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U-Th-Pb age and initial isotopic ratios of the coxcomb , and a K-Ar date of ilivone basalt from the Coxcomb mountains, southern California J.P. Calzia, E. DeWitt, and J.K. Nakata Isochron/West, Bulletin of Isotopic Geochronology, v. 47, pp. 3-8 Downloaded from: https://geoinfo.nmt.edu/publications/periodicals/isochronwest/home.cfml?Issue=47

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 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) & Resources and the Nevada Bureau of Mines & Geology.

All back-issue papers are available for free: https://geoinfo.nmt.edu/publications/periodicals/isochronwest This page is intentionally left blank to maintain order of facing pages. U-Th-Pb AGE AND INITIAL STRONTIUM ISOTOPIC RATIOS OF THE COXCOMB GRANODIORITE, AND A K-Ar DATE OF OLIVINE BASALT FROM THE COXCOMB MOUNTAINS, SOUTHERN CALIFORNIA

J. P. CALZIA U.S. Geological Survey, Menio Park, CA ED DeWITT U.S. Geological Survey, Denver, CO J. K. NAKATA U.S. Geological Survey, MenIo Park, CA

The Coxcomb Mountains are located in the southern biotite-hornblende granodiorite and muscovite from the Mojave Desert approximately 240 km east of Los Angeles, biotite-muscovite monzogranite yield K-Ar dates of 70.8 California (fig. 1). Most of the range is underlain by the and 68.8 m.y., respectively (Armstrong and Suppe, Coxcomb Granodiorite of Miller (1944). The granodiorite 1973). Biotite from the monzogranite yields a younger intruded the McCoy Mountains Formation of Harding and date of 54.9 m.y., but this biotite contained only 6.36% Coney (1985) and is overlain by Tertiary fanglomerate K2O, a low value compared to other biotite from the deposits that contain olivine basalt flows. This report sum Coxcomb Granodiorite (table 1). The biotite may have marizes previous K-Ar and Rb-Sr data from the grano been chloritized or contaminated by other minerals that diorite, and reports new U-Th-Pb and K-Ar dates from the contain very little potassium, and hence the date from the granodiorite and olivine basalt, respectively. The U-Th-Pb biotite may be artificially young. Other biotite from the data confirm the Late Cretaceous age of the granodiorite as monzogranite do not have reduced dates, thereby in previously determined by the K-Ar data and constrain the dicating that the cause of the 54.9 m.y. date is related to a minimum age of the McCoy Mountains Formation. The relatively local phenomenon or a poor mineral separate not new K-Ar date helps to define the minimum age of faulting a thermal event younger than about 65 m.y. as proposed in the southern Coxcomb Mountains. by Miller and Morton (1 980) for areas to the west of the Calzia (1982a) divided the Coxcomb Granodiorite into Coxcomb Mountains. four cognetic intrusive facies. Three of these intrusive Calzia (1 982b) reports a Late Jurassic age of 145 rn v facies are (from oldest to youngest) biotite-hornblende for the Coxcomb Granodiorite based on strontium isotooic granodiorite, porphyritic biotite granodiorite and monzo- data. The strontium isotopic data, listed in table 2 for the , and biotite-muscovite monzogranite. The biotite- first time, are scattered about the 70 m.y. referenc hornblende granodiorite intruded the McCoy Mountains isochrons and show little variation in the ®'Rb/®®Sr ratio Formation near the southern end of the range. A grada- (fig. 2). New U-Pb data, combined with the above K Ar tional contact between the biotite-hornblende granodiorite dates, suggest that the Late Jurassic age is erroneous. and the porphyritic biotite granodiorite and monzogranite, and a smooth continuum of major element geochemistry ANALYTICAL PROCEDURES between these two intrusive facies and the biotite- muscovite monzogranite suggest that these three facies U-Th-Pb. and sphene from the biotite-hornblenrio are related through continuous magmatic differentiation granodiorite were concentrated by separatory table heevf by fractionation of hornblende and (Calzia, liquid, magnetic, and hand-picking procedures. Dissolution 1982a,b). and chemical preparation of the samples was sl,nh?i The biotite-muscovite monzogranite intruded a fourth modified from Krogh (1973). Uranium, thorium aS lean facies of porphyritic biotite granodiorite. Chemical data concentrations were determined by standard -dih. suggest that the fourth intrusive facies crystallized from a tion techniques, using "®U, "^Th, and ®o®Pb spikeri a ^ more primitive and may be older than the other concentrates were dissolved by phosphoric acid load!^ intrusive facies. Intrusive relationships that would sub onto single filaments, and coated with silica gel Uranf stantiate this suggestion are absent. and thorium concentrates were dissolved by nitric arid L Previously determined K-Ar dates (table 1) from the loaded onto tdple fllameota fo, mesa 2?, Coxcomb Mountains suggest that the Coxcomb Grano isotope ratios were determined on a 30.5 cm riin^+i 1 diorite cooled in Late Cretaceous time. Biotite from the solid-source mass spectrometer at the U.S. Geofogfcal

