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465 Mineralogy and Petrology of Tertiary-Quaternary Volcanic Rocks in Caribou County, Idaho Donald W. Fiesingerl, William ABSTRACT Basaltic lavas of Caribou County, Idaho, comprise the extensive interconnected Blackfoot, Willow Creek, and Gem Valley lava fields with small valley flows occurring within Enoch, Upper, Wooley, and Slug Creek Valleys. These lavas consist primarily of olivine tholeiite, with minor tholeiite and tholeiitic trachybasalt. The olivine tholeiite is similar to that of the Snake River Plain, as it is olivine and hypersthene normative, does not contain calcium-poor pyroxene, and shows an affinity with alkali olivine basalt on the alkali-silica diagram. The tholeiite and tholeiitic trachybasalt are hypersthene and quartz normative with silica contents ranging from 48 to 53 percent. The trachybasalt is higher in total alkalies, Pros, and Fe/(Fe + Mg) than the tholeiite and has alkali feldspar as a groundmass phase. Flows of plagioclase basalt porphyry occur at Cinder Island within the Blackfoot Reservoir and at Nelson and King Canyons within the Bear River Range east of Gem Valley. This porphyry consists of approximately 30 percent plagioclase phenocrysts, 5 to 20 millimeters in length, set in an aphanitic to opaque groundmass. A potassium-rich alkali trachyte occurs at the southern end of Slug Creek Valley. It consists of olivine and diopsidic augite phenocrysts in a groundmass of alkali feldspar, olivine, and augite, with minor plagioclase, exsolved iron-titanium oxides, and traces of amphibole and biotite. Olivine tholeiite is considered to be the parental magma from which other basaltic lavas have originated. Fractionation of 40 to 60 percent crystals from olivine tholeiite will yield a liquid with the composition of tholeiite or trachybasalt. As removal of groundmass phases is required, filter pressing during late-stage crystallization is the proposed mechanism. ‘Department of Geology, Utah State University, Logan, 94322. *Andover Oil, Tulsa, Oklahoma 74172. ‘Terra Resources, Inc., En&wood, Colorado 80111. Utah D. Perkins*, and Barbara J. Puchy3 In contrast to this, plagioclase basalt porphyry can be derived by the removal of olivine and the accumulation of plagioclase, both common phenocryst phases. Although olivine tholeiite may be derived with IO percent partial melting of pyrolite, equilibrium between the derived olivine tholeiite and the residuum from partial melting cannot be demonstrated. Using mica peridotite to represent a more potassic mantle composition, alkali trachyte may be derived with 30 percent partial melting. These contrasting magmas are considered to be indirect evidence for mantle inhomogeneity or perhaps different mantle structures beneath the Basin and Range province and the Middle Rocky Mountain province in southeastern Idaho. INTRODUCTION Tertiary-Quaternary basaltic lavas in Caribou County, Idaho, comprise the extensive interconnected Blackfoot, Willow Creek, and Gem Valley lava fields near Soda Springs and the small valley flows to the east within Enoch, Upper, Wooley, and Slug Valleys. Rhyolite also occurs in the region, forming three conspicuous domes at the south end of Blackfoot Reservoir and two islands within the reservoir. This paper represents the synthesis of three projects: (1) a study of the mineralogy and petrology of the lavas in the vicinity of Gem Valley (Perkins, 1979), (2) a study of the mineralogy and petrology of the lavas in the Overthrust Belt (Puchy, l981), and (3) a study of basaltic volcanism in the Blackfoot Reservoir region as part of a geothermal evaluation made in cooperation with the Department of Geology and Geophysics at the University of Utah for the U. S. Geological Survey. The objectives of this paper are (a) to present the chemistry and mineralogy of volcanic rocks representing the various lava fields and valley flows in the region, (b) to compare the basaltic lavas of Caribou County with those of the eastern Snake River Plain, 466 Cenozoic and (c) to present a petrogenetic of these volcanic rocks. PREVIOUS Geology Idaho model for the origin WORK A comprehensive study of the geology of southeastern Idaho was undertaken by Mansfield (1927). As part of this study, he classified the lavas in this region as olivine basalt, because they contain olivine and feldspar phenocrysts in a groundmass of augite, feldspar, olivine, and magnetite, with minor apatite and hematite. In a later investigation, Mansfield (1929) also described the lavas in Portneuf Valley, the northern end of the Gem Valley lava field, as olivine basalt, with mineralogy similar to that mentioned above. In an investigation of late Pleistocene stratigraphy in the Gem Valley area, the lavas of the Gem Valley lava field have been described as porphyritic olivine basalt, having phenocrysts of plagioclase, olivine, and augite in a groundmass of plagioclase, olivine, augite, magnetite, and glass (Bright, 1963, 1967). Potassiumargon dates by Armstrong and others (1975) give an age of 0.1 + 0.03 million years for these basal&. Bright (1963, 1967) estimated an age of 33,500-27,000 years for one flow in Gem Valley on the basis of stratigraphic relationships with Lake Thatcher sediments dated by carbon-14 methods. A series of geophysical investigations have dealt with the origin, structure, and prebasalt topography of the Blackfoot and Gem Valley lava fields (Mabey and Armstrong, 1962; Mitchell and others, 1965; Oriel and others, 1965; Mabey and Oriel, 1970; Mabey, 1971). The geologic map of the Soda Springs quadrangle (Armstrong, 1969) and the preliminary geologic map of the Bancroft quadrangle (Oriel, 1968) delineate the extent of the basalt flows and the locations of numerous eruptive centers. The small valley flows to the east of the Blackfoot and Willow Creek lava fields have been reported by Mansfield (1927). The vesicular lava in Slug Valley has been briefly described by Cressman (1964) as olivine basalt no younger than Pliocene in age. The distribution of lavas in Caribou County is shown on the generalized composite geologic maps of Mabey and Oriel (1970) and Oriel and Platt (1980). These maps incorporate the work of Mansfield (1927, 1929), Armstrong (1969), Oriel(1968), and Cressman (1964). GEOLOGIC of SETTlNG Caribou County lies in both the northeastern part of the Basin and Range province and the western part of the Middle Rocky Mountain province (Figure 1). r / I IDAHO _,_.-.-. Figure I. Index map of southeastern County, Idaho, the Snake River Range and Middle Rocky Mountains - -.A Idaho, showing Caribou Plain, and the Basin and physiographic provinces. In general, the western portion of the county is dominated by fault block mountains composed of Paleozoic sedimentary rocks locally overlain by Tertiary sediments. The eastern portion of the county is part of the Overthrust Belt dominated by upper Paleozoic and Mesozoic sedimentary rocks involved in a westward-dipping imbricate thrust zone (Armstrong and Cressman, 1963). The distribution of lavas within Caribou County is shown in Figure 2. In parts of the Blackfoot, Willow Creek, and Gem Valley lava fields, the flows are well-exposed along the extensive northwest-southeast scarp system which runs through the area. The scarp trends indicated by Armstrong (1969) for the Soda Springs area have been observed to continue more than 10 miles to the north, beyond the Blackfoot Reservoir. Individual scarps may be traced visibly for many miles, and where covered by loess, their presence can be inferred from topography and alignment of sink holes. The scarp system is best exposed and most continuous immediately adjacent to and south of the Blackfoot Reservoir. A study of aerial photographs and ground checks undertaken as part of the present study indicates only minor deviation in the lateral extent of the basaltic lavas from that reported by Mansfield (1927, 1929), Oriel (1968), and Armstrong (1969). The occurrence of basaltic cinder and cinder cones, however, is more widespread than revealed in earlier studies. In the area south of Henry and southeast of the Blackfoot Reservoir, numerous cinder pits have been dug and at least six conspicuous topographic depressions in this area are probably cinder cones masked by loess. Throughout the entire region, loess overlies most Fiesinger and others-Volcanic flows and cinder cones. The western part of the Blackfoot lava field near Tenmile Pass has a thick covering