Orbicular Rocks of the Sandia Mountains, New Mexico
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TOMMY B. THOMPSON Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80521 DAVID L. GILES Nord Resources, Inc., 2300 Candelaria, N. E., Albuquerque, New Mexico 87107 Orbicular Rocks of the Sandia Mountains, New Mexico ABSTRACT (Figs. 2A and 2B). The core of type I orbicules consists of granular biotite monzonite (Fig. 2B) surrounded by continuous or incom- Orbicular rocks that occur within biotite-rich Precambrian granite plete biotite- and plagioclase-rich shells (Fig. 3A). The biotite mon- of the Sandia Mountains in central New Mexico are of three types: (1) zonite contains conspicuous pink microcline porphyroblasts. Bio- multishelled orbicules with alternating biotite- and plagioclase-rich tite is radially or concentrically oriented in the shells (Fig. 4A) but is shells, (2) plagioclase orbicules with or without a discontinuous random in the core material. Plagioclase tends to be anhedral and biotite shell near the orbicule margin, and (3) orbicules with granular in the core of the orbicule but occurs in the layers with a plagioclase cores surrounded by thin concentric bands of finely distinct feathery radial orientation (Fig. 3A). An outer rind consists crystalline biotite alternating with plagioclase. Cores of the or- of strikingly white oligoclase (Figs. 2A and 2B) with minor perthite. bicules consist of fragments of biotite monzonite, plagioclase, or Type I orbicules occur in a zone that is in sharp contact with the hornfels. enclosing biotite-rich granite (Figs. 2A and 2B). Locally, lenses of Petrographic data on fragment reactions during orbicule forma- pink, coarsely crystalline plagioclase separate the granite from the tion, an aplite dikelet that cuts the orbicule zone, spacing of orbicule orbicular rocks. Elsewhere, the granite and orbicular rocks are shells, and chemical analyses suggest that these orbicular rocks separated by a selvage zone that consists of biotite plates, 1 to 5 formed by reactions between xenoliths and magmatic fluids during mm long, parallel with the contact and dispersed in a granular crystallization of the granite. Key words: igneous and metamorphic plagioclase-quartz matrix. The biotite content of the granite di- petrology, orbicule, Sandia Mountains (New Mexico), minishes from 10 percent near the contact to less than 5 percent metasomatism, chemistry, petrography. one meter from the contact. The type 1 orbicule zone to the south- east is gradational into the type II orbicule zone (Fig. 1) INTRODUCTION Type I orbicules are ellipsoidal with an average major axis of 6 cm. Fragments of orbicular rocks were found in the Sandia Mountains The elongate orbicules appear to be oriented parallel to each other of central New Mexico for many years, but it was not until 1964 that and flattened on some flat outcrop surfaces of less than 0.3 m2. Else- the first outcrop was discovered. The exposure is about 28 m2 (300 ft2) where, they are randomly oriented. The outer shells appear to have in the NE'ANE'ANW'/t sec. 1, T. 11 N., R. 4 E. (Fig. 1). been molded plastically where orbicules are most crowded (Fig. 2B), but this may be a function of original fragment shape. A few blocks of ORIGIN OF GRANITE biotite monzonite are found within the type I zone, and these are The orbicular rocks occur within granite dated radiometrically at encased in biotite (Fig. 2C) and in turn by oligoclase. There is little or 1,350 m.y. B.P. by Aldrich and others (1957) and more recently at no difference in minerals between these monzonite blocks and the 1,420 m.y. B.P. by Brookins (1973). The granite is exposed on the small cores in type 1 multishelled orbicules. west face of the Sandia uplift. To the south and west of the orbicule Orbicules of type II consist of small plagioclase porphyroblasts. locality, quartz-feldspar gneiss and quartz-mica schist have sharp to Their major axes are less than 3 cm (Figs. 5A and 5B) and average 1.5 gradational contacts with the granite (Hayes, 1951; Lodewick, cm. Type II orbicules are distinctly smaller than type I orbicules. Some 1960). In the quartz-feldspar gneiss, there is an increase in microcline type II orbicules are enveloped in a discontinuous layer of biotite and decrease in quartz as the granite is approached (Lodewick, 1960; (Figs. 3B and 5B); others consist of irregular masses of granular Shomaker, 1965), so that the mode of origin of the granite has been plagioclase (Fig. 5B). The matrix consists nearly entirely of biotite and questioned. Shomaker (1965, p. 37) noted that aplite and pegmatite minor amounts of magnetite, apatite, and plagioclase (Fig. 4C). Type dikes in the granite were distributed in rectangular patterns and thus II corresponds to either the proto-orbicules or single-shell orbicules of were related to emplacement and cooling of the granite from a Leveson's terminology (1963, p. 1018). magma. Also, the