CR-21-C(PDF Report)
Petrography, Geochemistry, and Volcanogenic Development of the San Francisco Mountain Volcanic System, Northern Arizona
Richard F. Holm Northern Arizona University
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Recommended Citation. Holm, R.F., 2021, Petrography, Geochemistry, and Volcanogenic Development of the San Francisco Mountain Volcanic System, Northern Arizona. Arizona Geological Survey CR-21-C, 36 p., 5 appendices.
Cover image. San Francisco Mountain in winter. Photo by Ted Grussing. Petrography, Geochemistry, and Volcanogenic Development of the San Francisco Mountain Volcanic System, Northern Arizona
Richard Holm* 2021
“From all points of view San Francisco Mountain stands out with great distinctness, rising with graceful outline to a height of 12,700 feet above the sea, or over 5,000 feet above the surrounding country.” Henry Hollister Robinson
1
CONTENTS
….Page ACKNOWLEDGEMENTS……………………………………………………………………………………………3 ABSTRACT……………………………………………………………………………………………………...…….4 INTRODUCTION……………………………………………………………………………………………………..5 SAN FRANCISCO MOUNTAIN VOLCANIC SYSTEM……………………………………………….…...... 6 San Francisco Mountain…………………………………………………………………………..………….7 Satellite Silicic Volcanoes……………………………………………………………………..……………..8 CHEMISTRY AND CLASSIFICATION OF LAVAS……………………………………………..…………………8 Field and Map Classification…………………………………………………………………..……………11 PETROGRAPHY……………………………………………………………………………………….………….....11 Methods…………………………………………………………………………………………………...... 11 Porphyritic-Aphanitic Rocks……………………………………………………………….…………….....11 Phenocrysts and Microphenocrysts…………………………………………………………....…..11 Silica Ranges of Minerals………………………………………………………………….....…...12 Mineral Assemblages……………………………………………………………………………...14 Textures……………………………………………………………………………………………15 Porphyritic and Granular Phaneritic Rocks…………………………………………………………………16 Mineral Assemblages and Textures……………………………………………………………….17 Anatectic Rocks……………………………………………………………………………………………..20 North Sugarloaf Trachyte Dome……………………………………………………………………………21 Contaminated and Mixed-Magma Rocks…………………………………………………………………...22 Anatectic Textures…………………………………………………………………………………22 Glassy Globules……………………………………………………………………………………23 Antipathetic Phenocryst Populations……………………………………………………………...23 Mafic Maroon Xenoliths………………………………………………………………………… 24 VOLCANOGENIC DEVELOPMENT……………………………………………………………...... 25 Protocone……………………………………………………………………………………………………25 Volumes……………………………………………………………………………………………………..26 Geochronology……………………………………………………………………………………………...26 The Volcanic System and Surrounding Volcanoes…………………………………………………………28 The Volcanic Field………………………………………………………………………………………….30 Subterranean Processes……………………………………………………………………………………...30 Parental Magmas…………………………………………………………………………………..32 Rhyolites and the Granite System…………………………………………………………………………..32 Granophyric Residuum……………………………………………………………………………………...33 REFERENCES CITED……………………………………………………………………………………………….34
FIGURES
1. Map of San Francisco volcanic field………………………………………………………………………………..5 2. Google Map of San Francisco Mountain volcanic system…………………………………………………………6 3. Google Map of San Francisco Mountain……………………………………………………………………...……7 4. Rose diagram showing strikes of dikes in San Francisco Mountain………………………………………………..7 5. Total alkali-silica diagram of analyses of San Francisco Mountain volcanic system………………………………8 6. Total alkali-silica diagram of analyses of satellite silicic volcanoes……………………………………………….9 7. Harker diagrams of the San Francisco Mountain volcanic system……………………………………………….10 8. Diagram of K2O/Na2O vs SiO2……………………………………………………………………………………11 9. Histogram showing percent of phenocrysts and microphenocrysts……………………………………….………12 10. Diagram showing silica ranges and proportions of phenocrysts and microphenocrysts………………..……….12 11. Diagram showing silica ranges of minerals not shown in Figure 10…………………………………………….13 12. Photomicrograph of mugearite in crossed polarized light……………………………………………………….15 13. Photomicrograph of mugearite in plane polarized light………………………………………………………….15 14. Photomicrograph of benmoreite in plane polarized light………………………………………………………...15
2 15. Photomicrograph of benmoreite in plane polarized light………………………………………………………...15 16. Photomicrograph of benmoreite in plane polarized light………………………………………………………..16 17. Photomicrograph of trachyte in plane polarized light……………………………………………………………16 18. Geologic map of the Core Ridge area……………………………………………………………………………16 19. Photomicrograph of microdiorite in plane polarized light……………………………………………………….17 20. Photomicrograph of microdiorite in crossed polarized light…………………………………………………….17 21. Photomicrograph of alkali feldspar and quartz displaying micrographic texture………………………………..17 22. Photomicrograph of magnetite and orthopyroxene pseudomorph of olivine…………………………………….17 23. Photomicrograph of quartz monzodiorite in plane polarized light………………………………………………18 24. Photomicrograph of quartz monzodiorite in crossed polarized light…………………………………………….18 25. Photomicrograph of plagioclase crystal zoned to a mantle of alkali feldspar……………………………………18 26. Photomicrograph of granophyric residuum in quartz monzodiorite……………………………………………..18 27. Photomicrograph of olivine crystal replaced by serpentine and carbonate and mantled by pigeonite…………..18 28. Photomicrograph of pyroxene leucodiorite in plane polarized light……………………………………………..19 29. Photomicrograph of pyroxene leucodiorite in crossed polarized light…………………………………………..19 30. Photomicrograph of phyllosilicate pseudomorph of olivine crystal with mantle of inverted pigeonite………...19 31. Photomicrograph of crystals in Figure 30 in crossed polarized light……………………………………………19 32. Photomicrograph of inverted pigeonite with inclusions of altered olivine………………………………………19 33. Photomicrograph of inverted pigeonite in Figure 32 in crossed polarized light…………………………………20 34. Diagram of Zr vs SiO2...... 20 35. Diagram of Rb vs SiO2…………………………………………………………………………………………...20 36. Photograph of quarry face of Pumice of Fremont Peak………………………………………………………….22 37. Photograph of impact structure in scoria bed in Pumice of Fremont Peak………………………………………22 38. Photomicrograph showing anatectic texture in pyroxene gneiss in plane polarized light……………………….22 39. Photomicrograph of gneiss in Figure 38 in crossed polarized light……………………………………………..23 40. Photomicrograph of andesitic glassy globule in rhyolite vitrophyre…………………………………………….23 41. Photomicrograph of labradorite and augite xenocrysts in rhyolite………………………………………………23 42. Photograph of mafic maroon xenoliths in Older Dacite of Doyle Peak…………………………………………24 43. Photograph of a projection of a thin section of a mafic maroon xenolith……………………………………… 24 44. Photograph of andesitic agglomerate ……………………………………………………………………………25 45. Diagram showing growth history of San Francisco Mountain volcanic system…………………………………28 46. Total alkali-silica diagram of 314 analyses………………………………………………………………………29 47. Diagram of normative hy, di, ol, ne of 92 basalt analyses……………………………………………………….31 48. Diagram of Zr vs SiO2……………………………………………………………………………………………31 49. Diagram of Ba/Zr vs MgO……………………………………………………………………………………….31 50. Diagram of the granite system…………………………………………………………………………………...33
TABLES
1. Satellite Silicic Volcanoes……………………………………………………………………………………...…..8 2. Summary of Assemblages of Phenocrysts and Microphenocrysts in Modes of Aphanitic Rocks………………..14 3. Trace Elements of North Sugarloaf, Sugarloaf, and Selected Trachyte Lavas ……………………………………21 4. Petrographic Data of Mafic Maroon Xenoliths……………………………………………………………………25 5. Stratigraphy of Principal Volcanic Map Units…………………………………………………………………….27 6. Analyzed Rocks in San Francisco Mountain Volcanic System…………………………………………………...28 7. Analyzed Rocks in 10-km-Wide Ring…………………………………………………………………………….28 8. Definition and Classification of Basalts in 10-km-Wide Ring……………………………………………………28 9. Summary of Primitive Lavas……………………………………………………………………………………...32 10. Modal Compositions of Granophyre……………………………………………………………………………...34
ACKNOWLEDGEMENTS
Ed Wolfe, George Ulrich, and Richard Moore generously shared field data, lab data, thin sections, and ideas during the course of this project. The United States Geological Survey provided field support, thin sections, and chemical analyses. The Organized Research Committee at Northern Arizona University awarded grants for field support, and the Department of Geology at NAU provided a vehicle for field work, and laboratory equipment and facilities for the petrographic research.
3 ABSTRACT Mg-olivine drops out of the phenocryst assemblage first, but Fe-olivine rejoins San Francisco Mountain and eight the assemblage at higher silica contents. satellite silicic volcanoes at its base Pigeonite, clinopyroxene, orthopyroxene, constitute the San Francisco Mountain and hornblende drop out of the volcanic system. The central composite assemblages sequentially. Opaque oxides volcano is the largest edifice in the San and plagioclase are members of Francisco volcanic field, a basaltic field phenocryst assemblages through the on the southwestern margin of the entire range of silica contents. Colorado Plateau in northern Arizona. Solid-solution minerals change Satellite volcanoes range from single regularly in end-member proportions domes to clusters of as many as eight with increase in silica contents of host domes. Vent locations and many internal lavas. High-temperature end members, volcano structures (dikes) coincide with Fo, En, and An in olivine, orthopyroxene, pre-volcanic crustal structures on and plagioclase respectively, decrease northwest, north, and northeast trends. through the igneous-rock series. The volcanic system was active from Some rocks with anomalous trace- late Pliocene to late Pleistocene (2.78- element contents are unique and not part 0.091 Ma). Lava erupted in four stages, of the igneous-rock series; origin of these or compositional cycles, each beginning lavas by crustal anatexis is possible. with rhyolite or dacite domes. The first Other rocks with antipathetic phenocryst three stages ended with andesite populations or numerous mafic xenoliths stratovolcanoes, but the fourth stage appear to be of mixed-magma or erupted only minor andesite. contamination origins. One-hundred-forty chemical analyses of Compositions of rhyolites plotted on the San Francisco Mountain and twenty granite system suggest middle to lower analyses of the satellite volcanoes crustal depths of origin. characterize the lavas. On the total An eruption history of the volcanic alkali-silica diagram (TAS) the analyses system of about 2.7 million years and plot in a continuum from low-silica multiple compositional cycles imply a basaltic trachyandesite through long-lived magmatic system and episodic trachyandesite and trachyte to rhyolite. resupply of fresh magma from the Major-element oxides plot in regular and mantle. Magmatic differentiation by characteristic trends on Harker fractional crystallization of parental diagrams. basalt magma is judged to be the Most rocks have compatible phenocryst principal process for the origin of the assemblages that change in a regular way lavas of the San Francisco Mountain with increasing silica content of the host volcanic system. Mixing of magmas lava. These rocks form a mildly alkaline within the magmatic system and addition transitional igneous-rock series. Olivine of crystals and liquids from crystalline and clinopyroxene ("augite") typify the crustal rocks changed the petrography low-silica basaltic trachyandesites. With and chemistry of some lavas. progressive increase in silica, the sequence of crystallization is: *7550 North Snow Bowl Road, Flagstaff, AZ 86001 orthopyroxene and pigeonite, hornblende, [email protected] biotite, anorthoclase, sanidine, quartz.
