Fracturing of garnet crystals in anisotropic metamorphic rocks during uplift

Garnet crystals in granulite-facies mylonites and gneisses from the Morin shear zone in the Grenville Province (Québec) are characterized by pervasive, closely spaced, relatively straight, tensile fractures aligned systematically normal to the mylonitic foliation and lineation. The fractures developed preferentially in coarse grains (> 0.2 mm) or in grains with a large aspect ratio (> 2). Fractured segments of each garnet crystal have not been separated or filled with matrix minerals (quartz and feldspar) or with retrograde minerals (biotite, muscovite and chlorite). This suggests that the garnets were fractured at shallow crustal depths (T < 300 °C or < 15 km for a thermal gradient 20 °C/km). We propose that the garnet fractures were formed by the response of anisotropic metamorphic rocks to a horizontal extension during uplift and cooling of the metamorphic terrane within the upper crust. Using a modified shear-lag theory, we can explain why tensile fracturing took place preferentially in stiff garnet rather than in felsic material, and why the fractures are unequally spaced in a garnet grain. Our data suggest that the sequential tensile fractures of garnet grains record a progressive uplift process of metamorphic rocks within the upper crust.