Layered Series of the Wichita Complex, Oklahoma

NANCY SCOFIELD Department of Geology and Geological Engineering, Michigan Technological University, Houghton, Michigan 49931

ABSTRACT

The core of the Wichita Mountains of Oklahoma is a layered igneous intrusive mass composed of plagioclase cumulates, predominantly anorthosite, with some olivine-bearing anorthosite, gabbro, and olivine gabbro. Some chemical trends in the rocks indicate that cryptic layering is pres- ent but is the reverse of that found in most mafic layered intrusions. The anomalous position of highly calcic plagioclase near the top of the intrusion, inferred from field relations, coupled with possible reverse cryptic layering suggests a separation of anorthosite by flotation or rafting of plagioclase. Key words: igneous petrology, layered intrusion, geochemistry.

INTRODUCTION

The Wichita Complex in southwestern Oklahoma is a nearly horizontal complex of intrusive rocks of Cambrian age. It is composed of a core of mafic rocks, gab- broic in subsurface and anorthositic in out- crop, which are intruded and overlain by granitic sills and plutons. Locally, diorite and quartz diorite occur between gabbro and granite. On the basis of their exposed Wichita Granite Group Carlton Rhyolite Group extent and subsurface data, the lateral di- CAMBRIAN mensions of the granitic rocks are at least €wg Outcrop outcrop 65 km by 175 km and those of the mafic rocks are 40 km by 175 km (Fig. 1). The ggèwâjï;:; Subsurface Subsurface form of the complex is inferred to be elon- gate and lenticular, trending northwest. Raggedy Mountain Gabbro Group Navajoe Mountain The intrusive complex is surrounded and Basa It-Spilite Group CAMBRIAN overlain by an average of 900 m of rhyolitic Outcrop volcanic rocks that have essentially the same chemical composition as the granitic Subsurface r *.Cnb i 1 Subsurface only intrusive rocks and an even more wide- spread areal distribution. Volcanic rocks of Tillman Metasedimentary EARLY basaltic, spilitic, and andesitic composition Group CAMBRIAN are widely distributed in the subsurface or LATE north and south of the Wichita Mountains €tm— Subsurface only PRECAMBRIAN and are as thick as 300 m. The igneous

rocks of silicic and basic compositions are OK LAHOMA exposed over about 1,000 km2, with anor- P€m Meta sediments thositic rocks exposed in the more uplifted WWO^- Study Area PRECAM BRIAN WICHITA I»«.»» •.«•.! and eroded central part of the complex. MOUNTAINS y-»>€r'„V Rhyolite 1,100-1,400 million years The sequence of anorthositic rocks has ^ « S / -I • A _ been shown to be a layered intrusion Location Map Granite vV (Chase, 1950; Gilbert, 1960; Hunter, : 1962). This paper presents detailed petro- Figure 1. Basement-rock map of southwestern Oklahoma and adjoining parts of Texas (after Ham graphic and chemical information on the and others, 1964).

Geological Society of America Bulletin, v. 86, p. 732-736, 5 figs., June 1975, Doc. no. 50602.

