Multistage zircon and titanite growth and inheritance in an Archean gneiss complex, Winnipeg River subprovince, Ontario

The paper presents results from a U-Pb isotopic investigation of zircon and titanite from three phases of the Archean Sioux Lookout gneiss complex, at the margin of the gneissic plutonic Winnipeg River Subprovince (WRS) of the Superior Province. The data reveal an intricate data pattern caused by inheritance and multistage crystallization and record the timing of two major orogenic events followed by a protracted post-metamorphic evolution. A zircon age of 2889 +/- 3 Ma for a foliated pink granite correlates with that of similar units elsewhere in the Superior Province and reflects one of the first major episodes of crustal assembly in the pre-Kenoran evolution of the region. Tonalite and diorite from the same site yield zircon and titanite ages of 2704 +/- 2 and 2705 +/- 5 Ma, which correspond to the timing of major magmatism related to Kenoran collision of the WRS with the adjacent Wabigoon volcanic domains. Tonalite and diorite both contain abundant xenocrystic zircon. Moreover, the late kinematic diorite also includes xenocrystic titanite, which has a minimum age of 2730 Ma and coexists with younger, ca. 2705 Ma titanite and zircon. Two generations of titanite are also present in both the tonalite and the granite: the earliest one reflects the 2704 Ma plutonic-metamorphic event, whereas the second one indicates post-metamorphic crystallization at less than or equal to 2650 and 2627 Ma caused by superimposed hydrothermal activity and retrogression. The data support the concept that the titanite can remain a closed system to Pb diffusion at elevated temperatures as long as the crystals escape magmatic or metamorphic recrystallization. Conversely, they also show that titanite can easily be produced by low-temperature processes; hence, generalized interpretations of titanite ages in terms of regional cooling can be inappropriate.