of loess with few cinder cones and few scarps visible. The alignment of sinks through the loess suggests the presence of scarps at depth, which parallel the northwest-southeast trend observable to the east. Similarly, in the eastern part of the Blackfoot lava field, south of Henry and east of the Blackfoot River, volcanic features are subdued. This area lies approximately 300 feet higher in elevation than the remainder of the Blackfoot lava field and has a thick covering of loess. Scarps are not well-exposed, show little relief, and are discontinuous, This area corresponds with negative magnetic anomalies interpreted by Mabey and Oriel (1970) to represent reversely magnetized basalt that is older than the flows to the southwest. Mabey and Oriel (1970) also observed negative magnetic anomalies in Lower, Wooley, and Upper Valleys and at the southern end of Enoch Valley, which were interpreted to result from reversely magnetized basalt. Positive anomalies attributed to normally magnetized basalt were reported for the northern end of Enoch Valley and the western side of the Willow Creek lava field. Since there have been twenty or more reversals in the Earth’s magnetic field since Eocene time (Doell and Cox, 1962), specific ages of these lavas cannot be determined from magnetic polarity, but the presence of both normal and reverse magnetization shows that the flows are not all equivalent in age. The reversely magnetized flows would have to have an age greater than 0.7 million years (Mabey and Oriel, 1970). The dominant northwest-southeast scarp system is somewhat coincident with block fault patterns recognized within adjacent mountain ranges. This fault pattern probably originated in the Tertiary, and this style of faulting has continued to the present (Armstrong and Oriel, 1965). The occurrence of isolated eruptive centers in the Bear River Range, Soda Springs Hills, Reservoir Mountain, and Pelican Ridge and the alignment of cinder cones in Gem Valley also indicate an association between volcanism and faulting (Mansfield, 1927; Armstrong, 1969). Field evidence indicates continued faulting throughout the period of volcanic activity in the region. Age relationships between volcanism and scarp formation are generally not clear and can be established only locally. Cinder cones are commonly located on scarps, with no recognizable offset, indicating eruption along older, inactive scarps. Similarly, scarps project through (under?) the rhyolite domes south of Blackfoot Reservoir, but only the southwestern portion of China Hat shows evidence of possible offset by faulting. Other cinder cones have been truncated by scarps indicating post-eruption displacement. Ex- Rocks in Caribou County 467 amples include the cinder cones south and southwest of China Hat, the cinder cone on the eastern side of Reservoir Mountain, and cinder cones east of the Blackfoot Reservoir. Field evidence reveals that basaltic volcanism took longer than the emplacement of the rhyolite domes. Three small lobes of basalt occur on the lower slopes of Middle Cone, on the northwestern and southeastern sides. Associated with the basaltic lobe to the southeast is flow-banded, pumiceous rhyohte containing blebs of basalt (3 to 5 centimeters in diameter) and larger xenoliths of basalt and white quartzite up to 20 centimeters in length. Three basaltic xenoliths were examined petrographically. Mineralogies and textures are somewhat typical of fine-grained basaltic lavas from the region. Unusual features include the presence of amphibole and alkali feldspar(?) as groundmass phases in one, and the presence of quartz and plagioclase xenocrysts and rounded inclusions of vesicular brown glass with trichites in the others. Armstrong and others (1975) have dated the rhyolites of China Hat and Middle Cone at 0.1 million years, whereas Leeman and Gettings (1977) obtained dates of about 0.05 million years. The rhyolite of Sheep Island, within Blackfoot Reservoir, has been dated at 1.4 million years (S. H. Evans, University of Utah, personal communication). SAMPLING ANALYTICAL AND PROCEDURES Samples were taken from basaltic flow units and eruptive centers representing the various lava fields and valley flows within the region. After a prehminary petrographic study of more than sixty-five specimens, twenty-six samples were selected for further study. Samples selected are relatively free of oxidation, alteration, and vesiculation and represent diversity as suggested by their textures, mineralogies, and occurrences. Sample locations are shown in Figure 2; brief descriptions are presented in Table I. Whole rock chemical analyses were obtained using volumetric, gravimetric, and calorimetric methods described by Carmichael and others (1968). Mineral analyses were obtained using the ARL EMX electron probe micro-analyzer at the University of Utah, following standard procedures for silicate and oxide analysis. Phenocryst phases were analyzed systematically from core to rim, with an average of five spot analyses obtained from each of ten randomly selected phenocrysts or microphenocrysts. Groundmass phases were analyzed by taking two to three spot analyses from each of twenty to twenty-five randomly selected mineral grains. Correction procedures used on microprobe data follow the methods of Bence and Albee Cenozoic 468 Geology of Idaho more exact identification. Most lavas tend to be nonporphyritic to microporphyritic with typical basaltic mineralogy and textures. The nonporphyritic lavas generally have less than 5 percent phenocrysts; the microporphyritic lavas generally have more than IO percent phenocrysts. Phenocryst assemblages consist of plagioclase and olivine, commonly clustered in a glomeroporphyritic texture. The olivine phenocrysts are euhedral to subhedral, average 0.6 to 1.0 millimeter in diameter, and commonly have euhedral crystals of magnetite attached. Skeletal olivine crystals are found in some lavas. The plagioclase phenotrysts average 1.5 millimeters in length, and are commonly twinned according to the albite, Carlsbad, and Carlsbad-albite twin laws. Plagioclase and olivine commonly exhibit a continuous size gradation between phenocrysts and groundmass in most lavas. A variety of groundmass textures have been observed in the basaltic lavas including ophitic, subophitic, intergranular, and hyaloophitic. Flow-banded lavas show subparallel alignment of plagioclase phenotrysts or groundmass laths in a pilotaxitic texture. The groundmass assemblage consists of plagioclase, olivine, augite, iron-titanium oxides, and apatite. (1968) and Albee and Ray (1970) and were made using the computer program of Nicholls and others (1977). In all samples where iron was analyzed by microprobe, total iron is reported as FeO. Modal analysis consisted of mineral determination at approximately 1,600 points covering 2 to 3 square centimeters using a spot interval of 0.1 millimeter. PETROGRAPHY Colors and textures of lavas within the region are variable and range from gray to black for the denser lavas and gray to black to dark red for the more vesicular lavas. Samples collected from cinder cones tend to have distinctive flow banding and are commonly heterogeneous in mineral distribution, grain size, and grain orientation, whereas samples collected from scarps and thick flow units tend to be homogeneous in texture. Modal analyses of the twenty-six analyzed lavas are presented in Table 2. Undifferentiated groundmass consists of birefringent material too small for -1 \ , / ,/ /’ : I i ‘, , i ‘1 C’ 8’ I\.