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Petrology and geochemistry of volcanic rocks of the Cerro Galan caldera, northwest Argentina

  • P. W. Francis (a1), R. S. J. Sparks (a2), C. J. Hawkesworth (a3), R. S. Thorpe (a3), D. M. Pyle (a2), S. R. Tait (a4), M. S. Mantovani (a5) and F. McDermott (a3)...

At least 2000 km3 of relatively uniform dacitic magma have been erupted from the Cerro Galan caldera complex, northwest Argentina. Between 7 and 4 Ma ago several composite volcanoes predominantly of dacitic lava were constructed, and several large high-K dacitic ignimbrites were erupted. 2.2 Ma ago the > 1000km3 Cerro Galan ignimbrite was erupted. The predominant mineral assemblage in the ignimbrites is plagioclase-biotite-quartz-magnetite-ilmenite; the Cerro Galan ignimbrite also contains sanidine. Fe-Ti oxide minerals in the Cerro Galan ignimbrite imply temperatures of 801–816 °C. Plagioclase phenocrysts in the ignimbrites typically have rather homogeneous cores surrounded by complex, often oscillatory zoned, rims. Core compositions show a marked bimodality, with one population consisting of calcic cores surrounded by normally zoned rims, and a second of sodic cores surrounded by reversely zoned rims. The older ignimbrites do not show systematic compositional zonation, but the Cerro Galan ignimbrite exhibits small variations in major elements (66–69% SiO2) and significant variations in Rb, Sr, Ba, Th and other trace elements, consistent with derivation from a weakly zoned magma chamber, in which limited fractional crystallization occurred. The ignimbrites have 87Sr/86Sr = 0.7108–0.7181; 143Nd/144Nd = 0.51215–0.51225, and δ18O = + 10 to + 12.5, consistent with a significant component of relatively non-radiogenic crust with high Rb/Sr and enriched in incompatible elements. Nd model ages for the source region are about 1.24 Ga. 87Sr/86Sr measurements of separated plagioclases indicate that Anrich cores have slightly lower 87Sr/86Sr than less calcic plagioclases, suggesting a small degree of isotopic heterogeniety in different components within the magmas. Pb isotope data for plagioclase show restricted ranges (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb = 18.87–18.92, 15.65–15.69 and 39.06–39.16 respectively), and suggest derivation from Proterozoic crustal material(> 1.5 Ga).

Contemporaneous satellite scoria cones and lavas are high-K basalts, basaltic andesites and andesites with SiO2 = 51–57%; K2O = 2–3% and normative plagioclase compositions of An37–48, and may be derived from a mantle source containing both ‘subduction zone’ and ‘within plate’ components. 87Sr/86Sr ranges from 0.7055 to 0.7094 and 143Nd/144Nd from 0.51250 to 0.51290. Variation diagrams such as MgO: SiO2 show two trends, one indicating closed system fractional crystallization and the other crustal contamination. AFC modelling of the open system rocks indicates a parental mantle-derived mafic magma which is itself enriched in K, Rb, Ba, U, Ta/Sm, Ta/Th and Sr, and has 87Sr/86Sr = 0.705–0.706, while the contaminant need not be more radiogenic than the dacitic ignimbrites.

The Cerro Galan dacitic magmas are interpreted in terms of a deep and uniform region of the central Andean continental crust repeatedly melted by emplacement of incompatible-element-enriched, mantle-derived mafic magmas, a proportion of which may also have mixed with the dacite magmas. A component of the crustal material had a Proterozoic age. The magmas derived by crustal melting were also enriched in incompatible elements either by crystal/liquid fractionation processes, or by metasomatism of their source regions just prior to magma generation. Much of the crystallization took place in the source region during the melting process or in mid-crustal magma chambers. The magmas may have re-equilibrated at shallow levels prior to eruption, but only limited compositional zonation developed in high-level magma chambers.

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C. R. Bacon & J. Metz 1984. Magmatic inclusions in rhyolites, contaminated basalts and compositional zonation beneath the Coso Volcanic Field, California. Contributions to Mineralogy and Petrology 85, 346–65.

