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Title: From slab breakoff to triggered eruptions : tectonic controls of Caledonian post-orogenic magmatism
Author: Neilson, Joanne Claire
ISNI:       0000 0001 3440 727X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2008
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Six new U-Pb zircon dates for the Siluro-Devonian magmatism in the south-west Grampian Highlands, Scotland, combined with data from previous studies, reveal a 25 million year magmatic 'flare-up'. The new dates are: Lome Lava Pile 424.96 ± 0.65 Ma; Rannoch Moor Pluton 422.48 ±0.47 Ma; Gleann Chaman Fault-Intrusion, Glencoe 419.4 ±4.8 Ma; Clach Leathad Pluton, Glencoe 417.99 ± 0.46 Ma; Cruachan Intrusion, Etive 414.96 ± 0.38 Ma and Inner Starav Intrusion, Etive 408.12 ± 0.40 Ma. The magmatic 'flare-up' occurred after subduction of Iapetus oceanic lithosphere, and following limited subduction of Avalonian continental lithosphere beneath Laurentia. It was accompanied by dominantly strike-slip faulting, with uplift, the location of which reflected the position of a 'bum' and crustal weakening due to the magmatism. Magmas, dominated by intermediate to silicic compositions, were exceptionally diverse, mainly of high-K calcalkaline I-type revealing an enriched mantle signature (eNd-eSr) and increasing addition of (silicic) aluminous crustal material through time. Paucity of basalt with SiOl S50 wt.% is a key feature, with limited mafic magmatism represented by appinitic (s.l.) rocks. This study concludes that partial melting of enriched sub-continental lithosphere occurred as a consequence of asthenospheric upwelling that followed breakoff and sinking of the subducted oceanic lithosphere. The resulting appinitic melts underplated and intruded predominantly mafic lowermost crust formed by previous magmatic underplating. This mafic lower crust partially melted due to heat and volatiles derived from the mantle-derived (appinitic) magma and a diverse range of intermediate-composition melts formed, with some mixing at depth. The ascent of magma batches to shallow levels from this 'hot zone' was facilitated by active crust-penetrating strike-slip faults and shear zones, with partial melting and assimilation of the silicic mid to upper crust also occurring. Tectonic facilitation of magma ascent resulted in the common eruption of crystal-poor, 'superheated' magmas, often in unusually large volumes. As the 'hot zone' developed with time, there was progressively more input from the silicic crust and fewer appinitic magmas reached shallow levels. The closely spaced development of centred volcanoes with associated plutons let to progressive assembly of the South-west Grampian Batholith in -14 million years. Volcanic activity was inextricably linked and a precursor to pluton development at specific crustal locations. The volcanoes comprise a greater proportion of intermediate magma compositions than the successor plutons, and they reveal a far greater compositional diversity than them. The extreme compositional diversity and the striking paucity of basalt are fundamental characteristics of the magmatism in the 25 million-year flare-up; they indicate mainly crustal recycling rather than a major episode of crustal growth.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral