K-Ar and fission-track ages (dates) of volcanic intrusive, altered, and metamorphic rocks in the Mohave Mountains area, west- central Arizona J.K. Nakata, M.A. Pernokas, K.A. Howard, J.E. Nielson, and J.R. Shannon Isochron/West, Bulletin of Isotopic Geochronology, v. 57, pp. 21-32 Downloaded from: https://geoinfo.nmt.edu/publications/periodicals/isochronwest/home.cfml?Issue=57
Isochron/West was published at irregular intervals from 1971 to 1996. The journal was patterned after the journal Radiocarbon and covered isotopic age-dating (except carbon-14) on rocks and minerals from the Western Hemisphere. Initially, the geographic scope of papers was restricted to the western half of the United States, but was later expanded. The journal was sponsored and staffed by the New Mexico Bureau of Mines (now Geology) & Mineral Resources and the Nevada Bureau of Mines & Geology.
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K-Ar AND FISSION-TRACK AGES (DATES) OF VOLCANIC INTRUSIVE, ALTERED, AND METAMORPHIC ROCKS IN THE MOHAVE MOUNTAINS AREA, WEST-CENTRAL ARIZONA
J. K. NAKATA M. A. PERNOKAS U,S. Geological Survey, Menio Park, CA 94025 K. A. HOWARD J. E. NIELSON U.S. Geological Survey, Denver, CO 80225 J. R. SHANNON Current address: Crown Resource Corp., Republic, WA 99166
114^30' The Mohave Mountains and adjacent ranges of west-central Arizona (fig. 1) occupy a key area for unraveling the evolution of Tertiary tectonics in the SSS3S333SSS8S3SS$S!sOUTLINE OF FIGURE 2. «®S35S8S538S3S3S?3SSSS« Colorado River extensional corridor. The IA Powell Peak area lies in the east part of the north- BUCK MOUNTAINS trending, 100-km-wide corridor, and Tumarion flanks a belt of metamorphic core com Peak plexes that define the center of the cor ridor (Davis and others, 1980; Howard and John, 1987). Field studies in the Mohave Mountains area indicate a record of extension-related Tertiary intrusion, volcanism, sedimentation, detachment MOHAVE MOUNTAINS faulting, tectonic fragmentation, and tilting (Howard and others, 1982a, in Grossman press; Pike and Hansen, 1 982; Nakata, 1 982; Light and others, 1 983; Nielson, ARIZONA 1986; Nielson and Glazner, 1986; Howard and John, 1987; Nielson and Havasu City Lake Beratan, in press). The dating was carried Havasu out partly to support an appraisal of mineral-resource potential (Light and others, 1983). Preliminary K-Ar dates reported earlier (Light and others, 1 983; CALIFORNIA Nielson, 1986; Glazner and others, AUBREY 1 986) are here fully documented, and in HILLS some cases have been corrected. Correc BiaWlLLAMS tions follows discovery by Nakata of a MOUNTAINS spike calibration error. In this paper we report 57 conventional O I NEVADA K-Ar and 7 fission-track ages (dates) on \ 52 rock samples located in figure 2. We ARIZONA term those with mixed cooling histories \ WHIPPLE dates, following Armstrong (1966), to MOUNTAINS emphasize that they do not necessarily correspond to emplacement ages. Dated 10km rocks include 13 volcanic, 14 dike, 6 granitoid, and 9 gneissic rocks and 6 rocks that experienced alteration related FIGURE 1. Location map of the Mohave Mountains area, AZ. to mineralizing processes. Materials dated by K-Ar were: biotite, sanidine, plagioclase, muscovite, sericite, hornblende, and whole The ages on volcanic rocks (figs. 3a, 4)in most cases are rocks. Zircon was dated by fission tracks. Figure 3 divides consistent with or close to the crystallization age inferred the dated rocks into four sample groups and presents from regional geologic relations, although dates on some histograms of the ages, identified by the material dated. hornblende and sanidine separates seem too young. These ages help to calibrate the time of Tertiary deposi Tertiary and Mesozoic dikes and stocks (fig. 3b) yielded tion and deformation, constrain the timing of different biotite ages that in many cases are geologically consistent styles of Tertiary magmatism, and provide age information as emplacement ages but, whole-rock, plagioclase, and helpful for interpreting Tertiary uplift, Mesozoic alteration hornblende dates are nearly all variant. Secondary and intrusion, and cooling of Proterozoic rocks. We inter muscovite from altered gneiss yielded dates that can be pret ages of events from these dates within the framework interpreted as near the age of alteration, but finer grained of our working model established by field studies from I0W-K2O sericite and zircon yielded younger dates. Dates 1979 to 1987. determined from biotite and zircon (fission-track) on
[ISOCHRON/WEST, no. 57,July 1991] 22
114022*30- 114015*00" 114007*30-
KILOMETERS Amphibolrte
POWELL PEAK BUCK Augen FAULT 46 1^3 Basalt 14.6 MOUNTAINS Granite 597]. I ; (WRK-Ar) IB. K-Ar) Tonalite 863 (B. K-Ar). 73.8 ; 14 Andesite porphyry (Z.FT) 19.8 (WaK-Ar) 47 • 22J-* ' 13 Volcanic breccia O'orite » Tuff 18.0 (San. K-Ar) 48.0 (B. K-Af) 72 5 0 Obrite 32.0 (B. K-Ar) (H. K-Ar) 27 "/» Toff 17.6 Dacite 20.8 '- '(San, K-Ar) Aug0n^«\.^9 Granite 18.1^ (B. K-Ar) Gneiss \ (B, K-Ar). 16.6(Z. FT) 41 Gneiss 49.2 Rhyolite 15.1 -c* " ^ :. (B.K-Ar) (P. K-Ar)V .^ (Z- ^• 24 Dadte 19.8 (B. K-Ar) 19 ArKfesite 78.1 r,: * V*- •* **A10 19.1 (B, K-Ar). (Wa K-Ar) . ^ .• .;(B. K-Ar). 16.3 ;\'8\ Latite>
EXPLANATION
Lake Quaternary deposits Havasu City rr<~!r3 < > - * !1 deposits Perlhe < i.'' 7 ^ - ^ ^ 2 Tuff 12.7 V • (San. K-Ar) Pre-Teniary rocks (Cretaceous and Proterozoic)
Contact BILL WILLAMS MOUNTAINS Lo/r-angle fault- Hachures on upper plate 48 Gneiss 130 AUBREY HILLS ^ (B. K-Ar). 81.7 (Z. FT) Sample Flock^YP^ Mineral dated kxation^ j Age tMe) Method # 39 Microdiorite 24 DACITE 20.8 (B. K-AR) 26 Dacite 19. 332 (H. K-Ar) / (B. K-Ar)
Z- Sinriplifiod QooloGic index men of fha im u >a which was collected from the WhioDle Mountaine showing sample localities and dates except for sample number 26, not portrayed for graphic simplicltv The minnrai ot the map area. The northwest-southeast trending Mohave Mountains dike swarm Is sanldlne, Ser = serlclte, WR = whole rock, WM J°white B = blotlte, H = hornblende, P = plagloclase, San = mica, Z = zircon.
and zircon (fission-track) (Armstrong 1966; Turner and If cooling Is relatively slow and excess radiogenic products Forbes 1976; Harrison and others 1979; and Hurford are not incorporated by the minerals, those species with 1986). This order varies depending upon the extraction ower bloclung temperatures will give younger ages than technique used to determine the blocking temperature (Harrison and Fitzgerald 1986; Gaber and others 1988). those with higher blocking temperatures. A common order Other factors that complicate interpretation of the numerical of progressively decreasing closure or annealing tempera age include rate of cooling, mineral structure, and availability tures is; hornblende (K-Ar), muscovite (K-Ar), biotite (K-Ar) of excess radiogenic argon from external sources.
