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Fe-enrichment in tholeiitic pyroxenes: complex two-pyroxene assemblages in Mesozoic dolerites, southern Tasmania

Published online by Cambridge University Press:  01 May 2009

R. P. Hall ,
D. J. Hughes  and
L. Joyner
R. P. Hall
Affiliation:
Department of Geology, Portsmouth Polytechnic, Portsmouth PO1 3QL, U.K.
D. J. Hughes
Affiliation:
Department of Geology, Portsmouth Polytechnic, Portsmouth PO1 3QL, U.K.
L. Joyner
Affiliation:
Department of Geology, Portsmouth Polytechnic, Portsmouth PO1 3QL, U.K.
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Abstract

The pyroxene assemblages of four samples of ferrodolerite from the Mesozoic Mount Wellington sill of southern Tasmania are described. The pyroxenes of each sample define an Fe-enrichment trend equivalent to almost half of a Skaergaard-type pyroxene evolution trend. The calcic pyroxenes range from augite to ferroaugite (Ca30–40Mg40–17Fe15–45) and late-stage hedenbergite (Ca45 Mg5Fe50), and the pigeonites from intermediate to ferriferous types (Ca13Mg55–15Fe32–70). XMg numbers (Mg/(Mg + Fe)) of the calcic pyroxenes vary from 0.8 to 0.1, and of the calcium-poor pyroxenes (pigeonites) from 0.62 to 0.17. The chemical variation in the pyroxenes within individual samples is far greater than between the different samples. One sample contains an extremely wide range of pyroxenes which include some of the most ferriferous pigeonites ever recorded (Ca14Mg15 Fe71). Ti and Al contents are also highly varied, Ti:Al ionic ratios ranging from 1:15 to 1:2 with decreasing XMg. The augites have higher Ti and Al contents than pigeonites with corresponding XMg. The hedenbergites have erratic but generally low Ti contents, due to their late precipitation together with ulvospinel. The presence of such highly variable pyroxenes within individual samples and in some cases as single, complex grains makes the recognition of coexisting pyroxene pairs in such dolerites very difficult, but graphical pyroxene thermometers suggest a crystallization temperature range of 1100 to 850 °C.

The range of pyroxene compositions in such dolerites is strongly influenced by the oxygen fugacity (fO2) of the liquid from which they derive. The primary Fe-Ti oxide phase in these Tasmanian dolerites is ulvospinel (now oxidized to ilmenite ‘oxidation-exsolution’ lamellae in titanomagnetite hosts) clearly demonstrating the originally low fO2 of the parental tholeiitic magma of the Mount Wellington sill.

Type
Articles
Information
Geological Magazine , Volume 125 , Issue 6 , November 1988 , pp. 573 - 582
Copyright
Copyright © Cambridge University Press 1988

