0 Apses Q - modal quartz A - modail alkali feldspar P - modal plagioctese normalized lo 100 Fields AFG - alkali feldspar grange SVG - syejiograniie MZG - monzograuiie GRD-granQdionLe TON - tonalile graniie A p Fig. L2 The QAP classification of the granitoid family after Strcckeiscn (1976). Trondhjemitc (= plagiogranite) (not labelled) is defined as a leLtcotortalire (Barker, 1979), 1970- Mantle Subducted Crust presenr Slab 1930-1970 -I960 1760-1800 Neptunist [sedimentary Fractional Crystgll -notion Anotecne Direct Melts X Merasomofic Magrnafie (granitizafion Plutonist (igneous, fnefamorphic! what is the source, or what arc the sources of granitic magmai? orrlve at granitic compositions by fractional crystallization from bosoits, or ore granitoids direct melts? because of the room problem are granitoids magmotic or metosomotic? are granitic rocks Igneous, sedimentary or metamorphic ? Fig. 1.6 Schematic flow chart to show the evolution of the granite problem over the past 200 years, Philosophical dead-ends h;ive no exits; ideas that are largely discredited as general explanations, but which may still apply in specific cases, have dashed exit lines. Table 1.1 Tripartitc ehcnrticäl classification of granitic rocks The grani toid family * QAP 60% > Quartz > 20% Alkali-feldspar ?(Alkaii-fdd$půr + Plagioclase) = 0-1 Peraluminous Metatuminosis Peralkaline Definition (Shand, 1947) A > CNK** CNK > A >NK+* A < NK** Characteristic minerals aluminosilicates. cordicrite, orthopyroxenc, fayalitic olivine, aegirine, (Chapter 3] garnet, topaz r clinopyroxene, arfvedsonite, riebeckite tourmaline, spinel, cu mm ingtonite, corundum hornblende, cpidote Other common minerals biotite, muscovite biotite, minor muscovite minor biotite Oxide minerals ilmcnite, tapiolite magnetite magnetite Accessory minerals apatite, zircon, monazite apatite, zircon, titanite, apatite, zircon, titanite, allanite allanite, fluorite, cryolite* pyrochlore Other chemical features F/CI > 3 — low CaO, AL>03, H20, Ba, Sr, Eu high Si02> Fe/Mg, Na + KT Zr, Nb, Ta, ZREEs, v i F/Cl < 3 Moles biottte muscovite cordierite andalusite garnet Pera luminous A/CNK>1 pyroxene hornblende biolite Metaluminous A/CNK<1 A>NK aegjrme nebeckite arfvedsonite Peralkahne A/CNK<1 A6 3.52 3.20 5.09 p^o. 0.14 0.17 0.06 Torsi 99.32 99.16 99.52 A/CNK 1-10 0.93 0.86 nk/a 0.67 0.65 1.09 Table 4.4 Summary of the use of gcochcmical data to intcrprcc the origin and evolution of granitoid rocks. The more capitalization in the 'Use' column, the greater the degree of usefulness of the elements EU'mtntf Considerations Use Major clement concentrations Tract element concentrations Tract tltment ratios PROCESSES Source variation in major clement conecniraiioni normally reflects melc-crystal-fluid source differentiation processes and contamination, but effectiveness to reveal information even about different]anon declines as the magma becomes trapped at the low temperature invariant point (Chapter 6); only if granitoids are primary magmas (unlikely) can the bulk compositions yield sonic indirect information about the source region trace clement concentrations (ppm) are a function of their concentration in the source, the degree and style of par rial melting, and all of the subsequent processes of melt-cystal-fluid differentia lion wirh high degrees of partial me lung or' the source region (likely in the case of voluminous granitoids), trace clement ratios in flic melt fraction may be identical to those in the source and will remain so until some differentiation process removes one element relative to the other; identification of exactly which trace clement ratios in the granitoid are still reliable indicators of the source is problcmi.tK Processes Source Table 4,4 C&ntirnted Eiettttrnis Stable isoropic ratios Radiogenic isotypic