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

68 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY. 1992
DISCUSSION AND TECTONIC
IMPLICATIONS
Despite the unwise practice of using data from several
peraluminous and metaluminous magmatic suites that are
distinct and unrelated to each other. However, the similar
U-Pb ages derived from samples from these suites, the
present spatial proximity of the metagranitic bodies, and
plutons to calculate a composite regression and concordia their similar porphyritic textures suggests to us that the
intercept age, the Devonian age (391+20 Ma) of Dillon Horace Mountain plutons, Geroe Creek orthogneiss body,
and others (1987) for orthogneisses in the eastern Brooks and Baby Creek orthogneiss body once formed parts of a
Range is accurate. Our new data for each pluton are more large Early Devonian batholith that was dismembered durprecise;
much of our data are concordant, and other frac- ing the Brookian orogeny in the Jurassic or Early Cretations
are mostly less than 10 percent discordant. However, ceous. This interpretation is supported by the tabular
despite very careful hand-picking of very small samples, shapes of the metagranitic bodies and the presence of genwe
were unable to avoid some grains that contain inherit- tly dipping faults both above and below several of the
ance of older radiogenic Pb, in even the most pristine zir- bodies. The compositional differences between the
con grains. The age of inheritance is unknown, although orthogneisses may be due to assimilation of different
the only known older rocks are Late Proterozoic (about amounts of pelitic wall rocks. The assimilation of pelitic
700-750 Ma). Zircons in the hornblende-biotite ortho- material is supported by the broad range of the 207~b/206~b
gneisses (fig. 3A, B) contain only small amounts of inher- ages for the Baby Creek orthogneiss body discussed above.
ited material, as evidenced by 207~b/206~b ages 393 and Thus, we suggest that the various orthogneisses represent dif-
412 Ma. Zircon fractions in the two-mica orthogneisses ferent parts of a variably contaminated batholith.
(fig. 3C, D) have a much broader range of 207~b/206~b Although the Coldfoot and Hammond terranes have
ages (398 to 612 Ma), and we were unable to obtain con- different stratigraphies and different metamorphic and
cordant data from any fraction. Although these plutons are deformational histories, the almost exact agreement in ages
not S-type granites in the strict sense because their of the orthogneisses and felsic metavolcanic rocks begs the
mineralogic and chemical characteristics were later found question of the relationship between the two terranes.
to be nondiagnostic (Dillon, 1989, p. 183), they are Dillon and others (1987) and Dillon (1989) suggested that
peraluminous and probably were derived from pelitic sedi- the volcanogenic rocks that comprise the Nutirwik Creek
ments. Detrital zircons in sedimentary rocks may be de- unit represent the extrusive volcanic centers of the
rived from many provenances (for example, quartzite in metagranitic rocks in the Coldfoot terrane to the south.
the Yukon-Tanana terrane contains zircons that range in Balanced cross sections through the Brooks Range, which
age from 1.2 to 3.4 Ga; Mortensen, 1990), and thus, inherited regard the Coldfoot and Hammond terranes as basement
material in the zircons may be a mixture of many ages. for the imbricated passive continental deposits of the
The orthogneisses are strongly foliated, a deformation Endicott Mountains terrane to the north, support this view
that apparently occurred in the middle Mesozoic on the ba- by placing the Coldfoot terrane beneath the Hammond tersis
of K-Ar ages (Armstrong and others, 1986). Our U-Pb rane in their reconstruction (Oldow and others, 1987). The
data on sphene indicate that this metamorphism did not ex- Early Devonian Nutirwik Creek unit, however, is inticeed
about 550°C, because the sphenes have not been reset mately interleaved with, and possibly may have been deto
a Mesozoic age. The sphene and monazite may be ig- posited on, Ordovician to Devonian carbonate platform
neous minerals that cooled through their closure tempera- deposits. The Coldfoot terrane into which the metagranitic
tures shortly after emplacement of the magmas, or they rocks were emplaced, in contrast, consists partly of calcarmay
have formed during a penetrative deformation in the eous quartz schist of Silurian (?) and Early Devonian age.
Devonian. The coarse-grained, euhedral morphology of Taken at face value, these relations suggest that the volcathese
minerals suggests that they are igneous in origin. nic rocks were deposited in a different early to middle Pa-
The -390-Ma ages of the felsites may provide con- leozoic stratigraphic setting than the metagranitic rocks
straints for the age of the stratigraphic section. Sample and thus may not be consanguineous. Alternatively, if
90TM409 (precisely dated at 393+2 Ma) particularly looks they are assumed to be consanguineous, then large-dislike
an interbedded metavolcanic rock and thus would date placement faults must divide the igneous rocks from the
the time of deposition of the purple and green phyllites of dated metasedimentary rocks in either or both terranes.
the Nutirwik Creek unit. However, geologic mapping The Devonian metagranitic rocks of the central
shows that the purple and green phyllite is allochthonous Brooks Range are part of an extensive, discontinuous, linrelative
to other rocks of the Hammond terrane and hence ear belt of middle Paleozoic plutonic and volcanic rocks
its stratigraphic significance is not understood. All con- that occur in various accreted terranes in the North Ameritacts
may be a complex interleaving of fault slivers. can Cordillera (Rubin and others, 1990). In the Brooks
On the basis of their contrasting composition and min- Range, Devonian granitic rocks in the Coldfoot,
eralogy, Newbeny and others (1986) concluded that the Hammond, and North Slope terranes crop out in a belt of
metagranitic rocks of the Chandalar quadrangle comprise over 900 km. Elsewhere in Alaska and western Canada,