GEOLOGIC MAP OF THE HART PEAK QUADRANGLE, CALIFORNIA AND NEVADA by Jane E. Nielson and Ryan D. Turner INTRODUCTION The Hart Peak 1:24,000-scale quadrangle is located about 12 km southwest of Searchlight, Nevada, comprehending the eastern part of the Castle Peaks, California, and most of the Castle Mountains and the northwestern part of the Piute Range, in California and Nevada (appendix 1, figs. 1, 2). The Castle Peaks area constitutes the northeasternmost part of the northeast-trending New York Mountains. The Castle Mountains straddle the California-Nevada State line between the Castle Peaks and north-trending Piute Range (appendix 1, fig. 1). The southern part of the Piute Range, near Civil War-era Fort Piute, adjoins Homer Mountain (appendix 1, fig. 2) mapped by Spencer and Turner (1985). Adjacent and nearby 1:24,000-scale quadrangles include Castle Peaks, East of Grotto Hills, Homer Mountain, and Signal Hill, Calif.; also Tenmile Well and West of Juniper Mine, Calif. and Nev. (appendix 1, fig. 2). The oldest rocks in the Hart Peak quadrangle are Early Proterozoic gneiss and foliated granite that crop out in the northern part of the quadrangle on the eastern flank of the Castle Peaks and in the central Castle Mountains (Wooden and Miller, 1990). Paleozoic rocks are uncommon and Mesozoic granitic rocks are not found in the map area. The older rocks are overlain nonconformably by several km of Miocene volcanic deposits, which accumulated in local basins. Local dikes and domes are sources of most Miocene eruptive units; younger Miocene intrusions cut all the older rocks. Upper Miocene to Quaternary gravel deposits interfinger with the uppermost volcanic flows; the contact between volcanic rocks and the gravel deposits is unconformable locally. Canyons and intermontane valleys contain dissected Quaternary alluvial- fan deposits that are mantled by active drainage and alluvial fan detritus. PREVIOUS WORK The earliest maps and descriptions of the Castle Mountains and Castle Peaks include regional geologic mapping by Hewett (1956) and theses by Medall (1964) and Balkwill (1964). New geologic mapping in the Castle Mountains and Piute Range, undertaken by the U.S. Geological Survey between 1984 and 1991, produced preliminary stratigraphic summaries by Nielson and others (1987, 1993), and Nielson and Nakata (1993). Turner (1985) defined the Miocene section of the northern Castle Mountains: following the example of Bingler and Bonham (1973), he applied nomenclature of Longwell (1963) to parts of these stratigraphic sequences; we refer to these names but do not use them. Mapping of Thompson (1990) in the Castle Peaks area used the nomenclature of Miller and others (1986), which we also employ. Capps and Moore (1991) have proposed a nomenclature for Castle Mountains rocks that is unrelated to any previous usage, and therefore we do not use those unit names. GEOLOGIC SETTING The Hart Peak quadrangle straddles the boundary between the northern and southern parts of the Basin and Range Province, west of the Colorado River valley. In the adjacent Castle Peaks quadrangle (appendix 1, fig. 2), Early Proterozoic gneiss units record episodes of tectonism, metamorphism, and plutonism extending from 2300 Ma to about 1640 Ma (Wooden and Miller, 1990). Paleozoic rocks in the northern part of the New York Mountains (appendix 1, fig. 1) are steeply tilted and cut by myriad faults with north to northwest strikes (Burchfiel and Davis, 1977; Miller and others, 1986). Mesozoic (Cretaceous?) granite plutons invaded the older rocks (Beckerman and others, 1982; Miller and others, 1986). Miocene continental extension produced the volcanism and structural trends of Miocene rocks and faults in the Hart Peak quadrangle. Regional-scale detachment faults are exposed to the west in the Kingston Range (appendix 1, fig. 1; Reynolds, 1993), and detachment faults may underlie the Black Mountains of Arizona to the east of the map area (Faulds and others, 1990; Faulds, 1993). No low-angle normal (detachment) faults of regional scale crop out in the region depicted by figure 2, but local low-angle normal faults cut both the Miocene volcanic rocks and underlying gneiss in the northern part of the map area. High-angle normal faults in the map area were also caused by Miocene extension; these faults are oriented northwest, northeast, and east-northeast. Some of the Tertiary faults parallel the Mesozoic faults, therefore. The high-angle faults locally exhibit dip-slip offsets that generally are on the order of tens of meters. Geometric relations indicate that offsets on some faults may be as great as 300 m to 1 km; for example, the contact of Early Proterozoic rocks and overlying Tertiary rocks in the Hart Peak quadrangle crops out at elevations 300 to 500 m higher than exposures of the basal contact of Tertiary rocks on the east side of the Piute Range (Homer Mountain, West of Juniper Mine, and Tenmile Well quadrangles). This offset of the basal nonconformity over a lateral distance of 5 km likely is due to a high-angle normal fault, or zone of faults, with strikes parallel to the west side of the Piute Range; one such fault crops out in the Hart Peak quadrangle. Steep faults that offset basement rocks at the boundary between the Castle Mountains and Piute Range probably produce the steep gravity and magnetic gradients observed in this part of the quadrangle (U.S. Geological Survey, 1983; Mariano and others, 1986). The relation of exposed high-angle faults to hypothetical Miocene low-angle normal faults at depth, or to regional strike-slip faults such as the Las Vegas shear zone, remains unclear (Faulds and others, 1990). PRE-TERTIARY ROCKS Pre-Tertiary rocks in the Hart Peak quadrangle include Early Proterozoic leucocratic granite and granitic gneiss (Xlg) in the Castle Peaks and migmatitic gneiss (Xmg) in the central part of the Castle Mountains. The leucocratic gneiss represents granitic plutons that were intruded at about 1680 Ma, after the Ivanpah orogeny at about 1710 Ma (Wooden and Miller, 1990). The migmatitic gneiss may be equivalent to exposures of multiply metamorphosed older Early Proterozoic rocks in the nearby New York and Ivanpah Mountains (appendix 1, fig. 1; Wooden and Miller, 1990). One small outcrop of Paleozoic limestone (Pzl) is known in the Hart Peak quadrangle, in apparent fault contact with gneiss (Capps and Moore, 1991). OLDER TERTIARY ROCKS Regional Units The lowermost Tertiary (Oligocene? and Miocene) unit exposed in the Castle Peaks and Piute Range is locally derived arkosic sandstone and conglomerate. In the Hart Peak quadrangle the stratigraphically lowest Tertiary volcanic unit (Tps) is a sanidine-rich, sphene-bearing, ash-flow tuff of alkalic rhyolite composition (appendix 1, fig. 3, appendix 2, tables 1 and 2). In the Castle Peaks and Piute Range this tuff overlies the arkosic sandstone and conglomerate unit, whereas in the Castle Mountains, the unit overlies pre-Tertiary plutonic rocks. Turner and Glazner (1990) reported conventional K-Ar ages of 18.5±0.5 Ma (biotite) and 17.5±0.4 Ma (sanidine) from this ash-flow tuff unit (location 2, fig. 2); another sample (location 3, fig. 2) produced an age of 18.79±0.04 Ma by the single-crystal 40Ar/39Ar laser fusion technique (B.D. Turrin, oral commun., 1991; reported by Nielson and others, 1993). This relatively high precision laser-fusion age is indistinguishable from the 18.5±0.2-Ma age of the regionally widespread Peach Springs Tuff (Tps) of Young and Brennan (1974), which is also a sanidine-rich, sphene-bearing ash-flow tuff of alkalic rhyolite composition (Nielson and others, 1990). The regional correlation of isolated tuff outcrops with the Peach Springs Tuff is supported by its unusual paleomagnetic direction for rocks of Miocene age (Young and Brennan, 1974; Wells and Hillhouse, 1989). A sample of the lower tuff unit collected in the adjacent Castle Peaks quadrangle yielded this unusual Miocene paleomagnetic direction (Wells and Hillhouse, 1989), confirming that the Peach Springs Tuff is present locally. Two conventional K-Ar mineral ages of about 22 Ma were reported for a sample from the basal ash-flow tuff (location 3, fig. 2) by Capps and Moore (1991; their "Castle Mountains Tuff (member) of the Castle Mountains Volcanic rocks"). These ages are explained by incremental heating 40Ar/39Ar experiments (Nielson and others, 1990), which