The elemental compositions of terrestrial igneous rocks are reviewed with special emphasis on those elements that partition strongly into the liquids in mafic and ultramafic systems. Published data are supplemented by 79 new major- and trace-element analyses. The magmatism of ocean basins is considered in terms of a model that has the following main features: (i) density layering in the sub-lithospheric upper mantle, so that the more fertile source of ocean-island basalts (o.i.b.) underlies the less fertile source of mid-ocean ridge basalts (m.o.r.b.); (ii) the genesis of all mantle-derived magmas restricted to very small degrees of partial fusion; (iii) genesis of m.o.r.b. source mantle as residuum from the loss of a melt fraction (forming o.i.b. magmas and lithospheric veins) from o.i.b.-source mantle; (iv) subduction of o.i.b;- veined lithosphere, with a thin veneer of m.o.r.b. and sediments, to the 670 km seismic discontinuity, followed by re-heating of these components and their buoyant upwelling into the o.i.b.-source reservoir; (v) very little chemical communication across the 670 km discontinuity. All continental anorogenic magmatism (distant from subduction zones in space and time) seems to be related ultimately to the o.i.b.-source mantle reservoir, which therefore must extend beneath the lithospheric roots of continents. The minor sodic-alkalic magmatism of continents is effectively identical in composition to o.i.b. Some continental flood basalts are similar but the majority contain minor contamination (rarely more than 15%) from fusible sialic rocks. Although substantial amounts of sediments appear to be subducted, only a small proportion of them seems to re-appear in the products of island-arc and Cordilleran magmatism. Much larger sediment fractions enter the sparse ultrapotassic magmatism that occurs far behind some subcontinental subduction zones and also characteristically follows the subduction related magmatism of collisional orogenies. The remaining subducted sediments finally pass into the o.i.b.-mantle source reservoir. It is well established that, during and immediately after collisional orogeny, the fusion of sialic crust contributes substantially (or even occasionally exclusively) to batholithic magmatism. Nevertheless, the elemental variation in such magmas implies that the role of fractional crystallization in their genesis has tended to be underestimated in recent years. Mantle-derived mafic to ultramafic magmas appear to be directly or indirectly (as heat sources) involved at deep crustal levels in the parentage of most batholithic intermediate and acid magams. These mantle-derived liquids are subduction-related before continental collisions and then change to o.i.b., several million years after subduction ceases. Enhanced subduction of terrigenous sediments during the final stages of ocean closure leads to the large subducted sialic fractions which re-emerge in the ultrapotassic mafic magmas that characteristically appear immediately after a continental collision.
Some of the magmas that were extruded to form the 59 Ma, early, plateauforming, basalt-dominated floods in the British Tertiary Volcanic Province mixed with small amounts of sialic melt during their uprise through the continental crust, while others underwent concomitant fractional crystallization and crustal assimilation. Magma batches of the first group originated in the uppermost of whom have explored the concept of concomitant crystallization of mantle-derived basic magmas in crustal reservoirs and fusion of the surrounding sial [e.g. McBirney, 1979; Taylor, 1980; De Paolo, 1981; O'Hara and Mathews, 1981]. Patchett [1980] suggested that, in igneous provinces where stable long-lived magma reservoirs failed to develop, Bowen's fundamental principle might operate in a different way. asthenosphere and mostly ponded at the Moho, where Early basic dykes and/or sills might crystallize they crystallized fractionally to form a basalt-completely within the continental crust, and the benmoreite suite. The major element compositions latent heat ,released by this process might fuse of members of this series show that they underwent adjacent sial, which would be incorporated negligible further fractional crystallization between the Moho and the surface. Nevertheless, they paused long enough during their uprise to dissolve their high-pressure phenocrysts and, in many cases, to mix with small amounts of acid melt from lower crustal, granulite-facies, Archaean Lewisian leucogneisses. Basalts and tholeiitic andesites of the second group outcrop locally at the base of the lava piles in Mull and Skye. Their major element compositions show that they equilibrated within the upper third of the crust, under conditions approximating to anhydrous 1-atm cotectic equilibria. Most of these lavas are in SW Mull, where ProterozoicMoine metasediments form the uppermost crust. The major and trace element and isotopic compositions of the SW Mull basal lavas show that they assimilated substantial amounts of Moine metasediments progressively as they fractionated within the upper crust.Pb isotope data reveal that, before their final upper crust evolution, some of the SW Mull basal lavas had already mixed with small amounts of lower crust acid melts.
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