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					The Challis Volcanic Group

Of the several Eocene volcanic fields in the northwestern U.S., the Challis field is the largest
(Fig. 2). Beginning about 51 Ma, lava flows, tuffs, and volcaniclastic sediments were deposited
across central Idaho in a brief but intense volcanic episode that ended about 45 Ma, although
minor silicic intrusives continued to 40 Ma. At the same time, plutons of porphyry and pink
granite were intruded. Those intrusive rocks are most voluminous along the eastern margin of
the Idaho batholith (Link and Janecke, 1999). Plutons range in composition from diorite to
granite and have overall characteristics of anorogenic granitoids that form by intracontinental
rifting (Kiilsgaard and Bennett, 1995). Ore-forming fluids circulated around porphyries, dikes,
and domes, yielding veins rich in silver and gold; circulation of fluids was controlled by caldera
complexes and by faults and fracture zones of the Trans-Challis fault system (Fisher and
Johnson, 1995; Kiilsgaard and Bennett, 1995).

Petrochemistry and nomenclature
Although field names can be applied to volcanic rocks based on color and phenocryst
assemblage, definitive names await chemical analysis. The diagram used for "arc magmas" plots
K2O versus SiO2 (Fig. 3). You probably are familiar with the "normal" series basalt -> basaltic
andesite -> andesite -> dacite -> rhyolite. That series is called calc-alkaline. Challis volcanics
plot, for the most part, in two series with higher K2O content (Fig. 3): the high-K series from
high-K basalt -> high-K basaltic andesite -> high-K andesite -> high-K dacite -> high-K rhyolite,
and the shoshonite series from absarokite -> shoshonite -> latite -> trachyte. Contents of Na2O
tend to increase along with K2O and therefore the the high-K and shoshonite series tend to be
more "alkalic."

In modern arcs high-K and shoshonite series rocks occur further back from the trench than the
calc-alkaline volcanoes that characterize the main arc. Whether or not that was true in the
Eocene in western North America is a subject of debate.

The northern Challis volcanic field
The greatest volumes of extrusives and intrusives occur along or near the Trans Challis fault
zone (Fig. 1, to the northwest). The extrusives are largely explosive in origin. Eruptive centers
include the Thunder Mountain cauldron complex, the Van Horn cauldron complex north of
Challis, the Twin Peaks caldera, and a small caldera south of Salmon, ID.

Volcanism occurred in three stages: (1) Extrusion of dacite flows with phenocrysts of
plagioclase, pyroxene, and/or amphibole and biotite and of andesite to basalt flows with
phenocrysts of pyroxene or olivine and plagioclase (McIntyre et al., 1982). Flows issued mainly
from stratovolcanoes or dome complexes. Most of that activity ended by 49 Ma (Fisher and
Johnson, 1995). An equally complex assemblage of igneous rocks was intruded. (2) Rhyodacite
to rhyolite ash-flows tuffs erupted in explosive volcanic episodes, each accompanied by collapse
of the calderas and cauldrons mentioned above. This activity occurred between about 45 and 48
Ma (Fisher and Johnson, 1995). Subsidence of grabens in the Trans-Challis fault zone preceded
and was synchronous with volcanism. The tuff of Challis Creek was the most extensive,
erupting from the Twin Peaks caldera outward at least 100 km to the southeast (Fig. 1, Janecke
and Snee, 1993). (3) Intrusion of rhyodacite to rhyolite domes and plugs. Epithermal precious-
metal mineralization is associated with the domes.


The town of Challis was settled by ranchers and then miners supplying the Yankee Fork gold
mines. Rocks south and west of town are Challis volcanics and consist of dacite and andesite
lavas and ashflow tuffs, erupted mainly from centers in the Twin Peaks Caldera and Van Horn
cauldron. North-south side roads off the main east-west street through Challis provide access to
a section of east-dipping ashflow tuffs on the northwest side of town (Link and Janecke, 1999;
McIntyre and Hobbs, 1987).

The southern Challis volcanic field
In the southern field, roughly south of about 44o15' (Fig. 1), there are two contrasting provinces.
In the Lost River and Lemhi Ranges and in the Pioneer Mountains (to the west of the White
Knob Mountains), andesitic and dacitic flows dominate, and tuffs are small in volume (Janecke
and Snee, 1993). In most portions of the White Knob Mountains, by contrast, ash-flow tuffs are
dominant. In the White Knob Mountains the volume of volcanic deposits is smaller than
elsewhere in the Challis field, and the deposits were erupted over a shorter time interval of 49-47
Ma (Snider and Moye, 1989).

