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COMMENTS & REPLY

A More Informative Way to Name Plutonic Rocks—

Comment by Hogan

John P. Hogan, Dept. of Geological Sciences and Geological Engineering and Petroleum Engineering, Missouri University of
Science and Technology, Rolla, Missouri 65409, USA

Glazner et al. (GSA Today, Feb. 2019) (“color index” “M”) of the rock (e.g., and reproducible facts, and internally con-
propose major modifications to the leucocratic granodiorite). Trends in M are sistent rules. Alternatively, classification
International Union of Geological readily discernible using the IUGS schemes can be interpretive and based
Sciences (IUGS) classification of igneous scheme (Figure 13 in Streckeisen, 1976; upon inferred parameters. Each type of
rocks (see Streckeisen, 1976; Le Maitre, Figure 8 in Wones, 1980). (D) The iden- classification scheme serves an important
2002) to correct what they see as major tity of minor and accessory minerals is scientific purpose. Glazner et al. (2019)
shortcomings, including: (1) consanguine- included in the name according to their suggest transforming the objective IUGS
ous, mappable, igneous rock bodies can modal abundance, with greater abun- classification scheme into an interpretive
exhibit modal variation requiring multiple dance closer to the root name (e.g., classification scheme where well-
rock names to describe; (2) these names leucocratic, titanite, hornblende, biotite, established taxonomy are repurposed
rarely convey information regarding rock- granodiorite). (E) Preliminary rock to convey a vision of how igneous rocks
forming processes or tectonic settings; names, based upon “rough” estimation form. The example they use is “granodio-
and (3) their discussion imparts the per- (i.e., using charts as a visual aid) of modal rite” and propose this term to become syn-
ception that their “quantitative” approach abundance (e.g., a leucocratic, biotite, onymous with a subduction zone setting.
corrects these problems. In addition, they hornblende granitoid) are used until the Unfortunately, such classification schemes
portend the “qualitative” nature of the alkali feldspar/plagioclase ratio is con- tend to be transient, as leucocratic biotite-
IUGS system inhibits the use of artificial strained (Figure 6 in Streckeisen, 1976). muscovite granite is not universally
intelligence (AI) for data mining and (F) The IUGS procedure is quantitative, accepted as an S-type granite, nor are
analysis of igneous rocks. I present sev- readily reproducible, easily followed, and gradational lit-par-lit contacts considered
eral points for consideration and recom- understood by geoscientists worldwide. proof of granitization. In contrast, by keep-
mend against adopting Glazner et al.’s The IUGS classification scheme is ing observations (modal data) separate
(2019) suggestions. compatible with mapping heterogeneous from interpretation (e.g., zone refining) the
The strength(s) of the IUGS system are igneous rocks. Sedimentary strata com- integrity of names determined by correct
as follows: (A) It is a quantitative classifi- monly consist of multiple rock types, application of the IUGS classification
cation scheme based upon modal abun- interbedded at several scales, with sharp scheme (e.g., hornblende-biotite granodio-
dances of minerals in the rock sample, or gradational contacts, and are mapped rite) will continue to hold factual meaning
which can be measured, with associated as a “formation” without abandoning worldwide, well into the future, despite
uncertainties, following well-known pro- well-established sedimentary nomencla- any interpretation of the tectonic setting
cedures (see Chayes, 1956). Quantitative ture. Heterogeneous intrusive igneous or the processes by which a rock formed
modal analysis is accomplished at many rocks, composed of multiple rock types (see Streckeisen, 1976, p. 4, section
scales: thin-sections (Willis et al., 2017), with gradational contacts, can be mapped “Principles of Classification”).
hand-samples (Elliott, 1999), and map- as lithodemes or as suites (e.g., Tuolumne Scientific classification schemes are
scale or pluton/batholith-scale using Intrusive Suite; see Easton et al., 2016) challenged by imposing order on the
multiple hand samples (see Figure 7 in without abandoning well-established continuum that is the natural world. This
Hogan and Sinha, 1989). (B) Procedures IUGS nomenclature for igneous rocks. is evident in every science discipline
for classifying rocks using this data are The description of the physical character- including biology despite the oversimpli-
self- consistent and easily followed. A root istics of the rock types, nature of the con- fication of “doggish cats” and “cattish
name (e.g., granodiorite) is assigned using tacts, comprising mappable lithodemes or dogs” manufactured by Glazner et al.
normalized modal abundances of major suites, along with the IUGS names, are (2019). For example, lichens are living
minerals (e.g., quartz, alkali feldspar, included in its formal description. organisms that are neither a fungi nor a
plagioclase). (C) The IUGS classification The motivation for classification cyanobacterium but a blend (DePriest,
scheme does include the abundance and schemes is an important and fundamental 2004). Their form bears no resemblance
identity of minor and accessory minerals subject for all students investigating a topic to either organism(s) from either
in naming the rock. Modifiers reflect the of interest. Classification schemes can be Kingdom that make up the lichen. Many
modal abundance of mafic minerals largely objective, based upon measurable life forms are continuums and present

GSA Today, v. 29, https://doi.org/10.1130/GSATG406C.1. Copyright 2019, The Geological Society of America. CC-BY-NC.

40 GSA Today | June 2019