cycle detritus from highly differentiated granitoids similar to those from the Aldan Shield. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Proterozoic; Shales; Trace elements; Rare earth elements; Provenance
1. Introduction
The mineralogical and chemical composition of fine-grained sedimentary rocks are commonly
used as a sensitive indicator of provenance and weathering conditions and only in a few cases as a
tool of tectonic setting Ronov and Migdisov, 1971; Cullers et al., 1975, 1979; Taylor and
McLennan, 1985, 1991; Bhatia and Crook, 1986; Roser and Korsch, 1986, 1988; Ronov et al.,
1990; Cullers, 1994b; Cox and Lowe, 1995; Cox et al., 1995; Nesbitt et al., 1996; Cullers and Berend-
sen, 1998; Cullers, 2000. On a global scale, mu- drock chemistry reflects the average composition
of continental crust Taylor and McLennan, 1985. Most mudrocks, however, form in re-
stricted basin environments in specific tectonic settings that reflect the composition of the source
rocks Cox and Lowe, 1995. Elements concen- trated in basic rocks e.g. Sc, Cr, Co and ele-
ments concentrated in silicic rocks La, Th, REE, REE patterns, and Eu-anomaly size have been
used for provenance and tectonic determinations of mudrocks Cullers, 1994b; Mongelli et al.,
1996. Of course these signatures of the source rock may be modified by weathering, hydraulic
sorting and diagenesis Cullers et al., 1987; Condie et al., 1995; Nesbitt et al., 1996.
Although few studies deal with effects of basi- nal tectonic settings controlling the chemical com-
position of mudrocks, it is generally assumed that in more stable and evolved intracratonic settings
mudrocks are more homogenized and represent the average composition of continental crust in
the region Ronov et al., 1974; Bhatia, 1985; Taylor and McLennan, 1985; Cox and Lowe,
1995. Recent studies of the influence of grain-size and transportation distance in a given tectonic
environment on the chemical composition of sedi- ments show that some major element and trace
element concentrations and ratios, including REE patterns and negative Eu-anomaly size, are similar
to the source rock in mudrocks compared with the more variable chemical composition of sand-
stones in the same sedimentary sequences Cullers et al., 1975; Cullers, 1988, 1994a,b; Mongelli et
al., 1996. Thus, mudrock compositions provide more information not only about weathering con-
ditions and sediment recycling, but also regional tectonic settings compared with the more variable
composition of associated sandstones Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch,
1988; Sochava et al., 1994; Cullers, 1995.
The specific aims of this paper are as follows: 1 to examine secular variations in mudrock com-
position of
a single
sedimentary unit
the Lakhanda Group, 1.05 – 1.01 Ga on the edge of a
mature craton and 2 to examine the effect of the input of the composition of the sediment as a
result of the changing tectonic evolution.
2. Geology
2
.
1
. Possible source rocks The study area is located in the Uchur – Maya
Region on the southeastern edge of the Siberian platform Figs. 1 and 2 in which platform sedi-
mentation occurred during the Riphean – Vendian Meso-
to Neoproterozoic,
1.60 – 0.54 Ga
Semikhatov and Serebrykov, 1983; Semikhatov, 1991. The underlying basement complex in the
Aldan Shield include strongly metamorphosed and deformed Archean to Early Proterozoic mag-
matic and supracrustal volcaniclastic units of the Uchur block to the west and the Batomga block
to
the southeast.
Supracrustal shales
and metagraywackes predominate in the Uchur block
Neelov et al., 1971; Dook et al., 1986. The Uchur block probably formed during accretion of
several arc-related terrains onto the eastern edge of the Archean Siberian craton Glebovitsky and
Drugova, 1993; Kotov et al., 1995; Kovach et al., 1996.
The Batomga granite – greenstone block consists of granulite gneisses associated with a variety
metamorphosed ultrabasic to silicic igneous rocks, and younger bimodal metavolcanics, shales, calc-
silicate rocks with gabbro and granite intrusions Larin, 1997. The latest pre-Riphean magmatic
event in the Batomga block is the Ulkan volcano- plutonic anorogenic A-type rapakivi complex
1721 – 1703 Ma Larin, 1997.
The proportion of rock types in the eastern Aldan Shield varies across structural domains, but
granitic rocks low- and moderate-K, with smooth REE-spectra with a small, negative Eu-anomaly
predominate with smaller amounts of mafic and metasedimentary rocks. The Proterozoic sedi-
ments include both immature, arc-related wackes lower REE content than the granites with small
or no Eu anomalies and more evolved cratonic- type mudrocks moderately high REE content
and a negative Eu-anomaly close to the average of most Phanerozoic shales Kovach et al., 1999.
Some rare trachyrhyolites and subalkaline gran- ites contain high REE content, marked LREE
enrichment
La
N
= 327 – 539,
[LaYb]
N
= 10.1 –
11.7 and a large negative Eu-anomaly EuEu = 0.11 – 0.15 Larin, 1997.
2
.
2
. Sedimentation history The Riphean Mezo- to Neoproterozoic and
Vendian 1.65 – 0.54 Ga sediments of the Uchur – Maya region unconformably cover the Archean –
Proterozoic basement of the Aldan Shield. These sediments form the two main tectonic provinces.
One provenance is the mature continental block of the Uchur – Maya plate, and the other prove-
nance is the marginal trough of the Yudoma – Maya region Saleeby, 1981; Semikhatov and
Serebrykov, 1983; Semikhatov, 1991; Khudoley et al., 2000.
