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

GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
Figure 22. Th-Hf-Ta diagram showing fields of major
volcanic settings of Wood (1980). SRV, subduction-related
volcanics; WPV, within-plate volcanics; NMORB and
EMORB, normal and enriched ocean-ridge basalt. Letter
symbols, table 1.
the Gastineau at Bishop Point and other metabasalt units of
the area (LILE-group 1). Table 3 summarizes the chemical
comparisons of the metabasalt units. Signatures particu-
larly apparent for identifying the older parts of the
Gastineau Volcanics are (1) higher K20 content (fig. 3);
(2) higher P205 content (fig. 4); (3) generally higher TiO,
content (figs. 5, 20); (4) generally higher Sr and Rb (figs.
7, 9); (5) generally lower RbISr (fig. 8A, B); (6) higher Zr
(figs. 9, 18, 20); (7) more enriched chondrite-normalized
LREE patterns (fig. 10); (8) distinctly higher La (figs. 14,
15); (9) higher Th (figs. 15, 16); (10) generally higher Hf
(fig. 16); (1 1) generally higher Th/Yb and TaIYb (fig. 17);
(12) distinctly enriched MORB-normalized abundances of
elements between Th and P (fig. 21); and (13) distinctly
more abundant Nb and Ta (figs. 21, 22). Major- and trace-
element geochemical signatures such as these and as
shown in table 3 can aid geologic mapping of the abundant
greenstone and greenschist units of this region that are
generally difficult to distinguish by field and petrographic
characteristics alone.
INTERPRETATIONS AND CONCLUSIONS
Metabasalts of this study are closely comparable to
those of Wrangellia in other southeastern Alaskan and
nearby areas. The Wrangellia identification of the meta-
basalt of the Chilkat Peninsula (Plafker and others, 1989)
and chemical comparisons of this study clearly indicate
that at least the Triassic upper part of Wrangellia extends
much farther to the south (fig. 1) than previously docu-
mented. The discontinuity of the Wrangellia greenstone
belt between Lions Head Mountain and Blackerby Ridge
near Juneau (fig. 1) is due to plutonic intrusions in inter-
vening areas (Knopf, 191 1; Brew and Ford, 1985) and as
indicated in this study also to the involvement of Late
Cretaceous to early Tertiary amphibolite-facies Barrovian
regional metamorphism (Brew and others, 1989; Himmel-
berg and others, 1991). The amphibolites of western
Heintzleman Ridge show many geochemical similarites
with greenschist-facies metabasalts of the Chilkat Penin-
sula and Lions Head Mountain, and the metabasalt of the
Bishop Point area of the Gastineau Volcanics (tables 1-3;
figs. 2, 5, 9-13, 19-22). The amphibolites have a tholeiitic
composition similar to that of Triassic greenschist-facies
metavolcanic rocks of the area (figs. 2-6, 9-10, 17, 19-
22). Like the greenschist-facies rocks, they show back-arc-
basin, within-plate, or ocean-floor basalt composition (figs.
5, 20, 22). Their unusually high Ba content suggestive of
calc-alkaline nature (figs. 14, 18) may be an effect of alter-
ation. The amphibolites of the Juneau area lack the island-
arc character reported (Stowell and Hooper, 1991 ; Stowell
and others, 1992) for other amphibolites of the west mar-
gin of the metamorphic and plutonic belt of the Coast
Mountains (JA, figs. 15, 17, 20-22). MORB-normalized
geochemical patterns (fig. 21) are those of oceanic within-
plate basalt (OIB) that lack the Ta and Nb depletion rela-
tive to Th and Hf typical of island-arc basalt (Pearce,
1983; Condie, 1989). An absence of metamorphic equivalents
of the voluminous andesites or other more differentiated vol-
canic rocks typically associated with island-arc suites (Gill,
1981)-and lacking from MORB and intraplate oceanic-basalt
associations (Perfit and others, 1980bis a strong argument
against an evolved island-arc origin not only for the arnphibo-
lite but for all metabasalts of this study.
The two groups of REE patterns for the metabasalts of
this study (fig. 10) lie entirely within the spread of REE
patterns reported by Davis and Plafker (1985) for Triassic-
age Wrangellia samples from the Chilkat Peninsula and the
Nikoiai Greenstone. HREE patterns of all units of this
study and LREE patterns of the Upper Triassic part of the
Gastineau Volcanics approximate those of basalt of the
Karmutsen Formation given in Barker and others (1989).
The Permian(?) and Triassic(?) lower part of the Gastineau
Volcanics, however (fig. 10, GVn, GVc, GVs), differs lack
evidence of subduction origin (fig. 22). The many cherni-
cal differences between the Permian(?) and Triassic(?)
western, older part of the Gastineau Volcanics and the Up-
per Triassic Bishop Point area of the Gastineau (figs. 3-5,
7-10, 15-17, 20-22) suggest variations in tectonic setting
during the formation of this volcanic sequence.
The Wrangellia terrane of mainland southeastern
Alaska is found in our study to extend from the Chilkat
Peninsula at least as far south as the vicinity of Taku Inlet
(fig. I), and constitutes much of the Taku terrane shown
by others in studies described earlier. We believe that the
Wrangellia terrane extends even farther south, at least to
the vicinity of Tracy Arm, where little-studied Permian