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. 20, 2016

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 I NTERMEDIATE AND F ELSIC R OCKS International Union of Geological Sciences Foid Minerals Nepheline Leucite Kalsilite Analcime Sodalite

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

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

N ORMALIZING AND P LOTTING M INERAL M ODES 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%

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: Moderately serpentinized, modally layered, moderately foliated, medium-grained, subophitic, biotitic, oxide-bearing TROCTOLITE

P RIMARY P LANAR F EATURES Foliation Alignment of elongate or tabular mineral phases Scale - % aligned within 10  of a common plane non-foliated (<25%) (decussate) poorly foliated (25-50%) moderately foliated (50-75%) well foliated (75-90%) very well foliated (>90%) Layering Type: modal, isomodal, graded modal, grain size, textural, phase, combination Contrast (or demarcation): strong, moderate, weak, subtle Frequency: single layer, rhythmic, intermittent, irregular Scale: centimeter, decimeter, meter, inconsistent, variable Lateral continuity: continuous, discontinuous, lenticular, wispy Other descriptors: wavy, cross-bedded, schlerien, colloform, trough, corrugated, deformed, slumped, convoluted

Igneous Layering Cm-scale Rhythmic Layering Intermittent Graded Layering Wavy Layering Phase Layering PCFO PO

A BSOLUTE AND R ELATIVE G RAIN S IZE T ERMINOLOGY Generalized Scale: Fine (<1 mm) Medium (1-5 mm) Coarse (5-12 mm) Very Coarse/Pegmatitic (>12 mm) Detailed Scale: Very Fine (<0.2 mm) Fine ( mm) Medium Fine ( mm) Medium (1.5-3 mm) Medium Coarse (3-7 mm) Coarse (7-12 mm) Very Coarse (12-30 mm) Pegmatitic (>30 mm) Terms describing Distribution of Grain Size equigranular – generally equal sizes for all granular phases seriate – gradational range in grain size of all granular phases hiatial – bimodal range in grain size of all granular phases porphyritic – bimodal range in grain size of one (or rarely two) primary phase types modifiers based on contrast in grain size– weakly, moderately, strongly alternate terminology – "phase"-phyric (e.g., plagioclase-phyric)

B ULK R OCK T EXTURES OF M AFIC I GNEOUS R OCKS Bulk rock texture for mafic igneous rocks commonly based on pyroxene habit  ophitic – multiple lath-shaped crystals of plagioclase totally enclosed in crystals of pyroxene  subophitic – multiple lath-shaped crystals of plagioclase partially enclosed in crystals of pyroxene  intergranular – generally equigranular euhedral to anhedral primary minerals (need not be just augite and plagioclase), none enclosing the others Textural terms for other primary non-granular phases  poikilitic – one phase completely envelops many other more granular phases (e.g., plagioclase-poikilitic)  subpoikilitic – one phase partially envelops other more granular phases (e.g., olivine-subpoikilitic)

SubophiticIntergranularPrismatic Ophitic Oikocrysts PYROXENE-BASED BULK ROCK TEXTURES in MAFIC INTRUSIVE ROCKS

OPHITIC INTERGRANULAR

M ODAL R OCK C LASSIFICATION FOR G ABBROIC R OCKS Phinney (1972) Severson and Hauck (1990) Davidson (1969) Streckeisen (1976) LeMaitre (2002) Duluth Complex Schemes -Cotectic Troctolite 72%Pl 28%Ol

R ECOMMENDED M ODAL C LASSIFICATION ► it uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and Feox) ► it defines modal boundaries that bracket natural modal populations and cotectic proportions determined from experimental data ► it uses simple mafic mineral ratios (3:1 or 1:1) which are easy to estimate in the field. From Miller, Green, and Severson, 2002

M ODAL G ROUPS From Miller, Green, and Severson, 2002

S TILLWATER C OMPLEX C UMULATE C LASSIFICATION S CHEME

P ROPOSED “C UMULATE ” C LASSIFICATION FOR DC R OCKS (M ILLER, G REEN, AND S EVERSON, 2002) Main Attributes: It applies to rocks that show some igneous foliation or modal layering and therefore solidified under conditions in which mineral phases segregated from their parent magma It lists abbreviations of all minerals composing greater than about 2 modal percent in decreasing order of abundance regardless of their cumulus status It denotes granular (cumulus) mineral phases with upper case letter abbreviations and interstitial (intercumulus) mineral phases with lower case abbreviations Cumulus/Intercumulus Mineral Codes PP*/P/p- plagioclaseF/f- Fe-Ti oxide O/o- olivineA/a- apatite C/c- clinopyroxene (augite)-/b- biotite I/i- inverted pigeonite-/a- amphibole H/h- hypersthene, bronzite-/g- granophyre * used with anorthositic group rocks

E XAMPLES OF C UMULATE R OCK C ODES Cumulate Code Translation of Some Common Rock Types in the Duluth Complex Ophitic augite troctolitePOcf Subophitic augite-bearing oxide troctolitePOFc Olivine gabbroic anorthosite with poikilitic olivine PPoc Ophitic olivine gabbroPcOf Biotitic feldspathic dunite with poikilitic plagioclaseOpb Intergranular apatitic oxide olivine gabbroPCFOA Ophitic biotitic augite leucotroctolitePOcb Possible modifications of the code: - preface with grain size designator (e.g. mPOcf, mcPPcfab) -designate well developed foliation of plagioclase with a bar over the P or PP -designate poikilitic texture with a ^ over the phase abbreviation (e.g. cPPĥ) -designate subpoikilitic texture with a ´ over the phase abbreviation (e.g mfPCó)

PO PcOf O PCFO

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%

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