An assessment of the relative roles of crust and mantle in magma genesis: an elemental approach

Abstract

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.

Geochemistry of igneous rocks of Shimane Peninsula, formed within a Miocene back-arc rifting zone at the Japan Sea margin.

“…Moreover, the degree of incom patibility of each of the IE3 elements can be fur ther compared using similar reasoning (Figs. 6k n), resulting in the order of the incompatibility of the elements; This order of the IEs is almost consistent with those previously proposed for basaltic magmas by Wood et al (1979b), Thompson et al (1984) and Sun (1980).…”

Petrologia y Estructura del Complejo igneo-Metamorfico Alumine, Provincia de Neuquen, Argentina

“…8), comportamiento característico de las rocas de arco magmático asociadas a subducción (Thompson et al, 1984). Además, su diseño es similar al que muestra el conjunto de tonalitas y granodioritas con hornblenda y biotita del Batolito Patagónico y rocas graníticas del Cinturón Subcordillerano del noroeste Patagónico (Rapela et al, 2005).…”

Basaltos continentais do Cretáceo Inferior da bacia de Campos, SE do Brasil: compilação de dados e petrogênese

“…(Hergt et al, 1991;Peate, et al 1992) Figura 5 -Diagramas bivariantes mostrando a relação entre MgO e SiO2,TiO2,Al2O3 e FeOt (a,b,c,d) e MgO e Sr, Zr, Y, Nb, Cr e Ni (e, f, g, h, i, j Estas equações incluem os seguintes parâ-metros: (1) Composição do contaminante (C a para os elementos traços e ε a para razões isotópicas); (2) Composição do magma parental não contaminado (C O para os elementos traços e ε O para razões isotópicas); (3) Fração de massa do líquido residual (F); (4) O coefi- (Thompson et al 1984;à exceção do K, P e Rb, que são valores de Sun 1980, e Ba = 3,85 ppm de Hawkesworth et al 1984; b) Diagrama de elementos terras raras normalizados para condrito Nakamura (1974). (Rollinson, 1993).…”