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maximum composite vertical section of the from closely related tectonic and thermal all rocks as accessory minerals. Pyrite, some exposed layered series, evidence for reverse processes. of which is surrounded by pyrrhotite or cryptic layering, and a hypothesis of dif- goethite, and apatite are rare. Alteration is ferentiation for the anorthositic rocks. LAYERED SERIES minor. The rock types (usage of Johannsen, REGIONAL GEOLOGY The layered series of anorthositic rocks 1939) are anorthosite, olivine-bearing that crop out in the central Wichita Moun- anorthosite, gabbro, and olivine gabbro. The Wichita Mountain region, approxi- tains was subdivided into the troctolite, Anorthosite predominates, and the other mately 6,500 km2, is a complex uplifted anorthositic gabbro, and olivine gabbro rock types are close to anorthosite in com- mass bounded by faults, between the Hollis zones (stratigraphically lowest to highest) position. Olivine gabbro, in particular, is basin on the south and the Anadarko basin on the basis of mineralogical and textural heterogeneous, and thin sections from it are on the north (Ham and others, 1964). This features (Gilbert, 1960; Hunter, 1962). not representative of the rock as a whole. block is composed of granite and anortho- However, because of lateral variation in This heterogeneity is probably responsible site at the surface and is covered by clastic lithologies, the units were renamed the K, for a few thin sections that contain 30 to 50 sedimentary rocks of Permian age. L, and M zones, respectively (Spencer, percent modal pyroxene. Textures are those Radiometric age determinations (Deni- 1961). These rocks are intruded by many of plagioclase cumulates, ranging from ad- son and others, 1966; Muehlberger and dikes and sills of biotite olivine gabbro, cumulate anorthosite to heteradcumulate others, 1966; Burke and others, 1969) indi- olivine microgabbro, microdiorite, aplite, olivine-bearing anorthosite to mesocumu- cate that the mafic and granitic rocks of the and granite (Hunter, 1967). late olivine gabbro. Dominant textural Wichita Mountains crystallized 490 to 510 Three sections providing maximum influences are igneous lamination and ad- m.y. ago. Although field relations clearly stratigraphic exposure were selected for de- cumulus growth of anorthosite and inter- show that the granitic rocks are younger tailed study (Fig. 2), and named Lower, cumulus development of large, poikilitic (Ham and others, 1964), the ages of the Middle, and Upper sections (Scofield, clinopyroxene crystals in gabbro (Fig. 3). two rock types "are extremely close and are 1968), with thicknesses of 20, 65, and 60 Lower, Middle, and Upper Sections. essentially not resolvable upon the basis of m, respectively. The Lower section is within The Lower section, entirely within the K present isotopic evidence" (Denison and the K zone. The bottom 10 m of the Middle zone, consists primarily of anorthositic others, 1966, p. 175). The very close rela- section, also in the K zone, is overlain by 35 gabbro (90 to 95 percent modal plagio- tion in space and time of all the Wichita m of L zone, which in turn is capped by 20 clase) with and without olivine in rhythmic igneous rocks and the many characteristics m of M zone. The Upper section is within layers. Individual layers with abrupt transi- they have in common with comparable as- the M zone. tions are 8 to 20 cm thick. The K zone of semblages in similar geologic settings imply Of the 148 samples collected, 80 were the Middle section also exhibits rhythmic that the mafic and silicic rocks resulted petrographically examined in thin section, layering, but it is on a larger scale and has and 33 were chemically analyzed by x-ray gradual transitions. The L zone of the Mid- fluorescence1. Mineral compositions were dle section is composed entirely of anortho- determined by the following methods: site of greater than 96 percent modal OKLAHOMA ^ plagioclase by fusion (Foster, 1955; curve, plagioclase. The M zone at the top of the Schairer and others, 1956), universal stage, Middle section is rhythmically layered extinction angles (curves, Tobi, 1963), and olivine gabbro and anorthosite. In the .Study Area x-ray fluorescence, Si/Al ratios (Scofield, olivine gabbro, clinopyroxene crystals are ^"^WICHITA MOUNTAINS 1968); olivine by x-ray diffraction (Yoder equidimensional, about 2 to 5 cm in diame- and Sahama, 1957); clinopyroxene by uni- ter, and they give the rock a glomeropor- versal stage, 2V, refractive index (Deer and phyritic texture. The M zone of the Upper others, 1963, Vol. 2); and orthopyroxene section is similar to that in the Middle sec- R 1 7 w ^ \ by refractive index (Deer and others, 1963, tion but differs in that clinopyroxene crys- Vol. 2). tals are elongate, up to 20 cm in length, and widely separated. Petrography Modal analyses representative of the i o- range of rock types are given in Table 1, cvi Certain aspects of the petrography of and modal analyses for the 80 petrographi- the layered series have been described cally examined thin sections are available 5> 16 LU (Chase, 1950; Gilbert, 1960; Rotan, 1960; (see footnote 1). Norms were calculated for CO o Hiss, 1960; Spencer, 1961; Karns, 1961). the analyses presented in Table 1. All sam- Vo The rocks consist mainly of plagioclase ples except R12U2 and R34U1 are (An62_83), olivine (Fo66_74), clinopyroxene nepheline normative, all samples except t\ Ít (Wo38_45En4o_37Fs22-i8)> and orthopyroxene R29M2 and R34U1 are olivine normative, (En80-83)- The common compositional and R34U1 is the only sample that is quartz N \ range for plagioclase is An73_78, but a single normative. thin section may show a range as wide as An62_8o- Because of the large amount of Chemistry f! scatter, mineral compositions could not be Y®' $ 1 used as indicators of cryptic layering. The consistently high plagioclase content 1 22 z Opaque minerals, predominantly titanifer- is reflected in the chemical composition of y*\(3. ^ * ous magnetite and ilmenite, are present in the 33 analyzed samples (see footnote 1). 0 0.5 I Their mean composition and analyses rep- 1 I I 1 MILE Copies of GSA supplementary material 75-15 may resentative of the range of compositions are be ordered from Documents Secretary, Geological Soci- Figure 2. Locations of stratigraphie sections ety of America, 3300 Penrose Place, Boulder, Colorado given in Table 1. The mean analysis ap- sampled. 80301. proximates the composition of bytownite

