Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway

Abstract

The buoyancy and strength of sub-continental lithospheric mantle is thought to protect the oldest continental crust (cratons) from destruction by plate tectonic processes. The exact origin of the lithosphere below cratons is controversial, but seems clearly to be a residue remaining after the extraction of large amounts of melt. Models to explain highly melt-depleted but garnet-bearing rock compositions require multi-stage processes with garnet and clinopyroxene possibly of secondary origin. Here we report on orogenic peridotites (fragments of cratonic mantle incorporated into the crust during continent-continent plate collision) from Otrøy, western Norway. We show that the peridotites underwent extensive melting during upwelling from depths of 350 kilometres or more, forming a garnet-bearing cratonic root in a single melting event. These peridotites appear to be the residue after Archaean aluminium depleted komatiite magmatism.

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

“…In many cases, the timing of attainment of peak pressures in the peridotite bodies is cryptic, and difficult to relate to the timing of peak metamorphism in the host continental crust (e.g. Brueckner et al, 2002;Spengler et al, 2006). In the Svartberget peridotite body, in the northern Western Gneiss Region of Norway, diamond-bearing phlogopite-garnet websterites recording pressures of 5.5 GPa appear to yield Caledonian U-Pb zircon ages of ∼ 397 Ma, similar to the timing of leucosome formation in the host felsic gneiss (Vrijmoed et al, 2009).…”

Evidence of sub-arc mantle oxidation by sulphur and carbon

“…Because of the similarity in mineralogy to that previously reported, this new-found metasomatic assemblage is considered to be equivalent to the M3-3 stage of Scambelluri et al (2008). The Ugelvik peridotite records stages M1 and M2 but has not been affected by later metasomatism (Brueckner et al, 2002;Spengler et al, 2006). Compositional mapping of UGL01 garnet, a garnet megacryst from harzburgitic domain in the Ugelvik peridotite, shows an absence of hydrous minerals or MSI (Fig.…”

“…Subsequently, during the Caledonian orogeny, fluids derived from subducting slab locally promoted metasomatism (Stage M3), as recorded by the Bardane peridotite on Fjørtoft island (Brueckner et al, 2002;Van Roermund et al, 2002;Scambelluri et al, 2008). Other portions have not been infiltrated by slab fluids and preserve the original mantle assemblage (M1-M2), exemplified by the Ugelvik peridotite on the island of Otrøy (Brueckner et al, 2002;Spengler et al, 2006). Although there is debate on the origin of metasomatism at Bardane, the trace element and isotopic signature of the metasomatic assemblage are comparable to the effects of typical subduction zone fluids sensu lato (cf.…”

“…Determination of redox processes associated with fluid-mantle interaction cannot be indisputably resolved by studying the product of the arc system (i.e. arc lavas); it is best approached by examining rocks from the deep portion of mantle wedge close to the slab-mantle interface that have been altered by slab fluids, which are preserved as orogenic peridotites.The Western Gneiss Region of Norway hosts several orogenic peridotite bodies that represent a portion of transition zone mantle that upwelled, melted (Stage M1) and accreted to cratonic lithosphere (Stage M2;Spengler et al, 2006). Subsequently, during the Caledonian orogeny, fluids derived from subducting slab locally promoted metasomatism (Stage M3), as recorded by the Bardane peridotite on Fjørtoft island (Brueckner et al, 2002;Van Roermund et al, 2002;Scambelluri et al, 2008).…”

“…The Western Gneiss Region of Norway hosts several orogenic peridotite bodies that represent a portion of transition zone mantle that upwelled, melted (Stage M1) and accreted to cratonic lithosphere (Stage M2; Spengler et al, 2006). Subsequently, during the Caledonian orogeny, fluids derived from subducting slab locally promoted metasomatism (Stage M3), as recorded by the Bardane peridotite on Fjørtoft island (Brueckner et al, 2002;Van Roermund et al, 2002;Scambelluri et al, 2008).…”