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An Alkali-Basalt Through Trachyte Suite, Mesa Chivato Mount Taylor Volcanic Field, New Mexico: Summary

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An Alkali-Basalt Through Trachyte Suite, Mesa Chivato Mount Taylor Volcanic Field, New Mexico: Summary An alkali-basalt through trachyte suite, Mesa Chivato Mount Taylor volcanic field, New Mexico: Summary L. S. CRUMPLER Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721 INTRODUCTION The Mount Taylor field is unusual in that alkalic suites appear to be rare in the record of volcanism on the North American continent. The Mount Taylor volcanic field of west-central New Mexico is In general, alkalic suites are characteristic of aseismic oceanic arcs one of several dominantly basaltic Colorado Plateau marginal vol- (for example, Hawaiian arc), stable craton volcanism (for example, canic fields of Pliocene-Pleistocene age (Best and Brimhall, 1974), Tibesti region, North Africa), and continental rifts (for example, most of which include at least one large silicic composite volcano, Danakil depression, Africa) throughout the world where magmas as well as isolated intermediate and silicic vents. are presumably derived from depths on the order of the thickness of North of the Mount Taylor composite cone, volcanism is alkalic the lithosphere (Carmichael and others, 1974). The Mount Taylor and consists of two distinct petrologic suites: (1) an alkali basalt- field lies just beyond the western margin of the Rio Grande rift (Fig. hawaiite-mugearite-benmorite-trachyte suite similar to Hawaiian 1) and is thus the only known example of a continental rift-type alkalic volcanism (terminology from Irvine and Baragar, 1971) and volcanic suite near the Rio Grande rift or in the Basin and Range (2) a later series of porphyritic alkali basalts and aphyric hawaiites. Province. Figure 1. Location of the Pliocene- Pleistocene Mount Taylor volcanic field with respect to other late Cenozoic ex- trusive volcanic rocks and the Rio Grande rift, New Mexico. Map modified from Woodward and others (1975). A geologic map is included in the corre- sponding article in Part II. The complete article, of which this is a summary, appears in Part II of the Bulletin, v. 91, no. 5, p. 1293-1331. Geological Society of America Bulletin, Part I, v. 91, p. 253-255, 2 figs., May 1980, Doc. no. S00501. 253 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/5/253/3418886/i0016-7606-91-5-253.pdf by guest on 03 October 2021 254 L. P. CRUMPLER This is the first systematic field and petrologic study in the Mount In addition, northeasterly linear fissure lines with overlapping Taylor field (Crumpler, 1976, 1977). Previous work has centered cinder cones, pit craters, and maars characterize this second out- on the Mount Taylor volcano and immediate vicinity (Bassett and burst of volcanism. Northeast-trending faults displace older flows, others, 1963; Baker and Ridley, 1970; Hunt, 1938; Lipman and particularly along fissure vents, but locally, cinder cones rest on the Moench, 1972; Lipman and others, 1979; Crumpler, 1978) or the faults, and it is difficult in every case to tell whether the fault or the recent lavas of the Bandera-McCartys area to the south (Laughlin fissures came first. and others, 1972; Nichols, 1946), but there has been no previous A dominant northeast trend throughout the field is manifest in study of the complex northern half of the field (Mesa Chivato). the numerous faults and fissure lines as well as the northeast elon- More than 100 cinder-and-spatter cones, trachyte flow-domes, gation of the entire field. Since the boundary of the Colorado maars, and pit craters have been documented by detailed mapping Plateau and Rio Grande rift trends northeast near the Mount (1:24,000) on Mesa Chivato. (See the corresponding article in Part Taylor field, the northeast-trending fissures and faults probably II for geologic map.) The total number of vents within the entire reflect late Cenozoic tensile stresses at the boundary of these two field may be two to three times this number. major physiographic provinces. FIELD RELATIONS PETROLOGY AND PETROGENESIS Eruptions commenced with alkali basalt (basanitoid type) flows Major mineral constituents include olivine, augite, plagioclase, and were succeeded by hawaiite, mugearite, benmorite, and and opaque minerals (mostly titaniferous magnetite), along with trachyte in stratigraphic sequence, although there is local in- traces of phlogopite in the early alkali basalt and pleochroic brown terfingering of intermediate compositional types. Basaltic lavas apatite in the mugearite. Phenocrysts, megacrysts, and ultramafic were erupted throughout the area, whereas the more silicic differ- inclusions are common in the late alkali basalts and include entiates were erupted largely near a central dome complex. The ac- aluminous titaniferous augite, andesine, and olivine, in addition to tivity of the alkalic suite culminated with explosive eruptions of dunitic and pyroxenitic clots. highly fractionated, dark, fluid trachyte from vents