The petrology and geochemistry of the Ocate , north-central New Mexico

ROGER L. NIELSEN Department of Geology, University of Maryland, College Park, Maryland 20742 MICHAEL A. E>UNGAN Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275

ABSTRACT INTRODUCTION

The Ocate volcanic field in north-central New Mexico is on the The Ocate volcanic field is one of several dominantly basaltic eastern flank of the Sangre de Cristo Mountains. It is a suite of Pliocene-Pleistocene eruptive complexes that defines the Jemez trend or Pliocene-Pleistocene basaltic and intermediate lavas which were lineament in northern New Mexico (Fig. 1). The Ocate field lies between erupted in five pulses over a period of ~7 m.y. (8.3-0.8 m.y. B.P.) the Taos Plateau volcanic field on the west and the Raton-Clayton field to during rejuvenation of the Rio Grande Rift. Geochemical and petro- the east. These three volcanic fields developed contemporaneously and are graphic criteria sire used to define five major rock types: alkali olivine characterized by comparable temporal evolution trends and by similar (AOB), transitional olivine basalt (TOB), xenocrystic basaltic assemblages of volcanic rock types. The Ocate lavas range from mafic (XBA), olivine (OA), and . The timing and alkali to tholeiitic , to andesites and , representing a wide eruptive volume:; of these five lava types define a complex temporal range of major- and trace-element content. sequence in which the transitional basalts and intermediate rocks are The objectives of this study were to develop a model for the pedo- closely associated during the maximum in basaltic activity (4.5-2.0 genesis of the variety of magma compositions observed in the Ocate field m.y. B.P.). and to fit them into a regional model for the origin of Pliocene-Pleistocene The alkali olivine basalts have higher incompatible and com- volcanism in northern New Mexico. Toward that end, we have employed patible trace-element abundances and higher normative nepheline petrographic data, mineral and bulk chemistry, and have performed mass- (>2%) than do the TOB lavas. The two basalt types apparently were balance (Stormer and Nicholls, 1978) and phase-equilibria calculations derived from physically discrete or heterogeneous source regions. The (Nielsen and Dungan, 1983) in order to evaluate the roles of mixing and AOB lavas could be the products of smaller degrees of melting from fractional crystallization. Utilizing data from this investigation and from similar sources, source heterogeneity, and/or melting at greater depth. investigations on the other Pliocene-Pleistocene volcanic fields in northern The intermediate lavas were generated from basaltic parent New Mexico (Bachman, 1953; Dungan and others, 1983a; Stormer, 1972; magmas by varying degrees of fractionation, assimilation of crustal Phelps and others, 1983), we have attempted to evaluate the Ocate field in material, and miring with silicic melts derived by crustal anatexis. The terms of its place in the regional geology and to develop a large-scale XBA lavas and ilacites are characterized by disequilibrium phase as- model for the evolution of volcanism in New Mexico during the past semblages which include olivine coexisting with biotite and quartz, 10 m.y. melt inclusions of K- and Si-rich glass, and a bimodal distribution of

plagioclase phenocryst compositions with modes at An^ and An25- The high concentration of Mg, Ni, and Cr in the intermediate lavas is inconsistent with simple fractionation of basaltic parent magmas. The linear trends defined on variation diagrams are consistent with mixing Utah\Colo._ mafic and silicic end members. The lack of continuity between two 'ÁrTz'jÑMe*: tpvM hybrid types may be a reflection of contrasting subvolcanic environ- V RCVF ments in which Che hybridization took place. I OVF ! / SAxial Rift Basins The olivine andesites show little petrographic evidence for Figure 1. Location mixing. Chemical variations within this group define highly nonlinear map. trends more consistent with extensive crystal fractionation. Calculated I 0 IQOkm liquid lines of descent from available mafic parent magmas (assuming Late Cenozoic Volcanic Fields perfect fractions!) crystallization) do not satisfactorily produce the ob- Northern New Mexico and served intermediate compositions. If assimilation of a silicic crystal Southern Colorado component is paired with crystal fractionation, such that the mass of Raton-Clayton 7.5 - 0.01 m.y. crustal material is approximately equal to 50% of the fractionated Ocate 8.1 -0.8 m.y. Taos Plateau 4.5-1.8 solids, the major- and trace-element characteristics of the olivine an- desites can be successfully modeled.

Additional material for this article, Tables A and B, may be secured free of charge by requesting Supplementary Data 85-12 front the GSA Documents Secretary.

Geological Society of America Bulletin, v. 96, p. 296-312, 15 figs., 5 tables, March 1985.

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GEOLOGIC SETTING TABLE 1. APPROXIMATE AGES. VOLUME, AND CHARACTER OF THE FIVE MAJOR VOLCANIC EPISODES IN THE OCATE VOLCANIC FIELD

The Ocate volcanic field lies astride the physiographic and geologic boundary between the southern Sangre de Cristo range of the Rocky Mountains and the High Plains province to the east. Volcanism in the Taos Age (m.y.) 8.1 5.5 4.8-4.0 3.2 3.0 2.2-2.0 1.4-0.8 Plateau and in the Ocate and the Raton-Clayton volcanic fields was coin- Elevation to drainage 600-200 m 300-100 m 150-50 m 50-20 m 30-10 m Volume (km3) cident with a major tectonic rejuvenation of the northern Rio Grande Rift (approx.) 3.9 31 23 28 4.0

during late Cenozoic time. Volcanism along the Jemez lineament was Appro* volume % of each accompanied by regional uplift and activation of major high-angle faults. rock type In contrast to the Taos Plateau field, which erupted within an actively AOB 40 20 10 10 20 subsiding rift basin (San Luis Valley), the Ocate and Raton-Clayton lavas TOB 40 50 80 60 50 were erupted on the uplifted eastern flank of the rift. Subsequent erosion XBA 10 20 10 25 30 has inverted the topography. Broad lava flows cap mesa surfaces at eleva- OA 10 >10 > 5 > 5 tions up to 600 m above the present-day drainages; the oldest lavas occupy Dacite > 2 _ 2 the highest local elevations. The correlation between age of eruption and terrace level has been documented by O'Neill and Mehnert (1980), who demonstrated that the levels would exist, and the observed interfingering of flows from separate Ocate volcanics were erupted in a series of five pulses during the period volcanic centers would not be present. 8.3 to 0.8 m.y. B.P., using K/Ar whole-rock dates of the capping flows. In Table 1, we have listed estimates of the total volume of volcanic We concur with the conclusions of O'Neill and Mehnert that the volcan- rocks produced during the five episodes and the proportions of the five ism occurred in five pulses, although we acknowledge that it is possible rock types present in each magmatic pulse. Greater uncertainties are at- that volcanic activity was more continuous than the existing dates indicate tached to older episodes, particularly the first phase, which may have been (Fig. 2). This is particularly true for the youngest events, for which only substantially eroded. The much lower volume of lavas that erupted during one or two dates are available. There are remarkably few terrace levels this event, compared to subsequent events, must be taken as a minimum within the area, considering that the elevations of the five surfaces decrease value. greatly (as much as 1,500 m) from the northwest to the southeast. If AGE (m.y) volcanic activity had been continuous, rather than episodic, more terrace

Figure 2. Sketch map of the Ocate volcanic field, north-cen- tral New Mexico. The approxi- mate distribution of the lavas of each eruptive episode is shown by patterns indicated in the key. Heavy lines delineate the major fault zones (after O'Neill and Mehnert, 1980). Key to loca- tions: OM - Ortega Mesa; SM - Sierra Montuoso; LCF - Lost Cabin fault; RMF - Range Margin fault of the Sangre de Cristo Mountains; CM - Cerro Montoso; CP - Cerro Pelon; CN - Cerro Negro; MC - Maxon Crater; MR - Mora River; OC - Ocate; WM - Wagon Mound; CH - Charette Mesa; LG - La Grulla Mesa; CO - Cerro del Oro.

miles

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The temporal evolution pattern which emerges from these estimates ANALYTICAL METHODS of eruptive volumes clearly indicates that the middle three phases of the Ocate field were far more voluminous than were the first or last episodes. Major-element, whole-rock compositions were obtained by XRF The TOB lavas, which are the dominant basalt type in the field as a whole, analysis at the Johnson Space Center in Houston and at the Department of are, on the average, four times more abundant than are alkali basalts. The Geology at the University of Massachusetts, Amherst. Two different sam- development of large, composite volcanic centers occurs only in the two ple preparation procedures were followed. The samples were crushed, most voluminous phases: the second and the fourth. cleaned, and ground in a tungsten-carbide shatterbox. At the Johnson

An An

An

• CORE o RIM a XENOCRYST ° BASALT

Or Ab Or

An

Figure 4. Plagioclase compositions in terms of molar anorthite, albite, and orthoclase. Closed circles represent phenocryst cores, open circles represent phenocryst rims and groundmass, triangles are xenocrysts, and squares are plagioclase compositions from basaltic xenoliths.

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Space Center, separate splits of the powder were analyzed for water con- tent, fused with lithium tetraborate flux at 1000 °C in a Pt crucible, and pressed into discs. At Amherst, the samples were ignited to drive off the water, mixed with Johnson-Mathey spectroflux 105 in a ratio of 1:5, fused at 1020 °C in a Pt crucible, and pressed into discs. Standard rocks BCR-1 and GSP-1 were prepared as standards. The trace elements Cr, Ni, Rb, Sr, Y, Zr, Nb, and Ba were deter- mined on rock powders by energy dispersive XRF analysis at the Depart- ment of Geology, University of Maryland. The concentrations of Na, Fe, and the rare-earth elements (REE) were determined by standard nonsep- arative neutron-activation analysis at the Johnson Space Center. Mineral analyses were performed in thin section, utilizing the JEOL 733 electron microprobe at the Department of Geological Sciences, Southern Metho- dist University. The: department is equipped with the KRISEL automation system, which uses the Bence and Albee (1968) method of data reduction. Analytical precision (la) is less than ~5% relative for the XRF trace- element analyses and for most REE by INAA. Major-element XRF and microprobe analysis have an analytical precision of ~1% absolute. The detection limits on XRF trace-element analyses are ~ 10 ppm for Cr, Ni, and Ba, and between 1-5 ppm for Rb-Nb.

