Skip to main content

Time-evolution of magma sources in a continental back-arc setting: the Cenozoic basalts from Sierra de San Bernardo (Patagonia, Chubut, Argentina)


East of the Patagonian Andes, mafic volcanic rocks (mainly lava flows and scoriae) are exposed in the Sierra de San Bernardo fold belt and neighbouring areas (central Patagonia; 44.5–46° S, 69–71° W). They were erupted over a wide interval of time (late Eocene–Pleistocene; 14 new K–Ar ages), and show systematic chemical and Sr–Nd–Pb isotopic variations in time. The alkaline lavas (Mg number 57–66) erupted during the late Eocene and early Miocene, have an intraplate geochemical affinity, and have the highest 143Nd/144Nd and 206Pb/204Pb and the lowest 87Sr/86Sr ratios of the dataset. Their compositions indicate that their depth of equilibration in the mantle was greater than that of subsequent lavas. In contrast, the Plio-Pleistocene alkaline lavas (Mg number 58–71) are the most enriched in incompatible elements, still showing an intra-plate signature, and have the lowest 143Nd/144Nd and 206Pb/204Pb and the highest 87Sr/86Sr ratios. A distinctive group of early Miocene subalkaline lavas is characterized by slightly more evolved compositions (Mg number 56–59), coupled with very low incompatible element contents, flat LREE and fractionated HREE patterns (‘kinked’ pattern), and intermediate Sr–Nd–Pb isotope compositions. The Pleistocene basanites (Mg number 71–72) from the Cerro Ante monogenetic cone, on the easternmost slopes of the Patagonian Andes, have a marked orogenic geochemical signature and Sr–Nd–Pb isotope ratios that overlap with those of volcanic rocks from the adjacent active Andean arc. They originated in a mantle source extensively modified by the addition of materials from the subducting Pacific oceanic plates. We suggest that the wide chemical and isotopic variability of the Sierra de San Bernardo lavas reflects the upwelling of asthenospheric mantle beneath the study area, which induced lithospheric erosion and progressive involvement of enriched mantle domains in the genesis of magmas. In this context, late Eocene and early Miocene alkaline magmatism was dominantly sourced from the asthenospheric mantle, whereas Plio-Pleistocene alkaline magmas contain the largest proportion of an enriched lithospheric component. The peculiar compositional features of the early Miocene subalkaline lavas are interpreted in terms of high-degree mantle melting followed by melt–lithospheric mantle reaction processes. Based on current knowledge about the relative movement and decoupling between lithosphere and asthenosphere, we propose that the asthenosphere below the study area rose up to compensate for the westward drift of the mantle wedge coupled with the South American lithosphere.

