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A suite of lower crustal nodules collected from the potassic latite (6.5% K2O) of the Camp Creek locality (16 km ENE of Carefree, Arizona) is comprised predominantly of eclogites, garnet clinopyroxenites and amphibolites. The nodule mineralogy consists of varying amounts of garnet, diopsidic clino-pyroxene and pargasite with accessory rutile, apatite, plagioclase, phlogopite, sphene, corundum, spinel and Fe-Ti oxides. The majority of the nodules are Type B eclogites. The amphibolite assemblage varies from garnet-rich to plagioclase-bearing and contains a variety of alteration products and partial melting textures. Several inclusions show eclogite and amphibolite in contact, suggesting the two assemblages coexist at depth and probably grade into one another. Geo-thermometers based on Fe-Mg exchange ratios between garnet-clinopyroxene and garnet-biotite pairs at P=10 Kb indicate equilibration temperatures on the order of 700-900°C. The mineralogy and chemistry of the Camp Creek nodule suite and host-rock indicate a striking similarity to the Chino Valley occurrence (Schulze and Helmstaedt, 1979; Arculus and Smith, 1979).

We present oxygen and strontium isotopic data for olivine tholeiites, evolved (that is, differentiated and (or) contaminated) lavas, rhyolites, and crustalderived xenoliths from the Snake River Plain. These data show that the olivine tholeiites are fairly uniform in 6 180 (5.1 to 6.2) and 87Sr/ 86 Sr (0.7056 to 0.7076) and reveal no correlation between these ratios. The tholeiites are considered representative of mantle-derived magmas that have not interacted significantly with crustal material or meteoric water. The evolved lavas display a wider range in 6 180 (5.6 to 7.6) and 87Sr/86Sr (0.708 to 0.717) with positive correlations between these ratios in some suites but not in others. Crustal xenoliths have high and variable 87Sr/ 86 Sr (0.715 to 0.830) and 6 18 0 values that vary widely (6.7 to 9.2) and are a few permil greater than 6 18 0 values of the Snake River basalts. Thus, isotopic data for the evolved lavas are permissive of small degrees of contamination by crustal rocks similar to the most 18 Q-depleted xenoliths. The 6 180 enrichments in some evolved lavas also are consistent with crystal fractionation processes and do not necessarily recaiire bulk interaction with crustal rocks. Enrichment in 18 0 but not in 8 'Sr/ 86Sr in one suite of evolved lavas suggests that crustal contamination may not be essential to the petrogenesis of those lavas. Other suites of evolved lavas display large variations in 87Sr/86Sr that reflect at least some selective contamination with 87Sr. Bulk solid/liquid oxygen-isotope fractionation factors (a's) calculated for the evolved lavas from Craters of the Moon National Monument are comparatively large. These a's are dependent upon the nature and proportions of phases removed by crystal fractionation; basaltic lava a's differ from latitic lava a's in accordance with different phenocryst assemblages in these rocks. Snake River Plain rhyolites are isotopically distinct from both the analyzed crustal xenoliths and olivine tholeiites. Their origin remains poorly understood, but crustal or sub-crustal sources may be viable. In the first case, they must be derived by anatexis of material distinct from the analyzed crustal xenoliths. In the second case, they must be derived from material unlike the source for tholeiites. No cogenetic relation with the tholeiites seems likely on the basis of available data. INTRODUCTION The Snake River Plain (SRP) is a bimodal basalt-rhyolite volcanic province that extends for 500 km across southern Idaho from the Columbia River Plateau eastward to Yellowstone National Park. Structurally, the western SRP is considered to be a graben bounded by high-angle faults (Malde, 1959), whereas the eastern SRP is essentially a downwarp controlled in part by margin faults (Kirkham, 1931; Trimble and Carr, 1976). Stratigraphic relationships within the SRP (for example, Malde and Powers, 1962; Cam and Trimble, 1963; Stearns and others, 1938) and absolute ages of the volcanic rocks (Armstrong and others, 1975) indicate that volcanic activity became progressively younger toward the east. In any locality within the SRP, volcanism began with voluminous eruptions of high-silica rhyolitic flows and ash-flow tuffs, and was followed by dominantly basaltic eruptions. Initial rhyolitic volcanism ranges in age from about 15 m.y. old to about 2 m.y. old from west to east whereas the oldest basalts range in age from about 13 m.y. old in the west to less than 1. m.y. old in the east. The basaltic rocks are dominantly olivine tholeiites characterized by comparatively high iron enrichment, high TiO£ (average 2.7 weight percent) and P2 05 (0.6 percent), low Si0 2 (46.9 percent) and total alkalies (3.1 percent) (Powers, 1960a; Stone, 1967; Leeman, 1974). The