Locally preserved beneath the Challis volcanics are conglomerates of the Smiley Creek
conglomerate and equivalents, many of which grade upsection into volcanic rocks (Link and
Janecke, 1999).

Porphyry Peak Area
Geology of the Porphyry Peak area (Moye et al., 1988; Snider and Moye, 1989), in the White
Knob Mountains, should correspond most closely to the Penn State mapping area, because
Porphyry Peak is two miles northeast of Castle Rock, on the eastern edge of the mapping area.
First stage volcanics are latitic, andesitic, and dacitic tuff breccias and lava flows. In the
Porphyry Peak area, the flow/breccia section is 700 m thick, but individual flows and units vary
in thickness and extent because they were confined to paleovalleys (Moye et al., 1988). Snider
and Moye (1989) dated these as 48.8+0.17 Ma. Phenocrysts in andesite lavas are predominantly
olivine and clinopyroxene, with minor plagioclase; hornblende or orthopyroxene are present in
more felsic rocks. Mafic absarokite and high-K basalt lavas are dominated by olivine and
clinopyroxene phenocrysts. Because a dacite flow in the Penn State mapping area was dated at
49+1 Ma (Anastasio and Schmitt, 1998), volcanics in the Penn State area must belong to the first
stage, with the possible exception of the rhyolite unit.

Snider and Moye (1989) surmise that the early andesite lavas were erupted from fissure vents
rather than stratovolcanoes and that they formed extensive volcanic plateaus that formed
topographic barriers that impeded widespread distribution of later ash-flow sheets.

The second stage of volcanism involved up to 200 m of rhyolitic flow/dome complexes (not
necessarily the same rhyolite unit mapped by Penn State). It was followed, in a third stage, by
variable thicknesses of lithic-rich trachytic Tuff of Cherry Peak, dated at 48.9+0.5 Ma. That tuff
had a volume of more than 200 km ; it was thickest in the vicinity of its vent that sat within the
Lehman Basin cauldron complex that collapses as the tuff was erupted. (The Lehman Basin is
west of Mackay; its western rim is about five miles southeast of Porphyry Peak.) Soon after that
explosive event, the Boone Creek stock, a shallow, silicic intrusion, was emplaced.


Above the ash-flow Tuff of Cherry Creek are 200-300 m of regionally-extensive dacitic
flow/dome complexes and tuff breccias that are bracketed in age between 48.9 and 47.8 Ma,
constituting a fourth stage.

Elsewhere in the White Knob Mountains there are even younger dacite and minor rhyolite domes
with associated small lava flows. Some of these are associated with epithermal mineralization.
Hydrothermal solutions have altered Paleozoic carbonates to jasperoids or skarns.

Volcanism and tectonics The largest volumes of Challis volcanics, and the most
explosivity, are concentrated along the trans-Challis fault system. Movement in this extensional
system of high-angle faults and grabens occurred during Eocene time, synchronous with
volcanism. This broad zone of crustal extension may have been intermittently active, however,
since Precambrian time (Kiilsgaard and Fisher, 1995). The fault system occurs over a distance of
at least 165 miles in Idaho and has been linked to the Great Falls lineament in northern Montana.

Tertiary igneous rocks occur throughout the western United States as far east as the Black Hills
and western Texas. The classic view of their origin is that shallow subduction and imbrication of
the Farallon plate beneath the North America extended volcanism much farther from the forearc
than is observed anywhere today (e.g., Lipman and others, 1971). Alternative models suggest
that collision of the Pacific and North American plates triggered intracontinental rifting (e.g., Fox
and Beck, 1985) or that volcanism is related to rise of mantle diapirs in a broad extensional,
back-arc setting (e.g., Meen and Eggler, 1987). It is noteworthy that neither the Absaroka nor
Challis volcanics show any geochemical signature of a "normal" arc origin within an
asthenospheric wedge above a subducting plate. The source material for magmas in both
provinces was ancient subcontinental mantle lithosphere (Meen and Eggler, 1987; Norman,
1988; Norman and Mertzman, 1991).

