Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California

RICHARD LEE ARMSTRONG Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 JOHN SUPPE Department of Geology, University of California, Los Angeles, California 90024

ABSTRACT dates in a belt running through southern Arizona into southeastern California at a high K-Ar dates for 123 mineral separates from angle to the edge of the continent. Only very 91 samples of granitic rocks, and 3 samples of sparse dates of this- age are found in Nevada. Orocopia and Vitrefax Schist from Nevada, Abundant Laramide dates are not encountered western Utah, the Mojave Desert, the Colorado again until the Idaho, Boulder, and Coast Desert, and the northern and eastern Penin- Range batholiths. These important changes in sular Ranges of southern California, range in Cordilleran tectonics may be effects of a age from 218 to 19 m.y.; however, most of the change from dominantly dip-slip subduction dates lie between 160 and 50 m.y. A majority during Franciscan time to dominantly north- of the hornblende and muscovite dates are northeast strike-slip subduction during Lara- nearly concordant with, but generally a few mide time. Furthermore, the initiation of rift- m.y. older than, coexisting biotite dates; some ing of eastern North America in Triassic time, are highly discordant. changes in spreading rate, and reorientation of The oldest plutons gave Late Triassic to spreading directions in the Atlantic Ocean Early Jurassic dates (El Paso and San Ber- appear to correlate with the beginning of wide- nardino Mountains, 218 to 194 m.y.). Jurassic spread pluton emplacement in western North igneous activity reached a culmination slightly America, changes in the rate of magma genera- before 150 m.y. ago over a large area in north- tion in the Cordillera, and shifts of the Cordil- ern Nevada and western Utah and supplied ash leran tectonic pattern, respectively, during to the Morrison Formation of the Colorado Mesozoic time. Plateau. Several plutons of the northeastern Mojave Desert yielded similar dates. INTRODUCTION A similar major culmination of Mesozoic The idea that sea-floor spreading and plate pluton emplacement in early and middle Late tectonics can provide a model for the Mesozoic Cretaceous time is represented by scattered evolution of the western United States, and plutons in Nevada and probably by most of particularly California, has been taken up the plutons in the Peninsular Ranges and enthusiastically by many geologists, including southern deserts of California. Earliest Ter- Hamilton (1969a, 1969b) and Ernst (1970). tiary dates were obtained from far-southeastern The strongest aspect of this model lies in the California (50 to 66 m.y.). close petrologic similarity between the Meso- Most dates in the Cordilleran batholith belt zoic granitic plutons and present-day arc-type of the Southwest, which parallels the edge of magmas, together with the close correspon- the late Mesozoic continent, are between 160 dence of arc-magma chemistry with depth to and 75 m.y. and are essentially coeval with the Benioflf zone (see, for example, Hamilton, metamorphism in the parallel high-pressure, 1969b; Dickinson, 1970); these magmas seem low-temperature Franciscan belt to the west to be derived, at least in part, from the de- (approximately 150 to 75 m.y.). About 75 scending, subducted oceanic plate. Thus it m.y. ago, this pattern abruptly changed to a would appear that the time-space distribution Laramide one with abundant 75 to 50 m.y. of arc-type magmatism in the western United

Geological Society of America Bulletin, v. 84, p. 1375-1392, 4 figs., April 1973

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States would record the times and p.aces at cambrian time up i:d the present; it is here also which subduction was proceeding al< ng the that the Cordillera narrows to less than 600 km west edge of the American plate and : lie re by as opposed to 900 to 1,100 km to the north and provide a framework for understand: ng the south. Previously we had dated Mesozoic development of the Cordilleran mounl.iin belt igneous rocks in Nevada, Utah, and on the in general. Colorado Plateau in Arizona and Colorado We chose to contribute to the available geo- (Armstrong, 1966b, 1969, 1970b; Speed and chronometric data by reconnaissance of a Armstrong, 1971), and we had available a particularly important yet little-studi;d por- number of unpublished dates for Mesozoic tion of the Cordillera, the southern CE iforria plutons in Nevada and western Utah which we segment, which has been a distinctive region include in this report. Finally, Evernden and of intersecting orogenic trends from late Pre- Kistler (1970), in a paper filled with data,

Figure 1. Generalized basement geologic map of Index map gives YAG-prelix sample localities. Small southeastern California showing SC-69- prefix sample open circles show locations of previously published localities (adapted from the Geologic Map of Cali- Mesozoic K-Ar dates done ¡it Yale, fornia, Olaf P. Jenkins Edition, scale, 1:250,000).

