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Home , Keres Group, Rhyodacite

Petrology and geochemistry of the Paliza Canyon Formation and the Bearhead , Keres , , 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, , 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 (7.1 to 6.5 m.y.) belong to a high-K, calc- depleted in the former group and enriched in (Valles 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- volcan- tain augite, hypersthene, and 0.7056, 0.7073, and 0.7076; two samples of ism has been provided by these men, very little . Olivine is an important pheno- Paliza Canyon , 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 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 . The oldest group of pre- very restricted range in the Fe/Mg composi- by the andesitic of the Paliza Canyon Valles caldera rocks in the southern Jemez tion is found for the (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 , and it has Jemez Mountains of New Mexico by R. L. lite (Table 1). The evidence is compatible with two (¡¡anidine and plagioclase) and Smith and R. A. Bailey (for example, Smith, the basaltic andesite-dacite-rhyodacite sequence bipyramidal and biotite. Its Si02 con- tent ranges between 76.5% and 77.3%. The Bearhead Rhyolite lies on the two- surface, or below it, in the 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

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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 -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 . 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 (Carmichael and dacite sequence. The 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 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 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- , 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. tains plagioclases in the An5o to An2o range. Paliza Canyon Formation. Only one sample was Biotite. Biotite is a significant phenocrystic Oxide Minerals. Although titanomagnetite is found which contained only calcic augite. No phase in the rhyodacite of La Jara Canyon as ubiquitous, ilmenite occurs sporadically and in

visible trend in the Fe/Mg ratio is observed in well as in the Bearhead Rhyolite. Unlike the minor amounts throughout the suite. Ti02 de- the rocks from basaltic andesite to dacite pyroxenes and hornblendes, biotite appears to creases steadily from the Paliza Canyon basaltic (Fig. 1). A limited range in the Fe/Mg ratio in become Fe enriched and Ti depleted as silica andesite to rhyodacite, and FeO* correspond- pyroxenes is a common feature in volcanic-arc enrichment increases in the host rocks (Table B). ingly increases (Table C). The percentage of il- suites and is a reflection of the lack of iron The MgO/FeO changes from greater than unity menite lamellae in titanomagnetite grains in- enrichment in the host magmas (Beddoe- to less than unity from the rhyodacite to the creases to 40% in the dacites and rhyodacites. Stephens, 1982; Ewart, 1979). Cameron and rhyolites. No zoning was detected. others (1980) believed that high oxygen fugacity MAJOR-ELEMENT VARIATIONS may be responsible. Only a slight zoning of <2% 'Tables A, B, C, D, E may be secured free of charge MgO is exhibited by individual phenocrysts. by requesting GSA Supplementary Data 85-02 from In the classification of these rocks, terms con- Both A1203 and Ti02 have an approximate the Documents Secretary. sistent with the early work of Smith and others

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• Phenocrysts Domes of the high-silica Bearhead Rhyolite are remarkably homogenous in major-element oGroundmass grains composition. Si02 shows the largest variation, with a maximum range of only 1.0%. One anal- ysis of from the Peralta Tuff Member reveals a silica content of 76.7%, slightly lower than the rhyolite domes. Significant deviation of the Bearhead Rhyo- lite analyses from the trends of the Paliza Can-

yon samples occurs, especially for Na2(), K20, CaO, and A1203. A prominent gap in silica of -9% exists between the two formations. The rhyolite compositions plot between the thermal valley of the ternary system and the minimum in

the projection of the An3 (Ab-Or-Q) system (Fig. 3) and directly on the quartz-saturated, two-feldspar boundary curve (Fig. 3). The Per- alta Tuff analysis plots within the alkali-feldspar field. In the feldspar diagram, plagioclaie com- positions from the Paliza Canyon Formation Figure 2. Feldspar ternary diagrams showing compositions of plagioclases from the Paliza plot as smooth trends within the plagiock.se field Canyon Formation and plagioclase-sanidine pairs from the Bearhead and Canovas Canyon without intersecting the two-feldspar boundary Rhyolites. Triangles contain analyses from the following rock types: (1) basaltic andesites, (2) curve (Fig. 3). andesite, (3) dacite, (4) rhyodacite, and (5) rhyolite. Sample 116 is from the Canovas Canyon Rhyolite; samples 007, 078, and 091 are from the Bearhead Rhyolite. TRACE-ELEMENT VARIATIONS

