The geodynamic evolution of the Alpine orogen in the Cyclades (Aegean Sea, Greece): insights from diverse origins and modes of emplacement of ultramafic rocks
The geodynamic evolution of the Alpine orogen in the Cyclades (Aegean Sea, Greece): insights from diverse origins and modes of emplacement of ultramafic rocks
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基克拉泽斯群岛（希腊爱琴海）阿尔卑斯造山带的地球动力学演化：来自超镁铁质岩石不同起源和侵位模式的见解
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发表时间：
2007
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通讯作者：
A. Matthews
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作者：
Y. Katzir;Z. Garfunkel;D. Avigad;A. Matthews
基克拉泽斯阿尔卑斯造山带是研究地幔岩侵位的重要地区，高压变质岩和超镁铁岩共存。在基克拉迪群岛内，有三种不同的超镁铁质组合：（1）Evia和Syros上基克拉迪蓝片岩单元（CBU）的HP-LT蛇绿混杂岩;（2）Naxos上与混合岩化的浅色片麻岩有关的变橄榄岩，这代表了CBU最深的暴露水平;（3）Tinos上通过伸展拆离并置在CBU顶部的绿片岩相变质肢解蛇绿岩。大多数基克拉迪超镁铁岩在阿尔卑斯变质作用之前被蛇纹化，这表明在重新埋藏之前发生了剥蚀。然而，纳克索斯变质橄榄岩保留了残留的地幔组合和类似地幔的氧同位素比值，因此表明在碰撞和高压变质作用（M1）期间，从地幔直接侵位到俯冲大陆。因此，Naxos白纹片麻岩核中M1的条件受到超镁铁组合的限制，为550-650 °C和≥14 kbar。西北基克拉迪群岛蛇绿混杂岩的基性岩块具有广泛的化学成分，表明其来源于多变的海洋环境和连续的蚀变和交代作用。由于锡罗斯和依云混杂岩段的地球化学异质性相当，锡罗斯混杂岩中含石榴石的变质玄武岩记录的M1温度（450-500 °C）高于依云混杂岩中不含石榴石的绿帘石蓝片岩（400-430 °C）。因此，从埃维亚向东南逐渐加深（即更热）的俯冲板块暴露出来。相应地，M2叠印的温度也增加，从南埃维亚的pumpellyite轴承组合，通过锡罗斯的绿片岩上角闪岩，硅线石的片麻岩纳克索斯。CBU的不同P-T路径形成一个阵列，其中岩石序列埋藏越深，其折返路径就越“热”。这种模式是通过侵蚀或均匀伸展导致的构造增厚地壳的热模拟来预测的。
Abstract The Alpine orogen in the Cyclades, wherein both high-pressure metamorphic rocks and ultramafic rocks co-occur, is a key area in studying the emplacement of mantle rocks into the crust. Within the Cyclades three distinct ultramafic associations occur: (1) HP–LT ophiolitic mélanges of the Cycladic Blueschist Unit (CBU) on Evia and Syros; (2) meta-peridotites associated with migmatized leucogneisses on Naxos, which represent the deepest exposed levels of the CBU; (3) a greenschist-facies metamorphosed dismembered ophiolite juxtaposed on top of the CBU by an extensional detachment on Tinos. Most of the Cycladic ultramafic rocks were serpentinized prior to Alpine metamorphism, suggesting denudation prior to reburial. The Naxos meta-peridotites preserve, however, relict mantle assemblage and mantle-like oxygen isotope ratios, and thus indicate direct emplacement from the mantle into an underthrust continent during collision and HP metamorphism (M1). Thus conditions for M1 in the Naxos leucogneiss core are constrained by ultramafic assemblages to 550–650 °C and ≥14 kbar. Mafic blocks of the ophiolitic mélanges in the NW Cyclades span a wide range of chemical compositions indicating derivation from variable oceanic settings and sequential events of alteration and metasomatism. Given the comparable geochemical heterogeneity in the Syros and Evian mélange intervals, the garnet-bearing meta-basites of the Syros mélange record higher M1 temperatures (450–500 °C) than the garnet-free epidote blueschists of the Evian mélanges (400–430 °C). It follows that going southeastwards from Evia progressively deeper (i.e. hotter) levels of the subducted plate are exposed. Correspondingly, temperatures of the M2 overprint also increase from pumpellyite-bearing assemblages on southern Evia, through greenschists on Syros to upper-amphibolite, sillimanite-bearing gneisses on Naxos. The diverse P–T paths of the CBU form an array wherein the deeper a rock sequence is buried, the ‘hotter’ is its exhumation path. Such a pattern is predicted by thermal modelling of tectonically thickened crust unroofed by either erosion or uniform extension.