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(0.570×10-7~120.450×10-7 m3·kg-1,平均29.996×10-7 m3·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(0.002×10-3~2.109×10-3 Am2·kg-1,平均0.210×10-3 Am2·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.
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)(0.570×10-7-120.450×10-7 m3·kg-1,average 29.996×10-7 m3·kg-1)and the lowest natural remanent magnetization(NRM)(0.002×10-3-2.109×10-3 Am2·kg-1,average0.210×10-3 Am2·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.