radios oxygen and sulphur isotopic ratios should reflect the ratios in Ehe source region, but are highly vulnerable to contamination by, and re-equilibration with, the hose rocks no internal process of differentiation, except possibly for Soret diffusion or Jong times of evolution, should arTect the radiogenic isotopic mEios {Sr. NdP Pb)i therefore the ratios should reflect chest; of the source region ^except possibjy Cor small degrees of partial melting not considered appropriate for granitoids); external reaction with wall-rocks (contamination) may disturb isotopk ratios inherited from source Processes Source processes SOURCE Metamorphic fabric Fig. 2.3 Stylized relationship of granitic plutons to bedding and structure in country rocks. (The relationship between bedding and structure is arbitrary). A pre-tectonic pluton cuts bedding and deflects later metamorphic fabric, although marginal areas of the pluton may become deformed. A syn-tectonic pluton is conformable with bedding, and the metamorphic fabric penetrates the entire intrusive body. A post-tectonic pluton cuts both bedding and metamorphic fabric. Table 2.2 field characteristics relevant to the level of emplacement of granitoid intrusions. Fhitons intru intermediate levels show transitional characteristics _ Feature Contact relations with country rocks lJtaton shapes Contact fades of the granitoid Internal structures Textures Regional metamorphic grade of country rocks Thermal aureole Mig ma cites Other possible diagnostic (?) characteristics Shallow iiitnntom predominantly sharp and discordant discrete isotropic to mildly anisotropic plutons may be finer-grained internally controntd; structures unrelated to those in country rocks missive, may be porphyrinic, granophyric gtccnschist, lower anrphibolite prominent in rocks of suitabfe composition, e.g. pclites local; restricted to contacts miarolitic cavities; pegmatite dykes; hydcothcrmal alteration; granophyric textures; roof pensiants; breccia dykes; cogencric volcanic rocks nearby; abundant country rock xenoliths Dcep-irafcd intrusions predominantly diffuse and concordant domes, conformable sheets no chilled maigins externally imposed; structures similar to those in country rock foliated* aphyric to augen gneisses upper amphibolitcf granufitc obscure in most cases regional (see Fig. 15-12 in Compton^ 1983) DIAPIRISM CAULDRON SUBSIDENCE STOPING COUNTRY ROCKS MOVE DOWN (vertical cross-sections) 1 y DOMING COUNTRY ROCKS MOVE UP (vertical cross-sections) Models of ascent and emplacement 55 Fig- 2.14 Examples of experimentally modelled magma ascent (a) by diapirism and (b) by fracture exploitation (after Rambcre, I9&1) OTOGENIC ANDROGENIC Oteanic Island arc hi 6x«pr**e.S«»0'isn Wards-, Owtinerrt nvarjgin arc. rctwtai basin Ra^pes 3aSMS«i <* Catted Votoiic and wanitssbc \simv3£'z fcwaSs GaJtooo, V-tyP* qy«tt dtorfca in ■ni^'B aits Au-beenng pon^iyry-C'J PbjmJ mating rt [rartr« S^iM*nj-fliirimaJ grafts ring Partial .-ra&$o< rwititf-fS^^tiw) ontarpiaw: pits. er^aaaJ ccrtriu- 'On WtM cCHU'-WTiJl:* H« Cfoeru-dicKfri magma E;b»i: fiartung MtfoSS* Ooi-ns T1 S^war*"***1" p\«K*>asa-aga & 'any* oi 'eotauic aKJd*fli* iiros>-3' Corf |ft£j0Mi & - laden-.Twh Sadrnwrtation.* IWMhiuii 4 Oti«i IsCfcnJ ti-HeanCTi UK). Rage**, bw-prassun mataircfDJieari Wp-aiw. f»» Giana* S*f*J. a«galy ^-typ* sntrs« snsrfa. in adrv* s-M*1 zor*s Sri i» V) f*£rn, votfl 4 rapbresowlS 'riot. tonaln'tmagma j*ts hflii v-fc fto n** £ ^ cms Alki1! =u*fs- as ^^W«fi una R*otj srtf-pegntairie* Encra^Ks*: w qos»-os«g«vut*ing P^-.iaf r*f&4 d oti nans*, or arthyo«Li£ bul cendtiort tnt-sh rr*B3T!u ad d#pth- wUi SiiKfjCe'7. snfeTjy - tranai*r-of heat of ^acHganJc tueI. pfes Mfe are 19.1 The grarrtic rocks in rhe.r contrast tedon-o niches. After Pit*er. W.S. 0987): see also &ew, D.A. (1992).