4 INTRODUCTION satellite volcanoes at its base. Detailed field, petrographic, and geochemical data San Francisco Mountain is the largest are given for each analyzed sample. Deal volcano in the late Cenozoic San Francisco (1969) studied the western end of the Inner volcanic field in northern Arizona (Fig. 1). Basin with a 1:24,000 scale geologic map The structure is over 1,650 m high, and the and petrographic descriptions, including summit, at 3,850 m (12,633 ft) above sea modes, of the lavas and dikes. Wenrich- level, is the highest in the region. The Verbeek (l975) measured the section of lava mountain dominates the landscape for miles flows on the southeast slope of Humphreys around. Peak, made detailed petrographic and geochemical analyses of the samples, and interpreted the petrology of the magmas. Updike (1977) did a comprehensive geological study of San Francisco Mountain, including stratigraphy, petrography, geochemistry, and glacial geology. Wolfe et al. (1987a) published major and trace element chemical analyses, sample locations, K- Ar ages, and paleomagnetic data Figure 1. San Francisco Mountain is in the central part of the San Francisco with the 1:50,000 volcanic field (pink). Inset map shows the position of the volcanic field (SFVF) scale geologic map of on the southwest margin of the Colorado Plateau (blue); heavy black line is Mogollon Rim, the edge of the plateau. Clusters of lava domes of silicic and the central part of the intermediate rocks are green. Numbers are ages of rocks in millions of years. San Francisco volcanic field, which includes San Francisco Mountain and eight The volcanic geology of San Francisco satellite volcanoes. Holm’s 1988 1:24,000 Mountain and associated satellite silicic scale geologic map of San Francisco volcanoes has been described and Mountain and five satellite volcanoes interpreted in several field and laboratory includes detailed field and petrographic studies. Robinson (1913) devoted much of descriptions of 60 extrusive and intrusive his classic study of the volcanic field to map units. Arculus and Gust (1995) petrography, petrology, and geochemistry of included San Francisco Mountain in their the rocks. His report contains 23 high- field-wide survey of geochemical data and quality wet-chemical analyses, of which petrologic interpretations of the origin of the nine are of San Francisco Mountain and
5 lavas. Wenrich-Verbeek (1972) interpreted volcano (HP) is at the western base of San the mechanism of emplacement of the White Francisco Mountain. Horse Hills (Marble Mountain) intrusive dome, one of the satellite volcanoes. Kluth (1974) described lava-flow structures and analyzed flow mechanisms of the lava lobes on Elden Mountain, another satellite. Dennis (1981) described the petrography and stratigraphic positions of silicic fall deposits from San Francisco Mountain and Sugarloaf, a satellite volcano. This report on the San Francisco Mountain volcanic system uses 160 chemical analyses and petrographic data from 601 thin sections to update the classification of the rocks and supplement the map-unit descriptions of U.S. Geological Survey maps MF-1959 and I-1663 (Wolfe et al., 1987a; Holm, 1988). The report first reviews the field geology of San Francisco Mountain and eight satellite silicic volcanoes. This is followed Figure 2. Google Maps image of the San Francisco Mountain volcanic system. San Francisco by the current widely-used chemical Mountain is the high-relief structure in the center classification of volcanic rocks: total alkali- of the image. See text for definitions of the symbols. silica diagram, or TAS. New petrographic information includes modal analyses, Six satellite volcanoes lie in a northwest mineral compositions, texture descriptions, trending lane that includes the central and representative photomicrographs. conduit system of San Francisco Mountain Rocks are grouped into categories according (red overlay on Fig. 2). North Sugarloaf and to texture and inferred processes of origin. Sugarloaf are in a northeast trending The report concludes with a discussion of structural alignment that includes large dikes volcanogenic development and in the core of the composite volcano (Fig. interpretation of magma genesis. 18, p. 16), the axis of the large breach in the northeast quadrant of San Francisco SAN FRANCISCO MOUNTAIN Mountain, a linear negative aeromagnetic VOLCANIC SYSTEM anomaly northeast of Sugarloaf (Sauck and Sumner, 1970), and the O’Leary Peak silicic The volcanic system is defined as center 8 km northeast of Sugarloaf (Figs. 1 consisting of the San Francisco Mountain and 2). These two volcanic and geophysical composite volcano and eight satellite silicic alignments coincide with regional structures volcanoes at its base. Clockwise from in the northwest trending Cataract Creek northeast these satellites (and abbreviations) fault system and southwest projection of the are: North Sugarloaf (NS), Sugarloaf (SU), Doney structural system (Ulrich et al., Schultz Peak (SP), Elden Mountain (EM), 1984). Not evident in Figure 2 is the north Dry Lake Hills (DL), Hochderffer Hills trending Oak Creek Canyon fault system (HH), Kendrick Park (KP), White Horse that displaces Permian and Triassic strata Hills (WH) (Fig. 2). Hart Prairie shield and lava flows north and south of San
6 Francisco Mountain (Ulrich et al., 1984; conduit system, whereas silicic lavas Shoemaker et al., 1978). The volcanic typically constructed domes on the flanks system is at the intersection of these three and at the base of the central stratocones. regional structural elements. Buried rhyolite and dacite domes are exposed on the south and northeast sides of San Francisco Mountain Inner Basin, and partly buried domes are on the outer slopes of Humphreys, Fremont, San Francisco Mountain was constructed and Doyle Peaks (Fig. 3; Holm, 1988). on a platform of late Miocene, and possibly Core Ridge and the adjacent ridge to the younger, basalt lava flows that overlie south (Fig. 3) consist of lava flows, tuffs, Permian limestone and Triassic sandstone. tuff breccias, agglomerates, and agglutinates The oldest-known lava in the volcano is a that are intruded by large dikes and plugs. A rhyolite dome at Raspberry Spring (1.82 quartz monzodiorite dike on Core Ridge that Ma) in the Inner Basin (Fig. 3; Holm, 1988). supplied the stage 3 stratocone is about 1 km The dome is overlain by stratified lava flows northeast of the diorite feeder plug of the and pyroclastic deposits of andesite. This stage 2 stratocone. The older stratocone blocked southwest flow of stage 3 lavas. Most dikes exposed in the walls of Inner Basin are radial and project back to the volcano's core. Many radial dikes and small dikes on Core Ridge strike parallel and subparallel to regional faults and lineaments (Fig. 4; Dohm, 1995). Large dikes in the Core Ridge area strike east-northeast subparallel to the axis of Interior Valley (Fig. 4, Fig. 18, p. 16). The largest sector of dikes strikes north (Fig. 4)
Figure 3. Google Maps image of San Francisco Mountain. White dash line marks edge of Inner Basin and Interior Valley. RS: Raspberry Spring rhyolite dome. LM: Lockett Meadow dacite flow. sequence is repeated twice, which records three eruption stages that began with silicic Figure 4. Rose diagram showing the strikes of domes and ended with construction of dikes in San Francisco Mountain. Andesite dikes andesitic stratovolcanoes, the latest of which outnumber dacite dikes. Open double arrow shows has a K-Ar age of 0.43 Ma (Holm, 1988, direction of regional extensional stress(Wong and 2004) and Ar-Ar age of 0.51 Ma (Karatson Humphrey, 1989). Diagram from Holm, 2004). et al., 2010). The fourth eruption stage produced several silicic domes and flows The individual peaks on San Francisco (0.40 Ma), but only one andesite flow. Mountain are erosional remnants of the fully Andesite flows and pyroclasts erupted developed compound composite volcano, principally from vents above the central the stratocones of which predate the Inner
7 Basin (Fig. 3). The Inner Basin and Interior Sample locations, K-Ar ages, Valley developed sometime between paleomagnetic data, and compositions of the construction of the latest (stage 3) stratocone satellite volcanoes are on MF-1959 (Wolfe and the eruption of Sugarloaf at the mouth et al., 1987a). Sample location and Ar/Ar of Interior Valley (0.43-0.091 Ma). The data of Sugarloaf are in Morgan et al., 2010. likely process of origin is sector collapse of the northeast side of San Francisco CHEMISTRY AND CLASSIFICATION Mountain, which produced a large debris fan OF LAVAS down slope to the northeast (Holm, 2004). Erosion, mass wasting, and glaciation Chemical analyses of 135 samples from enlarged Inner Basin to its modern bowl San Francisco Mountain and analyses of 20 shape (Fig. 3). samples from the satellite volcanoes were extracted from Table 2 in MF-1959 (Wolfe Satellite Silicic Volcanoes et al., 1987a) and placed in a separate set of analyses; added to this data set are three Ages and compositions of the satellite analyses of lavas on MF-1960 (3832, 3833, volcanoes correlate well with the active 4802, Moore and Wolfe, 1987) and two history and geochemical characteristics of analyses of pyroclasts from a distal fall San Francisco Mountain. The oldest and deposit on MF-1956 (5828B, 5828C, Ulrich youngest satellites predate and postdate the and Bailey, 1987). These 160 analyses, K-Ar age range of San Francisco Mountain which include 19 with trace element (1.82-0.40 Ma), and with one or possibly abundances from Table 5 in MF-1959, are two exceptions (Fig. 2, SU, HH) reproduced here in Appendix 1. Normalized compositions of the satellites coincide with analyzes adjusted to 100 percent of major the compositions of the trachytes and oxides are in Appendix 2. rhyolites in the central volcano (Table 1). The normalized analyses are plotted on the total alkali-silica diagram (TAS diagram, Le Table 1. Satellite Silicic Volcanoes Maitre, 1989) in Figures 5 and 6 to classify Satellite Structure Composition Age Ma the lavas in the volcanic system. North lava dome trachyte 2.78 ± Sugarloaf 0.13 Sugarloaf lava dome. rhyolite 0.091 ± 0.002 Schultz lava dome trachyte 0.75 ± Peak 0.04 Elden four lava trachyte and ~0.53* Mountain domes trachydacite Dry Lake eight lava trachyte ~0.75** Hills domes Hochderffer lava dome rhyolite 1.64 ± Hills 0.11 Kendrick lava dome rhyolite 2.15 ± Park 0.13 White intrusive rhyolite, normal Figure 5. Total alkali-silica diagram with 140 Horse Hills dome benmoreite, polarity analyses from San Francisco Mountain (red dots) latite and 20 analyses from satellite volcanoes (blue * Average of 0.49 and 0.57 determined on two flows triangles). Samples of probable mixed magmas are on the southeast side of Elden Mountain. 3732P, Qrcr and 3733D, Qmgi (Appendices 2, 4, ** Estimate based on normal polarity, stratigraphy, and discussion on p. 23). and more subdued by erosion than Elden Mountain.
