Abstract
The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The “cratonic” basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in 87Sr/86Sr (0.70316 to 0.70512), 143Nd/144Nd (ɛNd) and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the “cratonic” basalts. In contrast, the “transitional” basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by 87Sr/86Sr=0.7039±0.0004, ɛNd, 206Pb/204Pb=18.60±0.08, 207Pb/204Pb=15.60±0.01, and 208Pb/204Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the “cratonic” basalts, are displaced to higher 87Sr/86Sr at a given 143Nd/144Nd and to higher 207Pb/204Pb at a given 208Pb/204Pb. The “transitional” basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the “cratonic” and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, δ18O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock δ18O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock δ18O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The “cratonic” basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, 87Sr/86Sr at a given 143Nd/144Nd, 207Pb/204Pb at a given 208Pb/204Pb, and δ18O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The “transitional” basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the “transitional” basalts earlier in the Cenozoic.
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Allegre CJ, Dupre B, Lambert B, Richard P (1981) The subcontinental versus suboceanic debate, I. Lead-neodymium-strontium isotopes in primary alkali basalts from a shield area: Ahaggar volcanic suit. Earth Planet Sci Lett 52:85–92
Baker PE, Rea WJ, Skarmeta J, Caminos R, Rex DC (1981) Igneous history of the Andean cordillera and Patagonian plateau around 46° S. Phil Trans R Soc London A 303:105–149
Barrazangi M, Isacks BL (1976) Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America. Geology 4:686–692
Barreiro BA (1984) Lead isotopes and Andean magmagenesis. In: RS Harmon, BA Barreriro (eds) Andean magmatism: chemical and isotopic constraints. Shiva, Cheshire, pp 21–30
Chen CY, Frey FA (1983) Origin of Hawaiian tholeiite and alkali basalt. Nature 302:785–789
Cohen RS, O'Nions RK, Dawson JB (1984) Isotope geochemistry of xenoliths from East Africa: implications for the development of mantle reservoirs and their interaction. Earth Planet Sci Lett 68:209–220
Cong BL, Zhang WH, Ye DM (1979) The study of the Cenozoic basalts in North China fault block. Acta Geol Sinica 59:112–123 (in Chinese)
Dalziel IWD (1988) Andean magmatism: the global tectonic setting. Geol Soc Am Annual Meeting Abstr Progr 20(7):A5
DeWit MJ (1977) The evolution of the Scotia arc as a key to the reconstruction of Gondwanaland. Tectoniphysics 37:53–81
Douglas BJ, Saul SL, Stern CR (1987) Rheology of the upper mantle beneath southernmost South America inferred from peridotite xenoliths. J Geol 95:241–253
Frey FA, Green DH, Roy SD (1978) Integrated models of basalts petrogenesis: a study of quartz tholeiites to olivine melilitites from southeastern Australia utilizing geochemical and experimental petrological data. J Petrol 19:463–262
Futa K, Stern CR (1988) Sr and Nd isotopic and trace element compositions of Quaternary volcanic centers of the southern Andes. Earth Planet Sci Lett 88:253–262
Geolos EM, Hayase K (1979) Estudio de las inclusions peridotiticas en un basalto de la region de Comallo y otras localidades de la Provincias de Rio Negro y Chubut. Actas Sexto Congreso Geologico Argentina 2:69–82
Gerlach DC, Frey FA, Moreno-Roa H, Lopez-Escobar L (1988) Recent volcanism in the Puheyue-Cordon Caulle region, southern Andes, Chile (40.5° S); petrogenesis of evolved lavas. J Petrol 29:333–382
Halpern M (1973) Regional geochronology of Chile south of 50° S latitude. Geol Soc Am Bull 84:2407–2422
Harmon RS, Barreiro BA, Moorbath S, Hoefs J, Francis PW, Thorpe RS, Deruelle B, McHugh J, Viglino JA (1984) REgional O-, Sr-, and Pb-isotope relationships in late Cenozoic calc-alkaline lavas of the Andean cordillera. J Geol Soc London 141:803–822
Hawkesworth CJ, Norry MJ, Roddick JC, Baker PE (1979) 143Nd/144Nd, 87Sr/86Sr and incompatible element variations in calc-alkaline andesites and plateau lavas from South America. Earth Planet Sci Lett 42:45–57
Hickey RL, Frey FA, Gerlach DC, Lopez-Escobar L (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. J Geophys Res 91:5963–5983
Hickey-Vargas RL, Moreno H, Lopez-Escobar L, Frey FA (1989) Geochemical variations in Andean basaltic and silicic lavas from Villarica-lanin volcanic chain (39.5° S): an evaluation of source heterogeneity, fractional crystallization and crustal assimilation, Contrib Mineral Petrol 103:361–386
Ha P, Frey FA (1984) Utilization of neutron activation analysis in the study of geologic materials. Atomkernenergie Kerntech 44:710–716
