June 2021CONTRIBUTED REPORT CR-21-CCONTRIBUTED REPORT CR-21-C

Arizona Geological Surveyazgs.arizona.edu | repository.azgs.az.gov

Petrography, Geochemistry, and Volcanogenic Development of the San Francisco Mountain Volcanic

System, Northern Arizona

Richard F. HolmNorthern Arizona University

Arizona Geological SurveyP.A. Pearthree, Arizona State Geologist and Director

Manuscript approved for publication in June 2021Printed by the Arizona Geological Survey

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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.

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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

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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

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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.

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ABSTRACT San Francisco Mountain and eight satellite silicic volcanoes at its base constitute the San Francisco Mountain volcanic system. The central composite volcano is the largest edifice in the San Francisco volcanic field, a basaltic field on the southwestern margin of the Colorado Plateau in northern Arizona. Satellite volcanoes range from single domes to clusters of as many as eight domes. Vent locations and many internal volcano structures (dikes) coincide with pre-volcanic crustal structures on northwest, north, and northeast trends. The volcanic system was active from late Pliocene to late Pleistocene (2.78-0.091 Ma). Lava erupted in four stages, or compositional cycles, each beginning with rhyolite or dacite domes. The first three stages ended with andesite stratovolcanoes, but the fourth stage erupted only minor andesite. One-hundred-forty chemical analyses of San Francisco Mountain and twenty analyses of the satellite volcanoes characterize the lavas. On the total alkali-silica diagram (TAS) the analyses plot in a continuum from low-silica basaltic trachyandesite through trachyandesite and trachyte to rhyolite. Major-element oxides plot in regular and characteristic trends on Harker diagrams. Most rocks have compatible phenocryst assemblages that change in a regular way with increasing silica content of the host lava. These rocks form a mildly alkaline transitional igneous-rock series. Olivine and clinopyroxene ("augite") typify the low-silica basaltic trachyandesites. With progressive increase in silica, the sequence of crystallization is: orthopyroxene and pigeonite, hornblende, biotite, anorthoclase, sanidine, quartz.

Mg-olivine drops out of the phenocryst assemblage first, but Fe-olivine rejoins the assemblage at higher silica contents. Pigeonite, clinopyroxene, orthopyroxene, and hornblende drop out of the assemblages sequentially. Opaque oxides and plagioclase are members of phenocryst assemblages through the entire range of silica contents. Solid-solution minerals change regularly in end-member proportions with increase in silica contents of host lavas. High-temperature end members, Fo, En, and An in olivine, orthopyroxene, and plagioclase respectively, decrease through the igneous-rock series. Some rocks with anomalous trace-element contents are unique and not part of the igneous-rock series; origin of these lavas by crustal anatexis is possible. Other rocks with antipathetic phenocryst populations or numerous mafic xenoliths appear to be of mixed-magma or contamination origins. Compositions of rhyolites plotted on the granite system suggest middle to lower crustal depths of origin. An eruption history of the volcanic system of about 2.7 million years and multiple compositional cycles imply a long-lived magmatic system and episodic resupply of fresh magma from the mantle. Magmatic differentiation by fractional crystallization of parental basalt magma is judged to be the principal process for the origin of the lavas of the San Francisco Mountain volcanic system. Mixing of magmas within the magmatic system and addition of crystals and liquids from crystalline crustal rocks changed the petrography and chemistry of some lavas. *7550 North Snow Bowl Road, Flagstaff, AZ 86001 richard.holm@nau.edu

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INTRODUCTION San Francisco Mountain is the largest volcano in the late Cenozoic San Francisco volcanic field in northern Arizona (Fig. 1). The structure is over 1,650 m high, and the summit, at 3,850 m (12,633 ft) above sea level, is the highest in the region. The mountain dominates the landscape for miles around.

The volcanic geology of San Francisco Mountain and associated satellite silicic volcanoes has been described and interpreted in several field and laboratory studies. Robinson (1913) devoted much of his classic study of the volcanic field to petrography, petrology, and geochemistry of the rocks. His report contains 23 high-quality wet-chemical analyses, of which nine are of San Francisco Mountain and

satellite volcanoes at its base. Detailed field, petrographic, and geochemical data are given for each analyzed sample. Deal (1969) studied the western end of the Inner Basin with a 1:24,000 scale geologic map and petrographic descriptions, including modes, of the lavas and dikes. Wenrich-Verbeek (l975) measured the section of lava flows on the southeast slope of Humphreys 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 with the 1:50,000 scale geologic map of the central part of the San Francisco volcanic field, which

includes San Francisco Mountain and eight satellite volcanoes. Holm’s 1988 1:24,000 scale geologic map of San Francisco Mountain and five satellite volcanoes includes detailed field and petrographic descriptions of 60 extrusive and intrusive map units. Arculus and Gust (1995) included San Francisco Mountain in their field-wide survey of geochemical data and petrologic interpretations of the origin of the

Figure 1. San Francisco Mountain is in the central part of the San Francisco volcanic field (pink). Inset map shows the position of the volcanic field (SFVF) 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 intermediate rocks are green. Numbers are ages of rocks in millions of years.

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lavas. Wenrich-Verbeek (1972) interpreted the mechanism of emplacement of the White 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 by the current widely-used chemical classification of volcanic rocks: total alkali-silica diagram, or TAS. New petrographic information includes modal analyses, mineral compositions, texture descriptions, and representative photomicrographs. Rocks are grouped into categories according to texture and inferred processes of origin. The report concludes with a discussion of volcanogenic development and interpretation of magma genesis. SAN FRANCISCO MOUNTAIN VOLCANIC SYSTEM The volcanic system is defined as consisting of the San Francisco Mountain composite volcano and eight satellite silicic volcanoes at its base. Clockwise from northeast these satellites (and abbreviations) are: North Sugarloaf (NS), Sugarloaf (SU), Schultz Peak (SP), Elden Mountain (EM), Dry Lake Hills (DL), Hochderffer Hills (HH), Kendrick Park (KP), White Horse Hills (WH) (Fig. 2). Hart Prairie shield

volcano (HP) is at the western base of San Francisco Mountain.

Figure 2. Google Maps image of the San Francisco Mountain volcanic system. San Francisco Mountain is the high-relief structure in the center of the image. See text for definitions of the symbols. Six satellite volcanoes lie in a northwest trending lane that includes the central conduit system of San Francisco Mountain (red overlay on Fig. 2). North Sugarloaf and Sugarloaf are in a northeast trending structural alignment that includes large dikes in the core of the composite volcano (Fig. 18, p. 16), the axis of the large breach in the northeast quadrant of San Francisco Mountain, a linear negative aeromagnetic anomaly northeast of Sugarloaf (Sauck and Sumner, 1970), and the O’Leary Peak silicic center 8 km northeast of Sugarloaf (Figs. 1 and 2). These two volcanic and geophysical alignments coincide with regional structures in the northwest trending Cataract Creek fault system and southwest projection of the Doney structural system (Ulrich et al., 1984). Not evident in Figure 2 is the north trending Oak Creek Canyon fault system that displaces Permian and Triassic strata and lava flows north and south of San

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Francisco Mountain (Ulrich et al., 1984; Shoemaker et al., 1978). The volcanic system is at the intersection of these three regional structural elements. San Francisco Mountain San Francisco Mountain was constructed on a platform of late Miocene, and possibly younger, basalt lava flows that overlie Permian limestone and Triassic sandstone. The oldest-known lava in the volcano is a rhyolite dome at Raspberry Spring (1.82 Ma) in the Inner Basin (Fig. 3; Holm, 1988). The dome is overlain by stratified lava flows and pyroclastic deposits of andesite. This

