ZIRCON-FORMING AND MONAZITE-FORMING METAMORPHIC REACTIONS AT MANITOUWADGE, ONTARIO

At the Manitouwadge volcanogenic massive sulfide camp, Ontario, a garn et-rich cordierite-orthoamphibole gneiss and a calc-silicate rock cont ain an unusually high abundance of zircon, monazite, allanite, fluorap atite, and titanite, and minor amounts of other accessory minerals (xe notime, zirconolite, Zr-bearing rutile, columbite, thorite, bastnasite and synchysite). The majority of the zircon grains in the garnet-rich cordierite-orthoamphibole gneiss are igneous in origin, and have surv ived a seafloor hydrothermal alteration and an upper-amphibolite-facie s regional metamorphism. Metamorphic zircon occurs mainly at the expen se of igneous zircon via a reaction such as (Zr,Hf,Y,REE)(Si,P)O-4 --> (Zr,Hf)SiO4 + (Y,REE)PO4. The occurrence of metamorphic zircon, titan ite and rutile in replacement assemblages after zirconolite is probabl y related to a reaction such as CaZrTi2O7 + 2Si(4+) + 4O(2-) --> ZrSiO 4 + CaTiSiO5 + TiO2. There are two compositionally distinct types of m onazite: Th-rich (3-12 wt% ThO2) and Th-poor (<1.5 wt% ThO2). However, available monazite U-Pb ages (Davis et al. 1994) show that both compo sitional varieties crystallized during the high-grade metamorphism and most likely formed from pre-existing REE-rich minerals. In addition, some grains of Th-poor monazite formed during retrograde metamorphism as replacement after metamorphic allanite, fluorapatite, and titanite. This study illustrates the importance of accessory minerals in contro lling the crystallization of zircon and monazite during prograde and r etrograde metamorphism. Moreover, the robust U-Pb dates obtained from zircon and monazite can be integrated with specific metamorphic P-T es timates from their related accessory minerals, which provides importan t insights into the tectonic evolution of metamorphic terranes.