Constraining conditions and mechanisms of the early stages of exhumation from within subduction zones is challenging. Although pressure, temperature, and age can be inferred from the exhumed rock record, it is generally difficult to derive each of these parameters from any single rock, thus demanding assumptions that diverse data from multiple samples can be safely combined into a single pressure-temperature-time (P-T-t) path that might then be used to infer tectonic context and mechanisms of exhumation. Here, we present new thermobarometric and geochronologic information preserved in a single sample from Syros, Greece, to deduce the conditions and rates of the earliest phase of exhumation as a part of the well-preserved high-pressure metamorphic rocks of the Cycladic Blueschist Unit (CBU). The sample studied here is a garnet-bearing, quartz-mica schist that records two distinct metamorphic events. Results from thermodynamic models and quartz-in-garnet elastic geobarometry show that metamorphic garnet cores formed as P-T conditions evolved from ∼485°C and 2.2 GPa to 530°C and 2.0 GPa, and that garnet rims formed as conditions evolved from ∼560°C and 2.1 GPa to ∼550°C and 1.6 GPa. Sm-Nd geochronology on garnet cores and rims yields ages of 45.3 ± 1.0 and 40.5 ± 1.9 Ma, respectively, thus indicating a 4.8 ± 2.1 Myr growth span. Given the decompression path calculated based on garnet core and rim P-T estimates, we conclude that the distinct phases of garnet growth preserve evidence of the initial exhumation of portions of the CBU.

中文翻译：

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深入了解希腊锡罗斯基克拉迪蓝片岩单元的早期发掘：分区石榴石年代学、热力学建模和石英弹性气压测量法的组合应用

从俯冲带内部剥落早期阶段的约束条件和机制具有挑战性。虽然压力、温度和年龄可以从挖掘出的岩石记录中推断出来，但通常很难从任何一块岩石中推导出这些参数中的每一个，因此需要假设来自多个样本的不同数据可以安全地组合成一个单一的压力-温度-时间（P - T - t) 路径，然后可以用来推断构造背景和挖掘机制。在这里，我们展示了保存在希腊锡罗斯的单个样本中的新的热气压和地质年代学信息，以推断作为基克拉迪蓝片岩单元保存完好的高压变质岩的一部分的最早挖掘阶段的条件和速率。 CBU）。此处研究的样本是含石榴石的石英云母片岩，记录了两个不同的变质事件。热力学模型和石榴石中石英弹性地质气压测量的结果表明，变质石榴石核形成为P - T条件从 ~485°C 和 2.2 GPa 发展到 530°C 和 2.0 GPa，并且随着条件从 ~560°C 和 2.1 GPa 发展到 ~550°C 和 1.6 GPa，石榴石边缘形成。石榴石核心和边缘的 Sm-Nd 年代学产生的年龄分别为 45.3 ± 1.0 和 40.5 ± 1.9 Ma，因此表明生长跨度为 4.8 ± 2.1 Myr。鉴于根据石榴石核心和边缘P - T估计计算的减压路径，我们得出结论，石榴石生长的不同阶段保留了部分 CBU 初始挖掘的证据。