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terrane. Igneous rocks and sedImenUry rocks derived
from igneous material make up about two thhds, and
pelltic and calcareous cocks as much as one-third, of
the total succession.
Because of intense deformation, e dafinfte stra-
tigraphy cannot be established for the Plngston terrane
in the study area. However, the abundance of meta-
volcanic rock and of rnetasedirnentary rock derived
from volcanic material, as well as the presence of
lesser schist, quartzite, and marble, suggests that the
original stratigraphy consisted of a succession of sub-
marine volcanic rocks and racks derived ftkorn volcanic
Grafil\lc cbcks
Flysen &@sfts
p. Plngston temd
rn. McKinley tenane
CRETACEOUS
, . . Hlph.angtd cult-lbnsd where mcsd
7L Strike and dlp of schlStDCity
CRETACEOUS OR JURASSIC (7)
F&me 45.--SlmpliEied geahgic map of parts of the
Mount Hayes C-5 and C-6 quadrangles, eastern Alaska
Range, Aleska. X denotes site of rnetarhycdacite
sample 80AAP039A, dated by U-Pb methods on zircon
at 373ff m.y. (~leinikoff and Nokleberg, 1983).
material, 6s well as lesser shale, chert or quarteite,
and limestone, probably deposited in a submarine
island-arc environment. The age of the Plngston ter-
rene in the Mount Hayes and Healy quadrangles is not
well established. Recent U-Pb geoehronologic studies
of a metarhyodacite from the Mount Hayes C-6 quad-
rangle indicate a 373fl-m.y. (Devonian) age of extru-
sion for the protolith of the metarhyodaeite (Meini-
koff and Nokleberg, 1983).
In determining the origin of the Pingston terrane,
the more important units are the metavolcenic mks
and the metavoloanic graywacke (table 14). me mapr
relict minerah in the rnetavolcanic rock8 are albite-
rich ph$ioclase, quartz, and potassium feldspar.
Evidence of an igneous origin for these rocks consists
of abundant complex twinning In plagioclase, local
normal and delicate oscUtory zoning in plagioclase,
local well-preserved euhedral outUnes of feldspar, and
sparse resorbed outlines and embayrnents in quartz.
The rnetrix and relict phenocrysts are intensely de-
formed. The matrix commonly consists of an intensely
deformed and schistose eggregate of mainly metamor-
phic mica, feldspar, epidote, and chlorite. The matrix
exhibits a well-developed schistosity defined by paral-
lel-alined mica and elongate quartz and feldspar, and
by e strong preferred orientation of quertz end feld-
spar crystallographic axes. The relict phenocrysts are
generally fractured, granulated, or crushed and have
been ger\eraLly rotated into orientations paraIfel to
schistosity. Locally, schistosity crosscuts the relict
phenocrysts. Metamorphism of the Pingston terme
under conditions appmximating upper greenschist
facies is indicated by the occurrence of metamorphic
epidote, chlorite, actinolite, biotite, end albite-rich
plagioclese In rocks of appropriate compositions (table
14). Hornblende in the metagabbro probably formed
during replacement of clinopyroxane during hydro-
thermal alteration. Partial replacement of hornblende
by chlorite and actinoUte indicates that the amphibole
has been metamorphosed under conditions approximet-
Ing greenschist facies.
Petrologic anaIysis indicates that the metavol-
canlc rocks of the Pingston terrane range in cornpsi-
tion mainly from andesite and dacite to rhyodacite,
latite, and trachyte. 'Local grading into keratophyre is
suggested by locally low quartz content (as low as 5
modal percent) and by abundant relict phenocrysts of
albite-rich plagioclase. Chemia analyses of the
metevolcanic rocks give the following ranges (in
weight percent) for selected oxides: Si02, 56.3 to 75.7;
CaO, 0.57 to 5.73; Na20, 2.75 to 4.50; and K20, 2.06
to 4.90.
The Pingston terrane is intensely deformed.
Bedding In former volcanic and sedimentary rock is
generally transposed to foliation that consists of de-
formed elongate lenses of variotis rock types. The
most common structures are isoclinal folds and a per-
vasive axial-plane schistosity. Schistosity, defined by
atined mica and deformed feldspar and quartz augen,
ocours subparallel to the transposed bedding except in
the cores of folds. Fold-axlal planes and schistyity
enerally strike west-northwest and dip 40480 S.
. 4). Adjscent to the Hinen Creek fault (fig. 45).
the vario- rock units commonly are intensely cleaved
and sheared. Along the fault is local structural irnbri-
cation of the schist of Jarvis Creek with various rock