8 The 140 analyses of samples from San In Figure 6 the two trachyandesites are Francisco Mountain form a mildly alkaline post-rhyolite pipe-like bodies at White transitional series from basaltic Horse Hills; one is benmoreite and the other trachyandesite to rhyolite. Sodium is latite. In the trachyte-trachydacite field exceeds potassium in amounts sufficient to one sample is trachydacite and the others are classify all basaltic trachyandesites as trachyte. mugearite and most trachyandesites as On Harker diagrams the oxides in the benmoreite; only eight trachyandesites have combined data set of 160 analyses display sufficient potassium to be classified as latite. regular trends (Fig. 7, next page). Fairly Three samples plot slightly below the alkali tight negative slopes across the diagrams are line in the andesite field. In the trachyte- displayed by TiO2, FeO, MgO, and CaO. trachydacite field most samples are deficient On the K2O diagram, some satellite in normative quartz and so classify as rhyolites, trachytes, and a latite contain trachyte; only two samples have enough Q higher contents of potassium than the San to classify as trachydacite. Five rhyolite Francisco Mountain series. The two samples have normative acmite and classify trachytes at just under 5 per cent K2O are a as peralkaline rhyolite (or comendite). The rhyolite lava flow (3732P) and its alkali four percent silica gap in the rhyolite field microgranite feeder plug (3733D); both are has no obvious explanation. The gap could mixed with a more mafic component that be real because no lavas in the volcanic lowered the silica content (Qrcr and Qmgi system fill in the gap, or it could be a result on I-1663, Holm, 1988, and Fig. 5). The of no samples from buried lavas. mugearites and benmoreites show some Five specimens in Figure 5 are phaneritic, scatter on the Al2O3 and P2O5 diagrams. and are classified with the modal Quartz- Several “younger andesites” (Qay on map I- Alkali feldspar-Plagioclase-Feldspathoid 1663) have unusually high P2O5. The (QAPF) diagram for plutonic rocks (Le inflection in the P2O5 curve at about 60 Maitre, 1989). Phaneritic lithologies are percent SiO2 is close to the appearance of leucodiorite, microdiorite, quartz apatite in the mineral assemblages (Fig. 11, monzodiorite, and microgranite. p. 13). The Na2O diagram has an open Twenty analyses of satellite volcanoes pattern of analyses, which trends in a broad range from trachyandesite to rhyolite (Fig. band of increasing Na2O to the rhyolites 6). All analyses coincide with the San where Na2O declines abruptly. Plagioclase Francisco Mountain series (Fig. 5). is the principal mineral that accommodates sodium. Variable water pressures in magma chambers can affect the character and extent of zoning in plagioclase, and possibly its fractionation, which together can affect the content of sodium in derivative magmas. The K2O/Na2O ratio increases significantly through the series to the rhyolites in which sanidine and anorthoclase are the principal feldspar phenocrysts (Fig. 8, page 11; Appendix 3).
Figure 6. Total alkali-silica diagram with 20 analyses from satellite silicic volcanoes.
9 Figure 7. Harker diagrams of the San Francisco Mountain volcanic system. Red dots: San Francisco Mountain. Blue triangles: satellite silicic volcanoes.
10 of Igneous Rocks ( Le Bas et al., 1986). Map units and map-unit descriptions on the geologic maps (MF-1959, I-1663) use the lithologic names described above. Lithologic names applied to samples in Appendices 1 and 2 are TAS names as determined on Figures 5 and 6. TAS names are used in the following sections of descriptive petrography.
PETROGRAPHY Figure 8. Diagram showing increase in K2O/Na2O ratio with increase in SiO2. Symbols same as in Figure 7. The rhyolite with highest ratio is Methods Sugarloaf dome (SU), and the latite with high ratio is a pipe at White Horse Hills (WH). Petrographic data were obtained from thin sections with traditional methods using a Field and Map Classification Leitz polarzing microscope, Swift electronic mechanical stage, and Leitz universal stage. Mapping of San Francisco Mountain and Most modes were obtained with more than the satellite silicic volcanoes was conducted one thousand points to ensure that sparse and completed in the 1970s. The published phenocrysts were counted. Plagioclase maps are MF-1959 (Wolfe et al., 1987a) and compositions were estimated with extinction I-1663 (Holm, 1988). For mapping angles measured on a-normal crystals and purposes the porphyritic-aphanitic rocks the curve in Deer, Howie and Zussman, were classified with hand-specimen and thin-section descriptions as basalt, andesite, 1963b, Figure 55, p. 138. Olivine and dacite, and rhyolite, largely using orthopyroxene compositions were estimated phenocryst assemblages as criteria. As with 2V angles measured with a Leitz chemical analyses became available, universal stage and the curves in Deer, lithologic boundaries based on silica content Howie and Zussman 1962, Figure 11, p. 22 were identified that correlated well with (olivine) and 1963a Figure 10, p. 28 mineral assemblages. With increasing (orthopyroxene). In most thin sections silica, these boundaries are: several crystals of each mineral were measured and the optical data averaged. basalt-andesite: 52 % SiO2, abrupt increase of plagioclase and diminished olivine and clinopyroxene; Porphyritic-Aphanitic Rocks andesite-dacite: 62 % SiO2, appearance of biotite and disappearance of olivine; Most rocks in the volcanic system have phenocrysts set in aphanitic matrices; which dacite-rhyolite: 70 % SiO2, appearance of alkali feldspar and quartz and disappearance range from holocrystalline through of orthopyroxene. hypocrystalline to holohyaline (glassy). The maps were still in editorial review and Holocrystalline matrices range from preparation for publication in 1986 when the microcrystalline to cryptocrystalline. total alkali-silica diagram (TAS) was recommended for volcanic rocks by the Phenocrysts and Microphenocrysts International Union of Geological Sciences Petrographic descriptions and modal (IUGS) Subcommission on the Systematics analyses of samples in the volcanic system
11 classify crystals larger than 0.5 mm as phenocrysts. Microphenocrysts are crystals between 0.5 mm and 0.05 mm. Crystals smaller than 0.05 mm, glass, and devitrified glass are grouped together as matrix. Modal analyses determined that phenocrysts and microphenocrysts together range from 5 to 56 percent (Fig. 9). Modal analyses of 55 porphyritic- aphanitic samples from the volcanic system are in Figure 10. Diagram showing the silica ranges of phenocrysts and Appendix 3. For microphenocrysts and their relative proportions when normalized to 100 percent. comparison at the low Colored lines show high-temperature end-member compositions of plagioclase silica end of the series (An), olivine (Fo), and orthopyroxene (En). n is number of crystals measured . modes of five samples of basalt from the eastern San Francisco each sample document the range of silica volcanic field are included in Appendix 3. values through which each mineral crystallized (Fig. 10). Phenocrysts and microphenocrysts counted in modes (Appendix 3) and normalized to 100 percent show relative proportions of the minerals through the entire range of silica values (Fig. 10). Five basalt samples from the eastern San Francisco volcanic field that range from 48.7 to 51.0 percent silica (Moore,
Figure 9. Histogram showing the percent of 1974) are included to show the petrographic phenocrysts and microphenocrysts counted in transition from basalt to basaltic modes of porphyritic-aphanitic samples from the trachyandesite. Note that Figure 10 shows San Francisco Mountain volcanic system (55), only the relative normalized proportions of O’Leary Peak silicic center (9), and five basalts phenocrysts and microphenocrysts through from the eastern San Francisco volcanic field. the range of silica values; the diagram does
not show mineral assemblages or modal Silica Ranges of Minerals percents in individual samples at specific Chemical analyses of 160 samples from silica values. For mineral percents go to the volcanic system (Appendices 1 and 2) Appendix 3. combined with thin section descriptions of
12 The mineral composition curves in Figure Orthopyroxene appears in mugearites at 10 were constructed with rolling averages at about 54 percent silica, with En-contents data points of one-half percent silica for around 70, and is present through the plagioclase and one percent silica for olivine trachytes to 67 percent silica and En34. and orthopyroxene. The five basalt samples Clinopyroxene (“augite”) and from Moore (1974) were selected because hornblende are present in assemblages they have microprobe data of the through a wide range of silica contents. compositions of the phenocrysts. Clinopyroxene drops out in the high-silica Plagioclase replaces olivine as the trachytes, but hornblende is present in dominant phenocryst phase in the transition decreasing amounts well into the rhyolites. from basalt to basaltic trachyandesite, and is Biotite appears in assemblages at 62 dominant through the series until it is percent silica, and is the principal mafic replaced by alkali feldspar in the rhyolites. mineral in the rhyolites. An-contents of phenocryst cores in the Anorthoclase joins the assemblages of the basalts range from 74 to 67; An-content in highest-silica trachytes, and together with the lowest silica basaltic trachyandesite is sanidine are the dominant feldspars in the 64. An-contents decline steadily through the high-silica rhyolites. series to 16 in the highest silica rhyolites; Quartz is present as a few scattered these rhyolites plot in field 3 of the QAPF crystals in some trachytes (see below), but is diagram recommended by the International a principal mineral only in the rhyolites. Union of Geological Sciences (Le Maitre, Opaque oxides are ubiquitous in all 1989). Plagioclase phenocrysts in some assemblages through the entire range of rhyolites have An-contents less than five and silica values. these are classified as alkali feldspar Other minerals were not included in rhyolite, which plot in field 2 of the QAPF Figure 10 because of small size (most diagram; examples are Sugarloaf and apatite and zircon), low abundance (quartz Hochderffer Hills (SU and HH in Figure in trachytes, ferrohedenbergite in rhyolites, 10). A third group of rhyolites lack tridymite and cristobalite in vesicles), or not plagioclase; these are peralkaline rhyolites readily identifiable (difficult to distinguish (comendite) characterized by phenocrysts of pigeonite from augite on a mode traverse) sanidine. (Fig. 11). The sodium-iron minerals Olivine decreases sharply in amount from riebeckite, aegirine, and aenigmatite were the basalts to the mugearites, declines not included in Figure 10 because of limited steadily through the mugearites and silica range (Fig. 11). benmoreites, and drops out of assemblages at the trachyte field boundary (~62.5% SiO2) (Fig. 10). Olivine reappears in trachytes (~65% SiO2) and is present in trachyte and rhyolite assemblages. Fo-contents in the basalts range from 84 to 73. Fo-content of olivine in the lowest silica mugearite is 75, and the end-member declines steadily to 47 at the benmoreite-trachyte boundary. Fo- Figure 11. Silica ranges of minerals not shown in contents of olivine decrease from 28 to 0 in Figure 10. See text above for discussion. Dash line high-silica trachytes through the rhyolites. indicates sparse amounts.