PETROGRAPHY AND MINERAL CHEMISTRY

Lavas of the Ocate volcanic field have been subdivided into five major rock types. Eiven though this classification into alkali olivine basalt, Figure 5. Photomicro- transitional olivine basalt, xenocrystic , olivine andesite, graphs of (a) Agua Fria dacite and dacite is based solely upon chemical criteria, lavas within each group with orthopyroxene reacting to show a set of distinctive mineralogical and petrographic characteristics. amphibole, (b) Cerro Montoso dacite orthopyroxene reacting Alkali Olivine Basalt to clinopyroxene, (c) oligoclase with melt inclusions, (d) quartz The alkali olivine basalts are characterized by normally zoned olivine with melt inclusions, (e) basaltic (0.5-2.0 mm) as t!ae predominant phenocryst phase (Fig. 3). Glomero- xenolith. Field of view is —2 crysts of olivine and plagioclase are common in samples (OC-38 and OC-206), as are mxrophenocrysts of calcic plagioclase. The total volume percent of phenocrysts ranges from 5%—15%. Clinopyroxene phenocrysts

(En^Wo^Fsu to En4oWo43FS|7) occur only in OC-206 (10% Ne nor- ture typical of these lavas is distinct from that of the AOB lavas. Phenocryst mative), the most silica-undersaturated lava from the area. Plagioclase populations range from predominantly olivine to predominantly plagio-

microphenocryst compositions range from An77 to An60 at the core (Fig. clase; maximum dimensions range from 1 to 5 mm. None of the TOB 4) to An55_45 at the rims. Groundmass plagioclase compositions range samples examined contains pyroxene phenocrysts. Clinopyroxene is pres- from An50 to An15l clustering near An45. ent only as a groundmass phase and ranges in composition (Fig. 3) from Spinels are present in AOB lavas in two forms: as small, euhedral En4gWo41Fsii to En4oWo37Fs23. TOB groundmass clinopyroxene is inclusions in the cores of olivine phenocrysts and as anhedral grains in the generally more Fe-rich than is groundmass clinopyroxene in AOB lavas. groundmass. The small, euhedral spinels are high-Al and high-Cr spinels Spinel in TOB lavas is compositionally bimodal. Euhedral spinels in 1 (Table A), ranging in Cr203 content from 13 to 18 wt %. The groundmass the cores of olivine phenocrysts are Cr-, Al-, and Mg-rich (Table A). spinels are Cr-poor titanomagnetite (Table A). Cr-spinel is absent from Anhedral groundmass spinel is titanomagnetite. No low-Ti magnetite or plagioclase or clinopyroxene phenocrysts and, in most cases, from the ilmenite were found in any mafic Ocate lava. A comparison with the AOB outer third of olivine phenocrysts. Cr-spinels shows that the Cr-spinel in the TOB lavas is significantly lower Xenocrysts of orthopyroxene and reversely zoned oligoclase and an- in magnetite and ulvospinel components. desine are present in small amounts (5%) in many AOB lavas. No individ- The rare plagioclase xenocrysts in TOB lavas exhibit reverse zoning

ual xenocrysts of quartz were observed in AOB lavas. Xenoliths of norite from An25 to An35 in the cores to An6o-5 in calcic overgrowths. The cores (0.1 to 3 cm in diameter), composed of andesine, orthopyroxene, and of these crystals are extensively resorbed and embayed. The embayments titanomagnetite, are present in some AOB lavas. are generally filled with groundmass material. Quartz xenocrysts are rounded and surrounded by pyroxene reaction rims. The pyroxene in the Transitional Olivine Basalt reaction rim is within the range of composition observed in the groundmass.

The TOB lavas are characterized by olivine (Fog2_72) and plagioclase (An67_62) phenocrysts in a groundmass of plagioclase, augite, titanomag- Xenocrystic Basaltic Andesite netite, and occasional olivine. The ophitic to diktytaxitic groundmass tex-

Olivine phenocryst cores in XBA lavas range from Fo83 to Fo72 (Fig. 'Tables A and II may be secured by requesting Supplementary Data 85-12 3) and are normally zoned to Fo55.45 at the rims. Unlike olivine pheno- from the GSA Documents Secretary. crysts in the TOB and AOB lavas, the olivine phenocrysts are usually

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9 The groundmass textures range from glassy to subophitic, the glassy groundmass textures being characteristic of the more silicic members of the 8 XBA lavas. The mineralogy of the groundmass varies greatly between 7 Nap samples but is commonly characterized by abundant brown glass, plagio- clase laths, clinopyroxene, and titanomagnetite, as well as minor orthopy- K¿0 6 roxene and alkali feldspar. The orthopyroxene and alkali feldspar are 5 found only in the more silicic XBA lavas, generally in those samples in 4 which the xenocrysts appear to be the most re-equilibrated. 3 Within the olivine phenocrysts are euhedral spinels similar in size and composition to those found in phenocrysts in TOB and AOB samples 5 (Table A). In fact, the most Cr-, Al-, and Mg-rich spinel analyzed in rocks 4 from the Ocate field was in an olivine phenocryst in an XBA sample (OC-57, Table A). 3 2 Olivine Andesites • -ir 1 The olivine andesites are compositionally and texturally variable. 0 Small phenocrysts (~ 1 mm) and microphenocrysts of olivine and plagio- • Aoe clase are set in very fine-grained groundmasses. Phenocryst abundances A TOB rarely exceed 5%. The olivine and plagioclase phenocrysts in the OA have • XBA core compositions in the range Fo 5_ o for olivine and An . Q for plagio- 10 ir OA 7 6 57 5 O Dacite clase (Figs. 3 and 4). The olivine phenocrysts are normally zoned to FeA « Rhyolite glass F055.50 at the rims and are usually resorbed in the more silicic OA. Some FeO inclusion plagioclase phenocrysts appear to be extensively resorbed and embayed. Core compositions of these crystals are not, however, significantly different from those of other phenocrysts. Occasionally, the cores of olivine or plagioclase phenocrysts are anomalously calcic or forsteritic compared to 0 the other phenocryst cores. This may indicate a more complex origin for 10 these phenocrysts, possibly related to an early mixing event. 8 The groundmass textures become progressively more fine-grained with increasing Si, and groundmass alkali feldspar is present in the more cP MgO 6 • DO O silicic members. Some of the more silicic OA contains microphenocrysts 4 TS** of augite and orthopyroxene. Two pyroxenes are generally present in the O groundmass. The absence of phenocrystic pyroxene in the more mafic 2 compositions suggests that augite and orthopyroxene followed olivine and • " IB plagioclase in the crystallization sequence. 45 50 55 60 65 70 75 80 Rare xenocrysts of oligoclase and quartz are present in a few OA. SiOz

Figure 6. Silica variation diagram for samples from the Ocate volcanic field.

resorbed, leaving a narrow rim of Fe-rich olivine at the edge. No ground- • AOB mass olivine was observed in the XBA lavas. Plagioclase core composi- ¿ TOB • XBA tions fall into two categories. Anhedral calcic plagioclase crystals, with «OA o DACITE core compositions between An60.65, exhibit normal zoning to An55^5 (Fig. 4). In some samples, the calcic plagioclase phenocrysts are mantled

with a narrow rim of sodic plagioclase (An40). Resorbed and embayed plagioclase crystals have core compositions in the range An^s- These oligoclase-cored plagioclase crystals are commonly characterized by

overgrowths of An60_55 plagioclase intergrown with inclusions of glass with high Si and K. The inclusions in the overgrowths are too small, however, for quantitative analysis which uses the electron microprobe.

Surrounding the overgrowths is a rim of An55.6o plagioclase. The embay- ments within the oligoclase crystals are most commonly filled with groundmass material. In some cases, however, the embayments are filled with clear to light brown glass. The composition of this glass is almost uniform from one embayment to another and is rhyolitic in nature (Table A). The other most common xenocrystic mineral is quartz. These rounded and embayed quartz xenocrysts are -1/5 as abundant as the Figure 7. AFM projection for Ocate whole-rock analyses. A =

oligoclase crystals and contain glass similar in composition to that included Na20 + K20; F = Fe203 + FeO; M = MgO, all in wt %. Line represents in the oligoclase xenocrysts (Table A). the tholeiite-calc-alkaline boundary from Irvine and Baragar (1971).

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• AOB a TOB Figure 8. Or-Ab-An diagram for • XBA OVF lavas. An = normative anorthite; Sub-alkaline •ir O A Ab = normative albite + 5/3 normative o Dacite Alkaline nepheline; Or = normative orthoclase. Potassic versus sodic series designation is from Irvine and Baragar (1971).

Or Ab

Figure 9. Chondrite normalized REE patterns. Data represent IN'AA analyses performed at the Johnson Space Center in Houston, Texas. Numbers on Agua Fría Peak FLEE patterns represent the silica content of those samples in wt %.

TABLE 2. CLASSIFICATION OF LAVAS FROM THE OCATE VOLCANIC FIELD The plagioclase xenocrysts are extensively resorbed and embayed, and the AOB TOB XBA OA Dacite quartz xenocrysts are mantled by reaction rims of pyroxene.

