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

ALASINO, P.; CASQUET, C.; GALINDO, C.; PANKHURST, R.; RAPELA, C.; DAHLQUIST, J.; RECIO, C.; BALDO, E.; LARROVERE, M.; RAMACCIOTTI, C.

GEOLOGICAL MAGAZINE

CAMBRIDGE UNIV PRESS

Año: 2020 p. 1 - 14

0016-7568

We report a study of whole-rock O?H?Sr?Nd isotopes of Ordovician igneous and metamorphicrocks exposed at different crustal palaeodepths along c. 750 km in the Sierras Pampeanas,NW Argentina. The isotope compositions preserved in the intermediate rocks (mostlytonalite) (average δ18O = þ8.7 ± 0.5?, δD = −73 ± 14?, 87Sr/86Srt = 0.7088 ± 0.0001 andεNdt = −4.5 ± 0.6) show no major difference from those of most of the mafic rocks (averageδ18O = þ8 ± 0.8?, δD = −84 ± 18?, 87Sr/86Srt = 0.7082 ± 0.0016 and εNdt = −4 ± 1.1),suggesting that most of their magmas acquired their crustal characteristics in the mantle.The estimate of assimilation of crustal material (δ18O = þ12.2 ± 1.7?, δD = −89 ± 21?,87Sr/86Srt = 0.7146 ± 0.0034 and εNdt = −6.9 ± 0.7) by the tonalite is in most samples withinthe range 10?20%. Felsic magmas that reached upper crustal levels had isotope values(δ18O = þ9.9 ± 1.5?, δD= −76 ± 5?, 87Sr/86Srt = 0.7067 ± 0.0010, εNdt = −3.5 ± 1.4) suggestingthat they were not derived by fractionation of the contaminated intermediate magmas,but evolved from different magma batches. Some rocks of the arc, both igneous (mostlygabbro and tonalite) andmetamorphic, underwent restricted interaction withmeteoric fluids.Reported values of δ18O of magmatic zircons from the Famatinian arc rocks (þ6 to þ9?) arecomparable to our δ18O whole-rock data, indicating that pervasive oxygen isotope exchangein the lower crust was not a major process after zircon crystallization.