TABLE 1. K-Ar data for the Coxcomb granodiorite and olivine basalt. Coxcomb Mountains, California.

Location Rock Mineral %K20 *°Arraci Sample Apparent N latitude, description dated (ave.) (10-^Omoles/gm) no. W longitude age (m.y.) Reference

biotite-muscovite biotite 8.99 024 SA^OO'Sl" 8.59 85 65.1 ±2.0 iiB^srai" monzogranite (1980) biotite 69-68 34°05'33" biotite-muscovite 6.36 5.10 73 54.9 ±1.5^ 115°24'28'' monzogranite Armstrong and Suppe (1973) muscovite 9.92 10.01 78 68.8 ± 1.0'

69-67 33°55'32'' biotite-hornblende biotite 8.10 8.42 79 115°18'07'' granodiorite 70.8 ±1,0' Armstrong and Suppe (1973) 015 33°55'53'' olivine basalt whole rock 1.20 0.08 86 115°23'55'' 4.5±0.29 This report

^Data corrected for decay constants listed in Steiger and Jager (1977).

[ISOCHRON/WEST, no. 47, December 1986] 115°30'

EXPLANATION

Qai I Alluvial deposits(Quaternary) Tb I Olivine basalt(Tertiary) I Fanglomerate deposits (Tertiary) Km 1 Biotite-muscovite monzogranite J (Cretaceous) Kgtn I Poiphyritic biotite granodiorite and 1 monzogranite (Cretaceous) I Biotite-hornblende granodiorite (Cretaceous) I P°fPhyritic biotite granodiorite (Cretaceous)3 4 "00 Kjan,| grariodiorite and monzogranite I 1 "Units^ln!t Kgm and® j Jg,monzogranite undivided (Jurassic) Monzogranite (Jurassic)

— Contact Gradational contact Fault, dotted where concealed ,69 67 Sample location and number

5 KILOMETERS

CALIF

AREA OF AAAP figure 1. GaneralKed geologic map of the Coxcomb Mountains, California (modified from Calzia and others, 1983). Survey, Denver, ColoraHn 1 1 208pb/206pb ratios are n r?R "'Pb/"8Pb and malized to 0.1194, and the 8'Sr/8®Sr ratios were further for the U-Th-Pb dates 10^? adjusted to an Eimer and Amend SrCOa value of 0.7080. year"'; _ q o.q' * H* ~ 1-55125 x 10"'® Uncertainties in the and strontium concentrations and the ®'Sr/®®Sr ratios are about ±1.0 and 0.008%, respectively. X(®^Rb) = 1.42 x 10"^^ year-\ K-Ar. A whole rock sample of olivine basalt was sized to less than 32 and greater than 64 mesh in Menio Park, Tor""' California. Potassium concentration was determined by energy dispersive x-ray fiuresc^nce'.'StrtS"^^ flame photometry with a internal standard. Argon ratios were determined on a 30.5 cm radius, 90®-sector concentration was determined by standard isotope dilution mass speedometer in Menio Park, California, using a triple and mass spectrometry techniques described by Dalrymple rhenium filament mode of ionization, automatic peak and Lanphere (1969). Constants used are: + >4 = switching, and digital output. The ®®Sr/®8Sr ratios were nor 0.581 X 10'^° year"\ = 4.92 x 10"^° year"\ and ^°K/K (total) = 1.167 x lO'^mol/moi.

