Abstract
The Sarmiento and Tortuga complexes are two mafic complexes located in southern Chile that represent the remnants of the mafic portion of the floor of an Early Cretaceous extensional back-arc basin. Basaltic dikes and lavas within each complex exhibit tholeiite differentiation trends whereby FeO*, FeO*/MgO, TiO2, P2O5, Zr, and Y increase together without significant increases in SiO2. In both complexes, as FeO*/MgO increases, REE abundance increases without significant change in Ce/Yb ratio, but with an increasing negative Eu anomaly. The Sarmiento complex contains intermediate icelandites and silicic dikes and lavas which are conspicuously absent in the Tortuga complex. These non-basaltic compositions have higher Zr, Y and REE contents than the associated basalts, but similar Ce/Yb ratios, suggesting co-genetic origin. Thick cumulate gabbro sequences in both complexes suggest shallow level crystal-liquid fractionation as a major cause of the observed wide range of chemical variations.
Significantly, in basalts from the Tortuga complex, incompatible elements (Zr, Y, REE) increase in abundance more rapidly with increasing FeO*/ MgO than in the Sarmiento complex. The rapid increase of incompatible elements relative to FeO*/ MgO in the Tortuga complex is best modeled by fractionation within an open magma chamber steadily replenished with new batches of undifferentiated magma. The observed chemical variations within the Sarmiento complex are best modeled by a magma chamber replenished only a limited number of times by a continuously decreasing volume of undifferentiated magma, followed, subsequent to the last input of new parental magma, by closed system fractionation which results in the formation of ferro-basalts, icelandites and silicic differentiates. Ferro-gabbros (FeO* >20 wt °/00) found within the gabbro unit of the Sarmiento complex closely approximate in composition the calculated crystal extracts required to evolve ferro-basalts into icelandites and the more silicic differentiates.
The difference between the nature of the postulated magma chambers within the spreading centers at which the Sarmiento and Tortuga complexes originated suggests that the zone of magma intrusion from the mantle may have been diffuse in the region where the Sarmiento complex formed and more localized in the region where the Tortuga complex formed. This is consistent with other geochemical and field evidence suggesting that the Sarmiento complex represents a less developed stage of evolution than the Tortuga complex of the mafic floor of the Mesozoic back-arc basin in southern Chile.
The apparent decoupling of major and trace element variations in ocean floor basalts may be explained by shallow level igneous fractionation without involving large proportions of clinopyroxene if the magma chambers within spreading centers at midocean ridges behave as open systems periodically replenished with batches of undifferentiated parental magma as is inferred for the Tortuga complex in southern Chile.
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References
Arth, J.G., Hansen, G.N.: Geochemistry and origin of the early Precambrian crust of northeastern Minnesota, Geochim. Cosmochim. Acta 39, 325–362 (1975)
Bruhn, R.L., Dalziel, I.W.D.: Destruction of the Early Cretaceous marginal basin in the Andes of Tierra del Fuego. In: Island arcs, deep sea trenches, and back-arc basins, Maurice Ewing Series, AGU, Washington, D.C. (M. Talwani and W.C. Pitman III, eds.) (1977)
Bruhn, R.L., Stern, C.R., Wit, M.J. de: The bearing of new field and geochemical data on the origin and development of a Mesozoic volcano-tectonic rift zone and back-arc basin in southern-most South America. Earth Planet. Sci. Lett. 41, 32–46 (1978)
Bryan, W.B., Moore, J.G.: Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near 36 ° 49′ N. Geol. Soc. Am. Bull, 88, 556–570 (1977)
Bryan, W.B., Thompson, G., Frey, F.A., Dickey, J.S.: Inferred geologic settings and differentiation in basalts from the Deep-Sea Drilling Project. J. Geophys. Res. 81, 4285–4304 (1976)
Carmichael, I.S.E.: The petrology of Thingmuli, a Tertiary volcano in eastern Iceland. J. Petrol. 5, 435–460 (1964)
Church, W.R., Riccio, L.: Fractionation trends in the Bay of Islands ophiolite of Newfoundland: polycyclic cumulate sequences in ophiolites and their classification. Can. J. Earth Sci. 14, 1156–1165 (1977)
Clague, D.A., Bunch, T.E.: Formation of ferrobasalts at East Pacific mid ocean spreading centers. J. Geophys. Res. 81, 4247–4256 (1976)
Clark, D.B., Loubat, H.: Mineral analyses from the peridotitegabbro-basalt complex at site 334, DSDP Leg 37. Initial report of the DSDP 37, 847–855 (1977)
