California Division of Mines Speial Report 53 Igneous and Metamorphic Rocks of Parts of Sequoia and Kings Canyon National Parks, California

Plutonic rocks underlie approximately 130 square miles of the mapped area, and occur as separate mappable bodies. Each mappable unit with the exception of the alaskite is designated by a local geographic name and a rock name or the term pluton.* Pluton is used in this report in much the same way that formation is used in sedimentary rocks. The plutons are mappable, relatively homogeneous units that can generally be designated by one of the standard rock names. Where the variation is greater than the limits of a standard rock name, the term pluton is used to save the use of a rather cumbersome compound name. The use of local geographic names for the plutons is intended merely as a convenient device for referring to mappable bodies. The Sequoia area is isolated from areas where detailed work has been done on Sierra Nevada plutonic rocks, and no attempt has been made to correlate the Sequoia rocks with those of other areas. It should be emphasized that the names employed in the Sequoia area are not proposed as permanent names.

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*Pluton was first defined by Hans Cloos (1928) as an all-inclusive, noncommital term to apply to plutonic masses. Cloos in addition proposed numerous modifying terms to subdivide plutons on the basis of form, size, and other features. Most of this nomenclature has not been adopted, but the term pluton is coming into general use for bodies of igneous rock formed by slow cooling, particularly where the form of the body is irregular or unknown.

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The petrographic data are summarized to limit repetitious discussion under the pluton sub-heading. The mineral composition of the plutonic rocks is summarized in table 1. This compilation eliminates the necessity for detailed descriptions of the amounts and ranges of the various minerals for each pluton. In addition the mineralogic range and average composition of some of the larger masses (the Giant Forest and Big Meadow plutons, and the Weaver Lake quartz monzonite) are shown by 76 thin section modal analyses that were made using the point count method of Chayes (1949, pp. 1-11). The description of minerals will also be limited to unusual or particularly important features. The textures throughout the granitic suite are generally hypautomorphic granular, medium- to coarse-grained, with locally porphyritic and seriate facies. The texture in the gabbroic rocks, on the other hand, is extremely variable. In general the gabbro is xenomorphic granular, medium grained; but locally it contains irregular poikilitic hornblende crystals as large as 6 inches across.

Table 1. Summary of mineral compositions of the plutonic rocks.

Sphene, magnetite, zircon, apatite, allanite are common accessory minerals; monzonite, garnet, and pyrite only locally present.

Throughout the plutonic suite, minor late- or post magmatic alterations are common. The plagioclase is saussuritized to some extent in all the rocks with the resultant development of epidote, clinozoisite, and calcite. Both the potash feldspar and plagioclase show some sericitization. The zoning of some plagioclase crystals is accentuated by thin, cloudy layers of fine sericite or clay minerals, and some plagioclase cores are completely altered to a mass of unidentified clay minerals. Most of the hornblende is in part altered to penninite (chlorite) and less commonly epidote and calcite are developed as patches in the hornblende. Thin, irregular sphene stringers in the hornblende are probably also the result of secondary reactions. The green hornblende that surrounds the crystals of brown hornblende and pyroxene in the gabbro might also be considered an alteration. This is merely a matter of where the line is drawn between late-magmatic and post-magmatic reactions. Biotite has the same alterations as the hornblende, but penninite is more common. Some of the brown biotite is rimmed with a green mineral that has the properties of biotite. The green mineral is probably a transitional stage in chloritization. A minor amount of serpentinization is found along the curving fractures in the olivine of the gabbro.

Uniformity of both texture and mineral content is characteristic of many of the plutons, particularly the smaller ones; but the larger plutonic bodies show marked variations, particularly in mineral content. Some of the variations may be due to missed contacts, but variations were also noted within mappable units. Variation in mineral content in the three most wide spread plutons is shown in figure 3. The method used to portray these data is that of Johannsen (1932). Each point on the diagram represents a modal analysis for which the quartz and feldspar have been recalculated to total 100 percent. The great advantage of the triangular diagram is that it permits the comparison of a great number of different specimens more rapidly and with more clarity than the table of modes (table 2). Another characteristic of these granitic rocks that can be shown by a triangular diagram is the hornblende-biotite ratio and the color index (percentage of dark minerals). Figure 3 also shows these features, using biotite, hornblende, and other minerals as the corners of a triangle. The combination of these two triangular plots permits a rapid comparison of the characteristics of several granitic masses. As a check on the homogeneity of hand specimens, two thin sections were cut at right angles from homogeneous, equigranular specimens that were considered representative of the major plutons. Figure 3 shows the variations in these specimens as determined by modal analyses. Variations of 10 percent or more in the major constituents of the same hand specimen suggest that whereas a large number of analyses permit statements on gross composition and range of composition, caution should be used in drawing conclusions based on the comparison of a few modes unless homogeneity of specimens has been established.

Table 2. Modes of granitic rocks.

¹ Location of specimens shown on figure 2.
² Specimens showing range in anorthite content indicate zoning determined on universal stage, for other specimens the most calcic plagioclase is indicated.
³ Number of regularly spaced mineral identifications made after the manner described by Chayes (1949, pp. 1-11) from a standard thin section (3/4 to 1 inch wide and about 1-1/2 inches long).
⁴ Duplicate analyses of selected thin sections resulted in variations of as much as 2 percent in the various constituents.

FIGURE 2.—Location map of modally analyzed granitic rocks. (click on image for an enlargement in a new window)