Deep komatiite signature in cratonic mantle pyroxenite

Abstract

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 the websterites to represent the first recognized deep plutonic crystallization products that formed from komatiite melts. The other pyroxenitic rocks are likely fragments of such crystallization products. An implication is that a mantle plume environment contributed to the formation of (one of) the worldwide oldest lithospheric mantle underneath the eastern Rae craton.