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 domes and ended with construction of andesitic stratovolcanoes, the latest of which has a K-Ar age of 0.43 Ma (Holm, 1988, 2004) and Ar-Ar age of 0.51 Ma (Karatson et al., 2010). The fourth eruption stage produced several silicic domes and flows (0.40 Ma), but only one andesite flow. Andesite flows and pyroclasts erupted principally from vents above the central

conduit system, whereas silicic lavas typically constructed domes on the flanks and at the base of the central stratocones. Buried rhyolite and dacite domes are exposed on the south and northeast sides of Inner Basin, and partly buried domes are on the outer slopes of Humphreys, Fremont, and Doyle Peaks (Fig. 3; Holm, 1988). Core Ridge and the adjacent ridge to the south (Fig. 3) consist of lava flows, tuffs, tuff breccias, agglomerates, and agglutinates that are intruded by large dikes and plugs. A quartz monzodiorite dike on Core Ridge that supplied the stage 3 stratocone is about 1 km northeast of the diorite feeder plug of the 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 4. Rose diagram showing the strikes of dikes in San Francisco Mountain. Andesite dikes outnumber dacite dikes. Open double arrow shows direction of regional extensional stress(Wong and Humphrey, 1989). Diagram from Holm, 2004). The individual peaks on San Francisco Mountain are erosional remnants of the fully developed compound composite volcano, the stratocones of which predate the Inner

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Basin (Fig. 3). The Inner Basin and Interior Valley developed sometime between construction of the latest (stage 3) stratocone and the eruption of Sugarloaf at the mouth of Interior Valley (0.43-0.091 Ma). The likely process of origin is sector collapse of the northeast side of San Francisco Mountain, which produced a large debris fan down slope to the northeast (Holm, 2004). Erosion, mass wasting, and glaciation enlarged Inner Basin to its modern bowl shape (Fig. 3). Satellite Silicic Volcanoes Ages and compositions of the satellite volcanoes correlate well with the active history and geochemical characteristics of San Francisco Mountain. The oldest and youngest satellites predate and postdate the K-Ar age range of San Francisco Mountain (1.82-0.40 Ma), and with one or possibly two exceptions (Fig. 2, SU, HH) compositions of the satellites coincide with the compositions of the trachytes and rhyolites in the central volcano (Table 1). Table 1. Satellite Silicic Volcanoes Satellite Structure Composition Age Ma North Sugarloaf

lava dome trachyte 2.78 ± 0.13

Sugarloaf lava dome. rhyolite 0.091 ± 0.002

Schultz Peak

lava dome trachyte 0.75 ± 0.04

Elden Mountain

four lava domes

trachyte and trachydacite

~0.53*

Dry Lake Hills

eight lava domes

trachyte ~0.75**

Hochderffer Hills

lava dome rhyolite 1.64 ± 0.11

Kendrick Park

lava dome rhyolite 2.15 ± 0.13

White Horse Hills

intrusive dome

rhyolite, benmoreite, latite

normal polarity

* Average of 0.49 and 0.57 determined on two flows on the southeast side of Elden Mountain. ** Estimate based on normal polarity, stratigraphy, and more subdued by erosion than Elden Mountain.

Sample locations, K-Ar ages, paleomagnetic data, and compositions of the satellite volcanoes are on MF-1959 (Wolfe et al., 1987a). Sample location and Ar/Ar data of Sugarloaf are in Morgan et al., 2010. CHEMISTRY AND CLASSIFICATION OF LAVAS Chemical analyses of 135 samples from San Francisco Mountain and analyses of 20 samples from the satellite volcanoes were extracted from Table 2 in MF-1959 (Wolfe et al., 1987a) and placed in a separate set of analyses; added to this data set are three analyses of lavas on MF-1960 (3832, 3833, 4802, Moore and Wolfe, 1987) and two analyses of pyroclasts from a distal fall deposit on MF-1956 (5828B, 5828C, Ulrich and Bailey, 1987). These 160 analyses, which include 19 with trace element abundances from Table 5 in MF-1959, are reproduced here in Appendix 1. Normalized analyzes adjusted to 100 percent of major oxides are in Appendix 2. The normalized analyses are plotted on the total alkali-silica diagram (TAS diagram, Le Maitre, 1989) in Figures 5 and 6 to classify the lavas in the volcanic system.

Figure 5. Total alkali-silica diagram with 140 analyses from San Francisco Mountain (red dots) and 20 analyses from satellite volcanoes (blue triangles). Samples of probable mixed magmas are 3732P, Qrcr and 3733D, Qmgi (Appendices 2, 4, and discussion on p. 23).

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The 140 analyses of samples from San Francisco Mountain form a mildly alkaline transitional series from basaltic trachyandesite to rhyolite. Sodium exceeds potassium in amounts sufficient to classify all basaltic trachyandesites as mugearite and most trachyandesites as benmoreite; only eight trachyandesites have sufficient potassium to be classified as latite. Three samples plot slightly below the alkali line in the andesite field. In the trachyte-trachydacite field most samples are deficient in normative quartz and so classify as trachyte; only two samples have enough Q to classify as trachydacite. Five rhyolite samples have normative acmite and classify as peralkaline rhyolite (or comendite). The four percent silica gap in the rhyolite field has no obvious explanation. The gap could be real because no lavas in the volcanic system fill in the gap, or it could be a result of no samples from buried lavas. Five specimens in Figure 5 are phaneritic, and are classified with the modal Quartz-Alkali feldspar-Plagioclase-Feldspathoid (QAPF) diagram for plutonic rocks (Le Maitre, 1989). Phaneritic lithologies are leucodiorite, microdiorite, quartz monzodiorite, and microgranite. Twenty analyses of satellite volcanoes range from trachyandesite to rhyolite (Fig. 6). All analyses coincide with the San Francisco Mountain series (Fig. 5).

Figure 6. Total alkali-silica diagram with 20 analyses from satellite silicic volcanoes.

In Figure 6 the two trachyandesites are post-rhyolite pipe-like bodies at White Horse Hills; one is benmoreite and the other is latite. In the trachyte-trachydacite field one sample is trachydacite and the others are trachyte. On Harker diagrams the oxides in the combined data set of 160 analyses display regular trends (Fig. 7, next page). Fairly tight negative slopes across the diagrams are displayed by TiO2, FeO, MgO, and CaO. On the K2O diagram, some satellite rhyolites, trachytes, and a latite contain higher contents of potassium than the San Francisco Mountain series. The two trachytes at just under 5 per cent K2O are a rhyolite lava flow (3732P) and its alkali microgranite feeder plug (3733D); both are mixed with a more mafic component that lowered the silica content (Qrcr and Qmgi on I-1663, Holm, 1988, and Fig. 5). The mugearites and benmoreites show some scatter on the Al2O3 and P2O5 diagrams. Several “younger andesites” (Qay on map I-1663) have unusually high P2O5. The inflection in the P2O5 curve at about 60 percent SiO2 is close to the appearance of apatite in the mineral assemblages (Fig. 11, p. 13). The Na2O diagram has an open pattern of analyses, which trends in a broad band of increasing Na2O to the rhyolites where Na2O declines abruptly. Plagioclase 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).

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Figure 7. Harker diagrams of the San Francisco Mountain volcanic system. Red dots: San Francisco Mountain. Blue triangles: satellite silicic volcanoes.

