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

Abstract

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.