ALBERT

Description: 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 UTh, 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 UTh, and pronounced Ti and Zr negative anomalies. They have measured 87Sr/86Sr isotopic ratios similar to Type-R1, but lower 144Nd/143Nd (0.5124310.512524) and higher 176Hf/177Hf (0.2829210.283014). 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 SrNd 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.

Description: The NeogeneQuaternary alkali-basalthawaiite 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 Na2O and Al2O3 relative to cpx; feldspar includes both alkali-feldspar (Or 1751) and plagioclase (An 2364). Bulk rocks have flat heavy rare earth element (HREE) patterns (1.22.3 times chondrite) and are variably enriched in light REE (LREE; LaN/YbN up to 6.6). The constituent clinopyroxenes are characterized by flat HREE distributions (814.5 times chondrite) and variable LREE enrichment with LaN/YbN up to seven, which generally conform to the bulk-rock chemistry. Samples relatively unaffected by metasomatism have clinopyroxene SrNd isotopic composition (87Sr/86Sr down to 0.7023, 143Nd/144Nd 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 87Sr/86Sr c. 0.7030, 143Nd/144Nd c. 0.5130, with 206Pb/204Pb 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 (87Sr/86Sr=0.7032, 143Nd/144Nd=0.5130, 206Pb/204Pb=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 northerncentral African lithosphere, suggesting a common regional sub-lithospheric component. The relatively low 3He/4He of the Gharyan xenoliths (5.36.5 Ra) indicates that this component originates within the upper mantle and is unrelated to the deep-seated mantle plume source of the EthiopianYemen 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 AfricanEurope collisional system.

Description: A detailed petrological study of mafic and ultramafic xenoliths from Cerro del Fraile (Southern Patagonia, Argentina) was developed to highlight (1) the mineralogical and geochemical composition of the lithospheric mantle beneath the area, (2) the nature of the metasomatizing agents that infiltrate the mantle wedge above the Antarctic plate, (3) the processes that allow the mantle to be refertilized, and (4) the nature of the material dragged down in the subduction zone and recycled within the South Patagonian sub-arc mantle. Major and trace element analyses of clinopyroxene and orthopyroxene in peridotitic and pyroxenitic rocks suggest that a proto-adakite, deriving from the melting of the subducting Antarctic plate, was responsible for the metasomatic features of the peridotitic rocks and the crystallization of the pyroxenites. A few composite xenoliths bridge the two processes—peridotite enrichment and pyroxenite crystallization—indicating that the variously depleted mantle reacts with the incoming melt to generate a newly fertile mantle domain. This reaction occurs under reducing conditions [log f O 2 (QFM) –1·21 to –0·34], unusual for mantle wedge settings. Heavy rare earth elements and Al 2 O 3 and MgO contents in pyroxenes indicate a partial melting degree varying from 10 to 25%. The peculiar enrichment in Zr (–Hf), Th and U of the pyroxenes indicates the melting of oceanic sediments, which include manganese nodules and, possibly, organic matter, in agreement with the estimated low oxygen fugacity conditions. Some geochemical analogies have been found between the calculated metasomatic melts and the Austral Volcanic Zone adakites. In this case, the amount of sediment involved in the genesis of the infiltrating melts is larger than that previously proposed for the genesis of the erupted Patagonian adakites. Chemical–physical conditions favouring the upward percolation through the mantle wedge of these SiO 2 -rich and viscous melts are also discussed.