Petrographic study of the pimampiro pluton, imbabura, ecuador

The Late Cretaceous evolution of the South American Margin is associated with subduction and accretion of oceanic lithosphere. The occurrence of I-type granitoids like the Pimampiro pluton during this epoch further suggest subduction beneath the South American Margin during the Late Cretaceous period. The petrography of the Pimampiro pluton and its mafic enclaves described in this work show that calcic amphibole megacrysts (> 30 mm) are dispersed on the pluton along with widespread phenocrystic hornblende (~ 1cm) and occasionally biotite as the major mafic mineralogy. Plagioclase feldspar in the pluton show anorthite contents that vary inside the andesine compositional field (An38-An45). Although previous studies have described the pluton as either a granodiorite (Litherland et al., 1994) or a hornblende-granodiorite (Chiaradia, 2013) our petrographic analysis determined a tonalite composition that along with the ubiquitous presence of hornblende in the pluton implies that it should be termed hornblende-tonalite under the IUGS systematics. The widespread hornblende minerals in the pluton place the Pimampiro tonalite on the Amphibole Calc-alkaline Granitoids (ACG) petrogenetic field of Barbarin (1999) which is consistent with subduction related origin during the Late Cretaceous, although geochemical analyses are needed to further confirm this hypothesis.