Fig. 6. The SiO
2
versus Al
2
O
3
concentrations of shales from the Belaya River open triangle and Maya River closed
triangle are plotted relative to the idealized composition of the observed minerals. Much of the variation in composition
may be accounted for by variation in quartz and clay miner- als-muscovite. The three samples with high FeO total and low
SiO
2
are skewed in the direction of hematite, opaque minerals and Fe-rich chlorite compositions. The FeO total vs. Al
2
O
3
concentrations of shales from the Belaya and Maya Rivers are plotted relative to the composition of the observed minerals
same symbols as in Fig. 6a. Again much of the variation in composition may be accounted for by variation in quartz and
clay minerals-muscovite. The Fe-rich samples are again skewed toward the hematite, magnetite, and Fe-rich chlorite
compositions.
primary and recycled clay material Cabanis and Lecolle, 1989; Cullers, 1994b, 1995. Ancient
weathering profiles are observed in some parts of the Lakhanda sequence. These profiles contain
mostly kaolinite with lesser hematite and chlorite Semikhatov and Serebrykov, 1983. The B 2 mm
fractions of samples in this study, however, were mainly illite – muscovite with smaller amounts of
kaolinite and chlorite. Elemental plots are also consistent with these phases, but the whole rock
samples have variations in major element compo- sitions that suggests a significant amount of
quartz and iron oxides may be present e.g. Fig. 6.
Also the high concentrations of Zr, Th, REE, Hf, and Th relative to the PAAS in the Lakhanda
shales could be due to the concentration of certain accessory minerals zircon, monazite, ilmenite, ru-
tile. For example, the observed correlations be- tween Zr, Hf, and TiO
2
tends to support this possibility. The correlation of Sc and Cr with
Al
2
O
3
and lesser correlation of Th and the REE with Al
2
O
3
suggests these elements may also be included in the clay minerals of the Lakhanda.
There are poor or no trends in the elemental compositions with time. The best of these correla-
tions is a slow increase in Na
2
O in both the Maya River and Belaya River samples with time.
5. Discussion
5
.
1
. Comparison of the composition of samples from the Belaya and Maya Ri6ers
The composition between the samples from the Belaya and Maya Rivers are compared by using
the Student t-test of the log
10
of the elemental ratios relative to Al
2
O
3
. Statistically comparing the log
10
of elemental ratios avoids the constant sum problem that insures that there must be some
correlations of elements since they must add to 100 percent. Comparing the log
10
of the elemental ratios converts the constant sum data with contin-
uous variables that range to infinity so the data may be compared using parametric tests such as
the Student t-test Cardenas et al., 1996. index of weathering values CIA = 0.67 – 0.89;
Table 1, especially shales from the Maya River area. These chemical characteristics are consistent
with formation in stable platform environments with intense chemical weathering of mostly silicic
source
rocks Ronov
and Migdisov,
1971; Sochava et al., 1994 with possible mixing of
The log of the ratios of TiO
2
to Al
2
O
3
is significantly higher and those of SiO
2
, MgO, Na
2
O, K
2
O, Rb, Ba, and Ni to Al
2
O
3
ratios are lower in the Maya River than the Belaya River
sections Fig. 7. Other elemental ratios between the two areas are statistically the same thus sug-
gesting similar source rock compositions for the Belaya and Maya River areas. The differences in
the composition of samples along the Belaya and Maya Rivers that may reflect the degree of weath-
ering, proximity to the source, or sedimentary sorting processes. For instance, the CIA chemical
index of alteration is thus significantly lower in the Belaya shales than the Maya shales, suggest-
ing much less weathering of the material in the Belaya shales than the material in the Maya
shales.
5
.
2
. Source rock composition — major elements Also shales along the Maya River suggest that
they were formed as platform sediments in a deeper shelf facies epicratonic and restricted
marine basin, whereas, those along the Belaya River probably formed in more open and active
environments of an upper shelf carbonate ramp that could have occurred closer to the source
Semikhatov and Serebrykov, 1983.
The present major element composition of mu- drocks or shales reflect changes through time,
including the changes due to diagenesis and meta- morphism Cox et al., 1995. The present chemical
composition can be used to suggest the original detrital mineralogy of the shales Cox et al.,
1995.
