NEW K/Ar AGES ON TERTIAY VOLCANIC ROCKS IN THE VALLE DEL CURA, PAMPEAN FLAT SLAB SEGMENT, ARGENTINA Vanesa D. Litvak1, Suzanne M. Kay2 and Constantino Mpodozis3 1
CONICET- Departamento de Ciencias Geológicas – Facultad de Ciencias Exactas y Naturales Universidad de Buenos Aires – Ciudad Universitaria Pabellón II – C1428EHA
[email protected] 2
Dept. Earth Atmospheric Sciences and INSTOC, Cornell University, Ithaca, NY, 14853, USA,
[email protected].
3
SIPETROL SA, Avenida Vitacura 2736, Los Condes, Santiago, CHILE,
[email protected]
Key words: Valle del Cura, Tertiary, volcanism, radiometric ages, Andes
INTRODUCTION The Valle del Cura region, in the High Andes of San Juan, Argentina located over the Pampean flat-slab, corresponds to the eastern extension of the El Indio metallogenetic belt in Chile (Fig. 1). The El Indio-Valle del Cura region, which is along the Argentine-Chilean border in the main Andean cordillera, is characterized by a thick sequence of Tertiary volcanic and volcanoclastic rocks that are associated with both epithermal and porphyry Au-Ag-Cu-type mineral deposits. The volcanic sequence was originally studied by Maksaev et al. (1984), Nasi et al. (1990), Kay et al. (1987, 1988, 1991) and Ramos et al. (1989) who defined the geological units and presented the first radiometric ages. Later on, and particularly during the last few years, new ages were presented that not only supplemented the previous ones, but also allowed new volcanic and volcaniclastic units to be defined (Martin et al. 1997a; Limarino et al. 1999; Bissig et al., 2001; Litvak and Page, 2002; Litvak et al. 2004; Mpodozis and Kay 2003). The purpose of this paper is to present four new K/Ar ages for volcanic rocks that crop out in the Valle del Cura region which help to improve the knowledge of the Tertiary stratigraphy and volcanic events of the region.
Figure 1: Valle del Cura-El Indio belt location, over the pampean flat slab segment of Central Andes, Chile and Argentina.
REGIONAL SETTING The Valle del Cura is located in the main Andean cordillera over the volcanically inactive Pampean flat-slab segment of the southern Central Andes, which is limited to the north by the Central Volcanic Zone and to the south by the Southern Volcanic Zone; both with modern volcanic activity (Fig. 1). The Miocene shallowing of the Nazca plate has been attributed to the collision of the Juan Fernández Ridge on the Nazca plate (Pilger 1981, 1984; Yáñez et al. 2001). The arrival of the east-west trending segment of the ridge under the El Indio-Valle del Cura belt corresponds with the eruption of the middle Miocene volcanic rocks, this perturbation pronounced the shallowing of the subduction zone, resulting in changes of the magmatic and tectonic style over the Pampean flat slab (Kay and Mpodozis 2002). The geology of the El Indio-Valle del Cura belt is dominated by a series of north-south trending basement blocks bounded by high angle reverse faults and intervening grabens filled by Tertiarry sequences (Maksaev 1984, Jones et al. 1996, Martin et al. 1997b). The studied area is located in the argentine sector of the El Indio-Valle del Cura belt, as shown in figure 2. The Carboniferous-Triassic basement consists of the arenites and lutites of the Agua Negra Formation, of Upper Carboniferous age (Polanski 1970), and to Permo-Triassic volcanics and sedimentary rocks of the Choiyoi Group (Sato and Llambías 1993) intruded by granitoids of the Colangüil batholith (Llambías and Sato 1990, 1995) The Cenozoic stratigraphy of the Valle del Cura region is characterized by a series of Paleogene and Neogene magmatic and sedimentary sequences. Table 1 shows the main Tertiary stratigraphic units that describes the Valle del Cura Cenozoic volcanic history. There are eight volcanic and volcaniclastic units from Lower Paleocene to Upper Miocene age, with a compositional variation that includes basalts, andesites and rhyolites.
