Magnetic susceptibility and partial anhysteretic remanence anisotropies in the magnetite-bearing granite pluton of Tourão, NE Brazil

Abstract

Rock magnetism and magnetic fabric studies were performed in the Tourão granite (western part of Seridó belt, NE Brazil). This pluton (350 km 2) is composed of K-rich calc-alkalic monzogranites and was emplaced between a set of NE- and E-trending dextral strike–slip shear zones. Electronic and optical microscopy, thermomagnetic curves, hysteresis data and coercivity spectra revealed that magnetite (fine- and coarse-grained) is the unique ferromagnetic mineral in these rocks. Anisotropy of low-field magnetic susceptibility (AMS) was measured in 83 sites, and for many sites from central and southern parts of the intrusive body a large scatter was noticed. Anisotropy of partial anhysteretic remanence (pAAR) was determined for 29 sites in the 0–50 mT (pAAR 0–50) and 19 sites in the 50–90 mT (pAAR 50–90) windows to isolate the fabric contribution of the remanence bearing grains (magnetite). In most sites AMS is dominantly carried by magnetite, whereas in some sites the paramagnetic contribution exceeds 60% of the bulk susceptibility. The pAAR 0–50 is assumed to be related to coarse-grained early crystallized magnetites, while the pAAR 50–90 is interpreted as due to fine needle-shaped magnetites hosted in biotites. The combined analysis of these magnetic techniques permits interpretation in terms of fabric formation in the Tourão granite and the inference that at least two successive kinematic events were recorded in this intrusive body devoid of any solid-state deformation overprint. The AMS and pAAR 0–50 fabrics for most of the sites correlate with the early NNE-trending magmatic fabric, whereas pAAR 50–90 reveals a different fabric, probably related to late partitioning of magmatic straining along the E-trending shear zones. In the southern part of the body, a completely different pAAR 0–50 fabric is correlated with the late intrusion of charnockite stocks.