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 separ ating Neoproterozoic pelitic metasediments from Meso- to Paleoproterozoic r ocks that constitute the Cachoeirinha belt of northeastern Brazil. An aureo le of pelitic gneisses containing sillimanite and andalusite indicates that emplacement occurred at 12-15 km depth. The objective of this work is a de tailed study of its magmatic structures, mainly by means of anisotropy of m agnetic susceptibility (AMS). Regularly spaced sampling (61 stations) yield ed a magnetic fabric map, which shows two domains: (1) a western lobe, whic h has lineations close to vertical and E-trending foliations that merge sou thwestward 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 magneti c fabric is mostly paramagnetic in origin, i.e., related to crystallographi c preferred orientation of biotite and hornblende. A low susceptibility mag nitude (<0.4 x 10(-3) SI), typically of the ilmenite-series granites, is fo und in porphyritic granites, diorites or their hybrid products. Hysteresis data indicate that the ferrimagnetic contribution to the bulk susceptibilit y does not reach 30%. The magma was emplaced into a chamber developed by lo cal 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 barr ier formed by the contact between phyllitic metapelites and its gneissic ba sement, which controlled the final emplacement structures of the pluton. (C ) 1999 Elsevier Science B.V. All rights reserved.