Si02(%) <51 <51 51-57 50-57 >60 Dacites Norm. >5% Ne <3% Ne Hy Hy Qtz The dacite lavas that erupted from all four major volcanic centers are Mg« >60 53-60 55-63 45-52 40-50 similar in major-element geochemistry. The Agua Fria Peak dacites K;0(%) >1.2 <1.2 1.4-2.4 1.5-2.6 3.5-4.7 (northwest corner of Ocate field) are, however, distinct from the Cerro (La/Sm) >5 <5 5.4-6.0 5.0-7.0 Montoso, Cerro Pelon, and Cerro Negro dacites in mineralogy and in Cr ppm >150 >150 >100 <100 0-70 trace-element chemistry. The Agua Fria dacites contain amphibole and Ni ppm >120 >70 >90 <50 10-60 plagioclase phenocrysts. The amphibole occurs as rims around resorbed Sr ppm >1000 <1000 500-700 700-1800 350-500 and embayed orthopyroxene crystals (Fig. 5). These amphiboles were Rb ppm 39-55 17-42 35-65 55-78 65-120 apparently formed by a reaction between the orthopyroxene and the Y/Zr <0.11 >0.11 0.12-0.14 0.05 0.12 0.08-0.16 magma. The plagioclase phenocrysts in the dacites from Agua Fria Peak La- >100 <80 90-120 120-250 150-180 range in composition from An60 to An25 (Fig. 3). Amphibole phenocryst compositions are given in Table A. The orthopyroxene in the cores of the "Chondrite normalized La. amphibole phenocrysts is En79Wo4Fsi7, similar to the orthopyroxene

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found in the norite xenoliths in AOB lavas. Biotite phenocrysts exhibit grained and commonly range from 1 mm to 3 cm in diameter. Plagio- slight normal zoning and a reaction rim of magnetite (Table A). clase and pyroxene core compositions are in the range An55.5o and As in the case of the XBA lavas, the plagioclase phenocrysts in the Ei168-70Wo3-4Fs27-29 (Table A; Figs. 3 and 4). Grain-boundary melting Cerro Montoso dacites are chemically and morphologically bimodal. The has occurred between pyroxene and plagioclase. The glass between the feldspars with resorbed, embayed texture are generally oligoclase and are crystals includes small (<5 micron) skeletal olivine. Both the glass and the

reversely zoned to An45.5o at the rim. Phenocrysts with anhedral mor- olivine are too small for analysis. The pyroxene and plagioclase crystals phologies are characterized by calcic cores, normal zoning, and possess have reaction rims where the minerals are in contact with the grain- thin overgrowths of more sodic plagioclase. Embayments in plagioclase boundary melt. The reaction rims are An65_55 and En44.46W043Fs10.14, are commonly filled with rhyolite glass that is similar in composition to the within the compositional range of the AOB groundmass. glass in the XBA plagioclase inclusions (Table A). Quartz crystals in both The coarse crystal size and the small size of the xenoliths prevented dacite types are extensively resorbed. The abundance of resorbed oligo- the determination of modal percentages or bulk chemistry. Given, how- clase is approximately ten times that of quartz and biotite. The embay- ever, the magnesian character of the pyroxene and the intermediate plagio- ments in quartz are usually filled with rhyolite glass. The glass from clase composition, an origin for these xenoliths as high-pressure cognates inclusions in dacites from Agua Fria Peak is significantly different from of the host magma cannot be ruled out. The preferred origin for the norites that found in inclusions in the Cerro Montoso dacites. is that of accidental inclusions of lower crustal material. This is supported Resorbed crystals of olivine and clinopyroxene are present only in the by the occurrence of many similar xenoliths in contemporaneous alkaline dacites from Cerro Montoso, Cerro Pelon, and Cerro Negro. The cores of rocks in the central and southern Rio Grande Rift (Riecker, 1979). the olivine range from Fo to Fo and are normally zoned to Fo . o at 82 75 45 5 Granite the rims. Clinopyroxene overgrowths on orthopyroxene (Fig. 5) are signif- icantly less calcic than are clinopyroxene overgrowths not associated with In spite of the widespread presence of oligoclase xenocrysts and the orthopyroxene (Fig. 3). common occurrence of quartz and biotite xenocrysts, only two xenoliths of Virtually all the large crystals in the dacites show evidence that they granitic rock were found in Ocate lavas. The granitic material consists of were out of equilibrium with the liquid and with one another at the time of two 1- to 2-cm crystals of oligoclase and K-feldspar with inclusions of eruption. We suggest that this disequilibrium assemblage arose by the quartz and biotite. The xenolith found in dacite OC-166 is a rounded mixing of mafic and silicic magmas shortly before eruption to produce crystal of K-feldspar with a lath of oligoclase crosscutting one edge. Inclu- complex hybrid dacites (Eichelberger, 1978; Sakuyama, 1981). This hy- sions of quartz and biotite are present only in the K-feldspar. No grain- pothesis is supported by the presence of fine-grained inclusions of basalt boundary melting was observed in the xenolith. The core of the K-feldspar (Fig. 5) within the dacites. These range in size from small aggregates of has the composition An2Ab2oOr7g and is zoned to An45Ab450rio at the calcic plagioclase and olivine to large pillow-like features (>50 cm) with rim. The core composition of the oligoclase crystal is An25Ab6oOri5, fine-grained textures that are indicative of a rapid cooling rate. These zoned to An47Ab45Or7 at the rim. Both rim compositions are close to the inclusions will be described in detail in a later section. groundmass plagioclase composition. The feldspar crystals are not zoned along the internal boundaries. The biotite inclusions in the K-feldspar are XENOLITHS similar to the biotite in the dacite liquid (Table A). No melt inclusions were observed in the xenolith. Norite The xenolith in the AOB lava consists of two oligoclase crystals with inclusions of quartz and biotite. Both crystals have reacted extensively

Xenoliths of norite composed of orthopyroxene, andesine, and along crystal faces in contact with the liquid. The crystal cores are An35.37 magnetite are found only in AOB lavas. They are coarse (0.2-4 mm) and are zoned to An6o^5 at the rim. The rarity of mineral pairs and xenoliths of silicic material may be attributed to the generation of melt along feldspar-quartz grain boundaries, leading to the rapid disaggregation of any granitic xenoliths. It has there- fore been concluded that the oligoclase, quartz, and biotite xenocrysts and xenoliths are the products of the partial fusion of granitic basement rock, initiated by the injection of basaltic magma into the crust.

KjO

Figure 11. Rb- Sr variation dia- gram for Ocate lavas. Symbols as 200 250 in Figure 6.

Figure 10. KzO-La variation diagram for OVF lavas. 60 . 80 100 120 Symbols are the same as for Figure 6. BHD

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Basalt 300 AOB ^ Xenoliths of basalt are a common constituent in all Ocate dacites, Q TOB with the exception of those erupted from Agua Fria Peak. They range in A size from 1-mm-wide mineral pairs to blocks over 50 cm in diameter. The ¿¿A 200 D - majority of the xenoliths are spherical in shape and are more fine grained • A • XBA than are their counterparts in basaltic flows. Bulk chemical and mineral ** A analyses indicate that these xenoliths are TOB lavas (Figs. 3 and 4). They Cr • A D • consist of olivine (Fog2.74) and plagioclase (An64.60) phenocrysts and a • • fine groundmass of plagioclase, pyroxene, titanomagnetite, and glass. Cr- 100 — spinel inclusions in olivine phenocrysts fall in the same compositional if DACITE •A- c range as those from TOB lavas. The groundmass of the dacite lavas * *0A contains numerous; individual minerals and mineral pairs of basaltic *** material. i 1 1 1 1 1 1 , , 0 One possible origin for these xenoliths is that of accidental inclusions of previously erupted basaltic lavas. The abundance, morphology, and • texture of the xenoliths suggest, however, that they have originated as 200 • AOB ~ quenched pillows of basalt produced by injection of a basaltic magma into AOB a relatively cooler silicic magma chamber (Eichelberger, 1975, 1978). A TOB • • XBA A Ä OA MELT INCLUSIONS • 150 O DACITIE- D Recent investigations of silicate melt inclusions (Anderson, 1976; * • Dungan and Rhodiss, 1978; Hibbard, 1981) in phenocryst phases have Ni • • demonstrated that they can place critical constraints on petrogenetic mod- A XBA

