Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example from the Isle of Rum, NW Scotland

Abstract

-Sr and Nd isotope ratios, together with lithophile trace elements, have been measured in a representative set of igneous rocks and Lewisian gneisses from the Isle of Rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of Palaeogene magmatism in the North Atlantic Igneous Province (NAIP). The Rum rhyodacites appear to be the products of large amounts of melting of Lewisian amphibolite gneiss. The Sr and Nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component. Concentrations of the trace element Cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced Cs and Rb loss prior to Palaeogene times, possibly during a Caledonian event. This depletion caused heterogeneity with respect to 87 Sr/ 86 Sr in the crustal source of silicic melts. Other igneous rock types on Rum (dacites, early gabbros) are mixtures of crustal melts and and primary mantle melts. Forward Rare Earth Element modelling shows that late stage picritic melts on Rum are close analogues for the parent melts of the Rum Layered Suite, and for the mantle melts that caused crustal anatexis of the Lewisian gneiss. These primary mantle melts have close affinities to Mid-Oceanic Ridge Basalts (MORB), whose trace element content varies from slightly depleted to slightly enriched. Crustal anatexis is a common process in the rift-to-drift evolution during continental break-up and the formation of Volcanic Rifted Margins systems. The 'early felsic-later mafic' volcanic rock associations from Rum are compared to similar associations recovered from the now-drowned seaward-dipping wedges on the shelf of SE Greenland and on the Vøring Plateau (Norwegian Sea). These three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place.

Bimodal magmatism produced by progressively inhibited crustal assimilation

“…For example, large-scale partial melting of Lewisian gneiss is documented in the early stages of the Rum and Skye central complexes, where early felsic magmas show a strong crustal component and are typically followed by less contaminated mafic compositions 15,16,21 . Likewise, assimilation of partial melts from hydrated basaltic crust has been shown as a source for Icelandic rhyolites 48 .…”

“…The r-value represents the ratio of AFC, with high r-values representing high levels of assimilation 43 . Mantle Sr and Nd end members are after Saunders et al 60 and Pb end members were taken from Ellam and Stuart 61 and Meyer 21 .…”

“…The co-occurrence of mafic and felsic magmas has been proposed to be the result of incomplete mixing in compositionally stratified magma chambers 20 and suggests a rapid return to mafic magmatism once felsic magmas have been generated. Mafic replenishments into felsic magma reservoirs are considered an important trigger for large caldera-forming events in the North Atlantic Igneous Province and elsewhere (for example, Rum 21 , Skye 18 , Askja 18 and perhaps Carlingford 22 ) and can therefore fundamentally alter the eruptive style and behaviour of a volcanic system.…”

“…1b). We test current models for the generation of felsic magmas by crustal melting 9,11 , and question why felsic magmatism is usually only seen in the early stages of individual BPIP centres 15,16,19,21 . We propose that the potential to produce large volumes of felsic magma by crustal assimilation rapidly decreases during the lifetime of an igneous centre as the local crust becomes progressively more refractory with each intrusion and subsequent crustal melting event.…”

Pre-breakup magmatism on the Vøring Margin: Insight from new sub-basalt imaging and results from Ocean Drilling Program Hole 642E

“…Geoffroy, 2005;Abdelmalak, 2010;Abdelmalak et al, 2012). Magmatic activity is typically expressed within the stretched continental crust by: (1) large wedges of seaward-dipping basaltic flows and tuffs extruded at the surface (Hinz, 1981;Eldholm, 1991;Planke et al, 2000;Menzie et al, 2002); (2) massive sill/dyke intrusions within the sedimentary basin (Svensen et al, 2004;Planke et al, 2005); (3) melt emplacements into the upper and mid continental crust by mafic to ultramafic intrusions (Karson and Brooks, 1999;Klausen and Larsen, 2002;Lenoir et al, 2003;Geoffroy et al, 2007;Meyer et al, 2009b); and (4) a highvelocity lower crustal body (LCB) at the base of the crust characterized by high Vp velocity (Vp> 7.0 km/s) (White et al, 1987;Kelemen and Holbrook, 1995;Holbrook et al, 2001;Mjelde et al, 2009a;Mjelde et al, 2009b).…”