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A More Informative Way to Name Plutonic Rocks a More Informative Way to Name Plutonic Rocks

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A More Informative Way to Name Plutonic Rocks a More Informative Way to Name Plutonic Rocks 2018 GSA Presidential Address, p. 16 VOL. 29, NO. 2 | FEBRUARY 2019 A More Informative Way to Name Plutonic Rocks A More Informative Way to Name Plutonic Rocks Allen F. Glazner, Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA; John M. Bartley, Dept. of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA; and Drew S. Coleman, Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA ABSTRACT a formidable entry barrier to students of granodiorites (Fig. 1). Thus, any classifi- The International Union of Geological the field. In a recent undergraduate text- cation based on discrete categories will Sciences (IUGS) system for rock classifi- book, Winter (2010, p. 32) lists 157 com- split continuously variable rock composi- cation, introduced more than 40 years mon igneous rock names, many of them tions at arbitrary boundaries. ago, has served geologists well but suffers unknown to practicing petrologists. Say An international effort to systematize from the problem of dividing a continuum “kugdite” to a geologist and you will the nomenclature of plutonic igneous of rock compositions into arbitrary bins. likely get a puzzled stare. rocks was started in the 1960s under the As a result, closely related rocks can be Classification of igneous rocks has leadership of Swiss petrologist Albert given unrelated names (e.g., granodiorite occupied and irritated petrologists for Streckeisen, and summaries of this work and tonalite), and the names themselves, centuries. Unlike biological classifica- (e.g., Streckeisen, 1974, 1976; LeBas and which were generally derived from the tions, which can place organisms into Streckeisen, 1991) are the standard refer- names of places or people, rarely contrib- discrete categories, rock classifications ences for current nomenclature. The prin- ute to understanding the processes that place sharp boundaries between objects cipal classification is based on a double generate the diversity of igneous rocks. that are completely gradational. A biolo- triangle (Fig. 2); this diagram, appropriate Here we propose a quantitative modifica- gist can classify something definitively for rocks with 10% or more quartz or tion to the IUGS system that reduces the as a dog or cat, knowing that there are no feldspathoid minerals plus feldspars, uses number of distinct names but more effec- doggish cats or cattish dogs, but a petrolo- the modal (volume) proportions of quartz tively communicates the inherent vari- gist cannot do so—there are plenty of (Q), alkali feldspar (A), plagioclase (P), ability of plutonic rocks. The system rec- granodioritic granites and granitic and feldspathoids (F) to name rocks. ognizes that mapped plutonic rock units are characterized by recognizable tex- tures and mineral assemblages, but that Cathedral Peak mineral proportions within those units Granodiorite can be highly variable. Adding quantita- tive data to rock names is an important step toward moving geologic field obser- vations into quantitative digital form and preparing them for advanced data mining and analysis. One thing quarks do have going for them: all Figure 1. Outcrop photo- their names are simple—something chemists, graphs of the Cathedral Peak Granodiorite and El biologists, and especially geologists seem Capitan Granite, Yosemite incapable of achieving when naming their own National Park, California, stuff. —Neil deGrasse Tyson, Astrophysics for USA. In spite of largely People in a Hurry equivalent mineral propor- El Capitan tions, one is termed a gran- INTRODUCTION ite and the other a granodi- Granite orite. This confusion is Why do we bother to name rocks? One lessened if name boundar- answer among many is that rock catego- ies are considered fuzzy rather than sharp. Pennies ries can efficiently convey important are 2 cm in diameter. information about a geologic setting just G384A as biological categories can convey the same for ecosystems. Say “zebra” to a biologist and they will likely think “African savanna”; say “granodiorite” to a geologist and they will likely think “subduction zone.” However, the sheer number of igneous rock names presents GSA Today, https://doi.org/10.1130/GSATG384A.1. Copyright 2019, The Geological Society of America. CC-BY-NC. Q The International Union of Geological quartzolite Sciences (IUGS) commission aimed to simplify the nomenclature, eliminate 90 90 synonymic terms (e.g., adamellite = quartz-rich quartz monzonite), and come up with granitioids fields and corresponding names that are 60 60 consistent with general usage. The IUGS e method relies on two parameters, the granodiorit tonalite modal percentage of quartz or feld- granite quartz monzodiorite, quartz-monzogabbro spathoid minerals (horizontal lines), and syeno- monzo- quartz alkali feldspar syenite granite granite e quartz diorite, the ratio of alkali feldspar to plagioclase quartz gabbro, alkali feldspar10 granit 35 65 90 (lines radiating from the quartz and feld- 20 20 quartz anorthosite alkali feldspar syenite quartz quartz monzodiorite, spathoid apices). These values are easy to syenite monzonite monzogabbro estimate in thin section, although less so 55 syenite monzonite in the field, where the plagioclase/alkali APfoid-bearing foid-bearing diorite, syenite monzonite feldspar distinction can be subtle. In con- foid-bearing 10 10 gabbro, alkali feldspar syenite 10 50 90 anorthosite trast, the modal proportions of mafic min- foid monzosyenite foid monzodiorite, foid syenite foid monzogabbro foid-bearing diorite, erals and their sum (color index) are gen- foid-bearing gabbro, erally easy to estimate in the field and foid-bearing anorthosite diagnostic of a given rock unit, although foid-bearing monzodiorite these are not used in the IUGS system. foid-bearing monzogabbro foid diorite, foid gabbro The IUGS diagram provides a convenient 60 60 way to assign a standardized name once 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 boundaries are arbitrary and not designed to follow petrologic processes, so closely related rocks National Park in California, USA (Fig. 1), 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 carve up related rocks (e.g., one map unit) into several different rock types. and granodiorite fields. Thus, any random 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. ABQ M 50 40 30 Color 20 index 10 0 Q P A P A Figure 4. Modal data for 403 quartz-bearing plutonic rocks from the Sierra Nevada batholith, plotted on the quartz-alkali feldspar-plagioclase (QAP) triangle and in a tetrahedron whose base is QAP and whose apex M represents mafic minerals. It is clear that the proportion of mafic minerals (color index) increases dramatically away from the center of the QAP triangle, toward the QP sideline. This trend is neglected by the International Union of Geological Sciences classification. Data from Bateman et al. (1984) and Bateman et al. (1988). separation-by-name of closely related of classification. Noting an analogy described in the literature are covered by rocks is an unfortunate consequence of between mineral assemblages and life just one or two dozen. We compiled the fitting observations into arbitrarily assemblages, he said: number of citations in GeoRef mentioning defined boxes. How artificial a classification of faunas 19 of the most common IUGS names over Figure 3 also shows that the color indi- would be which was based on the ratio of the period 1970–2018. “Granite” made up ces of rocks called El Capitan Granite foxes to hares, of hares to moles and so on! just under half of the total number of cita- range from near zero to more than 20, an To be sure it is by no means accidental that tions (~223,000) in this list, and the names the ratio of hares to foxes is 10 in a certain important point to which the name “gran- area and only 2 in another, but as compared were distributed as in Figure 5. The first ite,” as defined by the IUGS system, gives with the broad factors controlling life in the nine names account for 90% of the cita- no indication. In general, color index two areas it is relatively accidental… So it is tions (allowing for multiple counting of increases moving away from the center of not accidental that a rock is nearly pure oliv- citations listing more than one name). the quartz-alkali feldspar-plagioclase ine here and only 75 per cent olivine a few As is common with textual data, feet distant, but it is relatively accidental and (QAP) triangle, toward plagioclase (Fig.
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