Dr Helen Lang Dept p of Geology gy Geography g p y West Virginia University SPRING 2011 GEOL 285 PETROLOGY Classification of Igneous Rocks In 1979 a committee of the International Union of Geological Sciences IUGS IUGS chaired by Streckeisen proposed a now widely accepted classification of igneous rocks Plutonic Igneous Rocks are classified on the basis of their Mineral Content Minerals are usually large enough to see and identify y Mineral Content is expressed as the Mode where Mode of each mineral present Mode is plotted in IUGS classification It is called the IUGS or Streckeisen Classification and is based on the Percent of the felsic minerals Q P A Q Quartz A Alkali Feldspar P Plagioclase ag oc ase F Feldspathoids plotted on the diamondshaped diagram at right F What are the formulas and properties of Quartz Alkali Feldspar Plagioclase Feldspathoids are SiO2 deficient feldsparlike minerals also framework silicates Leucite KAlSi2O6 Isometric trapezohedron N h li NaAlSiO Nepheline N AlSiO4 SiO2 deficient NaAlSi3O8 2 SiO2 NaAlSiO4 Incompatible with Quartz free silica Occur only in unusual Si poor alkali rich igneous rocks Streckeisen QAPF Diagram with Plutonic Igneous Rock names How to plot on the Streckeisen QAPF Di Diagram Ignore mafic minerals if rock has 90 90 mafics Calculate and plot Q Q A P 100 counting up from A P side Or F F A P 100 counting down from A P side Igneous rocks will either have Quartz Q or Feldspathoids F but not both Q F incompatible Calculate and plot P A P 100 100 counting from A at the left toward P at the right connect diagonal line to Q corner Q Plotting Or F F A P 100 counting down from AP side id A 0 P P A P 100 F F A P Plot P A P 100 counting from A at the left toward P at the right connect diagonal line to Q corner Q Q Q A P P Plot Q Q A P 100 up from A A P P side F Examples with Quartz top triangle I Q 10 A 50 P 30 M 10 II III 25 15 0 10 60 25 15 50 Note that N h severall fields fi ld contain 2 or 3 names which do yyou use Diorite has C I 35 and Na rich plagioclase with An 50 Gabbro has C I 35 and C i h plagioclase Ca rich l i l with ih An 50 Anorthosite has 90 plagioclase 10 mafic minerals There is a Streckeisen QAPF Diagram for Volcanic Rocks but it is hard to use b because most mineral i l grains i in i volcanic l i rocks are too small to identify therefore Volcanic rocks are most commonlyy classified or named on the basis of their chemical compositions Igneous Rocks have a limited range of chemical compositions Th They re made d up mainly i l off a few f Major M j Elements El t generally 1 0 wt SiO2 Al2O3 Fe2O3 FeO MgO CaO Na2O K2 O 30 78 wt 30 78 0 10 wt 0 15 wt Igneous Rock Compositions Minor Elements generally 0 1 to 1 0 wt H2O structural water water bound in minerals H2O adsorbed water driven off at 110oC TiO2 P2O5 MnO O CO2 Cl F S Trace Elements in Igneous Rocks Present in amounts less than 0 1 wt or 1000 pp ppm p parts pper million Very low amounts But tell a lot about the rock rock ss history Include Rare Earth Elements REEs or l h id radioactive lanthanides di i elements l andd other h heavy elements Alkali metals Alkali earth metals Noble gases Nonmetals Halogens Transition metals also called Rare Earth Elements REEs Metalloids SiO2 is the most important and variable major element in magmas Primary subdivision of volcanic rocks is on the basis of SiO2 content picrobasalt basalt basaltic andesite andesite dacite rhyolite 41 45 wt SiO2 45 52 45 52 52 57 57 63 63 69 69 dep dep on wt Na2O K2O Alkali Content is also Important Some groups of igneous rocks are relatively low in alkalis Na2O and K2O called subalkaline Some groups of igneous rocks are relatively high in alkalis called alkaline These may be either sodic or potassic potassic depending which Na or K is dominant IUGS Volcanic Classification is Graphical Alkaline Subalkaline It is Traditional and useful to Classify Volcanic Rocks as follows Subalkaline Rocks Tholeiitic Series Calc Calc alkaline alkaline Series Series relatively common Alkaline Rocks Alkali Olivine Alk li Oli i Basalt B lt Series S i Nephelinites Leucitites Analcitites rare Each of the common series has different chemical and mineralogical characteristics and occurs in i different diff tectonic i environments i The Tholeiitic Series Subalkaline Mid ocean Ridge Basalts MORBs Iceland plateau or flood basalts Columbia p River Basalts some oceanic islands Hawaii major p of Archean ggreenstone belts component Not porphyritic may have a few Olivine or Pyroxene phenocrysts Basalt is dominant The Calc alkaline Series Subalkaline Subduction related S bd i l d volcanoes l andd plutons l Circum Pacific andesitic stratovolcanoes Cascades Andes Japan Strongly porphyritic dominantly Plagioclase phenocrysts also Olivine Opx or Hornblende phenocrysts Andesite is dominant Distinguish Tholeiitic from Calcalkaline Series on AFM diagram F FeO 0 9Fe2O3 oo o oo o o ooTholeiitic oo Tholeiitic series shows h F Feenrichment as SiO2 and alkalis A increases o o o ffelsic l i x x x x x x x x x oo mafic fi Calc alkaline x A Na2O K2O M MgO AFM Galapagos tholeiitic vs Cascades calc alkaline Cascades and Galapagos Molar AFM Mole 1 F 0 8 0 6 0 4 0 2 M 0 A 0 0 2 0 4 0 6 0 8 1 1 2 The Alkali Olivine Basalt AOB Series Alkaline Cap Hawaiian volcanoes volcanoes dominate some oceanic islands Tahiti St Helena Azores occur the in interior of island arcs arcs associated with continental rifting East African rift Plagioclase Plagioclase Augite or Olivine phenocrysts may be dominant Felsic F l i rocks k are trachytic t h ti Silica minerals are rare Feldspathoids are common in the groundmass IUGS Volcanic Classification is Graphical Alkaline Subalkaline Alkaline Subalkaline Distinguish Tholeiitic from Calcalkaline Series on AFM diagram F FeO 0 9Fe2O3 oo o oo o o ooTholeiitic oo Tholeiitic series shows h F Feenrichment as SiO2 and alkalis A increases o o o ffelsic l i x x x x x x x x x oo mafic fi Calc alkaline x A Na2O K2O M MgO