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Geolagy and Mineral Industries Bulletin 95, p.
57.
Penrose Field Confecence part1 icipants,
lites: Geotimes, v. 17, no, 12, p. 24-
New age data for the K&n&i M%
Ataaka
By William W. Patton, Jr., m t h J. 116- Merrim A
Lanphem, and Wvld L Jones
The Kaiyuh Mountains, si I the Nula]
quadrangle along the southeast margin of the Creta-
ceous Yukon-Koyukuk basin (area 3, fig. 131, are com-
posed of a Precambrian and (or) Paleozoic metamor-
phic complex that is overlain by a late Paleozoic and
early Mesozoic assemblage 6f mafic and dtramdie
rocks and intruded by Cretaceous granitic rocks (fig.
17). The mafie and dtramafic rocks make up a
reversely stacked dismembered ophblite assemblage
that appears to have been thrust southeastward across
the metamorphic complex from a root zone at the
margin of the Yukon-Koy~kuk basin (Patton and
others, 1977; Patton and Moll, 1961). This report
'
presents new potassium-argon and fossil age deter
nations for the metamorphlc complex, the ophio
assemblage, and the granitic intrusive rocks.
1. Two potassium-argon age determinations
metamorphic muscovite from g1aucophanebearing
schist in the metamorphic complex gave
ages of 134 m.y. and 136 m.y. (latest Jurassicearliest
Cretaceous) (samples 1 and 2, respectively,
fig. 17; table 4). These ages are thought
to reflect metamorphlc events associated with
overthrusting of the ophiolites. A similar interpretation
has been given for Early Cretaceous
metamorphic ages from the schist belt in the
southern Brooka Range (Turner and others, 1979;
Dillon and others, 1980).
2. Radiolarians were collected from chert inter
layered with basalt and gabbro in the lower
thrust sheet of the ophiolite mernblage (fig.
17). Pour of these dections were determined
to be Late Mississippian and (or) Early Pennsylvanian,
and one Late Triassic. These ages generally
accord with the Miasissippian to Triassic
fossI1 ages previously reported from interlayered
basalt, gabbro, and chert suites in ophiolite
assemblages elsewhere around the borders of the
Yukon-Koyukuk basin (Patton and others, 1977;
Rafker and others, 1978).
3. Potasslum-argon ages of 151 m.y. (Late Jurassic)
and 269 m.y. (Permian) were measured on hornblende
from hornblende pegmatite (sample 3, fig.
17, table 4) and hornblendite dikes (sample 5, fig.
17, table 4) cutting peridotite and layered gabbro
in the upper thrust sheet of the ophiorite assernblage.
The Jurassic age agrees with previously
published potassium+rgon eges on gabbro and
hornblende-bearing dikes from ophiolite assemblages
else where around the Yukon-Koyukuk
basin (Patton and others, 1977), but the Permian
age is anomalous and cannot be reconciled with
any previously reported ages. An age of 172 m.y.
(Middle Jurassic) was measured on hornble~A~
from garnet-beating amphtbollte (semple 1
lected near the base of the upper thrust sheet of
the ophiollte. Ages of 155 and 161 m.y. were obtained
previously on arnphlbollte associated with
other ophiolite assemblages elsewhere dong the
southeast margin of the Yukon-Koyukuk basin
(Patton and others, 1977).
4. A potassiurn-ergon age of 112 m.y. (Early Cretaceous)
was obtained on a biotite mineral separate
(sample 6, table 4) from the granitic pluton in
the southwestern the Kaiyuh Mountains (fig.
17). This rye is of interest because it suggests
that this pllrton may represent the south end of
the Mebzitna pluton, offset approximately 180
km to the west along the Kaltag fault. The
Melozitna pluton, north of and apparently cut off
by the fault, has yielded n potassium-argon age
of 111 m-y. and is compositionally similar to the
Kaiyuh Mountains pluton (Patton and others,
1978; Chapman and Patton, 1970). Both plutons
intrude lithologically similar metamorphic complexes.
A previous estimate of 130 km of strikedip
movement along this segment of the Kaltag
fault is based M right-lateral separation of the
southeast margin of the Yukon-Koyukuk basin
(Patton and Hoare, 1968). North of the fault,
this margin lies along the Melozitna River valley
a short distence west of the Melozltna pluton;
south of the fault, it appears to lie between the
"-'----'I Mountains and the Yukon River.
REFERENCES CITED
Chapman, R M., and Petton, W. W., Jr., 1978, Prelim-
inary summary of the geology in the northwest
part of the Ruby quadrangle, & Johnson, I