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Figure 3. A, sample R8M2, anorthosite, crossed nicols. Vermicular plagioclase at edges of titaniferous magnetite. In some wide (0.5 mm) coronas, ilmenite is intergrown with orthopyrox- "normal" plagioclase grains; that attached to upper crystal shows optical continuity with it at ene near outer edges of coronas. E, sample R4M3, olivine gabbro, crossed nicols. Poikilitic right, but offset twin plane near center of figure indicates a degree of optical discontinuity; ver- olivine (ol) enclosing plagioclase (pi). Generally, plagioclase is not enclosed in center of olivine micular growth adjacent to lower grain shows complete optical discontinuity. B, sample R12U2, grains, and olivine crystals are interpreted as primocrysts that continued to grow poikilitically gabbro, plane polarized light. Large poikilitic magnetite crystal (mt) and ophitic clinopyroxene from intercumulus material. Orthopyroxene coronas commonly extend the poikilitic habit. F, (cpx) totally surrounded by large and small plagioclase (pi) grains. C, sample R8M2, anortho- sample R12U2, gabbro, crossed nicols. Ophitic clinopyroxene (cpx) with surrounding plagio- site, plane polarized light. Interstitial clinopyroxene (gray) with associated magnetite and ilmen- clase (pi) crystals and smaller enclosed plagioclase crystals. In many thin sections, plagioclase ite (black). Apatite (ap) occurs as two slender needles within the pyroxene and two cross sec- crystals are approximately same size inside and outside of pyroxene crystals. Genetic significance tions. D, sample R3M3, olivine gabbro, plane polarized light. Orthopyroxene-magnetite inter- of difference is uncertain. Near center of clinopyroxene, corona of orthopyroxene (opx) sur- growth in upper part of figure adjacent to olivine (gray) in lower part; white area is plagioclase. rounding olivine (ol) may be seen. Olivine is generally surrounded by corona of orthopyroxene intergrowth with vermicular

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TABLE 1. REPRESENTATIVE CHEMICAL AND MODAL ANALYSES OF LAYERED SERIES Z.0

§ 1.5 Lower section Middle section Upper section CE

K zone L zone M zone à l0' P R1L5* R3L1 R2M2 R7M1 R28M1 R29M2 R12U2 R17U3 R34U1 Mean 0'

