Introduction to Igneous Rocks IN THIS LECTURE –The basis of classification –Major versus trace versus REE –Classification of Igneous Rocks Plutonic Volcanic –Other Classification Schemes

Classification of Igneous Rocks Why is it necessary to have a system for the classification of igneous rocks? Lots of different names used lots of difference people A classification scheme is therefore useful so that everyone knows if they are talking about the same rock.

The Basis of Classification

Major versus Trace versus REE Mineral compositions normally only specify the wt % oxide of the major elements, these being, Si, Ti, Al, Fe, Mg, Ca, Na, K, P and sometimes Cr, Ni, and S. For a whole rock composition, major elements (same of minerals) as well as trace elements and REE’s are often reported and can be used in the classification of igneous rocks and discrimination diagrams.

Classification of Igneous Rocks The primary classification of igneous rocks should be based on their mineral content or mode using the QAPF diagram –Q = modal volume proportion of quartz, tridymite, cristobalite –A = modal volume proportion of alkali feldspar, including orthoclase, microcline, perthite, anorthoclase, sanidine, and albitic plagioclase (An 0 to An 5 ) –P = Plagioclase (An 5 to An 100 ) and scapolite –F = feldspathoids or foids including nepheline, leucite, kalsilite, analcime, sodalite, nosean, haüyne, cancrinite, and pseudoleucite. –This diagram should not be used in rocks that have M > 90% where M = mafic and related minerals e.g. mica, amphibole, pyroxene, olivine, opaque minerals, accessory minerals (e.g. zircon, apatite, titanite, monazite), epidote, allanite, garnet, melilite, monticellite, and primary carbonate.

Classification of Igneous Rocks Plutonic Rock – an igneous rock with a phaneritic texture, i.e. a relatively coarse-grained (>3mm) rock in which individual minerals can be distinguished with the naked eye. Volcanic Rock – an igneous rock with an aphanitic texture, i.e. a relatively fine-grained (<1mm) rock in which most of the individual minerals cannot be distinguished with the naked eye. Therefore QAPF diagrams are useful for classifying plutonic rocks but not so much for volcanic rocks

Plutonic rocks M<90%

Volcanic Rocks M<90%

IUGS Classification of Gabbroic Rocks

IUGS Classification of Ultramafic Rocks

Geochemical Classification Volcanic Rocks Chemical Clasification of Volcanic Rocks using TAS (total Alkali-Silica Diagram) after le Bas et al., 1986

Other Classification Groups Other classification groups are: 1.Pyroclastic rocks and tephra Formed by fragmentation as a result of explosive volcanic eruptions or processes 2.Carbonatites Rocks that contain more than 50 % modal carbonate and have a plutonic or volcanic origin 3.Melilite-bearing rocks Rocks that contain more than 10% modal melilite and if feldspathoids are present melilite > feldspathoid 4.Kimberlites Group I and Group II 5.Lamproites –Based on mineralogical and geochemical criteria

Other Classification Groups Other classification groups are: 1.Leucite-bearing rocks Rocks containing little or no feldspar 2.Lamprophyres Several characteristic criteria 3.Charnockitic rocks Characterised by the presence of orthopyroxene (or fayalite + quartz) and in many rocks perthite, mesoperthite or antiperthite.