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

Figure 8a.-Geolaic sketch map of gabbro complex near Ketchikan, Alaska.
2:1, rarely as small as 6:l. Pine albite twinning is
common in most plqioclase crystals; Carlsbad
twinning is also common, and perlcline twinning occurs
in minor amounts in mast of the samples exernined.
The plagioclase crystals are unzoned to very strongly
zoned; zoning is gradational progressive (normal),
progressive concentric stepped, and locally oscillatory
with as many as 6 or more cycles. Cornpition of
most grains lies toward the calcic end of the range
An to An which is the optically measurable range
of ?Ke mor!hrongty zoned erpta. A few grains in
samples from throughout the complex have a clear
untwinned core area with high relief, yellow
interference colors, and irregular fractures. These
cores ere probably early-formed and partially resorbed
plagioclase of extremely oalcic composition, probably
close to pure anorthite. The plagioclase commonly has
myrrnekitic borders where it contacts potassium
feldspar.
Mafic mlnerals make up from about 12 to at
kast 50 percent of the rock, and the relative
proportions of different mafic minerals vary markedly
from place to place. With local exceptions, however, a
crudely sequential change in these relative proportions
appears to occur from the olivine-bearing core area to
the quartz-bearing outer areas. In the core area, the
maf ic assemblage includes olivine, ort hopyroxene,
clinopyroxene, hornblende, and biotite. Away from the
core area there is generally a diminishment and,
finally, loss of the maftc minerals olivine,
orthopyroxene, and clinopyroxene, commonly in that
order. The pyroxenes are anhedral to subhedral and
commonly form cores in brown hornblende or are
rimmed by brown hornblende, with sharp or gradational
boundaries between the two miner&. The brown
hornblende Is anhedral to subhedral and generally
within the mediumwained size range; it locally forms
large pokilitic crystals wIth many pyrnoxene
Inclusions. Biotite b strongly colored dark reddish
brown. Opaque minerals occur as discrete disseminated
anhedra in trace emounts to 7 percent of the rack in a
few places. Where the opaque minerals are abundant,
they also form densely packed inclusions in pyroxene.
Apatlte occurs in most samples a8 trace amounts of