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discussion, and interpretation, set the stage for fraction. The variations in date with changing a study such as ours by documenting the geo- K content are in opposite senses, and not sig- chronology of the Sierra Nevada region. With- nificant, but there is a consistent suggestion of out their contribution as well as those of other better Ar retention by the coarse-grained frac- workers in the southwest, particularly southern tion; this is as might be expected where diffu- Arizona, our paper would lack much of its sion loss of Ar is grain-size dependent. impact, and a regional summary would be The K-Ar dates we report for the West much more elusive. Riverside and Lakeview mountains plutons in southern California and the Ely area pluton in ANALYTICAL TECHNIQUES Nevada are in excellent agreement with other The laboratory techniques used in this study published dates for those bodies (Bishop, 1963; are the same as those described by Armstrong Morton, 1969; Evernden and Kistler, 1970; (1970a). Isotope dilution was used for Ar McDowell and Kulp, 1967). Discordant results analysis, atomic absorption spectrophotometry for Toana Pass, Cortez, and Snake Range for K analysis (except for a few samples from plutons in eastern Nevada appear to be related the eastern Great Basin that were done by to postemplacement alteration, Mesozoic re- flame photometry; Armstrong, 1966b). Ana- gional metamorphism, or heating by younger lytical precision of the dates reported is 2 per- igneous activity. cent (cr). All K analyses and many of the Ar The data of Evernden and Kistler (1970) have been run in duplicate. In four cases (SC- and discussion of errors in K-Ar dating by 69-37, -60, -89, and -163) where two biotite Engels and Ingamells (1970) emphasize the separates from the same rock were dated, the now well-documented effects of sample inhomo- dates agreed within 3 percent or better, indi- geneity on analytical precision. We have used cating that the above estimate for over-all pure separates of a narrow range in grain size precision is reasonable. The constants used for (usually 40 to 60 mesh per in.) to minimize calculation of dates are KAp = 4.72 X 10~10, this potential hazard. A few mineral separates, 10 40 4 KAe = 0.584 X 10- , and K = 1.19 X 10~ especially those from eastern Great Basin atom abundance. Sample descriptions, geo- plutons prepared before 1965, are of poorer graphic data, analytical results, and comments quality; these are noted on Table 1. Dates on are provided in Table l,1 and sample localities mineral mixtures from rocks with discordant are shown on Figure 1. minerals will lie between end member dates and are useful only as indications of the mini- CONCORDANCE, DISCORDANCE, mum age of the rock dated. Significant horn- AND SIGNIFICANCE blende-biotite discordance may be masked if the hornblende separates are even moderately Evernden and Kistler (1970) discussed as- contaminated by biotite. pects of the problem of K-Ar dating of plutonic rocks in considerable detail. They demon- For many specimens, we have dated biotite strated that a slight increase in K-Ar date may and hornblende or biotite and muscovite sepa- occur with decreasing K content in biotite rates to test internal concordance within speci- (which is associated with increased included mens. Approximately 66 percent of the mineral chlorite) and confirmed the observation of pairs agree within 5 percent and can be con- Evernden and others (1964) that K and Ar sidered concordant (this is about the same as are generally covariant in different size frac- the criterion of <5-m.y. difference used by tions of mica. Our samples SC-69-60 and SC- Evernden and Kistler to identify concordant 69-163 strikingly demonstrated the covariance: dates). Muscovite dates are consistently slightly for SC-69-60 K was 9 percent lower, the date 3 older than coexisting biotite. In only a few percent higher, in the coarse-grained fraction, cases are they distinctly greater (SC-69-7, -68, for SC-69-163 K was 13 percent higher, the and -163). Hornblende dates are similar to, or date 1 percent higher, in the coarse-grained distinctly greater than, the dates for coexisting biotite.

1 The key to concordance is rapid cooling For a complete version of Table 1, order NAPS no. 02036 from Microfiche Publications, Div. of Microfiche through a temperature range of very approxi- Systems Corp., 305 East 46th Street, New York, New mately 400 to 200 degrees. Concordance has York 10017. Enclose $5.00 for photocopies or $1.50 for been generally interpreted as evidence of em- microfiche. Make checks payable to Microfiche Publica- placement age or the result of drastic heating tions. by a later, adjacent pluton (for example,

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TABLE 1. K-AR DATES

Date, m.y. Date, m.y. Sample Mineral dated* and error (a) Sample Mineral dated* and error (or)

CALIFORNIA CALIFORNIA SC-69-lb B 112 ±2 SC-69-66 B 62.3 ± 1.2 SC-69-7 B 64.8 ± 1.3 M 65.3 ± .9 M 70.9 ± 1.4 SC-69-67 B 69.0 ± 1.0 SC-69-8 B 67.9 ± 1.4 SC-69-68 B 53.5 ± 1.5 SC-69-9 B 69.4 ± 1.0 M 67.1 ± 1.0 SC-69-10 B 71.1 ± 1.0 SC-69-69 B 66.9 ± .9 B-H Mixture 107 ± 1.5 H 66.9 ± .9 SC-69-11 H-B Mixture 91.0 ± 1.5 SC-69-70 B 141 ±3 SC-69-12 B 145 ±2 SC-69-71 B 154 ±2 SC-69-13 B 71.0 ± 1.0 SC-69-73 B 68.7 ± 1.4 SC-69-14 B 71.2 ± 1.4 SC-69-75 B 70.2 ± 1.0 SC-69-16 B 70.1 ± 1.0 SC-69-77 B 148 ±2 SC-69-17 H-B Mixture 82.0 ± 1.2 SC-69-78 B 71.8 ± 1.4 SC-69-18 B 72.1 ± 1.4 B, second spec .Tien 69.0 ± 1.4 SC-69-19 B 70.9 ± 1.4 SC-69-79 B 135 ±2 SC-69-20 B 68.5 ± 1.0 SC-69-80 B 136 ±2 H 77.7 ± 1.0 SC-69-81 B 18.9 ±.3 SC-69-21 B 72.0 ± 1.0 SC-69-87 B 142 ±2 SC-69-22 B 81.3 ± 1.2 SC-69-89 B, 203 ±4 H 194 ±3 First separa:; SC-69-23 B 78.3 ± 1.2 B, 203 ±4 SC-69-24 B 72.3 ± 1.4 Second separate SC-69-26 B 68.9 ± 1.4 H 218 ±3 SC-69-27 B 77.1 ± 1.1 SC-69-153 B 61.2 ±.9 SC-69-28 B 90.5 ±2 SC-69-154 B 74.7 ± 1.5 H 89.3 ± 1.8 SC-69-155 B 76.9 ± 1.5 SC-69-30 B 88.1 ± 1.3 H 88.1 ± 1.4 H 94.7 ± 1.5 SC-69-156 B 75.4 ± 1.5 SC-69-33 M 86.6 ± 1.3 SC-69-157 B 78.5 ± 1.6 SC-69-36 B 92.4 ± 1.8 SC-69-158 B 75.5 ± 1.1 H 91.6 ± 1.8 SC-69-159 B 70.9 ± 1.0 SC-69-37 B 90.2 ± 1.8 H 71.0 ±2 B, less pure 90.4 ± 1.8 SC-69-161 B 88.8 ± 1.3 SC-69-39 B 87.4 ± 1.7 SC-69-162 B 71.4 ± 1.0 H 89.4 ± 1.8 SC-69-163 B, 40-60 mesh 74.8 ± 1.5

