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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
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