Coleman, R.G., Peterman, Z.E.: Oceanic plagiogranites. J. Geophys. Res. 80, 1099–1108 (1975)
Dalziel, I.W.D., Wit, M.J. de, Palmer, K.F.: Fossil marginal basin in the southern Andes. Nature 250, 291–294 (1974)
Donaldson, C.H., Brown, R.W.: Refractory megacrysts and magnesian-rich melt inclusions with spinel in oceanic tholeiites: indicators of magma mixing and parental magma composition. Earth Planet. Sci. Lett. 37, 81–89 (1977)
Elthon, D.L.: The petrology and geochemistry of high magnesium lavas from the Tortuga ophiolite complex, Chile. G.S.A. Abstracts with Programs, 1978 Annual Meetings, 396 (1978)
Elthon, D.L., Stern, C.R.: Metamorphic petrology of the Sarmiento ophiolite complex, Chile. Geology 6, 464–468 (1978)
Gass, I.G.: Is the Troodos Massif of Cyprus a fragment of Mesozoic ocean floor? Nature 220, 39–42 (1968)
Gast, P.W.: Trace-element fractionation and the origin of tholeiitic and alkaline magma types. Geochim. Cosmochim. Acta 32, 1057–1116 (1968)
Hawkins, J.W.: Petrology and geochemistry of basaltic rocks of the Lau Basin, Earth Planet. Sci. Lett. 28, 283–298 (1976)
Katz, H.R.: Contrasts in tectonic evolution of orogenic belts in the southwest Pacific. J. Roy. Soc. New Zealand 3, 333–362 (1973)
Kay, R.: The rare earth geochemistry of alkaline basaltic volcanics. Ph. D. Thesis, Columbia University, New York (1970)
Kay, R., Hubbard, H.J., Gast, P.W.: Chemical characteristics and origin of ocean ridge volcanic rocks. J. Geophys. Res. 75, 1585–1613 (1970)
Kay, R., Senechal, R.G.: The rare earth geochemistry of the Troodos ophiolite complex. J. Geophys. Res. 81, 964–970 (1976)
Masuda, A., Nakamura, N., Taneka, T.: Fine structures of mutually normalized rare-earth patterns of chondrites. Geochim. Cosmochim. Acta 37, 239–248 (1973)
Mazzullo, L. J., Bence, A.E.: Abyssal tholeiites from DSDP Leg 34: the Nazca plate. J. Geophys. Res. 81, 4327–4352 (1976)
Moores, E.M., Vine, F.J.: The Troodos Massif, Cyprus, and other ophiolites as oceanic crust: evaluation and implications. Roy. Soc. Phil. Trans. London A268, 443–466 (1971)
Norrish, K., Chappell, B.W.: X-ray fluorescence spectrography, In: Physical Methods in Determinative Mineralogy. (J. Zussman, ed.) New York: Academic 1967
O'Hara, M.: Geochemical evolution during fractional crystallization of a periodically refilled magma chamber. Nature 266, 503–507, (1977)
Pankhurst, R.J.: Open system crystal fractionation and incompatible element variations in basalts. Nature 268, 36–38 (1977)
Roeder, P.L., Emslie, R.F.: Olivine-liquid equilibrium. Contrib. Mineral. Petrol. 29, 275–289 (1970)
Saunders, A.D., Tarney, J., Stern, C.R., Dalziel, I.W.D.: Geochemistry of Mesozoic marginal basin floor igneous rocks from southern Chile. Geol. Soc. Am. Bull. (in press)
Shaw, D.M.: Trace element fractionation during anatexis. Geochim. Cosmochim. Acta 34, 237–243 (1970)
Sigurdsson, H.: Generation of Icelandic rhyolites by melting of plagiogranites in the oceanic layer. Nature 269, 25–28 (1977)
Stern, C.R.: Geochemistry of Chilean ophiolites: Evidence for the compositional evolution of the mantle source of back-arc basin basalts. J. Geophys. Res (under review)
Stern, C.R., Elthon, D.L.: Vertical variations in the effects of hydrothermal metamorphism in Chilean ophiolites: Their implications for ocean floor metamorphism. Tectonophysics (in press)
Stern, C.R., Wit, M.J. de, Lawrence, J.R.: Igneous and metamorphic processes associated with the formation of Chilean ophiolites and their implication for ocean floor metamorphism, seismic layering, and magnetism. J. Geophys. Res. 81, 4370–4380 (1976)
Wagner, L.R.: The major element variation of the layered series of the Skaegaard intrusion and a re-estimation of the average composition of the hidden layered series of the successive residual magmas. J. Petrol. 1, 364–398 (1960)
Wagner, L.R., Brown, G.M.: Layered Igneous Rocks. Freeman, San Francisco (1967)
Watts, A.B., Weissel, J.K., Larson, R.L.: Sea-floor spreading in marginal basins of the western Pacific. Tectonophysics 37, 167–181 (1977)
Wit, M.J. de: The evolution of the Scotia Arc as a key to the reconstruction of southwestern Gonowanaland. Tectonophysics. 37, 53–81 (1977)
Wit, M.J. de, Stern, C.R.: Pillow talk. J. Volcanol. Geotherm. Res. 4, 55–80 (1978)
Wright, T.L., Fiske, R.S.: Origin of differentiated and hybrid lavas of Kilauea Volcano, Hawaii. J. Petrol. 12, 1–65 (1971)
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Stern, C. Open and closed system igneous fractionation within two chilean ophiolites and the tectonic implication. Contr. Mineral. and Petrol. 68, 243–258 (1979). https://doi.org/10.1007/BF00371545
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DOI: https://doi.org/10.1007/BF00371545