Preliminary results of Sm-Nd analisys of Yahgán and Lapataia Formations, their provenance.

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

Santiago de Chile

Congreso; GeoSur; 2007

Sm- Nd analyses were carried out for the first time in the Yahgán and Lapataiametasedimentary Fonnations in TielTa del Fuego Agentina. The Deformed Complex ofthe Fueguian Andes (Quartino el al., 1987; Acevedo, 1988) comprise severallithostratigraphic units related because deformational styles of thrusting and lowmetamorphic grade. The sequence starts with a low grade metamorphic basementnamed Lapataia schists of Palaeozoic age (Borello, 1969) recognized in Chile andmentioned outcropping in the south western area of Tierra del Fuego Island. To the top,the upper Jurassic-Iower Cretaceous Yahgán Fonnation (Kranck, 1932) is superimposedby the Porfiritic Series (Harrington, 1943) rhyolitic rocks and upper Cretaceous marinesediments. The metamorphic grade recognized in the Yahgán fonnation is zeolite facies,with prehnite and pumpellyte paragénesis. There has been continuous debate aboutwhether the Palaeozoic metasediments of Lapataia Formation, considered equivalent tothe Chilean basement, are representative of the Yahgán Cretaceous Formation and noPalaeozoic basement is cropping out in the Argentinian part of the Tierra del FuegoIsland. Provenance of detritic rocks in a sedimentary basin could be characterized byanalysis of Nd isotopes in the sedimentary rocks (De Paolo el al., 1991). Preliminarydata on metasediments from Lapataia Formation (samples LAPA 1, 2, 3, and 4) andYahgán Formation (Difu 11, MO 14 and PG 18) in a profile crossing the FueguianAndes gave CNd(t), between -1.04 and -4.82 for Lapataia Fonnation and -0.017 and -7.9for Yahgán. The Sm/Nd ratios are between 0.1290 and 0.1696 for Lapataia and 0.0963and 0.1560 for Yahgán, indicating a basic magmatic rock imput in the basin. TDM modelages are between 1.1 and 1.35 in both formations. These preliminary data do not showany isotopic differences between Lapataia and Yahgán Formations and suggest that bothmetasedimentary units represent the Upper Jurassic - Lower Cretaceous basin, and notbasement rocks - similar to the basement in Chile - cropping out in the ArgentinianFueguian Andes.