Iridescent Anthophyllite-Gedrite from Simiuttat, Nuuk District, Southern West Greenland: Composition, Exsolution, Age

K. A. Rodgers, P. D. Kinny, V. R. McGregor, G. R. Clark and G. S. Henderson
Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand
Department of Applied Physics, Curtin University of Technology, GPO Box U1987 Perth, Western Australia 6001
Atammik, DK-3912 Maniitsoq, Greenland. Denmark
Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand
Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada

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) Å; a single crystal has a = 18.546(7), b = 17.950(16), c = 5.280(1) Å, space group Pnma with some reflections being notably broader than others. Spot EMPA yielded composite analyses: AlIV = 0.89–1.3, Mg/(Mg+Fe2+) = 0.57–0.61, Na/AlIV = 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°, 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 μm, have Hf/Zr = 0.008–0.012, Hf+FeII c. 0.16. 207Pb/206Pb ages are from 2690 to 2770 Ma, averaging 2732±10 Ma. Coexisting, included Th-, La-, Ce-, Pr-, Nd-, Gd-, Y-monazites have 207Pb/206Pb ages from 2680 to 2720 Ma, averaging 2707±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.

Keywords: anthophyllite • gedrite • cell parameters • composition • iridescence • lamellae • age • nuummite

Mineralogical Magazine; December 1996 v. 60; no. 403; p. 937-947; DOI: 10.1180/minmag.1996.060.403.08
© 1996, The Mineralogical Society
Mineralogical Society (