Major-element discrimination of titanian andradite from magmatic and hydrothermal environments: an example from the Canadian Cordillera

Abstract

Titanian andradite is a diagnostic accessory phase of silica undersaturated alkalic plutonic and volcanic rocks but also occurs in skarn and hydrothermal alteration assemblages. We examine the extent and nature of Ti substitution in andradite as a function of environment, based on electron microprobe analyses of titanian andradite from magmatic and hydrothermal occurrences in the Canadian Cordillera. Although magmatic and hydrothermal titanian andradite forms under diverse conditions, the range of compositions reported is similar. All compositions are completely described by eight linearly independent Thompson-space components: Andradite (additive), TiSi-, TiMg (super 2+) [Fe (super 3+) ] (sub -2) , Fe (super 2+) Mg-, Al[Fe (super 3+) ]-, MgCa-, MnCa- and H 4 Si-. Titanium is accommodated in all igneous garnet via two exchange components: TiSi-, which is numerically equivalent to silicon deficiencies at the tetrahedral site, and TiMg[Fe (super 3+) ]-2 representing coupled substitutions involving octahedral cations. In natural samples of igneous Ti-bearing garnet, TiSi-tends to be twice as abundant as TiMg[Fe (super 3+) ] (sub -2) . In contrast, hydrothermal titanian andradite can have as much TiSi- as igneous varieties, but has zero or negative amounts of TiMg[Fe (super 3+) ] (sub -2) . This is also true of synthetic Ti-bearing garnet produced under high f O2 conditions. Experimental data on titanian andradite are used to demonstrate that composition can be directly related, at least semi-quantitatively, to the intensive parameters aSiO 2 and f O2 attending the formation of Ti-bearing garnet in both igneous and hydrothermal environments. Potentially, titanian andradite can be used to track changes in these two important parameters throughout the evolution of individual magmatic or hydrothermal systems.