Quaternary Sanukitoid-like Andesites Generated by Intracrustal Processes (Chacana Caldera Complex, Ecuador): Implications for Archean Sanukitoids

Resumen

High-Mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction-related rock types that have been found in modern arc settings and in Late Archean sequences, where they are associated with trondhjemite^tonalite^granodiorite (TTG) suites. Archean rocks with these geochemical features are known as sanukitoids and, despite their limited abundance, are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction-related high-Mg andesites and diorites. Understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes. The accepted petrogenetic models for modern, enriched, high-Mg andesites and their Archean equivalents, the sanukitoids, consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting, or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge. We present new data on the petrography, mineral chemistry and whole-rock geochemistry (major and trace elements and Sr^Nd^Pb isotopes) of andesitic rocks from an $30 ka lava flow (Yuyos flow) from the Chacana Caldera Complex, Eastern Cordillera of Ecuador. These rocks show a remarkable geochemical affinity with Archean sanukitoids, including high magnesium numbers (058^063) accompanied by high contents of incompatible elements (e.g. Th 17^23 ppm, U 6^75 ppm, Ba 1600^1800 ppm, Sr 1430^1565 ppm, La 749 4 ppm). Additionally, the sanukitoid-like andesites of Yuyos are Three distinct trench-parallel chains of volcanoes occur in Ecuador (Fig. Between the frontal and the main arc several volcanoes also occur in the Interandean Valley, such as Cusin, Mojanda and Ilalo (Fig. The crustal thickness beneath the Eastern Cordillera, where Chacana is situated, is 450 km based on gravimetric data