geochemistry of metasediments from the precambrian hapschan series, eastern anabar shield, siberia

Precambrian Research, 50 ( 1991 ) 37-47 37 Elsevier Science Publishers B.V., Amsterdam

Geochemistry of metasediments from the Precambrian Hapschan Series, eastern Anabar Shield, Siberia

K.C. Condie a, M. Wilks a, D.M. Rosen b and V.L. Zlobin b aDepartment of Geoscience, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA

blnstitute of the Lithosphere, Academy of Sciences of the USSR, Staromonetny per. 22, Moscow 109180, USSR

(Received March 21, 1990; accepted after revision September 11, 1990)

ABSTRACT

Condie, K.C., Wilks, M., Rosen, D.M. and Zlobin, V.L., 1991. Geochemistry of metasediments from the Precambrian Hapschan Series, eastern Anabar Shield, Siberia. Precambrian Res., 50: 37-47.

The Precambrian Hapschan Series in the Anabar Shield in northern Siberia is composed of garnet paragneisses (pelitic graywackes), marbles, calciphyres, and calc-silicate rocks (carbonates and carbonate-rich clastic sediments), and a minor amount of enderbite and mafic granulite. Trace element distributions in the paragneisses (especially REE patterns and low Cr and Ni) suggest felsic sources for the graywacke protoliths. With exception of Mn, Pb, and Sr trace elements in Hapschan carbonates and calc-silicates are housed chiefly in silicate contaminant phases. When normalized to A1203 to partially compensate for trace elements contained in Al-silicates, most trace elements are depleted in Hapschan carbonates relative to average Phanerozoic marine limestone (PML). Rb, Cr, Ni and Ti are strikingly depleted in Hapschan as well as other Precambrian high-grade carbonates relative to PML. The element depletions in Hapschan marbles may be ex- plained by some combination of loss during granulite-facies metamorphism or by removal in another depository such as BIF.

Geologic and geochemical data are consistent with deposition of the Hapschan Series in a cratonic or passive continental margin setting, during the Early Proterozoic. Basement to this series may be the high-grade rocks of the Daldyn and Upper Anabar Series from which a zircon U-Pb ion probe age of 3.3 Ga has been reported.

Introduction

The chemical composition of sediments provides an important clue to the provenance and tectonic setting in which they were deposited. Sediments are particularly important in characterizing and constraining tectonic settings and crustal composition during the Pre- cambrian (Wronkiewicz and Condie, 1989; Boak and Dymek, 1991; Condie and Wron- kiewicz, 1990). The Precambrian Hapschan Series in the Anabar Shield in northern Siberia is a widespread supracrustal succession. Al- though highly deformed and metamorphosed to the granulite facies, sedimentary units re- main intact in this series and provide a provenance window to the composition of the Early

Proterozoic crust of the Anabar Shield. In this study, we present new geochemical data from the Hapschan Series and discuss it in terms of metamorphism, provenance, and tectonic setting.

Geologic setting and age

The Hapschan Series is one of three metamorphosed and deformed series of Precam- brian rocks recognized in the Anabar Shield (Vishnevsky and Turchenko, 1986; Rosen, 1986, 1989) (Fig. 1, inset). Whereas the Hap- schan Series includes chiefly paragneisses, calc- silicates and marbles, the Daldyn and Upper Anabar Series, which may be older than the Hapschan Series, are composed chiefly of en-

0301-9268/91/$03.50 © 1991 - - Elsevier Science Publishers B.V.

3 8 K.C. CONDIE ET AL.

Fig. 1. Geologic map along part of the Nalim Rassokha River in northern Siberia. Inset shows location of the map area in the Anabar Shield.

derbites and mafic granulites. The most widespread rocks in the Daldyn and Upper Anabar Series are enderbites composed of antiperthi- tic plagioclase, hypersthene, quartz, magnetite, and in some cases, diopside and K-feldspar. Maflc granulites, composed of two pyroxenes, plagioclase and occasionally hornblende and/or garnet comprise deformed units up to several hundred meters thick within the enderbites. Ultramafic boudins composed of olivine, two pyroxenes and hornblende also occur locally within the granulite complex. Isolated layers of quartzite, metapelite, and BIF within the enderbites may represent xenoliths or infolded supracrustal rocks. Protoliths of the enderbites are problematic. Although they have been interpreted by Rosen ( 1986, 1988 ) as felsic metavolcanics, these rocks bear a striking

resemblance to Archean granulites on other shields that have been interpreted as TTG (tonalite-trondhjemite-granodiorite) (Rol- linson and Windley, 1980; Janardhan et al., 1982; Condie and Allen, 1984; Jahn and Zhang, 1984; Sheraton et al., 1984).

