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Petrogenesis of the Capitan dike swarm, Lincoln County, New Mexico.

ABSTRACT. -- The Capitan dike swarm in southwestern Lincoln County, New Mexico, contains six different rock types ranging from tephrite to rhyolite. With the exception of the rhyolite dikes, which are probably related to the adjacent Vera Cruz Mountains rhyolitic intrusions, the dikes have a silica range of 41.7 to 62.1 weight percent. Increase in silica is accompanied by increase in [K.sub.2]O, up to 4.7 weight percent, and [Na.sub.2]O, up to 6.8 weight percent.

Phenocryst minerals include plagioclase, clinopyroxene, and olivine. Magnetite, pyrite and apatite are ubiquitous accessory minerals. Plagioclase composition ranges from An50 to An70, and crystals show oscillations in composition. Clinopyroxene phenocrysts are also zoned and plot on or near the diopside-augite boundary.

Pearce element ratio plots indicate that plagioclase and clinopyroxene crystallization are responsible for the observed major element variation. Phenocryst-poor latite and trachyte are interpreted as the result of mineral separation, and phenocryst-rich tephrite porphyry and olivine diabase porphyry are the result of phenocryst accumulations in the magma chamber. Key words: New Mexico; petrogenesis; geochemistry.


In the last two decades, detailed petrologic studies of magma chambers have documented the widespread occurrence of compositionally zoned, generally silicic magmatic systems (Hildreth, 1981; Warren et al., 1989). More recently, the concept of open magmatic systems in which magma interacts with wallrock or is replenished during crystallization has gained acceptance (O'Hara and Matthews, 1981; McMillan and Dungan, 1986; Hill, 1988; Johnson et al., 1989). The Tertiary Capitan dike swarm appears to represent the emanations from a crystallizing and differentiating mafic magma chamber or chambers that were episodically replenished by influxes of basaltic magma.

Dikes of the Capitan dike swarm occupy an equidimensional area of approximately 170 square kilometers in the southwestern part of Lincoln County, New Mexico. They are best exposed in roadcuts along U.S. highway 380 for nine miles (14 kilometers) westward from the west edge of the village of Capitan (Fig. 1).

The dikes generally trend north-northeast and range in thickness from less than one meter to as much as 15 meters. They intrude shale and sandstone of the Mesaverde group of Cretaceous age as well as sandstone and mudstones of the Cub Mountain Formation (Kelley, 1971) of Tertiary age. Contact metamorphic effects in the adjacent wallrock are minor, even next to the larger dikes. Xenoliths of sandstone and shale are rare.


Other igneous rocks in the vicinity include the rhyolitic intrusions of Carrizo Peak (Pertl, 1984) and the Vera Cruz Mountains to the north of the dike swarm and the predominantly andesitic to trachytic flows, breccias, and agglomerates of the Sierra Blanca volcanic sequence (Thompson, 1966) to the south and west. To the east, igneous rocks decrease in abundance and are rare east of the Capitan Mountains.

Rock types present in the dikes range from tephrite to rhyolite. Their abundance and intimate association within a small area suggest that with the possible exception of the rhyolitic dikes, which are found only adjacent to the Vera Cruz Mountains, the dikes represent the successive emplacement of magma pulses derived from a fractionating alkali basalt magma during mid-Tertiary time.


To insure a representative sample for whole-rock chemical analyses, samples weighing at least two kilograms were broken into fragments of five to 10 centimeters and all pieces with thick weathering rinds or lithic fragments were discarded. The remaining sample was crushed to pea-sized fragments in a jaw crusher. A 10-gram to 20-gram alliquot was obtained by splitting and this material then was ground to minus 100 mesh in a ball mill.


For each sample, 1.500 grams of rock powder mixed with 15.00 grams of lithium tetraboarate flux was fused in a graphite crucible in a furnace at 1200 degrees C for 20 minutes. The resulting glass pellet was analyzed by X-ray fluorescence spectrometry in a Phillips PW 1404 X-ray spectrometer. USGS standard AGV-1 was used as an internal standard to check on accuracy of the analyses. Results for this internal standard were within 0.4 weight percent for Si[O.sub.2], and within 0.1 weight percent for all other oxides analyzed, of the consensus value reported by Gladney and Burns (1983).

