TERTIARY VOLCANIC DOMES near Jackson, California By Robert L. Rose Assistant Professor of Geology San Jose State College Special ^efuvit 60 California Division of Mines Ferry Building, San Francisco, 1959 STATE OF CALIFORNIA EDMUND G. BROWN, Governor DEPARTMENT OF NATURAL RESOURCES DeWITT NELSON, Director DIVISION OF MINES IAN CAMPBELL, Chief Special Report 60 Price 500 CONTENTS p a g e Abstract 5 Introduction 7 Stratigraphy 7 Calaveras group 7 Amador group 9 Cosumnes formation 9 Logtown Ridge formation 9 Mariposa formation 10 lone formation 10 Valley Springs formation 10 Mehrten formation 11 Pre-Tertiary intrusive igneous rocks 12 Volcanic domes of the Jackson quadrangle 13 Golden Gate Hill 13 Tunnel Peak 16 MeSorley dome 17 Hamby dome 17 Jackson Butte 17 Bibliography 21 Illustrations Figure 1. Index map showing location of Jackson Butte and area covered by figure 3 8 Figure 2. Golden Gate Hill dacite dome 14 Figure 3. Geologic map of Golden Gate Hill area 15 Figure 4. Jackson Butte dacite dome 18 Frontispiece. Jackson Butte, Amador County, from the south 6 Photo 1. Photomicrograph of augite meta-andesite from Logtown Ridge formation near Golden Gate Hill 9 2. Photomicrograph of devitrified biotite rhyolite welded tuff of the Valley Springs formation 10 3. Photomicrograph of hornblende rhyodacite, from the Mehrten formation near Golden Gate Hill 11 4. Photomicrograph of hornblende-pyroxene-olivine andesite from Mehrten formation 11 5. Photomicrograph of Tunnel Peak hornblende dacite 16 6. Tunnel Peak vent breccia 16 7. Jackson Butte from Mokelumne Hill 18 8. Photomicrograph of Jackson Butte hornblende-biotite dacite 18 9. Weathered surface of Jackson Butte dacite 19 10. Jackson Butte dacite breccia 19 11. Photomicrograph of Golden Gate Hill hornblende-biotite dacite 20 Tables Table 1. Stratigraphy of central part of Jackson quadrangle 8 Table 2. Petrographic summary of Tertiary volcanic rocks 13 (3 ABSTRACT The stratigraphic sequence near Jackson, California is composed of two major units: an older or bedrock series composed of Paleozoic and Mesozoic low-grade metamor- phosed eugeosynclinal sedimentary and volcanic rocks and a younger or superjacent series of Eocene to Pliocene nonmarine sedimentary and pyroclastic rocks. The older strata are steep dipping and have been tightly folded and intruded by a quartz diorite stock. The younger strata dip gently to the west and are tectonically undisturbed. Massive volcanic rock associated with the Mehrten formation, the youngest stratigraphic unit, forms five separate masses. Four of these are situated in a small area about 7 miles south of Jackson. The fifth, Jackson Butte, is located a few miles east of Jackson. All of these volcanic masses are structurally similar and consist of flow-banded felsophyre and breccia. In each the flow banding is moderate to steep-dipping. Near the margins it strikes parallel to the outer contacts and dips inward. In the center por- tions the flow banding is nearly vertical but variable in strike. These features and the spatial relationships indicate that these masses are endogeneous volcanic domes. Evi- dently they were emplaced as partly crystalline magma, too viscous to flow away from the vents. The largest and oldest of these volcanic domes is the Golden Gate Hill dome com- posed of hornblende-biotite dacite. Clasts of identical dacite are present in the basal Mehrten formation suggesting the dome is equivalent in age to basal Mehrten (Mio- Pliocene) or slightly older. The two small rhyodacite domes north of Golden Gate Hill are intrusive into the Mehrten formation; however, identical rhyodacite clasts are locally present in the upper part of the Mehrten formation indicating rhyodacite explosive volcanism (possibly at these sites) in late Mehrten time. The adjacent Tunnel Peak dome is intrusive into a vent breccia, composed of Mehr- ten debris and metamorphic clasts and hornblende dacite; hence it is younger or contemporaneous with Mehrten. The fifth volcanic dome, the Jackson Butte dome, composed of hornblende-biotite dacite, apparently intrudes the Mehrten formation but differs from the other domes in that it contains an abundance of xenoliths of metamorphic and plutonic rocks including peridotite xenoliths with prominent reaction rims. (5) a I ft, S o to V © 9 « a o a> v t-s CO H § fa TERTIARY VOLCANIC DOMES NEAR JACKSON, CALIFORNIA By Robert L. Rose INTRODUCTION The United States Geological Survey Jackson Folio by H. W. Turner, published in 1894, was the third Sierra Nevada geological folio and the first comprehensive ac- count of the geology of the Jackson area. Turner divided the Tertiary igneous rocks of this region into two carto- graphic units : an earlier sequence of rhyolite tuff and a later unit composed mainly of fragmental andesite ranging from andesitic sand to andesitic mudflow brec- cia. In addition he recognized three small bodies of "massive andesite" in the central part of the quad- rangle but included these in the andesite unit. In con- sidering the origin of these massive andesites, Turner said, "It is possible that these were vents through which the lava escaped and in which it congealed." Several years later in the text of the Mother Lode Folio, F. L. Ransome described these andesites and two adjacent masses that Turner had overlooked. These five occurrences of "massive andesite" were shown on the map of the Mother Lode Folio as "thick lava flows" although Ransome said, "It is probable, although not demonstrable, that the hornblende-andesite of Golden Gate Hill was erupted in place through the underlying rocks. ' ' Four of these five occurrences of massive andesite (Golden Gate Hill, Tunnel Peak, etc.), are located on the ridge south of the Mokelumne River a few miles southwest of Mokelumne Hill. The fifth occurrence forms the monolith of Jackson Butte which is about 2| miles northwest of Mokelumne Hill and approximated 7 miles N. 12° E. of Golden Gate Hill. During the summer of 1948 and spring of 1949 the writer mapped these "massive andesites" and adjacent rocks.* This field work was followed by a petrographic * This investigation was conducted at the University of California at Berkeley while the author was a graduate student. The pres- ent article is a condensation and partial revision of the original report : "The Tertiary volcanic domes of the Jackson quadran- gle" which was submitted to the Department of Geology as a Master's Thesis in 1949. The author gratefully acknowledges the assistance and encouragement of Professors N. L. Taliaferro, Howel Williams, and P. J. Turner of the University of Califor- nia. The writer is also indebted to Mr. Oliver E. Bowen of the California Division of Mines for the use of several thin sections. study of thin sections, crushed rock samples and mineral separations. The purpose of the project was to determine the nature of the "massive andesites" structurally and petrographically, their origin, and their relationships to the other volcanic rocks of the area. All mapping was done on aerial photographs : Jack- son Butte and Golden Gate Hill were mapped on en- larged aerial photographs at 335 feet = 1 inch and 550 feet = 1 inch respectively. The area of figure 3 was mapped on photographs with a scale of 1/20,000 feet. The petrographic work was done by standard methods and all extinction angles and optic axial angles were measured on a four-axis universal stage. Feldspar com- positions were estimated from the least index of re- fraction of cleavage fragments. Silica percentages of the volcanic rocks were estimated from the index of refraction of the corresponding glass prepared by fusing powdered rock samples in a carbon arc lamp according to a method described by Mathews (Mathews, W. H. 1951, pp. 92-101. Mathews' curve for the Markleeville rocks was used in the silica estimations as the Marklee- ville volcanic rocks are probably chemically similar to those of this area. STRATIGRAPHY The rocks of the central part of the Jackson quad- rangle can be divided into two major groups : The metamorphosed pre-Tertiary clastic, pyroclastic, and igneous rocks — the bedrock series — and the Tertiary continental clastic and pyroclastic rocks — the super- jacent series. The pre-Tertiary metamorphic rocks have been folded and faulted and intruded by plutonic rocks. They dip eastward at high angles whereas the overlying Tertiary rocks dip gently to the west. Calaveras Group The oldest stratigraphic unit of this area is the Paleo- zoic Calaveras group which is the only pre-Tertiary stratified unit recognized east of the Mother Lode fault. It is unconformably overlain by Tertiary volcanic rocks (7) 121° 30' I 2 1° I 5' CALIFORNIA DIVISION OF MINES I2I°00' [Special Report 60 I 20° 45' 38° 30 38° 15 10 miles Figure 1. Index map showing the