Total abundances of rare earth elements

(1970) were used, and in only a few cases were Na20 and K20 and depletion of CaO, A1203, (REE) and other trace elements in the Paliza these terms and the field terms modified. Ti02, MgO, FeO + Fe203, and P2Os occur. Canyon Formation rocks (Table D) are signifi- Rocks from tine Paliza Canyon Formation The scatter in these diagrams is most likely d ue cantly higher (generally >50% higher) than

range from 56% t.o 67% Si02 and generally de- to variation in phenocryst percentages among those of typical calc-alkaline suites from island fine smooth trends on the variation diagrams the samples, because the suite consists entirely of arcs but are similar to high-K, calc-alkaline as- (Table 2). With increasing silica, enrichment of porphyritic lavas. sociations such as those found in the western

TABLE 2. MAJOR-ELEMENT CHEMICAL ANALYSES AND MOLECULAR (NIGGLI) NORMS OF REPRESENTATIVE SAMPLES FROM KERES GROUP VOLCANICS

Rock Unit Tccr Trie Tpa, Tpa2 Tpd Tprd Tbp Tbr

Rock type r rd ba ba ba a a d rd r* r r r r r

Sample no. 116 171 099 174 132 045 080 039 055 042 007 126 147 165 078

Si02 76.23 68.97 55.33 56.10 56.28 60.29 62.25 62.60 66.36 73.18 76.72 76.34 77.34 76.65 76.77 AI2O3 12.80 16.21 18.15 17.35 17.60 16.10 17.80 16.45 16.38 12.72 12.45 12.58 12.12 11.94 12.50 Fe203 0.51 2.08 4.26 5.61 5.34 3.74 4.31 3.06 2.23 0.62 0.52 0.55 0.48 0.46 0.45 FeO 0.08 0.20 2.83 2.24 1.69 1.02 0.59 1.81 0.73 0.23 0.11 0.10 0.09 0.23 0.16 MgO 0.06 0.34 2.82 1.91 2.25 2.22 0.58 1.94 0.46 0.23 0.07 0.08 0.05 0.16 0.07 CaO 0.34 1.45 5.75 6.20 6.40 5.58 4.16 4.12 1.60 0.57 0.39 0.34 0.30 0.17 0.49 Na20 3.90 4.66 4.66 4.49 4.55 4.45 4.44 4.65 5.50 2.57 3.80 3.78 3.58 3.74 3.66 K2O 4.54 4.30 2.34 1.68 2.26 2.28 2.67 2.86 4.06 5.07 4.86 4.70 4.54 4.70 4.70 Ti02 0.10 0.43 1.34 1.31 1.33 0.95 0.90 0.89 0.66 0.15 0.10 0.11 0.11 0.16 010 p2o5 0.01 0.07 0.61 0.47 0.48 0.47 0.50 0.30 0.18 0.01 0.01 0.01 0.01 0.03 0.01 MnO 0.04 0.05 0.11 0.09 0.09 0.05 0.04 0.08 0.08 0.07 0.05 0.06 0.04 0.01 0.04 H2O* 0.38 0.85 0.71 1.40 0.99 1.04 1.38 0.52 0.66 3.77 0.37 0.38 0.40 0.70 0.49 H2CT 0.14 0.20 0.24 0.30 0.65 0.29 0.23 0.19 _0J8 0.45 0.05 0.13 0.11 0.17 0.10 Total 99.13 99.81 99.15 98.79 99.91 98.49 99.85 99.47 99.08 99.64 99.50 99.36 99.12 99.13 99.54 SiO/ 77.13 69.84 56.34 57.57 57.27 62.06 63.37 63.38 67.55 76.69 77.43 77.44 78.40 78.02 77.59

