LOWER CRUST EROSION DURING SOMÚN CURÁ POST-PLATEAU MAGMATISM?

CORDENONS, PABLO DAMIÁN

San Rafael

Encuentro; 14 Encuentro International Center For Earth Sciences; 2019

CNEA - UNCUYO

The Somún Curá Magmatic Province (SCMP) lies between 40º30?-43º20? S and 500-750 km east of the Chile trench, within the North Patagonian Massif. It is formed by a wide basaltic mesa of ~30?000 km2 in which the remnants of large bimodal volcanic complexes protrude. The magmatic activity spans from Eocene to Miocene times, with its peak during the Oligocene, and can be divided in pre-plateau, plateau and post-plateau stages (Kay et al., 2007). At the southernmost ranges of the SCMP, a basaltic plains extends between the Chacays and Negra de Telsen sierras, where oligocene (~27 Ma) Somún Curá Formation´s olivine basalts are covered by the miocene (~17) La Mesada Basalt, showing the passage from plateau to post-plateau stage. La Mesada Basalt differs from its predecessor in that it hosts orthopyroxene xenocrysts with an olivine disequilibrium crown and plagioclase xenocrystic clusters with irregular contours, lamellar polysynthetic twinning and sutured triple boundaries between grains, characteristics that might reflect a lower crust origin. Similar xenocrysts have been reported at Mallín Grande, Cañadón Pelado and La Colonia sierra, where the host lava was dated in ~21 Ma (Cordenons, 2017). As both Kay et al. (2007) and Cordenons (2017) discarded significant contamination by lithospheric assimilation during the SCMP emplacement, the systematic presence of these xenocrysts in miocene lavas in such extended area needs yet to be accounted for. One possible explanation could be the thermomechanical erosion of the lithosphere (Davies, 1994) by asthenospheric upwelling. If so, this would have required such a lithospheric thinning that the local North Patagonian Massif?s lower crust was directly exposed to the asthenosphere. The ascending magmas would have then transported upwards the released noritic fragments, rapidly enough or in such a small proportion that they had negligible impact on the magmas? chemistry. Implications for this hypothesis are not minor, and thus will be thoroughly analyzed in future works.Sponsored by UBACYT 20020170100554BA & PIP11220130100579CO