Fig. 7. The ratios of the log of the elemental concentration to Al
2
O
3
ratios in the Belaya relative to the Maya shales are compared.
Fig. 8. Samples of the Lakhanda shale are plotted in an A – CN – K diagram. Samples have no tendency to project back
to source rock compositions either parallel to the A – CN line implying weathering changes; the lighter lines or perpendicu-
lar to the heavier A – K line implying K-metasomatism; the heavier line. G, granite; Gn, granodiorite; T, tonalite.
average ICV = 0.62 range = 0.32 – 1.24, suggest- ing that most shales were compositionally mature
and were likely dominated by recycling. The few shales with ICVs greater than 1, however, suggest
that there may be periodic input of first cycle sediment in both sample sets.
Also K
2
OAl
2
O
3
ratios may suggest how much alkali feldspar vs. plagioclase and clay minerals
may have been present in the original shales Cox et al., 1995. In order from high to low values, the
K
2
OAl
2
O
3
ratios of minerals are alkali feldspars 0.4 – 1, illite 0.3, other clay minerals
nearly 0 Cox et al., 1995. Shales with ratios of K
2
OAl
2
O
3
greater than 0.5 suggest a significant quantity of alkali feldspar relative to other minerals
in the original shale; those with ratios of K
2
O Al
2
O
3
less than 0.4 suggests minimal alkali feldspar in the original shale Cox et al., 1995. The Belaya
shales of the Lakhanda Formation have an average ratio of K
2
OAl
2
O
3
= 0.22 range = 0.11 – 0.29 and
the Maya shales have an average ratio of K
2
O A
2
O
3
of 0.15 range = 0.10 – 0.23, suggesting min- imal alkali feldspar relative to other minerals in the
original shale. Another approach to the composition of the
original source rock is to plot molar ratios of Al
2
O
3
– CaO + Na
2
O – K
2
O in A – CN – K diagrams in order to potentially separate compositional
changes of shales and coexisting sandstones related to chemical weathering, transportation, diagenesis-
metamorphism, and source composition Fedo et al., 1995, 1997a,b. The CaO included in carbonate
or apatite is not included in the chemical plots. The A – CN – K diagams are useful in that the average
source rock composition and metasomatic effects can be inferred especially if a wide range of
compositions of shales and sandstones are available to be plotted. In such a system, plots of shales and
sandstones due to weathering trends plot parallel to the A – CN boundary, and they extract back to
a plagioclase – alkali feldspar horizontal line of the source composition Fig. 8 unless metasomatism
affects the rocks. The K-metasomatism of kaolinite weathered rocks can produce illitic rocks with
points plotted at right angles to the A – K side of the diagram Fig. 8. The Lakhanda Formation,
however, contains only shales and no sandstones so that plots in the A – CN – K diagrams produces no
One approach is to use the Index of Composi- tional Variability ICV = Fe
2
O
3
+ K
2
O + Na
2
O + CaO + MgO + TiO
2
Al
2
O
3
and the
ratio of
K
2
OAl
2
O
3
Cox et al., 1995. Non-clay minerals have a higher ratio of the major cations to Al
2
O
3
than clay minerals so the non-clay minerals have a higher ICV. For example, the ICV decreases in the
order of pyroxene and amphibole 10 – 100, biotite 8, alkali feldspar 0.8 – 1, plagioclase
0.6, muscovite and illite 0.3, montmoril- lionite 0.15 – 0.3, and kaolinite 0.03 – 0.05
Cox et al., 1995. Thus, immature shales with a high percent of non-clay silicate minerals will
contain ICV values of greater than one. These shales are often found in tectonically active settings
in first cycle deposits Van de Kamp and Leake, 1985. In contrast, more mature mudrocks with
mostly clay minerals ought to have lower ICV values of less than one Cox et al., 1995. Such
shales ought to form in cratons of quiesecent environments Weaver, 1989. They have also been
found, however, in some first cycle material that was intensely weathered Barshad, 1966. The
Lakhanda shales at the Belaya River contain an average ICV = 0.81 range = 0.46 – 1.08 and those
from the Maya River platform shales contain an
Table 3 The range of elemental ratios of shales in this study are compared with those of fine-fractions derived from silicic and basic source
rocks Belaya River
Maya River PAAS
b
Range of fine-fractions from Range of fine-fractions from sililic
sililic sources
a
sources
a
— platform — near source
0.36–0.67 0.32–0.83
0.70–1.02 0.66
EuEu 0.35–0.66
1.92–2.75 0.70–27.7
1.75–3.39 0.40–1.1
LaSc 2.4
0.79–1.54 ThSc
0.73–1.02 0.64–18.1
0.05–0.4 0.91
1.11–11.3 LaCo
1.37–12.0 1.4–22.4
– 1.65
0.52–5.68 0.30–7.5
0.44–4.77 –
ThCo 0.63
0.17–0.27 ThCr
0.067–4.0 0.17–0.25
0.002–0.045 0.13
a
From a summary in Cullers 2000.
