Late Miocene garnet-bearing andesites in the Northern Andes and their tectonic implications

Detailed petrographic analysis of these rocks reveals three distinct garnet types, differentiated by composition and texture, indicative of a complex, multistage magmatic evolution with significant magma mixing and mineral-liquid interactions. Type-3 garnets, found in the northernmost samples, crystallized at ~13 kbar and 1100°C in deep-seated magma chambers, while Type-1 and Type-2 garnets in the central and southern ACPB record shallower crystallization depths (5–7 kbar and ~900°C), consistent with rapid ascent and differentiation.

U-Pb zircon dating supports a south-to-north emplacement-crystallization gradient, with Type-1 garnet-bearing rocks emplaced at 11.75 Ma in the south, Type-2 rocks at 10.43 Ma, and Type-3 rocks at 8.45 Ma in the north. These garnet-bearing rocks formed under transient extensional conditions associated with the subduction of the Sandra Ridge beneath the South American Plate, facilitating the rapid ascent of deep hydrous magmas.

This study provides direct mineralogical evidence of the stratified magma storage system and dynamic magmatic processes that shaped the Northern Andean block during the Miocene.