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

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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

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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-

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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.

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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. The Santo Tomas- sion among the four volcanic centers (Hoffer, Black Mountain basalts have a high total 1969c). The correlation of the Black Moun- alkali content and fall clearly within the field tain, Little Black Mountain, San Miguel, and of alkali-olivine basalts, as well as displaying Santo Tomas flows is shown in Figure 4. increasing alkali contentwith increasing silica. The sequence indicates initial eruption at The relationship of potassium relative to Santo Tomas (ST-1), followed by simultane- sodium with increasing silica can be shown ous extrusion at Black Mountain (BM-l) with the Niggli k-values. The k-values, mo- and Santo Tomas (ST-2). The third period of lecular ratio K2O/ (K2O + Na2O), of the activity is represented by simultaneous ex- New Mexico basalts indicate a general in- trusion at all four centers (BM-2, LBM-1, crease of potassium relative to sodium with SM-1, and ST-3). Periods four, five, six, and increasing silica; this is characteristic of most seven are shown by flows BM-3, BM-4, BM-5, Cenozoic volcanic rocks in the western United and BM-6 at Black Mountain. States (Moore, 1962). The Niggli k-value of Hawley and Kottlowski (1969) report that 0.29 (extrapolated to 50 percent silica) ap- vertebrate remains found in ancestral Rio proximates those of other Cenozoic volcanic Grande gravel of the upper Santa Fe Group rocks in the same region, that is, Terlingua- that underlies the Black Mountain-Santo Solitario region K = 0.27, Tascotal Mesa Tomas volcanics, show that the basalts must k = 0.28, Black Range k = 0.25 (see Moore, be of middle Pleistocene age (Irvingtonian). 1962). K-values of 0.30 to 0.20 are typical of These fanuas indicate the basalts are younger Cenozoic volcanics in south-central New than 2.2 m.y. and possibly younger than 1.36 Mexico and West Texas. m.y. (Evernden and others, 1964). Recently, K-Ar dates have been obtained which indicate MINERAL VARIATIONS the basalts are younger than 0.3 m.y. (Kott- lowski and Hawley, 1970, written commun.; Introduction Fig. 4). Because these young age dates over- No systematic variations in mineralogy lap they do not support or reject the pro- occur either vertically or horizontally within posed sequence of extrusion (Fig. 4). How- the individual flows of the Santo Tomas- ever, they do resolve the question of-the Black Mountain basalts. The total mineralogy basalts being "too old" relative to the under- of all flows is similar, but differences in lying Irvingtonian vertebrate fossils, as indi- phenocryst and groundmass constituents cated by earlier obtained age dates (Hoffer, exist from flow to flow. 1969c). Variations and Extrusive Sequence CHEMISTRY Detailed analysis of the mineralogy indi- cates a unique pyroxene-olivine ratio for each Introduction of the flows and a definite correlation with Chemical analyses of each of the three the probable sequence of extrusion. For ex- major phenocryst types (Table l) indicate ample, at Black Mountain, the oldest flow undersaturation and show low SiO2 (averag- (BM-l) has a pyroxene-olivine ratio of 2.0, ing 46.8 percent), moderately high AlsOs whereas the youngest flow (BM-6) has a (average 16.1 percent), high total alkalis value of 4.4. The four intermediate flows give (NA2O + K2O = 6.2 percent), and lowTiO2 ratios between 2.3 and 3.7, with successively (0.56 to 1.34 percent). The undersaturation higher ratios matching the progressively is expressed by high normative nepheline (8 younger flows (BM-2 = 2.3, BM-3 = 2.9, to 12 percent), but only trace modal felds- BM-4 = 3.5, and BM-5 = 3.7; Fig. 3). In pathoid has been identified. addition, the pyroxene-olivine ratios of the San Miguel and Little Black Mountain flows Alkali-Silica Relationships are 2.4 and 2.2, respectively; phenocryst min- The relationship of total alkalis to silica eralogy suggests their correlation with flow content has been used by various authors to BM-2 at Black Mountain (pyroxene-olivine identify basalt types (Kuno, 1968). One ratio of 2.3). chemical distinction between the alkali-oli- The ratios of the Santo Tomas flows are

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TABLE 1. CHEMICAL ANALYSES, MODAL AND NORMATIVE MINERALOGY OF THE SANTO TOMAS-BLACK MOUNTAIN BASALTS

Type A Type B TypeC (Santo Tomas-3) (Black Mtn.-6) (San Miguel- 1) Olivine-rich plagioclase-rich Olivine and plagioclase-rich phenocrysts phenocrysts phenocrysts