[ISOCHRON/WEST, no. 47, December 1986] TABLE 2. Rb-Sr data for the Coxcomb granodiorite. Coxcomb Mountains, California (data from R. W. Kistfer, 1981, written communication)

Location Sample Rock N latitude, Rb (ppm) Sr (ppm) 87Rb/«®Sr 8^Sr/8®Sr no. description W longitude

03 33°53'22'' biotite-hornblende 62.9 805 0.226 0.70941 115oi9'56" granodiorite

16 33°69'33" porphyritic granodiorite 126 515 0.708 0.71050 115°24'35" and monzogranite

024 34O06'5r biotite-muscovite 98.8 352 0.812 0.71 171 115°3V3r' monzogranite

GR175 34°05'40" biotite-muscovite 150 323 1.34 0.71 167 115°24'35'' monzogranite

U-Th-Pb DATA zoic radiogenic lead and/or inherited zircon within the Late Cretaceous zircon. The amount of this inherited compo Four nonmagnetic size fractions of zircon from the nent increases as the size of the zircon increases, indicating biotite-hornblende granodiorite have greatly discordant that the coarse zircon contains a greater percentage of in U-Pb and Th-Pb dates that range from 122 to 744 m.y. herited material (Proterozoic seed crystals?) than the finer (table 3). Pb-Pb dates are much older and range from 1119 zircon, and/or that the seed crystals in the coarser zircon to 1485 m.y. The coarsest zircon yields the oldest U-Pb have lost less of their radiogenic lead to the Late date and acicular-shaped, finest-grained zircon yields the Cretaceous magma than have the smaller seed crystals. youngest date. Th-Pb dates also decrease with decreasing The actual cause of the discordance and the significance of grain size. The U-Pb data define a very linear discordia that the upper intercept date are being investigated by the has a lower intercept with the concordia curve of 70 ± 1 authors. m.y. (fig. 3). We interpret the 70 ± 1 m.y. date to be the Sphene from the biotite-hornblende granodiorite has con time of crystallization of Late Cretaceous zircon and the cordant U-Pb and Th-Pb dates of 75 m.y., but a sliahtlJ emplacement age of the granodiorite. discordant Pb-Pb date of 84 m.y. Common lead correction^ The progressively older U-Pb and Th-Pb dates for applied to the sphene dates (table 3) are the same as tho<5^ coarser-grained zircon indicates a component of Protero- used for the zircon (i.e. model lead isotopic ratios for 70

