Petrological and chemical evidence for polybaric differentiation across the preserved middle crustal plutonic column of the Famatinian arc, Argentina

CAMILLETTI, G.C.; OTAMENDI, J.E.; ESCRIBANO, F.A.; CRISTOFOLINI, E.A.

INTERNATIONAL JOURNAL OF EARTH SCIENCES

SPRINGER

Año: 2023 vol. 112 p. 2025 - 2047

1437-3254

This study investigates the formation of the middle layer of the Earth´s crust in the Early Ordovician Famatinian arc. We focus on the Valle Fértil–La Huerta batholith, which provides a unique exposure of continuous lower to middle crustal depths, spanning approximately 14 km. The assembly of this batholith took place over a period of approximately 6 million years (473–467 Ma). We collected whole-rock and mineral geochemical data at different emplacement pressures, ranging from < 4 to 7 kbar. Our results indicate that amphiboles crystallized within a temperature range of 774–873 °C but exhibited chemical disequilibrium with the surrounding rocks. Both whole-rock and mineral chemistry provide evidence of plagioclase and amphibole accumulation during magmatic differentiation. Examining the vertical distribution of plutonic rocks, we observe a compositional trend showing decreasing concentrations of MgO, Sr, Sc, and Dy/Yb with decreasing paleo-depths. A comparative analysis of the REE patterns yields that plutonic rocks in the shallow middle crust contain amphibole crystal cores inherited from deeper crustal levels. Moreover, the trace-element compositions of the amphiboles yield valuable insights into the coexistence of distinct magma compositions at different levels within the middle crust. Importantly, mantle-derived mafic rocks with high-Sr/Y ratios are attributed to plagioclase accumulation rather than the presence of garnet as a residual phase. Our study suggests that fractional crystallization played a significant role in the generation and evolution of the Famatinian intermediate crust. Further differentiation occurred through mixing between tonalitic magmas and intra-crustal metasedimentary-derived melts in the middle crust. The findings highlight the importance of variable melt extraction in shaping the petrological stratification of arc crustal columns.