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Fowler M.,University of Portsmouth | Millar I.L.,Kingsley Dunham Center | Strachan R.A.,University of Portsmouth | Fallick A.E.,Scottish Universities Environmental Research Center
Lithos | Year: 2013

The Neoproterozoic (c. 870. Ma) West Highland Granitic Gneiss, exposed in the Northern Highlands Terrane of Scotland, has elemental characteristics that are strikingly similar to those of the host Moine metasediments, which are thus consistent with an origin involving major Moine melting. Most of the constituent bodies have compositions significantly removed from minimum melts of pelites, and trace element constraints suggest variable but significant restite entrainment leading to less silicic bulk compositions with enhanced REE, Zr and Y. However, initial Nd and Hf isotope ratios are not coincident with contemporary Moine and imply a significant juvenile contribution. Close association with a regional suite of metabasites prompts consideration of mafic magma input, for which binary mixing models offer qualitative support. Quantitative difficulties with typical Moine metasediments are eased with radiogenic pelites or by partial melting of the mafic component. A possible alternative is currently unexposed Grenvillian felsic crust. Subsequent interaction of the granitic gneisses with meteoric water has significantly perturbed the oxygen and Sr isotope systems, the timing of which is equivocal but probably occurred during Caledonian events.The elemental characteristics of the West Highland Granitic Gneiss show many similarities with Scandinavian (rift-related?) granites of the same age, but since their geochemistry is largely inherited from the protolith it would be unwise to pursue palaeotectonic attribution on this basis. However, the probable incorporation of significant mantle-derived mafic magma of MORB-like affinity is consistent with an extensional setting. © 2013 Elsevier B.V. Source

Riley T.R.,British Antarctic Survey | Millar I.L.,Kingsley Dunham Center
Antarctic Science | Year: 2014

The recognition of a Mesoproterozoic large igneous province (LIP) across large parts of southern Africa has been strengthened by recent geochronology, geochemistry and petrology. The c. 1100 Ma Umkondo province has been recognized across parts of Botswana, Zimbabwe, South Africa and Mozambique where tholeiitic sills, dykes and rare lava flows have been correlated into a single magmatic province emplaced in the interval 1108-1112 Ma. The extension of the province into the Dronning Maud Land region of Antarctica has been suggested by several workers, but detailed analyses of geochemistry and petrogenesis are lacking, as are comparative studies. This study investigates 25 dykes and sills of the Borgmassivet intrusions which include several of the major diorite sills of the province, up to 300 m in thickness. The dykes and sills are also considered to be c. 1100 Ma and they were emplaced, in part, synchronously with the Ritscherflya Supergroup sedimentary sequence. The Borgmassivet intrusions are characterized by geochemical signatures that suggest the magmas were either extensively contaminated by continental crust or derived from an enriched lithospheric mantle source, where the enrichment was related to earlier subduction. The limited geochemical range of the Borgmassivet and Umkondo intrusions are probably not consistent with significant levels of crustal contamination. Furthermore, the trace element ratios indicate a source in the sub-lithospheric mantle, followed by gabbroic fractionation and interaction with lithospheric wall rocks. © Antarctic Science Ltd 2014. Source

Vaughan A.P.M.,British Antarctic Survey | Leat P.T.,British Antarctic Survey | Dean A.A.,British Antarctic Survey | Millar I.L.,Kingsley Dunham Center
Lithos | Year: 2012

