3-D MICROSTRUCTURAL ANALYSIS OF PAYER LAND PARAGNEISSES AND ORTHOGNEISSES, NORTH-EAST GREENLAND CALEDONIDES

The Payer Land gneiss complex exposed in the Greenland Caledonides preserves high-pressure – high-temperature granulites (peak P=1.5 GPa, T=850°C) composed of interlayered paragneisses and orthogneisses. This complex is separated from the overlying thick sequence of low-grade metasedimentary rocks of the Eleonore Bay Supergroup by an extensional detachment, which is composed primarily of mylonitized paragneiss. HP granulite facies metamorphism is approximately 405 Ma based on previous U-Pb zircon ages from anatectic paragneisses. Our 3-D microstructural analysis of numerous Payer Land gneisses has yielded information pertaining to overall shear sense, fabric formation and retrograde P-T path. Paragneiss samples contain the assemblage grt + ky + sil + qtz + bt + pl + kfs in varying proportions and exhibit a decrease in temperature and pressure as evidenced by the progressive decrease of kyanite as it breaks down to sillimanite. Direct replacement of kyanite by sillimanite suggests crossing of the Al2SiO5 phase boundary at high temperatures. The orthogneiss also exhibits a granulite facies assemblage of qtz + kfs + pl+ opx + grt with preferentially oriented retrograde biotite forming the foliation. The fabrics in the orthogneisses and paragneisses are all formed by retrograde biotite and sillimanite indicating fabric formation coeval with retrogression/thermal relaxation. Measured foliations are dominantly sub-horizontal throughout the gneiss complex with lineations trending towards the ENE. Winged porphyroclasts of garnet, feldspar, and kyanite show evidence of top-to-the-ENE kinematics. These measurements contrast with the observed NNE lineation and top-to-the-SW shear sense in the overlying detachment zone mylonites. The detachment mylonites also contain retrograde muscovite fish; whereas, no muscovite is present in the gneiss dome. These observations suggest that rocks within the gneiss dome have undergone a different retrograde deformational history than the detachment zone rocks. One possibility is that the gneiss complex rocks retain a record of an earlier contractional or extensional event than the detachment. Another alternative is that the gneiss dome rocks contain antithetic collapse fabrics formed simultaneously with the larger extensional detachment.