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JERRY M. HOFFER \ Department of Geological Sciences, The University of Texas atEl Paso, El Paso, ROBIN L. HOFFER J Texas 79968

Composition and Structural State of Inclusions from Alkali Olivine , Potrillo Basalt, Southern New Mexico

ABSTRACT area is covered by upper Quaternary alkali olivine basalt flows. In the western portion of Crystals of anorthoclase, lime-anorthoclase, the area, the West Potrillo Mountains, volcanic potash-, and plagioclase (Ab 42.7 to evidence is abundant, and includes cinder and 72.7, An 4.6 to 53.5, and Or 4.0 to 32.0 cinder-spatter cones, and maar craters. percent) are found as inclusions in flows, The West Potrillo Mountains are made up by as loose crystals on the flanks of cinder cones and are the type area for the lava herein named and maar craters, and in the cores of ejected the Potrillo Basalt. The flows, erupted from bombs in the upper Quaternary Potrillo cinder and cinder-spatter cones, are - Basalt (defined here), south-central New rich with of olivine; the lava Mexico. Nodules and crystals of olivine- shows little or no differentiation. Stratigraphi- -pyroxene, pyroxene, and cally, the basalt flows are underlain by Pleisto- are associated with the feldspar crystals in cene and gravel; Tertiary acidic to basic undifferentiated lava. flows, , and breccia; Cretaceous and Per- The structural state of the alkali feldspar mian limestone and ; lower Paleozoic crystals is high, whereas that of the plagioclase limestone; and Precambrian and appears to range from high to low. The feldspar monzonite. inclusions probably formed in a deep-seated high-temperature environment. OCCURRENCE Feldspar, pyroxene, amphibole, and olivine- pyroxene-spinel nodules occur as isolated INTRODUCTION crystals or within volcanic bombs on the flanks The occurrence of exotic nodules in alkalic of cinder cones and maar craters and as in- olivine basalt is common, and most of the clusions in basalt flows. To date, only the inclusions are of composition. Less olivine-pyroxene-spinel nodules have been common are feldspar inclusions; these have studied. Carter (1970) stated that these been reported from Antarctica, Africa, Austra- nodules represented a portion of the upper lia, Canary Islands, and Mongolia (Moun- brought to the surface during volcanic tain, 1925; Vlodavetz and Shavrova, 1953; eruptions. Boudette and Ford, 1966; Borley and others, The feldspar crystals occur most abundantly 1971). The are described as anortho- as loose crystals on the ground or as inclusions clase or lime-anorthoclase. in bombs associated with the older volcanoes Volcanic rocks in the Potrillo field contain in the field. The crystals are subhedral to abundant feldspar inclusions showing a great euhedral, white to colorless, and from several variety of compositions including alkali and millimeters to nearly 5 cm in diameter. The plagioclase feldspar. crystals show rounded edges and embayed faces. GEOLOGIC SETTING Feldspar crystals have also been found as The Potrillo volcanic region occupies an inclusions within several of the olivine basalt area of approximately 400 sq mi (1,025 sq km) flows. Here they are associated with pyroxene in south-central New Mexico (Fig. 1). The and olivine-pyroxene-spinel nodules.

Geological Society of America Bulletin, v. 84, p. 2139-2142, 4 figs., June 1973 2139

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NEW COMPOSITIOf OF FELDSPAR INCLUSIONS MEXICO OF THE POTRILLO BASALT

Kz0 Na20 CaO Or Ab An Sample (wt I) (wt t) (wt %) (mole %) (mole %) (mole t)

MR1 3.05 7.52 2 .00 25.4 63. 8 10.8

MR2 3.06 7..6 0 2..1 8 24.5 64. .0 11.5

MR3 4.37 11 .30 1 .20 26.2 69.. 2 4.6

2 OA 3.91 7 .20 1..2 7 32.0 61. 0 7.0

20B 3.57 7 .17 1..9 2 28.9 60. .7 10.4

11 AX 2.90 7 .49 2..9 0 22.6 62. .4 15.0

17 2.93 7 .04 4..1 0 21.7 57. 5 20.8

21 1.02 7..6 1 6. 11 7.4 63. 3 29.3

19 2.54 7. 62 8. 76 17.0 49. 4 33.6

KHZ 0.61 4. 94 10. .85 4.0 42. 7 53.3

See Figuri 1 for sample locations. Analyst: J. Renault, New Merico Bureau of Mines and Resources

inclusions in lava are plotted as points R (in basalt from Mongolia) and A (in an unspecified lava from Australia) in Figure 2. Both anortho- clase crystals are poorer in An content than those of the Potrillo Basalt (Vlodavetz and Shavrova, 1953; Kracek and Neuvonen, 1952). Figure 1. Index map of the Potrillo Basalt (includes West Potrillo Basalt, Aden-Afton Basalt, and Santo STRUCTURAL STATE Thomas-Black Mountain Basalt), south-central New The structural state or degree of disorder of Mexico. Dots are locations of samples. the feldspar crystals was determined, utilizing the methods of Wright (!'.968) for alkali feld- COMPOSITION spar and those of Smith and Gay (1958) for Feldspar inclusions were selected from ten TABLE 2. OPTICAL AND X-RHY PROPERTIES OF different localities for partial chemical analysis. FELDSPAR INCLUSIONS OF 'OTRILLO BASALT Each crystal was analyzed by x-ray fluorescence for K2O, Na20, and CaO. These results are (201)» (131)* Reflec- Reflec- expressed in mole percent Ab, An, and Or (see Sample Homogenei ty a-Index tion tion Table 1). The degree of zoning and unmixing of each analyzed feldspar crystal was investi- A' or'thoelase gated by observation of thin sections, x-ray MPI Homogeneous 1.536 Sharp

reflections, and the a-index (see Table 2). The MR2 Homogeneous 1. .532 Sharp

data indicate that most of the anorthoclase MR3 Perthitic 1. 527 Sharp crystals are homogenous and the plagioclase Split, 20B Perthitic 1. E32 crystals are moderately zoned. The a-index of sharp Portrillo anorthoclase is similar to those 20A Homogeneous 1.536 Sharp reported for anorthoclase from Antarctica (Boudette and Ford, 1966). A 11AX 1-534 Sharp variety of compositions are present; they in- 17 Slightly zoned 1.534 to 1. 536 Broad clude anorthoclase, lime-anorthoclase, potash Plagioclase andesine, and plagioclase (Fig. 2). The feldspar 19 Zonec?, An32-3s 1.544 to 1. 547 Broad crystals of plagioclase composition (samples 19, 21, and KHZ) occur as inclusions in volcanic 21 1.543 Sharp bombs, whereas the alkali feldspar crystals are KHZ Zoned, An52-56 Sharp

found as loose crystals. * £*iarp is < 0..'" 20 at 1top 1/10 peak height, and broad Two other occurrences of anorthoclase is > 0.1' re.

