n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

were measured on 8 12-in. mass spectrometer with on-
Une dtgital proc~lfg. three size fractions have
nearly idbntieal Pbj2&b ages, ranging from 368
to 372 m.y. (table 15); the U-Pb ages range from about
320 to 350 m.y. (table 15). This age variation does not
follow the ordinary dependency on uranium concentration,
which, in this sample, is extremely low, ranging
from about 120 to 154 ppm and increasing with decreasing
grain size. A best-fit line (Ludwig, 1980)
through the data points yields concordia intercepts of
373~7 m.y. and 256% m.y. (fig. 46). We interpret these
data to indicate that the protolith for the metftrhyod~cite
was erupted during the Late Devonim. The
lowerintercept age reflects either modern lead loss
due to dil~tancy (Goldich and Mudrey, 1972) or, possibly,
late Mcsozoic and Tertiary movement, metamorphism,
and deformation of rocks along the Hines Creek
fault.
Most of the rocks of the Pingston terrane to the
west in the Healy quadrangle or near Mount McKinley
have been assigned u late Paleozoic and Triassic age,
mainly on the besis of conodonts (Sherwood and
Craddock, 1979; Reed and Nelson, 1980; Jones and
others, 1981). The age of the relatively younger diabase
and gabbro is assumed to be middle or Late Mesozoic.
Before our study, no metavolcanic rock f1.orn the
Pingston terrane had been dated. Our data show that
an important component of the Pingston terrane consists
of former siliceous volcanic rock of Late Devonian
age. On the basis of lithology, metamorphic grade,
and structure, we interpret the metarhyodaclte to be
an important constituent of the terrae in the western
part of the Mount Hayes quadrangle (Nokleberg and
others, 1983). This report sug-gests that the age of at
least a part of the Pingston terrene is Late Devonian
and thereby extends the tige range of the terrane and
revises the stratigraphy of the terrane to Include
former siliceous volcanic rock.
REFERENCES CITED
Gilbert, W. G., 1917, General geology md geochemistry
of the nealy D-1 and southern Fairbanks A-l
quadrangles and vicinity, Alaska: Alaska Division
of Geological and Geophysical Surveys
Open-File Report 195, 13 p.
Gilbcrt, W. G., and Redman, EarI, 1977, Metamorphic
rocks of the Toklat-Teklanika Rivers area,
Almka: Alaska Division of Geology and
Gcophysical Surveys Geologic lieport 50, 13 g.
Goldick, S. S., und Mudrey, !\-I. G., Jr., 1972, Dilatancy
model for discordnnt U-Pb zircon ages, &
l'ugarinov, A. I., ed., Recent contributions to
geochemistry and t~nnlytical chemistry: New
York, John Wiley & Sans, p. 466-470.
Jones, D. L., Silberling, N. J., Berg, H. C., and Plafker,
George, 1981, Map showing tectonostra tigraphic
terranes of Alaska, columnar sections, and
summary description of terranes: U.S. Geologlcal
Survey Open-Pile lleport 81-792, 20 p., scale
1:2,500,000, 2 sheets.
Krogh, T. E., 1973, A low-contamination method for
hydrothermal decomposition of zircon and extr~ction
of U and Pb for isotopjc age determinations:
Geochlrnica et Cosrnochimica Acta, v. 37,
no. 3, p. 495-994.
Ludwig, K. R., 1980, Calculation of uncertainties of U-
Pb isotope data: Earth and Planetary Science
Letters, v. 46, no. 2, p. 212-220.
Nokleberg, W. J., BWmaster, C. L., and Schwab, C.
E., 1983, Stratigraphy, petrology, and structure
of the Pingston terrane, Mount Hayes C-5 and C-
6 quadrangles, eastern Alaska Range, & Coonrad,
W. L., and Elliott, R. L., eds., The United States
Geological Survey in Alaska: Accomplishn~ents
during 1981: U.S. Geological Survey Circular
868, p. 70-73.
Reed, B. L., and Nelson, S. W., 1980, Geologic map of
the Talkeetna quadrangle, Alaska: U.S. Geologi-
cal Survey MisceUaneous Investigations Series
Map 1-1174, 15 p., scale 1:250,000.
Sherwood, K. W., and Craddock, Campbell, 1979,
General geology of the central Alaska Range
between the Nenana River and Mount Deborah:
Alaska Division of Geological and Ccophysical
Surveys Open-P Ue Report AOF-116, 22 p.
Wahrhaftig, Clyde, Turner, D. L., Weber, P. R., and
Smith, T. E., 1975, Nature and timing of move-
ment on the Wines Creek strand of the Denali
fault system, Alaska: Geology, v. 3, no. 8, p.
463-466.
The Jeanie Point complex revisited
By Julie & ~Umolilin and Martha L. lldiller
The so-called Jeanie Point complex is a dlstinctive
paclrage of rocks within the Orca Group, a Ter
tiary turbidite sequence. The rocks cmp out on the
southeast coast of Montague Island, Prince William
Sound, approximately 3 kni northeast of Jeanie Point
(loc. 7, fig. 44). These rocks consist dominantly of
fine-grained limestone and lesser amounts of siliceous
limestone, chert, tuff, mudstone, argzllite, and smdstone
(fig. 47). The Jeanie Point rock elsO differ from
those typical. of the Orca Group in their fold style.
Thus, the Or- Group of the area is isoclinally folded
on a large scale (tens to hundreds of meters), whereas
the Jeanie Point rocks are tightly folded on a 1- to 3-
m-wavelength scale (differencm in rock competency
may be responsible for this variation in fold style).
Helwig and Bmmet (1981) first described these
rocks and interpreted them as an accreted terrane of
pelagic origin. They suggested "* * * the complex
could represent a separate terrane andlor a separate
basement to the Orca Group" (Helwig and Emmet,
1981, p. 28). Our studies suggest that the Jeanie Point
rocks are interbedded with the Orca Group and may
have formed in a smr!U ~narginal basin.
The Jeanie Point rocks crop out along the shoreline
on two prominent points and Include about 0.8 km
of tidal-flat and cliff exposures (fig. 48). The rocks
are structurally complex; a best estimate of their stratigraphic
thickness is 100 to 300 rn. Beds are cornrnonly
2 to 10 cm thick and, though folded, ore laterally
continuous, The dominant rock type Is micritic to
sllghtky coarser grained limestone containing few to
abundant planktic and scarce benthic fovaminifers.
Recrystallized and (or) replaced radiobrian tests are
present but not abundant in the micrite. A subordlnate
rock type consists of chert containing few to abundant