0.7120 -1 isocWfll. te^ete^ tAa 70 024 GR175

0.7110 - r.i 0.7109

won. CO ence \s^ (O VAa telSl 0.7100 70

00 03

r.= 0.7092 0.7090 -

0.7080

"Rb/»«8r FIOURE 2. Hot of Bb-Sr Isotopio tiato aoti oalcol.l.tl MtM ■•StfS, ..tloa Cn lor U» Coxoomb Granotliortt. of Ml.,, |, 944). (ISOCHRON/WEST, no. 47. December 1986] m.y. lower crust and mantle). Ideally, lead isotope ratios from co-existing potassium feldspar should be used for this correction. Also, the uncertainty in the age calculation for young sphene containing only 38 ppm uranium is about ± coin CDcs CM CO o CM ®0 cncM dcM CMoi CMCO T— (NJ CN CN in 3-4 m.y., greater than the uncertainty in the lower inter COCO CMOl cept for the zircon data. Because the sphene has a slightly discordant Pb-Pb date of 84 m.y., we believe that in heritance of Proterozoic radiogenic lead, as discussed oQ.| 1 r-in• o>'5t• • inco• o above, is partially responsible for U-Pb and Th-Pb dates I SS ^ o> ^ which are slightly older than the lower intercept age of 70 c ^ ' 22 nil CMCM «-»- « *5 ± 1 m.y. defined by zircon. c 3 O Rb-Sr DATA S I A p in o> ^ 9 9 E ^ I ^^ 6 r-: 9 A plot of the strontium isotopic data from the three in o CO en CO en CM CM CM CM CO CO o rv in in trusive facies indicates a substantial variation in initial X CO CO CM CM o strontium isotopic ratios. Calculated initial ratios, based on U the 70 m.y. age from the U-Pb and K-Ar data, vary from •c 0.7092 to 0.7109 (fig. 2). This variation may be caused o "^.00 CM in ^ by three different but nearly synchronous with dif 99 . c4<>i 00 Oi rt o CM CM ^ 3; *- in in c in in w CO CM CM T- ,- ferent initial strontium isotopic ratios, or by contamination S o of a granodiorite magma by Proterozoic material. Field, CO o •O chemical, and the U-Th-Pb data suggests that the latter is c CO p s o p cn in CO the more likely case. in CM CO 2 O) c GO CM O) CM CM CM c " 0 ^ K-Ar DATA

1E 5-g in 5 o CO CO 8 2 divine basalt interbedded with fanglomerate deposits CO o &- I I o> z 2 along the southwest side of the Coxcomb Mountains CM (fig. 1) yields a whole-rock K-Ar date of 4.5 ± 0.29 m.y.

(O (table 1). The basalt and fanglomerate deposits dip 25°- CO CO in 40° southwest and are adjacent to a northwest-trending <§ I M 00 oi p O CO o I i (O CM § fault that cuts the McCoy Mountains Formation and the r" CO CO CO ^ CO CO CO Coxcomb Granodiorite. Outcrops of the McCoy Mountains in Formation are unknown west of this fault. The fault, part of ■o c the Sheep Hole fault zone of Hope (1966), is overlain by CD c alluvial deposits that include an isolated outcrop of white, o o subhorizontal ash that correlates with the Bishop ash bed E a CD (A. M. Sarna-Wojoicki, 1982, written commun.; Merriam a o CD CO and Bischoff, 1 97 5). Sanidine from the Bishop ash yields a •d- K-Ar date of 0.75 m.y. (Dalrymple and others, 1 965). The c 9 o in field and K-Ar data suggest: (1) the Sheep Hole fault zone <0 3 o is the western limit of known outcrops of the McCoy o 00 o o Mountains Formation; and (2) this fault was active in the 3 C7> o CO last 4.5 m.y. and is older than the Bishop ash. o a> CO E •o a T- m DISCUSSION c S I ^ 9 CO II o CO2 00CO 'O* It is apparent from the U-Th-Pb and the Rb-Sr data that the Coxcomb Granodiorite either has been extensively con o taminated by Proterozoic lead and strontium, or that the O 2 CM in CO CM £ + + magma which crystallized to form the granodiorite actually c + O o O J2 CD Z. o was created from isotopically inhomogeneous Proterozoic in O CD CM CM crust. The U-Pb age calculations assumed that common £ I I ^ lead incorporated into the magma during its formation had an isotopic composition of model upper mantle or lower D c= 3 w .O N >. 0 O E crystal material at 70 m.y. If the magma actually was CO .1 "S § created from a 1760 m.y. protolith (the approximate age OQ C © T3 CO CM 6 3 of the nearest dated Proterozoic crystalline rocks) that con O 73 3 < •c 3 .t: r w 1 S O) © O) CO hs tained zircon —and that zircon did not lose all of its « C O 5 O in r- O o radiogenic lead to the magma — a much less radiogenic CO in CO component of common lead could have been incorporated into the magma. The second set of dates in table 3 indicate that even if a . E o 1760 m.y. material had melted to create the parent E s magma, the U-Pb dates would be at most 0.5% older. The O Q O • resulting lower intercept of the discordia derived from such U-Pb dates would be 69 ± 1 m.y., indistinguishable from the 70 ± 1 m.y. date calculated above. Even if 1 760 m.y.

