Composite magnetic fabrics and S–C structure in granitic gneiss of Cerro de los Viejos, La Pampa province, Argentina

The Cerro los Viejos gneiss, in La Pampa Province, Argentina, is a mylonitized granite which lies along the late Paleozoic deformed boundary zone between the Patagonia block and ‘mainland’ Gondwana to the north. Petrofabric and anisotropy of magnetic susceptibility (AMS) analyses were compared for this body. Unexpectedly, it was found that the Kmin poles deviate from the poles of the main foliation s1 by about 25°. Similarly, but not so noticeably, the Kmax poles trend NE/SW, with a near-horizontal plunge, whereas the principal lineation l1 plunges gently SW. This unusual situation is attributed to the existence of superimposed fabrics arising from S–C structures. Examination of oriented cores in a modified universal stage and of oriented thin-sections revealed that the Kmin poles are clearly associated with a younger but less obvious metamorphic ‘C’ foliation (s2). Biotite and magnetite, the main contributors to the AMS response, are distributed along both s1 and s2. The dominance of s2 in the AMS anisotropy suggests that magnetite is preferentially distributed on s2.