[ISOCHRON/WEST. no. 57, July 19911 23
WR
SAM 10 8
8
8 A. VOLCANIC ROCKS
B N P H
H
SAM H
WR WR WR WR 8 JH 1 1 1 1 1 i i i i ! I I I I I i 300 500 700 900 1100 1300 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95100 200 400 600 800 1000 1200
10
B. DIKES AND STOCKS
N 5
m —r—r n n Fi T—I—r —!—I—i—I—I—I—I—I—I—I—I / 300 500 700 900 1100 1300 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95100 200 400 600 800 1000 1200
10
0. ALTERED GNEISS
N 5
Isr] faTI (T1 "I I i I I i I I I I I 1 I I I 1 i I I iN/ l I I ; I I i ! I I j I 0 5 10 15 20 25 30 35 40 45 50 55 60 6570 75 80 8590 95l002OO^00400®00gOO^°°3QQ900Q0'Q'9°2O02°°
10 D. PROTEROZOIC ROCKS
N
HL n—r I I 1 n—r 1—r ^ . n . n , , , F! 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 ^°°200^°%0^°°600^°°800^°°1 OoV°1200^°°
AGE (Ma)
FIGURE 3. Histograms of dates, divided into four groups of samples. Abbreviations: (K-Ar) WR = whole rock, H = homblende, P plagioclase, San = sanidine, B = biotite, M = muscovite, Ser = sericite; (fission-track) Z = zircon.
[ISOCHRON/WEST, no. 57,July 1991] 24
GEOLOGIC FRAMEWORK VOLCANIC ROCKS
The Mohave, Buck, and Bill Williams Mountains expose Conventional K-Ar ages of volcanic rocks (figs. 3a and 4) Proterozoic gneisses and intrusive rocks, Mesozoic intrusive help to constrain the time of tilting and faulting as well as to rocks. Tertiary dikes, and Neogene volcanic and sedimentary calibrate the magmatism. Nielson (1986) divided the pre- rocks (fig. 2). Paleozoic and Mesozoic strata that are present Pliocene Tertiary section into four sequences numbered: I, II, elsewhere in the region were stripped prior to the middle III and IV, from oldest to youngest. Tertiary, so that the pre-Tertiary rocks are overlain noncon- The Peach Springs Tuff of Young and Brennan (1 974) is formably by volcanic and clastic rocks of Miocene and the only unit of regional extent found in the Mohave Moun Oligocenel?) age. In the Mohave Mountains a swarm of tains area. It is a single cooling unit of welded tuff that out northwest-trending dikes intrudes the pre-Tertiary rocks and crops in an area of at least 280 by 170 km (Glazner and lower parts of the Tertiary section. The dikes occupy about others, 1986; Wells and Hillhouse, 1989). The Peach 16 percent of the central Mohave Mountains east of Lake Springs has yielded a spread of K-Ar ages (Young and Havasu City, where their total thickness is about 2 km Brennan, 1974, Glazner and others, 1986), but studies (Nakata, 1982). using the ^°Ar/^®Ar technique indicate its emplacement age Angular unconformities in the lower Miocene section is 18.5 ± 0.2 Ma (Nielson and others, 1990). document progressive westward tilting due to extensional The youngest rocks dated are olivine basalt (sample no. 1) growth faults. The younger strata are little tilted or faulted, at 11.1 ± 0.3 Ma (whole rock), and an underlying silicic tuff in contrast to the older units, which commonly dip at steep (no. 2)dated at 1 2.7 ± 0.6 Ma (sanidine); both are included angles and are locally overturned. Tilting of fault blocks was in sequence IV. These rocks have southwest dips of 6° to unidirectional to the southwest. The Grossman Peak fault 15° and contain normal faults with down-to-the northeast and the probably equivalent Powell Peak fault are both nor displacements in contrast with much steeper to overturned mal faults with moderate dips. They juxtapose numerous beds in the underlying Miocene strata. The change in dip fault-bounded blocks 1 to 2 km thick in their hanging walls marks the waning stages of deformation following extreme against large rotated crustal blocks as much as 1 5 km extensional faulting and tilting that affected the older rocks. across. These large blocks form the central Mohave Moun Suneson and Lucchitta (1979, 1983) reported similar ages tains (Grossman block). Bill Williams Mountains, and Powell for basalt and rhyolite that occur a few kilometers to the Peak/Tumarion Peak area. The structurally equivalent southeast of samples no. 1 and 2, in the Gastaneda Hills; Whipple Mountains and Ghemehuevi faults, in exposures they reported a age of 8.6 Ma for an undeformed basalt flow near the Colorado River, project beneath these large rotated 8 km southwest of sample no. 1 (fig. 2). blocks(Howard and others, 1 982a, 1 987,in press; Howard A 14.6 ± 0.4 Ma whole-rock age was determined on a and John, 1987). steeply dipping olivine basalt lava flow in a fault block in the
WR 1 . 1 2 SAN 2 •- WH 3 4 DC _ SAN 5 , 5i—•—I 111 BIO CD 6 . 6 H PLAG 7 T"- BIO 8^_ , 8 BIO UJ 9' 9 HBLD 10 10, ■ DL 10 s 11 11, "r ,11, I HBLO, BIO < 12 12 CO BIO 13 13 WR 14 14 15 16
10 12 14 16 18 20 22 24
AGE (Ma) figure 4. K-Ar ages (error bars) for volcanic rocks, arranged by sample number along the ordinate in approximate order of increasing stratigraphic age. A geologically anomalous date of 48.0 ± 1.2 Ma (no. 13) is not shown.