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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 Sugarloaf dome (SU), and the latite with high ratio is a pipe at White Horse Hills (WH). Field and Map Classification Mapping of San Francisco Mountain and the satellite silicic volcanoes was conducted and completed in the 1970s. The published maps are MF-1959 (Wolfe et al., 1987a) and I-1663 (Holm, 1988). For mapping purposes the porphyritic-aphanitic rocks were classified with hand-specimen and thin-section descriptions as basalt, andesite, dacite, and rhyolite, largely using phenocryst assemblages as criteria. As chemical analyses became available, lithologic boundaries based on silica content were identified that correlated well with mineral assemblages. With increasing silica, these boundaries are: basalt-andesite: 52 % SiO2, abrupt increase of plagioclase and diminished olivine and clinopyroxene; andesite-dacite: 62 % SiO2, appearance of biotite and disappearance of olivine; dacite-rhyolite: 70 % SiO2, appearance of alkali feldspar and quartz and disappearance of orthopyroxene. The maps were still in editorial review and preparation for publication in 1986 when the total alkali-silica diagram (TAS) was recommended for volcanic rocks by the International Union of Geological Sciences (IUGS) Subcommission on the Systematics

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 Methods Petrographic data were obtained from thin sections with traditional methods using a Leitz polarzing microscope, Swift electronic mechanical stage, and Leitz universal stage. Most modes were obtained with more than one thousand points to ensure that sparse phenocrysts were counted. Plagioclase compositions were estimated with extinction angles measured on a-normal crystals and the �� curve in Deer, Howie and Zussman, 1963b, Figure 55, p. 138. Olivine and orthopyroxene compositions were estimated with 2V angles measured with a Leitz universal stage and the curves in Deer, Howie and Zussman 1962, Figure 11, p. 22 (olivine) and 1963a Figure 10, p. 28 (orthopyroxene). In most thin sections several crystals of each mineral were measured and the optical data averaged. Porphyritic-Aphanitic Rocks Most rocks in the volcanic system have phenocrysts set in aphanitic matrices; which range from holocrystalline through hypocrystalline to holohyaline (glassy). Holocrystalline matrices range from microcrystalline to cryptocrystalline. Phenocrysts and Microphenocrysts Petrographic descriptions and modal analyses of samples in the volcanic system

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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 Appendix 3. For comparison at the low silica end of the series modes of five samples of basalt from the eastern San Francisco volcanic field are included in Appendix 3.

Figure 9. Histogram showing the percent of phenocrysts and microphenocrysts counted in modes of porphyritic-aphanitic samples from the San Francisco Mountain volcanic system (55), O’Leary Peak silicic center (9), and five basalts from the eastern San Francisco volcanic field. Silica Ranges of Minerals Chemical analyses of 160 samples from the volcanic system (Appendices 1 and 2) combined with thin section descriptions of

each sample document the range of silica 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, 1974) are included to show the petrographic transition from basalt to basaltic trachyandesite. Note that Figure 10 shows only the relative normalized proportions of phenocrysts and microphenocrysts through the range of silica values; the diagram does not show mineral assemblages or modal percents in individual samples at specific silica values. For mineral percents go to Appendix 3.

Figure 10. Diagram showing the silica ranges of phenocrysts and microphenocrysts and their relative proportions when normalized to 100 percent. Colored lines show high-temperature end-member compositions of plagioclase (An), olivine (Fo), and orthopyroxene (En). n is number of crystals measured .

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The mineral composition curves in Figure 10 were constructed with rolling averages at data points of one-half percent silica for plagioclase and one percent silica for olivine and orthopyroxene. The five basalt samples from Moore (1974) were selected because they have microprobe data of the compositions of the phenocrysts. Plagioclase replaces olivine as the dominant phenocryst phase in the transition from basalt to basaltic trachyandesite, and is dominant through the series until it is replaced by alkali feldspar in the rhyolites. An-contents of phenocryst cores in the basalts range from 74 to 67; An-content in the lowest silica basaltic trachyandesite is 64. An-contents decline steadily through the series to 16 in the highest silica rhyolites; these rhyolites plot in field 3 of the QAPF diagram recommended by the International Union of Geological Sciences (Le Maitre, 1989). Plagioclase phenocrysts in some rhyolites have An-contents less than five and these are classified as alkali feldspar rhyolite, which plot in field 2 of the QAPF diagram; examples are Sugarloaf and Hochderffer Hills (SU and HH in Figure 10). A third group of rhyolites lack plagioclase; these are peralkaline rhyolites (comendite) characterized by phenocrysts of sanidine. Olivine decreases sharply in amount from the basalts to the mugearites, declines steadily through the mugearites and 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-contents of olivine decrease from 28 to 0 in high-silica trachytes through the rhyolites.

Orthopyroxene appears in mugearites at about 54 percent silica, with En-contents around 70, and is present through the trachytes to 67 percent silica and En34. Clinopyroxene (“augite”) and hornblende are present in assemblages through a wide range of silica contents. Clinopyroxene drops out in the high-silica trachytes, but hornblende is present in decreasing amounts well into the rhyolites. Biotite appears in assemblages at 62 percent silica, and is the principal mafic mineral in the rhyolites. Anorthoclase joins the assemblages of the highest-silica trachytes, and together with sanidine are the dominant feldspars in the high-silica rhyolites. Quartz is present as a few scattered crystals in some trachytes (see below), but is a principal mineral only in the rhyolites. Opaque oxides are ubiquitous in all assemblages through the entire range of silica values. Other minerals were not included in Figure 10 because of small size (most apatite and zircon), low abundance (quartz in trachytes, ferrohedenbergite in rhyolites, tridymite and cristobalite in vesicles), or not readily identifiable (difficult to distinguish pigeonite from augite on a mode traverse) (Fig. 11). The sodium-iron minerals riebeckite, aegirine, and aenigmatite were not included in Figure 10 because of limited silica range (Fig. 11).

Figure 11. Silica ranges of minerals not shown in Figure 10. See text above for discussion. Dash line indicates sparse amounts.

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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

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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 14. Benmoreite. SiO2 ~ 60, hyalopilitic texture. Qao. Plane polarized light.

Figure 15. Benmoreite. SiO2 ~ 61, hyalo-ophitic texture. Pigeonite mantles on orthopyroxene. Qay. Plane polarized light.

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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 the stage 2 stratocone, and the dike Qmi a feeder for the stage 3 stratocone (Holm, 1988). The youngest centrally-erupted lavas from San Francisco Mountain (Qd and Qay on I-1663) flowed down all flanks of the volcano except on the southwest side. This stratigraphy is interpreted as evidence that the youngest stratocone (eruption stage 3) was constructed northeast of the older stage 2 stratocone, which blocked and diverted the

Figure 18. Geologic map of the Core Ridge area in the Inner Basin of San Francisco Mountain. Map-unit symbols of plutons described in the text are circled (Qmi, Qmdi, Qpli). The northeast trending purple dike south of Core Ridge is about 2,350 feet long. Qcc is Central Complex of lavas, tuffs, breccias, agglomerates. Map from Holm, 1988. younger lava flows (Holm, 1987, 1988). The elevation of the two plugs (Qpli, Qmdi) on the smaller ridge south of Core Ridge is 11,104 ft, which is 3,006 ft (916 m) below the projected elevation of the summit

Figure 16. Benmoreite. Sample 3729I, SiO2=60.4, hyalopilitic texture. Qao. Plane polarized light.

Figure 17. Trachyte. Sample 3727, SiO2=63.8, intersertal texture. Qd. Plane polarized light.

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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 Microdiorite (Qmdi) (Appendix 3 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 was counted as orthopyroxene, and pseudomorphs of olivine were counted as olivine. Olivine is altered to brown phyllosilicates or replaced by magnetite and 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 19. Photomicrograph of microdiorite (Qmdi), hypidiomorphic texture. Plane polarized light. ol=magnetite+orthopyroxene pseudomorphs of olivine. Micro width of image is about 2.5 mm.

Figure 20. Photomicrograph of microdiorite in Figure 19 in crossed polarized light.

