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 are 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 andesite samples within the Amagá-Cauca-Patía Basin (ACPB) to constrain their petrogenesis, formation conditions, and tectonic setting. Mineral chemistry and trace element geochemical data indicate that the metaluminous magmas are the product of a mantle-derived parent magma influenced by partial melting of enriched lithospheric mantle sources. Textural evidence, including garnet and plagioclase resorption, amphibole diffusion zoning, and sieve textures, suggests an open-system evolution involving crustal assimilation, crystal entrainment, and magma recharge in shallow reservoirs.
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.

在汇聚边缘环境中，含石榴子石火山岩较为罕见，然而，它们构成了哥伦比亚中安第斯山脉北部晚中新世火山套的独立组成部分。本研究展示了来自阿马加-卡乌卡-帕蒂亚盆地（ACPB）七个流纹岩样品的新岩相学、地球化学和地质年代数据，以限定其成因、形成条件和构造环境。矿物化学和微量元素地球化学数据表明，金属铝质岩浆是由地幔源岩浆和富集地壳源岩部分熔融的产物。纹理证据，包括石榴子石和斜长石的吸收、角闪石的扩散分带以及筛状结构，暗示了涉及地壳同化、晶体捕获和浅层储库中岩浆补充的开放系统演化。这些岩石的详细岩相学分析揭示了三种不同的石榴子石类型，根据成分和纹理进行区分，表明了一种复杂的多阶段岩浆演化，具有显著的岩浆混合和矿物-液态相互作用。最北端样品中的3型石榴子石在深部岩浆房中于约13 kbar和1100°C下结晶，而中部和南部ACPB记录的1型和2型石榴子石在较浅的结晶深度（5-7 kbar和约900°C）下结晶，这与快速上升和分异相一致。U-Pb 锆石定年支持了从南到北的侵位-结晶梯度，南部1型石榴子石岩在11.75 Ma时侵位，2型岩在10.43 Ma，北部3型岩在8.45 Ma。这些含石榴子石岩在与南美洲板块下 Sandra Ridge 相关联的瞬时伸展条件下形成，促进了富含水的深部岩浆的快速上升。本研究为塑造中新世期间北部安第斯地块的分层岩浆储存系统和动态岩浆过程提供了直接的矿物学证据。