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301 The Australandean Volcanic Zone (South Patagonia), Rolf KILIAN Mineralogical and Petrological Institut of the University Wilhelmstr. 56. 7400 Ttibinaen. W-Germu of Tubingen. Abstract New geochemical data of volcanics of the Mt. Burney and Viedma review on petrology of the six volcanic centers of the Australandean 49 - 55’S). Volcanic products of recent activities field. The volcanics of the AVZ are acid andesites volcano of the AVZ shows small geochemic a special position, related ratios. Volcanics the north, and a Volcanic Zone (AVZ: of 1988 were observed on the Viedma iceto dacites; development. basafts are missing; every Volcanics of Cook island occupies show increasing K. Rb. Ba contents of Mt. Burney belong to a low-K lavas and pumices of Lautaro, contamination complete to volcanics of rift zones. From Mt. Burney in 52’S to Viedma volcano in 49’ S. the volcanics %r/*‘Sr volcano magma mixing as well as K/Rb andesites Viedma and Aguilera are med-K fused are discussed with crustal rocks and and dacites. More to dacites. Crustal for this SN development. Introduction The volcanism reports the beginning which in the south Patagonian are available. Volcanic of this century originated Swedish expedition Andes has remained eruptions on the Patagonian almost unknown by QUENSEL (1910). Ash deposits in volcanic centers of the inaccessible (1932-34) and examined and only few Ice Cap were first reported in the Argentine Andes, were collected at pampas, during a by SALMI (1941) and more recently by STERN (1990). Some rocks of the Lautaro. Aguilera, Mt. Burney and Cook island (fig. 1) have been geochemically volcano analysed by STERN et al. (1976, 1984) and FUTA Reclus (Sr57’S) 1988 and MARTINIC has been described by South Chilean 1988). In the area of the Patagonian a volcanic center and poducts of recent eruptions Tectonic & STERN (1988). The geologists Icefield, were described (HARAMBOUR on the Viedma glacier, by KILIAN (1990). setting The Austral Volcanic Zone (AVZ) extends between and Viedma volcanoes are situated at the northern 49’ and 55”s. The Lautaro. Aguilera end, Reclus, Mt. Burney and Cook Island are located more to the south (fig. 1). All these Cenozoic volcanoes convergent plate boundary plate and the continental between the oceanic Antarctic are situated near the South x.._____...-_ -- r 302 American plate. The convergence rate (3 cm/year. Minster et al. 1974). the tectonic . volcanic activity are lower than to the north of the plate triple junction, plate is subducted at a rate of 5 to 10 cm/year. volcanic gap of 300 km at the continental and where the Nazca South of the plate triple junction. there is a margin. Patagonlan Andes wmlcmlozekvokUnou cl A ---- area ot hvea~noll cmozokvwkMo Frwlur.zo~ __lA__ Ageofn-mowmk _._._.. Ff- cwslhmU.ye~ South American Fig. 1: Kanoaoic volcanoes and plate tectonic structure of the Patagonian Andes. Volcanic activity OQthe Viedma icefield of 1988 An extended melting, area was observed Hot mud flows were formed, valley net was formed of eruption deposits on the Viedma icefield (Patagonia) caused by the fall of hot pumices and large ash fractions The eruptions intensely at 49’225 periodic and 73’19’W in an elevation eruptions ice a large area of the glacier. By the hot mud flows a in the ice surface with a depth of up to 35 m. The probable was located of previous covering showing after volcanic euptions. were mapped; pumices center of 1100 m. Pyroclastic and lavas were examined. must have taken place recently between Sept. and Nov. 1988. . 