Magnetism of Granitic Gneiss from Chinese Continental Scientific Drilling Main Hole and Fluid Activities
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摘要: 通过对中国大陆科学钻探工程主孔花岗质片麻岩进行详细的岩石磁学研究及岩石矿物学分析表明: 花岗质片麻岩磁化率(0.570×10-7~120.450×10-7m3·kg-1, 平均29.996×10-7m3·kg-1)在主孔所有岩石中仅次于蛇纹石化石榴石橄榄岩, 而其天然剩余磁化强度(0.002×10-3~2.109×10-3Am2·kg-1, 平均0.210×10-3Am2·kg-1)则是所有岩性中最低的.磁化率随温度变化曲线、交变退磁曲线及磁滞回线特征表明, 花岗质片麻岩中磁性矿物组合主要成分为磁铁矿, 小部分样品中含有赤铁矿, 其中磁铁矿以多畴为主, 伪单畴磁铁矿仅在少量样品中出现.和同为完全退变质岩的角闪岩(完全退变质榴辉岩)相比, 花岗质片麻岩具有相似的磁性矿物组合, 但其磁铁矿的颗粒明显较大.多畴磁铁矿的形成, 可能和超高压变质岩折返过程中, 花岗质片麻岩较强的流体活动相关.部分分布于花岗质片麻岩主体岩性段外的样品, 具有较高的天然剩余磁化强度, 则可能反映了花岗质片麻岩及周围榴辉岩之间的流体交换.Abstract: Detailed magnetic studies and mineralogy analysis show that the granitic gneiss has the second highest (only second to the serpentinized garnet peridotite) low-field susceptibility (χ) (0.570×10-7-120.450×10-7m3·kg-1, average 29.996×10-7m3·kg-1) and the lowest natural remanent magnetization (NRM) (0.002×10-3-2.109×10-3Am2·kg-1, average 0.210×10-3Am2·kg-1). Temperature dependence of magnetic susceptibility, alternating field (AF) demagnetization and magnetic hysteresis properties suggest that the magnetic minerals in granitic gneiss are magnetite ± hematite, the magnetites are mainly multi-domain (MD), pseudo-single domain (PSD) magnetites are also presented in small amounts. The grain size of magnetites are obviously larger than that in the completely retrograded eclogites, which have the same magnetic mineral assemblage and experienced amphibolite facies retrograde metamorphism. The formation of MD magnetites are thought to be related with stronger fluid activities during the retrogression. Samples occurring out of the major gneiss subunit and adjacent to the eclogites, which have related high NRM, may reflect fluid movements between felsic and mafic UHPM rocks.
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Key words:
- granitic gneiss /
- CCSD main hole /
- rock magnetism /
- fluid activity /
- geomagnetism
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图 1 CCSD主孔位置及采样点位(Xu et al., 2009b)
TLF.郯庐断裂;JXF.嘉山-响水断裂;WQYF.武梁-青岛-烟台断裂
Fig. 1. Location map of CCSD main hole with the simplified lithological profile of the depth interval 100-2 000 m and sample location
图 7 正片麻岩Mrs/χ v.s. Bcr交会图(Perters and Dekkers, 2003)
Fig. 7. Mrs/χ vs Bcr plot of granitic gneiss
图 8 片麻岩样品Day氏图(Dunlop, 2002)
Fig. 8. Day-plot of gneiss samples
表 1 花岗质片麻岩样品密度、磁化率及天然剩余磁化强度统计
Table 1. The summary of density, susceptibility and NRM of granitic gneiss
类别 ρ (g·cm-3) χ(10-7 m3·kg-1) NRM(10-3 Am2·kg-1) Q 全体样品 范围 2.581~2.878 0.570~120.450 0.002~2.109 0.100~28.590 全体样品 Mean±SD 2.649±0.039 29.996±32.418 0.210±0.444 2.562±5.548 主体岩性段 范围 2.606~2.878 0.570~120.450 0.003~1.142 0.100~20.310 内样品 Mean±SD 2.645±0.040 22.988±25.478 0.098±0.252 1.573±3.582 主体岩性段 范围 2.581~2.693 0.590~115.240 0.002~2.109 0.150~28.590 外样品 Mean±SD 2.653±0.030 49.682±40.581 0.504±0.663 5.197±8.452 表 2 代表性样品磁滞回线参数
Table 2. Magnetic hysteresis parameters of selected samples
编号 深度(m) Mrs(10-3 Am2·kg-1) Ms(10-3 Am2·kg-1) Bc(mT) Bcr(mT) Mrs/χ(kAm-1) Mrs/Ms Bcr/Bc 187 1 023.30 9.256 4 101.900 0.402 10.3 0.738 0.002 25.622 204 1 103.10 2.431 302.732 1.080 14.3 2.524 0.008 13.241 207 1 113.85 3.820 1 326.527 0.515 12.1 0.667 0.003 23.495 210 1 134.09 10.795 2 914.471 0.594 12.6 1.084 0.004 21.212 219 1 175.38 5.510 1 201.784 0.747 17.3 1.042 0.005 23.159 222 1 191.81 0.635 19.326 5.050 66.2 11.437 0.033 13.109 224 1 200.00 2.125 249.313 1.370 19.4 1.667 0.009 14.161 227 1 216.50 1.941 8.124 26.600 35.0 27.190 0.239 1.316 229 1 227.48 1.178 26.673 6.970 92.9 9.567 0.044 13.329 230 1 242.50 4.843 258.580 2.820 56.3 3.527 0.019 19.965 239 1 305.17 1.774 282.314 1.140 24.2 1.491 0.006 21.228 240 1 313.26 0.573 5.487 16.600 196.0 14.111 0.105 11.807 243 1 343.19 0.805 227.930 0.613 21.3 0.734 0.004 34.747 249 1 394.38 0.876 165.494 0.916 20.9 1.155 0.005 22.817 251 1 420.00 2.073 639.551 0.621 17.0 0.829 0.003 27.375 252 1 422.50 6.375 886.693 1.080 14.4 1.420 0.007 13.333 255 1 443.50 3.532 997.163 0.700 12.7 0.836 0.004 18.143 259 1 475.00 5.466 578.362 1.740 16.2 2.672 0.009 9.310 266 1 514.50 8.757 1 999.339 0.731 14.5 1.004 0.004 19.836 268 1 523.20 2.716 271.584 1.610 16.2 2.125 0.010 10.062 274 1 552.65 10.774 310.391 4.820 22.0 6.149 0.035 4.564 278 1 591.50 18.740 476.798 6.120 27.8 11.053 0.039 4.542 318 1 843.57 3.910 82.713 9.280 111.0 10.576 0.047 11.961 注:Mrs.饱和等温剩余磁化强度; Ms.饱和磁化强度; Bc.矫顽力; Bcr.剩磁矫顽力; χ.低场磁化率. -
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