Felsic microgranitoid enclaves as records of solidification front collapse in silicic magma chambers: Insights from the Devonian Characato suite (Achala Intrusive Complex, Argentina)

Abstract

Felsic microgranitoid enclaves (FMEs) are underexplored components of granitic plutons that preserve key evidence of magma chamber processes. This study examines FMEs and the coexisting fine-grained units in relation to the porphyritic regional granites that host them within the Characato suite of the Achala Intrusive Complex, Argentina, integrating field observations, petrography, whole-rock geochemistry, mineral chemistry, and Nd isotope analyses.Our main conclusions indicate that the FMEs and associated fine-grained units represent autolithic fragments derived from solidification fronts within granitic magma chambers. Their finer grain size and relatively higher crystallization temperatures (e.g., ΔTZr ≈ 60 °C and ΔTBt ≈ 40 °C) suggest early crystallization under enhanced undercooling conditions near chamber margins.Field evidence -such as spatial association of FMEs with marginal fine-grained units, gradational contacts, ductile felsic dykes, and magmatic layering and deformation-indicates multiple episodes of solidification front collapse into the host magma. The observed diversity in enclave size, texture, and degree of interaction reflects a continuum of rheological states, ranging from crystal-rich suspensions to partially solidified mushes.Furthermore, the neodymium isotopic compositions and geochemical trends of the FMEs and marginal fine-grained units are consistent with those of the regional host granites. This supports a cogenetic and crustal origin, with the enclaves and associated lithologies representing more primitive crystallization products.This study emphasizes the dynamic role of solidification fronts as zones of instability, and magmatic recycling, contributing to the construction and internal evolution of felsic plutonic systems.