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Mineralogy and Petrology of the Santo Tomas-Black Mountain Basalt Field, Potrillo Volcanics, South-Central New Mexico

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Mineralogy and Petrology of the Santo Tomas-Black Mountain Basalt Field, Potrillo Volcanics, South-Central New Mexico JERRY M. HOFFER Department of Geological Sciences, The University of Texas at El Paso, El Paso, Texas 79900 Mineralogy and Petrology of the Santo Tomas-Black Mountain Basalt Field, Potrillo Volcanics, South-Central New Mexico ABSTRACT called the Santo Tomas-Black Mountain basalt field (Hoffer, 1969c). The Santo Tomas-Black Mountain basalts The Santo Tomas-Black Mountain basalt were erupted during the Quaternary from field includes four major centers, each with four centers. Six lava flows are present at one or more cones and associated flows Black Mountain, three at Santo Tomas, and (Hoffer, 1969a). From north to south, the one each at Little Black Mountain and San four volcanic centers are Santo Tomas, San Miguel. The basalts are grouped into three Miguel, Little Black Mountain, and Black major types of phenocryst mineralogy: (l) Mountain. The largest volume of lava has plagioclase abundant, (2) olivine abundant, been extruded from the Black Mountain and and (3) both olivine and plagioclase abundant. Santo Tomas centers where six and three All three types are alkali-olivine basalts, individual flows, respectively, have been showing high alkali-silica ratios and total mapped (Hoffer, 1969a). Each of the two alkali content increasing with silica. intervening centers, Little Black Mountain Seven periods of basaltic extrusion among and San Miguel, shows only a single flow. the centers have been established on the No flow from a given center coalesces with basis of field evidence, phenocryst mineralogy, those from neighboring centers, but all ap- and pyroxene-olivine ratios. K-Ar dates show pear to be closely related in time (Kottlowski, the basalts to be less than 0.3 X 106 m.y. old. I960). A complete description of the volcanic The basalts are thought to have originated features of this area has been presented pre- from a single small, shallow magma chamber viously (see Hoffer, 1969c, p. 107-112). which was under the influence of a high thermal gradient during differentiation. PETROGRAPHY INTRODUCTION AND LOCATION Texture The Potrillo volcanic field occupies an The Santo Tomas-Black Mountain basalts area of more than 400 sq mi in south-central are porphyritic and hypocrystalline. The New Mexico and northern Chihuahua, amount of glass varies from approximately 50 Mexico (Fig. l). Quaternary volcanism has percent in the quickly cooled, vesicular flow produced a series of olivine basalt flows, tops to less than 5 percent in flow interiors. cones, and maar (Kottlowski, I960; Dane Phenocrysts of plagioclase feldspar, olivine and Bachman, 1961;DeHon 1965a; Hawley and pyroxene comprise from 15 to 30 percent and Kottlowski, 1969; Hoffer, 1969a, 1969b, of the rock. The fine-grained groundmass 1969c). contains small plagioclase laths and small Geographically, the area has been divided anhedral grains of pyroxene, averaging less into three regions: (l) a western section than 0.1 mm in diameter, with minor amounts called the West Potrillo Mountains, com- of magnetite, olivine, and light-to-dark in- posed of numerous cinder and spatter cones terstitial glass. and related flows; (2) a central section con- Parallel to subparallel orientations of sisting of a series of maar (that is, Kilbourne phenocrysts are common. Also abundant are Hole, Hunt's Hole, and Potrillo Maar), cones, glomeroporphyritic accumulations of olivine and flows (DeHon, 1965a, 1965b; Reeves and and areas of ophitic intergrowth of plagio- DeHon, 1965); and (3) an eastern portion clase and pytoxene. Geological Society of America Bulletin, v. 82, p. 603-612, 5 figs., March 1971 603 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/603/3432665/i0016-7606-82-3-603.pdf by guest on 24 September 2021 604 J. HOFFER-POTRILLO VOLCANICS, NEW MEXICO Black Mtn. LEGEND 6 \\wa9 iPlogioclose I Pyroxene 0 10 20 30 40% Santo Tomas Little Block Mtn San Miguel Wxvwvvvt v^vvxxvywvi WA ' IQ 20 30 40% ip IQ 20 30 40% 01020 30 40% 0 10 20 30 40% Percentage Composition Figure 1. Index map of the Potrillo volcanic field showing the area of Santo Tomas-Black Mountain basalt field. anhedral grains, averaging 0.05 mm in Mineralogy diameter. Plagioclase feldspar is the most abundant Olivine is most abundant as subhedral to mineral in the Black Mountain-Santo Tomas euhedral phenocrysts displaying locally basalts; it comprises from 22 to 48 percent strong glomeroporphyritic development. Its of the total rock, occurring in both phenocryst abundance, as phenocrysts, ranges from 17 and groundmass portions (Fig. 2). The pheno- to 62 percent. As a groundmass constituent cryst plagioclase is generally subhedral to olivine is sparse, occurring as small subhedral euhedral with some crystals showing irregular to anhedral grains. Many of the phenocryst outline due to resorption reaction. These olivines have irregular cracks and fractures crystals are moderately zoned