location of Jackson Butte dome, and the area (stippled) covered by the geologic map of the Golden Gate Hill, Tunnel Peak, McSorley, and Hamby Tertiary volcanic domes (figure 3). Table 1. Stratigraphy of central part of Jackson quadrangle. Formation Approx.jnax. or General thickness Age group lithology in feet Mio-Pliocene Mehrten Andesitic conglomerate and sandstone, with minor rhyodacite breccia 250 I) I S C O N F O R M I T Y Mio-Pliocene Valley Springs Rhyolitic pyroclastic rocks and conglomerate 250 D I S C O N F O R M I T Y Eocene lone Quartz conglomerate 100 UNCONFORMITY Upper Jurassic Mariposa Mainly black slate 1300 Upper Jurassic ~"~ """""Logtown Ridge Meta-andesite, tuff and agglomerate ? Upper Middle Jurassic <*roup Cosumnes Mainly slate with interbeds of metasandstone, _° . metaconglomerate and chert 2500? Paleozoic Calaveras. UNCONFORMITY Phyllite, schist, limestone, etc.. 1959] TERTIARY VOLCANIC DOMES NEAR JACKSON and is intruded by the Mokelumne Hill quartz diorite. Irregular masses of Calaveras are present along the margins of the two small domes just east of Tunnel Peak and appear to have been dragged into their present posi- tions by the intrusive rocks. Many different rock types are present in the Calaveras, but gray phyllite is the most prominent constituent. Meta-chert, quartz-muscovite schist, and green schists are locally conspicuous and lenses of limestone are pres- ent on the northwest side of Chile Gulch. The various rocks of this unit dip steeply to the east and appear to be complexly folded ; however, original bedding has been obscured by metamorphism. Near Jack- son Butte Calaveras schists show two sets of lineations and an obvious triclinic megafabric suggesting two pe- riods of deformation. The rocks, which have been com- pletely recrystallized, belong to the green schist facies of regional metamorphism. No fossils were found in the Calaveras in this area but those that have come from the Calaveras group in adjacent areas indicate that it is in part Carboniferous. Permian fossils have been found in the western belt of the Calaveras ; strata in the north- ern Sierra Nevada that are possibly correlative with this unit have yielded faunas as old as Silurian. Thus the Calaveras may represent a large portion of the Paleozoic era. Amador Group On the Cosumnes River about 25 miles north of this area the Calaveras group is unconformably overlain by a group of Middle and Upper Jurassic metamorphosed clastic and pyroclastic rocks which have been called the Amador group by N. L. Taliaferro (1942, p. 81). The lower part of this assemblage of rocks is called the Cosumnes formation and the upper the Logtown Ridge formation. The two formations are distinct lithologic units and appear to represent continuous sedimentation during the upper Middle and Lower Upper Jurassic. Cosumnes formation At the type section along the Cosumnes River the up- per Middle Jurassic Cosumnes formation consists of metamorphosd tuff, chert, shale, sandstone, limestone, and conglomerate. The conglomerate, which is present at the base of the formation contains abundant debris of the underlying Calaveras rocks (Taliaferro, 1942). Metamorphosed sedimentary beds tentatively correlated with the Cosumnes formation are present in the vicinity of Tunnel Peak and to the south. They consist mainly of black slate with interbeds of fine-grained meta-sandstone, interbeds of pebble conglomerate up to 5 feet in thickness, and a few thin layers of brown chert near Beale Ranch. Slaty cleavage, visible in all of the fine-grained rocks, dips steeply to the northeast ; cleavage and bedding appear to be parallel in most places but locally they are appreci- ably discordant. The formation resembles the Mariposa formation except for presence of the meta-conglomerate, chert and sandstone interbeds. Assignment to Cosumnes formation is tentative and is largely based on lithologic features and the fact that it appears to underlie the Logtown Ridge formation east of Golden Gate Hill. However, this contact with Logtown Ridge could be a fault contact. The contact with the Logtown Ridge for- mation near the Beale Ranch and northwest of Golden Gate Hill has been interpreted as a fault contact on the basis of shearing, mineralization along and adjacent to the contact, and the apparent disagreement with contact relationships to the east. The base of the Cosumnes formation in this area is not exposed, consequently its total thickness is unknown; however the exposed thickness is probablv less than 3000 feet. Logtown Ridge formation The lower Upper Jurassic Logtown Ridge formation crops out in the western part of the area in two belts separated by a narrow infolded strip of Mariposa slate. Adjacent to the Mother Lode fault, south of State High- way 8, is a thin discontinuous strip of Logtown Ridge meta-andesite which apparently overlies the Cosumnes formation and is in fault contact with the Calaveras group. Outcrops of the Logtown Ridge are conspicuous in the field as they stand out in bold contrast to the less resistant Mariposa and Cosumnes slates. Both the upper and lower contacts are apparently sharp and conform- able. The dominant rock type is a metamorphosed, dark grayish-green augite andesite tuff which locally grades into an agglomerate with rounded to subangular frag- ments of vesicular and massive augite andesite. The metatuff consists of partly chloritized euhedral to sub- hedral greenish-black augite crystals ranging from less than 1 mm to more than 8 mm in diameter, embedded in a green microcrystalline ground-mass. The latter forms 50 percent to 80 percent of the rock and consists mainly of a uniaxial carbonate, (calcite or dolomite), chlorite, clinozoisite, sericite and albite, with variable amounts of pumpellyite, stilpnomelane and actinolite. No fossils were found in the Logtown Ridge formation in this area but ammonites indicative of a Jurassic age have been found in the tvpe section on the Cosumnes River (Taliaferro, 1942). " Photo 1. Photomicrograph of augite meta-andesite: Logtown Ridge formation near Golden Gate Hill. Large light grain near center is zoned augite with small dark areas of chlorite. Porphy- ritic texture of matrix and rectangular outlines of original plagio- clase phenocrysts are both vague. The rock is now composed mainly of a carbonate mineral (ankerite ?), albite, pumpellyite ?, chlorite, and sericite with relic augite crystals. Plane-polarized light. x30. 10 CALIFORNIA DIVISION OF MINES [Special Report 60 Mariposa Formation The youngest stratigraphic unit of the basement com- plex is the Upper Jurassic Mariposa formation which consists mainly of slate. It is present west of the Mother Lode fault in the western part of the map area. The fresh, unweathered slate is dull black and has a well-defined cleavage that appears to be parallel or nearly parallel to the bedding. The slate is less resistant than the Logtown Ridge formation and occupies topo- graphic depressions while the latter formation forms rocky ridges. Interbedded with the slate are thin layers of fine-grained meta-sandstone and meta-siltstone which are mainly less than 2 inches in thickness. The thickness of the exposed section in this area is about 1300 feet. Graded bedding and truncated cross-bedding are char- acteristic of this unit which overlies the Logtown Ridge formation. The contacts are not mineralized and appear to be normal depositional contacts. Molluscan and ammonite faunas of the Mariposa for- mation on the Cosumnes River and elsewhere indicate the age of the Mariposa is Upper Jurassic Oxfordian and possibly lower Kimmeridgian (Taliaferro, 1942). No fossils were found in the Mariposa in this area. lone Formation Quartzose gravels and sands totaling about 100 feet in thickness are present northeast of Golden Gate Hill on the north side of Chili Gulch in sections 13 and 14 where they unconformably overlie the bedrock complex, and are disconformably overlain by the Valley Springs rhyo- lite. These sediments are lithologically correlative with the pre-rhyolite auriferous gravels of the Mother Lode district and are therefore referred to the Eocene lone formation. The lower portion of the unit consists of coarse gravel with rounded to subangular boulders and cobbles of quartz and quartzite in a quartz sand matrix. Clasts of granodiorite, schist, and other non-resistant rock types are not common in the gravels. Quartzose sand and pebble conglomerate occur higher in the section and are interbedded with the coarse gravels. Current bedding and channeling are conspicuous features throughout the unit and all physical characteristics are consistent with a fluviatile origin for the deposit. Apparently these sedi- ments were deposited in channels of streams that were flowing into the Eocene sea that occupied