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MGS Report of Investigations 58, Chapter 1 by

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. Descriptive Terminology. Pyroxene-based (augite) bulk textural terminology

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MGS Report of Investigations 58, Chapter 1 by

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  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 (0.2-0.8 mm) Medium Fine (0.8-1.5 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 IntermediateMafic 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

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