Punctuated or continuous deformation during the exhumation of subducted crust? A linked tectono-metamorphic and geochronological study.

这个项目关注的是曾经深埋地壳的变质历史，并将这些日期直接与这些岩石返回地球表面时形成的变形结构联系起来。这些物质（所谓的高压（HP）变质岩/榴辉岩和蓝片岩）及其等效物（绿片岩，在低压条件下再结晶）是板块边界带垂直运动的重要记录者，因此是许多地球动力学模型测试的基础。重要的问题包括：HP状态是否在不同的时间经历较低P（绿片岩）状态？一个HP地形的不同部分是否会在不同的时间被挖掘出来，从而产生一个混合的HP碎片？掘出尸体的比率是多少？这些问题的答案直接影响到关于俯冲带过程的激烈辩论。这些地区是否经历了不同时期的局部构造分裂，在其他趋同的板块边界地区，岩石圈范围内的颈缩和HP岩石的挖掘被重新埋葬的时期所点缀？或者，物质是作为分离的“尖点”不断向下和向上循环的俯冲带。为了解决这些问题，有必要确定与变形和变质条件有关的矿物结构的年代。我们将使用铷锶法测定云母生长和再结晶的年代，我们在阿尔卑斯山绿片岩相条件下剪切的岩石的变形年代测定中证实了这种方法。现在是时候通过蓝片岩向绿片岩相的减压来明确地跟踪岩石了。我们将把重点放在锡罗斯岛上暴露出来的大西洋-基克拉迪斯带的壮观蓝片岩上。这是非常合适的，不仅因为在露头尺度上构造变质历史比在阿尔卑斯山脉更容易处理，而且因为峰顶温度不超过500℃，这是辉云岩中Rb-Sr体系的封闭温度。现有的模式表明，Syros蓝片岩的间断挖掘到绿片岩相条件（从20kbar， ~70km深度到6kbar， ~20km深度）。然而，这些模型依赖于峰值压力变质矿物的同位素年龄（例如锆石的U-Pb年龄）和冷却年龄（云母的Ar年龄，显然是岩石的最后一次经历~350C）。这两种方法都不能轻易地与特定的变形织物联系起来。Rb-Sr法可以。因此，我们的研究将首次确定，在单一露头的大范围挖掘条件下的减压率和相关变形率。微采样和质谱技术的进步现在允许进行必要的详细工作，以确定样品中显示重复矿物生长和结构叠印的织物的年代。我们将以爱丁堡的地区专业知识和样本收集为基础，专注于特征良好的结构。这些研究表明，Syros蓝片岩非常适合我们的研究目的：云母化学成分及其与变质组合的关系非常好地表征了在变形和减压过程中，云母生长和再结晶的广泛压力条件。白云石的化学成分对矿物生长过程中的压力条件很敏感。而辉长石通常定义了岩石中的变形结构。通过对比Syros上HP岩石的减压历史，我们将确定它们的挖掘是一个脉冲的、间断的过程还是连续的过程，以及HP地形的不同部分是否在不同的时间和不同的速度被挖掘出来。这些结果不仅有助于了解Syros上高压岩石的地质演化，还将为高压岩石的发掘提供直接测试，并为俯冲带过程的速率和动力学提供基础数据。

This project is concerned with dating the metamorphic history of once deeply-buried crust and relating these dates directly to the deformation structures that formed while these rocks returned towards the Earth's surface. These materials (so-called high pressure (HP) metamorphic rocks / eclogites and blueschists) and their equivalents (greenschists, recrystallised under lower pressure conditions) are important recorders of the vertical movements in plate boundary zones and thus underpin many tests of geodynamic models. Important questions include: Are HP conditions experienced at distinct times to lower P (greenschist) conditions? Do different parts of an HP terrain exhume at different times, creating an amalgam of HP fragments? What are the rates of exhumation? Answers to these questions impact directly on a vigorous debate concerning subduction zone processes. Do these zones experience distinct periods of local tectonic divergence within otherwise convergent plate boundary zones with lithosphere-wide necking and exhumation of HP rocks punctuated by periods of renewed burial? Or, is material continuously cycled down and up subduction zones as detached 'pips'. To address the questions it is necessary to date the mineral fabrics that link to deformation and metamorphic conditions. We will use the rubidium-strontium method to date phengite mica growth and recystallisation, a method proven by us in deformation age dating of rocks sheared under greenschist facies conditions in the Alps. It is now timely to extend the approach to track rocks explicitly through decompression / from blueschist into greenschist facies. We will focus on the spectacular blueschists of the Attic-Cyclades belt exposed on the island of Syros. These are highly suitable not only because the tectono-metamorphic history is much more tractable on the outcrop scale than in the Alps but also because peak temperatures did not exceed 500C, the closure temperature for the Rb-Sr system in phengite. Existing models imply punctuated exhumation of the Syros blueschists rocks to greenschist facies conditions (from 20 kbar, ~70km depth to c. 6 kbar, ~20km). However, the models rely on isotopic ages for peak pressure metamorphic minerals (e.g. U-Pb ages on the mineral zircon) and cooling ages (Ar ages on micas, apparently dating the last experience of ~350C for the rocks). Neither approach can readily be related to specific deformation fabrics. The Rb-Sr method can. Consequently our research will establish, for the first time, the rates of decompression and related deformation over a wide range of exhumation conditions charted by single outcrops. Advances in micro-sampling and mass spectrometry now allow the detailed work necessary to date fabrics within specimens that show repeated mineral growth and structural overprint. We will build on the regional expertise and sample collections held in Edinburgh to focus on well-characterised structures. These researches show the Syros blueschists to be ideal for our purpose: the mica chemistries and relationships to metamorphic assemblages are very well characterised showing a wide range of pressure conditions with phengite growth and recrystallisation during deformation and decompression. The chemical composition of phengite is sensitive to the pressure conditions during mineral growth. And phengite commonly defines the deformation fabric in the rocks. By contrasting the dated decompression history of the HP rocks on Syros we will establish whether their exhumation was a pulsed, punctuated process or was continuous, and whether different parts of the HP terrain exhumed at different times and different rates. Not only will these results help to understand the geological evolution of the HP rocks on Syros, they will provide direct tests of the competing models for the exhumation of HP rocks, and provide fundamental data on the rates and dynamics of subduction zone processes.