portions of the San Joaquin and Sacramento Valleys. Fossil and lith- ologic data indicate that the lone is in part correlative with the middle Eocene Domegine and Yokut formations of the Great Valley and the Coast Ranges. Valley Springs Formation Disconformably overlying the lone formation and overlapping on to the pre-Tertiary rocks is a Mio-Pli- ocene stratigraphic unit of variable thickness and l'ith- ology that is characterized by the presence of pyroelastic rhyolitic material and the absence of andesitic^detritus. Gale, Piper, and Thomas, (Gale, 1939, pp. 71-80) \pplied the name Valley Springs formation to this unrK and designated the west slope of Valley Springs Peak near the town of Valley Springs as the type locality of the formation. The type section has a thickness of 420 feet and colt sists mainly of tuff, conglomerate, sandstone and silt- stone. The formation, which is widespread on the western $\r Sns?' t«i *a Photo 2. Photomicrograph of devitrified biotite rhyolite welded tuff of the Valley Springs formation. Light-colored streaks are devitrified, flattened glass shards now composed of tridymite and feldspar. Dark areas are mainly magnetite. Plane-polarized light. x30. slopes of the Sierra Nevada, includes some of the lith- ologic units that have previously been mapped as part of the lone formation. In this area the Valley Springs formation consists mainly of conglomerate and rhyolite tuff. The conglom- erates are similar to the lone conglomerates and consist mainly of rounded pebbles of quartz, quartzite, schist, and granodiorite with various proportions of angular to subangular fragments of rhyolite. On the southeast side of Golden Gate Hill, Valley Springs conglomerate has a tuffaceous matrix and grades laterally into a rhyolite tuff. Elsewhere the conglomerate usually has a matrix of quartzose sand. The tuffs are pale-colored, crystal-vitric rhyolite tuffs. Some are well-bedded and appear to have been deposited subaqueously while others are subaerial deposits. The crystal fraction consists of quartz, sanidine, oligoclase and biotite and makes up 2-5 percent of the rock. The glassy fraction is partly devitrified but usually shows a good vitroclastic texture. The tuff on the southeast side of Golden Gate Hill is very coherent but notably porous. Microscopically the glass shards show fibrous rims appar- ently composed of tridymite and alkali feldspar. The coherence of this tuff is apparently due to the partial devitrification of the glass shards and cementation by feldspar and tridymite. Fenner, in 1948 (p. 879-894) proposed the name "sillar" for this type of pyroclastic rock. A fine-grained, hard, resistant, gray rhyolitic rock crops out conspicuously in sections 23 and 24. It is well- jointed with horizontal and vertical joints and has a maximum thickness of about 150 feet. Megascopically, the rock is porphyritie with small crystals of quartz, feldspar, and biotite embedded in a micro-crystalline groundmass which constitutes about 95 percent of the rock. This rhyolite member is underlain by a thin water- deposited tuff and is hard, coherent, and lithoidal to its upper surface where it is disconformably overlain by the Mehrten formation. Nowhere does the rock appear to be 1959] TERTIARY VOLCANIC DOMES NEAR JACKSON 11 vesicular nor does it have the contorted streaky flow structure that is common in rhyolitic lavas. Although H. W. Turner referred to this rock as a rhyolite lava, field relationships and the fabric of the rock indicate it is a devitrified welded tuff. The maximum thickness of the Valley Springs forma- tion in this area is about 250 feet. It disconformably overlies the lone formation and is disconformably over- lain by the Mehrten formation. The source of the Valley Springs formation is not known but presumably was to the east of this area in the Sierra Nevada. Some of the pyroclastic material was apparently transported by streams and deposited in the old stream channels. The rhyolitic sillar and welded tuff are probably products of glowing avalanches which evidently came from the east and deposited the fragmental rhyolite in various surface depressions. According to Axelrod (Axelrod, 1956, p. 62) paleobotanical evidence from numerous localities indicates the Valley Springs formation is Mio-Pliocene, about the same age as the Cierbo formation. Mehrten Formation Disconformably overlying the Valley Springs forma- tion and unconformably overlying the rocks of the bed- rock complex is a Mio-Pliocene to middle Pliocene group of andesitic, clastic and pyroclastic rocks that has been designated as the Mehrten formation by Piper, Gale, and Thomas (1939). The type section of the formation on the Mokelumne River near the Mehrten Dam is 183£ feet thick and consists mainly of andesitic sandstone with minor amounts of siltstone, clay, and breccia (Piper, 1939, p. 62). The formation is widespread in the Sierra Nevada and thickens toward the crest of the range where it consists mainly of andesitic breccia and con- glomerate. The Mehrten formation is the most wide- spread Tertiary stratigraphic unit in this area, covering most of the area above the 1400 foot contour line south- west of Mokelumne Hill. Coarse andesitic conglomerate with minor interbedded sandstone constitutes the bulk of the Mehrten in this area. The conglomerates consist mainly of subrounded to rounded pebbles and cobbles of gray or black porphyritic andesite embedded in a gray silty sand matrix. Occasional cobbles and boulders of quartz, granodiorite, and various metamorphic rocks are present and large angular boulders of andesite are locally common. The sandy matrix of the conglomerate, like the sandy interbeds, is gray in color, poorly sorted and composed mainly of angular grains of quartz, feld- spar, hornblende, gray rock particles, and silt grains. The pebbles are dark gray or black hornblende- augite-hypersthene andesite, hornblende-augite-hyper- sthene-olivine andesite, and hornblende-augite andesite. They are typically porphyritic with a pilotaxitic ground- mass and phenocrysts of plagioclase (zoned andesine to labradorite), hornblende and augite. Some pebbles also contain phenocrysts of hypersthene and olivine (with rims of iddingsite). In the Mehrten formation on the southeast side of Golden Gate Hill is a local basal zone about 40 to 60 feet thick that contains large blocks of reddish-brown horneblende-biotite dacite. These blocks are angular and vary in size from a few inches up to 4 feet in diameter. Locally they constitute about 95 percent of the breccia but this grades laterally into andesite conglomerate that Photo 3. Photomicrograph of hornblende rhyodacite : a frag. ment from the Mehrten formation near Golden Gate Hill. Pheno- crysts are green hornblende and plagioclase. This rock is chemically and petrographically like the rhyodacite forming the two volcanic domes near Tunnel Peak. x30. contains about 5 percent of this material and 10 to 30 percent of angular fragments of gray to pale brown recrystallized, welded tuff. The remainder of the frag- ments are well-rounded porphyritic andesite, megascopi- cally the same as the typical Mehrten pebbles. Ransome (1900) mapped this area as a massive lava flow and used the same symbol for it that he used for Golden Gate Hill. The angular fragments of hornblende-biotite dacite are identical in appearance with the rock that makes up the volcanic dome of Golden Gate Hill. The angularity and size of these fragments indicate that they have not been transported far. Their similarity to the rock forming Photo 4. Photomicrograph of hornblende-pyroxene-olivine an- desite: a pebble from the Mehrten formation. The phenocryst of olivine has prominent rims and veinlets of iddingsite. Small pheno- crysts with high relief are hypersthene and augite. Hornblende is represented by the dark rectangular areas now composed mainly of granular magnetite and pyroxene. Larger phenocrysts have core's of basaltic hornblende. Plane-polarized light. x30. 