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

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Composition and Structural State of Feldspar Inclusions from Alkali Olivine Basalt, Potrillo Basalt, Southern New Mexico 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 Feldspar Inclusions from Alkali Olivine Basalt, 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-plagioclase, 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 mole cinder-spatter cones, and maar craters. percent) are found as inclusions in lava 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 pyroxene- Basalt (defined here), south-central New rich with phenocrysts of olivine; the lava Mexico. Nodules and crystals of olivine- shows little or no differentiation. Stratigraphi- spinel-pyroxene, pyroxene, and amphibole cally, the basalt flows are underlain by Pleisto- are associated with the feldspar crystals in cene sand and gravel; Tertiary acidic to basic undifferentiated lava. flows, tuff, and breccia; Cretaceous and Per- The structural state of the alkali feldspar mian limestone and dolomite; lower Paleozoic crystals is high, whereas that of the plagioclase limestone; and Precambrian granite 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 peridotite composition. Less olivine-pyroxene-spinel nodules have been common are feldspar inclusions; these have studied. Carter (1970) stated that these mafic been reported from Antarctica, Africa, Austra- nodules represented a portion of the upper lia, Canary Islands, and Mongolia (Moun- mantle 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 feldspars 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/2139/3428984/i0016-7606-84-6-2139.pdf by guest on 29 September 2021 2140 HOFF2R AND HOFFER 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 Mineral 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 xenolith 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 xenoliths 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/2139/3428984/i0016-7606-84-6-2139.pdf by guest on 29 September 2021 FELDSPAR INCLUSIONS FROM POTRILLO BASALT, NEW MEXICO 2141 Figure 2. Composition of feldspar inclusions, Potrillo Basalt, plotted on diagram of Muir, 1962. Points R and A represent anorthoclases from lavas 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 orthoclase. 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 magma. 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/6/2139/3428984/i0016-7606-84-6-2139.pdf by guest on 29 September 2021 2142 HOFFER AMD HOFFER 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.
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