Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps
Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps
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延性剪切过程中的流体窜流：东阿尔卑斯山陶恩窗中花岗闪长岩转变为铝片岩
DOI：
10.1111/j.1525-1314.1991.tb00536.x
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发表时间：
1991
影响因子：
3.4
通讯作者：
J. Staude
中科院分区：
文献类型：
--
作者：
J. Selverstone;G. Morteani;J. Staude
奥地利Tauern窗核部的韧性剪切作用，在阿尔卑斯山造山过程中，使变花岗闪长岩在35-40公里深度内转化为硅不饱和石榴石-绿泥石-十字石片岩。从围岩到剪切带中心已识别出四个不同的带：区I-未蚀变的花岗闪长岩与从属的角闪岩；区II-黑云母-白色云母-石榴石片岩；区III-黑云母-多硅白云母片岩；区IV-无石英的石榴石-绿泥石-十字石片岩，其石榴石直径可达10厘米。全岩分析表明，剪切带中的SiO_2含量从>；65wt%急剧下降到<；35wt%；整个剪切带中的Ca、Na、Sr含量也在降低，而Al、Ti、Fe、Mg、P、Cr、Ni、Zn和Rb在剪切带中均有所增加。质量平衡计算表明，剪切作用伴随着近剪切带中心60%的体积损失。Tauern窗样品与花岗岩主岩中其他剪切带的对比表明，伴随着镁、铁和钛的增加的二氧化硅损失是体积损失剪切带的典型特征，但与剪切带中表现出的元素行为明显不同，后者被认为代表了大致等体积的行为。在这里研究的样品中，体积损失似乎是剪切过程中沟道化流体流动的结果，在IV区产生了±108cm3 cm−2的时间积分流体通量。这种大量的流体可能部分起源于北宾尼期俯冲带始新世运动期间变质闪长岩下携带的复理石的脱水。俯冲过程中形成的反向温度梯度将允许流体在上升和加热过程中从剪切带中清除大量二氧化硅。
Ductile shearing in the core of the Tauern Window, Austria, transformed metagranodiorite into Si-undersaturated garnet-chlorite-staurolite schist at a depth of c. 35–40 km during the Alpine orogeny. Four distinct zones have been recognized extending from the wallrock into the centre of the shear zone: Zone I—unaltered metagranodiorite with subordinate amphibolite; Zone II—biotite-white mica-garnet schist; Zone III—biotite-phengite schist; Zone IV—quartz-absent, garnet-chlorite-staurolite schist with garnets up to 10 cm across. Whole-rock analyses show a dramatic decrease in SiO2 from >65 wt% in Zone I to <35 wt% in Zone IV; Ca, Na, and Sr also decrease across the shear zone, whereas Al, Ti, Fe, Mg, P, Cr, Ni, Zn, and Rb all increase towards Zone IV. Mass-balance calculations indicate that shearing was accompanied by up to 60% volume loss near the centre of the shear zone. Comparison of the Tauern Window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in Mg, Fe, and Ti is typical for volume-loss shear zones, but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour. In the samples studied here, volume loss appears to have resulted from channellized fluid flow during shearing, producing time-integrated fluid fluxes of ± 108 cm3 cm−2 in Zone IV. This large volume of fluid may have originated, in part, from dehydration of flysch carried beneath the metagranodiorites during Eocene movement on the North Penninic subduction zone. Development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating.