Orogenic Andesites of the Trans-Mexican Volcanic Belt - Mattia Parolari (UNAM, Messico)

Seminario di Mattia Parolari (UNAM, Messico) High-Mg orogenic andesites are regarded as the building blocks of the continents, yet their genesis remains a petrologic paradox involving the interplay between mantle derivation and crustal differentiation. The Trans-Mexican Volcanic Belt (TMVB) serves as a strategic natural laboratory to resolve this controversy, as it produces a volumetrically dominant suite of orogenic andesites along a highly erosive convergent margin. Over the past decade, we have integrated isotopic tracing, zircon geochronology, and mass-balance modeling to explore whether the isotopic enrichments of the TMVB andesites result from a "canonical" process of intracrustal differentiation/assimilation or instead arise from variations in the nature and extent of subducted crustal material. We show that the along-arc isotopic diversity (Sr, Nd, Pb, Hf) of the TMVB andesites is not controlled by the composition of the overriding basement, but instead closely mirrors the geochemical signature of the tectonically ablated forearc lithologies incorporated into the mantle wedge. Isotopic modelling further indicates that approximately half of the mass of these andesitic magmas derives from the reworking of subducted crust—both continental (forearc) and oceanic (altered slab). Consistently, mass-balance calculations in the western TMVB (Jalisco) reveal a steady state in which magmatic addition is effectively counterbalanced by subduction erosion, implying that this margin is not a site of net continental growth. We argue that mechanically mixed subduction mélanges—composed of eroded forearc, sediments, and altered oceanic crust—rise as diapirs within the mantle wedge, imparting their diverse isotopic signatures to the mantle source. This mechanism is also confirmed by the presence of xenocrystic zircons in the Malinche volcano, which preserve age spectra typical of the forearc provenance, supporting the transfer of subducted continental crust to the arc source region. Likewise, trace element modeling suggests that fractionation is controlled by diverse residual mineral assemblages formed during the mélange-mantle reaction. Thus, the "building" of continents in this margin is fundamentally a process of metamorphic reworking of previously destroyed crust.