Bulletin of the Geological Society of America Vol

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 69, PP. 1191-1192. 1 PL. SEPTEMBER IQBS

RIEBECKITE GRANITE IN THE WICHITA MOUNTAINS, OKLAHOMA

BY W. T. HUANG

Dark-green riebeckite-rich granite occurs in mixture of magnetite and hematite with tlie Wichita Mountains of southwestern Okla- zircon. In one thin section, however, magnetite homa. The green granite forms irregular and hematite occur together as uniformly bodies within a large mass of reddish aegirite distributed interstitial grains up to 1.5 mm in and riebeckite rocks and crops out chiefly diameter and make up 6 per cent of the rock. in northeastern Elm Mountain in the east- Hornblende occurs in anhedral grains up to central part of T. 3 N., R. 15 W. Smaller 4 mm across. The striking pleochroism (X = areas of similar rocks are on Quanah Mountain blue, Z = green or brown) suggests that it is in the southeastern portion of T. 3 N., R. 14 W. rich in sodium. The hornblende occurs in This rock type has not been described in felted aggregates and irregular crystals with previous reports on the Wichita Mountains. clots of magnetite and ilmenite, suggesting Megascopically one can recognize quartz, alteration after pyroxene. In it, /3 ranges from pinkish potassium feldspar, white plagioclase, 1.659 (blue variety) to 1.679. green amphibole, and dark-green pyroxene, Aegirite or aegiritic pyroxene forms euhedral all having an average grain size of about 3 mm. to subhedral grains 0.2 mm across associated Under the microscope, the granite is holo- with hornblende and riebeckite. The mineral crystalline, hypautomorphic granular. Table 1 is strongly pleochroic with X = grass green is a modal composition based on Rosiwal and Z = yellowish green; 2V(+) = 62°. Some analyses of three thin sections. aegirite is partly altered to hornblende mixed Quartz occurs in anhedral grains mostly 1-4 with iron ores. mm in diameter, but ranging up to 6 mm. In the aegirite of one specimen strongly Tiny quartz grains within plagioclase in pleochroic astrophyllite occurs as an irregular myrmekitelike blebs and graphic intergrowths grain 0.1 by 0.4 mm. The color ranges from red are probably of replacement origin. to yellow. Many small dark-brown fibrous The potassium feldspar is orthoclase micro- aggregates associated with hornblende may perthite and has the same size range as quartz. also be astrophyllite. Small red granules, Optically it appears to be monoclinic (j_ X, probably of the same mineral, are associated ZAa = 4°; _L Z, XAb = 0°); 2V (-)= 54°, with clusters of riebeckite needles. corresponding to Or74(Ab-An)26 (MacKenzie Zircon is common in all the thin sections and Smith, 1956, p. 406). Numerous specks of examined; a few large euhedral crystals up to clay and some disseminated sericite makes the 6 mm long are zoned, similar to those found potassium feldspar turbid. The plagioclase is in pegmatite that cuts the Quanah granite unzoned oligoclase (An2s) concentrated at the (Larsen et al., 1953). In places large zircon margins of perthite grains. crystals occur together as interstitial grains. There are two amphiboles in the rock: Two generations of zircon can be recognized: riebeckite and hornblende. Strongly pleochoric the large euhedral zircon crystals are primary, riebeckite with X = blue, Y = yellow green, whereas the minute ones are secondary. The and Z = light yellow forms stubby prisms decrease in grain size from the primary to about 2 mm long and sheaves of very thin secondary is extremely abrupt. Of particular needles 0.2-0.5 mm long. The aggregates are interest is the close relationship between clustered in areas where they are associated secondary zircon and riebeckite. The zircon with biotite, aegirite, iron ores, and quartz. occurs only with altered riebeckite. The gra- The riebeckite has X || c, and low birefrin- nitic rocks of the Wichita Mountains are notable gence, with 13 about 1.688. for the occurrence of abundant zircon crystals, One of the two alteration products of rie- and zirconia content of the rocks must be high. beckite is a fine-grained yellowish pseudomorph Rockallite analyzed by Washington (1914, with higher birefringence but the same parallel p. 297) contained 1.17 per cent zirconia; since extinction; it partly or completely replaces zircon is rare in this rock, Washington con- some sheaves of riebeckite needles and may cluded that zirconia is present in the acmite, be acmite. A second alteration product is a and he calculated that it contains 2.6 per cent 1191

