INTEGRATING ZIRCON AND GARNET ISOTOPIC AND TRACE ELEMENT COMPOSITION TO CONSTRAIN THE TIMING AND DURATION OF HIGH PRESSURE METAMORPHISM IN A LARGE HOT OROGEN

The long duration and intensity of the Grenville orogeny in eastern Canada has produced deep crustal rocks with complex metamorphic histories, and an associated difficulty in the geological interpretation of radiometric ages. Previous geochronological analysis of high pressure (HP) metamorphic rocks yielded precise U-Pb zircon ages, with most falling between ~1085-1100 Ma; however, uncertainty has remained as to whether these ages represent the timing of HP metamorphism (associated with continental subduction and/or attainment of maximum crustal thickness) or the granulite/amphibolite facies overprint accompanying exhumation to a hot middle orogenic crust. Recent detailed study of HP rocks and their bounding gneisses, involving phase equilibrium modeling, in-situ analysis of zircon and garnet by LA-ICP-MS, and garnet Lu-Hf geochronology, has yielded much improved constraints on the timing and conditions of HP metamorphism. Zircon from five HP localities yield anchored discordia intercept and ²⁰⁷Pb/²⁰⁶Pb weighted average ages between 1085-1112 Ma, and typically have trace element compositions consistent with growth in a garnet-rich, plagioclase-poor eclogitic assemblage (i.e. no negative Eu anomaly and flat HREE trends). Titanium in zircon and Zr in rutile thermometry indicates that zircon crystallization temperature (647-750 °C) was close to that of rutile inclusions in garnet (704-740 °C), but lower than matrix rutile (742-803 °C). Additionally, in the sample that best preserves the HP assemblage and prograde garnet zoning, equilibrium REE partitioning between zircon and garnet is most closely approached near the rims of garnet crystals. Thus zircon crystallization appears to have occurred late in the garnet growth history. In pseudosections calculated for this sample, zircon crystallization temperatures intersect the HP assemblage field (Grt+Cpx+Ky+Rt+Qtz) and maximum garnet mode isopleths at pressures greater than ~1.7 GPa, further indicating that the ~1085-1100 Ma zircon ages date HP metamorphism. Preliminary garnet-whole rock Lu-Hf ages for two samples predate their corresponding zircon ages by ~10 Ma, suggesting the interval of garnet growth during prograde metamorphism (i.e. the episode of crustal thickening/tectonic burial) lasted at least that long.