Geology 406: Petrology

Lecture 4

Modal Classification of Magmas

(see end of Lecture 4 for continuation of alumina saturation lecture notes) Mode = volume %

Lots of different classification schemes defined, but we'll use the IUGS - International Union of Geological Standards

Uses modal amounts of common minerals plotted on triangles. Click here for a description of how to use these triangular diagrams

I. Felsic plutonic rocks (<90% mafic minerals, <60% feldspathoids) - Fig 3.2 in text, p. 52 in lab manual

II. Gabbroic rocks (>50% mafic minerals) - Fig 3.4 in text, p. 56 in lab manual

III. Ultramafic rocks (>90% mafic minerals) - Fig 3.3 in text, p. 59 in lab manual

IV. Volcanic rocks

Other Examples


30% quartz, 50% alkali feldspar, 20% plagioclase
6% quartz, 50% alkali feldspar, 44% plagioclase

Test yourself with an exercise: Click here

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