granite is remarkably homogeneous (Fitzsimmons, Near the contacts of the type II zone with the enclosing granite, 1961, p. 92), suggesting a homogeneous melt rather than some of the orbicules are larger with a major axis as much as 15 cm. metasomatic replacement of varied metamorphic rocks that occur The larger orbicules consist predominantly of plagioclase surrounded in the adjacent area. In addition, numerous xenoliths are present by several cloudy, concentric layers of biotite and magnetite. The zone within the granite, and they are usually in sharp contact or display of type II orbicules is in sharp contact with the granite, but to the narrow reaction rims with the granite. The available data appear to southeast, the type II zone biotitite (Fig. 1) grades into biotite dio- indicate that the granite was emplaced as a melt during a single rite consisting of 60 to 65 percent biotite with disseminated plagio- magmatic event. Metasomatism and partial assimilation of the in- clase porphyroblasts averaging 5 mm in length (Fig. 3C). Abundant truded rocks caused the gradational contacts along the periphery of foliated xenoliths in the biotite diorite are altered to biotite (Fig. the granite. 5C). The biotite diorite is in sharp compositional and color con- trast with the granite, which locally has as much as 10 percent bio- ORBICULAR ROCKS tite but averages less than 5 percent. Field Study and Description Many scattered multishelled orbicules occur in biotite-rich rock 20 Leveson (1966, p. 410) listed 18 occurrences of orbicular rocks in to 30 m east-southeastof the main orbicule outcrop. They occur in an the United States but was not aware ofthe New Mexico locality at that area approximately 15 m by 2 m and parallel the apparent trend time. He (1966, p. 421-422) presented a classification of orbicules (east-southeast) in Figure 1. These orbicules, termed type III, range that is used in this report. from those consisting of a single hornfels inclusion to those in which The relations of the three types of orbicules at the New Mexico the center ofthe orbicule is plagioclase surrounded by thin concentric locality are shown on Figure 1. Type I orbicules are multishelled with bands of finely crystalline biotite alternating with plagioclase (Figs. alternating and irregularly spaced layers of biotite and plagioclase 6A and 6B). Biotite is less abundant than in the type 1 orbicules of the Geological Society of America Bulletin, v. 85, p. 911-916, 6 figs., June 1974 911 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/6/911/3443843/i0016-7606-85-6-911.pdf by guest on 30 September 2021 912 THOMPSON AND GILES Sandoval County R. 4 E. Orbicular rocks: p€oj., type I 106 30 orbicule; pGo^, type II orbicule ,p€b ) / Biotite diorite v • I p€g Granite EME3 Schist 0 3 m Contact, dashed where approximately located. Sample and photograph location Strike and dip of foliation -^71 Geology by Tommy B. Thompson, 1964, 1969 Figure 1. Index and geologic map of the Sandia orbicule locality, New Mexico. Base adapted from U.S. Geological Survey, Sandia Crest, New Mexico, quadrangle, 1961. main outcrop to the northwest. Spacing of the scattered orbicules Petrographic and Chemical Studies varies from greater than 380 to 3 8 per m3. The orbicules average 6 cm in axial length like the type 1 orbicules at the main outcrop. The type III Type I orbicules consist of concentric layers of plagioclase and orbicules probably represent partially "arrested" orbicules that did biotite with accessory quartz, apatite, magnetite, rutile, sphene, and not react as completely as those at the main outcrop and therefore zircon. Not all layers are continuous, and biotite in some layers will apparently record stages of orbicule development. Some of the merge with biotite on an outer shell (Fig. 2B). Plagioclase:biotite xenoliths contain remnant foliation (Fig. 5C). ratios within the orbicules average 3.75:1. Total quartz consti- The age relations between the orbicule formation and the igneous tutes less than 5 volume percent of the orbicules. The biotite is re- and metamorphic history of the granite are relatively clear. A sing e markably homogeneous from the center to the rim of the orbicules episode of regional metamorphism occurred prior to emplacement of (Table 1), but it is nol noticeably poikilitic (Fig. 4A). Finely crystal- the granite (Fitzsimmons, 1961, p. 94). The igneous rocks belong to a line apatite, rutile, magnetite, and zircon are disseminated in the single Precambrian igneous event that concluded with emplace- biotite. Plagioclase composition, determined from extinction angles ment of aplite and pegmatite dikes (Shomaker, 1965). One aplite of albite twins and refractive indices, varies from calcic oligoclase dikelet cuts the orbicula- granite (Fig. 5A); this indicates that or- (An;a) in the centers of orbicules to sodic oligoclase (An,,,) in the bicules formed before th; final stage in the igneous cycle.