Figure 21. Photomicrograph of alkali feldspar and quartz in interstitial residuum in microdiorite (Qmdi) displaying micrographic texture. Crossed polarized light. Alkali feldspar is at extinction and quartz displays first-order white interference color. Micro width of image is about 0.35 mm.

Figure 22. Photomicrograph of magnetite and orthopyroxene pseudomorph of euhedral olivine crystal in microdiorite (Qmdi); pigeonite mantles the orthopyroxene. Plane polarized light. Micro width of image is about o.85 mm.

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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 criteria place the sample in field 9* of the QAPF diagram for the name quartz monzodiorite (Le Maitre, 1989).

Figure 23. Photomicrograph of quartz monzodiorite (Qmi) displaying hypidiomorphic texture. Plane polarized light. pig=pigeonite mantle on orthopyroxene. Micro width of image is about 2.5 mm.

Figure 24. Photomicrograph of quartz monzodiorite in Figure 23 in crossed polarized light.

Figure 25. Photomicrograph of plagioclase crystal (lower center) zoned to a mantle of alkali feldspar (a.f. at extinction), which is in optical continuity 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 26. Photomicrograph of granophyric residuum between two plagioclase crystals in quartz monzodiorite. Micrographic texture of quartz and 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 27. Photomicrograph of euhedral olivine crystal replaced by serpentine and carbonate and mantled by pigeonite, in quartz monzodiorite. Micro width of image is about o.85 mm.

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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. Minor biotite and cummingtonite are deuteric. Textures range from porphyritic and fine-grained hypidiomorphic in the margin of the plug to medium-grained hypidiomorphic-granular in the interior.

Figure 28. Photomicrograph of pyroxene leucodiorite; sample plots in QAPF field 10 (Le 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 29. Photomicrograph of pyroxene leucodiorite in Figure 28 in crossed polarized light. Micro width of image is about 2.5 mm.

Figure 30. Photomicrograph of phyllosilicate pseudomorph of olivine (ol) surrounded by mantle of inverted pigeonite (i pig). Plane polarized light. Micro width of image is about o.85 mm.

Figure 31. Photomicrograph of reaction texture between olivine and pigeonite in Figure 30 in crossed polarized light. Inverted pigeonite (i. pig ex) has exsolution lamellae of augite. Micro width of image is about 0.85 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.

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Figure 33. Photomicrograph of inverted pigeonite crystal in Figure 32 in crossed polarized light. Exsolution lamellae of augite are oriented along (001) of the original monoclinic pigeonite and along (100) of the inverted orthorhombic pyroxene. Micro width of image is about 0.35 mm. Anatectic Rocks Sugarloaf dome and tephra have zirconium and rubidium contents that deviate on x-y graphs from the patterns shown by the other analyzed rocks; these trace element contents are interpreted as resulting from partial melting of crustal rocks. 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 granodiorite, quartz monzonite, and granite. If such crustal rocks are partially melted, zircon is likely to be refractory and would store zirconium in the crystal residue (Watson, 1979). The anatectic magma would be high in silica and alkali elements, and if erupted would produce a rhyolite low in zirconium.

Figure 34. Zirconium (ppm) and silica (wt%) analyses in the San Francisco Mountain volcanic system. Red dots are San Francisco Mountain; blue triangles are satellite silicic volcanoes. Sample numbers: 2705A, peralkaline rhyolite (Qro); 2736, Elden Mountain (Qdem); 3713A, North Sugarloaf (Tdns); 3723, Sugarloaf tephra (Qts); 3723B, Sugarloaf dome (Qrs). Rubidium does not form its own mineral and is accommodated in potassium minerals like mica and K-feldspar. Sugarloaf dome and tephra have extraordinarily high contents of rubidium compared to the highly differentiated peralkaline rhyolite (Fig. 35).

Figure 35. Rubidium (ppm) and silica (wt%) analyses in the San Francisco Mountain volcanic system. Sample numbers: 2705A, peralkaline rhyolite (Qro); 2736, Elden Mountain; 3713A, North Sugarloaf (Tdns); 3723, Sugarloaf tephra (Qts); 3723B, Sugarloaf dome (Qrs) During crystallization and differentiation of magmas rubidium is concentrated in the liquid until the end stages when it is admitted into orthoclase, microcline, and

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biotite. During anatexis, rubidium will be enriched in the first partial melt of a crustal rock composed of K-feldspar and quartz, like granite. Such a magma, if erupted, could produce a rhyolite high in Rb, low in Zr, and low in calcium-bearing plagioclase. Sugarloaf dome and tephra also differ from other rhyolites in that they lack Ca-bearing plagioclase phenocrysts (Table 2, alkali feldspar rhyolites; Appendix 3, samples 56, 59). The plagioclase in Sugarloaf is albite <An5, which in the QAPF diagram for volcanic rocks places the rock in field 2 for the name alkali feldspar rhyolite (Le Maitre, 1989). The rhyolite dome at Hochderffer Hills has not been analyzed for trace elements, but its plagioclase is low-Ca albite, so the dome is also classified as alkali feldspar rhyolite (Appendix 3, Sample 52). Hochderffer Hills rhyolite might be anatectic in origin, but this needs to be investigated. The contrast in plagioclase compositions between rhyolites is apparent in Figure 10, where HH=Hochderffer Hills and SU=Sugarloaf. Plagioclase in both rhyolite domes has An-contents less than 5, but other rhyolites have An-contents of 16. Peralkaline rhyolites lack plagioclase (Appendix 3, Samples 53, 54, 57) Basalt magma that intrudes from the mantle is a source of heat for melting crustal rocks. Crustal chambers that stored basalt magma long enough for crystallization and accumulation of olivine, pyroxene, and plagioclase are documented by cumulous xenoliths in basalt cinder cones and lava flows throughout the san Francisco volcanic field (Stoeser, 1973). Heat released from such chambers could reasonably be expected to melt crustal rocks, at least partly. San Francisco Mountain and its satellite silicic volcanoes are surrounded by basalt vent structures that are broadly contemporaneous with the volcanic system (Wolfe et al., 1987a, Moore and Wolfe,

1987). The close spatial and temporal relationship between the volcanic system and neighboring basalt vents is demonstrated by silicic pumice and ash beds at the eastern base of San Francisco Mountain (Qpf on I-1663). Here, scoriaceous basalt lapilli from a nearby cinder cone is interlayered in silicic pumice that erupted from a vent near Fremont Peak (Figs. 36, 37 next page). The scoria bed, which contains thin layers of silicic ash, is between aphyric rhyolitic pumice below and porphyritic trachytic pumice above. North Sugarloaf Trachyte Dome North Sugarloaf trachyte is low in Zr like Sugarloaf rhyolite, but its Rb lies on the trend line of the igneous-rock series in the volcanic system (Fig. 34, 35). Like Rb, the other large-ion lithophile elements in North Sugarloaf (Ba, Sr) are similar to those in the other high-silica trachytes in the volcanic system (Table 3). North Sugarloaf appears to be part of the igneous-rock series, but its Zr content is anomalous. According to Watson (1979), zircon can nucleate with fewer than 100 ppm Zr in peraluminous and subaluminous melts. North Sugarloaf has considerably more normative corundum (c) than the other high-silica trachytes (Table 3), so zircon fractionation might explain the low Zr in the North Sugarloaf trachyte. Table 3. Trace Elements of North Sugarloaf, Sugarloaf, and Selected Trachyte Lavas Sample, SiO2 Ba Rb Sr Zr c 3723BSUr, 75.67 2 265 13 53 0.10 3723SUr, 74.68 2 275 5 24 1.31 3713ANSt, 68.58 1350 85 122 47 1.02 3732PSFt, 68.19 780 65 83 470 0.00 3729HSFt, 66.52 1150 58 540 270 0.02 2736EMt, 65.69 1000 51 530 300 0.81 DC58SFt, 64.67 980 38 510 300 0.00 3729SFt, 64.25 1050 52 560 315 0.00 Abbreviations: SUr, Sugarloaf rhyolite dome and tephra. NSt, North Sugarloaf trachyte. SFt, San Francisco Mountain trachyte. EMt, Elden Mountain trachyte. c, normative corundum. Data from Appendix 2.