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zirconia (1914, p. 300). He suggested that the prehnitization of gabbro in the Wichita Moun- distinction between the yellow-brown acmite tains (Huang, in press). and the green aegirite may be due to zirconia Minor accessories—apatite, sphene, biotite, or oxides of rare earths in the former and their muscovite, epidote, and magnetite—are dis- absence from the latter (1914, p. 301; Wash- tributed throughout the rock in euhedral to subhedral crystals. TABLE 1.—MINERAL COMPOSITION OF DARK-GREEN The dark-green riebeckite-rich granite was RIEBECKITE GRANITE, WICHITA MOUNTAINS (Volume per cent) located in the field because of a specimen pre- Quartz 20 sented by the late Professor J. Willis Stovall, Potassium feldspar 25 of the University of Oklahoma, who gave Plagioclase (An2s) 6 preliminary information on its occurrence. Riebeckite 25 Hornblende 5 Aegirite ] 2 Biotite ] References Cited Muscovite 1 Zircon 1 Huang, W. T., 1954, Petrography of the troctolite Other accessories 4 of the Wichita Mountains, Oklahoma: Am. Mineralogist, v. 39, p. 549-565 (in press), Prehnitization of gabbro: Texas ington and Merwin, 1927, p. 233, 248). A Jour. Sci. comparable view was expressed by Mathews Larsen Jr., E. S., Waring, C. L., and Berman, J., and Watson (1953, p. 445-446) regarding 1953, Zoned zircon from Oklahoma: Am. Mineralogist, v. 38, p. 1118-1125 alkali rhyolite dikes in northern British Colum- MacKenzie, W. S., and Smith, J. V., 1956, The bia. A similar explanation might fit the Wichita alkali feldspars, III, An optical and x-ray rocks. study of high-temperature feldspars: Am. Iron-rich olivine was found in 2 of the 20 Mineralogist, v. 41, p. 405-427 Mathews, W. H., and Watson, K. D., 1953, thin sections. The mineral occurs in irregular Spherulitic alkali rhyolite dikes in the Atsutla grains or broken euhedra, 0.3-0.5 mm across, Range, Northern British Columbia: Am. associated with clusters of riebeckite needles. Mineralogist, v. 38, p. 432-447 Many olivine grains were selectively replaced Trbger, W. E., 1956, Optische Bestimmung der by iron ore, a common feature of olivine altera- Gesteinsbildenden Minerale-Teil I, Bestim- mungstabellen: E. Schweizerbart'sche Ver- tion in the Wichita Mountains igneous rocks lagsbuchhandlung, Stuttgart (Huang, 1954 p. 554-557). 2V, was deter- Washington, H. S., 1914, The composition of mined to be 58° (universal-stage measurement rockallite: Geol. Soc. London Quart. Jour., of six grains). According to Trb'ger (1956, p. 37), v. 70, p. 294-301 this corresponds approximately to Fast, a fer- Washington, H. S., and Merwin, H. E., 1927, The rohortonolite. acmitic pyroxenes: Am. Mineralogist, v. 12, In one thin section radiating prehnite p. 233-252 sheaves, 1 to 2 mm long, make up almost 2 DEPARTMENT OF GEOLOGY, BAYLOR UNIVERSITY per cent of the rock. Wavy extinction is com- WACO, TEXAS mon. Pleochroism is from pale yellow to color- MANUSCRIPT RECEIVED BY THE SECRETARY OF THE less. The prehnite here recorded is like the SOCIETY, APRIL 8, 1958

PLATE 1.—PHOTOMICROGRAPHS FIGURE 1.—Riebeckite granite from Elm Mountain, NE 1/4 Sec. 24, T. 3 N., R. 14 W. Euhedral and altered riebeckite crystals occupy most of the photograph. The rest is potassium feldspar, plagioclase, quartz, and biotite. Note that zircon occurs only with altered riebeckite (lower central and right). Others are bubbles (upper). Plain light, X 36. FIGURE 2.—Enlargement of the altered riebeckite in Figure 1. The riebeckite is a mixture of magnetite, hematite with closely associated zircon (high relief). Plain light, X 245.

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PHOTOMICROGRAPHS

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