Recrystallization or cooling ages: in situ UV-laser 40Ar/39Ar geochronology of muscovite in mylonitic rocks
Recrystallization or cooling ages: in situ UV-laser 40Ar/39Ar geochronology of muscovite in mylonitic rocks
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DOI：
10.1144/0016-764903-110
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发表时间：
2004-07
影响因子：
2.7
通讯作者：
A. Mulch;M. Cosca
中科院分区：
文献类型：
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作者：
A. Mulch;M. Cosca
绿片岩相糜棱岩带白云母的粒内40Ar/39Ar年龄分布是变质历史的传感器。两个伸展剪切带的织构、成分和原位紫外激光烧蚀40Ar/39Ar数据表明，冷却年龄可以与新结晶或再结晶年龄区分。来自Pogallo剪切带（Ivrea带，南阿尔卑斯）的变形白云母斑岩碎屑和再结晶剪切带白云母显示出不同的粒内40Ar/39Ar年龄，内部年龄变化大于65 Ma。晶粒内40Ar/39Ar年龄范围较宽，与晶粒内组织控制的扩散主导氩损失相一致。相比之下，来自挪威南部Porsgrunn-Kristiansand剪切带元古代糜伦岩的白云母鱼的粒内40Ar/39Ar年龄仅显示出较小的年龄分散，年龄变化与白云母成分的变化有关。不同白云母显微结构的共生年龄-组成相关性与白云母的序贯新结晶和再结晶直接相关，并最终揭示了沿Porsgrunn-Kristiansand剪切带伸展变形过程中漫长的再结晶历史。
The intra-grain 40Ar/39Ar age distributions of muscovite from greenschist-facies mylonite zones are shown to be sensors of metamorphic history. Textures, compositions and in situ UV-laser ablation 40Ar/39Ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo- or recrystallization ages. Deformed muscovite porphyroclasts and recrystallized shear band muscovite from the Pogallo Shear Zone (Ivrea Zone, southern Alps) reveal variable intra-grain 40Ar/39Ar ages with internal age variations of more than 65 Ma. The wide range of 40Ar/39Ar ages within compositionally homogeneous grains is consistent with diffusion-dominated argon loss controlled by observable intra-grain microstructures. In contrast, intra-grain 40Ar/39Ar ages of muscovite fish from Proterozoic mylonite of the Porsgrunn–Kristiansand Shear Zone (Southern Norway) display only minor age dispersion, and age variations are correlated with changes in muscovite composition. The sympathetic age–composition correlations in different muscovite microstructures directly relate to sequential neo- and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the Porsgrunn–Kristiansand Shear Zone.