Mantle wedge peridotites: Fossil reservoirs of deep subduction zone processes: Inferences from high and ultrahigh-pressure rocks from Bardane (Western Norway) and Ulten (Italian Alps)

Abstract

The garnet websterites from Bardane (Western Gneiss Region, Norway) derive from cold Archean subcontinental lithosphere involved in Scandian continental subduction to ultrahigh-pressures. Subduction zone metamorphism was promoted by slab fluid infiltration into the cold overlying mantle wedge. The earliest subduction transformation (M3-1) consists of garnet/clinopyroxene exsolution from old pre-subduction orthopyroxene. This stage was likely coeval with fluid input and formation of phlogopite and dolomite rods in the exsolution structures. Magnesite formation after dolomite and entrapment of fluid-related diamond-bearing polyphase inclusions in corona structures around the exsolved orthopyroxenes point to pressure increase to 4.5 GPa (M3-2). Peak pressures of 6.5–7 GPa (c.a. 200 km depth) are witnessed by crystallization of majoritic garnet (M3-3), mostly in veins cutting all the above microstructures. When such veins infiltrate the corona domains, formation of majoritic garnet in coronas is enhanced. This multistage evolution thus envisages episodic fluid influx, favouring rock recrystallization and formation of microdiamond-bearing inclusions and of majoritic garnet veins. These mantle rocks thus record fluid circulation along grain boundaries and microfractures down to 200 km depth in subduction environments. The Ulten Zone peridotites are slices of Variscan mantle wedge. Infiltration of metasomatic subduction fluids favoured transition from spinel-facies to garnet + amphibole ± dolomite parageneses at pressures below 3 GPa. Formation of metasomatized garnet-bearing peridotite mylonites suggest channelled influx of subduction fluids. The incompatible element-enriched signature of all subduction minerals in Bardane indicate that previously depleted websterites have been refertilized by COH subduction fluids. Comparison with the Ulten Zone garnet + amphibole ± dolomite peridotites outlines striking similarities in the metasomatic style and in the COH fluid phase involved. Comparable geochemical fingerprints observed in HP and UHP Ulten and Bardane mantle rocks call for concomitant subduction of the continental crust, that would provide the incompatible element-enriched fluids. For Bardane this implies subduction of crustal slabs to 200 km depth, to release the fluid phases necessary for UHP metasomatism and for majoritic garnet crystallization. Mantle refertilization by crust-derived COH subduction fluids thus operates over a large depth range, down to 200 km depth. The textural features of the Bardane and Ulten Zone mantle rocks indicate that rock recrystallization and refertilization is concomitant with fluid infiltration along channelways, outside which the rocks preserve pre-subduction, long-lasting stories and pristine geochemical characteristics. Comparably with uprising magmas, the subducted continental crust appears to be a quite efficient carrier to the surface of ultradeep mantle tectonic ‘xenoliths’ representing major observatories of diverse metamorphic and geodynamic histories recorded by the Earth's mantle at variable depths.