K-Ar Apparent Ages, Peninsular Ranges Batholith, Southern California and Baja California

DANIEL KRUMMENACHER R. GORDON GASTIL Department of Geology, San Diego Suite University, San Diego, California 92182 JONATHAN BUSHEE* JOAN DOUPONT*

ABSTRACT Albian-Aptian age (Allison, 1955; Allen and others, 1960; Silver and others, 1963). These observations, however, only limit the More than 200 K-Ar apparent ages have been determined from maximum age of specific plutons. In western San Diego County, a minerals from the Peninsular Ranges batholith of southern variety of granitic clasts have been found in volcaniclastic strata California and northern Baja California. In general, the apparent that are believed to be Portlandian on the basis of paleontology. ages show a progressive decrease from about 120 m.y. in the And, near Guadalupe Valley, northern Baja California, volcanic- southwestern (coastal) part of the batholith to less than 70 m.y. in volcaniclastic strata rest on granitic rock (Ashley, 1972, unpub. the northeastern (desert) part. The gradients for biotite and data). We know, therefore, that some plutons are at least as young hornblende ages can be represented by contours of equal ages. Both as Aptian (112 m.y. B.P.) and that others are at least as old as Port- concordant and discordant hornblende-biotite pairs and minerals, landian (135 m.y. B.P.; time scale from Harland and others, 1964). from a variety of plutonic and metamorphic rock types, share in the Granitic rocks are overlain by Turonian strata (—90 m.y. B.P.) in apparent-age gradient. Ages for hornblende average 5 m.y. older the Santa Ana Mountains (Popenoe and others, 1960). Unfos- than the ages for coexistent biotite. Isotopic U-Pb and Pb-a mea- siliferous strata tentatively correlated with Popenoe's Turonian surements on zircon indicate ages greater than those calculated rocks are widespread in western San Diego County (Luzardi For- from K-Ar ratios of hornblende or biotite. It is believed that in the mation of Nordstrom, 1970), and possible equivalents have been Peninsular Ranges province, the U-Pb ages for zircon approximate found in northern Baja California (Redonda Formation of Flynn, the ages of emplacement, whereas concordant K-Ar ages may or 1970). may not approximate the ages of emplacement, depending on the Some of the undated Cretaceous strata in San Diego County and depth of emplacement and the rate of uplift and denudation. northern Baja California rest on amphibolite-grade metamorphic rocks; this indicates that considerable erosion of the batholith pre- INTRODUCTION ceded their deposition. John Minch (1972, oral commun.) believed that some of the clast types in the upper portion of the Rosario The Peninsular Ranges province is bounded by the Pacific bor- Group (Campanian-Maestrichtian, ~70 m.y. B.P.) were derived derland on the west, the Gulf of California on the east, the Trans- from basement rocks in the desert ranges that border the Gulf of verse Ranges of California on the north, and lat 28° N. in Baja California. So, although the evidence is fragmentary, it suggests California on the south. The K-Ar data presented in this paper were that the batholith was unroofed along the Pacific margin before 90 obtained (by us and by several students) from rocks in that part of m.y. B.P. and was unroofed as far to the east as the desert ranges the Peninsular Ranges which lies in Baja California north of lat 31° before 70 m.y. B.P. N. and in southern California to 50 km north of the international border. The work of Larsen and others (1958), Banks and Silver U-PB AND RB-SR AGES1 (1969), Silver and others (1969), Evernden and Kistler (1970), and Armstrong and Suppe (1973) is integrated with our work to dem- The first extensive mineral dating of the Peninsular Ranges was onstrate that the trends which we found to the south continue to done by the Pb-a method (Larsen and others, 1958). These authors the north (see Figs. 1 and 2). dated 24 rocks from southern California and the northern part of The objective of this paper is not to determine the emplacement Baja California. These determinations indicated ages of 92 to 136 ages of the Peninsular Ranges batholith but to interpret the spacial m.y.; the median age was about 110 m.y. The ages reported from pattern of K-Ar apparent ages. To make this interpretation, how- tonalite are generally older than those from more siliceous rocks, ever, it is necessary to consider the evidence for the age of em- but the two groups overlap considerably. Because of the difficulty placement. of obtaining appropriate minerals, no dates were reported for gab- bro. Bushee and others (1963) reported 15 analyses that confirmed STRATIGRAPHIC LIMITS FOR the results of Larsen and his colleagues. The Pb-a ages do not show THE AGE OF THE BATHOLITH a consistent pattern of age variation across the peninsula. Pb-a dates on zircons in plutonic rocks may be in error because of either In the Santa Ana Mountains at the northern end of the province, loss of lead (which makes ages too young) or inherited zircon granitic rocks intrude strata of Middle and Late Jurassic age (which makes ages too old). In the volcaniclastic western part of (Imlay, 1963, 1964). In San Diego County, the granitic rocks in- the Peninsular Ranges, inheritable zircons are rare and the Pb-a trude strata of Late Jurassic age (Fife and others, 1967). In north- dates can therefore be considered as minimum ages. Lepidolite from ern Baja California, rocks of the batholith intrude strata of 1 We are not suggesting that the credibility of nonisotopic ages compares with that * Present address: (Bushee) Department of Geology, Northern Kentucky State of, for example, isotopic U-Pb work on zircons. The point we are trying to make is that College, Covington, Kentucky 41011; (Doupont) Lawrence Livermore Laboratory, all of these methods yielded a range of ages compatible with the stratigraphic limita- Livermore, California 94550. tions, whereas K-Ar does not.

Geological Society of America Bulletin, v. 86, p. 760-768, 5 figs., June 1975, Doc. .. 50605.

760

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A = Actinoliie Me = Microcline gabbro

B = Biotit« Mu = Muscovite

H = Hornblende P = Plagiodose adamellìte and granodiorite

Ks = Potassium Feldspar Ph = Phlogopite

Z = Zircon undifferentiated tonalite

sphene-rich leucotonalite

K-Ar DATA FROM SAN DIEGO STATE UNIVERSITY

K-Ar DATA FROM ARMSTRONG AND SUPPE (1973)

AND EVERNDEN AND KISTLER (1970)

K-Ar DATA FROM SAN DIEGO STATE UNIVERSITY;

ISOTOPIC U-Pb ZIRCON, WRITTEN COMMUNICATION

FROM L.T. SILVER (1973)

KRUMMENACHER AND OTHERS, FIGURE 1 Geological Society of America Bulletin, v. 86, no. 6 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/760/3429075/i0016-7606-86-6-760.pdf by guest on 25 September 2021 / II70 00- ??6° 00'

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0 K-Ar DATA FROM SAN DIEGO STATE UNIVERSITY

95 > • K-Ar DATA FROM ARMSTRONG AND SUPPE (1973) Hl Vf"r>. i AND EVERNDEN AND KISTLER (1970) ... 85 1 ^t K-Ar DATA FROM SAN DIEGO STATE UNIVERSITY; 90 A* 84 ISOTOPIC U-Pb ZIRCON. WRITTEN COMMUNICATION

FROM l.T. SILVER (1973)

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KRUMMENACHER AND OTHERS, FIGURRE 2 Geological Society of America Bulletin, v. 8636, no. 6

