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Q A particular hand sample of the granodiorite, and the rock would be a
Cathedral Peak pluton might be called a hbl-bio 20,20,50 granodiorite.
hornblende-biotite 30,20,45 granodiorite, 2. A shonkinite (Johannsen, 1932, p. 355)
indicating a color index of 5 with biotite contains 3% quartz, 8% orthoclase,
> hornblende. The variation in mineral- 22% plagioclase, 65% hornblende, and
ogy in the El Capitan pluton could be 2% accessories. This would be a hbl
described as a range from biotite 3,8,22 monzonite. The sum of Q, A, and
30,50,20 granite to hornblende-biotite P is only 33, implying a large amount of
25,10,55 granodiorite. This expresses hornblende ± other phases.
granite grano- the variations observed in the felsic and 3. A “leucolitchfieldite” from Johannsen
diorite
tonalite
gabbro mafic mineralogy and color index far (1938, p. 181) contains 16% micro-
syenite monzonite diorite better than the unqualified names. cline, 55% plagioclase, 18% nepheline,
A P 8% muscovite, and 1% each magnetite
foid foid
syenite diorite Rocks with Feldspathoid(s) + Feldspars and biotite. This would be an 18,16,55
>10% (Lower Half of IUGS Diamond) nepheline syenite or a musc 18,16,55
Classification of feldspathoid-bearing nepheline syenite.
rocks is the same as with quartz-bearing 4. Boyd and McCallister (1976) gave a
rocks except that the identity of the peridotite mode as 59% olivine, 11%
feldspathoid(s) replaces “foid” (e.g., neph- orthopyroxene, 20% clinopyroxene,
eline syenite rather than foid syenite). and 10% garnet. This rock would be a
garnet 59,11,20 lherzolite.
Ultramafic Rocks 5. Boudreau (1988) listed modal mineral-
Olivine-pyroxene rocks with <10% ogy of rocks from the Stillwater
F felsic minerals are named via the olivine- Complex as Table 1.
orthopyroxene-clinopyroxene (OOC)
Figure 6. Proposed simplified names for rocks triangle (Fig. 7). The same simplification Complications
in the International Union of Geological Sci-
ences (IUGS) diamond. Boundaries are fuzzy, principles apply to these rocks: estimate There are many details. For example,
fields overlap, and names are redundant of the mineral proportions and then name the it is common in granitoids that the feld-
numeric values, which can be converted into
formal IUGS names if desired. rock with these numbers and the simpli- spars are difficult to distinguish in the
fied, blurred boundary classification in field; in such cases they can be lumped,
far fewer bins in which to put the rocks Figure 7. Preface the name with other with only two numbers reported, as bio-
(six versus sixteen). The bins simply indi- important minerals such as garnet or tite 35,60 granite. Modal data can be
cate broad QAP proportions. For example, spinel; their proportion is 100 minus determined with varying levels of preci-
granites and granodiorites are quartz-rich the sum of OOC. sion. Field estimates might be good to
and distinguished broadly by alkali feld- only the nearest 10%, whereas micro-
spar > plagioclase or plagioclase > alkali Gabbroic Rocks scopic estimates can be good to a per-
feldspar, respectively. Syenites are rich in Gabbroic rocks can be classified using cent; the approximate precision should
alkali feldspar and poor in plagioclase + a tetrahedron with apices of plagioclase, always be stated. A feldspathoidal rock
quartz. Diorites are rich in plagioclase and olivine, orthopyroxene, and clinopyrox- might contain two or more important
poor in alkali feldspar + quartz, and so on. ene (POOC; Fig. 8). The base of the feldspathoids, as in 30,20,40 sodalite-
For map scale, nomenclature likely skips tetrahedron is the OOC triangle of nepheline syenite, indicating sodalite +
Step 3 because the modal variation at that Figure 7; the apices are anorthosite, nepheline = 30 and sodalite < nepheline.
scale is too variable for quantification. dunite, orthopyroxenite, and clinopyrox-
Accordingly, the Cathedral Peak enite; troctolite lies along the plagioclase- SUMMARY
Granodiorite mentioned earlier would be olivine edge, and the interior is gabbro There are several advantages to this
just that, or the Cathedral Peak Biotite or norite depending on whether the method of naming plutonic rocks.
Granodiorite to reflect the dominant mafic dominant pyroxene is clinopyroxene or • It allows for overlap of names such as
mineral. This does not directly address the orthopyroxene. Hybrid names, such as “granite” and “granodiorite.” These names
fact that the Cathedral Peak body includes gabbronorite, and qualified names, such are redundant of the quantitative informa-
both granodiorites and granites as defined as olivine gabbro, are unnecessary (but tion and merely serve as a guide to the
by IUGS boundaries. However, the recog- can be used if desired) when the defining appropriate classification triangle or
nition that a granodiorite is defined as a mineralogy is given in the name (e.g., tetrahedron and to the overall rock type.
quartz-rich rock, with generally (but not 50,10,20,20 gabbro). • The abundances of the determinative
necessarily exclusively) plagioclase > minerals are given directly in the name,
alkali feldspar, is an improvement in repre- Examples and the QAP/FAP/OOC/POOC param-
senting the nature of the pluton. Addition 1. A rock has 20% quartz, 20% K-feldspar, eters can be calculated from the name.
of the mafic mineralogy to the name sig- 50% plagioclase, with the remainder • Thus, everything needed to derive
nificantly advances one’s knowledge of (10%) mafic minerals consisting of biotite the standard IUGS classification is in
what to expect in the field. > hornblende. The root name would be the name.

8 GSA Today | February 2019