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

Garnet-bearing volcanic rocks are rare in convergent margin settings, yet they constitute a distinct component of the Late Miocene volcanic suite in the Northern Andes along Colombia’s Central Cordillera. Here, we present new petrographic, geochemical, and geochronological data from seven porphyritic garnet-bearing andesites of the Amagá-Cauca-Patía Basin (ACPB) to constrain their petrogenesis, crystallization conditions, and tectonic context. We identify three distinct garnet types in the ACPB andesites, distinguished by their chemistry, textures, and associated mineral assemblages. Type‑1 and Type‑2 garnets, from the southern and central ACPB, crystallized at moderate depths (8–7 kbar and ~900 °C), consistent with rapid ascent and differentiation in mid‑crustal reservoirs. In contrast, Type‑3 garnets, found in the northernmost samples, formed at greater depths (~13 kbar and 1100 °C) within deep magma chambers. Textural evidence, including garnet and plagioclase resorption, amphibole diffusion zoning, and sieve textures, indicates that these melts evolved in an open system, involving processes such as crustal assimilation, crystal entrainment, and magma recharge in shallow reservoirs. Mineral chemistry and trace element data show that the metaluminous magmas come from a parental melt modified by partial melting of enriched lithospheric mantle. U – Pb zircon dating reveals a south-to-north progression of magmatism, with Type-1 garnet-bearing andesites emplaced at 11.75 Ma in the south, Type-2 at 10.43 Ma, and Type-3 at 8.45 Ma in the north. We propose that these garnet-bearing andesites formed under transient extensional conditions associated with the subduction of the Sandra Ridge beneath the South American Plate, which facilitated the ascent and preservation of deep-seated hydrous magmas. This study provides direct mineralogical evidence of stratified magma storage and dynamic magmatic processes that shaped the Northern Andean block during the Miocene.