\ I I’ \/ Figure 2. Map of Caribou County, Idaho, showing the general Enoch Valley and Wooley Valley lie east of Henry Upper distribution Valley lies of volcanic rock. south of Wayan. The Willow Creek Sample locations lava field lies north of Henry; are indicated by open circles. Fiesinger and others- Volcanic Plagioclase commonly dominates the groundmass, with laths being 0. I to 0.2 millimeter in length, and albite twinning being ubiquitous. Groundmass olivine occurs as equant anhedral grains, ranging from 0.1 to 0.5 millimeter in diameter. Augite is commonly in an ophitic to subophitic relationship with plagioclase. Single, optically continuous augite crystals greater than 2 millimeters in length have been recognized. The iron-titanium oxides generally average less than 0.3 millimeter in maximum dimension. Magnetite grains are characteristically equant, commonly occurring as euhedral cubic crystals, whereas the ilmenite grains are typically elongate or needlelike. In addition to these groundmass constituents, two samples (6 and 12) contain diffuse laths of alkali feldspar and have been classified as trachybasalt. Interstitial glass, containing abundant microlites, Table I. Locations of analyred samples from lavas of Caribou County, Rocks in Caribou County 469 is present in many lavas. Glass is the major constituent within the pillow lavas of Gem Valley (sample 5). The outer margins of these pillow structures are similar to those described by Moore (1970) for Hawaiian submarine eruptions. They contain an outer palebrown layer of sideromelane that becomes progressively opaque with an increase in microlites going inward from the rim. In contrast to these lavas, a distinctive basalt porphyry forms Cinder Island, within the Blackfoot Reservoir (sample 24), and the flows in Nelson and King Canyons within the Bear River Range, east of Gem Valley (samples 22 and 23). This porphyry consists of approximately 30 percent plagioclase phenocrysts, 5 to 20 millimeters in length, in an aphanitic to opaque groundmass. These phenocrysts are commonly fractured or fragmented, suggesting Idaho. 470 Cenozoic Geology that they were broken during emplacement. Twinning is similar to that described above for microphenotrysts, and extinction patterns indicate normal concentric zoning. There is no evidence of resorption. The lava at the southern end of Slug Valley (samples 25 and 26) has been previously described as ohvine basalt (Mansfield, 1927; Cressman, 1964) and it does have the appearance of basalt in hand specimen. In thin section, this lava is found to contain olivine and augite phenocrysts, up to 2 millimeters in Table 2. Modal analyses of laws from Caribou County, Idaho (volume of Idaho diameter, in a groundmass of alkali feldspar, olivine, and augite, with minor plagioclase, exsolved irontitanium oxides, and traces of amphibole and biotite. Petrographically, this lava is similar to a fine-grained shonkinite (Williams and others, 1954) and has tentatively been classified as alkali trachyte (Streckeisen, 1979). Xenocrysts of sodic plagioclase and quartz occur in many of the lavas and are commonly found throughout the Blackfoot lava field. The largest percent) Fiesinger and others- Volcanic plagioclase xenocryst, measuring 1 by 2 by 4 centimeters, was found in the lavas of Sheep Island in the Blackfoot Reservoir. Xenocrysts are more commonly less than 5 millimeters in maximum dimension. Quartz xenocrysts tend to be rounded and have finegrained reaction rims of augite. Plagioclase xenotrysts show resorption features such as embayments, honey-combed interiors, complex zoning, rounded to irregular outlines, and replacement by iron-titanium oxides. Xenocrysts of hypersthene, up to 3 millimeters in diameter and with compositions of Enso to En,,,, have been found in lavas from Gem Valley and Upper Valley. Xenoliths of silt are present in the lavas of Gem Valley and are probably derived from the sediments of Lake Thatcher (Bright, 1963). MINERALOGY FELDSPARS Plagioclase is the only feldspar within most samples of nonporphyritic and microporphyritic basalt, pillow basalt, and basalt porphyry. It coexists with alkali feldspar as groundmass in trachybasalt, and it is subordinate to alkali feldspar as groundmass in alkali trachyte. In general, zoning in plagioclase phenocrysts tends to be normal, from calcic cores to sodic rims. Groundmass plagioclase is less calcic than coexisting phenocrysts and is zoned to more sodic compositions. Representative partial analyses of plagioclase phenocrysts and groundmass are presented in Table 3; zoning trends are shown in Figure 3. Compositions of plagioclase phenocrysts and groundmass in basaltic lavas lie between Anso and An,5 with the average compositions of both within the labradorite field, AnTo to Am,,. Pillow basalt (sample 5) has the most calcic plagioclase phenocrysts, with core compositions near Anso (Figure 3b). Plagioclase is generally more calcic in nonporphyritic basalt than in microporphyritic basalt. The most extensive zoning is found in a basalt with diabasic texture from Gem Valley (Figure 3a). The large phenocrysts in the plagioclase basalt porphyry (samples 22, 23, and 24) show limited normal zoning from An-i,, to Anso. Sample 8 contains numerous plagioclase xenotrysts of more sodic composition than that of the coexisting phenocrysts (Figure 3~). The phenocryst zoning trend for this sample is more sodic than that of the coexisting groundmass suggesting that the phenocryst trend may include some xenocrysts approaching equilibration with the magma. Sample 18 is unusual in that the zoning trends of the coexisting phenocrysts and groundmass do not overlap, suggesting that the phenocrysts are out of equilibrium with Rocks in Caribou County 471 the magma (Figure 3i). Samples of trachybasalt (samples 6 and 12) have average plagioclase compositions that are more sodic than those of other basaltic lavas. The plagioclase is more extensively zoned in sample 6 than in sample 12. OLIVINE Olivine is present as both phenocrysts and groundmass in most samples of basaltic lava and alkali trachyte. It is restricted to the groundmass in trachybasalt (samples 6 and 12). The extent of zoning in both phenocrysts and groundmass is illustrated in Figure 4; average compositions are presented in Table 4. In general, zoning in olivine is normal, from forsteritic cores to fayalitic rims. In the basaltic lavas, phenocryst zoning generally overlaps that of the groundmass, with groundmass compositions being more iron-rich. Compositions of phenocrysts range from FOKJ to Fo40; coexisting olivines in the groundmass range in composition from Fo75 to Fo25. Some lavas show limited compositional variation, such as the pillow lava (sample 5), reflecting rapid extrusion and cooling. In four lavas, samples 2, 9, 17, and 19, phenocryst and groundmass compositions do not overlap, suggesting that the phenocrysts were out of equilibrium with the magma. Groundmass olivine in trachybasalt is more iron-rich than that in other basaltic lavas, with compositions ranging from Fo4s to Fo,o. The olivine phenocrysts in alkali trachyte are more magnesium-rich than those of most basaltic lavas, with compositions ranging from Foss to Fo72. AUGITE Compositions and zoning trends are illustrated in Figure 5; representative analyses are presented in Table 5. Augite is present in the groundmass of all basaltic lavas except the pillow basalt (sample 5). Zoning is more extensive and compositions more iron-rich in the microporphyritic lavas (Figure 5g) than in the nonporphyritic ones (Figure 5a-d). Compositions tend to be more calcic than the tholeiitic Skaergaard trend (Brown and Vincent, 1963), consistent with the lack of a coexisting calcium-poor pyroxene. Augite occurs as both phenocrysts and groundmass in the samples of alkali trachyte (samples 25 and 26) where it is noticeably diopsidic and zoned toward hedenbergite (Figure 5e, f). The two samples (6 and 12) of trachybasalt show distinct augite composition fields suggesting differentiation (Figure 5~). Sample 3 is diabasic, has cooled at a relatively slow rate, and shows a strong trend toward hedenbergite (Figure 5a). Sample 16 is aphanitic and 472 Cenozoic Geology hypocrystalline, has cooled rapidly, and shows a calcium-enrichment trend (Figure 5d). This unusual zoning of sample 16 is similar to that observed by Leeman and Vitaliano (1976) in some augites of the McKinney Basalt and by Smith and Lindsley (1971) in a flow from the Picture Gorge Basalt. Smith and Lindsley (197 I) proposed that this “quench trend” of calcium-iron substitution reflects a metastable crystalliquid partitioning during rapid crystallization. IRON-TITANIUM OXIDES Coexisting magnetite and ilmenite occur as groundmass phases in all lavas studied. Representative analyses are presented in Tables 6 and 7. Analyses were not made where these oxide phases were found of Idaho to be exsolved, oxidized, or extremely small and disseminated. Two generations of oxides are present in sample 19. The magnetite microphenocrysts (P) are noticeably enriched in CrzO,, AbOr, and MgO and depleted in CaO relative to the magnetite groundmass (G). The major difference in the coexisting ilmenites is the higher MgO content and lower CaO content of the microphenocrysts. These trends are consistent with those reported by Carmichael and others (1974) for fractionating tholeiitic magmas. Magnetite