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the Pala pegmatite has been dated several times by Rb-Sr methods Silver (in Armstrong and Suppe, 1973). An additional determina- and has yielded ages in the order of 104 ± 4 m.y. (Herzog and oth- tion of 117 m.y. (from L. T. Silver, oral commun.) is reported in ers, 1960). John Earl (1965) analyzed 33 lepidolite samples from this paper. These are 10 to 15 m.y. older than K-Ar ages for biotite pegmatite in seven districts in southern California and northern Baja from the same or nearby rocks. In the axial and eastern parts of the California. He used the total Rb-Sr method by x-ray fluorescence peninsula, we report one isotopic zircon age by L. T. Silver of 98 and obtained ages ranging from 92 to 117 m.y. for the seven dis- m.y., which is 6 m.y. older than the biotite and 9 m.y. older than tricts. Such large, complex pegmatite bodies are generally consid- the hornblende K-Ar ages from the same rock. No direct compari- ered to be a late magmatic phenomenon, but the nonisotopic Rb- sons are available for the desert ranges, but Silver (1972, oral com- Sr method cannot be corrected for original Sr and thus can yield mun.) has not found ages younger than 90 m.y., whereas K-Ar ages excessive ages. Silver and others (1969) and Banks and Silver are typically 65 to 80 m.y. The lower K-Ar ages could be explained (1969) reported isotopic U-Pb ages for northern Baja California by resetting in the southwestern part of the province, but it seems and the Peninsular Ranges of southern California. These range very unlikely that 80, 70, and even 60 m.y. ago, plutons intruded from 94 to 120 m.y., with the youngest ages being found in the and reset the eastern part of the peninsula because no such young eastern part of the peninsula. Coastal Sonora (previously adjacent) ages have been detected in the Peninsular Ranges province by either shows even younger ages (Anderson and others, 1969). These are U-Pb or Rb-Sr techniques. based on isotope ratios rather than isochrons and are therefore To understand the apparent-age gradient, more than 150 K-Ar susceptible to errors resulting from lead loss. However, for most dates were determined in a broad belt across the peninsula (see Fig. localities, it has been possible to determine and correct for dis- 1). Figures 1 and 2 combine our data with the data of Evernden and cordance (Banks and Silver, 1966). The belief that isotopic zircon Kistler (1970) and Armstrong and Suppe (1973). ages closely approximate emplacement age is supported by the ability of zircon to retain radiogenic lead under near-melting condi- ANALYTICAL PROCEDURE tions (Gastil and others, 1967). In summary, isotopic and nonisotopic Rb-Sr and U-Pb determi- The extraction and purification of the gases followed the usual nations indicate an age range in the order of 95 to 135 m.y. This is method used by the Berkeley group (see, for example, Evernden in agreement with the stratigraphic conclusion that most of the and Curtis, 1965): fusion of the sample in a Mo crucible using high granitic rocks were emplaced between the Portlandian and Turo- frequency induction, purification on CuO-Cu and zeolite traps, nian Ages. transfer with liquid nitrogen, and purification on a Ti trap. Extrac- tion lines and the Reynold's glass mass spectrometer were indepen- K-AR AGES dent. Blanks have been run regularly. They show an average contami- Concordant (variation of less than 5 percent between two or nation of 3 x 10~13 moles of atmospheric Ar. K was analyzed in more minerals in the same rock) K-Ar ages on hornblende-biotite duplicate on a Zeiss flame photometer until 1974 by J. Hampel in pairs have been reported by Evernden and Kistler (1970), Krum- the geochronometry laboratory at the University of California, menacher and Gastil (1970), Estavillo and Rogers (1970), Arm- Berkeley. Subsequent flame photometry was done at San Diego strong and Suppe (1973), and this paper. These concordant ages State University. range from 113 m.y. near the Pacific coast to 50 m.y. in southeast- ern California. Estavillo and Rogers (1970) reported nine ages K-AR GRADIENT ACROSS THE PENINSULA from gneiss, amphibolite, gabbro, tonalite, granodiorite, pegma- tite, and aplite from a single locality. The minerals — hornblende, The apparent ages for biotite are contoured in Figure 1. The con- biotite, muscovite, and potassium feldspar — all yield K-Ar appar- tours, beginning with 115 m.y. in the west and dropping to 65 m.y. ent ages of 80 ± 3 m.y. (Fig. 3). in the east, are surprisingly regular. There are contradictions in the In the western part of the peninsula, three isotopic zircon deter- data that make exact contouring to all data points impossible minations of 120, 119, and 109 m.y. were reported by Banks and without introducing contour detail that is inconsistent with the density of points. The average deviation of points from the con- tours in Figure 1 is 1.8 m.y. with a standard deviation of 2.4 m.y. The Elsinore and San Jacinto faults are shown in Figure 1 displac- ing the apparent-age contours. However, because the assumed off- sets are small relative to the data density, the contours can be drawn without these offsets. The data of Armstrong and Suppe show that east of the San Andreas fault, there is a terrane of mixed ages where very different K-Ar ages exist side by side. The diagram was not extended into this region. The apparent ages for hornblende are contoured in Figure 2. In general, they parallel those of biotite, although they are displaced 5 to 10 m.y. to the east. In detail, however, the contours are quite different; the most striking difference is a ridge of comparatively old ages extending from Tijuana, Baja California, to Mount Laguna, California. Along this ridge, there are no concordant hornblende-biotite pairs and the hornblende ages are as great as gneiss metagabbro aplite t» 83 m.y. biotite 81 m.y. biotite \ '.»v/U 79 m.y. biotite 143 m.y. Most of the hornblende-bearing rocks studied in this re- gion were gabbro (Miller, 1937; Everhart, 1951); an exception is younger pegma- schist 80 m.y metapegmatite tonalite at Dulzura, California (135 m.y.). In general, the tite, 79 m.y. hornblende 77 m.y. biotite muscovite hornblende ages appear to be less predictable than the biotite ages. Eight hornblende localities are located more than 5 m.y. off of the granodiorite contours as drawn in Figure 2. The average deviation of points 80 m.y. biotite from the contours is 2 m.y. with a standard deviation of 3 m.y. Figure 3. Intrusive relations in San Marias Pass, Baja California, Mexico. The anomalous hornblende ages are primarily from rocks where

