Accessory mineral chemistry of high Ba-Sr granites from northern Scotland: constraints on petrogenesis and records of whole-rock signature

The Rogart and Strontian high Ba–Sr plutons (Northern Highlands, Scotland) comprise a range of lithologies from felsic to ultramafic rocks. The latter are mantle-derived and their differentiation to produce the felsic components of the plutons is the result of fractional crystallization and variable assimilation of the surrounding Moine metasediments. New results presented here demonstrate that accessory mineral chemistry can provide further insight into their petrogenesis and highlight the petrological potential of apatite and titanite. The main accessory minerals titanite, apatite and zircon contain most of the rare earth elements (REE) in the high Ba–Sr plutons. Results for apatite and titanite show that careful imaging and in situ trace element analysis provide constraints on the petrogenetic history of the host-rock. In both plutons, apatite and titanite record in situ crystallization and fractionation. In Strontian, both apatite and titanite from the granitoids record a mixing event with mafic magma in their rim compositions. Apatite and titanite chemistries are sensitive to the nature of their host-rocks (felsic versus ultramafic) and some elements (e.g. Sr, V) closely reflect whole-rock chemistry and the degree of fractionation. In some cases, whole-rock trace element concentrations can be calculated based on accessory mineral chemistry. Thus, trace elements in accessory minerals can give direct access to the nature and crystallization history of plutonic rocks. This petrological tool may be helpful in provenance studies using accessory minerals, and because high Ba–Sr plutons have recently been equated with Archaean sanukitoids, this might also be important in constraining the temporal distribution of this important magma type.