Mineral compositions were determined on an automated CAMECA electron microprobe with accelerating potential set at 15 keV and sample current at 15 na, with a beam diameter of 10 mm. Compositions were calculated from corrected peak intensities using the method of Bence and Albee (1968).

Rare earth elements and Sc, U, and Th were analyzed by INAA at a commercial laboratory, using 50 grams of minus 100 mesh sample. Precision, based on comparison with an internal standard, is better than 10 percent for all elements except U, which is better than 17 percent.


Six different rock types can be distinguished. From oldest to youngest (Elston and Snider, 1964), they are diabase porphyry, olivine diabase porphyry, fine-grained diabase, latite or trachyte, tephrite porphyry, and rhyolite. Plagioclase is the dominant phenocryst phase and is found in all rock types. It attains its maximum size and abundance in the diabase porphyry (Fig. 2) where it constitutes up to 40 volume percent as megacrysts up to two centimeters in length. Clinopyroxene is also a major phenocryst phase. It forms generally equidimensional crystals up to six millimeters in diameter and is most abundant in the olivine diabase porphyry (Fig. 3) and tephrite, where it may constitute up to 40 percent by volume. Many of the larger clinopyroxene phenocrysts exhibit dark rims and are optically zoned. A few contain a number of small dark green spinel grains in their cores (Fig. 3). Olivine occurs as small phenocrysts in the more mafic members of the suite, constituting up to 15 volume percent in the olivine diabase.



The more alkalic members of the suite tend to be partially to completely altered, with widespread kaolinite, sericite and replacement of feldspar phenocrysts and mafic minerals by calcite or hematite, or both (Fig. 4). Hornblende occurs in some of the dikes as small needles in the groundmass or as large ragged grains. The latter occurrence probably represent xenocrysts. Biotite occurs as small flakes in the groundmass of some dikes of fine-grained diabase. Magnetite, apatite, and pyrite are prominent accessory minerals.


A total of 28 samples was analyzed for major element chemistry. They exhibit a silica range from 41.7 to 62.1 weight percent Si[O.sub.2] (Cepeda, 1990a) except for one sample of rhyolite, which contains 76.1 weight percent silica (Table 1). The suite is clearly alkalic; analyses plot well above the alkalic-tholeiitic boundary. On a total alkali-silica plot (LeMaitre, 1984) compositions span a broad range from tephrite to trachyte (Fig. 5), with a moderate amount of scatter. Increase in silica content is accompanied by an increase in [K.sub.2]O and [Na.sub.2]O. A plot of CaO/[Al.sub.2][O.sub.3] versus MgO shows a decrease in CaO/[Al.sub.2][O.sub.3] ratio with increasing differentiation (Fig. 6) suggesting clinopyroxene fractionation during differentiation. MgO contents range from 0.05 weight percent in the rhyolite to 7.75 in the tephrites. The tephrites are strongly enriched in MgO, Fe (calculated as FeO), CaO, and Ti[O.sub.2]. Because of the abundance of phenocrysts in this rock, it is unlikely that these compositions reflect true liquid compositions, but rather are the products of clinopyroxene and magnetite accumulation.

Some of the scatter in the chemical data, particularly the alkalies, may be the result of alkali metasomatism documented by Elston and Snider (1964). The effects of metasomatism are difficult to quantify but alteration is most extensive in the latites and trachytes.

The low totals listed in Table 1 are the result of loss on ignition of both [H.sub.2]O and S[O.sub.2]. Sulfur is contained in pyrite, a common accessory mineral, particulary in the more mafic rocks of the suite.

There is a general increase in Th and U content with increasing differentiation (Table 1). Evidence for cogenesis is provided by the limited range (3.1 to 4.8) in Th/U ratios. Rhyolite has the lowest Th/U ratio (3.1, one analysis) followed by tephrite (3.3, two analyses), latites and trachytes (3.4 to 3.7, three analyses), diabase porphyry (4.3, one analysis), olivine diabase porphyry (4.7, one analysis) and fine-grained diabase (4.2 to 4.8, three analyses).



