TEMPORAL RELATIONS AMONG THICKENING, HEATING, MELTING, AND EXHUMATION IN A LARGE, HOT OROGEN: INSIGHTS FROM HP MAFIC ROCKS IN THE CANADIAN GRENVILLE PROVINCE

Geodynamic models of large, hot orogens predict a protracted period of high-T conditions (e.g. T > 700 °C for >70 Myrs) across the orogenic infrastructure during and after the collisional phase. In the western Canadian Grenville Province, a type-large, hot orogen, this is supported by a ~50 Myr range in zircon ages from mid-crustal migmatites and hornblende ⁴⁰Ar-³⁹Ar cooling ages that are younger by another ~65 Myrs. A long-duration high-T thermal history should result in complete diffusional re-equilibration of major-element zoning in most garnet crystals and young ages in U-Pb thermochronometers with amphibolite-facies closure temperatures. However, exploratory thermal-chronological studies of HP mafic blocks within the orogenic core show characteristics consistent with relatively short high-T residence times and rapid cooling. Prograde growth zoning is well-preserved in ~4 mm garnet crystals and variably preserved in many ~1 mm crystals. Given that these samples yield peak T estimates of ~ 800 °C, the preservation of growth zoning suggests high-T durations of <10 Myrs for T_char = 750 °C and <30 Myrs for T_char = 700 °C. Steep Mn enrichment profiles along the garnet margins, associated with resorption and back-diffusion during decompression, also yield short timescales of cooling (~2 Myrs for T_char = 743 °C and ~7 Myrs for T_char = 663 °C). Rutile depth-profiles commonly show U-Pb age progressions from ~1080 Ma in the cores to ~980 Ma along the rims, indicating core-to-rim migration of the intracrystalline diffusional closure front (associated with cooling from ~650 °C to ~500 °C) over this timeframe. The older ages preserved in the rutile cores are commonly within 10 Ma of U-Pb ages from eclogite-facies zircon in the same samples (1085-1097 Ma), and suggest relatively rapid cooling (>10 °C/Myr) to <650 °C following HP metamorphism and heating. Leucosome within the interior of one HP complex contains large titanite with ~1100 Ma U-Pb ages, and HP assemblages (>16 kbar) in leucosome-rich high-strain margins of smaller blocks, suggest that melting occurred during or closely after subduction/maximum burial. Taken together, these data suggest that rapid cooling during syn-convergent exhumation to shallow crustal levels may have been aided by weakening (and buoyancy?) associated with melting deep within the orogen.