多成因白色云母(希腊纳克索斯)的岩石年代学

IF 3.5 2区 地球科学 Q1 GEOLOGY
Igor M. Villa, Johannes Glodny, Alexandre Peillod, Alasdair Skelton, Uwe Ring
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引用次数: 2

摘要

希腊基克拉泽斯中的纳克索斯保留了一个多聚同构的典型例子。岛的南部和北部记录了始新世和中新世之间不同的第三纪P–T历史,包括蓝片岩相事件、一个或多个角闪岩/绿片岩相叠加和接触变质作用。这些事件的年龄归属在文献中并不一致。在这里,我们提出了一种新的方法,将白云母(WM)的电子探针显微分析仪(EPMA)表征与39Ar‐40Ar–Rb‐Sr多时间测定相结合。纹理-岩相学-成分观察表明,多成因WM由五个不同的世代组成:前始新世残余岩、共生岩、高硅多硅土、低硅多硅石和白云母。先前由Rb‐Sr分析的四个WM样品的EPMA图谱显示,主要元素组成在μm范围内是不均匀的。每个WM由化学上不同的世代组成,记录了高压(HP)多硅土颗粒向白云母的亚微米级退化。通过39Ar‐40Ar步进加热分析了岛上N‐S导线的四个WM样品,比较了粗粒度和细粒度,以获得超定的K‐Ar系统。细筛级分比粗筛级分富含Cl。Cl/K年龄同位素相关图中的线性阵列显示了两个主要的WM世代(其中一个在ca。38 Ma和一个在<20时富含Cl 马)。来自南部Naxos的低品位样品的再结晶不太普遍,提供了较老的年龄,并保存了至少三代WM,包括一个具有前古新世K-Ar年龄的残余WM,这与WM在没有完全再结晶的情况下的高Ar保留率一致。Cl较差的最终成员WM的年龄接近HP事件的年龄,38 白垩纪云母残余物中的Ma。Ar继承在单颗粒尺度上是不均匀的。比较WM馏分的脱气速率排除了“多畴”扩散。由于没有样品是单矿物的,因此每个多成因云母的脱气速率由其组成矿物的不相关速率常数的质量平衡和控制。鉴于分区和复合云母的普遍性,这里详细介绍的方法可能有助于重建世界范围内的多相变质历史。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Petrochronology of polygenetic white micas (Naxos, Greece)

Petrochronology of polygenetic white micas (Naxos, Greece)

Naxos in the Greek Cyclades preserves a type example of polymetamorphism. The southern and northern parts of the island record different Tertiary P–T histories between Eocene and Miocene times, including a blueschist facies event, one or more amphibolite/greenschist facies overprint(s) and contact metamorphism. Age attributions for these events are inconsistent in the literature. Here, we propose a new approach that combines electron probe microanalyzer (EPMA) characterization of the white mica (WM) with 39Ar-40Ar–Rb-Sr multichronometry. Textural–petrographic–compositional observations reveal that the polygenetic WM consists of five different generations: pre-Eocene relicts, paragonite, high-Si phengite, low-Si phengite and muscovite. EPMA mapping of four WM samples, previously analysed by Rb-Sr, reveals major element compositions heterogeneous down to the μm scale. Each WM consists of chemically distinct generations, documenting submicron-scale retrogression of high-pressure (HP) phengite grains to muscovite. Four WM samples from a N-S traverse across the island were analysed by 39Ar-40Ar stepheating, comparing coarse and fine sieve size fractions to obtain overdetermined K-Ar systematics. Fine sieve fractions are richer in Cl than coarse ones. Linear arrays in Cl/K-age isotope correlation diagrams show two predominant WM generations (one Cl-poor at ca. 38 Ma and one Cl-rich at <20 Ma). A lower-grade sample from southern Naxos was less pervasively recrystallized, provides older ages and preserves at least three WM generations, including a relict WM with a pre-Palaeocene K-Ar age, consistent with the high Ar retentivity of WM in the absence of complete recrystallization. The age of the Cl-poor end-member WM approximates the age of the HP event, 38 Ma. Ar inheritance in Cretaceous mica relicts is heterogeneous at the single-grain scale. Comparing the degassing rates of the WM fractions rules out ‘multidomain’ diffusion. As no sample is monomineralic, the degassing rate of each polygenetic mica is instead controlled by the mass balanced sum of the unrelated rate constants of its constituent minerals. Given the commonness of zoned and composite micas, the approach detailed here is potentially useful for reconstructing polyphase metamorphic histories worldwide.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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