[ISOCHRON/WEST, no. 47, December 1986] 0.12 -

0.10 -

0.08 -

oo 00 CM., JQ Q. CO g 0.06

0.04 -

0.02 -

0.8

FIGURE 3. Concordia diagram for zircon from the biotite-hornbiende granodiorite (spi. no. 79-1). Concordia intercepts at 70 ± 1 and 1580 ± 4 m.y.

zircon from such a lower crustal source had completely SAMPLE DESCRIPTIONS melted (which they have not, as witnessed by the pattern of discordancy) and liberated all their radiogenic lead, the "^9-1 U-Pb lower intercept with concordia would be unaffected, as the Biotite granodiorite (33° 53'1 3" N,11 5° 1 7'52" w discordia defined by the data is essentially a mixing line Cox^comb Mountains 15' quad., CA). Light arav' between 70 m.y. and Proterozoic zircon. IT grained, generally massive (al' The U-Th-Pb data confirm the 70 m.y. age of the Cox thougli Miller, 1 944, describes a local poorly-devel comb Granodiorite originally suggested by K-Ar dates, and oped foliation). Mode (in volume percent)- 20 "^o/ preclude the 145 m.y. date reported by Calzia (1982b) ; 18.7% perthitic potassium feldspar witMnclJ based on Rb-Sr data. The granodiorite apparently was em- sions of quartz and plagioclase* 48 8% • placed and cooled very quickly, as K-Ar biotite dates from ^"3,-.,): 11.7% tnclulg'HS fresh material (indicative of the time when the pluton cooled below 225"'C) are identical to emplacement ages Collected by: Ed DeWitt. 3- for the body. The undeformed granodiorite discordantly in truded the folded and foliated McCoy Mountains Forma 2. 03 tion, thereby indicating that this formation must be older Biotite-hornblende granodiorite (33° 53'o o" than 70 m.y. 1 1 5° 1 9-56" W;Coxcomb Mountains 15'quad , CA^'

