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

摘要

Mapping of isograds related to regional amphibolite-facies metamorphism constrains a three-dimensional model of isogradic surfaces near Mount Cheadle in the northern Shuswap metamorphic complex (lat. 52 degrees 20'N, long. 119 degrees 05'W). Kyanite and sillimanite coexist in a lens-shaped zone, bounded by the kyanite-out and sillimanite-in isogradic surfaces, that is 50 km long, up to 10 km thick, and up to 20 km wide. Textural equilibrium, simple regular geometry of isogadic surfaces, and simple mineral assemblages suggest that metamorphism occurred at P-T conditions near those of the kyanite-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 was Early Cretaceous (monazite U-Pb geochronology) with quenching in the Late Cretaceous, possibly caused by motion on the basal thrust beneath the Malton complex. A younger generation of sillimanite grew in discrete outcrop-scale ductile shear zones, veins, and pods in a north-south-oriented belt (50 km by 20 km). U-Pb dales on zircon, monazite, and titanite indicate an age of the sillimanite overprint of 65-59 Ma. It may have resulted from the influx of hot fluids associated with widespread Late Cretaceous and Paleocene leucogranite emplacement concomitant with extensional faulting. [References: 59]