13 Mineral Assemblages Assemblages of phenocrysts and microphenocrysts are documented with thin section descriptions and the modal analyses in Appendix 3. In Table 2 minerals are listed left to right in order of decreasing percentage. Mineral assemblages are listed down in general order of increasing silica percent of host rock. Number in parentheses is number of samples with modes. Opaque oxide is present in all assemblages. Figures listed are photomicrographs of examples.
Table 2. Summary of Assemblages of Phenocrysts and Microphenocrysts in Modes of Aphanitic Rocks
Principal Minerals: ≥ 1% Minor Minerals: < 1% Selected Basalts from Eastern San Francisco Volcanic Field SiO2 = 48.7-51.0 (5) olivine + plagioclase + clinopyroxene plagioclase + olivine + clinopyroxene Porphyritic-Aphanitic Rocks in San Francisco Mountain Volcanic System Mugearite SiO2 = 52.2-56.6 (9) plagioclase + olivine clinopyroxene Figure 12 plagioclase + olivine + clinopyroxene orthopyroxene Figure 13 Benmoreite SiO2 = 57.3-62.3 (17) plagioclase + clinopyroxene + orthopyroxene + olivine pigeonite plagioclase + orthopyroxene + clinopyroxene + olivine pigeonite Figure 14 plagioclase +orthopyroxene + clinopyroxene olivine, pigeonite Figure 15 plagioclase + orthopyroxene + hornblende olivine, clinopyroxene Figure 16 plagioclase + hornblende + orthopyroxene biotite Latite SiO2 = 60.6 (1) plagioclase + hornblende + orthopyroxene Andesite SiO2 = 57.4-60.9 (3) plagioclase + orthopyroxene + clinopyroxene + olivine Trachyte SiO2 = 62.2-68.6 (16) plagioclase + hornblende + orthopyroxene plagioclase + hornblende + orthopyroxene + olivine clinopyroxene plagioclase + orthopyroxene + hornblende clinopyroxene plagioclase + hornblende + orthopyroxene clinopyroxene, biotite plagioclase + orthopyroxene + clinopyroxene Figure 17 plagioclase + hornblende + orthopyroxene + biotite olivine anorthoclase + biotite Rhyolite SiO2 = 74.6-76.1 (3) anorthoclase + plagioclase + biotite + quartz + sanidine hornblende anorthoclase + quartz + plagioclase quartz + anorthoclase + plagioclase + sanidine + biotite Alkali Feldspar Rhyolite SiO2 = 72.9-75.7 (3) anorthoclase + quartz + albite + biotite sanidine, hornblende sanidine + quartz + albite + anorthoclase + biotite olivine anorthoclase + sanidine + quartz + albite + biotite olivine Peralkaline Rhyolite (Comendite) SiO2 = 74.4-74.7 (3) aegirine-augite + quartz + sanidine + aenigmatite quartz + sanidine + aenigmatite + riebeckite aegirine-augite quartz + sanidine + aegirine-augite
14 Textures Photomicrographs of examples of textures and minerals of porphyritic-aphanitic rocks are shown in Figures 12 to 17. Abbreviations: cpx=clinopyroxene (“augite”), hbl=hornblende, ol=olivine, opx=orthopyroxene (“hypersthene”), p=plagioclase, pig=pigeonite. Map units are on I-1663 (Holm, 1988). Numbers are of samples in Appendices 1, 2, and 3. All of the microscope images are about 2.5 mm wide.
Figure 12. Mugearite, sample 3732J, SiO2=52.2, hypohyaline texture. Qao. Crossed polarized light. Figure 13. Mugearite, sample 2705D, SiO2=55.2, hyalo-ophitic texture. Qao. Plane polarized light.
Figure 15. Benmoreite. SiO2 ~ 61, hyalo-ophitic Figure 14. Benmoreite. SiO2 ~ 60, hyalopilitic texture. Pigeonite mantles on orthopyroxene. texture. Qao. Plane polarized light. Qay. Plane polarized light.
15
Figure 16. Benmoreite. Sample 3729I, SiO2=60.4, Figure 17. Trachyte. Sample 3727, SiO2=63.8, hyalopilitic texture. Qao. Plane polarized light. intersertal texture. Qd. Plane polarized light.
Porphyritic and Granular Phaneritic Rocks
Several dikes and plugs in the Core Ridge area of the Inner Basin have phaneritic textures that range from fine to medium grained. Large plutons on Core Ridge and the smaller ridge to its south include an irregularly-shaped dike of quartz monzodiorite (Qmi), a pyroxene leucodiorite plug (Qpli), and a microdiorite plug (Qmdi) (Fig. 18). No radiometric ages have been determined for these map units in the Core Ridge area. On the basis of field position, stratigraphy, petrography, and chemistry, the plugs Qpli and Qmdi are considered to be feeders for Figure 18. Geologic map of the Core Ridge area in the stage 2 stratocone, and the dike Qmi a the Inner Basin of San Francisco Mountain. Map- feeder for the stage 3 stratocone (Holm, unit symbols of plutons described in the text are 1988). circled (Qmi, Qmdi, Qpli). The northeast trending purple dike south of Core Ridge is about 2,350 feet The youngest centrally-erupted lavas from long. Qcc is Central Complex of lavas, tuffs, San Francisco Mountain (Qd and Qay on I- breccias, agglomerates. Map from Holm, 1988. 1663) flowed down all flanks of the volcano except on the southwest side. This younger lava flows (Holm, 1987, 1988). stratigraphy is interpreted as evidence that The elevation of the two plugs (Qpli, the youngest stratocone (eruption stage 3) Qmdi) on the smaller ridge south of Core was constructed northeast of the older stage Ridge is 11,104 ft, which is 3,006 ft (916 m) 2 stratocone, which blocked and diverted the below the projected elevation of the summit
16 of the stage 2 stratocone (14,110 ft, 4,300 m) and is the estimated depth of crystallization of the plugs. The large dike Qmi on Core Ridge is around 11,000 ft (3,353 m) in elevation, which is 4,420 ft (1,347 m) below the projected elevation of the summit of the stage 3 stratocone (15,420 ft, 4,700 m) and is the estimated depth of crystallization of the dike.
Mineral Assemblages and Textures Figure 20. Photomicrograph of microdiorite in Microdiorite (Qmdi) (Appendix 3 Figure 19 in crossed polarized light. Sample 61, Figs. 19-22) is composed, in descending order, of: plagioclase (An59-20), clinopyroxene, opaque oxide, orthopyroxene (En70), alkali feldspar, quartz, olivine, minor amounts of pigeonite, and trace amounts of apatite and ilmenite. Hornblende and biotite are variable, ranging from absent in some samples to minor in others. Hornblende mantles clinopyroxene, pigeonite mantles olivine and clinopyroxene, and biotite mantles olivine, hornblende, and opaque oxide. In counting the mode, pigeonite was counted as clinopyroxene, inverted pigeonite Figure 21. Photomicrograph of alkali feldspar and was counted as orthopyroxene, and quartz in interstitial residuum in microdiorite pseudomorphs of olivine were counted as (Qmdi) displaying micrographic texture. Crossed polarized light. Alkali feldspar is at extinction and olivine. Olivine is altered to brown quartz displays first-order white interference color. phyllosilicates or replaced by magnetite and Micro width of image is about 0.35 mm. orthopyroxene. Classification criteria place the rock in field 10 of the QAPF diagram for the name diorite; micro- notes the fine- grained texture (Le Maitre, 1989).
Figure 22. Photomicrograph of magnetite and orthopyroxene pseudomorph of euhedral olivine crystal in microdiorite (Qmdi); pigeonite mantles the orthopyroxene. Plane polarized light. Micro Figure 19. Photomicrograph of microdiorite width of image is about o.85 mm. (Qmdi), hypidiomorphic texture. Plane polarized light. ol=magnetite+orthopyroxene pseudomorphs of olivine. Micro width of image is about 2.5 mm.
17 Quartz monzodiorite (Qmi) (Appendix 3 Sample 63, Figs. 23-27) is composed, in descending order, of: plagioclase (An55-6), alkali feldspar, clinopyroxene, pigeonite and inverted pigeonite, quartz, orthopyroxene (En60), opaque oxide, olivine, apatite, and ilmenite. Hornblende and biotite range from minor to absent in different specimens. Orthopyroxene mantles olivine; pigeonite mantles orthopyroxene and olivine; and biotite mantles opaque oxide. Classification Figure 25. Photomicrograph of plagioclase crystal criteria place the sample in field 9* of the (lower center) zoned to a mantle of alkali feldspar QAPF diagram for the name quartz (a.f. at extinction), which is in optical continuity monzodiorite (Le Maitre, 1989). with alkali feldspar in granophyric residuum (g.r.) with quartz (Q); quartz displays first order white interference color. Crossed polarized light. Micro width of image is about 2.5 mm.
Figure 23. Photomicrograph of quartz monzodiorite (Qmi) displaying hypidiomorphic Figure 26. Photomicrograph of granophyric texture. Plane polarized light. pig=pigeonite residuum between two plagioclase crystals in quartz mantle on orthopyroxene. Micro width of image is monzodiorite. Micrographic texture of quartz and about 2.5 mm. alkali feldspar. Alkali feldspar in the residuum, at
extinction, is in optical continuity with mantles on
the plagioclase crystals. Image width about o.85 mm.
Figure 24. Photomicrograph of quartz monzodiorite in Figure 23 in crossed polarized Figure 27. Photomicrograph of euhedral olivine light. crystal replaced by serpentine and carbonate and
mantled by pigeonite, in quartz monzodiorite.
Micro width of image is about o.85 mm.
