Magmatic and mantle processes in the genesis of the Gardar Alkaline province, south Greenland
|Location||International Geological Congress,oslo 2008|
|Holding Date||27 September 2008|
The Mesoproterozoic Gardar Alkaline Province that straddles the Archaean-Palaeoproterozoic suture, formed in conjunction with pre-Grenville rifting of a super-continent. Despite Cenozoic uplift and erosion, thick syn-rift sequences of sub-aerial lavas and clastic sediments are preserved, together with major dyke swarms and ~ 15 alkaline plutons. The Province was the product of two principal tectono-magmatic episodes at a) ~1280-1260 Ma and b) ~1170-1140 Ma.
The abundance of anorthositic inclusions inthe mafic intrusions suggests derivation from a major anorthosite complex at depth. The Province is accordingly regarded as an integral part of theNorth American Proterozoic Anorthosite association, aberrant in its a) more alkaline character and b) relative shallowness of erosion. The anorthosite are inferred to have formed as plagioclase flotation cumulates from hawaiitic magmas at mid- to lower-crustal depths. The peralkalinity attained by some Gardar magmas is attributed to extreme plagioclase fractionation (cf. N.L. Bowen’s ’plagioclase effect’). At intermediate stages, plagioclase fractionation generated Fe-rich magmas (ferro-mugearites) which, following magnetite fractionation, gave rise to more buoyant benmoritic magmas from which the bulk of the plutons were derived.
Remarkable affinities between Gardar suites separated by ~100 Ma during which active plate tectonics may be asumned, imply that there was a strong lithospheric control on magma compositions. It is proposedthat the incompatible element content of the lithospheric mantle was raised by repetitive metasomatic events.
The Gardar magmas were distinguished by high halogen contents. It is suggested that metasomatic infiltration of fluor-carbonatite asthenospheric small-melt fractions promoted and facilitated fractional melting and fractional crystallisation processes. These resulted in extreme incompatible element concentrations, as in the Ilimaussaq and Ivigtut intrusions. These rare elements must have been scavenged from large volumes of the Earth’s mantle.