A GEOTECTONIC MODEL FOR THE LATE PALEOZOIC IN THE PROTO-ANDEAN MARGIN OF GONDWANA FROM ND ISOTOPIC DATA, SIERRAS PAMPEANAS (ARGENTINA).

DAHLQUIST, J.A.; ALASINO, P.H.; RAPELA, C.W.; CASQUET , C.; GALINDO, C.; PANKHUSRT, R.J.; BALDO, E.G.

Heidelberg

Simposio; 22nd Lateinamerika-Kolloquium.; 2011

Heidelberg Institute of Earth Sciences.

Late Devonian and Early Carboniferous granitoids in Sierras Pampeanas of Argentina (proto-Andean foreland) are A-type magmas that represent variable mixtures of asthenospheric mantle and continental lithosphere. The magma source was primarily (and dominantly for the Early Carboniferous granites) asthenospheric mantle with subsequent crustal contamination (Rapela et al., 2008; Dahlquist et al., 2010). Our Nd isotopic study reveals varying juvenile magma participation in the parental magma of granitic rocks from Late Devonian to the Early Carboniferous. Late Devonian granites are localized in the east, on the foreland, and the Carboniferous granites towards the west, probably near the paleo-trench. Isotopic data from Late Devonian granitoids of the large Achala batholith (Rapela et aI., 2008; Dahlquist et al., 2010) show ENd values of -4.0 to -6.2 (mean =-5.5) for the main granitic facies (379 Ma) and -1.3 to -1.9 (mean =-1.6) for smallscale tonalitic intrusive rocks (369 Ma), representing possible juvenile magma ascended by local conduits. Carboniferous granites represented by the Huaco granitic complex (354 Ma), the San Bias pluton (340 Ma), the Zapata granitic complex (323 Ma), and the Los Arboles pluton (335 Ma) have ENd values of +0.6 to -4.8, (mean =-1.9), -2.6 to -3.9 (mean =-3.3), -0.8 to -2.6 (mean =-1.6), -2.4 to -3.2 (mean =-2.8); respectively. Nd isotope composition thus suggests that the extension in the continental lithosphere (providing suitable conduits for ascending juvenile material and heat influx in the crust) as well as the mantle contribution to the A-type granitic magmas increased with time, starting in the foreland with the Late Devonian and continuing eastward with the Carboniferous magmatism. Sims et al. (1998) argued however that the Achala batholith was emplaced under compressional conditions. Our suggestion, based on structural considerations and the Nd isotope data, is that the Late Devonian parental magmas were generated in a dominantly extensional environment but were emplaced under transient compressional conditions. By the Early Carboniferous pure extensional conditions prevailed on the west, leading to the generation of the Carboniferous parental magmas.