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fault, and Its depositional basement is absent. Radio-
larlan chert, structurally asociated wlth graywacke
and argillite of the Mesozotc flysch belt south of Ram-
part, is Triassic; its presence may indicate that clastfc
sedimentation withln this belt commenced much
earlier than hitherto suspected, although .definite
depositional relations of the ohert and elastic rocks
are not yet proved.
REPEREN CES CVED
Brosg6,~. P., Lanphere, M. A., ReReiser, H. N., md
Chapman, R M., 1869, Probable Permian age of
the Rampart group, central Alaska: U.S. Geo-
logical Survey Bulletin 1294-B, p. B1-818.
Bberlein, a. D., Gassaway, J. S., and Beikrnen, LI. M,
compUers, 1977, Pwlirnlnary geologic map of
central Alaskaz US. Geological Survey Open-
File Report 77-168-A, scale 1;1,000,000.
1 Mertie, J. B., Jr., 1957, The Yukon-Tanana region,
I
Alaska: U.S. Geological Survey Bulletin 872, 276
P.
Patton, W. W., Jr., Tailleur, I. L., BrosgC, W. P., and
Lanphere, M. A., 1977, Preliminary report on the
! ophlolltes of northern and western Alaska, &
1 Coleman, R G., and Irwin, W. P., eds., North
American ophiolites: Oregon Department of
Geology and Mineral Industries Bulletin 95, p. 51-
I
Isotapic evidence h m &Ma1 zimzona for Early
Pmte~ic
cntgCal matadal, eastcentral Ahska
By Job N. Alehikoff, Helen 1. Foster, Warren J.
NokIebeg, and Cynthia r)rreel-Batwra
The ages of metarnorphio r6cks in east+entral
AIaska, especially in the Yukon-Tanma Upland, have
been the subject of much conjecture for many years.
MertIe (1937) concluded, on the bash of now-dlsproven
stratigraphic relations, that all the crystalline schist
with sedimentary protoliths (Birch Creek Schist of
former usage) is Precambrian. Although the sedlmentary
history of those rocks has yet to be.complete-
$ deciphered, there has been some progress both in
determining the we of the parent material (provenance)
and in bracketing the subsequent age(s) of
metamorphism. 'his study (area 2, fig. 23) reports
new isotopic analyses of detrital zircona that show
evidence for Early Proteroeolc sources of the sediment
composing rocks in and adjacent to the UpMd.
Zircons were extracted from four distinct
qumtzite units in three quadrangles, the Circle, Blg
DeAta and Mount Hyes (fig. '251, encomg&ssing an area
of about 18,500 km , or approximately a fourth of the
total area of the Upland. The zlrcon separates contain
rounded, pitted, and frosted grains that are unequivocally
detrital. Within each separate ere several digtinct
populations, distinguishable primarily by color
and shape. Because sedi~nentary rocks commonly are
composed of material derived from several sources, it
is llkely that these sepmates contain zircons of different
ages. We have endeavored to obtain, by careful
handpicking, small samples Oess than 5 mg) that probably
are composed of zircons from a aingle source and
thus, within eech group, are of the same age.
65' . 810 DELTA
Figure 25.--Sketch map showing sample locallties and
numbem In the Circle, Big Delta, and Mount Hayes
quadrangle., Aleska. Table 7 Zists sample data.
Sample 80AFr 253 (table 7) is a weakly foliated
grit characterized .by rounded silt- to sand+ize quartz
grains In a sericitic matrix., Rocks of this type are
found Ln the eastern and western Crazy Mountains and
In the northwestern part of the Circle quadrangle on
both sides of the Tintina fault zone, where they are
interbedded locally with green and meroon @Ute.
the grlt Is tentatively considered to be Cambrian(?{
because of the occurrence of the trace fosil Oldhamia
(Churkln and Brebb, 1985). Sample 80APr 255, from a
strongly follated w tzite and schist unlt, L composed
of flne-gralned quartz, minor biotite, and rn uscovite
containing scattered large (2-5 mrn) gray, blue, and
whlte quartz grains. This quartzltic unit, which ranges
from greenschiat- to epidote-amphlbolite facies, is
extensive in the central part of the Circle quadrangle
south of the Tlnth fault zone. Although these rocks
could be higher grade equivalents of grits almllar to
sample BOAR 253, the foliated quartzites have a more
complex deformational and metamorphic history than
the grit. Sample 79AFr 2017, from the southeatern'
pert of the Big Delte quadrangle, is a foliated gray
quartzite lnterbedded with qntutz-mica schist and con-
tainlng minor muscovite, biotite, and garnet. This
rock tentatively has been interpreted as a wallrock for
a Large orthoau en gneiss body of Rarly Mississippian
(350 m.y.1 age fueinikolf and others, 1BBI). Sample
BDAAP 057A Is a well-foliated pinkish qumtzite con-
taining minor muscovite end garnet, collected in the
central part of the Mount Hayes quadrangle north of
the Hlnes Creek strand of the Den& feult.
Within the largest size fraction of three of the
samples (80AFr 255 excepted), two zlrcon populations