High Initial Sr Isotopic Ratios and Evidence for Magma Mixing in the Pioneer Batholith of Southwest Montana

Arth, Joseph G.; Zen, E‐an; Sellers, G. A.; Hammarstrom, Jane M.

doi:10.1086/629040

Cited by 21 publications

(11 citation statements)

References 8 publications

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“…Initial Sr isotopic values for the Late Cretaceous Pioneer batholith range from 0.7110 to 0.7160 (Arth et al 1986;Mueller et al 1997), and Pb isotopes give a model age of 1.9 ± 0.2 Ga (Doe et al 1968;Zen 1992). These results suggest involvement of mainly Paleoproterozoic crust, as do inherited zircon ages of -1.4 and 1.7-1.8 Ga (SHRIMP-II) for the Grayling Lake granite and Uphill Creek granodiorite reported by Murphy et al (2002).…”

Section: Evidence For the Selway Terranementioning

confidence: 99%

Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

et al. 2006

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Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous-Tertiary magmatism. U-Pb zircon geochronological data from small exposures of pre-Belt supergroup basement along the western side of the Wyoming craton, in southwestern Montana, reveal crystallization ages ranging from -2.4 to -1.8 Ga. Rock-forming events in the area as young as -1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous-Eocene granitoids. Most of this lithosphere is primitive, gives ages -1.7-1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions.

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Section: Evidence For the Selway Terranementioning

confidence: 99%

Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

et al. 2006

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“…The Pioneer Batholith is characterised by iSr values of 0-7113-0-7160 (Arth et al 1986), whereas the Boulder on July 12, 2015 specialpapers.gsapubs.org Downloaded from Batholith is characterised by iSr values of 0-7055-0-7092 (Doe et al 1968). These two batholiths, so discriminated, are neatly separated on the ground by a system of high-angle faults known as the Trusty Gulch-Sawmill Gulch Fault System (Zen 1988b), as is shown on Figure 2. 2.…”

Section: The Late Cretaceous Batholiths Of Southwestern Montanamentioning

confidence: 99%

Using granite to image the thermal state of the source terrane

Zen

1992

Geological Society of America Special Papers

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It should be possible to infer the thermal state of the source terrane for granitic bodies, provided we have independent means to establish the chemical nature of this terrane. The chemical nature of the granitic rocks, including their degree of hydration, implies the solidus temperature. The concentration of the heat-producing radioactive elements in the granite (K, U, and Th) probably provides an upper estimate of their concentration in the source rock, which is an important thermal parameter. The depth and ambient temperature of the country rock into which the granite magma intruded provide useful boundary conditions for the thermal regime at the crustal level of anatexis. These constraints in turn form the bases for estimating the subcrustal thermal flux as well as the effective thermal interface for enhanced heat flow from below that resulted in anatexis. These inferences, in combination with other field-based parameters such as uplift rates and permissible time lapses for the geological events, permit realistic thermal modelling for the formation of granitic batholiths. The procedure is applied to the Late Cretaceous Pioneer and Boulder batholiths in southwestern Montana, U.S.A. The modelling results suggest that mantle upwelling, not subduction or thrust loading, caused anatexis. The isotopic chemistry of the granitic rocks rules out direct mixing of mantle magma, and field relations rule out crustal thinning as causes for partial melting.

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“…Cordilleran plutons is given byArth et al [ 1986] for plutons of the Pioneer batholith, Montana and byFleck and Criss [1985] for the western Idaho batholith. Evidence for magma mixing followed by fractional crystallization is given byKistler et al [1986] for plutons of the Sierra Nevada batholith, California.…”

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Isotopic and trace element variations in the Ruby batholith, Alaska, and the nature of the deep crust beneath the Ruby and Angayucham terranes

Arth¹,

Zmuda²,

Foley³

et al. 1989

J. Geophys. Res.

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Thirty‐six samples from plutons of the Ruby batholith of central Alaska were collected and analyzed for 22 trace elements, and many were analyzed for the isotopic compositions of Sr, Nd, O, and Pb in order to delimit the processes that produced the diversity of granodioritic to granitic compositions, to deduce the nature of the source of magmas at about 110 Ma, and to characterize the deep crust beneath the Ruby and Angayucham terranes. Plutons of the batholith show a substantial range in initial 87Sr/86Sr (SIR) of 0.7055–0.7235 and a general decrease from southwest to northeast. Initial 143Nd/144Nd (NIR) have a range of 0.51150–0.51232 and generally increase from southwest to northeast. The δ18O values for most whole rocks have a range of +8.4 to +11.8 and an average of +10.3‰. Rb, Cs, U, and Th show large ranges of concentration, generally increase as SiO2 increases, and are higher in southwest than in northeast plutons. Sr, Ba, Zr, Hf, Ta, Sc, Cr, Co, and Zr show large ranges of concentration and generally decrease as SiO2 increases. Rare earth elements (REE) show fractionated patterns and negative Eu anomalies. REE concentrations and anomalies are larger in the southwest than in the northeast plutons. Uniformity of SIR and NIR in Sithylemenkat and Jim River plutons suggests a strong role for fractional crystallization or melting of uniform magma sources at depth. Isotopic variability in Melozitna, Ray Mountains, Hot Springs, and Kanuti plutons suggests complex magmatic processes such as magma mixing and assimilation, probably combined with fractional crystallization, or melting of a complex source at depth. The large variations in SIR and NIR in the batholith require a variation in source materials at depth. The southwestern plutons probably had dominantly siliceous sources composed of metamorphosed Proterozoic and Paleozoic upper crustal rocks. The northeastern plutons probably had Paleozoic sources that were mixtures of siliceous and intermediate to mafic crustal rocks. The inferred sources could well have been the higher‐metamorphic‐grade lithologic equivalents of the exposed Proterozoic(?) to Paleozoic schists, orthogneisses, and metavolcanic rocks of Ruby terrane, the silicic portions of which are quite radiogenic. The deeper crustal sources that gave rise to most of the batholithic magmas are inferred to be similar under both the Ruby metamorphic terrane and the Angayucham ophiolitic terrane.

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High Initial Sr Isotopic Ratios and Evidence for Magma Mixing in the Pioneer Batholith of Southwest Montana

Cited by 21 publications

References 8 publications

Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

Using granite to image the thermal state of the source terrane

Isotopic and trace element variations in the Ruby batholith, Alaska, and the nature of the deep crust beneath the Ruby and Angayucham terranes

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