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Figure 36. Photograph of quarry face in Pumice of Fremont Peak (Qpf on I-1663) at eastern base of San Francisco Mountain. Dark layer is basalt scoria interlayered in the pumice and ash deposit. Thin layers of silicic ash in the scoria bed indicate simultaneous eruptions. Tape is extended to 1 m.

Figure 37. Photograph of impact structure in basalt scoria bed at left side of Figure 36. Lapilli size of scoria suggests the source is a nearby cinder cone. Tape, extended 1 m, rests on a silicic ash bed.

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 volcanic system are: anatectic textures, glassy globules, antipathetic phenocryst populations, and mafic maroon xenoliths. Anatectic Textures 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 glass (tan) from quenched liquid in which euhedral feldspar crystals have grown. Width of image is about 0.85 mm. Plane polarized light. In crossed polarized light the gneiss is seen to be partly disaggregated, and feldspar crystals have reacted with the anatectic melt

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to recrystallize and grow euhedral shapes (Fig. 39). If these euhedral feldspar crystals floated away in the trachyte magma they probably could not be distinguished from original phenocrysts.

Figure 39. Photomicrograph of pyroxene gneiss in Figure 38 in crossed polarized light. Image is overexposed. Glass from quenched anatectic liquid is isotropic. Glassy Globules Round glass in a vitrophyre of a different composition is interpreted as mixing of magmas. Such a globule is inside a round hypohyaline rhyolite xenolith that is in a mugearite lava flow in the Older Andesite of San Francisco Mountain (Qao) (Fig. 40).

Figure 40. Photomicrograph of a glassy globule in vesicular rhyolite vitrophyre. Globule has feldspar microlites in dark brown glass suggesting its composition is andesitic. Vitrophyre has phenocrysts of anorthoclase, sanidine, quartz, and ferrohedenbergite, and xenocrysts of olivine, augite, and plagioclase An57, all in clear glass matrix. Some plagioclase crystals (not in view) have partial shells of brown andesitic glass. Plane polarized light. Width of image is 2.5 mm.

Surface tension on a drop of the smaller volume liquid causes it to draw up in a sphere to present the smallest possible surface area to the larger volume liquid. 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); a few crystals typical of basalt or mugearite might account for the low silica content (SiO2=68.2, 68.4). In the center of Figure 41 a euhedral crystal of labradorite (An65) is surrounded sharply by a mantle of alkali feldspar, and crystals typical of rhyolite are nearby. Plagioclase of An65 likely crystallized originally in a magma with SiO2 content about 51.5 percent (An curve on Fig. 10, p. 12). Labradorite and augite from a mafic magma appear to have been stirred into a rhyolite magma.

Figure 41. Photomicrograph of probable mixed-magma lava flow in Rhyolite of Core Ridge (Qrcr). Phenocrysts are: ano, anorthoclase; sa, sanidine; Q, quartz. Xenocrysts are: p, plagioclase An65; cpx, augite sharply zoned to aegirine-augite. Mantle around plagioclase xenocryst is alkali feldspar (af). Some anorthoclase phenocrysts have cores of sodic oligoclase to albite (An20-10). Crossed polarized light. Width of image is about 4.5 mm.

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Mafic Maroon Xenoliths Conspicuous in many trachyte lavas in the volcanic system are mafic maroon xenoliths; they are most common in the lava domes and thick lava flows (Fig. 42)

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 contain thin laths of plagioclase and strongly acicular hornblende in a compact, felty texture. Other matrix minerals can include hypersthene, olivine, augite, apatite, and iron oxide. Hornblende may have strong alteration to opaque oxide. If xenoliths are vesicular, tridymite is a common occupant on vesicle walls. An example of a xenolith is illustrated in Figure 43. The matrix texture and minerals, especially acicular hornblende, contrast sharply with the matrices of the porphyritic-aphanitic lava flows (Figs. 12 to 17) and the porphyritic-phaneritic plugs and dikes (Figs. 19, 23, 28). Lavas that contain maroon xenoliths also have antipathetic phenocryst populations.

The xenoliths do not appear to be derived from lava flows or intrusions in the volcanic system or from subvolcanic intrusions. The felty arrangement of thin laths of plagioclase 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 43. Photograph of a projection of a thin section of mafic maroon xenolith. Lacy resorbed plagioclase phenocryst (PR) in a matrix of felty acicular hornblende (hbl), plagioclase, and iron oxide. Nonresorbed plagioclase phenocrysts (P) are clear. Plane light. Width of image is about 35 mm., None of the xenoliths were analyzed for whole-rock chemical composition. Silica percentages are estimated from end-member contents of olivine (Fo) and plagioclase (An) using the curves in Figure 10, p. 12. The estimated silica percents indicate that the xenoliths range in composition from basalt or trachybasalt to trachyandesite (Table 4). Some xenoliths contain antipathetic phenocrysts, which suggests magma mixing.

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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. VOLCANOGENIC DEVELOPMENT Volcanogenic processes and history of the San Francisco Mountain volcanic system are interpreted with the stratigraphy, structural geology, geomorphology, and regional volcanic geology displayed on geologic maps MF-1959 (Wolfe et al., 1987a), I-1663 (Holm, 1988), and I-1446 (Ulrich et al., 1984), and the petrographic descriptions, geochemistry, and radiometric age determinations of the rocks; relevant 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 deposits are between the inferred base of the dome and the estimated base of San Francisco Mountain at 7,200 feet (2,195 m) elevation, but one possibility is an initial

volcano. If this presumed volcano is an andesitic stratocone, and its summit is in the area of Core Ridge one mile west of Raspberry Spring dome, then the dome is on the flank of the stratocone. With these geographic positions and elevations the inferred stratocone is projected to a height of about 1,300 m (4,260 ft) and summit elevation of 3,495 m (11,464 ft). This restoration places map unit Qcc in Figure 18 (p. 16) in the summit area of the inferred stratocone. Some Central Complex deposits on Core Ridge support this idea (Fig. 44).

Figure 44. Photograph of andesitic agglomerate at 11,040 feet elevation (3,365 m) in the Central Complex on Core Ridge (Qcc). Central position, size of fusiform bomb, and structureless character of the agglomerate suggest proximal deposition.

26

If the inferred andesitic protocone does exist, and if it postdates the silicic dome at North Sugarloaf (2.78 Ma), then an early eruption stage is implied prior to Raspberry Spring dome (1.82 Ma) for a possible fifth eruption stage (Table 5, next page). Volumes Volumes of the satellite silicic volcanoes were calculated individually with area and height measurements, supplemented with the formulas for a cone and a cylinder. Estimation of the volume of a large volcano like San Francisco Mountain and the percentages of its different lithologies is fraught with uncertainty. San Francisco Mountain is partially dissected, and the upper 2,800 feet (850 m) are well exposed in the Inner Basin. Between elevations 10,000-9,800 feet (3,048-2,987 m) and the estimated base of San Francisco Mountain at 7,200 feet (2,195 m) the rocks are covered so assumptions have to be made about some of the volumes and compositions. San Francisco Mountain was calculated as a restored, precollapse volcano using map I-1663 lithologies. Volumes of map units exposed in the Inner Basin, on the outer slopes of San Francisco Mountain, and beyond its base below 7,200 feet, exclusive of the Older Andesite of San Francisco Mountain (Qao on I-1663) were calculated individually using area and thickness measurements. In this way the distal parts of flows were included, such as the Younger Andesite of San Francisco Mountain (Qay on I-1663) that forms Cedar Ridge 10 miles (16 km) north of the volcano (Qa2 on MF-1959 and MF-1960, Wolfe et al., 1987a, Moore and Wolfe, 1987). Volume of the Older Andesite of San Francisco Mountain (Qao on I-1663) was calculated with the formulas for a cone and a cylinder, and area and thickness measurements for flows beyond the base.