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the hornblende yields a younger age than companion or nearby been mapped by Miller (1935), R. H. Merriam (unpub. map on file biotite. Examples from Appendix Table 1 are BIB—5, B15—7, with the California Div. Mines and Geology), Brooks and Roberts B2G-311, B2G-304, BCI-410, and B1J-3. The first four are from (1954), Weber (1963), and Jensky (unpub. data, 1971), all of the western margin of the batholith where unrecrystallized whom interpreted it as a continuous rock unit from its western con- hornblende proved very difficult to find, but the latter two are from tact near Rancho La Posta, California, to the floor of the desert. the central part where this was not a problem. Microscopic re- This pluton is characterized by modal uniformity — mainly examination shows opaque exsolution and mottled colors (possible leucotonalite with only small amounts of potassium feldspar. The recrystallization) in B2G-304 and B1B-5, but in general, the sam- most distinctive phase, occurring at many places through the body, ples in question are normal-appearing hornblende. contains large idiomorphic books of biotite and well-formed Thirteen K-Ar ages were determined for plagioclase and four for prisms of hornblende and unusually abundant amber-colored potassium feldspar. The feldspar ages tended to be concordant with sphene crystals commonly as much as 0.5 cm long. The La Posta biotite and discordant with hornblende. Sample B1A—9 (see App. rock type is conspicuous along the eastern edge of the batholith as Table 1) shows plagioclase concordant with both biotite and far south as lat 28° N. Since the La Posta pluton has been inter- hornblende. preted as a single intrusion and is cut by several age contours, it provides evidence in support of the hypothesis that the K-Ar ages RELATION OF K-AR DATES TO MAPPING are related to regional uplift and erosion rather than to the time of emplacement. If similar K-Ar ages were found throughout this All of the area investigated has been mapped at least in recon- large body, they could be interpreted as being related to the cooling naissance with some areas mapped in greater detail (Gastil and history of the pluton per se. Because, however, K-Ar ages vary others, 1975). In making the sample selection, an attempt was across the pluton in the same sense that they vary across the entire made to obtain aerial coverage, a variety of granitic rock types, province, they appear to be unrelated to the time of emplacement. linear sequences across large plutons, and temporal rock-unit se- Most of the K-Ar ages obtained on the La Posta Quartz Diorite quences documented by crosscutting relationships. The following (Jensky, 1971, unpub. data) are clearly discordant. They show a examples illustrate the relation of K-Ar apparent ages of the gra- general decrease from about 90 m.y. on the southwest to 75 m.y. nitic and metamorphic map units. on the northeast for biotite and 100 m.y. to 85 m.y. for In San Matias Pass (Baja California), at the northern end of the hornblende. The gradient across this pluton is slightly less than the Sierra San Pedro Martir (lat 31°18' N., long 115°27' W.), Estavillo gradient across the entire province. Three localities within the La and Rogers (1970) determined the K-Ar apparent ages of nine min- Posta pluton yield concordant hornblende-biotite ages — two near erals in seven rocks (B9E-1 to B9E-7) collected from an area the western margin (85 and 83 m.y. and 89 to 91 m.y.) and one mapped by M. H. Rogers (1970, unpub. data). Figure 3 is a dia- toward the eastern margin (75 and 73 m.y.). Leon T. Silver (1973, grammatic illustration of the crosscutting relationships of the rocks written commun.) reported a U-Pb isotopic zircon age of 98 ± 2 involved. The country rock is coarse-grained schist and gneiss con- m.y. A north-trending, almost-continuous screen of metamorphic taining sillimanite with two superimposed deformation patterns. It rocks and small intrusions of muscovite granodiorite separate the is cut by a set of pegmatite dikes that bear a mutually intrusive prominent concentric foliation pattern in the western portion of the relationship to gabbroic dikes (now crystallized to amphibolite). pluton from the eastern part. The apparent ages do not decrease Similar amphibolite is marginal and gradational to a nearby body regularly but fall into two groups: biotite ages range from 83 to 91 of dated gabbro. The pegmatite and amphibolite show only one of m.y. in the western portion of the pluton and from 73 to 78 m.y. in the two deformation patterns found in the gneiss. All of the above the eastern portion, with a similar magnitude variation for rocks are cut in turn by granodiorite, aplite, and a younger pegma- hornblende. Thus, this pluton could be interpreted as two separate tite. We believe that all of the ages obtained for these rocks record, plutons of similar composition but different age, or it could be in- within our limits of precision, a single cooling event. terpreted as two halves of a single pluton cleaved by a vertical fault so that an upper portion of the pluton is exposed to the west and a The rocks in the area northeast of Guadalupe Valley, Baja much deeper portion to the east. In the latter case, the difference in California (Fig. 1), have been investigated by a number of students at San Diego State University (J. V. Kaiser, Jr., 1967, unpub. data; B. A. Peterson, 1967, unpub. data; H. M. Snyder, 1970, unpub. data; R. J. Ashley, 1972, unpub. data). At this locality, a sequence of graywacke, slate, marble, and quartzite is intruded by a number of small plutons some of which show chill boundaries, myrolitic cavities, and granophyric texture as evidence of emplacement at a shallow depth. These rocks were unroofed by erosion and covered by a sequence of volcanic and volcaniclastic rocks ranging in com- position from andesite to dacite. In the areas from which this vol- canic sequence has been stripped, an extensive swarm of dacite and andesite dikes are exposed; presumably they were feeder dikes for the overlying volcanic pile. Intruding all of the above rocks, includ- ing the dikes, is a tonalite pluton at least 10 km in diameter. Judging from its internal homogeneity and the zone of plastically deformed migmatitic rocks along its boundary, it was emplaced at considera- ble depth. This could only be true if the eruptions of volcanic rocks had collected to a thickness capable of providing this depth. Figure weakly metamorphosed dacite dike cutting older 4 is a diagrammatic illustration of the relationships in this area. Mesozoic volcanic rocks, granitic rocks, cut by The only dated representative of the older granitic rocks yielded undated younger tonalites ages of 98 m.y. (biotite) and 94 m.y. (hornblende). The volcanic 96 m.y. plagioclase dikes and the pluton that cuts them show ages of 96 to 103 m.y. for slates and meta- + + + + older granitic tonalite hornblende, biotite, and plagioclase. We do not believe that the graywackes, + + -I-+ rocks, 94 m.y. 98 8i 99 m.y. above events of intrusion, erosion, and reintrusion all occurred undated hornblende, biotite, 96 98 m.y. biotite & 103 m.y. within this short interval. hornblende The La Posta Quartz Diorite pluton (Miller, 1935) occupies an Figure 4. Stratigraphie and intrusive relations near Rancho Vallecitos, area of about 300 km2 near the eastern edge of the province. It has northeast of Guadalupe Valley, Baja California, Mexico.

