Exhumation and cooling histories for high-pressure blocks above the western-most ABT: Potential insights from garnet zoning and rutile U-Pb depth profiling

English Abstract:

Geodynamic models applied to the western Grenville Province predict a protracted period of high-temperature conditions (e.g. T > 700 °C for >70 Myrs) across the orogenic infrastructure during and after the Ottawan collisional phase. This is supported by ~50 Myr range in zircon ages from mid-crustal migmatites and hornblende ⁴⁰Ar-³⁹Ar cooling ages that are younger by another ~65 Myrs across much of the western Grenville. This type of long-duration high-T thermal history should result in complete diffusional re-equilibration of growth zoning in all but the largest garnet crystals and young ages in U-Pb thermo­chronometers with amphibolite-facies closure temperatures (T_c). However, exploratory thermal-chronological studies of HP mafic blocks within the Shawanaga Shear Zone (SSZ), the expression of the ABT along the Georgian Bay transect, show that prograde growth zoning (i.e. decreasing Mn and increasing Mg from core to rim) is well-preserved in ~4 mm garnet crystals and moderately preserved in many ~1 mm crystals. Given that these samples yield peak T estimates from 750-850 °C, the preservation of growth zoning suggests relatively short high-T durations (<5-10 Myrs at T_char = 750 °C and <7-30 Ma at T_char = 700 °C). Similarly, depth-profiled rutile crystals commonly show ²⁰⁷Pb/²⁰⁶Pb 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 (commonly within ~10 Ma of U-Pb zircon ages for the same samples (1085-1097 Ma)) suggests relatively rapid cooling within the interval of >750 °C to <650 °C following HP metamorphism. The rapid cooling (~10 °C/Myr) during the early Ottawan suggests syn-convergent exhumation to relatively shallow crustal levels followed by much slower cooling (~1.5 °C/Myr) during post-convergent residence in the thermally-matured middle-crust.