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history here is cc r to that in the
higher grade mks, ana epparenrly alifers from that to
the southwest In some places, however, the actual
west limit of metamorphism and deformation is known
to lie to the east of the boundary selected here.
The first part of this report describes the
northern Coast plutonic-metamorphic complex as it
now exists; the second part is a brief speculation on
the driving forces behind its evolution and their
relation to the accretion of the Chugach terrane
(Plafker and others, 1977). 17rroughout this report, the
reeder should bear in mind that most of the events
inferred from the geologic record occurred elsewhere
than at the present position of the northern Coast
plutonicmetamorphic! complex. Some of the earlier
events may have transpired thousands of kilometers
away, but no consistent paleomagnetic evidence exists
to establish control on movements.
DESCRIPTION
Throughout the northern region, the Coast
plutonic-metamorphic complex can be divided into
four more or less parallel wnes (fig. 791, described
here from southwest to northeast es the western
metamorphic zone, the central metamorphic zone, the
central granitic zone, and the eastern metamorphic
zone. These four wnes have been subdivided lmally
according to the proportions of intrusive and
metamorphic rocks and of different metamorphic-rock
types. The Ges of metamorphism and intrusion differ
in part between these zones.
The western metamorphic zone consists mostly
of progressively metarnor~hbsed (higher grade to the
northeast) low- to intermediate-temperature low- to
high-pressure faciesseries metamorphic rocks,
scattered mesdzonal to epkonal granitic bodies, and
rare concentrically zoned mafic-ultramafic messes.
Rare fossils indicate that the protolith ranges in age
from at least Permian to middle Cretaceous. Various
compositions are present: The Lower Permian rocks
are dominantly greenstone/greenschls t,
lirnestone/marble, and slate/phyllite/semischist; the
Upper Triassic rocks are geenstone/greenschist,
slete/phyllite, and minor lirnestone/marble; end the
~urassic(?) to midme Cretaceous rocks are
metaconglomerate/slate/phyUite/semischist derived
from a nysch wedge, and greenstone/penschist
derived from volcanic flows and breccia.
Various plutons are scattered throughout the
zone. The crosscutting concentrically zoned mafic-
ultramafic Mies that intrude these rocks, which were
described by Taylor (1967), belong to the Klukwan-
Duke bland belt of J3rew and Morrell (1980d; they are
probably 100 to 110 m.y. old (Lanphere and Eberlein,
1966). Generally concordant granitic bodies consist
dominantly of diorite, quartz diorite, rnonzodiorite,
and granodiorlte, probably 80 to 90 m.y. old; they
belong to the AdmiraIty-RevllIagigedo belt (Brew and
MorreU, 1980; Burrell, 1983) and have hornfels
aureoles associated with them. Crosscutting granitic
bodies, consisting mostly of biotite granite,
granodiorite, and quartz rnonzodiorite but including
significant amounts of syenite, rnonzonite, and
monzodiorite, belong to the Kuiu-EtoLin intrusive belt
(Brew and others, 1979; Brew and MorreU, 1980a; Brew
@
4
$so
0 50 100
CPG
150 200 KILOMETERS
Figure 79.-Northern Coast plut6hicmetarnorphic
complex (CPC), southeastern Alaska and northwestern
British Columbia, showing zones and units as foUows:
WM, western metamorphic zone; CM, central
metamorphic zone; CG, central granitic zone; EM,
eastern metamorphic zone; SL, Soko Volcanics; LE,
Level Mountain and Mount Edziza volcanic fields.
Boundaries are approximately located.
and others, 1981; Hunt, 1983); they are probably 20 to
25 m.y. old and are epizonaL
The central metamorphic zone consists of
abundant synkinernatic to postkinematic mesozonal to