Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece
Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece
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DOI：
10.5194/se-12-1335-2021
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发表时间：
2020-09
期刊：
影响因子：
3.4
通讯作者：
Miguel Cisneros;Jaime D. Barnes;W. Behr;A. Kotowski;D. Stockli;K. Soukis
中科院分区：
文献类型：
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作者：
Miguel Cisneros;Jaime D. Barnes;W. Behr;A. Kotowski;D. Stockli;K. Soukis
摘要。逆行变质岩为了解挖掘出的物质的压力-温度（P-T）演化提供了关键的信息，由此产生的P-T约束对俯冲界面的力学和热条件有直接的影响。然而，限制逆行变质岩的P-T条件在历史上一直具有挑战性，并导致了对这些岩石所经历的条件的争论。在这项工作中，我们结合弹性热气压法和氧同位素测温法，量化了暴露在希腊Syros的基克拉迪蓝片岩单元（CBU）的逆行变质岩的P-T演化。我们分别采用石榴石中的石英和绿帘石中的石英气压测定法来限制石榴石和绿帘石在俯冲峰值附近和挖掘过程中的生长压力。用石英和方解石的氧同位素测温法来估计挖掘过程中的温度，并改进压力估计。三个不同的压力组与不同的变质事件和织物有关：500-550°C之间高压石榴石生长在~ 1.4-1.7 GPa， 400 - 500°C之间逆行绿帘石生长在~ 1.3-1.5 GPa，以及第二阶段逆行绿帘石生长在~ 1.0 GPa和400°C之间。这些结果与从野外和微观构造观测推断出的不同变形阶段一致，记录了蓝片岩-榴辉岩相的递进俯冲和蓝片岩-绿片岩相条件下的退变。我们的新结果表明，在达到最大高压-低温条件后，CBU在减压过程中经历了冷却。这些P-T条件和构造观测结果与中新世岩心杂岩形成之前靠近冷却俯冲板块的俯冲通道内的挖掘和冷却一致。该研究还说明了将弹性热气压法与结构和微观结构约束相结合，以更好地了解高压-低温（HP/LT）变质地体中逆行矿物生长的p -t变形条件的潜力。
Abstract. Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes.