The petrogenesis of anorogenic felsic magmas and AMCG suites: Insights on element mobility and mutual cryptic contamination from polythermal experiments

The close association of mantle-derived mafic rocks and crust-derived felsic rocks in AMCG suites the world over is now interpreted in terms of delamination of the lithospheric keel of an orogen within a short time after the cessation of a major collision. The stage is set for the ascent of an asthenospheric diapir, which is accompanied by the ascent of a stream of H2O–CO2 representing regional degassing of the mantle in the ensuing extensional setting. The crust gets variably metasomatized prior to melting, and this episode of melting seems to involve almost complete melting rather than the expected films of leucosome. Results of polythermal experiments with a large array of target rocks (pulverized) + H2O show that it is possible to mobilize the major elements K, Na, Al, Si and Fe such that the transported fractions resembles an A-type granite or syenite. The open-system process increases in efficiency with increasing temperature and increasing pressure. A stream of such fluid interacting with gabbro or basic magma could create anorthositic and ultrabasic assemblages that are candidates for contamination of pools of basic magma in the uppermost mantle and lower crust. The same stream continues its buoyant rise and makes over the sterile granulitic lower and middle crust into a geochemically fertile protolith for the generation of A-type felsic magmas by wholesale anatexis.

► Novel hypothesis to account for widespread cryptic contamination of mantle by crust, and vice versa, in AMCG suites.
► Surprising new information about the mobility of petrogenetically important elements in H2O.
► Amplification of the role of open-system processes in explaining A-type granites and syenites.
► Explanation of the interplay between tectonic processes and petrogenetic processes.