Emplacement of the Anta Granite from Rio de Janeiro State (SE, Brazil); determined by magnetic fabrics

Abstract

Magnetic fabric and rock magnetism studies were performed on the three facies of the Anta granite (SE of Rio de Janeiro State, southern Brazil). This intrusion is roughly elliptical (∼ 14 km²), NE-SW oriented and composed of granodiorite, monzodiorite, quartz-diorite, syenogranite, and monzogranite. It shows solid-state deformation and tectonic foliation at its borders and is apparently isotropic in the central part where the internal fabrics are impossible or very difficult to measure through classical techniques. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The AMS and AARM fabrics are coaxial for all facies. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect in the AMS fabric. Several rock-magnetism experiments performed on one specimen from each facies show that, for all units, magnetic susceptibility and fabrics are carried by magnetite grains. Most of the magnetic foliations are steeply dipping in all facies, and are roughly parallel or sub-parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for almost the whole pluton, except for one site where it is steep (> 60°). Thin section analysis show that rocks from the borders of the Anta pluton are solid-state deformed, indicating that the borders of the pluton were affected by the regional strain during its emplacement. On the other hand, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the central part of the pluton to be determined, where igneous textures are preserved. This evidence allows us to interpret the magnetic fabrics observed in this part of the pluton as primary in origin (magmatic) and acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlain this area. Our data suggest that the regional deformation partially affected the emplacement of the Anta granite.

KEYWORDS:
Magnetic Fabrics: anisotropy of low-field magnetic susceptibility; anisotropy of anhysteretic remanent magnetization; granite; Anta; Rio de Janeiro