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Classification of Igneous Rocks

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Classification of Igneous Rocks Igneous rock ≠ magma ≠ melt Magma = melt + crystals + dissolved volatiles (gases) Why does the chemistry of an igneous rock usually not correspond to that of the parental magma? • Degassing/volatiles release • Hydrothermal alteration and low-grade metamorphism Vojtěch Janoušek: • Crystal accumulation Diversity and processes modifying composition • Presence of xenocrysts and xenoliths of the igneous rocks • Assimilation of such foreign material • Magma mixing Photo J. Žák Geochemical data Classification of igneous rocks Major elements Qualitative parameters • Expressed in wt. % • Average grainsize of the groundmass: coarse-, medium-, fine-grained, glassy • Concentration > arbitrary value (0.1 or 1 wt. % depending on the authors) • Usually quoted in the order Si, Ti Al, Fe, Mn, Mg, Ca, Na, K and P • Observation of minerals present (occurrence of Qtz implies silica saturation) • Form the main rock-forming minerals • Controlled by mineral stability limits (P–T conditions), simple mass balance Trace elements • Concentration < 0.1 %, usually expressed in parts per million, ppm • Substitute in major phases and/or occur as accessory minerals where they often form essential structural components (zircon ZrSiO4) Volatiles + – • H2O , H2O , F, Cl, CO2 vs. Loss on ignition, LOI • Sometimes are distinguished also minor elements (Ti, Mn, P…) • Some elements could be major in some systems, traces in other (K in mantle) Gill (2010) 1 Classification of igneous rocks Classification of igneous rocks Quantitative parameters Quantitative parameters • Percentages of minerals present (modal composition): • Chemical composition: colour index: ultramafic, mafic, felsic silica contents: ultrabasic, basic, intermediate, acid melanocratic, mesocratic, leucocratic (but relative terms) Total Alkali (Na2O + K2O) – Silica diagram (TAS) normative composition Gill (2010) Gill (2010) Modal vs. normative compositions Modal vs. normative compositions Modal composition Modal composition = real proportions of minerals in vol. % = ‚real‘ proportions of minerals in vol. % Normative composition • Point counting of standard thin sections or polished (stained) slabs CIPW norm (Cross, Iddings, Pirsson and Washington 1902) • Image analysis • proportions of ideal mineral end-members in wt. % • Powder X-ray diffraction • need to be divided by density and recast to 100% to get percentages in vol. % to (some extent) comparable with mode • calculation involves a hierarchical list of rules that tend to be often ambiguous • does not include hydrous minerals, yields phases often not matching modal mineralogy in the studied igneous rocks, especially acidic ones ‚Improved Granite Mesonorm’ (Mielke and Winkler 1979) Numerical techniques (least-squares or linear programming) • Using real (averaged) mineral analyses and recombining them to match – as much as possible – the whole-rock composition 2 Modal (QAPF) classification M < 90 % Ternary plots of plutonic rocks (Streckeisen 1976) Q = quartz A = alkali feldspar P = plagioclase (An >5) F = foids (nepheline, leucite, kalsilite, sodalite, nosean, haüyne, cancrinite, 70 % X, 20 % Y, 10 % Z analcime…) M = mafic minerals (micas, amphibole, olivine, pyroxene, accessories…) Q + A + P + F = 100 Le Maitre ed. (2002) M < 90 % QAPF classification QAPF classification (Streckeisen 1976) (Streckeisen 1976) Remarks 3 granite = syeno- (a) + monzo- (b) 5 tonalite (M > 10) trondhjemite, plagiogranite (M < 10) 9 monzodiorite (An < 50) Gabbroic monzogabbro (An > 50) rocks 10 anorthosite (M < 10) diorite (An < 50) gabbro (An > 50): separate scheme gabbro, troctolite, norite, … 9* quartz monzodiorite (An < 50) quartz monzogabbro (An > 50) 10* quartz anorthosite (M < 10) quartz diorite (An < 50) quartz gabbro (An > 50) Le Maitre ed. (2002) Le Maitre ed. (2002) 3 M > 90 % QAPF classification Field classification (Streckeisen 1976) (Streckeisen 1976) Ultramafic rocks Le Maitre ed. (2002) Le Maitre ed. (2002) Devising full petrographic name TAS classification of volcanic rocks (Le Bas et al. 1986) Root name y = Na2O + K2O x = SiO2 Qualifier(s) Analyses have to be recast to • Type mineral(s) present: 100% anhydrous (volatile-free) basis !!! muscovite–biotite granite (muscovite < biotite) do not list those that correspond to the rock type definition ol = CIPW normative olivine (e.g., diopside in basalt) q = CIPW normative value: 100Q / ( Q or ab an ) • Textural: vesicular, porphyritic, spherulitic, ophitic… • Nature of phenocrysts: plagioclase-phyric andesite olivine-augite-phyric basalt • Chemical features: low-K basalt, peraluminous granite Le Maitre ed. (2002) 4 TAS classification of volcanic rocks TAS classification of volcanic rocks (Le Bas et al. 1986) (Le Bas et al. 1986) Ol- and