basalts are relatively uniform in composition regardless of age or location. In addition, minor volumes of "evolved" lavas occur situated mainly on the margins of the SRP. These rocks are typified by lavas at Craters of the Moon (COM) National Monument, which range widely in composition (from ferrobasalt to tholeiitic latite) as the result of low pressure crystal fractionation (Leeman and others, 1976; Stone, 1970). The COM-type lavas may be related to olivine tholeiite parental magmas via high-pressure differentiation (Thompson, 1975; Leeman and others, 1976). However, the presence of crustal-derived xenoliths in some of the COM-type lavas, and anomalies in their strontium and lead isotopic compositions (Leeman and Manton, 1971; Leeman, 1974 and in preparation) suggest that they may also be contaminated by crustal material. The eastward younging of SRP volcanism, as well as geologic and geophysical information for this province, have led numerous authors to suggest that volcanism there is related to movement of the North American plate over a mantle hot-spot presently located beneath the Yellowstone volcanic complex (for example, Smith and Sbar, 1974; Suppe and others, 1975; Eaton and others, 1975; Thompson, 1977; Brott and others, 1978). To evaluate this model it is desirable to determine the source(s) of the SRP magmas. Here we report oxygen isotopic analyses of representative basalts and crustal xenoliths from the SRP for which strontium isotopic analyses are available. Analyzed samples include: (a) 19 olivine tholeiites covering a broad temporal and spatial distribution, (b) 26 evolved COM-type lavas from five localities, (c) 8 crustal xenoliths from three localities, and (d) 6 rhyolite flows and ash-flow tuffs. These data augment analyses by Muehlenbachs and Stone (1973) for nine other SRP samples and, in combination with strontium isotopic and major-element data, provide important constraints on possible source materials for SRP magmas and on processes by which they evolved. ANALYTICAL METHODS Identical splits of sample powders (about 200 mesh) were used in this study for both strontium and oxygen isotopic analyses. Many of the strontium isotopic analyses are taken from Leeman and Manton (1971), and the remaining samples were analyzed on a 6-inch-radius NBS design mass spectrometer with triple filament ionization located at the U.S. Geological Survey, Denver, Colorado. All isotopic ratios are normalized to a 87Sr/ 86Sr ratio of 0.1194, and 87 Sr/86Sr ratios are adjusted to a value of 0.7080 for the Eimer and Amend SrCOs standard. Withinand between-run precision for this ratio is about +0.0002 (two-sigma) or better. After drying the sample powders for one hour at 200°C, oxygen was extracted by reaction with BrF 5 at 550°C in nickel vessels (Clayton and Mayeda, 1963). Oxygen was converted to COz by combustion with a resistance-heated carbon rod (Taylor and Epstein, 1962). Isotopic analyses of CO were performed on a Nuclide 60 RMS mass spectrometer with modified source, collector, and emission regulator. The isotopic data are expressed as 6, the per mil deviation relative to SMOW (Craig, 1961). Twenty-two analyses of NBS-28 reference standard performed during this study yielded a mean 6 180 value of 9.53 + 0.36 (two-sigma), and three analyses of St. Peter sandstone yielded a mean value of 11.10 + 0.16 (two-sigma). The unusually large 2-sigma of NBS-28 reflects some severe Oz-extraction problems during the final stages of the study. The average 6 180 difference between replicate analyses of 22 individual samples (tables 1, 2, and 3) is 0.1 permil. RESULTS 01ivine tholeiites SRP olivine tholeiites were selected for analysis to provide representative spatial and temporal coverage. Our samples are divided into two age groups in table 1; lavas of the Snake River Group (that is, less than about 1 m.y. old), and those equivalent to the Glenns Ferry and Banbury formations of Malde and Powers (1962). The latter rocks range in age from about 3 to 10 m.y. old and, despite an attempt to sample fresh material, some of these rocks display incipient iddingsitic alteration of olivines. Both groups include samples from the western and eastern parts of the SRP. The Snake River Group lavas display a wide range in ICO content (0.14 to 0.83 weight percent), but narrow ranges in 6 180 (5.12 to 5.99) and 87Sr/ 86 Sr (0.7056 to 0.7068), and show little or no correlation between these compositional features. The older lavas also display a wide range in KzO (0.10 to 0.64) and similar ranges in 6 180 (5.06 to 6.21) and 87Sr/ 86Sr (0.7065 to 0.7076). Average 6 18 0 and 87 Sr/86Sr ratios (table 1) are slightly higher for the older group, but the differences are statistically insignificant. Other compositional features of these respective groups are similar except that the older lavas have higher average potassium/rubidium (580) than the younger group (470) (Leeman, 1974). The average

