Abstract
The alkaline volcanic rocks of the 1.8–0.9 Ma Auca Mahuida and post-mid-Pliocene Rio Colorado backarc volcanic fields east of the Andean Southern Volcanic Zone at ~37°–38°S have pronounced intraplate-like chemical signatures with some striking similarities to oceanic DM-EM1-like lavas of the south Atlantic Tristan da Cunha type. These backarc lavas are considered to have formed as a series of mantle batches typified by 4–7 % melting, with decompression melting initiating in a garnet-bearing mantle above a steepening subduction zone, and final equilibration occurring near the base of a ~65- to 70-km-thick lithosphere at temperatures of ~1,350–1,380 °C. Evolved Auca Mahuida mugearite to trachytic samples are best explained by crystal fractionation with limited mixing of partial melts of recently underplated basalts, in line with isotopic signatures that preclude significant radiogenic contamination in a preexisting refractory crust. Higher Ba/La and subtly higher La/Ta ratios than in nearby ~24–20 Ma primitive basalts or oceanic (OIB) lavas are attributed to the residual effects of slab fluids introduced during a shallow subduction episode recorded in the arc-like chemistry of the adjacent 7–4 Ma Chachahuén volcanic complex. Positive Sr, K and Ba spikes on mantle-normalized patterns of both primitive Auca Mahuida and ~24–20 Ma basalts, like those in EM-like OIB basalts, are attributed to mixing of continental lithosphere into the asthenosphere. In Patagonia, this mixing is suggested to have peaked as the South America continent accommodated to major late Oligocene plate convergence changes, as similar Sr, K and Ba spikes and DM-EM1 signatures are absent in ~50–30 Ma backarc lavas north of 51°S, and all of those south of 51°S. Introduction of an EM1-like component associated with lateral mantle flow of a Tristan da Cunha source is largely precluded by its Cretaceous age and distance to Patagonia.
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Acknowledgments
Initial funding was provided by a grant from REPSOL-YPF for the work in Kay, S.M., Tertiary to Recent magmatism and tectonics of the Neuquén Basin between 36.5°S and 38°S latitude, Report to REPSOL-YPF, 2001, 215 pp. Later, funding came from US NSF grant 01-26000 and a Cornell graduate fellowship to Jones. We thank Daniel Ragona, Tomas Zapata, Facundo Fuentes, and Linda Godfrey for assisting with field and analytical work; Beatriz Coira for XRF analyses; the Cornell University Keck Isotope Laboratory and Center for Materials Research for use of analytical facilities and Adriana Bermudez, W. M. White, Andres Folguera, Victor Ramos and two anonymous reviewers for discussion and comments.
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Kay, S.M., Jones, H.A. & Kay, R.W. Origin of Tertiary to Recent EM- and subduction-like chemical and isotopic signatures in Auca Mahuida region (37°–38°S) and other Patagonian plateau lavas. Contrib Mineral Petrol 166, 165–192 (2013). https://doi.org/10.1007/s00410-013-0870-9
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DOI: https://doi.org/10.1007/s00410-013-0870-9