HOT AND COLD GRANITES? IMPLICATIONS OF ZIRCON SATURATION TEMPERATURES AND PRESERVATION OF INHERITANCE

Abstract

Zircon saturation temperatures (TZr) calculated from bulk-rock compositions provide minimum estimates of temperature if the magma was undersaturated, but maxima if it was saturated. For plutons with abundant inherited zircon, TZr provides a useful estimate of initial magma temperature at the source, an important parameter that is otherwise inaccessible. Among 54 investigated plutons, there is a clear distinction between TZr for inheritance-rich (mean 766 ° C) and inheritance-poor (mean 837 ° C) granitoids. The latter were probably undersaturated in zircon at the source, and hence the calculated TZr is likely to be an underestimate of their initial temperature. These data suggest fundamentally different mechanisms of magma generation, transport, and emplacement. “ Hot” felsic magmas with minimal inheritance probably require advective heat input into the crust, are crystal poor, and readily erupt, whereas “ cold,” inheritance-rich magmas require fluid influx, are richer in crystals, and are unlikely to erupt.