Adjustments were made for the Hart Prairie shield volcano that underlies the west side of San Francisco Mountain, and the dacite and rhyolite domes and flows that underlie or are interlayered in the Older Andesite. The Pumice of Fremont Peak (Qpf on I-1663 and Qsfp on MF-1956) is not included in the erupted volume and composition estimates because of inadequate field data. Nevertheless, a sketchy estimate based on thickness, location, and deposit characteristics of the pumice exposures on I-1663 and MF-1956 (14 miles north of the vent), and assumptions about the densities of the rhyolitic and dacitic pumice, suggests the dense-rock equivalent of the pumice might be as much as 4 km3. Also not included in the erupted volume and composition estimates are rhyolite lavas and pyroclastic deposits penetrated by a bore hole between 7,580 and 7,320 feet elevation in Hart Prairie on the west flank of San Francisco Mountain. Cumulative volumes of the four eruption stages are in Table 5. The estimated composition percents of the 111.13 km3 of the volcanic system are: andesite 83.9%, dacite 14.6%, rhyolite 1.5%. The amount of andesite is probably overestimated, and the amounts of dacite and rhyolite are underestimated because the buried parts of San Francisco Mountain that could not be identified as dacite or rhyolite are assumed to be andesite. Regardless of these uncertainties, San Francisco Mountain is classified as an andesitic volcano. Geochronology Geochronology data for the volcanic system includes K-Ar ages determined in the 1970s, Ar-Ar ages determined in the 2000s, and paleomagnetic analyses (Table 5). For the same map unit, and even the same flow unit, the Ar-Ar ages are older

27

Table 5. Stratigraphy of Principal Volcanic Map Units of the San Francisco Mountain Volcanic System Eruption stage

Map units on MF-1959 and I-1663 Lithology Field position relative to San Francisco Mountain

K-Ar age, Ma Ar-Ar age, Ma Fission-track age, Ma (F-t) (N = normal polarity)

Height of stratocones. Cumulative volume of volcanic system. Collapse event (Holm, 2004).

4 Sugarloaf dome

Lockett Meadow flow Doyle Peak flows (4 flows) Humphreys Peak cryptodome & dike White Horse Hills dome

Rhyolite Dacite Dacite; minor andesite Dacite Rhyolite

Northeast base Northeast flank East flank Central intrusion Northwest base

0.22±0.02, 0.091±0.002 0.41±0.16, 0.530±0.058 0.40±0.03 (youngest flow) (N)

No new stratocone 111.13 km3, 100% Collapse of northeast sector (central valley)

3 Younger Andesite of San Francisco Mountain Elden Mountain domes Agassiz Peak flow Dacite of San Francisco Mountain Domes and flows of Reese and Fremont Peaks; distal pumice deposit Tuffisite dikes

Andesite flows Dacite Andesite Dacite flows Dacite, rhyolite, pumice, block and ash deposit Tuff breccia

Central stratocone Southeast base Southwest flank Central stratocone Northeast and South flanks North flank

0.43±0.03 (N), o.514±0.021, 0.505±0.009 0.49±0.06, 0.57±0.03 (N) 0.60±0.08 0.75±0.17 (N) 0.80±0.11 (F-t) (N)

Stage 3 stratocone, 2,500 m 107.76 km3, 97.0% Collapse of northeast sector (ancestral valley)

2 Older Andesite of San Francisco Mt Dry Lake Hills domes Dome and flows of Fremont Peak, and Core Ridge plug and flows Schultz Peak domes Humphreys Peak dome

Andesite flows and tuffs Dacite Peralkaline rhyolite Dacite Dacite

Central stratocone South base Central, and South and southeast flank Southeast base Northwest base

0.76±0.07 (N); 0.589±0.011, 0.556±0.013 (N) 0.70±0.10, 0.87±0.15 (N) 0.75±0.04 (N) (N)

Stage 2 stratocone, 2,100 m 98.40 km3, 88.5%

1 Older Andesite of San Francisco Mt. Hochderffer Hills dome Raspberry Spring dome Central Complex of San Fran. Mt. Kendrick Park dome North Sugarloaf dome

Andesite flows and tuffs Rhyolite Rhyolite Andesite pyroclastics Rhyolite Dacite

Central stratocone Northwest base Central Central Northwest base Northeast base

0.92±0.03 1.64±0.11 1.82±0.16 No date 2.15±0.13 2.78±0.13

Stage 1 stratocone, 1,700 m 49.68 km3 , 44.7% Protocone, 1,300 m 18.39 km3, 16.5%

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.

and younger than the K-Ar ages. The age differences are not considered important for a general discussion, so except for Sugarloaf dome the K-Ar ages are used for consistency. The oldest and youngest volcanoes in the volcanic system are North Sugarloaf (2.78 Ma) and Sugarloaf (0.091 Ma), and the other dated satellite 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,

which is about half of the dense-rock equivalent of a large cinder cone like Sunset Crater. Data from Table 5 plotted in Figure 45 (next page) show a low rate of eruption for the first 1.5 m.y. followed by an accelerated eruption rate during the early to middle Brunhes chron. The decline in stage 4 might signal exhaustion of the magma supply.

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Figure 45. Diagram showing the growth history of the San Francisco Mountain volcanic system from North Sugarloaf (2.78 Ma) to Sugarloaf (0.091 Ma). Dots are the stratocones that cap eruption stages 1, 2 , and 3. Diamond is the inferred protocone in early stage 1; protocone age is from Raspberry Spring dome (1.82 Ma). The Volcanic System and Surrounding Volcanoes The 160 chemical analyses of the San Francisco Mountain volcanic system tabulated in Appendices 1 and 2 are summarized in Table 6. Table 6. Lithologies and Number of Analyzed Samples in San Francisco Mountain Volcanic System TAS Name San Francisco

Mountain Satellite Sum &

Percent basaltic trachyandesite

12 0 12, 7.5

trachyandesite 82 2 84, 52.5 andesite 3 0 3, 1.9 trachyte 30 13 43, 26.9 rhyolite 8 5 13, 8.1 microdiorite* 2 0 2, 1.25 quartz monzodiorite*

2 0 2, 1.25

microgranite* 1 0 1, 0.6 Total 140 20 160, 100 * IUGS name for plutonic rocks on QAPF diagram.

