Identifying types of remanent magnetization in igneous contact zones

Abstract

The magnetization of host rock adjacent to an igneous intrusion (e.g., dyke) will be completely reset near the contact, partially reset in a zone of hybrid directions farther away, and unaffected at even greater distances. If the intrusion-related direction and the host direction are known, the hybrid zone magnetization can be analysed in terms of pTRM, VRM and CRM overprints of the host magnetization using orthogonal component plots of thermal demagnetization results. Straight-line segments representing the host and the intrusion-induced magnetization show a sharp intersection in cases of (1) a pure pTRM overprint, (2) a VpTRM overprint of one magnetic carrier, (3) two magnetic minerals with a VpTRM partial overprint of the high blocking temperature mineral and a complete VpTRM overprint of the low blocking temperature mineral, and (4) a CRM overprint where its blocking temperature range is entirely distinct from that of the host component. A gradual transition between both components during thermal demagnetization is expected for (a) two magnetic minerals both of which have a VpTRM partial overprint, (b) a VpTRM partial overprint of a continuous range of magnetic mineral compositions and (c) a CRM overprint in which the blocking temperature range partially overlaps that of the host component. Swings from one direction to the other and back are observed if the blocking temperature of one of the components completely overlaps that of the other, one of them being due to CRM.