Chemical Analyses Oxides U) SiOi 46.58 47.51 45.90 47.10 46.72 47.35 47.58 46.69 47.71 47.03 UJ AUO 28.50 31.80 28.60 32.10 33.40 34.50 31.00 29.70 33.20 31.27 c_> Fe!03+ 3.95 1.75 4.00 1.80 0.75 0.30 2.45 3.35 0.46 1.70 a. TiOî 0.020 0.510 0.015 0.330 0.000 0.000 0.000 0.065 0.025 0.09 LÜ O MnO 0.051 0.030 0.050 0.029 0.017 0.012 0.045 0.052 0.022 0.03 CL H CaO 13.20 14.45 13.70 14.22 15.00 15.55 13.70 14.00 14.80 14.41 MgO 4.85 1.15 5.63 0.20 0.40 0.00 3.00 3.80 0.00 1.67 K20 0.130 0.180 0.090 0.170 0.110 0.120 0.130 0.115 0.140 0.13 X Na20 2.40 2.91 2.07 2.98 2.70 2.73 2.35 2.53 2.62 2.63 0 P20 5 0.085 0.110 0.000 0.085 0.185 0.085 0.085 0.190 0.000 0.10 UJ HjO" 0.033 0.044 0.194 0.070 0.093 0.066 0.378 0.170 0.064 M+ 0.344 0.294 0.000 0.215 0.199 0.240 0.157 0.112 0.287 0.205 0.4-1 Total 100. l4 100.74 100.25 99.30 99.57 100.95 100.88 100.77 99.33 99.06 0.3- O Modal Analyses c 0.2- • • Minerals5 co & (*) UJ 0.1- PI 92.0 98.0 90.2 97.8 56.9 99.0 67.3 93.7 98.9 Q Cpx 0.9 1.6 1.1 1.8 36.5 1.0 23.4 0.9 0.6 X Opx 1.8 0.0 3.1 0.0 5.4 0.0 7.8 1.4 0.0 o 01 5.4 0.0 5.0 0.0 0.0 0.0 0.0 3.7 0.0 Op tr. 0.3 0.6 0.5 1.2 tr. 1.5 0.3 0.3 0

* L, M, or U 1n sample numbers refer to Lower, Middle, and Upper sections, respectively. Digits preceding these letters refer to relative stratigraphie position of outcrop; one is lowest. Digits following L, M, or U refer to relative vertical position of sample within that outcrop; one is lowest, t Total iron. S PI = plagioclase; Cpx = clinopyroxene; Opx = orthopyroxene; 01 = olivine; Op = opaque; and tr. = trace. EE . 0 • r-, n 16*00 l7bo is'oo — K ZONE—- «—L ZONE—- •—M ZONE— (Deer and others, 1963, Vol. 4), with a cated for the generation of anorthosite by ELEVATION IN FEET trace of mafic constituents, and the near Grout (1918, 1928) and Morse (1968). Figure 4. Plots of oxides versus elevation for uniformity is indicated by the narrow range Huang (1962, p. 183) specifically suggested anorthosites of Middle section. in the analyses from the different rock it for cumulates in the Wichita Mountains. types. Hess (1960) examined density relations X ANORTHOSITE To detect possible cryptic layering, the O OLIVINE-BEARING ANORTHOSITE of the fine-grained norite of the Stillwater • GABBRO weight percentages of the oxides were plot- Complex border facies as parental magma ^ 155- ted against elevation for samples from the and bytownite An . He concluded (1960, LLI 85 0 Middle and Upper sections. Discernible p. 85) that "the plagioclase probably would et 15.0- trends are shown in Figures 4 and 5. For the Ld not have floated upward" but also that the Q. Middle section, only anorthosite analyses density data led to no positive conclusion I— 145- are plotted. The weight percentages of total about the feasibility of flotation, because 1 iron, Ti02, and MnO show trends of de- density differences at the estimated temper- O creasing weight percentage oxide with in- ature of the beginning of crystallization W 14.0-1 creasing elevation. The trend of the weight (1125°C) are in the range 0.01 to 0.07 g/cc, percentage of MgO is not as well defined, with the crystals being denser. and the relative importance of one sample Even if conditions for true flotation of o (R1M2) with an analysis of 1.00 should be plagioclase crystals are not obtained, the D O 13.0 noted. Weight percentage of CaO for all narrower the density difference between 1600 1700 samples from the Upper section was plotted magma and crystals, the greater the proba- M ZONE- (Fig. 5) and the rock types differentiated. bility the crystals will be carried along by ELEVATION IN FEET CaO shows a general increase with eleva- upward currents. Wager and Brown (1967) tion. The iron-enrichment trends of the regarded plagioclase crystals of the Upper Figure S. Plot of CaO versus elevation for all Middle section and the CaO trend of the Border Group of the Skaergaard intrusion rocks of Upper section. Upper section are opposite of those found as primocrysts that were carried upward by in most mafic layered intrusions. The term convection currents and became stuck in "reverse cryptic layering" is thus proposed the congealing magma at the top of the in- cryptic layering. Morse's (1968) model ac- for such trends. trusion. The magma was assumed to be counts for igneous lamination by flow, slightly less dense than the crystals. coarse grain size by adcumulus growth, and DISCUSSION Wager and Brown (1967) noted that crystallization of intercumulus minerals by Upper Border Group rocks showed coarse- isolation of intercumulus liquids at times or The cumulate nature of the layered series ness of plagioclase crystals, some igneous places of high accumulation rates. All of is well established, and cumulates are gen- lamination, and few large primocrysts of these features have been described as erally postulated as having been formed by olivine and augite, and that interstitial characteristics of the layered anorthosites crystal settling in the magma chamber olivine, pyroxene, and magnetite formed of the Wichita Complex; however, the re- (Wager and Brown, 1967). However, after plagioclase crystals were oriented. verse cryptic layering of these rocks is flotation of crystals is also a possible Moreover, plagioclase became increasingly shown by specific bulk chemical trends mechanism of accumulation and was advo- calcic with height; that is, it showed reverse rather than in plagioclase compositions.

