Ductile shearing, hydrous fluid channelling and high-pressure metamorphism along the basement-cover contact on Sikinos, Cyclades, Greece

On the island of Sikinos in the Cyclades a schistose carapace separates a marble-blueschist cover sequence from underlying basement rocks. The basement ‘core’, comprising metapelitic gneisses, biotite-bearing granodiorites and aplites, becomes increasingly strained towards the carapace with progressive obliteration of earlier structures and intensification of a mylonitic foliation that becomes pervasive within the carapace. Granodiorites in the ‘core’ can be traced into microcline schists within the carapace, whereas metapelitic gneisses are converted to garnet-mica schists. The carapace is therefore a simple shear zone comprising basement rocks mylonitized during overthrusting of the cover. Biotite clusters in granodiorites of the basement ‘core’ are partially altered to phengite, whereas plagioclase shows incipient sericitization. In more strained rocks these hydration reactions show enhanced progress, until biotite and plagioclase are finally eliminated in the carapace. Rare glaucophane and chloritoid inclusions within garnets of metapelitic gneisses adjacent to the carapace are also attributed to hydration reactions. The association of higher strain with increased hydration in the basement suggests localization of fluids in the strained carapace zone, with limited percolation into underlying rocks. The restricted availability of water outside the carapace may be responsible for preservation of pre-Alpine assemblages in large parts of the Cycladic basement.

在基克拉泽斯群岛（Cyclades）的锡基诺斯岛（Sikinos）上，一片片状壳层（schistose carapace）将蓝片岩盖层序列（marble-blueschist cover sequence）与下伏基底岩石分隔开来。基底‘核部’由副变质片麻岩（metapelitic gneisses）、含黑云母花岗闪长岩（biotite-bearing granodiorites）以及细晶岩（aplites）组成，其变形程度向壳层方向逐渐增强，早期构造被逐步改造殆尽，同时糜棱面理（mylonitic foliation）不断强化，并在壳层内部广泛发育。基底核部的花岗闪长岩可被追索至壳层内的微斜长石片岩（microcline schists）中，而副变质片麻岩则转变为石榴云母片岩（garnet-mica schists）。因此，该壳层实为一条简单剪切带（simple shear zone），由盖层逆冲过程中发生糜棱岩化的基底岩石构成。基底核部花岗闪长岩中的黑云母集合体部分蚀变为多硅白云母（phengite），斜长石则呈现出初始绢云母化现象。在变形程度更强的岩石中，这些水化反应进程进一步加剧，直至壳层内的黑云母与斜长石最终被完全消耗。紧邻壳层的副变质片麻岩的石榴石中，罕见蓝闪石（glaucophane）与硬绿泥石（chloritoid）包裹体，同样可归因于水化反应。基底中变形程度越高、水化作用越强的现象表明，流体集中在变形强烈的壳层带内，仅少量渗透至下伏岩石中。壳层外部有限的水体供给，可能是基克拉泽斯群岛基底大片区域得以保留阿尔卑斯期前组合（pre-Alpine assemblages）的原因。