Ultramafic inclusions and high-pressure xenocrysts in submarine basanitoid, equatorial Mid-Atlantic Ridge
Tóm tắt
A basanitoid dredged from near St. Paul's Rocks represents the first reported occurrence of an abyssal lava containing ultramafic xenoliths and high-pressure xenocrysts. The xenoliths include three separate populations. A suite of clinopyroxene-poor spinel-harzburgite and spinel-lherzolite inclusions have tectonite textures and highly refractory mineral compositions; these nodules probably represent residua from partial melting events unrelated to the host basanitoid. One harzburgite inclusion apparently accumulated from a tholeiitic magma and is also an accidental inclusion. Ultrabasic mylonites comprise a third xenolith group whose members are comparable to lithologies exposed on St. Paul's Rocks. A diverse xenocryst assemblage including labradoritebytownite, andesine, high-Ca pyroxene, low-Ca pyroxene, aluminous Cr-spinel, Cr-free aluminous spinel and Fe-Ti oxides, suggests at least two petro-genetic affinities. Fe-rich low-Ca pyroxene and Ca-rich plagioclase, together with the cumulate harzburgite xenolith, indicate the presence of a differentiated tholeiitic suite in the area. Most xenocrysts, including Ca-rich pyroxene, spinel and andesine have alkalic affinities consistent with crystallization at elevated pressure over a wide range in temperature. Both spinel populations are interpreted as cognate precipitates with Cr-free Al-spinels crystallizing above about 10 kb and aluminous Cr-spinels co-precipitating with plagioclase at lower pressures. Compared to the olivine-dominated, low-pressure phenocryst assemblage, extraction of the generally basaltic, aluminous clino-pyroxene-dominated assemblage at high pressure should have a minor effect on the major element differentiation of the host basanitoid magma.
Tài liệu tham khảo
Aoki, K.: Andesine megacrysts in alkali basalts from Japan. Contrib. Mineral. Petrol. 25, 284–288 (1970)
Arculus, R.J.: Melting behavior of two basanites in the range 10 to 35 kb and the effect of TiO2 on the olivine-diopside reactions at high pressure. Carnegie Inst. Washington Yearb. 74, 512–515 (1975)
Babkine, J., Conquéré, F., Vilminot, J.-C.: Dunnées nouvelles sur les spinelles des basaltes alcalins: les ferripleonastes. C.R. Acad. Sci. [D] (Paris) 266, 1455–1458 (1968)
Beeson, M.H.: Petrology, mineralogy and geochemistry of the East Molokai Volcanic Series, Hawaii. U.S. Geol. Surv. Prof. Papers 961 53 pp. (1976)
Binns, R.A., Duggan, M.B., Wilkinson, J.F.G.: High pressure megacrysts in alkaline lavas from northeastern New South Wales. Am. J. Sci. 269, 132–168 (1970)
Bosley, G.D.: The Oceanic Islands. In: The Alkaline Rocks (H. Sorensen, ed.), pp. 311–330. London: John Wiley and Sons 1974
Boullier, A.M., Nicolas, A.: Classification of textures and fabrics of peridotite xenoliths from South African kimberlites. Phys. Chem. Earth 9, 467–475 (1974)
Bultitude, R.J., Green, D.H.: Experimental study of crystal-liquid relationships at high pressures in olivine nephelinite and basanite compositions. J. Petrol. 12, 121–147 (1971)
Coombs, D.S., Wilkinson, J.F.G.: Lineages and fractionation trends in unsaturated volcanic rocks from East Otago Volcanic Province (New Zealand) and related rocks. J. Petrol. 10, 440–501 (1969)
Dick, H.J.B.: Partial melting in the Josephine peridotite, I. the effect on mineral composition and its consequence for geobarometry and geothermometry. Am. J. Sci. 277, 801–832 (1977)
Frey, F.A.: Rare earth and potassium abundances in St. Paul's Rocks. Earth Planet. Sci. Lett. 7, 351–360 (1970)
Frey, F.A., Green, D.H.: The mineralogy, geochemistry and origin of lherzolitic inclusions in Victorian basanites. Geochim. Cosmochim. Acta 38, 1023–1059 (1974)
Frisch, T., Wright, J.B.: Chemical composition of high-pressure megacrysts from Nigerian Cenozoic lavas. Neues Jahrb. Mineral. Monatsh. 289, 289–304 (1971)
Green, D.H.: Composition of basaltic magmas as indicators of conditions of origin: application to oceanic volcanism. Philos. Trans. R. Soc. London, Ser. A. 268, 707–725 (1971)