* B = Biotite M = Muscovite H = L ornlilcnde

Evernden and Kistler, 1970, p. 9-16). We con- may occur. Rocks that hacl been previously sider it quite possible that concordant biotite- maintained above 400 to 500 degrees may yield hornblende and biotite-muscovite dates may concordant dates reflecting this change in have an even greater variety of significances tectonism. within the Mesozoic Cordilleran batholith belt, 3. Moderately rapid denudation (approxi- all ol which may be important to understand- mately 0.3 km/m.y. or greater) may produce ing the tectonic development of this region: concordance in rocks that had been maintained 1. Shallow plutons may cool rapidly upon above 400 to 500 degrees prior to the onset of emplacement and yield a close lower limit to uplift and denudation (Armstrong, 1966a). the time of crystallization, or plutons may yield 4. Heating and recrystallization as a conse- similar dates to adjacent younger plutons which quence of cataclastic deformation may give have heated them sufficiently to degas biotite, rise to concordant dates. muscovite, and hornblende. The few whole-rock strontium isotope iso- 2. The high regional heat flow of arc-type chrons available for Sierra N :vada plutons are magmatic provinces is a result of mass transfer in general agreement with the ages inferred for of heat at depth (Horai and Uyeda, 1969). the same plutons on the basis of the oldest Once this style of tectonism ceases, the high concordant biotite-hornblende pairs or oldest geothermal gradient and heat flow is unsup- hornblende dates. On the oi:her hand, several ported by conduction. Rapid regional cooling isotopic uranium-lead dates reported for south-

TABLE 1. (Continued)

Date, m.y. Date, m.y. Sample Mineral dated* and error (cr) Sample Mineral dated* and error (

CALIFORNIA CALIFORNIA SC-69-40 B 87.0 ± 1.3 B, 60-100 mesh 74.0 ± 1.5 H 84.0 ± 1.7 M 90.8 ± 1.3 SC-69-41 B 79.2 ± 1.1 YAG 757 M 58.2 ±.9 H 82.7 ± 1.2 YAG 758 B (6% chlorite) 137 ±3 SC-69-42 B 83.0 ± 1.7 YAG 760 B (6% chlorite, 145 ±3 SC-69-43 B 81.7 ± 1.2 5% hornblende) H 89.2 ± 1.8 YAG 762 B (5% chlorite) 81.6 ± 1.6 B, 2nd specimen 87.7 ± 1.4 NEVADA SC-69-44 B 75.3 ± 1.1 YAG 767 B 127 ±3 SC-69-46 B 68.3 ± 1.0 YAG 86 B-H 149 ±3 SC-69-47 B 63.6 ± 1.0 YAG 85 B-M 153 ±3 H 66.4 ± 1.0 (5% chlorite) SC-69-48 B 71.2 ± 1.0 YAG 133 B 151 ±3 H 73.5 ± 1.5 (7% adhering rock) SC-69-49 B 24.3 ±.5 YAG 131 H 134 ±3 M 29.9 ±.6 (5% chlorite, 5% SC-69-51 H 36.8 ± 1.2 rock) M 50.5 ±.8 YAG 127 H (3% chlorite, 125 ±3 SC-69-52 M 58.0 ± .8 5% rock) YAG 126 H (2% chlorite, 141 ±3 SC-69-53 B 62.6 ± 1.3 5% rock) H 61.2 ± 1.2 YAG 38 H (4% biotite, 106 ±2 SC-69-55 B (55a) 54.1 ±.8 3% rock) B (55b) 21.5 ±.5 YAG 181 M 106 ±2 SC-69-56 B 52.9 ±.8 YAG 184 M (5% biotite, 66.2 ± 1.4 SC-69-57 B 56.3 ±.8 7% chlorite) SC-69-59 B 63.4 ± 1.3 YAG 190M M-B 111 ±2 SC-69-60 B, 40-60 mesh 64.3 ± 1.3 (5% chlorite, B, 60-100 mesh 62.5 ± 1.3 5% rock) SC-69-62 B 92.8 ± 1.9 YAG 186 M 82.4 ± 1.0 SC-69-63 B 95.6 ± 1.9 YAG 187 M (5% rock) 81.6 ± 1.6 SC-69-64 B 93.6 ± 1.9 UTAH H 88.3 ± 1.9 YAG 88 B-H 140 ±3 SC-69-65 B 64.0 ± 1.3 (10% chlorite) M 65.6 ± 9 YAG 178 B (7% chlorite) 143 ±3

* B = Biotite M = Muscovite H = Hornblende

era California and eastern Nevada are generally of dates (Fig. 2) and regional date histograms 10 to 20 percent greater than nearby or closely (Figs. 3 and 4) are provided to accompany the related K-Ar dates, indicating distinctly differ- discussion. ent times of system closure for these cases. No single Mesozoic pluton in the southwestern Palinspastic Map (Fig. 2B) United States has been studied by all three Available geochronometric data for the four dating techniques. Such studies, when eventu- southwestern states are plotted on two maps, ally undertaken extensively, should yield a one on a present-day geographic base (Fig. 2A), variety of information about the tectonic the other representing a reconstruction of the history of the Cordillera, not just times of region at the end of the Cretaceous (Fig. 2B), magmatism. and presumably a more appropriate representa- tion of the structural setting of the Mesozoic DISCUSSION AND REVIEW plutons. We have followed the two-stage model In the following sections, we review the for the San Andreas fault system of Suppe geochronometric and stratigraphie evidence for (1970). The Sierra-Klamath block has been times of plutonism in the southwestern United shifted and rotated to close the Basin and States, considering each province in turn, and Range province. The map was developed using thereby discuss our new dates in a regional con- a cut-up tectonic map in order to avoid vio- text. Maps showing the geographic distribution lating any geometric constraints imposed by

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"i r 150 MILLION YEARS TRIASSIC JURASSIC CRETACEOUS ITERI Figure 3. Date histograms for various latitudinal Rb-Sr dates calculated for RbXg = 1.47 X 10_1Iyr_1, segments of the southwestern United States based upon the decay constant we prefer; the original dates, com- the selected dates used to construct Figure 2 (rejection puted using RbX,J = 1.39 X 10-11yr-1, are plotted as of low discordant values, averaging of closely spaced small arrows displaced 6 percent to the left of the larger data points to reduce the monograph effect). Isotonic dark arrows. The time scale differs significantly from U-Pb and whole-rock Rb-Sr dates are indicated by light those commonly used (for example, Harland and others, and dark arrows, respectively. The named intrusive 1964) and is based especially on Folinsbee and others epochs of Evernden and Kistler (1970) are shown at the (1970), Borsi and others (:1966), Borsi and Ferrara top of the diagram. The larger dark arrows are far (1967), and Armstrong and ]iesancon (1970).

