b'Don Emersons best of Exploration GeophysicsFeaturesusceptibility with increasing SiO 2 . This decrease becomesEmplacement depthvery rapid above ~74 wt % SiO 2 . M-types from the southwestPecherskiy (1965) noted a strong correlation between shallow Pacific have the highest susceptibilities in this extensiveemplacement depth and occurrence of magnetite for a wide collection of granitoid rocks from eastern Australia andvariety of granitoids in northeastern Russia. Figure 15 shows Oceania, unless they are altered. Both carbonate and pyrite- the percentage of moderately and strongly ferromagnetic pyrrhotite-chalcopyrite alteration are magnetite-destructivegranitoids versus estimated depth of emplacement for a in the M-type rocks. For silica contents of 70% to 74% bylarge number of plutons. There is a systematic increase weight, A-type granitoids exhibit a bimodal distribution ofin ferromagnetic proportion with decreasing depth of susceptibilities, similar to those of oxidised and reducedemplacement. The ferromagnetic proportion rises to 70% for I-types with equivalent SiO 2 . Above ~74 wt% SiO 2thesubvolcanic/epizonal granitoids. This interesting observation A-types exhibit a broad unimodal susceptibility distribution,does not appear to have attracted much attention, but other reflecting a rapid decrease from WFM to PM levels as SiO 2 studies lend some indirect support. Czamanske, Ishihara, and increases from 74 wt% to 78 wt%. Atkin (1981) explain a similar correlation in Japan by invoking Average susceptibilities for M-, I- and S-type granitoids areonset of second boiling in the residual melt in epizonal plutons. 40,000106, 8900106 and 410106 SI (3200 G/Oe, 700 G/ Dissociation of water and preferential diffusion of hydrogen Oe and 30 G/Oe) respectively. The corresponding medians areout of the pluton into fractured country rock is the oxidation 32,500106, 5600106 and 270106SI (2600 G/Oe, 450process that is postulated to produce the high magnetite G/Oe and 20 G/Oe). The median susceptibility of the limitedcontents of these plutons. However, Candela (1986) has set of A-type granitoids studied by Blevin (1994) is ~1000106shown that dissociation of water can only be an important SI (~80 G/Oe). For the ferromagnetic subpopulation of theoxidising process for iron-poor (1 wt % FeOT) granitoids. This slightly less felsic (70 wt % to 74 wt % SiO 2 ) varieties of A-type,mechanism may operate in Climax-type Mo porphyries, which the median susceptibility is an order of magnitude greater. are very felsic, but interaction with oxygenated meteoric waters Magnetite contents of granitoids show distinct provinciality,is a more probable explanation for the relatively oxidised nature along with other mineralogical and chemical characteristics,of at least some epizonal granitoids.reflecting distinctive compositions of lower crustal sourceThere is also a general correlation between the source regions (Chappell, White, and Hine 1988; Blevin 1994). Forrock, depth of generation and depth of emplacement of example, in most basement terranes I-type granitoids aregranitoids, which probably explains much of the empirical relatively oxidised magnetite-series rocks. In the Melbournetrend shown in Figure 15. Deep-seated, high temperature, Basement terrane, however, the I-type granitoids are reducedanhydrous magmas rise to shallow crustal levels, whereas and belong to the ilmenite-series. The infracrustal protolithlower temperature, hydrous magmas (produced by partial from which these rocks have been derived is therefore inferredmelting of muscovite-rich pelitic metasediments, for to be more reduced than elsewhere in the Lachlan Fold Belt.example) do not rise very far from their source regions, Granitoids belonging to individual suites, which are derivedproducing catazonal granitoids. The former type of magma is from fairly homogeneous source rocks, exhibit a systematicmore likely to produce magnetite-series granitoids, whereas correlation between magnetic susceptibility and compositionthe latter generally produces ilmenite-series granitoids, for that is much better defined than global relationships betweenreasons already explained.these variables.Tectonic settingOverall, mantle-derived granitoids, I-types derived from mafic crustal underplates and second-generation I-types derivedReferring to Maniar and Piccolis (1989) classification, most from oxidised I-type source rocks are magnetite series, whereasisland arc and oceanic plagiogranites, and more mafic I-types derived from reduced igneous rocks are ilmenite-series.continental arc granitoids, are ferromagnetic. Nearly all A-types resemble felsic I-types and have subequal magnetite- continental collision and post-orogenic granitoids are series and ilmenite-series populations. Most S-types are reduced ilmenite series granitoids, probably reflecting presence of carbon in their lower to middle crustal source material.LithologyThe overall proportion of ferromagnetic rocks within a given geological province or within a particular igneous rock series decreases from gabbro through to granite. This trend is apparent from Figure 13(a), which combines data from a wide range of areas and rock series, but is more clearly expressed within particular provinces or rock series. Mafic to felsic and intermediate to felsic associations are much more likely to be magnetite-series throughout, than compositionally restricted felsic associations. Alkaline intrusive rocks are often magnetite-series, with the exception of extreme compositions, such as peralkaline granites and agpaitic (peralkaline, undersaturated) nepheline syenites. In tholeiitic layered complexes, less evolved lower gabbros are paramagnetic to weakly ferromagnetic, whereas sufficiently evolved upper ferrogabbros and ferrodiorites, and associatedFigure 15.Proportion of ferromagnetic granitoids versus estimated granophyres, are usually strongly ferromagnetic. emplacement depth for granitoids from NE Russia (Pecherskiy 1965).59 PREVIEW APRIL 2020'