Slab breakoff:a model for caledonian,late granite syn-collisional magmatism in the orthotectonic (metamorphic)zone of scotland and donegal,Ireland

摘要

None of the existing models for calc-alkaline"Late Granite"(Siluro-Devonian) genesis in the metamorphic Caledonian orogenicbelt of Ireland and Scotland fully explains their spatial,age or chemical character.A consistent model must involve the closure of Iapetus Ocean,where slab breakoff is a natural consequence of attempted subduction of continental crust.Expected outcome is a long linear belt of high-K,calc-alkaline magmas,some with characteristic trace element signatures,specifically high Ba,Sr andZr.Other features include the critical magmatic association of coeval appinite and granite,rapid uplift,erosion and the low-grade regional metamorphism in the Southern Uplands.The linear heat pulse on breakoff is spatially,intensity and time limited producing small volume melts empalced as separated plutons,over a short time span.Magmatism in the Caledonian metamorphic belt is accurately accounted for by slab breadoff on collision of Baltica with the Scoto-Greenland margin during the Scandian orogeny,following lapetus Ocean closure.The two chemically,isotopically and areally distinctive granites where sequential partial melting of new,lamprophyric underplated crust,then shallower old crust,as heat conduction moved up through the cruston slab brekoff,produced magmas characteristic of the two suites.