Magnetic study of metamorphosed sedimentary rocks of the Hatrurim formation, Israel

Abstract

SUMMARY The Hatrurim formation is a ‘Mottled Zone’ complex of metamorphosed carbonate rocks, which is known as a complex subjected to combustion metamorphism (CM). Its magnetic characteristics and their relationships to the CM were studied using magnetic measurements by different techniques and on different scales. The magnetic features revealed are characterized by different spectral components and anisotropy determined at scales from hundreds of metres to a few centimetres. A very short wave of relatively high magnetization is associated with shallow small bodies of high-grade metamorphosed rocks of the Hatrurim formation. The magnetic susceptibility (κ )o fhigh-grade rocks is highly variable with marked heterogeneity and anisotropy. The frequency and magnitude of ground magnetic anomalies (up to 4000 nT) and the κ of high-grade rocks (up to 5600 × 10 −5 SI) are higher than those reported from other areas of CM. An approximate field method and conventional palaeomagnetic studies of oriented samples showed similar results, namely: (a) natural remanent magnetization of highgrade Mottled Zone rocks is heterogeneous and (b) the remanent magnetization of rocks within the Hatrurim basin usually exceeds their induced magnetization and has a similar direction. Relatively low and homogeneous magnetization is found in another type of slightly metamorphosed rock of the Hatrurim formation and in some cases also in adjacent outcrops of the underlying Mishash formation. This type appears to be the product of other sources not related to the CM process. Such sources can cause observed local aeromagnetic maxima that show some correlation with geological structure. The weak magnetization is also related to hydrothermally altered and weathered rocks of the Hatrurim formation. Thus, the magnetization of the Mottled Zone is due to a combination of near-surface/surface magnetic bodies (similar to the CM rocks in other regions) and magnetization induced by hidden magnetic sources possibly related to hydrocarbon fluxes or magmatic activity along the faults and fractures.