demonstrated that the Peach Springs Tuff may produce spuriously old ages due to contamination by xenocrysts from surface regolith that the ash-flow tuff incorporated during its deposition. In both the Hart Peak and Castle Peaks quadrangles, the lowermost part of this basal ash-flow tuff contains granite and gneiss xenoliths, and on this basis we propose that the conflicting results of conventional K-Ar and laser- fusion techniques demonstrate xenocrystal contamination of the Capps and Moore (1991) sample. We therefore conclude that the basal ash-flow tuff in the Hart Peak quadrangle is the Peach Springs Tuff. Local Units Volcanic and sedimentary rocks that overlie either the arkosic sandstone and conglomerate unit and (or) the Peach Springs Tuff in the Hart Peak quadrangle are locally erupted mafic to silicic, alkalic to subalkaline flows, tuff, and breccia of early and middle Miocene age, which are interbedded with volcaniclastic and minor epiclastic sedimentary rocks. The volcanic rocks of the Castle Peaks and Piute Range are predominantly of mafic and intermediate compositions (appendix 1, fig. 3). The Castle Mountains volcanic sequence includes a significant volume of rhyolite that is underlain by mafic and intermediate rocks resembling the Castle Peaks sequence, and which underlies and interfingers with mafic and intermediate flows characteristic of the Piute Range. Interfingering relations between the Castle Peaks and Castle Mountains volcanic rocks are not exposed, and relations elsewhere suggest that the sequences may be juxtaposed by a buried regional-scale fault (D.M. Miller, oral commun., 1995; queried on cross sections). The total thickness of the Castle Peaks volcanic sequence is 350 m, that of the Piute Range is as much as 600 m, and that of the Castle Mountains volcanic sequence may be 1 to 1.5 km. Castle Peaks Volcanic Rocks The lowest locally erupted unit of the Castle Peaks volcanic sequence is light-colored volcanic breccia (Tbr), consisting of monolithologic eruptive breccia and megabreccia. This breccia unit is locally rhyolitic and contains mafic dikes and interbedded finer grained heterolithologic volcaniclastic eruptive rocks. Stream channels that developed on the surface of the breccia unit are filled by gravel deposits, consisting of volcanic, granitic, and gneissic clasts, and very little matrix. White lithic tuff (Tcp) generally overlies the breccia unit, but locally the base of the lithic tuff is interbedded with the upper part of the breccia unit. Massive to brecciated pyroxene- bearing andesitic flows (Tap) cap the sequence (Thompson, 1990). Many dikes in the breccia unit are feeders for these capping flows, as well as for zones with andesitic composition within the breccia unit. Reliable isotopic ages have not yet been produced from samples of the Castle Peaks volcanic sequence. A clast from the volcanic breccia, sampled in the Castle Peaks quadrangle to the west, produced a K-Ar age of 14.7±0.4 Ma (biotite) and a highly imprecise 40Ar/39Ar total fusion age of 17.5±10.4 (sanidine; table 4). A sample of the Castle Peaks Tuff produced a relatively imprecise 40Ar/39Ar total fusion age of 21.4±1.6 (sanidine; table 4). The actual minimum age of the Castle Peaks volcanic sequence may be indicated by a silicic welded ash-flow tuff that we call the tuff of Barnwell, which is part of the capping andesite flow unit. The tuff of Barnwell contains an oxide- rich suite of heavy minerals (Gusa and others, 1987), and may be laterally equivalent to a subunit of the Wild Horse Mesa Tuff of McCurry (1988). High-resolution 40Ar/39Ar techniques have produced ages of 17.7 to 17.8 Ma for the Wild Horse Mesa Tuff (McCurry and others, 1995). Castle Mountains Volcanic Rocks The lowest locally erupted volcanic unit in the Castle Mountains (volcanic flows and breccia of the Castle Mountains, Tcm) is composed mostly of trachyandesite to trachybasalt flows and breccia and interbedded sedimentary rocks derived principally from mafic volcanic sources. In the north-central part of the Hart Peak quadrangle, the unit is conformably overlain by varicolored, upwardly fining, predominantly lacustrine, sedimentary strata composed of volcaniclastic and arkosic detritus (Tlss). The lacustrine sedimentary rocks are overlain by the tuff of Jacks Well (Tjw), a 16-Ma partly welded ash-flow tuff (tables 1, 3; Turner and Glazner, 1990), which underlies a thick unit of massive to layered, white, pink, and lavender rhyolite flows, tuff, and breccia (Tr). Most of this unit constitutes overlapping aprons of ejecta that erupted from vents now represented by northeast-alined rhyolite domes and dikes (Tir). A unit of layered tuff and sedimentary material (Tts) is mapped wherever it is distinguishable from the rhyolite flows, tuff, and breccia unit. Thick rhyolite flows (Trf), reworked volcaniclastic sandstone (Tvss), and lahar (Tvl) all are local units present either within or between thick local deposits of rhyolitic ejecta. Basalt flows (Tb) and sills (Tib) are interbedded with or overlie the rhyolite eruptive units. In the northern part of the quadrangle, the tuff of Juan (Tj) is a gently tilted welded ash-flow tuff deposit dated at 14.4±0.2 Ma (tables 1, 3) that overlies both the basalt flows and rhyolite units. In the southern part of the quadrangle, a major latite dike (Til) with K-Ar ages of 14.6 (plagioclase) and 14.3±0.4 and 14.1±0.4 Ma (biotite; table 3) intruded the rhyolite and related units (Capps and Moore, 1991). Thin basalt dikes (Tib) intruded the Miocene and older rocks. Capps and Moore (1991) reported K-Ar ages of 14.5 and 15.1 Ma for the latite dike, and ages between 16.5 and 14.9 Ma for rhyolite domes and dikes. An apparently untilted rhyolite dome (Tir), with steep, symmetrical flow-banding, produced an age of 12.8±0.2 Ma (table 1, 3). The rhyolite units (Tr, Tts) are thickest in the western part of the Castle Mountains, where domes are most abundant. The greatest thickness (about 1 km) is found 2 km north of Hart town site (SW 1/4 sec. 13, T. 14 N., R. 17 E.). Less than 2 km to the east (NE 1/4 sec. 18, T. 14 N., R. 18 E.), the Castle Mountains rhyolite sequence is represented by a thin (0.1 km) rhyolite tuff unit (Tts) that overlies Early Proterozoic gneiss and is overlapped in turn by basaltic lava of the Piute Range. The main exposures of the rhyolite unit (Tr) in the east part of the map area are eruptive and intrusive breccia that form the marginal carapace of a dome; for example, near Lewis Holes, Nevada (sec. 15, T. 30 S., R. 62 E.). Related dikes trend northeast, parallel to both a major fault zone and to the trend of domes on the west side of the Castle Mountains. Abundant zones of alteration, including silicified and gold-bearing veinlets, are present in tuff and breccia of the rhyolite and related units near Hart, which supported gold mining in the early 20th century. Pits in altered tuff (NW 1/4, SW 1/4 sec. 24, T. 14 N., R. 17 E.) have yielded china clay (kaolinite) since World War II. The Viceroy Gold Corporation began development of a major heap-leach gold mine south of the kaolinite pits (N 1/2 sec. 25, T. 14 N., R. 17 E.), in 1991. Piute Range Volcanic Rocks The Piute Range volcanic sequence in the Hart Peak quadrangle is composed predominantly of a unit of stubby andesite and trachyandesite flows and flow breccia (Ta); this unit is interleaved with and locally overlain by basalt, trachybasalt, and basaltic andesite flows and flow breccia (Tb). Both units include sparse dacite and rhyolite flows, as well as gravel-filled channels (Tg). Both units may be interbedded with a horizon of white air-fall tuff (Tpr). The predominantly andesitic flows (Ta) contain dikes and domes of the same lithology (Tia); the largest dome in the quadrangle, about 300 m in diameter, is hornblende trachyandesite in composition. Samples of units Ta and Tb obtained from all parts of the Piute Range (appendix 1, fig. 2, appendix 2, tables 3, 4) have produced substantial age ranges: Ta, 19.8 to 8 Ma; Tb, 12.2 to 10.7 Ma. A rhyolitic flow within unit Tb yielded an age of 13.3 Ma. Including the silicic types, most rocks of the Piute Range are generally dark colored, markedly in contrast with coeval leucocratic rhyolite units exposed in the Castle Mountains. YOUNGER TERTIARY SEDIMENTARY ROCKS Silicified rhyolite breccia of the rhyolite tuff, flows, and intrusive rocks unit (Tr) grades