Butalón igneous rocks, Neuquén, Argentina: Age, stratigraphic relationships and geochemical features

Abstract Three main groups of igneous rocks of different age are recognized in the Butalon area, including volcanic rocks of the Choiyoi Group (Permian–Triassic), stocks and dikes of the Granodiorita Varvarco (Upper Cretaceous–Paleocene) and stocks and dikes of the Tonalita Butalon (Paleocene–Eocene?). The Choiyoi Group consists of ignimbrites, breccias, tuffs and silicic lavas. Most are subalkaline and four groups can be distinguished on the basis of their trace element patterns. Most samples are enriched in Ba, but depleted in Nb, Sr, P, Ti and Eu. The subvolcanic rocks of the Granodiorita Varvarco and Tonalita Butalon are mainly calc-alkaline, metaluminous to peraluminous, with low to medium potassium contents. They have similar subparallel REE and multielement patterns with low La/Yb and Eu/Eu ∗ ratios. These patterns mimic those observed in Pleistocene–Holocene volcanic rocks, of similar SiO 2 content, from the CSVZ of the Andes. They are depleted in Nb and Ti, have normal to low P, and are enriched in Sr and Zr. Hornfels-skarn type hydrothermal alteration is produced by Tonalita Butalon in the adjacent igneous rocks. Magmatic-hydrothermal breccias are developed on top of some of the stocks. While some of the breccias exhibit Mo anomalies, Au and Ag anomalies are present in the Choiyoi Group.