Magnetic Fabrics and rock magnetism of the Permian La Esperanza Plutonic Complex, Northern Patagonia, Argentina

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

Heidelberg

Congreso; LAK 2011 22th International Colloquium on Latin American Earth Sciences; 2011

Deutsche Forschungsgemeinschaft (DFG)

Magnetic fabrics are investigated by anisotropy of magnetic susceptibility studies on the Permian isotropic La Esperanza Plutonic Complex (Llambías and Rapela 1984) exposed in a northernmost area of Patagonia, south of Argentina. This intrusive complex is formed by at least three facies –Prieto Granodiorite, Giménez Granite and Donosa Granite- whose emplacement mechanisms are not clear yet. The younger pluton of the complex, the Donosa Granite, is a biotite rich K-feldspar megacrystic granite which crops out as an elongated N-S trending body formed by small discrete outcrops around Estancia La Esperanza (40°24.363´S/ 68°28.585´W). The age of Donosa can be bracketed by the age of its host, the Prieto Granodiorite, which has been accurately dated as 273±2Ma and the age of the cover, the Dos Lomas volcanics 264±2Ma, both obtained using U-Pb SHRIMP (Pankhurst et al., 2006). Nearly 150 oriented specimens were studied from 14 equidistributed sites on the Donosa Granite to determine its magnetic fabric pattern. Magnetic susceptibility (κ) measurements indicate that average magnetic susceptibilities for Donosa Granite are close to 3 E-03 SI, yielding a ferromagnetic character, whereas its host, Prieto Granite exhibits κ reaching up to 1.5x10-2 SI. Microscopic observations, blocking temperature analysis, thermomagnetic curves (from about -195°C to 700°C) and bulk susceptibility measurements at room temperature for the Donosa Granite point out that in every case low Ti- and/or pure multidomain magnetite seems to be not only the most important carrier of the susceptibility but also a primary phase. Donosa Granite data analysis indicates that 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 statistics), indicates prolate to isotropic to oblate magnetic fabrics depending on the location of the site. Magnetic directional data reveal a consistent SE-NW trending foliation arrangement with variable dips mostly towards N along the southern and central parts of the pluton and a SW-NE trending foliation with subhorizontal dips towards SE in the northern part. However, steady NW-SE lineation data is consistent in every site with subhorizontal dippings (