upward into a horizon of bentonitic clay in the area of Hightower Well (sec. 14, T. 14 N., R. 17 E.) in the western Castle Mountains. This probable paleosol (R.E. Reynolds, oral commun., 1994) is overlain by crystal-rich sandstone that contains lenses of coarse gravel. The sandstone coarsens upsection to predominantly gravel with pebble- and cobble-size clasts and very little matrix (Tg); this gravel unit can be mapped continuously from the area of Hightower Well westward into the Castle Peaks quadrangle, where basalt lava flows and a silicified rhyolite tuff are interbedded in the basal part of the gravel unit. Clasts of the gravel unit are mostly Early Proterozoic gneiss and granite, Paleozoic limestone and marble, and Mesozoic granite. The gravel unit generally contains a low proportion of volcanic clasts, although horizons composed entirely of clasts derived from the Castle Peaks volcanic sequence are found locally. Similar deposits of the gravel unit fill channels developed on or within flows in the Piute Range. A younger gravel unit (QTg) of similar description, conformable with and locally indistinguishable from the Tertiary gravel unit, is also mapped in the area between the Castle Peaks and Castle Mountains in the Hart Peak quadrangle. QUATERNARY ALLUVIAL AND PLAYA DEPOSITS Several generations of unsorted alluvial-fan and stream-channel deposits are found throughout the ranges and intervening valleys. Older fan deposits (Qoa) are highly dissected and have surfaces that have been stripped of soil, exposing calcified zones. Alluvium of intermediate age can be distinguished morphologically, either as fan (Qia1) or channel deposits (Qia2). In the Piute Range (but not in Hart Peak quadrangle) the an older and younger generation of each intermediate unit can be distinguished by truncation and surface erosion. The intermediate age unit is less eroded than the older fan unit and has featureless surfaces and thick but poorly defined soil profiles. The intermediate-age channel deposits have bar-and-swale surfaces, but also may show significant soil development. The older fan and channel deposits are overlain by alluvium of present-day active channels (Qya). STRUCTURE DIPS AND UNCONFORMITIES Exposures of lower and middle Miocene volcanic and sedimentary rocks in the northwest corner of the map area (easternmost Castle Peaks), dip gently southeast toward the Castle Mountains, whereas the thick sequence of Miocene volcanic units of the northwestern Castle Mountains generally dip 30° to 40° W. toward the Castle Peaks. Miocene flows of the Piute Range mostly dip W. 15° or less in the Hart Peak quadrangle. East of Hart town site in the southern part of the Castle Mountains (NW 1/4 sec. 19, T. 14 N., R. 18 E.), the Peach Springs Tuff and a lower unit of mafic flows and breccia (Tcm) have measured dips as much as 65° W; farther to the north, the Peach Springs Tuff dips around 40° W. In both the southern and northern parts of the Castle Mountains, the overlying rhyolite units (Tjw, Tr, Tts, Tj) generally dip more moderately (between 45° and 25° W.). Injection of domes locally steepened or reversed dips of intruded strata and also deflected the strike of units (for example, units Tlss and Tjw exposed north of small dome in the middle of cross section A–A'). South and west of Quail Spring (SW 1/4 sec. 4, T. 14 N., R. 18 E.), mafic flows of the volcanic flows and breccia of the Castle Mountains (Tcm) unit dip gently southward. In this part of the map area, the variations of dip and strike in this unit resemble the nose of an anticlinal fold with northeast-trending hinge line and southwest plunge (Turner and Glazner, 1990). No unconformities of greater than local scale can be distinguished within any of the Miocene volcanic sequences in the Hart Peak or nearby quadrangles. However, local buttress and angular unconformities between domes and eruptive units are common within the rhyolite tuff, flows, and intrusive rocks (Tr) unit in the Castle Mountains and basalt and andesite units (Tb, Ta) in the Piute Range. FAULTS The Castle Mountains are divided into north and south parts by faults in the central part of the quadrangle (sec. 8, T. 14 N., R. 18 E.) with steep dips and west- to northwest-strikes. These faults cut the lower volcanic section and exposures of underlying migmatitic gneiss. Most other faults in the Castle Mountains strike north-northeast and west-northwest and dip steeply to the east, although low-angle normal faults in the lower part of the Castle Mountains volcanic sequence (for example, near Jacks Well, sec. 4, T. 30 S., R. 62 E.) have moderately low dips to the east. High- angle faults generally cut low-angle faults, although one major high-angle, oblique-slip fault north and northeast of Jacks Well turns into a low-angle fault as the strike changes from east to north (Turner, 1985). Faults within Piute Range rocks generally trend north and northeast, and all apparently have steep dips. Steep northeast-striking normal faults that crop out in the Castle Peaks, west of the quadrangle, drop basement gneiss units and overlying Miocene rocks down to the northwest (Miller and others, 1986; Thompson, 1990). The relation between those faults and faults that cut Castle Peaks rocks in the northwest corner of the quadrangle is unknown. Reconstruction of Cretaceous granite plutons and faults south of the New York Mountains (fig. 1) suggests about 10 km of strike slip offset on the northeast-striking Cedar Canyon fault (D.M. Miller, oral commun., 1995), which also shows dip slip displacement of early Miocene volcanic units down to the southeast (Miller, 1995a). If the inferred magnitude of strike slip offset on the Cedar Canyon fault is correct and of Miocene age, projected traces of related faults within early Miocene deposits and underlying rocks in the area of the Hart Peak quadrangle may also have strike-slip offsets. We therefore have tentatively inferred a fault (unmapped; represented on cross sections), now covered by middle Miocene and younger deposits, to explain the differences between the Castle Peaks and Castle Mountains, both of exposed Early Proterozoic rocks and of the lower Miocene volcanic units. This inferred fault would be found in the area where the oppositely-directed dips of the Castle Peaks and Castle Mountains (see above) must intersect. A zone of alteration and severe brecciation in the area of Hightower Well (sec. 14, T. 14 N., R. 17 E.) may represent shearing by normal faults that formed during Miocene extensional tectonism and associated volcanism (Capps and Moore, 1991). These faults define crustal blocks in the Castle Mountains area that were dropped down to the southwest, thus creating a subsiding half-graben basin which ponded drainages—as indicated by the unit of lacustrine deposits—and collected eruptive materials. Faults on the west side of the Piute Range may also have a large local displacements. Other fault displacements appear relatively small, however, and the map area appears less extended than the neighboring Eldorado and Black Mountains of the Lake Mead region (Anderson, 1971; Anderson and others, 1972; Bohannon, 1979; Faulds and others, 1990). INTERPRETATION STRATIGRAPHIC RELATIONS BETWEEN MIOCENE SEQUENCES The locally erupted volcanic sequences in the Hart Peak quadrangle all formed after deposition of the Peach Springs Tuff at about 18.5 Ma. All Miocene deposits older than 12.8 Ma are tilted (Turner and Glazner, 1990). The timing of volcanic activity and deformation coincides with major episodes of extensional faulting in the nearby Black and El Dorado Mountains of Nevada (fig. 1; Faulds and others, 1994). Both volcanic vents for the Castle Mountains flows and ejecta and feeder dikes of the Piute Range flows are abundant in the quadrangle. Feeder dikes for capping andesitic flows (Tap) in the Castle Peaks also are common but feeders for the underlying units are less conspicuous; the coarse clast sizes of the Castle Peaks' volcanic breccia (Tbr) deposit indicate that the sources must have been local intrusive and eruptive domes. The volcanic sequences of the Castle Mountains and Piute Range formed coevally, for the most part, in close proximity on an irregular topography. The relation between the Castle Peaks and Castle Mountains volcanic sequences is less clear. Ages determined on volcanic units from all parts of the Piute Range