q 33.44 16.91 4.95 9.49 6.71 12.70 20.58 13.44 14.23 36.76 33.08 33.75 36.48 34.62 34.33 an 1.72 17.79 22.00 22.98 21.35 17.73 6.81 15.75 6.83 2.94 1.90 1.72 1.53 0.67 2.48 ab 35.78 40.83 42.43 41.68 41.68 41.13 42.30 42.23 49.88 24.57 34.73 34.64 32.96 34.51 33.54 or 27.41 16.15 14.02 10.26 13.62 13.86 25.68 17.09 24.23 31.89 29.22 28.34 27.50 28.53 28.34 di 0.00 0.00 2.31 4.52 6.28 6.09 0.00 2.36 0.00 0.00 0.00 0.00 0.00 0.00 0.00 hy 0.17 1.64 6.74 3.19 3.20 3.27 0.95 4.24 1.28 0.68 0.20 0.23 0.14 0.45 0.20 ol 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 il 0.14 1.00 1.89 1.89 1.89 1.36 0.39 1.25 0.93 0.22 0.14 0.16 0.16 0.23 0.14 rt 0.00 0.14 0.00 0.00 0.00 0.00 0.11 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 mt 0.07 0.00 4.09 2.77 1.39 0.52 0.00 2.56 0.51 0.41 0.17 0.15 0.08 0.23 0.26 ap 0.00 1.07 1.29 1.02 1.02 1.01 0.15 0.63 0.38 0.02 0.02 0.00 0.00 0.06 0.00 hm 0.31 3.08 0.28 2.20 2.87 2.34 1.47 0.45 1.23 0.19 0.26 0.29 0.29 0.18 0.15 c 0.95 1.39 0.00 0.00 0.00 0.00 1.57 0.00 0.51 2.32 0.28 0.73 0.86 0.52 0.56

'Pumice analysis. All other analyses are from whole-rock samples. + Si02 content calculated HjO-frce with analysis normalized to total 100.00%. ba = basaltic andesite; a = indesite, d = dacite; rd = rhyodacite; r - rhyolite Tccr = Canovas Canyon Rhyolite, Trie = rhyodacite of La Jara Canyon, Tp - Paliza Canyon Formation, Tb = Bearhead Rhyolite; Tbp = Peralta Tuff Member of Bearhead Rhyolite.

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AN/ ' ' 7 7 7 \AN Figure 3. Normative compositions of the Keres Group volcan- 1/ V ics plotted in the feldspar and quartz-feldspar ternary systems. Symbols are identical to those in Figure 2. Curve in Figure 3a represents the two-feldspar boundary curve; in Figure 3b, curve (1) represents the quartz-feldspar and two-feldspar boundary curves at 1-kb water vapor pressure (after James and Hamilton, 1969). Curve labeled "2" in Figure 3b is after Tuttle and Bowen (1958). Sample 042 is from the Peralta Tuff Member of the Bear- head Rhyolite.

