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Q The International Union of Geological
quartzolite Sciences (IUGS) commission aimed to
simplify the nomenclature, eliminate
90 90
synonymic terms (e.g., adamellite =
quartz monzonite), and come up with
quartz-rich
granitioids fields and corresponding names that are
consistent with general usage. The IUGS
60 60 method relies on two parameters, the
alkali feldspar granite syeno- granite monzo- granodiorite tonalite quartz monzodiorite, modal percentage of quartz or feld-
spathoid minerals (horizontal lines), and
quartz-monzogabbro
quartz alkali feldspar syenite granite granite quartz diorite, the ratio of alkali feldspar to plagioclase
quartz gabbro,
(lines radiating from the quartz and feld-
20 10 35 65 90 20 quartz anorthosite spathoid apices). These values are easy to
alkali feldspar syenite quartz quartz monzodiorite,
syenite monzonite monzogabbro estimate in thin section, although less so
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monzonite
syenite
A foid-bearing foid-bearing P in the field, where the plagioclase/alkali
foid-bearing 10 syenite monzonite 10 diorite, feldspar distinction can be subtle. In con-
gabbro,
alkali feldspar syenite 10 50 90 anorthosite trast, the modal proportions of mafic min-
foid diorite, foid gabbro foid-bearing monzodiorite diagnostic of a given rock unit, although
foid monzosyenite foid monzodiorite, erals and their sum (color index) are gen-
foid monzogabbro foid-bearing diorite, erally easy to estimate in the field and
foid-bearing gabbro,
foid-bearing anorthosite
foid syenite
these are not used in the IUGS system.
way to assign a standardized name once
60 60 foid-bearing monzogabbro The IUGS diagram provides a convenient
the mode has been estimated, and this is
foidolite both a strength and a significant problem.
As an example of the problems that this
90 90 kind of classification can cause, consider
the data in Figure 3, taken from Bateman
F (1992). Modal data from the well-known
Cathedral Peak Granodiorite of Yosemite
Figure 2. The International Union of Geological Sciences classification double triangle. Field National Park in California, USA (Fig. 1),
boundaries are arbitrary and not designed to follow petrologic processes, so closely related rocks
can scatter across several fields. The plethora of names and positions of boundary lines are diffi- are split rather evenly between the granite
cult to remember, particularly because they mean little in terms of process. They also serve to and granodiorite fields. Thus, any random
carve up related rocks (e.g., one map unit) into several different rock types.
hand sample or outcrop of the Cathedral
Peak pluton might be a granite or a grano-
diorite or straddle the arbitrary boundary
between them. This unit was designated
a granodiorite because one or the other
name had to be chosen, even though the
pluton includes both. The nearby El
Capitan Granite (Fig. 1) is also more-or-
less evenly divided between the granite
and granodiorite fields (with several
points in the tonalite field), but it is offi-
cially a granite. Rocks of the Kuna Crest
suite, a set of medium-grained rocks with
high color index, scatter over the grano-
diorite, tonalite, quartz monzodiorite/
quartz monzogabbro, and quartz diorite/
quartz gabbro fields. This sort of artificial

Figure 3. Modal data for plutonic units from
Yosemite National Park, California, USA, from
Bateman (1992). Many of the main units scat-
ter evenly across the granite (monzogranite)
and granodiorite fields, and petrologic vari-
ability in the granodiorite of Kuna Crest
crosses the common point of four fields,
meaning that the rock could be called any of
the four (really, six; see text). Q—quartz;
A—alkali feldspar; P—plagioclase.

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