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age could be due to the difference in the time of cooling produced native 2) might result from movement of the geothermal gradients by the difference in time or rate of uplift and erosion. The El Pinal within the Earth either independent of changes in erosional levels pluton (Duffield, 1968) is of the La Posta rock type and lies just to or from progressive uplift and consequent erosion. The age of uplift the southwest of the La Posta pluton. Six biotite ages range from 90 (and cooling) might or might not be a function of the same dia- to 96 m.y. Four hornblende ages range from 93 to 99 m.y. Thus, strophic events that resulted in the plutonic emplacement. the biotite ages are somewhat older than those in the adjacent La Posta pluton, but the hornblende ages are similar. A third iarge plu- EFFECTS OF COOLING ON ton of the same composition is located to the southeast of the La HORNBLENDE AND BIOTITE Posta pluton. The apparent ages range from 103 m.y. for hornblende and 84 m.y. for biotite on the western edge of the The concordance of two or more dates derived from minerals pluton to 83 m.y. for hornblende and 75 m.y. for biotite at the with different closing temperatures (for example, hornblende and desert (eastern) edge. biotite) has commonly been cited as evidence that the age pair in Even with the above examples, there is not sufficient data to be question corresponds to the age of emplacement (Evernden and able to say conclusively that the apparent ages are not related to the Kistler, 1970). In the foregoing paragraphs, we have presented evi- individual plutons. It does appear, however, that within a given dence which makes us believe that this commonly held view may petrographic rock type (referred to as granitic "formations" north not be correct in the case of the Peninsular Ranges batholith of of the international border), there is a systematic decrease in age to southern and Baja California. the northeast. Hart (1964) and Hanson and Gast (1967) made direct compari- In southern California north of the international border, several sons between the effects of reheating hornblende and biotite by an of these granitic formations have been mapped and correlated igneous intrusion at depth over geologic intervals of time. They across a large portion of the batholithic province. To test the age were concerned with contact-metamorphic heating whereas we are variation within plutonic formations, we selected the Woodson concerned with regional heating. If Hart's model A (1964, p. 510) Mountain Granodiorite and the San Marcos Gabbro (Miller, 1937; for an infinite dike is rotated so that it represents a horizontal Larsen, 1948; Merriam, 1946, 1958; Everhart, 1951). These two rather than a vertical heat source, his values can be used to approx- formations Were selected because they are widely distributed within imate the temperatures at which hornblende and biotite will be- the batholith and were interpreted by the investigators who come open and closed systems with reference to Ar. Thus, Hart's mapped them as late (Woodson Mountain Granodiorite) and early Figures 2 and 5 (1964, p. 502, 510) show that hornblende begins to (San Marcos Gabbro) in the intrusive sequence. Four biotite ages close at about 500°C and is essentially closed at 450°C. Biotite, on from rocks mapped as Woodson Mountain Granodiorite range from the other hand, begins to close at about 430°C and is completely 104 m.y. (Mount Woodson, California, lat 32°59'42" N., long closed by about 230°C. Hart studied a pluton 4 km in exposed 116°58'50" W.) to 89 m.y. (near Mount Laguna, California, lat diameter emplaced at a depth of at least 1 km. This is the same ex- 32°57'57" N., long 116°30'32" W.). All the gabbro samples col- posure size as the average Sierra Nevadan or Peninsular Ranges lected in the southwestern portion of the province proved to be un- pluton (Gastil and others, 1975). The slower cooling of deep em- datable because of the fact that the original minerals had been re- placement and regional metamorphism may allow lower tempera- placed by mixtures of low-grade metamorphic minerals unsuitable tures of effective Ar closure. for dating. Two concentrates of fibrous amphibole (B1A-1, For the sake of calculation, we used a geothermal gradient that B2G—302) were analyzed, but the ages obtained were young rela- reaches 500°C at a depth of 10 km; this is considerably steeper than tive to other minerals in nearby younger rocks. The eight gabbro average continental gradients but is consistent with metamorphic samples from north of the international border showed apparent terranes of the Abukuma type (Turner, 1968, Figs. 8-4 and 8—8). hornblende ages ranging from 143 to 103 m.y. without any sys- We used a fast rate of 1 km/m.y. for denudation of the uplifted tematic age variation across the province. Plagioclase ages from mountains (Clark and Jäger, 1969; Schümm, 1963). With these these rocks ranged from 97 m.y. (in the rock with hornblende values in mind, several alternative cooling models can be evaluated showing an age of 143 m.y.) to 74 m.y. (in the rock with (Table 1). These models are as follows: hornblende showing an age of 104 m.y.) and, as with the 1. The plutonic rock is emplaced in an environment where the hornblende, showed no relation to the gradient across the province. ambient geothermal temperature is less than 230°C. The rock cools The two western granodiorite samples yielded concordant biotite rapidly to the original temperature of the country rook regardless and potassium-feldspar ages, but none of the mineral pairs from of whether any uplift and erosion takes place. The result is concor- gabbro were concordant. dant hornblende-biotite ages only slightly younger than and, for The hornblende ages from gabbro of southern San Diego County practical purposes, the same as the date of emplacement. and immediately adjacent Baja California present an important de- 2. The plutonic rock is emplaced at depths where the ambient viation to the general contoured pattern (see Fig. 2). Nevertheless, temperature is between 450° and 230°C, and the emplacement is it should be noted that hornblende from gabbro further south in not followed by rapid uplift and erosion. The hornblende age may be northern Baja California and further north in southern California only slightly younger than the age of emplacement, but biotite will decreases in age to the northeast the same as in other rocks. remain open; it will not close until the rock cools below 230°C. Moreover, the hornblende from rocks other than gabbro within the This will result in a discordant age. The amount of discordance is anomalous belt yielded an apparent age consistent with that of the dependent on the delay in uplift and the rate of denudation. gabbro. Therefore, our tentative conclusion is that the bulge in 3. The plutonic rock is emplaced in an environment above hornblende contours reflects a real irregularity in the regional clos- 500°C (more than 10 km deep). If the area is not uplifted and the ing ages for hornblende. regional geothermal gradient does not drop, both minerals will re- main open indefinitely; neither records the date of emplacement. INTERPRETATION OF THE APPARENT-AGE GRADIENT 4. The plutonic rock is emplaced in an environment between 450° and 230°C (5 to 9 km deep); emplacement is followed by Gradients in K-Ar apparent ages such as those that we observe in rapid uplift. With denudation of 1 km/m.y., a pluton emplaced at 9 the Peninsular Ranges could theoretically be due to (1) sequential km would reach the closing temperature of biotite in about 4 m.y. emplacement of igneous rocks resulting in sequential cooling of (not taking into account the heat of the pluton itself). Thus for both the intrusive rocks and the surrounding metamorphic rocks, rocks 100 m.y. old, the resultant ages would qualify under the (2) transgressive regional cooling related to factors other than the definition of "concordant" used by Evernden and Kistler (1970). emplacement and cooling of individual intrusions, or (3) a combi- 5. The plutonic rocks are emplaced in an environment above nation of (1) and (2). Transgressive regional cooling fronts (alter- 500°C (more than 10 km deep); uplift is delayed but when it oc-

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TABLE 1. MODELS FOR PLUTON COOLING TABLE 3. COMPARISON OF ISOTOPIC U-Pb ZIRCON AND K-Ar BIOTITE AND HORNBLENDE AGES

Model Emplacement Metamorphic grade Concordance of K-Ar Concordance of hornblende Uniformity of K-Ar dates Locality and Isotopie Biotite Hornblende depth for hornblende and biotite versus whole- in the area rock type zircon* (m.y. ) (m.y.) versus biotite rock Rb-Sr or Isotopic (m.y.) U-Pb zircons

Sha I low Good Good May be varied BCI 410 Moderate Green schist, low Poor Poor Biotite and feldspar Twenty mi Ies east amphibolI te uniform; hornblende may of Tecate on Mexi- differ can Highway 1 leucotonali te Amph i boI i te ( No K-Ar ages possible. greater Zircons, if present, might BCl 411 give crystallization ages. Eleven mi les east of Ensenada on road Green schist, low Biotite and feldspar to Ojos Negros amphiboli te uniform; hornblende may differ

AmphibolIte or * Dates provided by Leon Silver, California Institute of greater Technology.

