Abstract The Neogene–Quaternary alkali-basalt–hawaiite lavas of the Gharyan volcanic field (NW Libya) contain mantle xenoliths. These mostly consist of protogranular spinel lherzolites with superimposed metasomatic textures represented by reaction patches where primary orthopyroxene (opx), clinopyroxene (cpx) and spinel (sp) are the main reacting phases. The secondary parageneses include clinopyroxene (cpx2), olivine (ol2) and feldspar (feld) as reaction rims around opx, spongy-textured clinopyroxene with recrystallized portions (cpx2±feldspar), and brown spinel destabilized in a higher Cr/(Cr+Al) black vermicular aggregate (sp2) generally associated with feldspar microlites. Cpx2 are typically depleted in Na 2 O and Al 2 O 3 relative to cpx; feldspar includes both alkali-feldspar (Or 17–51) and plagioclase (An 23–64). Bulk rocks have flat heavy rare earth element (HREE) patterns (1.2–2.3 times chondrite) and are variably enriched in light REE (LREE; La N /Yb N up to 6.6). The constituent clinopyroxenes are characterized by flat HREE distributions (8–14.5 times chondrite) and variable LREE enrichment with La N /Yb N up to seven, which generally conform to the bulk-rock chemistry. Samples relatively unaffected by metasomatism have clinopyroxene Sr–Nd isotopic composition ( 87 Sr/ 86 Sr down to 0.7023, 143 Nd/ 144 Nd up to 0.5139) that approaches the depleted mantle (DM), suggesting that the lithospheric mantle beneath the area underwent a long-term depletion probably by pre-Palaeozoic extraction of basic melts. The remaining samples approach 87 Sr/ 86 Sr c . 0.7030, 143 Nd/ 144 Nd c . 0.5130, with 206 Pb/ 204 Pb up to 19.66. These data imply that the causative agents of metasomatism were Na-alkali silicate melts with a clear HIMU affinity, in accordance with the isotopic signature of the host lavas ( 87 Sr/ 86 Sr=0.7032, 143 Nd/ 144 Nd=0.5130, 206 Pb/ 204 Pb=19.60). This prevalent HIMU geochemical signature is comparable with that recorded in Cenozoic alkaline basic lavas and associated mantle xenoliths from other occurrences of the northern–central African lithosphere, suggesting a common regional sub-lithospheric component. The relatively low 3 He/ 4 He of the Gharyan xenoliths (5.3–6.5 R a ) indicates that this component originates within the upper mantle and is unrelated to the deep-seated mantle plume source of the Ethiopian–Yemen plateau basalts. Therefore, the Cenozoic volcanic districts of the Saharan belt could be related to smaller-scale shallow mantle upwellings (also referred to as ‘hot fingers’) triggered by intraplate reactivation of regional tectonic lineaments within the Pan-African cratonic basement, as a foreland reaction of the African–Europe collisional system.

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 . 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.