GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 2Classification of Igneous Rock

or

How many ways can you skin a rock? Jan. 23, 2009

GOALS OF A ROCK CLASSIFICATION SCHEMEPractical especially for field useDescriptive minimal genetic termsSystematic allows for easy information retrievalHierarchical allows for greater levels of detailNatural defines boundaries that agree with nature

Most 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 MINERALSMelanocraticMafic MineralsLIGHT MINERALSLeucocraticFelsic Minerals

Color Index-CI=% Dark Minerals

Synonymous TermsSynonymousTermsAcidBasicSpinel

A SIMPLE (BUT LIMITING) CLASSIFICATION SCHEME

CLASSIFICATION OF PHANERITIC MAFIC IGNEOUS ROCKS RECOMMENDED BY THE IUGS**International Union of Geological SciencestroctoliteOl gabbrogabbro

PLOTTING 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 IUGS

MAFIC 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 CLASSIFICATION

MODIFIERS 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 TROCTOLITE

Ophitic Ophitic-Subophitic Subophitic Intergranular Why PYROXENE-BASED BULK-ROCK TEXTURES?PlOlCpx

CLASSIFICATION OF INTERMEDIATE AND FELSIC ROCKSRoot RockNameAttach mafic mineral prefixto root rock name for intermediate rocks...

or ferro if uncertain of mafic mineral phase

From Streckeisen (1976)FelsicIntermediate(ferro-)

THE MAFIC TO INTERMEDIATE TRANSITIONTypically 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). ProblemsCant see AN content in hand samplesEven DC felsic rocks are dominated by pyx over hb and bio

Suggested Field Criteria for DC Intermediate MaficDark Mineral Habitprismatic, subprismaticgranular to poikilitic Felsic Mesostasis >5 %

CLASSIFICATION OF VOLCANIC ROCKS RECOMMENDED BY THE IUGSBasalt/Andesite Distinctions

Basalt AndesiteSiO2< 52% > 52%

CI > 35% < 35%Foid MineralsNephelineLeuciteKalsiliteAnalcimeSodalite

CHEMISTRY-BASED TAS CLASSIFICATION OF APHANITIC OR METAMORPHOSED IGNEOUS ROCKS

CIPW NORMATIVE CALCULATIONSPSEUDO-MINERALOGYCalculated from whole rock geochemical analyses;distributes major elements among rock-forming minerals

CLASSIFICATION OF PYROCLASTIC ROCKS

*****************