Petrogenesis of garnet-bearing two-mica leucogranites from the late Paleozoic Deuco pluton, Nahuelbuta Batholith, south-central Chile

Rabbia O.; Hernández L.; Steenken A.; Aragón E.; Demichelis A.; Hidalgo A. · Journal of South American Earth Sciences (2026) · DOI: 10.1016/j.jsames.2026.106124

Resumen

The Deuco pluton at the southern end of the Late Paleozoic Nahuelbuta Batholith (south-central Chile) comprises small, leucocratic, garnet-bearing two-mica granite bodies with an unusual trondhjemitic affinity: high SiO2 (73-76 wt%), calcic yet strongly peraluminous (ASI = 1.14-1.32), high Na2O/K2O (1.2-2.4), and elevated Sr/Y (44-73) and La/Ybn (25-80), but low K, Rb, Th, U, and Ti. Whole-rock geochemistry, mineral chemistry, and Eu-anomaly modeling show that (i) the least evolved samples carry positive Eu/Eu∗ and high Sr consistent with accumulation of An40-50 plagioclase, whereas (ii) the slight decrease in Eu/Eu∗ among the most evolved samples is reproduced by modest Rayleigh plagioclase fractionation. Almandine-spessartine garnet (Alm59-66 Sps29-36 Prp2-4 Grs1-3) is texturally equilibrated, euhedral, and compositionally zoned in a manner typical of magmatic growth, not xenocrystic entrainment. Primitive-mantle-normalized patterns display HREE-Y and strong Ti depletions, implying a garnet (± rutile ± zircon) residue with minor/no residual plagioclase (high Sr/Y). We infer a high-pressure (>1 GPa) dehydration partial-melting origin for the parental melt, a high-SiO2, HREE-depleted tonalitic–trondhjemitic liquid lacking a significant Eu/Eu∗, derived from low-K mafic (meta)igneous lower crust. Subsequent upper-crustal plagioclase accumulation and limited fractionation produced the observed spread in Eu/Eu∗, Sr, and Na2O/K2O without invoking extensive AFC. Oxygen-fugacity indicators (garnet–biotite assemblages; absence of titaniferous magnetite) point to ≈ NNO to NNO+1 conditions. Our preferred age for the emplacement of the Deuco pluton is 298.2 ± 1.2 million years ago (Ma), which is consistent with field observations.Overall, Deuco-type magmatism aligns closely with lower-crust melting in a thickened continental-margin arc. However, this scenario contrasts with a forearc accretionary wedge setting leading us to refrain from completely ruling out subducted slab melting.Finally, unlike other garnet-bearing lithologies within this batholith, the low total REE, especially MREE-HREE, of Deuco-type rocks limits the potential for residual REE resources in these granites.

Instituciones: Universidad Nacional Jorge Basadre Grohmann