12 CALIFORNIA DIVISION OF MINES [Special Report 60 Golden Gate Hill and their close proximity to it sug- gests that they are probably talus fragments that have been incorporated in the Mehrten formation forming a local "breches d'ecroulement." This apparently fixes the age of Golden Gate Hill as either pre-Mehrten or equivalent to basal Mehrten. Stratigraphically above this breccia zone is a promi- nent lens of pale yellowish-gray, biotite rhyolite tuff. It is rather well-bedded and appears to be of fluviatile origin. It is similar in appearance to the water-deposited tuffs of the Valley Springs formation but appears to be an interbed in the Mehrten formation. Andesite mudflow breccias that are common elsewhere in this formation are not abundant in this area. Where present they are composed of angular blocks of horn- blende-pyroxene andesite enclosed in a tuffaceous matrix. In the upper part of the Mehrten formation on the sides of Golden Gate Hill is a thin, inconspicuous layer of rhyodacite breccia. This breccia is about 50 feet below the top of the Mehrten formation and is probably about 15 feet thick. It consists of angular fragments of pale gray rhyodacite and small platy fragments of fine- grained phyllite embedded in a fine-grained grayish- white tuffaceous matrix. Material similar to this also occurs near the top of the formation in Section 23 a short distance northwest of the highway and about 600 feet north of the center of the plug, near the center of Sec- tion 23. At Golden Gate Hill the rhyodacite fragments are seldom more than an inch long but in Section 23 they are commonly 1 inch to 3 inches in length. The rhyoda- cite is very fine-grained and porphyritic with small black crystals of hornblende embedded in a dense, gray, microvesicular groundmass. Microscopically the material is porphyritic with phenoerysts of hornblende (about 10 percent) and oligoclase embedded in a cryptofelsitic groundmass and small grains of cristobalite lining the microsveicles. The silica content of this rock is about 70 percent according to the index of refraction of the fused material. Megascopically and microscopically the rhyodacite clasts are identical with the material com- posing the two small volcanic domes just east of Tunnel Peak and the volcanic dome on the Licking Fork of the Mokelumne River just south of West Point. The local occurrence of this rhyodacite breccia in the Mehrten and its proximity to the two rhyodacite domes east of Tunnel Peak suggest that the rhyodacite clasts may have come from volcanic vents now occupied by the domes. PRE-TERTIARY INTRUSIVE IGNEOUS ROCKS Three pre-Tertiary intrusive igneous bodies have been recognized in this area : an augite andesite sill, a quartz diorite stock, and an irregular mass of horn- blendite which is perhaps a basic border phase of the quartz diorite. All three are more or less metamorphosed and were apparently emplaeed before or during the latest regional metamorphism. Augite Andesite. West of Tunnel Peak is a thin sill of metamorphosed augite andesite that is intrusive into the Cosumnes formation. Megascopically it is a massive, dark green, porphyritic rock with abundant phenoerysts of saussuritized plagioclase and a minor amount of black augite enclosed in an aphanitic groundmass. The feldspar phenoerysts are euhedral to subhedral and about half an inch in length. Microscopically the rock appears to consist of subhedral to euhedral pale green augite partly altered to chlorite and altered plagioclase in a fine- grained matrix of secondary minerals. Outlines of the original feldspar phenoerysts are visible due to the presence of small residual patches of plagioclase grains. The secondary minerals replacing the plagioclase and the groundmass are mainly clinozoisite (or zoisite), chlorite, actinolite, and albite (?). Chlorite partly re- places augite and sphene occurs as a microgranular aggregate, replacing skeletal and tabular micropheno- crysts of ilmenite. The intrusive mass is lens-shaped, nearly vertical, about 50 feet thick and about 500 feet long, and is concordant to the slaty cleavage of the intruded Cosumnes formation. Mokelumne Hill Quartz Diorite. Part of an irregu- lar-shaped stock of quartz diorite crops out in the area and is well-exposed in sections 13 and 24. However, the major part of this stock is north and east of Mokelumne Hill. According to the Mother Lode folio (Ransome, 1900), the stock has a maximum width of about 3 miles and a length of about 5.3 miles. The stock is somewhat elongated parallel to the structural trend of the region but only the western boundary is apparently concordant with the enclosing rocks. The stock is mainly composed of gray quartz diorite which weathers to a friable gray or grayish-brown rock. Typically, it is medium-grained hypidiomorphic-granu- lar, and is composed mainly of quartz, saussuritized plagioclase, biotite, and hornblende with a small amount of altered orthoclase. The amount of ferromagnesian minerals is variable but biotite commonly makes up about 5 percent of the rock with hornblende ranging from about 10 to 15 percent. The biotite is usually partly altered to green chlorite. Although the bulk of this stock is moderately uniform in composition the northern and eastern borders are quite variable and grade into hornblende gabbro and hornblendite. About 1 mile southeast of Jackson Butte at the contact with Calaveras quartz-garnet-mica schist, the intrusive rock is a garnetiferous hornblendite (Ran- some, 1900, p. 4). Irregular masses of hornblendite and hornblende gabbro ocur in the canyon of the Mokel- umne River and also at the Le Roi Mines about 2\ miles southeast of Jackson Butte. In the western part of the stock the rock has pro- nounced foliation approximately parallel to the schistos- ity of the adjacent Calaveras rocks. In the central and eastern parts the foliation is not as prominent as it is in the west. Thin dikes of aplite and pegmatite are present in the margins of the stock. They are mineral- ogically simple but locally the aplites contain garnet and the pegmatites about 1 mile south of Mokelumne Hill contain black tourmaline crystals up to 2 inches in length. The age of the quartz diorite is questionable. Ransome (1900, p. 6) referred to it as "metadiorite" and re- garded it as of "Jura-Trias or early Cretaceous" age. Knopf (1900, p. 18) said the "metadiorite" of the Mother Lode District is "probably ... of two ages, the earlier being of pre-Mariposa age and the later of 1959] TERTIARY VOLCANIC DOMES NEAR JACKSON 13 post-Mariposa age." Those of pre-Mariposa age he con- sidered to have been emplaced at the end of Carboni- ferous time shortly after the first metamorphism of the Calaveras rocks. His main evidence for this is apparently the restriction of the "metadiorites" to the Calaveras rocks and their foliated structures. Apparently he con- sidered the Mokelumne Hill stock to have been emplaced in pre-Mariposa time; however, his evidence is not con- vincing. The foliated structure of the quartz diorite may be due to the flowage of a partly crystalline magma during emplacement rather than dynamic metamor- phism. It seems probable that the Mokelumne quartz diorite is of post-Mariposa age and is possibly related to the granitic rocks of Sierra Nevada foothills, the Valley batholith of Taliaferro. Hornblenditc. About 0.4 mile southeast of Jackson Butte is a small dike-like mass of hornblendite that is in- trusive into the Calaveras. It is about 1200 feet long with a maximum width of about 400 feet. The fresh horn- blendite is black, coherent, and tough. On weathering it gives a deep red soil. The texture of the rock is al- lotriomorphic to hypidiomorphic granular. Typically, hornblende constitutes 90-95 percent of the rock, the remainder being calcite, clinozoisite, chlorite, and pyrite. This grades into a material composed of about 50 percent hornblende and 50 percent saussuritized feldspar. The grain size of the rock is variable, hornblende crystals are from 0.25 inch to 2 inches in length. Microscopically, the hornblende is seen to be pleochroic in shades of green with ZAC about 22°. The hornblende is partly replaced by chlorite ; calcite occurs interstitially, and pyrite occurs as thin veinlets. Clinozoisite occurs in varying amounts as fine-grained anhedral material ap- parently replacing basic feldspar. The hornblendite resembles other hornblendite out- crops along the borders of the Mokelumne Hill quartz diorite, and although it is about half a mile from the nearest outcrop of the quartz diorite it is probable that it is a basic border phase of the Mokelumne Hill stock. The Calaveras rocks adjacent to the hornblendite have been changed into quartz muscovite schist, apparently by contact metamorphism. The effects of metamorphism are rather constant between the hornblendite and the exposed Mokelumne Hill quartz diorite suggesting that this hornblendite is possibly a basic border phase of the quartz diorite. Xenoliths of hornblendite that occur in the dacite of Jackson Butte indicate that hornblendite is present at depth at this point. VOLCANIC DOMES OF THE JACKSON QUADRANGLE Five individual occurrences of massive volcanic rock are present in the central part of the Jackson quadran- gle. The structure of each of these and the relationships to their surroundings indicate they are endogenous vol- canic domes. Four of the domes occur close together in a small area just southwest of Mokelumne Hill. The largest, and also the oldest, is Golden Gate Hill. To the northeast, adjacent to the Mother Lode fault, are three smaller domes. The