Crustal contamination in northern Andean volcanics

Abstract

Late Cenozoic andesitic rocks of the Colombian Andes are characterized by small but systematic and well-correlated variations in 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and δ 18 O that reflect significant crustal contamination. The range in isotopic ratios measured in northern Andean andesites is most pronounced for oxygen, relatively much more subtle for Sr and Nd. Pb isotopic data, so diagnostic of crustal contamination in central Andean lavas, exhibit minimal variation in the northern Andean volcanics. Trace element trends and phenocryst populations suggest dominantly mafic mineral fractionation, although plagioclase fractionation may be significant in later-stage differentiation. An assimilation-fractional crystallization model is constructed to match measured isotopic data. The results of that modelling indicate that the andesitic magmas of Galeras and Ruiz volcanoes may have assimilated up to 10–20% of crustal material. Correlated variations in measured isotopic ratios can be modelled satisfactorily by assuming a crustal contaminant of composition δ 18 O = 10 per mil, 87 Sr/ 86 Sr = 0.710–0.712 (Sr ≈ 90ppm), and 143 Nd/ 144 Nd=0.5120–0.5122 (Nd ≈ 20–30 ppm). We estimate that the composition of the primary magma (prior to crustal contamination) was in the range δ 18 O = 6.5 per mil, 87 Sr/ 86 Sr = 0.704, and 143 Nd/ 144 Nd = 0.51285, values that may suggest an additional crustal component was present in the primary magma. If we assume the added crustal component was derived from partial melting of subducted continental sedimentary material and/or oceanic crust (source contamination), then model calculations suggest that crustal material comprises less than 10 to 15 wt% of the primary magma. This model-dependent effect of source contamination is to shift isotopic ratios to the right on the Sr–Nd plot (see also Hawkesworth et al. 1977 ), whereas crustal contamination of the magma during ascent through the Andean crust appears to have produced isotopic trajectories that cut downward and through the mantle array.

Provenance of Amazon Fan muds: constraints from Nd and Pb isotopes

“…The Andes mountains run the entire length of the western margin of South America ( Fig. 1), and consist of Paleozoic to Holocene volcanic and plutonic rocks representing accreted island arc, backarc, and continental arc suites with varying amounts of crustal incorporation, metamorphosed basement, and thrusted sedimentary rock (James and Murcia, 1984;McCourt et al, 1984;Megard, 1984;Dalziel, 1986;Miller and Harris, 1989).…”

Late Pliocene and Early Pleistocene Variability of the REE and Nd Isotope Composition of Caribbean Bottom Water: A Record of Changes in Sea Level and Terrestrial Inputs During the Final Stages of Central American Seaway Closure

“…Sediments from the continental and metamorphic basement in the Eastern and Central Cordillera have very unradiogenic compositions, (ε Nd ≤ −13), whereas those with a magmatic arc source in the Western Cordillera are more radiogenic (−12 to −9 ε Nd ) (Nie et al, ). Jurassic plutonic and volcanic units in the Colombian Andes are generally more radiogenic than the basement complexes, ranging from −10.3 to +4.0 ε Nd (James & Murcia, ; Quandt et al, ). Therefore, a preformed signal transported by sediments from the Magdalena River could explain the full range of ε Nd values in our Plio‐Pleistocene records.…”

The Andes Cordillera: A Synthesis of the Geologic Evolution

“…In turn, the Ph-isotope ratios, which are strongly diagnostic of crustal contamination in the southern Peruvian volcanics, exhibit minimal variations in the southwestern Columbia lavas (James and Murcia 1984).…”