A metamorphic study was conducted on metapelites of the Boundary Ranges and Florence Range metamorphic suites in the Yukon-Tanana terrane of north-western British Columbia. A new inverse garnet fractionation modelling technique was developed as the software program G FRAC. Through the combined application of G_FRAC and THERIAK-DOMINO, and its integration with Lu-Hf garnet-whole rock age dating of select samples, prograde and retrograde P-T points were estimated for Early Jurassic metamorphism of these two suites. The Boundary Ranges Suite experienced two garnet growth events in this period at 201.7±2.8Ma and 192.4±0.4Ma with the later event reaching peak conditions of ∼ 550◦C and 700 to 800MPa. The Florence Range suite experienced a synchronous garnet growth event between 190.9±3.3Ma and 185.3±1.1Ma reaching a peak pressure of ∼1000MPa at ∼620◦C and a peak temperature of 690 to 730 ◦C at 600 to 750MPa before cooling to ∼610◦C and ∼670MPa.