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Coarse-grained inclusions in Upper Palaeozoic alkali basalts, occurring in over 60 localities in the British Isles, include samples of the underlying mantle and lower crust. The ultramafic assemblages imply an extremely heterogeneous upper mantle involving a tectonized spinel lherzolite and harzburgite matrix transected by younger bodies of wehrlite, websterite, clinopyroxenite, orthopyroxenite, garnet-pyroxenite and assorted kaersutite- and biotite-rich ultramafites. Associated megacrysts include anorthoclase, sanidine, clinopyroxene, kaersutite, Ti-biotite, garnet, Ti-magnetite, Mg-ilmenite, apatite, zircon and corundum. These originated in part from disaggregation of pegmatoidal bodies of alkaline felsic rocks, pyroxenites, lherzites and glimmerites. Plagioclase-pyroxene rocks (basic granulites) are probably samples of the lower crust, with quartzo-feldspathic gneisses possibly derived from shallower levels in a chemically-zoned crust. Other inclusions described include apatite-magnetite rocks, biotite- and hornblende-albite rocks and unfoliated tonalites, diorites, trondhjemites and granites. A high-grade (granulite-facies) feldspathic basement is inferred to underlie all the major N British structural provinces, including the Midland Valley and Southern Uplands. Anorthosite xenoliths imply that anorthositic rocks compose some part of the 'basement' in S and central Scotland.

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Synopsis The mould of a track from SE Arran, and several in situ trackways and individual tracks, as well as a partial trackway on a loose block of Triassic sandstone, from western Arran, represent the first verifiable fossil tracks of Chirotherium from the Triassic of Scotland and support a Scythian (Lower Triassic) age for the base of the Auchenhew Beds. The grouping of the I-IV toes with toe V behind and lateral to the group is characteristic of Chirotherium-like tracks. A comparison with European and American Triassic trackways suggests that the tracks belong to the species Chirotherium barthii Kaup, 1835, first described from Hildburghausen, Germany.

Ultramafic xenolith lithologies representative of the mantle beneath the Midland Valley of Scotland comprise magnesian peridotite (predominantly spinel lherzolite) and cumulate wehrlites and clinopyroxenites. The lherzolites are typical of the worldwide type I (Cr-diopside) xenolith suite; their textures and mineral chemistry record a complex thermal and deformational history. The petrographical and mineralogical features of the wehrlite-clinopyroxenite suite can be interpreted within the context of a sequence of cpx + ol ± sp cumulates that have undergone a protracted period of subsoli dus re-equilibration. Mineral compositions are similar to those of type II (Al-augite) ultramafic xenolith suites.Although xenolith populations imply widespread lower crustal heterogeneity, meta-igneous basic granulites form a major component of this region beneath the Midland Valley. They are principally composed of pi + cpx + mt ± opx ± ap; a modal continuum exists from clinopyroxenite to anorthosite, skewed towards plagioclase-rich lithologies. Garnet is rare. Locally, evidence indicates re-equilibration from garnet granulite precursors. Rock densities range from c. 2·8 to c. 3·2 gm . cm−3, implying P-wave velocities in the range 6·5–7·5 km . s−1 consistent with the known seismic properties of the lower crustal layer beneath the Midland Valley. Phase relations are also consistent with equilibration of primary assemblages at depths of 20–35 km. Retrograde reactions indicate that portions of the crust may have had a complex pressure-temperature-time evolution. Major element compositions of the granulites are broadly basaltic, ranging from ne to hy normative with the mean corresponding to alkali olivine basalt. They are distinct, chemically, from basic granulites of the Lewisian complex of NW Scotland.