Geothermometry and geobarometry studies suggest that the highest temperatures and pressures of granulite grade metamorphism were 850-950°C and 10-11 kbar, respectively, as established from garnet-diopside assemblages in mafic granulites and hypersthene-sillimanite, hypersthene-cordierite, and sapphirine- sillimanite assemblages in pelitic gneisses (Luts, 1974). Studies of high-density CO2 fluid inclusions reflect similar maximum P - T con- ditions (860-890°C; 9-11 kbar; Rosen and Sonyushkin, 1987). More typical of the Ana-

METASEDIMENTS FROM THE PRECAMBRIAN HAPSCHAN SERIES, SIBERIA 39

bar Shield as a whole, however, are temperatures of 780-850 °C and pressures of 7-8.5 kbar (Vishnevsky, 1978 ).

Conventional U - P b isotopic studies of pris- matic zircons from enderbites of the Daldyn Series yield an age of 3.26 Ga, which is gener- sio2

Ti02 ally interpreted as the age of the igneous pro- A1203

tolith (Bibikova et al., 1988). Ion probe Fe203(T) (SHRIMP) U - P b ages of single zircons from MgO

CaO this population are 3.32 + 0.10 Ga (Bibikova Na20 et al., 1988), which agrees well with the con- K20 cordia age. A second population of rounded MnO

P205 zircons exhibits two concordia intercept ages. LOl

The oldest at 2.76 _+ 0.02 Ga, may represent the Total

age of granulite-facies metamorphism, and the second at 1.97 __ 0.02 Ga, a later metamorphic Rb

Ba or metasomatic event. A Nd model age of ~ 2.4 Cs Ga from the Hapschan Series (Rosen, 1988) Sr suggests that this series is Early Proterozoic in Pb Th age. u

Samples for this investigation were collected Sc

from the Hapschan Series in a relatively small v area in the northern part of the Anabar Shield Cr along the Nalim Rassokha River (Fig. 1 ). In Ni this area, the Hapschan Series includes garnet y paragneisses with interlayered marbles, calci- Zr phyres and calc-silicate rocks. The latter three Nb

Ta rock types occur as dismembered beds that range from a few centimeters to 200 m thick.

Analytical methods

A total of 24 samples of metasediments from the Hapschan Series were selected for chemical analysis in this study, and a representative group of these analyses is given in Tables 1 and 2. All chemical analyses were done in the geochemical laboratories at New Mexico Tech.

Major and some trace elements (Rb, Sr, Ba, Nb, Y, Zr, Ni, V, Pb) were determined by X-ray fluorescence using an automated Rigaku 3064 XRF spectrometer following methods similar to those described by Norrish and Hut- ton ( 1969 ) and Norrish and Chappell ( 1977 ). Calibration curves are made using interna- tional rock standards. Other trace elements

TABLE 1

Representative chemical analyses of garnet paragneisses from the Hapschan Series (samples 11-17 ), Anabar Shield.

11 12 13 14 17

59.03 60.02 64.42 66.53 69.38 0.52 0.69 0.74 0.66 0.51

21.3 15.0 15.7 15.1 16.0 4.46 9.61 7.44 6.70 3.85 2.55 5.91 3.10 3.07 1.20 4.99 3.45 2.30 1.97 3.73 4.73 2.65 2.72 2.46 3.40 1.72 1.57 2.90 2.31 1.37 0.06 0.11 0.09 0.09 0.11 0.05 0.08 0.19 0.10 0.09 0.57 0.24 0.26 0.36 0.36