[ISOCHRON/WEST, no. 57,July 1991] 25
northern Mohave Mountains (no. 3). The basalt overlies and xenocrystic biotite is suspected. Based on ages that we con dips the same direction as volcanic rocks of sequence I and sider the most reliable, the age of the part of the volcanic is overlain by moderately dipping arkosic sandstone of section lower than the Peach Springs Tuff (sequence I of sequence III. Similar olivine basalt flows both underlie and Nielson, 1986)is concluded to be about 1 9 to 22 Ma. This overlie the Peach Springs Tuff nearby in the same fault age span matches that for fresh rocks of similar stratigraphic block. This association suggests that the dated basalt may position dated from the Turtle and Stepladder Mountains, 50 be close in age to the Peach Springs age of 18.5 Ma km west of the study area (Howard and others 1982b; reported by Nielson and others (1 990). If the 14.6-Ma K-Ar Nielson and Glazner, 1986). age correctly dates the basalt, alternatively then its steep dip We conclude, based on these data and on biotite ages implies that this fault block tilted since 14.6 Ma. This age of from dies and stocks (see below), that a suite of basaltic, tilting is not inconsistent with the time of tilting of laterally andesitic, dacitic, latitic, and rhyolitic rocks were intruded equivalent units in adjacent areas; for example, 55 km to the and erupted in the early part of the extensional event northnorthwest in the Sacramento Mountains 14.6 Ma, and between about 19 and 22 Ma. After the rhyolitic Peach the Dead Mountains later than 12.2 Ma (Spencer, 1985). Springs Tuff was deposited across the area at 18.5 Ma, Major tilting ceased by 13 Ma 20 km to the south in the magmatism in the area produced mainly olivine basalt and Whipple Mountains region (Davis and others, 1980; Dickey rhyolite through the late stages of deformation from 1 5 to and others, 1 980). 1 2 Ma and the cessation of deformation between 8 and Major tilting of some large fault blocks, including the 12 Ma. Grossman block of the central Mohave Mountains (fig. 2), occurred earlier. Gently dipping (100) rocks dated at 17.9 DIKES AND STOCKS ± 0.7 Ma (plagioclase, no. 7) and 19.9 ± 0.5 Ma (biotite, no. 8) unconformably overlie vertical to moderately tilted Dike rocks yielded a wide variety of dates, only a few of older Miocene strata (sequence I) that are part of that which seem consistent with geological evidence for the age upended block. Within the estimated precision (±0.5 Ma) of emplacement. Most of the dates appear to be too old, and the age on no. 8 overlaps a biotite age on the underlying se may reflect excess argon derived from the Proterozoic host quence (19.2 ± 0.5 Ma, no. 10), which suggests that the rocks and incorporated in the younger intrusive rocks. For unconformity age is within the overlap, about 1 9.5 Ma. The nonvolcanic rocks we regard the K-Ar ages on potash-rich section containing samples 7 and 8 lacks the Peach Springs mica as minimum ages and relatively insensitive to excess Tuff, which elsewhere is an important regional marker argon. (Glazner and others, 1986). A 1 5.1-Ma plagioclase age is taken as the emplacement The Peach Springs Tuff in other fault blocks of the study age of a distinctive thick rhyolitic dike (no. 1 5) that cuts area commonly dips at moderate angles, and overlies Miocene volcanic and sedimentary rocks. Its trend, outcrop sequence I with slight angular discordance. Within the map style, and subvolcanic character are different from dikes in area the Peach Springs Tuff yielded K-Ar sanidine dates of the Mohave Mountain dike swarm. 1 7.6 ± 0.4 and 1 8.0 ± 0.5 Ma (nos. 4 and 5), and a rock The Mohave Mountains dike swarm is interpreted as tentatively identified as belonging to the Peach Springs Tuff having intruded at about 20 Ma on the basis of (a) similar yielded a biotite age of 1 8.7 ± 0.7 Ma (no. 6). The young biotite ages (19.8 ± 0.5, 19.8 ± 0.2, 20.8 ± 0.5 Ma, K-Ar dates on the sanidine may reflect incomplete recovery nos. 24, 25, 27) on dacitic dikes, (b) a biotite age of (19.8 of argon during extraction because of high sample ±0.5 Ma, no. 26) on a similar dike from the possibly cor viscosities, as suggested by McDowell (1983) and G. B. relative Chambers Well dike swarm in the Whipple Moun Dalrymple (written communication, 1 986). tains, California, and (c) intrusion of the swarm into the Volcanic rocks that are or thought to be stratigraphically lower part of the volcanic section of sequence I. A diorite below the Peach Springs Tuff yielded K-Ar dates ranging dike in Proterozoic rocks, which cuts other dikes of the from 1 6.3 ± 0.4 to 48 ± 1.2 Ma (nos. 9-1 4), but the most Mohave Mountains swarm, yielded hornblende dates of reliable lie between 1 8 and 21 Ma. A basalt (no. 9) dated at 22.6 ± 0.7 (no. 16), 26.9 ± 0.7 (flux added to no. 16), 17.9 ± 0.7 Ma occurs in sequence I where the Peach and 21.7 ± 0.7 (no. 1 7). These dates (no. 1 6 and no. 1 6 Springs Tuff is absent. It may be a sill that post-dates erup w/flux) are older than seem likely for the emplacement age tion of the Peach Springs. A "turkey-track"-textured because the dike swarm is intrusive into part of sequence I. porphyry with altered composition (no. 14) yielded a Other intermediate-composition dikes from the Mohave geologically reasonable whole-rock age of 19.8 ± 0.6 Ma. Mountains dike swarm yielded discordant hornblende, Three rhyolitic rocks (nos. 10, 11, 12) yielded biotite ages plagioclase, and whole-rock dates ranging from 11.3 ± 0.3 of 19.2 ± 0.5, 19.1 ± 0.6 and 20.5 ± 1.6 Ma, but con to 78.1 ±2.0 Ma (nos. 18-23). We do not interpret these sistently younger hornblende dates of 1 6.3 ± 0.4, 1 7.2 ± as emplacement ages. A small body of quartz diorite (no. 0.7 and 18.8 ± 0.5 Ma; repeated extractions showed 28)that grades into a mafic dike of the dike swarm yielded a similar results. Miller and Morton (1 980)found that 