Ductile‐brittle transition in pre‐Alpine amphibolite facies mylonites during evolution from water‐present to water‐deficient conditions (Mont Mary nappe, Italian Western Alps)
Ductile‐brittle transition in pre‐Alpine amphibolite facies mylonites during evolution from water‐present to water‐deficient conditions (Mont Mary nappe, Italian Western Alps)
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从有水条件到缺水条件演化过程中前高山角闪岩相糜棱岩的韧性-脆性转变（意大利西阿尔卑斯山玛丽山推覆体）
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
B. Cesare
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作者：
G. Pennacchioni;B. Cesare
在奥阿尔卑斯玛丽山推覆体（意大利西阿尔卑斯山脉）中，由前阿尔卑斯时代的粗粒上角闪岩相变长岩发育而成的糜棱岩-超长岩离散带。同糜伦岩矿物组合为石英-黑云母-白云母-斜长石-石榴石-硅石-钛铁矿-石墨，通过Grt1+Kfs+H2O=Bt2+Ilm2+Qtz+Ms±Sil的模式水化反应形成。晶粒尺寸减小约3个数量级的同时，除了硅线石外，所有矿物都发生了广泛的再结晶，石榴石和黑云母的成分也发生了变化。变形发生在510-580°C的低压条件下（0.25-0.45 GPa），对应于前阿尔卑斯变质演化的最新阶段。前高寒期糜棱岩化条件接近于脆性-韧性转变，这主要表现在同糜棱岩化生成的伪石质和石英粒度压力测量推断出的高差应力。在相对较高温度下的脆性-延展性行为，以及石英集合体中缺乏退火织构，表明在糜棱化过程中存在缺水条件。这些都是通过同步运动水化反应和吸附在动态再结晶引起的大大增加的晶界面积上的水的逐步消耗来完成的。
In the Austroalpine Mont Mary nappe (Italian Western Alps) discrete zones of mylonites–ultramylonites developed from coarse‐grained, upper amphibolite facies metapelites of pre‐Alpine age. The syn–mylonitic mineral assemblage is quartz–biotite–muscovite–plagioclase–garnet–sillimanite–ilmenite–graphite, and formed via the model hydration reaction: Grt1+Kfs+H2O=Bt2+Ilm2+Qtz+Ms± Sil .Grain‐size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite, and by compositional changes of garnet and biotite. Deformation took place at temperatures of 510–580 °C under low‐pressure conditions (0.25–0.45 GPa) and corresponds to the latest stages of pre‐Alpine metamorphic evolution. The pre‐Alpine mylonitization conditions were close to the brittle‐ductile transition, as indicated by syn–mylonitic generation of pseudotachylytes and high differential stress inferred from quartz grain‐size piezometry. The brittle‐ductile behaviour at a relatively high temperature, and the absence of annealing textures in quartz aggregates, are suggestive of water‐deficient conditions during mylonitization. These were accomplished through progressive consumption of water by syn–kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization.