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

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 southeast 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

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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. The bio- rinds. Microprobe data (Table 1) indicate that the plagioclase may tite monzonite and biotite diorite represent a large xenolith within be as calcic as andesine (An3;>) but ranges downward to oligoclase the granite. The xenolith was subjected to synmagmatic physical (An2(i). and chemical changes that resulted in the formation of the orbicu- Type 111 orbicules east-southeast of the main outcrop exhibit what lar rocks. we have termed arrested reactions. Orbicules contain a wide range of

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Figure 2. (A) Contact (indicated by lines) of type I orbicule with granite (loc. 1 on Fig. 1). Scale in millimeters. (B) Typical appearance of type: 1 orbicules at the Sandia Moun- tains locality. Note the random orientations (loc. 3 on Fig. 1). (C) Biotite encasing a biotite monzonite fragment within the type I orbicule zone (loc. 4 on Fig. 1).

xenoliths and products ranging from those with distinct xenolith centers rimmed by plagioclase and minor biotite to those with plagioclase centers (that is, no original material) rimmed by plagioclase and finely crystalline biotite crystals with distinct concentric arrangement (Fig. 4B). The xenoliths consist of biotite monzonite, biotite schist, and hornfels. Only those orbicules with hornfels xenoliths show a gradual loss of xenolith identity when com- paring several orbicules (Fig. 6A). The hornfels xenoliths consist of albite (An6), quartz, biotite, muscovite, epidote, chlorite, and accessory apatite, magnetite, and sphene. This assemblage is a typical albite-epidote-hornfels facies caused by contact metamorphism. Surrounding and replacing the xenolith is a reaction zone contain- ing poikiloblastic oligoclase with finely crystalline quartz, minor apatite, and scattered remnants of epidote and muscovite. Type II orbicules consist mostly of oligoclase but may have a single discontinuous biotite shell (Fig.3B). Refractive indices of biotite from both type I and type II orbicules are identical. The type II orbicules "float" in a matrix of biotite, magnetite, and sphene. The orbicules constitute approximately 36 percent of the rock volume within the zone. The mode of the biotitite that contains type II orbicules (Fig. 3B) is 62 percent biotite and 38 percent plagioclase. Thus, there is no significant mineralogic change between type II orbicules and matrix and the adjacent biotitite. Similarly, no significant difference exists between the biotite monzonite of type I cores and the and Asquith (1971), who believed that rock fragments (mainly igne- adjacent and enclosing biotite monzonite matrix between type I or- ous) served as nuclei for orbicule growth. Circulation of the orbicules bicules. Whole-rock analyses of four type 1 orbicules (Table 2) within a converting intrusive magma to alternately hot and cool re- reflect the predominant plagioclase-biotite content. (The charac- gions accounted for rhythmic deposition of biotite and feldspar. teristics of the three orbicule types at the Sandia Mountains locality The variety of orbicule types in proximity to each other, the lack of are summarized in Table 3.) zoning in the plagioclase, the lack of shell continuity, the differing proportions of biotite and plagioclase within and between zones, and Petrogenesis evidence of metasomatic reaction with hornfels cores in limited cases Several modes of origin of these orbicular rocks seem possible: suggest that other mechanisms were operative. No clear evidence of A magmatic origin for the orbicules was suggested by Daugherty chilling occurs at the margins of the orbicular zones with the granite.

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Figure 3. Orbicule types and bictite sketches. (A) Type I orbicule with biotite-rich (shaded or dotted) and >lagioclase-rich layers. Radial orientation is shown dia- ¡rarnmatically. Quartz (Q) is present only at the outer >art of the orbicule. Small rectangle outlines are of ihotomicrograph in Figure 4A. IV: Type II orbicules vith partial biotite shell (on the left) or as plagioclase Figure 4. (A) Photomicrograph of biotite-plagioclase »orphyroblasts in a biotitc-maftnetite-apatite matrix boundary in type I orbictJe. Note radial orientation of Figure 5. (A) Type II orbiculi: with crosscutting aplite (shaded) . Arrows point to edge of slabbed specimen. biotite and poikilitic texture. (B) Photomicrograph of (loc. 5. on Fig. 1). (B) Typical rrpe D orbicules at Sandia C) Biotite diorite (shaded) with disseminated plagi- biotite-magnetic selvage in type HI orbicule. Note radial Mountains locality. Massive plagioclase orbicules with iclase porphyroblasts. Arrow points to edge of slabbed veinlets of magnetite. (C) P tiotomicrograph of edge of type thin concentric layers of biotite within a matrix of biotite pecimen. II orbicule and host biotite-magnetite-apatite matrix. (Iocs. 5 and 7 on Fig. 1).