with calcic cores with irregular edges; these boundaries are (Aneo-es) and more sodic exteriors (An^eo). possibly due to resorption. A high 2V (80 The groundmass plagioclase, averaging 0.05 to 85 degrees) and a negative sign indicate a mm, is unzoned and less calcic than the high magnesium content, Foso— FOe». phenocrysts averaging An4o (andesine). Subhedral to anhedral magnetite, ranging Pyroxene is present as phenocrysts, but in size from 0.08 to 0.05 mm, occurs as in- more abundantly as groundmass crystals. clusions within olivine and pyroxene and as Phenocryst pyroxene is generally subhedral, scattered subhedral crystals in the ground- averages 0.5 mm and is typically moderate mass associated with the glass. The opaque- to dark brown in color with a moderage 2V ness of the glass is probably a function of the (40 to 60 degrees). The color and 2V suggest amount of included opaque oxides. Minor titanium-rich augite. The groundmass py- amounts of interstitial potassium feldspar roxenes, also dark brown, occur in small and traces of feldspathoid (analcime?) have Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/603/3432665/i0016-7606-82-3-603.pdf by guest on 24 September 2021 PETROGRAPHY 605 X Santo Tomas 0 San Miguel + Little Black Mountain • Black Mountain Extrusion Sequence Figure 2. Mineralogy of the Santo Tomas-Black Mountain basalts. been seen in the groundmass. The presence flows were accomplished on the basis of of the feldspathoid was verified by methyl normal field criteria at each center where blue staining. certain flows contained unique phenocryst Secondary calcite and opal fill vesicles near mineralogies. the top of the flows. The flows can be classified into three groups The samples examined appear to be essen- (Fig. 3): type A: olivine rich; type B: plagio- tially unaltered, but a few, especially near clase rich; and type C: both olivine and the top of the flows, show evidence of minor plagioclase. Figure 3 represents a plot of alteration. This consists of locally small total phenocryst mineralogy for the flows brownish borders around some of the olivine based on plagioclase, pyroxene, and olivine phenocrysts (iddingsite?) and iron oxide content and shows the relationships among stains from weathering. the three phenocryst types. All flows appear similar in total mineralogy Some flows, for instance flow 4 (BM-4) (Fig. 2). and flow 6 (BM-6) at Black Mountain, are very distinct and can be identified and traced Phenocryst Mineralogy easily in the field on the basis of phenocryst The most distinctive characteristic of the mineralogy. Other flows, although not as individual Black Mountain-Santo Tomas distinct megascopically, can be identified by flows is their phenocryst mineralogy. The determining the relative percentages of plag- mapping and identification of the individual ioclase, pyroxene, and olivine by point count- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/603/3432665/i0016-7606-82-3-603.pdf by guest on 24 September 2021 606 J. HOFFER—POTRILLO VOLCANICS, NEW MEXICO interior, and vesicular base is nearly constant with only a slight increase in total pheno- crysts in the central, more dense portion of the flow. A slight increase in phenocryst olivine and pyroxene and total olivine and pyroxene with a corresponding decrease in plagioclase was observed in the central sec- tion of the flow (Hoffer, 1969b). These varia- tions could indicate a small amount of crystal settling. It is more likely, however, that the variations are caused by longer periods of crystallization in the more insulated interior of the flow. Lateral variations in mineralogy were not found within any flow in the Santo Figure 3. Phenocryst mineralogy of the Tomas-Black Mountains area. Santo Tomas-Black Mountain basalts. SEQUENCE OF FLOWS Oq the basis of the topographic positions ing. Little variation in phenocryst or total of flows and other field relationships, the rock mineralogy was detected within any flow. sequence of extrusion has been established at Small mineralogical variations can be seen Black Mountain and Santo Tomas (Hoffer, in the San Miguel flow from which samples 1969c). Once the sequence was determined were obtained for each of six sections along at these multiple extrusive centers correla- the flow (Hoffer, 1969b). The mineralogy of tions were sought for the entire extrusive the phenocrysts in the vesicular top, dense sequence among the four centers. The unique Block Mt LEGEND Composition of Phenocrysts .02510.1 my V////ffff( Plagioclase .12610.1 my I I Olivine Q Pyroxene 0 50 100% * - K-Ar dates (Denison, Mobil Reieorch, 19701 Santo Tomas nttititA 551 011610.116 mv 0.16110.08 my 0.12710.13 my Figure 4. Sequence of extrusion of the Santo Tomas-Black Mountain basalts. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/603/3432665/i0016-7606-82-3-603.pdf by guest on 24 September 2021 MINERAL VARIATIONS 607 phenocryst mineralogies of the individual vine, high-alumina, and tholeiite basalts is flows suggest a method of correlation rep- based on the progressively higher alkali con- resented by seven periods of basaltic extru- tent with increasing silica.
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