Trace element characteristics of clinozoisite pseudomorphs after lawsonite in talc-garnet-chloritoid schists from the Makbal UHP Complex, northern Kyrgyz Tian-Shan
Trace element characteristics of clinozoisite pseudomorphs after lawsonite in talc-garnet-chloritoid schists from the Makbal UHP Complex, northern Kyrgyz Tian-Shan
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DOI：
10.1016/j.lithos.2014.10.008
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发表时间：
2015-06
期刊：
影响因子：
3.5
通讯作者：
R. Orozbaev;T. Hirajima;A. Bakirov;A. Takasu;K. Maki;Kenta Yoshida;K. Sakiev;A. Bakirov;T. Hirata;M. Tagiri;A. Togonbaeva
中科院分区：
文献类型：
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作者：
R. Orozbaev;T. Hirajima;A. Bakirov;A. Takasu;K. Maki;Kenta Yoshida;K. Sakiev;A. Bakirov;T. Hirata;M. Tagiri;A. Togonbaeva
在吉尔吉斯斯坦天山北方马克巴尔超高压杂岩中的石榴石和滑石-石榴石-绿帘石片岩基质中发现了由斜锆石+蓝晶石+石英±钠长石±多硅白云母组成的多相矿物集合体。这些矿物的纹理被解释为假晶后，硬毛钠石，我们重建的组成PMAs的斜生石+蓝晶石+石英，与硬毛钠石一致。岩石学研究表明，硬铝石在超高压峰期（P = 28-33 kbar，T = 530-580 °C）是稳定的，在P = 16-20 kbar，T = 510-580 °C的等温减压过程中，硬铝石分解为PMAs。微量元素特征的斜生石颗粒的PMA（前硬硅钙石）显示出一个平面的稀土元素（REE）的硬硅钙石标准化模式，与典型的报告的稀土模式的硬硅钙石，虽然丰富的稀土元素不同的样品。因此，石榴子石中的PMAs中的斜生石的稀土元素含量可能是继承自前硬铝石，因为分解反应与基质隔离发生。基质中分散的斜生石颗粒具有高的轻稀土富集在重稀土的辉铜矿标准化模式，符合典型的绿帘石模式。研究结果表明，早古生代（480-509 Ma），Makbal杂岩中的滑石-石榴石-绿帘石片岩埋藏较深（> 100 km），地温梯度较低（< 6 °C/km）。在等温减压阶段，伴随着流体释放的过程中，劳森石分解和斜生石形成反应，这意味着流体不仅可以在俯冲过程中产生，而且可以在超高压岩石在冷俯冲环境中折返过程中产生。
Polyphase mineral aggregates (PMAs) composed of clinozoisite + kyanite + quartz ± chlorite ± paragonite ± phengite have been found within garnet and in the matrix of talc-garnet-chloritoid schists from the Makbal ultrahigh-pressure complex in the northern Kyrgyz Tian-Shan. These mineral textures are interpreted as pseudomorphs after lawsonite, and we reconstructed the compositions of PMAs of clinozoisite + kyanite + quartz, consistent with lawsonite. Petrological study demonstrated that lawsonite was stable during the prograde to the UHP peak stage (P = 28–33 kbar and T = 530–580 °C) and decomposed to the PMAs during isothermal decompression around P = 16–20 kbar and T = 510–580 °C. Trace element characteristics of the clinozoisite grains in the PMAs (former lawsonite) show a flat rare earth element (REE) chondrite-normalized pattern, comparable with the typical reported REE pattern of lawsonite, although the abundance of REE varied from sample to sample. Thus, the REE content of clinozoisite in the PMAs included in garnet was likely inherited from the former lawsonite as the decomposition reaction took place isolated from the matrix. Discrete clinozoisite grains in the matrix have high light REE enrichment over heavy REE in the chondrite-normalized pattern, consistent with the typical epidote pattern. Our results indicate that the talc-garnet-chloritoid schists in the Makbal complex were buried to great depth (> 100 km) with a low geothermal gradient (< 6 °C/km) during the Early Paleozoic (480–509 Ma). Lawsonite decomposition and clinozoisite-forming reactions accompany fluid release during the isothermal decompression stage, implying that the fluids can be generated not only during subduction, but also during exhumation of ultrahigh-pressure rocks in cold subduction settings.