Rb–Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity
Rb–Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity
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蒂诺斯岛（希腊基克拉泽斯）的铷-锶同位素研究：变质作用的额外时间限制、渗透控制套印和变形活动的程度
DOI：
10.1017/s0016756898008681
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发表时间：
1998
影响因子：
2.3
通讯作者：
L. Franz
中科院分区：
文献类型：
--
作者：
M. Bröcker;L. Franz
本研究提供了新的Rb-Sr年龄数据有关的变质演化的Attic-Cycladic结晶带，代表一个复杂的多变质火山岩内的阿尔卑斯造山带的Hellenides。可以区分两大类构造单元。上部单元部分的变质作用通常被认为是白垩纪事件。与此相反，该组较低的单位经历了第三纪高压变质，其次是中压叠加。我们专注于蒂诺斯岛，在基克拉泽斯群岛中发现的岩石单元的代表性光谱暴露在三个构造单元中：上部单元，中间单元和基底单元。大量的岩石学和构造学研究记录了影响阿提卡-基克拉迪结晶带的构造变质和岩浆事件的完整范围。蛇绿岩上部单元的千屈岩和千屈岩的Rb-Sr表观年龄（多硅白云母-全岩）在c. 92和21马。较老的年龄与基克拉泽斯其他地方的上单元岩石的白垩纪日期不同。建议研究的序列属于侏罗纪蛇绿岩的Hellenides，而不是白垩纪的发生。传播到更年轻的年龄是有关的非普遍的复兴和重置的Rb-Sr系统在构造并列的上部单元的中间单元。最年轻的年龄，到目前为止，从上单元（21马）的样品被认为是近似的构造并列的时间，可能发生在区域绿片岩相插曲生产普遍的叠印在结构较低的构造单元。上单元的主要千枚岩/变辉长岩/蛇纹岩序列被解释为一个侵位相关的韧性剪切带，经历了在脆性条件下的改造。在中间单元，Rb-Sr年龄梯度从顶部（c. 40 Ma）到底部（C。22 Ma），这被解释为代表了该单元下部流体渗透和叠加的更大影响，可能受变形强度的变化控制，这可能与构造并置到基底单元上有关。我们认为，同变质叠加的所有三个构造单元发生在渐新世-中新世绿片岩事件。在构造叠加和主体花岗岩侵入后，伸展变形仍在继续，如来自主体未变形岩体边缘部分的韧性变形含石榴子石的浅色花岗岩的Rb-Sr全岩等时线（15.1 ~ 0.6 Ma）所示。Rb-Sr测年技术的应用没有提供明确的证据表明，以前出版的始新世K-Ar和40 Ar-39 Ar年龄的高压多硅白云母从较低的单位显着污染过量氩。
This study presents new Rb–Sr age data concerning the metamorphic evolution of the Attic-Cycladic Crystalline Belt which represents a complex polymetamorphic terrane within the Alpidic orogenic belt of the Hellenides. Two major groups of tectonic units can be distinguished. Metamorphism in parts of the upper units is commonly considered as a Cretaceous event. In contrast, the group of lower units experienced Tertiary high-pressure metamorphism which was followed by a medium-pressure overprint. We focus on the island of Tinos where a representative spectrum of the rock units found in the Cyclades is exposed in three tectonic units: the Upper Unit, the Intermediate Unit and the Basal Unit. The complete range of tectono-metamorphic and magmatic events affecting the Attic-Cycladic Crystalline Belt is documented by numerous petrological and tectonic studies. Phyllites and phyllonites from the ophiolitic Upper Unit yielded Rb–Sr apparent ages (phengite–whole-rock) between c. 92 and 21 Ma. The older age differs from the Cretaceous dates reported for upper unit rocks elsewhere in the Cyclades. It is suggested that the sequence studied belongs to the Jurassic ophiolites of the Hellenides rather than to Cretaceous occurrences. The spread to younger ages is related to non-pervasive rejuvenation and resetting of the Rb–Sr system during tectonic juxtaposition of the Upper Unit over the Intermediate Unit. The youngest age obtained so far for a sample from the Upper Unit (21 Ma) is believed to approximate the timing of tectonic juxtaposition which probably occurred during a regional greenschist-facies episode producing a pervasive overprint in the structurally lower tectonic unit. The major phyllite/meta-gabbro/serpentinite sequence of the Upper Unit is interpreted as an emplacement-related ductile shear zone which experienced reworking under brittle conditions. In the Intermediate Unit, a gradient in Rb–Sr ages from top (c. 40 Ma) to the bottom (c. 22 Ma) was recognized, which is interpreted to represent greater effects of fluid infiltration and overprinting in the lower parts of this unit, possibly controlled by variable intensity of deformation which might be related to tectonic juxtaposition onto the Basal Unit. We suggest that synmetamorphic stacking of all three tectonic units took place during an Oligocene–Miocene greenschist event. Extensional deformation continued after tectonic stacking and after intrusion of the main granite, as is indicated by a Rb–Sr whole-rock isochron (15.1∓0.6 Ma) for a ductilely deformed garnet-bearing leucogranite from the marginal parts of the main undeformed pluton. Application of the Rb–Sr dating technique provided no unequivocal evidence that previously published Eocene K–Ar and 40Ar–39Ar dates for high-pressure phengites from the lower units are significantly contaminated with excess argon.