18 Pyroxene leucodiorite (Qpli) (Appendix 3 Sample 65, Figs. 28-33) is composed, in descending order, of: plagioclase (An53-21), clinopyroxene, orthopyroxene (inverted pigeonite, En54), olivine, opaque oxide, pigeonite, alkali feldspar, quartz, apatite. Alkali feldspar mantles plagioclase and occurs interstitially with quartz. Olivine, altered to phyllosilicates and carbonate, is mantled by pigeonite and inverted pigeonite.
Clinopyroxene is mantled by pigeonite. Figure 30. Photomicrograph of phyllosilicate Minor biotite and cummingtonite are pseudomorph of olivine (ol) surrounded by mantle deuteric. Textures range from porphyritic of inverted pigeonite (i pig). Plane polarized light. and fine-grained hypidiomorphic in the Micro width of image is about o.85 mm. margin of the plug to medium-grained hypidiomorphic-granular in the interior.
Figure 31. Photomicrograph of reaction texture between olivine and pigeonite in Figure 30 in crossed polarized light. Inverted pigeonite (i. pig Figure 28. Photomicrograph of pyroxene ex) has exsolution lamellae of augite. Micro width leucodiorite; sample plots in QAPF field 10 (Le of image is about 0.85 mm. Maitre, 1989). Texture is hypidiomorphic-granular. ol=phyllosilicate pseudomorphs of olivine; pig=inverted pigeonite mantle on olivine. Plane polarized light. Micro width of image is about 2.5 mm.
Figure 32. Photomicrograph of inverted pigeonite (i. pig) with inclusions of phyllosilicate alterations of olivine (ol). Pyroxene cleavage is oriented E-W. Plane polarized light. Micro width of image is about 0.35 mm. Figure 29. Photomicrograph of pyroxene leucodiorite in Figure 28 in crossed polarized light. Micro width of image is about 2.5 mm.
19
Figure 33. Photomicrograph of inverted pigeonite Figure 34. Zirconium (ppm) and silica (wt%) crystal in Figure 32 in crossed polarized light. analyses in the San Francisco Mountain volcanic Exsolution lamellae of augite are oriented along system. Red dots are San Francisco Mountain; blue (001) of the original monoclinic pigeonite and along triangles are satellite silicic volcanoes. Sample (100) of the inverted orthorhombic pyroxene. Micro numbers: 2705A, peralkaline rhyolite (Qro); 2736, width of image is about 0.35 mm. Elden Mountain (Qdem); 3713A, North Sugarloaf
(Tdns); 3723, Sugarloaf tephra (Qts); 3723B, Anatectic Rocks Sugarloaf dome (Qrs).
Sugarloaf dome and tephra have zirconium Rubidium does not form its own mineral and rubidium contents that deviate on x-y and is accommodated in potassium minerals graphs from the patterns shown by the other like mica and K-feldspar. Sugarloaf dome analyzed rocks; these trace element contents and tephra have extraordinarily high are interpreted as resulting from partial contents of rubidium compared to the highly melting of crustal rocks. differentiated peralkaline rhyolite (Fig. 35). Because of its high valence and ionic radius zirconium tends to remain in a crystallizing magma until its concentration is high enough for zircon to nucleate and grow. Zircon is most common in higher silica rocks like trachyte and rhyolite, and their plutonic counterparts (Fig. 11). In Sugarloaf dome and tephra zirconium is abnormally low compared to the trend of the igneous-rock series and the highly differentiated peralkaline rhyolite (Fig. 34). Zircon is a common accessory mineral in quartz- and feldspar-rich rocks like Figure 35. Rubidium (ppm) and silica (wt%) granodiorite, quartz monzonite, and granite. analyses in the San Francisco Mountain volcanic If such crustal rocks are partially melted, system. Sample numbers: 2705A, peralkaline rhyolite (Qro); 2736, Elden Mountain; 3713A, zircon is likely to be refractory and would North Sugarloaf (Tdns); 3723, Sugarloaf tephra store zirconium in the crystal residue (Qts); 3723B, Sugarloaf dome (Qrs) (Watson, 1979). The anatectic magma would be high in silica and alkali elements, During crystallization and differentiation and if erupted would produce a rhyolite low of magmas rubidium is concentrated in the in zirconium. liquid until the end stages when it is admitted into orthoclase, microcline, and
20 biotite. During anatexis, rubidium will be 1987). The close spatial and temporal enriched in the first partial melt of a crustal relationship between the volcanic system rock composed of K-feldspar and quartz, and neighboring basalt vents is like granite. Such a magma, if erupted, demonstrated by silicic pumice and ash beds could produce a rhyolite high in Rb, low in at the eastern base of San Francisco Zr, and low in calcium-bearing plagioclase. Mountain (Qpf on I-1663). Here, Sugarloaf dome and tephra also differ scoriaceous basalt lapilli from a nearby from other rhyolites in that they lack Ca- cinder cone is interlayered in silicic pumice bearing plagioclase phenocrysts (Table 2, that erupted from a vent near Fremont Peak alkali feldspar rhyolites; Appendix 3, (Figs. 36, 37 next page). The scoria bed, samples 56, 59). The plagioclase in which contains thin layers of silicic ash, is Sugarloaf is albite 21 Contaminated and Mixed-Magma Rocks Rocks from magmas contaminated by assimilation of country rocks, and rocks from mixtures formed by stirring together magmas of different compositions are difficult or impossible to distinguish by petrographic methods unless field and microscopic evidence remains. Four lines of evidence for addition of external components to certain magmas in the Figure 36. Photograph of quarry face in Pumice of volcanic system are: anatectic textures, Fremont Peak (Qpf on I-1663) at eastern base of glassy globules, antipathetic phenocryst San Francisco Mountain. Dark layer is basalt populations, and mafic maroon xenoliths. scoria interlayered in the pumice and ash deposit. Thin layers of silicic ash in the scoria bed indicate Anatectic Textures simultaneous eruptions. Tape is extended to 1 m. Some metamorphic xenoliths in trachyte lava flows of the Younger Dacite of Reese Peak (Qdry on I-1663) contain evidence for partial melting of crustal rocks, at least locally. Figure 38 shows a pyroxene gneiss xenolith, possibly of granulite facies, with an anatectic texture. Foliation defined by the pyroxene is oriented north-south in the photomicrograph. Figure 38. Photomicrograph of pyroxene gneiss. Left side of image shows gneissic banding of pyroxene (high relief) and plagioclase with granoblastic texture. Right side shows interstitial Figure 37. Photograph of impact structure in basalt glass (tan) from quenched liquid in which euhedral scoria bed at left side of Figure 36. Lapilli size of feldspar crystals have grown. Width of image is scoria suggests the source is a nearby cinder cone. about 0.85 mm. Plane polarized light. Tape, extended 1 m, rests on a silicic ash bed. In crossed polarized light the gneiss is seen to be partly disaggregated, and feldspar crystals have reacted with the anatectic melt 22 to recrystallize and grow euhedral shapes Surface tension on a drop of the smaller (Fig. 39). If these euhedral feldspar crystals volume liquid causes it to draw up in a floated away in the trachyte magma they sphere to present the smallest possible probably could not be distinguished from surface area to the larger volume liquid. original phenocrysts. Antipathetic Phenocryst Populations Some rocks contain crystals whose compositions suggest equilibrium with the host rock, and other crystals whose compositions suggest original growth in a magma of a different composition. For example, Rhyolite of Core Ridge and its feeder plug (Qrcr and Qmgi on I-1663) are dominated by phenocrysts typical of rhyolite, but plot in the trachyte field on the TAS diagram (Fig. 5, p. 8, 3732P, 3733D); Figure 39. Photomicrograph of pyroxene gneiss in a few crystals typical of basalt or mugearite Figure 38 in crossed polarized light. Image is might account for the low silica content overexposed. Glass from quenched anatectic liquid (SiO2=68.2, 68.4). In the center of Figure is isotropic. 41 a euhedral crystal of labradorite (An65) is surrounded sharply by a mantle of alkali Glassy Globules feldspar, and crystals typical of rhyolite are Round glass in a vitrophyre of a different nearby. Plagioclase of An65 likely composition is interpreted as mixing of crystallized originally in a magma with SiO2 magmas. Such a globule is inside a round content about 51.5 percent (An curve on Fig. hypohyaline rhyolite xenolith that is in a 10, p. 12). Labradorite and augite from a mugearite lava flow in the Older Andesite of mafic magma appear to have been stirred San Francisco Mountain (Qao) (Fig. 40). into a rhyolite magma. Figure 40. Photomicrograph of a glassy globule in vesicular rhyolite vitrophyre. Globule has feldspar Figure 41. Photomicrograph of probable mixed- microlites in dark brown glass suggesting its magma lava flow in Rhyolite of Core Ridge (Qrcr). composition is andesitic. Vitrophyre has Phenocrysts are: ano, anorthoclase; sa, sanidine; Q, phenocrysts of anorthoclase, sanidine, quartz, and quartz. Xenocrysts are: p, plagioclase An ; cpx, ferrohedenbergite, and xenocrysts of olivine, augite, 65 augite sharply zoned to aegirine-augite. Mantle and plagioclase An , all in clear glass matrix. 57 around plagioclase xenocryst is alkali feldspar (af). Some plagioclase crystals (not in view) have partial Some anorthoclase phenocrysts have cores of sodic shells of brown andesitic glass. Plane polarized oligoclase to albite (An ). Crossed polarized light. Width of image is 2.5 mm. 20-10 light. Width of image is about 4.5 mm. 23 Mafic Maroon Xenoliths The xenoliths do not appear to be derived Conspicuous in many trachyte lavas in the from lava flows or intrusions in the volcanic volcanic system are mafic maroon xenoliths; system or from subvolcanic intrusions. The they are most common in the lava domes felty arrangement of thin laths of plagioclase and thick lava flows (Fig. 42) and acicular hornblende resembles the textures produced by rapid cooling in controlled experiments (Lofgren, 1980). One possibility is that the xenoliths are megadrops of mafic magma stirred into cooler bodies of silicic magma where they crystallized rapidly in an internal static state. Intrusion of a hotter mafic magma from below into a cooler silicic magma might produce enough disruption to form the xenoliths, stir them into the silicic magma, and give the resulting mixture enough energy to erupt. Such a scenario was described in detail by Eichelberger (1978). Figure 42. Photograph of an outcrop of the Older Dacite of Doyle Peak (Qddo). Xenoliths that range in size from 10 cm to a few millimeters, and xenocrysts of labradorite, augite, and olivine, are so pervasive that the trachyte was not analyzed for chemical composition. The xenoliths range from porphyritic to aphyric, and have variable proportions of plagioclase, augite, and olivine phenocrysts. The matrices are also variable, but most Figure 43. Photograph of a projection of a thin contain thin laths of plagioclase and strongly section of mafic maroon xenolith. Lacy resorbed acicular hornblende in a compact, felty plagioclase phenocryst (PR) in a matrix of felty texture. Other matrix minerals can include acicular hornblende (hbl), plagioclase, and iron hypersthene, olivine, augite, apatite, and oxide. Nonresorbed plagioclase phenocrysts (P) are clear. Plane light. Width of image is about 35 mm., iron oxide. Hornblende may have strong alteration to opaque oxide. If xenoliths are None of the xenoliths were analyzed for vesicular, tridymite is a common occupant whole-rock chemical composition. Silica on vesicle walls. An example of a xenolith percentages are estimated from end-member is illustrated in Figure 43. The matrix contents of olivine (Fo) and plagioclase (An) texture and minerals, especially acicular using the curves in Figure 10, p. 12. The hornblende, contrast sharply with the estimated silica percents indicate that the matrices of the porphyritic-aphanitic lava xenoliths range in composition from basalt flows (Figs. 12 to 17) and the porphyritic- or trachybasalt to trachyandesite (Table 4). phaneritic plugs and dikes (Figs. 19, 23, 28). Some xenoliths contain antipathetic Lavas that contain maroon xenoliths also phenocrysts, which suggests magma mixing. have antipathetic phenocryst populations. 