Petrology of Acandí´'s Batholith and associated bodies in Unguía, Chocó, Colombia

Abstract

The Acandi’s batholith is a large intrusive body with a wide compositional variation (Gonzalez and Londoño, 2002) dating from the Oligocene (Restrepo and Toussaint, 1976), and intruded by subvolcanic andesites and dacites bodies and basalt dikes. Due to the compositional difference of this body in the Western Cordillera, it is performed integration and interpretation of new petrographic, geochemical and isotopic data from samples collected in the northern Gulf of Urabá between the towns of Acandí and Unguía it is done in Choco department. For this area, batholith rocks are gabbros: melanocratic rocks, massive, equigranular fine to medium grained, subhedral, phaneritic texture; and the subvolcanic bodies and dikes are: leucocratic, massive and inequigranular rocks, fine to medium grained, subhedral, with sporphyritic texture and aphanitic melanocratic rocks (basalts). Detailed petrography shows that the rocks from the Batolith are constituted by plagioclase, pyroxene and amphibole, with disseminated pyrite, apatite, magnetite and ilmenite as accessory phases. The batholith rocks are holocrystalline, melanocratic, inequigranular, subhedral, from fine to granular roughly, with the presence of ophitic and subophitic textures, with reaction rings (coronas) and zoning in plagioclase. The intrusive subvolcanic rocks that crosscut the Batholith are afanitic basalts, dacites, and andesites with porphiritic textures, having plagioclase and hornblende as main minerals. They are inequigranular, very fine to coarse grained, with glomeroporphyritic texture and also vesicular and amygdular textures. Geochemical and isotopic analyses indicate that both batholiths and intrusive rocks are sub-alkaline from the calc-alkaline series of low to medium K, metaluminous with light rare earth elements enrichment in relation to the heavy rare earth elements. The geochemical and Nd and Sr isotopic data suggest that these rocks are formed in island arcs, associated with subduction zones, being later added to a continental margin.