藏北龙木错双湖缝合带蓝片岩实验与热力学模拟：俯冲带变质条件估计及其对石榴石稳定性的影响,Geological Journal

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藏北龙木错双湖缝合带蓝片岩实验与热力学模拟：俯冲带变质条件估计及其对石榴石稳定性的影响
Geological Journal ( IF 1.4 ) Pub Date : 2021-12-13 , DOI: 10.1002/gj.4337
Neng Gong ₁ , Xiao‐Chao Che ₂ , Guo‐Li Yuan ₁ , Gen‐Hou Wang ₁ , Toshiaki Tsunogae ₃ , Zhi‐Bo Liu ₄

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蓝片岩的压力-温度（PT）路径通常用于理解俯冲洋壳的演化。石榴石在计算变质条件中起着关键作用，但有时在蓝片岩中不存在。然而，高 P 实验模拟可以作为约束 PT 条件的有用工具。在藏北龙木错-双湖缝合带中段，发现了嵌在大理岩中的蓝片岩块体，岩心富含石榴石，而接触大理岩的边缘则不存在。这两种蓝片岩含有相似的矿物组合（青光石、绿帘石和白云石），并显示出相当的玄武岩特征。拟截面模型应用于计算 ca 峰值变质条件的含石榴石蓝片岩。1.9 GPa 和 530°C。至于没有石榴石的蓝片岩，在设计的 93 wt% 玄武岩和 7 wt% 方解石的混合物上进行了实验（1.2-2.3 GPa 和 500-800°C）。结果表明，绿光石和重晶石中的总Fe含量与压力呈正相关，与温度无关，可以作为一个气压计。因此，峰值条件被限制在 2.3 GPa 和 500–600°C。因此，蓝片岩等效的变质条件表明古特提斯洋壳俯冲到至少70公里。此外，实验和 TX（CaCO 结果表明，绿光石和重晶石中的总Fe含量与压力呈正相关，与温度无关，可以作为一个气压计。因此，峰值条件被限制在 2.3 GPa 和 500–600°C。因此，蓝片岩等效的变质条件表明古特提斯洋壳俯冲到至少70公里。此外，实验和 TX（CaCO 结果表明，绿光石和重晶石中的总Fe含量与压力呈正相关，与温度无关，可以作为一个气压计。因此，峰值条件被限制在 2.3 GPa 和 500–600°C。因此，蓝片岩等效的变质条件表明古特提斯洋壳俯冲到至少70公里。此外，实验和 TX（CaCO₃ ) 基于玄武岩岩性构造的假断面支持蓝片岩中石榴石的缺失主要是由方解石的混合造成的。综上所述，本研究为通过实验模拟探索变质条件提供了有效尝试，并评估了俯冲带玄武质地壳与上覆含碳酸盐沉积物相互作用的深远影响。

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Experiments and thermodynamic modelling on the blueschists in the Longmu Co-Shuanghu Suture Zone, North Tibet: Estimation of the metamorphic conditions and implications for garnet stability in the subduction zone

The pressure–temperature (P-T) path of blueschist is usually applied to understanding the evolution of the subducted oceanic crust. Garnet plays a key role in calculating the metamorphic conditions, but sometimes it is absent in the blueschists. Nevertheless, the high-P experimental simulation may serve as a useful tool to constrain the P-T conditions. In the middle of the Longmu Co-Shuanghu Suture, North Tibet, blueschist blocks embedded in the marble were discovered, where garnet is abundant in the core but absent in the marble-contacting margin. These two kinds of blueschist contain similar mineral assemblage (glaucophane, epidote, and phengite) and show comparable basaltic character. Pseudosection modelling was applied to the garnet-bearing blueschist that calculated the peak metamorphic condition of ca. 1.9 GPa and 530°C. As for the garnet-absent blueschist, experiments (1.2–2.3 GPa and 500–800°C) were performed on the designed mixture of 93 wt% basalt and 7 wt% calcite. The results show that the total Fe content in the glaucophane and barroisite is positively correlated with the pressure but not with temperature, which could be regarded as a geobarometer. Accordingly, the peak condition was constrained at 2.3 GPa and 500–600°C. Therefore, the equivalent metamorphic conditions of the blueschists indicate that the Palaeo-Tethys oceanic crust subducted into at least 70 km. Furthermore, both the experiments and the T-X (CaCO₃) pseudosection, constructed based on basaltic lithology, support that the absence of garnet in the blueschist is predominantly caused by the mixing of calcite. In summary, this study provides a valid attempt in exploring metamorphic conditions by experiment simulation, and evaluates the profound influence of the interaction between the basaltic crust and overlying carbonate-bearing sediments in the subduction zone.

更新日期：2021-12-13

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