- Paliza Canyon Fm. o rhyodacite • dacite 100' A andesite T bas. andesite

United States (comparative data from Jakes and White, 1972; Ewart, 1979). With increasing silica, rocks from the Paliza 1.0 1 il i il Canyon Formation are enriched in the incom- La Ce Sm Eu Dy Yb Lu patible elements Ba, Rb, Hf, Th, and U, and Bearhead Rhyolite correspondingly depleted in the compatible ele- Figure 4. Chondrite-nor- a 147 o 007 ments Sc, V, and Sr. Samples from the Bearhead malized REE plot of Paliza • 126 »078 Rhyolite deviate sharply from the trends estab- Canyon Formation members lished by members of the Paliza Canyon Forma- (upper) and the Bearhead range of Paliza Canyon Fm. tion, especially for Ba, Hf, Th, and U, which are Rhyolite (lower). Normaliz- depleted in the rhyolite, and for V, which is ing factors from Leedey enriched in the rhyolite. Variation between chondrite (Masuda and oth- samples of the Bearhead Rhyolite is small; how- ers, 1973) divided by 1.20. ever, phenocryst-rich sample 078 is enriched in Th and U and depleted in Sc, relative to the phenocryst-poor samples. Chondrite-normalized REE patterns for the Paliza Canyon Formation have steeply negative slopes. They are light REE (LREE) enriched, Sm Eu Yb Lu relative to heavy REE (HREE), and show La Ce Dy La/Yb ratios ranging from 23 to nearly 15 (Fig. 4). Although there is some overlap in the patterns of the intermediate samples with similar 1979); however, the Bearhead Rhyolite does not lite, the modeling calculations were divided into silica contents, there is an over-all increase in show the extreme depletion in Ba that many of three steps: (1) basaltic andesite to dacite, total REE with increasing silica (Fig. 4). these high-silica rhyolite provinces exhibit. (2) dacite to rhyodacite, and (3) rhyodacite to The Bearhead Rhyolite samples are depleted Samples from the Coso rhyolites and the Bande- Bearhead Rhyolite. in La through Dy, relative to the Paliza Canyon lier Tuff (Smith and Bailey, 1966) have Ba con- Because rocks of the Paliza Canyon Forma- rocks, and have equal to enriched abundances of tents <100 ppm (compare with 400 to tion are porphyritic, compositions determined the HREE, Yb and Lu. The rhyolite patterns 600 ppm). by linear regression were modeled in order to have flatter slopes (La/Yb = 5.7) and strong overcome the effects of phenocryst enrichment MODELING negative Eu anomalies, with Eu/Eu* (observed (method used by Reid, 1979). Regression lines Eu/Eu value predicted by a smooth chondrite- In order to test quantitatively the petrogenesis were fitted through all element plots, and com- normalized REE pattern) averaging 0.36. The of the rocks of the Paliza Canyon Formation positions were calculated at silica contents cor- over-all REE pattern is typical of high-silica and the Bearhead Rhyolite, least squares mixing responding to specific samples. Results of the rhyolites (Bacon and others, 1981). The abun- calculations of Wright and Doherty (1970) for major-element modeling (using average mineral dances of REE and other trace elements are sim- the major elements and various models for the compositions) were then employed to estimate ilar to many high-silica rhyolite provinces, such trace elements have been used. Because of the trace-element concentrations, using distribution as the Coso (Bacon and others, large range in silica values and the differences in coefficients of Bornhorst (1980), Le Roex 1981) and the Long Valley rhyolites (Hildreth, phenocrystic phases in basaltic andesite to rhyo- (1980), and Arth (1976).

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TABLE 3. PETROGENETIC MODELING OF PALIZA CANYON FORMATION element composition of the rhyodacite. The frac- tionating assemblage consists of plagioclase, STEP 1: Parent = basaltic andesite to Daughter = dacite

(57.875 Si02) (63.65SSi02) augite, hypersthene, and titanomagnetile in the ratio 71:15:5:9. As in step 1, the agreement for Variable Wt% (proportion) X Deviation^ all major elements except P2O5 is within analyt- ical error. The proportions of fractionating dacite 68.30

plagioclaie (An5g) 20.71 (66.91) phases are again consistent with assemblages olivine 1.04 (3.36) found in the parent samples. The petrologic mix augite 5.60 (18.09) titanomafnelile 3.60 (11.63) calculated in step 2 shows that hypersthene be- 99.25 comes an important fractionating phase at this Daughter stage. Except for Th, the results of the trace- observed calculated element modeling (Table 3), using the same

S: 15 10 ± 0.2+ 10 proportions of fractionating phenocrysts as in V 201 100 i 6 98 the major-element mix, are excellent. Rb 39 66 ± 7 56 S- 893 669 ± 40 668 Bi 711 1185 ± 200 974 REE modeling for the Paliza Canyon Forma- Hf 3.5 7.6 ± 0.5 4.9 tion was done in stepwise fashion, using the Ti 5.0 11 ± 0.4 7.2 U 1.6 3.7 ± 0.1 2.3 same assemblages and proportions as were used in the major-element modeling. In general, the STEP 2: Parent » ilacite to Daughter = rhyodacite fit between the calculated and the observed de- (63.61 V.iiOj) (67.7IS Si0 ) 2 rivative composition (Fig. 5) is within analytical

Variable Wffi (proportion) X Deviation^ error (Table D) for all rare-earth elements ex- cept Lu. rhyodacite 76.65 22.82 0.11 The final step in the petrologic modeling, that plagioclase (An41) 16.21 (71.03) augite 3.43 (15.03) of attempting to derive the compositions of the hypersthene 1.21 (5.30) titanomagnetite 1.97 (8.63) Bearhead Rhyolite from the rhyodacite of the 99.47 Paliza Canyon Formation by crystal fractiona- Parent Daug' hier tion, was unsuccessful. This was especially true ppm observed observed calculated for the trace elements which yielded an average