24 Table 4. Petrographic Data of Mafic Maroon Xenoliths Map unit Sample SiO2 by Fo SiO2 by An Probable Lithology Qddh 6-15-76-5 51.6, 61.0 51.7 basalt or trachybasalt or basaltic trachyandesite 6-17-76-1 59.5 trachyandesite 6-18-76-1 59.5 trachyandesite Qdso 7-31-75-1 50.2 basalt or trachybasalt 8-5-75-5 59.2 51.5, 56.7 basalt or trachybasalt or basaltic trachyandesite, and trachyandesite 8-5-75-4 48.4 basalt or trachybasalt Qdem 6-13-77-1 48.0 basalt or trachybasalt Qdff 8-5-75-2 50.0 basalt or trachybasalt Notes: 1. Map units on I-1663: Qddh: Dacite of Dry Lake Hills. Qdso: Older Dacite of Schultz Peak. Qdem: Middle Dacite of Elden Mountain. Qdff: Dacite Lava Flows of Fremont Peak. 2. Silica estimates are determined with 2V of olivine and extinction angles of plagioclase and measured compositions of Fo and An plotted on curves in Figure 10, p. 12. volcano. If this presumed volcano is an VOLCANOGENIC DEVELOPMENT andesitic stratocone, and its summit is in the area of Core Ridge one mile west of Volcanogenic processes and history of the Raspberry Spring dome, then the dome is on San Francisco Mountain volcanic system are the flank of the stratocone. With these interpreted with the stratigraphy, structural geographic positions and elevations the geology, geomorphology, and regional inferred stratocone is projected to a height of volcanic geology displayed on geologic about 1,300 m (4,260 ft) and summit maps MF-1959 (Wolfe et al., 1987a), I-1663 elevation of 3,495 m (11,464 ft). This (Holm, 1988), and I-1446 (Ulrich et al., restoration places map unit Qcc in Figure 18 1984), and the petrographic descriptions, (p. 16) in the summit area of the inferred geochemistry, and radiometric age stratocone. Some Central Complex deposits determinations of the rocks; relevant on Core Ridge support this idea (Fig. 44). information is in Table 5 (p. 27). Protocone The rhyolite dome at Raspberry Spring in the Inner Basin is the oldest known volcanic map unit in San Francisco Mountain (1.82 Ma; Fig. 3, Table 5). The preserved top of the dome is at elevation 10,240 feet (3,121 m). Similar silicic domes in the San Francisco volcanic field have an average height of 1,250 feet (381 m), which, if applied here, places the base of Raspberry Spring dome at about 9,000 feet (2,743 m) elevation. It is unknown what lavas and Figure 44. Photograph of andesitic agglomerate at deposits are between the inferred base of the 11,040 feet elevation (3,365 m) in the Central dome and the estimated base of San Complex on Core Ridge (Qcc). Central position, Francisco Mountain at 7,200 feet (2,195 m) size of fusiform bomb, and structureless character elevation, but one possibility is an initial of the agglomerate suggest proximal deposition. 25 If the inferred andesitic protocone does Adjustments were made for the Hart Prairie exist, and if it postdates the silicic dome at shield volcano that underlies the west side of North Sugarloaf (2.78 Ma), then an early San Francisco Mountain, and the dacite and eruption stage is implied prior to Raspberry rhyolite domes and flows that underlie or are Spring dome (1.82 Ma) for a possible fifth interlayered in the Older Andesite. eruption stage (Table 5, next page). The Pumice of Fremont Peak (Qpf on I - 1663 and Qsfp on MF-1956) is not included Volumes in the erupted volume and composition estimates because of inadequate field data. Volumes of the satellite silicic volcanoes Nevertheless, a sketchy estimate based on were calculated individually with area and thickness, location, and deposit height measurements, supplemented with characteristics of the pumice exposures on I- the formulas for a cone and a cylinder. 1663 and MF-1956 (14 miles north of the Estimation of the volume of a large volcano vent), and assumptions about the densities of like San Francisco Mountain and the the rhyolitic and dacitic pumice, suggests percentages of its different lithologies is the dense-rock equivalent of the pumice fraught with uncertainty. San Francisco might be as much as 4 km3. Also not Mountain is partially dissected, and the included in the erupted volume and upper 2,800 feet (850 m) are well exposed in composition estimates are rhyolite lavas and the Inner Basin. Between elevations 10,000- pyroclastic deposits penetrated by a bore 9,800 feet (3,048-2,987 m) and the hole between 7,580 and 7,320 feet elevation estimated base of San Francisco Mountain at in Hart Prairie on the west flank of San 7,200 feet (2,195 m) the rocks are covered Francisco Mountain. so assumptions have to be made about some Cumulative volumes of the four eruption of the volumes and compositions. San stages are in Table 5. The estimated Francisco Mountain was calculated as a composition percents of the 111.13 km3 of restored, precollapse volcano using map I- the volcanic system are: andesite 83.9%, 1663 lithologies. dacite 14.6%, rhyolite 1.5%. The amount of Volumes of map units exposed in the Inner andesite is probably overestimated, and the Basin, on the outer slopes of San Francisco amounts of dacite and rhyolite are Mountain, and beyond its base below 7,200 underestimated because the buried parts of feet, exclusive of the Older Andesite of San San Francisco Mountain that could not be Francisco Mountain (Qao on I-1663) were identified as dacite or rhyolite are assumed calculated individually using area and to be andesite. Regardless of these thickness measurements. In this way the uncertainties, San Francisco Mountain is distal parts of flows were included, such as classified as an andesitic volcano. the Younger Andesite of San Francisco Mountain (Qay on I-1663) that forms Cedar Geochronology Ridge 10 miles (16 km) north of the volcano (Qa2 on MF-1959 and MF-1960, Wolfe et Geochronology data for the volcanic al., 1987a, Moore and Wolfe, 1987). system includes K-Ar ages determined in the Volume of the Older Andesite of San 1970s, Ar-Ar ages determined in the 2000s, Francisco Mountain (Qao on I-1663) was and paleomagnetic analyses (Table 5). For calculated with the formulas for a cone and a the same map unit, and even the same flow cylinder, and area and thickness unit, the Ar-Ar ages are older measurements for flows beyond the base. 26 Table 5. Stratigraphy of Principal Volcanic Map Units of the San Francisco Mountain Volcanic System Eruption Map units on MF-1959 and I-1663 Lithology Field position relative K-Ar age, Ma Height of stratocones. stage to San Francisco Ar-Ar age, Ma Cumulative volume of volcanic Mountain Fission-track age, Ma (F-t) system. (N = normal polarity) Collapse event (Holm, 2004). 4 Sugarloaf dome Rhyolite Northeast base 0.22±0.02, 0.091±0.002 No new stratocone Lockett Meadow flow Dacite Northeast flank 0.41±0.16, 0.530±0.058 111.13 km3, 100% Collapse of northeast sector Doyle Peak flows (4 flows) Dacite; minor andesite East flank 0.40±0.03 (youngest flow) (central valley) Humphreys Peak cryptodome & dike Dacite Central intrusion White Horse Hills dome Rhyolite Northwest base (N) 3 Younger Andesite of San Francisco Andesite flows Central stratocone 0.43±0.03 (N), o.514±0.021, Stage 3 stratocone, 2,500 m Mountain 0.505±0.009 107.76 km3, 97.0% Elden Mountain domes Dacite Southeast base 0.49±0.06, 0.57±0.03 (N) Agassiz Peak flow Andesite Southwest flank 0.60±0.08 Dacite of San Francisco Mountain Dacite flows Central stratocone 0.75±0.17 (N) Collapse of northeast sector Domes and flows of Reese and Dacite, rhyolite, pumice, Northeast and South 0.80±0.11 (F-t) (N) (ancestral valley) Fremont Peaks; distal pumice block and ash deposit flanks deposit Tuffisite dikes Tuff breccia North flank 2 Older Andesite of San Francisco Mt Andesite flows and tuffs Central stratocone 0.76±0.07 (N); 0.589±0.011, Stage 2 stratocone, 2,100 m 0.556±0.013 98.40 km3, 88.5% Dry Lake Hills domes Dacite South base (N) Dome and flows of Fremont Peak, Peralkaline rhyolite Central, and South 0.70±0.10, 0.87±0.15 (N) and Core Ridge plug and flows and southeast flank Schultz Peak domes Dacite Southeast base 0.75±0.04 (N) Humphreys Peak dome Dacite Northwest base (N) 1 Older Andesite of San Francisco Mt. Andesite flows and tuffs Central stratocone 0.92±0.03 Stage 1 stratocone, 1,700 m Hochderffer Hills dome Rhyolite Northwest base 1.64±0.11 49.68 km3 , 44.7% Raspberry Spring dome Rhyolite Central 1.82±0.16 Central Complex of San Fran. Mt. Andesite pyroclastics Central No date Protocone, 1,300 m Kendrick Park dome Rhyolite Northwest base 2.15±0.13 18.39 km3, 16.5% North Sugarloaf dome Dacite Northeast base 2.78±0.13 Notes:. 1 Stratocone heights are maximums obtained by slope and contact projections to narrow summits; broader summits result in lower heights. Average base elevation of the stratocones is 2,195 m (7,200 ft). Base elevation plus height of stratocones gives summit elevations. 2. K-Ar ages and polarity data are from Wolfe et al., 1987a. 3. Ar-Ar ages are from Karatson et al., 2010, and Morgan et al., 2010. 4. Fission-track age from Ulrich and Bailey, 1987. 5. Lithology names are those used on MF-1959 (Wolfe, et al., 1987a) and I-1663 (Holm, 1988). 