b'Don Emersons best of Exploration GeophysicsFeatureFigure 8.IUGS classification of plutonic rocks, based on the QAPF double triangle (Le Bas and Streckeisen, 1991). Q = quartz, A = alkali feldspar, P = plagioclase, F = feldspathoid (foid), M = mafic minerals, BI = biotite, HB = hornblende. Rock qualifiers are specified in terms of modal percentages. Ultramafic rocks, for which mafic minerals constitute 90% to 100% of the rock, are classified separately.potash in feldspars consumes six molecules of silica. whereasalkali-lime index into one of four categories: alkalic (ALI 51), CaO only consumes two. Thus alkaline rocks, with relatively highalkali-calcic (ALI = 51-56), calc-alkalic or calc alkaline (ALI = 56-61) Na and/or K for their silica content, have no excess silica to formand calcic (ALI 61), as shown in Figure 11(a).a free silica phase and are undersaturated. The thermodynamic parameter, silica activity, is strongly dependent on the alkaliExamples of igneous rock series representing each of the ALI content for this reason. As an example, alkali basalts are silicacategories include: tholeiitic basalts (calcic); basalt andesite-undersaturated and are characterised chemically by normativerhyolite series (calc-alkalic); alkali basalt-phonolite series (alkali nepheline. Undersaturated magnesian olivine is relativelycalcic), and alkali syenite complexes (alkalic). The ALI provides abundant in these basalts, whereas tholeiitic basalts are silicaa measure of the maturity of volcanic arcs, with igneous rock saturated, with hypersthene in the norm. Olivine, if present, isseries tending to evolve from early mantle derived, calcic in a reaction relationship to ferromagnesian pyroxene and wasmagmatism, through calc-alkalic orogenic magmatism, therefore out of equilibrium with the tholeiitic magma. reflecting crust-mantle interactions, to post orogenic alkali-calcic or anorogenic alkalic magmatism.An important geochemical classification of igneous rock series is based upon Peacocks (1931) alkali-lime index (ALI), which isFigure 11(b) shows a major difference in the behaviour of iron a measure of the relative alkalinity of a rock series derived byduring differentiation of tholeiitic and calc-alkaline magmas. igneous differentiation from a parental magma. With increasingOn a ternary plot of MgO, total iron and alkalis (AFM diagram) differentiation, accompanied by increasing silica content, CaOtholeiitic magmas show a pronounced initial iron enrichment decreases while Na 2 O and K 2 O increase. There is a value of silicatrend, reflecting early crystallisation of Mg rich olivine and content, therefore, where the trend of CaO plotted against SiO 2 pyroxenes. This trend is typical of many layered mafic complexes intersects the trend of Na 2 O + K 2 O versus SiO 2 . This SiO 2 value (in(e.g. the Skaergaard, Stillwater and Bushveld Complexes), for weight per cent) is the alkali-lime index, and is lower for morewhich the parental mantle-derived magma is anhydrous and alkaline rock series. Rock series are classified on the basis of theirrelatively reduced. The initial oxygen fugacity in such magmas APRIL 2020 PREVIEW 52'