Mantle diversity beneath the Colombian Andes, Northern Volcanic Zone: Constraints from Sr and Nd isotopes

In order to provide mantle and crustal constraints during the evolution of the Colombian Andes, Sr and Nd isotopic studies were performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in the Granatifera Tuff, a deposit of Cenozoic age, in which mantle- and crustal-derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions. Garnet-bearing xenoliths are the most abundant mantle-derived rocks, but websterites (garnet-free xenoliths) and spinel-bearing peridotites are also present in minor amounts. Amphibolites, pyroxenites, granulites, and gneisses represent the lower crustal xenolith assemblage. Isotopic signatures for the mantle xenoliths, together with field, petrographic, mineral, and whole-rock chemistry and pressure-temperature estimates, suggest three main sources for these mantle xenoliths: garnet-free websterite xenoliths derived from a source region with low P and T (16 kbar, 1065 degrees C) and MORB isotopic signature, Sr-87/Sr-86 ratio of 0.7030, and Nd-143/Nd-144 ratio of 0.5129. Garnet-bearing peridotite and websterite xenoliths derived from two different sources in the mantle: i) a source with intermediate P and T (29-35 kbar, 1250-1295 degrees C) conditions, similar to that of sub-oceanic geotherm, with an OIB isotopic signature (Sr-87/Sr-86 ratio of 0.7043 and Nd-143/ 144 Nd ratio of 0.5129); and ii) another source with P and T conditions similar to those of a sub-continental geotherm (>38 kbar, 1140-1175 degrees C) and OIB isotopic characteristics (Sr-87/Sr-86 ratio=0.7041 and Nd-143/Nd-144 ratio=0.5135). (C) 2005 Elsevier B.V. All rights reserved.