bracket those of the Castle Mountains volcanic sequence, but mafic and intermediate-composition eruptive rocks of the Piute Range in the Hart Peak quadrangle are age equivalents of only the upper volcanic and intrusive units (Tts, Tr, Tb, Til, Tir) of the Castle Mountains. The tentative correlation of the tuff of Barnwell, a part of the capping andesite flow (Tap) unit in the Castle Peaks, with the 17.8-Ma tuff of Wild Horse Mesa at the west side of Lanfair Valley (fig. 1), suggests that the entire Castle Peaks volcanic sequence may be coeval with Castle Mountains units Tcm and Tlss, both of which are older than 16 Ma. MIOCENE DEPOSITIONAL ENVIRONMENTS Castle Mountains Deposits The total thickness of major rhyolite units (Tts, Tr) changes from about 1 km near Hart town site to less than 100 m near Quail Spring, about 6.5 km to the northeast. In the area of Quail Spring the lowermost Miocene units, principally the Peach Springs Tuff and basal volcanic flows and breccia (Tcm), have tilts of as much as 65°, whereas overlying units have uniformly moderate dips (30° to 40°). The abrupt variation in thickness, as well as the lower proportion of rhyolitic rocks in the Castle Peaks area, west of the Castle Mountains, indicate that the rhyolite units accumulated in a half-graben basin that shoaled to the east. In addition, the steeper dips of the older units (Tps, Tcm) show that they underwent a greater amount of tilting than the overlying voluminous rhyolite eruptive units, substantiating the inference that the younger units were deposited as the basin subsided and deepened. We agree with interpretations of Capps and Moore (1991), that the southern part of the Castle Mountains is a volcano-tectonic depression created by the eruption of rhyolite and tuff units equivalent to units Tr and Tts. We also agree with Capps and Moore (1991) who located the western edge of this depression at the zone of sheared and silicified volcanic rocks near Hightower Well. We do not concur with Capps and Moore's (1991) interpretation that the volcano-tectonic depression is limited to the southern part of the Castle Mountains, however. The basis for this disagreement is: 1) the volume of rhyolite eruptive units (Tts, Tr) in the northern part of the quadrangle is about equivalent to that in the area near Hart and Hightower Well, 2) domes (Tir) are nearly as abundant north of Hightower Well, although not as large or as concentrated as farther south, and 3) the generally north trending alignment of latite and rhyolite intrusions shows no measurable change from the southern to the northern Castle Mountains. The zone of shearing and silicification does not crop out in the northern part of the Castle Mountains, however, possibly due to a lower volatile content of late intrusions. The west- and northwest- striking faults in the central Castle Mountains likely define an intrabasinal boundary. Piute Range Deposits The change in elevation of the contact of basal Miocene rocks on Early Proterozoic rocks in the Hart Peak and adjacent quadrangles, shows that faults must offset pre-Miocene basement rocks 300 m to 500 m down to the east. Geophysical data also support the interpretation that Piute Range faults with north to northeast strikes and apparently steep dips in the east part of the Hart Peak quadrangle have displacements that are down to the east. These faults likely are members of a fault system that formed the western boundary of an eastern volcano-tectonic half- graben basin. In the eastern basin, Piute Range lavas accumulated coeval with eruption of Castle Mountains rhyolite ejecta and domes in the western basin. The presence of adjacent, coeval volcano-tectonic basins is shown in aeromagnetic data (fig. 4; U.S. Geological Survey, 1983). A positive magnetic anomaly with northeast trend in the northeastern part of the quadrangle is generated by exposed and near-surface gneiss of the Castle Mountains. South of Hart town site, a negative magnetic anomaly trends east, and trends northeast over the westernmost outcrops of Piute Range flows (fig. 4). Immediately east of Quail Spring, the steep magnetic gradient between these positive and negative anomalies trends northeast; this gradient probably corresponds to a steep contact between the migmatitic gneiss (Xmg) unit, including intrusive rhyolite domes and dikes (Tir) on the northwest side, and relatively less magnetic volcanic rocks of the Piute Range, which must thicken abruptly on the southeast side. That abrupt thickening most likely is due to the fault or faults that dropped the basement rocks down to the east while the Piute Range volcanic units were erupted. Gravity measurements (Mariano and others, 1986) similarly support offset of the basement rocks down to the east by a fault (or faults) that strikes parallel to the west side of the Piute Range. The apparent buttress contact between the migmatitic gneiss (Xmg) unit and the volcanic flows and breccia of the Castle Mountains (Tcm) unit (cross section B–B') suggests that the Piute Range basin began forming toward the end of eruption of unit Tcm in the Castle Mountains. This supposition is supported by the abundance and continuity of rhyolite dikes and domes that intruded that contact, and which crop out from south of Quail Spring to north of Lewis Holes. Most of the intrusions appear to postdate fault movements, except for one in the area southwest of Quail Spring in the central Castle Mountains (E 1/2, NW 1/4 and W 1/2, NE 1/4, sec. 8, T. 14 N., R. 18 E.) that is pervasively sheared by northwest-striking faults. Fault-Basin Origin of the So-Called Castle Mountains Anticline The outcrop pattern that has been interpreted as an anticline in the Castle Mountains (Bingler and Bonham, 1973; Turner and Glazner, 1990) probably is not due to compressional folding, but more likely is an artifact of differential subsidence during volcanic eruptions that was driven by regional extensional faulting in the nearby Eldorado Mountains (Faulds and others, 1990, 1994). As noted above, formation of the volcano-tectonic basin of the Castle Mountains produced the westerly dips of Miocene rocks, and most dips of rocks in the Piute Range are also to the southwest. Very low southerly and easterly dips (toward the Piute Range) are measured on volcanic flows and breccia of the Castle Mountains (Tcm) unit of the lower part of the Castle Mountains volcanic sequence, in a belt that extends from southwest of Quail Spring to Lewis Holes. Units Tcm, Tts, and Tvl at the south end of this belt (NE 1/4 sec. 18, T. 14 N., R. 18 E.) are very thin and directly overlie migmatitic gneiss; the unit and basal contact dip very gently to the southeast beneath olivine-bearing basaltic flows (Tb). The section is truncated to the east by a north-striking fault. This attenuated section of gently south- or east-dipping units probably does not constitute an east- dipping fold limb, but more likely is the remnant of a thinner accumulation of volcanic units on a ridge of pre-Tertiary basement rocks. The basement ridge separated the western and eastern volcano-tectonic depressions of the Castle Mountains and Piute Range, respectively (Nielson and others, 1993). POST-VOLCANIC DEPOSITS Upper Miocene sandstone and gravel deposits (Tg) overlie volcanic rocks of the Castle Peaks and Castle Mountains on a distinct angular unconformity. The basal part of the gravel deposits unit generally is derived from gneiss and granite of the basement complex. Thus, in Miocene time, as today, high elevations at the heads of drainages provided detritus composed mostly of pre-Tertiary rocks. The local concentrations of volcanic clasts from the uppermost part of the Castle Peaks volcanic sequence indicate that some of the Castle Peaks andesite flows (Tap) also occupied high topographic positions and that erosion of the unit began in late Miocene time. Depositional and structural relations of the gravel deposits unit (Tg), exposed in the Hart Peak and adjacent quadrangles, support continued episodes of faulting in Pliocene or latest Miocene time. Clasts of gray, chert-bearing Paleozoic limestone are present in the gravel deposits near Hightower Well and southeast of Hart town site, in association with Mesozoic granitic rocks. These clasts probably were derived from the area of the Mescal Range and Ivanpah Mountains, although clasts of some Mesozoic granite types may have come from