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The wide range of plagioclase compositions REFERENCES CITED Jones, V. L., and Lyons, P. L., 1964, Vertical could be explained by the variability of up- intensity magnetic map of Oklahoma: ward currents. Burke, W. H., Otto, J. B., and Denison, R. E., Oklahoma Geol. Survey Map GM—6, scale In addition, geophysical evidence, 1969, Potassium-argon dating of basaltic 1:750,000 (with text). specifically very large positive gravity and rocks: Jour. Geophvs. Research, v. 74, p. Karns, A.W.W., 1961, Ophitic pyroxene from 1082-1086. magnetic anomalies (Lyons, 1964; Jones the Raggedy Mountains area, Wichita Chase, G. W., 1950, The igneous rocks of the Mountains, Oklahoma [M.S. thesis]: Nor- and Lyons, 1964), indicates a large gab- Roosevelt area, Oklahoma [M.S. thesis]: man, Univ. Oklahoma, 68 p. broic mass at depth. The best geologic evi- Norman, Univ. Oklahoma, 108 p. Lyons, P. L., 1964, Bouguer gravity-anomaly dence suggests that the exposed part of the Deer, W. A., Howie, R. A., and Zussman, J., map of Oklahoma: Oklahoma Geol. Survey layered series represents the upper part of 1963, Rock-forming minerals (Vols. 2 and Map GM-7, scale 1:750,000 (with text). the intrusion (Ham and others, 1964). If a 4): London, Longmans, 379 p., 435 p. Morse, S. A., 1968, Layered intrusions and anor- basaltic parental magma is assumed, such a Denison, R. E., Hetherington, E. A., Jr., and thosite genesis, in Isachsen, Y. W., ed., basic composition is not expected near the Kenny, G. S., 1966, Isotopic-age dates from Origin of anorthosite and related rocks: top of the intrusion. Therefore, it is a basement rocks in Oklahoma: Oklahoma New York State Mus. and Sci. Service reasonable possibility that this layered Geology Notes, v. 26, p. 170-176. Mem. 18, p. 175-187. series of anorthositic rocks of the Wichita Foster, W. R., 1955, Simple method for the de- Muehlberger, W. R., Hedge, C. E., Denison, termination of the plagioclase feldspars: R. E., and Marvin, R. F., 1966, Geochron- Complex differentiated by the rafting of Am. Mineralogist, v. 40, p. 179-185. ology of the midcontinent region, United plagioclase crystals by upward currents. Gilbert, M. C., 1960, The geology of the western States; 3. Southern area: Jour. Geophys. Glen Mountains, Oklahoma [M.S. thesis]: Research, v. 71, p. 5409-5426. SUMMARY Norman, Univ. Oklahoma, 48 p. Rotan, P. M., 1960, Preferred orientation of Grout, F. F., 1918, Two-phase convection in plagioclase in basic rocks, Raggedy Moun- All rocks investigated from the Wichita igneous magmas: Jour. Geology, v. 26, p. tains, southwestern Oklahoma [M.S. Complex are anorthositic. Some contain as 481-499. thesis]: Norman, Univ. Oklahoma, 62 p. much as 7.1 percent modal olivine. Pyrox- 1928, Anorthosites and granite as differen- Schairer, J. F., Smith, J. R., and Chayes, F., 1956, ene