b
From Taylor and McLennan 1985.
clear trend back to the source composition or indication of metasomatism. Most shales of the
Lakhanda plot parallel and along the A – K line suggestive of intense chemical weathering high
CIA. If K-metasomatism produced these rocks, then they could have formed from tonalites to
basalts. This interpretation is not consistent with other trace element characteristics discussed in the
next section e.g. EuEu, ThSc, and REE pat- terns. If weathering produced these rocks then
they could have been produced from varied amounts of mostly granodiorite to granite. The
low K
2
OAl
2
O
3
ratios of these shales suggests that the amount of granite in the source may have been
minimal unless the original K
2
O was removed from the system by other processes. This interpre-
tation is more consistent with the trace element composition of these rocks as discussed below.
5
.
3
. Source rock composition — trace elements Elemental ratios critical of provenance LaSc,
LaCr, LaCo, ThSc, ThCr, ThCo, and EuEu are not significantly different between the Maya
and Belaya River samples. Moreover, these ratios and the size of the negative Eu-anomaly size are
fairly similar to platform sediment or fine-grained Holocene sediment that has been interpreted to
have been derived from silicic source rocks such as granodiorite to granite rather than basic rocks
Table 3. The higher La or Th relative to Cr, Co, or Ni ratios and the more negative Eu-anomaly of
most of the Lakhanda shales relative to sediment averages like the PAAS, however, suggests that the
Lakhanda shales may on the average be derived from somewhat more differentiated granitoids
than those that make up the PAAS.
In addition, the low K
2
OAl
2
O
3
ratios, the A – CN – K plots along with the moderately negative
Eu anomalies, ThSc ratios, and La – Th – Sc plots Fig. 9 of the Lakhanda shales are most consis-
tent with mostly a granodiorite rather than a granite source. For example, a Holocene source in
the Wet Mountains, USA, is composed of mostly granodiorite with minor granite, tonalite, and ba-
sic rocks B 15. Fine-grained stream sediment
Fig. 9. Lakhanda shales plot in a fairly narrow region in a La – Th – Sc plot in the granodiorite field rather than basalt or
granite.
draining this area is producing the same range of negative Eu-anomaly size and ThSc ratios as the
Lakhanda shales thus supporting a dominately granodiorite source Cullers et al., 1987; Cullers,
1994a. The ThSc ratios and the negative Eu- anomaly size are also in the range of values for
old upper continental crust although the Eu- anomaly size is somewhat more negative than is
usually observed McLennan et al., 1993.
Samples 52 – 103 and 52 – 119 in the lower por- tion of the Belaya River samples contain lower
EuEu values of 0.35 than the other samples so they could have been derived from a source with
more granite with large negative Eu anomalies. The only presently observed differentiated gran-
ites are those found in the Proterozoic Ulkan complex of the eastern Aldan Shield e.g. Eu
Eu = 0.11 – 0.15 in addition to the more com- mon less differentiated granitoids and older
recycled mudrocks derived from them. No UPb detrital zircons have been analyzed but sandstones
throughout the Riphean have UPb ages of zir- cons which are almost entirely Proterozoic in age
Khudoley et al., 2000.
6. Summary
