Temperature, fluid content and rheology of localized ductile shear zones in subsolidus cooling plutons
Temperature, fluid content and rheology of localized ductile shear zones in subsolidus cooling plutons
复制标题
固相线以下冷却岩体局部延性剪切带的温度、流体含量和流变学
DOI：
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发表时间：
2020
影响因子：
3.4
通讯作者：
G. Pennacchioni
中科院分区：
文献类型：
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作者：
A. Ceccato;P. Goncalvès;G. Pennacchioni
温度和流体含量是控制大陆地壳岩石流变和应变局部化的关键参数。在这里，我们通过热力学模型确定的T-MH2O的局部韧性剪切在冷却过程中的三个不同的花岗岩类岩体：Rieserferner和Adamello岩体在意大利阿尔卑斯山，和爱迪生湖岩体在内华达山脉-美国。剪切带利用了前兆节理、伴生脉和蚀变带。用Perple_X计算了MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-Fe2O3体系的T-MH2O和P-T相图截面。相图部分显示脆性前体（接头、脉）的成核在流体饱和条件下在T>> 450 ° C下发生。局部韧性剪切可能发生在420和460 ° C之间的温度范围内，在封闭系统中从最初的流体饱和状态发展到流体欠饱和状态。在这个温度范围内，花岗岩类岩石可能会发生一系列退变质反应，用较弱的层状硅酸盐取代承载的长石。变质反应发生在空间上与变质构造相关联，导致局部剪切。温度降低和流体欠饱和条件可能通过减缓变质反应、热激活位错蠕变过程、流体介导的变形机制和弱化机制来阻碍剪切带中的渐进应变调节。多相花岗岩类超糜棱岩和糜棱岩石英脉受系统流体欠饱和条件的不同影响，这是局部剪切过程中不同的主导变形机制和同运动共生的结果。冷却岩体中的局部韧性剪切作用有效地发生在有限的温度范围内（420 - 460 ° C），其中剪切带的应变调节能力由冷却速率、变质反应和变形机制的动力学以及有限量的可用流体的消耗之间的负反馈控制。
Temperature and fluid content are critical parameters that control rock rheology and strain localization in the continental crust. Here, we determine by thermodynamic modelling the T‐MH2O of localized ductile shearing during cooling of three different granitoid plutons: the Rieserferner and the Adamello plutons in the Italian Alps, and the Lake Edison pluton in the Sierra Nevada—USA. Shear zones exploited precursor joints, associated veins and alteration zones. T‐MH2O and P−T phase diagram sections were computed with Perple_X in the system MnO−Na2O−CaO −K2O−FeO−MgO−Al2O3−SiO2−H2O−Fe2O3. The phase diagram sections show that the nucleation of the brittle precursors (joints, veins) occurred at T» 450°C at fluid‐saturated conditions. Localized ductile shearing likely occurred at temperature ranging between 420 and 460°C evolving from initially fluid‐saturated to fluid‐undersaturated conditions in a closed system. In this temperature range, granitoid rocks are potentially subject to a series of retrograde metamorphic reactions replacing the load‐bearing feldspars with weaker phyllosilicates. Metamorphic reactions occurred in spatial association with the precursory structures, leading to localized shearing. Decreasing temperature and fluid‐undersaturated conditions likely hampered progressive strain accommodation in shear zones by slowing down metamorphic reactions, thermally activated dislocation creep processes, fluid‐mediated deformation mechanisms and weakening mechanisms. Polyphase granitoid ultramylonite and mylonitic quartz veins have been affected differently by the fluid‐undersaturated conditions of the system, as consequence of different dominant deformation mechanisms and syn‐kinematic paragenesis during localized shearing. Localized ductile shearing in cooling plutons effectively occurs in a limited temperature range (420–460°C) in which the strain accommodation capacity of the shear zone is controlled by the negative feedback between the cooling rate, the kinetics of metamorphic reactions and deformation mechanisms, and the consumption of the limited amount of available fluids.