Magnetic susceptibility and partial anhysteretic remanence anisotropies inthe magnetite-bearing granite pluton of Tourao, NE Brazil

Rock magnetism and magnetic fabric studies were performed in the Tourao gra nite (western part of Serido belt, NE Brazil). This pluton (350 km(2)) is c omposed of R-rich calc-alkalic monzogranites and was emplaced between a set of NE- and E-trending dextral strike-slip shear zones. Electronic and opti cal microscopy, thermomagnetic curves, hysteresis data and coercivity spect ra revealed that magnetite (fine- and coarse-grained) is the unique ferroma gnetic mineral in these rocks. Anisotropy of low-held magnetic susceptibili ty (AMS) was measured in 83 sites, and for many sites from central and sout hern 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 i solate the fabric contribution of the remanence bearing grains (magnetite). In most sites AMS is dominantly carried by magnetite, whereas in some site s the paramagnetic contribution exceeds 60% of the bulk susceptibility. The pAAR(0-50) is assumed to be related to coarse-grained early crystallized m agnetites, while the pAAR(50-90) is interpreted as due to fine needle-shape d magnetites hosted in biotites, The combined analysis of these magnetic techniques permits interpretation i n terms of fabric formation in the Tourao 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) fa brics 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. (C) 1999 Elsev ier Science B.V. All rights reserved.