An ¹⁸O/¹⁶O Study of Mesozoic and Early Tertiary Granitic Batholiths of the Southwestern North American Cordillera

Citation

Solomon, George Cleve (1989) An ¹⁸O/¹⁶O Study of Mesozoic and Early Tertiary Granitic Batholiths of the Southwestern North American Cordillera. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/T831-NP65. https://resolver.caltech.edu/CaltechETD:etd-01192007-082647

Abstract

Abundant evidence from previous studies indicates that, as long as samples are collected well away from pluton margins, the whole-rock δ¹⁸O value of an unaltered granitic pluton is not likely to vary by more than ±0.5 per mil from the original ¹⁸O/¹⁶O composition of its source rocks. Therefore, granitic plutons may be viewed as "remote-sensing probes" which sample deep portions of the continental crust or upper mantle, and ¹⁸O/¹⁶O studies of such plutons can provide detailed information on lithologic boundaries at depth. This thesis presents approximately 350 new ¹⁸O/¹⁶O analyses of whole-rock and quartz powders from Mesozoic and Cenozoic granitic plutons in the Northern Great Basin (NGB) and Southern Basin and Range (SBR) provinces of the western United States. The samples were collected along two broad, regional traverses eastward from the Sierra Nevada Batholith (SNB) and the Peninsular Ranges Batholith (PRB) in California: (1) the NGB traverse from western Nevada, near Carson City, eastward to the area around Salt Lake City, Utah; (2) the SBR traverse in southeastern California (SECA), eastward from the Central and Eastern Transverse Ranges across the Mojave Desert to the Colorado River, and then southeastward into southern Arizona. Where available, wholerock major-element geochemistry, [ε_Nd, and (⁸⁷Sr/⁸⁶Sr)_i analyses of the same samples by other workers are integrated with these ¹⁸O/¹⁶O analyses. In addition, several hundred whole-rock ¹⁸O/¹⁶O analyses and, where available, Nd and Sr isotopic data, have been taken from the literature and combined with the new results to compile a data base that provides virtually complete reconnaissance coverage of the batholithic terranes in the Cordillera of southwestern North America.

Samples in the southern Arizona part of the SBR traverse were collected from Jurassic, late Cretaceous, and early Tertiary granitic plutons emplaced well within mapped boundaries of the > 1.5 Ga craton. The Jurassic plutons are metaluminous, alkali-calcic, epizonal syenites, monzodiorites and granodiorites (avg. whole-rock δ¹⁸O: +6.7 to +7.4). The late Cretaceous plutons are metaluminous hornblende-bearing monzogranites and granodiorites (+7.4 < δ¹⁸O < +9.9). The early Tertiary (Laramide) plutons are all peraluminous, leucocratic, two-mica granites (+8.2 < δ¹⁸O < +9.0), which exhibit synkinematic and post-kinematic features. The Cretaceous suite is sliqhtly more ¹⁸O-enriched and less oxidized than the Jurassic suite. The peraluminous two-mica granites, which are mineralogically closest to typical S-type plutons (as defined in SE Australia), have distinctly lower δ¹⁸O values than most S-type granitic rocks throughout the world. We therefore classify all of these Arizona granites as basically I-type; there is no isotopic evidence for a major, pelitic, S-type source in southern Arizona. The two-mica granites probably represent highly fractionated "first-melts" of cratonal basement, while the epizonal Jurassic and Cretaceous plutons probably formed from "drier" melts that originated deeper in the crust.