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 basaltic trachyandesite 16 10.4 basaltic andesite 5 3.25 trachyandesite 11 7.2 trachyte 9 5.8 rhyolite 3 1.95 Total 154 100 The basalts in the ring around the volcanic system are subdivided into chemical types on the basis of their CIPW normative compositions following the classification of Holm (1994). Alkali basalts are slightly more abundant than transitional and subalkali basalts combined (Table 8). Table 8. Definition and Classification of Basalts in 10-km-Wide Ring Around San Francisco Mountain Volcanic System Basalt Type and Definition Number Percent basanitic alkali basalt, ne > 5 2 2.1

alkali basalt, ne > 0 to 5 46 49.4

transitional basalt, ol, di/hy > 2 generally hy < 9

22 23.7

subalkali basalt, ol, di/hy<2 generally hy > 9

22 23.7

quartz subalkali basalt, Q>0 1 1.1

Total 93 100

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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 passing from the mantle through the crust. Primitive features of primary lavas are: Mg# in the range of 65-72 (Mg-number=100Mg/(Mg+Fe)), MgO >8%, and An-content >50. The three basalts with primitive features are on MF-1960 (Moore and Wolfe, 1987); their vent numbers and data are: Medicine Crater V3818 (next to North Sugarloaf), transitional basalt, Mg#=70.0, MgO=12.52, An=57.2. V3814 (between Sunset Crater and O’Leary Peak), subalkali basalt, Mg#=68.6, MgO=12.54, An=64.6. V4931 (analysis 4836B, between O’Leary Peak and Strawberry Crater), subalkali basalt, Mg#=72.3, MgO=14.34, An=55.6. All of the other basalts in Table 8, and all of the trachybasalts in Table 7, have geochemical characters or petrographic characters, or both, that indicate variable degrees of differentiation and possible mixing or contamination, such as Mg-numbers less than 65, low An-contents, quartz xenocrysts with pyroxene jackets, plagioclase crystals with different An-contents, and olivine crystals with different Fo-contents. The analyzed rocks in the 10-km-wide ring around the San Francisco Mountain volcanic system have geochemical compositions that place them in a sodium-rich transitional igneous rock series. All of the trachybasalts are hawaiite. Most of the basaltic trachyandesites are mugearite; only one is a shoshonite. Most of the trachyandesites are benmoreite; only one is a latite. All of the trachytes and rhyolites are at the O’Leary Peak silicic center; all rhyolites have Ca-plagioclase. The combined data sets of San Francisco Mountain volcanic system (160 analyses) and the 10-km-wide ring (154 analyses) are shown on the TAS diagram in Figure 46.

Figure 46. TAS diagram showing the total alkali and silica distribution of 314 analyses from the San Francisco Mountain volcanic system and a 10-km-wide ring around the volcanic system. The 314 analyses in Figure 46 form a coherent and continuous series from basalt to rhyolite, and only a few outliers have higher or lower alkali elements. The ring analyses merge with and overlay the volcanic system analyses (compare Figure 46 with Figures 5 and 6, p. 8 and 9). In the 10-km-wide ring around the volcanic system basalt is common and widespread (Table 7; Wolfe et al., 1987a). Basalt erupted on the periphery of the volcanic system at Medicine Crater (next to North Sugarloaf), on the north side of White Horse Hills, and on the west side of Hochderffer Hills. Hawaiite built the Hart Prairie shield volcano at the west base of San Francisco Mountain (Fig. 2), and erupted at Fern Mountain on the north side of Hart Prairie. Within the volcanic system mugearite erupted on the east side of Hochderffer Hills, and basaltic andesite and mugearite erupted between White Horse Hills and Hochderffer Hills. The data in Figure 5 and Table 6 document that basalt is unknown in San Francisco Mountain. The geologic map of the Flagstaff 2-degree sheet (map I-1446: Ulrich et al., 1984) gives a bird’s eye view of the broad distribution and abundance of basaltic cinder cones in the east half of the San Francisco volcanic field. Notable is the absence of parasitic cinder cones on San Francisco Mountain and the paucity of cinder cones in the margin of the volcanic system; as noted above, the only cinder cones within the volcanic system are two mugearites and one basaltic andesite between White Horse Hills and Hochderffer Hills.

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The Volcanic Field The San Francisco volcanic field has at least 833 small volcanoes, mostly basaltic cinder cones, but also cinder cones of trachybasalt, basaltic trachyandesite, trachyandesite, and basaltic andesite (Appendix 5.2, 5.4). Small domes and flows of trachyte and rhyolite, some in polygenetic vent structures, are scattered through the volcanic field, but clusters of silicic to intermediate domes and lava flows form only a few large volcanic centers (Fig. 1); San Francisco Mountain is the largest of these centers. Magma was supplied to many volcanoes over a broad area, but was erupted in large volumes only locally. The San Francisco Mountain volcanic system might contain as much as 22 percent of the estimated 500 km3 of erupted lava in the volcanic field (Wolfe, 1992). Examination of the five MF-series geologic maps of the San Francisco volcanic field (Ulrich and Bailey, 1987; Wolfe et al., 1987a; Newhall et al., 1987; Wolfe et al., 1987b; Moore and Wolfe, 1987) and Appendix 5 produces these observations: 1. Basalt is the most abundant and widespread rock type in the volcanic field. 2. Most mafic lavas have evolved compositions; only 40 of 408 basalt, basanite, and melanephelinite analyses have primitive compositions. 3. San Francisco Mountain is the largest volcano in the volcanic field. 4. Basalt is unknown in San Francisco Mountain. Subterranean Processes A simple explanation for these four observations is that basalt magma from the mantle penetrated the crust widely across the volcanic field, but stalled and differentiated in many local magma chambers. In a chamber below San Francisco Mountain basalt magma differentiated to derivative magmas that erupted to build the silicic domes and andesitic stratocones. The large volume of San Francisco Mountain implies either that the volcano was constructed above a productive mantle source for basalt magma or that the intersection of three regional crustal fracture systems below San Francisco Mountain is a favorable conduit for rising magma, or both. Low density and high viscosity magmas derived from basalt magma might be a barrier that traps rising fresh basalt magma. Episodic resupply of new basalt brings thermal and kinetic energy to the magmatic system, which could trigger top-down evacuation of

chemically graded derivative magmas and a silicic to andesitic eruption stage. These ideas cannot be proven, and evidence for them is largely circumstantial, but they are supported by these points: 1. Most basaltic cones and flows have evolved compositions, and rocks from magmas linked by systematic and predictable changes in chemistry are scattered through the volcanic field, which implies that differentiation likely is a common process. 2. Lavas progressively richer in silica and alkali elements and poorer in magnesium, iron, and calcium than basalt are successively reduced in abundance and volume from basalt to rhyolite (compare Tables 6 and 7, Appendix 5.4, and volumes estimated for San Francisco Mountain on page 26). These abundance and volume data argue against origin of intermediate rocks by mixing of basalt and rhyolite. 3. Analyzed basalts in the 10-km ring around the San Francisco Mountain volcanic system have the potential to differentiate to silica-rich magmas. The average basalt, blue square on Figure 47 (next page), plots in the ol-di-hy field. Crystal fractionation of olivine and clinopyroxene from the average composition will drive derivative liquids toward increase in hypersthene and, ultimately, silica oversaturation. The analyses in Figure 47 spread from the average toward hy. Felsic and intermediate rocks with normative nepheline have not been found in the San Francisco volcanic field. 4. Stoeser (1973, his Table 4.1) lists xenoliths in vent structures and lava flows at 31 localities across the San Francisco volcanic field. The xenoliths of interest are coarse grained, have cumulate textures, and are dominated by olivine and clinopyroxene. The xenolith suite is composed primarily of wehrlite, olivine clinopyroxenite, clinopyroxenite, and dunite, but includes small amounts of websterite, gabbro, anorthosite, and troctolite. Stoeser (1973) interpreted the xenoliths as originating by crystal accumulation in multiple basalt magma chambers at different crustal depths. Fractional crystallization would produce derivative magmas. Later magmas intersected the layered intrusions and pieces of solidified cumulates were carried as xenoliths to eruption sites. 5. Crystal fractionation by incomplete reaction is illustrated by textures of the quartz monzodiorite dike in Core Ridge. Although the quartz monzodiorite has normative quartz (6.74%) olivine was an early phase on the liquidus. At lower

31

Figure 47. Plot of normative hy, ol, di, ne of 92 basalt analyses in the 10-km-wide ring around the San Francisco Mountain volcanic system. One analysis with normative quartz is not plotted. Blue square is average composition. hy=hypersthene, ol=olivine, di=diopside, ne=nepheline temperature the olivine became unstable and reacted with the magma to form a rim or mantle of low-calcium pyroxene that stopped the reaction under the hypabyssal cooling conditions (Fig. 27). 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 coincide on the curve, which implies that parental magmas in each stage were similar in composition. Sugarloaf is not related to the samples that

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 exhibits nearly identical ratios of Ba/Zr through several percent of MgO of samples from the San Francisco Mountain volcanic system. Sugarloaf is not part of the differentiated suite. Map unit Qrcr might be a mixed magma (Fig. 41). See discussion of North Sugarloaf on p. 21. The Ba/Zr ratios of analyzed rocks from the four eruption stages coincide on the line, which implies that parental magmas in each stage were similar in composition.