Sc 10 7.2 ±0.1 7.5 deviation of >50% between calculated £.nd ob- V 100 45 i 5 46 served compositions. Fractionation of REE- Rt' 66 86 i 8 84 Sr 669 508 ± 30 516 enriched accessory minerals, such as apatite, Ba 1185 1422 s 200 1376 Hf 7.6 10 ± 0.9 9 would generate depleted-REE patterns, but Th II 19 ±0.7 14 these models require unreasonably high percen- U 3.7 5.1 ±0.2 4.7 tages of apatite and produce patterns which are •Total Fe as FeO; + : 1 standard deviation. X = % fractionation. still dissimilar to those of the rhyolites.

Sr-ISOTOPE DATA The crystallization of -31% of the basaltic- cal error, except for Hf, Th, and U (Table 3). andesite magma will yield the major-element The trace-element model can be significantly Three Bearhead Rhyolite and two Paliza composition of the dacite (Table 3). The frac- improved by assuming a total fractionation of Canyon basaltic-andesite samples were analyzed tionating assemblage consists of plagioclase, oli- 40% instead of 31 %. A difference of 9% fraction- for Rb and Sr and for initial isotope ratios vine, augite, and titanomagnetite in the respec- ation between a major- and trace-element (Table E) by John Bristow of de Beers, Kimber- tive proportions 67:3:18:12. The mineral pro- model is considered to be within reasonable ley, South Africa; and R. A. Armstrong, af Ber- portions from the step 1 mix are generally agreement by Cameron and Hanson (1982). nard Price Institute of Geophysics, University of consistent with proportions found in the basaltic The deviation in P2O5, between the calculated Witwatersrand, South Africa. Clearly, the Bear- andesite samples. One important point to ob- and observed compositions, can be narrowed by head Rhyolite (initial ratios between 0.7056 and serve is that hypersthene is not one of the frac- adding apatite to the fractionating assemblages, 0.7076) cannot be derived directly by fractional tionating phases. This suggests that hypersthene even though it is a trace constituent in the basal- crystallization from a parent Paliza Canyon ba- was a late-forming liquidus phase, a suggestion tic andesites and andesites (especially at —62% saltic andesite (initial ratios of 0.7041 and that is supported petrographically by the lack SiC>2). Th, U, and Hf generally show the largest 0.7044). The basaltic andesite has initial ratios of hypersthene in some of the basaltic andesite deviations among the trace elements. These typical of other Pliocene-Pleistocene in samples. Except for P2O5, the agreement among deviations may be due to inaccurate distri- New Mexico, but the Bearhead Rhyolite must the major elements between the calculated and bution coefficients or to post-solidification remo- come from a Rb-enriched source rock. These observed parent composition is within analytical bilization of these elements by hydrothermal results are consistent with the major- and trace- error, as illustrated by the low sum of the processes. element data and the modeling. 2 squares of the deviations (R = 0.20). The The result of step 2, in which the rhyodacite agreement between observed and calculated composition is derived from the dacite composi- DISCUSSION trace-element concentrations for the derivative tion, is equally successful (Table 3). In this cal- composition, assuming Rayleigh fractionation culation, -23% crystallization of the parent The quantitative petrogenetic modeling of and using the above mix, is also within analyti- dacite magma is required to achieve the major- major- and trace-element data is consistent with

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1000. Figure 5. Comparison of calculated and observed REE patterns from petrogenetic modeling, steps 1 and 2. For step 1, the lowest distribution coefficients within the range of andesitic values were used; for step 2, typical andesitic values.