generally ranges from a trace to 15 per- tiates of a diabase sill on Pigeon Point, Refractive indices of plagioclase glasses: Minnesota: Geol. Soc. America Bull., v. 39, Carnegie Inst. Washington Year Book 55, cent. Igneous lamination is ubiquitous, p. 555-578. p. 195. rhythmic layering occurs on several scales, Ham, W. E., Denison, R. E., and Merritt, C. A., Scofield, N., 1968, Vertical variation in the and reverse cryptic layering is suggested for 1964, Basement rocks and structural evolu- layered series, Raggedy Mountain Gabbro total iron, Ti02, and MnO in the Middle tion of southern Oklahoma: Oklahoma Group, Kiowa County, Oklahoma [M.S. section and CaO in the Upper section. The Geol. Survey Bull. 95, 302 p. thesis]: Norman, Univ. Oklahoma, 155 p. occurrence of such a basic composition in Hess, H. H., I960, Stillwater igneous complex, Spencer, A. B., 1961, Geology of the basic rocks the inferred upper part of the intrusion, Montana: A quantitative mineralogical of the eastern portion of the Raggedy coupled with possible reverse cryptic layer- study: Geol. Soc. America Mem. 80, 230 p. Mountains, southwestern Oklahoma [M.S. ing, suggests a hypothesis of origin of the Hiss, W. L., 1960, Ferromagnesian minerals in thesis]: Norman, Univ. Oklahoma, 46 p. basic igneous rocks, Raggedy Mountains anorthositic rocks by flotation or rafting of Tobi, A. C., 1963, Plagioclase determination Area, Wichita Mountains, Oklahoma [M.S. with the aid of the extinction angles in sec- plagioclase crystals. thesis]: Norman, Univ. Oklahoma, 104 p. tions normal to (010). A critical compari- Huang, W. T., 1962, Petrology: New York, son of current albite-Carlsbad charts: Am. ACKNOWLEDGMENTS McGraw-Hill Book Co., 480 p. Jour. Sci., v. 261, p. 157-167. Hunter, H. E., 1962, Layered basic intrusive Wager, L. R., and Brown, G. M., 1967, Layered Most of this study was undertaken for rocks of the Wichita Mountains, S. W. igneous rocks: San Francisco, W. H. partial fulfillment of the requirements of the Oklahoma [abs.]: Am. Mineralogist, v. 47, Freeman and Co., 588 p. Master of Science degree at the University p. 192. Yoder, H. S., and Sahama, G., 1957, Olivine of Oklahoma. G. T. Stone directed that part 1967, Raggedy Mountain Gabbro Group: X-ray determinative curve: Am. Miner- Geol. Soc. America South-Central Section alogist, v. 42, p. 475-491. of the investigation. W. I. Rose, Jr., R. E. Field Trip Guidebook, p. 34-41. Denison, and S. C. Nordeng critically re- Johannsen, A., 1939, A descriptive petrography MANUSCRIPT RECEIVED BY THE SOCIETY MAY 2, viewed the manuscript. The photomicro- of the igneous rocks, Vol. 1 (2nd ed.): 1974 graphs were taken by Virginia Doane of Chicago, University of Chicago Press, REVISED MANUSCRIPT RECEIVED OCTOBER 11, Michigan Technological University. 318 p. 1974

Primed in U.S.A.

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