Reassessment of the volume of the Las Aguilas Mafic-Ultramafic intrusives, San Luis, Argentina: an alternative geophysical model

ZAFFARANA, C.; GEUNA, S.E.; POMA, S.

Foz do Iguassu

Congreso; 2010 Meeting of the Americas, American Geophysical Union; 2010

A layered complex of mafic-ultramafic rocks crops out in the Sierra de San Luis, known as Las Aguilas Group. The volume of this intrusion has been considered to be very important (about 250 km3, Kostadinoff et al. 1998), although the outcrops are limited to small lenses of about 1x3 km. The volume was estimated from forward modelling of an intense gravity anomaly centered over the Sierra de San Luis, with an approximated width of 15 km and an amplitude of 300 u.g. In this work we have revalued the volume of this intrusion, measuring the density of the rocks and performing an alternative 2.5D modelling. The model was carried out taking into account the geological constraints, which in this case are: a) the outcropping width of the several metamorphic complexes outcropping in the area and mafic-ultramafic lenses, and b) the measured densities. The geology of the Sierra de San Luis is conformed by three main basement units of Late Proterozoic-Early Paleozoic metamorphic rocks and Palaeozoic granitoids, which are clearly separated by ductile shear zones. The three main basement units are, from east to west: the Conlara Metamorphic Complex, the Pringles Metamorphic Complex (PMC), and the Nogolí Metamorphic Complex. The mafic-ultramafic lenses are hosted by the PMC. They appear as small, elongated shaped, fragmented bodies aligned along a belt of NNE-SSW strike, concordant with the main structural trend. We found the average density of the Nogolí and Conlara Metamorphic Complexes to be 2.67 t/m3. This value was considered the background value against which we contrasted the other densities of our model. Density contrasts for PMC and the granitoids resulted in 0.17 and -0.05 t/m3, respectively, while maficultramafic bodies showed the highest contrast of about 0.40 t/m3. These density contrasts were used for modelling. The extension along strike of the three metamorphic complexes was established in about 100 km, in agreement with the approximate length of the Sierra de San Luis. The mafic-ultramaficrocks were incorporated to the model with thickness of 1-2 km and an extension along strike of 20 km. In our model, most of the anomaly was explained by the lateral contrast between Pringles Metamorphic Complex and the surrounding, less dense metamorphic complexes. The maximum vertical gravity gradients of the Sierra de San Luis coincide very precisely with the boundaries of the PMC observed in the surface: to the east, the Río Guzmán shear zone (which is the contact area with the Conlara Metamorphic Complex of lower density), and to the west, the contact area with La Escalerilla granite, hosted by the Nogolí Complex. The obtained model fits the anomaly and the imposed geological constraints, and it does not require appealing to near-surface ultramafic bodies larger than the size observed in surface outcrops. Kostadinoff, J., Bjerg, E. A., Delpino, S., Dimieri, L., Mogessie, A., Hoinkes, G., Hauzenberger, C. A. & Felfernig, A. 1998. Gravimetric and magnetometric anomalies in the Sierras Pampeanas of San Luis. Revista de la Asociación Geológica Argentina 53(4), 549-552.