Effect of Temperature on Fe-Mg Partition of Garnet during the High Pressure and High Temperature Metamorphism of Pelitic Rock
doi: 10.11858/gywlxb.2015.05.002
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摘要: 在700~780 ℃、2.1~2.9 GPa温压范围内, 对天然泥质岩体系进行了高温高压实验, 对体系中生成的石榴子石的Fe-Mg组成受温度的影响进行了研究。实验产物中石榴子石主要以MgO和FeO为主要组成。石榴子石中镁铝榴石和铁铝榴石含量高于88%(质量分数), 钙铝榴石和锰铝榴石含量低于12%。石榴子石中FeO和MgO含量与温度呈现明显线性关系, 但与压力没有呈现线性关系。研究结果确定了具有复杂化学组成的天然泥质岩体系中石榴子石Fe-Mg组成特征随温度的变化趋势, 为确定泥质岩原岩组成的榴辉岩相岩石的变质温度提供了实验研究结果。Abstract: The effects of temperature on Fe-Mg compositions of garnet in a natural pelitic system were studied experimentally at temperatures from 700 to 780 ℃ and pressures from 2.1 to 2.9 GPa.The garnets formed in the run products were mainly characterized by magnesian and ferreous compositions.The contents of pyrope and almandine are more than 88%(mass fraction) in all the garnet particles analyzed, whereas grossular and spessartine are less than 12%.The concentrations of FeO and MgO of garnet in the run products show significant linear relationship with experimental temperatures but which is not observed with pressures.The results determine the Fe-Mg partition trend with temperature in a natural pelitic rock with complex chemical compositions, which is useful for the identification of metamorphic temperature for eclogite facies rocks with a pelitic protolith composition.
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Key words:
- Fe-Mg partition /
- garnet /
- pelite /
- geothermometer
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Table 1. Phases in the starting material and high pressure experimental run products
Sample No. p/(GPa) T/(℃) t/(h) Minerals (Microprobe & XRD) LS26 Starting material Qz, Chl, Ms, Ilm, Ab G17 2.1 720 351 Qz, Grt, Amp, Ms, Rt G12 2.4 710 351 Qz, Grt, Amp, Jd, Ms G4 2.8 750 301 Qz, Grt, Jd, Ms, Ky, Rt G19 2.9 700 356 Qz, Grt, Jd, Ms, Rt, St G16 2.9 780 382 Coe, Grt, Ms, Ky, Rt, Melt Table 2. Selected electron microprobe analyses of garnet in high pressure experimental run products
Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O Total Grs/(%) Pyp/(%) Alm/(%) Sps/(%) G17 36.45 0.74 20.29 35.44 1.88 2.53 2.64 0.12 100.10 8 10 78 4 36.86 0.59 21.25 35.11 0.53 4.08 1.50 0.15 100.07 4 16 78 1 36.79 0.40 21.03 35.40 0.61 4.02 1.37 0.24 99.86 4 16 79 1 37.05 0.58 20.86 35.36 0.47 3.96 1.53 0.23 100.03 4 16 79 1 36.78 0.99 20.79 35.47 0.91 2.96 2.36 0.14 100.40 7 12 79 2 G12 36.65 0.39 21.30 34.80 1.03 3.17 2.28 0.18 99.80 7 13 78 2 36.50 0.91 21.08 35.13 1.48 3.06 1.97 0.17 100.30 6 12 78 3 36.85 1.31 21.09 35.52 0.84 2.83 2.15 0.21 100.80 6 11 80 2 36.44 0.76 21.26 34.96 1.44 3.00 2.15 0.19 100.20 6 12 78 3 36.59 0.39 21.04 35.03 1.64 2.88 2.03 0.17 99.77 6 11 78 4 G4 36.75 0.61 21.04 33.25 0.64 5.23 0.93 0.29 98.74 3 21 74 1 36.68 1.11 20.84 33.05 0.66 5.31 0.93 0.31 98.89 3 21 73 1 36.62 0.59 20.84 33.54 0.67 5.37 1.06 0.23 98.92 3 21 73 2 40.33 0.50 21.47 30.99 0.68 6.35 1.01 0.39 101.71 3 25 69 2 37.17 0.47 21.18 32.57 0.73 6.18 0.90 0.26 99.46 3 24 70 2 G19 36.24 0.61 20.63 36.52 1.35 2.64 1.28 0.22 99.49 4 11 81 3 36.90 0.50 20.90 35.10 0.76 4.02 1.14 0.16 99.47 3 16 78 2 36.19 0.59 20.48 36.72 1.44 2.58 1.30 0.11 99.42 4 10 82 3 35.87 0.59 20.60 36.34 1.41 2.66 1.30 0.14 98.91 4 11 82 3 36.61 0.50 20.75 36.29 0.77 3.61 1.04 0.08 99.65 3 14 80 2 G16 35.90 0.92 20.58 32.38 1.62 3.96 2.71 0.09 98.18 8 16 72 4 36.15 0.77 20.87 32.44 0.81 6.07 1.31 0.27 98.68 4 24 69 2 40.81 0.57 20.22 30.38 0.50 7.00 0.85 1.50 101.83 2 27 62 1 40.63 0.30 19.57 27.09 0.17 7.35 0.54 0.49 96.14 2 31 64 0 37.91 0.56 20.63 31.14 0.51 7.33 0.99 0.24 99.31 3 29 66 1 -
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