TABLE 2. RELATION OF OEPTH OF EMPLACEMENT AND RATE OF UPLIFT AND EROSION TO H0RNBLEN0E-BI0TITE DISCORDANCE textural criteria for the depth of plutonic emplacement. This may not be true where shallow plutons are emplaced in metamorphic

Depth Ambient Never Long-delayed Slow uplift Immediate rapid terranes of deep-seated origin. If there is a significant delay in (km) temperature uplifted uplift followed and erosion uplift and erosion CC) by rapid uplift dropping below 500°C, model 5 should be distinguishable from models 1 and 4 by reference to isotopic U-Pb ages from zircon and

Concordant Concordant Concordant Concordant whole-rock Rb-Sr ages. The zircon U-Pb and whole-rock Rb-Sr sys- (0 (1) (1) (1) tems, closing at higher temperatures, will record ages closer to the 230 - actual time of emplacement (Tables 1 and 2 summarize the criteria Discordant Discordant Discordant Concordant for choosing among the five models). (2) (2) (2) W We can limit the cases in which model 1 is appropriate to plutons WO that show shallow-emplacement textures and very weakly No age Concordant Discordant Concordant recorded (5) (3"*2) (5) contact-metamorphosed host rocks. Such rocks do occur in the (3) Santa Ana Mountains, the westernmost part of San Diego County, and coastal areas of Baja California (for example, near Punta Note: The geothermal gradient is assumed to be 50°C/km. Rapid erosion means I kjn/m.y. or more. Data based on Hart (196M- Concordant means ±5% of apparent age China, Baja California; Allison, 1955). We do not know of any value. Model numbers (from Table 1) are in parentheses. isotopic determinations on rocks in the Santa Ana Mountains. An attempt to separate datable minerals from the rocks of several plu- curs, is rapid. Because neither mineral begins to close until the tons in the coastal zone led to the discovery that ferromagnesian temperature drops below 500°C, it will make no difference how far minerals were replaced by chlorite-epidote-actinolite (deuteric?) below 10 km the pluton is emplaced nor how rapidly denudation minerals. The only K-Ar data we have from the zone of very weak takes place during the interval that the pluton is deeper than 10 km. metamorphism is sample C1J-1 (lat 33°00'54" N., long Once the depth of burial becomes less than 10 km, however, the 117°09'56" N.), north of San Diego where the apparent age on results will be the same as in model 4. Unlike model 4, however, plagioclase from a (Upper Jurassic?) volcanic porphyry is 121 m.y. model 5 provides a situation in which plutons can yield a concor- Leon T. Silver and others (1970, written commun.) have obtained dant age many millions of years younger than the date of emplace- isotopic U-Pb ages from zircons from the Punta Cabra area; the ment. ages were 127 ± 5 m.y. for the volcanic rocks of the middle Cre- As examples of these models, we suggest (model 1) much of the taceous Alisitos Formation and 116 ± 2 m.y. for granodiorite that Basin and Range province where concordant Precambrian to intrudes the Alisitos Formation. The three westernmost biotite ages Mesozoic and Tertiary ages are found side by side; (model 2) the (see Fig. 1), samples SC-69-lb near Corona, B2G-310 near La Sierra Nevada foothill belt and the continental borderland of Mesa, and B1F—1 near Tijuana, yield apparent ages of 112, 113, California where ages are clearly and consistently discordant; and 114 m.y., respectively, but are within the zone of low-grade re- (model 3) beneath the ocean basins where emplacement is rarely gional metamorphism. The three westernmost isotopic U-Pb zircon followed by uplift sufficient to close the rocks to Ar; (model 5) ages north of the international border are 120, 119, and 109 m.y. Salinia and the Transverse Ranges province where concordant ages (Silver and Banks, reported in Armstrong and Suppe, 1973; see clearly younger than the age of emplacement are common (Evern- Table 3). Although we do not have K-Ar and isotopic U-Pb zircon den and Kistler, 1970; Armstrong and Suppe, 1973). data from the same rocks within the unmetamorphosed or weakly An alternate interpretation for many discordant ages, as well as metamorphosed zone, the fact that some plagioclase and biotite some concordant ages, is resetting where one or more minerals is ages approximate the zircon ages suggests that they were emplaced partially or totally cleansed of Ar by the heat of a later intrusion (or under conditions close to that of model 1. of burial). Province-wide resetting, however, would require either a Rocks emplaced under the conditions imposed by model 2 could swarm of younger plutons or deep and widespread reburial. Scat- include Evernden and Kistler's (1970) locality 245 east of Ocean- tered plutons emplaced in a terrane that is cooler than the closing side (lat 33°07'15" N., long 117°08'22" W.), locality B2G-309 at temperature of biotite are not likely to produce widespread reset- Dulzura (lat 31°03'36" N., long 114°49'12" W.), B1K-3 north of ting. Plutons intruded into a terrane that is above this temperature Guadalupe Valley (lat 32°17'50" N, long 116°31'54" W.). How- may only be adding to the discordance in mineral pairs that are al- ever, the fact that there are so few discordant ages in the low-grade ready destined to be discordant. In any case, any intrusions that are metamorphic zone suggests that we are more likely dealing with responsible for resetting rock ages should be recognizable, as the model 4 rather than model 2. indicated ages of such intrusions should be as young or younger Rocks having histories of models 4 and 5 are distinguished from than those of the rock they reset. Plutons as young as the younger each other by the structure and petrography of the plutons them- K-Ar ages have been recognized in the Peninsular Ranges. selves and by the metamorphic rocks in which they are emplaced. The plutons that correspond to models 1, 4, and 5 can generally They are distinguished from rocks of model 2 by the occurrence of be distinguished from one another by the grade of metamorphism both concordant and discordant ages. The discordance between at their contact with the surrounding rocks and by mineralogic and K-Ar ages and isotopic U-Pb ages from zircons has been determined

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from the same sample at locality BCI—410 southeast of Campo zones of mixed ages (model 1) from those of uniform ages (data and at locality BCI—411 east of Ensenada (see Table 2). compiled from Evernden and Kistler, 1970, and Armstrong and Of course, it is possible that plutons might be intruded under Suppe, 1973). The areas of uniform ages probably conform to model 1 or 2 into high-grade metamorphic rocks that had been model 4 along their margins and model 5 in their axial portions. If previously uplifted and deeply eroded. In such a case, there should the apparent-age differences were due merely to depth of erosion, be a wide difference in the K-Ar ages yielded by the plutons and the the youngest ages would appear in the axial zone. Instead, they ages of surrounding metamorphic rocks. Such situations are found occur along the eastern edge of the Gulf of California well east of in portions of the Basin and Range and Rocky Mountains prov- the zone of highest metamorphism, and no amount of strike slip of inces where ages ranging from Tertiary to Precambrian are found the gulf will place them in the axial zone. side by side. In the Peninsular Ranges province, however, plutonic rocks yield ages similar to those of the enclosing metamorphic SIGNIFICANCE OF APPARENT AGES SET rocks. BY RAPID UPLIFT AND EROSION INTERPRETATION OF SIGNIFICANCE OF THE K-AR GRADIENT Where K-Ar closing ages are identified as having been set by events as described in models 4 and 5, it is possible to date the ac- Although there may be isolated situations in which the ages ob- tual uplift (and erosion) of mountains. Thus, these are the ages of tained approach the conditions of models 1 and 2, we believe that orogeny in the strict meaning of the word. The appearances of most of the localities can best be explained by models 4 and 5. The coarse detritus or plutonic-metamorphic detritus in the strati- few isotopic U-Pb ages from zircons (Leon T. Silver, 1972, written graphic record (for example, Dickinson, 1970, 1971) relate to the commun.) suggest that the plutons in the eastern part of the penin- uplift and erosion of mountains, not necessarily to the ages of sula were emplaced later than those in the western part; some of plutonism or metamorphism. Thus, the axial portion of the Penin- those in western Sonora (probably once continuous), still later sular Ranges batholith was emplaced perhaps 95 to 115 m.y. ago (Anderson and others, 1969). Thus, we are looking at two progres- and uplifted about 80 m.y. ago to shed the debris found in the sions: first, a west-to-east progression of the age of intrusion (oldest Campanian-Maestrichtian Rosario Group. This view was already in the west), which at least partially parallels that reported by held in 1962 when Hurley and others wrote "... the ages com- Evernden and Kistler (1970) in the Sierra Nevada and by Hutchi- monly measured by K-Ar and Rb-Sr on biotite may actually reflect son (1970) in the Coast Ranges batholith of British Columbia; and the time of major uplift and erosion, which coincides with that of second, an east-to-west progression of uplift and erosion (deepest sedimentation, and not the initial period of metamorphism and erosion in the east-central portion of the Peninsular Ranges prov- igneous intrusion in the orogenic belt." ince; Armstrong and Suppe, 1973). Figure 5 delineates the general SIGNIFICANCE OF APPARENT-AGE GRADIENTS

