TERMINOLOGY, NOMENCLATURE, AND TERMINOLOGY, NOMENCLATURE, AND CLASSIFICATION OF KEWEENAWAN CLASSIFICATION OF KEWEENAWAN IGNEOUS ROCKS OF NORTHEASTERN IGNEOUS ROCKS OF NORTHEASTERN

MINNESOTA MINNESOTA

MGS Report of Investigations 58, Chapter 1by

James D. Miller, Jr., John C. Green, and Mark J.Severson

Descriptive TerminologyDescriptive Terminology

Pyroxene-based (augite) bulk textural terminologyPyroxene-based (augite) bulk textural terminology► ophiticophitic – multiple lath-shaped crystals of plagioclase totally – multiple lath-shaped crystals of plagioclase totally

enclosed in crystals of pyroxene enclosed in crystals of pyroxene ► subophiticsubophitic – multiple lath-shaped crystals of plagioclase – multiple lath-shaped crystals of plagioclase

partially enclosed in crystals of pyroxene partially enclosed in crystals of pyroxene ► intergranularintergranular – generally equigranular euhedral to anhedral – generally equigranular euhedral to anhedral

primary minerals (need not be augite and plagioclase), none primary minerals (need not be augite and plagioclase), none enclosing the othersenclosing the others

Terms for other primary non-granular phasesTerms for other primary non-granular phases► poikiliticpoikilitic – one phase completely envelops many other more – one phase completely envelops many other more

granular phases (e.g., plagioclase-poikilitic)granular phases (e.g., plagioclase-poikilitic)► subpoikiliticsubpoikilitic – one phase partially envelops other more – one phase partially envelops other more

granular phases (e.g., olivine-subpoikilitic)granular phases (e.g., olivine-subpoikilitic)

Poikilitic (Ophitic) Subpoikilitic Subhedral Granular

Ameboidal Anhedral Granular Euhedral Granular

Mineral Textures

Absolute and Relative Grain Size Absolute and Relative Grain Size TerminologyTerminology

Generalized scale:Generalized scale:FineFine (<1 mm) (<1 mm) MediumMedium (1-5 mm) (1-5 mm) CoarseCoarse (5-12 (5-12

mm)mm)

Very Coarse/PegmatiticVery Coarse/Pegmatitic (>12 mm) (>12 mm)

Detailed scale: Detailed scale: Very FineVery Fine (<0.2 mm) (<0.2 mm) FineFine (0.2-0.8 mm) (0.2-0.8 mm)

Medium FineMedium Fine (0.8-1.5 mm) (0.8-1.5 mm)

MediumMedium (1.5-3 mm) (1.5-3 mm) Medium CoarseMedium Coarse (3-7 mm) (3-7 mm)

CoarseCoarse (7-12 mm) (7-12 mm) Very CoarseVery Coarse (12-30 mm) (12-30 mm)

PegmatiticPegmatitic (>30 mm) (>30 mm)equigranular – generally equal sizes for all granular phasesseriate – gradational range in grain size of all granular phaseshiatial – bimodal range in grain size of all granular phasesporphyritic – 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)

Planar FeaturesPlanar FeaturesAlignment of elongate or tabular mineral phasesAlignment of elongate or tabular mineral phases(igneous lamination, fluxion structure, (igneous lamination, fluxion structure, foliation)foliation)

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 Layering (not typically observed at the scale of a thin section)

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

Modal Rock Classification Modal Rock Classification SchemesSchemes

Phinney (1972a)Severson and Hauck (1990)

Davidson (1969a)

LeMaitre (1989)Streckeisen (1976)

Elements of a Practical Elements of a Practical Classification Scheme for DC Classification Scheme for DC

Mafic RocksMafic Rocks► it uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and Feox)it uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and Feox)► it defines modal boundaries that bracket natural modal populations and it defines modal boundaries that bracket natural modal populations and

cotectic proportions determined from experimental datacotectic proportions determined from experimental data► it uses simple mafic mineral ratios (3:1 or 1:1) which are easy to it uses simple mafic mineral ratios (3:1 or 1:1) which are easy to

estimate in the field.estimate in the field.

