Petrology and geochemistry of the Paliza Canyon Formation and the Bearhead Rhyolite, Keres Group, Jemez Mountains, New Mexico •™EAU I Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 ABSTRACT however. Removal of the observed pheno- 1960a, 1960b; Smith and Bailey, 1966, 1968; crysts found in the rhyodacite would form a Smith and others, 1970; Bailey and others, Basaltic andesite, andesite, dacite, and rhyolite enriched in Ba, Hf, Th, U, and light 1969; Ross and Smith, 1961) have resulted in rhyodacite of the Paliza Canyon Formation rare earth elements (LREE) and depleted in V many discoveries which have contributec. to our (9.1 to 8.5 m.y.) and the Bearhead Rhyolite and Sc; the Bearhead Rhyolite, however, is knowledge of ash-flow tufls, resurgent calderas (7.1 to 6.5 m.y.) belong to a high-K, calc- depleted in the former group and enriched in (Valles caldera is the classic example), and the alkaline association in the southern Jemez the latter. These conclusions are supported by long volcanic activity which occurred in the Mountains, New Mexico. The majority of the initial ^Sr/^Sr ratios: three samples of Jemez Mountains. Although our basic under- rocks of the Paliza Canyon Formation con- Bearhead Rhyolite have initial ratios of standing of the older pre-Valles caldera volcan- tain augite, hypersthene, and plagioclase 0.7056, 0.7073, and 0.7076; two samples of ism has been provided by these men, very little phenocrysts. Olivine is an important pheno- Paliza Canyon basaltic andesite, on the other work integrating detailed field mapping with cryst in the basaltic andesite, but it is absent hand, have ratios of 0.7041 and 0.7044. llie geochemical studies has been accomplished on in the other rock types. In the rhyodacite, most likely source for the Bearhead Rhyolite these groups of rocks which document more hornblende is a major phenocryst at the ex- is lower crustal material which may have than 8 m.y. of volcanism prior to the eruption of pense of the py roxenes. Characteristically, a been fused partially from the heat provided the Bandelier Tuff. The oldest group of pre- very restricted range in the Fe/Mg composi- by the andesitic magmas of the Paliza Canyon Valles caldera rocks in the southern Jemez tion is found for the pyroxenes (augite from Formation. Mountains, the Keres Group, is the target of this 0.33-0.29) and hornblende (0.41-0.47) study. We will present mineral chemistry, throughout the whole range of rock types, INTRODUCTION major- and trace-element compositions, and from basaltic andesite (-56% Si02) to rhyo- computer modeling of the data for the Paliza dacite (67% Si02). On the other hand, the The careful and significant studies in the Canyon Formation and for the Bearheac. Rhyo- Bearhead Rhyolite lacks pyroxene, and it has Jemez Mountains of New Mexico by R. L. lite (Table 1). The evidence is compatible with two feldspars (¡¡anidine and plagioclase) and Smith and R. A. Bailey (for example, Smith, the basaltic andesite-dacite-rhyodacite sequence bipyramidal quartz and biotite. Its Si02 con- tent ranges between 76.5% and 77.3%. The Bearhead Rhyolite lies on the two-feldspar surface, or below it, in the sanidine field; the TABLE 1. DESCRIPTION AND STRATIGRAPHY OF THE KERES GROUP VOLCANICS IN THE STUDY AREA Paliza Canyon rocks plot within the plagio- Unit Age (m.y.) Brief description clase volume of the granite system. Both major- and trace-element trends in the Paliza Bearhead Rhyolite 7.1-6.5 Flow and dime member: flow-banded, phenocrysts of bipyramidal quartz, sanidine, plagioclase, biotite, opaque mineral; trace of zircon and sphene. Canyon Formation do not project to the Peraiia Tuff member, crystal-poor, lithic-rich, poorly welded ash-flow tuff with interbediled air-fall Bearhead Rhycilite composition. Major- and and water-reworked tuffs; phenocrysts of sanidine, plagioclase, quartz, biotite, opaque minerals. trace-element modeling suggests that the se- Paliza Canyon Formation 9.1-8.5 Rhyodacite member: flows, domes, porphyritic with plagioclase, hornblende, minor opaques, hypersthone, traces of biotite, apatite, zircon, and quartz. quence from basaltic andesite to rhyodacite in Dacite member: flows, domes, coarsely porphyritic with plagioclase. hornblende, two pyoxenes, opaques, apatite. the Paliza Canyon Formation can be formed Upper Andesite member: flows, domes, aphyric andesite with platy cleavage, plagioclase. by simple fractional crystallization. The hornblende, minor augite and hypersthene. Lower member: flows and interbedded la hare, andesite, and olivine-bearing basaltic andesite; Bearhead Rhyolite cannot be formed from phenocrrsts of plagioclase, hypersthene, augite, apatite, opaque minerals, ± divine. the Paliza Canyon rhyodacite by this process, Rhyodacite of La Jara Canyon ? Flow and dome sequence: flow-banded rhyodacite, coarsely porphyritic with plagioclase. biotite, and minor hornblende, augite, zircon, apatite, rare quartz. Canovas Canyon Rhyolite 10.2 Dome merr;ber: flow-banded, sparsely phyric rhyolite with sanidine, quartz, biotite, plagioclase, and trace opiiques and zircon. Similar chemically and petrographically to the Bearhead Rf yolite above. Flow and Tuff member -, interbedded flows and tuffs, phenocrysts of plagioclase, biotite, opaques, *Present addres: Shell Oil Company, P.O. Box rare quartz and sanidine. 