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

U-Pb AGES OF ZIRCON, MONAZITE, AND SPHENE FROM BROOKS RANGE, ALASKA 6 1
orogen in the southern Brooks Range (Coldfoot and
Harnrnond terranes). Subsequent uplift, closure of the K-
Ar system, and erosion of the metamorphic core, all per-
haps from extension as a consequence of the crustal
thickening, occurred principally in mid-cretaceous time.
The metamorphic core of the Brooks Range orogen
consists of a southern belt of schist (schist belt) and a
northern assemblage of schist and phyllite (central belt of
Till and others, 1988; fig. 1). The stratigraphy of both of
these units is largely obscured by extensive deformation
and metamorphism. On the basis of contrasts in texture
and lithology, the schist belt was assigned to the Coldfoot
terrane and the schist and phyllite assemblage to the
Harnmond terrane (Jones and others, 1987; Moore and oth-
ers, 1992; Moore, 1992). Nonetheless, the contact be-
tween the Coldfoot and Hamrnond terranes is ambiguous
in many places (Till and others, 1988), although in the
TACT comdor near Wiseman, the contact has been lo-
cated at the south-dipping Wiseman or Minnie Creek
thrust (Oldow and others, 1987; Grantz and others, 1991;
Moore and others, 1992). On the basis of new mapping by
the TACT project we now consider the contact to be lo-
cated about 10 kilometers to the north of the Wiseman
thrust on a north-dipping fault that extends east-west along
the northern margin of the metagranitic rocks in the
Chandalar quadrangle. The Coldfoot terrane was
emplaced as a crustal wedge by thrusting either into, or
beneath, the Harnmond terrane (Oldow and others, 1987;
Moore and others, 1992).
Stretching over 700 km westward from the Canadian
border is a belt of nine groups of deformed and metamor-
phosed granitic bodies of Devonian age (Dillon and others,
1980, 1987). The metagranites occur along the contact be-
tween the Coldfoot and Hammond terranes and have been
assigned to either or both of those units by various work-
ers (for example, Moore and others, 1992). On the basis
of the new mapping by the TACT project and high pres-
sure-low temperature metamorphic assemblages in the
plutons (B. Patrick, oral commun., 1990), all of the
metagranitic bodies in the TACT study area (fig. 2) are
here regarded as part of the Coldfoot terrane. Felsic and
silicic metavolcanic rocks, inferred to be cogenetic with
the metagranitic rocks (Dillon and others, 1980, 1987), are
present in both terranes, but in neither terrane do they have
a close spatial relationship. Compositionally, the meta-
granitic bodies consist of granite and granodiorite that is
largely peraluminous, but locally metaluminous (Newberry
and others, 1986). The felsic metavolcanic rocks of the
study area typically are albite-quartz phyllite that is struc-
turally interleaved with carbonate rocks and quartz-rich
schist and phyllite. Samarium-neodymium (Nelson and
others, in press) and lead data (Aleinikoff and others,
1991) from outside the study area indicate variable in-
volvement of Precambrian crust in the genesis of the
metagranitic rocks.
COLDFOOT TERRANE
Along the TACT transect, the Coldfoot terrane consists
chiefly of pelitic and semipelitic, quartz-mica schist
that contains minor lenses of metabasite and albite schist
(fig. 2). Locally map-scale units of calcareous schist and
marble, quartz-poor greenschist, amphibole-rich schist,
metabasite, quartzite, and metagranitic rocks also are
present along the northern margin of the Coldfoot terrane.
Low-angle discordances indicate that gently to moderatedipping
faults bound most of these units, including the
metagranitic rocks, but in a few places intrusive contacts
may be preserved. In most places, the Coldfoot terrane is
characterized by a penetrative foliation and complete recrystallization.
Metamorphic assemblages throughout the
Coldfoot terrane are chiefly greenschist facies, but evidence
of an earlier high pressure-low temperature metamorphism
is widely preserved in some schists (Till and
others, 1988; Gottschalk, 1990; Till and Moore, 1991) and
in the metagranitic rocks (B. Patrick, oral commun., 1990).
The protolith ages of the schists are largely unknown, although
conodonts of Early Devonian age, and Silurian or
Devonian age have been recovered from dolomitic
calcschist at three localities southwest of Wiseman (A.G.
Harris, written commun., 1990). Quartz-mica schist in the
TACT corridor has yielded white mica with 40~r/39~r
ages as old as 120-130 Ma (Blythe and others, 1990), and
metagranitic rocks in the Chandalar quadrangle have yielded
K-Ar ages of 100-125 Ma (Brosgt and Reiser, 1964).
Orthogneisses sampled for U-Pb dating include three
large bodies in the Chandalar quadrangle about 20 to 40
krn east of the Dalton highway and a small metagranitic
body on the west side of the Middle Fork of the Koyukuk
River, about 10 km south of Wiseman (Brosgt and Reiser,
1964; Dillon and others, 1986; Dillon and Reifenstuhl,
1990) (fig. 2; table 1). The Chandalar bodies were subdivided
on the basis of composition into the Horace Mountain
plutons and Baby Creek batholith by Newbeny and
others (1986). The Horace Mountain plutons are a string
of small metagranitic bodies along the northern margin of
the Coldfoot terrane that are largely metaluminous, porphyritic,
biotitekhornblende granite and hornblende-biotite
granodiorite (Newberry and others, 1986). These metagranitic
rocks are bounded by faults that dip northward at
moderate angles. The larger metagranitic bodies to the
southwest were included in the Baby Creek batholith
which consists dominantly of peraluminous, porphyritic,
biotitekgarnetkwhite mica granite (Newberry and others,
1986). The Baby Creek batholith of Newberry and others
(1986) consists of at least two large metagranitic bodies
that are separated by east-west-trending thrust faults. To
the north is the Geroe Creek orthogneiss body, which consists
of white mica +_biotite&hornblende granite and granodiorite.
To the south are orthogneiss bodies at Baby Creek
and Twin Lakes, which appear to be physically connected