Classification of igneous rocks


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Classification of Igneous rocks

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Classification of igneous rocks

  1. 1. Classification of igneous rocksPhaneritic: crystals visible with naked eye Plutonic or intrusive rocks PhaneriticAphanitic: crystal too small for naked eye Volcanic or extrusive rocks Aphanitic
  2. 2. Porphyritic: two different,dominant grain sizesLarge Crystals = phenocrysts;Small Crystals = groundmass PorphyriticFragmental: composed ofdisagregated igneous materialPyroclastic rocks Pyroclastic
  3. 3. PegmatiticPegmatitic: very large xtals (cm to 10s of cm); i.e.,slowly cooledForms veins or layers within plutonic body GlassyGlassy: non-crystalline; cools very fast (e.g.,obsidian) Volcanic rocksVesicular: Vesicular vesicles (holes, pores, cavities) form asgases expand Volcanic rocks
  4. 4. Classification of Igneous Rocks Pyroclastic rocksClassification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975)Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size ofthe material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.
  5. 5. Compositional terms for igneous rocksFelsic: feldspar + silica ~55-70% silica, K-feldspar > 1/3 of feldspars present light-colored silicate minerals — Continental crustIntermediate: between felsic and mafic ~55-65% silica, plag > 2/3 of feldspars present Na-rich plag predominates over Ca-rich plagMafic: magnesium + ferric iron ~45-50% silica; Ca-rich plag dominant feldspar dark silicate minerals — Oceanic crustUltramafic: >90% mafic minerals, silica < 45%, few or no feldspars Mantle-derived
  6. 6. Classification of common igneous rocksComposition Phaneritic Aphanitic Color index (% dark minerals)Felsic Granite Rhyolite 10 Syenite Trachyte 15 Monzonite Latite 20Intermediate Granodiorite Dacite 20 Diorite Andesite 25Mafic Gabbro Basalt 50Ultramafic Peridotite 95
  7. 7. Mineralogical terms of Igneous Rocks
  8. 8. Mineralogical terms of Igneous Rocks
  9. 9. Classification of Igneous Rocks. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J.Petrol., 27, 745-750. Oxford University Press.
  10. 10. (a) Q The rock must contain a total of at least 10% of the minerals below. Quartzolite Renormalize to 100% 90 90 Quartz-richClassification of Granitoid 60 60 Phaneritic ite n Gra To Grano- na Granite ar diorite lite dsp Igneous Rocks Fel li Alka Alkali Fs. 20 20 Qtz. Diorite/ Quartz Syenite Quartz Qtz. Gabbro Quartz Quartz Plutonic rocks Alkali Fs. Syenite 5 Syenite Syenite Monzonite Monzodiorite Monzodiorite 5 Diorite/Gabbro/ 10 35 Monzonite 65 90 Anorthosite A (Foid)-bearing (Foid)-bearing (Foid)-bearing P Syenite Monzonite Monzodiorite 10 10 (Foid)-bearing Diorite/Gabbro (Foid)-bearing (Fo Alkali Fs. Syenite id) ro (Foid) (Foid) bb Sy Monzosyenite Monzodiorite Ga en ite id) (Fo 60 60 Figure. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + (Foid)olites feldspathoids). After IUGS. F
  11. 11. Q Classification of Aphanitic Igneous Rocks 60 60 Volcanic rocks Rhyolite Dacite 20 20 Trachyte Latite Andesite/Basalt 35 65 A (foid)-bearing (foid)-bearing (foid)-bearing P Trachyte Latite Andesite/Basalt 10 10 Phonolite TephriteA classification and nomenclature of volcanicrocks. After IUGS. 60 60 (Foid)ites F
  12. 12. Classification of Igneous Rocks Plagioclase Gabbroic Anorthosite Figure. A classification of the phaneritic 90 rocks igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. ro T ro Olivine bb cto Ga Ultramafic lite Olivine Dunite gabbro 90 rocks Peridotites We te hr i urg Plagioclase-bearing ultramafic rocks lite Lherzolite rzbPyroxene Olivine Ha (b) 40 Orthopyroxenite Olivine Websterite Pyroxenites 10 (c) 10 Websterite Clinopyroxenite Orthopyroxene Clinopyroxene