Geol 2312 Igneous and Metamorphic PetrologyGoals of a Rock Classification Scheme“Essential” MineralsA SIMPLE (but limiting) CLASSIFICATION SCHEMEClassification of Phaneritic Mafic Igneous Rocks Recommended by the IUGS*Plotting on Ternary DiagramsClassification of Ultramafic Igneous Rocks Recommended by IUGSMafic Rock Classification Schemes Applied to the Duluth ComplexElements of a Practical Classification Scheme for Duluth Complex Mafic RocksPreferred Modal ClassificationModifiers – For a Complete Rock DescriptionSlide 14Classification of Intermediate and Felsic RocksThe Mafic to Intermediate TransitionClassification of Volcanic Rocks Recommended by the IUGSChemistry-Based TAS Classification of Aphanitic or Metamorphosed Igneous RocksCIPW Normative Calculations Pseudo-mineralogyClassification of Pyroclastic RocksGEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 2Classification of Igneous Rock orHow many ways can you skin a rock? Jan. 23, 2009GOALS OF A ROCK CLASSIFICATION SCHEMEPractical – especially for field useDescriptive – minimal genetic termsSystematic – allows for easy information retrievalHierarchical – allows for greater levels of detailNatural – defines boundaries that agree with natureMost common schemes based on mode (vol. %) of “essential” primary minerals ...(When observation of primary minerals is not possible (e.g., too fine, altered) chemistry is commonly used)...with modification by texture and structural features and occurrence of “accessory” and minor minerals“ESSENTIAL” MINERALSDARK MINERALS•Melanocratic•Mafic MineralsLIGHT MINERALS•Leucocratic•Felsic MineralsColor Index-CI=% Dark MineralsSynonymous –TermsSynonymousTermsAcidBasicSpinelA SIMPLE (BUT LIMITING) CLASSIFICATION SCHEMECLASSIFICATION OF PHANERITIC MAFIC IGNEOUS ROCKS RECOMMENDED BY THE IUGS**International Union of Geological SciencestroctoliteOl gabbrogabbroPLOTTING ON TERNARY DIAGRAMSRock AMineral X – 59.5%Mineral Y – 17%Mineral Z – 8.5%Mineral A – 12%Mineral B – 3%Normalization to XYZ onlyX+Y+Z = 85%Xn = 59.5/85 = 70%Yn = 17/85 = 20%Zn = 8.5/85 =10%CLASSIFICATION OF ULTRAMAFIC IGNEOUS ROCKS RECOMMENDED BY IUGSOlivineClinopyroxeneOrthopyroxeneLherzoliteHarzburgiteWehrliteWebsteriteOrthopyroxeniteClinopyroxeniteOlivine WebsteritePeridotitesPyroxenites90401010DuniteMAFIC ROCK CLASSIFICATION SCHEMES APPLIED TO THE DULUTH COMPLEXPhinney (1972a)Severson and Hauck (1990)Davidson (1969a)LeMaitre (1989)Streckeisen (1976)ELEMENTS OF A PRACTICAL CLASSIFICATION SCHEME FOR DULUTH COMPLEX MAFIC ROCKS- uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and Feox)- defines modal boundaries that bracket natural modal populations and cotectic proportions determined from experimental data- uses simple mafic mineral ratios (3:1 or 1:1) which are easy to estimate in the field. From Miller (1986)PREFERRED MODAL CLASSIFICATIONMODIFIERS – FOR A COMPLETE ROCK DESCRIPTIONAlterationFoliation/LayeringAbsolute Grain SizeBulk Rock Texture –based on pyroxene habitAccessory/Minor MineralsFor example:Serpentinized, modally layered, medium-grained, subophitic, biotitic, oxide-bearing TROCTOLITEOphitic Ophitic-Subophitic Subophitic Intergranular Why PYROXENE-BASED BULK-ROCK TEXTURES?PlOlCpxCLASSIFICATION OF INTERMEDIATE AND FELSIC ROCKSRoot RockNameAttach mafic mineral prefixto root rock name for intermediate rocks...or “ferro” if uncertain of mafic mineral phaseFrom Streckeisen (1976)FelsicIntermediate(“ferro-”)THE MAFIC TO INTERMEDIATE TRANSITIONTypically based on average An content (> or < An50)Hyndman (1972) suggests also considering: rock associations (diorite with more granitoid rocks, gabbro with more mafic rocks)mafic mineral assemblage (diorite = hornblende or biotite  pyroxene; gabbro = pyroxene  olivine  hornblende)plagioclase color (diorite whitish or nearly so; gabbro - greenish gray to gray) Blatt and Tracy (1995) suggest using diorite for rock that contains hornblende over pyroxene and that contain less than 35 modal percent high-temperature mafic minerals (olivine, pyroxene). ProblemsCan’t “see” AN content in hand samplesEven DC felsic rocks are dominated by pyx over hb and bioSuggested Field Criteria for DC Intermediate MaficDark Mineral Habit prismatic, subprismatic granular to poikilitic Felsic Mesostasis >5 % <5 %CLASSIFICATION OF VOLCANIC ROCKS RECOMMENDED BY THE IUGS(foid)-bearing Trachyte(foid)-bearing Latite(foid)-bearing Andesite/Basalt(Foid)ites10606035651020206060FAPQRhyolite DaciteTrachyte Latite Andesite/BasaltPhonoliteTephriteBasalt/Andesite DistinctionsBasalt AndesiteSiO2< 52% > 52%CI > 35% < 35%Foid MineralsNephelineLeuciteKalsiliteAnalcimeSodaliteCHEMISTRY-BASED TAS CLASSIFICATION OF APHANITIC OR METAMORPHOSED IGNEOUS ROCKSCIPW NORMATIVE CALCULATIONSPSEUDO-MINERALOGYCalculated from whole rock geochemical analyses;distributes major elements among rock-forming mineralsCLASSIFICATION OF PYROCLASTIC