The Mount Morgan pyritic copper-gold deposit of central eastern Queensland, Australia, exhibits both syngenetic stratiform processes and epigenetic hydrothermal processes. These facies of mineralization lie vertically one above the other, thus facilitating a chronological study of ore-forming sequences. Remnants of a former stratiform sulphide horizon are followed by a massive sulphide breccia pipe and this is intruded by a latite stock which has yielded sulphide-bearing quartz veins injected into the ore of the breccia pipe.

Major glaciation in the Sierra Nevada of California resulted in the deposition of till which underlies latite 2.7 x 10(6) years old and overlies andesite 3.1 x 10(6) years old. This till, herein called the Deadman Pass till, is the oldest Pleistocene glacial deposit that has been found in temperate latitudes.

The 34 Ma Grizzly Peak Tuff in west-central Colorado preserves evidence for distinct compositional layers in a high-level magma chamber. Intracaldera tuff consists of a single cooling unit, at least 2.7 km thick, zoned from a high-silica rhyolite base to an eroded low-silica rhyolite top. Two horizons of heterogeneous tuff beneath wedges of caldera-collapse breccia contain fiamme of dacite to mafic latite along with the rhyolitic fiamme that make up the rest of the tuff. Compositional zoning defined by fiamme is a step function rather than a continuous gradient. Seven petrographic groups of fiamme each have distinct compositions separated by compositional gaps. As the same clusters and gaps are found in collections of fiamme from widely separated stratigraphic levels, step-function zoning must be intrinsic to the chamber rather than a consequence of the tapping process. We interpret the seven fiamme groups as seven separately converting layers in a density-stratified magma reservoir tapped by eruption of the Grizzly Peak Tuff. Excluding the thin heterogeneous tuff horizons, ∼3 km of caldera fill is drawn from the three most silicic layers, suggesting that the three rhyolitic layers were each at least 1 km thick in the magma chamber.

