Jerry M. Hoffer

Compositional diversity in late Cenozoic mafic lavas in the Rio Grande rift and Basin and Range province, southern New Mexico

This study reports major- and trace-element compositions of late Cenozoic (5 Ma and younger) mafic volcanic rocks from southern New Mexico. Stratigraphic sampling is reported for two of the volcanic fields, the Potrillo and the Jornado del Muerto volcanic fields; reconnaissance sampling, for an additional seven fields. These are among the first chemical analyses for some of these volcanic fields, and they provide insight into the dynamics of melt generation in this continental rift zone. The lavas fall into two groups: (1) an alkaline suite of basanite, alkali basalt, and trachybasalt with low SiO2 and high abundances of incompatible elements (TiO2, Nb, Rb, and Sr); (2) a subalkaline suite of subalkali basalts with high SiO2 and low abundances of the incompatible elements. Compositions of volcanic rocks from the Geronimo volcanic field, the Taos Plateau volcanic field, and the transition zone in central New Mexico are similar to those reported in this study, confirming that these are general characteristics of lavas throughout the region. For many of the large fields, the volcanic rocks belong exclusively to one compositional group or the other. Furthermore, there appear to be no patterns to the distribution of these compositional groups; for example, the Potrillo volcanic field, which is composed exclusively of alkaline lavas, and the Jornado del Muerto volcanic field, which is dominantly subalkaline, both occur in the axis of the Rio Grande rift. We see no evidence for evolution in melt chemistry during the past 5 m.y. In the Potrillo volcanic field, for which we have the best documentation of stratigraphic changes in composition, incompatible elements increase upsection and are correlated with decreasing Mg number, suggesting that magmas underwent differentiation before their final ascent. Phenocrystic olivine and plagioclase, which are liquidus phases at low pressure, imply a shallow depth for this crystallization event. Processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution. Correlation between incompatible-element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types.

Petrology and Mineralogy of the Campus Andesite Pluton, El Paso, Texas

The Campus Andesite represents a small igneous pluton located in the Rio Grande valley and crops out on the campus of The University of Texas at El Paso. The igneous mass is post-Cretaceous in age, showing intrusive contacts with shale and marl of the Boquillas Formation and is surrounded by Quaternary lake or alluvial deposits (or both). Absolute age of the pluton, based on a K-Ar date, is 47.1 ± 2.3 m.y. Texturally, the intrusive is porphyritic with phenocrysts averaging 2 to 3 mm in length and set in an aphanitic groundmass. The phenocrysts, composed predominantly of plagioclase (andesine), comprise approximately 40 percent of the rock. The mineralogy of the andesite consists of phenocrysts of plagioclase, biotite, and hornblende with groundmass constituents of plagioclase, K-feldspar, and minor quartz and magnetite. Two chemical analyses of the Campus Andesite indicate a chemical composition intermediate between an andesite and a dacite. Mineralogically, the rock is classified as a porphyritic andesite.

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

The Santo Tomas-Black Mountain basalts were erupted during the Quaternary from four centers. Six lava flows are present at Black Mountain, three at Santo Tomas, and one each at Little Black Mountain and San Miguel. The basalts are grouped into three major types of phenocryst mineralogy: (l) plagioclase abundant, (2) olivine abundant, and (3) both olivineand plagioclase abundant. All three types are alkali-olivine basalts, showing high alkali-silica ratios and total alkali content increasing with silica. Seven periods of basaltic extrusion among the centers have been established on the basis offieldevidence, phenocryst mineralogy, and pyroxene-olivineratios. K-Ar dates show the basalts to be less than 0.3 × 10 6 m.y. old. The basalts are thought to have originated from a single small, shallow magma chamber which was under the influence of a high thermal gradient during differentiation.

Composition and Structural State of Feldspar Inclusions from Alkali Olivine Basalt, Potrillo Basalt, Southern New Mexico

Crystals of anorthoclase, lime-anorthoclase, potash-plagioclase, and plagioclase (Ab 42.7 to 72.7, An 4.6 to 53.5, and Or 4.0 to 32.0 mole percent) are found as inclusions in lava flows, as loose crystals on the flanks of cinder cones and maar craters, and in the cores of ejected bombs in the upper Quaternary Potrillo Basalt (denned here), south-central New Mexico. Nodules and crystals of olivine-spinel-pyroxene, pyroxene, and amphibole are associated with the feldspar crystals in undifferentiated lava. The structural state of the alkali feldspar crystals is high, whereas that of the plagioclase appears to range from high to low. The feldspar inclusions probably formed in a deep-seated high-temperature environment.

The San Miguel Lava Flow, Dona Ana County, New Mexico

The San Miguel lava flow is a porphyritic olivine basalt erupted from a small cinder cone on the eastern edge of the La Mesa surface in south-central New Mexico 10 miles south of Las Cruces. The elongate pattern of the flow and its present position indicate extrusion of the flow followed a pre-existing drainage channel into the Rio Grande Valley. Texturally the flow is divisible into highly vesicular and scoriacious top and bottom zones with a more dense, columnar interior. The phenocryst mineralogy, consisting of approximately equal amounts of plagioclase and olivine with subordinate pyroxene, suggests correlation with one of the flows from nearby Black Mountain.

Discrimination of topaz rhyolites by major-element composition: a statistical routine for geochemical exploration

Abstract The recognition of topaz-bearing, calc-alkaline, and peralkaline rhyolites at an early stage of an exploration program may be of both geologic interest and of economic significance due to the different mineral deposits characteristically associated with each of these rock suites. Such discrimination could result in better definition of target areas and commodities to explore within a selected region. A geochemical database of major element analyses of calc-alkaline, peralkaline (comendites), and topaz rhyolites from western North America was assembled. Multiple discriminant function analysis assigned each sample statistically to the calc-alkaline, topaz, or peralkaline groups, using only major element composition. The assignment correctly identified 90% of the rhyolites as members of the groups to which they actually belong. In effect, then, major-element composition can serve as a proxy for fluorine content either when no fluorine analysis is available or when it is suspected that fluorine was not preserved in the rock due to its loss during emplacement or subsequent devitrification. We have included a simplified procedure to use our discrimination model to identify silicic rocks of unknown affiliation. This model makes it possible to evaluate the economic potential of both new or previously explored areas by analyzing data files containing major-element chemical analyses.

Herradura: A New Lava Flow Feature

Lava ridges which are horseshoe-shaped in plan occur on the flanks of several volcanic cones in the La Mesa Basalts, Quaternary alkaline olivine basalts in south-central New Mexico. I propose to call these newly described features herradura (Spanish: horseshoe). They formed by extrusion of lava through an already existing sloping crust of lava.

Tectonic Implications of a Basaltic Dike, Southern Franklin Mountains, El Paso County, Texas

A basaltic dike cuts Paleozoic strata and a Cenozoic landslide breccia in the southern Franklin block of the Franklin Mountains. Strike-slip displacement of this dike records mostly right lateral but some left lateral movements along major faults in the range. These major faults are genetically related with the boundary faults of the range. The dike has been emplaced following exogenic gravity gliding of large masses of west-dipping strata on the western dip slope of the range, thus indicating the following general geologic history: contemporaneous uplift and tilting of the range, with some internal faulting and gravity gliding of large blocks, dike emplacement, and strike-slip displacement along the internal faults. The Franklin Mountains are bounded by the Texas lineament on the southern end and by the Rio Grande trench on their westerly side, hence this strike-slip displacement in major faults of the range is of significance in the analysis of the regional tectonics of these two major lineaments.