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O–H–Sr–Nd isotope constraints on the origin of the Famatinian magmatic arc, NW Argentina

2020; Cambridge University Press; Volume: 157; Issue: 12 Linguagem: Inglês

10.1017/s0016756820000321

1469-5081

Pablo H. Alasino, César Casquet, Carmen Galindo, R. J. Pankhurst, Carlos W. Rapela, Juán A. Dahlquist, Clemente Récio, Edgardo G. Baldo, Mariano A. Larrovere, Carlos D. Ramacciotti,

High-pressure geophysics and materials

Abstract We report a study of whole-rock O–H–Sr–Nd isotopes of Ordovician igneous and metamorphic rocks exposed at different crustal palaeodepths along c. 750 km in the Sierras Pampeanas, NW Argentina. The isotope compositions preserved in the intermediate rocks (mostly tonalite) (average δ 18 O = +8.7 ± 0.5‰, δD = −73 ± 14‰, 87 Sr/ 86 Sr t = 0.7088 ± 0.0001 and εNd t = −4.5 ± 0.6) show no major difference from those of most of the mafic rocks (average δ 18 O = +8 ± 0.8‰, δD = −84 ± 18‰, 87 Sr/ 86 Sr t = 0.7082 ± 0.0016 and εNd t = −4 ± 1.1), suggesting that most of their magmas acquired their crustal characteristics in the mantle. The estimate of assimilation of crustal material (δ 18 O = +12.2 ± 1.7‰, δD = −89 ± 21‰, 87 Sr/ 86 Sr t = 0.7146 ± 0.0034 and εNd t = −6.9 ± 0.7) by the tonalite is in most samples within the range 10–20%. Felsic magmas that reached upper crustal levels had isotope values (δ 18 O = +9.9 ± 1.5‰, δD= −76 ± 5‰, 87 Sr/ 86 Sr t = 0.7067 ± 0.0010, εNd t = −3.5 ± 1.4) suggesting that they were not derived by fractionation of the contaminated intermediate magmas, but evolved from different magma batches. Some rocks of the arc, both igneous (mostly gabbro and tonalite) and metamorphic, underwent restricted interaction with meteoric fluids. Reported values of δ 18 O of magmatic zircons from the Famatinian arc rocks (+6 to +9‰) are comparable to our δ 18 O whole-rock data, indicating that pervasive oxygen isotope exchange in the lower crust was not a major process after zircon crystallization.