Effects of Retrogression of Ultrahigh-Pressure Eclogites on Magnetic Susceptibility and Anisotropy
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摘要: 系统测量了中国大陆科学钻探(CCSD) 主孔10 0~ 2 0 0 0m超高压榴辉岩的磁化率及其各向异性, 并进行了相应的岩石学和矿物学分析.研究结果表明, 不同退变质程度榴辉岩的磁化率特征具有明显差别: 新鲜-弱退变质榴辉岩对应超高压榴辉岩相, 平均体积磁化率κ=1.4× 10-3SI, 磁化率各向异性度Pj=1.0 9, 磁性载体主要是顺磁性矿物石榴子石和绿辉石; 中等-强退变质榴辉岩对应角闪岩相, κ =5.1× 10-3SI, Pj=1.2 0, 磁性载体主要是退变质所产生的磁铁矿和钛铁矿-钛磁铁矿; 完全退变质榴辉岩对应绿帘角闪岩相-绿片岩相, κ =1.3× 10-3SI, Pj=1.08, 磁性载体主要来自顺磁性矿物角闪石、绿帘石、黑云母等.该研究为超高压变质岩石地区磁异常的解释提供了基础数据和重要的科学约束, 同时也为大陆深俯冲峰期变质岩石和折返阶段岩石退变质程度提供了磁化率鉴别特征.Abstract: The magnetic susceptibility and anisotropy of the ultrahigh-pressure (UHP) eclogite from Chinese Continent Scientific Drilling (CCSD) 100-2 000 m mainhole and their relationship with petrology and mineralogy are systematically investigated. The results show that eclogites of different retrogressions have specific magnetic characters. Fresh to weakly retrograded eclogites which correspond to UHP eclogite facies have relatively low magnetic susceptibility (κ=1.4×10-3 SI) and low anisotropy (Pj=1.09), the paramagnetic mineral omphacite and garnet being the principal carriers. For moderately to strongly retrograded eclogites corresponding to amphibolite facies, because of the formation of magnetite and ilmenite by retrogression, the magnetic susceptibility and anisotropy increases up to κ=5.1×10-3 SI and Pj=1.20. However, for the totally retrograded eclogites corresponding with epidote amphibolite to greenschist facies, magnetite disappears while ilmenite and titanomagnetite changes to sphene. Thus the magnetic susceptibility and anisotropy reduces to κ=1.3×10-3 SI and Pj=1.08, respectively. This research provides fundamental data and constrains for the interpretation of geophysical magnetic surveying on the UHP metamorphic terranes. Furthermore, the magnetic characters show us the insight into the identification of the peak metamorphic rocks during subduction and the degree of retrogression during exhumation.
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Key words:
- magnetic susceptibility /
- anisotropy /
- UHP eclogite /
- retrogression /
- CCSD /
- Donghai
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图 1 CCSD主孔100~2 000 m榴辉岩的磁化率与密度对应关系
a.新鲜钛磁铁矿榴辉岩; b.新鲜榴辉岩; c.弱退变质榴辉岩; d.中等退变质榴辉岩; e.强退变质榴辉岩; f.完全退变质榴辉岩; N.样品数量. (以下图解沿用此类符号定义)
Fig. 1. Relationship between magnetic susceptibility and density of 100-2 000 m eclogites from CCSD main hole
图 2 榴辉岩磁化率各向异性度(Pj) 与形状因子(T) 对应关系
Fig. 2. Relationship between the anisotropy intensity (Pj) and the shape parameter (T) of eclogites
图 3 榴辉岩磁化率各向异性度(Pj) 随平均磁化率(κ) 的变化关系
Fig. 3. Anisotropy intensity (Pj) varies with the mean susceptibility (κ) of eclogites
图 4 超高压榴辉岩P-T轨迹与磁化率变化关系(张泽明等, 1999)
Fig. 4. P-T path and magnetic susceptibility of ultrahigh-pressure eclogites
表 1 CCSD主孔退变质榴辉岩分类及其代表性样品的矿物组成
Table 1. Classification of retrograded eclogites and mineral compositions of core samples from CCSD main holes
表 2 CCSD主孔榴辉岩的密度、磁化率及其各向异性度统计
Table 2. Statistics of density and magnetic susceptibility and anisotropy of eclogites from CCSD main hole
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