Table 1: Main Tertiary volcanic units of the Valle del Cura region.* Age
Stratigraphic units
PLIOCENE Vacas Heladas Ignimbrite Tambo Formation MIOCENE
Cerro de las Tórtolas Formation (andesites
(dacitic tuffs)
(dacitic tuffs)
and dacites)
Escabroso Formation (mainly andesites, basaltic andesites and volcanic breccias)
Tilito Formation OLIGOCENE
(mainly rhyolites, dacitic tuffs and volcanic breccias)
Las Máquinas Basalts (basalts)
Valle del Cura Formation EOCENE PALEOCENE
(rhyolitic flows, dacitic tuffs and volcanic breccias)
Basaltos Río Frío (basalts)
* Based on Maksaev et al. (1984); Ramos et al. (1989), Kay et al. (1987, 1991, 1999); Martin et al. (1997a), Limarino et al. (1999), Bissig et al. (2001) Litvak and Page (2002) Litvak et al. (2004) and Litvak (2004).
Figure 2: Simplified regional geology of the Valle del Cura region, San Juan province, Argentina.
NEW RADIOMETRIC DATA The new data presented here consist of four K/Ar ages on volcanic rocks that crop out in the Valle del Cura, as shown in figure 2 and table 2. The first sample (CO283) corresponds to a volcanic breccia clast that crops out on the southern margin of Despoblados River, near Veladero. This age corresponds with ages of the Tilito volcanic event described by several authors (Maksaev 1984, Nasi et al. 1990, Martin et al. 1997a, Kay et al. 1987, 1991, 1999; Bissig et al. 2001) and confirms the assignment of those rocks to the Upper Oligocene-Lower Miocene (or younger) unit as presented by the regional mapping of the area (Cardó et al. 2001). The next two samples (CO284 and CO285) are volcanic rocks that crop out in the Despoblados area, near the Cordillera de la Brea. They are porphyritic andesites with hornblende as the main mafic phenocryst and pyroxene as a subordinate phase. Their geological and petrographic characteristics are like those of the Cerro de las Tórtolas Formation. Previous regional mapping of the area assigned these outcrops to the Choiyoi Group (Malizia et al. 1997) or to the Cerro de las Tórtolas Formation (Cardó et al. 2001), but no radiometric age was available until Winokur (2004) who presented a K/Ar age of 13,1 ± 0,9 Ma. The results presented here are consistent with that value and can be regionally correlated with the Cerro de las Tórtolas Formation. In particular, the petrographic features of the Despoblados andesites and the new age presented here are consistent with the geological features shown by the younger Cerro de las Tórtolas Formation volcanic rocks, which were originally defined as the upper section of the unit (Kay et al. 1987). The last sample (CO286) is an aphanitic basalt that crops out in the southern extreme of Valle del Cura (Fig. 2). The sample is from the Upper Oligocene-Lower Miocene Las Máquinas Basalt group, and belongs to the series of volcanic necks that intrude the Eocene volcanic rocks near the entrance of Las Máquinas creek.
Table 2: Radiometric ages results (K/Ar) from Tertiary volcanic rocks in the Valle del Cura.* Sample
Stratigraphic Unit
CO283
Tilito Formation
CO284 CO285 CO286
Cerro de las Tórtolas Fm. Cerro de las Tórtolas Fm. Las Máquinas Basalt
Location 29°26’48” 69°50’30” 29°27’33” 69°47’39” 29°29’16” 69°46’17” 29°58’04” 69°45”17”
%K
rad. Ar. nl/g
Ar. Atm. %
Age Ma
Error 2 sig.
7.206
7.289
46
25.8
± 0.8
2.143
0.679
59
8.1
± 0.5
1.874
0.878
30
12.0
± 0.4
0.833
0.716
23
22.0
± 0.8
* Sample CO283 on biotite, others are whole rock analysis. Ages were determined at the SERNAGEOMIN Geochronological Laboratory in Santiago, Chile.
