Zirconium/hafnium fractionation and rare earth element systematics in sub-cratonic garnet pyroxenites, Norway

Abstract

Ten previously studied garnet pyroxenites and one eclogite from orogenic peridotites in the Western Gneiss Region of Norway were analysed for whole-rock major and trace and mineral trace elements to characterise the evolutionary stages of the East Greenland sub-cratonic mantle. A continuous range of whole-rock MgO contents (17–30 wt.%) correlates inversely with $$\hbox {Na}_2\hbox {O}$$ Na 2 O , $$\hbox {Al}_2\hbox {O}_3$$ Al 2 O 3 , and CaO contents. In contrast, rare earth element (REE) compositions allow the samples to be devided into two types. Type I samples have relatively flat primitive mantle (PM) normalised REE patterns (0.4$$-$$ - 6.8 times PM values) with minor fractionation of middle–heavy REEs. Type II samples are simlar to type I samples but have higher light REE contents (6.1–57 times PM values) that reflect metasomatic enrichment. Whole-rock Zr contents (1–$$27\,\upmu \hbox {g}\,\hbox {g}^{-1}$$ 27 μ g g - 1 ) and Zr/Hf ratios (17–39) are positively correlated. Type I samples have low Zr/Hf ratios that mostly differ from type II samples, indicating that metasomatism modified the initially sub-chondritic Zr/Hf ratios. Garnet Zr–Zr/Hf–Y–Ti systematics suggest that metasomatic enrichment transformed the type I samples into type II samples, consistent with processes observed in other cratonic areas (e.g., the northern East European Platform and the Kaapvaal Craton). The formation of type I pyroxenite in dunite from a melt at $$\sim 100\,\text {km}$$ ∼ 100 km depth implies the sub-chondritic Zr/Hf ratios were inherited from that melt. Such melts are thought to form by melting of garnet-bearing, melt-depleted mantle, which is consistent with models for recycling of Archaean palaeo-oceanic crust throughout the mantle prior to the formation of the sub-cratonic lithosphere beneath East Greenland.