XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S7_019 Evolution of < 10 Ma Valle Ancho Region Lavas, Southern End of the Central Andean Volcanic Zone (~27.5°S) Mpodozis, C 1, Kay, S.M. 2. (1) Antofagasta Minerals, Ahumada 11, Santiago, Chile. (2) Dept. EAS and INSTOC, Snee Hall, Cornell University, Ithaca, NY 14853, USA.
[email protected] Introduction The Valle Ancho region in Catamarca, Argentina near 27.5° S latitude (Fig. 1) is a relatively low elevation, northwest-trending structural corridor that lies between the > 6000 meter high Pliocene to Holocene Ojos del Salado and Tres Cruces volcanic complexes to the north and the ~ 6-4 Ma Nevado Pissis and < 3 Ma Incapillo caldera to the south [1-3]. The basement of the Valle Ancho region is composed of Precambrian gneisses and late Paleozoic volcano sedimentary sequences forming a series of tectonic blocks bounded by northwest to north-northwest trending sub-vertical faults. This basement is overlain by Tertiary units, including Miocene to Pliocene lavas and domes that record the Neogene eastward expansion of arc volcanism and the southward passing of the subducted Juan Fernandez Ridge. Here, and based on fifty major and trace elements analysis, together with 32 previously unreported, K/Ar ages (listed in Table 1 and plotted in Figure 1) we discuss the main geochemical features of the post middle Miocene volcanic rocks of the Valle Ancho region Latest Miocene to Pliocene Volcanism The oldest late Miocene volcanism in the Valle Ancho region is recorded by altered dacitic domes, porphyries and ignimbrites (68-72% SiO 2) dated at 10.7±0.8, 9.9±1.8 and 9.4±0.5 Ma, and minor mafic flows (52-53% SiO 2) dated at 9.5±0.5 and 9.1±0.6 Ma.