Dr. Helen Lang Dept. of Geology & Geography West Virginia University

SPRING 2015 GEOL 285: PETROLOGY Classification of Igneous Rocks

In 1979, a committee of the International Union of Geological Sciences (IUGS), chaired by Streckeisen, proposed a now widely accepted classification of igneous rocks Plutonic Igneous Rocks are classified on the basis of their Content • are usually large enough to see and identify • Mineral Content is expressed as the Mode, where Mode = % of each mineral present • Mode is plotted in IUGS classification It is called the IUGS Q (or Streckeisen) Classification and is based on the Percent of the minerals A P Q = Quartz A = Alkali P = F = Feldspathoids plotted on the diamond- F shaped diagram at right What are the formulas and properties of: •Quartz?

• Alkali Feldspar?

• Plagioclase? Feldspathoids are SiO2-deficient feldspar- like minerals (also framework silicates)

Leucite - KAlSi2O6 Isometric, trapezohedron

Nepheline - NaAlSiO4

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 names How to plot on the Streckeisen (QAPF) Diagram • Ignore minerals (if rock has <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, counting from A at the left toward P at the right (connect diagonal line to Q corner) Q Plotting

• Plot Q/(Q+A+P) * 100, up from A-P side – Or F/(F+A+P) * 100, counting down from A-

P side Q/Q+A+P A P • Plot P/(A+P) * 100, 0P/A+P 100 F/F+A+P counting from A at the left toward P at the right (connect diagonal line to Q corner)

F Examples with Quartz (top triangle)

I II III Q 10 25 15 A 50 0 10 P 30 60 25 M 10 15 50 Note that several fields contain 2 or 3 names, which do you use?

Diorite has C.I. < 35 and Na-rich plagioclase with An<50 Gabbro has C.I. >35 and Ca-rich plagioclase with An>50 Anorthosite has >90% plagioclase (<10% mafic minerals) There is a Streckeisen (QAPF) Diagram for Volcanic Rocks, but it is hard to use because most mineral grains in volcanic rocks are too small to identify; therefore, Volcanic rocks are most commonly classified (or named) on the basis of their chemical compositions Igneous Rocks have a limited range of chemical compositions • They’re made up mainly of a few Major Elements (generally >1.0 wt%)

–SiO2 30-78 wt%

–Al2O3

–Fe2O3 –FeO –MgO –CaO

–Na2O 0-10 wt%

–K2O 0-15 wt% Alkali metals Noble gases Alkali earth metals Nonmetals Halogens

Transition metals

Metalloids also called Rare Earth Elements (REEs) Igneous Rock Compositions • Minor Elements (generally 0.1 to 1.0 wt%) + –H2O structural water, bound in minerals - o –H2O adsorbed water, driven off at 110 C

–TiO2

–P2O5 –MnO

–CO2 –Cl –F –S Trace Elements in Igneous Rocks

• Present in amounts less than 0.1 wt% or <1000 ppm (parts per million) • Very low amounts • But tell a lot about the rock’s history • Include Rare Earth Elements (REEs, or lanthanides), radioactive elements and other heavy elements SiO2 is the most important and variable major element in magmas

• Primary subdivision of volcanic rocks is on the

basis of SiO2 content: – picrobasalt 41-45 wt% SiO2 – 45-52 – basaltic 52-57 – andesite 57-63 – 63-69

– >69 (dep. on wt% Na2O+K2O) Alkali Content is also Important

• Some groups of igneous rocks are relatively

low in alkalis (Na2O and K2O), and are called subalkaline • Some groups of igneous rocks are relatively high in alkalis, called alkaline – These may be either sodic or 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* * relatively common – Calc-alkaline Series* • Alkaline Rocks – Alkali-Olivine Basalt Series* – Nephelinites, Leucitites, Analcitites (rare) • Each of the common series has different chemical and mineralogical characteristics and occurs in different tectonic environments The Tholeiitic Series

• Subalkaline • Mid-ocean Ridge (MORBs), Iceland, plateau or flood basalts (Columbia River Basalts), some oceanic islands (Hawaii), major component of Archean greenstone belts • Not porphyritic, may have a few Olivine or Pyroxene phenocrysts • Basalt is dominant The Calc-alkaline Series

• Subalkaline • Subduction-related volcanoes and plutons, Circum-Pacific andesitic stratovolcanoes – Cascades – Andes – Japan • Strongly porphyritic, with dominantly Plagioclase phenocrysts, also Olivine, Opx or Hornblende phenocrysts • Andesite is dominant Distinguish Tholeiitic from Calc- alkaline Series on AFM diagram

F =FeO+0.9Fe2O3

Tholeiitic series shows Fe- o o o enrichment as SiO2 oo (and alkalis, A) oTholeiitico o oo increases o o o o felsic x x x xox xx o mafic x Calc-alkalinex x A M =MgO =Na2O+K2O AFM Galapagos (tholeiitic) vs. Cascades (calc-alkaline)

Cascades and Galapagos Molar AFM

Mole% F

1

0.8

0.6

0.4

0.2 0 M A 0 0.2 0.4 0.6 0.8 1 1.2 The Alkali-Olivine Basalt (AOB) Series • Alkaline • Cap Hawaiian volcanoes, dominate some oceanic islands (Tahiti, St. Helena, Azores), occur the in interior of island arcs, associated with continental rifting (East African rift) • Plagioclase, Augite or Olivine phenocrysts may be dominant • Felsic rocks are trachytic • Silica minerals are rare, Feldspathoids are common in the groundmass IUGS Volcanic Classification is Graphical

Alkaline

Subalkaline Alkaline

Subalkaline Distinguish Tholeiitic from Calc- alkaline Series on AFM diagram

F =FeO+0.9Fe2O3

Tholeiitic series shows Fe- o o o enrichment as SiO2 oo (and alkalis, A) oTholeiitico o oo increases o o o o felsic x x x xox xx o mafic x Calc-alkalinex x A M =MgO =Na2O+K2O