Corresponding author
Author for correspondence:
Hide All
Albarede, F. 1992. How deep do common basaltic magmas form and differentiate? Journal of Geophysical Research 97, 1099711009.
Baker, P. E., Rea, W. J., Skarmeta, J., Caminos, R. & Rex, D. C. 1981. Igneous history of the Andean cordillera and Patagonian plateau around latitude 46°S. Philosophical Transactions of the Royal Society of London A303, 105–49.
Balogh, K. 1985. K–Ar dating of Neogene volcanic activity in Hungary. Experimental technique, experiences and methods of chronological studies. ATOMKI Rep. D/1, pp. 277–88.
Cande, S. C. & Leslie, R. B. 1986. Late Cenozoic tectonics of the Southern Chile Trench. Journal of Geophysical Research 91, 471–96.
Cembrano, J., Hervé, F. & Lavenu, A. 1996. The Liquiñe-Ofqui fault zone: a long-lived intra-arc fault system in southern Chile. Tectonophysics 259, 5566.
Chelotti, L. A. 1997. Evolución tectónica de la Cuenca del Golfo San Jorge en el Cretácico y Terciario; algunas observaciones desde la interpretación sísmica. Boletín de Informaciones Petroleras 49, 6282.
Conceição, R. V., Mallmann, G., Koester, E., Schilling, M., Bertotto, G. W. & Rodriguez-Vargas, A. 2005. Andean subduction-related mantle xenoliths: isotopic evidence of Sr–Nd decoupling during metasomatism. Lithos 82, 273–87.
de Ignacio, C., López, I., Oyarzun, R. & Márquez, A. 2001. The northern Patagonia Somuncura plateau basalts: a product of slab-induced, shallow asthenospheric upwelling? Terra Nova 13, 117–21.
D'Orazio, M., Agostini, S., Innocenti, F., Haller, M. J., Manetti, P. & Mazzarini, F. 2001. Slab window-related magmatism from southernmost South America: the late Miocene mafic volcanics from the Estancia Glencross area (~52°S, Argentina–Chile). Lithos 57, 6789.
D'Orazio, M., Agostini, S., Mazzarini, F., Innocenti, F., Manetti, P., Haller, M. J. & Lahsen, A. 2000. The Pali Aike Volcanic Field, Patagonia: slab-window magmatism near the tip of South America. Tectonophysics 321, 407–27.
D'Orazio, M., Gonzalez-Ferrán, O., Innocenti, F., Mazzarini, F., Mazzuoli, R., Tonarini, S. & Adorni-Braccesi, A. 1999. Alkaline basaltic volcanism in the Weddell Sea side of the northernmost Antarctic Peninsula: Sr–Nd isotope and trace-element characteristics. 8th International Symposium on Antarctic Earth Sciences, Wellington, Abstract volume, p. 92.
D'Orazio, M., Innocenti, F., Manetti, P. & Haller, M. J. 2004. The Cenozoic back-arc magmatism of the southern extra-Andean Patagonia (44.5–52°S): A review of geochemical data and geodynamic interpretations. Revista de la Asociación Geológica Argentina 59, 525–38.
D'Orazio, M., Innocenti, F., Manetti, P., Haller, M. J., Di Vincenzo, G. & Tonarini, S. 2005. The Late Pliocene mafic lavas from the Camusú Aike Volcanic Field (~50°S, Argentina): evidences for geochemical variability in slab window magmatism. Journal of South American Earth Sciences 18, 107–24.
D'Orazio, M., Innocenti, F., Manetti, P., Tamponi, M., Tonarini, S., González-Ferrán, O., Lahsen, A. & Omarini, R. 2003. The Quaternary calc-alkaline volcanism of the Patagonian Andes close to the Chile Triple Junction: geochemistry and petrogenesis of volcanic rocks from the Cay and Maca volcanoes (~ 45°S, Chile). Journal of South American Earth Sciences 16, 219–42.
Espinoza, F., Morata, D., Pelleter, E., Maury, R. C., Suárez, M., Lagabrielle, Y., Polvé, M., Bellon, H., Cotten, J., De la Cruz, R. & Guivel, C. 2005. Petrogenesis of the Eocene and Mio-Pliocene alkaline basaltic magmatism in Meseta Chile Chico, southern Patagonia, Chile: evidence for the participation of two slab windows. Lithos 82, 315–43.
Fitzgerald, M. G., Mitchum, R. M., Uliana, M. A. & Biddle, K. T. 1990. Evolution of the San Jorge Basin, Argentina. American Association of Petroleum Geologists Bulletin 74, 879920.
Foley, S. F., Venturelli, G., Green, D. H. & Toscani, L. 1987. The ultrapotassic rocks: characteristics, classification, and constraints for petrogenetic models. Earth Science Reviews 24, 81134.
Futa, K. & Stern, C. R. 1988. Sr and Nd isotopic and trace element composition of Quaternary volcanic centers of southern Andes. Earth and Planetary Science Letters 88, 253–63.
