Isotopic composition of Late Mesozoic basic and ultrabasic rocks from the Andes (23–32°S) – implications for the Andean mantle

Abstract

Mesozoic mantle-derived magmatic rocks from the Jurassic–Early Cretaceous active continental margin of N Chile near 24°S, and from the Cretaceous rift system of NW Argentina near 26°S (central rift) and 32°S (southern rift) are used to constrain the isotopic composition of mantle reservoirs before the onset of the Cenozoic Andean orogeny. They are largely uncontaminated by assimilation of the local Palaeozoic basement as seen in the major and trace element patterns and Sr and Nd isotopic composition of most samples, but small amounts of crustal contamination are detected in the Pb isotope composition. Two different types of mantle sources have been identified. (1) For rocks from the Jurassic arc, now at the continental margin at 24°S, and for rocks from the central rift at 26°S, a depleted mantle source with 143Nd/144Nd(t) 0.5127–0.5128, 87Sr/86Sr(t) ca. 0.7033 may be inferred. Pb isotopic ratios of most samples from the magmatic arc are overall less radiogenic than those from the rift (206Pb/204Pb(t) ca. 18.0–18.4 vs. 18.1–19.1, 207Pb/204Pb(t) 15.55–15.59 vs 15.58–15.64 and 208Pb/204Pb(t) 37.8–38.6 vs. 38.4–38.9). (2) In the southern rift at ca. 30–32°S [Sierras Los Condores region (LC) and Sierras Chajan region (Ch)], an enriched mantle source has been identified, which is similar to the composition of the mantle lithosphere beneath the Brazilian Shield. Samples from LC and Ch show some isotopic differences because of varying degrees of crustal contamination in the LC samples. The LC samples have a higher Th/U ratio than the Ch samples and different Pb isotopic ratios 206Pb/204Pb(t) 18.0–18.6 (LC) vs. 17.7 and 17.8 (Ch), 207Pb/204Pb(t) 15.50–15.66 (LC) vs. 15.50 (Ch) and 208Pb/204Pb(t) 38.6–39.2 (LC) vs. ca. 37.5 (Ch). The Nd isotopic ratios of LC and Ch samples are similar 143Nd/144Nd(t) 0.5123–0.5122 (LC) vs. 0.5123 (Ch), whereas their Sr isotopic ratios differ 87Sr/86Sr(t) 0.7054–0.7079 (LC) vs. ca. 0.704 (Ch). Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00410-001-0344-3.