Insights from igneous reaction space: a holistic approach to granite crystallisation

Abstract

ABSTRACT: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.

An Early Cretaceous garnet-bearing metaluminous A-type granite intrusion in the East Qinling Orogen, central China: Petrological, mineralogical and geochemical constraints

“…Miller and Stoddard (1981) and Abbott (1981) discussed and reviewed how garnet could crystallize at the expense of biotite in MnO-and Al 2 O 3 -rich evolved magma. However, Hogan (1996) argued that late crystallization of garnet reflects increasing Al in the melt but does not necessarily require high Mn activities for the melt. Positive relationship of SiO 2 and MnO, rather higher w(MnO)/ w(FeO t þ MgO) ratios (0.1e0.5) of the Erlangmiao granite, and MnO-rich garnet support that the Erlangmiao granite crystallized from a highly evolved MnO-rich magma.…”

Magmatic garnet in the Triassic (215 Ma) Dehnow pluton of NE Iran and its petrogenetic significance

“…Most occurrences of magmatic garnet are in felsic pegmatites (SiO 2 ≥70 wt.%) associated with peraluminous to metaluminous granitoids (e.g. Allan and Clarke 1981;Miller and Stoddard 1981;du Bray 1988;Hogan 1996;Dahlquist et al 2007). Garnet may occur in granite for several reasons, for example as a restite mineral or xenocryst, as a partial melting product, and as primary magmatic or secondary metasomatic phases (Clarke and Rottura 1994).…”