Abstract

Golden iridescent, 1-100 mm crystals of alternating lamellae of anthophyllite and gedrite constitute the bulk of orthoamphibolite pods within 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) Aa; a single crystal has a = 18.546(7), b = 17.950(16), c = 5.280(1) Aa, space group P nma with some reflections being notably broader than others. Spot EMPA yielded composite analyses: Al IV = 0.89-1.3, Mg/(Mg+Fe 2 ) = 0.57-0.61, Na/Al IV 0.22-0.26. AFM imaging of {210} cleavage surfaces, showed a uniform corrugated morphology parallel to [001]; wavelength was 190-350 nm, mean 250 nm, amplitude 3 nm. A plan view resembles TEM images of (010)-parallel exsolution textures of orthoamphiboles. A second set of corrugations may crosscut the [001]-parallel ridges at 20-25 degrees , akin to reported lamellar intergrowths developed parallel to both (010) and (120). Unequivocal evidence linking topography with lamellae is absent. In contrast to the conventional multi-layer reflector model, the ridged surface provides an additional origin for iridescence, acting as a diffraction grating. Included zircons, 50-10 micrometers, have Hf/Zr = 0.008-0.012, Hf+Fe II c. 0.16. 207 Pb/ 206 Pb ages are from 2690 to 2770 Ma, averaging 2732+ or -10 Ma. Coexisting, included Th-, La-, Ce-, Pr-, Nd-, Gd-, Y-monazites have 207 Pb/ 206 Pb ages from 2680 to 2720 Ma, averaging 2707+ or -12 Ma. The included crystals grew during a late Archaean metamorphism that produced overgrowths on zircons within gneisses to the north, but with Simiuttat grains showing a more complex history. The lamellae may have developed at the same time, or during a reheating c. 2550 Ma, or in a subsequent Proterozoic metamorphism.

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