Geologic Studies in Alaska by the U.S. Geological Survey, 1992

140 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
overlap for ophiolitic rocks of the Orca Group with addi-
tional samples plotting in the MORB region. Clearly there
is a significant range of values for the Orca rocks both
locally and regionally.
STRATIGRAPHIC GEOCHEMICAL VARIATION
Several of the geochemical values from this study
were evaluated to see if there were any significant changes
in values from samples stratigraphically low to high in the
Drier Bay basalt section. At all sites, the stratigraphic up
direction is to the west. The data were divided into three
geographic groupings (fig. 2): group 1 consists of samples
1, 2, 3 from the north side of Drier Bay, group 2 consists
of samples 7 and 8 from the south side of Drier Bay, and
group 3 consists of samples 4, 5, 6 from near the mouth of
Drier Bay. Within each group the low-numbered sample
is the stratigraphically higher one, and group 3 samples are
the stratigraphically highest group from this study (fig.
2A). Geochemical values examined consist of both major
oxides (Si02, Na20, K20, FeO*, Ti02) and trace elements
(Ba and Zr). Groups 1 and 2 show either increases or
decreases of the elements stratigraphically, although the
same element may not change the same way
stratigraphically (FeO* decreases up section in group 1,
but increases in group 2). In group 3, three of the ele-
ments (K20, FeO, and Ti02) decrease up section and the
other three elements show no trend. None of the elements
shows the same trend in each of the groups. Therefore, we
find no systematic geochemical changes within the strati-
graphic section sampled.
TECTONIC INTERPRETATION
Table 2 summarizes the tectonic classification from
the discrimination and selected variation diagrams pre-
sented above. Five of the figures show plots suggesting
that the rocks formed in a midocean-ridge setting. The
rest of the figures suggest some overlap between arc,
MORB, or primitive arc tectonic settings. The data from
Knight Island show similar geochemical variations as ob-
served in other ophiolites in Prince William Sound (Crowe
and others, 1992). This overlap of MORB and arc-like
chemistry is permissive of a back-arc basin basalt (BABB)
(Hawkesworth and others, 1977; Dick, 1982; and Donato,
1991). Crowe and others (1992) suggested, however, that
the variation of basalt chemistry in the ophiolites of Prince
William Sound was due to the contamination of their mag-
mas by sediments in a ridgeltrench setting.
How then do we resolve the arc, back-arc, and ridge
settings? Petrographic (Dumoulin, 1987) and isotopic
evidence (Farmer and others, 1989) from the sedimentary
rocks of the Orca Group suggests that they were derived
from an eroding arc inboard of their present position, pos-
sibly the Coast Plutonic Complex of southeastern Alaska
and British Columbia. Seismic studies of deep crustal
structure beneath the Chugach terrane shows evidence for
underplated oceanic crust and upper mantle(?), but no clear
evidence for a subducted island arc (Plafker and others,
1989). In other words, geologic evidence suggests an in-
board arc but no former existence of an island arc out-
board of Prince William Sound.
The ophiolites of the Orca Group show a great com-
positional diversity. Basalt, basaltic andesite, diorite, and
plagiogranite compositions are reported (this report and
Crowe and others, 1992). These variations are consistent
with sediment contamination of a MORB magma such as
in a ridgeltrench setting (Lull and Plafker, 1990).
As mentioned above, the present-day setting of the
ophiolitic rocks in Prince William Sound is in an accre-
tionary prism or fore-arc setting. It seems incompatible
with the strong MORB-like geochemical signature and the
presence of sheeted dikes that these rocks could have
formed in a contractional fore-arc region. It appears more
likely that the ophiolites were derived from a subducting
oceanic ridge in a setting similar to the present-day Juan
de Fuca ridge.
Several workers have called upon sediment contami-
nation to account for variable volcanic rock compositions
in an accretionary setting (Moore and others, 1983; Cole
and Basu, 1992; Crowe and others, 1992). What is not
always addressed is where or how the contamination took
place. For Knight Island the possibilities are: (1) con-
tamination of ridge magmas before subduction, (2) con-
tamination of ridge magmas after subduction, or (3)
contamination in a partially subducted ridge segment.
Possibility 1 seems unlikely because of the difficulty of
getting the sedimentary deposits from the surface down
into a magma zone. Possibility 2 seems unlikely because
it would require that the ophiolite form in a subduction
zone below the accretionary prism and therefore it would
be impossible for 5,000 m of pillow lavas to form. Possi-
bility 3 seems reasonable, as discussed next.
The composition of rocks along the Endeavor Seg-
ment of the Juan de Fuca Ridge are of two types (Karsten
and others, 1990). These types are geochemically classi-
fied as E-MORB (enriched-MOD) or transitional MOD.
The enriched rocks were collected north of the Cobb Off-
set. The enriched composition described by Karsten and
others (1990) differ from the Knight Island rocks in that
they contain greater than MORB values for more elements
(fig. 13) and are chemically similar to the compositions of
rocks forming in a within-plate setting (Pearce, 1983).
Although the compositions of the enriched rocks
found along the Endeavor Segment of the Juan de Fuca
Ridge do not match those from Knight Island, the mecha-
nism suggested by Karsten and others (1990) for their em-
placement could explain the Knight Island compositions.