Grain size reduction due to fracturing and subsequent grain-size-sensitive creep in a lower crustal shear zone in the presence of a CO2-bearing fluid
Grain size reduction due to fracturing and subsequent grain-size-sensitive creep in a lower crustal shear zone in the presence of a CO2-bearing fluid
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在含 CO2 流体存在的情况下，下地壳剪切带中的破裂和随后的晶粒尺寸敏感蠕变导致晶粒尺寸减小
DOI：
10.1016/j.jsg.2016.11.001
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发表时间：
2017
影响因子：
3.1
通讯作者：
K.
中科院分区：
文献类型：
--
作者：
Okudaira;T.;Shigematsu;N.;Harigane;Y.;Yoshida;K.
为了了解下地壳的流变弱化，我们研究了挪威北方古元古代艾兹峡湾斜长岩中的糜棱岩。斜长糜棱岩带的厚度从几毫米到几米不等，包括超糜棱岩和原糜棱岩。含氯角闪石、方柱石等同运动变质矿物。热力学分析表明，在含CO2的条件下，在600 °C和700 MPa下发生同变形水化反应。原糜棱岩中含有许多破碎的斜长石碎斑岩。碎斑岩的裂隙被细粒斜长石充填，表明破碎作用是碎斑岩粒度减小的一种普遍机制。细粒多角形基质斜长石的钙长石含量与碎斑斜长石的钙长石含量不同，表明基质颗粒在同运动变质作用中成核长大。糜棱岩基质中的斜长石聚集体没有表现出明显的晶体学择优取向，这意味着主要的变形机制是粒度敏感的蠕变。因此，在下地壳斜长糜棱岩，粒度减小发生通过压裂，而不是通过动态重结晶，导致粒度敏感蠕变。斜长石晶界处次要相的同运动再结晶抑制了斜长石的生长，有助于晶粒尺寸敏感蠕变的发展。
To understand rheological weakening in the lower continental crust, we studied mylonites in the Paleoproterozoic Eidsfjord anorthosite, northern Norway. The zones of anorthositic mylonites range from a few millimeters to several meters thick, and include ultramylonites and protomylonites. They contain syn-kinematic metamorphic minerals, including Cl-bearing amphibole and scapolite. Thermodynamic analysis reveals that syn-deformational hydration reactions occurred at ∼600 °C and ∼700 MPa under CO2-bearing conditions. The protomylonites contain many fragmented plagioclase porphyroclasts. The fractures in porphyroclasts are filled with fine-grained plagioclase, suggesting that fracturing is a common mechanism of grain size reduction. The anorthite contents of fine-grained polygonal matrix plagioclase are different from those of porphyroclastic plagioclase, suggesting that the matrix grains nucleated and grew during syn-kinematic metamorphism. Plagioclase aggregates in the matrices of mylonites do not exhibit a distinct crystallographic preferred orientation, which implies that the dominant deformation mechanism was grain-size-sensitive creep. Consequently, in the lower crustal anorthositic mylonites, grain size reduction occurred via fracturing, rather than through dynamic recrystallization, leading to grain-size-sensitive creep. The syn-kinematic recrystallization of minor phases at plagioclase grain boundaries may suppress the growth of plagioclase and contribute to the development of grain-size-sensitive creep.