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

俯冲地壳折返过程中是间断变形还是连续变形？

• 批准号: NE/D008263/1
• 负责人: Robert Cliff
• 金额: $ 5.49万
• 依托单位: University of Leeds
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FD008263%2F1
• 关键词: Punctuated; continuous; deformation; during; exhumation

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.

该项目致力于测定曾经深埋的地壳的变质历史，并将这些日期与这些岩石返回地球表面时形成的变形结构直接联系起来。这些物质(所谓的高压变质岩/榴辉岩和蓝片岩)及其等价物(绿片岩，在较低压力条件下重结晶)是板块边界带垂直运动的重要记录，因此是许多地球动力学模型检验的基础。重要的问题包括：是否在不同的时间经历了降低P(绿片)条件的HP条件？HP地形的不同部分会在不同的时间挖掘出来，形成HP碎片的混合体吗？掘出遗骸的速度是多少？这些问题的答案直接影响到一场关于俯冲带过程的激烈辩论。这些带是否经历了不同的局部构造分异时期，在原本汇聚的板块边界带中，整个岩石圈的高压岩石的颈缩和折返被重新埋藏的时期所打断？或者，物质是作为分离的‘点’在俯冲带上下循环的吗？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组构的年代。我们将使用Rb-锶方法测定白云母生长和重结晶的年龄，这是我们在阿尔卑斯山绿片岩相条件下剪切岩石的变形年龄测定中证实的方法。现在是时候推广通过减压显式追踪岩石/从蓝片岩相到绿片岩相的方法了。我们将重点关注暴露在锡罗斯岛上的阁楼-Cyclade带的壮观蓝片岩。这不仅是因为构造变质历史在露头尺度上比阿尔卑斯山更容易处理，而且还因为峰温不超过500℃，即多硅白云母中Rb-Sr系统的闭合温度。现有的模型表明，Syros蓝片岩岩石点状折返到绿片岩相条件(从20Kbar，~70公里深到约6Kbar，~20公里)。然而，这些模型依赖于峰值压力变质矿物的同位素年龄(例如矿物锆石上的U-Pb年龄)和冷却年龄(云母上的Ar年龄，显然是岩石最后一次经历的~350℃年龄)。这两种方法都不能很容易地与特定的变形织物联系起来。Rb-sr方法可以。因此，我们的研究将首次确定在单一露头绘制的大范围出土条件下的减压和相关变形速率。显微取样和质谱学的进步现在允许进行必要的详细工作，以确定样品中显示反复矿物生长和结构叠印的织物的年代。我们将在爱丁堡举行的地区专业知识和样本收集的基础上，专注于具有良好特色的结构。这些研究表明，Syros蓝片岩非常适合我们的目的：云母化学和与变质组合的关系非常好地描述了广泛的压力条件，在变形和减压过程中多硅白云母生长和重结晶。白云母的化学成分对矿物生长过程中的压力条件十分敏感。多硅白云母通常定义岩石中的变形组构。通过对比Syros上高压岩石的年代减压史，我们将确定它们的折返是脉冲的、点状的还是连续的，以及高压地形的不同部分是否以不同的时间和不同的速度折返。这些结果不仅有助于了解Syros上高压岩石的地质演化，还将直接检验相互竞争的高压岩石折返模式，并提供俯冲带过程的速率和动力学的基础数据。