Metamorphic framework and plutonic styles in the Prince Rupert region of the Central Coast Mountains, British Columbia

Abstract

The metamorphic framework in Prince Rupert – Skeena region of the Coast Mountains of British Columbia comprises schist, gneiss, and migmatite displaying progressive regional metamorphism that overlaps the Barrovian and Idahoan Facies Series. Although part of the circum-Pacific metamorphic zone, the Coast Mountain metamorphic belt is apparently not paired. Plutonic rocks, which were probably an integral part of the early metamorphic framework, have apparently been mobilized during metamorphism and continued to move out of their original environment while metamorphism waned, some even deforming the pre-existing fabric.Within the framework, four main plutonic styles have been recognized:1) Autochthonous, migmatitic, plutonic complexes.2) Para-autochthonous, steep-walled (tadpole) plutons.3) Para-autochthonous, tongue-shaped, recumbent plutons.4) Allochthonous, intrusive plutons.Quartz diorite and granodiorite are the most common plutonic rocks. Diorite and quartz monzonite are less common: gabbro and especially granite are rare.In the course of moving from the sites of generation to the zones of emplacement, the plutonic rock became:1) more homogeneous.2) less migmatitic and impoverished in inclusions.3) less foliated.4) more acidic, more biotite-rich.5) a rock containing plagioclase of lower average anorthite content and more complex oscillatory zoned crystals.The complex oscillatory zoning appears in a gross way to reflect the variable history accompanying (pulsative ?) movement during crystallization.Time of emplacement of most of the plutonic rock is not known. The potassium–argon age dates (between 53° N and 55° N) display a consistent pattern, with a westerly zone yielding the oldest dates (84 to 140 m.y.), a median zone, intermediate dates (64 to 79 m.y.) and the eastern zone, youngest dates (chiefly 40 to 50 m.y.). These dates may reflect sequential emplacement from west to east but some evidence also suggests that they may reflect sequential uplift and unroofing from west to east.