1. TERMINOLOGY, NOMENCLATURE, AND CLASSIFICATION OF KEWEENAWAN IGNEOUS ROCKS OF NORTHEASTERN MINNESOTA
MGS Report of Investigations 58, Chapter 1 by
James D. Miller, Jr., John C. Green, and Mark J.Severson

2. Descriptive Terminology
Pyroxene-based (augite) bulk textural terminology
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 augite and plagioclase), none enclosing the others
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)

3. Mineral Textures Poikilitic (Ophitic) Subpoikilitic Subhedral Granular
Ameboidal Anhedral Granular
Euhedral Granular

4. Absolute and Relative Grain Size Terminology
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)
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)

5. Planar Features
Alignment of elongate or tabular mineral phases
(igneous lamination, fluxion structure, 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 (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

6. Modal Rock Classification Schemes
Streckeisen (1976)
LeMaitre (1989)
Phinney (1972a)
Davidson (1969a)
Severson and Hauck (1990)

7. Elements of a Practical Classification Scheme for DC Mafic Rocks
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.
Data from Miller (1986)

8. Proposed Modal Classification

9. Modal Groups

10. Cumulate Classification Schemes

11. Proposed Cumulate Classification for DC rocks
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 - plagioclase F/f - Fe-Ti oxide
O/o - olivine A/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

12. Example of Cumulate Rock Codes
Cumulate Code Translation of Some Common Rock Types in the Duluth Complex
Ophitic augite troctolite POcf
Augite-bearing oxide troctolite POFc
Olivine gabbroic anorthosite with poikilitic olivine PPoc
Ophitic olivine gabbro PcOf
Biotitic, feldspathic dunite with poikilitic plagioclase Opb
Intergranular, apatitic oxide olivine gabbro PCFOA
Ophitic biotitic augite leucotroctolite POcb
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)

13. Classification of Intermediate and Felsic Rocks
From Streckeisen (1976)
Root
Rock
Name
Attach mafic mineral prefix
to root rock name for intermediate rocks...
or “ferro” if uncertain of
mineral phase

14. The Mafic to Intermediate Transition
Typically based on 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).
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
Mafic mineral habit prismatic, subprismatic granular to poikilitic
Felsic mesostasis >5 % <5 %

15. Determining QAP modal abundances in granophyric rocks
Problem: 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)

16. Field-based criteria for distinguishing felsic from intermediate Keweenawan intrusive rocks

17. Lithostratigraphic Classification for Keweenawan Rocks of NE Minnesota