Early Cretaceous kyanite-sillimanite metamorphism and Paleocene sillimanite overprint near Mount Cheadle, southeastern British Columbia: geometry, geochronology, and metamorphic implications

Mapping of isograds related to regional amphibolite-facies metamorphism con strains a three-dimensional model of isogradic surfaces near Mount Cheadle in the northern Shuswap metamorphic complex (lat. 52 degrees 20'N, long. 11 9 degrees 05'W). Kyanite and sillimanite coexist in a lens-shaped zone, bou nded by the kyanite-out and sillimanite-in isogradic surfaces, that is 50 k m long, up to 10 km thick, and up to 20 km wide. Textural equilibrium, simp le regular geometry of isogadic surfaces, and simple mineral assemblages su ggest that metamorphism occurred at P-T conditions near those of the kyanit e-sillimanite equilibrium curve. Reconstruction of isotherms in the kyanite + sillimanite zone suggests that the metamorphic field gradient was about 14 degrees C.km(-1). A 5 km thick, staurolite-free kyanite zone adjacent to the sillimanite-in isograd suggests a pressure range of about 1.5 kbar (1 kbar = 100 MPa) for Bathozone 5 of D.M. Carmichael. Regional metamorphism w as Early Cretaceous (monazite U-Pb geochronology) with quenching in the Lat e Cretaceous, possibly caused by motion on the basal thrust beneath the Mal ton complex. A younger generation of sillimanite grew in discrete outcrop-s cale ductile shear zones, veins, and pods in a north-south-oriented belt (5 0 km by 20 km). U-Pb dales on zircon, monazite, and titanite indicate an ag e of the sillimanite overprint of 65-59 Ma. It may have resulted from the i nflux of hot fluids associated with widespread Late Cretaceous and Paleocen e leucogranite emplacement concomitant with extensional faulting.