Anastasio, D.J. and J.G. Schmitt, 1998, Early Eocene upper crustal shortening coincident with
   midcrustal extension during gravitational collapse of the Sevier hinterland, Idaho-Montana
   thrust belt: Geol. Soc. Amer. Abstracts with Programs 30, no. 7, p. A73.
Dover, J.H., 1983, Geologic map and sections of the central Pioneer Mountains, Blaine and
   Custer Counties, central Idaho: U.S. Geological Survey Miscellaneous Investigations Series
   Map I-1319.
Fisher, F.S. and K.M. Johnson, 1995, Challis volcanic terrane, in Geology and Mineral Resource
   Assessment of the Challis 1ox2o quadrangle, Idaho: U.S. Geological Survey Professional
   Paper 1525, p. 41-43.
Janecke, S.U. and L.W. Snee, 1993, Timing and episodicity of Middle Eocene volcanism and
   onset of conglomerate deposition, Idaho: J. Geology 101:603-621.
Kiilsgaard, T.H. and E.H. Bennett, 1995, Eocene plutonic terrane, in Geology and Mineral
   Resource Assessment of the Challis 1ox2o quadrangle, Idaho: U.S. Geological Survey
   Professional Paper 1525, p. 44-47.
Kiilsgaard, T.H. and F.S. Fisher, 1995, Trans-Challis fault system terrane, in Geology and
   Mineral Resource Assessment of the Challis 1ox2o quadrangle, Idaho: U.S. Geological Survey
   Professional Paper 1525, p. 50-52.
Lewis, R.S. and T.H. Kiilsgaard, 1991, Eocene plutonic rocks in south central Idaho: Jour.
   Geophys. Res. 96:13,295-13,311.


Link, P.K. and S.U. Janecke, 1999, Geology of east-central Idaho: geologic roadlogs for the Big
  and Little Lost River, Lemhi, and Salmon River valleys, in S.S. Hughes and G.D. Thackray,
  eds., Guidebook to the Geology of Eastern Idaho: Idaho Museum of Natural History,
  Pocatello, p. 295-334.
Lipman, P.W., H.J. Prostka, and R.L. Christiansen, 1971, Evolving subduction zones in the
  western United States, as interpreted from igneous rocks: Science, 174:821-825.
McIntyre, D.H. and S.W. Hobbs, 1987, Geologic map of the Challis Quadrangle, Custer and
  Lemhi Counties, Idaho: U.S. Geological Survey Geologic Quadrangle Map GQ-1599, scale
Meen, J.K. and D.H. Eggler, 1987, Petrology and geochemistry of the Cretaceous Independence
  volcanic suite, Absaroka Mountains, Montana: clues to the composition of the Archean sub-
  Montanan mantle: Geol. Soc. Amer. Bull.: 98:238-247.
Moye, F.J., W.R. Hackett, J.D. Blakley, and L.G. Snider, 1988, Regional geologic setting and
  volcanic stratigraphy of the Challis volcanic field, central Idaho, in P.K. Link and W.R.
  Hackett, eds., Guidebook to the Geology of Central and Southern Idaho: Idaho Geological
  Survey Bulletin 27, p. 87-97.
Norman, M., 1988, Challis volcanics from subcontinental lithospheric mantle?: Geol. Soc. Amer.
  Abstracts with Programs 20:xxxxxxxx
Norman, M.D. and S.A. Mertzman, 1991, Petrogenesis of Challis volcanics from central and
  southwestern Idaho: trace element and Pb isotopic evidence: Jour. Geophys. Res., 96:13,279-
Snider, L.G. and F.J. Moye, 1989, Regional stratigraphy, physical volcanology, and geochemistry
  of the southeastern Challis volcanic field: U.S. Geological Survey Open-File Report 89-639
  (Geology and Mineral Deposits of the Hailey and Western Idaho Falls Quadrangles, Idaho), p.
Wilson, A.B. and B. Skipp, 1994, Geologic map of the eastern part of the Challis National Forest
  and vicinity, Idaho: U.S. Geological Survey Miscellaneous Investigations Series Map I-2395.



Figure 2. (from Moye et al., 1988) Distribution of Eocene volcanic rocks in the northwestern
United States and southern British Columbia. Major fields are labeled, showing approximate age
span of most intense volcanism. Regional structures are shown in relation to distribution of
volcanic fields; Trans-Challis Fault Zone is from Bennett (1986) and Great Falls Tectonic Zone
is from O’Neill and Lopez (1985)

Figure 3. A Peccarillo and Taylor (1976) plot of Challis volcanics. Symbols are from the
central Lost River and Lemhi Ranges (Janecke and Snee, 1993). The outlined field contains
about 50 analyses of mafic to intermediate lavas from Mertzman (written communication in
Moye et al., 1988; Norman and Mertzman, 1991) and about 32 felsic rocks (McIntyre and others,



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