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References

Abbey, S. 1980. Studies in ‘standard samples’ for use in the general analysis of silicate rocks and minerals. Geostandards Newsletter 4 (2), 163–90.CrossRefGoogle Scholar
Beaty, D. W. & Albee, A. L. 1978. Comparative petrology and possible genetic relations among the Apollo 11 basalts. Proceedings of the 9th Lunar and Planetary Science Conference, 359–463.Google Scholar
Compston, W., McDougall, I. & Heier, K. S. 1968. Geochemical comparison of the Mesozoic basaltic rocks of Antarctica, South Africa, South America and Tasmania. Geochimica et Cosmochimica Acta 32, 129–49.CrossRefGoogle Scholar
Deer, W. A., Howie, R. A. & Zussman, J. 1978. Rock-forming Minerals. Vol. 2A. Single-Chain Silicates, London: Longman, 668 pp.Google Scholar
Edwards, A. B. 1942 a. Differentiation of the dolerites of Tasmania. I. Journal of Geology 50, 451–80.CrossRefGoogle Scholar
Edwards, A. B. 1942 b. Differentiation of the dolerites of Tasmania. II. Journal of Geology 50, 579–610.CrossRefGoogle Scholar
Gibb, F. G. F., Stumpfl, E. F. & Zussman, J. 1970. Opaque minerals in an Apollo 12 rock. Earth and Planetary Science Letters 9, 217–24.CrossRefGoogle Scholar
Haggerty, S. L. 1976 a. Opaque mineral oxides in terrestrial igneous rocks. In Mineralogical Society of America Short Course Notes. Vol. 3. Oxide Minerals (ed. Rumble, D.), pp. Hg101–39. Blacksburg: Southern Printing Co.Google Scholar
Haggerty, S. L. 1976 b Oxidation of opaque mineral oxides in basalts. In Mineralogical Society of America Short Course Notes. Vol. 3. Oxide Minerals (ed. Rumble, D.), pp. Hg1–100. Blacksburg: Southern Printing Co.Google Scholar
Haggerty, S. L. & Meyer, H. O. A. 1970. Apollo 12: opaque oxides. Earth and Planetary Science Letters 9, 379–87.CrossRefGoogle Scholar
Hall, R. P., Hughes, D. J. & Friend, C. R. L. 1985. Geochemical evolution and unusual pyroxene chemistry of the ‘MD’ tholeiite dykes swarm from the Archaean craton of southern West Greenland. Journal of Petrology 26 (2), 253–82.CrossRefGoogle Scholar
Hall, R. P., Hughes, D. J. & Friend, C. R. L. 1986. Complex sequential pyroxene growth in tholeiitic hypabyssal rocks from southern West Greenland. Mineralogical Magazine 50, 491–502.CrossRefGoogle Scholar
Hall, R. P., Hughes, D. J., Friend, C. R. L. & Snyder, G. L. 1987. Proterozoic mantle heterogeneity: geochemical evidence from contrasting basic dykes. In Geochemistry and Mineralization of Proterozoic Volcanic Suites (eds. Pharaoh, T. C., Beckinsale, R. T. and Rickard, D. T.), pp. 7–19. Geological Society of London Special Publication 33.Google Scholar
Heier, K. S., Compston, W. & McDougall, I. 1965. Thorium and uranium concentrations and the isotopic composition of strontium in the Tasmanian dolerites. Geochimica et Cosmochimica Acta 29, 643–59.CrossRefGoogle Scholar
Hill, R. & Roeder, P. 1974. The crystallization of spinel from basaltic liquid as a function of oxygen fugacity. Journal of Geology 82, 709–29.CrossRefGoogle Scholar
Huebner, J. S. & Turnock, A. C. 1980. The melting relations at 1 bar of pyroxenes composed largely of Ca-, Mg-, and Fe-bearing components. American Mineralogist 65, 225–71.Google Scholar
LeBas, M. J. 1962. The role of aluminium in igneous clinopyroxenes with relation to their parentage. American Journal of Science 260, 267–88.CrossRefGoogle Scholar
Lindh, A. 1973. Relations between iron–titanium oxide minerals and silicate minerals in a dolerite intrusion. Neues Jahrbuch für Mineralogie Abhandlungen 120 (1), 31–50.Google Scholar
Lindsley, D. H. 1976. Experimental studies of oxide minerals. In Mineralogical Society of America Short Course Notes. Vol. 3. Oxide Minerals (ed. Rumble, D.), pp. L61–88. Blacksburg: Southern Printing Co.Google Scholar
Lindsley, D. H. 1983. Pyroxene thermometry. American Mineralogist 68, 477–93.Google Scholar
McDougall, I. 1961. Optical and chemical studies of pyroxenes in a differentiated Tasmanian dolerite. American Mineralogist 46, 661–87.Google Scholar
McDougall, I. 1962. Differentiation of the Tasmanian dolerites: Red Hill dolerite-granophyre association. Geological Society of America Bulletin 73, 279–316.CrossRefGoogle Scholar
McDougall, I. 1964. Differentiation of the Great Lake dolerite sheet, Tasmania. Journal of the Geological Society of Australia, 11 (1), 107–32.CrossRefGoogle Scholar
McDougall, I. & Lovering, J. F. 1963. Fractionation of chromium, nickel, cobalt and copper in a differentiated dolerite-granophyre sequence at Red Hill, Tasmania. Journal of the Geological Society of Australia 10 (2), 325–38.CrossRefGoogle Scholar
Mathison, C. I. 1987. Pyroxene oikocrysts in troctolitic cumulates – evidence for supercooled crystallization and postcumulus modification. Contributions to Mineralogy and Petrology 97, 228–36.CrossRefGoogle Scholar
Nisbet, E. G. & Pearce, J. A. 1977. Clinopyroxene composition in mafic lavas from different tectonic settings. Contributions to Mineralogy and Petrology 63, 149–60.CrossRefGoogle Scholar
Nwe, Y. Y. 1976. Electron probe studies of the earlier pyroxenes and olivines from the Skaergaard intrusion, East Greenland. Contributions to Mineralogy and Petrology 55, 105–26.CrossRefGoogle Scholar
Osborn, E. F. 1959. Role of oxygen pressure in the crystallization and differentiation of basaltic magma. American Journal of Science 257, 609–47.CrossRefGoogle Scholar
Pearce, J. A. & Cann, J. R. 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letters 19, 290–300.CrossRefGoogle Scholar
Saxena, S. K., Sykes, J. & Eriksson, G. 1986. Phase equilibria in the pyroxene quadrilateral. Journal of Petrology 27 (4), 843–51.CrossRefGoogle Scholar
Statham, P. J. 1976. A comparative study of techniques for quantitative analysis of X-ray spectra obtained with a Si(Li) detector. X-ray Spectrometry 5, 16–28.CrossRefGoogle Scholar
Sweatman, T. R. & Long, J. V. P. 1969. Quantitative electron microprobe microanalysis of rock-forming minerals. Journal of Petrology 10, 332–79.CrossRefGoogle Scholar
Tasmania Department of Mines (TDM) 1976. 1:500 000 Geological Map of Tasmania.Google Scholar
Wager, L. R. & Brown, G. M. 1968. Layered Igneous Rocks, London: Oliver & Boyd. 588 pp.Google Scholar