M. L. Brown & I. Parsons 1981. Towards a more practical two-feldspar geothermometer. Contributions to Mineralogy and Petrology 76, 369–77.

D. J. DePaolo 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters 53, 189202.

B. Deruelle 1982. Petrology of the Plio-Quaternary volcanism of the south-central and meridional Andes. Journal of Volcanology and Geothermal Research 14, 7124.

B. Deruelle , R. S. Harmon & S. Moorbath 1983. Combined Sr-O isotope relationships and petrogenesis of Andean volcanic rocks of South America. Nature 302, 814–16.

N. W. Dunbar , P. R. Kyle & C. J. N. Wilson 1989. Evidence for limited zonation in silicic magma systems, Taupo volcanic zone, New Zealand. Geology 17, 234–6.

P. W. Francis , L. J. O'Callaghan , G. A. Kretschmar , R. S. Thorpe , R. S. J. Sparks , R. N. Page , R. E. de Barrio , G. Gillou & O. E. Gonzalez 1983. The Cerro Galan ignimbrite. Nature 301, 51–3.

P. W. Francis , R. S. Thorpe , S. Moorbath , G. A. Kretschmar & M. Hammill 1980. Strontium isotope evidence for crustal contamination of calc-alkaline volcanic rocks from Cerro Galan, north-west Argentina. Earth and Planetary Science Letters 48, 257–67.

A. L. Grunder & D. R. Boden 1987. Comment on '…Magmatic Conditions of the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado' by Whitney and Stormer, 1985. Journal of Petrology 28, 737–46.

R. S. Harmon , R. S. Thorpe & P. W. Francis 1981. Petrogenesis of Andean andesites from combined Sr-O isotope relationships. Nature 290, 396–99.

C. J. Hawkesworth , M. Hammill , A. R. Gledhill , P. van Calsteren & G. Rogers 1982. Isotope and trace element evidence for late stage intra-crustal melting in the high Andes. Earth and Planetary Science Letters 58, 240–54.

C. J. Hawkesworth & R. Ellam 1989. Chemical fluxes and wedge replenishment rates along recent destructive plate margins. Geology 17, 46–9.

W. Hildreth 1979. The Bishop Tuff: Evidence for the origin of compositional zonation in silicic magma chambers. In Ash Flow Tuffs (ed. C. E. Chapin , W. E. Elston ); pp. 4376. Geological Society of America Special Paper no. 180.

W. Hildreth 1981. Gradients in silicic magma chambers: implications for lithospheric magmatism. Journal of Geophysical Research 86, 10153–92.

W. Hildreth & S. Moorbath 1988. Crustal contribution to arc magmatism in the Andes of Central Chile. Contributions to Mineralogy and Petrology 98, 455–89.

P. K. Hormann , H. Pichler & W. Zeil 1978. New data on the young volcanism in the Puna of NW Argentina. Geologische Rundschau 62, 397418.

H. E. Huppert & R. S. J. Sparks 1988 a.The generation of granitic melts by intrusion of basalt into continental crust. Journal of Petrology 29, 599624.

T. E. Jordan , B. L. Isacks , R. W. Allmendinger , J. A. Brewer , V. A. Ramos & C. J. Ando 1983. Andean tectonics related to geometry of the subducted Nazca plate. Geological society of America Bulletin 94, 341–61.

J. Klerkx , S. Deutsch , H. Pichler & W. Zeil 1977. Strontium isotopic composition and trace element data bearing on the origin of Cenozoic volcanic rocks of the central and southern Andes. Journal of Volcanology and Geothermal Research 2, 4971.

P. W. Lipman 1971. Iron-titanium oxide phenocrysts in compositionally zoned ash flow sheets from southern Nevada. Journal of Geology 79, 438–56.

B. D. Marsh 1981. On the crystallinity, probability of occurrence and rheology of lava and magma. Contributions to Mineralogy and Petrology 78, 8598.

D. S. Musselwhite , D. J. DePaolo & M. McCurry 1989. The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountain Volcanic Center, eastern California. Contributions to Mineralogy and Petrology 101, 1929.

