Journal of South American Earth Sciences 18 (2005) 125–145 www.elsevier.com/locate/jsames The upper mantle beneath Patagonia, Argentina, documented by xenoliths from alkali basalts E.A. Bjerga,*, T. Ntaflosb, G. Kuratb, G. Dobosic, C.H. Labudı´aa aDepartamento de Geologı´a, CONICET—Universidad Nacional del Sur, San Juan 670, B8000ICN Bahı´a Blanca, Argentina bDepartment of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria cHungarian Academy of Sciences, Laboratory for Geochemical Research, Budao¨rsiu´t 45, H-1112 Budapest, Hungary Received 1 March 2003; accepted 1 September 2004 Abstract Mantle xenoliths in alkali basaltic lavas (with ocean-island basalt chemical signatures) and cinder cones occur in several areas of Patagonia. A representative suite of mantle xenoliths was collected in the region between latitudes 408 and 528S and longitudes 678 and 718W in the Rı´o Negro, Chubut, and Santa Cruz provinces, Argentina. Mantle xenoliths in Patagonia display distinguishing peculiarities compared with those of other worldwide occurrences. The lithospheric mantle beneath Patagonia, as inferred from chemical variation diagrams, has experienced only minor melt extractions in the garnet peridotite field and more extensive melt extractions in the spinel lherzolite field. Variably intensive cryptic and modal metasomatism affected the lithospheric mantle in this region. Textural evidence shows that the mantle is moderately to strongly tectonized and recrystallized on both the local and the regional scale, with an overall predominance of deformed textural types. Mineral equilibrium indicates a strongly elevated geotherm similar to the southeast Australia and oceanic geotherms, which is not normal for a continental intraplate tectonic setting. Therefore, the properties of the Patagonian samples are probably related to the presence of rising mantle plume(s) in an extensional tectonic setting. q 2004 Published by Elsevier Ltd. Keywords: Alkali basalts; Garnet; Metasomatism; Patagonia; Upper mantle; Xenoliths Resumen Asociados a lavas basa´lticas alcalinas (con signatura quı´mica OIB) y conos de cineritas se han reconocidos xenolitos mante´licos en varias areas de Patagonia. Un conjunto representativo de xenolitos del manto fueron recolectados en la regio´n comprendida entre las latitudes 408 a 528S y las longitudes 678 a718W en las provincias de Rı´o Negro, Chubut, y Santa Cruz, Repu´blica Argentina. Los xenolitos mante´licos de Patagonia presentan algunas caracterı´sticas peculiares en comparacio´n con las correspondientes a los hallados en otras partes del mundo. El manto lito´sferico que la subyace ha experimentado solo extracciones menores de fundidos en el campo de estabilidad de las peridotitas granatı´feras y mayores en el correspondiente a las lherzolitas espine´licas, tal como lo indican los diagramas de variacio´n quı´mica. El manto litosfe´rico de esta regio´n ha sido afectado en forma variable por metasomatismo modal y crı´ptico. Las evidencias texturales indican que el manto esta tectonizado y recristalzado en grados que varı´an de moderado a intenso, tanto a escala local como regional, con un claro predominio de los tipos texturales deformados. Los equilibrios minerales indican la presencia de una geoterma extremadamente alta, similar a la geoterma de South East Australia y las geotermas ocea´nicas. Esto no es normal para un ambiente tecto´nico de intraplaca. Por ello sugerimos que las caracterı´sticas evidenciadas por los xenolitos de Patagonia son muy probablemente el resultado de la presencia de pluma(s) de manto ascendente(s), en un ambiente tecto´nico extensional. q 2004 Published by Elsevier Ltd. 1. Introduction The geologic evolution of South America remains a * Corresponding author. Fax: C54 291 4595148. E-mail address: [email protected] (E.A. Bjerg). matter of discussion because the available geologic 0895-9811/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.jsames.2004.09.002 126 E.A. Bjerg et al. / Journal of South American Earth Sciences 18 (2005) 125–145 information is far from complete. Most xenolith localities in 3. Sampling localities South America have been discovered in the past 25 years, and Patagonia in particular offers an outstanding opportu- A representative suite of peridotitic xenoliths was nity to gather information about the nature of the collected in several localities of the Rı´o Negro, Chubut, subcontinental lithospheric mantle at this latitude. and Santa Cruz provinces (Fig. 1). In Rı´o Negro, samples Upper mantle xenoliths in Patagonian basalts in Argen- were collected along a 190 km regional profile extending 0 0 0 0 tina have been reported by Villar (1975), Gelo´s and Hayase between 41801 S–70815 W and 40830 S–68803 W at the (1979), Skewes and Stern (1979), Gelo´s and Labudı´a following localities: Comallo, Laguna Frı´a, Traful, and (1981), Labudı´a et al. (1984, 1990), Stern et al. (1986, Prahuaniyeu. Along this profile, xenoliths occur in late 1999), Stern (1989), Bjerg et al. (1991, 1994), and Varela Pliocene–Early Pleistocene alkali lava flows, necks, and et al. (1995). In recent years, Stern et al. (1990), Barbieri cinder cones, which postdate Miocene meseta lava flows. At (1998), Kilian et al. (1998a,b, 2002), Ntaflos et al. (1998, Comallo, mostly small (2–4 cm), deep red, oxidized 1999, 2000, 2001), Barbieri et al. (1999), Laurora et al. xenoliths occur in a cinder cone, whereas unaltered (1999, 2001), Dobosi et al. (1999), Bjerg et al. (1999a,b, websterites, harzburgites, and lherzolites are present in the 2000, 2002), Vannucci et al. (2002), Mazzucchelli et al. lava flow, which occurs on top of the cinder cone. At the (2002), and Rivalenti et al. (2002) have contributed to a Traful basaltic neck and Laguna Frı´a lava flows, unaltered better understanding of the petrology and geochemistry of harzburgites and lherzolites reach 30 cm in diameter. the upper mantle, the metasomatic processes that took place Prahuaniyeu is the farthest northern locality in Patagonia (and still take place) in the upper mantle, and the origin of where garnet xenoliths have been discovered. The dominant the fluids responsible for metasomatism. On the basis of a types are spinel peridotites, as well as garnet peridotites, study of xenoliths from northern Patagonia, Varela et al. transitional spinel garnet peridotites, and few pyroxenites. At this locality, xenoliths occur in both a cinder cone and a (1997) have shown that metasomatism and tectonization small lava flow. were significant processes that affected xenoliths, many of In Chubut (Ch) province, samples were collected in which originated at a depth close to the garnet stability field. the area of Paso de Indios (438490S–698020W). Websterites, We provide the results of our studies of upper mantle harzburgites, lherzolites, and dunites, up to 10 cm in size, xenoliths from Pliocene–Pleistocene alkali basalts in an area are hosted by Oligocene alkali basalt necks and lava flows. between latitudes 408–528S and longitudes 678–718W. In Santa Cruz, mantle xenoliths were collected at the These data, together with those reported previously, provide Gobernador Gregores (DUB), Tres Lagos (TL), and Pali some insight into the upper mantle underneath Patagonia Aike (PA) localities. Some of the largest Patagonia xenoliths and its geochemical evolution in recent geological times. (up to 40 cm) occur at Gobernador Gregores (488340S– 708110W) in a deeply eroded cinder cone (2.5 km diameter). The collected samples comprise spinel harzburgites, lherzo- lites, and wehrlites. Amphibole, secondary calcite, and 2. Geological setting phlogopite-bearing wehrlites, lherzolites, and harzburgites contain glassy patches up to 1 cm in diameter. Xenoliths The Andean Cordillera represents the western continen- from Gobernador Gregores, compared with xenoliths from tal margin of the South American plate whose geologic the other localities, are unusually coarse grained. evolution is related to the subduction of the Nazca and At Tres Lagos (498110S–718200W), the xenolith sizes Antarctic plates below the South American plate. South of range between 4 and 20 cm, and they occur in a cinder 398S, the Andean Cordillera can be divided in two major cone. Most show deep red colors due to oxidation. regions (Fig. 1). The southern volcanic zone (SVZ) extends Unaltered xenoliths are found associated with basalt dikes to 468S and is followed by a gap in volcanic activity due to (w40–50 cm wide), which cut the cinder cone, and small the Chile Triple Junction (Ramos, 1999). The gap separates lava flows related to this cinder cone. the SVZ from the Austral volcanic zone, which extends At Pali Aike, a large volcanic field extending along the from 498 to 538S(Stern and Kilian, 1996). southernmost E–W border between Argentina and Chile In Patagonia, Pliocene and Quaternary alkalic lavas (518490S–698410W), xenoliths between 2 and 8 cm occur in carrying ultramafic xenoliths erupted east of the stratovol- three tephra levels. Harzburgites are dominant, followed by canos of the actual orogenic arc and occupy a backarc spinel lherzolites, wehrlites, orthopyroxenites, and garnet- tectonic environment. These lavas were classified as bearing lherzolites. cratonic by Stern et al. (1990) and have geochemical characteristics similar to intraplate oceanic island basalts. The regional geology of Patagonia, which is beyond the 4. Analytical methods scope of this article, has been reported by several authors; for a synthesis of the regional geology, see Caminos (1999) Electron