An ancient remanent magnetization carried by ilmeno-hematite in the Devonian Achala Batholith, Córdoba, Argentina

GEUNA, SILVANA; ESCOSTEGUY, LEONARDO; MIRO, ROBERTO

Sapporo

Congreso; General Assembly of the International Union of Geodesy and Geophysics (IUGG); 2003

TheAchala batholith (31o 30" Lat.S. 64o45 ´ Long.W. Córdoba. Argentina) is a major magmatic complex of the SierrasPampeanas, with a maximum extension of 100x50 km. It is a post-collisional intrusion of Devonian age,emplaced following the Ordovician Famatinian Orogeny in metamorphic rocksof lhe Pampean cycle(Upper Proterozoic).The area was peneplanized during the Carboniferous, previously to the deposition of Upper Paleozoic sequencesin local troughs. Since then, the peneplain has been justdisrupted by small block rotationsrelated toAndeanTertiary tectonics. Paleomagnetic sampling was carried out in 42 sites distributedthroughout lhe Achala batholith. Overall, the batholith is weakly magnetic, with a mean magneticsusceptibility of 1.5x 10-4 (SI), mainly due to the content of paramagnetic minerals (essentially biotite). Seventeen sites showed unstable magnetic behaviour. The remaining 25 sites have ilmeno-hematite as magnetic carrier of a stableremanence, sometimes accompanied by magnetite in subordinate proportions. Thermal demagnetizationup to 620-640oC applied to samples of these sites isolated steeply dipping, double polarity remanence directions. Thepreliminary mean direction is Dec. 173o, Inc. 76o, a95 6o. The paleomagnetic pole is Lat. 59oS, Long. 301oE, implying a paleolatitude of 63o forthe Achala batholith. Althoughthe magnetization isolated clearly reflects an ancient magnetic field, further work remains to bedone to establish whether it is a primary remanence acquired on cooling (in the Devonian),or a chemical remanence due to alteration- weathering, related topeneplanization during the Carboniferous. In any case, it will contribute to the poorly defined South AmericanPaleozoic apparentpolar wanderpath.