Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example form the Isle of Rum, NW Scotland

Sr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to 87Sr/86Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustalmelts and and primarymantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The ‘early felsic–later mafic’ volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place

The origins of carbonatites and related rocks from the Gr\uf8nnedal-\ucdka Nepheline Syenite complex, South Greenland: C-O-Sr isotope evidence

The Gr\uf8nnedal-\ucdka ring complex (1299 \ub1 17 Ma) in the Gardar province, South Greenland is composed of a range of layered nepheline syenites which were intruded at a late stage by xenolithic syenite and a plug of carbonatite. The complex was subsequently intruded by a variety of basic dykes, including olivine dolentes, kersantites, vogesites, spessartites, camptonites and an aln\uf6ite, and then extensively faulted. The nepheline syenite magmas, produced by fractional crystallisation of basic magmas, show a range in \u3b413C (-3.86 to -7.57\u2030) and \u3b418O (8.27 to 15.12\u2030), distinctly different to the carbonatites which form a tight group with average \u3b413C= -4.31 \ub1 0.22 \u2030, (1 s.d.) and average \u3b418O= 7.18 \ub1 0.41\u2030(1 s.d.). Initial 87Sr/86Sr isotope ratios (typically 0.703) suggest the syenites and carbonatites have not assimilated crustal rocks, and therefore the C and O isotope variation within each group is a result of isotopic evolution during fractional crystallisation. A suite of lamprophyre dykes (\u3b413C -3.86 to -7.86\u2030 and \u3b418O 9.12 to 10.81\u2030) form a coherent group whose stable isotope compositions overlap part of the syenite field, and again are distinctly different from the carbonatites. A single aln\uf6ite has \u3b413C = -3.32\u2030 and \u3b418C= 12.34\u2030. C and O isotope ratios are consistent with origins of syenitic and lamprophyric magmas from a similar source. Despite geochemical evidence which suggests a genetic link between nepheline syenites and carbonatites, C and O isotopic evidence shows that they are not related directly by liquid immiscibility. Comparisons are made between similar rock types from Gr\uf8nnedal-\ucdka and from the Gardar Igaliko Dyke Swarm. The possible role of F in controlling \u3b413C and \u3b418O during crystallisation of calcite from carbonatite magmas is discussed

Lateral versus vertical emplacement in shallow-level intrusions? The Slieve Gullion ring-complex revisited

Recent studies on shallow-level arcuate intrusions have identified numerous examples of horizontal mineral fabrics. These are commonly interpreted as reflecting considerable lateral flow during magma emplacement, thus querying established 'semi-vertical' ring-dyke models. We question the recent lateral emplacement model proposed for the Paleocene Slieve Gullion Ring-complex, NE Ireland, where the absence of steep fabrics in parts of the ring-complex has been used to support a shallow, semi-horizontal sheet intrusion mechanism. We argue that such simple flow models cannot be applied to explosive ring-fissure eruptions and that fabric data alone do not warrant rejection of the ring-dyke model. Moreover, the apparent 'absence of steep intrusive contacts' along the intrusion's perimeter is re-addressed and we present numerous examples of outcrops (27) with steep-sided geometries. The Camlough Breccias are reinterpreted as the product of gas-driven tuffisites injected along the active ring fault (rather than of purely tectonic origin). Crucially, the porphyritic microgranite and porphyritic rhyolite ring-dyke rocks exhibit geochemical and petrographic signatures of contamination by the geographically restricted Palaeozoic Newry granodiorite and are best explained through crustal interaction vertically beneath the ring-complex. Subsequently, these silicic magmas rose into ignimbrite feeders along a caldera ring-fault system that was emplaced into near-surface vent-filling breccias

A new exposure of a caldera fault segment at the Slieve Gullion Igneous Centre: implications for the emplacement of the early ring-complex

A contact zone between porphyritic microgranite of the early ring-complex of the Paleocene Slieve Gullion igneous centre and Lower Palaeozoic metasedimentary rocks of the Longford–Down inlier was exposed along a 300m section excavated during construction of the new M1 motorway. The outcrop displays a thin sliver of Longford–Down metasediment in contact with porphyritic microgranite in a steep, and locally intensely crushed, contact zone that dips away from the ring-complex. This outcrop pattern is reminiscent of a 'caldera-superfault'. Given the recent discussion on sheet versus ring-dyke emplacement of the early ring-complex at Slieve Gullion, this new evidence argues in favour of the traditional ring-dyke model with magma ascending along an active ring fracture associated with caldera subsidence

Picritic magmas and the Rum ultramafic complex, Scotland

Three small picritic dykes, intruded at a late stage in the evolution of the Rum basic-ultra-basic complex, Inner Hebrides, shed new light on the nature of the magmas responsible for the main complex. The magmas are of transitional (mildly alkalic) type, generated by relatively small-fraction (6-7%) melting of a depleted mantle source. Melting is tentatively deduced to have commenced at +/- 100 kin, straddling the garnet-spinel transition. Of the three samples, one (M9) is remarkable for the preservation of very primitive characteristics, with olivines of Fo(93) containing highly aluminous spinels; these appear unique within the British Tertiary Volcanic Province. Sr, Nd and Ph isotopes indicate only minor (less than or equal to4%) contamination with Precambrian crustal lithologies, reflecting the rapidity of ascent of the magma batches. The Rum picrites have Os-187/Os-188 ratios and trace element characteristics comparable to those of recent picrites from Iceland, suggesting minimal temporal change of the depleted parts of the Iceland plume over 60 Ma. Movements of the Long Loch Fault may have been instrumental in causing decompression melting of the spreading Iceland plume-head and facilitating ascent of the melts to near-surface levels