The Riphean of the Uchur – Maya basin con- tains sediment sampled in this study; it is a trans-
gressive sedimentary series. The total thickness of these strata in the Uchur – Maya basin is 4 – 4.5
km, and it increases in thickness to 11 – 12 km to the
east in
the Yudoma – Maya
trough Semikhatov and Serebrykov, 1983; Semikhatov,
1991; Khudoley et al., 2000.
Fig. 1. Index map of the area in which samples were collected along the southeastern edge of the Siberian platform heavy
outlined area.
Fig. 2. Location of the sections from which samples were collected along the Maya River platform sediment; sections
77, 78, and Lh and the Belaya River near source; sections 51 and 52.
Fig. 3. Stratigraphic section along the Maya River with the main lithologies identified.
record for the Lakhanda carbonate sequences dis- play a marked shift to heavy d
13
C
carb
values from +
4 to + 6 in PDB scale Podkovyrov and Vino- graadov, 1996 that is similar to that of other
similar Mesoproterozoic successions of that age. They also contain low
87
Sr
86
Sr ratios 0.70519 – 0.70566 that probably reflect platform marine
sedimentation with an input of ocean water en- riched in a juvenile Sr isotopic component
Semikhatov et al., 1998.
2
.
3
. Lakhanda Group The Lakhanda Group was sampled in this
study. The Lakhanda Group carbonates are in- terbedded with multi-colored shales and minor
siltstones – sandstones that were deposited in epi- cratonic shallow marine and upper shelf environ-
ments.
They form
a gently
north- and
northeast-dipping rock sequence 400 – 550 m thick in central to eastern parts of the Maya
basin, and they thicken Fig. 3 eastwardly 1000 – 1200 m in thrust-faulted and folded successions
in the Yudoma – Maya trough Sklyrov, 1981; Semikhatov and Serebrykov, 1983; Khudoley et
al., 2000. In the Maya sections, an erosional unconformity with remnants of an ancient iron-
rich, kaolinite weathered crust precedes the Maya platform sequence. Samples of this sequence were
sampled along the Maya River Figs. 2 and 3. Here the sequence is subdivided into two forma-
tions: the Neruen Formation in the lower portion and Ignikan Formation in the upper portion
Semikhatov and Serebrykov, 1983. In the Neruen Formation 60 – 65 terrigenous rocks,
30 – 35 carbonaceous, fine-grained terrigenous sediments are abundant in the lowermost unit
45 – 50 m and upper unit 110 – 125 m. These two units are separated by interbedded multi-col-
ored stromatolitic and clastic limestones and dolomitic limestones 70 – 80 m.
The lower unit of the Neruen Formation Fig. 3, section 77 includes iron-rich, kaolinite-bearing
calcareous shales with rare siltstones, dolomitic shales and siderite beds or concretions at the base.
The overlying sequence within the lower unit Fig. 3, section 77 consists of varigated shales with
minor stromatolitic dolomites, quartz sandstones, The upper sequences of the Riphean include the
Lakhanda and the Ui Groups sediments that have traditionally been attributed to Upper Riphean
960 – 750 Ma Semikhatov and Serebrykov, 1983. However, more recent UPb ages of mafic
sills cutting the Lakhanda and Ui sediments give ages of 1010 – 965 Ma so that their age must be
older than 1.0 Ga Rainberd et al., 1998. Thus, these units must really be of Mezoproterozoic
Middle Riphean age. The observed isotopic
siltstones and siderite concretions. These beds of the lower part of the Neruen Formation were
deposited in a transgressive, high energy marine sequence, most likely tidal flat to distal sublitoral,
near the provenance Semikhatov and Sere- brykov, 1983. These beds were enriched in heavy
minerals, especially zircon.
The upper terrigenous unit of the Neruen For- mation in the central Maya sections Fig. 3, sec-
tion 78 contains varigated shales with minor laminae of siltstones, rare sandstones, siderite
lenses, and stromatolitic limestones Semikhatov and Serebrykov, 1983. The mudstones are inter-
preted to have formed in low-energy, partly anoxic, outer shoal environments Semikhatov
and Serebrykov, 1983. The source rock of clay material was probably, as with the lower unit, the
adjacent Aldan Shield.
The Belaya River section is the one that was studied in the Uchur – Maya trough. It is located
in the north-east part of the region, along the Belaya River on the eastern flank of the Gornos-
takh Anticline Figs. 1 and 4. The section sec- tion
52 is
composed of
mostly platform
carbonate rocks up to 82 in the Neruen and 95 – 97 in the Ignikan Formations, and minor
shales and siltstones, and sandstones Semikhatov and Serebrykov, 1983; Podkovyrov and Vino-
graadov, 1996; Semikhatov et al., 1998. The se- quence is divided into seven transgressive –
regressive carbonate – shale units. The limestones and dolomites were formed in a marine shoal in
subtidal and tidal environments. Black thinly lam- inated shales in lower units represent predomi-
nantly low-energy subtidal, distal ramp and open marine
environments, whereas,
multi-colored shales with thin horizontal and low-angle cross-
bedding, small-scale flazer lamination and small symmetrical ripple marks in upper units were
deposited in low-energy tidal, lagoonal and, prob- ably, supratidal settings.
3. Sampling and methods