els. This is particularly true of magmas that have undergone mixing. The 100 - Ik A TOB D resorbed nature of the oligoclase and quartz xenocrysts in many Ocate O lavas provides abundant opportunities for melt entrapment. The embay- A ments range in size from 5 to 250 ¿i in diameter and most are open to the • host liquid. The glass inclusions of particular interest to this investigation DACITE O are clear to light brown glasses found in embayments in oligoclase-cored 50 * plagioclase and quartz in XBA and dacite lavas (Fig. 5). These silicic * * OA * glasses are homogeneous and include small laths of alkali feldspar. The oo° percentage of the crystals in the glass inclusions ranges from 0 to 10 -tr-Mrfr O volume percent. 1 . 1 1 1 1 1 1 1 The composition of the glass is close to the low pressure minimum in 48 50 52 54 56 58 60 62 64 66 68 the system QTZ-AEl-OR (Tuttle and Bowen, 1958) and is characterized by a narrow compositional range. The primary exception is the composi- S i02 tion of melt inclusions in the Agua Fria Peak dacites (OC-59, Table A) Figure 12. Cr and Ni-silica variation diagram for Ocate lavas. which are significantly lower in Mg, Fe, and Ti, as compared to inclusions from other dacites. The high silica intent of the rhyolite glasses precludes their origin as partial melts of plagioclase, because their silica content (74-78 wt %) is higher than that in any feldspar. Alternatively, these glasses may be resid- BULK CHEMISTRY ual liquids, trapped as the plagioclase crystals grew. The inclusions are present, however, only in phases that are apparently out of equilibrium Representative major- and trace-element bulk analyses for each of the with the melt. An alternative process for the formation of the silicic glasses compositional groups are given, along with normative percentages, in has been suggested by Watson (1982), who reported that the liquids Table B.2 The major-element chemical trends of the Ocate mafic lavas are produced along the interface between basaltic liquid and quartz crystals continuous; there are no substantial gaps in composition separating the are enriched in K. relative to the basalt. In the Ocate suite, however, the different mafic rock types (Fig. 6; Table B). The calculated norms for the liquids produced along basalt-molten feldspar interfaces are not K en- mafic rocks range from ~ 10% nepheline normative to >2.5 quartz norma- riched. Diffusive processes cannot, therefore, explain the observed pres- tive. When the classification scheme of Irvine and Baragar (1971) is ap- ence of rhyolitic inclusions in oligoclase. The hypothesis preferred by the piied, the hypersthene normative compositions fall into the calc-alkaline authors is that the inclusions are samples of the rhyolitic melt generated by field on an AFM plot (Fig. 7) and into the average rock composition of an the partial fusion of the country rock and trapped in the embayments prior Or-Ab-An plot (Fig. 8). The nepheline normative compositions straddle to mixing. The difference in the composition of the glasses in the Agua Fria dacites can be attributed to the partial melting of a parent lower in mafic minerals. 2See footnote 1.

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i i i are consistently higher in Ni and Cr compared to OA lavas. The dacites 30 have a range of Cr and Ni content similar to that of the OA lavas. The distribution of the elements Y, Zr, and Nb exhibits significantly more overlap than does the distribution of the major elements or REE (Fig. 13). Correlation with respect to rock type can, however, still be made. The basaltic lavas may be divided into the AOB and TOB groups (based upon trace-element characteristics and normative mineralogy) (Table 4). The more silicic mafic lavas also fall into two major groups. Y 20 Lavas with >60 wt % Si02 are dacites. The rocks with silica contents between 51 and 60 wt % Si02 would not be easily recognizable as distinct subgroups on the basis of the Irvine and Baragar (1971) classification. The significantly different Mg contents, Ni and Cr content, REE patterns, Rb and Sr content, and petrographic character of these lavas suggest, however, that such a division is valid. The compatible-element contents of the i i I intermediate-composition lavas provide the most diagnostic criteria for the 100 200 300 Zr classification. The OA lavas are those samples with significantly lower Mg, I I Ni, and Cr. The XBA lavas have higher values for those elements (Tables T0B B and 2). The REE patterns of all XBA lavas show low LREE enrichment 0.20 (La/Sm = 5.4-6.0). The OA lavas have a comparable but wider range of ( V LREE enrichment (La/Sm = 5.0-7.5). \ A \ \ A The distribution of La-K and Rb-Sr concentrations for the OA falls into two groups (Figs. 10 and 11). One group is characterized by relatively Y/Zr \ Ay-^pf XBA^J>OPj? higher values of La, K, and Sr. The other group has lower La, K, and Sr and plots near the compositions of the XBA lavas. From field relations, it 0.10 can be inferred that the OA possessing the higher La, K, and Sr contents ® AOB V(••• • are associated with AOB lavas, and the lower La, K, and Sr OA are associated with TOB lavas. Even though this latter group of OA has some

• ' ^OA L trace-element similarity to the XBA lavas, they are classified as OA on the basis of lower compatible-element content. Nb The major-element chemistry of the dacites is coherent and readily discernable from the other Ocate groups. An internal division of the da- Figure 13. (a) Y-Zr variation diagram for OVF lavas. Key to cites may be made on the basis of the REE patterns in addition to Y, Zr, dacites: C = Cerro Montoso; P = Cerro Pelón; N = Cerro Negro; A = and Nb content. The dacites from Agua Fria Peak have lower heavy rare Agua Fría Peak, (b) Y/Zr-Nb variation diagram. Symbols same as in earth element (HREE) patterns. These dacites, designated by an A inside Figure 6. the hexagons in Figure 13, also have trace-element compositions different from those of dacites sampled from elsewhere in the Ocate field.

the sodic and potassic series line on the Or-Ab-An plot. The Ocate dacites PETROGENESIS OF THE BASALTIC MAGMAS plot near the average dacite-rhyodacite boundary on the Or-Ab-An plot and are in the calc-alkaline field of the AFM diagram. The AOB and TOB lavas are the most primitive in the Ocate field. In the interest of creating a more detailed classification for the Ocate They have the lowest Si, highest Mg#s, and highest compatible-element lavas than that derived from only normative composition, a classification contents of any rock type present. The TOB and AOB overlap in composi- scheme (Table 2) was developed, based on both the major-element charac- tion for many elements. In particular, the TOB with normative nepheline teristics and the minor- and trace-element concentrations. Two types of is similar in many respects to the least-undersaturated AOB lavas. patterns may be distinguished in the most mafic lava types on the basis of Experimental investigations of the fusion of peridotite (Mysen and light-REE (LREE) enrichment (Fig. 9). All basaltic samples characterized Kushiro, 1977; Presnall and others, 1978, 1979; Olafsson, 1980; Jaques by a LREE-enriched pattern (La/Sm >5.0) have nepheline normative and Green, 1980) indicated that alkali olivine, transitional, and tholeiitic major-element compositions. The basaltic lavas with lower LREE patterns basalts can all be produced by the melting of a peridotite under different (La/Sm <5.0) contain both nepheline- and hypersthene-normative lavas. conditions. These investigators have demonstrated that the partial melt of a Figure 10 is a plot of whole-rock K versus chondrite-normalized La (LaN) peridotite becomes increasingly alkaline with decreasing degree of partial for Ocate lavas. The K content is representative of the major-element, melting or with increasing pressure. large-ion lithophile elements (LIL), and La is a representative of the It is a characteristic of distillation processes that incompatible- LREE. The lavas designated as AOB and TOB fall into two coherent element concentrations in the liquid are very strongly affected by variation groups. This is also evident in Figure 11: the AOB possesses a significantly in the degree of partial melting at low percentages of partial melting. The higher concentration of Rb and Sr than does the TOB. The AOB lavas are same is true for compatible-element concentrations for low percentages of also characterized by a higher Ni but similar Cr content compared to TOB fractional crystallization. Given this and the major- and trace-element data lavas. Both AOB and TOB lavas have a wide range of Cr and Ni content from the Ocate field, the AOB lavas are probably products of a smaller over a small range of silica content (Figs. 12A and 12B). The XBA lavas degree of partial melt of a peridotite compared to that of the TOB lavas.

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As the degree of partial melting increases, the parent becomes depleted in nants has also contributed to the observed variety within the TOB and incompatible elements, and their concentration in subsequent partial melts AOB lavas. drops. For example, the AOB lavas have high concentrations of the in- compatible elements Zr and Nb and have a low Y/Zr ratio (Fig. 13). Y is ORIGIN OF THE INTERMEDIATE HYBRID MAGMAS a more compatible element than is Zr or Nb, partitioning into pyroxene or garnet (Pearce and Norry, 1979). With increased partial melting, the Nb Published petrogenetic models for the origin of intermediate- and Zr concentrations should drop and the Y/Zr ratio should increase. composition magmas fall into four categories: (1) the fractional crystalliza- This is consistent with the observed trends. The highly compatible ele- tion of a basaltic parent magma; (2) partial melting of a hydrous mantle or ments are not greatly affected by variation in the degree of partial melting. subducted oceanic crust; (3) assimilation of a crustal contaminant by an Variation of the degree of partial melting at low percentages melting will ascending, initially basaltic magma; and (4) mixing of basaltic magma and therefore create a group of melts with a wide range of incompatible- melted silicic crust. element contents and a narrow range of compatible-element concentra- Fractional crystallization of basaltic magma has been modeled by the tions. The observed trends indicate a wide range of Ni and Cr content in removal of a variety of phases under a wide range of physical conditions. the basalts from the Ocate field, however. This can be attributed to a small The two currently favored models are the fractional crystallization of degree of fractional crystallization of olivine and Cr-spinel, both of which amphibole at elevated pressure, temperature, and PH2O (Boettcher, 1973; are phenocryst phases. This small degree of fractionation would not signif- Cawthorn and O'Hara, 1976; Allen and Boettcher, 1978), and the frac- icantly increase the incompatible-element concentration in the evolved tionation of magnetite at elevated oxygen fugacity (Gill, 1981). For many lavas; therefore, none can be considered equivalent to the parent magma of basaltic compositions, however, perfect fractional crystallization of basalt either of these groups. It must also be kept in mind that the 8-m.y. time has been demonstrated to produce ferrobasalts, not andesites, at low pres- span of Ocate volcanism makes the choice of any single composition as a sure (Roeder, 1974, 1975; Grove and others, 1982). parental magma highly tenuous. Kushiro (1972) suggested that partial melting of hydrous peridotite The strong correlation between Mg and Si within the AOB type would produce andesitic magmas by the expansion of olivine stability. cannot be explained by fractional crystallization. If the most Mg-rich lava Subsequent experimental work has contradicted that hypothesis (Ring- is taken as the parent for the apparently more-evolved samples, the Si wood, 1974; Mysen and others, 1974; Green, 1976). They found that the increase is not nearly sufficient in the calculated derivatives to match the liquids produced were silica enriched but otherwise dissimilar to andesites. observed trend. Minor crustal contamination of AOB magmas as they The partial melting of eclogite to produce andesitic magmas was found to fractionated within the crust could produce these changes in Si (that is, be inconsistent with the expected trace-element abundances in andesites