Data from Miller (1986)

Proposed Modal ClassificationProposed Modal Classification

Modal GroupsModal Groups

Cumulate Classification Cumulate Classification Schemes Schemes

Proposed Cumulate Classification for DC Proposed Cumulate Classification for DC rocksrocks

Main Attributes:Main Attributes:► It applies to rocks that show some igneous foliation or modal It applies to rocks that show some igneous foliation or modal

layering and therefore solidified under conditions in which layering and therefore solidified under conditions in which mineral phases segregated from their parent magmamineral phases segregated from their parent magma

► It lists abbreviations of all minerals composing greater than It lists abbreviations of all minerals composing greater than about 2 modal percent in decreasing order of abundance about 2 modal percent in decreasing order of abundance regardless of their cumulus status regardless of their cumulus status

► It denotes granular (cumulus) mineral phases with upper case It denotes granular (cumulus) mineral phases with upper case letter abbreviations and interstitial (intercumulus) mineral letter abbreviations and interstitial (intercumulus) mineral phases with lower case abbreviationsphases with lower case abbreviations

Cumulus/Intercumulus Mineral CodesPP*/P/p - plagioclase F/f - Fe-Ti oxideO/o - olivine A/a - apatiteC/c - clinopyroxene (augite) -/b - biotiteI/i - inverted pigeonite -/a - amphiboleH/h - hypersthene, bronzite -/g - granophyre* used with anorthositic group rocks

Example of Cumulate Rock Example of Cumulate Rock CodesCodes

► Cumulate Code Translation of Some Common Rock Cumulate Code Translation of Some Common Rock Types in the Duluth Complex Types in the Duluth Complex

► Ophitic augite troctoliteOphitic augite troctolitePOcfPOcf► Augite-bearing oxide troctoliteAugite-bearing oxide troctolite POFcPOFc► Olivine gabbroic anorthosite with poikilitic olivine Olivine gabbroic anorthosite with poikilitic olivine

PPocPPoc► Ophitic olivine gabbroOphitic olivine gabbro PcOfPcOf► Biotitic, feldspathic dunite with poikilitic plagioclaseBiotitic, feldspathic dunite with poikilitic plagioclase OpbOpb► Intergranular, apatitic oxide olivine gabbroIntergranular, apatitic oxide olivine gabbro PCFOAPCFOA► Ophitic biotitic augite leucotroctoliteOphitic biotitic augite leucotroctolite POcbPOcb

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)

Classification of Intermediate and Felsic Classification of Intermediate and Felsic RocksRocks

Root RockName

Attach mafic mineral prefixto root rock name for intermediate rocks...

or “ferro” if uncertain of mineral phase

From Streckeisen (1976)

The Mafic to Intermediate The Mafic to Intermediate TransitionTransition► Typically based on An content (> or < AnTypically based on An content (> or < An5050))

► Hyndman (1972) suggests also considering: Hyndman (1972) suggests also considering: rock associations (diorite with more granitoid rocks, gabbro rock associations (diorite with more granitoid rocks, gabbro

with more mafic rocks)with more mafic rocks) mafic mineral assemblage (diorite = hornblende or biotite mafic mineral assemblage (diorite = hornblende or biotite

pyroxene; gabbro = pyroxene pyroxene; gabbro = pyroxene olivine olivine hornblende) hornblende) plagioclase color (diorite whitish or nearly so; gabbro - plagioclase color (diorite whitish or nearly so; gabbro -

greenish gray to gray) greenish gray to gray) ► Blatt and Tracy (1995) suggest using diorite for rock that Blatt and Tracy (1995) suggest using diorite for rock that

contains hornblende over pyroxene and that contain less than contains hornblende over pyroxene and that contain less than 35 modal percent high-temperature mafic minerals (olivine, 35 modal percent high-temperature mafic minerals (olivine, pyroxene).pyroxene).

ProblemsProblemsCan’t “see” AN content in hand samplesCan’t “see” AN content in hand samplesEven DC felsic rocks are dominated by pyx over hb and bioEven DC felsic rocks are dominated by pyx over hb and bio

Suggested Field Criteria for DCSuggested Field Criteria for DC

IntermediateIntermediate MaficMaficMafic mineral habitMafic mineral habit prismatic, subprismaticprismatic, subprismatic granular to granular to poikilitic poikilitic Felsic mesostasisFelsic mesostasis >5 % >5 % <5 % <5 %

Determining QAP modal Determining QAP modal abundances in granophyric abundances in granophyric

rocksrocksProblem: micrographic texture and alteration commonly makes accurate modal estimations difficult to impossible.

Solution: calculate CIPW norm from whole rock analysis and use values for An, Ab, and Or (Problem #2 – what to do with Ab being a component in both alkali feldspar and plagioclase feldspar; Solution – partition based on microprobe analysis of feldspars or estimate 25% of Albite is in Alk Feldspar)

Field-based criteria for distinguishing felsic Field-based criteria for distinguishing felsic from intermediate Keweenawan intrusive from intermediate Keweenawan intrusive

rocksrocks

Lithostratigraphic Classification for Keweenawan Rocks of NE MinnesotaLithostratigraphic Classification for Keweenawan Rocks of NE Minnesota