Heulandite, clinoptilolite, mordenite, analcime, thomsonite(?), erionite, and chabazite(?) occur with other authigenic minerals in the thick Miocene volcaniclastic sequence of the southern Desatoya Mountains, Nevada. The mineral assemblages formed diagenetically in an open system probably at depths of 2,500 m or less and in a variety of rock types, including rhyolite, quartz latite, dacite, latite, and andesite. The authigenic minerals developed mainly through the breakdown of the inherently unstable glassy components, and the original porosity and permeability of the rocks were major factors controlling the degree of alteration. Determination of the temperatures of diagenesis depends greatly on ascertaining the time of diagenesis and, thus, the geothermal gradient at that time. The paragenetic sequence of events between primary and secondary materials and among authigenic mineral species is (1) partial replacement of glass by clay minerals, (2) solution of remaining glass, (3) precipitation of heulandite and/or clinoptilolite (± silica) and/or other authigenic minerals, and (4) deposition of opal or chalcedony in cavities or veinlets. Twenty-seven electron microprobe analyses are given for single crystals and crystal clusters of heulandite, clinoptilolite, and thomsonite(?). Chemical comparisons between (1) petrographically identified glass and its adjacent alteration products and (2) unaltered and/or partially altered and completely altered whole rocks from the same unit indicate that Si, Ca, Na, K, and H 2 O were mobile components. It is inferred from petrographic observations that Fe and Mg also were mobile. Na, and possibly some K, apparently were lost during diagenesis. Mineral development and chemical changes seem to have followed the general pattern for leaching in an open system.

The chemical and thermal evolution of a single magmatic system is recorded in the Paintbrush Tuff, a series of four ash flow sheets, that were erupted from the same caldera within 0.6 Ma. Glassy pumice samples were collected from the tops and bottoms of the ash flow sheets. Because these samples closely approximate magma compositions, their chemical and mineralogical compositions were used to quantitatively evaluate magma mixing and fractional crystallization as potential differentiation mechanisms. Prior to eruption of the first ash flow sheet, the Topopah Spring Member (>1200 km3), an abrupt compositional interface existed in the magma chamber between high-silica rhyolite and quartz latite magmas. The subsequent Pah Canyon Member ( 1000 km3) to be erupted, and it consists of three compositional modes, a higher-silica rhyolite, rhyolite, and quartz latite. The high-silica rhyolite of the Yucca Mountain Member is an early eruptive phase of the higher-silica rhyolite of the Tiva Canyon Member, and their origins are the same. The rhyolite most likely formed by a combination of fractional crystallization and magma mixing of the Pah Canyon magma and the quartz latite of the Tiva Canyon Member. Alternatively, the rhyolite may have formed by fractional crystallization and magma mixing of higher-silica rhyolite and quartz latite of the Tiva Canyon Member, or by fractional crystallization from the Tiva Canyon Member. The origin of the quartz latite has not been determined. Quantitative modeling indicates that all of the chemical variation within the Pah Canyon, Yucca Mountain, and Tiva Canyon members can be accounted for by fractional crystallization or magma mixing operating alone or in conjunction; no other processes need be invoked. Volume estimates of the magma bodies based on these quantitative models predict much larger volumes than the sizes of the ash flow sheets would indicate. A recurring theme in the evolution of the Paintbrush Tuff is the interaction of quartz latitic magmas with high-silica magmas in conjunction with crystal fractionation.

Two hundred chemical analyses made of igneous rocks from seven of the Cape Verde Islands and from Fernando de Noronha show that these Atlantic islands are predominantly composed of plagioclase-free limburgite grading into plagioclase-bearing basanite. Minor amounts of nepheline latite, nepheline monzonite, and nepheline syenite occur as intrusive stocks together with flows and plugs of phonolite. The high average Sr, Ti, P, total Na + K, and normative Ne contents distinguish the series from the ankaramite-alkali basalt-trachyte suites found in most oceanic volcanic islands. Nepheline latite and nepheline syenite rocks of the Cape Verde Islands and Fernando de Noronha could have formed by the subtraction of a highly mafic olivine-bearing kaersutitic clinopyroxenite from a limburgitic magma. Comparison with published results from other oceanic islands and the sea floor shows that the Cape Verde Islands and Fernando de Noronha are the most alkalic parts of the oceanic crust. A plot of Sr versus TiO 2 for oceanic and sea-floor islands clearly delineates the extreme character of these island groups. This representation of magma type is proposed as being of greater value than the conventional alkali versus silica diagram for oceanic island comparisons.

