loading page

Late Miocene garnet-bearing andesites in the Northern Andean Block and their tectonic implications
  • +4
  • Susana Hoyos,
  • Marion Weber,
  • Jose Duque Trujillo,
  • Andres Cardenas,
  • Elizabeth Cottrell,
  • Alejandro Beltran-Trivino,
  • Albrecht von Quadt
Susana Hoyos
Universidad EAFIT

Corresponding Author:[email protected]

Author Profile
Marion Weber
Universidad Nacional de Colombia Sede Medellín
Author Profile
Jose Duque Trujillo
Universidad EAFIT
Author Profile
Andres Cardenas
Universidad EAFIT
Author Profile
Elizabeth Cottrell
National Museum of Natural History, Smithsonian Institution
Author Profile
Alejandro Beltran-Trivino
EAFIT University
Author Profile
Albrecht von Quadt
ETH Zurich Institute of geochemistry and Petrology
Author Profile


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.