35 per biotite age of 21.5 ± 0.5 Ma, in agreement with most other cent of their 45 biotite-hornblende pairs from the southern biotite ages from dikes and with the geologically interpreted Mojave Desert, California, yielded younger hornblende dates emplacement age. because at standard extraction temperature (1,200° to Evidence of Laramide-age (Cretaceous to earliest Tertiary) 1,400° C) the refractory hornblende was not completely magmatism is provided by ages on a rhyolite dike and a fused. Like theirs, our samples were run before the avail granitoid porphyry apophysis. Except for its northeast-trend, ability of water-cooled extraction bottles which allow higher the rhyolite dike (no. 31), resembles rocks of the middle fusing temperatures. All our samples appeared fused, but Tertiary dike swarm. It yielded an age of 62.3 ± 1.6 Ma on dates are systematically younger than biotite by as much as biotite. The porphyry (no. 32), which yielded a biotite age of 1 5 percent, suggesting incomplete recovery of argon. A 72.0 ± 1.8 Ma, is satellitic to a granodiorite pluton at whole-rock K-Ar age of 21.5 Ma was reported by W. Rehrig Powell Peak, which is tentatively correlated to biotite for a mafic lava flow low in sequence I (Pike and Hansen, granodiorite of Late Cretaceous age in the Chemehuevi 1982; Nielson, 1986; Nielson and Beratan, 1990). One Mountains (Howard and others, 1982a; John, 1988; John silicic breccia (no. 1 3) yielded an anomalously old date on and Mukasa, 1 990). Further independent support for an age biotite of 48 ± 1.2 Ma, from which contamination by of about 72.0 Ma (no. 32) for the Powell Peak pluton is a IISOCHRON/WEST, no. 57, July 1991] 26
73.8 ± 7.7-Ma zircon fission-track date on a nearby PROTEROZOIC ROCKS Proterozoic rock which yielded a biotite date of 863 ± 21.6-Ma (no. 47). We infer that the ambient temperature in Dates determined on altered Proterozoic rocks (fig. 3d) the local Proterozoic rocks was colder than the blocking can be interpreted as mixed cooling ages. The regional Pro temperature for biotite, but was thermally perturbed by terozoic rock suite is known to comprise Early Proterozoic emplacement of the pluton and its satellites at about 72-74 gneisses and granitoids and Middle Proterozoic granitoids Ma. and diabase (Lanphere 1964; Howard and others, 1982b; Ages on a small diorite stock and associated younger Anderson, 1983; in press; Wooden and others, 1988). granite, which resembles Late Cretaceous granites in the Granodiorite, tentatively correlated with the Middle Pro region, are harder to interpret (nos. 30 and 29). The diorite terozoic granodiorite of Bowmans Wash of Anderson yielded a hornblende date of 32.0 ± 1.0 Ma and the granite (1 983), yielded a fission track date on zircon of 46.4 ± 5.4 yielded a biotite (K-Ar) date of 18.1 ± 0.5 Ma and zircon Ma (no. 43). Rocks assigned to the Early Proterozoic by fission-^ack date of 1 6.6 ± 1.7 Ma. The dates are younger Howard and others (1982a; in prep.)(samples 40 to 42 and than mica and zircon cooling ages in most of the rest of the 44 to 52) yielded hornblende K-Ar dates ranging from Grossman Peak fault block, and suggest the possibility of 104 ± 2.6 to 1372 ± 34 Ma, biotite K-Ar dates ranging Tertiary emplacement. from 49.2 ± 10.5 to 863 ±21.6 Ma, and zircon fission- xAi^rWilliams Mountains yieldedwhich a cuts hornblende an augen date gneiss of 332 in the ± 16Bill track dates ranging from 22.6 ± 1.9 to 81.7 ± 8.7 Ma. Ma (no. 39), which is not a crystallization age because These dates are taken to imply cooling of some rocks as Paleozoic sections in nearby regions exhibit no evidence of recently as the Miocene. An analysis of cooling and uplift magmatism (Stone and others, 1983). The dike could be patterns is in progress. Tertiary, Mesozoic, or most likely Precambrian in age. ANALYTICAL METHODS altered rocks K-Ar Ages were measured on white mica from six intensely The K-Ar age determinations were made in the isotope ^tered and sericitized Proterozoic rocks associated with laboratories of the U.S. Geological Survey at Menio Park, California, using the methods described by Dalrymple and Lanphere (1969). Argon was extracted on an ultrahigh vacuum system by fusion; the reactive gases were then scrubbed by an artificial molecular sieve, Cu-CuO and Ti metals. 91.4 ± 2.3, 89.7 ± 2 2 92 6 dates of The spectrometry was performed on a Nier-type, 1 5-cm radius, 60° sector and a multichannel, 23-cm radius, 90 sector mass spectrometers, both operated in the static mode. Argon was analyzed by comparing the liberated gas to a pure ^®Ar spike of known volume and composition with the muscovite dates Thp c ^ compared added during fusion. The decay constants used are those youngest date (55 Ma)Ss a J n ^^e published by Steiger and Jager (1977): cent, whereas the next older P®^" '^°K/K 1.67 X 10-^mol/nno' mediate KzO content of ft q °"® ® ®" X C^oKb-) 4.962 X Harrison 1 988 suggest that low and X(^°Ke) + V (^°Ke) 0.581 x 10"^° V' illite or hydromuscovite. The '^® Flame photometry with a lithium internal standard wa through lower blockino tom,, I '"®y record cooling used to analyze potassium using the procedure descri mu^vlte. If .0, a ZaltiTrr """ by Cremer and others (1984). All samples were run temperature for argon retention mav ho if .®®"®'^® blocking duplicate to check precision date for the sericite sample (no ^'ss'on-track <'°Ar») as a percent of total arg®" content. Zircons in the sample seem to°r '"^®™ediate-K20 (E Ar) may differ by as much as 17 percent from rep jca tions. The fission-track age of 78 + q ^ popula- arialysis of the same sample on different extraction I""® ' neyertheless slightly exceeds tho o * • although crude with different bakeout times, spike sets and the totaj gaata tha. ,hia a.-icita ^"9- amount of sample used. These factors contribute varV'^y annealing temperature for fission t ^ ''®'ow the atmospheric components and thereby lead to an apP®""® Theorder lower-KjO of 2000 (Harrisonsericite (no. and 33) otheVfm,; i?7o'u^''®°"'^^"rford, 1986).