DURATION OF GARNET GROWTH AND P-T-T PATHS FROM SM-ND DATING AND ISOCHEMICAL PHASE DIAGRAMS: COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA, CANADA

Textural and compositional discontinuities in garnet may result from changing reactions producing garnet, changing P-T conditions, and/or temporally discrete metamorphic events. These discontinuities are typically used to define core and rim sampling domains. Identifying the cause of a discontinuity, while often difficult, adds crucial context to core-rim garnet ages and P-T-t paths.

Garnet Sm-Nd ages from the southern Coast Mountains batholith, British Columbia, Canada illustrate this dating technique. Two samples with ≥ 1cm garnet show abrupt changes in inclusion mineralogy and inclusion trail orientation, with rutile and ilmenite in the core and only ilmenite in the rim. The textural/mineralogical boundary also corresponds to a compositional change, most notably an increase of Ca in the rim and a less pronounced increase in Mn at the core-rim boundary. Garnet core-rim Sm-Nd dating indicate similar timing of garnet growth for both samples: cores are 72.3±1.2 and 72.3±1.5 Ma, rims are 67.1±1.6 and 64.4±1.0 Ma. Based on isochemical phase diagram modeling these ages most likely correspond to a temperature and pressure increase for both samples: 1) 535±35°C 5.5±1.0 kbar to 640±25°C 6.3±0.5 kbar and 2) 510±60°C 4.5±1.3 kbar to 610±15°C 5.5±0.5 kbar. These pressure-temperature trajectories account for the observed core-rim changes in the garnets. The increase in temperature would cause the loss of rutile, as rutile is stable at lower temperatures than ilmenite in this pressure range and bulk composition. Garnet mode shows growth-resorption-growth of garnet along the P-T paths, which explains the increase in Mn at the core-rim boundary for both samples. Additionally, modeling of garnet end-member isopleths shows that Ca increases along the paths. Because modeling results can explain garnet compositional zoning and textural zoning along a single P-T path, these garnets likely grew during one metamorphic event. In this case, the ages approximate initial and final growth, and the difference the duration of garnet growth. The garnets record a continuously increasing P-T setting indicating burial, likely caused by crustal shortening from 72 to 64 Ma.