Gerlach, D. C., Frey, F. A., Moreno-Roa, H. & López-Escobar, L. 1988. Recent volcanism in the Puyehue–Cordon Caulle region, Southern Andes, Chile (40.5°S): petrogenesis of evolved lavas. Journal of Petrology 29, 333–82.
Gorring, M. L. & Kay, S. M. 2000. Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism. Contributions to Mineralogy and Petrology 140, 5572.
Gorring, M. L. & Kay, S. M. 2001. Mantle processes and sources of Neogene slab window magmas from southern Patagonia, Argentina. Journal of Petrology 42, 1067–94.
Gorring, M. L., Kay, S. M., Zeitler, P. K., Ramos, V. A., Rubiolo, D., Fernandez, M. I. & Panza, J. L. 1997. Neogene Patagonian plateau lavas: Continental magmas associated with ridge collision at the Chile Triple Junction. Tectonics 16, 117.
Gorring, M. L., Singer, B., Gowers, J. & Kay, S. M. 2003. Plio-Pleistocene basalts from the Meseta del Lago Buenos Aires, Argentina: evidence for asthenosphere–lithosphere interactions during slab window magmatism. Chemical Geology 193, 215–35.
Gripp, A. E. & Gordon, R. G. 2002. Young tracks of hotspots and current plate velocities. Geophysical Journal International 150, 321–61.
Guivel, C., Morata, D., Pelleter, E., Espinoza, F., Maury, R. C., Lagabrielle, Y., Polvé, M., Bellon, H., Cotten, J., Benoit, M., Suárez, M. & de la Cruz, R. 2006. Miocene to Late Quaternary Patagonian basalts (46–47°S): geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction. Journal of Volcanology and Geothermal Research 149, 346–70.
Hart, S. R. 1984. The DUPAL anomaly: a large-scale isotopic anomaly in the southern hemisphere. Nature 309, 753–6.
Hart, S. R., Hauri, E. H., Oschmann, L. A. & Whitehead, J. A. 1992. Mantle plumes and entrainment: isotopic evidence. Science 256, 517–20.
Hechem, J. J. & Strelkov, E. 2002. Secuencia sedimentaria mesozoica del Golfo San Jorge. In Geología y Recursos Naturales de Santa Cruz (ed. Haller, M. J.), pp. 129–47. Relatorio del XV Congreso Geológico Argentino, Buenos Aires, Argentina.
Hervé, F., Davidson, J., Mpodozis, E. & Covacevich, E. V. 1981. The late Palaezoic in Chile: stratigraphy, structure and possible tectonic framework. Anais da Academia Brasileira de Ciências 53, 361–73.
Hickey, R., Frey, F. A. & Gerlach, D. 1986. Multiple sources for basaltic arc rocks from the Southern Volcanic Zone of the Andes (34°–41°S): trace element and isotopic evidence for contributions from subducted oceanic crust, mantle, and continental crust. Journal of Geophysical Research 91, 5963–83.
Hickey-Vargas, R., Moreno-Roa, H., López-Escobar, L. & Frey, F. A. 1989. Geochemical variations in Andean basaltic and silicic lavas from the Villarrica–Lanin volcanic chain (39.5° S): an evaluation of source heterogeneity, fractional crystallization and crustal assimilation. Contributions to Mineralogy and Petrology 103, 361–86.
Hirose, K. & Kushiro, I. 1993. Partial melting of dry peridotite at high pressures: determination of compositions of melts segregated from peridotite using aggregates of diamond. Earth and Planetary Science Letters 114, 477–89.
Hole, M. J., Kempton, P. D. & Millar, I. L. 1993. Trace-element and isotopic characteristics of small-degree melts of the asthenosphere: evidence from the alkalic basalts of the Antarctic Peninsula. Chemical Geology 109, 5168.
Hole, M. J., Saunders, A. D., Rogers, G. & Sykes, M. A. 1995. The relationship between alkaline magmatism, lithospheric extension and slab window formation along continental destructive plate margins. In Volcanism associated with extension at consuming plate margins (ed. Smellie, J. L.), pp. 265–85. Geological Society of London, Special Publication no. 81.
Homovc, J. F., Conforto, G. A., Lafourcade, P. A. & Chelotti, L. A. 1995. Fold beld in the San Jorge Basin, Argentina: an example of tectonic inversion. In Basin Inversion (eds Buchanan, J. G. & Buchanan, P. G.), pp. 235–48. Geological Society of London, Special Publication no. 88.
Kay, S. M., Ardolino, A. A., Gorring, M. L. & Ramos, V. A. 2007. The Somuncura large igneous province in Patagonia: interaction of a transient mantle thermal anomaly with a subducting slab. Journal of Petrology 48, 4377.
