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Magnetic Structure of Archean Kongling Group from Yangtze Craton,South china

Journal of Earth Science 1998年第2期17卷 52-59页

作者： liu qingsheng department of applied geophysics, china university of geosciences, {2. 43.074 gao shan faculty of Earth sciences, china university of geosciences, {2. 43.074 xu qidong faculty of Earth resources, china university of geosciences, w

Densities and various magnetic parameters (susceptibility, saturation magnetization, saturation isothermal remanent magnetization and intrinsic coercivity) were measured for 2. representative rock samples of different lithologies from the Archean Kongling amphibolite to granulite facies terrain of the Yangtze craton. Metasedimentary rocks and tonalitic trondhjemitic granodioritic granitic (TTGG) gneisses show that values of susceptibility κ and saturation isothermal remanent magnetization SIRM are higher than those of amphibolites and gabbros. The felsic gneisses have averages of κ =(1 163±375)×10 -6 SI, SIRM =(18.2.±8.38) A/m and R 1=0.083 3± 0.005 7 and the metasedimentary rocks κ =(1 2.6±82.)×10 -6 SI, SIRM =(2..70±10.91) A/m and R I=0.071 4±0.02. 2. In contrast, mafic rocks have average κ =(764±316)×10 -6 SI, SIRM = (10.46±3.94)A/m and R 1=0.036±0.009 4, and are dominated by a mixed paramagnetic and ferrimagnetic behavior. Thermal magnetic analyses indicate that magnetite and maghemite of low coercivity are the major carriers of remanent magnetism in the metaclastic sedimentary rocks and TTGG gneisses. The amphibolite and gabbro contain minor amounts of magnetite and pyrrhotite. Magnetism of metaclastic sedimentary rocks and TTGG gneisses is highly heterogeneous; variation coefficients of κ and SIRM are as high as 67 % and 53 % for the former and 32.% and 46 % for the latter. Mineral compositions suggest that biotite may be responsible for the higher magnetism of the metasedimentary rocks. The highest variations in κ, SIRM and R I exhibited by metasedimentary rocks can also be interpreted by their largest absolute variations in biotite mass fraction relative to mafic rocks and felsic gneisses. The average ratio ( Q ) of natural remanent magnetization to induced magnetization of felsic gneisses and metasediments is 0.47 . Ratios ( REM ) of natural remanent to sat

关键词： magnetic structure amphibolite granulite facies Yangtze craton.

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