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.