Reaction progress and timing of retrogression of eclogite-facies rocks, Danmarkshavn, North-East Greenland Caledonides

Abstract

Eclogites are commonly preserved in the cores of mafic lenses that are surrounded by amphibolite-facies margins. The amphibolites are produced on the retrograde path with the addition of fluid to an otherwise anhydrous assemblage; the fluids are expected to enhance zircon growth. We studied two high-pressure (H P ) mafic/ultramafic lenses from the North-East Greenland eclogite province (a boudinaged garnet-clinopyroxenite and a garnet-websterite) in order to establish the timing of retrogression. We focused on the garnet-clinopyroxenite outcrop to establish the role of the fluid during retrograde metamorphism and to determine the progress of the metamorphic reactions that define the transformation of the H P assemblage to amphibolite. We collected three samples representing different degrees of amphibolitisation, and used them to determine the reaction progress between the different retrograde stages. Reactions (1) and (2) show increasing fluid availability with increasing retrogression: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[3.33 Cpx + 0.56 Grt + 0.105 Plag + 1.17 H_{2}O = 1.17 Amp\] \end{document}(1) \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[2.40 Cpx + 1.99 Grt + 6.33 Qtz + 1.06 Rt + 2.02 H_{2}O = 2.02 Amp + 0.59 Plag + 0.63 Ilm\] \end{document}(2) We determined U-Pb SHRIMP ages and collected zircon trace-element data on metamorphic rims of zircons for both lithologies, from both the H P samples and the surrounding amphibolite rims. These ages and trace elements, combined with chronological and structural data previously published, allow us to interpret the retrograde path for these rocks and the Danmarkshavn area in general. The age of the metamorphic zircon is the same in the center and at the margin of the mafic and ultramafic pods, with ages spanning between 390 and 360 Ma and younger ages (about 330–340 Ma) for the garnet-clinopyroxenite. The oldest Paleozoic ages in the mafic samples likely record the timing of H P metamorphism or initial retrogression. Pegmatite emplacement into strain shadows between mafic boudins was ongoing at 375 Ma. Fluid circulation accompanying deformation and pegmatite emplacement is recorded by the 375 Ma zircon growth in the garnet-websterite. Additional zircon growth is observed both in the garnet-clinopyroxenite and garnet-websterite as well as in a boudin-neck pegmatite at ca . 360 Ma. Continued zircon growth at 340, 330, and 315 Ma is recorded in the garnet-clinopyroxenite, the adjacent boudin-neck pegmatite and the pegmatite dike that cross-cuts amphibolite-facies structural fabrics. All of the younger episodes of zircon growth are interpreted to reflect fluid influx linked to continued retrograde metamorphism, deformation, and pegmatite emplacement in the Carboniferous.