99.98 99.34 99.82 99.36 100.01

11 36 73 40 30 220 310 380 420 220 <0.5 <0.5 0.3 0.2 0.3 604 213 336 230 230

15 10 15 10 22 1.8 3.1 7.8 2.0 1.6

<1 <1 1.2 1.3 <1

5.1 13 18 12 25 100 103 130 120 62 140 100 130 109 152 38 13 23 16 10

3.6 16 36 24 20 19 118 214 202 NA

1.9 6.5 15 12 NA 0.04 0.6 0.8 0.7 1.1

La 53 15 43 38 21 Ce 97 32 87 79 31 Sm 3.3 2.1 8.0 5.3 2.1 Eu 2.8 0.68 1.6 1.2 1.1 Tb 0.15 0.43 1.1 0.70 0.50 Yb 0.20 1.7 3.8 3.0 2.3 Lu ~0.03 0.26 0.60 0.45 0.40

CIA 53 55 57 60 54 La/Y 15 0.94 1.2 1.6 1.1 Sc/Cr 0.03 0.23 0.13 0.08 0.15 Ti/Zr 163 35 21 20 NA La/Sc 10 1.1 - 2.4 3.2 0.86 Cr/Th 77 32 17 75 95

Fe203 (T) = total Fe as Fe203. Trace elements in ppm. Major oxides in wt%. CIA (Chemical index of alteration)=[AlzO3/(A1203 + C a O + N a 2 0 + K 2 0 ) ] × 100 using molecular proportions; CaO is CaO in silicate minerals, excluding carbonates and apatite). NA = not analyzed for.

40 K.C. CONDIE ET AL.

TABLE2

Representative chemical analyses ofcalc-silicate rocks, calciphyres and marbles from the Hapschan Series, Anabar Shield. (Num- bers refer to sample nos. )

Calc-silicate rocks Calciphyres Marbles

18 19 20 21 22 23 24 25 26 27 28 29

SiO2 46.76 49.75 49.96 51.47 54.29 20.76 17.03 28.96 0.64 1.44 1.74 6.52 TiO2 0.43 0.66 0.33 0.98 0.44 0.54 0.10 0.41 0.10 0.10 0.04 0.12 A1203 12.1 14.2 11.4 15.3 9.67 7.04 6.19 7.81 5.79 2.13 1.01 1.37 Fe30~(T) 2.35 5.91 2.26 7.03 3.70 3.19 1.85 3.10 0.65 0.67 0.02 0.99 MgO 0.61 3.02 0.73 2.87 1.78 0.99 2.13 0.83 2.16 0.34 18.6 3.43 CaO 32.5 19,1 29.5 16.7 23.1 38.3 40.7 35.6 47.7 54.7 34.4 46.9 Na20 1.29 2,40 1.40 1.80 0.09 1.40 0.62 1.18 0.24 0.32 <0.1 0.26 K20 0.38 1.45 0.55 0.82 1.80 1.01 0.78 1.53 0.35 0.41 0.01 0.51 MnO 0.04 0.08 0.07 0.11 0.10 0.04 0.01 0.01 0.01 0.01 0.02 0.08 P2Os 0.03 0.04 0.04 0.10 0.10 0.02 0.01 0.03 0.01 0.01 0.04 0.02 LOl 2.92 2.78 3.76 2.71 3.40 26.2 29.8 20.0 41.5 39.8 44.0 39.53

Total 99.43 99.42 100 .01 99.84 99.45 99.48 99.22 99.46 99.18 99.90 99.90 99.75

Rb 15 12 16 NA 29 20 20 45 2.4 6.0 5.1 6.8 Ba 80 560 240 520 310 360 280 230 210 220 240 180 Cs 0.5 0.4 0.3 0.2 1.0 1.3 0.8 0.6 2.4 0.5 NA 0.5 Sr 698 550 1156 711 849 693 903 1840 792 2850 138 1340 Pb 7.3 6.1 4.3 2.0 9.0 8.2 8.5 3.0 5.5 1.2 7.0 5.0 Th 0.40 0.50 1.0 2.0 7.5 2.8 4.1 0.5 0.6 0.4 <0.1 1.0 U 0.3 0.4 0.7 0.9 3.7 2.4 0.8 0.3 0.4 0.4 0.09 0.06

