Insights from Igneous Reaction Space: A Holistic Approach to Granite Crystallisation
Petrological investigations of granite commonly reveal multiple periods of growth punctuated by resorption for many of the constituent minerals. Complementary to such textures are mineral compositional heterogeneity manifested by zoning or grain to grain variability. These features ultimately reflect changes in the intensive parameters or activities of components during melt solidification. Such complexities of granite crystallisation can be simultaneously modelled in a reaction space constructed from the set of linearly independent reactions describing the equilibria among all phases and components in the system of interest.
The topology of the linearly independent reactions that define the reaction space for garnetmuscovite-biotite granites yields the following insights: (1) there is no one unique reaction that produces or consumes aluminous minerals (e.g. garnet); (2) minerals can alternate as reactants or products in different reactions accounting for textures indicating multiple periods of crystallisation separated by resorption; (3) mineral compositions are regulated by the reaction(s) producing them and vary as the stoichiometry of the reaction(s) producing them varies; (4) resorption of early crystallising garnet is likely to reflect decreasing pressure, presumably during magma ascent; (5) late crystallisation of garnet, at the expense of biotite, reflects an increase in melt aluminosity and does not necessarily require high Mn activities for the melt and (6) increasing melt H2O, at H2Oundersaturated conditions, favours the formation of biotite-muscovite granite.
Application of the reaction space method to other granite types holds considerable promise for elucidating reactions that regulate mineral assemblages and compositions during crystallisation.
J. P. Hogan, "Insights from Igneous Reaction Space: A Holistic Approach to Granite Crystallisation," Earth and Environmental Science Transactions of the Royal Society of Edinburgh, vol. 87, no. 1-2, pp. 147-157, Royal Society of Edinburgh, Jan 1996.
The definitive version is available at http://dx.doi.org/10.1017/S0263593300006568
3rd Hutton Symposium. The Origin of Ganites and Related Rocks (1995: Aug. 26-Sep. 2, College Park, MD)
Geosciences and Geological and Petroleum Engineering
Keywords and Phrases
Biotite; Compositional Variability; Garnet; Granite; Muscovite; Reaction Space
Article - Conference proceedings
© 1996 Royal Society of Edinburgh, All rights reserved.