USING ACCESSORY MINERAL COMPOSITION AND PHASE EQUILIBRIUM MODELING TO CONSTRAIN THE TIMING AND CONDITIONS OF HIGH-PRESSURE METAMORPHISM

The timing and conditions of high pressure (HP) metamorphism is of primary importance in understanding lithosphere-scale geodynamics; however, acquiring high fidelity P-T-t data is commonly hampered by the poor preservation of equilibrium mineral assemblages and uncertainties in linking P-T and chronometric data from the same sample. Within the western Grenville Province, Ontario, Canada, variably retrogressed eclogite bodies typically retain vestages of their HP metamorphic assemblage (e.g. garnet porphyroblasts, kyanite, and rutile) within a matrix dominated by incomplete decompression reactions (e.g. pseudomorphous diopside + sodic plagioclase intergrowths after omphacite, and concentric coronas of aluminous minerals (corundum, spinel, anorthite, and muscovite) surrounding kyanite. Despite the lack of preservation of original HP mineral compositions, portions of the P-T paths can be constrained by combining petrographic observations of relict HP phases with accessory mineral thermometry and phase equilibrium modeling.

Titanium-in-zircon constrains crystallization temperatures to ~678-736 °C, whereas Zr-in-rutile yield crystallization temperatures of ~705-740 °C (inclusions in garnet) and 742-764 °C (matrix). Zircon yields U-Pb ages between 1085-1097 Ma, and exhibits REE characteristics consistent with crystallization in an eclogite-facies (garnet-rich, plagioclase-poor) mineral assemblage. REE partitioning between zircon and garnet suggests zircon growth coincided with the latter stages of garnet growth. Intersection of zircon and rutile crystallization temperature with the calculated stability field for the HP assemblage (Grt+Cpx+Ky+Rt+Zrn±Hbl) yields P ≥ 15 kbar. Thus, HP metamorphism apparently occurred at ca. 1090 Ma, at minimum depths of ~53 km and T ~700±50 °C, yielding a geothermal gradient of < 15 °C/km. Widespread high T-medium P metamorphism in the surrounding gneisses apparently occurred shortly after (ca. 1090-1080 Ma), suggesting a rapidly evolving thermal field during exhumation of HP rocks to mid-crustal depths. Ongoing diffusion modeling of chemical gradients in garnet crystals, reaction coronas, and symplectites should yield tighter constraints on the duration of HT residence and cooling/exhumation rates.