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

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 δ18O = +8.7 ± 0.5‰, δD = −73 ± 14‰, 87Sr/86Srt = 0.7088 ± 0.0001 and εNdt = −4.5 ± 0.6) show no major difference from those of most of the mafic rocks (average δ18O = +8 ± 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 = +12.2 ± 1.7‰, δD = −89 ± 21‰, 87Sr/86Srt = 0.7146 ± 0.0034 and εNdt = −6.9 ± 0.7) by the tonalite is in most samples within the range 10–20%. Felsic magmas that reached upper crustal levels had isotope values (δ18O = +9.9 ± 1.5‰, δD= −76 ± 5‰, 87Sr/86Srt = 0.7067 ± 0.0010, εNdt = −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 δ18O of magmatic zircons from the Famatinian arc rocks (+6 to +9‰) are comparable to our δ18O whole-rock data, indicating that pervasive oxygen isotope exchange in the lower crust was not a major process after zircon crystallization.

Thermal maturation of a complete magmatic plumbing system at the Sierra de Velasco, Northwestern Argentina

“…Liu et al 2014). Similar δ 18 O compositions (δ 18 O = þ8) have been observed for mafic rocks from the Ordovician Famatinian arc (Alasino et al 2016(Alasino et al , 2020Rapela et al 2018), suggesting an invariable nature of δ 18 O composition for the sublithospheric mantle since at least Ordovician times. The anomalous δ 18 O value of þ4.8 ‰ is not taken into account since it may be influenced by the presence of calcite-rich vesicles that resulted from circulation of water-rich fluids.…”

Devonian to Early Carboniferous Retreating—Advancing Subduction Switch in the Northwestern Patagonia Accretionary Orogen: U‐Pb and Lu‐Hf Isotopic Insights

“…On the other hand, the northern segment exhibits a latitudinal segmentation (Figure 8c). A widespread early Carboniferous magmatic episode with positive εNd trend is likely associated with an extensional setting recorded between 28° and 32°S (Figure 8c; Alasino et al., 2012). In contrast, the section between 32° and 35°S is characterized by the absence of magmatic records, basement exhumation, and reverse brittle‐ductile shear zones in the Pampean Range, Frontal Cordillera and San Rafael Block (e.g., Bense et al., 2014, 2017; Löbens et al., 2017).…”

“…In contrast, extensional regimes are linked to retreating orogen stages triggered by steeply dipping oceanic lithosphere subduction. In such a setting, continental arc magmatism record trenchwards migration, crustal thinning and a significant mantle‐derived juvenile magmatic input that produces εHf and εNd positive signatures (e.g., Alasino et al., 2012; Collins et al., 2011; Du et al., 2021; Folguera & Ramos, 2011; Gianni et al., 2018; Kemp et al., 2009; Li et al., 2011; Nelson & Cottle, 2018; Oriolo et al., 2023; Wu et al., 2021). Throughout the subduction evolution, each regime can operate at different timescales, from ca.…”