Geologic Studies in Alaska by the U.S. Geological Survey, 1992

U-Pb AGES OF ZIRCON, MONAZITE, AND S ;PHENE FROM BROOKS RANGE, ALASKA
METAFELSITES OF THE HAMMOND TERRANE
Two samples of fine-grained, foliated, porphyritic
rocks (identified as felsites in the field) were collected
from the Nutirwik Creek unit of the Hammond terrane to
test their temporal correlations with granitic orthogneisses
in the Coldfoot terrane. One or both of the samples may
be metavolcanic in origin and thus may also provide con-
straints for the age of the stratigraphic section. Isotopic
data from zircons from eight samples of fine-grained felsic
rocks in the southern and eastern Brooks Range have
yielded a composite discordia with an upper intercept age
of 396+20 Ma (Dillon and others, 1987). Two of their
0.065 plagioclase felsite
2 0.063
N
\
a
0.061
weighted average of
PW % ages = 393 + 2 ~a
0.059
0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.50
207 pb) 235 u
INTERCEPTS AT
1015 k 103 and 388.7 k 3.2 Ma
(MSWD = 1.4)
Figure 4. Concordia plots of U-Pb data from metafelsites of the
Hammond terrane, eastern Brooks Range. Sample localities
shown on figure 2. MSWD, mean square of weighted deviates.
samples, from Whiteface Mountain and Wiehl Mountain,
are about 25 km west and 30 km south, respectively, of
our sample 90TM409.
Sample 90TM409 is a buff-colored, strongly foliated
felsic rock. It is about 25-40 cm thick and is interlayered
with gray, green, and purple shale, and interfolded (or
faulted) with limestone and marble. It is compositionally
homogeneous and contains augen of partly polygonized
quartz phenocrysts, albitized feldspar phenocrysts, and volcanic
rock fragments in a fine-grained, foliated matrix of
metamorphic quartz, feldspar, calcite, and white mica.
The lithologic characteristics and field occurrence have led
to the interpretation that this rock is volcanic (extrusive) in
origin, although it is possible, given the superimposed deformation
and very limited outcrop, that it is a sill. Zircons
from our sample are euhedral, light pink, adamantine,
and have Yw of 2-4 (table 1). We analyzed three fractions,
all of which are concordant (fig. 4A), despite having
fairly high uranium concentrations of 431-987 ppm. The
weighted average of the 207~b/206~b ages is 39322 Ma,
interpreted as the time of crystallization of the rock.
Sample 90TM497 is a plagioclase porphyry (table I)
interpreted in the field as a hypabyssal felsite plug, flow,
or sill. It contains feldspar and amphibole phenocrysts in a
fine-grained groundmass and has been metamorphosed under
apparent static conditions to albite, epidote, quartz,
white mica, and pale amphibole. Zircons from this sample
are similar in morphology to zircons in metafelsite sample
90TM409 (euhedral, light pink, adamantine) except that
they are more prismatic with llw of 4-8 (table 1). One
fraction is concordant at about 385 Ma; two other fractions
have 207~b/206~b ages of 400 and 469 Ma (table 2).
The isotopic data define a chord with a lower-intercept age
of 389+3 Ma (fig. 4B); the upper-intercept age of
1,015+103 Ma may be geologically meaningless because
the oldest known rocks in the central and eastern Brooks
Range are Late Proterozoic (Dillon and others, 1987; Karl
and others, 1989; Karl and Aleinikoff, 1990). Thus, the
age of the sample may be best estimated by the 206~b/238~
age (385+2 Ma) of the concordant fraction.
In summary, a11 six samples dated in this study are
Early to Middle Devonian in age. The ages of the hornblende-biotite
orthogneisses are identical within uncertainty
(391+_1 and 393+_5 Ma) and the rocks may be
related. The two-mica orthogneisses were more difficult
to date, because of inheritance of radiogenic Pb, high uranium
contents, and poorly preserved zircon grains. However,
concordant monazite and two discordant zircon
fractions in the Baby Creek orthogneiss body constrain the
emplacement age to be between 381 and 398 Ma. A
slightly discordant zircon fraction in the Middle Fork
Koyukuk River orthogneiss body suggests a maximum
crystallization age of about 390 Ma. The ages of the two
metafelsites are 393+2 Ma and probably about 385-390
Ma, respectively.
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