The selvage zone along the northwest contact is here interpreted as c. ment of minibasic front1; migrating outward appears to be at least one metasomatic aureole. mechanism that was operative during orbicule formation. Type III orbicules contain hornfels xenoliths with reaction prod- On the other hand, there are type I orbicules with unaltered biotite ucts; this suggests that orbicule formation is related, at least in part, monzonite in their centers (Fig. 2C). These suggest that the xenoliths to xenolith reaction with a melt or aqueous fluid. The hornfels con- could have been coated with successive layers of plagioclase and sist of albite, biotite, muscovite, epidote, and chlorite. They are sur - biotite precipitated from a highly variable fluid but not a silicate rounded with reaction products that include oligoclase, biotite, melt. These fluids apparently had no suspended crystals in them, or apatite, sphene, magnetite (Fig. 4B), and pyrite. The following the crystals would have been trapped within orbicules. According generalized reactions describe the crystallization of observed to this hypothesis, the xenoliths had to be suspe nded in the fluid or products: between adjoining fragments where some crowding prevented 1. Albite + epidote 4- quartz—> oligoclase + apatite. complete or separate shell crystallization. The evidence supports 2. Chlorite + epidote + muscovite—> biotite + magnetite. restriction of fluid flow, and the orbicules just have grown in a sta- These are general reactions for progressive metamorphism in re- tic environment. No layering is found on the adjoining granite; this sponse to increasing temperature and require abundant volatiles in- suggests that it was no': yet crystalline during orbicule formation. A cluding water, phosphorus, and minor sulfur. In fact, these general similar origin for comb layering and orbicular texture has recently reactions fit across and help define the boundary between the albite- been advocated by Moore and Lockwood (1973). Several stages of epidote-amphibolite facies and the amphibolite facies. Reaction ol xenolith availability are apparent as shown by varying numbers of the albite-epidite-hornfels core to an amphibolite (nonhornfels) as- shells within orbicules. This is consistent with orbicular granodio- semblage would take place with progressive metamorphism under rite in Taylor Valley, Antarctica (Palmer and others, 1967), where

increasing PHl0 (Turner, 1968, p. 366) at temperatures in the range there also are differences in reaction between xenoliths and mag- 400° to 450°C. Progressive destruction of xenoliths and the develop- matic fluid from one location to the next.

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TABLE 1. MICROPROBE ANALYSES OF BIOTITE AND PLAGIOCLASE IN TYPE I ORBICULES, SANDIA MOUNTAINS, NEW MEXICO

Biotite range Plagioclase range from 15 samples from 10 samples («) (8)

S1O2 35.2-37.4 61.2-63.2 A120J 15.1-16.1 21.7-23.7 FeO 15.8-17.4 0.03-0.41 MgO 12.1-13.5 0.02 MnO 0.40-0.60 n.d. CaO 0.02-0.11 5.5-7.1 Ti02 2.10-3.00 n.d. K;0 9.1-9.4 0.05-0.20 NazO 0.02-0.13 7.3-8.9

Note: Analyses by T. E. Bunch: n.d. means not detected.

TABLE 2. WHOLE-ROCK ANALYSES OF TYPE I ORBICULES

Sample number Average of values for samples 244 23 4 1 + 3 244, 23, and 4 (wt *) (wt %) (wt %) (wt %) (wt t)

Si02 56.70 57.80 58.40 56.30 57.60 AI2O3 21.40 21.20 21.70 21.10 21.40 FeO (total) 4.02 3.99 3.93 3.99 3.98 MgO 3.17 (3.17) (3.17) (3.17) 3.17 CaO 3.18 3.11 3.09 3.22 3.13 MnO 0.13 (0.13) (0.13) (0.13) 0.13 KiO 4.45 4.58 4.54 4.57 4.52 Na20 4.68 (4.68) (4.68) (4.68) 4.68 T1O2 0.71 0.71 0.69 0.64 0.70 Total 98.44 99.37 100.33 97.80 99.31

Note: Analyses by x-ray fluorescence and atomic absorption spectrophotometry, values in parentheses are from sample no. 244. Analyst, D. L. Giles.

TABLE 3. CHARACTERISTICS OF ORBICULAR ROCKS Figure 5. (C) Xenolith altered to biotite in biotite diorite (loc. 8 on Fig. 1).

