Dissolution precipitation creep as a process for the strain localisation in mafic rocks
Dissolution precipitation creep as a process for the strain localisation in mafic rocks
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溶解沉淀蠕变作为镁铁质岩石应变局部化的过程
DOI：
10.31223/x57c9s
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发表时间：
2021
影响因子：
3.1
通讯作者：
M. Battisti
中科院分区：
文献类型：
--
作者：
Amicia L. Lee;H. Stünitz;M. Soret;M. Battisti
未蚀变的镁铁质岩石由机械强度高的矿物（如辉石、斜长石和石榴石）组成，只有在高温（>800°C）下才能通过晶体塑性机制变形。然而，许多镁铁质岩石在低温条件下确实表现出非脆性机制的广泛变形。在这种情况下，变形通常由矿物反应辅助。在这里，我们表明，溶解-沉淀蠕变（扩散蠕变的一种类型）在上角闪岩相条件下的辉长岩透镜体变形中起着重要作用。在南Arnøya出露的Kågen辉长岩由几乎未变形的辉长岩透镜体组成，其周围包裹着剪切边缘。剪切在690 ± 25 °C的温度和1.0至1.1 GPa的压力下进行。本文分析了低应变辉长岩向高应变边缘的显微构造和组构演化。显微结构和晶体学择优取向（CPO）的数据表明，溶解-沉淀蠕变是占主导地位的变形机制，辉长岩的溶解发生在单斜辉石和斜长石的反应阶段，和沉淀发生在新的矿物的形式：新的斜长石和单斜辉石（具有不同的组成），角闪石，石榴石。角闪石显示出强烈的CPO，这主要是由其在拉伸方向上的优先生长控制的。单斜辉石的同步变形和矿物反应表明，镁铁质岩石在一般的相变弱化过程中，即矿物反应和变形的相互作用下，通过扩散蠕变而发生力学弱化。弱化直接与流体辅助的转变过程有关，该过程在比位错蠕变低得多的应力和温度下促进强矿物的扩散蠕变变形。最初坚固的岩性可以变弱，只要在变形过程中可以进行反应;转变过程本身是镁铁质（和其他）岩石中重要的弱化机制，有助于在低差应力和低应力指数下变形。
Unaltered mafic rocks consist of mechanically strong minerals (e.g. pyroxene, plagioclase and garnet) that can be deformed by crystal plastic mechanisms only at high temperatures (>800°C). Yet, many mafic rocks do show extensive deformation by non-brittle mechanisms when they have been subjected to lower temperature conditions. In such cases, the deformation typically is assisted by mineral reactions. Here we show that dissolution-precipitation creep (as a type of diffusion creep) plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro exposed on south Arnøya is comprised of almost undeformed gabbro lenses with sheared margins wrapping around them. The shearing has taken place at temperatures of 690 ± 25 °C and pressures of 1.0 to 1.1 GPa. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and crystallographic preferred orientation (CPO) data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene (with different composition), amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the stretching direction. Synchronous deformation and mineral reactions of clinopyroxene suggest that mafic rocks can become mechanically weak during a general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation; the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses and low stress exponents.
影响因子：
3.4
作者：
Giuntoli, Francesco;Menegon, Luca;Warren, Clare J.
通讯作者：
Warren, Clare J.