Evidence of an early NS magmatic event in the Paleoproterozoic Teofilandia granitoids (Sao Francisco Craton, Brazil):: a combined microstructural and magnetic fabric study

The Teofilandia area, south of the Serrinha block (Sao Francisco Craton, NE Brazil), marks a region where the boundary between the Serrinha block and the Itabuna-Salvador-Curaqa belt rotates from NS to EW. The granitoids of Teofilandia are made of granodiorites and trondjhemites dated at 2.13 Ga. To the south and west, they are hosted by biotite-bearing "basement" orthoaneiss. An oriented sampling of 157 sites (covering 220 km(2)) was subjected to a systematic microstructural and magnetic fabric study, complemented by pressure determinations using the Al-in-homblende barometer. The highly variable magnetic susceptibilities, from 50 to 21 000 muSI, reflect both the variable primary magnetite-content of these rocks, and the variable intensity of the late hydrothermalism that is responsible for gold mineralization hosted by the metabasalts in the north of the area. Three main types of microstructures have been distinguished along with some transitions (magmatic, high-temperature-gneissic, and fluid-present, low-temperature-high-stress mylonitic). Combined microstructural and magnetic fabric data reveal that a NS-trending, primary magmatic structure has been locally shielded from two subsequent EW-trending, HT-gneissic and low-T-mylonitic events. Our tectonic reconstruction proposes that the preserved NS magmatic lineations were formed in a submeridian magmatic arc parallel to the western border of Serrinha block and along which an oblique convergence took place. The subsequent EW-trending events are equated with (i) the already identified EW-compression that accreted the Serrinha block to the Craton, and (ii) the late event that we correlate with ore deposits in the metabasalts, dated between 2.08 and 2.03 Ga. (C) 2004 Elsevier B.V. All rights reserved.