CONCLUSIONS The new ages presented here complement the available radiometric database for the Valle del Cura Tertiary volcanic sequences. They are consistent with previous published ages, and all four can be correlated not only with the volcanic units previously defined in the area, but also with those in El Indio belt in Chile. The new ages confirm the assignments of some of the volcanic units in the region to the Tilito and Cerro de las Tórtolas Formation. Importantly, the new middle Miocene age for the Cerro de las Tórtolas Formation in the Despoblados area shows that these outcrops can be correlated with the younger volcanic flows of Cerro de las Tórtolas Formation. Rocks of this age have only previously been identified in the uppermost areas of Cerro de las Tórtolas volcano at the southern extreme of Valle del Cura.
ACKNOWLEDGMENTS This research was supported by grants from the University of Buenos Aires (UBACYT X127), CONICET (PIP 02382) and Fondecyt 1990009. We thank Victor Ramos for the review of the paper.
REFERENCES Bissig, T., Clark, A. H., Lee, J. K. W. and Heather, K.B., 2001. The Cenozoic history of volcanism and hydrothermal alteration in the Central Andean flat-slab region: New 40Ar-39Ar constrains from the El Indio-Pascua Au (-Ag, Cu) belt, 29°20-30°30” S. International Geology Review, 43: 312-340. Cardó. R., Díaz, I. N., Poma, S., Litvak, V. D., Santamaría, G. and Limarino, C. O., 2001. Memoria Hoja Geológica 2969-III, Malimán, Servicio Geológico Minero Argentino 67 pp. (Inédito) Jones, J. P., Martínez, R. and Rossello, E., 1996. La Faja volcánica terciaria del Valle del Cura (San Juan, Argentina): sus alcances y potencial para localizar World Class Deposits. XIII Congreso Geológico and III Congreso de Exploración de Hidrocarburos Actas 3: 55-71, Buenos Aires. Kay, S.M., Maksaev, V. A., Moscoso, R., Mpodozis, C., Nasi, C. and Gordillo, C. E., 1988. Tertiary Andean Magmatism in Chile and Argentina between 28°S and 33°S: Correlation of magmatic chemistry with changing Benioff zone. Journal South American Geology Earth Sciences, 1: 21-38. Kay, S.M., Maksaev, V. A., Moscoso, R., Mpodozis, C. and Nasi, C., 1987. Probing the evolving Andean lithosphere: Mid-Late Tertiary Magmatism in Chile (29°-30°30'S) over the modern zone of subhorizontal subduction. Journal of Geophysical. Research, 92(B7): 6173-6189. Kay, S.M., Mpodozis, C., Ramos, V. R. and Munizaga, F., 1991. Magma source variations for mid-late Tertiary magmatic rocks associated with shallowing zone and thickening crust in the central Andes (28° to 33°S). En: Harmon, R. S. and Rapela, C. W. (Eds.): Andean magmatism and its tectonic setting. Geological Society of America, Special Paper 265: 113-137. Kay, S.M. and Mpodozis, C., 2002. Magmatism as a probe to Neogene shallowing of the Nazca plate beneath the modern Chilean flat-slab. Journal of South American Earth Science, 15: 39-57. Kay, S.M., Mpodozis, C. and Coira, B., 1999. Neogene magmatism, tectonism and mineral deposits of the Central Andes (22º-23º S Latitude). En Skinner, B.J., (Ed.). Geology and Ore Deposits of the Central Andes: Society of Economic Geologists Special Publication, 7: 27-59. Limarino C.O., Gutiérrez. P. R., Malizia, D., Barreda, V., Page, S., Ostera, H. and Linares, E., 1999. Edad de las secuencias paleógenas y neógenas de las cordilleras de la Brea y Zancarrón, Valle del Cura, San Juan. Revista de la Asociación Geológica Argentina, 54(2): 177-181. Litvak, V.D., 2004. Evolución del volcanismo terciario en el Valle del