Green, D.H.: Conditions of melting of basanite magma from garnet peridotite. Earth Planet. Sci. Lett. 17, 456–465 (1973)
Green, D.H., Hibberson, W.: Experimental duplication of conditions of precipitation of high pressure phenocrysts in a basaltic magma. Phys. Earth Planet. Inter. 3, 247–254 (1970)
Green, D.H., Ringwood, A.E.: The genesis of basaltic magmas. Contrib. Mineral. Petrol. 15, 103–190 (1967)
Green, T.H.: Experimental fractional crystallization of quartz diorite and its application to the problem of anorthosite origin. In: Origin of Anorthosite and Related Rocks (Y.W. Isachsen, ed.), Vol. 18, pp. 23–29. New York: State Mus. Sci. Service Mem. 1969
Irvine, T.N.: Chromium spinel as a petrogenetic indicator. Part 2, petrologic applications. Can. J. Earth Sci. 4, 71–103 (1967)
Irving, A.J.: Megacrysts from the Newer Basalts and other basaltic rocks of southwestern Australia. Bull. Geol. Soc. Am. 85, 1503–1514 (1974)
Jackson, E.D., Wright, T.L.: Xenoliths in the Honolulu Volcanic Series, Hawaii. J. Petrol. 11, 405–430 (1970)
Keil, K., Fodor, R.V., Bunch, T.E.: Contributions to the mineral chemistry of Hawaiian rocks. II. Feldspars in rocks from Haleakala and West Maui volcanoes, Maui, Hawaii. Contrib. Mineral. Petrol. 37, 253–276 (1972)
Kushiro, I.: Decrease in viscosity of some synthetic silicate melts at high pressures. Carnegie Inst. Washington Yearb. 75, 611–714 (1976)
Kushiro, I.: Viscosity and structural changes of albite (NaAlSi3O8) melt at high pressures. Earth Planet. Sci. Lett. 41, 87–90 (1978)
Loney, R.A., Himmelberg, G.R., Coleman, R.G.: Structure and petrology of the alpine-type peridotite at Burro Mountain, California, USA. J. Petrol. 12, 245–309 (1971)
Melson, W.G., Hart, S.R., Thompson, G.: St. Paul's Rocks, equatorial Atlantic: petrogenesis, radiometric ages, and implications on seafloor spreading. Geol. Soc. Am. Mem. 132, 241–272 (1972)
Melson, W.G., Jarosewich, E., Cifelli, R.: Alkali olivine basalt dredged near St. Paul's Rocks, Mid-Atlantic Ridge. Nature 215, 381–382 (1967)
Melson, W.G., Thompson, G.: Glassy abyssal basalts, Atlantic floor near St. Paul's Rocks: petrography and composition of secondary clay minerals. Bull. Geol. Soc. Am. 84, 703–716 (1973)
Menzies, M.A.: Rare-earth geochemistry of fused ophiolitic and alpine lherzolites, 1. Othris, Lanzo and Troodos. Geochim. Cosmochim. Acta 40, 645–656 (1976)
Menzies, M.A., Allen, C.: Plagioclase lherzolite-residual mantle relationships within two eastern Mediterranean ophiolites. Contrib. Mineral. Petrol. 45, 197–213 (1974)
Mysen, B.O., Holloway, J.R.: Experimental determination of rare earth fractionation patterns in partial melts from peridotite in the upper mantle. Earth Planet. Sci. Lett. 34, 231–237 (1977)
Nicholls, I.A., Ringwood, A.E.: Effect of water on olivine stability in tholeiites and the production of silica-saturated magmas in the island arc environment. J. Geol. 81, 285–300 (1973)
Sigurdsson, H., Schilling, J.-G.: Spinels in Mid-Atlantic ridge basalts: chemistry and occurrence. Earth Planet. Sci. Lett. 29, 7–20 (1976)
Sinton, J.M.: Equilibration history of the basal alpine-type peridotite, Red Mountain, New Zealand. J. Petrol. 18, 216–246 (1977)
Sinton, J.M.: Petrology of (alpine-type) peridotites from Site 395, DSDP Leg 45. In: Initial Rept. Deep Sea Drilling Project (W.G. Melson, et al., eds.), Vol. 45, pp. 595–601. Washington: U.S. Government Printing Office 1978
Sinton, J.M.: Petrology and evolution of the Red Mountain ophiolite complex, New Zealand. Am. J. Sci. (in press, 1979)
Thompson, R.N.: Some high-pressure pyroxenes. Mineral. Mag. 39, 768–787 (1974)
Velde, B., Kushiro, I.: Structure of sodium alumino-silicate melts quenched at high pressure; infrared and aluminous K-radiation data. Earth Planet. Sci. Lett. 40, 137–140 (1978)
Waff, H.S.: Pressure-induced coordination changes in magmatic liquids. Geophys. Res. Lett. 2, 193–196 (1975)
Wright, J.B.: The iron-titanium oxides of some Dunedin (New Zealand) lavas, in relation to their paleomagnetic and thermo-magnetic character (with an appendix on associated chromiferous spinel). Mineral. Mag. 36, 425–435 (1967)