Low-silica latite highly enriched in large-ion-lithophile elements and moderately potassic low-Si andesite were erupted in central Peru during late Cenozoic time. FeO*/MgO ratios of 0.93 to 1.25 at 53 wt percent SiO/sub 2/ indicate a definitely calcalkalic character. The combination of low FeO*/MgO ratios, low SiO/sub 2/, and high Cr, Co, and Ni with large-ion-lithophile and light rare-earth elements makes it very unlikely that the parent magmas were produced by high-pressure partial melting of subducted ocean-floor basalt (eclogite). The data are more compatible with small degrees of partial melting of ultramafic material or mixtures of basalt and ultramafic mantle. The initial melts probably contained 52 to 53 percent SiO/sub 2/ and had a content of large-ion-lithophile elements nearly as high as that of the rocks. /sup 87/Sr//sup 86/Sr ratios of from 0.7042 to 0.7051 and low to very low Rb/Sr ratios indicate an isotopically variable source region that, at least in part, had earlier been depleted in Rb relative to Sr.

The Magdalena District in central New Mexico comprises the Magdalena Range and Granite Mountain which are block mountains of the Basin Range type. They consist of Carboniferous sedimentary formations on a pre-Cambrian basement and are covered by Tertiary volcanic rocks. Volcanism in the District was preceded by considerable faulting, tilting, and uplifting very probably in early Tertiary time. The initial volcanic extrusion consisted of latite flows and tuffs now resting unconformably on the Granite Mountain fault-block in the northern part of the District. Subsequent extrusions consisted of alternations of andesites, rhyolites, and latites, and have a minimum thickness of approximately 4,000 feet. Graben faulting, probably in compensation for the effusion of the large volumes of lava, followed and some of the major faults were subsequently the sites of the intrusion of monzonitic and granitic stocks. Late lamprophyre and white rhyolite dikes in large numbers intruded the intrusive as well as earlier extrusive and sedimentary rocks.

During late Miocene (14.8 m.y.) to early Pliocene (10.1 m.y.) time, local latite and widespread basaltic flows accumulated with associated continental sedimentary deposits in north-central Arizona. Some of these rocks were displaced and tilted by normal faults, and new drainage, now occupied in part by basalt flows of late Pliocene age (average 5 m.y., maximum of 6 m.y.), was established at the southern margin of the Colorado Plateaus. The time of faulting and uplift of the Colorado Plateaus in this region is thus bracketed between about 10.1 and 6 m.y. ago. Five analyzed basalts fall within the tholeiitic basalt field of the alkali-silica diagram and twenty are in the alkalic basalt field; in general, they are similar to other alkali-olivine basalts from the western United States.

Cryptocrystalline magnesite occurs as relatively small dense, white masses in a Tertiary volcanic tuff, here named the Currant tuff, in White Pine and Nye counties, 29 miles southwest of Ely, Nevada. The deposits are small, but some of the magnesite is of very high quality with almost no iron or aluminum. However, much of the magnesite is mixed with dolomite and calcite, and a serpentinelike mineral occurs in one of the deposits in sufficient abundance to be of possible use in ceramics. In scattered areas the tuff has been altered by solutions rich in magnesium and bicarbonic acid to dolomite, magnesite, magnesium silicate, and calcite. Mineral and chemical characteristics are the same throughout the entire rock assemblage of flows and tuffs, suggesting that all the rocks have been derived from the same parent magma. The tuff formation, which ranges in thickness from a few feet to over 400 feet, occurs between two groups of volcanic flows here referred to as the lower volcanics and the upper volcanics. The lower volcanics is composed mainly of flows of hypersthene dacite, but in the southwestern part of the area flows of a hypersthene andesite of balsaltic habit occur between the dacite and the overlying Currant tuff. The upper volcanics which overlie the Currant tuff consist of porphyritic quartz latite and an overlying massive latite crystal tuff. The Currant tuff containing the magnesite has both unmodified and reworked tuff members. The formation of hydrothermal dolomite, magnesite, and deweylite is discussed in detail, and the physical chemistry of solutions containing calcium, magnesium, and carbon dioxide is applied in an explanation of the origin of magnesium-rich solutions.