J. S. Myers 1975. Cauldron subsidence and fluidization: mechanisms of intrusion of the coastal batholith of Peru into its own volcanic ejecta. Geological Society of America Bulletin 86, 1209–20.

D. C. Noble , T. A. Vogel , P. S. Peterson , G. P. Landis , N. K. Grant , P. A. Jezek & E. H. McKee 1984. Rare element enriched, S-type ash flow tuffs containing phenocrysts of muscovite, andalusite and sillimanite, southeastern Peru. Geology 12, 35–9.

J. A. Pearce , N. B. W. Harris & A. Tindle 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25, 956–83.

A. Peccerillo & S. R. Taylor 1975. Geochemistry of Upper Cretaceous volcanic rocks from the Pontic chain, northern Turkey. Bulletin of Volcanology 39, 557–69.

M. Pichavant , D. J. Kontak , J. V. Herrera & A. H. Clark 1988 a.The Miocene-Pliocene Macusani volcanics, S.E. Peru. I. Mineralogy and magmatic evolution of a two-mica aluminosilicate-bearing ignimbrite suite. Contributions to Mineralogy and Petrology 100, 300–24.

M. Pichavant , D. J. Kontak , J. V. Herrera & A. H. Clark 1988 b.The Miocene-Pliocene Macusani volcanics, S.E. Peru. II. Geochemistry and origin of a felsic peraluminous magma. Contributions to Mineralogy and Petrology 100, 325–38.

C. W. Rapela , L. M. Hearman & R. H. McNutt 1982. Rb–Sr geochronology of granitoid rocks from the Pampean ranges, Argentina. Journal of Geology 90, 574–82.

G. Rogers & C. J. Hawkesworth 1989. A geochemical traverse across the north Chilean Andes: evidence for crustal generation from the mantle wedge. Earth and Planetary Science Letters 91, 271–85.

R. L. Rudnick , W. F. McDonough , M. T. McCulloch & S. R. Taylor 1986. Lower crustal xenoliths from Queensland, Australia: Evidence for deep crustal assimilation and fractionation of continental basalts. Geochimica et Cosmochimica Acta 50, 1099–115.

R. L. Smith 1979. Ash flow magmatism. In Ash Flow Tuffs (ed. C. E. Chapin , W. E. Elston ) pp. 528. Geological Society of America Special Paper no. 180.

R. S. J. Sparks , P. W. Francis , R. D. Hamer , R. J. Pankhurst , L. O. O'Callaghan , R. S. Thorpe & R. N. Page 1985. Ignimbrites of the Cerro Galan caldera, NW Argentina. Journal of Volcanology and Geothermal Research 24, 205–48.

J. S. Stacey & S. D. Kramers 1975. Approximation of terrestrial lead isotope evolution by a two stage model. Earth and Planetary Science Letters 26, 207–21.

J. C. Stormer , J. A. Whitney & M. Dorais 1987. Reply to a comment on ‘Magmatic conditions of the Fish Canyon Tuff…’ Journal of Petrology 28, 747–54.

R. S. Thorpe , P. W. Francis & S. Harmon 1981. Andean andesites and crustal growth. Philosophical Transactions of the Royal Society, Series A, 301, 305–20.

R. S. Thorpe , P. W. Francis & S. Moorbath 1979. Rare earth element and strontium isotope evidence concerning the petrogenesis of north Chilean ignimbrites. Earth and Planetary Science Letters 42, 359–67.

R. S. Thorpe , P. W. Francis & L. J. O'Callaghan 1984. Relative roles of source contamination, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks. Philosophical Transactions of the Royal Society, Series A, 310, 673–92.

J. A. Whitney & J. C. Stormer 1985. Mineralogy, petrology and magmatic conditions from the Fish Canyon Tuff, Central San Juan volcanic field, Colorado. Journal of Petrology 26, 726–62.

J. A. Whitney & J. C. Stormer 1986. Model for the intrusion of batholiths associated with the eruption of large volume ash-flow tuffs. Science 231, 483–5.

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