Sc 4.1 14 3.2 12 6.4 7.7 2.8 6.6 0.4 1.2 0.20 1.1 V 34 41 17 100 63 63 32 53 5.0 10 3.0 11 Cr 13 74 45 128 21 48 80 58 1.6 5.0 1.3 7.3 Ni 8.0 26 220 80 13 16 19 23 5.0 5.0 5.0 11

Y 12 9.1 22 14 22 8.2 12 12 2.4 5.4 1.5 2.0 Zr 41 67 51 70 136 73 44 53 16 13 I I 17 Nb 8.6 13 5.5 19 9.7 7.4 7.3 7.5 5.9 4.9 3.4 5.8 Hf 1.8 2.4 1.3 2.6 5.4 1.6 5.5 1.8 0.15 0.30 NA 0.60 Ta 0.8 I.l 0.1 1.1 0.5 0.2 0.3 0.02 0.02 0.1 NA 0.2

La 1.5 2.5 11 25 38 13 49 22 3.3 21 0.69 5.6 Ce 4.4 7.2 25 56 71 25 85 44 4.2 36 1.14 11.4 Sm 1.9 1.8 2.9 4.0 4.9 2.2 4.7 3.5 0.30 1.8 0.09 0.70 Eu 0.30 0.30 0.60 0.60 1.1 0.50 1.6 0.6 0.1 0.3 0.02 0.20 Tb 0.30 0.21 0.52 0.43 0.63 0.29 0.54 0.40 0.04 0.20 0.01 0.10 Yb 1.4 0.90 2.4 1.3 2.1 0.70 0.70 1.04 0.07 0.50 0.03 0.20 Lu 0.19 0.22 0.36 0.20 0.32 0.10 0.10 0.16 0.01 0.07 <0.01 0.03

FezO3(T) =total Fe as Fe203. Trace elements in ppm. Major oxides in wt%. NA = not analyzed for.

M E T A S E D I M E N T S F R O M T H E P R E C A M B R I A N H A P S C H A N S E R I E S , S I B E R I A 41

(Th, U, Sc, Cr, Co. Hf, Ta, 7 REE) are determined by instrumental neutron activation, using a Nuclear Data 6600 y-ray spectrometer in conjunction with two Ge detectors. Analytical methods are similar to those proposed by Ja- cobs et al. ( 1977 ) and Lindstrom and Korotev (1982). Calibration is based on NBS Fly Ash standard 1633a, and precision and accuracy are better than 5% for most major and trace elements, and better than 10% for all elements.

Petrographic features

In the Nalim Rassokha River area (Fig. 1 ), the most widespread rocks are pink to gray, medium- to coarse-grained garnet paragneisses composed chiefly of quartz, plagioclase, Mg-Fe-rich garnet, and biotite, and in some instances, hypersthene, cordierite, or sillimanite are present. Of the rocks analyzed, garnet and plagioclase occur with hypersthene in samples 11 and 12 and they are associated with biotite in sample 17. In samples 13 and 16, garnet is the only marie phase present. Pla- gioclase (Anso_7o) contains 30-50% of anti- perthitic K-feldspar. Only sample 13 contains individual K-feldspar grains, which comprise 10% of the rock. All of the paragneisses contain accessory magnetite, zircon, and apatite, and in addition, sample 17 contains graphite.

Carbonate-bearing supracrustal rocks in the Hapschan Series of the study area are white to light-green and medium-grained. For discussion purposes, they are divided into diopside marbles ( > 85% carbonate), calciphyres ( 15- 85% carbonate), and calc-silicate rocks ( < 15% carbonate). Most marbles are composed of calcite with small amounts of diopside, scapol- ire, or forsterite with phlogopite. In sample 28, dolomite also occurs with the calcite. Calci- phyres contain diopside, plagioclase (Anso_7o), scapolite with 0.9-1.7% sulfate (Zlobin, 1988 ), calcite, and in some cases, K-feldspar. Calc-silicate rocks include the same mineral assem- blage, with wollastonite partially replacing scapolite in some rocks. Sphene is a typical

accessory mineral, and sample 28 also contains accessory spinel.