within the basalt porphyry (sample 24) is noticeably enriched in A1203 and MgO, containing 3.55 percent Al203 and 2.64 percent MgO compared with a range of 1.8 to 2.4 percent A1203 and a range of 0.5 to 2.4 percent MgO in magnetites of the other basaltic lavas studied. These values of Al203 and MgO tend to straddle the typical values reported for titanomagnetites of basic Figure 3. Zoning trends of feldspars in lavas of Caribou County, Idaho (molecular percent An, Ab, Or). Open circles indicate average phenocryst compositions; filled circles indicate average groundmass compositions. For normative feldspars, filled circles indicate olivine tholeiites; open circles indicate tholeiite, trachybasalt, and alkali trachyte. A-G q nonporphyntic basalt; H and I q microporphyritic basalt; and J-L = plagioclase basalt porphyry. Ab Fiesinger and others- Volcanic lavas with low silica activity, and tholeiitic lavas (Carmichael and others, 1974). Compositions of the magnetite-ulvospinel solid solution range from 30 to 54 molecular percent magnetite whereas compositions of the coexisting hematite-ilmenite solid solution range from 88 to 96 molecular percent ilmenite. PETROLOGY CHEMISTRY AND CLASSIFICATION Chemical analyses and CIPW norms of lavas analyzed for this study are presented in Table 8. The silica contents range from 45 to 53 percent for basaltic lavas and from 53 to 56 percent for alkali trachyte. Using the normative classification of Yoder and Tilley (1962), the basaltic lavas are saturated to oversaturated with silica; they contain normative Rocks in Caribou Counry 473 hypersthene with olivine or quartz and are classified as olivine tholeiite and tholeiite respectively. It should be noted that no calcium-poor pyroxene is present in the groundmass of any of these lavas, nor is there any evidence of olivine reaction to form a calcium-poor pyroxene. The occurrence of normative quartz in samples 4, 22, and 24, and the low amount of normative olivine in sample 17 (less than 1.0 percent) is attributed to oxidation of iron. These four samples have FeO/(FeO + Fe203) ratios ranging from 0.36 to 0.60 compared with a range of 0.73 to 0.87 for all other basaltic lavas. The high silica content and normative quartz content of sample 18 is attributed to the assimilation of pumiceous rhyolite from Middle Cone or crustal rock. Trachybasalt (samples 6 and 12) and basalt of Upper Valley (samples 15 and 16) contain normative hypersthene and quartz and are classified as tholeiitic. The designation of most basaltic lavas as olivine 474 Cenozoic Geology tholeiite, using normative rather than mineralogical criteria, is consistent with other studies of lavas from the Snake River Plain, such as Tilley and Thompson (1970), Thompson (1975), Leeman and Manton (1971), and Leeman and Vitaliano (1976). Characteristics of the Snake River Plain olivine tholeiite include MgO between 6.4 and 11.2 percent and phenocryst of olivine * plagioclase set in a groundmass of olivine, plagioclase, augite, and irontitanium oxides (Thompson, 1975). The association between the various volcanic rocks, and the change in normative constituents as a function of ferrous-ferric ratio, are illustrated on the normative olivine-diopside-quartz (Ol-Di-Qtz) diagram (Figure 6). The indicator ratio (I.R.) of Coombs (1963) may be calculated from normative constituents ((Hy + 2Qz)/Hy + 2(Qz + Di)] or determined from this diagram by projecting a line from the 01 apex, through the sample data point, to the Di-Qtz join. There is an apparent clustering of samples of nonporphyritic olivine tholeiite (filled circles) with I.R.‘s between 0.0 and 0.3, in the field of mildly alkaline and transitional basalt. This is comparable with the I.R.‘s reported for Snake River Plain basaltic lavas (Stout and Nicholls, 1977). The differentiation index (D.I.) of Thorton and Tuttle (1960), defined as the sum of normative quartz, orthoclase, albite, nepheline, leucite, and kaliophilite, ranges from 26 to 31 for samples of nonporphyritic olivine tholeiite (Table 8). This range is also comparable with that of Snake River Plain basaltic lavas (Stout and Nicholls, 1977). Samples designated as tholeiitic have D.I.‘s ranging from 36 to 44, whereas alkali trachyte samples have D.l.‘s greater than 48. The alkali-silica variation diagram has been used by Macdonald and Katsura (1964) in their study of Hawaiian lavas to distinguish alkali-olivine basalt from tholeiitic basalt. Figure 7 shows that most lavas have an alkali-olivine basalt affinity, with only three samples in or near the tholeiite field. These three samples have been discussed previously, with samples 15 and 16 being classified as tholeiitic and sample 18 containing xenocrysts of alkali feldspar and quartz. The AFM diagram (Figure 8) indicates the relationships between the lavas of Caribou County, the Snake River Plain olivine tholeiites, and the lavas from Craters of the Moon. In general, the samples of nonporphyritic olivine tholeiite from Caribou County (filled circles) lie within or near the field of Snake River Plain olivine tholeiites. The three samples of plagioclase basalt porphyry (22, 23, and 24; open circles with horizontal bars) and two samples of tholeiite (I5 and 16; filled circles with vertical bars) are more alkali-rich than the other olivine tholeiites. The alkali and iron enrichment of trachybasalt (samples 6 and 12; filled circles with horizontal bars) is of Idaho also apparent, as these two samples plot close to the Craters of the Moon field. Characteristics of the Craters of the Moon-type lavas include high Ti02, FeO, P205. and total alkalies (Leeman and others. 1976). Samples 6 and 12 are noticeably higher in PzOS, total alkalies, and Fe/(Fe + Mg) than the other analyzed samples, but they contain alkali feldspar in the groundmass which has not been reported previously for basaltic lavas of the Snake River Plain, and they contain normative quartz. In addition, the groundmass of these lavas is hypocrystalline rather than glassy to opaque as reported Nonporphyritic Basalt UI I 2 I -m I 3 I I . 1 4 5 . I 7 I l 1 I 8 9 , 1 - -m I I= IO . I I- II 0 . I I3 I . I I .I ml 6 - I2 I 14 ., 1 IS 1 L I6 Mieroporphyrilic I Barolt I7 -m 16 I I l I I9 . I -ml L 20 . I 21 Plagioclorr Bosoll t 1 - . I . Porphyry 22 - 23 24 I Alkali Trachyte 25 w 26 1 I Fo I 20 Figure 1 I I I I 60 olivines in lavas of 1 I 80 I I FO Caribou County, circles above horizontal bars indicate average phenocryst compositions; filled circles below horizontal bars indicate average groundmass compositions. Idaho 4. Zoning trends of (molecular percenr I I I 40 Fo, Fa). Filled Fiesinger and others- Volcanic Rocks for the ferrobasalt of Leeman and others (1976), and the high-iron lavas of Stout and Nicholls (1977). The two samples of alkali trachyte (25 and 26; open diamonds) lie completely away from the basaltic lavas indicating the lack of a petrogenetic relationship. The scatter of the remaining samples (18, 19, and 8; open circles) is attributed to the occurrence of quartz and alkali feldspar xenocrysts. Similar relationships are also indicated on the iron enrichment diagram (Figure 9). There is a cluster of data points for nonporphyritic olivine tholeiite (filled circles), and a distinct separation of tholeiite (filled circles with vertical bars), trachybasalt (filled circles with horizontal bars), plagioclase basalt porphyry (open circles with horizontal bars), and alkali trachyte (open diamonds). The microporphyritic lavas (open circles) which contain xenocrysts of quartz and alkali feldspar show considerable scatter. The uniqueness of alkali trachyte is indicated as it lies well away from any iron enrichment trend of the basaltic lavas. Table 4. Representative Sample average Weight Fe0 Number microprobe Per+ analyses of olivines Oxides in Caribou Counfy 475 GEOTHERMOMETRY Coexisting magnetite-ulvospinel and ilmenite-hematite solid solutions in sixteen samples representing tholeiitic lavas have been analyzed permitting the application of the geothermometer of Buddington and Lindsley (1964). Microprobe analyses were recalculated using the method of Carmichael (1967a). and the temperatures and fugacities were calculated using the equations derived by Powell and Powell (1977). Results of these calculations are presented in Table 9 and Figure 10. The temperatures of equilibration and corresponding oxygen fugacities plot close to the fayalite-magnetite-quartz buffer curve, indicating internal buffering of oxygen by the crystal-liquid assemblage (Carmichael and Nicholls, 1967). Temperatures range from a high of 1lOl’C to a low of 805” C with a corresponding range in oxygen fugacity (-logfoz) of 8.6 to 15.0. Sample 19, containing both microphenocryst and groundmass oxides, shows a in lavas from Weigbt Percent Fe Caribou County, End ?