^^^® discrepancy in the radiogenic argon percentages from one the youngest apparent age mav J !®'"® with extraction to the next. Howeyer, the total atmospheric components are subtracted and the final age calculati temperature yet. ' ®y '^®®ord a lower cooling are based on radiogenic Ar concentration measured The muscovite dates between qn on,.. n m^es/gram which differ by no more than 3 pe^®®"^- concordant. Because they occur at rti« '^® ®'^® crudely The 2.5-3 0 percent (±)error represents a conservativ tural depths, we suspect that thev ma Pre-tilting struc- estimate of the overall analytical precision for samples w of the alteration event rather thL ^ ®PP''°*''^®^® tbe age greater than 10 percent radiogenic argon. This is based on depth.would Anbe ageexpected of alteration^f to vary systenH®.""^.®' aSuTfAn m structural^^ich empirical tests over a period of 14 years at the U-S. sistent with dates obtained from nh!. . be con Geological Suryey Isotope laboratory in MenIo Pa^k, tains and from an upper plate ^^®^"rtle Moun- to^19ftn^h others 1985). All samples f®" P" g Mountains (Dayis and others 198fv w ® ^bipple to diaftaidigital readouts decreased P®fc®nt:the error at to this 2.5 time percent. a change Po 1982b): both localities are near estnr'a?°'~^''' °^^®'®' tions of the Mohaye Mountains fault ? Pt®"®*tension posi- Laml sam d®terminations on two splits others 1982a). '^°""ta.ns fault blocks (Howard and ^.52 5-3 3.0 O^Prn'o percent estimates®"°'^ reflects at ± ^the standard same deviation -ndand 68-pe,c.nt confiaeno, lev.t Howa^S [ISOCHRON/WEST, no. 57,July 19^ ^' 27
between replicate analysis were usually less then 2 per (sequence I of Nielson, 1986) and underlies steeply cent. For those samples with greater than 3 percent error dipping arkosic sandstone and conglomerate between replicate analysis the dates were averaged and a (sequence III of Nielson, 1 986) that contains clasts of standard deviation was calculated and reported to ± 1 the olivine basalt. Intergranular to diktytaxictic tex standard deviation. ture. Glass content less than 5%. Rock generally fresh. Olivine rimmed by iddingsite. Age is revised Fission Track from 14.1 Ma age reported by Nielson (1986). (whole rock) 14.6 ± 0.4 Ma The external detector method was used to date the zircons (Naeser, 1976, 1979), which were mounted in A. JP80MH-192 K-Ar Teflon and etched in a eutectic melt of KOH-NaOH Peach Springs Tuff (34°37'49"N, 114°21'10"W; (Gleadow and others, 1 976) at 21 5°C for 30-50 hours. S22,T1 5N,R20W; Franconia 7.5' quad., Mohave Co., The Teflon mounts were covered with a muscovite detec AZ). Analytical data: K2O = 9.09%, 9.06%, 9.08%; tor and irradiated along with neutron dose monitors 40Ar* = 2.34 X 10-^0 mol/gm, 2.30 x 10"^° mol/gm; '*°Ar*/E^°Ar = 72%, 64%. Argon analysis: (U-doped glasses SRM 962 also covered with muscovite J. K. Nakata. Collected by: J. E. Nielson. Comments: detectors) in the U.S. Geological Survey TRIGA reactor at Salmon-pink crystal-lithic tuff breccia, unwelded, Denver, Colorado. The neutron dose was determined using sequence IV; occurs above bedded tuff (altered track densities in the muscovite detectors and the Cu pumice fragments). Sanidine is dominant crystal. Age calibration for SRM 962 (Carpenter and Reimer, 1974). is revised from 1 6.7 Ma and 1 7.8 Ma ages reported by The errors shown for the fission-track dates are ± 2 Nielson (1 986). standard deviations. (sanidine) 17.6 ± 0.4 Ma b.P81MH-2 K-Ar ACKNOWLEDGMENTS Peach Springs Tuff (34°37'54"N, 114°21'21"W; S22,T1 5N,R20W; Franconia 7.5' quad., Mohave Co., Many individuals contributed to this study. They include: AZ). Analytical data: K2O = 9.36%, 9.35%, 9.32%, D. Sorg and B. Hamachi who prepared minerals and rocks; 9.46%; "^^Ar* = 2.40 x 1 0"^°mol/gm, 2.48 x 10"^° M. Dyslin, L. Espos, P. Klock, S. MacPherson, S. Pribble, mol/gm; '^oAr^/L^^Ar = 67%, 87%. Argon analysis: and D. Vivit analyzed samples for potassium contents; M. A. Pernokas. Collected by: J. E. Nielson. Com E. Koleman, M. Pringle, J. Saburomaru and J. Von Essen ments: Strongly welded crystal-lithic rhyolite tuff, discussed extraction procedures; G. Reimer and C. Naeser sequence II. Phenocrysts are sanidine and lesser facilitated the fission-track work; and K. Parish prepared the biotite, opaques, hornblende, and sphene. Sanidine is illustrations. The manuscript was improved by the helpful euhedral and fresh. Date is revised from 1 7.4 Ma date suggestions provided by Chuck Naeser and Marvin reported by Nielson (1986). (sanidine) 18.0 ± 0.5 Ma Lanphere. 6. JP81MH-159 K-Ar Silicic tuff (34°30'41"N, 114°05'11"W; SAMPLE DESCRIPTIONS S31 ,T14N,R17W; Standard Wash 7.5' quad., Mohave Co., AZ). Analytical data: K2O = 8.04%, ^.P81MH-8a K-Ar 8.05%;'^^Ar* =2.22 x 1 0"^° mol/gm, 2.1 4 x IQ-io divine basalt (34°30'44"N, 114°04'27"W; S31,T14N,R17W; Buck Mountains SE 7.5' quad., mol/gm; '^°Ar*/E^°Ar = 37%, 54%. Argon analysis: Mohave Co., AZ). Analytical data: K2O = 1.326%, M. A. Pernokas. Collected by: J. E. Nielson. Com 1.317%, 1.337%; '^^Ar* =2.16 x 10"'^ mol/gm, ments: In fault block that exposes bedded rocks of 2.09 X 10-11 mol/gm; ^^Ar^/L^^Ar = 45%, 34%. sequence I of Nielson (1986). Flattened pumice Argon analysis: M. A. Pernokas. Collected by: M. A. fragments show no preferred orientation. Probably the Pernokas. Comments: Gently dipping basalt flow, Peach Springs Tuff, sequence II. sequence IV; caps Black Mountain. Holocrystalline (biotite) 18.7 ± 0.7 Ma subophitic basalt containing phenocrysts of olivine, 1. JP81MH-388B K-Ar clinopyroxene, and plagioclase. Age is revised from Silicic tuff (34°33'50"N, 114°19'27"W; 10.6 Ma age reported by Nielson (1986). SI 1,T14N,R20W; Lake Havasu City N 7.5' quad., (whole rock) 11.1 ±0.3 Ma Mohave Co., AZ). Analytical data: K2O = 1.19%, 2. JP82MH-23 K-Ar 1.17%, 1.18%; '^^Ar* = 3.06 x 10"^^ mol/gm; Silicic tuff (34°27'36"N, 114°33'36"W; ^oAr*/r^°Ar = 3\%. Argon analysis: A. Pernokas. 