EASTERN BASIN AND RANGE PROVINCE (1966), Bateman and Wahrhaftig (1966), Bate- man and Eaton (1967), and Hamilton (1969a, 1969b). Stratigraphic evidence in the Klamath Mountains brackets the principal time of pluton emplacement as post-Middle Jurassic and pre-Early Cretaceous (Lanphere and others, 1968; Murphy and others, 1969; Jones 160 150 120 90 60 30 0 and Irwin, 1971). The maximum age of many IS Frtnehìt Cr««k RhfOlft» Klamath plutons is not well defined. 1 U8G6 2 Lomont MILLIONS OF YEARS 6.P. The youngest "pre-batholithic" strata on the western side of the Sierra Nevada are Late Figure 4. Histogram of Mesozoic and Tertiary Jurassic. Eastward the maximum age limit in- granitic pluton dates for the Basin and Range province creases to Early Jurassic in the Devils Postpile of Nevada and Utah. area and Middle Triassic in the Inyo Mountains existing structures. The final map is the most (Bateman and Wahrhaftig, 1966); however, satisfactory of several successive approxima- many of these rocks are volcaniclastic and may tions, and it should be reasonably good as there be comagmatic with older parts of the batho- is only a limited amount of freedom in such a lith. The oldest overlying strata are Late cartographic exercise. Our reconstruction is Cretaceous (Coniacian and younger) along the more conservative than a similar effort by western side. Eocene auriferous river gravels Hamilton (1969a) who advocates even more overlie the northern parts of the batholith, drastic closure of the Basin and Range, Cascade, indicating that something close to the present and Modoc Plateau regions. level of exposure was attained by early Tertiary time (Bateman and Wahrhaftig, 1966). Large portions of the map undergo little change in area. The Basin and Range has been The geochronology of the Sierra-Klamath everywhere reduced in area; the closure across region, as illustrated in Figures 2 and 3, is de- northern Nevada is approximately 180 km, rived from Lanphere and others (1968), Kistler roughly 34 percent of its original width, and and others (1965), Hurley and others (1965), decreases to about 115 km or less toward the Putnam and Alfors (1965), McKee and Nash south (similar values were estimated by (1967), McKee (1968), Ross (1969), and Evern- Hamilton and Myers, 1966, p. 529, and by den and Kistler (1970). Four whole-rock stron- ProfFett, 1971). The only region that occupies tium isochrons (Evernden and Kistler, 1970) a greater area on the reconstruction is the and numerous Pb-a dates (Larsen and others, Transverse Ranges (Fig. 1), which is an area of 1958) support the pattern displayed by the high mountains, deep basins, and related thrust K-Ar results. faults of late Cenozoic age which are a conse- The axial region of the composite batholith, quence of the bend of the San Andreas fault trending north-northwest through the High system. Sierra, contains the youngest plutons that be- The several major shear zones near the Cali- long to the Cathedral Range intrusive epoch fornia-Nevada border are treated as transform (79 to 90 m.y.) of Evernden and Kistler (1970). faults connecting zones of extension within On either side of and as inclusions in this axial different parts of the Basin and Range. The region are plutons emplaced at earlier times in Las Vegas Valley shear zone, for example, the Mesozoic. The two oldest yet dated (224, transforms extension that is concentrated south 246 m.y.) are of Triassic or perhaps older age of the shear zone in southern Clark County in the eastern Klamath Mountains. Somewhat (Anderson, 1971) to synchronous extension younger are plutons of Evernden and Kistler's north of the shear zone in western Nevada. Lee Vining epoch (210 to 195 m.y.) known from a small area in the eastern Sierra. We Sierra Nevada-Klamath Mountains Region have encountered a pluton of similar latest A well-defined Mesozoic chronology exists Triassic-earliest Jurassic age just north of the for the region north of the San Andreas and Garlock fault (SC-69-89: 203 m.y. Bi„ 218 Garlock faults and west of the California- m.y. Hbl.). Plutons of later Jurassic and Early Nevada border. Recent geologic syntheses Cretaceous age lie east and west of the axial covering this region are by Davis (1966), Irwin zone and are widespread in the Klamath Moun-

