Cenozoic Basaltic volcanic fields at the rear of Southern Andes, between Argentino and Viedma Lakes (Patagonia Argentina)

MAZZARINI F; D'ORAZIO M; HALLER MJ; INNOCENTI F; MANETTI P; MEISTER CM

Chieti

Congreso; FIST, GeoItalia 2001; 2001

FIST

Intra-plate geodynamic settings as well as passive and active continental margins share the occurrence of OIB-type magmatism. Particularly, in West Antarctica and southern South America, Tertiary-Quaternary mafic volcanism occurs just at the rear of the active Pacific margin of Antarctic Peninsula and Patagonia, respectively (Hole & Larter, 1993; Gorring et al., 1997; D'Orazio et al., 2000). In this note we report preliminary geologic, structural and geochemical data on basalts cropping out in southern Patagonia, south of latitude 49 "S in the area between Argentino and Viedma lakes. Since late Paleozoic up to Quaternary the Pacific margin of South America experienced several phases of compression, rifting and wrenching in response to changing in plate tectonics. Paleozoic, Mesozoic and Cenozoic metamorphic, sedimentary and volcanic sequences form the main cordillera that, eastward, through a fold and thrust belt connects with the Magellan foreland basin characterized by up to 8000 m of sedimentary and volcanic sequences (see Diraison et al., 2000 for a review). Transition from the cordillera to the foreland is marked by widespread, Cenozoic, mafic volcanics (Gorring et al., 1997; D'Orazio et al., 2000). The surveyed area extends eastward from the cordillera for - 150 km and expands for ~100 km in NS direction. Main morphologic features are the Argentino and Viedma lakes whose terminations, characterized by widespread glacial and fluvio-glacial deposits, occur in the westemmost sector of the studied area. The Rio Santa Cruz originates from Argentino Lake and runs to the Atlantic Ocean, whereas the Rio La Leona emanates from Viedma Lake running southward into the Argentino Lake. In the studied area, Cenozoic volcanics rest upon both Mesozoic sedimentary sequences and Tertiary continental deposits (Mercer, 1976; Panza & Nullo, 1994). The studied volcanics consist of alkaline and subalkaline basalts, basaltic andesites and trachibasalts forming several large mesas built up by tabular lava flows and characterized by elevations ranging from 1500 m close to the cordillera (e.g. Meseta del Viento) to 150-200 m for the easternmost ones. Several volcanic cones, generally outlined by yellow tones in the RGB FCC Landsat TM image (bands, 7, 5, 4), eruptive fractures and associated lavas are scattered over the mesas. The channeled lava flows seem to fill paleo-valleys and, especially in the easternmost area, flowed from the mesas into the present Rio Santa Cruz valley (e.g. Condor Cliff). Satellite lineament analysis was performed in order to constraint the regional tectonic framework. Preliminary data suggest the occurrence of two main fracture trends directed NW-SE and NE-SW, respectively. These two fracture systems seem to control the erosional morphologies of mesas as well as of glacial deposits. Textural, tonal and spectral features of different types of surfaces, allowed us to recognize seven homogeneous geomorphic units: lava mesas, sedimentary mesas, lava flows, scoria cones and related lava flows, glacial deposits, playas and desert areas. Sampled lavas vary between to extreme compositions: - products belonging to the most recent (as evidenced from morphological features) volcanic fields (Camusú Aike) are alkaline and relatively evolved. They exhibit a clear OIB-type trace element distribution; - older mesa-forming lavas from the Meseta de las Vizcachas are subalkaline and more primitive. They are characterized by high LILE/HFSE ratios and negative Nb-Ta anomalies, type of subduction-related magmas. Samples from other volcanic fields (e.g. Cerro Frailes, basal Camusú Aike lavas) show intermediate geochemical features. These geochemical variations are related to the evolution of mantle magma source of this area. The oldest lavas originated from sources modified by subduction-derived components released from the oceanic pacific plates subducting under South America, whereas youngest lavas were produced by an fertile asthenospheric source replacing the subduction-modified supra-slab mantle. Such an evolution of the mantle magma sources is compatible with a process of slab window opening under this sector of southern South America following the ridge-trench collision occurred along its pacific margin since 14 Ma (Gorring et al., 1997; D'Orazio et al., 2000).