&mt& La MgO CIO Fa 27.1 41.6 22.3 27.7 43.8 22.3 39.0 35.0 23.0 38.6 34.0 19.7 38.6 24‘7 0.39 0.49 0.42 0.55 0.69 0.73 0.87 38,d 58.9 31.6 39.3 65.0 31.9 53.3 61.1 40.2 67.4 59.4 34.3 67.4 43. I 0.6 0.8 0.1 1‘1. 1.1 1.3 100.4 52.2 31.4 44.7 14.7 26.7 20.9 0.66 0.71 0.69 65.9 53.0 63.4 33.1 46.7 36,d 1.1 1.1 1.1 36.5 27.8 36.5 20.1 ii.38 0.50 0.67 0.71 33.4 51.2 36.0 63.6 63.8 48.5 63.8 35.0 28.7 30.4 42.8 25.8 33.6 31.8 21.9 0.26 0.39 0.78 0.71 53.4 40.7 43. I 60.6 17.5 22.6 42.8 38.3 0.50 0.43 24.9 32.0 Idaho. Motecutar FTi wTQtd Percent F0 End Members La Nonporphytitic Basalt 3(P) 3(G) W) 5(G) 6(G) W) w3 12(G) IS(P) 13(G) lo%1 99.9 99.1 99.5 100.4 99.1 30.3 50.0 24.3 31,2 56.0 24.5 d6.4 69. I 49.2 75.1 68.0 42.9 1d.S 52.3 0.6 0.8 0.6 0.8 1.1 1.0 1.3 1001 100.8 100.9 66.0 43.6 54.1 33.0 33.4 44.8 1.1 I.0 I.1 0.6. 0.8 1.0 I.1 99.8 100.3 100.8 99.7 21.6 41.8 27.9 55.0 71.8 57.4 71.1 0.5 0.8 0.9 43.9 1.1 45.0 $8.6 55.3 38.2 0.4 0.6 1.2 1.1 100.8 99.9 99.8 99.9 43,8 32‘2 34.5 31.7 53.8 67.2 64.4 47.2 0.4 0.6 1.1 1.1 74.7 66.8 0.8 0.7 100.4 99.5 18.6 24.7 80.7 74.7 0.7 0.6 0‘8 -. Microporphyritic Basalt 17(P) 17(G) 18(P) 18(G) 25.0 36.1 23.4 44.8 PlagioclaseBasalt Porphyry 2X’? 39. I 23(G) 24(p) 24~~1 Alkali Trachyte 25~) I 2W’) (P) = phenocryst (G) = groundmass I I 476 Cenozoic Geology decrease in temperature and oxygen fugacity going from microphenocrysts to groundmass as is expected. Reported iron-titanium oxide equilibration temperatures for Snake River Plain olivine tholeiites and Craters of the Moon high-iron lavas range from 1095” C to 915” C, and corresponding oxygen fugacities (-log fo2) range from 9.6 to 12.8 (Stout and Nicholls, 1977). The presence of groundmass olivine and augite permits the application of a second geothermometer, that of Powell and Powell (1974). Calculated temperatures, using average olivine and augite groundmass compositions and an assumed pressure of one bar, range from lOl3O C to 993” C (Table 9). The apparent poor correlation between the olivine-clinopyroxene geothermometer and the iron-titanium oxide geothermometer reflects the uncertainty of both methods (* 30”) and the use of average compositions for zoned groundmass phases. In addition, the uncertainty of the olivine-clinopyroxene geothermometer increases with increasing pressure, 5” * 5’ per kilobar (Powell and Powell, 1974). of Idaho PETROGENESIS As previously indicated in the discussion of chemistry and classification, the analyzed lavas from Caribou County do not comprise a coherent population. To evaluate the presence of a liquid line of descent, lavas to be considered should be fine grained and nonporphyritic. An assumption is made that these lavas represent samples from an evolving magmatic system. Because of the chemical similarity in the samples of nonporphyritic olivine tholeiite collected from diverse parts of Caribou County, their low silica content, and high MgO content, these lavas most likely approximate a parental magma (Figure 1 I). Processes of assimilation and low-pressure fractionation can then be evaluated using whole-rock chemical analyses from Table 8, mineral analyses from Tables 3,4, 5,6, and 7, and the interactive computer program and procedures outlined by Stormer and Nicholls (1978). The quality of fit for each test is expressed as the sum of squares of residuals, where the residual of each component oxide is obtained by subtracting the n .Hd n 20 19 Y!Y- En Figure 5. Zoning trends of augiles in laws of Caribou County, zoning within individual laws; small filled circles indicate intrusion (Brown and Vincent, 1963). A-D q nonporphyritic 18 VFS Idaho (molecular percent Wo, En, Fs). Irregular oval fields indicate extent of average compositions. Curve is zoning trend of augites from the Skaergaard basalt; G = microporphyritic basalt: and E and F = alkali trachyte. Fiesinger and others- Volcanic calculated difference between parent and daughter from the observed difference between parent and daughter. The smaller the value of the sum of squares term, the better the fit; tests with values less than one are given further consideration. Acceptability as a petrologic model depends on consistency between the calculated amounts of fractionated/assimilated phases and petrographic observations. Representative fractionation schemes are presented in Table 10. Tholeiite and Trachybasalt Models tested tc determine the relationship between parental olivine tholeiite and daughter products, tholeiite and trachybasalt, consisted of the fractionation of olivine, plagioclase, augite, and irontitanium oxides and the assimilation of rhyolite (unpublished analyses from W. P. Nash, University Table 5. Representative (P) = phenocryst (C) = groundmass average microprobe analyses (weight percent) Rocks in Caribou County 477 of Utah). As shown in Table 10, tholeiite, represented by sample 16, and trachybasalt, represented by sample 6, can be derived from olivine tholeiite (sample 10) by fractionating olivine, plagioclase, augite, and irontitanium oxides. The transition from tholeiite sample 15 to tholeiite sample 16 can also be accounted for by fractionation of these same phases. The transition from trachybasalt sample 6 to trachybasalt sample 12 requires the fractionation of feldspar, olivine, augite, and ilmenite and the addition of a small amount of magnetite. Similar testing with rhyolite included as an additional phase made no significant difference in residuals or in the amounts of other phases, showing that assimilation was not a significant factor in the formation of these lavas. Most fractionation models require the removal of all analyzed phases found within these basaltic lavas, rather than just the early formed microphenocrysts, of au&es in laws from Caribou County, Idaho. 478 Cenozoic Geology of Idaho Fiesinger 01 and others- Volcanic in Caribou Counrj 479 011 HY Figure 6. Normative olivine-diopside-quart7 (01.Di-Qtz) diagram for laws of Caribou County, Idaho. Symbols used: filled circles-nonporphyritic basalt; open circles-microporphyritic basalt; open circles with horizontal bars-plagioclase basalt porphyry; filled circles with vertical bars-tholeiite; filled circles with horizontal bars-trachybasalt; open diamondsalkali trachyte; lines with arrows indicate changes in normative constituents with increase in ferrous-ferric ratio. Indicator ratio of Coombs (1963) is shown on the Di-Qtz join. 46 Rocks Figure 8. AFM diagram for laws of Caribou County, Idaho. A = weight percent K20 + NaO; F = weight percent Fe0 + FeO,; M = weight percent MgO. Symbols used: filled circles-nonporphyritic basalt; open circles-microporphyritic basalt; filled circles with vertical bars-tholeilte; filled circles with horizontal bars-trachybasalt; open circles with horizontal bars-plagioclase basalt porphyry; open diamonds-alkali trachyte. SRP indicates distribution of Snake River Plain olivine tholeiites and COM indicates distribution of Craters of the Moon laws (Leeman and others, 1976). 52 Figure 7. Alkali-silica diagram for lavas of Caribou County, Idaho. Symbols used: filled circles-nonporphyritic basalt: open circles-microporphyritic basalt; filled circles with vertical bars-tholeiite; filled circles with horizontal bars-trachybasalt; open circles with horizontal bars-plagioclase basalt porphyry. Diagonal line is alkali olivine basalt-tholeiite boundary of Macdonald and Katsura (1964). Figure 9. Iron enrichment diagram for laws of Caribou County, Idaho. Symbols used: filled circles-nonporphyritic basalt; open circles-microporphyritic basalt; filled circles with vertical bars-tholeiite; filled circles with horizontal bars-trachybasalt; open circles with horizontal bars-plagioclase basalt porphyry; open diamonds-alkali trachyte. 