Figure 5 delineates the terrane of homogeneous biotite ages of DATA FIOH EVERNDEN AND KISTIER (1970) the Sierra Nevada, Salinia, the Transverse Ranges, and Peninsular AND ARMSTRONG AND SUPPE (1973) Ranges from the terrane of mixed ages in eastern California, Nevada, and Arizona. Apparent-age gradients within such homogeneous terranes in British Columbia (Hutchison, 1970), the Klamath Mountains-Sierra Nevada (Evernden and Kistler, 1970), the Peninsular Ranges province (Armstrong and Suppe, 1973, and in this paper), and in the Andes (James, 1971) show a progressive de- crease in cooling ages toward the continent. This provides the stu- dent of regional tectonic structures with an additional criterion for the recognition of tectonic orientation in ancient orogenic belts. Because the cooling-age contours mark the surface trace of a set of inclined and roughly parallel planes, the contours provide excellent criteria for palinspastic reconstruction.

ACKNOWLEDGMENTS

ZONES OF During 1969 and 1971, this work was financially supported by BIOTITE Undergraduate Research Participation Grants from the National COOLING AGE Science Foundation. Coparticipants in the research were Willard Libby, then of San Diego State University and now with the Geological Survey of Western Australia, and the following stu- dents: Randal Ashley, David Barthelmy, Patricia Bell, Toni Calla- way, William Estavillo, Martin Frazer, John Garcia, Judith Gassa- way, Wallace Jensky, Frank Kingery, Wayne Mattox, Arthur BOUNDARIES Ravenscroft, John Robinson, Henry Snyder, and Michael Sommer. COOLINO ZONES Data and advice were generously provided by Wendell Duffield —— STRUCTURAL and Leon T. Silver. We are indebted to G. H. Curtis for permitting COVER the use of the mass spectrometer at the Department of Geology and BETWEEN ZONED Geophysics at the University of California at Berkeley when we AND MIXED were in the process of setting up the laboratory at San Diego State COOLINO PROVINCES University. Figure 5. Distribution of biotite isochrons in California and western Following our initial reconnaissance work, we held a discussion Nevada. The term "cooling age" does not discriminate between minerals with Leon T. Silver, Paul Damon, and Garniss Curtis. We grate- that date intrusion and those that reflect subsequent events, such as delayed fully acknowledge the ideas offered at that meeting; some may be cooling or reheating. reflected in this paper.

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* 0 * Field K-Ar Latitude Longltude t SIze K Ar* Ac Rock types' Field K-Ar Latitude Longitude t Size K Ar'0, AgeS rad Rock types' sample analysis 1 sample analysIs (W.) (N.) (w.) (Standard (wt. %) (m: y.) (N.) (standard (wt. %) rad (m.y. ) no. no. mesh) (W no. no. mesh) (»)

+ BIA-I 816 3r30'42" 1I6°34'08" U A 80/150 0.383 55 95.9 1.7 quartz gabbro BlS-8 904 32°16'20" 115°57'07" U B 60/80 7.409 87 71.1 ± 0.5 granodlor1 te

BIA-4 818 3r34'29" 116*30'22" u H 80/150 0.470 72 105.2 1.1 tonal 1 te BlS-10 905 32*22'10" 115°58'48" U B 60/80 7.315 87 77.7 ± 0.5 tonaii te 1 BIA-7 825 32°06'54" 116*30 18" u H 80/150 0.277 57 113.3 t 1.9 gabbro B1S-12 906 32°17'37" 116*02 '28" u B 60/80 8.574 88 67-7 ± 0.5 do 1 BIA-9 835 32*02'27" 116*37'25" u P 60/80 0.243 53 98.1 ± 1.9 granod ior1 te BIS- 13 902 32*20 12" 116*02'21" u B 60/80 6.765 89 83.8 ± 0.6 quartz diorite

do 836 do do u B 60/80 7-384 83 100.1 ± 0.8 do do 899 do do u H 100/150 0.458 63 102.5 ± 1.4 do

do 837 do do u H 60/80 0.582 49 103.6 ± 2.3 do B1S-16 897 32°23'55" 1I6*06'49" u Mu 60/80 8.461 67 85.9 ± 1.0 apiitic granodioril

BlA-10 840 32*07'14" 116°30'42" p B 80/150 7.310 82 105.0 ± 0.8 do B1S-25 909 32°15'0O" 116*02'06" u H 100/150 0.678 63 90.9 ± 1.2 tonai i te

do S».l do do p H 80/150 0.750 69 104.9 ± 1.2 do BIS-103 901 32*25'22" 115°52'21" u B 60/80 5.530 83 75.2 ± 0.6 do

BlA-14 852 32*06'24" 116"33'36" u B 80/150 6.959 87 104.8 t 0.7 do do 900 do do u H 100/150 0.589 61 83.4 ± 1.2 do

do 851 do do p H 80/150 0.584 75 110.1 i 1.0 do BIS- 104 910 32*24'51" 115*53'39" u H 100/150 0.317 49 95.3 ± 2.1 gabbro

BlB-3 891 31*52'33" 1I6*33'09" u B 60/100 6.802 86 106.4 ± 0.8 do BIS- 105 908 32*32'20" I15*53'07" u B 60/80 7.359 81 74.8 ± 0.6 tonai 1 te

do 892 do do u H 60/100 0.815 71 100.9 ± 1.1 do B1S-106B 876 32°35'18" H5*44'00" p B 45/80 7.411 79 69.1 ± 0.6 do

BlB-4 858 3r52'04" 116°34'39" p B 80/150 7.293 92 103.4 ± 0.7 do do 879 do do u P 80/100 0.256 41 78.3 ± 2.3 do

do 857 do do p H 80/150 O.632 77 107.3 ± 1 .0 do B1S-106C 819 do do u B 45/80 7.530 93 68.4 ± 0.4 quartz gabbro