Geochemical results

Garnet paragneisses

As shown in Table 1, the garnet paragneisses are variable in composition, both in terms of major and trace elements. On the whole, they are felsic with SiO2 ranging from 59-69% and A1202 from 15-21%. Major element contents favor graywacke protoliths for the paragneisses as evidenced by the distribution of data on the log(SiO2/A1203)-log( [ CaO + Na20 ] / K20 ) graph of Garrels and McKenzie ( 1971 ), on the Fe203T + TiO2 + CaO-SiO:-AI203 and MgO-K20-Na20 plots of De la Roche ( 1966 ), and on the normative mineral diagram of Ro- sen and Dimroth (1982). The relatively high SIO2/A1203 ratios (3-5) yet low K20/Na20 ratios ( _< 1 ) (Fig. 2) also favor a graywacke affinity, although it is possible that the latter ratio may have decreased due to K-loss during granulite-facies metamorphism. Low CIA values (53-60) (Table 1 ), however, do not favor significant K loss, as such loss would raise CIA indices. Compared to most graywackes, the Hapschan paragneisses are relatively high in AI203 ( 1 5 - 2 0 % compared to 3-15% in graywackes), which is reflected also in the some- what low 8iO2/A1203 ratios (3-5 compared to 4-6 in graywackes). This observation suggests

IO , i ' ' ' ' 1 i I i i ,

ca

i s ) •

i i i ] i i i L I i i I I i i

O l 05 1 .0 5 I 0

K20 /Ne20

Fig. 2. K20/Na20-SiO2/AI203 diagram showing the distribution ofparagneisses from the Hapschan Series. Some data are unpublished analyses of O. Rosen. Fields after Wronkiewicz and Condie (1987).


that the Hapschan sediments were fine-grained graywackes (graywacke pelites) in which clay minerals were concentrated.

The variable trace element contents of the paragneisses may reflect provenance differences, mineral fractionation during turbidity- current deposition, or metamorphic effects. Of the elements known to be depleted in rocks during some granulite-facies metamorphism (Rb, Cs, U, Th, Pb and sometimes K), only Cs is usually low and may have been lost by this mechanism. The relatively low Cr and Ni contents of these rocks are different from most Ar- chean graywackes and pelites (Taylor and McLennan, 1985; Wronkiewicz and Condie, 1989). This suggests that felsic granitoids or felsic volcanics predominated in the source areas and that komatiites were minor. The relatively low Cr/Th ratios ( < 75) support this conclusion in that Th losses during metamorphism would increase this ratio. The Ti/Zr, La/Sc, La/Y, and Sc/Cr ratios of the paragneisses (Table 1 ) suggest that their graywacke protoliths may have had affinities to Phanero- zoic graywackes deposited at active or passive continental margins using the criteria of Bha- tia ( 1986 ).

Although REE contents are also variable, four out of the five paragneiss samples have similar REE patterns with light REE enrichment and relatively unfractionated heavy REE (Fig. 3 ). All but one of these four have negative Eu anomalies and in this respect they are like REE patterns of pelites from cratonic successions (Taylor and McLennan, 1985; Wronkiewicz and Condie, 1989 ). Only sample 11 deviates significantly from the other samples exhibiting a large positive Eu anomaly and extreme heavy REE depletion. This REE pattern is reminiscent of those characteristic of Archean TTG and may have been inherited from a TTG source. Unlike REE distributions in graywackes from many Archean green- stones (Taylor and McLennan, 1985), the Hapschan paragneiss REE patterns are complex and exhibit Eu anomalies. These data

I r r i l r i r l r i x~

~ ' x HAPSCHAN GARNET i o o ~ ~ PARAGNEISSES

> 50 '1~ "\ ~

_ "~. " , , ~ _ ' / t

C3

I

J I

LO Ce

I I I J

Sm Eu

i i i l ' "7- - Tb Yb Lu

Fig. 3. Chondrite-normalized REE distributions in paragneisses from the Hapschan Series, northeastern Anabar Shield. Data given in Table 1.

support the conclusions of Gibbs et al. ( 1986 ), Wronkiewicz and Condie ( 1987, 1989), Boak and Dymek (1991), and Condie and Wron- kiewicz (1990), which suggest that both Ar- chean and post-Archean sediments may have Eu anomalies, and that no "standard" REE pattern characterizes REE distributions of either age.

