Deep komatiite signature in cratonic mantle pyroxenite

• Main Authors: Spengler, Dirk, van Roermund, Herman L.M., Drury, Martyn R.
• Other Authors: Structural geology and EM, Structural geology & tectonics
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: Archean; Cratonic mantle; Garnet-pyroxenite; Komatiite; Pyroxene exsolution; Geology; Geochemistry and Petrology; Greenland; Midsundvatnet; Norway; East Greenland
• Online Access: https://dspace.library.uu.nl/handle/1874/365197

We present new and compiled whole-rock modal mineral, major and trace element data from extremely melt depleted but pyroxenite and garnet(-ite)-bearing Palaeoarchean East Greenland cratonic mantle, exposed as three isolated, tectonically strained orogenic peridotite bodies (Ugelvik, Raudhaugene and Midsundvatnet) in western Norway. The studied lithologies comprise besides spinel- and/or garnet-bearing peridotite (dunite, harzburgite, lherzolite) garnet-clinopyroxenite and partially olivine-bearing garnet-orthopyroxenite and -websterite. Chemical and modal data and spatial relationships between different rock types suggest deformation to have triggered mechanical mixing of garnet-free dunite with garnet-bearing enclosures that formed garnet-peridotite. Inclusions of olivine in porphyroclastic minerals of pyroxenite show a primary origin of olivine in olivine-bearing variants. Major element oxide abundances and ratios of websterite differ to those in rocks expected to form by reaction of peridotite with basaltic melts or silica-rich fluids, but resemble those of Archean Al-enriched komatiite (AEK) flows from Barberton and Commondale greenstone belts, South Africa. Websterite GdN/YbN, 0.49-0.65 (olivine-free) and 0.73-0.85 (olivine-bearing), overlaps that of two subgroups of AEK, GdN/YbN 0.25-0.55 and 0.77-0.90, with each of them being nearly indistinguishable from one another in not only rare earth element fractionation but also concentration. Websterite MgO content is high, 22.7-29.0 wt%, and Zr/Y is very low, 0.1-1.0. The other, non-websteritic pyroxenites overlap-when mechanically mixed together with garnetite-in chemistry with that of AEK. It follows an origin of websterite and likely all pyroxenite that involves melting of a garnet-bearing depleted mantle source. Pyroxene exsolution lamellae in the inferred solidus garnet in all lithological varieties require the pyroxenites to have crystallized in the majorite garnet stability field, at 3-4 GPa (90-120 km depth) at minimum 1,600°C. Consequently, we interpret ...