SiO2 47.32 49.49 46.45 A12O3 14.72 17.91 15.72 Fe2O3 3.18 2.10 3.83 FeO 7.09 7.80 6.72 MgO 8.70 5.04 9.01 CaO 10.76 8.93 10.56 6° Na20 3.95 4.42 3.91 K2O 1.97 2.57 1.85 TiO2 1.26 0.56 1.34 P205 0.41 0.90 0.33 MnO 0.11 0.26 0.13 99.47 99.98 99.85 Ap 0.9 2.1 0.8 11 2.5 1.2 2.4 Or 11.7 15.2 10.9 Ne 11.7 8.5 12.0 > V3 Ab 11.9 21.7 10.9 '•£ "rt An 16.7 21.4 20.0 E C Mg 4.6 3.0 5.6 ^i Fo 8.6 6.4 9.6 Z Fa 3.7 6.8 3.4 Wo-14.3 Wo-6.9 Wo-12.7 Di 27.6 En- 9.5 13.7 En- 3.4 24.4 En- 8.8 Fs- 3.8 Fs- 3.4 Fs- 2.9 99.9 100.0 100.0 ~rt "O jn Flag. 35 39 36 Pyrox. 31 27 30 Oliv. 10 6 12 Mag. 8 9 8 — c H Glass 16 19 14 100 100 100 Normative minerals by CIPW classification system by C. Ondrick and J. Holloway (1967), Pennsylvania State University, and modified by D. V. LeMone and M. Adams for CDC 31008 (1969), The University of Texas at El Paso; program deposited at Computations Center, University of Texas at El Paso. Analyst: Booth, Garrett, and Blair, Inc., Ambler, Pennsylvania.

not the same as those of the Black Mountain Santo Tomas-Black Mountain basalts indi- flows. Instead, they range from 2.05 (oldest cate an origin from a common magma source. flow ST-1) to 3.0 (youngest flow ST-3), but The north-south alignment of the four centers still show a pattern of decrease with greater suggests eruption from one fracture in the age (Fig. 5). crust extending to a magma source but with The increases with time of pyroxene to intermittent eruption localized at four places olivine ratios would be expected if the indi- along the fracture. vidual flows were derived from the same With chemical data available for only three differentiating magma at depth; the earliest of the eleven flows modal data have been flows would be richest in olivine (lowest extrapolated to reflect the chemistry of the pyroxene-olivine ratio) and the later flows unanalyzed flows. Therefore, any petrologic richer in pyroxene (highest pyroxene-olivine conclusions are, at present, only tentative. ratios). Variation in the percentage of phenocryst compositions, oscillation of silica and alkali PETROLOGY values, and increasing pyroxene-olivine ratios The similarity in over-all chemistry and with time indicate that the magma was under- mineralogy of the individual flows of the going differentiation at depth. Alternation of

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Santo Tomas

San Miguel Put tie Black Mountain Black s Mountain0 West Potrillo Mountains Kilbourne Hole

Hunts Hole

New Mexico Mexico (J 5 Miles Figure 5. Pyroxene-olivine ratio and sequence of extrusion.

phenocryst types with cottesponding oscilla- allowing gravitational separation of olivine tions in the values of silica content and total and plagioclase during initial crystallization. alkalis for successive eruptions is apparent, The apparently large thermal gradient, which whereas the pyroxene-olivine ratios display a seems to have developed during cooling, general increase in successively younger flows. suggests the magma source must have been For example, the first eruptive period shows small. Initially, the parent olivine basalt basalt of high silica and alkali content, then magma would have been introduced into both values decrease in the next two periods this small shallow chamber within the crust of extrusion. Succeeding extrusions display at high temperature. Rapid conduction of an increase in silica and alkali content (fourth heat from the margins of the magma chamber through fifth extrusion), followed by a de- to the surrounding rocks would set up a crease (sixth extrusion), and finally another thermal gradient with lower temperatures oc- increase (seventh period). curring near the margins of the magma. Based upon the available evidence it is With the development of a thermal gradient suggested that the Santo Tomas-Black Moun- the areas of initial crystallization would be tain basalts originated from a common small, near the edges of the magma reservoir, with magma source. The magma chamber was olivine and plagioclase the first minerals to probably shallow, less than 30 km deep, thus crystallize. During crystallization the heavier

