Magnetic Fabrics of the Permian Prieto Pluton (Northern Patagonia, Argentina): Integration of Petrography and Magnetic Mineralogy

MARTÍNEZ DOPICO, C.; RAPALINI, A.E.; LÓPEZ DE LUCHI, M. G.

Foz de Iguacu

Congreso; Meeting of the Americas; 2010

American Geophysical Union

Magnetic fabrics were investigated by anisotropy of magnetic susceptibility (AMS) studies on the apparently isotropic Prieto granodiorite (PG) from the Permian La Esperanza Plutonic Complex (LEPC) exposed in a northwestern area of the North Patagonian Massif. This intrusive complex is formed by at least three facies whose extension and emplacement mechanisms are not clear. The oldest facies, Prieto granodiorite, is a biotite-hornblende equigranular to inequigranular granitoid which was recently dated by U-Pb SHRIMP in zircon in 273±2 Ma (Pankhurst et al., 2006 Earth Sci.Rev. 76, 235-257). Nearly 250 oriented specimens were studied from 28 sites on the PG to determine its magnetic fabric pattern. Petrographic and polished section observations to characterize lithology, texture and microstructures were complemented with rock magnetic experiments to better constrain the AMS interpretations. Rock magnetic experiments included measurements of continuous low-field thermomagnetic curves (from about -195°C to 700°C) and thermal remanence studies. Low Ti- and/or pure Magnetite seems to be not only the most important magnetic carrier but also a primary phase which shares syn-emplacement deformation as indicated by the microstructures in the granodiorite. PG is ferromagnetic with mean susceptibilities reaching up to 1.5x10-2 SI. Anisotropy degree is generally low for a ferromagnetic body. The analysis of the shape parameters of the susceptibility ellipsoids (i.e. Pj vs T, Jelinek), indicates mostly oblate to isotropic magnetic fabrics. Magnetic directional data reveal a consistent concentric arrangement along most of the borders of the pluton. A consistent E-W trending foliation with variable dips towards the north characterizes its northern and southern sections. The central areas of the intrusion show low-angle foliation planes suggesting a dome-like architecture for the main intrusive body. However a narrow N-S trending zone across the whole intrusion shows sub-vertical N-S trending foliations with intermediate lineations which could be interpreted as feeder dykes. Anywhere else lineations tend to be shallow to subhorizontal with no systematic orientations.