Evolution of the blueschist and greenschist facies rocks of Sifnos, Cyclades, Greece
Evolution of the blueschist and greenschist facies rocks of Sifnos, Cyclades, Greece
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希腊基克拉泽斯群岛锡夫诺斯岛蓝片岩和绿片岩相岩石的演化
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发表时间：
1984
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通讯作者：
M. Schliestedt
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作者：
A. Matthews;M. Schliestedt
西夫诺斯岛的变质作用以始新世发育一致的高压蓝片岩地体和早中新世绿片岩相叠置为特征。这项研究记录了仍然保存在岛上北部的蓝片岩组合的变质演化，以及随后在中部和南部转变为绿片岩的过程。氧同位素地质测温是基于对石英、辉石和磁铁矿的标定(Matthews等人)。白云母(ΔQZ−Mu=1.55×106t−2)、绿帘石(δQZ−Ep=1.56+1.92ΔPS)106t−2)和金红石(ΔQZ−Ru=4.54×106t−2)等矿物的氧同位素分析得到了一致的分馏，与岩石类型无关。平均温度为455摄氏度，尽管从不同的地质温度计得出的温度散布表明，平衡发生在略微变化的物理化学条件下。矿物和全岩分析表明，在共同变质流体存在的情况下，没有发生普遍的平衡作用。相对于蓝片岩组合，西夫诺斯中部绿片岩的矿物和全岩系统地富含~(18)O。化学数据表明，绿片岩叠置伴随着钙离子和二氧化碳的交代富集。岩石学、同位素和化学证据表明，变质作用是由富含18O-CO2的水溶液渗入控制的。有理由认为这与在整个旋回中观察到的中新世岩浆活动有关。分离蓝片岩和绿片岩单元的大理岩可能是流体渗入蓝片岩的屏障，对蓝片岩的保存起到了重要作用，蓝片岩变质作用的压力估计为14±2kbar，对应深度约50公里。Sifnos的构造样式和地层学暗示了大陆边缘序列的俯冲。很明显，相当大的构造凹陷可能与大陆碰撞和逆冲有关。
The metamorphism on the island of Sifnos is characterized by the Eocene development of a coherent highpressure blueschist terrane and an early Miocene greenschist facies overprint. This study documents the metamorphic evolution of the blueschist assemblages, still preserved in the northern parts of the island, and their subsequent transformation into greenschists in the central and southern parts.The oxygen isotope geothermometry is based on calibrations for quartz, pyroxenes and magnetite (Matthews et al. 1983a) augmented by revised calibrations for the minerals muscovite (ΔQz−Mu=1.55×106T−2), epidote (δQz−Ep= 1.56+1.92Δps)106T−2), and rutile (ΔQz−Ru=4.54×106T−2).Oxygen isotope analyses of minerals from the Blueschist unit of northern Sifnos give consistent fractionations which are independent of rock type. An average temperature of 455° C was obtained, although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions. Analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred.The minerals and whole rocks of the greenschists of central Sifnos are systematically enriched in 18O relative to the blueschist assemblages. Chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of Ca2+ and CO2. The petrologic, isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18O-CO2 enriched aqueous solutions. It is reasonable to assume that this is connected with the Miocene magmatic activity observed throughout the Cyclades. The marbles separating the Blueschist from the Greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation.The pressure of the blueschist metamorphism is estimated at 14±2 kbar, corresponding to a depth of ca. 50 km. The structural style and stratigraphy of Sifnos are suggestive of the subduction of a continental margin sequence. It is clear that the considerable tectonic depression may be associated with continental collision and underthrusting.