Garnet-bearing volcanic rocks are extremely rare at convergent margins, with few known occurrences worldwide [ref 1-2]; however, such rocks are common within the late Miocene volcanic rock suite of the Northern Andean block (NAB) along the Central Cordillera, Colombia [ref 3-5]. They have been linked to pre-existing zones of crustal weakness that channeled magmas to the upper crust in a short period of time [ref 5-6]. Here we present new geochronological and petrographic data to constrain the timing and petrogenesis of these unusual rocks. We obtained mineral chemical analyses from 7 porphyritic-andesite samples from the eastern flank of the Central Cordillera and the Cauca-Patia Basin, Colombia. Garnet phenocrysts are almandine in composition, ranging from 23 to 29 wt.% FeO, 6 to 8 wt.% CaO, and 1 to 4 wt.% MnO. In some samples, garnets are homogeneous with no reaction rims and lacking inclusions; however, in other samples, garnets show re-absorption rims and inclusion assemblages similar to the rock matrix (plag, amph, ± bt) as well as rare plagioclase coronas. The high Ca and low Mn contents of the NAB garnet cores are consistent with crystallization at ~1.2GPa, based on phase equilibrium experiments of [ref 7], while garnet rim assemblages are congruent with a second stage of crystallization at ~0.8GPa under water-undersaturated conditions. Finally, a pre-eruption dehydration stage is evidenced by the presence of breakdown rims in amphibole crystals. Our new U-Pb in zircon ages reveal that NAB garnet-bearing volcanic rocks formed between 9 and 8 Ma. Taken together, our data suggest a rapid ascent of the NAB magmas associated with the onset of regional volcanism and extension, and possibly the development of the Caldas Tear, a slab window within the Nazca Plate. [1] Green & Ringwood (1968) CMP. [2] Harangi et al. (2001) Journal of Petrology. [3] Orrego (1975) UNAL Colombia. [4] García (1983) UNAL Colombia. [5] Bissig et al. (2017) EG. [6] Weber et al. (2018) SGC (in press). [7] Alonso-Pérez et al. (2006) CMP.