造山作用的演化机制提供关键参数。然而，此次碰撞形成的最高级变质岩的变质年龄仍存在争议。本研究对采自角闪岩相矽线石及含石榴石片岩中的独居石（monazite）开展U-Th-Pb测年，结果显示峰期变质作用发生于5.5~4.7 Ma。鉴于独居石年龄较新且样品中含有大量普通铅，本研究开发了一套全新的分析方案，可同时校正²³⁰Th不平衡与²⁰⁷Pb普通铅的影响。本次得到的变质年龄估算结果，与基于板块重建得到的东帝汶弧-陆碰撞起始时间的估算结果一致。该变质事件正是此次弧-陆碰撞的产物。