Tunnel Peak dacite dome is just west of the Mother Lode fault and two rhyodacite domes (the south- ernmost will be called the Hamby dome and the northern one the McSorley dome for convenience) are situated just east of the Mother Lode fault. All three lie close to the fault and it is possible that the magmas which consolidated to form these domes came up along the fault. About 7 miles north of this area is the isolated volcanic dome of Jackson Butte. It is in the eastern belt of the Calaveras rocks about 2 miles east of the Mother Lode fault. The structure of the bedrock of this area is complex and virtually unknown. In the Big Trees quadrangle, just east of its western boundary and near the confluence of the Licking and South Forks of the Mokelumne Kiver, is a small dome that is structurally and petrographically similar to the two domes near Tunnel Peak but the field relationships of this occurrence were not studied in detail. Golden Gate Hill Golden Gate Hill is a conspicuous landmark about 4 miles southeast of Mokelumne Hill, standing about 400 feet above the surrounding area. It is irregular in shape Table 2. Petrographic summary of Tertiary volcanic rocks. Plagioclase Approxi- < Cristo- Ground Refractive index mate Si content Approximate Volume Hornblende Biotite Pyroxenes balite mass fused by weight composition (percent) (percent) (percent) (percent) (percent) (percent) rock (percent) Golden Gate Hill An 45-50 An 41-45 26 22 23 basaltic 26 basaltic and brown 5 4 Rare xenocrysts Common xenocrysts and reaction rims 1 3 45 45 1.533 1.528 61 62 Tunnel Peak An 40-42 20 22 brown and greenish brown Rare hypersthene 3 55 1.527 62 Rhyodacites from Hamby, McSorley, An 13-22 50 10 green -- Rare hypersthene 5 35 1.506 70 and Licking domes Rhyodacite clasts from upper part of An 15-20 50 10 green -- .- 5 35 1.504 70 Mehrten formation Andesite clasts from Mehrten formation An 45-65 26 2 basaltic Augite 5 hypersthene 2 ! Tridy- mite present 65 1.546 57 Valley Springs rhyolite tuff 1 _ An 14 1 present Tridy- mite abun- dant quartz 1 97 1.487 74 1 Sanidine present. 2 Olivine 2 percent. 14 CALIFORNIA DIVISION OF MINES [Special Report 60 and somewhat elongated in a north-south direction with a length of about 2700 feet. Its eastern side is steep and the north, northwest, and northeast sides are covered by dense brush. Golden Gate Hill is made up of gray and brown, por- phyritie hornblende-biotite dacite that is either massive with a prominent planar structure or brecciated. The planar structure is largely due to the orientation of the crystalline constituents ; the hornblende crystals lie with their long axes roughly in the plane of the banding but are not otherwise parallel. The biotite crystals are paral- lel to the flow banding as are the tabular plagioclase crystals. Orientation of the hornblende and biotite is megascopically conspicuous but the orientation of the plagioclase is only microscopically apparent. Near the borders of the dome the flow-banding paral- lels the margins and dips inward. In some places the flow banding rapidly increases in dip from moderately steep at the margin to nearly vertical a few feet within the dome. In the central portion of the dome the attitude of the planar flow structure shows no apparent relation- ship to the margins. At three different places within the dome there are small domical structures where the flow planes come up steeply and arch over forming small, rather symmetrical, secondary domes. The brecciated areas are irregularly distributed and are not confined to the margins of the dacite mass. Typically, the breccia is dark, reddish-brown but locally it is light gray. The grain size of the breccia is variable. Some breccia consists of angular fragments less than 2 inches in length cemented by similar material and at some places this grades into much coarser material. Elsewhere it appears to grade into well-banded unbrecciated dacite. On the northeastern side of the dome are prominent spires of breccia composed of blocks of dacite averaging about a foot and a half in length with some clasts as much as 4 feet long. The breccia is cemented by material of the same color and fabric as the fragments of which it is made and the fragments are conspicuously dis- oriented. Near the base of the hill, surrounding the dacite, is the Mehrten formation which unconformably overlies the pre-Tertiary rocks. The contact between Cosumnes and Logtown Ridge formations passes under Golden Gate Hill. The fact that fragments of dacite, identical to that which forms Golden Gate Hill, are present in the basal part ef the Mehrten formation indicates that the dome is either pre-Mehrten or equivalent to the earliest Mehr- ten in this area. Apparently the Mehrten formation was deposited about the base of Golden Gate Hill and the contacts shown on the map are not intrusive contacts but are depositional contacts. The structure of the dacite as revealed by the attitude of the flow-banding, is essentially that of an endogenous dome (Williams, 1932). Evidence indicates that the dome was protruded as a steep-sided mass at least 400 feet above the surface of the surrounding land. In order to be protruded in this manner and not flow away from the center of eruption, the magma must have been ex- tremely viscous. Some parts were undoubtedly more viscous than others and differential movement of the material shattered and brecciated the more viscous dacite. Fluid magma cemented the shattered dacite to form the /p. 65 w 75 x -"■^ «S A \\ BO \ 1 ^^ /XBS t 70,/ / + X v 60 + as I < \ 60, I I ' 601 I > I '60/ i«0 / -f- Horizontal Flow Layers \ ♦ "4 60) Inclined Flow Loyers \\^*VP ^ / \ Vertical Flow Layers \ \«? 73 ■\«o, / i "¥ 60) r- 1000 FE ET -I Scale Figure 2. Golden Gate Hill dacite dome. breccia. Spines of breccia were probably formed and the collapse of these evidently produced abundant talus which was incorporated in the basal Mehrten on the east side of the dome. Megascopically, the rock of Golden Gate Hill is gray to reddish-brown and porphyritic with conspicuous phenocrysts of hornblende and biotite embedded in an aphanitic groundmass. Clusters of small black hornblende crystals are common and inclusions of various rock types (e.g. peridotite, meta-andesite) constitute several per- cent of the rock. Microscopically, the rock is seen to be porphyritic with phenocrysts of basaltic hornblende, biotite, and plagio- clase, and microphenocrysts of apatite and magnetite embedded in a cryptofelsitic to glassy groundmass. Finely divided hematite is abundant in the groundmass of the reddish dacite and cristobalite occurs as small, spherical grains in microvesicles. Basaltic hornblende in euhedral crystals up to 5 mm long makes up about 25 percent of the rock. It is com- monly zoned in shades of brown with rims of granular magnetite. Biotite occurs as sharp euhedral crystals, rarely more than 1 mm in width, constituting about 5 percent of the rock. Plagioclase (calcic andesine about An 48 ) consti- tutes about 25 percent of the rock and occurs as small euhedral to subhedral, slightly zoned crystals. It ranges TERTIARY VOLCANIC DOMES NEAR JACKSON 15 to Mokelumne Hill f GEOLOGIC MAP OF THE GOLDEN GATE HILL AREA CALAVERAS COUNTY CALIFORNIA ATION Geology by R.L.Rose, 1949 Strike of vertical foliation Strike and dip of cleavage Tunnel Peak vent breccia Mehften formation Valley Springs formation lone formation ,H»mqd*/f Mokelumne Hill quartz d i o rile I Mariposa formation Logtown Ridge formation Cosumnes formation Calaveras group Figure 3. 