The samples in the Transverse Ranges and the SECA part of the SBR traverse were obtained from Triassic monzonites and syenogranites, Jurassic granodiorites and monzogranites, and late Cretaceous granodiorites, monzogranites, and two-mica granites. Most of these plutons are alkalicalcic to alkalis and were intruded upward through Precambrian basement rocks, with the exception of Cretaceous calc-alkaline monzogranites and rare two-mica granites emplaced west of cratonal basement rocks in the San Bernardino Mountains (SBM) and San Gabriel Mountains (SGM). The Triassic plutons (e.g. Mt. Lowe pluton) have uniformly low δ¹⁸O values (+6.7 to +8.0). The Jurassic and Cretaceous magmas had δ¹⁸O values between +6.7 and +9.3, including the two-mica granites of the Old Woman Mountains (+7.2 to +9.3), Cadiz Valley Batholith (+7.7 to +9), Chemehuevi Mountains (+7.8), and eastern SBM (+8.8 to +8.9). As in southern Arizona, these "cratonal" two-mica granites have lower δ¹⁸O values than typical S-type plutons. In contrast, the Cretaceous plutons emplaced west of mapped cratonal basement in the SBM and SGM have high δ¹⁸O values, between +8.5 and +10.8. This east-west change in primary whole-rock δ¹⁸O marks a fundamental, regional ¹⁸O/¹⁶O boundary, which we believe can be used to map the western edge of the craton in the Cordillera of the southwestern USA. The whole-rock δ¹⁸O values of the plutons in the fault-reconstructed SGM terrane, the SBM terrane, and in the Little San Bernardino Mountains (LSB) can be contoured in a systematic fashion, and these contours are subparallel to the aforementioned regional ¹⁸O/¹⁶O boundary.

Nearly all of the Jurassic plutons in the SBR traverse were emplaced into shallow volcanic centers, and they show characteristics related to calderas, including hydrothermal alteration by heated low-¹⁸O meteoric waters. The altered plutons have δ¹⁸O values ranging from -3.4 to +5.7, and where sampling density permits, contouring of δ¹⁸O values reveals map patterns similar to those found at other meteoric-hydrothermal centers throughout the world. The best-studied of these Jurassic centers in this work is the Rodman-Ord Mountains (ROM) area, where the distinctive ¹⁸O/¹⁶O map patterns produced by the Jurassic hydrothermal events have been used to estimate approximately 3 to 4 km of left-lateral strike-slip displacement along the late Cenozoic Camp Rock Fault. These SBR calderas are apparently part of a major Jurassic rift-system that extends from southeastern Arizona to the California-Nevada border. The low δ¹⁸O values of the altered Jurassic plutons in SECA indicate that the paleoclimate in that portion of the rift was typical of mountainous regions today.

The Oligocene to Jurassic plutons in the NGB traverse in Nevada and Utah are the same samples analyzed by Farmer and DePaolo (1983) in their Nd-Sr isotopic study of NGB plutonism: (1) calc-alkaline, metaluminous granodiorites and monzogranites intrude eugeoclinal Paleozoic allochthonous terranes between the SNB and the Roberts Mountain Thrust; (2) calc-alkaline, metaluminous to peraluminous granodiorites, monzogranites and two-mica granites intrude miogeoclinal terranes between the Roberts Mountain Thrust and the first outcrops of > 1.5 Ga cratonal basement going east; (3) alkali-calcic monzodiorites, granodiorites, and monzogranites intrude cratonal shelf sediments deposited on > 1.5 Ga craton in northeastern Nevada and western Utah. The primary, whole-rock δ¹⁸O values in the first of the above groups exhibit the same geographic systematics discovered by Taylor and Silver (1978) for the PRB in southern and Baja California. There is a sharp, north-trending ¹⁸O/¹⁶O boundary in western Nevada, analogous to the longitudinal "¹⁸O-step" down the center of the PRB. West of this boundary, the NGB plutons have δ¹⁸O values that are uniformly lower than +8.5, and east of this boundary the plutons have δ¹⁸O > +8.5, ranging up to +13.2. The highest ¹⁸O/¹⁶O areas coincide with the second of the above groups, particularly where two-mica granite plutons occur. Just east of the Utah border, the third group of plutons exhibits δ¹⁸O values < +9, and farther inland, δ¹⁸O decreases to values as low as +6.7. This eastern boundary is inferred to be the same one we observe in the eastern Transverse Ranges in SECA.