480 Cenozoic Geology olivine with magnetite, and plagioclase, as observed in thin section. As mentioned previously, augite does not occur as distinctive phenocrysts, but rather as an ophitic to subophitic groundmass phase. This suggests that a process such as filter pressing may have been in operation, rather than gravity settling or flowage differentiation. This process would drive off interstitial liquid, leaving a crystal mush of microphenocrysts and groundmass crystallization products. When deriving the tholeiite and trachybasalt, Table 8. Whole-rock ‘Total includes chemical 4.95 percent analyses normative (weight hematite percent) and CIPW of Idaho estimated amounts of fractionated crystals range from 40 to 60 percent; that is, 40 to 60 percent of the parent magma would have crystallized at the time of separation of the daughter lavas. This is not unlike the 65 to 70 percent crystallization required to derive the iron-rich Craters of the Moon lavas of Stout and Nicholls (1977). The fractionation of olivine and plagioclase, the microphenocrysts commonly found in olivine tholeiite, has been proposed by Leeman and Vitaliano norms of lava from Caribou County, Idaho Fiesinger and others- Volcanic (1976) to account for the variation in composition of the McKinney Basalt, and also by Tilley and Thompson (1970) to account for the variation in their suite of Snake River Plain lavas. Similarly, olivine and plagioclase fractionation can account for the variation within the samples of olivine tholeiite from Caribou County. But there is still the possibility that the slight scatter of data for olivine tholeiite may result from the presence of more than one liquid line of descent or the presence of more than one parental magma. Age relationships have not been sufficiently Table 8. continued. Rocks in Caribou County 481 established to conclude that all samples of olivine tholeiite in Caribou County are contemporaneous, and magnetic data discussed previously certainly indicates otherwise. Contaminated Lavas The presence of numerous xenocrysts of quartz and feldspar, possibly derived from the incorporation of rhyolite, together with differentiation, may account for the compositional variation among lava samples 482 Cenozoic Geology 18, 19, and 8. Sample 18, for example, can be accounted for by adding approximately 25 percent rhyolite to parental olivine tholeiite (sample 7). Sample 19 can be derived by fractionating a small amount of olivine and magnetite from olivine tholeiite (sample 9) and adding approximately 9 percent rhyolite (Table IO). Sample 8, in turn, can be derived by fractionating olivine, augite, plagioclase, and irontitanium oxides and adding less than 5 percent rhyolite (Table 10). Sample 18, however, is the only lava close to rhyolite, but the assimilation of 2.5 percent rhyolite in the near-surface environment leaving less than I percent xenocrysts is probably unrealistic. Alternatively, the incorporation of quartzofeldspathic crustal rocks at depth would have to be considered for all three samples, but this process cannot be evaluated at this time. Plagioclase Basalt Porphyry The texture of plagioclase basalt porphyry, represented by samples 22, 23, and 24, indicates the accumulation of plagioclase phenocrysts. Using olivine tholeiite (sample 9) as the parent, plagioclase basalt porphyry (sample 24) can be derived by fractionating olivine, augite, and less than I percent iron-titanium oxides and adding I5 percent plagioclase (Table IO). The accumulation of plagioclase in the plagioclase basalt porphyry may result from processes such as flowage differentiation, bubble transport, or the of ldaho buoyancy of plagioclase in a more dense magma. Both olivine and plagioclase occur as dominant phenocrysts in microporphyritic lavas in the region, but only plagioclase is selectively concentrated in these rocks suggesting that bubble transport or density contrast may be significant. Densities of olivine tholeiite were calculated over a range of temperature from 1000°C to 1200°C using the procedure of Bottinga and Weill (1970). Densities of plagioclase over this same temperature interval were determined for compositions Anso and AnTo using data presented in Campbell and others (1978). This range in composition encompasses that of the average plagioclase in the three analyzed samples of plagioclase basalt porphyry. Calculations for anhydrous conditions yield liquid densities consistently greater than that of plagioclase. For calculations with water contents greater than I molecular percent, the density contrast is reversed, with the density of the magma being less than that of plagioclase (Table 11). For density contrast to have been effective in producing the plagioclase basalt porphyry, a low water content must be assumed for the differentiating magma. This is consistent with the lack of resorption features within the plagioclase phenocrysts. Table 9. Temperatures County, Idaho. (P) (G) = phenocryst = groundmass 9IO II P - 13- 800 900 1000 1100 Figure IO. Temperatures of iron-titanium oxide equilibration for lavas of Caribou County, Idaho. Symbols used: filled circlesnonporphyritic basalt; open circles-microporphyritic basalt; filled circles with vertical bars-tholeiite; P-phenocryst; Ggroundmass. Curve indicates fayalite-magnetite-quartz buffer. of equilibration for lava from Caribou Fiesinger Table IO. Representative I Is. I+- . .. ‘;-‘,--m* F.0‘3. . . 12II 7 fractionation and others- schemes for 0I 0 00 ---------0 0 et ** 0 t 5 4 6 derivation Volcanic of lava Rocks from in Caribou Caribou County County, 483 Idaho. I , Figure I I. MgO variation diagram for lavas of Caribou microporphyritic basalt; filled circles with vertical horizontal bars-plagioclase basalt porphyry. County, bars-tholeiW Idaho. Symbols used: filled circlesPnonporphyritic filled circles with horizontal barsPtrachybasalt: basalt; open open circlescircles with Cenozoic 484 Table I I. Densities plagioclase. * z-I.5 Alkali molecular @m/cc) percent of olivine tholeiite and Geology coexisting H20 Trachyte The high MgO, silica, and alkali content, and low total iron content of the samples (25 and 26) of alkali trachyte from Slug Valley preclude the derivation of this lava from olivine tholeiite. Comparing these samples with other potassium-rich alkaline rocks, they are most similar to orenditic rocks, saturated rocks with K20 in excess over NazO (Sahama, 1974). The lavas from Slug Valley are quartz normative with a KzO:NazO ratio of approximately 2: I; but they are noticeably less potassic than the “average” orendites of Sahama (1974), and they contain magnetite which is uncommon in rocks of orenditic affinity (Carmichael, 1967b). Orenditic rocks are rare and occur in a few restricted areas, such as the Leucite Hills in Wyoming, the Fitzroy Basin of Western Australia, and southeastern Spain, where they are confined to volcanic or subvolcanic environments and lack plutonic counterparts (Sahama, 1974). Carmichael (1967b) considered the orenditic lavas to be of mantle origin, and Prider (1960) proposed the derivation of the Fitzroy Basin rocks to be from a mica peridotite magma. In a recent review of hypotheses for the origin of highly potassic magmas, Gupta and Yagi (1980) concluded that the origin is related to partial melting of a phlogopite peridotite-bearing upper mantle. If a similar origin is to be assumed for the Slug Valley lavas, the amount of partial melting can be determined using an addition-subtraction diagram (Bowen, 1928). To represent the mantle composition, mica peridotite analyses from Best and others (1968) were used. These peridotites are part of an association of highly potassic, silica-poor igneous rocks located east of Salt Lake City, Utah, at the western end of the east-west trending Uinta Arch. The related volcanic rocks are reported to be similar to those of the Leucite Hills (150 miles northeast of the Utah locality) and range in age from upper Eocene to lower Pliocene (Best and others, 1968). Partial melting on the order of 30 to 35 percent is required to derive the Slug Valley lavas from this mica peridotite. of Idaho Olivine Tholeiite The origin of olivine tholeiite, considered to be the parental magma from which tholeiite, trachybasalt, and plagioclase basalt porphyry have been derived, must now be evaluated. Considering the studies of Snake River Plain olivine tholeiite, Stout and Nicholls (1977) proposed partial melting of a pyrolite mantle; Leeman and Vitaliano (1976) and Leeman (1976) proposed partial melting of a spinel-peridotite mantle, or derivation from a deeper source and equilibration with a spinel-peridotite mantle; and Thompson (1975) proposed partial melting of a relatively iron-rich spinel-lherzolite upper mantle. Thus, the percentages of partial melting were calculated, using the pyrolite of Green and Ringwood (1967) and a spine1 