CONICET | Buscador de Institutos y Recursos Humanos

A map of bulk magnetic susceptibility was obtained onEl Hongo trondhjemite, a small Cambrian pluton intruding the metamorphicbasement in Eastern Sierras Pampeanas, Argentina, based on systematic magneticsusceptibility measurement at 450 sites using a SM30 susceptibility meter.Samples were collected on 58 sites and their bulk magnetic susceptibility wasmeasured in laboratory with a Bartington MS2 system. Point-to-point comparisonshowed differences, that were attributed to the effect of roughness of the surveyedsurfaces, and to the development of a weathered cap. However, the difference wassystematic and in accordance with expected values predicted by manufacturertables, whereby, once corrected with the appropriate factor, the obtainedvalues with SM30 susceptibility meter were regarded as representative of freshrocks. The resulting map was interpreted in terms of variation in abundance ofmagnetite, which is present in the rocks as a magmatic mineral, altered tohematite (martitized) in varying degrees. The map revealed that El Hongotrondhjemite is a weakly magnetic pluton, with a typical bulk susceptibility ofabout 500 x 10-6 SI, which would correspond toan abundance of magnetite below 0.2 vol%, but with conspicuous variations. Lowsin the outer sector and in the vicinity of metamorphic xenoliths wereinterpreted as due to destruction of magnetic minerals linked to reactionsbetween magma and host rock. A distinct concentric pattern in the western areacould indicate the presence of a separate intrusion. Finally, alternate highsand lows in susceptibility follow the undulations in regional schistosity,which in turn would have controlled the emplacement of the pluton. Thus we providea good example of the utility of magnetic susceptibility mapping in granitoid terrains,as an expeditious way for preliminary mapping that could guide further and moredetailed research.

enviar mensaje