Magnetic fabric and pluton emplacement in a transpressive shear zone system: the Itaporanga porphyritic granitic pluton (northeast Brazil)

The Itaporanga granitic pluton intruded into the lithological contact separating Neoproterozoic pelitic metasediments from Meso- to Paleoproterozoic rocks that constitute the Cachoeirinha belt of northeastern Brazil. An aureole of pelitic gneisses containing sillimanite and andalusite indicates that emplacement occurred at 12-15 km depth. The objective of this work is a detailed study of its magmatic structures, mainly by means of anisotropy of magnetic susceptibility (AMS). Regularly spaced sampling (61 stations) yielded a magnetic fabric map, which shows two domains: (1) a western lobe, which has lineations close to vertical and E-trending foliations that merge southwestward as the transcurrent Igarassi shear zone is approached (ISZ), and (2) an eastern lobe, showing SE-trending lineations associated with inward-dipping magmatic foliation situated in the center of the lobe. The magnetic fabric is mostly paramagnetic in origin, i.e., related to crystallographic preferred orientation of biotite and hornblende. A low susceptibility magnitude (<0.4 x 10(-3) SI), typically of the ilmenite-series granites, is found in porphyritic granites, diorites or their hybrid products. Hysteresis data indicate that the ferrimagnetic contribution to the bulk susceptibility does not reach 30%. The magma was emplaced into a chamber developed by local WNW-ESE extension induced by an overall N-S shortening. Melt ascent may have been facilitated by the pre-existence of a steep fault zone bounding crustal discontinuities. The magma must have ponded along a mechanical barrier formed by the contact between phyllitic metapelites and its gneissic basement, which controlled the final emplacement structures of the pluton. (C) 1999 Elsevier Science B.V. All rights reserved.