60775, New Orleans, Louisiana 70160. Additional material for this article (Tables A-E) may be secured by requesting Supplementary Data 85-02 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 96, p. 108 -113, 5 figs., 3 tables, January 1985. 108 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/1/108/3430210/i0016-7606-96-1-108.pdf by guest on 25 September 2021 PETROLOGY AND GEOCHEMISTRY, JEMEZ MOUNTAINS, NEW MEXICO 109 of the Paliza Canyon Formation being generated Di Hd by simple fractional crystallization but that the Bearhead and (possibly) Canovas Canyon Rhy- olites have a different origin. STRATIGRAPHY AND PETROGRAPHY The oldest sequence of volcanic rocks in the southern Jemez Mountains was named the "Keres Group" by Bailey and others (1969). This group is composed of two subgroups: an older, bimodal basalt-rhyolite sequence of the basalt of Chamisa Mesa and the Canovas Can- yon Rhyolite, and a younger, basalt-andesite- dacite-rhyodacite sequence of the Paliza Canyon En Fs Formation and the Bearhead Rhyolite. One rock unit not previously described is herein in- Figure 1. Pyroxene quadrilateral diagram showing compositions from Paliza Canyon For- formally named the "rhyodacite of La Jara mation lavas. Outlined fields represent ranges of pyroxene analyses; filled triangle is average Canyon" (Table 1). Table 1 also shows addi- olivine composition from basaltic andesite sample 099. tional subdivisions of the Paliza Canyon Forma- tion into rhyodacite, upper andesite, lower basaltic andesite and andesite member, and the two-fold decrease in clinopyroxenes from basal- Feldspar. Feldspar is ubiquitous throughout lahar member. Based on the field occurrence tic andesite to dacite. This is common in igneous the suite. Plagioclase is the dominant phenocrys- and the petrography and nominal chemical data, rock suites and probably reflects the rising silica tic phase in the basaltic andesite through rhyo- it appears that the Canovas Canyon Rhyolite is activity of the host magma (Carmichael and dacite sequence. The rhyolites have both plagio- very similar to the Bearhead Rhyolite. The others, 1974, p. 273-275). No substantial differ- clase and sanidine in subequal amounts (Fig. 2). rhyodacite of La Jara Canyon, a flow and dome ence in composition was observed between In one sample of Bearhead Rhyolite, sanidine sequence exposed in the southern part of the phenocrystic and groundmass pyroxenes. phenocrysts are rimmed by plagioclase. Ruiz Peak area and previously mapped as Paliza Amphibole. Amphibole is a major phase in The plagioclase phenocrysts in the basaltic Canyon andesite (Smith and others, 1970), is a the rhyodacite member of the Paliza Canyon andesite and in the andesite from the lower sec- distinctly different unit with alternating dark and Formation, although it occurs as a minor con- tion of the Paliza Canyon Formation have a light pink bands and a coarsely porphyritic tex- stituent in the basaltic andesites and upper an- similar range of composition between An45_7o, ture with plagioclase, biotite, and minor horn- desite member of the Paliza Canyon Formation but the basaltic andesites have plagioclases blende. None of the Paliza Canyon rocks is and in the rhyodacite of La Jara Canyon. Ac- which are weakly zoned. The normal and re- strongly banded nor so coarsely porphyritic. cording to a classification scheme of Deer and verse zones are generally <10 mole percent others (1978), which uses Al4 versus alkalis, the maximum; in the andesites, however, some of MINERAL CHEMISTRY amphibole appears to be pargasitic hornblende the larger plagioclase grains have a variation be- (Table A).1 Hornblende grains in the basaltic tween core and rim of up to 20 mole percent. Olivine. Olivines present in the basaltic an- andesite are kaersutite, containing >5% Ti02- Plagioclase microphenocrysts from the upper desite of the Paliza Canyon Formation are per- The Fe/Mg ratios of the hornblende grains have andesite member are only slightly zoned, rang- vasively iddingsitized. Only one sample was a limited range (0.41-0.47), and no apparent ing from An5o to An6o from rim to core. In the found which contains olivine crystals with unal- trend from basaltic andesite to rhyodacite is ob- dacites, the plagioclase phenocrysts are normally an tered cores between F076.6 d F076 7 (Fig. 1). served. Ti02 does decrease, however, toward zoned, with a large compositional range span- Pyroxene. Calcic augite and bronzite-hyper- the more silica-rich rock. A decrease in Ti02 of ning the labradorite to oligoclase field sthene occur commonly as phenocrysts in all >15% was found from core to rim in one (An65_25). The Paliza Canyon rhyodacite con- rocks, from basaltic andesite to dacite, in the sample.