Late Miocene garnet-bearing andesites in the Northern Andean Block and
their tectonic implications
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
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SGC (in press).  Alonso-Pérez et al. (2006) CMP.