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

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 TiO2 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, TiO2 and Zr contents. Both groups of clinopyroxenes present analogous isotopic features. They have measured 87Sr/86Sr values from 0.70652 to 0.70826, 144Nd/143Nd from 0.512093 to 0.512687 and 176Hf/177Hf 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 87Sr/86Sr ranging from 0.70330 to 0.70383, 144Nd/143Nd from 0.512643 to 0.512761 and 176Hf/177Hf 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 87Sr/86Sr isotopic ratios similar to Type-R1, but lower 144Nd/143Nd (0.512431–0.512524) and higher 176Hf/177Hf (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. 550 Ma, which may be related to the opening of the Iapetus Ocean following the break-up of the Rodinia supercontinent. These data indicate a complex metasomatic history of the Scottish lithospheric mantle, which relate different geological events, most probably prior to the juxtaposition of the various tectonic blocks that nowadays constitute the Northern Highland Terranes.