FRACTURING OF GARNET CRYSTALS IN ANISOTROPIC METAMORPHIC ROCKS DURINGUPLIFT

Garnet crystals in granulite-facies mylonites and gneisses from the Mo rin shear zone in the Grenville Province (Quebec) are characterized by pervasive, closely spaced, relatively straight, tensile fractures ali gned systematically normal to the mylonitic foliation and lineation. T he fractures developed preferentially in coarse grains (> 0.2 mm) or i n grains with a large aspect ratio (> 2). Fractured segments of each g arnet crystal have not been separated or filled with matrix minerals ( quartz and feldspar) or with retrograde minerals (biotite, muscovite a nd chlorite). This suggests that the garnets were fractured at shallow crustal depths (T < 300 degrees C or < 15 km for a thermal gradient 2 0 degrees C/km). We propose that the garnet fractures were formed by t he 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 fe lsic material, and why the fractures are unequally spaced in a garnet grain. Our data suggest that the sequential tensile fractures of garne t grains record a progressive uplift process of metamorphic rocks with in the upper crust. (C) 1997 Elsevier Science Ltd.