NEW AL-IN-HORNBLENDE CRYSTALLIZATION PRESSURES FROM THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA

This study uses Al-in-hornblende pressures to investigate the relation between magmatism and crustal deformation in the southern Coast Mountains batholith of British Columbia. Widespread magmatism from ~85 to 60 Ma has been attributed to crustal extension based on Hf values in zircon, yet structural studies suggest this magmatism occurred during crustal shortening. Al-in-hornblende geobarometry of plutons from 167 to 60 Ma helps constrain the style of crustal deformation during Late Cretaceous magmatism, as crystallization pressures should decrease in plutons emplaced during crustal extension. New Al-in-hornblende pressure estimates for five samples are 3.9, 4.9, 5.6, 5.7, and 6.8 kbars. Combined with five published Al-in-hornblende results and with U-Pb zircon ages, these data show that pluton crystallization pressures vary both with distance across the batholith and pluton age. The highest crystallization pressures (4.9 - 6.8 kbars) occur in the core of the batholith in the youngest sampled plutons (85- 60 Ma). On the western side of the core, older plutons (167 to 95 Ma) yielded crystallization pressures between 2.5 and 3.8 kbars. East of the core, a 60 Ma pluton yielded a crystallization pressure of 3.9 kbars. These results suggest that plutons in the core of the batholith were emplaced at mid-crustal levels between 85 and 60 Ma and exhumed after 60 Ma. The crystallization pressures show no evidence of pluton emplacement in thinning crust and thus do not support the model of Late Cretaceous crustal extension based on geochemical data. Overall, the Al-in-hornblende data are compatible with crustal thickening during Late Cretaceous batholith formation.