16 CALIFORNIA DIVISION OF MINES [Special Report 60 in size up to 0.5 mm in length and is twinned according to the carlsbad, albite, and pericline laws. Apatite occurs as euhedral prisms and six-sided basal sections which are often included in plagioclase and magnetite occurs as small cubic crystals embedded in the groundmass which is either glassy or cryptofelsitic. Specimens of fresh gray rock were finely pulverized and fused in a carbon arc yielding a glass with an index of refraction of about 1.533 which indicates a silica con- tent of about 61 percent ± 2 percent. (Mathews, 1951). For calc-alkaline rocks this corresponds to a dacite. Nockolds (1954) gives the silica content of the average dacite as 63.6 percent and andesite 54 percent. Even though the rock does not contain phenocrysts of quartz it is termed a dacite as the silica content indicates an excess of 10 percent normative quartz. Xenoliths and xenocrysts constitute about 5 percent by volume of the Golden Gate Hill dacite. Quartz, augite, sodic plagioclase and enstatite are present as xenocrysts while several rock types occur as xenoliths. The most abundant type of xenolith is a vesicular horn- blende-plagioclase vitrophyre which occurs as ellipsoidal masses up to 18 inches in length. Aphanitic, reddish- brown, subangular xenoliths several inches long are present in the western part of the dome. These are ap- parently fragments of the Logtown Ridge formation. A few small dunitic xenoliths about ^ inch in diameter were found. These have double reaction rims, an inner rim of white fibrous enstatite and an outer zone of black ba- saltic hornblende. Weathered inclusions like this are common ; they consist of a central ocherous area up to 15 mm in diameter, an inner zone of fibrous orthopyro- xene up to 5 mm thick, and an outer black rim of ba- saltic hornblende often as much as 5 mm in thickness. Tunnel Peak About a mile north of Golden Gate Hill is a small volcanic complex about 2000 feet in diameter that makes up the topographic prominence known locally as Tunnel Peak. This complex consists of an irregular mass of hornblende dacite surrounded by tuff breccia. The tuff breccia is gray to pale brown, massive, and composed of angular fragments of slate, hornblende da- cite and rounded pebbles and cobbles of Mehrten ande- site embedded in a tuffaceous matrix. The slate frag- ments average about 1 inch in length, are black when fresh but weather to a greenish color, and are mega- scopically identical with the adjacent slate. The horn- blende dacite fragments average about 3 inches in length but a few are 12 to 15 inches long. They are megascopic- ally and microscopically identical with the rock that forms the central portion of the Tunnel Peak complex. The matrix of the breccia is a medium-grained horn- blende dacite tuff apparently of the same composition as the dacite fragments and Tunnel Peak dome. The quantity of the large fragments in the breccia varies considerably. On the south side of Tunnel Peak the large angular fragments constitute about 65 percent of the rock. On the east side of Poorman Gulch the angular fragments rmike up about 10 to 15 percent of the breccia. The contact between the Mehrten and the vent breccia is nearly vertical and apparently the breccia intrudes the Mehrten. Fragments of Tunnel Peak hornblende dacite are unknown in the Mehrten formation. The pres- ^&a^W*^ S&A-sv'^J Photo 5. Photomicrograph of Tunnel Peak hornblende dacite. Section approximately parallel to flow banding. Hornblende and plagioclase phenocrysts in a cryptofelsitic groundmass ; rare hy- persthene. x30. V '4 1/ &*'«£ ;v*'- V'J \ i. >> •*r A ' ► J. • A ■ . , * *K . :*& Photo 6. Tunnel Peak vent breccia. Large subrounded clast just above and to the left of the pick head is a worn fragment of Mehrten andesite. Small tabular clasts are mainly fragments of Cosumnes slate. Pale angular clasts are Tunnel Peak dacite. ence of Mehrten andesite pebbles in the breccia indicates that it was formed after the deposition of the Mehrten. Evidently the breccia was formed by shallow volcanic explosion or a series of explosions when the area was covered by the andesitic sediments of the Mehrten for- mation. The central and northeastern part of Tunnel Peak is composed of well-banded, gray hornblende dacite with minor amounts of hornblende dacite breccia. The dacite is porphyritic with small phenocrysts of plagioclase and hornblende embedded in an aphantic groundmass. Under the microscope the rock appears to be made up of pheno- crysts of brown hornblende and andesine feldspar (about An 4 o) and microphenocrysts of apatite and mag- netite embedded in a cryptofelsitic groundmass. The rock is microvesicular with small cristobalite grains lining 1959] TERTIARY VOLCANIC DOMES NEAR JACKSON 17 the cavities ; in some specimens these are accompanied by small crystals of tridymite. The silica content of this rock is about 63 percent corresponding to dacite accord- ing to Nockolds (1954, p. 1015). The conspicuous band- ing or layering of the rock is due to the distribution and orientation of the phenocrysts. The plagioclase crystals are tabular parallel to the side pinacoid and lie with their side pinacoids in a common plane. The long axes of the hornblende crystals lie in this plane with no ap- parent lineation of the crystals. This banding was ap- parently produced by the laminar flowage of a partly liquid viscous magma at the time of emplacement. The flow banding near the margins of the dacite mass strikes tangent to the outer contact of the dacite and usually dips toward the center of the mass. In the cen- tral portion of the mass the banding is irregular and often contorted but usually steeply dipping. The struc- ture of this igneous mass indicated by the attitude of the flow bands shows that this body of rock is not a remnant of a lava flow but is an endogenous volcanic dome. It was evidently forced up through the Tunnel Peak vent breccia and possibly stood 100 feet or more above its surface. McSorley Dome Just northeast of Tunnel Peak is a small hill composed mainly of gray, aphanitic hornblende rhyodacite previ- ously referred to as the McSorley dome. Much of the hill is covered by talus and brush but enough rock is exposed to indicate that the structure of this mass of rhyodacite is that of a volcanic dome similar to Golden Gate Hill. Near the margins of the mass the banding dips toward the dome and strikes parallel to its outer contact. Much of the rock is brecciated and cemented by similar material. A band of Calaveras phyllite nearly surrounds the rhyoda- cite and separates it from the Mehrten formation. Ap- parently this mass of Calaveras rock was dragged into its present position by the intrusive or protrusive rhy- odacite, which must have been emplaced in a very viscous state. Possibly the dome had a cover of rock of the Mehrten formation at the time of emplacement as its present top is only about 60 feet above the top of the Mehrten formation. The fact that it has deformed the Mehrten rocks and dragged up the mass of Calaveras ma- terial between the Mehrten and the rhyodacite clearly indicates that it was emplaced after deposition of the Mehrten formation. Megascopically the rhyodacite is well-banded, pale gray and porphyritic with very small crystals of hornblende embedded in an aphanitic groundmass. Under the micro- scope the rock appears to consist of phenocrysts of green hornblende, microphenocrysts of plagioclase (oligoclase about An 2 2) and minute apatite and magnetite crystals all enclosed in a cryptofelsitic groundmass. Minute vesi- cles and crevices are lined with very small globular grains of cristobalite. The index of refraction of the fused dacite is about 1.507 which indicates a silica con- tent of about 70 percent corresponding to a rhyodacite according to Nockolds (1954). Hamby Dome East of Tunnel Peak and south of McSorley dome is a small dome that is composed of rhyodacite, petrograph- ically very similar to the last described rock. The rhyoda- cite outcrops on both sides of the road but is partly covered by talus and roadfill and much of it is covered by dense brush. It is, however, well exposed in the road cut. Two small masses of Calaveras rock have evidently been dragged up along the sides of the dome. One is exposed in the road cut at the northeast margin of the dome and the other at the southwest. The adjacent Mehrten formation dips away from the dome at a moder- ate angle. The dome has an elongated shape with a maximum length of about 1350 feet. It is composed of well-banded, gray rhyodacite and rhyodacite breccia. Its structure is similar to that of other domes of this region ; that is, the flow banding at the margins strikes roughly parallel to the contact and dips inward. In the central part the banding is vertical or nearly so. Much of the dome is apparently composed of autobrecciated dacite that is cemented by a small amount of identical material. Megascopically and microscopically the rock of this dome is identical to that composing the McSorley dome. The refractive index of the fused rock is essentially the same as that for the rock of the adjacent rhyodacite dome. The structure of the dome indicates 'that it is an endogenous volcanic dome that was emplaced as a viscous, perhaps largely crystalline mass. Evidently it was em- placed in late Mehrten or post-Mehrten time and about the same time as the adjacent McSorley dome. Originally the dome probably protruded above the Mehrten surface 50 or 60 feet. Jackson Butte About 1\ miles N. 20° E. of Mokelumne Hill and about the same distance S. 75° E. of Jackson stands an irregu- larly shaped rocky prominence called Jackson Butte. It has a maximum diameter of about 2100 feet and its sum- mit stands 2310 feet above sea level and about 600 feet above the surrounding area. Jackson Butte is steep- sided ; the north and northeastern sides are covered with dense brush. A thick accumulation of talus about the base of the knob obscures the contacts between the dacite of Jackson Butte and the