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sampled is related to widespread mafic-dike
emplacement, subsequeht work on both Prince of
Wales and Annette Isbds suggests otherwise. h
particular, even though samples from both islands show
similar discordance patterns, crosscutting ma fic dikes
are rare on Annette Mand in comparison with Prince
of Wales Island. Furthermore, although restriction of
the main dike swarms to the Paleozoic rocks suggests
a Paleozoic age, it is unlikely that dike empIacernent
could account for a latestage disturbance.
KASAAN PENINSULA
~rpn-E dike -
- -
--
Fgure 74.-Zircon 206Pb/238~ and 207~b/207F% qes
on cock samples from localities Cflg. 2724
southern Alexander terrane. Large dots, Pb/ in23v;
ages, with diameters ro ' ating analytical
uncerteintien; small dots, 2qbl''Pb ages, with Line
lengths %gingt3palytical uncertainties. Links
between Pb/ U dots denote multtple-fraction
analyses from cogenetic sat&.
The oldest rnetaignwus rocks dated ate the
trondhjernite of Klakas Inlet. The five fractions show-
ing apparent concordance and the systematics of the
discordant fractions indicate igneous ages in the range
450-500 m.y. The Klakas Inlet trondhjemite occurs as
o kilometerscale homogeneous mass and as meter-
scale dikes and screens within sheeted mafic dikes.
Contradictory intrusive relations (mafic dikes cutting
trondhjemite and trondhjemite dikes cutting mafic
dikes) suggest that the Klakas Inlet mafic dikes are
cogenetic with the trondhjemite. Detailed relations
between the trondhjemite/mafic-dike assemblage and
the adj~cent Wales Group are unclear awing to com-
plex intrusive patterns along their contact zone thet
may involve younger igneous rocks.
The youngest rocks dated are the trondhjemitlc
plutons and dikes of Annette Island. Concordant and
discordant ages from these rocks indicate igneous
crystallization at about 400 m.y. 5.P. Detailed
mapping by Gehrels and others (1 983) indicates that
the Annette Island trondhjemite intrudes both meta-
dioritic and Wales Group-type assemblages. A quartz
dioritic phase of the metadioritic wallrock assemblage
yields an age erray suggesting an igneous age in the
range 400-500 m.y.
DetaiIed age constraints cannot be placed at this
time on the trondhjemite and quartz diorite samples
from the Kasaan Peninsula and Clover Polnt-Island
Polnt weas. The Kasaan Peninsula trondhjerni te
sample is from a series of dikes and pods that occur in
gebbro-diorite of locaUy sheeted basaltic
dikes. The 2'b/'Pb we on the Vondhjemlte
sugg& that igneous crystdlization took place about
400 m.y. B.P. Quartz diorite of the Island Point area
to the south is the youngest phase from a complex
mixture o et ' ritic, gabbroic, and pymxenitic
mks; the io'Pb/wl?b ages suggest M igneous age in
the range 400-450 m.y.
Sand- to bouldersize debris of diorite, gabbro,
and pyroxenite from the Island Point assemblage con-
stitutes an ancient talusslope breccia exposed from
the vicinity of Clover Point for about 2 km north-
ward. Important additional elastic components consist
of rare trondhjemite and mafiedike-rock clasts
and blocks f &wed?) Devonian limestone.
Diseordanl 20'Pb/2 Pb ages on zircon from a large
trondhjernite cobble suggest an igneous Rge of about
450 rn.y., not unlike that of the Klakas trondhjemite.
The Klakas Inlet trondhjemite and its related mafic
dlke rock were unroofed and shed as coarse detritus
during Early Devonian time, as is shown by a wne of
highly shattered trondhjernite along the eastern part of
Klakas lsland that grades into an ancient talus breccia
similar to that exposed at Clover Point. The KLakas
Inlet breccia is depositionally overlain by Lower
Devonian grit and shale. Although the age of the
Clover Point breccia is not hown, crosscutting mafic
dikes that are widespread in Paleozoic rack on Prince
of Wales Island elso crosscut the breccla. The breccia
is tentatively considered an expresion of the same
mid-Paleozoic tectonic event that unroofed the Klakas
Inlet trondhjemite and rocks of the rnetoigneoks
complex on Annette Island (Gehrals and others, 1983).
The metaigneous rocks described above and by
Gehrels and others (1983) ma9 constitute a major and
fundamentally important lithologic constituent of the
southern Alexander terrane. Published field and age
data (Mac~evett, 1963) Lanphere and others, 1964;
Berg, 1973; Kmh and others, 1977) and our own recon-
naissance work suggest that such metaigneous rocks
underlie parts of Gravina and Duke Islands, the Cape
Fox areB on the mainland, and southernmost Prinm of
Wales Island. Furthermore, dioritic, gabbrolc, and
pyroxeaitic members of the metaigneous complex are
apparently of the proper age and composition to be