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

foliated and inequigranular end contain seriate
plagioclase, although locaI variation to porphyritic as
well as equigranular texture occun. The mineralogy
of this body is the same as that of the crowdedplaglbcIase
bodies. Biotite and hornblende are the
dominant maf ic minerals, although their proportions
vary. Primary epidote, sphene, and apatite are also
present. The composition of body 4 range from
granodjorite to tonalite, and the color index from 24 to
35. Rocks from the east margin contain anhedral redorange
garnets associated wlth the mafic minerals.
Body 5, on central Mitkof Island, is anomalous in
comparison with the surrounding stocks. Hornblende Is
generally the dominant mafic mineral, although biotite
is locally present in equal amounts. Hornblende occurs
as anhedral crystals, commonly clustered togeLher. In
about half the rocks studied, the hornblende has relict
pyroxene cores, and much of it is poikilitic, with
enclosed quartz, feldspar, and opaque minerals.
Primary epidote is a minor phase in some rocks.
Phgioclase, as small twinned and zoned laths grown
together, forms the bulk of the gmundrnm. These
rocks are inequigranuhr foliated rnonzodiorfte to
quartz diorlte; the color index rang- from 34 to 55.
An arc of inequigranular crowded-plagloclase quartz
rnoneodiorite and quartz diorite crops out along the
north edge of this body.
Body 6; on the northern Lindenberg Peninsula, is
much like body 5. The rocks ere mostly foriated
inequigranular wtz monaodiorite, and the color
index ranges from 52 to 51. Hornblende, the dominant
maflc mineral, ranges from small &mete rmbhedral
crystals to larger anhedral crystals, wlth inclusions of
quartz and feldspar and commonly relict pyroxene
cores. Primary epidote L present in very smd
amounts. Plagioclase is twinned and Boned, and
commonly exhbits a seriste texture. Sphene and
apatite are common accessory minerals.
Mapping is incomplete southeast of Mitkof
Island, and the plutons there have been compared with
the Mitkof Island and Lindenbem Peninsula ~lutons by
reconnaissance mappis only. %cks observed in the
southeastern Dart of the area. include epidote(sarnet)hornblende-bibtite
graflodiorite, ton&e, and-qwtz
diorite with ineguigranular to porphyritic textures.
A recent potassium-argon age determhation on
hornblende from body 4 yielded an age of 80.1 m.y.
(Marvin hnphere, written commun., 1982). Bodies 1
through 4 we correlated with similar porphyritic and
inequigranular rocks in the Bradfield Canal 1:250,000scale
quadrangle, which were also dated at
approximately 90 m.y. by potassium-argon methods (R.
D. Koch, oral commun., 1982).
The Mitkof Island and Lindenberg Peninsula
plutons may represent a complex intrusive sequence.
Bodies 1 through 4, though exhibiting Iimited textural
differences that range from crowded porphyry to
inequigranular, have similar mineralogles and ate
likely to be coeval; they are herein considered to
constitute a distinct group. Bodies 5 and 6 compose a
second distinct group; pbkilitic hornblende with
pyroxene cores, an ineguigranular texture, and a
plagkalase-dominant ground mass distinguish them
from bodies 1 through 4. Despite these observed
differences, however, the presence of primary epidote
and the generally similar compositions and structural
settings indicate that these two groups of plutonlc
bodies may be genetlcaUy related.
BBFERENCE CITED
Brew, D. A., and Morrell, R. P., 1980, Intrusive rooks
and plutonic belts of Wutheastern Alaska,
U.S.A .: U.S. Geological Survey Open-Pile Report
80-18, 34 p.
Late Oligocene gabbta near Ketehilrsn, m u ~ e m
Alaska
Bp Richard D. Koch aod Bsymond L. Klliott
A large elongate gabbro complex, here referred
to informdy as the gabbro of Ketchikan takes, is
exposed over an irregular area, 7 by 16 km, in the
vicinity of Ketchikan Lakes just north and east of the
town of Ketchikan (area 8, fig. 72; fig. 82). Originally
believed to be mainly granodiorite and quartz dlorite
with a small area of leucogabbro (Berg and others,
1978), this heterogeneous pluton consists enthely of
various gabbrofc rocks.
Composition varies considerably across this
complex, with a crude eonation, with a small slightly
offcenter core area oi olivine-bearing twbpycoxene
gabbro, wholly enclused by an area of biotite-
hornblende-twepyroxene gabbro. A third,
discontinuous zone of quartz-bearing gabbro underlies
two large areas at the northwest and southeast ends of
the complex. A few samples from this outermost zone
also contain minor amounts of potassium feldspw.
Compositions vary within, as well as among, these
zones. Contacts between the zones were not observed
in the fleld and are presumed to be gradational.
Quartz-free gabbmlc rocks from the ollvhe-
bearing core zone and from much of the surrounding
mea contain biotite, hy persthene, hornblende, and
auglte. The color index for these rocks ranges from
about 15 to 50 and is generally 30 or less; thus, the
rocks are lagely leucogabbro. Relative abundances of
the rnallc miner- vary considerably from place to
place; clho yroxene exceeds orthopymxene tn all but
one sample P norite).
The color index for the quartz-bearing gabbro
ranges from about 12+ to 55 and is mostly less than 30
Oeucogabbro). Many of these quartz-bearing rocks
contain clinopyroxene, hornblende, biotite, and little
or no orthopgroxene; a few contain only hornblende
and bIotIte as rnafic minerals.
Most of the gabbro is massive, tine to medium
gained, serlate to relatively equigrandar, with a
hypidlomorphic-grmular texture. Foliation, formed by
alinement of plagimlase and mafic minerals, is poorly
developed in a few places, and diabasic and rarer
aUotriomorphl~anular textures also occur.
Quartz, where it occurs, forms small interstitial
grains hat constitute less than 5 percent of the rock.
Potassium feldspar ~eurs in only a few places, 8s
smell untwlnned anhedral interstitial grains.
F'lagioolase forms anhedral to euhedral crystah,
but the texture of almost all the gabbro 18 dominated
by subhedral phgioclase as equant to, more commonly,
elongate laths with a length-to-width ratio of 411 to