Geology Reference
In-Depth Information
Plagioclase
10-65
0-10
65-90
90-100
percent total feldspar
0
Alkali
10
Grano-
diorite
Granite
Granite
20
30
40
50
60
Figure 3.12
Plot of increasing content of plagioclase feldspar (left to right), as
a percentage of total feldspar, against mafic mineral content for granitoid rocks,
defined as those with quartz as between 20 and 60% of the salic minerals.
with experience, can be used in the field quite successfully, especially in
conjunction with Figures 3.13 - 3.14.
Consider first those rocks where quartz is typically 20 - 60% of the felsic
minerals: these can be named accurately as alkali granite, granite, granodi-
orite or tonalite, if the relative proportions of alkali feldspar and plagioclase
are estimated (cf. Figure 3.12). Where this is not possible, any sample with
