b'Don Emersons best of Exploration GeophysicsFeatureFigure 9.(a) Generalised plot of mineral composition for the full range of plutonic rock types (after Washington and Adams 1951). (b) Average trends in plagioclase composition (expressed as average and range of anorthite contents (%)), mafic mineral contents and hornblende/biotite ratio in granitoid rocks, showing systematic variation with position in the QAP diagram (after Hyndman, 1972).is too low to precipitate magnetite. The early-crystallising spinelferromagnetic due to copious amounts of cumulus magnetite. phase in these intrusions is chromite. As fractional crystallisationAlthough the unaltered ultramafic cumulates are paramagnetic, proceeds, ferrous iron is increasingly sequestered in silicatesserpentinisation, particularly of olivine-rich layers, frequently and removed from the residual melt, whereas nearly all theproduces secondary magnetite and produces susceptibilities in ferric iron remains in the melt as the magma compositionthe MFM range.evolves along the iron-enrichment trend. Thus the Fe3+/Fe2+ratio steadily increases in the melt until the point is reached whenCalc-alkaline series, typified by orogenic andesites and their magnetite can precipitate. The differentiation trend then turnsplutonic equivalents, and minor related mafic and silicic rocks, towards the alkali apex of the AFM diagram as iron is removedshow a quite different trend, with early depletion in iron and from the melt in magnetite. In layered complexes, therefore, thepronounced silica enrichment. This is thought to reflect more primary ultramafic rocks near the base and the overlying lowerhydrous magmas associated with crust-mantle interactions gabbros are magnetite-free and have susceptibilities in thein a subduction zone, with more oxidised parental magma paramagnetic range. Overlying, more differentiated, gabbros,and early, and continuing, crystallisation of Fe-Ti oxides norites and anorthosites have higher susceptibilities, increasingand hydrous phases, such as hornblende. This leads to a upwards, due to the presence of intercumulus magnetic andpronounced depletion in iron in the more evolved members of the upper ferrogabbros and ferrodiorites are very stronglya calc-alkaline series, whereas fractionated rocks derived from 53 PREVIEW APRIL 2020'