lherzolite analysis from Bacon and Carmichael (1973) as hypothetical mantle compositions (Table 12). For a pyrolite mantle, approximately 10 percent partial melting is required, whereas for a spine1 lherzolite mantle, less than 2 percent melting is required. The amount of partial melting required before liquid will separate from the mantle depends on such variables as water content in the mantle and conduit geometry. As the percentages of partial melting for the pyrolite mantle are greater than that for a lherzolite mantle, the segregation of a larger volume of melt should occur more readily. As demonstrated by Stout and Nicholls (1977), the residual material derived from partial melting of pyrolite has a composition similar to spine1 lherzolite. Thus, if the mantle is similar to pyrolite, spine1 lherzolite represents depleted mantle. Regardless of the interpretation, spine1 lherzolite nodules are not associated with tholeiitic magmas, nor have they been found in lavas of the Snake River Plain or in those of the present study. To pursue the issue further, the residual material was recast as a mineral assemblage of olivine, clinopyroxene, orthopyroxene, and spinel, that is, a spine1 lherzolite using the method of Nicholls (1977b). The temperature of equilibration was assumed to be 1200” C and the pressure of equilibration was taken to be 12 kilobars (12 x 10’ kPa) based on a crustal thickness of approximately 40 kilometers for this area (Smith, 1978). The resulting calculated mineral comTable 12. Percentage of partial melting olivine tholeiites from hypothetical lherzolite composition. required to derive selected mantles of pyrolite and Fiesinger and orhers- Volcanic positions and modal amounts of these minerals are relatively insensitive to moderate changes in pressure and temperature. The next step is to estimate pressure and temperature conditions of equilibration between the parental magma and the calculated residual assemblage. The procedure used was that of Nicholls (1977a), based on the thermodynamics of heterogeneous equilibria using activity-composition relationships in silicate melts and solid phases (see also Carmichael and others, 1977; Evans and Nash, 1979). In this procedure, pressure-temperature curves are calculated representing equilibration between the activities of components in the solid residuum and activities of those same components in the magma. Theoretically, the intersection of these curves would then indicate the pressure and temperature of equilibration. As many as five pressure-temperature curves were calculated for a given magma-residuum pair. Equilibrium, based on curve intersections, was not indicated at any reasonable pressures and temperatures. It is thus concluded that pyrolite is not a realistic source composition for the olivine tholeiite. CONCLUSIONS The volcanic rocks of Caribou County, Idaho, consist primarily of olivine tholeiite, sharing many characteristics with the olivine tholeiite of the Snake River Plain. Petrographic similarities include the presence of olivine and plagioclase phenocrysts, the lack of calcium-poor pyroxene, the presence of olivine in the groundmass, and the occurrence of ophitic to subophitic groundmass augite. Chemically, the similarity is apparent when analyses are plotted on AFM and the alkali-silica diagram, and on a comparison of the differentiation index and Coomb’s indicator ratio. Minor occurrences of tholeiite and tholeiitic trachybasalt may be genetically related to parental olivine tholeiite by fractionation of olivine, plagioclase, augite, and iron-titanium oxides, leaving 40 to 60 percent residual liquid. As this requires removal of all crystalline phases rather than just microphenocrysts, filter pressing within a compressed near-surface magma chamber is proposed for the mechanism of formation. Plagioclase basalt porphyry, containing 30 percent plagioclase phenocrysts, occurs at Cinder Island in the Blackfoot Reservoir and at Nelson and King Canyons in the Bear River Range east of Gem Valley. Its derivation from parental olivine tholeiite may be accomplished by fractionating 5 percent olivine and Rocks in Caribou County 485 accumulating I5 percent plagioclase. Density calculations indicate an appropriate density contrast between liquid and plagioclase under anhydrous conditions, for plagioclase crystals to rise through the magma. The lack of resorption features and limited zoning suggest that water pressure was probably very low during emplacement. Alkali trachyte, which is high in MgO, silica, and alkalies, occurs at the southern end of Slug Creek Valley and is unrelated to other lavas in the area. It is chemically similar to orendites and may be related to the Leucite Hills in Wyoming (130 miles to the southeast). The origin of olivine tholeiite and alkali trachyte has been evaluated with the partial melting of various hypothetical mantle compositions. The derivation of olivine tholeiite would require less than 2 percent melting of spine1 lherzolite or approximately IO percent melting of pyrolite. The migration of magma would be facilitated with larger amounts of melt, favoring a pyrolite model. Equilibration studies between melts and residuum from pyrolite, recast as spine1 lherzolite representing depleted mantle, show that pyrolite, however, is not a reasonable source composition. The origin of the alkali trachyte, with its high KzO content, necessitates a potassic source, such as mica peridotite. Mica peridotite is associated with potassic lavas at the western end of the Uintas (130 miles to the south) and was used as the hypothetical mantle composition. The derivation of alkali trachyte requires 30 percent partial melting of such mica peridotite. The occurrence of these two different magmas, both of which are assumed to be mantle derived, indicates contrasting source regions or in homogeneities in the mantle. This may be indirect evidence for differences in the mantle, either in composition or structure, between the Basin and Range province and the Middle Rocky Mountain province. ACKNOWLEDGMENTS Financial support for this work was provided in part by U. S. Geological Survey Grant No. l4-080001-G-545; a summer Fellowship, Utah State University (to Perkins); Research Assistantships, Utah State University (to Perkins and Puchy); and the College of Science, Utah State University (computer costs). We thank William P. Nash and Stan Evans, University of Utah, for use of the electron microprobe, unpublished analyses of rhyolite, and age dates. Cenozoic Geology REFERENCES Albee, A. L. and Lily Ray, 1970, Correction factors for electron probe microanalysis of silicates, carbonates, phosphates and sulphates: Analytical Chemistry, v. 42, p. 1408-1414. Armstrong, F. C., 1969, Geologic map of the Soda Springs quadrangle, southeastern Idaho: U. S. Geological Survey Miscellaneous Geological Investigations Map I-557. Armstrong, F. C. and E. R. Cressman, 1963, The Bannock thrust zone, southeastern Idaho: U. S. Geological Survey Professional Paper 374-J. 22 p. Armstrong, F. C. and S. S. Oriel, 1965, Tectonic development of the Idaho-Wyoming thrust belt: American Association of Petroleum Geologists Bulletin, v. 49, no. 11, p. 1847-1866. Armstrong, R. L., W. P. Leeman, and H. E. Malde, 1975, K-Ar dating, Quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho: American Journal of Science, v. 275, p. 225-251. Bacon, C. R. and I. S. E. Carmichael, 1973, Stages in the P-T path of ascending basalt magma: an example from San Quintin, Baja California: Contributions to Mineralogy and Petrology, v. 41, p. l-22. Bence, A. E. and A. L. Albee, 1968, Empirical correction factors for the electron microanalysis of silicates and oxides: Journal of Geology, v. 76, p. 382-403. Best, M. G., L. F. Henage, and J. A. S. Adams, 1968, Mica peridotite, Wyomingite, and associated potassic igneous rocks in northeastern Utah: American Mineralogist, v. 53, p. 1041-1048. Bottinga, Yan and D. F. Weill, 1970, Densities of liquid silicate systems calculated from partial molar volumes of oxide components: American Journal of Science, v. 269, p. 169-182. Bowen, N. L., 1928, The evolution of the igneous rocks: Princeton, Princeton University Press, 332 p. Bright, R. C., 1963, Pleistocene lakes Thatcher and Bonneville, southeastern Idaho: University of Minnesota Ph.D. dissertation, 292 p. , 1967, Late Pleistocene stratigraphy in Thatcher Basin, southeastern Idaho: Tebiwa, v. 10, no. 1, p. 1-7. Brown, G. M. and E. A. Vincent, 1963, Pyroxenes from the late stages of fractionation of the Skaergaard Intrusion, east Greenland: Journal of Petrology, v. 4, p. 175-197. Buddington, A. F. and D. H. Lindsley, 1964, Irontitanium oxide minerals and synthetic equivalents: Journal of Petrology, v. 5, p. 310-357. of Idaho Campbell, I. H., P. L. Roeder, and J. M. Dixon, 1978, Plagioclase buoyancy in basaltic liquids as determined with a centrifuge furnace: Contributions to Mineralogy and Petrology, v. 67, p. 369-377. Carmichael, I. S. E., 1967a, The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates: Contributions to Mineralogy and Petrology, v. 14, p. 36-64. 