Geochronology and tectonic implications of magmatism and metamorphism, southern Kootenay Arc and neighbouring regions, southeastern British Columbia. Part II: Mid-Cretaceous to Eocene

Abstract

A north–south-trending belt of amphibolite facies regional metamorphism parallels the Purcell Trench, transects the Kootenay Arc, and is, in part, fault bounded. Towards the axis of this belt progressively higher pressure metamorphic mineral assemblages are exposed in the contact aureoles of post-kinematic, mid-Cretaceous (~100 Ma) plutons and in metapelites. Contours of K–Ar biotite dates for plutonic rocks (55–95 Ma) are regular, are broadly conformable with metamorphic isograds, appear to cross internal intrusive contacts of post-kinematic plutons, and young towards the highest pressure and temperature regional metamorphic zones. Within the sillimanite zone most micas yield K–Ar dates between 40 and 55 Ma; Rb–Sr muscovite dates for deformed and undeformed pegmatites and for muscovite-bearing monzogranite and granodiorite fall between 53 and 84 Ma. U–Pb zircon dates for the Kaniksu batholith and nearby gneiss of uncertain origin yield a lower concordia intercept of 94 Ma. Micas from mid-Jurassic and mid-Cretaceous plutons yielding conventional K–Ar dates between 55 and 100 Ma also yield plateau-shaped 40Ar/39Ar age spectra that are indicative of normal closure to Ar diffusion due to cooling during this time interval.Contrasting isotopic cooling curves for plutonic rocks in the Purcell Anticlinorium and in the metamorphic infrastructure imply that these regions had different thermal histories. Combined with metamorphic mineral assemblage data and interpreted in terms of uplift and erosion, these curves support a tectonothermal model for the development of the Kootenay Arc and Purcell Anticlinorium that involves (1) mid-Cretaceous emplacement of post-kinematic plutons into a tectonically dormant supra-structure accompanying renewed heating, deformation, and metamorphism in the deepest levels of an evolving infrastructure; (2) slow cooling from mid- to Late Cretaceous time; (3) uplift and erosion of the continental terrace wedge and post-kinematic plutons and parts of the mid-Cretaceous infrastructure in latest Cretaceous–earliest Tertiary time as these rocks were thrust eastward over a steplike feature in the basement leading to the formation of the Purcell Anticlinorium; and (4) rapid uplift and cooling of the metamorphic infrastructure in Eocene time.