817,T13N,R17W; Parker Dam 15' quad., Mohave Collected by: J. E. Nielson. Comments: Overlies the Co., AZ). Analytical data: K2O = 9.03%, 9.06%; flow sampled as JP81MH-378. Gently dipping. Ashy ^oAr* = 1.66 X 10"'° mol/gm; ^oAr*/E^°Ar = 63%. matrix contains relatively fresh plagioclase and fused- Argon analysis: J. K. Nakata. Collected by: J. E. appearing biotite. Silicified. Flow-banded. Nielson. Comments: Flow-banded tuff, sequence IV, (plagioclase) 17.9 ± 0.5 Ma not welded, with sanidine as sole crystals; no lithic Q. JP81MH-378 K-Ar clasts present. Andesitic flow (34°35'25"N, 114°21'08"W; (sanidine) 12.7 ± .6 Ma S33,T1 5N,R20W; Havasu City N 7.5' quad., Mohave 3.H81MH-18 K-Ar Co., AZ). Analytical data: K2O = 8.34%, 8.33% Olivine basalt (34°40'55"N, 114°18'42"W; 8.32%; '^oAr* = 2.34 x 10"^® mol/gm; S36,T1 4N,R20W; Franconia 7.5' quad., Mohave Co., '*°Ar*/E^°Ar = 69%. Argon analysis: M. A. Pernokas. AZ). Analytical data: K2O = 1.846%, 1.861%, Collected by: J. E. Nielson. Comments: Fine-grained 1.854%; ^°Ar* = 3.87 x 10"'' mol/gm, 3.95; x groundmass with large (6-mm) phenocrysts and 10-^^ mol/gm; ^oAr*/E^°Ar = 62%, 64%. Argon clusters of phenocrysts. Contains relatively unaltered analysis: M. A. Pernokas. Collected by: K. A. Howard. biotite in addition to altered plagioclase and Comments: Steeply dipping basalt, sequence II. clinopyroxene and emphibole. Overlies intermediate-composition volcanic rocks (biotite) 19.9 ± 0.5 Ma
[iSOCHRON/WEST, no. 57,July 1991] 28
B.JP81MH-14a K-Ar AS considered much aider than geologically reasonable Basalt (34°39'42"N, 114°14'34"; S3, and may indicate the presence of xenocrystic biotite. T13N,R19W; Standard Wash 7.5' quad., Mohave (biotite) 48.0 ±1.2 Ma Co., A2. Analytical data: K2O = .93%,.92%; ""Ar* ^A,P80MH-187 K-Ar = 2.33 X 10-" mol/gm, 2.44 x 10"" mol/gm; Latite (34° SyBS" N, 114°21'08"W; S22, ""At*IT.*"At = 37%, 41%. Argon analysis: J. K. T1 5N,R20W; Franconia 7.5' quad., Mohave Co., AZ). Nakata. Coilected by: J. E. Nielson. Comments: Flow Analytical data: K2O = 3.04%, 3.06%, 3.08%, or dike in sequence I of Nielson (1986). Porphyritic 3.03%;'^°Ar* =8.72 x 1 0"^^ mol/gm, 8.75 x 10"^^ olivine basalt; holocrystalline groundmass. Plagioclase mol/gm; '*°Ar*/E'*°Ar = 86%, 88%. Argon analysis: laths, clinopyroxene and olivine In granular opaques M. A. Pernokas. Collected by:J. E. Nielson. Comments: and alterations Indicate chlorite. Olivine Is fine-grained. Platioclase-pyroxene-phyric "turkey-track" or "jack- Age Is revised from 17.8 Ma age reported by Nielson straw" porphyry; holocrystalline. Apatite very abun (1 986). dant. Plots in latite field using normative minerals, may (whole rock) 17.9 ± 0.7 Ma have had andesitic affinities before alteration. High K2O \0.P81MH-296 K-Ar may indicate potassium enrichment from metasomatism. Rhyollte flow (34°35'31" N, 114°22'01"W; (whole rock) = 19.8 ± 0.6 Ma S33N,T1 5N,R20W; Lake Havasu City N 7.5' quad., \%.P81MH'1 K-Ar Mohave Co., AZ). Anaiytical data: (blotlte) K2O = Rhyolitic dike (34°36'01"N, 114°21'12"W; S28, 8.54%,8.55%; "Ar* =2.37 x 1Q-"* mol/gm, 2.03 T1 5N,R20W; Lake Havasu City N 7.5' quad., Mohave X 10"" mol/gm; *°Ar*IT,''°At = 48.9%, 13.8%; Co., AZ). Analytical data: K2O = 0.679%, 0.679%, (hornblende) K2O = 0.866%,0.858%; "Ar* = 2.03 0.676%; -^oAr* = 0.158 x 10"^° mol/gm; X 10"" mol/gm, 2.02 x 10"" mol/gm; ''OAr*/S'"'Ar '*oAr*/E'*°Ar = 53%. Argon analysis: M. A. Pernokas. = 1 4%,10%. Argon analysis: J. K. Nakata. Coilected Collected by: M. A. Pernokas. Comments: South ^y' 3- Nielson. Comments: In sequence I of Nielson western pinnacle of a line of pinnacles formed by (1986). Medium-grained matrix with large plagioclase 1 00-m-thick dike, on west side of Arizona highway phenocrysts and glomerophenocrysts; mafic pheno- 95. Flow-banded rhyollte containing phenocrysts of crysts are relatively fresh; hornblende Is green. plagioclase, K-feldspar, quartz, biotite, hornblende, (biotite) 19.2 ± 0.5 Ma sphene, and apatite. Plagioclase is euhedral and fresh; (hornblende) 16.3 ± 0.4 Ma grain size 1 -2 mm. Age is revised from 1 6.2 Ma age 11. P80MH-223 K-Ar reported by Nielson (1986). Rhyolltic perllte (34°37'31"n, 114°20'02"W; (plagioclase) = 15.1 ± 0.4 Ma S20'T1 5N,R20W; Franconia 7.5' quad., Mohave Co., 1 6. JN'81MH'90-2 K-Ar AZ). Analytical data: (blotlte) K2O = 8.34%, 8.32%; Diorite dike (34°33'39"N, 1 14°06'20"W; 812, t ^ mol/gm, x 10"° mol/gm; T14N,R1 8W; Buck Mountains SE 7.5' quad., Mohave 63.6%; (hornblende) K2O = Co., AZ). Analytical data: (hornblende) K2O = •A 9/°' = 2.08 X 10-"mo)/gm, 1.93 x 0.782%, 0.789%; ^°Ar* = 2.57 x 10"^° mol/gm, 10 mol/gm; ""Ar'/E-'OAr = 19%, 22%. Argon (with flux) 3.06 X 10"^' mol/gm; '^°Ar*/i:^°Ar = analysis: M. A. Pernokas. Coilected by:.S. E. Nielson. 14%; (with flux) 25%. Argon analysis: J. K. Nakata. Cornments:8ase of rhyollte flow In sequence I of Collected by: K. Nakata. Comments: Northeast trend Nielson (1 986). Dark, perlitic glass, containing 2-mm- ing dike that cuts other nearby dacitic and andesitic sized biotite. Previously reported as sample JP81 -1 23 dikes. Fine- to medium-grained hypidiomorphic granular (Nielson, 1986). texture. Hornblende is fresh with some alteration to (biotite) 19.1 ±0.6 Ma chlorite. Second analysis used flux and H20-cooled (hornblende) 17.2 ± 0.7 Ma bottle for fusion, and therefore would be expected to be 1 2. JP80MH-139 K.Ar more accurate. Both dates are suspected to be older Rhyolltic perllte (34°28'1 4" N, 114° 1 2'02" W; than the geologic age of emplacement. rA 9^' Standard Wash 7.5' quad., (hornblende) 22.6 ± 0.7 Ma R 46or R 4R0/ (blotlte) K2P = (with flux) 26.9 ± 0.7 Ma V iO"oL^/ = 2.44 X 10-mol/gm, 2.35 1 7. JN-81MH'90'2A K-Ar 75% 66^ 'fino; ^ 10-'° mol/gm;""ArVE^'Ar = Diorite dike (34°33'39"N, 114°06'20"W; SI 2, T14N,R1 8W; Buck Mountains SE quad., Mohave Co., fKZ). Analytical data: K2.O = 0.78%,0.78%; -^oAr* = 2.46 X 10-^^ mol/gm; ^oAr^/E'^oAr = 12%. Argon D Collected by: J. E. Nielson. analysis: J. K. Nakata. Collected by: J. K. Nakata. Nielson (19Rfi?