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Partial melting and phase relations in metapelitic granulites

The cornerstone of the thesis is the set of sixty four experiments carried out on three synthetic, mineral-mix, KFMASH (K<SUB>2</SUB>O-FeO-MgO-Al<SUB>2</SUB>O<SUB>3</SUB>-SiO<SUB>2</SUB>-H<SUB>2</SUB>O) compositions, with bulk X<SUB>Mg</SUB>'s of 0.62, 0.74 and 0.86. Magnesian compositions are used so that univariant equilibria, many of which do not act on iron-rich compositions, can be constrained. The experiments were conducted in the pressure-temperature range 5-12.5 kilobars and 840-1000°C. The assemblages stable in this range were subsets of garnet-cordierite-biotite-osumilite-orthopyroxene-sillimanite-K-feldspar-quartz-melt. The results of the experiments represent a close approach to equilibrium and several key reactions were reversed. The results are used to derive a petrogenetic grid of KFMASH univariant reactions and pseudosections for each of the bulk compositions. In the KFMASH system, the position of reactions on the grid does not depend on the rock bulk composition or the activity of water. This provides an excellent, fixed reference for consideration of lower crustal processes and mineral assemblages. In particular, the fundamental granulite-facies KFMASH invariant point, which involves the BDMR's and the reaction separating garnet-cordierite and orthopyroxene-sillimanite-quartz assemblages, has been positioned between 890-910°C, 8.7-9.5kb. The current and previous data on BDMR's are accounted for by a simple model in which the melting interval is inversely proportional to the bulk X<SUB>Mg</SUB> and its absolute position in temperature is controlled by the concentration of titanium and fluorine in biotite. The reaction separating garnet-cordierite and orthopyroxene-sillimanite-quartz assemblages has been constrained to run from 900°C, 8.8kb to 1000°C, 7.8kb. This is at lower pressure than previous estimates as it accounts for vapour-undersaturation in cordierite and the fact that melt must be produced when cordierite breaks down. Osumilite has a large stability field at temperatures above approximately 900°C and is likely to be more common than is currently thought.
Date January 1993
CreatorsCarrington, Damian Peter
PublisherUniversity of Edinburgh
Source SetsEthos UK
Detected LanguageEnglish
TypeElectronic Thesis or Dissertation

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