Timing, relations and cause of plutonic and volcanic activity of the Siluro-Devonian post-collision magmatic flare-up in the Grampian Terrane, Scotland [abstract]

Siluro-Devonian high-K calc-alkaline magmatism in the Grampian Terrane initiated at ~430 Ma, just when subduction of oceanic lithosphere ceased. It followed subduction of the leading edge of continental Avalonia beneath the margin of Laurentia and it persisted for 22-25 million years. Previous U-Pb zircon age data record the earliest plutonic crystallisation at ~429-428 Ma (Rogers and Dunning 1991). New highprecision ID-TIMS U-Pb dating of magmatic zircons yields 425.0 ± 0.7 Ma for the Lorn Lava Pile, 422.5 ± 0.5 Ma for the Rannoch Moor Pluton, 419.4 ± 4.8 Ma for a Fault-Intrusion at Glencoe caldera volcano, 418.0 ± 0.5 Ma for the Clach Leathad Pluton, which cuts Glencoe, 415.0 ± 0.4 Ma for the Cruachan Intrusion of the Etive Pluton and 408.1 ± 0.4 Ma for the Inner Starav Intrusion of the Etive Pluton. The Etive Pluton was assembled in ~8 million years and together with the Clach Leathad and Rannoch plutons constitutes the Lochaber Batholith, which formed in 14 million years. The Etive Dyke Swarm was mostly formed within ~3-4 million years and was part of the plumbing of a large volcano (Etive Volcano; ≥2000 km3) that was intruded by the Etive Pluton and then almost entirely removed by uplift and erosion. The postcollision magmatism is described in terms of a magmatic flare-up that resulted from breakoff of the subducted oceanic lithosphere and consequent rise of asthenosphere, which led to partial melting of LILE- and LREE-enriched lithospheric mantle and mafic-to-intermediate lowermost crust. Large volumes (1000s of km3) of andesite and dacite were erupted repeatedly throughout the flare-up but were rapidly removed by contemporaneous uplift and erosion. This essential volcanic counterpart to the ‘Newer Granite’ plutons has not previously been fully recognised; it is probable that centred volcanoes preceded the plutons widely, from Shetland to Donegal. Most intermediate magmas formed by partial melting of mafic-to-intermediate lowermost crust that derived its high Ba-Sr qualities via a previous partial melting of the LILEenriched lithospheric mantle, possibly at ~1.8 Ga. This crustal recycling was caused by heat and volatile addition from underplated mantle melts (parental appinitelamprophyre magmas) and involved uplift related to the rise of asthenosphere through the slab break.