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Mafic and Ultramafic Igneous Rocks
Definitions, Descriptions and Compositions
21
Geology
Undergraduate 4
04/26/2012

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Term
Mafic Rocks
Definition
Plagioclase felspar >2/3 total feldspar
Calcic plagioclase An50-100
Potash felspar <10% total feldspar
Quartz <10%; Feldspathoids <10%
Chiefly pyroxene, olivine
Saturated and Unsaturated
CI usually >40
Volcanic: Gabbro, Norite, Troctolite, Anorthosite, Alkaline Gabbro, Diabase
Plutonic: Tholeiitic Basalts and Diabases, Alkali Olivine Basalts, Hawaiite, Mugearite
Term
Ultramafic Rocks
Definition
Little or no Feldspar (Plagioclase 0-10%)
Chiefly Pyroxene and/or Olivine
Chiefly Serpentine, Iron Oxides, Hornblende, Biotite
Undersaturated
CI 90-100
Plutonic: Peridotite, Pyroxenite, Hornblendite
Term
Basalt
Definition
Volcanic, holocrystalline-hypocrystalline, rarely vitreous, with aphanitic groundmass; plagioclase (labradorite) 40-60%; mafics (clinopyroxene, olivine) 55-35%
Term
Tholeiitic Basalt
Definition
Plagioclase and pyroxene principal constituents; Mg olivine present as phenocrysts; groundmass pyroxenes commonly pigeonite, or subcalcic augite, more rarely hypersthene. Interstitial siliceous glass or quartz may be present. Oceanic varities have >2.5% TiO2; continental varieties =1% TiO2
Term
Olivine Tholeiite
Definition
Plagioclase and pyroxene principal constituents. Pyroxenes are augite and hypersthene or pigeonite. Considerable olivine may be present, which may have reaction rims of pyroxene. Oceanic varieties have >2.5% TiO2; continental =1% TiO2
Term
High-Al Basalt
Definition
Plagioclase and pyroxene principal constituents. Pyroxene is augite, rarely pigeonite or hypersthene. Typically non-porphyritic; olivine common, with or without reaction rim of pyroxene. Al2O3 17%, TiO2 = 1%; intermediate characterristics between tholeiites and alkali types
Term
Alkali-Olivine Basalt
Definition
Abundance of Mg olivine, commonly in two generations, phenocrysts and groundmass; one pyroxene - titanaugite (slight purple pleochroism). Feldspathoids, alkali feldspar, phlogopite or kaersutite may be present; no reaction rims of pyroxene on olivine. Total alkali content higher and silica lower than types above.
Term
Picrite Basalt
(Oceanite)
Definition
Basaltic rocks rich in olivine phenocrysts.
Term
Ankaramite
Definition
Basaltic rich in both augite and olivine phenocrysts.
Term
Spilite
Definition
Basic rock containing plagioclase of highly sodic composition (albite); produced by metasomatic alteration of basalts; mafic minerals generally partly or completely altered to serpentine, chlorite, epidote.
Term
Hawaiite
Definition
Fine-grained non-porphyritic or sparsely porphyritic rock; when present, phenocrysts include labradorite, titanaugite, and olivine; groundmass dominated by lathes of andesine with interspersed granules of augite, olivine and magnetite.
Term
Mugearite
Definition
Plagioclase most abundant constituent (65 - 70%), and dominates texture by sub-parallel alignment. Microphenocrysts of plagioclase (when present), and larger groundmass plagioclase have cores of albite-twinned sodic andesine with clear rims of Ca-anothoclase; numerous microphencrysts of olivine; Ca-pyroxene is usually inconspicuous; magnetite plentiful.
Term
Plutonic gabbro
Definition
Intrusive, holocrystalline varying from coarse to medium grained; plagioclase plus augite; olivine or hornblende prominent in correspondingly designated varities.
Term
Diabase
Definition
Intrusive, normally holocrystalline usually containing mafics (various pyroxenes, less common olivine) 65-25%. plagioclase 30-70%. Fine- to medium-grained and even-grained, often ophitic. Occurs as shallow intrusives such as sills, dikes, laccoliths and plugs.
Term
Norite
Definition
Intrusive, holocrystalline, plagioclase plus hypersthene, typically with subordinate augite; olivine present in some rocks.
Term
Troctolite
Definition
Intrusive, holocrystalline, predominatly plagioclase plus olivine.
Term
Anorthosite
Definition
Intrusive, holocrystalline, plagioclase the dominant or only mineral, minor hypersthene or augite common.
Term
Peridotite (Varies based on type of mafic mineral)
Definition
Holocrystalline, contains mafics (olivine, pyroxene, hornblende) 85-95%; ores (magnetite, ilmenite, chromite) 10-30%; calcic plagioclase <5%.
Term
Varities of Peridotite
Definition

Mafic Mineral                        Rock

Olivine                                                    Dunite

Olivine + Orthopyroxene                                    Harzburgite

Olivine + Clinopyroxene (Augite)                           Weherlite

Olivine + Orthopyroxene + Clinopyroxene                    Lherzolite

Orthopyroxene + Clinopyroxene                              Websterite

Pyroxene (Orthopyroxene, augite +/- Olivine +/- Hornblende)Pyroxenite

Hornblende +/- Olivine +/- Pyroxene                        Hornblendite

Garnet + Enstatite + Diopside                              Garnet Peridotite

Olivine + Pyroxene + Plagioclase (subordinate)        Plagioclase-Peridotite

Term
Kimberlite
Definition
Holocrystalline, Mg olivine, phlogopite, Cr-diopside, enstatite, Mg garnet, and ilmenite in groundmass of serpentinized olivine and calcite.
Term
Komatiite
Definition
Volcanic rock of peridotitic or closely related composition; devitrified glass a characteristic constituent; spinifex texture (produced by rapid quenching from high temperature); two varities: peridotitic komatiites - rich in olivine phenocrysts and chrome spinel, in a groundmass of finely crystallized pyroxene; Pyroxenitic komatiites - olivine less abundant and subcalcic augite more plentiful; neither rock contains plagioclase; essentially restricted to the Pre-Cambrian.
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