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Book Chapter

The Idaho batholith and associated plutons, Idaho and Western Montana

By
Donald W. Hyndman
Donald W. Hyndman
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Published:
January 01, 1983

The 39,000-km2 Idaho batholith lies 600 km east of the present Pacific coastline and east of the Columbia River basalt plateau. The batholith is Late Cretaceous in age and is emplaced immediately east of the Triassic Seven Devils volcanic arc, an apparently allochthonous terrane which may be part of the recently recognized "Wrangellia terrane" of western Canada and southern Alaska. Locus of a Late Cretaceous subduction zone related to the Idaho batholith is not yet defined but must lie west of the Seven Devils arc.

Country rocks of the Idaho batholith are Proterozoic Belt metasediments and pre-Belt basement orthogneisses. Pre-batholithic, sillimanite-zone, regional dyna-mothermal metamorphism, apparently Jurassic or Cretaceous in age, is broadly concentric to the northern half of the batholith, extending for a few to several kilometers beyond the contact.

The Idaho batholith is dominantly medium-grained, massive to moderately foliated, muscovite-biotite granite and granodiorite. Gneissic tonalite, rich in biotite and hornblende, forms a 12-16 kilometer-wide western border zone of the batholith, and tonalite or trondhjemite form satellitic plutons for 50-70 kilometers to the west. Granodiorite makes up a 10-20 kilometer-wide border zone against exposed country rocks of most of the batholith and surrounds the voluminous granite of the batholith interior. Such granodiorite has not been documented adjacent to the pre-Belt basement rocks of the northwest-trending Salmon River arch, which divides the Idaho batholith into a northern Bitterroot lobe and a southern Atlanta lobe. Granodiorite may have formed a broad shell over most of the batholith which may have originally extended across the deeply eroded Salmon River arch.

The borders of a few large separate intrusions have been partly documented, especially in the interior of the Bitterroot lobe, but for most of the batholith, separate major bodies are as yet unknown. Foliation in the western tonalitic border zone of both lobes dips 50 to 70 degrees eastward under the batholith. To the east in the main body of the batholith, the foliation weakens and gradually arches to nearly horizontal in the interior. Southwestern and northeastern border zones of the deeper northern part of the batholith are marked by large alternating sheets of granitic and high-grade country rocks. Large, tabular or contorted inclusions and nebulous schlieren are abundant towards the interior.

The north-trending Bitterroot dome of the northeastern Bitterroot lobe appears to have formed a mushroom-shaped diapir into the country rocks, then rose isostatically in response to eastward unloading of the 6000 km2 Sapphire tectonic block. The base of the flanks of this part of the batholith has been mapped on the southwest and northeast.

Chemically, the Idaho batholith appears related to the volumetrically minor "sodic series" recognized by Tilling in the Boulder batholith to the east. Radiometric ages in both batholiths appear to be similar. The hornblende-bearing tonalites of the western border and more-mafic satellitic plutons to the west, north, and northeast of the Idaho batholith appear to have the "I-type", "magnetite-series" mineralogy of Chappell and White and of Ishihara, respectively. The muscovite-bearing main units of the batholith appear to have "S-type" and probably "ilmenite-series" mineralogy.

Sources for magmas forming the tonalitic western border zones of the Idaho batholith were probably mafic-rich rocks of the upper mantle or of subducted oceanic crust or young continental margin volcanic rocks. Magmas forming the granodiorite-granite main body of the batholith wre probably derived by partial melting of Precambrian continental basement rocks. The main body of the Bitterroot Lobe of the batholith appears to have been emplaced at a depth of 15 to 20 km, and much of the magma was probably generated at not much greater depth.

Metallic mineral deposits are essentially absent from the deeper Bittrroot lobe. Correlation of magnetite-series granitic rocks and Cu-Mo mineralization and ilmenite-series granitic rocks with Sn-W mineralization can be partly documented for the Atlanta lobe and for plutons east of the batholith, but data are sketchy and, in part, appear contradictory.

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Contents

GSA Memoirs

Circum-Pacific Plutonic Terranes

J. A. Roddick
J. A. Roddick
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Geological Society of America
Volume
159
ISBN print:
9780813711591
Publication date:
January 01, 1983

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