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tains. As recognized by Evernden and Kistler sedimentary rocks (Willden, 1964); in central (1970), many of the dates are actuaLy Nevada, Middle Cr;:taceous sedimentary rocks younger than crystallization times clue to overlie an intrusive-volcanic complex in the alteration and heating (in some cases, oemon- Cortez Range (Muff'er, 1964, p. 21). Many of strably related to emplacement of adjacent these Mesozoic strat ¿graphic sections contain younger plutons), but the broad pattern is volcanic rocks which may be comagmatic with well established and many of the plutons are Cordideran granitic rocks. Dated plutons in accurately dated. Utah and Nevada provide direct evidence de- As implied above, Evernden and Kistler fining the ages of structures (for example, Arm- (1970, p. 16-21) developed a concept of regu- strong and Hansen, 1966; Kistler and Willden, larly spaced intrusive epochs for the Sierra 1969) in a region where the Mesozoic and Ter- Nevada regions; each of these epochs is 10 to tiary structural history was one of continued 15 m.y. long and spans the time of emplace- and complex movements. ment of a well-known plutonic complex in the In addition to the dates we report for this central Sierra Nevada for which it is named region, Figures 2 and 3 contain data gleaned (Fig. 3). Kistler and others (1971) and Shaw from Schilling (1965), Gilluly and Masursky and others (1971) further elaborated on these (1965), Coats and others (1965), McDowell epochs and developed a general theory to (1966), Marvin (1968), Hausen and Kerr explain episodic magma generation. This epi- (1968), Lee and others (1968), McDowell and sodic concept has been accepted and extended Kulp (1967), Kistler and Willden (1969), by others (Damon, 1971; Dickinson, 1970; Marvin and others (197G), Krueger and Schil- Hamilton, 1969a), but there is little in the ling (1971), Speed and Armstrong (1971), available data to compel the conclusion that Silberman and McKee (1971), and Smith and such epochs are of any regional importance. others (1971). The epochs are simply not implicit in the data Our new data are largely for the eastern originally presented for the Sierra-Klamath Basin and Range province north of the 35 th region and are controverted by the results parallel; they reinforce the previous conclusion from surrounding regions (Silberman and Mc- (Armstrong, 1966b, 1970b) that almost all Kee, 1971; Smith and others, 1971). This is not intrusive igneous rocks in that region belong to say that there is no broad order and system to two distinct populations: Tertiary (40 to 5 to the dates on Cordilleran plutons. The imme- m.y.) and Mesozoic (160 to 75 m.y.; see Fig. diately obvious features of the chronologic 4). Only a few scattered plutons in Nevada fall data are broad culminations of magmatic within the 40- to 75-m.y. range that is of such activity close to the end of the Jurassic and in great importance in the Basin and Range the middle of Late Cretaceous time. We will province of Arizona and New Mexico (Damon treat this further in the text below. and Mauger, 1966; Damon, 1971) and, as we shall see, perhaps in southeastern California. Nevada and Utah A neat spatial pattern of dates is not evident In Nevada, the youngest widespread strati- on Figure 2A. Granitic rocks of a given age do graphic units cut by Mesozoic plutons are not always have a narrow distribution as marine Late Triassic and marine and continen- claimed by Kistler and others (1971). Plutons tal lowermost Jurassic sedimentary rocks older than 118 m.y. are scattered across the (Silberling and Wallace, 1969). Speed and northern and western two-thirds of Nevada; Jones (1969) described sedimentary and vol- two of this Late Jurassic-Early Cretaceous canic rocks as young as Middle Jurassic that group occur in Utah. Notable is the concentra- predate emplacement of plutonic rocks and tion of dates near 152 m.y. (a t; me not included deformation in the Stillwater and Clan Alpine in any of Kistler and Evetnden's epochs). ranges of west-central Nevada. Low-grade re- 1 'hese igneous centers were probably the source gional metamorphism and emplacement of gra- oi the volcanic ash found in the Upper Jurassic nitic rocks occurred there more than once Morrison Formation of the Colorado Plateau. during deposition of a Middle Jurassic through The distribution of this ash caused Imlay (in lowermost Cretaceous sequence of sedimentary McKee and others, 1956) to infer volcanoes in and volcanic rocks that contains several uncon- and neer western Utah on his paleogeographic formities (Noble, 1962, 1963). In northwestern maps for Oxfordian through early Portlandian Nevada, emplacement of plutons apparently (Lste Jurassic) times. both predates and postdates Lower Cretaceous The oldest pluton yet dated in this region

is 202 m.y. (hornblende) for granodiorite from ment rock displaced along the San Andreas the southern Cedar Mountains (Speed and fault system. Armstrong, 1971). Compton (1966) interpreted the crystalline An approximately east-trending row of mid- basement of Salinia as being a block tilted Cretaceous igneous centers (Eureka-Seligman- eastward such that higher grade rocks of more Ely-Snake Range) extends across eastern Ne- deep-seated origin are exposed along its western vada. The significance of the belt is uncertain; side. Granulite facies rocks suggest many ki- however, it, as well as the Laramide Colorado lometers of uplift and denudation during Late Mineral Belt (Armstrong, 1969), could con- Cretaceous time before the crystalline rocks ceivably be plume traces (Morgan, 1971). The were overlain by Campanian or Maestrichtian detailed study of the Ely area by McDowell (late Upper Cretaceous) sedimentary rocks (1966) and McDowell and Kulp (1967) leaves (Page, 1966). The Late Cretaceous K-Ar dates no doubt that igneous activity occurred there for Salinia are generally younger to the west at about 110 m.y.; our single date of 106 m.y. and consistent with deeper burial and pre- is in agreement. The Eureka and Seligman sumed later cooling (Compton, 1966; Evern- samples were altered, however, and may well den and Kistler, 1970). An early Late Creta- be younger than times of emplacement. The ceous (~110-m.y.) Rb-Sr isochron reported rocks of the Snake Range show a complex by Evernden and Kistler (1970) and a chemical history of igneous activity, regional metamor- Th-Pb age of 106 m.y. for monazite (Hutton, phism, and cataclastic deformation (Misch, 1959) suggest that the 70 to 92 m.y. K-Ar 1960; Misch and Hazzard, 1962; Nelson, 1969). dates reflect cooling, presumably through rapid Alteration and metamorphism are evident in denudation prior to the onlap of the latest Cre- the younger samples on which we report, those taceous sediments. The true age of the granitic less than 100 m.y. old being distinctly gneissic. rocks is probably similar to the age of the As a result of intensive studies in the Snake western part of the Peninsular Ranges and Range, Lee and others (1970) have concluded Mojave Desert against which Salinia apparently that the reduced dates there are the result of originally lay (Wiebe, 1970; Suppe, 1970). Tertiary cataclastic deformation. Our earlier interpretation had been that slow cooling of The few K-Ar dates available for the San deeply buried rocks that had reached high Gabriel block are 61 to 83 m.y. and discordant temperatures during Mesozoic regional meta- (Hsu and others, 1963; Evernden and Kistler, morphism was the explanation for young dates 1970). Silver (1971) reports a 220-m.y. (Late (Armstrong, 1966a; Armstrong and Hansen, Triassic) U-Pb isotopic date for the Mount 1966). Lowe-Parker Mountain plutonic complex and also reports plutonism during the periods 160 Plutons of Late Cretaceous age (85 to 100 to 170 and 75 to 90 m.y. but gives no details. m.y.) are most concentrated in the north- Included with the Transverse Ranges in the western third of Nevada where they form a palinspastic reconstruction (Suppe, 1970) are continuation of the core of the Sierra Nevada the rocks of the Mount Pinos and Alamo (Kistler and others, 1971; Smith and others, Mountain blocks, which give dates similar to 1971). those of the San Gabriels. An isolated 145- An important negative result of our study m.y. K-Ar date for the Willows Diorite of is the discovery that the presence of numerous Santa Cruz Island was reported by Weaver Tertiary granitic plutons is restricted to the (1969); because of the similarity of the schist on Basin and Range province in Nevada, Utah, Santa Cruz Island to the Pelona and related Arizona, and the Death Valley region of Cali- schist, this date has been placed with the fornia. We were surprised to find few Tertiary Transverse Ranges on the palinspastic map. plutons in the Mojave region. Evidently the Yielding similar dates are the quartz gabbros of Tertiary history of the Mojave block differs the Gold Hill (144 m.y.) and Logan (156 m.y.) significantly from the history of other parts fault slices along the San Andreas rift in central of the Basin and Range province. California (Ross, 1970).