^ How m sequence I of Comments: Same dike as JN-81MH-90-2. Fine abundant nian* perlitic groundmass with grained hypidiomorphic granular texture. SiZ Phenoory.,. and (hornblende) 21.7 ± 0.7 Ma ^Q.BLM-139'8 K-Ar (blotlte) 20.5 ± 1.6 Ma Andesitic dike (34°34'30"N, 114°08'47"W; S9, (hornblende) 18.8 ± 0.5 Ma ^3. JP81MH-361 ^ . T14N,R1 8W; Crossman Peak 7.5' quad., Mohave Co., AZ). Analytical data: K2O = 1.426%, 1.434%; ^°Ar* ISt15N rSow^^^^ <34°37'57"N, 114°18'34"W; = 8.9 X 10"^^ mol/gm; ^oAr*/E'^°Ar = 57%. Argon analysis: J. K. Nakata. Collected by: J. K. Nakata. Com ments: Trachytic-textured (turkey-track); mafic minerals are altered to chlorite. Date Is considered to be older than the likely geological age of emplacement. rrvstals recrvstaiiiTo,? plagioclase and biotite (plagioclase) 42.7 ± 1.1 Ma pTag7oSrbS^ I'T ""T-r ^9. BLM-163-35 K-Ar Andesitic dike (34°35'45"N, 1 1 4° 1 1 '1 5"W; S31, altered plagioclase, and clasts of tuff, sequence I. Date T15N,R18W; Crossman Peak 7.5' quad., Mohave
[ISOCHRON/WEST, no. 57,July 1991] 29
Co., AZ). Analytical data: K2O = 2.382%, 2.464%; Biotite is interleaved with chlorite and opaques, and ^oAr* = 2.83 X 10'^° mol/gm, 2.74 x 10"^° less commonly included with hematite and sphene. mol/gm; '*°Ar*/L'^°Ar = 78%, 78%. Argon analysis: (biotite) 19.8 ± 0.5 Ma J. K. Nakata. Collected by: J. K. Nakata. Comments: Date is considered to be older than the likely geological Dacite^dfke (34°32'29"N, 114°12'21"W; S24, age of emplacement. T14N R19W; Grossman Peak 7.5' quad., Mohave Co., (whole rock) 78.1 ± 2.0 Ma AZ). Analytical data: K^O = 8.02%, 8.01%, 7.85%, 20. BLM'163A K-Ar 8 00%-"Ar* = 2.24 x 10^'" mol/gm, 2.20 x 10"'° Andesltic dike (34°35'1 8"N, 1 14°11 S26, mol/gm'; *<>Ar*l'L*°f 30. H80MH-311 K-Ar 114°14'05"W; S27,T1 4N,R1 9W; Crossman Peak Diorite (34°37'33"N, 114°16'42"W; S20, 7.5' quad., Mohave Co., AZ). Analytical data: K2O = T15N,R19W; Franconia 7.5' quad., Mohave Co., AZ). 10.58%, 10.56%, 10.53%, 10.55%; ^^Ar* = 1.44 Analytical data: K2O = 0.47%, 0.46%, 0.49%, X 10"® mol/gm, 1.41 x 10"® mol/gm; '*°Ar*/E^°Ar 0.49%; ^oAr* = 0.22 x 10"^® mol/gm, 0.22 x lO"'® = 96%, 94%. Argon analysis: M. A. Pernokas. Col mol/gm; '^°Ar*/r'*°Ar = 19%, 17%. Argon analysis: lected by: M. A. Pernokas. Comments: Borders a quartz M. A. Pernokas. Collected by: K. A. Howard. Comments: vein near the Pittsburg mine. Highly altered medium- to Medium-grained hornblende diorite in small stock. In coarse-grained gneiss. Contains much sericite, clays, truded by and associated with granite sampled at station and less chlorite and hematite. White mica occurs as (no. 29) H80MH310. Color index 52. Hornblende is euhe- fine-grained aggregated crystals (0.1-1 mm grain size). dral to subhedral, and has brown cores and green rims. (white mica) 91.4 ± 2.3 Ma (hornblende) 32.0 ± 1.0 Ma 30.P82MH-21D K-Ar 3^. P81MH-20 K-Ar Muscovite-quartz vein (34°31'14"N, 114°14'05"W; Rhyolite dike (34°32'26"N, 114°09'49"W; S20, S27,T14N,R1 9W; Crossman Peak 7.5' quad., Mohave T14N,R18W; Crossman Peak 7.5' quad., Mohave Co., Co., AZ). Analytical data: K2O = 11.12%, 10.91%, "^20 = 8.91%, 8.90%, 8.87%, 10.93%, 10.92%; ^®Ar* = 1.46 x 10"® = x 10"® 8.90%; ^°Ar* = 8.10 x 10-^° mol/gm, 8.14 x lO"^® mol/gm, 1.44 x 10"® mol/gm; ^0Ar*/i:^0Ar = 92%, mol/gm; '^°Ar*/L^oAr = 86%, 84%. Argon analysis: 95%. Argon analysis: M. A. Pernokas. Collected by: M. A. Pernokas. Collected by: M. A. Pernokas. Com- M. A. Pernokas. Comments: White mica concentration ments: ENE-trending dike 3.5 m thick. Cuts gneiss in the along margins of a small quartz vein in augen-gneiss host Jupiter mine area. Microcrystalline matrix and 30% rock; bottom back side of stope, Pittsburg mine area. phenoc^sts of quartz, plagioclase, biotite, microcline, White mica averages 0.2 mm in grain size, includes very and sphene. Biotite is medium-grained (1-2 mm), sub- fine opaques, and occurs as subhedral to anhedral grains. hedral, and intergrown with muscovite and abundant in- (white mica) 89.7 ± 2.2 Ma cludions of apatite, zircon, and calcite. 37. K81MH-36 K-Ar (biotite) 62.3 ± 1.6 Ma Sericitized gneiss (34°32'06"N, 114°10'14"W; 22. H81MH-154 S20,T14N,R1 8W; Crossman Peak 7.5' quad., Mohave porphyry (34°40'29"N, Co., AZ). Analytical data: K2O = 10.48%, 10.49%, Mnhav [iSOCHRON/WEST, no. 57, July 1991] 31 A'[,H82MH'16 K-Ar T16N,R20y2W; Topoc 7.5' quad., Mohave Co., AZ). Granulitic gneiss (34°36'11"N, 114°1 1'05"W; S31, Analytical data: K-Ar (biotite) K2O = 8.48%, 8.49%, T1 5N,R1 8W; Grossman Peak 7.5' quad., Mohave Co., 8.46%, 8.42%; ^°Ar* = 1.32 x 10"® mol/gm, 1.38 x AZ). Analytical data: K2O = 8.71%, 8.74%; ^^Ar* = 10-® mol/gm; ^®Ar*/E^®Ar = 89%, 98%. Argon 7.62 X 10-^0 mol/gm, 4.91 x 10"^° mol/gm; analysis: M. A. Pernokas. Fission-track (zircon, 7 grains) ^°Ar*/L^°Ar = 84%, 82%. Argon analysis: M. A. Ps = 4.28 X 10® tracks/cm^ (1507); Pi = 3.48 x 10® Pernokas. Collected by: K. A. Howard. Comments: tracks/cm^ (614); d = 1.01 x 10^® n/cm^; U = 107 Along jeep trail, several dike widths from dike sampled ppm. Counted by: J. R. Shannon. Collected by: K. A. as H82MH-15 (no. 27). Howard. Comment: Medium-grained biotite tonalite. (biotite) 49.2 ± 10.5 Ma K-Ar (biotite) 863 ±21.6 Ma fission-track (zircon) 73.8 ± 7.5 Ma 42. P81MH-6 K-Ar and fission-track Granodiorite augen gneiss (34°37'12"N, 48 H81BW-25 K-Ar and fission-track 114°17'17"W; S19,T15N,R19W; Lake Havasu City 'Gneiss (34°24'20"N, 114°07'14"W; S3, N 7.5' quad., Mohave Co., AZ). Analytical data: K-Ar T12N,R18W; Parker Dam 15' quad., Mohave Co., AZ). Analytical data: K-Ar (biotite) K2O = 8.97%, 8.91%, (biotite) K2O = 8.26%, 8.19%, 8.26%, 8.22%; ^°Ar* 4oaj.