Two-stage exhumatfon of midcrustal arc rocks, Coast Mountains, British Columbia

摘要

New geologic data from the Central Gneiss Complex along Douglas Channel help delineate the burial and exhumation of the Late Cretaceous to Eocene Coast Mountains magmatic arc. Arc plutonism was on going between ～90 and 60 Ma, replaced by formation of dikes to 52 Ma. Supracrustal rocks were buried to midcrustal levels by 90 Ma, and garnet and kyanite grew from ～90 through 70 Ma. Subsequent exhumation was nearly isothermal and took place in two stages. The first occurred from ～70 to 59 Ma as the arc contracted obliquely and sillimanite replaced kyanite. Exhumation was slow (～0.5 mm/yr), probably accomplished by erosion aided by coaxial crustal thinning. Exhumation rates about doubled in the second stage of exhumation, after 59 Ma and before ～52 Ma. Penetrative deformation ended prior to intrusion of the Quottoon pluton at 59 Ma and ～6.5 kbar. Production of cordierite rims on garnet at 4.5 kbar and ～700℃ signaled the end of near-isothermal decompression prior to 52 Ma. Rapid cooling (100℃/10~6 years) of the Central Gneiss Complex followed cordierite growth; temperatures dropped to ～250° by ～48 Ma. The increase in exhumation rate and the subsequent rapid cooling are attributed to excision of >6 km of crust on a detachment system on the northeastern side of the Central Gneiss Complex. Comparison to other parts of the Coast Mountains arc, the Sierra Nevada, and Fiordland, New Zealand, shows that the amount and tempo of exhumation vary greatly within and between arcs, suggesting that the processes accommodating exhumation vary significantly.