Geology Reference
In-Depth Information
Italian and Mexican potassic rocks, which are as strongly depleted in Nb
Ta as
calc-alkaline rocks (Fig.
5.20
b). On the other hand, the Chinese potassic rocks have
a similar con
-
guration in spidergram patterns to lamproites from Gaussberg and
Smoky Butte. Both display moderate depletion in Nb
Ta and U
Th and enrichment
-
-
in Ba, K, LREE and Zr
Hf (Fig.
5.20
c), although Chinese lavas are in general less
enriched in strongly incompatible elements than the lamproites.
Remarkable differences also exist between potassic rocks from Wudalienchih,
Erkeshan and Keluo and typical late-Cenozoic Chinese alkali basalts. The former
have higher K
2
O, LILE and LREE than the latter, which in contrast to potassic
rocks, also display OIB-like incompatible element patterns (Fig.
5.20
c).
The
87
Sr/
86
Sr ratios of potassic rocks from northeastern China range from
0.70500 to 0.70565. Olivine leucitites, some leucite basanites and trachybasalts,
which contain mantle xenoliths generally have lower ratios (0.70500
-
0.70527) than
-
xenolith-free leucite basanites and trachybasalts (0.70533
0.70549). Six samples
-
from this region have
143
Nd/
144
Nd ratios of 0.51250
0.51232 (
ε
Nd
of
2.8 to
6.3)
-
−
−
and display a negative correlation with
87
Sr/
86
Sr (Fig.
5.21
).
The Pb isotopic compositions of potassic rocks from China have the following
ranges:
(
206
Pb/
204
Pb),
(
207
Pb/
204
Pb)
16.607
-
17.058
15.352
-
15.447
and
37.204 (
208
Pb/
204
Pb). They plot on the left side of the
'
36.511
Geochron and above
the Northern Hemisphere Reference Line (NHRL; Hart 1984, Fig.
5.22
). Both
207
Pb/
204
Pb and
208
Pb/
204
Pb ratios of olivine leucitites and xenolith-bearing leucite
basanites and trachybasalts usually have higher
206
Pb/
204
Pb ratios (17.058
-
16.789)
-
than those which are xenolith free (16.900
16.607).
-
On a
87
Sr/
86
Sr versus
ε
Nd
diagram (Fig.
5.21
) the Chinese potassic rocks plot on
the less enriched end of a trend de
ned by potassic rocks with moderately high
Fig. 5.21 The
ε
Nd
versus
87
12
Sr/
86
Sr for K-rich rocks
from northeast China and
other potassic rocks.
(N
-
MORB data from White
et al. 1987; after Zhang et al.
1995)
WEK potassic rocks
Average N - MORB
West and Central Europe
MANTLE ARRAY
8
Italy
4
New South Wales
0
East African Rift
Micaceous Kimberlites
- 4
- 8
S.E. Spain
- 12
West Australia &
Gaussberg
Northwest
U.S.A
- 16
- 20
- 24
- 28
0.702
0.706
0.710
0.714
0.718
0.722
86
87
/
Sr
Sr
Search WWH ::
Custom Search