Orbicule Host Orbicule form Core of Plots of the distance (log A) of a given shell edge from the origin type (matrix) orbicule

of growth of the orbicule versus the number («) of shell increments Type I Biotite Multishelled with complete separation of Biotite involved in growth out to A„ yield curves that are concave upward. monzonite biotite and plagioclase into shells; monzonite closely spaced to adjoining orbicules According to Leveson (1963), this type of shell geometry is indica- (see Fig. 3) tive of metamorphic or metasomatic origin. Type II Biotitite Proto-orbicule plagioclase porphyroblasts Plagioclase with or without a single biotite shell; Although the nature of core material and nearly pure rinds on closely spaced (see Fig. 4) type I orbicules suggests that some mechanism of direct precipita- Type III Biotite Multishelled; fine development of shells Hornfels diorite with incomplete separation of biotite and inclusions tion might have been operative, the systematics for this are not plagioclase; orbicules closely spaced clear. We have particular difficulty in visualizing an exogenic (380 per m3j to widely spaced (38 per m3) "hypogene" fluid from depth capable of causing such widely con- trasting rhythmic precipitation. On the other hand, clear evidence exists in nearby type III orbicules of orbicule formation by progressive metasomatic destruction of xenolithic cores or "seed" material. Also, the nature and mode of occurrence of type II orbicules seem much more amenable to an origin by metamorphic (metasomatic) differentiation processes than by direct precipitation alone. Consequently, in view of the close and gradational spatial relation of the three orbicule types, we favor an overall metasomatic origin. Perhaps the fundamental difference between type I and types II and III (Table 3) lies in the combination of differing source rock (xenoliths) subjected to a gradient in metamorphic intensity or in situ fluid availability from one place to another within a semiclosed system. If type I orbicules reflect direct precipitation, perhaps the type II and type III zones contributed constituents that migrated in episodes or as fronts and accumulated in a pocket along the northern contact (type I zone).

ACKNOWLEDGMENTS J. Paul Fitzsimmons of the University of New Mexico did some early work on type 1 orbicules and introduced us to the locality. Microprobe analyses were done by T. E. Bunch of Ames Research Center. Thanks are due to the University of New Mexico, Harvard University, and Oklahoma State University for use of analytical facilities.

— —• Figure 6. (A) Type III orbicules exhibiting gradual assimilation (from left to right) of hornfels xenoliths. (B) Type III orbicule. Scale in millimeters.

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REFERENCES CITED Bull., v. 74, no. 8, p. 1015-1040. Aldrich, L. T., Wetherill, G. W., ard Davis, G. L., 1957, Occurrence of 1966, Orbicular rocks: A review: Geol. Soc. America Bull., v. 77, no. 1350 million-year-old granitic rocks in western United States: Geol. 4, p. 409^126. Soc. America Bull., v. 68, no. 5, p. 655—656. u-ocewick, R. B., 1960, Geology and petrography of the Tijeras Gneiss, Bernalillo County, New Mexico [M.S. thesis]: Albuquerque, New Brookins, D. G., 1973, Summary and interpretation of radiometric age de- Mexico Univ., 58 p. terminations from the Sandia Mountains, north-central New Mexico: Moore, J. G., and Lockwood, J. P., 1973, Origin of comb layering and or- Geol. Soc. America, Abs. with Programs (Rocky Mtn. Sec.), v. 5, no. bicular structure, Sierra Nevada batholith, California: Geol. Soc. 6, p. 467. America Bull., v. 84, no. 1, p. 1-20. Daugherty, F. W., and Asquith, G. !$., 1971, Orbicular rocks from Sandia Palmer, D. F„ Bradley, John, and Prebble, W. M., 1S67, Orbicular Mountains, Bernalillo County, New Mexico: Geol. Soc. America, Ab- granodiorite from Taylor Valley, South Victoria Land, Antarctica: Geol. stracts with Programs (Rocky Mtn. Sec.), v. 3, no. 6, p. 375-376. Soc. America Bull., v. 78, no. 11, p. 1423-1428. Fitzsimmons, J. P., 1961, Precambrian rocks of the Albuquerque country, Shomaker, John, 1965, Geology ot the southern portion ot the Sandia granite, in Guidebook of the Albuquerque country, New Mexico Geological Sandia Mountains, Bernalillo County, New Mexico [M.S. thesis]: Albu- Society, 12th Field Conf., 1961: p. 90-96.' querque, New Mexico Univ., 80 p. Hayes, P. T., 1951, Geology of the Precambrian rocks of the northern end Turner, F. J., 1968, Metamorphic petrology: New York, McGraw-Hill Book of the Sandia Mountains, Bernalillo and Sandoval Counties, New Co., 403 p. Mexico [M.S. thesis]: Albuque-que, New Mexico Univ., 54 p. Leveson, D. J., 1963, Orbicular rocks of the Lonesome Mountain area, MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 20,1973 Beartooth Mountains, Montana and Wyoming: Geol. Soc. America REVISED MANUSCRIPT RECEIVED NOVEMBER 27, 1973

Printed in U.S.A.

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