We use the ¹⁸O/¹⁶O data from the NGB and SBR traverses, combined and augmented with literature-derived data on the PRB, SNB, and Idaho Batholith to provide a framework for viewing the subcrustal distribution of petrotectonic assemblages in much of the western United States. In conjunction with the Nd-, Pb- and Sr-isotopic signatures, the ¹⁸O/¹⁶O data are used to map isotopic variations in the source regions of these plutons. This method yields a well-constrained model for the continental crust (especially when compared with earlier models that do not take into account the ¹⁸O/¹⁶O values). Such studies are particularly helpful in constraining rock-types in these source regions, because ¹⁸O/¹⁶O variations in rocks arise in a totally different manner than do the radiogenic isotope variations, which are mostly dependent upon age and upon various trace element concentrations.

As discovered in the PRB by Taylor and Silver (1978), the δ¹⁸O values of granitic rocks in the western United States define a series of sharp isotopic boundaries, independent of pluton lithologies, between different geographic groupings of granitic plutons. These are extremely well defined for the Cretaceous magmatic arc, for which three north-trending belts of plutons exist: (1) a Western Zone (WZ) of low-¹⁸O plutons with +5.5 < δ¹⁸O < +8.5; (2) a Central Zone (CZ) of high-¹⁸O plutons with δ¹⁸O between +8.5 and +13.2; and (3) an Eastern Zone (EZ) with variable δ¹⁸O, typically lower than +9.0, but locally exhibiting plutonic centers with δ¹⁸O > +9.0 (commonly associated with metamorphic core complexes). When (⁸⁷Sr/⁸⁶Sr)_i values are taken into account, the Central Zone in the NGB must be divided into two geographic and geochemical entities; one lying west of a north-trending (⁸⁷Sr/⁸⁶Sr)_i "step" (< 0.7080 to the west and > 0.7100 to the east), and one between this ⁸⁷Sr/⁸⁶Sr "step" and the CZ-EZ boundary. The westernmost part is here termed the Central V-type subzone (CZ-V), and the eastern part is termed the Central S-type subzone (CZ-S). The CZ-S subzone is not present (except on a very small, local scale) south of approximately latitude 37°N, but it makes up approximately half of the Central Zone in the NGB, and dominates the CZ in the Idaho Batholith, north of the NGB. In contrast, the CZ-V subzone extends along the entire length of the Cordillera in the western USA, although it is very narrow north of 40°N latitude in the western portions of the Idaho Batholith.

The three geographic ¹⁸O/¹⁶O zones have boundaries coincident with several fundamental geologic features. The WZ occurs west of the quartz diorite line of Moore (1959) while the CZ is centered on the thickest portions of the late Precambrian-early Phanerozoic (0.3 to 1.5 Ga) Cordilleran geosyncline. The CZ in general lies east of the quartz diorite line, and west of the western limits of > 1.5 Ga Precambrian crystalline basement. The CZ-V subzone lies within the area of the geosyncline characterized by accreted terranes and dominated by eugeoclinal lithologies, whereas the CZ-S subzone appears to be associated with late Proterozoic miogeoclinal metasedimentary rocks. The EZ is located east of the western limit of older (> 1.5 Ga) crystalline basement and east of the thick geosynclinal sedimentary section. The EZ hosts most of the major porphyry copper deposits of the region, whereas the CZ hosts the known tungsten-skarn deposits.