The compositon of plagioclase phenocrysts determined by microprobe analysis ranges from An50 to An70 (Fig. 7). There is no consistent chemical trend from core to rim. Composition oscillates between core and rim and within the same sample some grains may have more Ab-rich cores than rims. Variation within any individual grain is as much as seven percent anorthite.

Compositions of clinopyroxene phenocrysts plot on the diopside-augite boundary (Subcommittee on Pyroxenes, 1988) for pyroxenes from the tephrite porphyry dikes. Pyroxenes from olivine diabase porphyry plots just inside the augite field adjacent to the diopside-augite boundary (Fig. 8). In both rock types, pyroxenes show oscillations in compositon. Rims are slightly more Mg-rich in some grains, but this tendency is reversed in other grains. There is a slight tendency toward Mg-rich cores and more calcic rims in the grains analyzed. Maximum variation within any individual grain is less than two percent. Olivine grains were not analyzed.



Six whole rock samples were analyzed for rare earth elements. Plots of the analyses, normalized to chondritic values (Anders and Ebihara, 1982) are shown in Figure 9 and Figure 10. All of the whole rock plots show a steep, generally straight pattern with a negative slope. Lanthanum to lutetium ratios range from 144, olivine diabase porphyry, to 379, tephrite porphyry. The steep pattern with a negative slope is characteristic of alkalic basalts produced by partial melting of garnet bearing material (Ragland, 1989), where the HREE are retained in garnet relative to LREE.

All plots, except that for latite, show either a slight positive Eu anomaly or none at all. The slight positive anomaly is probably the result of plagioclase accumulation, whereas the negative anomaly of the latite suggests that plagioclase separation was responsible, at least in part, for its formation.





Systematic variations in mineralogy and major elements within this suite suggest that most of the compositional variations can be explained by crystal fractionation. To test this hypothesis, Pearce element ratio analysis was used to determine the mineral phases that would be required to produce the observed major element variations (Cepeda, 1990b). The interactive computer program PEARCE. PLOT (Stanley and Russell, 1989) facilitated the analysis. The first step in the process was a test of conserved constituents. As shown on Figure 11, Ti, K, and P are the closest to conserved elements among the more mafic members of the suite. The more alkalic rock types do not satisfy this condition; thus they were deleted from further analysis.

Utilizing a plot that tests for olivine, clinopyroxene, and plagioclase fractionation (Fig. 12) a best-fit straight line through the data points has a slope of 0.259. The ideal slope for olivine fractionation on this diagram is 1.0, for clinopyroxene fractionation, 0.25, and for plagioclase, 0.0. The slope of 0.259 suggests that clinopyroxene fractionation is a major influence on the fractionation trend, although it is possible that it could be the result of a combination of olivine and plagioclase fractionation. It is possible to test for plagioclase and clinopyroxene fractionation (Russell and Nichols, 1988) with the use of a different set of parameters. On this diagram (Fig. 13) the expected slope for clinopyroxene fractionation is [infinity], and for plagioclase it is 1.0. The slope of the best-fit line produced by PEARCE. PLOT is 1.738. The relative proportions of crystallizing plagioclase (Plag) and clinopyroxene (Cpx) can be calculated using the equation, m(slope) = (Cpx + Plag)/Plag. A slope of 1.738 indicates that 43 percent of the observed fractionation is due to fractionation of clinopyroxene and 57 percent is due to plagioclase fractionation.


The composition of the crystallizing plagioclase can be calculated using a third set of parameters (Russell and Nichols, 1988). On this diagram (Fig. 14), m(slope) = 2[X.sub.Ab]/(2-[X.sub.Ab]), where [X.sub.Ab] is the mole fraction of albite in plagioclase. For a slope of 0.39 as depicted in Figure 12, the corresponding plagioclase composition is An67. The large amount of scatter evident in the diagram may be due in part to sodium metasomatism, although the mafic rocks are fairly fresh. It also may be due to variations in plagioclase composition during crystallization. The plagioclase composition determined by Pearce element ratio analysis is well within the compositional range determined by microprobe analysis.