sources in the southern New York Mountains. Possible drainage routes between the Mescal Range and Ivanpah Mountains and Castle Peaks- Castle Mountains area are presently obstructed by the topographic barrier of the New York Mountains. Also, present-day drainages emerging from the southern New York Mountains are deflected by the western flank of the Castle Mountains and flow southeast, opposite to the direction required for transport of clasts into areas where the Tertiary gravel unit is exposed in the Castle Peaks and Castle Mountains (Nielson, 1995). These relations all show that the topographic barriers must postdate deposition of the Miocene gravel unit (Miller, 1995b; Nielson, 1995); thus, the earliest time that the topographic barriers could have formed is late Miocene. The topographic barriers probably were created by late Tertiary faulting episodes that caused relative uplift of the mountain ridges or relative subsidence of the valleys. Later faulting events may have continued, providing wide exposure of basement rocks and generating sources of detritus that was shed into the drainage systems. Those surficial deposits are represented by the undeformed younger gravel unit (QTg), which is conformable with and locally indistinguishable from, the older Quaternary alluvium (Qoa). Another topographic barrier was formed by the linear western boundary of Piute Range flows, which blocked Lanfair Valley drainages after the cessation of volcanism in the late Miocene. By early Pleistocene, thick playa deposits accumulated at this buttress (Nielson, 1995). Subsequently, cross-cutting drainages—for example, the east-trending canyon leading to the end of Old Homestead Road (secs. 4–16. T. 14 N., R. 18 E.; fig. 2)—were superimposed on the Piute Range. Immediately south of the Hart Peak quadrangle, some streams of the western Piute Range form the headwaters of Piute Gorge, an east-trending superimposed canyon of the southern Piute Range. These streams are prevented from merging with the major south-flowing washes of eastern Lanfair Valley by a low terrace of intermediate alluvium (Qia1) units. The terrace was formed by incision of the intermediate alluvium unit, which could another indication of continued faulting, and relative offset of valleys and mountain ridges, after Miocene time (Nielson, 1995). DESCRIPTION OF MAP UNITS SURFICIAL DEPOSITS Qya Younger alluvium (Holocene)—Clay, sand, pebbly sand, and cobble- to pebble-size gravel. Close to mountain fronts and in canyons, matrix is clay-rich and clasts are mostly volcanic rock. Elsewhere, matrix predominantly sand and clasts are about equal proportions of granite, gneiss, and volcanic rock. Forms in active stream channels and flanking bar-and-swale zones. Estimated thickness 2 m or less Intermediate alluvium (Holocene and Pleistocene)—Sand, pebbly sand, and gravel deposits. Consists of: Unit 2 (Holocene and Pleistocene)—Divided into: Qia2b Younger deposits (Holocene)—Sandy overbank deposits in broad alluvial valleys. Matrix predominantly sand; where inventoried, clasts are composed of granite, gneiss, and volcanic rocks in about equal proportions. Grades laterally into active stream deposits (Qya) or low terrace deposits (Qia2a, mapped in East of Grotto Hills quadrangle to the south). Limited bar-and- swale morphology with weak surface imbrication formed by network of thin stream-channel deposits containing pebble- to cobble-size clasts. Exposed thickness to 2 m Qia1 Unit 1 (Holocene? and Pleistocene)—Reddish, predominantly unsorted sand interspersed with clast-supported horizons of pebble- and cobble-size angular to subangular clasts composed of about equal amounts of granite, gneiss, and volcanic rock. Locally well developed soil at least 50 cm thick, sandy in upper 10 cm but clay-rich and vesicular below 20 cm; contains patchy calcareous zones. Forms terraces 2 to 4 m above deposits of active washes (Qya). Deposits overlap and, in places, partly bury dissected ridges of older alluvium (Qoa). Terraces in the broad valleys merge laterally into deposits of intermediate alluvium (Qia2a, Qia2b). Surfaces have no or poorly preserved bar-and-swale morphology. Surface pavements poorly developed and unvarnished, in part due to high proportion of granitic clasts and in part to mechanical erosion by range cattle. Exposed thickness to 4 m Qoa Older alluvium (Pleistocene)—Clast- and matrix-supported gravel deposits. Consists of clay-rich matrix containing coarse sand grains and septa of calcium carbonate, as well as cobbles of angular to subangular granite or gneiss; common local concentrations of volcanic rocks; uncommon pebbly zones and large boulders. Soils thin or absent in most places. Surfaces light- colored due to litter of fragments from petrocalcic horizon at shallow depth (10 to 12 cm maximum depth), as shown by concentrations of small pebbles around ant hills. Forms steep-sided spurs at mountain fronts and wide alluvial ridges 5 to 6 m above active deposits of stream channels (Qya). Surfaces display no depositional morphology; local clast concentrations interpreted as lag deposits. In general, varnish is observed on only 10 percent of surfaces; side slopes may have better pavement development and higher proportion (60 percent) of varnished clasts. Exposed thickness to 6 m QTg Gravel deposits (Pleistocene and Pliocene?)—Light-colored sandstone, siltstone, and rounded to angular pebble- and cobble-size clasts. Unit poorly exposed; matrix composition and texture poorly known. Forms low rolling hills that are difficult to distinguish from, and probably depositionally continuous with, older alluvium (Qoa). Unit mapped on the basis of hill slope concentrations of rounded to subrounded cobbles. Exposed thickness to 5 m VOLCANIC AND SEDIMENTARY ROCKS [Compositions of selected volcanic units are listed in table 1] Castle Peaks Tg Gravel deposits (Miocene)—Subangular to rounded pebble- to cobble-size clasts of granite, gneiss, and volcanic and sedimentary rocks, in matrix of immature coarse- to medium-grained crystal-lithic sand; deposit clast-supported in most exposures. Grades locally into poorly consolidated sandstone and siltstone with gravel-filled channels. Granite clasts include foliated and unfoliated types; foliated garnetiferous cobbles probably derived from Proterozoic terranes; undeformed leucocratic clasts from Cretaceous sources. Local concentrations of clasts derived from underlying pyroxene andesite porphyry (Tap). Sedimentary-rock clasts include cobbles of gray Paleozoic limestone with stringers of brown chert. Matrix crystals are predominantly quartz, biotite, feldspar, and rarely pyroxene grains. Overlies pyroxene andesite porphyry (Tap) unit and underlies Pleistocene and Holocene deposits. Exposed thickness 10 to 30 m Tap Pyroxene andesite porphyry (Miocene)—Vesicular, red-brown, blue-gray, or greenish-black, andesite porphyry flows and volcanic breccia. Flows are blocky in outcrop, commonly brecciated. Intruded by mafic or andesitic dikes. Contains 20 to 30 percent phenocrysts of plagioclase and square, deep-green clinopyroxene crystals (5 mm to 10 mm avg. dimension), in a microcrystalline matrix of plagioclase laths ± hornblende (Thompson, 1990). Blue-gray or purplish dikes locally are plagioclase rich and can be traced into flows; upward, massive-textured feeder dikes grade into eruptive breccia. Although rocks are plagioclase-phyric, compositions are trachyandesite, transitional to andesite; elevated alkali contents may be due to alteration (fig. 3; Thompson, 1990). Generally overlies volcanic breccia (Tbr) unit, locally overlies tuff of Castle Peaks (Tcp). Caps softer underlying tuff and breccia. Erodes as cliffs or steep slopes. Thickness <1 to 90 m Tcp Tuff of Castle Peaks (Miocene)—Bedded to massive, white to tan, pumice-rich rhyolite tuff of ash-flow origin (fig. 3); crops out locally as a single 1-m-thick air-fall tuff related to eruption of the ash-flow deposit. Contains sanidine, plagioclase, and sparse biotite crystals (Thompson, 1990). Highly imprecise total fusion 40Ar/39Ar age on bulk sanidine of 21.4 ±1.6 Ma (table 4), suggests tuff is contaminated, perhaps by surface detritus during deposition. Overlies and locally interbedded with uppermost part of volcanic breccia (Tbr) unit; underlies andesite porphyry flows (Tap) wherever both units are exposed. Air-fall component of the tuff probably preserved