BI8-5 859 31*53'39" 1I6*35'31" p H 80/150 0.464 62 103.6 ± 1.4 tonal 1 te do 880 do do u P 80/100 0.258 34 60.1 ± 2.4 do

BIB-6 893 32*02'48" 116"08'24" u B 60/80 7.543 92 91.8 ± 0.6 granodi or i te B1S-106D 814 do do u Mu 45/80 8.745 83 62.2 ± 0.5 quartz-plag ioclase do 894 do do u H 60/80 0.665 57 95.3 1.6 do pegmatI te plag lociase-quartz do 881 do do u p 45/80 0.551 44 106.0 ± do BlB-7 116°07' 50" 40 ± 2.6 2.9 895 32°02'47" u P 60/100 0.656 84.9 pegmati te BIX-4 842 32*00'58" 116*25'12" u H 100/150 0.374 64 104.2 ± 1.4 gabbro do 896 do do u B 60/100 6.391 85 87.2 ± 0.6 do B1X-5 863 32*01'03" 116*24'30" u II 60/100 0.272 53 104.8 ± 2.0 tonai i te BIB-18 889 31"58'02" I16*00'00" u B 50/100 6.877 90 89.0 ± 0.6 granod i ori te B1X-11 830 3I°58'33" 116*22'47" u B 60/100 6.976 93 107.8 ± 0.7 granod iori te do 888 do do p H 35/60 0.828 54 82.7 ± 1.5 do B2G-I09 955 3ro3'36" 114*49'12" u B 35/100 7.337 92 85.0 ± 0.6 tonai i te B1B-20 886 31°54'51" 116*05'40" u B 50/100 7.600 91 97.8 ± 0.6 do do 954 do do p H 35/100 0.382 55 80.0 1 1.4 do do 887 do do u P 80/100 0.362 66 104.3 ± 1.3 do B2G-121 983 32*01'06" 115°14'27" u Ks 35/60 11.945 92 75.9 1 0.5 granodiori te B1B-26 811 31°41'45" 116°00'00" p Mu 18/35 8.890 73 91.3 ± 0.9 do 948 do do p B 35/60 7.478 52 76.9 ± 1.5 do B1C-11 834 31*54'18" 115°54'51" u Mu 60/80 8.309 85 86.8 ± 0.6 adame 11 i te B2G-221A 1040 32*09'26" 115*47'36" p B tonaii te BIC-12 812 31 °55'56" I15*54'27" p B 18/35 7.570 85 89.O i 0.6 granod i or i te do 1039 do do p H 60/100 0.842 37 98.9 ± 3.5 do B1C-13 854 32*07'54" ii5°56'58" u B 50/100 6.369 90 92.8 ± 0.6 tonal 1 te . B2G-222 961 32*07'09" I15°45'51" u Mu 60/100 8.237 65 81.9 ± 1.0 adamel1i te do 855 do do u H 60/80 0.949 84 94.0 0.7 do do 957 do do u B 60/100 7.109 91 77.9 ± 0.5 do B1D-5 873 32*59'42" I16"58'50" p Me 18/35 8.213 85 100.7 ± 0.7 adame 111 te B2G-225 956 32*29'03" 116*!2'53" u B 60/100 7.643 67 92.5 ± 1.1 quartz diorite do 874 do do p B 35/100 6.981 93 104.1 ± 0.7 do do 964 do do u H 60/100 0.878 74 98.0 ± 0.9 do B1D-6 875 32*57'57" 116"30'32" p B 45/80 7.138 91 88.9 0.6 folîated B2G-302 1017 32*22'01" 116*52 '57" u A 60/100 0.078 20 85.0 ± 6.8 gabbro granodîorlte B2G-304 993 32°30'01" 116*43'01" p B 60/100 6.911 86 113.3 ± 0.8 quartz diorite B1D-7 828 32°50' 30" 116"36"36" u B 45/80 7.641 94 99.7 ± 0.6 adame 111 te do 992 dò do p H 60/100 0.454 61 100.4 ± 1.4 do do 890 do do u Me 100/150 9.111 93 90.5 ± 0.6 do B2G-306 1059 32*34'00" 116*36'05" p P 100/140 O.O89 35 86.3 i 3.3 gabbro B1D-8 829 32*49'02" I16*51'36" u B 45/80 7.553 93 100.8 1 0.6 granod1 or 1 te do I06I do do p H 100/140 0.093 38 126.0 1 4.2 do do 878 do do u Me 80/100 8.101 89 97.0 ± 0.7 do B2G-307 1022 32*30'39" 116*36'06" p B 60/100 7.331 96 100.6 ± 0.6 granodiori te B1F-1 845 32*24'16" 116"54i59" u P 80/100 0.332 64 100.7 ± 1.3 granod¡or 1 te do 1023 do do p H 60/100 0.797 85 IO8.6 ± 0.8 do do 833 do do u B 80/100 6.724 93 114.2 ± 0.7 do B2G-308 988 32*38129" 116*46'36" u B 60/100 7.410 85 107.3 ± 0.8 tonaii te B1F-8 827 32*19'29" 116°I7'25" u P 80/100 0.093 26 74.8 ± 4.3 gabbro quartz di or i te do 987 do do u H 60/100 0.335 57 134.9 ± 2.2 do

do 826 do do u H 100/150 0.233 52 88.8 ± 1.8 do B2G-310 990 32°46'30" 116°59'03" u B 60/100 7.058 83 112.0 ± 0.8 do