* = 1.72 X 10"® mol/gm, 1.18 x 10"® mol/gm; = 7.98 X 10-10 mol/gm, 8.13 x lO'io mol/gm; '*oAr*/E^oAj. = 74%^ 89%. Argon analysis: M. A. ^oAr*/E'i°Ar = 89%, 84%. Argon analysis: M. A. Pernokas. Fission-track (zircon, 7 grains) Ps = 3.72 x Pernokas. Fission-track (zircon, 7 grains) Ps = 8.358 x 10® tracks/cm^ (1190); Pi = 10.10 x 10® tracks/cm^ 10® tracks/cm^ (1465); Pi = 6.20 x 10® tracks/cm^ (1619); d = 1.03 X 10^® n/cm^; U = 304 ppm. (544); d = 1.02 X 10^® n/cm^; U = 1 89 ppm. Counted Counted by: J. R. Shannon. Collected by: M. A. by: J.'r. Shannon. Collected by: K. A. Howard. Comment: Pernokas. Comments: Medium grained, seriate gneiss. Medium-grained biotite granite gneiss. ^ ^ Color index 10. Biotite occurs as subhedral books, and is K-Ar (biotite) 130 ± 3.3 Ma fresh except for a few percent interleaved with chlorite. fission-track (zircon) 81.7 ± 8.7 Ma K-Ar (biotite) 66.7 ± 1.7 Ma AQ PR1BK 12 K-Ar fission-track (zircon) 22.6 ±1.9 Ma Augen gneiss (34°41'32"N, 114°08'1 9"W; S28, 43. P81MH-9 fission-track T16N R18W; Buck Mountains NE 7.5' quad., Mohave Granodiorite (34O25'01 " N, 1 1 4°09'08" W; Co AZ). Analytical data: K2O = 8.70%, 8.75%, S16,T13N,R18W; Lake Standard Wash 7.5' quad., 8 86%, 8.75%; "Ar* = 1.5638 x 10"' mol/gm, Mohave Co., AZ). Analytical data: (zircon, 7 grains) T.56163 X 10-'mol/gm; ^oArVE^Ar = 89%, 87%. Ps = 3.43 X 10® tracks/cm^ (839); Pi = 4.40 x 10® Argon analysis: M. A. Pernokas. Collected by. M. A. tracks/cm^ (539); d = 1.00 x 10^® n/cm^; U = 137 Pernokas. Comments: Inequigranular biotite grano ppm. Counted by: J. R. Shannon. Collected by: M. A. diorite augen gneiss containing medium- to coarse Pernokas. Comments: Dark, medium- to fine-grained grained feldspar augen. Biotite and minor epidote are in biotite-hornblende granodiorite, probably Proterozoic in fine-grained aggregates. Biotite is interleaved with small amount of chlorite. . ^ — age. Color index 1 5. Abundant opaques. (biotite) 120 ± 3.0 Ma (zircon) 46.4 ± 6.4 Ma 44. PS;/W/y-22 K-Ar BO P81BK-13 K-Ar Porphyritic granite gneiss (34°29'10"N, Amphibolite (34°41'50"N, 114°08'22"W; S28, 114°13'46"W; SI 0,T13N,R19W; Standard Wash 7.5' T16N R18W; Buck Mountains NE 7.5' quad., Mohave quad., Mohave Co., AZ). Analytical data: K2O = 7.97%, Co AZ). Analytical data: K2O = 1.029%, 1.024%, 7.96%; ^oAr* = 1.30 x 10"® mol/gm, 1.33 x 10"® 1 029% 1.026%; ^°Ar* =3.15 x 10"® mol/gm, mol/gm; ^®Ar*/E^oAr = 87/, 93%. Argon analysis: M. A. 2.93 X 10"® mol/gm; ^oAr*/E^®Ar = 94%,94%. Pernokas. Collected by: M. A. Pernokas. Comments: analysis: M. A. Pernokas. Collected by: M. A. Pernokas. Medium- to coarse-grained biotite syenogranite gneiss. Comments: Medium grained. Contains quartz and minor Biotite is olive-colored, anhedral, fine-grained (0.1-1 opaques, biotite, apatite, and sphene. Hornblende is pale mm), and includes opaques along cleavage. to light greenish brown, and occurs as fresh, subhedral to anhedral grains (0.5-1 mm grain size). (biotite) 111 ± 2.8 Ma (hornblende) 1372 ± 34 Ma 45. H83MH-67 K-Ar Amphibolite (34°33'22"N, 114°05'55"W; S13, 51.8/BK-7A K-Ar and fission-track T14N,R18W; Buck Mountains SE 7.5' quad., Mohave Granodiorite gneiss (34°39'34"N, 114°09'27"W; S9, Co., AZ). Analytical data: K2O = 0.412%, 0.41 5 T15N,R18W; Buck Mountains 7.5' quad., Mohave Co., ^oAr* = 6.3896 x lO"" mol/gm;'^^Ar^/E^oAr = 32%. AZ). Analytical data: K-Ar (biotite) K2O = 7.66%, Argon analysis: K. Nakata. Collected by: K. A. Howard. 7.72%, 7.70%;'^^Ar* =3.15 x 1 0"® mol/gm, 3.14 x Comments: More than 3 dike widths from the nearest 10"® mol/gm; ^°Ar*/E^°Ar = 90%, 91%. Argon anal dike. Medium grained. Color index 50. Bears opaques, ysis: M. A. Pernokas. Fission-track (zircon, 7 grains) quartz, epidote. Amphibole is green-brown and is in Ps = 7.77 X 10® tracks/cm^ (1497); Pi = 5.84 x 1 0® ^QSregates consisting of fine (0.1 mm)subhedral grains. tracks/cm^ (562); d = 1.03 x 10^® n/cm^; u = 176 (hornblende) 104 ± 2.6 Ma ppm. Counted by: J. R. Shannon. Collected by: K. A. Howard. Comments: Medium-grained hornblende-biotite ^6.P81MH'10 K-Ar granodiorite gneiss. Biotite is unstrained and fresh. Granite (34°38'24"N, 114°21'27"W; S16, K-Ar (biotite) 264 ±6.6 Ma T15N,R20W; Franconia 7.5' quad., Mohave Co., AZ). fission-track (zircon) 81.6 ± 8.6 Ma Analytical data: K2O = 9.06%, 9.03%; "^^Ar* — 9.41 X 10"® mol/gm, 9.16 x 10"® mol/gm, 9.09 x 10"® 52. H87MH-30 K-Ar mol/gm; -^^Ar^/E^^Ar = 97%, 97%, 97%. Argon Granite gneiss (34°33'13"N, 114°12'33"W; 813, analysis: M. A. Pernokas. Collected by: M. A. Pernokas. T14N,R1 9W; Grossman Peak 7.5' quad., Mohave Co., Comments: Medium-grained monzogranite containing AZ). Analytical data: K2O = 7.15%, 7.14%; ^oAr* = microcline. Color index 5. Biotite occurs as fresh books, 1.24 X 10"® mol/gm; -^oAr^/E^^Ar = 71%. Argon locally associated with minor muscovite; some biotite analysis: J. K. Nakata. Collected by: K. A. Howard. shows pleochroic halos. Comments: Leucocratic biotite granite gneiss. (biotite) 597 ±11 Ma Weathered. Red-brown biotite (1 -mm grain size) is part 47. 81MH-155 K-Ar and fission-track ly intergrown with chlorite. Tonalite (34°40'48"N, 114°24'43"W; S36, (biotite) 116 ± 3 Ma [iSOCHRON/WEST, no. 57, July 1991] 32 REFERENCES John, B. E. (1988) Structural reconstruction and zonation of a mid- Anderson, J. L. (1983) Proterozoic anorogenic granite plutonism of continental magma chamber—The felsic Chemehuevi Mountains North America: Geological Society of America Memoir 167, p. plutonic suite: Geology, v. 16, p. 613-617. 133-154. Lanphere, M. A. (1964) Geochronologic studies in the eastern Anderson, J. L., in press, Proterozoic anorogenic granites of the Mojave Desert, California: Journal of Geology, v. 72, p. 381- southwestern U.S.: Arizona Geological Society, Digest, v. 17. 399. Armstrong, S. L., 1966, K-Ar dating of plutonic and volcanic rocks Light, T. D., Pike, J. E., Howard, K. A., McDonnell, J. 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