METAMORPHIC PRESSURE-TEMPERATURE CONDITIONS DURING TRANSPRESSION WITHIN THE ROOTS OF A TRANSPRESSIVE MAGMATIC ARC

We describe a sequence of suprcrustal rocks deformed during sinistral transpression and batholith construction in the Coast plutonic complex of western British Columbia. We have applied conventional geobarometers and geothermometers to metamorphic country rocks from a nearly 100 km long transect of the Grenville Channel Shear Zone (GCSZ) of western British Columbia. The GCSZ separates a panel of rocks intruded by the 109 Ma Capitan Cove pluton from a panel intruded by the 90 Ma Ecstall pluton. The GCSZ is subvertical and northwest striking and contains rocks with protomylonitic to ultramylonitic fabrics. Lineations have moderate to shallow plunges. Folds adjacent to the GCSZ are en echelon and rotate in a counterclockwise sense into parallelism with the shear zone as the core of the shear zone is approached. Outcrop scale and micro-scale kinematic indicators consistently indicate a sinistral sense of shear. Rocks at Kennedy Island and in Kumealon Inlet contain sillimanite. Some of the sillimanite is rimmed by cordierite consistent with decompression at high temperature. To the northeast in the Prince Rupert area, sillimanite is absent and kyanite is observed in the aureole of the Ecstall pluton. Peak pressures in the Prince Rupert area are ~.8 GPa. P-T estimates from Kumealon Inlet, farther to the south are 600 C and .6 GPa. Along strike to the south at Lowe Inlet Al in hornblende barometry indicates pressure of ~.55 GPa. These P-T estimates indicate that pressure decreases from ~.8 GPa at the northern margin of the Ecstall pluton to ~.55 GPa near the southern margin of the pluton along the northeast side of the GCSZ. To the west of the GCSZ rocks contain no aluminosilicates, however P-T estimates from garnet bearing gneisses indicate temperatures of 610 C and .6 GPa on Porcher Island. Aluminum in hornblende barometry from the Captain Cove pluton indicates pressures between .63 and .73 GPa. Plutons on Gill Island record pressures of .6 GPa along the western side of GCSZ. The overall consistence of P-T estimates from the western side of the shear zone suggests this panel was exhumed as a coherent crustal block. In contrast rocks to the northeast of the GCSZ appear to have been differentially exhumed, with the deepest seated rocks occurring along the northern margin of the Ecstall pluton.