摘要
峨眉山大火成岩省位于中国西南部,在晚二叠纪约260 Ma喷发出巨量的大陆溢流型玄武岩.对于大火成岩省的岩浆喷发,在地下必定有一个相应的大规模岩浆聚集和运移系统.地球物理方法是探测岩石圈内部的有效方式.峨眉山大火成岩省为镁铁质岩浆喷发,由于镁铁质-超镁铁质岩石一般具有强磁性,因此,在喷发结束之后,地下岩浆系统如果被镁铁质岩浆填充,冷却固化成为岩石圈的一部分,很有可能会引起磁异常.本文使用区域磁异常数据来对峨眉山大火成岩省的深部构造进行研究.该区域的磁异常由一系列离散的异常组成,通过3D磁化率反演可以得到磁性体的空间分布.由于磁异常中具有明显的剩磁,直接使用经典的反演方法会有较大误差,我们首先将磁异常转换为弱敏感于磁化方向的磁异常模量,再使用模量数据进行3D反演,得到地下空间内磁异常源的分布.经过分析认为这些离散分布的磁异常源反映了岩石圈内部的镁铁质-超镁铁质侵入体.侵入体的位置可能反映了底侵和内侵的镁铁质岩浆固化形成的侵入体,代表镁铁质岩浆房位置或者岩浆运移的主要通道.
The Emeishan large igneous province is located in the southwest of China.At~260 Ma in the late Permian period,it erupted massive continental flood basalts.For such an event,there must be a large-scale underground magmatic system for magma accumulation and migration in the subsurface.After the eruption is over,the underground magmatic system might be filled with mafic magma and becomes a part of the lithosphere.The solidified mafic-ultramafic intrusions,which are usually strongly magnetized,may cause magnetic anomalies.Thus the magnetic method can be used to detect such a system in the earth.In this research,magnetic anomaly grid data is applied to image the deep structure of the Emeishan large igneous province.The magnetic anomalies in this area consist of a series of discrete ones with obvious remanence,implying that the direction of magnetization is inconsistent with that of the current geomagnetic field.Since the classical 3 Dinversion method is based an assumption that there is only induced magnetization,the inversion result would be inaccurate due to the existence of remanence.Thus,we first transformthe magnetic anomalies to magnetic amplitude data,which is less sensitive to the direction of magnetization.Then we use the amplitude data for 3 Dinversion to obtain the distribution of magnetic anomaly sources within the lithosphere.These underground discrete magnetic sources may indicate the mafic-ultramafic intrusions beneath the Emeishan large igneous province.These intrusions might be formed during underplating or intraplating in the Emeishan large igneous province,which might be magma chambers or magma transport channels filled with mafic magma.After cooling,these mafic-ultramafic intrusions would have produced magnetic anomalies observed now.