lower Ni, higher K.) without obscuring other chemical differences. (Gill, 1974,1981). In addition, for realistic amounts of water (<2% H20), In conclusion, the compositional difference between the TOB and the compositions of the melts are not greatly different from those produced AOB lavas can te attributed to the production of the TOB parental under anhydrous conditions (Presnall and others, 1979). magmas by a larger degree of partial melting in the mantle. We also point Models involving assimilation of crustal material in a basaltic host, or out that a variation in the depth of melting could provide additional mixing of basalt with rhyolite generated by crustal fusion, have been based complexity to the picture. Overprinted on the geochemical patterns pro- upon pétrographie observation or the association of basalt and rhyolite in duced by partial melting are the effects of subsequent fractionation. The the field (Kuno, 1950; Bryan, 1968; Anderson, 1976; Eichelberger, 1975, result is an apparently random distribution of compatible and incompati- 1978, 1981; Sakuyama, 1981). Bryan (1968) successfully modeled the ble trace-element concentrations. In addition, contamination of the basalts petrogenesis of the Paricutin volcanics by assuming fractional crystalliza- during ascent through the crust by a potentially wide variety of contami- tion of olivine and plagioclase paired with assimilation of crustal material.

TABLE 3. RESULTS OF MIXING CALCULATIONS ON XBA AND DACITES USING PROGRAM XLFRAC2

Samp's ""• Wl % in Wl iS in silicic Sid. basalt and -dac mixed product component error

basalt silicic olig. rhyolite

OC- 18 OC- 55 19 81 34 66 0.66 2.22 OC- 21 OC- 85 20 80 34 66 0.59 1.72 OC-191 OC- 79 23 77 38 62 0.47 1.09 OC- 18 OC- 59 24 76 36 64 0.38 .71 OC- 21 OC- 83 28 72 36 64 0.53 1.38 OC-196 OC-166 31 69 32 68 0.213 0.23 OC- 21 OC-114 37 63 27 73 0.34 0.58

OC-191 OC- 64 70 30 27 73 0.114 0.065 OC- 21 OC- 78 74 26 16 84 0.44 0.96 OC- 21 OC- 75 79 21 22 78 0.12 0.07 OC-191 OC- 97 82 18 12 88 0.29 0.43 OC- 70 OC- 57 85 15 2 98 0.19 0.18 OC-196 OC-115 89 11 14 86 0.21 0.23 OC-191 OC- 56 90 10 3 97 0.27 0.36 OC- 21 OC-131(OA) 62 38 48 52 0.94 3.50

Note: data are from Stormer and Nicholls (1978). Mixing components are basalt (sample no. given in the first column) with a silicic component composed of oligoclase and rhyolite glass (Table 2).

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In an investigation of the Medicine Lake Highlands volcanics, Grove and of granite has been reported by Dodge and Calk (1978), Kovach and others (1982) developed a model involving fractional crystallization of Marsh (1981), A1 Rawi and Carmichael (1967), and others. Many of these parental basalt followed by the mixing of the fractionated magma with a investigations reported the presence of biotite, embayed quartz, and K- silicic magma derived from the partial melting of the crust. The theoretical feldspar, in addition to plagioclase and glass in naturally occurring melted basis and the consequences of the paired fractional-crystallization-assimi- granitic material. The proportion of each phase is dependent upon the bulk lation process have been discussed by Allegre and Minster (1978) and composition of the system. In the Ocate xenocrystic lavas, plagioclase is elaborated on by DePaolo (1981). The following section examines the the dominant mineral phase in the hypothesized contaminant. This would petrogenesis of the Ocate olivine andesites, dacites, and xenocrystic basal- suggest that the crustal parent was intermediate rather than granitic. This is tic andesites in light of these concepts. supported by the significant Fe and Ca contents in the rhyolite glass inclusions (Table A). Xenocrystic Basaltic Andesite Dacite

The xenocrystic character of the basaltic andesites is consistent with The similarities of the major-element composition of the two dacite an origin due to mixing of basaltic and silicic end members. In addition, rock types suggest that the same process is responsible for the origin of the existence of the disequilibrium assemblage precludes the production of both the Agua Fria and Cerro Montoso dacites. Both dacite groups are the XBA lavas by a simple fractional-crystallization process. The Capulin characterized by the presence of unassimilated and partially assimilated lavas of the Raton-Clayton field are similar in many petrographic and basaltic material, partially melted oligoclase, orthoclase, biotite, quartz, geochemical characteristics to the Ocate XBA lavas. Stormer (1972) and inclusions of rhyolite. This disequilibrium assemblage has led to the argued against mixing as the process responsible for the Capulin lavas of conclusion that the Ocate dacites are the product of the mixing of mafic the Raton-Clayton field because he did not find evidence of a contaminant magma and silicic material made up of crystals and melt to create an enriched in Si and K. He could not, therefore, incorporate it into mixing intermediate hybrid. calculations. Instead, Stormer (1972) proposed that the quartz xenocrysts Fractional crystallization of andesites was not considered to be a valid were high-pressure cognate phenocrysts and that the magnesian olivine, mechanism for the production of the Ocate dacites. In addition to the also present in the Capulin lavas, was a product of low-pressure crystalliza- disequilibrium apparent in the mineralogy, the incompatible trace ele- tion. The results of subsequent experimental studies on basaltic systems at ments are higher in many of the andesites than in the dacites, and the high pressures (Helz, 1976; Bender and others, 1978) have demonstrated compatible elements Cr and Ni are higher in the dacites than in the OA. that quartz is not an equilibrium-near-liquidus phase in basalts at high The relative proportions of the mixed components in the dacites was pressure. Additional evidence cited against crustal contamination was the calculated using the program XLFRAC2 (Stormer and Nicholls, 1978). low 87Sr/86Sr ratio reported by Leeman (1970) for a Capulin flow The mixed components include transitional olivine basalt, rhyolite glass, (0.7048). There has been no independent determination of the isotopic and oligoclase. For each dacite modeled, the basaltic mixing component character of the possible contaminants. If, however, lower crustal rocks was selected from the compositions of basalts associated with that dacite in with low Rb/Sr were involved, the 87Sr/86Sr ratio of the assimilated the field. The results, listed in Table 3, indicate that a composition corre- material would not casue a major increase in the Sr-isotopic composition sponding to the Ocate dacites may be produced by a mixture of between of the mixed lavas. 18%-30% basaltic material and between 70%-82% silicic material. The In order to evaluate the viability of mixing as a mechanism for the average proportion of oligoclase to rhyolite in the silicic component was production of the XBA lavas, mass-balance calculations were performed 35% oligoclase and 65% rhyolite glass. using the Stormer and Nicholls (1978) XLFRAC2 program (Table 3). The proportion of oligoclase to rhyolite glass required in the silicic Each XBA lava was modeled using a basaltic mixing end member chosen component by mass-balance constraints is much less for the XBA lavas from basaltic-lava compositions associated with that XBA in the field. than for the dacites (Table 3). Also illustrated is the progressive drop in the Where possible, the silicic components were chosen from microprobe proportion of oligoclase to rhyolite and an increase in the fraction of basalt analyses of phases in the individual lavas. In cases where no inclusions of in the mixture. silicic melt were found in a rock, an average glass composition was used. In other localities where the site of mixing has been exposed, field The silicic components tested were oligoclase, rhyolite glass, quartz, and observations of the interaction between mafic intrusion and granitic host biotite. The quartz and biotite proved to be of negligible importance, rock suggest that the mechanism involved in the formation of some ande- consistent with their relative rarity as xenocrysts in XBA lavas. In all cases sitic mixed rocks may be different than that for the formation of dacitic tested, the sum of biotite and quartz in the calculated contaminant was mixed rocks (Dodge and Calk, 1978; Reid and Mayne, 1981; Reid, 1982). <5%. In subsequent calculations, quartz and biotite were dropped in order In some of the dacite mixed rocks in the Sierra Nevada, mixing has been to reduce the number of mixed components. The calculations indicate that inferred to occur as basalt is injected into dikes or small magma chambers the Ocate XBA lavas can be produced by the addition of a 10%-30% of silicic melt (Reid and Mayne, 1981; Eichelberger, 1975, 1978, 1981). silicic contaminant to a basalt. The composition of the silicic component The difference in liquidus temperature of the two magmas causes the ranges from 3% to 27% oligoclase and from 97% to 73% rhyolite glass. basaltic magma to quench. Reid and Mayne (1981) suggested that pro- The composition of this contaminant is consistent with the expected gressive re-equilibration and disaggregation of the basalt, coupled with material produced by the fusion of granitic or granodioritic crustal mate- mechanical mixing, lead eventually to homogeneous dacite or granodiorite rial initiated by the thermal flux related to the emplacement of a mafic for a suite of mixed rocks in Yosemite National Park in the Sierra Nevada. body into the crust (A1 Rawi and Carmichael, 1967; Dodge and Calk, The associated mafic rocks are generally not xenocrystic but range widely 1978; Kovach and Marsh, 1981; Hildreth, 1981). The combination of in color and composition, possibly representing contamination with silicic sodic plagioclase and "rhyolitic" glass as the primary products of the fusion material. The contamination of the mafic magma is localized along the

