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

210 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
but distinctly different from N-MORB (fig. 17). All units
except the high-Ba metabasalt of the Bishop Point area and
amphibolite show BaIZr values of back-arc-basin basalt,
and the western subareas of the Gastineau Volcanics form
Th, IN PARTS PER MILLION
Figure 16. Variation between average Hf and Th content
of metabasalt units, showing MORB (normal and
transitional) and Icelandic basalt fields of Wood and
others (1980). Letter symbols, table 1.
Figure 17. Variation between Yb-normalized average log
Th and Ta content of metabasalt units, showing fields of
volcanic-arc basalts, N-MORB, and intraplate basalt; and
tholeiitic (TH), alkaline (ALK), and calc-alkaline (CA)
[transitional (TR)] compositional fields of Pearce (1982,
1983). Band between solid lines shows field of basalt
from nonsubduction settings of Pearce (1983). Western
subareas of the Gastineau Volcanics (GVs, GVn, and
GVc) are in field of enriched mantle source of Wilson
(1989). Letter symbols, table 1.
a distinct Ba and Zr group (fig. 18). Because other chemi-
cal indicators clearly show tholeiitic composition for the
metabasalt of the Bishop Point area and amphibolite, the
calc-alkaline island-arc character of these units apparent in
figures 14 and 18 is inferred to reflect Ba mobility.
Zr, IN PARTS PER MILLION
Figure 18. Variation between average Ba and Zr content of
metabasalt units, showing fields of island-arc basalt, back-
arc-basin basalt (BABB), and MORB of Saunders and
Tarney (1991). Letter symbols, table 1.
Figure 19. Variation between Ti/100, Zr, and Y times 3 of
metabasalt units, showing field of within-plate basalt (WPB)
and overlapping fields of low-K tholeiite (LKT), ocean-floor
basalt (OFB), and calc-alkaline basalt (CAB) of Pearce and
Cann (1973). Field of LKT-OFB-CAB limited to ocean-floor
basalt (= back arc) by Hawkins and others (1985). Letter
symbols, table 1.