Garnet porphyroblast-bearing leucosomes in metapelites: mechanisms, phase diagrams, and an example from Broken Hill, Australia

  • Roger Powell
  • Jon Downes
Part of the The Mineralogical Society Series book series (MIBS, volume 2)


Mafic porphyroblasts are commonplace in leucosomes in metamorphic rocks at uppermost amphibolite facies and in the granulite facies. Large, 5 cm or more in diameter, garnet porphyroblasts are common in leucosomes in low-to medium-pressure granulite facies metapelites such as those in the Broken Hill area, Australia. Although there is significant consensus regarding the origin of those porphyroblasts — that they form as a consequence of incon-gruent melting reactions in which hydrous minerals are consumed — there are various interesting questions which need to be answered. For example: if the porphyroblasts are melting products, why then are they preserved during cooling when the melt crystallizes and the hydrous mineral becomes stable again? Intuitively, one would expect complete, or at least major retrogression of the porphyroblasts. This is not usually observed, and is generally absent in the garnet porphyroblasts at Broken Hill.


Partial Melting Bulk Composition Alkali Feldspar Hydrous Mineral Equilibration Volume 
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.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Grant, J. A. 1985. Phase equilibria in partial melting of pelitic rocks. In Migmatites, J. R. Ashworth (ed.), 86–144. Glasgow: Blackie.CrossRefGoogle Scholar
  2. Hensen, B. J. 1971. Theoretical phase relations involving cordierite and garnet in the system MgO-FeO-Al2O3-SiO2. Contributions to Mineralogy and Petrology 33, 191–214.CrossRefGoogle Scholar
  3. Holland, T. J. B. & R. Powell 1990. An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K2O-Na2O-CaO-MgO-MnO-FeO-Fe2O3-Al2O3-TiO2-SiO2-C-H2-O2. Journal of Metamorphic Geology 8, in press.Google Scholar
  4. Le Breton, N. & A. B. Thompson 1988. Fluid-absent (dehydration) melting of biotite in metapelites in the early stages of crustal anatexis. Contributions to Mineralogy and Petrology 99, 226–37.CrossRefGoogle Scholar
  5. Phillips, G. N. 1980. Water activity changes across an amphibolite-granulite facies transition, Broken Hill, Australia. Contributions to Mineralogy and Petrology 75, 377–86.CrossRefGoogle Scholar
  6. Powell, R. 1978. Equilibrium thermodynamics in petrology. London: Harper & Row.Google Scholar
  7. Powell, R. 1983. Fluids and melting under upper amphibolite facies conditions. Journal of the Geological Society of London 140, 629–33.CrossRefGoogle Scholar
  8. Powell, R. & T. J. B. Holland 1988. An internally consistent dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program. Journal of Metamorphic Geology 6, 173–204.CrossRefGoogle Scholar
  9. Stüwe, K. & R. Powell 1989a. Low-pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area. Journal of Metamorphic Geology 7, 465–83.CrossRefGoogle Scholar
  10. Stüwe, K. & R. Powell 1989b. Metamorphic evolution of the Bunger Hills, East Antarctica: evidence for substantial post-metamorphic peak compression with minimal cooling in a Proterozoic orogenic event. Journal of Metamorphic Geology 7, 449–64.CrossRefGoogle Scholar
  11. Stüwe, K. & R. Powell 1989c. Metamorphic segregations associated with garnet and orthopyroxene porphyroblast growth: two examples from the Larsemann Hills, East Antarctica. Contributions to Mineralogy and Petrology, in press.Google Scholar
  12. Thompson, A. B. 1982. Dehydration melting of pelitic rocks and the generation of H2O-undersaturated granitic liquids. American Journal of Science 282, 1567–95.CrossRefGoogle Scholar
  13. Vielzeuf, D. & J. R. Holloway 1988. Experimental determination of the fluid-absent melting relations in the pelitic system. Contributions to Mineralogy and Petrology 98, 257–76.CrossRefGoogle Scholar
  14. Waters, D. J. 1988. Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa. Journal of Metamorphic Geology 6, 387–404.CrossRefGoogle Scholar

Copyright information

© J.R. Ashworth, M. Brown & contributors 1990

Authors and Affiliations

  • Roger Powell
  • Jon Downes

There are no affiliations available

Personalised recommendations