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perimeter of the intrusion and near disaggregating granitic xenoliths. The Table 3. The results of these calculations for an average dacite are given in silicic contaminant, having a lower liquidus temperature than the basalt, is Table 4. The calculated silicic contaminant in the Agua Fria dacites has not quenched. Watson (1982) has demonstrated that diffusion is not rapid significantly lower HREE than do the calculated components suspected of enough to homogenize a mixed mafic-silicic magma in the time available. being involved in the genesis of the other dacites. The progressive character The higher viscosity of the silicic melt may prevent the two melts from of the drop in HREE with increased silica content (Fig. 9) is further mixing. If, however, there is mechanical mixing taking place in the dikes, support for mixing as a mechanism for the formation of the dacites. The as suggested by Reid and Mayne (1981) and Eichelberger (1981), the presence of amphibole and pyroxene in the Agua Fria Peak dacites raises silicic contaminant can be mixed with the basalt into a homogeneous the possibility that the fractionation of those phases is responsible for the intermediate magma. low HREE content. The pyroxene/melt and amphibole/melt distribution Dodge and Calk (1978) reported that the partially fused granodiorite coefficients for the HREE (Irving, 1978) are significantly larger than for

surrounding a trachybasalt plug in the Sierra Nevada is depleted of Si02 the LREE. Fractional crystallization of amphibole would therefore result and K20, relative to unfused granodiorite. This implies that the K, Si-rich in a relatively HREE-depleted pattern, as observed in the Agua Fria Peak partial melt of the granodiorite migrated out of the wall rock and was dacites. Amphibole is, however, present only in the dacites, whereas the preferentially assimilated into the basalt. Such a difference in interaction drop in HREE is recognizable in XBA lavas from Agua Fria Peak (Fig. 9). mechanism could cause the effective bulk composition of the silicic con- The fractional crystallization of pyroxene or amphibole would also have a taminant to be different between the XBA and dacite lavas in the Ocate strong negative effect on the Cr and Ni content, which was not observed. field. Even though a reaction relationship does not prove that orthopyroxene or A related, alternative explanation for the discrepancy between the amphibole did not fractionate from the system, it indicates a significant calculated silicic contaminant for the XBA and dacites involves the more residence time in the magma chamber. rapid diffusion of K and Si across basalt-rhyolite interfaces. In an experi- The presence of hydrous mineralogy in the Agua Fria dacites suggests

mental investigation of basalt-felsic mineral interaction, Watson (1982) a higher PH2Q in that system than in the Cerro Montoso dacites. At higher discovered that Si and K diffuse into the basaltic liquid from the felsic PH,0, amphibole forms by reaction with orthopyroxene and silicate melt. phase more rapidly than do other components. This is consistent with the The orthopyroxene composition is in the same range (Fig. 4) as the ortho- concept of a basaltic magma scavenging the wall rock for those elements pyroxene in the norite xenoliths in the AOB lavas. Similar orthopyroxene and becoming preferentially enriched in K and Si. crystals in the Cerro Montoso dacites are mantled by clinopyroxene rather If the contamination of the intermediate phase of volcanism in the than by amphibole. Ocate field is due: to the preferential assimilation of the high K-Si partial The energy required for the production of the observed quantity of melt of crustal material (leaving a largely feldspathic residuum), the result silicic rock by fusion of the crust was assumed to be derived by liquid mass might be a significantly lower Sr and a higher Rb content in the contami- transport and from the heat of crystallization of mafic magma bodies nant. This is due to the high distribution coefficient for Sr and the low injected into the lower crust. Based upon the relative volumes of mafic, distribution coefficient for Rb between feldspar and silicate melt (Long, intermediate, and dacite lavas, the volume of silicic melt required by mass 1978; Leeman and Phelps, 1981). The Rb and Sr contents of the XBA balance is ~ 10% of the volume of the mafic magma. Even without assum- lavas (Fig. 11) are colinear with those of the dacites. The relative positions ing that a large proportion of the mafic magma never reached the surface of the XBA and dacite compositions are significantly different for Rb and (Hildreth, 1981), we believe the relative volumes to be in accord with our Sr compared with the major elements (Fig. 6): the Sr and Rb contents of hypothesis. the XBA and dacites are closer than the major-element compositions. This may be explained by the assimilation of a silicic contaminant in the XBA that was higher in Rb and lower in Sr than the silicic material incorporated TABLE 4. CALCULATED SILICIC CONTAMINANT FOR OCATE DACITES AND XBA in the dacites. Silicic Dacite OC-59 The trace-element and petrographic dissimilarities between the Agua components XBA average Dacite average Agua Ftia Peak Fria and Cerro Montoso dacites (documented in Figs. 9-13) can be attrib- rhyolite and oligoclase rhyolite. qtz. rhyolite and rhyoliie and uted to their silicic component forming from the melting of different types biotite, oligoclase oligoclase oligoclase

of basement rocks. This hypothesis is supported by the geochemical evi- Oxides dence presented above and by the geographic separation of the Agua Fria (WIS) SiO, 75.85 72.20 72.53 72.79 dacites (Fig. 2). The Cerro Montoso dacites are located in the center of the AIA 13.47 15.83 15.86 15.68 TiO, 0.30 0.35 0.21 0.09 Ocate field, 30-40 km from Agua Fria Peak. Based on the similarity in FeO" 1.31 1.64 1.14 1.15 composition of dacites of different ages among the Cerro Montoso dacites, MgO 0.03 0.07 0.02 0.02 CaO 1.28 1.88 2.14 2.07 it is suggested that the silicic portion of the Cerro Montoso dacites origi- Na,0 2.77 3.77 3.80 3.73 5.28 4.70 4.50 4.56 nated from a similar source material. In contrast to the major elements and KjO

REE, the trace elements Y and Zr (Fig. 13) show a significant difference Trace elements* La 80 63 62 52 among the Cerro Montoso dacites. The dacite data are designated C, N, or Lu 0.39 0.42 0.42 0.20 P for data from Cerro Montoso, Cerro Negro, and Cerro Pelon, respec- Rb 130 113 112 no Sr 275 461 463 455 tively. The linear correlation of the Cerro Montoso-type dacites with Nb 31 35 35 li. Y 29 25 26 13 respect to Y and Zr indicate that they could be genetically related, perhaps Zr 185 191 190 170 by different degrees of partial melting of the same source material. •Trace elements values are calculated assuming the major-element proportions calculated using XLFRAC2 (Stormer Trace-element abundances for the silicic contaminant can be calcu- and Nicholls, 1978). lated, assuming the proportions of basalt and silicic material given in

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Sakuyama (1981) attributed the origin of a similar suite of rocks from upon mineral-melt equilibria by stoichiometry, it is thus possible to calcu- Japan to the mixing of a primitive mafic magma with a silicic magma late the equilibrium temperature and composition of any phase for which produced by differentiation of the mafic parent. Several characteristics of distribution functions were calculated. The Iiquidus phase for a given melt the XBA and dacite lavas suggest that the silicic magma in the Ocate field composition was determined by comparing the calculated equilibrium is not a differentiation product of TOB or AOB lavas. First is the lack of a temperatures for each phase. The phase with the highest calculated continuous trend, as documented by the wide gap between the XBA, OA, temperature is the designated Iiquidus phase. The major advantage of this and dacites. In addition, in contrast to the Japanese lavas, the silicic model over calculations such as those of Roeder and Emslie (1970), phenocrysts and silicic-melt inclusions are relatively constant in Allegre and Minster (1978), and Shaw (1970) is that it takes into account composition. the temperature dependence of mineral-melt equilibria and calculates the modeled Iiquidus phase rather than assuming what it is. Olivine Andesite

The geochemical and petrographic characteristics of the OA indicate 9 that their origin is not due to simple mixing of basaltic and silicic compo- nents. Few samples of OA contain >5% xenocrystic material, and most of 8 what is present is largely re-equilibrated. The compatible-element contents 7 Nap of the olivine andesites are low relative to the xenocrystic lavas with 6 comparable silica contents (Table 3). Mass-balance calculations, using KjO XLFRAC2 (Stormer and Nicholls, 1978) and the same end members as 5 for the XBA and dacite calculations, yield residuals for OA which are 4-5 4 times greater than those for comparable XBA lavas (Table 3). Although the compatible elements in the OA are depleted, the incompatible ele- 3 ments (K, Ba, Nb, Zr, Y, Rb, Sr, REE) are enriched with respect to XBA 5 lavas (Figs. 9-13). 4 O Stormer (1972) hypothesized that the compositionally similar Sierra AFC Grande andesites of the Raton-Clayton field were formed by the partial icp 3

melting of an upper-mantle garnet lherzolite at high PH2O- Subsequent 2 experimental studies (Mysen and others, 1974; Green, 1976) have demon- 1 strated that the partial melts of lherzolite at high PH2Q are silicic but otherwise not andesitic. The origin of the Ocate olivine andesites by such a 0 process is not supported by the mineralogic or geochemical data. First, • AOB there is no evidence of hydrous mineralogy in any olivine andesite. Second, 4 TOB o XBA the hypothesis does not explain the origin of the two separate olivine 10 •ir OA andesite groups with similar major-element compositions but with signifi- FeA O Dacite cantly different trace-element contents (Table B; Fig. 9). R Rhyoiite glass FeO An alternative hypothesis is that the Ocate OA originated by frac- AFC inclusion tional crystallization. Perfect fractional crystallization of basalt has, how- ever, been demonstrated to produce ferrobasalt rather than andesite under most conditions (Roeder, 1974, 1975; Grove and others, 1982). The frac- tional crystallization of magnetite, as suggested by Gill (1981), is unlikely 0 for the Ocate OA, because magnetite is not a phenocryst phase in these 10 andesites. Fractional crystallization, paired with assimilation of silicic 8 crustal material, has been suggested as a process responsible for the petrogenesis of intermediate rocks that evolved in a continental environ- MgO 6 ment (Allegre and Minster, 1978; DePaolo, 1981; Grove and others, 4 1982). In order to test the fractional crystallization and assimilation process 2 as a possible mechanism for the formation of OA, the calculation of the " n » B 75 80 evolutionary compositional paths of liquids undergoing fractionation was 45 50 55 60 65 70 accomplished utilizing the crystallization model of Nielsen and Dungan SiOz (1983). This model uses single-component, major-element distribution functions to calculate phase equilibria for plagioclase, olivine, pyroxene, Figure 14. Calculated liquid lines of descent for an AOB (OC- spinel, and ilmenite in mafic systems. The compositional dependence in- 171) and a TOB (OC-191) for the processes of fractional crystalliza- herent in most single-component distribution functions has been largely tion (FC) and fractional crystallization paired with assimilation (FCA) eliminated by the application of a two-lattice-melt component activity in a ratio of 2% fractional crystallization for every 1% assimilation. model. Given a melt composition, compositionally independent, single- Method of calculation is described in detail in Nielsen and Dungan component-distribution functions, and the additional constraint placed (1983).