Irvine TN, Barager WRA (1971) A guide to the chemical classification of rocks. Can J Earth Sci 8:523–548
Irving AJ, Green DH (1976) Geochemistry and petrogenesis of Newer basalts of Victoria and South Australia. J Geol Soc Australia 23:45–66
Kay SM, Ramos VA, Rapela CA (1988) Temporal and spacial variations in the distribution and geochemistry of Cenozoic Patagonian basalts in Argentina. Geol Soc Am Abstr Progr 20:A6
Labudia, CH, Berg EA, Gregori DA (1984) Nodulos de composicion ultrabasica de las lavas alcalinas de la localidad de Praguaniyue, Provincia de Rio Negro. Actas Noveno Congreso Geologico Argentina 2:547–553
McDonough, WF, McCulloch MT, Sun SS (1985) Isotopic and geochemical-systematics in Tertiary-Recent basalts from southcastern Australia and implications for the evolution of the subcontinental lithosphere. Geochim Cosmochim Acta 49:2051–2067
Morris JD, Hart SR (1983) Isotopic and incompatible element constraints on the genesis of island arc volcanics, Cold Bay and Amak island, Aleutians. Geochim Cosmochim Acta 47:2015–2030
Morris JD, Tera F, Harmon RS, Lopez-Escobar L, Klein J, Middleton R (1985) 10Be in lavas from the Andean Southern Volcanic Zone (35°–40° S): evidence for sediment subduction. Commun Depart Geol Univ Chile 35:157–160
Munoz J (1981) Inclusiones ultramaficas del manto superior en Meseta Las Vizcaches, Ultima Esperanza, Magellanes, Chile. Rev Geol Chile 13–14:63–78
Munoz J, Stern CR (1988) The Quaternary volcanic belt of the southern continental margin of South America: transverse structural and petrochemical variations across the segment between 38° S and 39° S. J South Am Earth Sci 1:(2) 147–161
Munoz J, Stern CR (1989) Alkaline magmatism within the segment 38°–39° S of the Plio-Quaternary volcanic belt on the southern South American continental margin. J Geophys Res 94:4545–4560
Munoz J, Stern CR, Bermudez A, Delpino D, Dobbs MF, Frey FA (1989) El volcanismo Plio-Cuaternario a traves de los 38° y 39° sur se los Andes. Rev Assoc Geol Argentino (in press)
Niemeyer H (1979) Nodulos maficos y ultramafico en basaltos alcalinos de la Meseta Buenos Aires, Lago General Carrera, Provincia De Aysen, Chile. Assoc Geol Argentina Rev 33:63–75
Nullo FE, Proserpio C, Ramos VA (1978) Estratigrafia y tectonica de la vertiente esta de hielo continental Patagonico, Argentina-Chile. Actas VII Congreso Geologico Argentino 1:455–470
Peng, ZC, Zartman RE, Futa F, Chen DG (1986) Pb-, Sr-, and Nd-isotopic systematics and chemical characteristics of Cenozoic basalts, eastern China. Chem Geol 59:3–33
Perry FV, Baldridge WS, DePaolo DJ (1987) Role of asthenosphere and lithosphere in the genesis of Late Cenozoic basaltic rocks from the Rio Grande Rift and adjacent regions of the southwestern United States. J Geophys Res 92:9193–9213
Ramos VA (1988) Late Proterozoic-Early Paleozoic of South America — a collisional history. Episodes 11:3:168–174
Rapela CW, Spalletti LA, Meridio JC, Aragon E (1988) Temporal evolution and spatial variation of early Tertiary volcanism in the Patagonian Andes (40° S–42°30′S). J South Am Earth Sci 1:75–88
Skewes MA, Stern CR (1979) Petrology and geochemistry of alkali basalts and ultramafic inclusions from the Pali-Aike volcanic field in southern Chile and the origin of the Patagonian plateau lavas. J Volcanol Geotherm Res 6:3–25
Stern CR (1989) 87Sr/86Sr of mantle xenolith bearing Plio-Quaternary alkali basalts of the Patagonian plateau lavas of southern-most South America. Rev Assoc Geol Argentina (in press)
Stern CR, Futa K, Zicheng P (1983) Pb, Sr and Nd isotopic composition of alkali basalts of the Patagonian plateau lavas, South America. Geol Soc Am Annual Meeting Abstr Progr 15:696–697
Stern CR, Futa K, Saul S, Skewes MA (1986) Nature and evolution of the subcontinental mantle lithosphere below southern South America and implications for Andean magma genesis. Rev Geol Chile 27:41–53
Stern CR, Saul S, Skewes MA, Futa K (1989) Garnet peridotite xenoliths from the Pali-Aike basalts of southernmost South America. In: “Kimberlites and Related Rocks”. Geologic Society of Australia, Special Publ 14, Blackwell, Carlton, Australia, vol 2, pp 735–744
Tyrrell GW (1932) The basalts of Patagonia. J Geol 40:374–383
Zindler A, Hart S (1986) Chemical geodynamics. Ann Rev Earth Sci 14:493–571
Zindler A, Staudigel H, Batiza R (1984) Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity. Earth Planet Sci Lett 45:249–262
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Stern, C.R., Frey, F.A., Futa, K. et al. Trace-element and Sr, Nd, Pb, and O isotopic composition of Pliocene and Quaternary alkali basalts of the Patagonian Plateau lavas of southernmost South America. Contr. Mineral. and Petrol. 104, 294–308 (1990). https://doi.org/10.1007/BF00321486
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DOI: https://doi.org/10.1007/BF00321486