~(40)Ar/~(39)Ar thermochronometric constraints on the tectonic evolution of the Clachnacudainn complex, southeastern British Columbia

摘要

~(40)Ar/~(39)Ar thermochronometry from theClachnacudainn complex indicates that the thermal evolution ofthe complex was controlled primarily by the intrusion ofgranitoid plutons in mid- and Late Cretaceous times.Hornblendes from the eastern part of the complex cooled belowtheir Ar closure temperature (ca. 500 deg C) shortly afterintrusion of the mid-Cretaceous plutons; those from the westernpart of the complex have latest Cretaceous cooling dates,indicating cooling of these hornblendes after intrusion of theleucogranite plutons at ca. 71 Ma. Micas from the southernClachnacudainn complex exhibit a pattern of progressivecooling toward lower structural levels, where Late Cretaceousand younger intrusions occur. The occurrence of LateCretaceous - Paleocene mica cooling dates in both the hangingwall and footwall of the Standfast Creek fault refutes thehypothesis that there has been significant Tertiary extensionalexhumation of the Clachnacudainn complex along the StandfastCreek fault. Furthermore, the widespread distribution of LateCretaceous - Paleocene mica cooling ages suggests that animportant volume of Late Cretaccous - early Tertiary intrusiverocks must be present in the subsurface beneath the Clachnacudainn complex.