Brittle-ductile microfabrics in naturally deformed zircon: Deformation mechanisms and consequences for U-Pb dating
Brittle-ductile microfabrics in naturally deformed zircon: Deformation mechanisms and consequences for U-Pb dating
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自然变形锆石中的脆性-韧性微纤维：U-Pb 定年的变形机制和后果
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发表时间：
2012
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通讯作者：
M. Whitehouse
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作者：
S. Piazolo;H. Austrheim;M. Whitehouse
摘要对大锆石颗粒（0.8 ~ 1.5 mm）进行了电子背散射衍射、阴极发光和放射性成因U-Pb定年研究，结果表明锆石晶粒存在晶内变形、破裂、晶粒尺寸减小和U-Pb年龄的较大分布。样品来自麻粒岩相斜长岩中的角闪岩相变形带（挪威卑尔根弧）。大尺寸锆石晶粒表现出三种主要的晶格畸变类型：(I)在<001>附近旋转，取向变化约0.3°/μm，与（100）平行；（II）位于晶粒边缘的高度畸变的半圆形区域，畸变程度至少为0.8 ~ 1°/μm；(3)形成宽达100 μm的变形区的低角度边界网络。II型和III型扭曲表现出明显的干扰，否则均匀的CL特征。带有滑移系统的晶体塑性变形[010]（100）导致I型变形。流变硬晶粒接触处的应力集中导致局部晶体塑性变形，少量微破裂形成II型变形。III型变形是由晶体塑性变形形成的，通常伴随着夹杂物使用几种滑移系统。I型和II型变形分别表现为原始约900 Ma锆石颗粒的轻微和中度重置，这是由于沿位错壁的管道扩散增强所致。在II型畸变中，加速的晶格扩散通过高度畸变的晶格，结合异常高的边界体积比，引起明显的化学干扰和年龄重置到410 Ma。细粒聚集体包含具有低内部变形和振荡带状CL特征的颗粒（z -颗粒）或具有高内部变形和扰动CL特征的颗粒（d -颗粒）。Z型和d型晶粒分别由非均质形核和生长形成，并沿应变硬化低角边界破裂。z -颗粒呈簇状化学特征，年龄为437±11 Ma，可直接确定加里东期角闪岩相改造的年代。
Abstract We present an electron backscatter diffraction, cathodoluminescence, and radiogenic U-Pb dating study of large zircon grains (0.8-1.5 mm) that show evidence of intracrystalline deformation, fracturing, grain size reduction and a large spread in U-Pb ages. The samples are from an amphibolite facies deformation zone within granulite facies anorthositic rocks (Bergen Arc, Norway). Large zircon grains show three main lattice distortion types: (I) distortions with rotations around <001> and an orientation change of ~0.3 °/μm subparallel to (100); (II) highly distorted, half circular shaped zones located at grain edges with at least 0.8-1°/μm distortions; and (III) low-angle boundary networks forming deformation zones up to 100 μm wide. Types II and III distortions exhibit significant disturbances of the otherwise homogeneous CL signature. Crystal plastic deformation with the slip system [010](100) resulted in type I distortions. Stress concentrations at grain contacts between rheologically hard grains caused localized crystal plastic deformation with minor amount of microfracturing forming type II distortions. Type III distortions formed by crystal plastic deformation often associated with inclusions using several slip systems. Distortions of types I and II show minor and moderate resetting of the original ca. 900 Ma zircon grains, respectively, due to enhanced pipe diffusion along dislocation walls. In type II distortions, accelerated lattice diffusion through the highly distorted crystal lattice, combined with exceptionally high boundary to volume ratio, caused significant chemical disturbance and age resetting to 410 Ma. Fine-grained aggregates contain grains with low internal deformation and an oscillatory zoned CL signature (Z-grains) or high internal deformation and a disturbed CL signature (D-grains). Z- and Dgrains are interpreted to have formed by heterogeneous nucleation and growth, and fracturing along strain-hardened low-angle boundaries present within types I and II, respectively. Z-grains show a clustered chemical signature with a 437 ± 11 Ma age interpreted to directly date the Caledonian amphibolite facies reworking.