Kay, S. M. & Copeland, P. 2006. Early to middle Miocene backarc magmas of the Neuquén Basin: Geochemical consequences of slab shallowing and the westward drift of South America. In Evolution of an Andean margin: a tectonic and magmatic view from the Andes to the Neuquén Basin (35°–39° S lat) (eds Kay, S. M. & Ramos, V. A.), pp. 185213. Geological Society of America, Special Paper no. 407.
Kempton, P. D., Hawkesworth, C. J., Lopez-Escobar, L., Pearson, D. G. & Ware, A. J. 1999. Spinel ± garnet lherzolite xenoliths from Pali Aike: Part 2, Trace element and isotopic evidence bearing on the evolution of lithospheric mantle beneath southern Patagonia. In The J. B. Dawson volume (eds Gurney, J. J., Gurney, J. L., Pascoe, M. D. & Richardson, S. H.), pp. 415–28. Proceedings of the 7th International Kimberlite Conference, Red Rood Design, Cape Town, South Africa.
Klemme, S. & O'Neill, H. St. C. 2000. The near-solidus transition from garnet lherzolite to spinel lherzolite. Contributions to Mineralogy and Petrology 138, 237–48.
Kogiso, T., Tatsumi, Y. & Nakano, S. 1997. Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts. Earth and Planetary Science Letters 148, 193205.
Kushiro, I. 2001. Partial melting experiments on peridotite and origin of mid-ocean ridge basalt. Annual Review of Earth and Planetary Science 29, 71107.
Lizuain, A., Ragona, D. & Folguera, A. 1995. Mapa Geológico de la Provincia del Chubut, República Argentina. Secretaría de Minería, Dirección Nacional del Servicio Geológico, Scale 1:750.000. Buenos Aires, Argentina.
López-Escobar, L., Kilian, R., Kempton, P. & Tagiri, M. 1993. Petrography and geochemistry of Quaternary rocks from the Southern Volcanic Zone of the Andes between 41°30′ and 46°00′S, Chile. Revista Geológica de Chile 20, 3355.
López-Escobar, L., Parada, M. A., Hickey-Vargas, R., Frey, F. A., Kempton, P. D. & Moreno, H. 1995. Calbuco Volcano and minor eruptive centers distributed along the Liquiñe–Ofqui Fault Zone, Chile (41°–42° S): contrasting origin of andesitic and basaltic magma in the Southern Volcanic Zone of the Andes. Contribution to Mineralogy and Petrology 119, 345–61.
Marshall, L. G., Cifelli, R. L., Drake, R. E. & Curtis, G. H. 1986. Vertebrate paleontology, geology and geochronology of the Tapera de López and Scarritt Pocket, Chubut Province, Argentina. Journal of Paleontology 60, 920–51.
Massaferro, G. I., Haller, M. J., D'Orazio, M. & Alric, V. I. 2006. Sub-recent volcanism in Northern Patagonia: a tectonomagmatic approach. Journal of Volcanology and Geothermal Research 155, 227–43.
McDonough, W. F. & Sun, S. S. 1995. The composition of the Earth. Chemical Geology 120, 223–53.
Ntaflos, Th., Bjerg, E. A., Labudia, C. H. & Kurat, G. 2007. Depleted lithosphere from the mantle wedge beneath Tres Lagos, southern Patagonia, Argentina. Lithos 94, 4665.
Pankhurst, R. J. & Rapela, C. R. 1995. Production of Jurassic rhyolite by anatexis of the lower crust of Patagonia. Earth and Planetary Science Letters 134, 2336.
Pankhurst, R. J., Rapela, C. W., Fanning, C. M. & Márquez, M. 2006. Gondwanide continental collision and origin of Patagonia. Earth Science Reviews 76, 235–56.
Pearce, T. H. 1978. Olivine fractionation equations for basaltic and ultrabasic liquids. Nature 276, 771–4.
Plá Cid, J., Nardi, L. V. S., Gisbert, P. E., Merlet, C. & Boyer, B. 2005. SIMS analyses on trace and rare earth elements in coexisting clinopyroxene and mica from minette mafic enclaves. Contributions to Mineralogy and Petrology 148, 675–88.
Quartino, B. J. 1958. El basalto olivínico del Cerro El Pedrero, Chubut. Revista de la Asociación Geológica Argentina 12, 233–64.
Ramos, V. A. & Kay, S. M. 1992. Southern Patagonian plateau basalts and deformation: backarc testimony of ridge collisions. Tectonophysics 205, 261–82.
Rivalenti, G., Mazzucchelli, M., Laurora, A., Ciuffi, S. I. A., Zanetti, A., Vannucci, R. & Cingolani, C. A. 2004. The backarc mantle lithosphere in Patagonia, South America. Journal of South American Earth Sciences 17, 121–52.
Rivalenti, G., Mazzucchelli, M., Zanetti, A., Vannucci, R., Bollinger, C., Hémond, C. & Bertotto, G. W. 2007. Xenoliths from Cerro de los Chenques (Patagonia): An example of slab-related metasomatism in the backarc lithospheric mantle. Lithos 99, 4567.