Subduction-related Mesozoic mafic dykes in eastern Palmer Land, Antarctic Peninsula, record the development of an intra-oceanic arc terrane, the Dyer Arc, probably of late Permian-Triassic age, represented by a tholeiitic dyke group. Arc rocks were deformed in late Triassic-early Jurassic and mid-Cretaceous times on the Gondwana margin. Eruption of syn- to post-mid-Cretaceous orogenesis magmatism is represented by a calc-alkaline group of dykes. The tholeiitic dykes intruded immature granitic crust of the Eastern Zone sub-terrane of the Central Domain, one of the magmatic terranes of the Pacific margin of Gondwana. The calc-alkaline dykes straddle the major tectonic boundary between the Central Domain and the continental margin Eastern Domain. 132 dykes were studied, 82 of which are calc-alkaline, 48 tholeiitic and two shoshonitic, across an area of approximately 4000km 2. Tholeiitic dykes strike broadly NNW-SSE, transposed into parallelism with the strike of the tectonic boundary during Cretaceous orogenesis whereas calc-alkaline dykes strike tightly ESE-WNW at a high angle to the boundary. The tholeiites pre-date late Triassic deformation and metamorphism, and are interpreted as dominantly Triassic in age, whereas Ar-Ar dated calc-alkaline dykes are younger (~100Ma and ~97Ma) and field relations indicate that they overlap with the waning phase of the mid-Cretaceous Palmer Land Event. The tholeiites have trace element abundances similar to, but more depleted than, those of modern intra-oceanic arcs, as recorded by Zr/Hf ratios. Nb/Yb versus TiO 2 */Yb and Sm/Yb versus La/Sm plots are used to model depths of partial melting. The tholeiites were mostly generated at shallow depths corresponding to 2.5 and certainly less than 3.0GPa in largely garnet-free mantle. The calc-alkaline magmas were generated at pressures greater than 3.0 to possibly >3.5GPa, in the garnet zone. The deepening of the mantle source, and onset of calc-alkaline magmatism are interpreted to have been caused by deformation of the intra-oceanic arc terrane on the Gondwana margin, and lithospheric thickening during the Palmer Land Event. © 2012 Elsevier B.V. Source

Dickson A.J.,University College London | Leng M.J.,Kingsley Dunham Center | Maslin M.A.,University College London | Rohl U.,University of Bremen
Quaternary Science Reviews | Year: 2010

Variations in the strength of coastal upwelling in the South East Atlantic Ocean and summer monsoonal rains over South Africa are controlled by the regional atmospheric circulation regime. Although information about these parameters exists for the last glacial period, little detailed information exists for older time periods. New information from ODP Site 1085 for Marine Isotope Stages (MIS) 12-10 shows that glacial-interglacial productivity trends linked to upwelling variability followed a pattern similar to the last glacial cycle, with maximums shortly before glacial maxima, and minimums shortly before glacial terminations. During the MIS-11/10 transition, several periodic oscillations in productivity and monsoonal proxies are best explained by southwards shifts in the southern sub-tropical high-pressure cells followed by abrupt northwards shifts. Comparison to coeval sea-surface temperature measurements suggests that these monsoonal cycles were tightly coupled to anti-phased hemispheric climate change, with an intensified summer monsoon during periods of Northern (Southern) Hemisphere cooling (warming). The timing of these events suggests a pacing by insolation over precession periods. A lack of similar regional circulation shifts during the MIS-13/12 transition is likely due to the large equatorwards shift in the tropical convection zone that occurred during this extreme glaciation. © 2010 Elsevier Ltd. Source

Meek A.,British Museum | Henderson J.,University of Nottingham | Evans J.,Kingsley Dunham Center
Journal of Analytical Atomic Spectrometry | Year: 2012

Glass is one of several early modern industries where the development from small-scale workshop to large-scale industry offers a valuable insight into wider socio-economic trends. Previously, medieval and early modern forest (wood ash) glass has been studied using a range of analytical techniques. However, characterisations of production centres and exchange systems for forest glasses are difficult to verify, in part because very few examples of raw glass from furnace sites have been investigated. Compositional analysis can provide evidence for the raw materials used and can sometimes provide compositional groupings specific to sites. However, strontium and neodymium isotope determinations can actually provenance the glass by linking the geological ages, or sources, of raw materials to production sites. Using EPMA fifty-six raw glass samples from four sites in two English production areas in operation during the 14 th and 16 th centuries have been analysed. These analyses have shown that the main products of these sites were potash-rich, wood or bracken ash glasses. However, the chemical composition does not provide sufficient resolution to identify differences between the two English regions or the sites within these regions. A subset of five raw glass samples from each of these sites have also been analysed using TIMS to determine strontium and neodymium concentration and isotope ratios. The results allow glasses produced in each region to be differentiated, and can even be used to divide glass production sites within the regions. © 2012 The Royal Society of Chemistry. Source

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