Salinian and San Gabriel Mountains Mojave Region Fault Blocks Previous studies in this region have provided Salinia, Gold Hill, the San Gabriel Moun- a few K-Ar and isotopic U-Pb dates (Hewett tains, and isolated Santa Cruz Island are con- and Glass, 1953; Bishop, 1963; Adams and sidered together as miscellaneous slices of base- others, 1967; Hewett and others, 1969; Evern-

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den and Kistler, 1970; Fleck, 1970). Strati- Salton Sea Region graphic dating of the Mesozoic igneous and Increasingly younger late Mesozoic dates are tectonic chronology is presently unavailable, encountered as far southeastern California is but Burchfiel and Davis (1971) and Bu:'chfel approached. Many dates in that region are in and others (1970) were able to date Mesozoic the 50- to 70-m.y. Laramide range, which is thrusting in the Clark and Hunter Mountain well represented in Arizona, yet almost entirely areas using K-Ar dates on plutons that c. t (or absent in the rest of the Southwest (Fig. 2). were cut by) thrust faults. Lanphere (196z ) and Most dates in this tegion are not markedly Lanphere and others (1964) demonstrated that discordant, suggesting that either (1) this late Mesozoic (64 to 156 m.y., or older) re- region was involved in Laramide magmatism gional metamorphism had affected deep-seated along with Arizona, c : (2) the exposed level of metamorphic rocks over a widespread area in the crust in this region remained hot later the eastern Mojave Desert and in the Panr.mint than tlie Cordilleran belt in general yet cooled Range and vicinity. Hamilton (1964) has rapidly, which also might be a manifestation studied the effects of this Mesozoic meta nor- of Laramide orogenesis. In either case, many phism associated with formation of nappes in- plutons are probably significantly older than volving Paleozoic shelf sediments and Pre- the dates they yield. cambrian basement in the Maria Mountains. Metamorphosed upper Paleozoic marine sedi- Another interpretation of these young dates, mentary rocks are known from scattered locali- not involving Mesozoic tectonics or magma- ties throughout much of the Mojave block tism, is possible. All the samples from the (Dibblee, 1967). The youngest strata that pre- vicinity of the Salton Sea have relatively low date granitic plutons in this region are volcanic K-Ar dates, and these dates generally decrease rocks of Jurassic(?) age in the east near southern toward the Salton Trough from both east and Nevada (Hewett, 1956; Barca, 1966) and west. Two possible explanations of this are: (1) Triassic(P) metavolcanics in the west (Dib :>lee, the immediate vicinity of the Salton Trough 1967; Bowen and Verplank, 1965; DeLisle and is an area of exceptional Tertiary uplift and others, 1965). All of these volcanic rocks c<;ulc consequent deep erosicn; and (2) the area has be comagmatic with the older batholkhic been subject to an abnormally high heat flux rocks. An uninformative upper limit for the some time during the Tertiary. Kyanite schist age of coarse-grained plutonic rocks is provided in the Cargo Muchacho Mountains is perhaps by Tertiary sedimentary and volcanic rocks. evidence of an unusually deep level of exposure. Both uplift and heating may share responsi- Available dates for the region of scattered bility for the Ar loss; bcth are closely associated plutons north and east of the San Bernardino with continental rifting. This interpretation Mountains are essentially bimodal in time, concurs with recent c.iscussion of the Salton often concordant, and in a few cases they may Trough as an active rift zone (Elders and others, approximate crystallization times. The older 1971). group of K-Ar dates ranges from 135 to 154 m.y.; several related U-Pb dates are somewhat older, 150 to 165 m.y., suggesting moderate Ar Orocopia Schist loss from the micas and hornblendes we ana- lyzed. Older still are two K-Ar hornblende Dates on the Orocopia Schist are highly dis- dates of 190 and 200 m.y. for the Clark Moun- cordant (50 to 24 m.y.) with some being tain area (Sutter, in Burchfiel and Da\is, younger than superjacent middle Eocene ma- 1971). Concerning the other dates, both K-Ar rine sedimentary rocks (Crowell and Susuki, and U-Pb fall in the 64- to 95-m.y. range, fnd 1959). Some hypabysss.l dikes were noted in represent igneous activity and the effects of the region and may be the source of the dis- deep burial and high heat flow in the reg on cordance, if it is not due to deep burial or high during Late Cretaceous time. In the San Ber- Tertiary heat flow, or both. It is interesting to nardino and Pinto Mountains, dates on grani :ic note that the correlative Pelona Schist of the rocks are notably discordant. A single 194-m.y. San Gabriel Mountains has been intruded by K-Ar hornblende date (SC-69-22) suggests tie mid-Tertiary igneous reeks (Hsu and others, presence of Early Jurassic or older igneous 1963); this adds strength to Crowell's (1962) activity. Jurassic plutonism is indicated by 150- proposed displacement of these general base- and 175-m.y. U-Pb dates and 145- and 163- ment terrains along the San Andreas fault m.y. K-Ar dates. system (Fig. 1).

