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An 18O/16O study of Mesozoic and early Tertiary granitic batholiths of the southwestern North American Cordillera

Solomon, G. 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.


Abundant evidence from previous studies indicates that, as long as samples are collected well away from pluton margins, the whole-rock [delta]18O 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, [epsilon]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 [delta]18O: +6.7 to +7.4). The late Cretaceous plutons are metaluminous hornblende-bearing monzogranites and granodiorites (+7.4 < [delta]18O < +9.9). The early Tertiary (Laramide) plutons are all peraluminous, leucocratic, two-mica granites (+8.2 < [delta]18O < +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 [delta]18O 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 [delta]18O values (+6.7 to +8.0). The Jurassic and Cretaceous magmas had [delta]18O 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 [delta]18O values than typical S-type plutons. In contrast, the Cretaceous plutons emplaced west of mapped cratonal basement in the SBM and SGM have high [delta]18O values, between +8.5 and +10.8. This east-west change in primary whole-rock [delta]18O 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 [delta]18O 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 [delta]18O values ranging from -3.4 to +5.7, and where sampling density permits, contouring of [delta]18O 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 [delta]18O 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 [delta]18O 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 [delta]18O values that are uniformly lower than +8.5, and east of this boundary the plutons have [delta]18O > +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 [delta]18O values < +9, and farther inland, [delta]18O 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 [delta]18O 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 < [delta]18O < +8.5; (2) a Central Zone (CZ) of high-18O plutons with [delta]18O between +8.5 and +13.2; and (3) an Eastern Zone (EZ) with variable [delta]18O, typically lower than +9.0, but locally exhibiting plutonic centers with [delta]18O > +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 [delta]18O = +6 to +7, (87Sr/86Sr)i ~ 0.702 to 0.704, and [epsilon]Nd ~ +2 to +7; (2) "eugeosynclinal" sediments and altered volcanic rocks (SAV-type sources) with [delta]18O = +10 to +13.5, (87Sr/86Sr)i ~ 0.705 to 0.710, and [epsilon]Nd = -2 to -9; (3) "miogeosynclinal" continental margin sediments (MCM), with [delta]18O > +10, (87Sr/86Sr)i > 0.715, and [epsilon]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 [delta]18O values of +6.0 to +8.0 in the SBR and +7.5 to +9 in the NGB, and [epsilon]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 [delta]18O = +8 to +10, (87Sr/86Sr)i > 0.715, and [epsilon]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/86SDr)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 ([delta]18O = +7 to +9) and SBR ([delta]18O = +6 to +8) bear on the structure of the craton. The [delta]18O 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-180 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 [delta]18O of the LCC/SCL underneath the SBR was lowered relative to the equivalent zone in the NGB.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Major Option:Geological and Planetary Sciences
Thesis Committee:
  • Taylor, Hugh P. (chair)
  • Rossman, George Robert
Defense Date:7 April 1989
Record Number:etd-01192007-082647
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:237
Deposited By: Imported from ETD-db
Deposited On:30 Jan 2007
Last Modified:25 Dec 2012 14:58

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