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

RECONNAISSANCE GEOCHEMISTRY OF BASALTS OF THE TAKU AND WRANGELLIA TERRANES 225
Discriminant plots also may be used to characterize
these basalts. For instance, the MnO-Ti02-P205 plot of
Mullen (1983) (fig. 6) shows the Permian basalts as ocean-
island alkalic type (OIA) and the Triassic ones as either
MORB or ocean-island tholeiite. The Th-Hf-Ta plot of
Wood (1980) (fig. 7) also shows the Permian samples as
OLA type, but it groups the Triassic ones as E-MORB or
within-plate tholeiitic (WIP) basalt. The use of several
other of the various discriminant plots found in the litera-
l! 2mt TAKU TERRANE. PERMIAN AGE
40 - TAKU TERRANE, TRIASSIC AGE
lo -
8 -
t WRANGELLIA
. .
\\
TERRANE, TRIASSIC AGE
Figure 5. Modified spider diagram of average compositions of
Permian and Triassic basalts of the Taku terrane and of Triassic
basalt of the Wrangellia terrane. Chondritic normalizing values
from Thompson (1985). Note that Ba, Rb, and K are omitted.
1
ture on tholeiites, however, generally tends to give contra-
dictory results (see, for instance, Barker and others' (1989)
discussion of the Karmutsen Formation's basalts of the
Wrangellia terrane).
The average major elements of 23 basalt samples, not
individually dated, from the Juneau area given by Ford and
Brew (1988) generally are like those of the Taku terrane.
However, their average values of 0.39 percent for P205
and 1.74 percent for Ti02 are intermediate to our Permian
and Triassic samples (table I), which suggests that their 23
samples include both ages of basalt.
COMPARISON WITH TRIASSIC BASALTS
OF CHILKAT PENINSULA
Triassic basalts underlie much of Chilkat Peninsula
and extend northwestward into southwestern Yukon (fig.
1). These basalts and related metasedimentary rocks were
considered by Monger and Berg (1987) to be part of the
Taku terrane. On the basis of similarities in stratigraphy,
age, faunal affinities, and geochemistry, Plafker and
Hudson (1980), Davis and Plafker (1985), and Plafker and
others (1989) correlate these basalts with the widespread
Triassic basalts that characterize the Wrangellia terrane in
southern Alaska (Nikolai Greenstone) and coastal British
Columbia (Karmutsen Formation). Our studies of the
rocks as found at Berners Bay and Lynn Canal (see figure
1 and table 1) support this correlation, and are also consis-
tent with Plafker and Hudson's (1980) and Ford and
TiO, Hfl3
Figure 6. Mn0-Ti02-P205 discriminant plot of Mullen (1983). Figure 7. Th-Hf-Ta discriminant plot of Wood (1980).
CAB, calk-alkaline basalt; IAT, island-arc tholeiite; MORB, Abbreviations as in figure 6, and E-MORB, enriched MORB; N-
midocean-ridge basalt; OIA, ocean-island alkali basalt; OIT, MORB, normal MORB; WIP, within-plate basalt. Symbols
ocean-island tholeiite. Symbols explained in figure 1. explained in figure 1.