Schilling, M., Conceição, R. V., Mallmann, G., Koester, E., Kawashita, K., Hervé, F., Morata, D. & Motoki, A. 2005. Spinel-facies mantle xenoliths from Cerro Redondo, Argentine Patagonia: petrographic, geochemical, and isotopic evidence of interaction between xenoliths and host basalt. Lithos 82, 485502.
Scoppola, B., Boccaletti, D., Bevis, M., Carminati, E. & Doglioni, C. 2006. The westward drift of the lithosphere; a rotational drag? Geological Society of America Bulletin 118, 199209.
Shervais, J. W. 1982. Ti–V plots and the petrogenesis of modern and ophiolitic lavas. Earth and Planetary Science Letters 59, 101–18.
Spera, F. J. & Bohrson, W. A. 2001. Energy-constrained open-system magma processes I: general model and energy-constrained assimilation fractional-crystallization (EC-AFC) formulation. Journal of Petrology 42, 9991018.
Steiger, K. H. & Jäger, E. 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–62.
Stern, C. R., Frey, F. A., Futa, K., Zartman, R. E., Peng, Z. & Kyser, T. K. 1990. Trace-element and Sr, Nd, Pb and O isotopic composition of Pliocene and Quaternary alkali basalt of the Patagonian Plateau lavas of southernmost South America. Contributions to Mineralogy and Petrology 104, 294308.
Stern, C. R. & Kilian, R. 1996. Role of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone. Contributions to Mineralogy and Petrology 123, 263–81.
Stern, C. R., Kilian, R., Olker, B., Hauri, E. H. & Kyser, T. K. 1999. Evidence from mantle xenoliths for relatively thin (< 100 km) continental lithosphere below the Phanerozoic crust of southernmost South America. Lithos 48, 217–35.
Sturm, M. E., Klein, E. M., Graham, D. W. & Karsten, J. 1999. Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He–Pb–Sr–Nd isotope systematics. Journal of Geophysical Research 104, B3, 5097114.
Sudo, A. & Tatsumi, Y. 1990. Phlogopite and K-amphibole in the upper mantle: implication for magma genesis in subduction zones. Geophysical Research Letters 17, 2932.
Sun, S. S. & McDonough, W. F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins (eds Saunders, A. D. & Norry, M. J.), pp. 313–45. Geological Society of London, Special Publication no. 42.
Sylwan, C. A. 2001. Geology of the Golfo San Jorge Basin, Argentina. Journal of Iberian Geology 27, 123–57.
Taylor, S. R. & McLennan, S. M. 1995. The geochemical evolution of the continental crust. Reviews in Geophysics 33, 241–65.
Thybo, H. 2006. The heterogeneous upper mantle low velocity zone. Tectonophysics 416, 5379.
Tiepolo, M., Zanetti, A., Oberti, R., Brumm, R., Foley, S. & Vannucci, R. 2003. Trace-element partitioning between synthetic potassic richterites and silicate melts, and contrasts with the partitioning behaviour of pargasites and kaersutites. European Journal of Mineralogy 15, 329–40.
Todt, W., Cliff, R. A., Hanser, A. & Hofmann, A. W. 1996. Evaluation of a 202Pb–205Pb double spike for high-precision lead isotope analysis. In Earth processes: reading the isotopic code (eds Hart, S. R. & Basu, A.), pp. 429–37. American Geophysical Union, Geophysical Monograph vol. 95. Washington, DC, USA.
Vernieres, J., Godard, M. & Bodinier, J.-L. 1997. A plate model for the simulation of trace element fractionation during partial melting and magmas transport in the Earth's upper mantle. Journal of Geophysical Research 102, 24771–84.
Wagner, T. P. & Grove, T. L. 1998. Melt/harzburgite reaction in the petrogenesis of tholeiitic magma from Kilauea volcano, Hawaii. Contributions to Mineralogy and Petrology 131, 112.
Xu, Y. G., Ma, J. L., Frey, F. A., Feigenson, M. D. & Liu, J. F. 2005. Role of lithosphere–asthenosphere interaction in the genesis of Quaternary alkali and tholeiitic basalts from Datong, western North China Craton. Chemical Geology 224, 247–71.
Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

Geological Magazine
  • ISSN: 0016-7568
  • EISSN: 1469-5081
  • URL: /core/journals/geological-magazine
Please enter your name
Please enter a valid email address
Who would you like to send this to? *



Full text views

Total number of HTML views: 2
Total number of PDF views: 25 *
Loading metrics...

Abstract views

Total abstract views: 194 *
Loading metrics...

* Views captured on Cambridge Core between September 2016 - 19th August 2018. This data will be updated every 24 hours.