foid-normative = undersaturated Saturation line Qtz-normative MacDonald (1974) = saturated Subdivision to alkaline/alkali and subalkaline/subalkali series Irvine and Baragar (1971) Le Maitre ed. (2002) http://academic.sun.ac.za/earthSci/honours/modules/igneous_petrology.htm Subdivision of the subalkaline series Subdivision of the calc-alkaline magmas y = K2O A = Na2O + K2O x = SiO2 F = FeOt M = MgO Both in wt. % All in wt. % Definition of tholeiitic and calc-alkaline series Peccerillo and Taylor (1976) Irvine and Baragar (1971) 5 Subdivision of the calc-alkaline magmas Alternatives for weathered samples Alternative to TAS Winchester & Floyd (1977) Le Maitre ed. (2002) modified by Pearce (1996) Alternatives for weathered samples Al vs. (Ca) + Na + K balance Alternative to the Al O A/CNK > 1: Peraluminous rocks ACNK/[.%] 23 mol SiO2–K2O plot • have excess Al over the amount CaO Na22 O K O needed to form feldspars • contain biotite, muscovite, B basalt Al O alumosilicates (kyanite, sillimanite basaltic andesite A/[.%]NK 23 mol or andalusite), cordierite, garnet, BA/A tourmaline, topaz or corundum and andesite Na22 O K O dacite and D/R* Shand (1943) A/CNK < 1 and A/NK < 1: rhyolite* Metaluminous rocks • presence of Ca- and/or alkali-rich * latites and trachytes also phases such as amphiboles and fall in the D/R field Feldspars pyroxenes indicates an Al deficit KAlSi3O8 –1/2K2O : 1/2Al2O3 A/CNK ~ 1: Subaluminous rocks NaAlSi O –1/2NaO : 1/2Al O 3 8 2 2 3 A/NK < 1: Peralkaline rocks CaAl2Si2O8 –1CaO : 1Al2O3 • contain aegerine, riebeckite, arfvedsonite or aenigmatite Hastie et al. (2007) 6 Al vs. (Ca) + Na + K balance Simple recalculations FeOt FeO0.89981 Fe23 O [ wt .%] Niggli (1948) defined simple cationic values. Several of them, si, al, fm, c, alk, k, mg, ti, p, c/fm, and qz are still in use. MgO mg# 100 [ mol .%] FeO MgO MgO Mgmol# 100 [ .%] FeOt MgO Mg# or mg# represent useful index of fractionation for binary plots showing differentiation trends. Multicationic classifications Multicationic classifications Example of calculation Millications Millications Cationic Wt. % MW n (per 100 g C proportions milli n 1000 of rock) MW SiO2 73.60 60.09 x1 1.225 1225 TiO2 0.10 79.90 x1 0.001 1 Al2O3 13.17 101.96 x2 0.258 258 MWα molecular weight Fe2O3 0.99 159.69 x2 0.012 12 number of atoms FeO 1.61 71.85 x1 0.022 22 in the oxide formula MgO 0.06 40.30 x1 0.001 1 nα (e.g., 2 for Na2O, CaO 0.70 56.08 x1 0.012 12 and again 2 for Na2O 3.69 61.98 x2 0.119 119 Al2O3) K2O 5.38 94.20 x2 0.114 114 R1 = 4Si - 11 (Na + K) – 2 (Fe + Ti) = 4900 - 11(233) – 2(12 + 22 + 1) = 2267 R2 = 6 Ca + 2 Mg + Al = 72 + 2 + 258 = 332 De la Roche et al. (1980) De la Roche et al. (1980) Batchelor and Bowden (1985) Batchelor and Bowden (1985) 7 Multicationic classifications Multicationic classifications Label Plutonic rocks Debon and Le Fort (1984) go gabbro, diorite, anorthosite mzgo monzogabbro, monzodiorite mz monzonite s syenite dq qtz diorite, qtz gabbro, qtz anorthosite mzdq qtz monzodiorite, qtz monzogabbro mzq quartz monzonite sq quartz syenite to tonalite, trondhjemite gd granodiorite, granogabbro ad adamellite gr granite P = K–(Na + Ca) proportion of K-feldspar among feldspars De al Roche et al. (1980) Q = Si/3–(K+Na+2Ca/3) quartz contents Multicationic classifications Multicationic classifications Example of calculation Debon and Le Fort (1984) Millications I muscovite > biotite Wt. % MW n (per 100 g II Peraluminous biotite > muscovite domain of rock) biotite (± minor III SiO2 66.95 60 x1 1116 amphibole) TiO2 0.35 80 x1 4 biotite, amphibole, ± IV pyroxene Al2O3 16.16 102 x2 317 t clinopyroxene, ± Fe2O3 2.95 160 x2 37 V Metaluminous amphibole, ± biotite domain MnO 0.10 71 x1 1 unusual mineral MgO0.6840x117 VI associations (carbonatites...) CaO3.9656x171 Na2O 4.27 62 x2 138 K2O2.9794x263 A = Al – (K + Na + 2Ca) Q = Si/3 – (K + Na + 2Ca/3) = 124 peraluminosity P = K – (Na + Ca) = –146 A = Al – (K + Na + 2 Ca) = –26 B = Fe + Mg + Ti B = Fe + Mg + Ti =58 Debon and Le Fort (1984) maficity 8 Processes determining/modifying the Partial melting composition of magmatic rocks Main factors determining the melt chemistry • composition of the source, • Modal and chemical composition of the source • depth ( pressure), • presence/lack of fluid phase and its composition (H O, CO , F, Cl etc.) • Partial melting 2 2 [fluid-present vs. dehydration melting of Amp, Bt, Mu], (+ restite unmixing) • composition of the restite and/or peritectic phases (Amp/Crd/Grt), • Differentiation • mechanism of partial melting (equilibrium/batch vs. fractional; (e.g., fractional crystallization, in the latter process small amounts are continuously drained from the source), crystal accumulation) • degree of partial melting, • Partial melting • Assimilation (or AFC) • temperature, •Segregation • Hybridization Melting can be triggered by:
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