adjacent Calaveras formation. Largely concealed beneath the talus on the west side is a small amount of andesitic gravel of the Mehrten for- mation. Evidently the early miners thought these gravels were present under the massive dacite as there are nu- merous old mining adits at the base'of the dome. Some of the adits are in the gravel, others are in the talus or in the underlying Calaveras rocks but only four of these were found to be accessible. Several inaccessible shafts penetrate the talus and apparently were driven to con- nect with the adits. Jackson Butte is made up of well-banded hornblende- biotite dacite and its brecciated equivalent and is sur- rounded by the Calaveras group. The contact of the dacite and the adjacent rock is exposed only on the south side of the butte, elsewhere it is obscured by talus. The massive dacite of Jackson Butte has a well-defined platy flow structure. In many places it is marked by alternate bands with slightly different color and/or texture. The flow structure is due to the orientation of the xenoliths, xenocrysts, and phenocrysts. The hornblende crystals and the prismatic xenoliths lie with their axes in the plane of the banding but show no linear parallelism ; the biotite crystals and the tabular xenoliths are parallel to the flow planes. Megascopically the orientation of feld- spar crystals is not apparent but microscopically they appear to have their broad surfaces parallel. This platy 18 CALIFORNIA DIVISION OF MINES [Special Report 60 * ytw/ Mehrten formotion ^^//y, laveros group -♦— Horizontal JC* Vertical s^ Inclined Flo« Laytrs SCALE 1000 FEET Figure 4. Jackson Butte dacite dome. flow structure is accentuated by weathering, the less re- sistant layers forming grooves and depressions and the resistant parts stand out as ridges and knobs. A psuedo- vesicular appearance is commonly produced by the par- tial removal of non-resistant xenoliths and the rock has a well-defined parting parallel to the flow structure. Near the margins of the dome the flow banding strikes approx- imately parallel to the contact and dips towards the center of the dome. The dip is moderate to steep at the margins but increases towards the interior. In the central portion of the dome there is no apparent relationship of the flow banding to the marginal contacts and the flow bands are often extremely contorted. In some places they form numerous small closely spaced "drag-folds". The jointing of the dacite is somewhat irregular, with the main joint set usually parallel and a secondary set nearly perpendicular to the flow banding and either vertical or steeply dipping. In some places the major joints are perpendicular to the flow banding. Much of the dacite is brecciated and although the breccia is common near the borders of the dome, it is not restricted to the border zones. Most of the breccia is reddish-brown in color and consists of large angular fragments cemented by similar material. The fragments vary in size from an inch or less to more than four feet in length but most of them range from 2 to 12 inches. /OC?* Photo 7. Jackson Butte from JMokelumne Hill ; view across the canvon of the Mokelunine River. 5ft? V<« ,«■ 4^ ^^^H ? * c*^'$K± jM *1~ Photo 8. Photomicrograph of Jackson Butte hornblende-biotite dacite with a xenoeryst of olivine surrounded by a reaction rim of basaltic hornblende. Plane-polarized light. x30. In some places bands of breccia alternate with massive dacite and locally they appear to grade into it. On the western side of the dome much of the breccia is compara- tively fine-grained and gray in color. 1959] TERTIARY VOLCANIC DOMES NEAR JACKSON 19 The lack of fragments of Jackson Butte dacite in the Mehrten formation at Jackson Butte and elsewhere indi- cates that the Mehrten is older than the dacite. This is further supported by the fact that adjacent Mehrten is covered by dacitic talus. The highest Mehrten in this area is about 465 feet below the crest of the butte. At the elevation of 1835 feet, according to a map by Gale (Gale, 1939, pi. 4) this is about the elevation of the original surface of the Mehrten formation in this area. The structure of Jackson Butte is that of an endogene- ous volcanic dome. The sides are steep, the marginal flow bands dip toward the center of the dome and strike parallel to the contact. Evidently Jackson Butte was squeezed up through the Calaveras rocks and the Mehr- ten formation as a very viscous magma to form a steep- sided, prismatic mass of volcanic rock. The top of the dome originally stood at least 400 feet above the sur- rounding surface. At the time of emplacement the dacite was undoubtedly a viscous, partly crystalline material. Differential movement between layers of different vis- cosity apparently caused the brecciation and the brecci- ated material was cemented by dacite that was still fluid. The emplacement occurred some time after the Mehrten formation was deposited in this area, possibly in middle or upper Pliocene. Megascopically, the rock composing Jackson Butte is fine-grained, gray to reddish-brown, and porphyritic, with small crystals of hornblende and biotite embedded in a gray aphanitic groundmass. More conspicuous than the phenocrysts are the abundant xenocrysts and xeno- liths. The foreign fragments which are up to 8 inches long constitute about 10 to 15 percent of the volume of the rock. Microscopically, the dacite is porphyritic, with pheno- crysts of hornblende, biotite, and plagioclase, and micro- phenocrysts of apatite and magnetite embedded in a cryptofelsitic groundmass. The hornblende occurs as small euhedral to subhedral crystals up to 1.5 mm long and makes up about 25 percent of the rock. In the gray dacite it is pleochroic in yellow and brown with an ex- tinction angle of about 1*4°. It often has narrow black rims of magnetite. Tn the red dacite the amphibole is 4?*"\* f ■ V -- basaltic hornblende with a brown to reddish-brown pleochroism, small exinction angle, very strong bire- fringence, and borders of magnetite. Biotite constitutes about 5 percent of the rock and occurs as euhedral crystals up to 1 mm wide. It is pleo- chroic from dark brown to yellow brown and commonly has narrow opaque rims of magnetite. Plagioclase (An 4 i_ 4g ) makes up about 20 to 25 percent of the rock and occurs as small, sharp, euhedral, twinned crystals ranging up to about 0.3 mm in length. The groundmass of all varieties of the dacite is cryptofelsitic but that of the reddish-brown dacite is cloudy with hematite dust. Samples of the fresh gray dacite apparently free from inclusions were pulverized and fused in a carbon arc. The resulting glasses had an average refractive index of about 1.528 indicating a silica content of 63 percent±3 percent. For calc alkaline rocks this silica content repre- sents at least 12 percent normative quartz, hence the rock is termed dacite even though it contains no quartz phenocrysts. The xenoliths and xenocrysts in the Jackson Butte dacite are abundant and varied. Hornblendite and horn- blende gabbro xenoliths are common and appear to be evenly distributed throughout the rock. The majority of them are less than 2 inches long but they range up to about 7 inches. The hornblendite consists mainly of an- hedral crystals of hornblende up to 0.5 inch in length. In the gray dacite the hornblende is pleochroic in brown and has an extinction angle of about 14°. In the red dacite it is deeply-colored reddish-brown basaltic horn- blende with a small extinction angle. The hornblende gabbro fragments are commonly saussuritized and con- sist essentially of hornblende or basaltic hornblende and clinozoisite or iron-poor epidote. Some xenoliths are schistose and consist essentially of anhedral to subhedral hornblende and subhedral calcic labradorite. Microscopically, one xenolith was found to have a hypidiomorphic granular texture with subhedral to an- hedral brown hornblende, anhedral labradorite, some quartz and orthoclase ( ?), with small grains of second- ary epidote and sericite. Some of the hornblendite is garnetiferous with euhedral red garnets up to 0.25 inch in Photo 9. "Weathered surface of Jackson Butte dacite showing flow banding. Ovoid pits are from partial removal of ultrabasic xenoliths by weathering. Photo 10. Jackson Butte dacite breccia. Note the angularity of the fragments. 