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Chapter 5Major IntrusionsClassificationThe major intrusions within the district comprise granodiorite, granite and quartz syenite. Theyhave been subdivided, on the basis of texture, into medium , fine to medium, and fine grainsizedivisions (after Strange 1984). Megacrystic texture in granite is shown by an overprint on mapsheets 9, 13 and 14.Modal mineral analysis is the primary method of igneous rock classification (Figure 8). Whererocks are too fine for modal determination, they are classified according to bulk rock chemicalcomposition using a total alkali-silica (TAS) diagram. The prefix micro- is used for plutonicrocks that are very fine grained, but not so fine that a volcanic origin is likely.Whole rock major and trace element analyses have been determined on 95 samples from majorintrusions in the district. Representative analytical data are presented in Appendix 2.Q = quartzA = alkali feldspar,including albite(AnO-An5)P = plagioclase feldspar,(An5-An100)M < 90% (mafic minerals)ALKALI FELDSPARGRANITEalkali feldsparrhyoliteQUARTZSYENITEquartz trachyteQUARTZMONZONITEquartz latite/ /SYENITE trachyte / MQNZONITE latiteDIORITE, GABBROandesite, basalt. \ \ 6V10 35 65 90Figure 8 - General ClassificationStreckeisen, 1974)and Nomenclature of Major and Minor Intrusive Rocks (after61