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. The silicic rocks crop out along the main northwest-trending Valle Ancho fault, and the mafic flows along the north-northeast trending Los Aparejos fault zone that marks the eastern limit of Tertiary volcanism in this region. The mafic flows show a marked HFSE depletion (La/Ta~55) and high Cr (>500 ppm) and Ni (>170 ppm) contents. The silicic rocks show less HFSE depleted (La/Ta=25-30) and moderately steep REE patterns (La/Yb ~ 29). This volcanic episode was followed by the emplacement of mafic andesitic lavas (56-59% SiO 2) along northwest and north north-west trending faults in Valle Ancho and near Laguna Verde (Figure 1), which yield ages of 7.7±0.5, 7.69±0.3 and 6.6±0.7 1 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Ma. These lavas show arc-like HFS element depletion (La/Ta = 45 to 60), moderate to steep REE patterns (La/Yb=24-35) and high Cr (294-65 ppm) and Sr (749-820 ppm) contents. Andesitic flows (60-63% SiO 2, La/Ta = 48-73, La/Yb = 39-57, 630-1070 ppm Sr, 38-59 ppm Cr) with ages of 7.2±0.8 to 7.0±0.6 Ma also erupted along these fault systems. These eruptions occurred to the east of the distinctive Dos Hermanos center located over the Argentine–Chilean border south of the Valle Ancho (Figure 1) which includes lavas with La/Yb~60, La/Ta~95, 1200 ppm Sr, ~ and 50 ppm Cr; [1-3]). From 6.6 to 3.5 Ma, important andesitic to mafic dacitic volcanism took place at Nevado Pissis, Cerro Nacimiento de Jagüe and Cerro Negro volcanic centers to the south of the Valle Ancho (Figure 1). The oldest lavas of this group, dated at 6.6±07 to 6.2±0.5 Ma are mostly silicic andesites (62-63% SiO 2) from the Nevado Pissis volcanic complex. Mafic andesites erupted from Cerro Negro (59-60% SiO 2) at the southern end of asystem of north northwest trending faults yield ages of 5.1±0.3, 4.5±0.6, 5.5 ±0.4 and 5.3±0.3 Ma. The youngest lavas from this group are andesitic to dacitic (59-64.5% SiO2) flows from Nevado Pissis and Cerro Nacimiento de Jagüe, five of which have ages from 4.6±0.3 to 4.1±0.4 Ma and one, 3.5±0.5 Ma. These rocks present variable La/Tb (36-76) and La/Yb (23-53) ratios at 641-893 ppm Sr and 30-130 ppm Cr. Overall the 7-3.5 Ma andesites with 60-62% SiO 2 show a wide range of La/Yb (25-55) and La/Ta ratios from 36 to 78 which show a positive correlation with Na and Sr contents [4]. After 3.5 Ma, most volcanism shifted outside of the Valle Ancho region to the Ojos del Salado, Tres Cruces and Cerro Bonete but remained active at Cerro Bayo (Figure 1) formed by lavas with 60-67% SiO 2, dated at 2.9±0.2, 2.6±0.2 and 1.1±0.1 Ma The youngest volcanics from the Valle Ancho region are flows and domes (~65% SiO 2) dated at 2.4±0.2 and 1.7±0.2 Ma. These younger units show a smaller HFSE depletion (La/Ta = 33 to 46) and lower La/Yb ratios (25-37). Discussion Late Miocene arc volcanism in the Valle Ancho region began as the Copiapó ignimbrites (~10-9) erupted during the last stages of volcanism in the Maricunga Belt to the west as the northwest trending arm of the Juan Fernandez Ridge began to subduct under the Valle Ancho region producing a transient shallowing of the Nazca plate [see 4]. This shallowing is reflected in the strong HFSE depletion (high La/Ta ratios) in the ~ 9.5 Ma basaltic andesitic lavas erupted in the eastern Valle Ancho. Lower La/Ta ratios in ~10-9 Ma silicic rocks reflect hybrid crust-mantle melts produced as mantle wedge generated magmas entered and partially melted the crust. Continued strong HFSE depletion found in 7.7 to 3.5 Ma volcanic rocks, which erupted along faults in the Valle Ancho, reflect the passage of the axis of the subducted part of the Juan Fernandez ridge bellow this region. Associated shallowing of the slab caused regional contraction resulting in magma evolution at variable crustal levels and the production of andesitic eruptions, whose volume peaked as frontal arc volcanism shut off in the Maricunga Belt by 7 Ma. By ~ 4- 3.5 Ma, when the subducting Juan Fernandez Ridge had passed to the south, volcanism 2 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 essentially ceased in the Valle Ancho region. After this time, volcanism became concentrated in the dominantly dacitic to rhyodacitic dome complexes at Ojos del Salado and Nevados Tres Cruces to the north [3] and Cerro Bonete to the south [2]. The silicic magmas in these centers reflect crustal melting related to decompression mantle melts produced in the thickening mantle wedge after the passage of the Juan Fernandez ridge. The overall chemistry of the Valle Ancho magmas reflects their evolution in thick continental crust (Fig 1) at a time of enhanced forearc subduction erosion during slab shallowing and frontal arc migration [1 to 4]. References [1] Mpodozis, C., Kay, S.M., Gardeweg, M.. Coira, B. (1997) Geología de la región de Valle- Ancho-Laguna Verde (Catamarca, Argentina): Una ventana al basamento del extremo sur de la zona Volcánica de los Andes Centrales. VIII Congreso Geológico Chileno , Actas , 1689-1693 [2] Goss, A., Kay, S.M., Mpodozis, C. (2009) The Incapillo Caldera and dome complex (~28º S): A stranded magma chamber over a dying Andean arc, Journal of Volcanology and Geothermal Research (in press). [3] Kay, S.M., Coira, B.L., Mpodozis, C. (2008) Neogene to Recent evolution of the Puna plateau and the southern Central Volcanic Zone, Geological Society of America Field Guide Series, 13, 117-181. [4] Kay, S.M., Mpodozis, C. ( 2002) Magmatism as a probe to the Neogene shallowing of the Nazca plate beneath the modern Chilean flat-slab, Journal of South American Earth Sciences , 15, 39-59. 3 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Figure 1. Map of the Valle Ancho region from [1] showing dated samples and preliminary crustal thicknesses from Ps receiver functions using data from the PUNA08 passive seismic array (E. Sandvol, D. Comte, S.M. Kay and others, personal communication, 2009). 4 .