Petrology of the Betulia Igneous Complex, Cauca, Colombia

Abstract

The Betulia Igneous Complex (BIC) is a group of Late-Miocene (11.8 ± 0.2 Ma) hypabyssal intrusions of intermediate to felsic composition located in the SW of the Colombian Andes. These bodies have a calc-alkaline tendency and are related to the subduction of the Nazca plate under the South American plate. Diorites, quartz diorites and tonalities have porphyritic and phaneritic textures and are composed of plagioclase, amphibole, quartz, biotite, and orthoclase. Plagioclase is mainly of andesine-type and the amphiboles were classified mainly as magnesiohornblendes, actinolites, and tschermakites.BIC rocks have a narrow range of SiO2 content (59–67wt%) and exhibit an enrichment of LILE and LREE relative to HFSE and HREE, respectively. These features are attributed to enrichment of LILE from the source and retention of HFSE (mainly Nb, Ta, and Ti) by refractory phases within the same source. The depletion of HREE is explained by fractionation of mineral phases that have a high partition coefficients for these elements, especially amphiboles, the major mafic phase in the rocks. Nevertheless, the fractionation of garnet in early stages of crystallization is not unlikely. Probably all BIC units were generated by the same magma chamber or at least by the same petrologic mechanism as shown by the similar patterns in spider and REE diagrams; fractional crystallization and differentiation processes controlled the final composition of the rocks, and crystallization stages determined the texture.Isotopic compositions of BIC rocks (87Sr/86Sr: 0.70435–0.70511; 143Nd/144Nd: 0.51258–0.51280; 206Pb/204Pb: 19.13–19.31; 207Pb/204Pb: 15.67–15.76; 208Pb/204Pb: 38.93–39.20) indicate a source derived from the mantle with crustal contamination. The model proposed for the BIC consists of fluids from the dehydration of the subducted slab (Nazca plate) and subducted sediments that generated partial melting of the mantle wedge. These basaltic melts ascended to the mantle–crust boundary where they were retained due to density differences and began to produce processes of melting, assimilation, storage, and homogenization (MASH zone). At this depth (∼40–45 km), fractional crystallization and differentiation processes began to produce more felsic magmas that were able to ascend through the crust and be emplaced at shallow depths.