20 CALIFORNIA DIVISION OF MINES [Special Report 60 Photo 11. Photomicrograph of Golden Gate Hill hornblende- biotite daeite showing large euhedral phenocrysts of basaltic horn- blende, thin platy biotite grains, and microphenocrysts of plagio- clase enclosed in a cryptofelsitic groundmass. Plane-polarized light. x30. diameter. The hornblende of these inclusions varies from brown hornblende to basaltic hornblende. All of the exotic hornblende in the red daeite appears to be basaltic horn- blende. The brown hornblende that is common in the gray rock approaches basaltic hornblende in some of its optical properties. All of the varieties of hornblendic rocks have been found as border phases of the Mokel- umne Hill quartz diorite, and some of them occur just south of Jackson Butte as described on an earlier page. The only major difference between the inclusions and the outcropping rocks is the difference in the optical prop- erties of the hornblende ; pleochroism, 2 V, etc. This dif- ference in properties is due to the heating of the horn- blende of the xenoliths. Basaltic hornblende is said to form from green hornblende at about 750°C. The domi- nance of basaltic hornblende in the red daeite either in- dicates that it reached a higher temperature than the gray daeite or that the partial pressure of oxygen in the red rock was greater than in the gray rock. Evidently the red daeite with the basaltic hornblende reached a temperature of at least 750°C, the transformation tem- perature of hornblende. Xenoliths of peridotite that vary from dunite to harz- burgite are very common. They range in size up to an inch in diameter, are usually rounded, and are pale green. Olivine constitutes 80 to 95 percent of the perido- tites, with enstatite and augite making up most of the remainder. Olivine grains are anhedral and range up to 4 mm in length. Several determinations on the universal stage indicate that they are somewhat variable with 2 V ranging from 84° positive to 88° negative. Thus they are rich in magnesia. The enstatite occurs in gray to brown crystals up to 5 mm in length with variable posi- tive 2 V ranging from 72° to 84° corresponding to En 93 to En 88 . In some specimens dark brown crystals of augite are present. Reaction rims are commonly present on the peridotite xenoliths and these vary from a fraction of a millimeter up to 2 mm in width. Commonly they con- sist of fibrous hornblende with the fibers approximately perpendicular to the surface of the inclusion. In some cases the peridotite fragments have double reaction rims, an inner white fibrous rim of orthopyroxene, ranging from hypersthene to enstatite, and an outer rim of horn- blende. Where the inclusion has only olivine grains at the contact both reaction rims are usually complete. Where pyroxene crystals are present at the borders, the inner zone of orthopyroxene is missing. Clots of fibrous, radiating, pale yellow orthopyroxene are common and some of them are as much as four inches in diameter. They usually have narrow reaction rims of hornblende. Although there is no residual olivine visible in most of these clots, apparently they were originally dunite and have been completely replaced by orthopyroxene which has partly been replaced by hornblende. Less common than the hornblendic and peridotitic xenoliths are those that are rich in feldspar. They are rather coarse-grained, with feldspar crystals averaging about 7 mm in length, but some are much coarser- grained. Although they all appear to be pegmatitic, they are quite variable in composition. Some consist mainly of plagioclase and quartz, others are wholly plagioclase. In all specimens examined microscopically the plagio- clase was found to be medium oligoclase ranging from An 16 to An 24 . Xenoliths of various types of metamorphic rocks are also common. They are mainly subangular to subrounded fragments of quartz-biotite-garnet schist, quartz-musco- vite schist, phyllite, and quartzite. Many of these appear to have been picked up at a rather shallow depth. A medium-grained quartz-biotite schist was studied micro- scopically and was found to consist of the various min- erals, in order of abundance, quartz, biotite, garnet, epi- dote, sphene, brown hornblende, apatite, and magnetite. The rock is schistose with anhedral quartz forming a granular mosaic matrix for the other minerals. The bio- tite is subhedral and pleochroic in dark reddish-brown and yellow-brown with rims of finely granular magnetite. Xenocrysts of the rock-forming minerals are very common. Hornblende, olivine, enstatite, augite, quartz, oligoclase, and garnet have all been identified and prob- ably many others are also present. They exhibit the same features relative to the enclosing rock as do the xenoliths. Quartz and oligoclase are deeply embayed and the quartz has not been observed to possess reac- tion rims of any kind. Olivine has fibrous reaction rims of hornblende or hornblende and orthopyroxene. En- statite and augite are rimmed by massive brown horn- blende (non -fibrous), that apparently has its c and b axes parallel, or nearly so, to those of the replaced min- eral. Garnet occurs as rounded grains, and hornblende, which is the commonest xenocryst, varies from the brown variety to deeply pleochoic basaltic hornblende. The olivine and pyroxene crystals illustrate Bowen's discontinuous reaction series. Olivine has reacted with the magma to form fibrous rims of orthopyroxene with an iron content comparable to that of the original min- eral. The orthopyroxene has further reacted with the magma to produce brown hornblende which in some cases, has been changed to basaltic hornblende. As would be expected in a rock of this environment, equilibrium has not generally been reached and residual grains of olivine and pyroxene are the rule rather than the ex- ception. 1959] TERTIARY VOLCANIC DOMES NEAR JACKSON 21 Quartz fragments are commonly embayed like the quartz phenocrysts of rhyolites but the fragments do not have reaction rims of pyroxene as is commonly the case in a basalt. Certainly, if the enclosing rock were just saturated or undersatu rated in silica it would be expected that the quartz inclusions would have reaction rims. The silica content of the enclosing rock is approxi- mately 63 percent and it is thought from this approxi- mate value and the relative stability of the various foreign minerals that the host rock probably contains more than 10 percent normative quartz and is thus a dacite rather than an andesite. The source of most of the xenoliths is obvious. Frag- ments of metamorphic rock were derived from the under- lying Calaveras group. The hornblendite and related materials were probably derived from a basic border phase of the Mokelumne Hill quartz diorite which is evi- dently present beneath the dome. The ultrabasic xeno- liths are the only types that are difficult to account for. Possibly they were derived from an underlying perid- otite mass that is too deep to have been serpentinized. BIBLIOGRAPHY Axelrod, D. I., 1956, Mio-Pliocene floras from west-central Nevada: Univ. California, Geol. Sci. Pub., vol. 33, pp. 1-322. Bowen, O. E. Jr., Crippen, R. A. Jr., et al., 1948, Geologic maps and notes along Highwav 49: California Div. Mines Bull.* 141, pp. 35-87. Durrell, Cordell, 1944, Andesitic breccia dikes near Blairsden, California : Geol. Soc. America Bull., vol. 55, pp. 255-272. Fenner, C. N., 1948, Incandescent tuff flows in souther Peru : Geol. Soc. America Bull., vol. 59, pp. 879-893. Gale, H. S., Piper, A. M., et al., 1939, Geology and groundwater of the Mokelumne area, California : U. S. Geol. Survey Water- Supply Paper 780. Gilbert, C. M., 1938, Welded tuff in eastern California: Geol. Soc. America Bull., vol. 49, pp. 1829-1862. Jenkins, O. P. et al., 1943, Manganese in California : California Div. Mines Bull. 125. Knopf, A., 1929, The Mother Lode system of California: U. S. Geol. Survey Prof. Paper 157. Mathews, W. H., 1951. A useful method for determining approxi- mate composition of fine-grained igneous rocks : Am. Mineralogist, vol. 36, pp. 92-101. Nockolds, S. R., 1954, Average chemical composition of some igneous rocks : Geol. Soc. America Bull., vol. 65, pp. 1007-1032. Ransome, F. L., 1900, U. S. Geol. Survey Geol. Atlas, Mother Lode district folio, no. 63. Rose, R. L., 1949, Tertiary volcanic domes of the Jackson quadrangle, California : Unpublished M. A. thesis, University of California at Berkeley. Taliaferro, N. L., 1942, Geologic history and correlation of the Jurassic of southwestern Oregon and California : Geol. Soc. America Bull., vol. 53, pp. 71-112. Taliaferro, N. L., 1943, Manganese deposits of the Sierra Nevada, their genesis and metamorphism : California Div. Mines Bull. 125, pp. 277-333. Turner, H. W., 1894a, The rocks of the Sierra Nevada: U. S. Geol. Survey, 14th Ann. Rept., pt. 2. Turner, H. W., 1894b, U. S. Geol. Survey, Geol. Atlas, Jackson folio, no. 11. Williams, Howel, 1929, Geology of the Marysville Buttes, Cali- fornia : Univ. California Dept. Geol. Sci. Bull., vol. 18, no. 5, pp. 103-220. Williams, Howel, 1932, The history and character of volcanic domes : Univ. California Dept. Geol. Sci. Bull, vol. 21, no. 5, pp. 51-146. printed in California state punting office A95316 4-59 3,500 :