SCHEMATIC STRUCTURE SECTION A-A'
0 , , 2000
r •• t
N.
~
Jolntln!f ' no .. structure Porpnyrltlc lranoc11orlte 1. sttppltd.
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very poorly developed. These three joint directions are not restricted to the porphyritic granodiorite, but the wide spacing of the joints on these directions is found only in 'rocks having porphyritic and sub- porphyritic texture.
PETROGRAPHY OF THE MAJOR INTRUSIVE TYPES
The petrography of the border rock and of the porphyritic, sub- porphyritic and nonporphyritic textural varieties of granodiorite are
discussed in this chapter under three headings: Mineralogy, Composition and Texture. Because of the similarities of the three textural types, the mineralogy and composition are described together under common
headings. Where significant differences in mineralogy and composition do exist between textural types, th.~se differences are discussed. The
section under the heading Texture contains a separate discussion for each textural type of granodiorite.
Mineralogy
Plagioclase. In all textural varieties of the Alta granodiorites, plagioclase occurs in subhedral to euhedral crystals which show both oscillatory and progressive zoning (Fig. 5). The plagioclase crystals can be divided in two parts: a central part comprising approximately 85 percent of the crystal in which the composition ranges from An 30 to An 55 and averages An 37; and a thin rim comprising approximatelt 15 percent of the crystal in which the composition ranges from An 21 to An 26 and averages An 24. The composition of the multiple-zoned part of the plagioclase was estimated by measuring the average extinction angle in
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c .
Figure 5. Photomicrographs of zoned plagioclase crystals.
Crossed nicols, xlO.
A. A-69-P B. -309-N
C. -3-5
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crystals with favorable orientation. These data are shown in Table 5.
The break in composition between the central part of the grain and the rim is generally sharp. A modal analysis of porphyritic granodiorite A-69-P in which the oligoclase rims were counted separately from the oscillatory zoned andesine core shows that 15 percent of the plagioclase phenocrysts is present as these sodic rims. In the nonporphyritic rocks, the oligoclase rims most likely comprise a greater percentage of the plagioclase abundance.
In five thin sections, traverses were made with a magnification of X450 across favorably oriented plagioclase crystals to study in detail the changes in composition which make up the oscillatory and progressive
zoning. Extinction angles were measured at many points on the traverse and plotted as a function of distance from the rim of the grain (Fig. 6).
Where possible the extinction angles were correlated with plagioclase composition. The details of the zoning are complex, but, in general, the composition changes from An 40 at the center of the grain to An 30 near the rim. Superimposed on this gradation are the irregularities in composition described as oscillatory zoning, whi:ch consists of narrow zones having up to 15 percent more anorthite than the intervening broader zones of less calcic plagioclase. The outline of the zones generally have the euhedral shape of plagioclase, but rounded forms are also common.
The inner contact of anorthite-rich zones is commonly very sharp and embays the adjacent less calcic zone. In a short distance, these calcic
zones grade outward to the next, less calcic zone.
Quartz forms myrmekitic intergrowths with the sodic rims on the plagioclase grains. These intergrowths are found only in the non-
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TABLE 5. COMPOSITION OF PLAGIOCLASE IN GRANODIORITE
Sample
A-3-S A-5-N A-58-S
A-69-P phenos A-69-P gmass
A-76-N A-91-B A-95-N A-135-B A-137 -N A-167-P A-183-P A-186-P A-2l9-L A-220-P A-226-P A-309-N
Central part 35 35 43 37 (30)
36 38 37 37 39 39 37 39 42 37 33 36
0/0 anorthite
Rim Remarks
23 44 max
21 25
indices in the range of quartz indices
25 31 to 40 in main part of xl 25 30 to 39
20 55 max
45 max
Figure 6. Zoning in plagioclase feldspar. Extinction angles in five zoned crystals from four rocks are plotted as a function of distance in mm from the rim of the crystal. Curves are based on observations on the width and abruptness of the various zones as well as the measured and plotted points.
A. A-3-S
B.
A-3-SC.
A-137-ND.
A-69-PE.
A-309-N+10
0
Xli
0/o
A .
-10
10
X/\
0\0
10
E.
O~~7-~---t~-
G9 1.0
-40-
-~-. C.': TE..
20 30
-
2.0 ---.":. :--- --'-,----
30 40
mIn
RL : C::::TER
Figure 6. t oning in plagiocla sa f eldspar.
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porphyritic granodiorite, and only on grain contacts with orthoclase.
Nearly all plagioclase crystals contain very small euhedral crystals of apatite, K-feldspar, biotite or hornblende. The abundance of these inclusions rarely exceeds one percent of the plagioclase volume.
In addition to these minerals, fine-grained colorless sericite and yellow- green to colorless epidote-clinozoisite are found as aggregates,
commonly localized in the more calcic zones in the plagioclase. The abundance of these aggregates ranges from two to thirty percent of the plagioclase abundance. There is no correlation of the abundance of these plagioclase alteration minerals with textural type or with location in the stock.
Orthoclase. The K-feldspar in the Alta granodiorites has the optical and X-ray properties of very slightly perthitic orthoclase. The perthitic structure cannot be seen in all thin sections, however. The albite lamellae are rarely over 0.005 mm wide. X-ray diffraction patterns were made of four K-feldspar concentrates from rock samples A-58-S, A-69-P, A-95-N and A-ll2-N. The perthitic nature of the feldspar is indicated by an additional peak to the K-feldspar pattern at 280 2Q (Cu-Ka) which is made up of reflections from (040), (002) and (220) (MacKenzie, 1954). The patterns do not show a separation of either the (130) or the (131) reflections and therefore the K-feldspar in the
granodiorite is monoclinic and will be referred to as orthoclase in the remainder of the text.
The phenocrysts of orthoclase in the porphyritic rocks have weak optical zoning. Extinction angles were measured in one unusually
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large (10 mm) grain in A-57-P. The crystal is oriented with 010 parallel to the plane of the thin section. Using Tuttle's (1952, p. 563) correlation of extinction angle on 010 and composition, the data were expressed in terms of composition of the feldspar and plotted as a fU:lction of distance of the point of measurement from the rim of the grain. The resulting graph (Fig. 7) shows a progressive increase in
1.5 c~r