1967b, The mineralogy and petrology of -> the volcanic rocks from the Leucite Hills, Wyoming: Contributions to Mineralogy and Petrology, v. 15, p. 24-66. Carmichael, I. S. E., J. Hampel, and R. N. Jack, 1968, Analytical data on the USGS standard rocks: Chemical Geology, v. 3, p. 59-64. Carmichael, I. S. E. and Jim Nicholls, 1967, Irontitanium oxides and oxygen fugacities in volcanic rocks: Journal of Geophysical Research, v. 72, p. 4665-4687. Carmichael, I. S. E., Jim Nicholls, F. J. Spera, B. J. Wood, and S. A. Nelson. 1977, High-temperature properties of silicate liquids: applications to the equilibration and ascent of basic magma: Philosophical Transactions of the Royal Society of London, Series A, v. 286, p. 373-431. Carmichael, I. S. E., F. J. Turner, and John Verhoogen, 1974, Igneous Petrology: McGraw-Hill, 739 p. Coombs, D. S., 1963, Trends and affinities of basaltic magmas and pyroxenes as illustrated on the diopside-olivine-silica diagram: Mineralogical Society of America, Special Paper I, p. 227-250. Cressman, E. R., 1964, Geology of the Georgetown Canyon-Snowdrift Mountain area, southeastern Idaho: U. S. Geological Survey Bulletin 1153, 105 p. Doell, R. R. and Allan Cox, 1962, Determination of the magnetic polarity of rock samples in the field: U. S. Geological Survey Professional Paper 450-D, p. 105-108. Evans, S. H. and W. P. Nash, 1979, Petrogenesis of xenolith-bearing basalts from southeastern Arizona: American Mineralogist, v. 64, p. 249-267. Green, D. H. and A. E. Ringwood, 1967, The genesis of basaltic magmas: Contributions to Mineralogy and Petrology, v. 15, p. 103-190. Gupta, A. K. and Kenzo Yagi, 1980, Petrology and genesis of Ieucite-bearing rocks: Springer-Verlag, 252 p. Leeman, W. P., 1976, Petrogenesis of McKinney (Snake River) olivine tholeiite in light of rare-earth element and Cr/Ni distribution: Geological Society of America Bulletin, v. 87, p. 1582-1586. Leeman, W. P. and M. E. Gettings, 1977, Holocene rhyolite in southeast Idaho and geothermal poten- Firsinger and others- VoLanic tial: Transactions of the American Geophysical Union, v. 58, no. 12, p. 1249. Leeman, W. P. and W. I. Manton, 1971, Strontium isotopic composition of basaltic lavas from the Snake River Plain, southern Idaho: Earth and Planetary Science Letters, v. 11, p. 420-434. Leeman, W. P. and C. J. Vitaliano, 1976, Petrology of McKinney Basalt, Snake River Plain, Idaho: Geological Society of America Bulletin, v. 87, p. 1777-1792. Leeman, W. P., C. J. Vitaliano, and Martin Prim, 1976, Evolved lavas from the Snake River Plain: Craters of the Moon National Monument, Idaho: Contributions to Mineralogy and Petrology, v. 56, p. 35-60. Mabey, D. R., 1971, Geophysical data relating to a possible Pleistocene overflow of Lake Bonneville at Gem Valley, southeastern Idaho: U. S. Geological Survey Professional Paper 750-B, p. 122-127. Mabey, D. R. and F. C. Armstrong, 1962, Gravity and magnetic anomalies in Gem Valley, Caribou County, Idaho: U. S. Geological Survey Professional Paper 450-D, p. 73-7.5. Mabey, D. R. and S. S. Oriel, 1970, Gravity and magnetic anomalies in the Soda Springs region, southeastern Idaho: U. S. Geological Survey Professional Paper 646-E, I5 p. Macdonald, G. A. and Takashi Katsura, 1964, Chemical composition of Hawaiian lavas: Journal of Petrology, v. 5, p. 82-133. Mansfield, G. R., 1927, Geography, geology, and mineral resources of part of southeastern Idaho: U. S. Geological Survey Professional Paper 152, 448 p. 1929, Geography, geology, and natural Qrces of the Portneuf quadrangle, Idaho: U. S. Geological Survey Bulletin 803, I10 p. Mitchell, C. M., F. F. Knowles, and F. A. Petrafeso, 1965, Aeromagnetic map of the Pocatello-Soda Springs region, southeastern Idaho: U. S. Geological Survey Geophysical Investigations Map GP-521. Moore, J. G., 1970, Pillow lava in a historic lava flow from Hualalai volcano, Hawaii: Journal of Geology, v. 78, p. 239-243. Nicholls, Jim, 1977a, The activities of components in natural silicate liquids, in D. G. Fraser, editor, Thermodynamics in Geology: D. Reidel Publishing Co., p. 327-348. , 1977b, The calculation of mineral compositions and modes of olivine-two pyroxene-spine1 assemblages: Contributions to Mineralogy and Petrology, v. 60, p. 119-142. Nicholls, Jim, D. W. Fiesinger, and V. G. Ethier, 1977, Fortran IV programs for processing routine Rocks in Caribou Counry 487 electron microprobe data: Computers and Geosciences, v. 3, p. 49-83. Oriel, S. S., 1968, Preliminary geologic map of the Bancroft quadrangle, Caribou and Bannock Counties, Idaho: U. S. Geological Survey open-file map. Oriel, S. S., D. R. Mabey, and F. C. Armstrong, 1965, Stratigraphic data bearing on the inferred pull-apart origin of Gem Valley, Idaho: U. S. Geological Survey Professional Paper 525-C p. 1-4. Oriel, S. S. and L. B. Platt, 1980, Geologic map of the Preston lo x 2” quadrangle, southeastern Idaho and western Wyoming: U. S. Geological Survey Miscellaneous Investigations Series Map I-l 127, scale 1:250,000. Perkins, W. D., 1979, Petrology and mineralogy of Quaternary basalts, Gem Valley and adjacent Bear River Range, southeastern Idaho: Utah State University M.S. thesis, 91 p. Powell, Marjorie and Roger Powell, 1974, An olivine-clinopyroxene geothermometer: Contributions to Mineralogy and Petrology, v. 48, p. 249-263. Powell, Roger and Margorie Powell, 1977, Geothermometry and oxygen barometry using coexisting iron-titanium oxides: a reappraisal: Mineralogical Magazine, v. 41, p. 257-263. Prider, P. T., 1960, The leucite lamproites of the Fitzroy Basin, Western Australia: Journal of the Geological Society of Australia, v. 6, p. 71-118. Puchy, B. J., 1981, Mineralogy and petrology of lava flows (Tertiary-Quaternary) in southeastern Idaho and at Black Mountain, Rich County, Utah: Utah State University M.S. thesis, 73 p. Sahama, T. G., 1974, Potassium-rich alkaline rocks, in H. Sorensen, editor, The Alkaline Rocks: J. Wiley and Sons, p. 96-109. Smith, Douglas and D. H. Lindsley, 1971, Stable and metastable augite crystallization trends in a single basalt flow: American Mineralogist, v. 56, p. 225-233. Smith, R. B., 1978, Seismicity, crustal structure, and intraplate tectonics of the interior of the western Cordillera, in R. B. Smith and G. P. Eaton, editors, Cenozoic tectonics and regional geophysics of the western Cordillera: Geological Society of America Memoir 152, p. I I I-144. Stormer, J. C. and Jim Nicholls, 1978, XLFRAC: a program for the interactive testing of magmatic differentiation models: Computers and Geosciences, v. 4, p. 143-159. Stout, M. Z. and Jim Nicholls, 1977, Mineralogy and petrology of Quaternary lavas from the Snake River Plain, Idaho: Canadian Journal of Earth Sciences, v. 14, p. 2140-2156. 488 Cenozoic Geology Streckeisen, Albert, 1979, Classification and nomenclature of volcanic rocks, lamprophyres, carbonatites, and melilitic rocks: recommendations and suggestions of the 1. U. G. S. Subcommission on the Systematics of Igneous Rocks: Geology, v. 7, p. 331-335. Thompson, R. N., 1975, Primary basalt and magma genesis, II: Snake River Plain, Idaho, U.S.A.: Contributions to Mineralogy and Petrology, v. 52, p. 213-232. Thornton, C. P. and 0. F. Tuttle, 1960, Chemistry of igneous rocks: part I, differentiation index: American Journal of Science, v. 258, p. 664-684. of Idaho Tilley, C. E. and R. N. Thompson, 1970, Melting and crystallization relations of the Snake River basalts of southern Idaho, U.S.A.: Earth and Planetary Science Letters, v. 8, p. 79-92. Williams, Howel, F. J. Turner, and C. M. Gilbert, 1954, Petrography, an Introduction to the Study of Rocks in Thin Section: W. H. Freeman and Co., 406 p. Yoder, H. S. and C. E. Tilley, 1962, Origin of basaltic magmas: an experimental study of natural and synthetic rock systems: Journal of Petrology, v. 3, p. 342-532.