Cura sobre el segmento de subducción horizontal Pampeano, provincia de San Juan. Tesis Doctoral. Universidad de Buenos Aires, 270 p. (Inédito). Litvak V.D. and S. Page, 2002. Nueva evidencia cronológica en el Valle del Cura, provincia de San Juan. Revista de la Asociación Geológica Argentina, 57(4): 483-486. Litvak, V.D., Poma, S. and C.O. and Limarino, 2004. Volcanismo piroclástico de edad miocena media en el Valle del Cura, provincia de San Juan: nuevos datos geológicos y geocronológicos. Revista de la Asociación Geológica Argentina, 59(3): 514-517. Llambías, E. J. and A. M. Sato, 1990. El batolito de Colangüil (29°-31° S), Cordillera Frontal de Argentina. Estructura y marco tectónico. Revista Geológica de Chile, 17(1): 89-108. Llambías, E. J. and A. M. Sato, 1995. El batolito de Colangüil: transición entre orogénesis y anorogonésis. Revista de la Asociación Geológica Argentina, 50(1-4): 111-131. Maksaev, V., Moscoso, R., Mpodozis, C. and Nasi, C., 1984. Las unidades volcánicas y plutónicas del Cenozoico superior entre la Alta Cordillera del Norte Chico (29°-31° S), Geología, alteración hidrotermal y mineralización. Revista Geológica de Chile, 21: 11-51. Malizia, D.,. Limarino, C.O., Sosa-Gomez, J., Kokot, R., Nullo, F. and Gutierrez, P., 1997. Descripción de la Hoja Geológica Cordillera del Zancarrón, escala 1: 100.000. Secretaría de Minería de la Nación, 280 p. Buenos Aires. (Inédito). Martin, M.W., Clavero, J.R. and Mpodozis, C.M. 1997a. Eocene to Late Miocene magmatic development of El Indio belt, 30° S, North-Central Chile. VIII Congreso Geológico Chileno, Actas 1: 149-153. Antofagasta. Martin, M.W., Clavero J.R. and Mpodozis, C.M., 1997b. Eocene to Late Miocene structural development of El Indio belt, 30° S, North-Central Chile. VIII Congreso Geológico Chileno, Actas 1: 144-148. Mpodozis C. and Kay, S.M., 2003. Neogene tectonics, ages and mineralization along the transition zone between the El Indio and Maricunga mineral belts (Argentine and Chile 28°-29°). X Congreso Geológico de Chile, Actas CD: 1 pp. Nasi, C.P., Moscoso, R.D. and Maksaev, V.J., 1990. Hoja Guanta, Región de Coquimbo. Servicio de Geología y Minería, Carta Geológica de Chile, 67: 140 pp.
Polanski, J., 1970. Carbónico y Pérmico de la Argentina. Editorial Universitaria de Buenos Aires, Manuales, 216 pp. Buenos Aires. Pilger, R.H., 1981. Plate reconstructions, aseismic ridges, and low angle subduction beneath the Andes. Geological Society of America Bulletin, 92: 448-456. Pilger, R.H., 1984. Cenozoic plate kinematics, subduction and magmatism: South American Andes. Journal of the Geological Society of London, 141: 793-802. Ramos, V.A., Kay, S. M., Page, R. and Munizaga, F., 1989. La Ignimbrita Vacas Heladas y el cese del volcanismo en el Valle del Cura, provincia de San Juan. Revista de la Asociación Geológica Argentina, 44(1-2): 336-352. Sato, A.M. and Llambías, E.J., 1993. El Grupo Choiyoi, provincia de San Juan: equivalentes efusivos del batolito de Colangüil. 12° Congreso Geológico Argentino y 2° Congreso de Exploración de Hidrocarburos, 4: 156-165. Winocur, D., 2004. Geología y estructura de la región de los Despoblados, Provincia de San Juan. Trabajo Final de Licenciatura, Universidad de Buenos Aires, 125 pp. (Inédito). Yáñez, G.A, Ranero, C.R., von Huene, R. and Díaz, J., 2001. Magnetic anomaly interpretation across the southern central Andes (32°–34°): the role of the Juan Fernández Ridge in the late Tertiary evolution of the margin. Journal of Geophysical Research, 106: 6325-6345.
