b'Don Emersons best of Exploration GeophysicsFeatureFigure 10.(a) QAP fields and differentiation trends for seven distinctive plutonic rock series. (b) QAP fields of peralkaline granites, plagiogranite and T-granitoids (tonalite/trondhjemite). (c) Spatially related plutonic rock series show clear mineralogical and chemical correlations with tectonic environment and relative time of emplacement e.g. evolution of calc-alkaline series in orogenic belts from the oldest (low-K tonalitic series) through the medium-K granodioritic series to the youngest (high-K monzonitic series). (d) Fields of the QAP plot typically occupied by I-, S- and A-type granitoids (after Bowden et al. 1984).tholeiitic magma are relatively iron-rich. While calc-alkalineclassification based on source rock has been extended to volcanics are subduction-related, calk alkaline granitoids areinclude M-type (mantle-derived) granitoids and A-type not necessarily directly associated with subduction, but are(anorogenic, alkaline, anhydrous and, somewhat cynically, often derived from partial melting of calc-alkaline source rocksambiguous) granitoids, with distinctive characteristics. produced during an earlier tectonic cycle. Tholeiitic magmasA-type granitoids are inferred to be derived by partial melting are associated with a variety of tectonic settings. These mainly,of F and/or Cl-enriched dry granulitic residue remaining in the but not always, correspond to tensional regimes and include:lower crust after earlier extraction of an orogenic granitic melt mid-ocean ridges; mantle plume-related intraplate oceanic(Whalen, Currie, and Chappell 1987).islands; and anorogenic continental settings, includingSelected characteristic features of these four granitoid types flood basalts, major dolerite dyke or sill swarms and layeredcan be drawn from Pitcher (1983) and Bowden et al. (1984). gabbroic complexes. They include:Source rock classification of granitoids I-type: metaluminous; calc-alkaline to alkali-calcic, relatively Chappell and White (1974) recognised two categories ofquartz-poor monzogranites, granodiorites and tonalites; 53% calc-alkaline granitoids with very distinctive mineralogical,to 76% SiO 2 ; high Na/K, high Ca for mafic varieties; hornblende-chemical and geological features, which were interpretedbearing (except most felsic members).as reflecting different source rocks. S-type granitoids areS-type: strongly peraluminous; alkali-calcic to calc alkaline, derived from partial melting of (meta)sedimentary rocks, andrelatively quartz-rich monzogranites, granodiorites and I-type granitoids from igneous source material. S may alsotonalites; 65% to 74% SiO 2 ; low Na/K, Ca and Sr; with stand for Supracrustal and I may represent Infracrustal.peraluminous minerals (muscovite, cordierite, garnet or S-type granitoids are characterised by metasedimentaryandalusite); often biotite-rich.inclusions (microgranitoid enclaves), whereas I-types contain hornblende-rich, mafic inclusions of igneous appearance.A-type: peralkaline to metaluminous; alkalic to alkali calcic Chappell and White interpret these inclusions as restite,syenogranites, alkali granites and quartz syenites; mostly 70% residual source material. Linear inter-element variationto 78% SiO 2 ; high Na + K, Fe/Mg, F + Cl and low Ca, Sr; accessory trends are regarded as due to restite unmixing. Alternativeminerals such as fayalite, hedenbergite, ferrohastingsite, interpretations involving magma mixing have been suggested,annite, fluorite, sodic pyroxenes, perthitic or rapakivi-textured but are not relevant to the present topic. This first orderfeldspars.APRIL 2020 PREVIEW 54'