Metamorphic titanite–zircon pseudomorphs after igneous zirconolite

The formation of metamorphic zircon after baddeleyite is a well-known reaction that can be used to date the metamorphism of igneous silica-undersaturated rocks. By contrast, metamorphic minerals formed after igneous zirconolite have rarely been reported. In this paper, we document metamorphic titanite inline-formula+ zircon pseudomorphs formed from the metamorphic breakdown of igneous zirconolite in syenodiorite and syenite, in the southeastern Sveconorwegian Province, Sweden. Water-rich fluid influx during tectonometamorphism in epidote–amphibolite-facies metamorphic conditions caused the release of silica during a metamorphic reaction involving igneous feldspar and pyroxene and the simultaneous breakdown of igneous Zr-bearing phases. Typical titanite inline-formula+ zircon intergrowths are elongated or platy titanite crystals speckled with tiny inclusions of zircon. Most intergrowths are smaller than 15 inline-formulaµm; some are subrounded in shape. Locally, bead-like grains of titanite and zircon are intergrown with silicate minerals. The precursor igneous zirconolite was found preserved only in a sample of near-pristine igneous syenodiorite, as remnant grains of mainly inline-formula< 2 inline-formulaµm in size. Two somewhat larger crystals, 8 and 12 inline-formulaµm, allowed semiquantitative confirmation using microprobe analysis. Analogous with zircon pseudomorphs after baddeleyite, titanite inline-formula+ zircon pseudomorphs after zirconolite potentially offer dating of the metamorphic reaction, although the small size of the crystals makes dating with today's techniques challenging. The scarcity of reports of zirconolite and pseudomorphs reflects that they are either rare or possibly overlooked.