Metamorphism in the Solitude Range, southwestern Rocky Mountains, British Columbia: comparison with adjacent Omineca Belt rocks and tectonometamorphic implications for the Purcell Thrust

Abstract

Rocks of the Solitude Range, British Columbia, have been metamorphosed from chloritoid–chorite-zone to kyanite-zone conditions. The grade of metamorphism increases southwestward toward the Rocky Mountain Trench (RMT) and the Omineca Belt. Isograds crosscut lithologies and trend more northerly than deformation 2 (D2) structures and the RMT. They are thought to have been quenched syn- to post-D2. Pelitic (Mahto Formation) and calc-pelitic (Tsar Creek unit) rocks contain assemblages that reflect the increase in metamorphic grade. Physical conditions of metamorphism are estimated to be approximately 450–540 °C from the garnet to the kyanite zone; pressures averaged 6–7 kbar (1 kbar = 100 MPa). The pressures, temperatures, and metamorphic assemblages are very similar to those of the Adamant Range, which lies across the Purcell Thrust, to the southwest. This is in contrast with the Big Bend area, to the northwest, where differences in pressure across the Purcell Thrust (PT) have been documented. Two possible models to explain these contrasting relationships are presented. One model suggests that there was post-movement heating on the PT, which reduced the metamorphic contrast across the PT. The second model suggests that a combination of thrust and normal faulting, including warping of isobaric surfaces, has produced an apparently unbroken metamorphic sequence across the PT.