Gasser Deta, Bruand Emilie, Rubatto Daniela, Stüwe Kurt
Department of Earth Science, University of Graz, Universitätsplatz 2, A-8010 Graz, Austria.
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.
Lithos. 2012 Mar;134-135(3-3):108-122. doi: 10.1016/j.lithos.2011.12.003.
Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U-Pb geochronology. However, linking monazite U-Pb ages with the evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U-Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~ 550-650 °C and ≥ 3.4 kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U-Pb SHRIMP analysis of the second and third monazite generations yields ages of ~ 55-50 Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle.
独居石是各种变质岩和岩浆岩中常见的副矿物，广泛应用于铀铅地质年代学。然而，将独居石的铀铅年龄与岩石的演化联系起来并非总是那么简单。我们研究了阿拉斯加南部始新世楚加奇变质杂岩中一套变质沉积序列中独居石的行为，该序列从绿片岩相千枚岩到上麻粒岩相深熔片麻岩。我们研究了不同岩石总体成分和变质程度样品中独居石颗粒的结构、化学成分和铀铅年代，特别关注独居石与其他含稀土元素矿物（如褐帘石和磷钇矿）之间的关系。在绿片岩相千枚岩中，存在碎屑和变质褐帘石，而独居石不存在。在较低的麻粒岩相片岩（约550 - 650°C和≥3.4千巴）中，小的、中钇的独居石广泛分布（Mnz1），表明独居石在石榴石和红柱石生长之前和/或同时生长。在深熔片麻岩中，新的低钇、高钍独居石（Mnz2）从部分熔体中结晶出来，在初始冷却和石榴石回吸过程中形成了第三代高钇、低钍独居石（Mnz3）。对第二代和第三代独居石进行铀铅SHRIMP分析得出年龄约为55 - 50百万年。在含白云母和/或绿泥石的逆行剪切带中，独居石变得不稳定，并被褐帘石和/或褐帘石/绿帘石/磷灰石冠状物覆盖生长。这项研究记录了在一个相对短暂的变质循环中多阶段、复杂的独居石生长和溶解过程。
