Petrogenesis of the Loch Bà ring-dyke and Centre 3 granites, Isle of Mull, Scotland

The Loch Bà ring-dyke and the associated Centre 3 granites represent the main events of the final phase of activity at the Palaeogene Mull igneous complex. The Loch Bà ring-dyke is one of the best exposed ring-intrusions in the world and records intense interaction between rhyolitic and basaltic magma. To reconstruct the evolutionary history of the Centre 3 magmas, we present new major- and trace-element, and new Sr isotope data as well as the first Nd and Pb isotope data for the felsic and mafic components of the Loch Bà intrusion and associated Centre 3 granites. We also report new Sr, Nd and Pb isotope data for the various crustal compositions from the region, including Moine and Dalradian metasedimentary rocks, Lewisian gneiss, and Iona Group metasediments. Isotope data for the Loch Bà rhyolite (87Sr/86Sri = 0.716) imply a considerable contribution of local Moine-type metasedimentary crust (87Sr/86Sr = 0.717–0.736), whereas Loch Bà mafic inclusions (87Sr/86Sri = 0.704–0.707) are closer to established mantle values, implying that felsic melts of dominantly crustal origin mixed with newly arriving basalt. The Centre 3 microgranites (87Sr/86Sri = 0.709–0.716), are less intensely affected by crustal assimilation relative to the Loch Bá rhyolite. Pb-isotope data confirm incorporation of Moine metasediments within the Centre 3 granites. Remarkably, the combined Sr–Nd–Pb data indicate that Centre 3 magmas record no detectable interaction with underlying deep Lewisian gneiss basement, in contrast to Centre 1 and 2 lithologies. This implies that Centre 3 magmas ascended through previously depleted or insulated feeding channels into upper-crustal reservoirs where they resided within and interacted with fertile Moine-type upper crust prior to eruption or final emplacement.

Sandbagging versus Sandbag Replacement Systems: Costs, Time, Helpers, Logistics

The classic aid in operative flood defence is the sandbag. Over the past few decades, though, so-called sandbag replacement systems (SBRS) have also been available for flood fighting. The use of sandbags is time-consuming as well as highly intensive in terms of materials and personnel. In contrast, the use of SBRS entails higher investment costs. However, SBRS are reusable and require lower costs for helpers and logistics, so that the higher investment costs are offset by repeated use. In fictitious but realistic scenarios, the use of sandbags and sandbag replacement systems is compared in order to enable a comparison of requirements such as deployment costs, time and helpers. Three different linear SBRS were compared to a 1.0-metre high and 100-metre long sandbag dam. Furthermore, the dyke defence measures load drain and ring dyke were compared with the comparable SBRS available on the market. All three SBRS clearly show time saving and logistical advantages. Under the assumed conditions, the higher investment costs of the SBRS are already amortised with one subsequent reuse.

The geometry and emplacement of the Pilanesberg Complex, South Africa

The circular 625 km2 alkaline Pilanesberg Complex, South Africa, contains coeval eruptive and several distinctive intrusive syenitic and foyaitic components, concentrically arranged at the surface. However, owing to poor outcrop the relationships between the different intrusive rocks, and their shape in the third dimension cannot be convincingly determined in the field. The original interpretation was a laccolith, whereas later models suggested a funnel shape, and appealed to ring-dyke and cone-sheet emplacement mechanisms. However, the radial widths of these coarse-grained bodies are over 1 km and so cannot have been emplaced as ring dykes or cone sheets, which are usually quite thin and fine grained. Creating the space for emplacement and removal of pre-existing country rocks for each postulated subsequent intrusive event presents a major challenge to this latter hypothesis. Extensive previously published and new field relationships are re-evaluated here to suggest that the body is a gently inward-dipping sheet and that subsequent injections of magma merely pumped up an existing and evolving magma chamber rather than intruded into solid rocks. A Bouguer gravity anomaly model is presented that supports the concept of a shallow, flat-bottomed body rather than one that continues to significant depth. There are many analogies with the Kangerlussuaq Intrusion, Greenland.

Kûngnât, revisited. A review of five decades of research into an alkaline complex in South Greenland, with new trace-element and Nd isotopic data

The Kûngnât Complex (1275±1.8 Ma) in the Gardar Alkaline Province, South Greenland, cuts Archaean gneisses and comprises two intersecting syenitic stocks and a gabbroic ring-dyke. The magmas, with increasingly more primitive compositions, were emplaced successively by ring-faulting and roof stoping. The syenites are orthocumulates (cumulus alkali feldspar, olivine, pyroxene, titanomagnetite and apatite; intercumulus phases include alkali amphibole, biotite, quartz and calcite). In the well dissected earlier stock, a 2.2 km-thick layered sequence displays graded modal layering, feldspar lamination and cryptic layering. Modal layering in both stocks is directed mainly inwards at 35° – 50°. Heterogeneous nucleation of the cumulus assemblage, close to steep thermal boundary layers, is inferred. The modal layering is ascribed primarily to gravitational sorting aided by the large density differential between a) feldspar and b) Fe-rich silicates and oxides. Episodic collapse of cumulus + melt slurries contributed to inward-dipping crystal pediments on the chamber floors. The Ring-Dyke (up to 100 m wide) is nearly continuous through 360°. Kûngnât exhibits a compositional nearcontinuum from olivine gabbro through syenite intermediaries to alkali granite, ascribed to protracted assimilation/fractional crystallization processes. The most radiogenic Nd isotope data from Kûngnât (εNdi values between –3.3 and –1.0) point to a lithospheric mantle source, whereas the most unradiogenic values imply enrichment in LREE by crustal contamination of the magmas.