Lecture 2 Classification of Igneous Rocks Friday, January 28th, 2005 Classification of Igneous Rocks Figure 2-1a. Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall. 1 Classification of Igneous Rocks Ratio of Y to Z = 100Y(Y + Z) = 100*20(20 + 10) = 67 Figure 2-1b. Method #2 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall. Q (a) The rock must contain a total of at least 10% of the minerals below. Quartzolite Renormalize to 100% 90 90 Quartz-rich Granitoid Classification of 60 60 Tonalite Granite Grano- Igneous Rocks diorite Alkali Feldspar Granite Alkali Fs. 20 20 Qtz. Diorite/ Quartz Syenite Quartz Quartz Quartz Qtz. Gabbro Foids are feldspathoids Alkali Fs. Syenite Monzonite Monzodiorite 5 5 Diorite/Gabbro/ Syenite Syenite Monzodiorite such as nepheline and 10 35 Monzonite 65 90 Anorthosite A (Foid)-bearing (Foid)-bearing (Foid)-bearing P leucite Syenite Monzonite Monzodiorite 10 10 (Foid)-bearing Diorite/Gabbro (Foid)-bearing (Foid) Syenite Alkali Fs. Syenite (Foid) (Foid) Monzosyenite Monzodiorite (Foid) Gabbro Figure 2-2. A classification of the phaneritic igneous 60 60 rocks. a. Phaneritic rocks with more than 10% (quartz + (Foid)olites feldspar + feldspathoids). After IUGS. F 2 Let’s try an example Modal analysis of a rock gives the following mineral proportions Quartz = 26.9 K-feldspar = 23.7 Plagioclase = 39.8 Biotite = 5.3 Hornblende = 4.3 Re-summing Qtz = 29.8 Kspar = 26.2 Plag = 44.0 100P/(P + A) = 63 Rock is a granite! How about another? Modal analysis of a rock gives the following mineral proportions Quartz = 8.5 K-feldspar = 5.1 Plagioclase = 59.6 Biotite = 16.1 Hornblende = 10.9 Re-summing Qtz = 11.6 Kspar = 7.0 Plag = 81.4 100P/(P + A) = 92 Rock could be Qtz diorite or Qtz gabbro 3 Classification of Igneous Rocks Plagioclase Anorthosite Figure 2-2. A classification of the phaneritic 90 igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. Troctolite Olivine Gabbro Olivine Dunite gabbro 90 Wehrlite Peridotites Plagioclase-bearing ultramafic rocks Lherzolite Pyroxene Olivine Harzburgite (b) 40 Orthopyroxenite Olivine Websterite Pyroxenites 10 Websterite (c) 10 Clinopyroxenite Orthopyroxene Clinopyroxene Q Classification of Igneous Rocks 60 60 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 Tephrite Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS. 60 60 (Foid)ites F 4 Classification of Igneous Rocks Alkaline -alkaline or calc Tholeiitic Figure 2-4. 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. An actual example from the Hawaii Scientific Drilling Project The dashed line separates alkalic from tholeiitic basalts Total alkali = SiO2 x 0.37 + 14.43 5 We can calculate an alkalinity index from the previous equation. It measures how far a sample plots above or below the alkali – tholeiite line. In other words, how alkaline or tholeiitic it is. Alkalinity Index = (Na2O + K2O) – (SiO2 x 0.37 + 14.43) This example shows the alkalinity index with depth in rocks from the Hawaii Scientific Drilling Project Classification of Igneous Rocks Figure 2-5. Classification 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 of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298. 6 Some Useful Textural Terms ● Grain Size – determined by cooling rate of a magma (also influenced by volatile content). Fine < 1 mm Medium 1 – 5 mm Course > 5 mm ● Aphanitic – a rock that is so fine-grained that individual minerals cannot be distinguished without a microscope. ● Phaneritic – all essential minerals are clearly visible ● Porphyritic – a rock containing large phenocrysts in a finer grained groundmass ● Grain Shapes euhedral – grains with well-developed crystal faces subhedral – grains with a few well-developed crystal faces anhedral – grains that have no distinct shape 7.