The Cenozoic Mafic Magmatism of the Extra-Andean Patagonia (44.5°S-52°S): Sources and Geodynamic Significance

D'ORAZIO M; INNOCENTI F; HALLER MJ; MANETTI P; TOMARINI S; BRUNI S

Florencia

Congreso; 32nd Internationa Geolological Congress; 2004

The voluminous effusion of basaltic plateau lavas is one the most prominent similarities and the dissimilarities of the mass movement classification systems event; occurred during the Cenozoic geological history of the extra-Andean Patagonia. At latitudes comprised between 44.5º S and 52º S, the volcanic rocks range from mildly silica -undersaturated basanites and tephrites- up to quartz normative basaltic andesites, with very scarce s1ightly products (up to hawaiite). Trace elements and Sr-Nd Isotopes revealed that the large majority of volcanics, regardless their age, have a well-marked within-plate geochemical affinity. A smaller group of lavas reveal a tendency towards the typical signatures of subduction-related magmas (e.g. high LILE/HFSE and LREE/HFSE ratios). Typically these products occur very close or above the easternmost side of the Andean cordillera. On the base of Sr-Nd isotopes Patagonian lavas can be divided in two groups: one is characterized by depleted Sr-Nd isotope compositions (87Sr186Sr - 0.7032 143Ndl144Nd - 0.51290) and essentially contains the products from the southernmost lava fields (e.g. Pali Aike). The other group is characterized by more enriched Sr-Nd isotope compositions (87Sr/86S 0.7036-0.7048 143Nd/144Nd 0.51264-0.51280) and contains the large majority of Cenozoic Patagonian lavas. The prevalence over a long time span and in a dominantly back-arc setting, of magmas with dell-defined Intraplate signature against magmas with orogenic imprints, can be interpreted according to two alternative geodynamic models The generation of these magmas can be related to the ridge-trench collision events occurred in Cenozoic times along the Pacific margin of the cordillera.  These collisions lead to the opening of slab-windows beneath the South American Plate allowing the upwelling and decompressional melting of asthenosphere: 1n particular the collision between the Chile Ridge and the Chile Trench occurred starting from 16 Ma originated a slab-window responsible for the genesis of the Late Miocene to Present volcanic, rocks comprised between ~46.5º S and 52º S whereas the Oligocene-Early Miocene lavas between 44.5º S and 46.5º S could de related to previous ridge-trench collision events. Alternatively, the Patagonian within-plate magmas can be originated by melting of the subcontinental mantle in response to the local convection in the mantle wedge induced by the subduction of the oceanic lithosphere along the Pacific margin of the continent.