rarely due to depositional disturbance by local eruption of either volcanic breccia unit or pyroxene andesite porphyry (Tap) unit. Erodes easily; preserved in slopes only where protected by overlying units. Thickness <1 to 5 m Tbr Volcanic breccia (Miocene)—Gray to dark-bluish-gray monolithologic to hetero- lithologic volcanic breccia and megabreccia composed of seriate clasts of glassy to holocrystalline, vesicular to massive, rhyolite and trachyandesite. Monolithologic and volcaniclastic, whitish-tan to pink, matrix-supported, chaotic breccia composed of sand-size matrix of comminuted volcanic rocks, crystals, pumice, and volcanic ash that contains pebble-size pumice lumps, cobble-size volcanic bombs, and blocky clasts. Blocks and matrix composition are biotite-phyric rhyolite and dacite; locally contains zones of darker hornblende+biotite (±pyroxene) trachyandesite. Average size of blocks, 30 to 50 cm; may be as large as 10 m diameter. Bombs have breadcrust rinds and radial joints; blocks generally equant shapes and display chatter marks, radial internal cooling joints, internal flow-banding, and fluted joint surfaces. Divergent ages produced from a blocky clast collected in the Castle Peaks quadrangle, by two isotopic techniques (table 4): conventional K-Ar age of 14.7±0.4 on biotite probably too young due to alteration; extremely imprecise total fusion 40Ar/39Ar age on bulk sanidine of 17.5±10.4 Ma suggests contamination of magma prior to eruption. Overlies Proterozoic gneiss (Xlg); contact has 150 m of relief. Always underlies pyroxene andesite porphyry (Tap) unit; generally underlies, but may be locally interbedded with, tuff of Castle Peaks (Tcp). Forms steep-sided ridges and buttes. Thickness 30 to 100 m Castle Mountains Tg Gravel deposits (Miocene)—Subangular to rounded pebble- to cobble-size clasts of granite, gneiss, and volcanic and sedimentary rocks, in matrix of immature coarse- to medium-grained crystal-lithic sand. Basal part of unit is poorly consolidated crystal-lithic sandstone and siltstone with gravel-filled channels. Crystals in basal sandstone and matrix are predominantly quartz, biotite, feldspar, and rarely pyroxene. Clast types include foliated and unfoliated gneiss and granite, gray Paleozoic limestone with stringers of brown chert, and greenish metavolcanic rocks. Foliated garnetiferous granite probably derived from Proterozoic terranes, and undeformed leucocratic granite clasts and metavolcanic rocks from Mesozoic sources. Proportions of volcanic and sedimentary clasts generally low, increasing in upper part of unit. Near Hightower Well the basal interval of sandstone and siltstone overlies bentonitic paleosol at top of rhyolite tuff, flows, and intrusive rocks unit (Tr) (sec. 14, T. 14 N., R. 17 E.). Underlies late Tertiary(?) and Quaternary gravel and fluvial deposits (QTg, Qya). Gently dipping, forms rolling hills. Exposed thickness 5 to 100 m Tj Tuff of Juan (Miocene)—Welded rhyolite tuff. Lower part of unit contains black vitrophyre 10 m thick, with well-developed color-banding parallel to subhorizontal flow planes; contains biotite and potassium feldspar in gray- brown glassy matrix of volcanic ash. Upper part contains sanidine and plagioclase feldspar, biotite, and hornblende crystals in light- to medium- gray, loosely indurated matrix. K-Ar age is 14.4±0.2 Ma (location 5, table 3). Overlies tuff, volcanic breccia, and sedimentary deposits (Tts), basalt flows (Tb), or rhyolite tuff, flows, and intrusive rocks (Tr) units. Forms cliffs. Thickness to 40 m Tb Basalt flows (Miocene)—Vesicular and scoriaceous, porphyritic to aphyric, fine- grained to glassy dark gray to black flows, locally reddened by oxidation. Unit mostly consists of basalt and basaltic andesite, locally includes andesite and trachyandesite; rarely contains flow breccia, cinders, and scoria. Porphyritic flows normally contain 10 to 15 percent phenocrysts. Basaltic andesite may contain only plagioclase phenocrysts; basalt also contains sparse olivine and rare pyroxene. Individual flows 3 to 4 m thick commonly have massive cores and well-defined 1- to 2-m thick breccia zones at upper and lower margins. Overlies and interbedded with rhyolite tuff, flows, and intrusive rocks (Tr); tuff, volcanic breccia, and sedimentary deposits (Tts); and gravel (Tg) units, and underlies tuff of Juan (Tj). Forms steep cliffs or steep-sided ridges. Thickness 3 to 50 m Tbts Basalt flows, rhyolite tuff, and sedimentary rocks (Miocene)—Thin basaltic flows interbedded with air-fall tuff, tuff breccia, ash-flow deposits, rhyolite flows, and fluvial sedimentary rocks; unit also includes feeder dikes of basalt flows. Unit mapped wherever silicic tuff and breccia ejecta and tuffaceous sedimentary rocks (Tts) too few to separate from interbedded basalt flows (Tb); also present southwest of Hart Peak where gravel of unit Tg is interbedded with basalt and air-fall tuff. Generally forms gentler slopes than those underlain by units Tts and Tb. Thickness 5 to 50 m Tr Rhyolite tuff, flows, and intrusive rocks (Miocene)—Rhyolite ejecta of white, pink, and red rhyolite tuff and breccia: includes airfall and ash-flow tuff (welded and unwelded), extrusive tuff breccia, and pumice breccia; locally includes intervals of bedded tuff and sedimentary deposits. Also includes rhyolitic flows and flow breccia fed by local intrusions. Contains 10 to 25 percent phenocrysts, mostly of biotite, sanidine, and quartz. Lithologically equivalent thick intervals of bedded tuff and sedimentary rocks mapped separately as unit Tts; thick rhyolite flows mapped separately as unit Trf. Laterally continuous bedded deposits grade into local eruptive breccia zones forming marginal carapaces of intrusive domes and larger dikes. Flows and smaller intrusions have well-defined 1- to 3-m-thick glassy chilled margins. In southern part of Castle Mountains, domes (Tir) are largest and most abundant, and mineralization, silicification, and alteration of tuff and breccia deposits to kaolinite is most intense. Uppermost breccia interval near Hightower Well (sec. 14, T. 14 N., R. 17 E.) marked by development of bentonitic paleosol. Unit ages (reported by Capps and Moore, 1991): rhyolite flow near Hart Peak, 16.3±0.5 Ma; tuff subunits near Hart town site, 14.7±0.3 Ma, 14.9±0.3 Ma, 15.7±0.6 Ma; rhyolite breccia east of Hart, 14.2±0.3 Ma; rhyolite breccia near Hightower Well, 14.9±0.3 Ma. Overlies the volcanic flows and breccia of the Castle Mountains unit (Tcm), tuff of Jacks Well (Tjw), or lahar (Tvl). Locally underlies basalt flows (Tb) or basalt flows, rhyolite tuff, and sedimentary rocks (Tbts) units. Forms steep slopes, locally cliffs. Apparent thickness is 10 to 250 m in northern part of Castle Mountains, perhaps as much as 1 km in southern part. True thicknesses difficult to estimate because internal faults are poorly-exposed Tts Tuff, volcanic breccia, and sedimentary rocks (Miocene)—Well-bedded silicic air-fall tuff and tuff breccia, pumice breccia, ash-flow tuff and flow breccia, as well as minor volcaniclastic sedimentary rocks, tuffaceous sedimentary materials, and volcanic conglomerate. In the northern part of the Castle Mountains, sedimentary rocks more common in lower part of unit and eruptive rocks more common in upper part. Sedimentary rocks are: siltstone; fine- to medium-grained, buff-colored sandstone; pebble to cobble conglomerate with abundant cobble-size and larger white pumice clasts grading upward to light-yellow-tan tuffaceous sandstone and siltstone, volcanic conglomerate, and ash-flow tuff. Eruptive rocks consist of orange- tan, thinly stratified air-fall tuff; welded lithic tuff; black, glassy, perlitic, vitrophyre flows; and fine-grained air fall tuff that locally contains black glassy bombs. Ages reported for tuff and tuff breccia (Capps and Moore, 1991) range from 14.2±0.3 to 15.7±0.6 Ma. Overlies volcanic flows and breccia of the Castle Mountains (Tcm) unit; in northern part of Castle Mountains, underlies and interbedded with basalt flows (Tb) unit. Forms steep to moderately steep slopes. Thickness 3 to 100 m Trf Rhyolite flows (Miocene)—Pink and lavender, biotite- and sanidine-bearing, thick, stubby, aphanitic and vitrophyric rhyolite flows related to adjacent domes; rarely thick enough to be mapped separately from rhyolite tuff, flows, and intrusive rocks (Tr) unit. Forms steep slopes mantled with talus. Individual