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

U-Pb AGES OF ZIRCON, MONAZITE, AND SPHENE FROM
DEVONIAN METAGRANITES AND METAFELSITES,
CENTRAL BROOKS RANGE, ALASKA
By John N. Aleinikoff, Thomas E. Moore, Marianne Walter, and Warren J. Nokleberg
ABSTRACT
Six samples of metaigneous rocks from the Coldfoot
and Hammond terranes of the central Brooks Range have
been individually dated by the U-Pb method. Zircons
from two hornblende-biotite orthogneisses (Geroe Creek
orthogneiss body and Horace Mountain plutons) yield
simple isotopic systematics that indicate ages of 391k1 and
3934 Ma, respectively. Zircons in two 2-mica
orthogneisses (Baby Creek orthogneiss body and Middle
Fork Koyukuk River orthogneiss body) have complex sys-
tematics, including significant inherited components and
modem Pb loss; we infer poorly constrained ages of Early
to Middle Devonian for these bodies. Zircons from two
metafelsites from the Hammond terrane have tightly con-
strained ages of 393+2 and 389k3 Ma. The data confirm
previous studies by J.T. Dillon who suggested, based on
composite discordias composed of data from several
samples, that the ages of most of the igneous rocks of the
Brooks Range are about 390+_20 Ma. These granitic rocks
are part of a large, discontinuous belt of middle Paleozoic
plutonic and volcanic rocks that occur throughout the
North American Cordillera.
INTRODUCTION
The central and eastern Brooks Range of northern
Alaska has been divided into several lithotectonic zones,
including, from south to north, the Angayucham terrane,
Coldfoot terrane (also called the schist belt), the Hammond
terrane (also called the central belt), the Endicott Moun-
tains terrane, and the North Slope terrane. Excluding the
Angayucharn terrane (of oceanic origin), the other four ter-
ranes comprise the major part of the Arctic Alaska
superterrane (Moore, 1992; fig. 1). The Coldfoot and
Hammond terranes both are of Paleozoic age and of conti-
nental origin but have different protolith compositions and
metamorphic and deformational histories, permitting their
designation as separate lithotectonic blocks or units
(Moore and others, 1992).
The most prominent belt of plutonic rocks in northern
Alaska is located near the boundary between the
Hammond and Coldfoot terranes in the southern Brooks
Range (fig. 1). These rocks consist of deformed and meta-
morphosed, medium- to coarse-grained metaluminous to
peraluminous granitic rocks. Metamorphosed hypabyssal
to extrusive felsic igneous are also locally present in the
southern Brooks Range, but because of poor age and struc-
tural control, the relation of the felsic rocks to the
metagranites is uncertain.
The age of magmatic activity in the southern Brooks
Range has been a matter of controversy for decades.
Mertie (1923) was the first to conclude that some of the
granites were emplaced in the Paleozoic. Biotite and horn-
blende from many plutons in the southern Brooks Range
have yielded Cretaceous K-Ar ages (Turner and others,
1979). However, these minerals define metamorphic fab-
rics in strongly foliated rocks, leading to the interpretation
that the rocks were deformed and metamorphosed in the
mid-Cretaceous (Nelson and others, 1979). Dillon and
others (1980), using the U-Pb zircon and Rb-Sr whole-
rock methods, obtained Devonian ages from several plu-
tons in the southern Brooks Range and concluded that the
plutons were emplaced in the Devonian and metamor-
phosed in the mid-Cretaceous. Although Dillon and others
(1980) found no evidence for Cretaceous magmatism,
Tailleur (1984) concluded that field relations require Cre-
taceous emplacement ages for the plutons, and suggested
that the granites contain Devonian xenocrystic zircons.
Subsequent U-Pb zircon dating of plutons in the central
and eastern Brooks Range by Dillon and others (1987)
confirmed the earlier geochronology indicating Devonian
ages for most of the plutons; they also found three bodies
that were emplaced in the Late Proterozoic.
Although the U-Pb zircon data of Dillon and others
(1980, 1987) strongly suggest ages of 370-390 Ma for the
foliated plutonic rocks of the southern and central-eastern
Brooks Range, a major weakness in both studies is that U-
Pb isotopic data from zircon fractions of several different
bodies (including both metaluminous and peraluminous
plutons) were plotted together. Plutons in the eastern