6. Satellite silicic volcanoes are in bold font. 7. See text for discussion of Protocone, p. 25. which is about half of the dense-rock equivalent of and younger than the K-Ar ages. The age a large cinder cone like Sunset Crater. differences are not considered important for a Data from Table 5 plotted in Figure 45 (next general discussion, so except for Sugarloaf dome page) show a low rate of eruption for the first 1.5 the K-Ar ages are used for consistency. m.y. followed by an accelerated eruption rate The oldest and youngest volcanoes in the during the early to middle Brunhes chron. The volcanic system are North Sugarloaf (2.78 Ma) and decline in stage 4 might signal exhaustion of the Sugarloaf (0.091 Ma), and the other dated satellite magma supply. volcanoes and dated lava flows in San Francisco Mountain fill much of the age gap between them (Table 5). The ages indicate a long-lived magmatic system that probably was replenished episodically to supply the eruption stages. The average rate of eruption during the active period of about 2.7 m.y. is modest, only 0.041 km3 per one thousand years, 27 The basaltic trachyandesites are all mugearite. The trachyandesites are mostly benmoreites, but include eight latites. The trachytes include two trachydacites. The rhyolites include Ca-plagioclase bearing rhyolite, alkali feldspar rhyolite, and peralkaline rhyolite (comendite). The San Francisco Mountain volcanic system is surrounded by several hundred volcanoes (Wolfe et al., 1987a; Moore and Wolfe, 1987). Most of these volcanoes are basalt, but compositions span the silica spectrum from basalt to rhyolite. A data set from a 10-kilometer-wide ring around San Francisco Mountain volcanic system contains 154 chemical analyses of volcanoes and lava flows, including the silicic center of O’Leary Peak about 10 km northeast of San Francisco Mountain; these analyses are summarized in (Table 7). Table 7. Analyzed Samples in 10-Kilometer- Wide Ring Around San Francisco Mountain Volcanic System TAS Name Number Percent basalt 93 60.4 trachybasalt 17 11.0 Figure 45. Diagram showing the growth history of the San basaltic trachyandesite 16 10.4 Francisco Mountain volcanic system from North Sugarloaf basaltic andesite 5 3.25 (2.78 Ma) to Sugarloaf (0.091 Ma). Dots are the stratocones trachyandesite 11 7.2 that cap eruption stages 1, 2 , and 3. Diamond is the trachyte 9 5.8 inferred protocone in early stage 1; protocone age is from rhyolite 3 1.95 Raspberry Spring dome (1.82 Ma). Total 154 100 The Volcanic System and Surrounding The basalts in the ring around the volcanic system Volcanoes are subdivided into chemical types on the basis of The 160 chemical analyses of the San Francisco their CIPW normative compositions following the Mountain volcanic system tabulated in Appendices classification of Holm (1994). Alkali basalts are slightly more abundant than transitional and 1 and 2 are summarized in Table 6. subalkali basalts combined (Table 8). Table 6. Lithologies and Number of Analyzed Table 8. Definition and Classification of Basalts in Samples in San Francisco Mountain Volcanic 10-km-Wide Ring Around San Francisco Mountain System Volcanic System TAS Name San Francisco Satellite Sum & Basalt Type and Definition Number Percent Mountain Percent basanitic alkali basalt, ne > 5 2 2.1 basaltic 12 0 12, 7.5 trachyandesite alkali basalt, ne > 0 to 5 46 49.4 trachyandesite 82 2 84, 52.5 transitional basalt, ol, di/hy > 2 22 23.7 andesite 3 0 3, 1.9 generally hy < 9 trachyte 30 13 43, 26.9 rhyolite 8 5 13, 8.1 subalkali basalt, ol, di/hy<2 22 23.7 microdiorite* 2 0 2, 1.25 generally hy > 9 quartz 2 0 2, 1.25 quartz subalkali basalt, Q>0 1 1.1 monzodiorite* Total 93 100 microgranite* 1 0 1, 0.6 Total 140 20 160, 100 * IUGS name for plutonic rocks on QAPF diagram. 28 The 154 analyzed rocks in the 10-km-wide ring around the volcanic system are a proxy for the broader San Francisco volcanic field in a smaller and more easily managed data set in the immediate neighborhood of the volcanic system. The percents of the different lithologies in the 10-km-wide ring are very similar to the basalt types and TAS names of 653 analyzed samples from known small vent structures and lava flows in the part of the volcanic field covered by the five MF-series maps (compare Tables 7 and 8 with Appendix 5.4). Only three of the basalts in Table 8 have geochemical characteristics of primary lavas, which are lavas that might have arrived at an eruption site with no or only minor change in chemistry while Figure 46. TAS diagram showing the total alkali and silica passing from the mantle through the crust. distribution of 314 analyses from the San Francisco Mountain volcanic system and a 10-km-wide ring around Primitive features of primary lavas are: Mg# in the the volcanic system. range of 65-72 (Mg-number=100Mg/(Mg+Fe)), MgO >8%, and An-content >50. The three basalts The 314 analyses in Figure 46 form a coherent with primitive features are on MF-1960 (Moore and and continuous series from basalt to rhyolite, and Wolfe, 1987); their vent numbers and data are: only a few outliers have higher or lower alkali Medicine Crater V3818 (next to North elements. The ring analyses merge with and Sugarloaf), transitional basalt, Mg#=70.0, overlay the volcanic system analyses (compare MgO=12.52, An=57.2. Figure 46 with Figures 5 and 6, p. 8 and 9). V3814 (between Sunset Crater and O’Leary In the 10-km-wide ring around the volcanic Peak), subalkali basalt, Mg#=68.6, MgO=12.54, system basalt is common and widespread (Table 7; An=64.6. Wolfe et al., 1987a). Basalt erupted on the V4931 (analysis 4836B, between O’Leary periphery of the volcanic system at Medicine Crater Peak and Strawberry Crater), subalkali basalt, (next to North Sugarloaf), on the north side of Mg#=72.3, MgO=14.34, An=55.6. White Horse Hills, and on the west side of All of the other basalts in Table 8, and all of the Hochderffer Hills. Hawaiite built the Hart Prairie trachybasalts in Table 7, have geochemical shield volcano at the west base of San Francisco characters or petrographic characters, or both, that Mountain (Fig. 2), and erupted at Fern Mountain on indicate variable degrees of differentiation and the north side of Hart Prairie. Within the volcanic possible mixing or contamination, such as Mg- system mugearite erupted on the east side of numbers less than 65, low An-contents, quartz Hochderffer Hills, and basaltic andesite and xenocrysts with pyroxene jackets, plagioclase mugearite erupted between White Horse Hills and crystals with different An-contents, and olivine Hochderffer Hills. crystals with different Fo-contents. The data in Figure 5 and Table 6 document that The analyzed rocks in the 10-km-wide ring basalt is unknown in San Francisco Mountain. The around the San Francisco Mountain volcanic geologic map of the Flagstaff 2-degree sheet (map system have geochemical compositions that place I-1446: Ulrich et al., 1984) gives a bird’s eye view them in a sodium-rich transitional igneous rock of the broad distribution and abundance of basaltic series. All of the trachybasalts are hawaiite. Most cinder cones in the east half of the San Francisco of the basaltic trachyandesites are mugearite; only volcanic field. Notable is the absence of parasitic one is a shoshonite. Most of the trachyandesites are cinder cones on San Francisco Mountain and the benmoreite; only one is a latite. All of the trachytes paucity of cinder cones in the margin of the and rhyolites are at the O’Leary Peak silicic center; volcanic system; as noted above, the only cinder all rhyolites have Ca-plagioclase. cones within the volcanic system are two The combined data sets of San Francisco mugearites and one basaltic andesite between Mountain volcanic system (160 analyses) and the White Horse Hills and Hochderffer Hills. 10-km-wide ring (154 analyses) are shown on the TAS diagram in Figure 46. 29 The Volcanic Field chemically graded derivative magmas and a silicic to andesitic eruption stage. The San Francisco volcanic field has at least 833 These ideas cannot be proven, and evidence for small volcanoes, mostly basaltic cinder cones, but them is largely circumstantial, but they are also cinder cones of trachybasalt, basaltic supported by these points: trachyandesite, trachyandesite, and basaltic andesite (Appendix 5.2, 5.4). Small domes and flows of 1. Most basaltic cones and flows have evolved trachyte and rhyolite, some in polygenetic vent compositions, and rocks from magmas linked by structures, are scattered through the volcanic field, systematic and predictable changes in chemistry are but clusters of silicic to intermediate domes and scattered through the volcanic field, which implies lava flows form only a few large volcanic centers that differentiation likely is a common process. (Fig. 1); San Francisco Mountain is the largest of 2. Lavas progressively richer in silica and alkali these centers. Magma was supplied to many elements and poorer in magnesium, iron, and volcanoes over a broad area, but was erupted in calcium than basalt are successively reduced in large volumes only locally. The San Francisco abundance and volume from basalt to rhyolite Mountain volcanic system might contain as much (compare Tables 6 and 7, Appendix 5.4, and as 22 percent of the estimated 500 km3 of erupted volumes estimated for San Francisco Mountain on lava in the volcanic field (Wolfe, 1992). page 26). These abundance and volume data argue Examination of the five MF-series geologic maps against origin of intermediate rocks by mixing of of the San Francisco volcanic field (Ulrich and basalt and rhyolite. Bailey, 1987; Wolfe et al., 1987a; Newhall et al., 3. Analyzed basalts in the 10-km ring around the 1987; Wolfe et al., 1987b; Moore and Wolfe, 1987) San Francisco Mountain volcanic system have the and Appendix 5 produces these observations: potential to differentiate to silica-rich magmas. The 1. Basalt is the most abundant and widespread rock average basalt, blue square on Figure 47 (next type in the volcanic field. 2. Most mafic lavas have page), plots in the ol-di-hy field. Crystal evolved compositions; only 40 of 408 basalt, fractionation of olivine and clinopyroxene from the basanite, and melanephelinite analyses have average composition will drive derivative liquids primitive compositions. 3. San Francisco Mountain toward increase in hypersthene and, ultimately, is the largest volcano in the volcanic field. 4. Basalt silica oversaturation. The analyses in Figure 47 is unknown in San Francisco Mountain. spread from the average toward hy. Felsic and intermediate rocks with normative nepheline have Subterranean Processes not been found in the San Francisco volcanic field. 