New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/15 Differentiation and alkali metasomatism in dike swarm complex and related igneous rocks near Capitan, Lincoln County, New Mexico Wolfgang E. Elston and Henry I. Snider, 1964, pp. 140-147 in: Ruidoso Country (New Mexico), Ash, S. R.; Davis, L. R.; [eds.], New Mexico Geological Society 15th Annual Fall Field Conference Guidebook, 195 p. This is one of many related papers that were included in the 1964 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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The prominent dike swarm between the villages of The most important properties of the dike rocks Capitan and Nogal has attracted the attention of almost are summarized in table 1 and photomicrographs of typi- every geologist who has travelled through the area. It cal examples are shown in figure 1. Detailed studies to extends from the vicinity of Sierra Blanca some 40 miles determine exact composition of the minerals have not to the north-northeast, into the Jicarilla Mountain area. been made, and would be difficult because of alteration. The longest individual dikes are tens of feet wide and about 3 miles long, but the majority are much smaller. Their best development is around Indian Divide and the Field Relationships area immediately to the southwest. As shown in figure 2, the dikes are irregular in Air photos of the outcrop belts of the Mancos Shale, shape. They are intrusive-magmatic rather than meta- Mesaverde Group and Cub Mountain Formation show somatic, judging by the disruption of invaded rocks, hundreds of low NNE-trending ridges, marked by prom- sharp contacts, fine-grained chilled borders, and flow-lin- inent tree lines. On the ground, each ridge turns out to eation of crystals. Rhyolite has a distinct tendency to be a composite dike, made up of many individual dikes form sills in incompetent shale beds; other rock types with complex cross-cutting relationships. The ridges are tend to form dikes controlled by fractures. covered by innumerable dark boulders formed by spher- Most relative ages could be established by cross- oidal weathering of various types of diabase. Outcrops cutting relationships, but even where two dikes were par- are poor except in cuts along U.S. Highway 380 and the allel the younger could usually be recognized by its fine- abandoned railroad between Carrizozo and Capitan. grained chilled border. Also, irregular apophyses, a few The dikes show features of more than local interest. inches long, of the younger dike locally invade the older The older dikes are various types of diabase; the younger dike. dikes include both quartz-bearing rhyolite and nephe- The relationship of Type 2 to Type 1 is based on a line-bearing phonolite. The plagioclases of altered dia- single weathered outcrop (figure 2A). Microscopically, base dikes generally show replacement of labradorite by the Type 2 olivine diabase porphyry is finer-grained at oligoclase, a feature more commonly associated with the the contact than a foot away from it, indicating chilling marine spilitic volcanic rocks of eugeosynclines than with against pre-existing Type 1 rock. These age relationships dikes in a continental environment. Secondary potassi- are important because Type 2 is the most mafic rock in um feldspar occurs in some of the dikes but seems to be the complex. As for the remaining age relationships, de- more common in the nearby Carrizo Mountain stock. tailed examination of composite dikes in 21 road and There, late magmatic or post-magmatic orthoclase pro- railroad cuts failed to show any exceptions to the order gressively replaces plagioclase phenocrysts near the roof of intrusion given here. Away from artificial cuts only of the intrusion. a few outcrops show conclusive cross-cutting relation- The only previous publication on the dikes (Patton, ships, and these also follow the same sequence. In none 1951) is confined to a brief description of olivine-free of the composite dikes examined were all seven rock diabases. It lacks an discussion of field occurrences. This types seen; the sequence was pieced together from seve- article is based on field work done by Elston for the ral occurrences. The sketches in figure 2 show all rock New Mexico Bureau of Mines and Mineral Resources types except Type 7 phonolite, but a phonolite dike cuts in 1956, followed by examination of 123 thin sections by a Type 6 latite dike 0.1 mile east of the roadcut shown Elston in 1957-58 and by Snider in 1962-63. in figure 2C. DIKE ROCKS Differentiation Trends The dikes consist of seven rock types, which are, in The dikes do not follow a simple differentiation order of age: (1) labradorite-olivine diabase porphyry, trend from mafic to felsic or mafic to alkalic. If the (2) olivine diabase porphyry, (3) diabase, (4) horn- field relationships have been interpreted correctly, lab- blende-biotite diabase, (5) rhyolite, (6) latite (grading radorite-olivine diabase porphyry (Type 1) is older than into trachyte), and (7) phonolite. For brevity, these the more mafic olivine diabase porphyry (Type 2). Pos- rocks are referred to as Types 1 through 7 throughout sibly, Type 1 is representative of the parent magma and this paper. Type 2 has been enriched in early-formed crystals. The diabases, especially Types 3 and 4, are the Further differentiation would follow a clearly alkalic most abundant; rhyolite is locally abundant but absent trend ending in phonolite (Type 7) if it were not for 140 Texture Trachytic, with sparse phenocrysts of oligoclase and orthoclase. 0? OOi 0 Alteration Calcite pseudomorphous after pheno- crysts of a ferromagnesian mineral (py- P. roxene?). Secondary cancrinite and so- dalite may be present in traces around nepheline. Texture Phenocrysts (sparse in some specimens) augite up to 3.0 mm long, sparse oligo- In M clase. Groundmass felted intergrowth of tl CO ctl K-spar and albite. Alteration Secondary quartz and calcite in cavities, green biotite flakes probably from altera- tion of augite; kaolinite, sericite, chlorite, zeolite (scolecite ?) present in some spe- cimens. Texture Fine-grained (0.125 mm) flow-banded 0 0 granular aggregate of quartz and K-spar, )? 10 sparse phenocrysts of embayed quartz, 0, 0 10 CD 1-1 altered albite. Alteration Potassium feldspar partly unmixed to perthite, sericitized. Some xls. rimmed with albite. Sparse albite phenocrysts, seriticized. Secondary calcite abundant. Texture Diabasic, plagioclase phenocrysts sparse, less than 2.5 mm long. Alteration Generally fresher than diabase, but ande- sine-labradorite commonly replaced by oligoclase. Biotite formed by reaction from hornblende, secondary orthoclase, chlorite, serpentine, zeolite present. 2 Texture Fine-grained, felty groundmass, pheno- Ca crysts less than 2 mm long, less abund- In 00 ant than in other diabase types. C.? E ti - o Alteration Intensely altered. Plagioclase sericitized, 02 v-I olivine replaced by calcite and chlorite; a) N augite by secondary green biotite, more rarely by chlorite with magnetite rims. 0 Zeolite present in some specimens, also •0 trace of secondary orthoclase. P. Cacd Texture Diabasic; phenocrysts 2.5 mm long, make In 0 up 50-60% of rock, are less conspicuous in 6 outcrops than in labradorite-olivine dia- .2i a) base porphyry. .0 0 Alteration Tends to be deeply weathered. In fresh • b-0 0 specimens, olivine has magnetite rims, is .0 )42 partly serpentinized. t.) 0.) • :E Ca Texture Diabasic; conspicous phenocrysts, mainly E) >% [1] N O labradorite laths 5.0 mm long, make up I 1 ."" a) a) o IfJ Ili 30-40% of rock. „, co 1-1 .10 0 Alteration Olivine partly replaced by calcite and 0 serpentine, some crystals have magnetite :5 centers. q 0 ;.-1 o o .cn cd E 1.