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Iridescent anthophyllite-gedrite from Simiuttat, Nuuk district, southern West Greenland composition, exsolution, age

Published online by Cambridge University Press:  05 July 2018

K. A. Rodgers ,
P. D. Kinny ,
V. R. McGregor ,
G. R. Clark  and
G. S. Henderson
K. A. Rodgers
Affiliation:
Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand
P. D. Kinny
Affiliation:
Department of Applied Physics, Curtin University of Technology, GPO Box UI987 Perth, Western Australia 6001
V. R. McGregor
Affiliation:
Atammik, DK-3912 Maniitsoq, Greenland. Denmark
G. R. Clark
Affiliation:
Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand
G. S. Henderson
Affiliation:
Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada
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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+FeIIc. 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

anthophyllitegedritecell parameterscompositioniridescencelamellaeagenuummite
Type
Mineralogy
Information
Mineralogical Magazine , Volume 60 , Issue 403 , December 1996 , pp. 937 - 947
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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