flows as much as 60 m thick Tvl Lahar of volcanic origin (Miocene)—Chaotic volcaniclastic breccia (lahar). Varies from clast-supported, having volcanic blocks as much as 1.5 m in maximum dimension, to matrix-supported structure with clast sizes ranging from 5 to 50 cm. Largest clasts concentrated in uppermost part of unit; many have fluted or breadcrust surfaces and radial cooling joints. Purplish granulated matrix of sand-size andesite, and white volcanic ash containing feldspar, pyroxene, and biotite crystals. In outcrops east of Hart, angular clasts include leucocratic rhyolite, white pumice, and oxidized andesitic scoria; subrounded clasts of gneiss and granite also present. In northeastern part of quadrangle (NW 1/4 sec. 2 and NE 1/4 sec. 3, T. 30 S., R. 62 E.), angular clasts of dark hornblende trachyandesite more abundant than rhyolitic clasts. Generally preserved where interbedded with resistant rhyolite tuff, flows, and intrusive rocks (Tr) unit, or capped by basalt flows of unit Tb. Forms gentle slopes. Thickness 5 to 50 m Tvss Volcaniclastic sandstone and conglomerate (Miocene)—Red, tan, or pinkish bedded sandstone and pebbly sandstone. Matrix is pumice, ash, and rounded quartz grains; spherical concretions developed locally. Clasts are subrounded volcanic pebbles and cobbles; maximum diameter, 9 cm. Beds are lensoid, normally graded, and cross bedded. Overlies tuff of Jacks Well (Tjw); forms locally thick interbeds in basal part of rhyolite tuff, flows and intrusive rocks (Tr) unit. Resistant beds form prominent ridges. Thickness 10 to 300 m Tjw Tuff of Jacks Well (Miocene)—Light-gray to grayish-pink, nonwelded, crystal-lithic, ash-flow tuff. Contains sanidine, biotite, quartz, sphene, and minor pumice fragments in homogeneous ashy matrix. Sanidine age of 16.1±0.4 by K-Ar technique (location 4, table 3; Turner and Glazner, 1990); age of 16.8±0.5 Ma reported by Capps and Moore (1991). Unit could be derived either from local eruption or from source outside Castle Mountains. Unit correlated by Bingler and Bonham (1973) with the Tuff of Bridge Spring, dated at 15.9±0.4 Ma (Faulds and others, 1990), which crops out to the north and east, mostly in Nevada. Overlies lacustrine and fluvial sedimentary rocks (Tlss) unit in the north- central part of the quadrangle. Underlies volcaniclastic sandstone and conglomerate (Tvss) unit and underlies tuff, volcanic breccia, and sedimentary deposits (Tts) unit, or rhyolite tuff, flows and intrusive rocks (Tr) unit. Forms classic hogback ridges of low relief that are locally intruded by rhyolitic domes (Tir), which affect dip and strike of unit. Thickness 3 to 25 m Tlss Lacustrine and fluvial sedimentary rocks (Miocene)—Grayish-yellow, light-gray, pale-greenish-yellow, and light-red; fine-grained; clastic; lacustrine siltstone grading upward into medium-grained fluvial deposits. The gray and pale- yellow basal lake deposits consist chiefly of silt and mudstone interbedded with minor limestone, arkosic sandstone, and water-laid tuff. Upper part of unit is interbedded fine-grained sandstone and granule to cobble conglomerate, having both parallel stratification and tabular and trough cross-stratification. Clast lithologies include scoriaceous basalt, nonvesicular basaltic andesite, and minor fragments of gneiss and granite. Overlies volcanic flows and breccia of the Castle Mountains (Tcm) unit, and underlies tuff of Jacks Well (Tjw) in northern part of Castle Mountains; overlain at southernmost exposure by rhyolite tuff, flows, and intrusive rocks (Tr) unit. Easily eroded, forms gentle slopes or badlands topography. Thickness 2.5 to 350 m Tcm Volcanic flows and breccia of the Castle Mountains (Miocene)—Black, gray, or dark-purplish trachyandesite porphyry, with less common trachybasalt and dacite flows, flow breccia, and pyroclastic breccia. Upper part of unit includes poorly sorted but well-bedded sedimentary intervals containing both high-grade metamorphic and volcanic clasts. Individual bedding horizons show both parallel and trough cross-stratification; subrounded to subangular clasts vary in size from very coarse sandstone to open-framework boulder conglomerate. Flows are 2 to 5 m thick and commonly have massive cores grading into oxidized flow breccia tops and bases. Compositions are trachyandesite with minor dacite (table 1, fig. 3): trachyandesite contains olivine altered to red clay, plagioclase feldspar, and green pyroxene in aphanitic, light- to dark-green-gray matrix; dacite contains sanidine, biotite, and pyroxene. Unit probably equivalent in age, but is not directly correlative, to the Patsy Mine Volcanics (Longwell, 1963) mapped elsewhere in Nevada, contrary to interpretation of Bingler and Bonham (1973). Overlies either erosional surface developed on Early Proterozoic migmatitic gneiss (Xmg) unit or outcrops of the Peach Springs Tuff (Tps). Generally underlies rhyolitic ejecta (Tts, Tr), but in the northern Castle Mountains underlies either lacustrine and fluvial sandstone and conglomerate (Tlss) or volcanic lahar (Tvl) units. Contains ubiquitous rhyolite intrusions (Tir) related to eruption of unit Tr, and basalt dikes (Tib) related to eruption of unit Tb. Forms low rolling hills and steep sided valleys. Thickness 2.5 to 110 m Tps Peach Springs Tuff of Young and Brennan (1974) (Miocene)—Welded ash-flow tuff of alkali rhyolite composition with dominant adularescent sanidine and essential accessory sphene. Regionally extensive unit may also contain plagioclase, hornblende, biotite, and allanite. Age determined on sanidine from pumice sampled at Kingman, Arizona, is 18.5±0.2 Ma (Nielson and others, 1990). Ages determined on samples from the Castle Mountains include conventional K-Ar ages of 18.5±0.5 Ma (biotite) and 17.5±0.4 Ma (sanidine) (location 2, table 3; Turner and Glazner, 1990), and a single- crystal 40Ar/39Ar laser fusion age of 18.79±0.04 Ma (location 3, table 3; Nielson and others, 1993). Nonconformably overlies Early Proterozoic granite and gneiss (Xlg, Xmg); basal part commonly contains xenolithic fragments of gneiss and granite regolith. Also underlies basal units in the Castle Peaks and Piute Range (units Tbr and Ta; see below). Highly faulted and brecciated; forms thin resistant ledges in area east of Hart town site, where dips are as much as 65°. Thickness 15 to 50 m Pzl Limestone (Paleozoic)—Gray, dolomitic, chert-bearing limestone that contains microscopic algal and possible echinoderm fragments. Isolated, fractured and altered outcrop of about 3 m by 10 m exposed in gully north of Nevada- California State line (sec. 16, T. 30 S., R. 62 E). Structural relations are obscure. If not part of a reworked deposit, limestone may be in fault contact with Early Proterozoic gneiss (Xmg) as proposed by Capps and Moore (1991). Piute Range Tg Gravel deposits (Miocene)—Pebble- and cobble-size granite, gneiss, volcanic, and sedimentary clasts in matrix of immature coarse- to medium-grained crystal-lithic sand. Prominent clast types include foliated and unfoliated gneiss and granite, gray Paleozoic limestone with stringers of brown chert, garnetiferous granite, undeformed leucocratic granite clasts, vein quartz, granite, and minor scoria and basalt. Proportions of volcanic and sedimentary rocks generally small. Fills channels eroded into andesite and basalt flows (Ta, Tb). Thickness 5 to 100 m Tb Basalt flows (Miocene)—Vesicular and scoriaceous, highly to moderately porphyritic, dark gray to black, fine-grained to glassy flows and flow breccia, cinders, and scoria, locally reddened by oxidation. Unit consists dominantly of basalt and basaltic andesite or trachyandesite compositions but locally includes few rhyolite flows. Basalt flows contain 10 to 15 percent phenocrysts, predominantly plagioclase, with sparse olivine and rare pyroxene. Dispersed basaltic andesite and trachyandesite flows and breccia generally are plagioclase-rich and contain hornblende phenocrysts; rhyolite flows also contain biotite. Dikes have steeply dipping or vertical flow- banding and high proportions of flow-aligned plagioclase crystals. Ages of samples from unit listed in tables 3 and 4 (see also Nielson and Nakata, 1993). Thickness as much as 410 m Tbts Basalt flows, rhyolite tuff, and sedimentary rocks (Miocene)—Rhyolite flows, air-fall and ash-flow tuff and tuff breccia, and sedimentary rocks interbedded with thin basalt or andesite