182 Peter Sdlle and Graham Shields
0.820
r
0.780
O~ 0.7LO
Jr
.<
0.700
0
0.850
&800
0.750
0,4 - G 5 um
J
9
A
3
6
9
02,:q3vm
J
O.?O0
,
'
:
0
5
10
t S
OmgO0
Ol- 02u~
~ "
/
O.~SO
0.@00
0.750
0.7O0
0
6
12
18
24
= ? R b / ~ 6 S r
0.7610 ~,
0.140 h
0
0820
0.780
O.740
0.700
0
|
|
I
2
&
6
t.
8
12
eTRb / 16Sr
9 Le,achate
U~treated
o Re~id~e
Fi~. 6.Z3. Rb-Sr isochron diagrams for various illite grain-size fractions from samples close
to the Precambnan/Cambrian bounda~, The clay mineral fractions represent mixtures of
authigenic and detrital illites. The clay fractions 0. I-0,2 ~.m mainly contain authigenic clay
minerals and define an age of 533 _+. 0.2 Ma. This age is considered to represent a minimum
sedimentation age close to the Precambrian-Cambnan boundary, (after Gorokhov et al.
t994)
This investigation shows once more that the interpretation of Rb-Sr isotope data
on clay mineral fractions is problematic because of the difficulty of separating
authigenic clay mineral fractions from detrital ones. Only accompanying studies
of the detailed clay mineralogy and general geology can help us towards a
satisfactory interpretation of isotopic ages.