Late Miocene garnet-bearing andesites in the Northern Andean Block and their tectonic implications
Hoyos Muñoz, Susana María
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Garnet-bearing volcanic rocks are rare at convergent margins, with few known occurrences worldwide. However, they are a common feature within the Late Miocene volcanic rock suite of the Northern Andean Block (NAB) along the Central Cordillera of Colombia. Here, we present new petrographic, geochemical and geochronological data from 7 porphyritic-andesite samples from the eastern flank of the Central Cordillera and the Cauca-Patia Basin (Colombia), in order to determine their petrogenesis, constrain their timing, and interpret the possible tectonic scenario. Our results show that garnet phenocrysts are almandine in composition, ranging from 23 to 29 wt% FeO, 6 to 8 wt.% CaO, and 1 to 4 wt.% MnO. On the basis of petrographic and geochemical characteristics primary garnets on the NAB volcanic rocks are classified into three distinct groups. Type-1 are homogeneous garnets with no reaction rim, lacking inclusions, type-2 are slightly zoned garnets with reabsorption rims, and mineral inclusion assemblage similar to the rock matrix (plagioclase, hornblende and in some samples biotite) and type-3 are heterogenous garnets with a light-colored core and a darker reddish overgrowth rim and a plagioclase corona. Furthermore, our data shows high Ca and low Mn contents on the NAB garnet cores which suggest crystallization at ~1.2GPa, based on phase equilibrium experiments by Alonso-Pérez et al. (2009). On the other hand, garnet rim assemblages are congruent with a second stage of crystallization at ~0.8GPa in water-undersaturated conditions. Moreover, a pre-eruption dehydration stage is evidenced by the presence of breakdown rims in amphibole phenocrysts. Finally, the new U/Pb in zircon ages reveal that NAB garnet-bearing volcanic rocks formed between 9 and 8 Ma. Consequently, our results suggest a rapid ascent of the NAB magmas associated with the onset of regional extension and volcanism, and the development of the Caldas Tear, a slab window within the Nazca Plate.