REFERENCES CITED

Arth, J. G., 1976, Behavior of trace-elements during magmatic processes—A summary of theoretical models and their application: U.S. Geological Survey Journal of Research, v. 4, p. 41 -47. Bacon, C. R., Macdonald, R„ Smith, R. L., and Baedecker, P. A., 1981, Pleistocene high-silica rhyolites of the Coso volcanic field, Inyo County, California: Journal of Geophysical Research, v. 86, p, 10223-10241. - Parent; bas. andesite } Derivative » rhyodaclte Bailey, R. A., Smith, R. L., and Ross, C. S., 1969, Stratigraphic nomenclature of volcanic rocks in the Jemez Mountains, New Mexico: U.S. Geologi- observed• observed o cal Survey Bulletin 1274-P, 29 p. calculated • Beddoe-Stephens, B-, 1982, The petrology of the Rossland volcanic rocks, southern British Columbia: Geological Society of America Bulletin, v. 93, p. 585-594. Bornhorst, Theodore J., 1980, Major- and trace-element geochemistry and 1.0 mineralogy of upper Eocene to Quaternary volcanic rocks of the La Ca Mogollon-Datil volcanic field, southwestern New Mexico [Ph.D. dis- Sm Eu Dy Yb Lu sert.]: Albuquerque, New Mexico, University of New Mexico, 431 p. Cameron, K. L., and Hanson, G. N., 1982, Rare-earth element evidence con- cerning the origin of voluminous mid-Tertiary rhyolitic ignimbrites and a fractional crystallization process to derive the pear feasible for the Bearhead Rhyolite. Samples related volcanic rocks. Sierra Madre Occidental, Chihuahua, Mexico: range of compositions within the Paliza Canyon of underlying Precambrian from Geochimica et Cosmochimica Acta, v. 46, p. 1489-1503. Cameron, M„ Bagby, W. C.. and Cameron, K. L., 1980, Petrogenesis of volum- Formation. The modeling and the Sr-isotope drill cores obtained during operations of the Los inous mid-Tertiary ignimbrites of the Sierra Madre Occidental, Chihua- hua, Mexico: Contributions to Mineralogy and Petrology, v. 74, data also indicate that the Bearhead Rhyolite Alamos Hot Dry Rock Geothermal Program are p.271-284. cannot be derived by fractional crystallization of significantly enriched in REE, relative to the Carmichael, I.S.E., Turner, F. J., and Verhoogen, J„ 1974, Igneous petrology: New York, McGraw-Hill, 739 p. the most differentiated Paliza Canyon member. Bearhead Rhyolite (W. A. Laughlin, 1982, per- Deer, W. A., Howie, R. A., and Zussman, J„ 1978, An introduction to the Several lines of evidence besides the Sr-isotope sonal commun.). Partial fusion of a more REE- rock-forming minerals: London, Longman Group Limited, 528 p. Elston, W. E., and Bornhorst, T. J., 1979, The in context of data indicate that the Bearhead Rhyolite and depleted source is required. Lower- to middle- regional post-40 m.y. volcanic and tectonic events, in Riecker, R. E„ ed., Rio Grande rift: Tectonics and magmatism: Washington, D.C., Paliza Canyon Formation are not comagmatic. crustal silicic rocks are a possible source. The American Geophysical Union Special Publication, p. 416-438. Ewart, A., 1979, A review of the mineralogy and chemistry of Tertiary-Recent Deviations on chemical variation diagrams sup- heat necessary to partially melt these rocks could dadtic, rhyolitic, and related salic volcanic rocks, in Barker, F„ ed., port this hypothesis. Clearly, the Paliza Canyon have come from the intermediate magmas of the Trondhjemites, dacites, and related rocks: Amsterdam, The Nether- lands. Elsevier, p. 13-121. Formation lavas were crystallizing within the Paliza Canyon Formation, which equilibrated at Hildreth, W„ 1979, The Bishop Tuff: Evidence for the origin of compositional zonation in silicic magma chambers, in Chapin, C. E., and Elston, plagioclase volume in the Q-Ab-An-Or system, temperatures in the 900 to 1000 °C range, well W. E., eds„ Ash-flow tuffs: Geological Society of America Special whereas the Bearhead Rhyolite was on the two- above the granitic solidus. Paper 180. p. 43-75. Jakei, P., and White, A.J.R.. 1972, Major and trace element abundances in feldspar surface or below it. Pétrographie evi- volcanic rocks of orogenic areas: Geological Society of America Bul- dence in which sanidine phenocrysts are rimmed