n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

ecrSigtallized catadastic matrix of guartz+pIagio-
olase+biotite+white mica and minor potassium feld-
spar. The porphyraclasts (augen) in rocks from both
areas are fairly densely packed and show the effects of
bunching up and deformation of preexisting lath-
shaped megacrysts (fig. 29A). Inoluslons of biotite,
arranged in concentric zones within augm simllac to
those In the metaplutonlc body, were aLso observed In
the sill Uoc. 1, fig. 28).
0 .5 1 t 5 2 KILOHEI~S
- - - 1
K,
CONTOUR INTERVAL 500 FEET
EXPUMATION
lncllned Iolallon snowlng MWng and Dlunpa
I Pigtaea8.--GeologlcsketchmapofpartofBigDel&
G2 quadrangle, shwing relations between bands of
augen gneiss and intervening metasedimentary rocks.
Stippling denotes thick orthoaugen gneiss sill at
locality 1 (fig. 29A); dashed Unea ouUtne thin
orthoaugen gneiss sills at locality 2. Rne-grairred
augen gneiss occurs at locality 3 (flg. 29g).
Twa thin bands of wen gneiss (appro% I m
thick) in which the augen are smaller but aimfiar to
those in the thick sill crop out nearby (dashed Ues,
loc. 2, fig. 28); these bands Bre also tentatively inter
preted as sills. Augenlikc clots of potassium CeldspX,
less than 1 cm long, occur in some layers of pelitic
schist and biotite gnelss between and below the two
thfn augen gneiss dUa; these porphyroblastic clots
probably formed during metamolphism.
A third, and most enigmatic, textural type of
augen gneiss lies above the thick sill b e. 3, fig. 281, in
an area compsed of the metasedimentmy-rock
sequence previously described. This augen gneiss
occurs as thin (approx 1 m thick) lagers in which nearly
perfect eyeshaped feldspar augen (1-3 cm long, avg
approx 1.5 am long) are evenly scattered in a fine-
grained matrix of quartz+plagioclese+potasslurn feld-
spar+biotite+white mica (fig. 29B). The augen gnelss is
werlaIn by biotite-hornblende schist that contains
scattered clots and thin laminae of feldspar. Foliation
in the augen gneiss parallels that in the adjacent rocks
and the layering. The texture of this augen gneiss,
however, differs from that of the metaplutonic augen
gneis and the nearby sLU (compare figs. 29A and 2 93
in that! (1) The augen are almost perfectly eye shaped
and do not appear to be broken and sheared leth-
shaped megacrysts; (2) the augen are less abundant (10
percent versus 25 percent), and the potasslum fdd-
spar/plagioclase ratio of the matrb hlgher (0.64 versus
0.321, than in the nearby sill) (3) no concentric zonas of
mlneral inclusions were observed in any of the augenj
and (4) the augen appear to be eye shaped in all visble
sections and & not have a randomly oriented euhedral
outhe in a plane perpendicular to bth foliation and
lineation, as In certah excellent exposures of the
m etaplutonic augen gneiss.
These textural differences can be explained by
two dUferent roechanfsms for formation of the augen
at the third locality. In the first mechanism, the tex-
tural differences could be Punctlons of the degree of
penetrative deformation. The augen gneiss layer at
the third locality could have been intruded as a sill,
and, as a result of severe cateclasis, the orlgfnal
phenwrysts (which may have been relatively small
originally) were oomminuted Into their prent shape.
Perhaps the scattered, sparsely distributed,
well-formed augen were the largest phenocrysts in the
original rock, and smaller ones are now part of the
matrix. The survival of the largest megacrysts as
more ldiornorphic coherent augen and the grmulatfon
of the smaller ones are features that ate commonly
observed in the Large body of orthoaugen gneiss.
Mdes of augen gneh from the am boc. 1, fig. 28) and
the fine-grained layer (loc. 3) were determined to
compare bulk compositions and to evaluate the possi-
bility that the textural differences are due to com mi-
nution and (or) to variations in phenocryst size and
abundance. The modes for the sill (total kf=Sl, pl='29,
qz=30, bt=D, mu=i percent) and the augen gneiss layer
(total kf=28, pl=25, qz=36, bt=9, mu=2 percent) are
simb and consistent with the hypothesis that the
twt~lral differences are due to comminution alone.
More than half (57 percent) of the total amount of
potassium feldspar in the enigmatic layer is in the
recrysWeed cataclastic matrix, and much less of the
totd (31 percent) in the matrix of the sin sample.
In the second mechnnlsm, the fine-gralned augen
gneiss could be porphyroblastIc arkosic meta9edlmen-
tary layers, rather than metalgneous W. Sldkulty
of modal compositions would be explained by the fact
that analyses of granitic and arkosic rocks are com-
monly difficult to distinguish. The incipient clots of