Cerro Porquesa

19°59′S 68°46′W[1]

False colour satellite image of the Cerro Porquesa lava domes (grey-violet) in the centre.

Cerro Porquesa (Aymara[2]) is an approximately 4,600 metres (15,100 ft) high[1] (800 metres (2,600 ft) above base)[3] rhyodacite lava dome in the Andes.[4] It has a 1.5 by 3 kilometres (0.93 mi × 1.86 mi) oval outline, and is 800 metres (2,600 ft) high. The lava dome is accompanied by block-and-ash flows and lava flows[5] and the presence of two volcanic cones has been reported.[6] The lava dome is of Pliocene/Pleistocene age[7] with little glacial features on the younger domes indicating young ages.[1] The Quebrada de Piga, which flows to Salar de Huasco, originates at Cerro Porquesa.[8]

The dome was formed in at least three different eruption stages, with each stage contributing about two or three different lobes. Further, a rhyolitic ignimbrite with 69.5% SiO2 may be linked to the domes. It fills a valley in the south of the complex about 2 metres (6 ft 7 in) thick.[1] This ignimbrite is dated 0.73±0.16 and 0.63 +0.92/-0.63 mya by potassium-argon dating in biotite, although with low precision.[3] Another 0.28 mya old ignimbrite was originally attributed to Porquesa but instead comes from a 4 kilometres (2.5 mi) wide caldera in Bolivia.[9] Three dates have been obtained from the domes, 0.63±0.63 mya, less than 1 mya and 1.1±0.4 mya.[5] The volcanic complex was probably active after 280,000 years ago but not in the Holocene. Some of the ignimbrites have been offset by faults.[9]

This lava dome is located 20° in a volcanic gap named Pica gap. In this gap volcanic activity younger than 2 mya isn't found and where lead (Pb) isotope ratios in rocks change with the radiogenicity of the isotope ratio decreasing northward. Porquesa has intermediary isotope ratios. The lower ratio was principally imparted by the low-radiogenic Pb upper crust.[4] Samples and the appearance of the domes in aerial photographs indicate a homogeneous composition with about 68% SiO2.[1] Their main component is dacite, with biotite, hornblende and plagioclase phenocrysts.[5]

References
  1. Worner, Gerhardt; Moorbath, Stephen; Horn, Susanne; Entenmann, Jürgen; Harmon, Russel S.; Davidson, Jon P.; Lopez-Escobar, Leopoldo (1994). "Large- and Fine-Scale Geochemical Variations Along the Andean Arc of Northern Chile (17.5°– 22°S)". Tectonics of the Southern Central Andes Structure and Evolution of an Active Continental Margin. Berlin, Heidelberg: Springer Berlin Heidelberg. pp. 77–88. doi:10.1007/978-3-642-77353-2_5. ISBN 978-3-642-77353-2.
  2. Duque, Carlos E. (1981). "Presentación Cartográfica de toponimia Indígena Chilena.(Area Septentrional)". Revista Signos (in Spanish). 13 (17).
  3. Wörner, Gerhardt; Hammerschmidt, Konrad; Henjes-Kunst, Friedhelm; Lezaun, Judith; Wilke, Hans (December 2000). "Geochronology (40Ar/39Ar, K-Ar and He-exposure ages) of Cenozoic magmatic rocks from Northern Chile (18-22°S): implications for magmatism and tectonic evolution of the central Andes". Revista Geológica de Chile. 27. Retrieved 8 December 2015.
  4. Wörner, Gerhard; Moorbath, Stephen; Harmon, Russel S. (1992). "Andean Cenozoic volcanic centers reflect basement isotopic domains". Geology. 20 (12): 1103–1106. doi:10.1130/0091-7613(1992)020<1103:ACVCRB>2.3.CO;2.
  5. Polanco, E.; Gardeweg, M. (2000). Preliminary study of the volcanic stratigraphy of Upper Cenozoic at Pampa Lirima and Cancosa, 1st Region highland, Chile (19 45'–20 00'S and 69 00'–68 30'W). Proceedings of the 9th Chilean Geology Congress (in Spanish). Puerto Varas, Chile. pp. 326–327 – via ResearchGate.
  6. Nakada, Setsuya (1 April 1991). "Magmatic processes in titanite-bearing dacites, central Andes of Chile and Bolivia". American Mineralogist. 76 (3–4): 548–560. ISSN 0003-004X.
  7. Worner, Gerhardt; Lopez-Escobar, Leopoldo; Moorbath, Stephen; Horn, Susan; Entenmann, Jürgen; Harmon, Russel S.; Davidson, Jon D. (1992). "Variaciones geoquimicas, locales y regionales, en el frente volcanico cuaternario de los Andes centrales (17°30'-22°00'S), norte de Chile". Andean Geology. 19 (a). Archived from the original on 22 March 2017. Retrieved 8 December 2015.
  8. Armijo, Luis S (1919). "Regadío de la Pampa del Tamarugal". Anales del Instituto de Ingenieros de Chile (in European Spanish) (7): 298. ISSN 0716-324X.
  9. Gardeweg, M.; D., Sellés (2013). "Geología del área Collacagua–Rinconada, Región de Tarapacá". Carta Geológica de Chile, Serie Geología Básica (in Spanish). SERNAGEOMIN – via ResearchGate.

"Porquesa". Global Volcanism Program. Smithsonian Institution.

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