letin, v. 83, p. 29-40. ACKNOWLEDGMENTS James, R. S„ and Hamilton, D. L., 1969, Phase relations in the system NaAl- by plagioclase suggests that crystallization of the Si3Og - KAlSi3Og - CaAI2Si2Og - Si02. at 1 kilobar water vapour pressure: Contributions to Mineralogy and Petrology, v.21,p. 111-141. initial Bearhead Rhyolite liquid may have This work was partially supported by grants Le Roex, A. P.. 1980, Geochemistry and mineralogy of selected Atlantic Ocean started below the two-feldspar surface, within basalts [Ph.D. dissert.]: Capetown, South Africa, University of from the New Mexico Geological Society, So- Capetown. the alkali-feldspar volume. Because the two liq- Masuda, A., Nakamura, N., and Tanaka, T., 1973, Fine structures of mutually ciety of Sigma Xi, and the Geological Society of normalized rare-earth patterns of chondrites: Geochimica et Cosmo- uids initiated on opposite sides of the two- America; and by scholarships from the Depart- chimica Acta, v. 37, p. 239-248. feldspar boundary, they cannot be related Reid, D. L., 1979, Petrogenesis of calc-alkaline metalavas in the mid- ment of Geology, University of New Mexico. Proterozoic Haib volcanic subgroup, lower Orange River region: Geo- through fractional crystallization. logical Society of South Africa Transactions, v. 82, p. 109-131. Funds for the operation of the electron micro- Ross, C. S-, and Smith. R. L., 1961, Ash-flow tuffs: Their origin, geologic In recent papers, a popular theory which ap- probe were covered by NASA grants NGL 32- relations, and identification: U.S. Geological Survey Professional Paper 366,81 p. pears to account for generation of high-silica 004-63 and 32-004-064 (K. Keil, principal Smith, R. L., 1960a, Ash flows: Geological Society of America Bulletin, v. 71, p. 795-842. rhyolites involves partial fusion of crustal mate- investigator). Assistance on the major- and 1960b, Zones and zonal variations in welded ash flows: U.S. Geological rial (Elston and Bornhorst, 1979; Ewart, 1979; trace-element analyses were kindly provided by Survey Professional Paper 354-F, p. 149-159. Smith, R. L., and Bailey, R. A„ 1966, The —A study of ash-flow Bacon and others, 1981). A partial fusion origin John Husler of the Department of Geology, eruption cycles from zoned magma chambers: Bulletin Volcanologique, ser. 2, v. 29, p. 83-104. for the Bearhead Rhyolite is supported by its University of New Mexico; and by Rick Warren 1968, Resurgent cauldrons: Geological Society of America Memoir normative composition, which plots near the of Los Alamos National Laboratory. Special 116, p. 613-662. Smith, R. L„ Bailey, R. A., and Ross, C. S., 1970, Geologic map of the Jemez ternary minimum in the quartz-feldspar system. thanks are extended to Dr. John Bristow of de Mountains, New Mexico: U.S. Geological Survey Miscellaneous Inves- tigations Map 1-571. Using the two-feldspar geothermometer Beers, Kimberley, South Africa, for the many Stormer, J. C., 1975, A practical two-feldspar geothermometer: American (Stormer, 1975), estimated temperatures range discussions and for arranging the Sr-isotope Mineralogist, v. 60, p. 667-674. Tuttle, O. F., and Bowen, N. L„ 1958, Origin of granite in the light of experi- between 678 and 701 °C for the Bearhead analysis. L. A. Fernandez, J. Dostal, J. Nichols, mental studies: Geological Society of America Memoir 74, 153 p. Wright, T. L., and Doherty, P. C„ 1970. A linear programming and least Rhyolite—consistent with that of the minima in and D. Francis made many constructive criti- squares computer method of solving petrologic mixing problems: Geo- the granite system (Fig. 3). An origin by fusion cisms to improve the manuscript greatly. The logical Society of America Bulletin, v. 81, p. 1995-2008. of upper-crustal rocks exposed or found in drill encouragement given by R. L. Smith and R. A. MANUSCRIPT RECEIVED BV THE SOCIETY SEPTEMBER 6, 1983 REVISED MANUSCRIPT RECEIVED JUNE 1, 1984 cores in the Jemez area does not, however, ap- Bailey is gratefully acknowledged. MANUSCRIPT ACCEPTED JUNF. 11. 1984

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