- 112 -
Project 8607C
A U-Pb Zircon Date for the Ennadai Group
by J.R. Chiarenzelli1 and R. Macdonald
Chiarenzelli, J.R. and Macdonald, R. (1986): AU-Pb zircon date for the Ennadai Group; in Sunmary of Investigations
1986, Saskatchewan Geological Survey; Saskatchewan Energy and Mines; Miscellaneous Reporr-86-4.
The Ennadai Group is the name adopted for an
assemblage of metavolcanic and associated
supracrustal rocks which occur in the vicinity of
Hattle Lake, near the northern Saskatchewan
border northeast of Stoney Rapids (NTS area
64M-14) (Fig. 1). The rocks at Hattle Lake
represent the southernmost outcrop of an
assemblage which extends northwards into the
Northwest Territories through Snowbird Lake
(Taylor, 1973). Similar rocks occur in a separate
basin further northeast of Snowbird Lake where
they have been termed the Hanik Group (Eade,
1974), and the Ennadai Group possibly also
correlates with the Kaminak Group which has
yielded late Archean dates (Stockwell, 1963).
Geology
The rocks around Hattle Lake were described
briefly by Macdonald (l 984). In summary, they
comprise mafic to intermediate metavolcanics with
many pillowed flows, and a variety of volcaniclastic
rocks. These rocks lie generally in a regional-scale
synform and are succeeded in the core of this
synform by volcanogenic metasediments, including
iron formation. Although the metamorphic grade is
generally about lower amphibolite facies,
deformation is relatively low. Contiguous granitic
rocks appear to be representative of high-level
plutons, and there is no indication of a basement.
Sample Data
The sample for age determination was collected
from a rare rhyolitic rock which occurs in a band
about one to two metres thick within the flow
succession immediately southeast of Hattle Lake
itself. The sample is unaltered, pinkish grey, fine
grained and contains potassium feldspar and quartz
phenocrysts. There is little sign of deformation.
Analytical Technigues
The analytical data (Table l) were determined at
the Isotope Geochemistry Laboratory, K.U. Center
for Research, University of Kansas. Techniques
used were similar to those given in Van Schmus et
al. (in press). Two zircon fractions were
air-abraided. Results are reported using Ludwig's
!university of Kansas, Lawrence, Kansas.
~ Phanerozo ic
D
D
Athabasca Group
(Helik ianl
Pre - Athabasca basement
_ . . - Major shear zone
Figure l - Location of the Ennadai Group (solid
triangle).
(l 980, 1983) Model 1 solution (essentially York,
1969) and ages are quoted at the 2-sigma level.
Results·
The sample yielded euhedral to anhedral zircons
averaging 0.2 mm in size, the bulk of which are
stubby dipyramids. Almost all show fine concentric
growth zoning typical of felsic volcanic rocks, and
about half are heavily fractured. Some zircons
contain needle-like inclusions of an unidentified
mineral.
These zircons give an upper intercept age of
2682±5. 9 Ma (Fig. 2). The age is consistent with
predictions and with the ages obtained from similar
- 113 Table 1 - Analytical Data from Zircon Fractions
Radiogenic Ratios
Pb Isotopic Composition
Pb - 204 Pb-207 Pb-208 Pb - 206 Pb-20 7 Pb-20 7
Pb - 206 Pb-206 Pb:...206 U- 238 U- 235 Pb - 206
Concentratio n s
Pb
u
( ppm)
(ppm)
Frac t i o,1
Ages
Pb- 206 Pb-207 Pb- 20 7
U-23 8 U- 235 Pb-20 6
M( - 2)a a
176
106
0 . 00010 0 . 18398 0 , 18264 0 , 5130 12 , 928 0 , 18278
2669
2674
2678
M(-l) aa
203
123
0 , 00035 0 . 18669 0 , 19720 0 , 5063 12 , 730 0 , 1823 7
2641
2660
2675
NM(-2)
168
106
0 . 00194 0 . 20447 0 , 23833 0 , 4957 12 . 366 0 . 18093
2595
2632
2661
M( - 2)
179
105
0 . 00045 0 , 18670 0 . 18902 0 . 4931 12.319 0 . 18120
2584
2629
266 3
M(O)
227
12 7
0 . 00029 0 . 18240 0 . 18702 0 , 4749 11. 709 0 , 17883
2502
2581
264 2
rocks in the Northwest Territories (Stockwell, op.
cit.), and is interpreted as the primary
crystallization age of the rhyolite.
References
Eade, K.E. (1974): Geology of the Kognak River
area, District of Keewatin, Northwest
Territories; Geol. Surv. Can., Mem. 337.
Acknowledgment
Stockwell, C.H. (1963): Second report on structural
provinces, orogenies and time-classification of
the Canadian Precambrian Shield; Geo 1. Surv.
Can., Pap. 62-17 , pl23-137.
The authors thank J .A. McNutt and D. J iricka
(Saskatchewan Mineral Development Corporation)
who facilitated the collection of the sample and
provided geological input during the field visit.
Taylor, F.C. (1963): Geology of the Snowbird Lake
area; Geol. Surv. Can., Mem. 333.
.s
HU0-84-29
•4
=,
(l)
g]
.3
.........
.0
CL
~ .2
/
C\J
/
/
/
/
/
/
/
/
/
/
/
/
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/
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Van Schmus, W.R., Bickford, M.E., Lewry, J .F. and
Macdonald, R. (in press): U-Pb geochronology in
the Trans - Hudson Orogen, northern
Saskatchewan; Can. J. Earth Sci.
.JI
York, D. (1969): Least squares fitting of a straight
line with correlated errors; Earth Planet. Sci.
Lett., v5, p320 - 324.
Ludwig, K.R. (1980): Calculation of uncertainties
of U-Pb isotopic age data; Earth Planet. Sci.
Lett., v46, p212-220 .
•1
INTERCEPTS AT
2682. 2 ! 5. 9 and 950 ! 160 Ma
QL--.....L...----L~_._---L~..L--L.~L-.....L...--l'--.........---'~.....____.____,
0
2
6
8
10
12
207pb;235u
Figure 2 - Concordia diagram for zircon fractions from a
rhyolite in the Ennadai Group. Open circles are
air-abraided points.
14
( 1983): Plotting and regression
programs for isotope geochemists, for use with
HP-86/87 microcomputers; U.S. Geol. Surv.,
Open File Rep. 83-849, 94p.