303 Petrography Minerals in volcanics electronmicroprobe Viedma, biotite (0.5 - 1.5 mm) and additionally a second xonation often occurs. corroded Some xenolithes, phenocrysts of Mt. Burner show slightly K-dacites often of volcanics of Lautaro. generation Large plagioclase of idiomoph anorthite phenocryst anorthite xonation (an 63) of plagioclase contamination. Plagioclase an 42 in many contents of orthoclase. and indicate are often rim corroded. inverse and xonation in the range between consisting occur in lavas of Viedma and Aguilera by microscope (0.5 - 2 mm) and a few phenocryst middle xone with the highest messurements. biotite and amphibole Hypersthene phenocrysts (1.5 - 4 mm), amphibole normally shows small anorthit inverse show a strongly our microprobe were determined (65). Characteristic are plagioclase Plagiocla& to an 60. A slightly pyroxene, and Mt. Burney messurements and Aguilera hypersthene. cues of Viedma and Plagioclase, phenocrysts of lavas in the range an 45 to 65. is the most commun mafic mineral with higher Mg/Fe ratio (mg* 65) than in of Viedma. Ceochemical and petrological aspects The volcanics of the AVZ range from 59 to 68 wt.% SiO, and are acid andesites in the K,O/SiO, diagramm (fig. 2). The analyses of the Viedma. Lautaro and dacites and Aguilera volcanoes show the highest KsO levels of the AVZ. To the south, the K,O, Rb, Ba. and Zr levels of rocks from the volcanoes Reclus and Mt. Burney decrease latter case to a low-K series. High-Al calcalkaline and correspond basalts and basic andesites, types of the SVZ (33 - 46’ S), are missing in the AVZ. Apparently individual volcanic sugesstion of FUTA development primitive center of the AVZ in the K,O/SiO, & STERN (1988) and STERN uniform diagramm K calcalcaline the south. Wide ranged calcalcaline volcanics of continental Aguilera volcanoes change to low-K volcano. This SN development contamination, indicated margin (fig. 2) confirm becoming series are missing. Orogenic type in 49’ S of Lautaro, the is no local more medium- Viedma and series of island arc type in 52’ S from the Mt. Burney to more continent by xenolithes typical rock plots of each et al. (1984). that there of the melts, but marked regional NS change in the chemistry towards in the related series is considered in the lavas of Viedma and Aguilera due to crustal volcanoes. But also magma mixing with small amounts of partial fused crustal rocks can not be excluded. Volcanics of the Cook island (tab. 1) located more to the south (5s’S) show more relations to MORB type than to erogenic and Also, contents series (FUTA & STERN as well as Sr. La concentrations KsO, Ba, Rb. Cs levels. 1988), with relatively but low TiO,, high MgO HREE, Zr and very low ._-I” ._ .“_ .*_ ..-__._ . 304 3.5 KZO lccy3 I l Reclus clururrl 2 Mt. Bumey Fig. 2: K,O/SiOs diagramm with distribution and plots of volcanics of the Southandean Volcanic Zone. References FUTA. K. & STERN, Ch., R. (1988): Sr and Nd isotopic and trace element compositions of Quaternary volcanic centers of the southern Andes. - Earth Planet. Sci. Lett.. 88 (1988): 253-262. Amsterdam (Elsevier). HARAMBOUR. S.M. (1988): Sobre et hallazgo del mitico volcan Reclus, ex Mano de1 Diabolo, Hielo Patagonico Sur. Magallanes, Chile. - Rev. Geol. de Chile, 15 (2):173 - 179; Santiago. KILIAN, R. (1990): Volcanic eruptions MARTINIC, on the Viedma icefield of 1988 (Patagonia). M. (1988): Actividad volcanica hist6rica Geol. de Chile 15, 2: 181 - 186; Santiago. STERN, C.R. (1990): Tephrochronology -.- (in print) of Southernmost en la regi6n de Magallanes. Patagonia. - Nat. geograph. - Rev. Res. 6: STERN, C.R., FUTA, K. & MUEHLENBACHS. K. (1984): Isotope and Trace Element Data for Orogenic Andesites from Austral Andes. - in: ‘Andean Magmatism Chemical ed.: Harmon R.S. & Barreiro B.A.: 31 - 48 S.; and Isotopic Constraints”. Cheshire, UK (Shiva). STERN, C.R., FUTA, K.. SAUL, S. & SKEWES. M.A. (1986): Nature and Evolution of the Subkontinental Mantle Lithosphere below Southern South America and implications for Andean Magma Genesis. - Rev. geol. de Chile. 27: 41 - 53; Santiago, Chile. STERN, C.R., SKEWES, M.A. & DURAN, A.M. (1976): Volcanismo Orogenico Austral. - I. Congr. geol. Chileno. S. 195 - 210; Santiago, Chile. en Chile