The combined results of the Pearce element ratio analysis and the petrographic characteristics of the individual dike rock types make it possible to sketch out the processes responsible for the evolution of this magmatic system. Using the earliest rock type (diabase porphyry) as a parental magma, it is likely that evolution of rock compositions was produced by crystallization of plagioclase and clinopyroxene. Separation of clinopyroxene and plagioclase phenocrysts resulted in phenocryst-poor magma enriched in Na, K, and Si and depleted in Mg and Fe relative to the parental magma, such as the phenocryst-poor latites and trachytes. Assuming that the clinopyroxene and possibly olivine settled somewhat toward the bottom of the chamber or chambers, the latite-trachyte magma would be in the upper part of the chamber and would contain sparse plagioclase phenocrysts as its only phenocryst phase. The dominant phenoocryst phase in the central portions of the chamber was plagioclase, and the porphyritic nature of the diabase porphyry suggests that the initial magma probably resided in the chamber for some period of time before being emplaced as dikes. The lower portions of the chamber accumulated the crystallizing olivine and clinopyroxene phenocrysts, and produced the phenocryst rich olivine diabase porphyry and tephrite porphyry. The chamber well may have been density-stratified with silicic magma towards the top and olivine and clinopyroxene phenocryst-enriched magma towards the bottom. The middle portion probably was enriched in neutral density plagioclase phenocrysts. Thus, the porphyritic rock types are the result of mineral accumulation, and the aphyric rock types are the result of mineral separation. Alternatively, some type of filter pressing mechanism may have operated to produce crystal-rich magma. The present level of erosion does not permit the determination of the size, number, or configuration of the magma chamber(s) in which this process occurred.

The fine-grained diabase is similar chemically to the diabase porphyry, and this generally aphyric rock may be closer to a true parental liquid. Its position in the middle of the intrusion sequence suggests that a fresh influx of basaltic magma, close to the liquidus temperture was added to the system during fractional crystallization. The observed oscillations in chemical composition in both the cliniopyroxene and plagioclase also suggests repeated influx of fresh magma into the chamber.
TABLE 1. Chemical analyses of Capitan dike swarm, Lincoln County, New

 Si[O.sub.2] [Al.sub.2][O.sub.3] Fe[O.sub.1] Ti[O.sub.2]

CN-5 45.50 17.10 9.62 1.50
CN-8 47.70 19.20 8.73 1.57
CN-11 48.20 17.40 9.17 1.81
CN-16 49.10 18.50 8.37 1.64
CN-19 48.50 20.10 7.02 1.24
CN-24 55.50 17.30 5.22 1.14
CN-29 45.80 17.60 8.82 1.37
CN-39 46.40 18.50 8.73 1.73
CN-49 49.80 18.00 6.84 1.53
CN-57 47.40 20.50 7.65 1.36
CN-59 45.20 17.10 9.62 1.70
CN-68 51.10 16.60 7.92 1.99
CN-93 42.60 13.90 10.70 2.18
CN-95 54.90 17.00 5.31 1.17
CN-100 50.60 18.50 6.75 1.17
CN-104 57.10 16.40 5.76 1.06
CN-111 50.80 15.60 7.20 1.55
CN-112 58.80 17.80 3.96 1.05
CN-95 41.70 11.90 9.80 2.96
CN-35 49.40 18.20 7.47 1.49
CN-36 49.70 20.50 6.03 1.14
CN-45 45.30 15.80 8.28 1.35
CN-98 54.50 16.60 6.03 1.13
CN-116 62.10 16.10 4.14 0.79
CN-137 76.10 13.50 0.99 0.15
CN-139 46.50 15.10 9.08 1.51
CN-140 48.60 17.50 7.11 1.48
CN-142 55.30 15.70 6.39 1.79

