Preliminary results of Sm-Nd analisys of Yahgán and Lapataia Formations, their provenance. Tierra del Fuego, Argentina.

ESCAYOLA, M.; ACEVEDO, R.; GONZÁLEZ GUILLOT, M.; PIMENTEL, M.

Santiago de Chile

Congreso; GeoSur 2007; 2007

Pontificia Universidad Católica de Chile

Sm-Nd analyses were carried out for the fírst time in the Yahgán and Lapataia metasedimentary Formations in Tierra del Fuego Agentina. The Deformed Complex of the Fueguian Andes (Quartino et al., 1987; Acevedo, 1988) comprise several lithostratigraphic units related becausedeformational styles of thrusting and low metamorphic grade. The sequence starts with a low grade metamorphíc basement named Lapataia schists of Palaeozoic age (Borello, 1969) recognized in Chile and mentioned outcropping in the south western área of Tierra del Fuego Island. To the top, the upper Jurassic-lower Cretaceous Yahgán Formation (Kranck, 1932) issuperimposed by the Porfiritic Series (Harrington, 1943) rhyolitic rocks and upper Cretaceous marine sediments. The metamorphic grade recognized in the Yahgán formation is zeolite facies, with prehnite and pumpellyte paragénesis. There has been continuous debate about whether the Palaeozoic metasediments of Lapataia Formation, considered equivalent to the Chilean basement, are representative of the Yahgán Cretaceous Formation and no Palaeozoic basement is cropping out in the Argentinian part of the Tierra del Fuego Island. Provenance of detritic rocks in a sedimentary basin could be characterized by analysis of Nd isotopes in the sedimentary rocks (De Paolo et al., 1991). Preliminary data on metasediments from LapataiaFormation (samples LAPA 1, 2, 3, and 4) and Yahgán Formation (Difu 11, MO 14 and PG 18) in a pro file crossing the Fueguian Andes gave sNd(t), between -1.04 and -4.82 for Lapataia Formation and -0.017 and -7.9 for Yahgán. The Sm/Nd ratios are between 0.1290 and 0.1696 for Lapataia and 0.0963 and 0.1560 for Yahgán, indicating a basic magmatic rock imput in the basin. TDM model ages are between 1.1 and 1.35 in both formations. Thesepreliminary data do not show any isotopic differences between Lapataia and Yahgán Formations and suggest that both metasedimentary units represent the Upper Jurassic - Lower Cretaceous basin, and not basement rocks - similar to the basement in Chile - cropping out in the Argentinian Fueguian Andes.