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. 732 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/732/3428968/i0016-7606-86-6-732.pdf by guest on 27 September 2021 LAYERED SERIES OF THE WICHITA COMPLEX, OKLAHOMA 733 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/732/3428968/i0016-7606-86-6-732.pdf by guest on 27 September 2021 .1 mm 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.