Pleistocene latite and basalt flows crop out along the Upper Truckee Canyon, located immediately downstream from Lake Tahoe, and throughout the Truckee Basin, an intermontane basin at the north end of the canyon. They were extruded over a topography somewhat similar to that of the present from numerous, widely spaced vents. The flows postdate the major part of the Pliocene-Pleistocene deformation of the Sierra Nevada and appear to predate the oldest recognized glaciation in this part of the Sierra Nevada. They are correlated with the Lousetown Formation of western Nevada. Large-scale deformation followed the volcanism. During upwarping of the area east of the Truckee Basin, the Truckee River maintained an antecedent course and eroded a deep canyon. Because no sediments in the Truckee Basin can be correlated with this period of uplift, it is concluded that the cutting of the canyon proceeded at a rate equalling or exceeding that of the uplift. All movements appear to predate the oldest recognized glaciation.

The Silver Peak volcanic center is part of a late Tertiary volcanic province in western Nevada and eastern California characterized by slightly alkaline, potassic lavas. The rocks of the Pliocene Silver Peak center have an alkali-lime index of 55.5, a K2O/K2O + Na2O ratio of 0.5 at 60 percent SiO2, and range in composition from trachybasalt to rhyolite, with latite the most voluminous. Chemical and mineralogical variations in the intermediate and silicic lavas are compatible with differentiation by crystal fractionation. The differentiation trend is similar to calc-alkaline suites with progressive enrichment in silica and alkalis but no relative enrichment in iron. Relatively high water pressures during crystallization are indicated by the ubiquitious presence of biotite or hornblende. The parent magma of the intermediate and silicic part of the series is believed to have a composition of trachyandesite with approximately 56 percent silica. The potassic nature of trachyandesite in this province is tentatively related to depths of a Tertiary subduction zone beneath western North America. Associated trachybasalts of the Silver Peak center are nepheline normative and characterized by high strontium contents, low Rb/K and Rb/Sr ratios, and relatively high Sr87/Sr86 ratios (.7061). These are believed to represent a separate parent magma derived from material previously depleted in rubidium.

The rocks exposed in the disseminated copper deposit at Bingham, Utah, consist of folded Pennsylvanian quartzites, limestones, and dolomites within which have been formed granite, actinolite syenite, granite porphyry with associated aplites, and biotite-quartz latite porphyry. Field and petrographic evidence shows that the granite porphyry and biotite-quartz latite porphyry are magmatic. That the granite and actinolite syenite may have been formed by granitization is strongly suggested by: (1) the presence of a microgranitic rock in the granite, recognized as being granitized quartzite, and (2) the replacement of quartzite by feldspar, observable only with the aid of the microscope, that occurs along the contact of the granite. Other chemical, textural, compositional, and other evidences support this hypothesis. The hydrothermal activity attending the ore deposition took place in seven stages following the formation of the igneous-appearing rocks. Except for the first two, which can be separated on good microscopic evidence, all stages can be differentiated by structural field relations. Stage I saw the formation of kaolinite and illite. During Stage II hydrothermal biotite and sericite were widely developed. In Stage III, chlorite and hydrothermal biotite were formed, but in restricted areas. In Stage IV, quartz and sericite formed over wide areas, while during Stare V quartz was deposited in open fissures without accompanying alteration. Stage VI is the sulfide stage, and Stage VII saw the development of sericite and allophane in open fissures.

