The tempo and conditions of metamorphism during magmatism, southern Coast Mountains batholith, British Columbia, Canada

Abstract

Garnet Sm‐Nd ages and pressure–temperature–time (P–T–t) paths for pelitic and semipelitic metamorphic rocks in the Mount Waddington area constrain the relationship between metamorphism and high‐flux magmatism recently identified in the southern Coast Mountains batholith. Amphibolite facies metamorphism in this area occurred at 99–90, 80, and 72–65 Ma at varying conditions of ~3 to 8 kbar and ~450°C to 685°C, partly associated with high magmatic fluxes at 85–70 and 61–48 Ma. P–T–t paths indicate increases in pressure and temperature of 2 kbar and 120°C at c. 80 Ma and up to 3 kbar and 165°C at 72–65 Ma. Garnet growth at c. 72 and 80 Ma was synchronous with a high‐flux event at 85–70 Ma. However, c. 65‐, 90‐, and 99‐Ma garnet growth cannot be directly attributed to nearby plutons and occurred during lulls between high magmatic flux events. The simplest explanation for the observed P–T–t paths for metamorphism is a single regional‐scale event between 99 and 64 Ma, synchronous with and in response to crustal contraction. Rock textures and pressure–temperature (P–T) isochemical phase diagrams indicate that most sampled rocks reached near‐solidus conditions with little or no melting. Although partial melting of sedimentary rocks has been proposed to contribute toward high‐flux magmatism in the northern and central Coast Mountains, the timing of metamorphism and high‐flux events differs in the Waddington area, suggesting that they are unrelated. Therefore, we conclude that metamorphism and partial melting of rocks exposed at the current level of exhumation played little or no role in construction of the batholith in the Waddington–Raleigh area of the southern Coast Mountains batholith.