The Thetford Mines ophiolite in southern Quebec was obducted in Early Ordovician time during the closing of the proto-Atlantic. The tectonized peridotite lower unit of the ophiolite is intruded by felsic dikes and pods including isolated lenses of massive rodingite, small bodies of strongly deformed diorite, and younger, less deformed quartz monzonite. These intrusions are found only near the base of the ophiolite, do not intrude the surrounding country rock, and are rootless; for these reasons they are considered to have been emplaced in the ophiolite before it reached its present location.The younger group of intrusions consists of biotite–muscovite quartz monzonite and leuco–quartz monzonite. Analyzed samples have high K₂O contents, high (K₂O × 100)/(Na₂O + K₂O) ratios, and high initial strontium ratios (0.7171–0.7179), indicating that the magma source region was continental and that these felsic rocks formed by partial melting of continental sediments. Whole-rock and mineral isochron ages suggest that the felsic intrusions are about 456 ± 4 Ma old and that they were metamorphosed about 418 ± 7 Ma ago.The detachment of the ophiolite occurred about 491 ± 3 Ma ago and is recorded by the age of the metamorphic aureole beneath the ophiolite. The felsic dikes were intruded some 35 Ma years later during the Taconic Orogeny. The lengthy time between detachment and final nappe emplacement recorded by the felsic dikes may be a requirement for the formation of abundant asbestiform chrysotile.