4. Stoeser (1973, his Table 4.1) lists xenoliths in A simple explanation for these four observations vent structures and lava flows at 31 localities across is that basalt magma from the mantle penetrated the the San Francisco volcanic field. The xenoliths of crust widely across the volcanic field, but stalled interest are coarse grained, have cumulate textures, and differentiated in many local magma chambers. and are dominated by olivine and clinopyroxene. In a chamber below San Francisco Mountain basalt The xenolith suite is composed primarily of magma differentiated to derivative magmas that wehrlite, olivine clinopyroxenite, clinopyroxenite, erupted to build the silicic domes and andesitic and dunite, but includes small amounts of stratocones. The large volume of San Francisco websterite, gabbro, anorthosite, and troctolite. Mountain implies either that the volcano was Stoeser (1973) interpreted the xenoliths as constructed above a productive mantle source for originating by crystal accumulation in multiple basalt magma or that the intersection of three basalt magma chambers at different crustal depths. regional crustal fracture systems below San Fractional crystallization would produce derivative Francisco Mountain is a favorable conduit for magmas. Later magmas intersected the layered rising magma, or both. Low density and high intrusions and pieces of solidified cumulates were viscosity magmas derived from basalt magma carried as xenoliths to eruption sites. might be a barrier that traps rising fresh basalt 5. Crystal fractionation by incomplete reaction is magma. Episodic resupply of new basalt brings illustrated by textures of the quartz monzodiorite thermal and kinetic energy to the magmatic system, dike in Core Ridge. Although the quartz which could trigger top-down evacuation of monzodiorite has normative quartz (6.74%) olivine was an early phase on the liquidus. At lower 30 delineate the curve (see discussion of anatectic rocks on pages 20-21). Figure 48. Plot of Zr (ppm) and SiO2 (wt %) of 19 samples from the San Francisco Mountain volcanic system. NS, North Sugarloaf; PR, peralkaline rhyolite (Qro); SU, Sugarloaf dome and tephra; EM, Elden Mountain. See Figure 34 for sample numbers. One stage 2 sample is concealed 7. Similar ratios of trace elements that are concentrated in derivative liquids through a range of SiO2 or MgO is consistent with differentiation by fractional crystallization from a common parental magma (Wilson, 1989, p. 352). Figure 49 Figure 47. Plot of normative hy, ol, di, ne of 92 basalt exhibits nearly identical ratios of Ba/Zr through analyses in the 10-km-wide ring around the San Francisco several percent of MgO of samples from the San Mountain volcanic system. One analysis with normative Francisco Mountain volcanic system. Sugarloaf is quartz is not plotted. Blue square is average composition. not part of the differentiated suite. Map unit Qrcr hy=hypersthene, ol=olivine, di=diopside, ne=nepheline might be a mixed magma (Fig. 41). See discussion temperature the olivine became unstable and of North Sugarloaf on p. 21. The Ba/Zr ratios of reacted with the magma to form a rim or mantle of analyzed rocks from the four eruption stages low-calcium pyroxene that stopped the reaction coincide on the line, which implies that parental under the hypabyssal cooling conditions (Fig. 27). magmas in each stage were similar in composition. The lowest temperature fractionated liquid in the dike crystallized interstitially as granophyre, or microgranophyre (Fig. 26). These textures imply that efficient fractional crystallization of large bodies of intermediate magma could produce eruptible batches of rhyolite magma. 6. Curvilinear trends of trace element parts per million through a range of SiO2 or MgO values is consistent with differentiation by fractional crystallization (Wilson, 1989, p. 84). Figure 48 displays a curvilinear trend of Zr that increases to very high content in the highly differentiated peralkaline rhyolite. Zirconium contents of analyzed rocks from the four eruption stages Figure 49. Plot of Ba/Zr ratios (ppm of elements) against coincide on the curve, which implies that parental MgO (wt %) of 18 samples from the San Francisco magmas in each stage were similar in composition. Mountain volcanic system. One sample of stage 2 is Sugarloaf is not related to the samples that concealed. Sample PR in Figure 48 did not plot because of a lack of MgO in the analysis (Appendix 1, 2705A). 31 Parental Magmas Because the trace element data in Figures 48 and Geologic mapping, K-Ar ages, and paleomagnetic 49 indicate similar parental magma compositions data indicate that the volcanogenic development of for the four eruption stages, new batches of the volcanic system was contemporaneous with primitive magma might have mixed and blended in basaltic volcanism in the surrounding San a magma chamber to an average composition Francisco volcanic field (Wolfe et al., 1987a; similar to the average in Figure 47. Moore and Wolfe, 1987; Tanaka et al., 1986; Figs. 36 and 37, p. 22). Thus, it is feasible that some Rhyolites and the Granite System basalt magmas entered the crust below the location of the San Francisco Mountain volcanic system. Rhyolites in the San Francisco Mountain volcanic Some, if not all, of the parental basalts could have system contain 73 to 76 percent SiO2 (normalized) been primary, or nearly primary magmas from the and greater than 90 percent normative Q+ab+or. mantle. Out of 408 analyzed basalts, basanites, and Compositions of the rhyolites are comparable to melanephelinites in the mapped part of the San many granites, so the rhyolites can be evaluated Francisco volcanic field 40 have geochemical with the experimental results of Tuttle and Bowen characteristics of primitive lavas (Appendix 5.5). (1958), Luth et al. (1964), and Winkler (1974) on Thirty-five of the primitive basalts are on the the granite system (quartz-albite-orthoclase-water). Southwest, Central, and East sheets of the MF- Plotted on the granite system in Figure 50 are seven series maps; the Northwest and SP Mountain rhyolites from the four eruption stages, and the sheets together have only five primitive basalts. interstitial microgranophyre in the quartz Data from Appendix 5.5a, reproduced in Table 9, monzodiorite dike in Core Ridge (Fig. 26, p. 18). indicate increasing numbers of primitive lavas with In the granite system increasing PH2O shifts the greater saturation in silica across the volcanic field liquidus temperature minima and ternary eutectics from west to east. away from quartz and toward albite as the temperature lowers from 770ºC at 0.5 kb to 625ºC Table 9. Summary of Primitive Lavas in at 10 kb in the anorthite-free system, and from Southwest, Central, and East MF-Series Maps 695ºC at 2 kb to 645ºC at 10 kb in the system with in San Francisco Volcanic Field. Data from Ab/An=2.9 (Winkler, 1974). Addition of anorthite Appendix 5.5a to the experimental system shifts the temperature Primitive Lava Southwest Central East minima and ternary eutectics away from albite and MF-1958 MF-1959 MF-1960 toward quartz and orthoclase (Fig. 50). Melanephelinite 2 0 0 The rhyolites in Figure 50 plot in two groups. Basanite 1 0 0 Group one samples, 1, 2, 4, 5, 6, lie on a line that is Basanitic alkali 5 3 1 parallel with the anorthite-free data points, but basalt closer to the anorthite-bearing data points. These Alkali basalt 5 4 5 rhyolites might have originated at water pressures Transitional 1 1 3 of about 2 to 4.5 kilobars, perhaps at middle crustal basalt depths of 8 to 18 km. The suggestion is that group Subalkali basalt 0 1 3 one rhyolites are derivatives from mafic parental Total 14 9 12 magmas by processes of fractional crystallization. Group two samples, 3 and 7, lie on a line parallel to The totals in Table 9 show that the Central sheet the anorthite-bearing system, and near the data has the fewest samples of primitive basalts, even points for the system with Ab/An=2.9. Group two though MF-1959 covers the largest area of the three rhyolites might be from anatectic melts that MF-series maps (Wolfe et al., 1987a, p. 30). The originated in the lower crust (see p. 20 and 21 for large area covered by the volcanic system might discussion of Sugarloaf and Hochderffer Hills). account for the apparent “deficiency” of primitive Except for sample 2, the apparent depths of origin basalts on the Central sheet if rising primitive of the rhyolites are within or near the magmas supplied a large, active, and growing compressional wave low-velocity region that magmatic system in the lower to middle crust. If Stauber (1982) identified at 9 to 34 km below sea this is true, then the data in Table 9 suggest level under San Francisco Mountain. multiple basalt types, but skewed toward hypersthene normative, fed the magmatic system. 32 Figure 50. Comparison of rhyolite and microgranophyre compositions representative of the four eruption stages with liquidus temperature minima and ternary eutectics at various pressures in the system Q-Ab-Or-H2O. Shaded part of inset diagram shows composition area displayed; Q, quartz; Ab, albite; Or, orthoclase. Black diamonds are data points of anorthite-free system under different kilobars (kb) of water pressure; black dots are data points for anorthite-bearing system. Experimental data from Winkler, 1974, Table 18-2. Plot numbers, rock units, and sample numbers are: 1. Kendrick Park dome (KP), 3604; 2. Raspberry Spring dome (RS , Qrro), 3734A; 3. Hochderffer Hills dome (HH), 3616A; 4. Rhyolite of Fremont Peak dome (Qrf), 3733; 5. Pumice of Fremont Peak (Qpf), 3819B; 6. White Horse Hills dome (WH), 3612; 7. Sugarloaf dome (SU, Qrs), 3723B; 8. interstitial microgranophyre in quartz monzodiorite dike (Qmi, Fig. 26, p. 18). Granophyric Residuum and modal analyses of the quartz monzodiorite and assumptions about the distribution of normative The residual interstitial microgranophyre in the An, Ab, and Or between plagioclase and alkali quartz monzodiorite dike in Core Ridge is sample 8 feldspar (Appendix 2 sample 3733C, Appendix 3 in Figure 50. The dike intruded about 1.3 km samples 64 and 63). The modal composition of the below the projected elevation of the summit of the microgranophyre is comparable to compositions of stage 3 stratocone. The density of the stage 3 cone granophyre from other locations (Table 10). above the dike is unknown, but a density range of The quartz monzodiorite contains 6.74 percent 2.0 to 2.5 gives a lithostatic pressure of 254 bars to normative quartz (Appendix 2, 3733C) and 1.2 318 bars (0.25-0.32 kb). Late to post-magmatic percent modal olivine (Appendix 3, sample 63). hornblende and biotite occur as small rims on Olivine crystals are mantled by pigeonite (Fig. 27) opaque oxide, but the late melt probably was not and plagioclase is strongly zoned (Fig. 25, p. 18). water saturated (PH2O 33 Table 10. Modal Compositions of Granophyre REFERENCES CITED in Weight Percent. Foreign data from Dunham, 1965. Arculus, R.J. and Gust, D.A., 1995, Regional Location Quartz Feldspar petrology of the San Francisco volcanic Tasmania (6) 44.6 55.4 field, Arizona, USA: Journal of Petrology, Rhum (9) 43.8 56.2 v. 36, no. 3, p. 827-861. 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