Abstract Garnet Lu–Hf and Sm–Nd ages from the Shetland Caledonides provide evidence of a polyorogenic history as follows: (1) c. 1050 Ma Grenvillian reworking of Neoarchaean basement; (2) c. 910 Ma Renlandian metamorphism of the Westing Group; (3) c. 622–606 Ma metamorphism of the Walls Metamorphic Series but of uncertain significance because the eastern margin of Laurentia is thought to have been in extension at that time; (4) Grampian I ophiolite obduction at c. 491 Ma followed by crustal thickening and metamorphism between c. 485 and c. 466 Ma; (5) Grampian II metamorphism between c. 458 and c. 442 Ma that appears to have been focused in areas where pre-existing foliations were gently inclined and thus may have been relatively easily reworked; (6) Scandian metamorphism at c. 430 Ma, although the paucity of these ages suggests that much of Shetland did not attain temperatures for garnet growth. There is no significant difference in the timing of Caledonian orogenic events either side of the Walls Boundary Fault, although this need not preclude linkage with the Great Glen Fault. However, the incompatibility of Ediacaran events either side of the Walls Boundary Fault may indicate significant lateral displacement and requires further investigation.

Abstract The island of Seram, eastern Indonesia, incorporates Miocene ultrahigh-temperature (UHT; >900°C) garnet–sillimanite granulites that formed by extensional exhumation of hot mantle rocks behind the rolling-back Banda Arc. UHT metamorphic conditions are supported by new Zr-in-rutile thermometry results and the Miocene age of the UHT event is confirmed by closely-matched heavy rare earth element (HREE) abundances between garnet and c. 16 Ma zircon. Monazites also record identical U–Pb ages, within uncertainty. However, these geochronometers do not date peak UHT metamorphism; instead, they date retrograde, garnet-consuming (Zr- and rare earth element (REE)-liberating) reactions that produced the granulites’ post-peak cordierite + spinel reaction microstructures. Zircons shielded within garnet did not crystallize c. 16 Ma rims and so were unaffected by the entire UHT event. Miocene UHT metamorphism overprinted a Late Triassic–Early Jurassic upper-amphibolite facies event that grew garnet cores and 216–173 Ma zircon. In the Miocene, these garnet cores were overgrown by peritectic garnet rims during UHT metamorphism, with some rutiles recording c. 900°C Zr-in-rutile temperatures. Garnet Lu–Hf ages of 138 Ma – produced by core–rim mixing – demonstrate that a component of Hf 4+ produced since c. 200 Ma was retained through the c. 16 Ma UHT event. Accordingly, UHT conditions must have been very short-lived and exhumation of the granulite complex very rapid.

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.

Abstract The account of the Old Red Sandstone (ORS) given by Wally Mykura (1991) in the third edition of this book remains an authoritative account containing many details which will not be repeated here. Wally Mykura wrote at a time of active controversy concerning the relation of the Grampian Block to Devonian successions south of the Highland Boundary Fault. His untimely death robbed him of the opportunity to discuss problems which have been reconciled to some extent by recent work and revision of absolute ages for the Devonian (Tucker et al . 1998). The chapter title follows tradition, recognizing that both the base and the top of the Devonian cannot generally be defined in Scotland. The Stonehaven Group of ORS facies is now thought to be as old as the Mid-Silurian (Marshall 1991) and the top of the Upper ORS passes conformably up into Carboniferous strata. The term Old Red Sandstone (discussion in Waterston 1965) was originally derived from the ‘Oelter rother Sandstein’ of Werner, and was applied to the Permian red sandstones of Germany. Phillips (1818) used ORS in its present sense, and Murchison (1839) considered it should be regarded as a system. However, it was Sedgwick & Murchison (1839) who established the Devonian system for marine strata in SW England and included the ORS within the Devonian. Old Red Sandstone is now used as a facies term and, whilst the rocks are mostly of Devonian age, there are notable exceptions. The distribution of ORS strata ( Fig. 8.1 ) shows

The Velay granite pluton (Massif Central, France) is the youngest (304 ± 5 Ma) and largest (∼6,900 km 2 ) of the major Massif Central monzogranites/granodiorites and was formed nearly 50 Ma after the cessation of Hercynian continental collision (Pin & Duthou 1990). It is a highly heterogeneous pluton consisting of I-type, high-Sr granites (Sr= 500–900 ppm) with low ε Sr (304) (+35 to +41) and high ε Nd (304) (−3 to −5), at its centre, grading into S-type and mixed I–S-type heterogeneous granites of more normal Sr content (100–420 ppm) and higher ε Sr (304) (+40 to +210) and lower ε Nd (304) (−3·8 to −7.3) at its margins. The metasedimentary lower crust of the Massif Central was underplated/intruded by mafic mantle-derived magmas between 360 Ma and 300 Ma. From 300–280 Ma (Downes et al. 1991) underplating led to partial melting and granulite facies metamorphism of the material (represented by felsic and mafic meta-igneous lower crustal xenoliths, ε Sr (304) = −11 to + 112, ε Nd (304) = +2·2 to 8·2, Downes et al 1990). The partial melts assimilated mainly schist but also felsic gneiss and older granite country rock material (ε Sr (304) = +100 to +300, ε Nd (304) = −5 to −9) to produce the heterogeneous granites. Plagioclase and biotite were accumulated at the base of the intrusion which was intruded to high levels to form the high-Sr granites.