flows and dikes. Unit mapped wherever silicic tuff and breccia ejecta and tuffaceous sedimentary rocks (Tts) too few to separate from interbedded basalt flows (Tb) unit. Generally forms gentler slopes than unit Tb. Thickness 30 to 150 m Tpr Tuff of the Piute Range (Miocene)—White biotite tuff composed of pebble-size biotite-bearing pumice in biotite-rich matrix of rhyolite ash. Poorly sorted tuff rarely contains large (30 cm across), lithic and pumice clasts. Where reworked, displays normal grading and crossbedding. Biotite-rich tuffaceous sandstone typically composes upper part of unit. Locally contains augen gneiss, dacite and andesite blocks, commonly underlain by soft-sediment sag features. Overlain and underlain by andesitic flows, breccia, and sedimentary rocks (Ta) unit. Forms steep slopes beneath cliffs. Thickness 3 to 10 m Ta Andesitic flows, breccia, and sedimentary rocks (Miocene)—Dark- to light-gray, generally fine-grained to porphyritic andesite, trachyandesite, and dacite flows and flow breccia. Andesite and trachyandesite flows have fine-grained matrix with approximately 10 percent phenocrysts, mostly composed of plagioclase (grain size as much as 3 mm across), with less common pyroxene (avg. 0.2 mm across) and rarer olivine. Flow bases commonly brecciated, locally pillowed, with granular glassy margins. Ages of samples from unit listed in tables 3 and 4 (see also, Nielson and Nakata, 1993). Interleaved with lenses of gravel and sandstone interbeds too small to map as separate gravel unit (Tg); gravel consists of Paleozoic limestone, gneiss (including garnetiferous types), vein quartz, granite, and minor scoria and basalt. Coarsest deposits found in channels within flows; locally, flows also filled stream channels. May be interleaved with either basalt flows (Tb), basaltic flows, rhyolite tuff and sedimentary rocks (Tbts), and gravel deposits (Tg) units, and overlain locally by basalt flows (Tb) and older alluvium (Qoa) units. Forms steep cliffs with 50 to 350 m of relief. Dips are gentle; relief generally corresponds to thickness, not accounting for paleotopographic variations of underlying units or fault repetition of indistinguishable flows Tps Peach Springs Tuff of Young and Brennan (1974) (Miocene)—Welded ash-flow tuff of alkali rhyolite composition with dominant adularescent sanidine and essential accessory sphene; locally may contain plagioclase, hornblende, biotite, and allanite. Highly faulted and locally brecciated; basal volcanic unit underlying Piute Range lava deposits in West of Juniper Mine quadrangle, east of the map area. Crops out in southeastern part of Hart Peak quadrangle as low cliffs within andesite flows and flow breccia (Ta), indicating the presence of multiple, otherwise undetectable, faults. Thickness 15 to 50 m INTRUSIVE IGNEOUS ROCKS Tib Basalt sills and dikes (Miocene)—Aphanitic basalt and basaltic andesite dikes and rare holocrystalline sills intruded into both gneiss and overlying volcanic rocks. Dikes, generally less than 3 m wide, form resistant ridges and spines. In northern part of map area, a dark-gray, homogeneous, fine- to medium- grained holocrystalline sill intrudes tuff, volcanic breccia, and sedimentary deposits (Tts) unit. Sill is locally coarse grained and contains hornblende and plagioclase feldspar; forms steep slope, thickness as much as 50 m Tia Andesitic intrusive rocks (Miocene)—Black, gray, or purplish domes, as well as sills of hornblende- and (or) biotite-plagioclase andesite, trachyandesite, and oxidized intrusive breccia. Domes have carapaces of hard, silicified breccia, locally composed of angular black and bright-red fragments in dark-gray matrix. A steep-sided dome 300 m across, located in northern Piute Range on the north side of major east-west canyon, forms a promontory within basaltic and andesitic flows and breccia (Tb, Ta). Tir Rhyolite domes and dikes (Miocene)—White to light-pink or lavender, intrusive domes and dikes of biotite rhyolite. Interiors of thicker domes are strongly flow-banded: margins grade into zones of intrusive breccia consisting of platy fragments of flow-banded rhyolite; flow banding dips steeply. Compositions listed in table 1 (see also, Turner and Glazner, 1990). Capps and Moore (1991) reported ages of 14.9±0.4 Ma to 16.5±0.7 Ma for rhyolite dikes and domes in southern part of Castle Mountains and near Hart Peak; conventional K-Ar age of 12.8 ±0.2 Ma determined on small, apparently untilted, dome with symmetrical radial dips in northern Castle Mountains (location 7, table 3) establishes upper age limit of local deformation. Intrudes rhyolite flows, tuff, and sedimentary rocks (Tr, Tts, Tvss) units. Smaller dikes and sills present in volcanic flows and breccia of the Castle Mountains (Tcm) and underlying migmatitic gneiss (Xmg) units. Domes form steep-sided prominences and peaks of the Castle Mountains; most prominent are Hart Peak (sec. 31, T. 15 N., R. 18 E.) and a broad unnamed peak (NW 1/4 sec. 6, T. 14 N., R. 18 E.). Southern part of Castle Mountains contains densest concentration of dikes and domes Til Latite dike (Miocene)—Lead-gray vitrophyric porphyry dike discontinuously exposed for more than 4 km in southwest part of quadrangle; main continuous exposure 2.8 km long. Dark aphanitic matrix contains 10 to 20 percent phenocrysts; locally concentrated; composed of plagioclase, hornblende, biotite, and quartz. K-Ar ages (14, 15, table 3) of 14.3±0.4 and 14.1±0.4 Ma (biotite), and 14.6±0.5 Ma (plagioclase) compare favorably with those reported by Capps and Moore (1991: 14.5±0.5 and 15.1±0.4 Ma on biotite). Forms steep-sided ridges or spines within host unit of rhyolite tuff, flows, and intrusive rocks (Tr); contains banded chill zone at contact with oxidized tuff. Dike segments strike north-northeast; dips near vertical; width varies from 10 to 150 m. North of northwest-striking fault zone, dike forms thin, discontinuous, parallel ridges ranging from 20 to 50 m in width. METAMORPHIC ROCKS Castle Peaks Xlg Leucocratic granite and granitic gneiss (Early Proterozoic)—Porphyroclastic to megaporphyroclastic biotite-rich augen gneiss, locally banded and intruded by foliated to unfoliated garnet-bearing leucogranite. Granitic assemblage consists of leucogranite, hornblende-biotite granite, biotite quartz monzonite and granodiorite. Aplite dikes, as much as 3 m wide, intrude medium-grained hornblende-biotite granite Castle Mountains Xmg Migmatitic gneiss (Early Proterozoic)—Varicolored foliated and compositionally layered metamorphic rocks of amphibolite to granulite facies. 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Thompson, S.M., 1990, Miocene volcanic rocks of the Castle Peaks area, New York Mountains, eastern San Bernardino County, California: Los Angeles, California State University, Los Angeles, M.S. thesis, scale 1:24,000, 71 p. Turner, R.D., 1985, Miocene folding and faulting of an evolving volcanic center in the Castle Mountains, southeastern California and southern Nevada: Chapel Hill, University of North Carolina, M.S. thesis, 56 p. Turner, R.D., and Glazner, A.F., 1990, Miocene volcanism, folding, and faulting in the Castle Mountains, southern Nevada and eastern California, in Wernicke, B.P., ed., Basin and Range extensional tectonics near the latitude of Las Vegas, Nevada: Geological Society of America Memoir 176, p. 23–35. U.S. Geological Survey, 1983, Aeromagnetic map of the Kingman-Trona area, California: U.S. Geological Survey Open-File Report 83–663, 2 sheets, scale 1:250,000. Wells, R.E., and Hillhouse, J.W., 1989, Paleomagnetism and tectonic rotation of the lower Miocene Peach Springs Tuff: Colorado Plateau, Arizona to Barstow, California: Geological Society of America Bulletin, v. 101, no. 6, p. 846–863. Wooden, J.L., and Miller, D.M., 1990, Chronologic and isotopic framework for Early Proterozoic crustal evolution in the eastern Mojave Desert region, SE California: Journal of Geophysical Research, v. 95, no. B12, p. 20,133–20,146. Young, R.A., and Brennan, W.J., 1974, Peach Springs Tuff: its bearing on structural evolution of the Colorado Plateau and development of Cenozoic drainage in Mohave County, Arizona: Geological Society of America Bulletin, v. 85, no. 1, p. 83–90. Map credit: Geology mapped by Jane E. Nielson, 1983–92; and Ryan D. Turner, 1982–84; assisted by Jay S. Noller, 1984–86; Jacqueline Huntoon, 1982; and Cynthia A. Ardito, 1983