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COMMENTS & REPLY

A More Informative Way to Name Plutonic Rocks—

Comment by Frost et al.

B. Ronald Frost, Carol D. Frost, Dept. of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA;
J. Lawford Anderson, Dept. of Earth and Environment, Boston University, Boston, Massachusetts 02215, USA; Calvin G. Barnes,
Dept. of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA; and Marjorie Wilson, School of Earth & Environment,
Leeds University, Leeds LS2 9JT, UK

In a recent paper, Glazner et al. (GSA ≥10% quartz (the most common group) a more precise description of the rock. This
Today, Feb. 2019) proposed a major change are described by only 23 names, many of suggestion indicates a confusion of preci-
in the terminology of plutonic rocks, which share the same root name. These sion and accuracy. Modes are difficult to
whereby a simplified rock name is prefixed names need not be memorized because determine in the field where, as Glazner et
with the mode. In this classification, a they are present in the various IUGS dia- al. (2019) observe, the distinction between
granite might be named 20,20,50 granite. grams for rock names, a diagram that is alkali feldspar and plagioclase can be sub-
Glazner et al. (2019) proposed this classifi- easily pasted into field notebooks. tle. Field estimation of modes is unlikely to
cation system for three reasons. First, they The argument of Glazner et al. (2019) be better than ±10%. With this precision, a
maintain that rock terminology is too com- that rock names are determined by rock classified as a 25,25,40 granite would
plex; they note that at least 157 igneous rock “arbitrary” boundaries is not compelling. occupy a large field in the QAP diagram
names exist. The second is that the bound- These boundaries are not arbitrary: (Fig. 1). For this reason, Streckeisen (1976)
aries in the International Union of Geo- The IUGS commission spent many years suggested a preliminary classification in
logical Sciences (IUGS) classification are developing a system that conformed, as which granitic rocks may be named with
arbitrary and hence are confusing when much as possible, with existing classifica- the termination “-oid,” as in granitoid.
applied to plutonic rock units that show a tion systems. Furthermore, the rock terms Point-counting a minimum of 1000 points
range in composition. Third, the IUGS sys- have meaning in the sense that geologists on stained slabs or thin sections produces a
tem of classification is qualitative, and the know what to expect of a rock described more accurate determination of quartz,
quantitative data from which the classifica- as tonalite instead of granite. Glazner et plagioclase, and alkali feldspar abundances
tion is derived are discarded once the name al. (2019) support their arguments with that are used to identify the appropriate
is determined. To solve these problems the observation that two of the plutons in IUGS rock name. However, as Glazner et
Glazner et al. (2019) propose that the petro- the Sierra Nevada batholith, the Cathedral al. (2019) observe, only 5% of the analyses
logic community discard the IUGS classifi- Peak Granodiorite and the El Capitan archived in the NAVDAT database have
cation system and substitute a system with a Granite, contain rocks that look the same associated modal data. This means that
limited number of rock names that are pre- (their Fig. 1). Thus they conclude that the their quantitative classification system, in
fixed by the modal abundance of major names “granodiorite” and “granite” are in addition to being of limited value in the
phases (such as quartz, alkali feldspar, and error. However, the error is not in the field, is not likely to be widely applied.
plagioclase [QAP] in felsic rocks). They names of the individual rocks, it is in the A further problem with modal classifi-
maintain that this is a simpler classification assumption that the Cathedral Peak cation is that even when mineral propor-
and that it lends itself to a more quantitative Granodiorite contains only granodiorite tions are accurately determined there
classification scheme. whereas the El Capitan Granite contains remains an inherent, irreducible uncer-
We take exception to Glazner et al.’s only granite. Plutons are rarely homo- tainty. First, the abundance of feldspars in
(2019) proposal and instead recommend geneous over distances greater than a few a granitoid is dependent on the cooling
that geologists continue to use the IUGS tens of meters: they contain rocks with a history of the rock. A rock that cooled
classification system for naming plutonic range of compositions. Whereas each relatively swiftly and contains sodium-
rocks. Their first justification, that there rock named by the IUGS classification is bearing orthoclase will have a different
are too many obscure terms in igneous valid, the assumption that the pluton ratio of alkali feldspar to plagioclase than
petrology, was a problem recognized by name (i.e., Cathedral Peak Granodiorite) one that cooled slowly and contains
the IUGS commission (Streckeisen, 1976; classifies all rocks within the pluton as sodium-poor microcline and plagioclase
Le Maitre et al., 2002). Hence, the IUGS granodiorite, as implied by Glazner et al. with sodic rims. Furthermore, crystalliza-
rock names replaced a plethora of obscure (2019), is simply false. tion of any muscovite or biotite in a rock
terms. The IUGS classification scheme Glazner et al. (2019)’s third point is that will deplete the orthoclase component
involves only 55 names for common plu- appending numbers that reflect modal from the feldspar matrix. Similarly, horn-
tonic igneous rocks. Of these, rocks with abundances to a simple name will result in blende crystallization will deplete a

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