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feldspar that developed in quartz-mka schist and
gneiss near the contacts of the two ban& of atgen
gneiss (pashed lines, lw. 2) might represent the
begi~ing stages of the same phenomenon. The association
of those clots with the narrow bands considered
to be sills may indicate that intrusion of the augen
gneiss sills, formation of porphyroblasts, and cataclasis
were all closely related in tlme The perfect eye
shape of most augen could be evidence Chat growth of
porphyroblasts and modification of their shape by
cataclasis was contemporaneous. Petrographic examination
of the fine-grained augen gneiss in question, as
well as study of the metamorphic textures of the catactasized
rocks in the upland h general, indicate8 that
cataclasis and neocrystallieatlon were closely linked.
However, because the timing of regional roetamorphism
and cetaclasis subsequent to intrusion of the
augen gneiss (about 350 m.y. B.P. for the large body of
augen gneiss) is still uncertain wit1 .lin about a 150-m.y.
F i i 29.--Slabs of augen gnek from localities in Big
Delta C-2 quadrangle. 4 ZXtt~oaugen gnelsq from
thick sill at locality 1 (fig, 28). Note fracturing and
cataclastic deformation of originally idiomorphic
megamysts, and apparent rotation of some smaller
megacrysts. Largest auge is 12 cm long. B Enigmatic
fine-grained augen gneiss from locality 3Tlfig. 28). In
outcrop, augen are more evenly spaced than in this
sample. Diagonal-stacking alincment of augen, which
h common in coarse-grained orthoaugen gnelss, may
be evidence for porphyritic-sill protolith. Largest
auge is 1.25 cm long.
L-.
interval (Aleinikoff and others, 19811, a genetia
relation between intrusion of the augen gneiss prote
lith and possible porphyroblastic formation of other
augen gneiss layers is only speculative.
The various layers at the localities described are
examples of the textural end members in the por
phyroclastic to possibly porphyroblastic origin of augen
gneiss in the upland. The silt of augcn meiss, which is
the first such inkusive layer that we have recognized,
may be similar to other layers of augen gneiss whoso
interlayering with metesedimentary mcks was pre-
viously Interpreted to suggest a sedirrlentary proto-
llth. The incipient clots of feldspar in pelitic rnetased-
imentary rocks near the narruw bands of augen gneiss
are evidence that the formation of feldspar porphyro-
blasts in metasedimentary rocks may be a plausible
explanation for same augen gneiss layers, and the
augen gneiss layer at the last locality described is n
good example of the equivoeel textures of the I1grey"
area between the two end members.
REFERENCES Cl"l'%D
Aleinikoff, J. N., Dusel-Bacon, Cynthia, Foster, H. F.,
and Futa, Kiyoto, 1981, Proterozofc zircon from
rtugen gneiss, Yukon-Tanana Upland, east-central
Alaska: Geology, v. 9, no. 10, p. 469473.
Duel-Bacon, Cynthia, and Aleinikoff, J. N., 1980, Pro-
terozoic cataclastic augen gneiss in ?he south-
eastern part of the Big Delta qua&angle,
Yukon-Tanma Upland, east+enbal Alaska
[abs.]: Geological Society of America Abstracts
with Programs, v. 12, no. 3, p. 104-105.
Holdaway, M. J., 1971, Stability of andalusite and the
aluminum silicate phase dlagram: American
Journalof Science, v. 271, no. 2, p. 97-131.
Raceelemeot evidence fat tbe tectonic aZIhrfties of
some amphibolites from the Pnbn-'I'anana Upland,
eest+entral BLaslce
Bg Cynthia -1-&eon
Amphibolite occurs throughout the Yukon-
Tanana Upland, interlayered with metasedim entary
and metaigneous rocks of Precambrian(?) and Paleozolc
age. There has been considerable speculation as
to the orlgins of the various nmphibolite layers and
their geologic significance. Amphibolite may result
from: (I) Metamorphism of basic igneous rocks, (21
metamorphism of calcareous or dolomitic shele, or (3)
metasomatism involving the exchange of significant
amounts of nonvolatlIe constituents (Preto, 1970). On
the bnsis of traceelement evidence obtained in the
preliminary study reported here, basaltic protoliths are
indic~ted for amphibolite of the Yukon-Tenana Uplmd.
In the amphibolite-grade terrane of the southeastern
part of the Big Delta quadrangle (area 7, fig.
231, amphalite is corn rnonly interlayered with
quarl-zofeldspathic gneiss and schist that are near the
margins of, and probably wallrock for, a batholith-size
body of orthoaugen gneiss of Late Devonian-Early Mississippian
age (Aleinikoff and others, 1981). Samples
from amphibolite layers at thee different localities
around the augen gneiss pluton were cllernicdy