Inherited zircon and titanite U-Pb systems in an archaean syenite from southwestern Australia: Implications for U-Pb stability of titanite

Inherited zircon and titanite have been identified in a syenite from the Archaean of southwestern Australia. Conventional and SHRIMP analyses on euhedral zoned zircon and zoned rims on complex grains define a crystallisation age of 2654 +/- 5 Ma for the syenite. In addition, SHRIMP analyses on zircon cores and unzoned subhedral zircons show that zircon has a ca. 3250 Ma inherited component. Conventional U-Pb ages on titanite also fall between ca. 3250 Ma and ca. 2650 Ma, demonstrating that inherited titanite as well as zircon is present in the syenite. The syenite has been affected by regional upper amphibolite facies metamorphism at an estimated temperature of 625-650 degrees C. Retention of the inherited radiogenic Pb in the titanite is evidence that the closure temperature for titanite is greater than 650 degrees C. The presence of inherited titanite and zircon also demonstrates a crustal source component for the syenite and indicates it originated by partial assimilation of crustal rocks by a potassic magma at < 750 degrees C, rather than from a homogeneous high T magma.