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Perfect fractional crystallization is modeled by removing an incre- TABLE 5. CALCULATED PHASE COMPOSITIONS FOR AN AOB (OC-171) AND A TOB (OC-191) UNDERGOING FRACTIONAL CRYSTALLIZATION AND ASSIMILATION ment of the calculated equilibrium liquidus phase from the system, then

recalculating the liquidus phase and temperature of the residual liquid. 10S Fractional crystallization, paired with assimilation, was modeled by add- TOB AOB ing an increment of granitic-melt composition taken from the XBA mix- r = 0.0 0.5 1.0 0.0 0.5 1.0 ing calculations (Table 4) for each increment of fractionation. The ratio of Oxides the number of moles of fractionated material, variable r of DePaolo (wl %) (1981), was evaluated from 0.0 (perfect fractional crystallization) to 2.0 in Si02 50.10 51.48 52.71 48.50 50.01 51.33 increments of 0.25. Two initial primitive bulk compositions were chosen A1203 16.59 16.41 16.23 16.92 16.71 l(.51 Ti02 1.65 1.58 1.52 1.67 1.60 1.53 on the basis of highest Mg number, an AOB lava (OC-171), and a TOB FeO 10.64 10.13 9.70 10.48 9.93 9.49 lava (OC-191). MgO 6.33 6.00 5.73 6.67 6.35 607 CaO 9.53 9.08 8.66 10.29 9.78 9.33 Figure 14 illijstrates the calculated liquid lines of descent superim- NajO 3.55 3.51 3.47 3.50 3.46 3.42 KjO 1.34 1.55 1.74 1.45 1.67 1.86 posed upon the major-element trends. The calculated liquidus phases, predominantly olivine, plagioclase, and spinel, match the observed pheno- ol(Fo) 79 78 78 80 80 80 pi (An) 63 64 65 63 64 65 cryst assemblage and composition within 3 mole % for all components.

Figure 15 illustrates the calculated liquid lines of descent for K20-LaN, 20S

representing the incompatible major and trace elements. Both K20 and TOB AOB Lajsi were considered perfectly incompatible for the purposes of these r = 0.0 0.5 1.0 0.0 0.5 .0 calculations, assuming that a low distribution coefficient (D) produces Si02 50.30 53.17 55.47 48.48 51.62 5-1.08 approximately the same results as a distribution coefficient of 0 for frac- AI2O3 15.95 15.61 15.33 16.27 15.91 15.57

Ti02 1.85 1.68 1.54 1.87 1.69 1.55 tional crystallization. The results of the calculations (Table 5) for the cases FeO 11.21 10.10 9.22 11.10 9.92 9.11 of fractional crystallization and of fractional crystallization paired with MgO 5.74 5.14 4.66 6.14 5.49 5.01 CaO 9.58 8.59 7.88 10.41 9.32 3.45

assimilation are given at 10% intervals, together with the calculated Na20 3.60 3.52 3.45 3.54 3.47 3.40 mineralogy. K2O 1.42 1.68 2.27 1.63 2.05 2.38

Comparison of the results in Figures 14 and 15 demonstrates that ol (Fo) 76 76 76 78 77 76 perfect fractional crystallization does not produce results compatible with pl (An) 61 63 64 61 64 65

the observed com]x>sitions. The compatible elements are depleted and the 3

incompatible elements are enriched for a given silica content compared to TOB AOB the natural suite. r = 0.0 0.5 1.0 0.0 0.5 1.0 Examination of the calculated trends demonstrates that the process of Oxides fractional crystallization, paired with assimilation, produces the closest (wl %) approach to the natural trends. A range of assimilation/fractional crystalli- sio2 50.53 55.10 58.23 48.44 53.45 56.83 AI2O3 15.17 14.75 14.42 15.53 15.03 14.69

Ti02 2.09 1.78 1.55 2.11 1.79 1.57 FeO 11.81 9.94 8.67 11.75 9.80 8.50 MgO 5.04 4.20 3.68 5.46 4.54 3.97 CaO 9.69 8.07 6.95 10.62 8.84 7.60

Na20 3.64 3.53 3.44 3.58 3.48 3.40

K20 1.71 2.35 2.79 1.85 2.47 2.90

ol (Fo) 73 72 72 75 75 74 pl (An) 59 62 65 59 62 65

40%

TOB AOB r = 0.0 0.5 1.0 0.0 0.5 1.0

Si02 51.95 57.83 61.30 49.28 55.94 59.75

AI2O3 14.16 13.81 13.56 14.59 14.10 13.99 Ti02 2.04 1.69 1.46 2.16 1.76 1.54 FeO 11.50 9.12 7.73 11.62 8.93 7.79 MgO 4.22 3.24 2.72 4.65 3.57 3.10 CaO 10.09 7.61 6.16 11.14 8.38 6.78

Na20 3.76 3.57 3.44 3.67 3.51 3.39 K2O 1.99 2.82 3.31 2.16 2.96 3.42

ol (Fo) 70 69 68 72 72 71 pl (An) 56 62 64 55 60 M

Noie: phase equilibria are calculated utilizing the mineral-melt relations of Nielsen and Dungan (1983). r = moles assimilated/moles Iractionaled (dePaolo, 1981 ).

zation ratios was tested, and a ratio of 0.5 was found to produce the results Figure IS. Calculated liquid lines of descent for an AOB and a with the smallest difference between the calculated and the observed com- TOB composition undergoing fractional crystallization (FC) or frac- positions. These results are applicable only to the Ocate field. Obviously, tional crystallization paired with assimilation (FCA). Method of calcu- other magma systems are influenced by other petrogenetic processes. lation same as for Figure 14. The wide range of REE contents in the olivine andesite lavas may be

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explained by their derivation from different primitive parental composi- tic basalts near the rift axis is consistent with the generation of larger tions. Figures 14 and 15 illustrate the difference in the calculated paths for volumes of melt by larger degrees of partial melting at shallow depths. In the AOB and TOB starting compositions. The greatest difference between rift flanks, on the other hand, alkaline lavas become progressively domi- the calculated paths is evident in La^ content. Both the calculated lines nant. The association of alkaline and tholeiitic basalts in the Ocate and pass near or through OA compositions. The line from the TOB starting Raton-Clayton fields suggests two possibilities: (1) the source regions were composition intersects the OA composition with lower REE contents, and separated by a large depth interval, or (2) significantly different degrees of the AOB starting composition produces calculated paths that pass through partial melting occurred at the same depth. The increase in incompatible the OA points with high REE contents. For example, OA lava OC-71, elements within the same rock type, as one proceeds away from the rift, possessing a high REE content, is associated with AOB lava OC-70 in a may be due to slightly greater depth and/or a lower degree of partial flow package on Sierra Montuoso mesa, and OA lava OC-19, with lower melting to the east. REE content, is associated with TOB lava OC-21 in flows from Cerro Pelon (Table B). SUMMARY AND CONCLUSIONS

