G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 2 Classification of Igneous Rock or How many ways can you skin a rock? Jan. 23, 2009

G OALS OF A R OCK C LASSIFICATION S CHEME 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 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

“E SSENTIAL ” M INERALS DARK MINERALS Melanocratic Mafic Minerals LIGHT MINERALS Leucocratic Felsic Minerals Color Index-CI =% Dark Minerals Synonymous – Terms Synonymous Terms Acid Basic Spinel

A SIMPLE ( BUT LIMITING ) CLASSIFICATION SCHEME

C LASSIFICATION OF P HANERITIC M AFIC I GNEOUS R OCKS R ECOMMENDED BY THE IUGS* *International Union of Geological Sciences troctolite Ol gabbro gabbro

P LOTTING ON T ERNARY D IAGRAMS Rock A Mineral X – 59.5% Mineral Y – 17% Mineral Z – 8.5% Mineral A – 12% Mineral B – 3% Normalization to XYZ only X+Y+Z = 85% Xn = 59.5/85 = 70% Yn = 17/85 = 20% Zn = 8.5/85 =10%

C LASSIFICATION OF U LTRAMAFIC I GNEOUS R OCKS R ECOMMENDED BY IUGS Olivine Clinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites Dunite

M AFIC R OCK C LASSIFICATION S CHEMES A PPLIED TO THE D ULUTH C OMPLEX Phinney (1972a) Severson and Hauck (1990) Davidson (1969a) LeMaitre (1989)Streckeisen (1976)

E LEMENTS OF A P RACTICAL C LASSIFICATION S CHEME FOR D ULUTH C OMPLEX M AFIC R OCKS - 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)

P REFERRED M ODAL C LASSIFICATION

M ODIFIERS – F OR A C OMPLETE R OCK D ESCRIPTION Alteration Foliation/Layering Absolute Grain Size Bulk Rock Texture –based on pyroxene habit Accessory/Minor Minerals For example: Serpentinized, modally layered, medium-grained, subophitic, biotitic, oxide-bearing TROCTOLITE

Ophitic Ophitic-Subophitic SubophiticIntergranular Why PYROXENE-BASED BULK-ROCK TEXTURES? Pl Ol Cpx

C LASSIFICATION OF I NTERMEDIATE AND F ELSIC R OCKS Root Rock Name Attach mafic mineral prefix to root rock name for intermediate rocks... or “ferro” if uncertain of mafic mineral phase From Streckeisen (1976) Felsic Intermediate (“ferro-”)

T HE M AFIC TO I NTERMEDIATE T RANSITION Typically based on average An content (> or < An 50 ) 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). Problems Can’t “see” AN content in hand samples Even DC felsic rocks are dominated by pyx over hb and bio Suggested Field Criteria for DC Intermediate Mafic Dark Mineral Habitprismatic, subprismaticgranular to poikilitic Felsic Mesostasis >5 % <5 %

C LASSIFICATION OF V OLCANIC R OCKS R ECOMMENDED BY THE IUGS Basalt/Andesite Distinctions Basalt Andesite SiO 2 52% CI > 35% < 35% Foid Minerals Nepheline Leucite Kalsilite Analcime Sodalite

C HEMISTRY -B ASED TAS C LASSIFICATION OF A PHANITIC OR M ETAMORPHOSED I GNEOUS R OCKS

CIPW N ORMATIVE C ALCULATIONS P SEUDO - MINERALOGY Calculated from whole rock geochemical analyses; distributes major elements among rock-forming minerals

C LASSIFICATION OF P YROCLASTIC R OCKS