 CaO MgO [Na.sub.2]O [K.sub.2]O [P.sub.2][O.sub.5] TOTAL

CN-5 11.60 7.17 3.00 1.10 0.47 97.06
CN-8 8.40 3.70 4.70 2.00 0.93 96.93
CN-11 8.50 3.92 4.20 2.70 1.23 97.13
CN-16 7.50 3.21 4.20 2.90 1.20 96.62
CN-19 8.30 2.73 4.20 3.20 0.95 96.24
CN-24 3.90 2.98 4.90 4.40 0.44 95.78
CN-29 12.00 6.85 2.50 1.20 0.47 96.61
CN-39 9.90 4.12 3.30 2.40 0.67 95.75
CN-49 5.10 2.65 5.00 4.70 0.81 94.43
CN-57 9.40 3.23 4.10 2.30 0.87 96.81
CN-59 7.90 4.63 4.30 2.30 1.49 94.24
CN-68 8.30 4.55 4.00 2.00 0.77 97.23
CN-93 13.60 7.75 2.50 1.00 1.15 95.38
CN-95 4.50 3.15 4.90 4.40 0.46 95.79
CN-100 4.70 2.97 5.30 3.90 0.81 94.70
CN-104 4.90 3.50 5.10 3.10 0.51 97.43
CN-111 6.90 4.13 6.50 0.30 0.52 93.50
CN-112 2.50 1.27 6.20 4.60 0.35 96.53
CN-95 13.90 7.49 3.70 1.70 1.35 94.50
CN-35 6.60 3.03 5.20 2.50 0.99 94.88
CN-36 8.40 2.30 4.00 3.10 0.82 96.79
CN-45 10.90 8.15 2.80 1.20 0.47 95.31
CN-98 5.70 3.21 5.40 2.90 0.64 96.91
CN-116 2.20 1.74 6.80 2.10 0.26 96.75
CN-137 0.10 0.05 4.80 3.50 0.01 99.23
CN-139 10.90 6.21 3.60 1.40 0.78 96.06
CN-140 6.50 2.91 5.50 1.70 0.77 93.05
CN-142 4.10 1.94 4.40 4.30 0.87 95.52

 CaO/[Al.sub.2][O.sub.3] [Th.sup.2] [U.sup.2] [Sc.sup.2] Th/U

CN-5 0.68 -- -- -- --
CN-8 0.44 8.7 1.8 11 4.8
CN-11 0.49 -- -- -- --
CN-16 0.41 9.3 2.2 10 4.2
CN-19 0.41 -- -- -- --
CN-24 0.23 -- -- -- --
CN-29 0.68 3.3 0.7 25 4.7
CN-39 0.54 -- -- -- --
CN-49 0.28 13 3.1 7.8 4.2
CN-57 0.46 -- -- -- --
CN-59 0.46 -- -- -- --
CN-68 0.50 -- -- -- --
CN-93 0.98 10 3.0 24 3.3
CN-95 0.26 21 6.5 9.3 3.3
CN-100 0.25 -- -- -- --
CN-104 0.30 -- -- -- --
CN-111 0.44 -- -- -- --
CN-112 0.14 -- -- -- --
CN-95 1.17 -- -- -- --
CN-35 0.36 -- -- -- --
CN-36 0.41 10 2.3 6.5 4.3
CN-45 0.69 4.8 1.3 29 3.7
CN-98 0.34 18 5.3 10 3.4
CN-116 0.14 5.4 1.5 6.8 3.6
CN-137 0.01 19 6.1 2 3.1
CN-139 0.72 -- -- -- --
CN-140 0.37 -- -- -- --
CN-142 0.26 -- -- -- --

(1) Total Fe as FeO.
(2) In parts per million.


I thank Bill Laughlin, Los Alamos National Laboratories, for allowing access to the electron microprobe, and Peggy Snow for kindly instructing me in its use. Jack Weaver provided access to the chemistry laboratory of Owens-Corning Fiberglass in Amarillo for the whole-rock chemical analyses. Killgore Research Center, West Texas State University provided funding for this project and equipment and space to pursue the research. Reviews by Calvin Barnes and an anonymous reviewer substantially improved the manuscript.


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Killgore Research Center and Department of Biology and Geosciences, West Texas State University, Canyon, Texas 79016
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Author:Cepeda, Joseph C.
Publication:The Texas Journal of Science
Geographic Code:1U8NM
Date:May 1, 1991
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