Cenozoic Mafic Magmatism in the Sierra San Bernardo and Río Genoa-Senguerr Valley, Chubut, Argentina

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

Florencia

Congreso; 32nd International Geological Congress; 2004

Oligocene, Miocene and Pliocene mafic igneous rocks occur in back-arc setting, in the Sierra San Bernardo (SSB) and Rio Genoa-Senguer Valley (RGSV) (44-46º S 69-71º W).  The Oligocene –Miocene magmatic activity is represented in the SSB by gabbroic rocks, occurring as small intrusions and large tabular bodies, as well as by basalts forming conspicuous mesetas. These latter can either preserve their original sub-horizontal position or be tectonically displaced. Along the RGSV occur a series of Pliocene (?) spatter cones and their lava flow fields. Some of these cones (e.g. Cerro de los Chenques) bear mantle xenoliths of variable nature and size.  The magmatic rocks of the whole area can be divided into three groups. 1) basaltic andesites; 2) transitional basalts, and (3) trachybalts and basanites. The alkaline rocks can be furtherly divided into sodic and potassic groups. Petrographically the volcanic rocks vary between two extreme types: lavas from the mesetas of SSB are porphyritic, holocrystalline with olivine phenocrysts and a groundmass constituted by labradoritic- bytownitic plagioclase, purple brown Ti-augite and Fe-Ti oxides. Lavas erupted from the scoria cone of RGSV are subaphyric, sometimes qlass-bearing, with olivine as the only phenocryst phase.  The primitive nature of the studied rocks and their incompatible elements ratios highlight a strong within-plate geochemical signature (e.g. La/Nb<1. Ba/Ta<130; Ba/La<15) without the occurrence of continental crust assimilation and/or subduction-related components. Moreover, the low concentration of HREE and high Sm/Yb ratio of some lavas is indicative of melting of a primitive source in presence of residual garnet. Within the studied area a notable exception is constituted by the volcanic cone of Cerro Ante, located in close proximity to the Andean Cordillera. The primitive alkaline lavas erupted by this cone are characterized by a strong subduction-related signature, revealed by high LILE/HFSE (e.g. La/Nb>3) ratios and related to the peculiar geographic/geodynamic position of this volcanic occurrence. The SSB and RGSV are located outside the vertical projection of the slab-window supposed to occur to the southeast of the Chile Triple Junction. Hence the causes of mantle melting in this area should be different; possibly they are related to previous ridge-trench collisions occurred during Eocene times along the Pacific margin of South America.