Peninsular Ranges heat flow related to the Salton Rift may all have been factors in reducing the K-Ar dates Stratigraphic evidence defining the age of for southeastern California below those ob- granitic rocks in the Peninsular Ranges is more tained by U-Pb and Rb-Sr methods. restrictive than elsewhere in central and southern California. Granitic rocks intrude the Bed- Arizona ford Canyon Formation, Santa Monica Slate, Geochronometric data for Arizona are largely and Santiago Peak Volcanics, which have from the University of Arizona (Damon and yielded Middle and Late Jurassic fossils (Imlay, Mauger, 1966; Bikerman, 1967; Livingston 1963, 1964; Fife and others, 1967). The batho- and others, 1968; Damon, 1968, 1971), but lith is overlain along its western margin by some additional data are available (Bain, 1952; Late Cretaceous (Turonian and younger) sedi- Creasey and Kistler, 1962; Hayes and others, mentary rocks (Popenoe and others, 1960). In 1965; Rose and Cook, 1966; Moorbath and the Peninsular Ranges of northwestern Baja others, 1967; Drewes, 1968; Armstrong, 1969). California, the Aptian volcaniclastic Alisitos Arizona is remarkable for the widespread and Formation is cut by granitic rocks which are abundant latest Cretaceous-early Tertiary in turn overlain by Campanian-Maestrichtian Laramide dates which are quite rare in the rest marine sedimentary rocks (Silver and others, of the Southwest. A few older plutons and 1963, 1969). The "prebatholithic" volcanic widespread volcanic rocks of Late Triassic units may very well be comagmatic with earlier through earlier Cretaceous age are known in Cordilleran granitic rocks. Early Tertiary southern Arizona. The southwestern part of gravel caps the batholith in many places the state is poorly known geologically and its (Minch, 1971), suggesting that something close geochronometry remains largely undone; how- to the present level of erosion was attained by ever, the pattern of Laramide K-Ar dates that time. (Fig. 2) may suggest some continuity at that Isotopic U-Pb dates on zircons from granitic time from southeastern California through rocks of northwestern Baja California bracket southwestern and southeastern Arizona. igneous activity between 100 and 115 m.y.; on the east side of Baja California, dates are Regional Dating Patterns of the younger, 94 to 96 m.y.; and those on the east Southwestern United States side of the Gulf of California are younger still, 79 to 97 m.y. (Silver and others, 1969; Ander- The many hundreds of dates we have re- son and others, 1969). Likewise, K-Ar dates viewed are of importance to the geologic decrease from 108 to 63 m.y., from west to histories of local areas within the Southwest; east across Baja California, with most dates beyond that, when taken in aggregate, these near 80 m.y. (Krummenacher and Gastil, dates give insight into the general tectonic de- 1970; Estavillo and Rogers, 1970). The K-Ar velopment of the Cordillera. We think this is dates are generally 10 to 30 percent below com- so even given the probable variety of meanings parable isotopic U-Pb dates. individual dates may have, as was discussed For the southern California portion of the earlier. As more U-Pb and Rb-Sr studies be- Peninsular Ranges, Banks and Silver (1966) come available, the insight into Cordilleran report isotopic U-Pb dates of 109, 119, and 120 tectonics will be more profound. m.y. for zircons from granitic rocks at Crest- The basic concepts we apply in examining more, Woodson Mountain, and Mount Rubi- regional K-Ar patterns are suitably modest: doux, respectively. Pala lepidolite has been (1) if extensive dating has been done in a dated by Rb-Sr at 103 m.y. (Herzog and others, region, the youngest dates place a younger limit 1960). As in Baja California, K-Ar dates re- on magmatism; (2) dates give a general impres- ported by Evernden and Kistler (1970), Reyn- sion of the time span of magmatism in regions olds (1957), Morton (1969), Krummenacher of mixed, concordant dates on scattered and Gastil (1970), and our new analyses show mesozonal plutons; and (3) older parts of the a decrease from 112 to 75 m.y. from west to record are naturally more obscure and tend to east across the range, with dates as low as 64 be blurred together. We make use of the to 68 m.y. east of the Santa Rosa mylonite palinspastic map and the histograms taken zone. The depth of erosion apparently increases from it (Figs. 2B and 3). The histograms are from west to east. Later Mesozoic igneous for longitudinal segments roughly normal to activity, greater uplift, and possibly higher the edge of the Mesozoic continent.

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Prior to the Mid-Jurassic (>160 m.y.), rela- Paired Metamorphic Bel ts tively few dates are available and the record is enticingly obscure; Mesozoic intermediate volcanic rocks of this age range are fairly wide- Parallel to and west of the batholith belt is spread. While one date in the Klamath Moi n- the Franciscan terrain, characterized by high- tains may be as old as Permian, and some pressure, low-temperature blueschist metamor- Permian volcanic rocks are known from the phism (Fig. 2A). Contrasting paired basement Cordillera (McKee and others, 1967), wide- belts such as these exist in a number of places spread intermediate magmatism seems to have around :he Pacific margin and are the eroded begun in the Late Triassic (Chinle time), ap- remains of arc-trench subduction zones (for proximately 220 m.y. ago (Fig. 3; also see example, Hamilton, 1969a, 1969b; Dickinson, McKee and others, 1959). Granitic plutons of 1970; Ernst, 1970; Suppe, 1972). Extensive about this age are scattered along the Cordillera K-Ar dating throughou t this belt from Oregon in at least seven separate localities. Five are dis- to Baja California show; that the high-pressure cussed in this paper, one is in western Id; no metamorphism spans a pproximately the time (Armstrong and Besancon, 1970), and several range 150 to 70 m.y. and is essentially coeval are in southern British Columbia (White £.nd with the times of most extensive magmatism others, 1967, 1968). Incidentally, and perh-.ps of the batholith belt (Suppe and Armstrong, not by coincidence, this is almost exactly the 1971). Franciscan sedimentation spans essen- time of rifting and Newark Group basaltic vol- tially the same time rs.nge. In addition to the canism in the eastern United States (Arm- broadly coeval nature of the paired belts, there strong and Besancon, 1970), which marks the are intriguing hints of more detailed relations beginning of the westward drift of North between events in the adjacent belts. For ex- America relative to Africa. The implication is, ample, in southern California and northern of course, that histories of these two different Baja California, most dates on Franciscan meta- boundaries of the North American plate are morphic rocks lie in t ae range of 90 to 110 not entirely independent. m.y., which is the range for most dates on granitic rocks in the adjacent Peninsular From Late Jurassic through Late Cretaceous Ranges. Similarly, many of the higher grade (160 to 70 m.y.) time, abundant and widespread blueschists of the Franciscan yield dates in the dates are available. There are more in Late 130 to 150-m.y. range which has already been Jurassic (160 to 130 m.y.) and Late Cretaceous noted as a major time of magmatism. All of (105 to 75 m.y.), with somewhat fewer in the these results lend considerable support to the Early Cretaceous (Fig. 3). About 150 to l55 concept relating paired basement belts to sub- m.y. ago, magmatism spread over a large part duction zones. of the Cordillera; subsequently, the dates are more and more consolidated toward the com- All of the data thus point to the termination posite axial portion of the batholith belt that of the Late Jurassic an:l Cretaceous subduction runs from northwestern Nevada through the zone, which genetically linked blueschist meta- High Sierra, the western Mojave Desert, San morphism and emplacement of granitic rocks, Bernardino Mountains, and ultimately into roughly 75 m.y. ago, and the onset of Laramide the southern California-Peninsular Range tectonics at that time. This has been suggested batholith (Fig. 2). This pattern of abundant as the time of the first period of offset along dates in a belt approximately parallel to the the San Andreas fault ;;ystem by Suppe (1970). edge of the continent abruptly ceases at s. It is probable that a major change in plate period dated as about 70 to 75 m.y. ago. At margins occurred at this time. One possible approximately this point in time, there is f. explanation is that subduction changed from switch to the Laramide pattern for which the Franciscan pattern with a large component abundant 50- to 75-m.y. dates form a belt f rom of dip-slip to a pattern with a large component southern New Mexico across southern Arizona of strike-slip (north-northeast). Under those toward southeastern California (see Figs. 2B conditions, California would experience domi- and 3). The belt then turns northward to in- nantly strike-slip, while Mexico and British clude very sparse dates of this age in Nevada. Columbia would experience a large component Abundant dates of this age are not encountered of dip-slip and thus 1 neir extensive Laramide again until the Idaho, Boulder, and Coast magmatism (see Fig. 4 of Hamilton, 1969a). Range batholiths are reached. Subduction with a large component of strike-