EVOLUTION OF THE OCATE VOLCANIC FIELD The Ocate volcanic field is characterized by a suite of lavas ranging from alkali olivine basalt to dacite. These lavas have been subdivided into The composition, volume, and location of lavas in the Ocate volcanic five groups. Two of these are basaltic (alkali olivine and transitional oli- field varies as a function of time (Table 1). The most voluminous volcanic vine basalt), two are mafic andesites (olivine andesite and xenocrystic output in the Ocate field occurred during the period 4.5-2.0 m.y. B.P. basaltic andesite), and one is dacite, separated by a gap of 7% Si02 from The volcanic output during this period was fairly constant, ranging from the andesites. 23-35 km3 for the three most voluminous episodes. The first and last epi- The composition of the mafic lava types in the Ocate field is interme- sodes, 8.1-5.0 and 1.4-0.8 m.y. B.P., were significantly less voluminous diate between the compositions of the mafic phase of volcanism in the (4 km3). Taos Plateau and the Raton-Clayton fields. The observed regional trend of Compositionally, the temporal trends demonstrate an increase in increasing undersaturation away from the axis of the Rio Grande Rift may TOB volume % from 8.1 to 3.2 m.y. B.P., followed by a drop in TOB indicate the existence of a regional thermal regime with highest heat flux output. Alkali basalt volcanism was at a maximum in the first and last centered along the rift at the time of volcanic activity. episodes; there was a minimum during the third episode (3.2-3.0 m.y. Recent experimental investigations have demonstrated that AOB and B.P.). Dacites were erupted only during the second and fourth episodes. transitional basalts can be produced at different depths and/or by varying This pattern of volcanism is consistent with a systematic increase in heat degrees of partial melting in the mantle (Mysen and Kushiro, 1977; Olafs- flux in the upper mantle from 8.1 to 3.2 m.y. B.P., followed by a cooling son, 1980; Presnall and others, 1978,1979; Jaques and Green, 1980). The trend that continues today. An increase in heat available in the mantle origin of the variety of basaltic magma compositions in the Ocate field has could produce a higher degree of partial melting at shallower depth, thus therefore been attributed to a lower degree of partial melt and/or greater producing a less alkaline magma. depth of melting for AOB compared to TOB lavas. Subsequent fractiona- During the middle episodes, an increase in the volume of mafic tion and contamination during ascent through the crust have resulted in magma introduced into the crust resulted in greater degree of melting in the observed evolved character of the Ocate basalts. the crust, thereby producing the dacites. The limited extent of the dacites Mixing of two magmas, one mafic and the other the silicic product of and their association with the large volcanic centers support the contention the fusion of crustal rocks, is the dominant process responsible for the that they are related to the heating of the crust by the introduction of large variety of intermediate rock compositions observed in the Ocate field. quantities of magma into shallow chambers. The OA lavas are most Mass-balance calculations demonstrate that magmas similar to the XBA common in the first episode, when the XBA lavas are most voluminous and dacites can be produced by the mixture of basaltic and silicic end during the final episode. This may be indicative of the decrease in depth of members. This is supported by the xenoliths and rhyolite glass inclusions in the production of the intermediate lavas with time. The trace-element the quartz and plagioclase xenocrysts. The results of the mass-balance characteristics within a given group do not, however, change with time. A calculations indicate that the proportions of the silicic mixing components, 5-m.y.-old AOB is similar to AOB lavas that erupted 3 m.y. or 1 m.y. ago. oligoclase and rhyolite glass, are different for XBA and for dacite lavas. This calculated difference in the composition of the contaminant in the REGIONAL RELATIONSHIPS XBA versus the contaminant in the dacites, published field observations of mixed rocks in other localities (Reid and Mayne, 1981; Reid, 1982), and The temporal relationships characteristic of the Ocate field are also experimental results (Watson, 1982) suggest that the XBA and the dacites present in the neighboring Taos Plateau and Raton-Clayton fields (Lip- may have originated by different mixing processes. The formation of XBA man, 1969; Dungan and others, 1983b; Lipman and Mehnert, 1979). In lavas by the contamination of a basaltic parent by wall-rock interaction all three fields, the most voluminous period of volcanism is 4.5-2.0 m.y. may result in the preferential assimilation of the most mobile components. B.P. This voluminous period of volcanism is characterized by the highest The presence of basaltic xenoliths in the Ocate dacites is consistent with an percentage of transitional or tholeiitic basalt and by the eruption of dacite origin by the injection of basaltic magma into a silicic magma chamber and rhyolite lavas. The final period of volcanism is characterized by (Reid, 1982; Eichelberger, 1975, 1978). smaller volumes of lava in which xenocrystic intermediate lavas and more Evidence of simple mixing, both chemical and petrographic, is largely alkaline primitive lavas predominate. absent from OA lavas. Mixing calculations identical to those performed In addition to the progressive alkaline nature of the lavas from west for the XBA and the dacites produce higher residuals compared to the to east, the incompatible minor- and trace-element concentrations within a results for the XBA lavas. Combined fractional crystallization and assimi- given rock type also increase from west to east. The dominance of tholeii- lation is the process that produces the smallest calculated-observed differ-

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D. A., and others, eds.. Guide to some volcanic terrains in Washington, Idaho, Oregon and northern California: ence. This process, and other processes involving fractional crystallization, U.S. Geological Survey Circular 838, p. 183-189. were modeled assuming a primitive mafic magma as a starting composi- Gill, J. B,, 1974, Role of undertbrust oceanic crust in the genesis of a Figian calc-alkaline suite: Contributions to Mineralogy and Petrology, v. 43, p. 29-43. tion and assuming the composition of the silicic mixing component from 1981, Orogenic andesites and plate tectonics: New York, Springer-Verlag, 390 p. the dacite mixing calculations as the assimilated material. The optimum Green, D. H., 1976, Experimental testing of "equilibrium" partial melting of peridotite under water-saturate, high pressure conditions: Canadian Mineralogist, v. 14, p. 255-268. ratio of assimilation to fractional crystallization for most OA lavas was Grove, T. L., Gerlach, D. C., and Sando, T. W., 1982, Origin of calc-alkaline series lavas at Medicine Lake by fractionation, assimilation and mixing: Contributions to Mineralogy and Petrology, v. 80, p. 160-182.

found to be 0.5. Helz, R. T., 1976, Phase relations of basalts in their melting range at PH - = 5 kb. Part II. Melt compositions: Journal of Petrology, v. 17, p. 139-193. Hibbard, M. J., 1981, The magma mixing origin of mantled feldspars: Contributions to Mineralogy and Petrology, v. 76, ACKNOWLEDGMENTS p. 158-170. Hildreth, W., 1981, Gradients in silicic magma chambers: Implications for lithospheric magmatism: Journal of Geophysi- cal Research, v. 86, p. 10153-10192. Irvine, T. N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Without the iissistance of D. Blanchard, Susan Wentworth, and oth- Journal of Earth Science, v. 8. p. 523-548. ers at the Johnson Space Center, Houston, Texas, in providing the equip- Irving, A. J., 1978, A review of experimental studies of crystal/liquid trace element partitioning: Geochimica et Cosmo- chimica Acta, v. 42, p. 743-770. ment and expertise;, this project could not have been done. We also would Jaques, A. L„ and Green, D. H., 1980, Anhydrous melting of peridotite at 0-15 kb pressure and the genesis of tholeiitic basalts: Contributions to Mineralogy and Petrology, v. 73, p. 287-310. like to thank Stuart Nelson for his help as R. L. Nielsen's field assistant, for Kovach, L. A., and Marsh, B. D., 1981, Magma flow rate and partial fusion of wallrock, Huntington Lake, California: cheerfully toting rocks across the desert for beans. Geological Society of America Abstracts with Programs, v. 13, no. 7, p. 490. Kuno, H., 1950, Petrology of Hakone volcano and the adjacent areas, Japan: Geological Society of America Bulletin, v. R. L. Nielsen was supported on a stipend from the R. A. Welch 61, p. 957-1019. Kushiro, I., 1972, Effects of water on the composition of magmas formed at high pressures: Journal of Petrology, v. 13, Foundation, Houston, Texas (No. 781) during this investigation. Field p. 311-334. support was provided by a grant from the New Mexico Bureau of Mines Leeman, W. P., 1970, The isotopic composition of Sr in late Cenozoic basalts from the Basin and Range Province, western : Geochimica et Cosmochimica Acta, v. 34, p. 857-872. and Mineral Resources. Funds for maps and thin sections were provided Leeman, W. P., and Phelps, D., 1981, Partitioning of rare earths and other trace elements between sanadine and coexisting volcanic glass: Journal of Geophysical Research, v. 86, no. B11, p. 10193-10199. by the New Mexico Geological Society and by the Geological Society of Lipman, P. W., 1969, Alkalic and tholeiitic basaltic volcanism related to the Rio Grande depression, southern Colorado America. Supporl for microprobe analysis and transportation was pro- and northern New Mexico: Geological Society of America Bulletin, v. 80, p. 1343-1353. Lipman, P. W., and Mehnert, H. H., 1979, The Taos Plateau volcanic field, northern Rio Grande Rift, New Mexico, in vided by NSF Grant NSF-EAR8026451-01 and by a grant from the Reicker, R. E., ed., Rio Grande Rift: Tectonics and magmatism: Washington, D.C., American Geophysical Union, p. 289-311. Institute for the Study of Earth and Man, Southern Methodist University. Long, P. E„ 1978, Experimental determination of partition coefficients for Rb, Sr and Ba between alkali feldspar and Support for the computer time required for this study was supplied by the silicate liquid: Geochimica et Cosmochimica Acta, v. 42, p. 833-846. Mysen, B. O., and Kushiro, I., 1977, Compositional variations of coexisting phases with degree of melting of peridotite in computer centers of Southern Methodist University and the University of the upper mantle: American Mineralogist, v. 62, p. 843-865. Mysen, B. O , Kushiro, I., Nicholls, I. A., and Ringwood, A. E„ 1974, A possible mantle origin for andesite magmas: Maryland. Discussion and replies: Earth and Planetary Science Letters, v. 21, p. 221-229. Nielsen, R. L., and Dungan, M. A., 1983, Low pressure phase equilibria in natural mafic silicate systems: Contributions to Mineralogy and Petrolog), v. 84, p. 310-326. REFERENCES CITED Olafsson, M., 1980, Partial melting of peridotite in the presence of small amounts of volatiles, with special reference to the low velocity zone [M.S. thesis]: University Park, Pennsylvania, Pennsylvania State University, 59 p. Allegre. C. J., and Minster, J, F , 1978, Quantitative models of trace element behavior in magmatic processes: Earth and O'Neill, J. M., and Mehnert, H. H., 1980, Late Cenozoic physiographic evolution of the Ocate volcanic field, noth-central Planetary Science Letters, v. 38, p. 1-25. New Mexico: U.S. Geological Survey Open-File Report 80-928, 44 p. Pearce, J. A., and Norry, M. J . 1979, Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks: Allen, J. C., and Boeucher, A. 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