Composite magnetic fabric deciphered using heating treatment

In a number of AMS studies, the presence and deciphering of composite magnetic fabrics is of major importance for a correct interpretation of the data. On the basis of several examples from intrusive rocks (diorites and dolerites) we show that the use of laboratory heatings can help to extract at least one component of the composite magnetic fabrics usually present. The procedure includes comparison of the fabrics measured after stepwise laboratory heating with the fabrics determined by tensor difference and by linear regression analysis. In the diorite samples, the measured AMS results from the superimposition of different component fabrics and does not correspond exactly to any of these fabrics. In these dykes, isolated magnetic fabric during thermal treatment corresponds to that of the main magnetic mineral (Ti-poor titanomaghemite) and reveals an unknown structure. In volcanic flow or doleritic dykes, a "parasitic" fabric related to late or post-magmatic evolution superimposed to the flow fabric can produce important scattering of the AMS principal directions. Decomposition of magnetic fabric during thermal treatment allows isolation of the flow fabric. © StudiaGeo s.r.o. 2007.