A date of 19.9 m. y. (early Hemingfordian) from a welded rhyolite tuff at the top of the type section of the Valley Springs Formation, and one of 25.7 m. y. (early Arikareean) from the basal welded rhyolite tuff in the Sonora Pass area, indicate that the Valley Springs Formation was deposited before late Miocene, and that rhyolitic volcanism in the Sierra Nevada began by earliest Miocene time. Three dates of 8.8–9.0 m. y. (early Hemphillian) on biotite-augite latite, which stratigraphically overlies the Table Mountain latite in the Stanislaus drainage, show that the Table Mountain latite is a member of the Mehrten Formation, which contains the late Hemphillian Oakdale fauna and the early Clarendonian Two Mile Bar fauna. A 3.5-million year date on basalt filling a canyon cut in the Chagoopa surface at Little Kern River, a 9.5-million year date on basalt from Jose Basin in Fresno County, and dates of 3.5–3.6 m. y. from basalts in the Upper San Joaquin drainage indicate that the Chagoopa surface and the Mountain Valley stage are at least as old as early Pliocene, and that the Sierra Nevada had a minimum relief of 4000 feet at that time. Basalt beneath McGee Till on McGee Mountain gives a date of 2.6 m. y., and basalt on Coyote Flat gives a date of 9.6 m. y. Basalt beneath Sherwin Till in Owens Gorge has an age of 3.2 m. y. A rhyolite tuff and an overlying basalt in Deep Springs Valley, White Mountains, give ages of 10.9 and 10.8 m. y., respectively.

The Sierra Blanca igneous complex of south-central New Mexico consists of the Oligocene Sierra Blanca Volcanics which have been intruded by Oligocene monzonite to quartz syenite stocks, sills, and dikes. The Sierra Blanca Volcanics are composed of four formations: Walker Andesite Breccia, Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte. The Walker Andesite Breccia comprises the major volume of the eruptive rocks. It is characterized by monolithic blocks transported and frozen in a matrix of plagioclase microlites, enstatite, hornblende, and hematite. A few K-Ar dates indicate that volcanism began approximately 35 m.y. ago and continued until about 25 m.y. ago. The Walker Andesite Breccia is succeeded by flows, tuff layers, and welded ash flows of the Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte, which are preserved as widely separated remnants. The younger volcanic formations give evidence of a more explosive period of volcanism compared with the Walker Andesite Breccia stage. Four hypabyssal stocks intrude the Sierra Blanca Volcanics: Rialto stock, Chaves Mountain stock, Bonito Lake stock, and Three Rivers stock. Three are multiple intrusives. Alkali enrichment is pronounced in one of the stocks. K-Ar dates of the stock range from 34.4 m.y. to 25.8 m.y. The volcanic and intrusive rocks show progressive mineralogical and chemical changes typical of alkali-calcic series. The oldest volcanic rocks contain zoned plagioclase (An 77 to An 47 ) and enstatite, but in successive volcanic rocks plagioclase becomes more sodic (An 32 ) and hornblende becomes the dominant ferromagnesian mineral. FeO(total Fe)/MgO ratios increased from 1.45 to >5 during magma differentiation. Fractional crystallization with removal of calcic plagioclase and enstatite during volcanism appears to have been responsible for differentiation and generation of the late-stage silicic intrusive phases. The large volume of andesitic to trachytic volcanic rocks and intrusive syenitic rocks along with the lack of cogenetic basalt suggest that the parent magma at Sierra Blanca was andesitic. Major structural planes of weakness, which intersect in the Sierra Blanca area, would allow magma generated in the mantle to move into the crust and erupt on the surface. Similar controls appear to have been operational along the Rocky Mountain trend and parallel to the Rio Grande depression. Other andesite breccia sheets present along the trend include the Absaroka volcanic field, Wyoming, the Thirtynine Mile volcanic field, Colorado, and the Red River volcanic-intrusive complex, New Mexico. They are Oligocene in age and have many similar characteristics.