The isotopic data suggest that the Cordilleran granitic plutons are derived from varying proportions of the following major end-member components (largely by simple two-component mixing): (1) upper mantle and/or subducted oceanic crust, either an Oceanic Island Arc (OIA), or MORB-type source, with δ¹⁸O = +6 to +7, (⁸⁷Sr/⁸⁶Sr)_i ~ 0.702 to 0.704, and ε_Nd ~ +2 to +7; (2) "eugeosynclinal" sediments and altered volcanic rocks (SAV-type sources) with δ¹⁸O = +10 to +13.5, (⁸⁷Sr/⁸⁶Sr)_i ~ 0.705 to 0.710, and ε_Nd = -2 to -9; (3) "miogeosynclinal" continental margin sediments (MCM), with δ¹⁸O > +10, (⁸⁷Sr/⁸⁶Sr)_i > 0.715, and ε_Nd < -9; (4) some type of "model lithospheric component" in the lower continental crust (LCC, > 1.5 Ga) and/or upper mantle (SCL), having evolved, crustal characteristics and (⁸⁷Sr/⁸⁶Sr)_i of about 0.705 to 0.710, with δ¹⁸O values of +6.0 to +8.0 in the SBR and +7.5 to +9 in the NGB, and ε_Nd = -6 and -12 in the NGB and -4 and -10 in the SBR; and (5) mid-level continental crust of the craton (MCC) with δ¹⁸O = +8 to +10, (⁸⁷Sr/⁸⁶Sr)_i > 0.715, and ε_Nd < -12 to as low as -20. The latter values depend on the age of the crust.

The simplest way to characterize each geographic ¹⁸O/¹⁶O zone is by simple two-component melt-solid or solid-solid mixing of source-region materials, although the lack of specific isotopic data on the actual end-members precludes a rigorous evaluation of the relative importance of assimilation-fractional crystallization (AFC) processes. Western Zone: OIA-SAV with OIA dominant. Central V-type subzone: OIA-SAV with SAV dominant. Central S-type subzone: dominantly MCM with minor SAV, LCC, and OIA(?). Eastern Zone: dominantly LCC/SCL with widely varying proportions of some other end-members, such as MCC and/or a modified OIA mantle component (i.e., one that is older and more LIL-enriched than Cretaceous OIA or MORB, and thus one with a relatively high Sr content and high ⁸⁷Sr/⁸⁶Sr ratio).

Previous workers place the "edge of the craton" beneath the -0.706 (Kistler and Peterman, 1978) or -0.708 (Farmer and DePaolo, 1983) (⁸⁷Sr/⁸⁶Sr)_i boundaries. However, we suggest that, in the NGB the (⁸⁷Sr/⁸⁶Sr)_i "step" (0.708) is not the edge of the craton, but instead is probably a structural discontinuity that has juxtaposed an accreted terrane of eugeosynclinal volcanic and volcanogenic sedimentary rocks on the west against a late Precambrian sedimentary terrane on the east. The sharpness of this boundary implies that it is either the edge of an ancient rift-zone (Kistler and Peterman, 1978), a strike-slip fault, or a suture zone. The hypothetical late Precambrian metasedimentary basin that we infer east of the ⁸⁷Sr/⁸⁶Sr "step" could represent an in-filled pull-apart basin, which opened during southward transport of the "Mojavia" terrane of Bennett and DePaolo (1987), thus explaining the east-trending boundary between the CZ and EZ that extends all the way across south-central Nevada.

The isotopic differences inferred for the deep continental crustal sources (LCC/SCL) in the NGB (δ¹⁸O = +7 to +9) and SBR (δ¹⁸O = +6 to +8) bear on the structure of the craton. The δ¹⁸O of the LCC/SCL component in SECA is similar to that in southern Arizona, implying that the ¹⁸O/¹⁶O composition of LCC/SCL in the SBR was acquired after any of the hypothetical tectonic movements that shifted "Mojavia" from the NGB southward into southeastern California (Bennett and DePaolo, 1987). This means that: (1) a relatively low-¹⁸O source could have underplated the SBR (including Mojavia) after tectonic emplacement of "Mojavia" athwart the southern Arizona region; or (2) previous fusion events at 1.4 Ga and 1.1 Ga could have modified the SBR deep sources, such that the δ¹⁸O of the LCC/SCL underneath the SBR was lowered relative to the equivalent zone in the NGB.

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