Basic and ultrabasic volcanic rocks from the Argyll Group (Dalradian) of NE Scotland

The Dalradian Supergroup of the Scottish Highlands is a largely metasedimentary succession of Neoproterozoic to Early Cambrian age, metamorphosed during the Caledonian Orogeny. The rocks were deposited on the Laurentian margin during and following the break-up of Rodinia. This rift setting is evidenced, in the upper half of the succession, by the presence of several volcanic sequences. A significant development of these volcanic rocks occurs in the NE Grampian Highlands. There, the Blackwater Formation, within the Argyll Group of the Huntly area, is dominated by basic and ultrabasic metavolcanic rocks, with intercalated metasedimentary rocks. The rocks were metamorphosed in the amphibolite facies, with the Mg contents of their dominant amphiboles apparently reflecting those of the whole-rock protoliths. The protoliths ranged from ultrabasic picritic types (MgO up to 35 wt%) through high-magnesia basalts to basaltic andesites and andesites. The magmas feeding the volcanism were of tholeiitic affinity, broadly similar to metavolcanic rocks elsewhere in the Dalradian. Higher-Nb and lower-Nb groups can be distinguished. The inferred parental magmas were basalts with MgO c. 10 wt%. The ultrabasic rocks formed by accumulation of olivine and minor Cr-spinel within the parental basalts. Some more evolved rocks show evidence of minor (5–10 wt%) accumulation of Fe–Ti oxides. Primary magmas of the Blackwater metavolcanic rocks were generated from a mantle source, or sources, relatively enriched compared to the MORB source. It is speculated that all the Dalradian metavolcanic rocks represent varying degrees of mixing of magmas from this source and a depleted, MORB-like, mantle source

More on the infrared renormalization group limit cycle in QCD

We present a detailed study of the recently conjectured infrared renormalization group limit cycle in QCD using chiral effective field theory. It was conjectured that small increases in the up and down quark masses can move QCD to the critical trajectory for an infrared limit cycle in the three-nucleon system. At the critical quark masses, the binding energies of the deuteron and its spin-singlet partner are tuned to zero and the triton has infinitely many excited states with an accumulation point at the three-nucleon threshold. We exemplify three parameter sets where this effect occurs at next-to-leading order in the chiral counting. For one of them, we study the structure of the three-nucleon system in detail using both chiral and contact effective field theories. Furthermore, we investigate the matching of the chiral and contact theories in the critical region and calculate the influence of the limit cycle on three-nucleon scattering observables.Comment: 17 pages, 7 figures, discussion improved, results unchanged, version to appear in EPJ

Archaeological signatures of landscape and settlement change on the Isle of Harris

Between 2004 and 2011, a programme of archaeological investigation by the University of Birmingham on the Isle of Harris, a distinctive island forming part of the Western Isles of Scotland, has allowed the archaeological remains of this enigmatic place to be further characterised and understood. Despite intensive archaeological interest in the archipelago for a number of decades, the Isle of Harris has been overlooked and only now are we beginning to identify the archaeological resource and make comparisons to the wealth of published data from islands such as the Uists, Barra and Lewis. This paper highlights some generic overall patterns of archaeological signatures on the Isle which has been identified through a range of archaeological methods including field walking, intrusive excavation, aerial reconnaissance, geophysical and topographical survey, and documentary research. Several key case studies will be introduced including upland shieling complexes and mulitperiod settlement sites on the west coast machair systems. The purpose of the paper is not to present a gazetteer of the results of the work to date, but to highlight some of the key findings with a view to demonstrating that the Isle of Harris is directly comparable with the archaeologically rich landscapes of the other islands

Geochemical evolution of Dalradian metavolcanic rocks : implications for the break-up of the Rodinia supercontinent

Neoproterozoic basaltic magmatism in the Dalradian Supergroup of Scotland and Ireland was associated with the break-up of the Rodinia supercontinent. Magmas were erupted in rift-related basins along a strike length of at least 700 km and during a time period of c. 80 Ma. New major and trace element analyses of metabasalts from several formations are presented to trace the variations in magma compositions in time and space. The primary magmas resulted from variable degrees of mixing of melts derived from mantle sources similar to those of normal and enriched mid-ocean ridge basalts; some younger lavas also show evidence of contamination with continental crust. In contrast to speculations about magmatism elsewhere in Rodinia, the evidence here suggests that there was no involvement of a mantle plume in basalt generation. For example, the Scottish promontory of Laurentia drifted rapidly southwards through c. 25° over the duration of the magmatism, with no evidence of significant elevation above sea level, as might be expected from involvement of a plume. Generation of the primary magmas might have taken place predominantly through decompression melting in depleted upper mantle containing enriched streaks and blobs. Both the Dalradian lithostratigraphy and the metabasaltic compositions are consistent with extreme lithospheric stretching and possibly rupture during the earliest phase of magmatism, whereas generation of later magmatism appears to have been associated with major fault systems, possibly on a foundering continental margin. Supplementary material: Chemical analyses of Dalradian metavolcanic rocks (major elements recalculated to 100%, anhydrous) are available at www.geolsoc.org.uk/SUP18468

The Palaeogene Bracken Bay-Straiton dyke: composition and controls on intrusion

The Bracken Bay–Straiton dyke, a member of the Palaeogene Mull dyke swarm, is one of the Solitary dykes of southern Scotland and northern England recognized by Geikie (1897). It shows a remarkable (unique?) change of strike direction, deviating from the regional NW–SE trend to propagate along the Southern Upland Fault for 16 km before apparently resuming its southeasterly course in the Southern Uplands. The dyke is a clinopyroxene–plagioclase–phyric quartz tholeiite, showing little petrographic variation along strike. However, geochemical data indicate that the dyke segment south of the Southern Upland Fault is not comagmatic with the main dyke; it is here distinguished as the Kello Water dyke. The reason for the dyke’s deviation at the fault is uncertain but may be related to a significant role for crustal heterogeneities, particularly Caledonian fractures which had earlier been utilized by the Late Carboniferous quartz dolerite dyke swarm

Chevkinite-group minerals in UK Palaeogene granites: underestimated ree-bearing accessory phases

Chevkinite-group minerals were sought in 26 granitic bodies from the Palaeogene sequences of Scotland and Northern Ireland, ranging from biotite ± amphibole granites through fayalite-ferrohedenbergite granites to alkali amphibole granites. Chevkinite-(Ce) was found, and analyzed by electron microprobe, in 20 of the bodies, confirming that it is a ubiquitous accessory phase in these rocks. The abundance of the chevkinite is highest in the latter two granite types, sometimes forming more than 50 crystals per thin section. Textural evidence indicates that they crystallized from early- to late-magmatic stages. Compositional variation within and between crystals in the same rock may have resulted partly from fractional crystallization and/or magma mixing and partly from element mobility during metamictization. In some rocks, subsequent hydrothermal alteration affected the composition of the chevkinite-(Ce), in extreme cases leaving totally pseudomorphed material. Apart from mirroring a small enrichment of the LREE in the peralkaline host rocks, there are no systematic changes in chevkinite composition with different bulk-rock compositions