Palaeozoic subduction-related and kimberlite or carbonatite metasomatism in the Scottish lithospheric mantle

Abstract

Mantle xenoliths entrained in late Carboniferous to mid-Permian silica-poor basic magmas from northern Scotland were investigated using major and trace element compositions of minerals and isotopic compositions of whole-rocks and clinopyroxenes. The aim of the study was to gain more precise information on the nature of the lithospheric mantle beneath this terrane, including evidence for its metasomatic modification and evolution. This study concerns peridotites from two localities in the ENE and WSW of the Scottish Northern Highlands Terrane: Rinibar (South Ronaldsay, Orkney) and Streap Com'laidh (near Glenfinnan). Two groups of clinopyroxenes can be distinguished both at Streap Com'laidh (Type-S1 and Type-S2) and Rinibar (Type-R1 and Type-R2) based on different trace element contents and isotopic ratios. Type-S1 is characterized by an almost flat profile from middle rare earth elements (MREE) to heavy REE (HREE) accompanied by an overall light REE (LREE) enrichment. It shows the highest Th and U, coupled with low Sr, Zr and TiO 2 contents. Type-S2 exhibits humped LREE-enriched patterns and a steep decrease from Nd to Yb. It has the lowest Th and U, coupled with the highest Sr, TiO 2 and Zr contents. Both groups of clinopyroxenes present analogous isotopic features. They have measured 87 Sr/ 86 Sr values from 0.70652 to 0.70826, 144 Nd/ 143 Nd from 0.512093 to 0.512687 and 176 Hf/ 177 Hf from 0.282727 to 0.283088. These isotopic features could be explained by the addition in the mantle wedge of a slab component, made up of altered oceanic crust plus a moderate quantity of subducted sediments. The most recent subduction event in the geological history of Scotland is at about 400 Ma. It may have been during this convergent stage that the metasomatism affecting the sub-Streap lithospheric mantle occurred. Type-R1 is characterized by the lowest concentrations of Ba, Rb, Sr, LREE and U-Th, associated with remarkably high levels of Ti and Zr. These clinopyroxenes have measured 87 Sr/ 86 Sr ranging from 0.70330 to 0.70383, 144 Nd/ 143 Nd from 0.512643 to 0.512761 and 176 Hf/ 177 Hf from 0.282705 to 0.282899. In contrast, Type-R2 shows the highest concentrations of Ba, Rb, Sr, LREE and U-Th, and pronounced Ti and Zr negative anomalies. They have measured 87 Sr/ 86 Sr isotopic ratios similar to Type-R1, but lower 144 Nd/ 143 Nd (0.512431-0.512524) and higher 176 Hf/ 177 Hf (0.282921-0.283014). Calculated melt in equilibrium with Type-R1 is very similar to inferred primary kimberlitic magmas and the clinopyroxene trace element profiles may have resulted from an efficient chemical exchange between a percolating melt and the peridotite host rock. On the other hand the calculated Type-R2 melt overlaps the field of Proterozoic carbonatites. Significantly, at the age of 550 (+50) Ma, the two groups have almost identical Sr-Nd compositions, similar to average depleted mid-ocean ridge basalt mantle (DMM) at 550 Ma. This strongly suggests contemporaneous overprinting of DMM by kimberlitic and carbonatitic metasomatic agents at c. ...

The mantle of Scotland viewed through the Glen Gollaidh aillikite

“…5). The LREE-enriched trend also mirrorsalthough with lower absolute REE concentrations -eclogitic, megacrystic or cognate (CPP) clinopyroxene from further north in the Northern Highland Terrane at Rinibar (Bonadiman et al 2008;Upton et al 2011), further south at Gribun, and at Loch Roag, Isle of Lewis (Hebridean Terrance; Upton et al 1999 (Fig. 4).…”

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

“…However, while a deep (garnet-or diamond-bearing) cratonic lithospheric mantle keel may be lacking in Scotland, the spinel lherzolites xenoliths from terranes north of the GGF represent the shallow regions of this cratonic SCLM (see Upton et al, 2011 and references therein). These localities provide an opportunity to investigate the pre-Mesozoic shallow lithospheric mantle, as entrained in basaltic magmatism rather than kimberlites (Bonadiman et al, 2008). By contrast the Loch Roag xenolith suite is entrained in an Eocene monchiquite dyke (Faithfull et al, 2012) and represents mantle potentially modified by the Icelandic plume.…”

“…The host dyke is dated by whole-rock K-Ar methods at 252 Ô10 Ma (Baxter and Mitchell, 1984). Xenoliths from Rinibar have been investigated previously for their silicate and spinel traceelement compositions, in addition to whole-rock Sr, Nd and Hf isotopes (Bonadiman et al, 2008) (see Table 1). The spinel lherzolites still largely record protogranular textures (Bonadiman et al, 2008) with fine-grained (40.5 mm) silicates and spinels and protogranular to porphyroclastic textures (Mercier and Nicholas, 1975).…”

“…Xenoliths from Rinibar have been investigated previously for their silicate and spinel traceelement compositions, in addition to whole-rock Sr, Nd and Hf isotopes (Bonadiman et al, 2008) (see Table 1). The spinel lherzolites still largely record protogranular textures (Bonadiman et al, 2008) with fine-grained (40.5 mm) silicates and spinels and protogranular to porphyroclastic textures (Mercier and Nicholas, 1975). However, the mineral crystal sizes are typically less than what would be expected in cratonic peridotite xenoliths (2À10 mm; Pearson et al, 2003 and references therein).…”

“…Lherzolites from Streap are protogranular but distinctly fine grained (200À400 mm); olivine crystals are fresh, <0.5 mm with rounded grain boundaries and occasionally with undulose extinction. Orthopyroxene may also be kinked (Bonadiman et al, 2008).…”