Lu-Hf geochronology on cm-sized garnets using microsampling: New constraints on garnet growth rates and duration of metamorphism during continental collision (Menderes Massif, Turkey)
Lu-Hf geochronology on cm-sized garnets using microsampling: New constraints on garnet growth rates and duration of metamorphism during continental collision (Menderes Massif, Turkey)
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DOI：
10.1016/j.epsl.2015.09.015
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发表时间：
2015-12
影响因子：
5.3
通讯作者：
A. Schmidt;A. Pourteau;O. Candan;R. Oberhänsli
中科院分区：
文献类型：
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作者：
A. Schmidt;A. Pourteau;O. Candan;R. Oberhänsli
摘要本文研究了土耳其门代雷斯地块南部云母片岩中石榴石环带晶体的Lu-Hf年代学。选定的样品是四个3-5厘米大的石榴石巨晶，其中几个连续的石榴石壳已被取样与微锯和分析的年龄。结果被用来提取石榴石的生长速率，并提高了时间的限制，阿尔卑斯山的年龄叠印的泛非基底在Menderes地块。利用石榴石贫Lu边缘成分的两点石榴石等时线测定了样品石榴石壳的Lu-Hf年龄。这产生了一致的减少年龄信息从核心到边缘段的个别石榴石晶体和计算的等时线年龄提出了一个时间框架之间的增长42.6±1.9和34.8±3.1 Ma。在调查石榴石的主要元素配置文件的特点分区模式指示的变质条件：瑞利分馏钟形锰和减少Fe/（Fe+ Mg）记录的石榴石的核心边缘组成。因此，所获得的Lu-Hf年龄记录了石榴石核心的早期石榴石生长的时间。其中两个大型石榴子石晶体的最内核也测得了58.83±0.69 Ma和50.16±0.84 Ma的等时线年龄，这似乎记录了早期成核事件。然而，这一观点与所观察到的石榴石的主元素分布不一致，因此应谨慎解释。石榴石生长期的终止是通过计算径向生长速率的基础上的石榴石的大小和连续壳获得的Lu-Hf年龄。这些速率的外推可能限制了石榴石生长的总持续时间终止于31±6 Ma。单个晶体计算的生长速率的比较显示了各种缓慢和快速生长的石榴石，和类似的结果已经获得了与Rb-Sr和Sm-Nd同位素系统。新的数据提供了一个精确的年龄测定在南部的Menderes地块，到目前为止，这是放置在63和27 Ma之间的云母Rb-Sr和Ar-Ar定年的基础上Barrovian变质作用。这项研究提供了新的限制至关重要的西南安纳托利亚和爱琴海地区的构造演化的理解，因为它产生了一个较短的轮廓阿尔卑斯年龄大陆碰撞。
Abstract This study shows Lu–Hf geochronology of zoned garnet crystals contained in mica schists from the southern Menderes Massif, Turkey. Selected samples are four 3–5 cm large garnet megacrysts of which several consecutive garnet shells have been sampled with a micro-saw and analyzed for dating. The results are used to extract growth rates of garnet, and also to improve the time constraint for Alpine-aged overprint of the Pan-African basement in the Menderes Massif. Lu–Hf ages of the sampled garnet shells are determined by two-point garnet-only isochrons using the garnets' Lu-depleted rim compositions. This yields a consistent decrease of age information from core to rim segments of individual garnet crystals and the calculated isochron ages propose a time frame of growth between 42.6±1.9 and 34.8±3.1 Ma. Major element profiles in the investigated garnets characterize zoning patterns indicative of prograde conditions: Rayleigh fractionated bell-shaped Mn and decreasing Fe/(Fe+ Mg) are recorded by the garnets' core to rim compositions. Therefore the obtained Lu–Hf ages record timing of early prograde growth for the cores of the garnets. Two of the large garnet crystals also yield isochron ages of 58.83±0.69 and 50.16±0.84 Ma in their innermost cores, which appear to record an early nucleation event. This view, however, is not in concordance with the observed major element profiles of these garnets, and therefore is interpreted with caution. Termination of the garnet growth period is determined through the calculation of radial growth rates based on the size of the garnets and the Lu–Hf ages obtained for consecutive shells. Extrapolation of these rates potentially constrains the total duration for garnet growth terminating at 31±6 Ma. Comparison of the growth rates calculated for individual crystals shows a variety of slow and fast growing garnets, and similar results have been previously obtained with the Rb–Sr and Sm–Nd isotope systems. The new data provides a precise age determination for prograde Barrovian metamorphism in the southern Menderes Massif, which so far was placed between 63 and 27 Ma on the basis of mica Rb–Sr and Ar–Ar dating. This study provides new constraints crucial to the understanding of the tectonic evolution of southwest Anatolia and the Aegean realm, as it yields a shorter outline for Alpine aged continental collision.