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suggested by coplanar foUaticm and the lower Intercept Goldich, 8. S., and Mudrey, M. G., Jr., 1975, Dilatency
(fig. 261, or after it cooled. The hear array of the model for discordant U-Pb zircon eges, in
wtmium-r detrital zircons, from widespread areas Tugarlnov, A. I., ed., Recent wntrtbutions
of the Yukon-Tanana Upland, suggests that thls 350- geochemistry and analytical chemistry: New
my.-B.P. event was a largescale, regional occurrence York, John Wiley 6( Sons, p, 468-470.
with major significeme to the geology of the region. Jones, D. L,, SUberUrrg, N. J., Berg, R C., and Piafker,
This conclusion is substantinted by many other 8ges of George, 1981, Map showiw tectonostratigaphlc
approxlmatelg 350 m.y. (J. N. Aleinikoff, unpub. data, terranes of Alaska, columnar see tlons, and
1981) on metaplutonic and metavolaac rocks in the summary description of terrmes: U.S. Geologiupland.
a Survey Open-File Report 81-792, 20 p, scale
Although the h v e sequence of events may be 112,500,000, 2 sheets.
egpltcable to one or more of the samples descrbed, it Ludwig, K. R., 1980, Calculation of uncertainties of U-
is urilikely to be applicable to all of them because the Pb isotope data: Earth and Planetary Science
Yukon-Tanana Upbnd probably is a cornpite terrae htters, v. 46, no. 2, p. 212-220.
(Churkin and others, 1982). The samples are separated Mertie, 3. B, Jr., 1937, The Yukon-Tanana region,
by at least one major fault system (the Tintina) and Alaska1 U.S. Geological Survey Bulletin 872, 276
probably more. Although dl the samples contain P.
detrital Proterozoic zircons, not all these zircons may Steiger, R H., and Jwer, EmWe, compilers, 1977,
have come from the same source(s), and so the sedi- Subcommission on Geochronologyl Convention
mentary rocks may not represent a single depodtional on the use of decay constants In geo- and cosmosequence.
However, the accwrence of these zlrcons in ahconology: Earth and Planetary Science
rnetasedirnentary rocks from different terranes is sig- Letters, V. 36, no. 3, p. 359-382.
nificant because the ege of Wt 2,250 m.y. for the Ternpelman-Wult, I). J., 1976, The Yukon Crystalline
provenance is uncommon for rocks of western North Terra.net Enigma In the Canadian CoWera;
America. Thus, tectonic models for the accretion of Geologic& Society of America Bulletin, v. 87,
terranes In Alaska should account for the source-rock no. 9, p. 1343-1357.
age data. As yet, no Proterozoic metalgneous rocks
have been found in the upland. Although no co~ection
with the upland is presently apparent, we note that a Uraniumlead iabtopec ages of drum fmm Jillidte
thermal event at 2,300 m.y. B.P. la recorded in the and implicatiom, for Pal80zolc metamorphism,
area straddling the boundmy between the Bear and BisDeltaguadrad-tralALardca
Slnve structural provinces in the Northwest Territories
of Canada, approximately 1,500 km to the east (filth Jbh N. Ahinkoff, Cynthia lhsel-lhcm, and Heh and others, 1977). To the southeast, in the Yukon L. Fodhr
Territory and British Columbia, Tempelman-Kluit
(1976) invoked e model that Includes Late Proterozoic As part of a continuing geochtonologic study of
to Devonian sedimentation, followed by Late Devo- igneous and metamorphic rocks in east-central Alaska,
den-Early Winsippian plutonism. The current con- we have dated four sarnplea of sillimanite gneiss and
Uept of microplate accretionary tectonics in Alaska one sample ot a crosscutting granlte by the uraniumbee
Jones and others, l98U implies that terranes which lead method. These samples were selected from some
in the past were fuxbposed may now be separated b of the highest grade metamorphic rocks in the Yukonvlut
distanced. Thus, we suggest that the terraned Tanana Upland (area 1, fig. 23) whase protolith is conthat
now compose the upland might have originated sidered to have been sedimentary rocks of Precamseveral
hundred kilometers to the east and (or) south. brian or Paleomlo age (Dusel-Bacon md Poster,
1983). Previous investigations (Aleinikoff and others,
1 REFERENCES CM'ED
1981) showed that early Protemlc crustal materiel
I
was involved In the formation of some of the Paleozoic
Alcinikotf, J. N., Dusel-Bacon, Cynthh, Foster. H. L., and Mesomic igneous rocks in this part of Alaska, but
1 and Futa, Kiyoto, 1981, Proter~zolc zircon from more data aw needed on the age of metamorphism.
augen gneiss, Yukon-Tahana Upland, east-aentral Pour of the five rocks sampled were collected
Alaska: Geology, v. 9, no. 10, p. 468-473.
from a nearly c i r w body of sillimanite gneiss,
Chwkin, Michael, Jr., and Brabb, E. R, 1965, Occur approximately 600 km in area, in the Big Delta quadrence
and stratigraphic significance of Oldhamla rangle. We consider this body to be a gnek dome on
a Cambrian trace fwil, in east-entrm the basis of structure, mineral assemblages, end geoin
Geolagical Survey research 1965: U.S. thermometry (-1-Bacon and Poster, 1983). Polia-
Geological Survey Professional Paper 525-D, p. tion is subhorizontal in the central part of the body
D120-D124.
and dips outward at the margins; quartzite and marble
ChurMn, Michael, Jr., Foster, H. L., Chapman, R M., are infolded locally. Metarnotphio grade increases
and Weber, F. R., 1982 , Terranes and suture inward from pelitlc schist that contains all three
zones in east*entral Aleska: Journal of Geo- durninasilicate polymorphs In apparent equilibrfum, to
physical W c h , v. 87, no. 5, g. 3718-3730. sUlimanite gneiss in the central part. Partial melting
Prith, Rosaline, Prith, R. A., and Doig, Ronald, 1977, mey have occurred near the center, as suggested by
The geachranology of the granitic rocks along garnet-biotite equilibrium temperatures and the occur
the Bear-Slave Structural Province kundary, rence of migmatite.
northwest Canadian Shield: Canadian Journal of The fifth sample was collected from an eastkth
Sclenced, v. 14, no. 6, p. 1958-1973. west-trending aree of sillimenite gneiss that crops out