IRIDESCENT ANTHOPHYLLITE-GEDRITE FROM SIMIUTTAT, NUUK DISTRICT, SOUTHERN WEST GREENLAND - COMPOSITION, EXSOLUTION, AGE

Golden iridescent, <1-100 mm crystals of alternating lamellae of antho phyllite and gedrite constitute the bulk of orthoamphibolite pods with in quartz-cordierite gneisses of the Akulleq terrane at Simiuttat, SW Greenland. X-ray powder diffraction powder gave a = 18.526(7), b = 17. 979(15), c = 5.285(23) Angstrom; a single crystal has a = 18.546(7), b = 17.950(16), c = 5.280(1) Angstrom, space group P-nma with some refl ections being notably broader than others. Spot EMPA yielded composite analyses: Al-IV = 0.89-1.3, Mg/(Mg+Fe2+) = 0.57-0.61, Na/Al-IV 0.22-0 .26. AFM imaging of {210} cleavage surfaces, showed a uniform corrugat ed morphology parallel to [001]; wavelength was 190-350 nm, mean 250 n m, amplitude 3 nm. A plan view resembles TEM images of (010)-parallel exsolution textures of orthoamphiboles. A second set of corrugations m ay crosscut the [001]-parallel ridges at 20-25 degrees, akin to report ed lamellar intergrowths developed parallel to both (010) and (120). U nequivocal evidence linking topography with lamellae is absent. In con trast to the conventional multi-layer reflector model, the ridged surf ace provides an additional origin for iridescence, acting as a diffrac tion grating. Included zircons, 50-10 mu m, have Hf/Zr = 0.008-0.012, Hf+Fe-II c. 0.16. Pb-207/Pb-206 ages are from 2690 to 2770 Ma, averagi ng 2732+/-10 Ma. Coexisting, included Th-, La-, Ce-, Pr-, Nd-, Gd-, Y- monazites have Pb-207/Pb-206 ages from 2680 to 2720 Ma, averaging 2707 +/- 12 Ma. The included crystals grew during a late Archaean metamorp hism that produced overgrowths on zircons within gneisses to the north , but with Simiuttat grains showing a more complex history. The lamell ae may have developed at the same time, or during a reheating c. 2550 Ma, or in a subsequent Proterozoic metamorphism.