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Contributions to Mineralogy and Petrology

, Volume 18, Issue 2, pp 163–174 | Cite as

Origin of garnet phenocrysts in calc-alkaline rocks

  • Trevor H. Green
  • A. E. Ringwood
Article

Abstract

A large number of garnet phenocrysts from Palaeozoic rhyodacites and granodiorite porphyrites from Central and Northeastern Victoria have been analyzed using the electron microprobe. These garnets, from an area of several thousand square miles, are very uniform in composition (dominantly almandine, with subordinate pyrope and minor grossular and spessartine). They show minor zoning with a very thin outer rim slightly richer in almandine and spessartine than the remainder of the phenocryst. They are surrounded by a complex intergrowth of cordierite and hypersthene forming a reaction rim. Resorbed quartz phenocrysts are typically associated with the garnet phenocrysts. The uniform composition, the conspicuous size and the subhedral-euhedral form of the garnet phenocrysts indicate that they crystallized directly from the acid calc-alkaline magma at an early stage of its crystallization. High pressure experimental work on a natural garnet-bearing rhyodacite glass demonstrates that almandine-rich garnet and quartz are near-liquidus phases at 18 and 27 kb \(\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)\), but garnet does not appear until well below the liquidus at 9 kb. A comparison of the composition of the experimentally crystallized garnets with the natural garnets suggests that these acid calc-alkaline magmas began to crystallize at pressures between 9 and 18 kb, i.e. at depths corresponding to the lower crust or upper mantle.

Keywords

Crystallization Quartz Experimental Work Mineral Resource Electron Microprobe 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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References

  1. Atherton, M. P., and W. M. Edmunds: An electron microprobe study of some zoned garnets from metamorphic rocks. Earth and Planetary Sci. Letters 1, 185–193 (1966).Google Scholar
  2. Banno, S.: Notes on rock-forming minerals: zonal structure of pyralspite garnet in Sanbagawa Schists in the Bessi area, Sikoku. J. Geol. Soc. Japan 71, 185–188 (1965).Google Scholar
  3. Cox, P. T.: Geology of the Rakaia George district. Trans. Proc. New Zealand Inst. 56, 91–111 (1926).Google Scholar
  4. Edwards, A. B.: On the occurrence of almandine garnets in some Devonian igneous rocks of Victoria. Proc. Roy. Soc. Victoria 49, 40–50 (1936).Google Scholar
  5. Green, D. H., and A. E. Ringwood: An experimental investigation of the gabbro to eclogite transformation and its petrological applications. Geochim. et Cosmochim. Acta 31, 767–833 (1967).Google Scholar
  6. Green, T. H.: Thermal metamorphism in the Trial Harbour district, Tasmania. J. Geol. Soc. Australia 13, 405–418 (1966).Google Scholar
  7. - High pressure experimental investigations on the origin of high-alumina basalt, andesite and anorthosite. Aust. Nat. Univ., Unpubl. Ph. D. Thesis (1967).Google Scholar
  8. —, and A. E. Ringwood: Origin of the calc-alkaline igneous rock suite. Earth and Planetary Sci. Letters 1, 307–316 (1966).Google Scholar
  9. —: Genesis of the calc-alkaline igneous rock suite. Contr. Mineral. and Petrol. 18, 105–162 (1968).Google Scholar
  10. —, A. Major: Friction effects and pressure calibration in a piston-cylinder apparatus at high pressure and temperature. J. Geophys. Research 71, 3589–3594 (1966).Google Scholar
  11. Hills, E. S.: Cauldron subsidences, granitic rocks and crustal fracturing in S. E. Australia. Geol. Rundschau 47, 543 (1959).Google Scholar
  12. Makarov, N. N., and V. A. Suprychev: Xenogenic garnet (pyrope-almandine) from volcanic rocks of the Crimea. Doklady Akad. Nauk S.S.R. (Engl. Transi.) 157, 64–67 (1964).Google Scholar
  13. Miyashiro, A.: Pyralspite garnets in volcanic rocks. J. Geol. Soc. Japan 61, 463–470 (1955).Google Scholar
  14. Oliver, R. L.: The origin of garnets in the Borrowdale Volcanic series and associated rocks, English Lake District. Geol. Mag. 93, 121–139 (1956).Google Scholar
  15. Ringwood, A. E.: Geology of the Deddick-Wulgulmerang area, East Gippsland. Proc. Roy. Soc. Victoria 67, 19–66 (1955).Google Scholar
  16. Smith, J. V.: X-ray-emission microanalysis of rock-forming minerals. I. Experimental techniques. J. Geol. 73, 830–864 (1965).Google Scholar
  17. Turner, F. J., and J. Verhoogen: Igneous and metamorphic petrology. New York: McGraw-Hill Book Co. 1960.Google Scholar

Copyright information

© Springer-Verlag 1968

Authors and Affiliations

  • Trevor H. Green
    • 1
    • 2
  • A. E. Ringwood
    • 1
  1. 1.Department of Geophysics and GeochemistryAustralian National UniversityCanberraAustralia
  2. 2.Hoffman LaboratoryHarvard UniversityCambridgeUSA

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