which form an Major elements in the alkalic suite follow the typical alkalic annulus around the central dome complex. Because the vents are trend (Fig. 2) similar to that reported for the Hawaiian alkalic suite. arranged in a circular pattern, a ring fracture is suggested, but there The basalts are unusually enriched in Ti02 relative to the average is no field evidence indicative of three-dimensional structure. Basin and Range basalt of Leeman and Rogers (1970), and in this Following the last eruptions of trachyte, renewed activity in the respect they are more akin to typical oceanic alkali basalts than field consisted of eruptions of alkali basalt, bearing abundant continental basalts. megacrysts, and aphyric hawaiites which locally overflowed es- Sr and Ba lineages are exemplary of trends in alkalic suites carpments formed in the lavas of the alkalic suite. The number of elsewhere which are interpreted as resulting from fractional crys- cinder-and-spatter cones was increased at least tenfold during this tallization (Zielinski and Frey, 1970; Bishop and Wooley, 1973; time; six maars, each 1 km in diameter or larger, were also formed. Weaver and others, 1973). The limited area over which the alkalic suite was erupted and the general order of eruption both strongly suggest that the rocks were evolved from a single magmatic source, F which was successively sampled by eruptions during its compo- sitional evolution. The origin of the abundant megacrysts in the late basaltic rocks is uncertain, but they may be fragmented ultramafic nodules originating in the mantle above the magma source. This is supported by the mineralogic similarity of the megacrysts and nodules common elsewhere in the volcanic field. 87Sr/86Sr ratios range from 0.7051 to 0.7061 for basalts and hawaiites and from 0.7039 to 0.7049 for trachytes (Brookins and others, 1977); the higher ratios for basaltic rocks possibly result from increased assimilation of co-liquidus crustal materials during the ascent of the basaltic magmas. Because of the higher viscosity of trachytic liquids, mixing of assimilated country rock may not take place as effectively during ascent as in basaltic liquids, thereby re- ducing bulk contamination of rising trachytic magmas relative to basalts. CONCLUSIONS The Mount Taylor field alkalic suite shows a similarity to vol- canism of aseismic oceanic islands, continental rifts, and other areas with major vertical lithospheric discontinuities, and is of a Figure 2. AMF diagram (A = Na20 + K20, M = MgO, F = petrologic type previously unknown in the continental United FeOr ) of representative rocks. Trend line shown is for early alkalic States. The strong fissure pattern and fault scarps imply that the suite only (solid circles); late alkali basalts and hawaiites, crosses. origin of the field is r elated to deep fracturing of the continental B = basalt, H = hawaiite, M = mugearite, Bm = benmorite, T = lithosphere along the margins of the Colorado Plateau through trachyte. Pleistocene time. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/5/253/3418886/i0016-7606-91-5-253.pdf by guest on 03 October 2021 ALKALI-BASALT THROUGH TRACHYTE SUITE, NEW MEXICO: SUMMARY 255 ACKNOWLEDGMENTS New Mexico, University of New Mexico, 131 p. and geologic map. 1978, Mount Taylor composite volcano, New Mexico: Geology of the north and west flanks: Geological Society of America Abstracts with This study was guided and encouraged by W. E. Elston while the Programs, v. 10, p. 101. author was at the University of New Mexico and supported by Hunt, C. B., 1938, Igneous geology and structure of the Mount Taylor vol- NASA Grant NGR 32-004-062 from the Planetary Geology Pro- canic field, New Mexico: United States Geological Survey Professional gram Office. Felix Chayes and Vernon E. Swanson provided com- Paper, 189-B, p. 51-80. plimentary and critical suggestions regarding the original manu- Irvine, T. N., and Barager, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of script. Peter Lipman and Harold Mehnert of the U.S. Geological Earth Sciences, v. 8, p. 525—544. Survey offered preliminary results of field and K-Ar studies of the Laughlin, A. W., Brookins, D. G., and Causey, T. D., 1972, Late Cenozoic Mount Taylor field which were beneficial to chronologic interpre- basalts from the Bandera lava field, Valencia County, New Mexico: tations. Geological Society of America Bulletin, v. 83, p. 1543-1552. Leeman, W. P., and Rogers, T. W., 1970, Late Cenozoic alkali-olivine basalts of the Basin-Range Province, U.S.A.: Contributions to REFERENCES CITED Mineralogy and Petrology, v. 25, p. 1-24. Lipman, P. W., and Moench, R. H., 1972, Basalts of the Mount Taylor vol- Baker, Ian, and Ridley, W. I., 1970, Field evidence, K, Rb, Sr data bearing canic field, New Mexico: Geological Society of America Bulletin, on the origin of the Mount Taylor volcanic field, New Mexico, U.S.A.: v. 83, p. 1335-1344. Earth and Planetary Science Letters, v. 10, p. 106—114. Lipman, P. W., Pallister, J. S., and Sargent, K. A., 1979, Geologic map of Basset, W.
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