Sheets of granite are confined to shear zones in the tectonite peridotite unit of the Thetford Mines ophiolite complex. Their orientation is parallel to the thrust plane at the base of the complex. The granite, which is calc-alkaline and rich in potassium, does not belong to the ophiolite sequence. We show that it was tectonically incorporated within the partially serpentinized peridotite during thrusting and emplacement of the complex.During this process, the granite was strongly deformed and recrystallized at low pressure (500 bar (50 MPa)) and low temperature (500 °C or less), which gave rise to the mineral assemblage orthoclase, albite, hydromuscovite, and hydrothermal biotite. This episode was followed by a rodingite alteration, characterized by the formation of grossular, which was associated with the development of economic chrysotile within the enclosing peridotite. The K–Ar ages of biotite, muscovite, and feldspar from the granite cluster around 450 Ma. These cooling ages suggest that the emplacement and alteration of the granite and peridotite had ended by Late Ordovician time.

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