Replacement reactions and deformation by dissolution and precipitation processes in amphibolites
Replacement reactions and deformation by dissolution and precipitation processes in amphibolites
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DOI：
10.1111/jmg.12445
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发表时间：
2018-12-01
影响因子：
3.4
通讯作者：
Warren, Clare J.
中科院分区：
文献类型：
--
作者：
Giuntoli, Francesco;Menegon, Luca;Warren, Clare J.
在碰撞环境中，中下地壳的变形是通过随岩石成分、流体含量、压力和温度变化的变形机制发生的。这些机制在推覆体堆积和挖掘过程中调节了大的构造搬运距离。研究表明，压裂和流体流动触发了溶蚀-沉淀和溶蚀-沉淀耦合蠕变过程，这是角闪岩中糜棱岩微观结构形成的原因。该构造发育于下西韦推覆体（斯堪的纳维亚卡利多尼德）的地壳厚度约500米。角闪岩呈糜棱质叶状，包裹着钠长石斑岩碎屑，在全色阴极发光（CL）中呈暗色。钠长石斑岩碎屑被沿（001）解理面优先发育的裂缝解剖破碎，呈片状边缘，具凹状和半岛状特征。两代钠长石/少辉长石，明亮的CL，吸收和过度生长的斑岩碎屑，封闭裂缝。电子背散射衍射结果表明，这两代钠长石/寡晶长石以钠长石卟斑碎屑为代价，以假晶和拓扑为代价生长，并在其周围外延生长。这两代钠长石/低晶长石也以平行于糜棱岩面理的新母细胞生长。角闪石晶体经历了类似的微观结构演变，证明了锈蚀的角闪石铁芯被铁闪石铁芯边缘包围，这些铁芯保持了相同的晶体取向。定向偏差图突出了角闪孔的定向偏差与垂直于其c轴的裂缝位移之间的关系。在这两种矿物中均未观察到晶体塑性。斜长石和角闪石表现出晶体学上的优先取向，这是母晶的拓扑生长和具有相似晶体学取向的新晶的形核的结果。角闪洞和斜长石的热压特征限制了糜棱岩片理发育为绿帘石角闪岩相（600℃，0.75 ~ 0.97 GPa）。结果表明，在中下地壳水平，晶界处富h2o流体的存在有利于溶蚀-沉淀耦合取代反应，有利于斜长石和角闪洞中溶蚀-沉淀蠕变变形，而不是位错蠕变变形。
The deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition, fluid content, pressure, and temperature. These mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation. Here, we show that fracturing and fluid flow triggered coupled dissolution-precipitation and dissolution-precipitation creep processes, which were responsible for the formation of a mylonitic microstructure in amphibolites. This fabric is developed over a crustal thickness >500 m in the Lower Seve Nappe (Scandinavian Caledonides). Amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence (CL). The albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the (001) cleavage planes and display lobate edges with embayments and peninsular features. Two albite/oligoclase generations, bright in CL, resorbed and overgrew the porphyroclasts, sealing the fractures. Electron backscattered diffraction shows that the two albite/oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them. These two albite/oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation. The amphibole crystals experienced a similar microstructural evolution, as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores. Misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c-axis. No crystal plasticity is observed in either mineral species. Plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation. Amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies (similar to 600 degrees C, 0.75-0.97 GPa). Our results demonstrate that at middle to lower crustal levels, the presence of H2O-rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution-precipitation and favours deformation by dissolution-precipitation creep over dislocation creep in plagioclase and amphibole.