A field and geochronometric study of the Horseranch Range, north-central British Columbia, was carried out to determine the cooling and uplift history of the area and to assess the possibility that the range may be a metamorphic core complex.
40Ar-39Ar cooling ages from hornblende, muscovite, and biotite in a variety of rock types and structural positions, and apatite fission track data indicate rapid cooling from about 525 to 50 °C or less in the middle to late Eocene epoch. Uplift and cooling took place by oblique-normal dextral strike-slip along a northwest-striking, moderately to steeply southwest-dipping mylonite zone associated with regional mid-Cretaceous to Oligocene, northwest-striking, dextral strike-slip faulting. Uplift by oblique-normal slip differs from dip-slip displacement reported for metamorphic core complexes in the southern Omineca Belt. The peak of regional metamorphism at 650 °C and 5 to 7 kb was late Early Cretaceous or younger in age and predated mylonitization. There is no evidence of resetting of older minerals. This is in contrast to the nearby Swannell Ranges, which implies to us that differential uplift took place in the northern Omineca Belt, probably due to control of bounding faults.