[ISOCHRON/WEST, no. 47, December 1986] Gray, medium grained. Mode: 18.4% quartz, 1 7.4% REFERENCES perthitic microcline with inclusions of quartz and Armstrong, R. L., and Suppe, John (1973) Potassium-argon plagioclase, 50.4% andesine (An35_43), 12.3% geochronometry of Mesozoic igneous rocks in Nevada, Utah, minerals including subhedral biotite with inclusions of and southern California: Geological Society America Bulletin, sphene and an unidentified partially resorbed mineral V. 84, p. 1375-1392. with amphibole habit, and 1.5% accessory minerals Calzia, J. P. (1982a) Geology of granodiorite in the Coxcomb including zircon, resorbed sphene, magnetite, and Mountains, southeastern California: Mesozoic-Cenozoic tec apatite. Analytical data:see table 2. Collected by: P. tonic evolution of the Colorado River region, California, Calzia. Arizona, and Nevada, Cordilleran Publishers, San Diego, p. 173-180. (1982b) Petrology and magmatic history of the Rb-Sr Coxcomb granodiorite. Coxcomb Mountains, southeastern Porphyritic monzogranite {33° 59'33" N, California: Geological Society of America Abstracts with Pro 11 5®24'35" W; Coxcomb Mountains 1 5' quad., CA). gram, V. 1 4, p. 1 53. Gray, medium grained; magmatic foliation expressed Calzia, J. P., and Morton, J. L. (1980) Compilation of isotopic y alignment of biotite and by compositional layering ages within the Needles 1 o x 2° quadrangle, California and 0 biotite, quartz, and microcline. Euhedral microcline Arizona: U.S. Geological Survey open-file report 80-1303. p enocrysts include zones of plagioclase inclusions Calzia, J. P., Kilburn, J. E., Simpson, R. W., Jr., Allen, C. M., Leszcykowski, A. M., and Causey, J. D. (1983) Mineral re 97^0? ®dge of the phenocryst. Mode: source potential map of the Coxcomb Mountains Wilderness 23.6% microcline (in groundmass), Study Area (CDCA-328), San Bernardino and Riverside Coun andesine (An3o_34) with more calcic ties, California: U.S. Geological Survey Miscellaneous Field trace amounts of zircon, Studies Map MF-1603-A. apatite. Analytical data: see table 2. Dalrymple, G. B., Cox, Allan, and Doell, R. R. (1965) Potassium- Collected by: Calzia. argon age and paleomagnetism of the Bishop Tuff, California: Geological Society America Bulletin, v. 76, p. 665 — 674. 4. 024 Dalrymple, G. B., and Lanphere, M. A. (1969) Potassium-argon dating: San Francisco, W. H. Freeman and Co., 258 p. monzogranite (34°06'51"N, Harding, L. E., and Coney, P. J. (1 985) The geology of the McCoy to rnedinm '-3'^® 15' quad., CA). Tan, fine Mountains Formation, southeastern California and southwest ouartz ®qo'gfanular. Mode: 29.6% ern Arizona: Geological Society American Bulletin, v. 96, (An ' \ '•.u' "^'^''ocline, 41.3% zoned andesine p. 755-769. minerals 1° sericite, 2.5% mafic Hope, R. A. (1 966) Geology and structural setting of the eastern Transverse Ranges, southern California: University of Califor muscovite and fine-grained euhedral nia, Los Angeles, Ph.D. thesis, 1 58 p. 2. Collected byfj p, cLI^ia ®®® 1®'''® Krogh, T. E. (1973) A low-contamination method for hydro- thermal decomposition of zircon and extraction of U and Pb for 5. GR 175 isotopic age determinations: Geochimica et Cosmochimica Acta, V. 37, p. 485-494. 1i°502^3l®^'1® monzogranite (34°05'40"N, Lee, D. E. (1984) Analytical data for a suite of granitoid rocks not available, see Let n^984Wo^' from the Basin and Range province: U.S. Geological Survey ment a&nrhcx^- * 984) for major and trace ele- Bulletin 1 602, 54 p. Merriam, Richard, and Bischoff, J. L (1 975) Bishop Ash: A wide tecTe/Co e! 2- Co/- spread volcanic ash extended to southern California: Journal Sedimentary Petrology, v. 45, p. 207-21 1. 6. 015 Miller, F. K., and Morton, D. M. (1980) Potassium-argon geo- K-Ar chronology of the eastern Transverse Ranges and southern Mojave Desert, southern California: U.S. Geological Survey comb®Moumain?T5"q"d cl)° ^ Professional Paper 1 1 52, 30 p. Miller, W. J. (1944) Geology of the Palm Springs-Blythe strip. Sample collected iutt°"r k mudstone interbeds."p- Riverside County, California: California Journal of Mines and Geology, v. 40, p. 1 1-72. Analytical data-sees®e tabletahi 1. Collected'°wermost by: J. P.interbed. Calzia. Steiger, R. H., and Jager, E. (1977) Subcommission on geo- (whole rock) 4.5 ± 0.29 m.y. chronology: convention and use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, V. 36, p. 359-362.

IISOCHRON/WEST, no. 47, December 1986]