Figure 49. Plot of Ba/Zr ratios (ppm of elements) against MgO (wt %) of 18 samples from the San Francisco Mountain volcanic system. One sample of stage 2 is concealed. Sample PR in Figure 48 did not plot because of a lack of MgO in the analysis (Appendix 1, 2705A).

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Parental Magmas Geologic mapping, K-Ar ages, and paleomagnetic data indicate that the volcanogenic development of the volcanic system was contemporaneous with basaltic volcanism in the surrounding San Francisco volcanic field (Wolfe et al., 1987a; Moore and Wolfe, 1987; Tanaka et al., 1986; Figs. 36 and 37, p. 22). Thus, it is feasible that some basalt magmas entered the crust below the location of the San Francisco Mountain volcanic system. Some, if not all, of the parental basalts could have been primary, or nearly primary magmas from the mantle. Out of 408 analyzed basalts, basanites, and melanephelinites in the mapped part of the San Francisco volcanic field 40 have geochemical characteristics of primitive lavas (Appendix 5.5). Thirty-five of the primitive basalts are on the Southwest, Central, and East sheets of the MF-series maps; the Northwest and SP Mountain sheets together have only five primitive basalts. Data from Appendix 5.5a, reproduced in Table 9, indicate increasing numbers of primitive lavas with greater saturation in silica across the volcanic field from west to east. Table 9. Summary of Primitive Lavas in Southwest, Central, and East MF-Series Maps in San Francisco Volcanic Field. Data from Appendix 5.5a Primitive Lava Southwest

MF-1958 Central MF-1959

East MF-1960

Melanephelinite 2 0 0 Basanite 1 0 0 Basanitic alkali basalt

5 3 1

Alkali basalt 5 4 5 Transitional basalt

1 1 3

Subalkali basalt 0 1 3 Total 14 9 12 The totals in Table 9 show that the Central sheet has the fewest samples of primitive basalts, even though MF-1959 covers the largest area of the three MF-series maps (Wolfe et al., 1987a, p. 30). The large area covered by the volcanic system might account for the apparent “deficiency” of primitive basalts on the Central sheet if rising primitive magmas supplied a large, active, and growing magmatic system in the lower to middle crust. If this is true, then the data in Table 9 suggest multiple basalt types, but skewed toward hypersthene normative, fed the magmatic system.

Because the trace element data in Figures 48 and 49 indicate similar parental magma compositions for the four eruption stages, new batches of primitive magma might have mixed and blended in a magma chamber to an average composition similar to the average in Figure 47. Rhyolites and the Granite System Rhyolites in the San Francisco Mountain volcanic system contain 73 to 76 percent SiO2 (normalized) and greater than 90 percent normative Q+ab+or. Compositions of the rhyolites are comparable to many granites, so the rhyolites can be evaluated with the experimental results of Tuttle and Bowen (1958), Luth et al. (1964), and Winkler (1974) on the granite system (quartz-albite-orthoclase-water). Plotted on the granite system in Figure 50 are seven rhyolites from the four eruption stages, and the interstitial microgranophyre in the quartz monzodiorite dike in Core Ridge (Fig. 26, p. 18). In the granite system increasing PH2O shifts the liquidus temperature minima and ternary eutectics away from quartz and toward albite as the temperature lowers from 770ºC at 0.5 kb to 625ºC at 10 kb in the anorthite-free system, and from 695ºC at 2 kb to 645ºC at 10 kb in the system with Ab/An=2.9 (Winkler, 1974). Addition of anorthite to the experimental system shifts the temperature minima and ternary eutectics away from albite and toward quartz and orthoclase (Fig. 50). The rhyolites in Figure 50 plot in two groups. Group one samples, 1, 2, 4, 5, 6, lie on a line that is parallel with the anorthite-free data points, but closer to the anorthite-bearing data points. These rhyolites might have originated at water pressures of about 2 to 4.5 kilobars, perhaps at middle crustal depths of 8 to 18 km. The suggestion is that group one rhyolites are derivatives from mafic parental magmas by processes of fractional crystallization. Group two samples, 3 and 7, lie on a line parallel to the anorthite-bearing system, and near the data points for the system with Ab/An=2.9. Group two rhyolites might be from anatectic melts that originated in the lower crust (see p. 20 and 21 for discussion of Sugarloaf and Hochderffer Hills). Except for sample 2, the apparent depths of origin of the rhyolites are within or near the compressional wave low-velocity region that Stauber (1982) identified at 9 to 34 km below sea level under San Francisco Mountain.

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Granophyric Residuum The residual interstitial microgranophyre in the quartz monzodiorite dike in Core Ridge is sample 8 in Figure 50. The dike intruded about 1.3 km below the projected elevation of the summit of the stage 3 stratocone. The density of the stage 3 cone above the dike is unknown, but a density range of 2.0 to 2.5 gives a lithostatic pressure of 254 bars to 318 bars (0.25-0.32 kb). Late to post-magmatic hornblende and biotite occur as small rims on opaque oxide, but the late melt probably was not water saturated (PH2O<Ptotal). Deuteric phases are serpentine, talc, carbonate, and a brown phyllosilicate. The calculated normative composition of the microgranophyre is Q=43.6, Ab=26.2, Or=30.2; this estimate is based on the chemical, normative,

and modal analyses of the quartz monzodiorite and assumptions about the distribution of normative An, Ab, and Or between plagioclase and alkali feldspar (Appendix 2 sample 3733C, Appendix 3 samples 64 and 63). The modal composition of the microgranophyre is comparable to compositions of granophyre from other locations (Table 10). The quartz monzodiorite contains 6.74 percent normative quartz (Appendix 2, 3733C) and 1.2 percent modal olivine (Appendix 3, sample 63). Olivine crystals are mantled by pigeonite (Fig. 27) and plagioclase is strongly zoned (Fig. 25, p. 18). Both textures indicate incomplete reactions that contributed to crystal-liquid fractionation. Alkali feldspar occurs as mantles on plagioclase, interstitially as discrete crystals with quartz, and in micrographic texture with quartz in

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).

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Table 10. Modal Compositions of Granophyre in Weight Percent. Foreign data from Dunham, 1965. Location Quartz Feldspar Tasmania (6) 44.6 55.4 Rhum (9) 43.8 56.2 San Francisco Mountain* 43.6 56.4 Lake District 43.5 56.5 Rhum 43.1 56.9 Dillsburgh, PA (7) 41.6 58.4 Slieve Gullion 39.7 60.3 Skye 38.2 61.8 *Appendix 3, sample 64 is in volume percent microgranophyre, which is 3.5 percent of the mode of the quartz monzodiorite (Fig. 26). The granophyric residuum demonstrates the capacity of the quartz monzodiorite magma to differentiate to low-temperature granitic magma, and gives testimony to the significance of fractional crystallization as an important process in the volcanogenic development of the San Francisco Mountain volcanic system. The quartz monzodiorite itself is one stage in a differentiation series descended from more mafic magmas.

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