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Figure 2. Composition of feldspar inclusions, Potrillo Basalt, plotted on diagram of Muir, 1962. Points R and A represent anorthoclases from in Mongolia and Australia, respectively. plagioclase. The results are shown in Figure 3 (alkali feldspar) and Figure 4 (plagioclase). The alkali feldspars possess a high degree of disorder. Samples MR1, MR2, and 20B are Figure 4. Structural states of plagioclase inclusions, intermediate between the standards S63-30 Potrillo Basalt. "Or" = mole percent . and high sanidine, whereas samples 11 AX, 20A, and 17 are intermediate between the standards content in samples 19 (17 percent) and 21 Puye and high albite. (7.4 percent) is responsible for the apparent The plagioclase feldspar crystals appear to low structural states of the Y versus composi- range in structural state from high (KHZ) to tion plot in Figure 4. low (19). The decrease in structural state cor- DISCUSSION AND ORIGIN The occurrence and composition of anortho- clase inclusions or crystal lapilli associated with basaltic rocks has been reported from Mongolia and Antarctica. The material from Mongolia is a monoclinic form of anorthoclase formed at a high temperature (Vlodavetz and Shavrova, 1953). Loose crystals of feldspar collected on the slopes of Mt. Erebus, in Antarctica, are of lime-anorthoclase (Mountain, 1925). Potash- oligoclase, potash-andesine, and lime-anortho- clase have been reported as microphenocrysts and groundmass crystals in differentiated Pacific alkaline lava (Barth, 1931; Macdonald, 1942; Aoki, 1959; Muir and Tilley, 1961). The occurrence and texture of the Potrillo anortho- clase and plagioclase crystals indicate that they are inclusions and were carried to the surface in an undifferentiated . A preliminary analysis of the alkali feldspar ?» ¡04 crystals in the Potrillo Basalt indicates an Figure 3. Structural states of alkali feldspars, origin in a high-temperature environment, Potrillo Basalt. probably at great depth. This is indicated by

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three facts: (1) The alkali feldspars are asso- chemistry of plagioclase and alkali feldspars ciated with high temperature, mantle-derived, (Bowen volume): Am. Jour. Sci., v. 250, p. olivine-pyroxene-spinel inclusions; (2) the 293-312. feldspar crystals are of high structural state; Macdonald, G. A., 1942, Potash oligoclase in and (3) no known anorthoclase occurs in the Hawaiian lavas: Am. Mineralogist, v. 27, p. prebasalt rocks at shallow depth through which 793-800. Mountain, E. D., 1925, Potash-oligoclase from Mt. the magma passed. Erebus, Antarctic, and anorthoclase from Mt. ACKNOWLEDGMENT Kenya, East Africa: Mineralog Mag., v. 20, p. 331-345. The investigation was supported by National Muir, I. D., 1962, The paragenesis and optical Aeronautics and Space Administration Grant properties of some ternary feldspars: Norsk NGR 44-012-147. Geol. Tidsskr., v. 42, p. 477-492. Muir, I. D., and Tilley, C. E., 1961, and their place in series: Jour. REFERENCES CITED Geology, v. 69, p. 186-203. Smith, J. V., and Gay, P., 1958, The powder Aoki, K., 1959, Petrology of alkali rocks of the Iki Islands and Higashi-matsuura district, Japan: patterns and lattice paiameters of plagioclase Tohoku Univ. Sci. Rept., ser. 3, v. 6, p. 261— feldspars, II: Mineralog. Mag., v. 31, p. 744- 310. 762. Vlodavetz, V. I., and Shavrova, N. N„ 1953, On Barth, T.F.W., 1931, Mineralogical petrography of Pacific lavas: Am. Jour. Sci., v. 21, p. 401. anorthoclase from a lava in Darigan volcanic Borley, G. D., Suddaby, P., and Scott, P., 1971, region, southeast Mongolia: Akad. Nauk SSSR, Some xenoliths from alkalic rocks of Teneriffe, Mineralog. Abs. 13, p. 480. Canary Islands: Contr. and Wright, T. L., 1968. X-ray and optical study of Petrology, v. 31, p. 102-114. alkali feldspar: Pt. II, An x-ray method for Boudette, E. L., and Ford, A. B„ 1966, Physical determining the composition and structural properties of anorthoclase from Antarctica: state from measurement of 2d values for three Am. Mineralogist, v. 51, p. 1374-1387. reflections: Am. Mineralogist, v. 53, p. 88-104. Carter, J. L., 1970, Mineralogy and chemistry of 's based on the partial MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER fusion-partial crystallization model: Geol. 20, 1972 Soc. America Bull., v. 81, p. 2021-2034. REVISED MANUSCRIPT RECEIVED JANUARY 17, Kracek, F. C., and Neuvonen, K. J., 1952, Thermo- 1973

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