Palaeomagnetism of the Devonian Achala Batholith, Córdoba, Argentina

GEUNA, S.E.; ESCOSTEGUY, L.D.; MIRÓ, R.

MENDOZA

Congreso; GONDWANA XII; 2005

The peraluminous Achala batholith (31o30´ S, 64o45´ W, Córdoba, Argentina) is a major magmatic complex of the Sierras Pampeanas, with a maximum extension of 100x50 km. It was emplaced at a shallow level in a Late Precambrian to Early Palaeozoic, amphibolite- to subordinate granulite-facies metamorphic?plutonic basement. The contacts of the batholith are regionally sharp and discordant (Lira and Kirschbaum, 1990). The batholith is an elliptical shaped body that trends N-NE, and is constituted by five co-magmatic suites. The Achala suite is the most extensive, and is best represented by its main facies, a porphyritic to coarse-grained equigranular monzogranite, dated in 368 ± 2 and 399 ± 25 Ma by U?Pb on zircons and Rb?Sr on whole rock, respectively (Dorais et al., 1997; Rapela et al., 1982). Uplift and fracturing of the batholith began in the Carboniferous. It is presently exposed in a series of asymmetric, eastward-tilted blocks, with a very gently sloping eastern side interpreted as a Gondwanian peneplain (Rabassa et al., 1996). Palaeomagnetic sampling was carried out on the main facies of the Achala suite, with forty-two sites sampled in the porphyritic to equigranular monzogranite. Overall, it is weakly magnetic, with a mean magnetic susceptibility of 15x10-5 (SI), mainly due to the content of biotite and ilmeno-hematite. Seventeen sites showed unstable magnetic behaviour. The remaining 25 sites have ilmeno-hematite as magnetic carrier of a stable remanence, sometimes accompanied by magnetite in very subordinate proportions. The ilmeno-hematite is an abundant accessory mineral, which appears as exsolved intergrowths with (hemo)ilmenite. The hematite-rich member is usually the host of disc-shaped rods of exsolved ilmenite-rich member. Thermal demagnetization up to 620-640oC isolated steeply dipping, double polarity remanence directions. The preliminary mean direction is Dec. 179o, Inc. 73o, a95 7.4o. The palaeomagnetic pole is Lat. 63oS, Long. 296oE. The remanence must have been locked at the moment of ilmeno-hematite exsolution, near 390oC, the eutectoid of the hematite-ilmenite system after Robinson et al. (2004). The Achala palaeomagnetic pole approximately coincides with the 400-370 Ma segment of the Gondwana apparent polar wander path (APWP) proposed by McElhinny et al. (2003), suggesting that cooling under 400oC was achieved very early after emplacement, and that the area was not significantly affected by subsequent deformation. The weak magnetism of Achala monzogranite facies contrasts with the magnetic character of other Devonian porphyritic, batholithic monzogranites in the region as Renca, El Talita and Las Chacras (Pinotti et al., 2004 and references therein). The virtual absence of magnetite together with the abundant presence of exsolved ilmeno-hematite point to cooling under highly oxidizing conditions, as responsible of the weak magnetic character of Achala monzogranite.