昆明碳酸盐岩红色风化壳磁化率高于玄武岩风化壳的现象及原因探析

The phenomenon and reason analysis of the magnetic susceptibility of carbonate red regolith higher than that of basalt red regolith in Kunming

一般认为强磁性母岩风化壳的磁化率要高于弱磁性母岩的风化壳.然而,在昆明及周边地区强磁性的玄武岩,尽管其红色风化壳的磁化率平均值在1 687.86×10^(-8)m^3/kg左右,但却系统地比弱磁性的碳酸盐岩的红色风化壳低了近1/2.XRF(X-rag fluorescence)元素分析结果显示,玄武岩风化壳的平均全铁质量分数为18.91%,碳酸盐岩风化壳为18.57%,二者几乎相当,铁含量的差异不是导致产生这种现象的主要原因.磁化率随温度变化曲线和磁滞回线分析结果显示,碳酸盐岩风化壳和玄武岩风化壳的主要载磁矿物基本相同,均为矫顽力低的伪单畴磁铁矿和磁赤铁矿.但在频率磁化率方面碳酸盐岩红色风化壳却比玄武岩风化壳高出很多,结合碳酸盐岩红色风化壳有比玄武岩风化壳更低的SiO_2/Al_2O_3值,认为风化程度的差异是导致碳酸盐岩红色风化壳磁化率高于玄武岩风化壳的重要原因.

It is generally believed that the magnetic susceptibility of regolith from the strong magnetic source rock is higher than that from weak magnetic source rock.However,the average magnetic susceptibility of red regolith from the strong magnetic basalt,which is about 1 687.86×10^(-8) m^3/kg,is lower than that from weak magnetic carbonate in Kunming and its surrounding areas.XRF(X-ray fluorescence) element analysis shows that the average total iron content of red regolith from basal and carbonate are 18.91% and 18.57% respectively and the two value are almost equal,suggesting that the differences in iron content is not the main cause of this phenomenon.The normalized thermal magnetic curves and hysteresis loops analysis showed that low coercivity pseudo-single domain magnetite and maghemite are the main magnetic carrier minerals in carbonate and basalt red regolith.But the frequency magnetic susceptibility of carbonate regolith is much higher than that of basalt regolith.In combination with the fact that the SiO_2/Al_2O_3 of carbonate red regolith is lower than that of basalt red regolith,we believe that the difference in weathering degree is the main reason that the magnetic susceptibility of carbonate red regolith is higher than that of basalt red regolith.