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/760/3429075/i0016-7606-86-6-760.pdf by guest on 25 September 2021 B1F-3 822 32°19'00" 116°17'25" P B 60/80 7.067 88 93.2 ± 0 6 quartz gabbro do 991 do do B H 60/100 0.506 44 112.7 - 3.0 do do 823 do do P H 150/200 0.811 79 96.4 ± 0 8 do B2G-311 979 32°47'51" 117°or33" P H 60/100 0.560 74 101.8 t 1.0 adamel11 te o B1F-IO 853 32"18'27" 116°17'24" u B 100/150 7.808 87 90.2 ± 0 6 biot i te schi st B6B-13 520 31 30'30" 115°11'34" u B 60/100 7.577 83 64.5 ± 0.4 adamel11 te B1 F— 3 847 32°15'34" 116°19'29" u B 60/100 7.705 93 94.1 ± 0 6 tonal 1 te B6B-160 519 31°00'42" 115°07'27" u B 60/100 6.402 74 80.9 ± 0.6 lamprophyre do 844 do do u H 60/100 0.912 85 99.2 ± 0 7 do B6G-54 803 31°38'02" 116°28'22" u B 60/100 7.431 88 103.5 ± 0.7 tonai ite B1F-15 846 32°I4' 13" 116°I4'55" u P 80/100 0.213 46 77.3 ± 1 9 tonali te B8B-21 521 31'22'15" 115° 15 '00" u B 60/IOO 7.294 86 78.7 ± 0.5 granodiori te do 838 do do u B 80/100 6.023 83 95.1 ± 0 7 do B9A-1 573 31°34'33" 116°00'18" u H O.215 94.9 ± 2.5 amph 1 boi i te B1F-16 839 32°11'26" 116°13'09" u B 60/100 7.030 78 95.9 ± 0 8 quartz diorite B9E-1 501 31'18'27" 115°27'26" u B 60/IOO 8.222 90 77.3 ± 0.4 meta-pegmat1 te do 843 do do u H 60/100 0.654 76 99.3 ± 0 9 do B9E-2 567 31°18'39" 1I5"26'30" u B 60/100 6.336 29 79.2 ± 1.0 aplitic granodiorite B1 J-l 802 33°O0'54" 117°09'56" u P 150/230 0.122 25 121.0 7 5 andesite porphyry B9E-3 503 do do u B 7.110 75 80.8 0.4 amph1 boii te hornblende-blot i te + 115°27'26" B1J-2 813 32"39'29" 116°05' 48" u B 45/60 7.501 89 72.8 0 6 tonal 1 te B9E-4 568 3I°18'39" u H 60/100 0.262 47 80.0 ± 0.4 schi st B1J-3 849 32°34'22" 116'26'54" u B 80/100 6.957 86 85.6 ± 0 6 do BSE-5 509 31°19'13" 115°27'51" u B 7.500 89 81.4 ± 0.4 quartz diorite do 850 do do u H 80/100 0.733 74 83.6 ± 0.8 do B9E-6 505 31°18'39" 115"26'30" u Mu 24/60 8.644 75 79.0 ± 0.4 pegmati te B1J-4 882 32=33,33,, 115° 53' 39" u B 60/100 7.401 84 72.5 ± 0 5 do B9E-7 507 31" 18' 27" 115°27'26" u B 60/100 6.920 88 82.9 t 0.4 gnei ss do 883 do do u H 60/100 0.774 56 79.7 ± 1 4 do B9E-8 569 3I°18'39" 115°26'30" u B 60/100 7.596 90 80.0 ± 0.4 grani te BIJ-5 884 32°20'47" 116°17'33" u B 60/100 7.432 74 90.1 i 0 9 do B9E-100 572H 31"34'09" 1I6°00'28" u H 0.954 79 83.7 ± 1.5 tonali te do 885 do do u H 60/100 0.679 77 97.2 ± 0 9 do B9H-I 601 32°16'59" 116-31'35" u B 6.999 56 98.7 ± 1.8 do 1 BIK-3 861 32°17'50" ) 16°31 54" P B 45/80 7.528 91 98.0 ± 0 6 tonal i te do 600 do do u A 0.613 30 96.1 ± 2.3 do do 860 do do p H 80/150 0.725 74 103.8 ± 1 0 do B9S-1 511 31°41'08" 116°07'55" u 8 60/100 7.599 90 92.1 t 0.4 quartz diorite 1 B1K-7 856 32°22 59" 116°28'07" p H 80/150 0.274 40 104.8 ± 3 2 tona 11 te BC1-4I0 969 u B 7.282 91 91.2 ± 0.6 do BIK-8 862 32*25'17" 116°23'59" p H 80/150 0.273 59 98.4 ± 1 5 do 1038 u H 0.500 74 88.S 0.8 do B1R-5 821 32°47'O0" 1I6°51'51" u P 60/100 0.236 45 87.8 ± 2 2 gabbro BC1-411 1037 p B 6.607 91 104.5 ± 0.7 do do 824 do do p H 60/100 0.156 37 103.4 ± 3 6 do 968 u H 0.498 54 105.7 ± 1.9 do B1R-7 820 32°50'02" 1)6°43'54" u P 60/100 0.025 8 58.3 ± 13 7 do FB-1 515 32°17'52" 115°20'00" u B 60/100 5.603 90 62.6 ± 0.4 granod i ori te do do do do u P do do 11 58.4 ± 9 5 do G-1 51 4B 32°41'33" 116°03'11" u B 60/100 7.336 91 75.1 ± 0.4 tonali te a a do 817 do do P H 60/100 0.141 39 100.5 ± 3 3 do JKT 918 32 17' 02" 115 21'03" u B 6.953 50 67.1 ± 1.4 granodiori te BIR-10 831 32°59121" ]16'35'02" u P 60/100 0.066 30 74.1 ± 3 6 do JSR-1 769 32°37'51" 116°26'12" u B 45/60 7.562 86 90.8 ± 0.6 granod ior1 te do 832 do ' do p H 60/100 0.380 68 104.5 ± 1 2 do do 770A do do u H 45/60 O.491 50 102.7 ± 2.2 do B1 Ft-11 869 32°53'15" I16"38'51" u P 50/60 0.058 21 78.8 ± 5 9 do JSR-3 777 32°36'48" 116°12'32" u B 45/60 7.494 90 87.3 ± 0.6 adamel1i te do 870 do do p H 50/60 0.225 57 112.0 ± 1 9 do JSR-6 76OA 32°42'26" 116"01'52" u B 45/60 7.169 73 76.7 0.7 tonali te B1R-14 877 32°54121" 116°27'21" p Ph 35/60 6.023 85 92.3 ± 0 7 do do 764 do do u H 45/60 0.843 60 85.6 ± 1.3 do do 872 do do p H 35/60 0.272 56 110.0 ± 1 9 do do 765 do do u B 45/60 7.169 82 77.6 i 0.6 do BIR-15 864 32°49'27" 116"30'03" u P 50/60 0.215 43 72.1 ± 2 0 do JSR-7 776 32°40'45" 116°17'31" u B 45/60 6.992 89 88.5 ± 0.6 granod i ori te do 865 do do p H 50/60 0.262 52 IO7.8 ± 2 1 do NS 919 32° 16 ' 42" 115°2i'22" u B 18/60 5.107 42 64.7 ± 1.8 schist B1B-l6 866 32-43'56" 116"34'39" u P 18/35 0.022 11 96.9 ± 15 3 gabbro pegmatite S2G-224B 1060 32°35'18" 115°44'00" u H 60/100 0.479 67 73.0 0.9 gneiss do 867 do do u H 18/35 0.070 26 143.3 ± 8 2 do BC1-911 1337 32° 15 ' 00" 116°35'50" u H 45/60 0.61 38 94.2 i 3.0 granod ior i te B1S-4 848 32°12'42" 115°58'57" u B 60/100 7.916 90 88.9 ± 0 6 . sch i st BCI-911 1338 32'15'00" 116°35'50" u B <60 4.39 71 97-9 ± 1.0 granod ior i te do 868 do do u Mu 100/150 7.751 91 96.8 ± 0 6 do B4G-806 1339 31°47'20" II5°49'40" u H 60/100 1.6 52 97.0 ± 1.9 tonal Ite B1S-5 898 32°13'10" 115"59'06" u B 60/80 7.505 66 91.2 ± 1 1 aplitic granodiorite B4G-806 1336 3I"47'20" 115°49'40" u B 60/100 6.59 92 89.7 ± 0.6 tonai ite B1S-6 903 32°15'41" 115°58'07" u B 60/80 7.176 84 77.1 ± 0 6 tonali te DRH-77417 1230 32°44'40" 116°42'15' u H 0.184 12 129.0 ± 18.6 gabbro

* P s picked; U = unpicked. t A • actinolite; B = biotite; H = hornblende; Mc = microcline; Mu = muscovite; Ks = potassium feldspar; P = plagioclase; Ph = phlogopite. § ± value only reflects uncertainty from percent radiogenic Ar. i Rock names follow definitions of Gastil and others (1975, Fig. 15).

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