slip is perhaps to be expected at this time ac- Agency Symp., Monaco, p. 453-461. cording to the model of Atwater (1970). Anderson, R. E., 1971, Thin-skin distension in Finally, we would like to note that recent Tertiary rocks of southeastern Nevada: Geol. attempts at a detailed spreading history for the Soc. America Bull., v. 82, p. 43-58. Atlantic Ocean show a possible synchroneity Anderson, T. H., Silver, L. T., and Cordoba, D. A., 1969, Mesozoic magmatic events of the north- of Atlantic spreading events and the history ern Sonora coastal region, Mexico: Geol. Soc. of Cordilleran magmatism and tectonics America, Abs. with Programs for 1969, Pt. 7 (Coney, 1972). We have mentioned the corre- (Ann. Mtg.), p. 3-4. lation between the upper Triassic Newark Armstrong, R. L., 1966a, K-Ar dating of plutonic Group rift-magmatism along the Atlantic coast and volcanic rocks in orogenic belts, in Potas- of North America and the beginning of wide- sium argon dating: New York, Springer- spread plutonism in the Cordillera. Vogt and Verlag, p. 117-133. others (1971) and Pitman and Talwani (1972) 1966b, K-Ar dating using neutron activation all agree that unusually rapid spreading took for Ar analysis: Granitic plutons of the eastern Great Basin, Nevada and Utah: Geochim. et place in later Cretaceous time, and Steiner and Cosmochim. Acta, v. 30, p. 565-600. Helsley (1972) point out that rapid movement —— 1969, K-Ar dating of laccolithic centers of the of North America away from Europe must have Colorado Plateau and vicinity: Geol. Soc. occurred during later Jurassic-Early Cretaceous America Bull., v. 80, p. 2081-2086. time. These periods when rapid Atlantic spread- 1970a, Geochronology of Tertiary igneous ing has been suggested are approximately the rocks eastern Basin and Range province, same as the times of culmination of mag- Nevada, Utah, and vicinity: Geochim. et matism we observe in the Cordillera. It is Cosmochim. Acta, v. 34, p. 203-232. premature to say a conclusive correlation can 1970b, K-Ar dating using neutron activation for Ar analysis: Comparison with isotope dilu- be demonstrated; the spreading time scales are tion Ar analyses: Geochim. et Cosmochim. uncertain and disputed, general agreement not Acta, v. 34, p. 233-236. yet being achieved. Nevertheless it is worth- Armstrong, R. L., and Besancon, fames, 1970, A while keeping this relation under continued Triassic time scale dilemma; K-Ar dating of examination, as it suggests a close link between Upper Triassic mafic igneous rocks, eastern phenomena on opposite edges of shifting crustal U.S.A. and Canada, and post-Upper Triassic plates. plutons, western Idaho, U.S.A.: Eclogae Geol. Helvetiae, v. 63, p. 15-28. ACKNOWLEDGMENTS Armstrong, R. L., and Hansen, E. C., 1966, Cordil- Pierre Biscaye, Harold Masursky, and L.J.P. leran infrastructure in the eastern Great Basin: Am. Jour. Sci., v. 264, p. 112-127. Muffler aided Armstrong in sample collection Atwater, Tanya, 1970, Implications of plate tec- in Nevada in 1961. The Rev. Paul N. Taylor tonics for the Cenozoic tectonic evolution of assisted in sample preparation and in all aspects western North America: Geol. Soc. America of the analytical work. Most of this research Bull., v. 81, p. 3513-3536. was completed while Suppe was at Yale, the Bain, G. W„ 1952, The age of the "Lower Creta- remainder as a National Science Foundation ceous" from Bisbee, Arizona, uraninite: Econ. Postdoctoral Fellow at the University of Cali- Geology, v. 47, p. 305-315. fornia at Los Angeles. The hospitality of the Banks, P. O., and Silver, L. T., 1966, Evaluation Geology Department there during the tenure of the decay constant of uranium 235 from of this fellowship is gratefully acknowledged. lead isotope ratios: Jour. Geophys. Research, This research was supported by the National v. 71, p. 4037-4046. Barca, R. A., 1966, Geology of the northern part Science Foundation Grants G14192, GP5383, of Old Dad Mountain quadrangle, San Ber- GA1670, and GA1694. The mass spectrometer nardino County, California: California Div. was provided by grants from the Research Mines and Geology Map Sheet 7. Corporation and the Sheffield Scientific School Bateman, P. C., and Eaton, J. P., 1967, Sierra of Yale University. Nevada batholith: Science, v. 158, p. 1407- 1417. REFERENCES CITED Bateman, P. C., and Wahrhaftig, C., 1966, Geology of the Sierra Nevada: California Div. Mines Adams, J.A.S., Burchfiel, B. C., and Sutter, J. F., and Geology Bull. 190, p. 107-172. 1967, Absolute dating of mountain building Bikerman, Michael, 1967, Isotopic studies in the events, in Radioactive dating and methods of Roskruge Mountains, Pima County, Arizona: low-level counting: Internat. Atomic Energy Geol. Soc. America Bull., v. 78, p. 1029-1036.

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