Variscan relicts in Alpine high‐P pelitic rocks from Samos (Greece): evidence from multi‐stage garnet and its included minerals
Variscan relicts in Alpine high‐P pelitic rocks from Samos (Greece): evidence from multi‐stage garnet and its included minerals
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萨摩斯岛（希腊）高山高磷泥质岩中的瓦力斯遗迹：来自多阶段石榴石及其所含矿物的证据
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发表时间：
2007
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通讯作者：
D. Rhede
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作者：
A. Feenstra;K. Petrakakis;D. Rhede
萨摩斯岛北方的变斑石榴石片岩在其基质中含有富钙石榴石+多硅白云母+钠云母±绿帘石组合，该组合在第三纪早期变质作用期间在2530°C和219 kbar下平衡。 这些高压/低温（HP-LT）变质岩石也表现出更古老，更高温度变质作用的矿物学和显微结构证据。显示生长分区的大型厘米级富铁石榴石发育出不连续的、厚度<0.5 mm、富Ca和贫Mn的过度生长，其成分与小型（<1 mm）高P基质石榴石相匹配。 由于不连续的石榴石边缘与阿尔卑斯山高磷矿物处于结构和化学平衡，因此发现石榴石变斑晶的中心部分在第三纪变质之前形成。这得到独居石包裹体电子探针U-Th-Pb测年的支持，部分重置华力西年龄在360和160 Ma之间。 独居石-磷钇矿和石榴石-白云母测温法应用于前阿尔卑斯山石榴石中的包裹体，得到的温度为600-625 °C（3-8 kbar）。 棱柱状富铝假晶（可能在蓝晶石/硅线石之后）和石榴石中由白色钾钠云母+石英±钠长石±钾长石组成的包裹体（被解释为中间钾钠长石的可能替代物）进一步支持华力西斜长角闪岩相条件。 因此，萨摩斯变质岩在华力西期经历了比阿尔卑斯变质作用更高的温度。前阿尔卑斯山石榴石和阿尔卑斯山石榴石之间的扩散弛豫非常有限;两者都充满了阿尔卑斯山石榴石沿着过度生长和裂缝。在阿尔卑斯俯冲带变质作用期间，流体介导的粒间元素迁移，通过变形增强，在将华力西石榴石转化为钙铝榴石更丰富的组合物方面显得至关重要。在这样的条件下，溶解-再沉淀似乎是一个更有效的机制，用于修改石榴石组合物比扩散。角闪岩相条件是典型的华力西基底遗迹暴露在基克拉迪群岛和多德卡尼斯群岛以及克里特岛东部。所研究的萨摩斯变质岩包括这些基底产状的东北延伸。
Porphyroblastic garnet schists from northern Samos contain in their matrix the assemblage Ca‐rich garnet + phengite + paragonite ± chloritoid equilibrated at ∼530 °C and ∼19 kbar during early Tertiary metamorphism. These high‐pressure/low‐temperature (HP‐LT) metapelitic rocks also exhibit mineralogical and microstructural evidence of an older, higher temperature metamorphism. Large, centimetre‐sized Fe‐rich garnet showing growth zoning developed discontinuous, <0.5 mm thick, Ca‐rich and Mn‐poor overgrowths, compositionally matching small (<1 mm) high‐P matrix garnet. Because the discontinuous garnet rims are in textural and chemical equilibrium with Alpine high‐P minerals, the central parts of the garnet porphyroblasts were found to have formed prior to the Tertiary metamorphism. This is supported by electron microprobe U‐Th‐Pb dating of monazite inclusions yielding partly reset Variscan ages between 360 and 160 Ma. Monazite‐xenotime and garnet‐muscovite thermometry applied to inclusions in the pre‐Alpine garnet yielded temperatures of 600–625 °C (at 3–8 kbar). Prismatic Al‐rich pseudomorphs, possibly after kyanite/sillimanite, and inclusions in garnet composed of white K‐Na mica + quartz ± albite ± K feldspar, interpreted as possible replacements of an intermediate K‐Na feldspar, further support Variscan amphibolite facies conditions. The Samos metapelites thus experienced higher temperatures during the Variscan than during Alpine metamorphism. Diffusional relaxation was very limited between pre‐Alpine garnet and Alpine garnet; both were filled with Alpine garnet along overgrowths and fractures. Fluid‐mediated intergranular element transport, enhanced by deformation, appears crucial in transforming the Variscan garnet into a grossular richer composition during Alpine subduction‐zone metamorphism. At such conditions, dissolution–reprecipitation appears to be a much more effective mechanism for modifying garnet compositions than diffusion. Amphibolite facies conditions are typical for Variscan basement relics exposed in central Cycladic and Dodecanese islands as well as in eastern Crete. The Samos metapelites studied comprise a north‐eastern extension of these basement occurrences.