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olivine crystals would tend to sink and the eralogies of the flows. As differentiation pro- lighter plagioclase to float in the magma. ceeded, successive groundmass portions and, Extrusion of the magma from a source near hence, total rock, would be richer in pyroxene the top (or side) of the chamber would yield and poorer in olivine because of the small a liquid rich in plagioclase crystals followed percentage of total mineralogy contributed by magma rich in olivine crystals. The gravi- by the phenocrysts. tational separation of the early phenocrysts, If the above sequence is correct, variations producing olivine and plagioclase-rich zones, in the composition of the phenocryst minerals would account for the two oldest basalt flows should be present; the intratelluric crystals of being richest in plagioclase and olivine pheno- plagioclaseandolivinein successively younger crysts, respectively. With progressive cooling flows should be richer in sodium and iron, the interior portion of the magma would respectively. However, at present, no detailed cool sufficiently to allow crystallization of information is available concerning the com- both olivine and plagioclase. However, it is position of the individual minerals. Mineral- suggested that sufficient time was not avail- ogical study of the plagioclase and olivine able before eruption of the younger magmas crystals is currently in progress. for a significant gravitational separation of olivine and plagioclase to occur. Therefore, ACKNOWLEDGMENTS eruption of this phase of the magma would Thanks are expressed to Dr. A. C. Waters, produce a lava with almost equal amounts of University of California, Santa Cruz, Dr. John olivineandplagioclasephenocrysts, represent- M. Hills, The University of Texas at El Paso, ing the thitd period of extrusion. and Dr. John W. Hawley, U.S. Department The same sequence apparently was repeated of Agriculture, University Park, New Mexico, later in the magma chamber; however, this for critically reading the manuscript. Ap- time the concentrated plagioclase was erupted preciation is also expressed to Mr. James F. rapidly in the two successive extrusions, four Williams, Department of Geological Sciences, and five. The sequence was then continued The University of Texas at El Paso, for by the eruption of a porphyritic olivine basalt several stimulating discussions concerning (extrusion six) after the olivine phenocrysts the petrology of the basalts. were concentrated by gravitational settling Funds for the investigation were furnished in the magma chamber. by a University Research Institute Grant, The latest eruption apparently repeated the 1968-1969, The University of Texas at El Paso. above sequence, involving sufficient time for gravity separation of plagioclase and olivine REFERENCES CITED and then a tapping of the upper portion of Dane, C. H.; and Bachman, G. O. Prelimi- the magma to produce a lava rich in plagio- nary geologic map of southwestern part of clase phenocrysts. With the extrusion of this New Mexico: U.S. Geol. Surv., Misc. flow (flow six at Black Mountain) eruption Geol. Inv., Map 1-344, 1961. of magma at the surface ceased. DeHon, R. A. Maar of the LaMesa: N. M. Geol. Soc., Guidebook, Southwestern New One fact not readily explained with this Mexico II, p. 204-209, 1965a. model is the apparent regular increase in the DeHon, R. A. The Aden Crater lava cone: The pyroxene olivineratio in successively younger Compass, Vol. 43, p. 34-40, 1965b. flows. The phenocrysts, which represent only Evernden, J. F.; Savage, D. E.; Curtis, G. H.; about 10 percent of the rock, were in dis- and James, G. T. Potassium-argon dates equilibrium with the differentiating magma. and the Cenozoic mammalian chronology This is evidenced by the corroded borders of North America: Amer. J. ScL, Vol. 262, and zonation of the plagioclase and the em- p. 145-198, 1964. Hawley, J. W. Geomorphic surfaces along the bayments in olivine. The magma, which later Rio Grande Valley from El Paso, Texas to crystallized as groundmass, would show suc- Caballo Reservoir, New Mexico: N. M. cessively higher pyroxene/ olivine ratios with Geol. Soc., Guidebook, Southwestern New time, regardless of the phenocryst distribu- Mexico II, p. 188-198, 1965. tions, as long as the phenocrysts remained Hawley, J. W. K-Ar ages of late Cenozoic ba- nonreactive. Tapping the magma in areas salts in Dona Ana County, New Mexico where high concentrations of intratelluric oli- [abstr.J: N. M. Geol. Soc., Guidebook, vine or plagioclase, or both, existed would Defiance-Suni Mt., Taylor region, p. 226, then explain the observed phenocryst min- 1967.

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Hawley, J. W.; and Kottlowski, F. E. Quater- Basalts: the Poldervaart treatise on rocks nary geology of the south-central New of basaltic composition, Interscience Pub- Mexico border region: N. M. Bur. Mines lishers, p. 862, New York, 1968. Min. Resour., Spec. Publ., Border stra- Kottlowski, F. E. Reconnaissance geologic tigraphy, Circ. 104, p. 89-115, 1969. map of the Las Cruces thirty-minute quad- Hoffer, J. M. A preliminary note of the Black rangle: N. M. Bur. Mines Min. Resour., Mountain basalts of the Potrillo field, south- Geol. Map 14, I960. central New Mexico: N. M. Bur. Mines Moore, J. G. K/Na ratio of Cenozoic igneous Min. Resour., Spec. Publ., Border stratig- rocks of the western United States: Geochim. raphy, Circ. 104, p. 116-121, 1969a. Cosmochim. Acta, Vol. 26, p. 101-130, Hoffer, J. M. The San Miguel lava flow, Dona 1962. Ana County, New Mexico: Geol. Soc. Reeves, C. C., Jr.; and DeHon, R. A. Geology Amer., Bull., Vol. 80, p. 1409-1414,1969b. of Potrillo Maar, New Mexico and northern Hoffer, J. M. Volcanic history of the Black Chihuahua, Mexico: Amer. J. Sci., Vol. Mountain-Santo Tomas basalts, Potrillo 263, p. 401-409, 1965. Volcanics, Dona Ana County, New Mexico; MANUSCRIPT RECEIVED BY THE SOCIETY APRIL N. M. Geol. Soc., Guidebook, northern 6, 1970 Chihuahua, Mexico, p. 107-112, 1969c. REVISED MANUSCRIPT RECEIVED OCTOBER 21, Kuno, H. Differentiation of basalt magmas: in 1970

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