Carbonates and calc-silicates

In terms of mineral and element concentrations, there is a complete spectrum from marble through calciphyre to calc-silicate rocks (Table 2). Marbles range from limestones to dolostones (molec. Mg/Ca= 0-0.75 ). As with most carbonate rocks, correlations between mineral contents and element concentrations and between major and trace element concentrations suggest that with exception of Ca, Mg, Fe, Mn, Pb, Sr and Ba, most elements are contained in non-carbonate, chiefly aluminum silicate phases (Veizer, 1983; Rock, 1987; Mun- yanyiwa and Hanson, 1988). Linear correlations in the Hapschan carbonates between A1203 (or S i O 2 ) and Y, heavy REE, Sc, Zr, Nb, Zr, Ti and Ni suggest that these trace elements were originally concentrated in clay

M E T A S E D I M E N T S F R O M T H E P R E C A M B R I A N H A P S C H A N S E R I E S , S I B E R I A 43

minerals and now reside in metamorphic AI- silicates. The high Sr/Ca ratios ( > 10 -3) and low Mn contents (< 800 ppm) of all of the marbles, calciphyres, and calc-silicates suggest that the carbonates were not altered during either diagenesis or metamorphism, employ- ing the criteria of Veizer ( 1983 ) and Veizer et al. ( 1989a, b).

The distribution of trace elements in Hap- schan carbonates and calc-silicates normalized to an average Phanerozoic marine limestone (PML) is shown in Fig. 4. Elements are arranged in this graph from left to right in order of increasing value for average Hapschan marble. Relative to PML the elements Rb through Zr are depleted and Ba through Nb are enriched in the marble. Rb, Cr, Mn, Ni, and Ti are especially depleted in the marble. The role of silicates in hosting trace elements is attested to by the increased concentration levels of most trace elements in average Hapschan calciphyre and calc-silicate rock. The negative anomalies at Mn, Pb, and Sr in these rocks probably in- dicate that these three elements are contained chiefly in carbonates. With few exceptions, calciphyre and calc-silicate element patterns are similar, suggesting similar pre-metamorphic mineral distributions in these rocks.

I 0 ^ Colc iphyre / ", t

C0 c - S c o t e / , ~ , , A ~ , _ , I ,.x..,,///x i ~t £" / "b- /" ~ , / 1 L o \ ..... x.,, __~.-'

Rb Cr Mn Ni Ti Y Pb Zr Eu Fe Ce LO Be AI Sr Nb

Fig. 4. Normalized element distributions in average marble, calciphyre, and calc-silicate rock from the Hapschan Series. Normalization values for average Phanerozoic marine limestone compiled by the senior author (K.C.C.) as follows: (in %): MNO=0.084; Fe203(T)=0.54; A1203=1.0; TiO~=0.2; (in ppm): Rb=20, Cr=15; Ni= 15; Y=5, Pb=7; Zr=20; Eu=0.2; Ce= 10; La=5; Ba=85; St=400; Nb= 1.5.

a

ii i 1 i i i i p i r i I I +. iN

50 I - : \ H A P S C H A N MARBLE~

I + \ \ \ z 7 3 \ \

t.J I\~\'X x. x 2 9 \

N .\\ \ /~_÷~ 3 5 = < \. ~+/ ~. AI203 2.1

\ " ~ ~ S iO2 = 1 . 4 4 ) " - . 2 6 "13"x

q "-. ~,o~ AIzO3 = 1.4 o ~ "x-'-x .... --"SiOz = 6"5

1.0 --- 28 ""~x " o - - . ~ --- ~ -- . .A,~% : s.a _- o ~ 0.5 ~ . ~ "-- SiO 2 = "--x. -064 --

" N " ~ ' ~ . ~ A I z 0 3 = 1.0 " '~

0.1 I I I I I i I I I J I I I LO Ce Sm Eu T b Yb Lu

b ' ' ' ' ' ' ' ' ' ' ' ' ' /

too HAPSCHAN CALCIPHYRES

~F" "-. z4.

o z

, i i I I I I ~ I I i b I I I " ,oo ;~c.~

~. HAPSCHAN CALC-SILICATES Z

Z 50 -- "'~ 22

IO

F . . . . . / ~ ,~... . . . . . . . . . . SI 1.1-I,8 % / + . . . . . --=--~-->_~

5

~- L ] L I I i i i I I i I i !

La Ce Sm Eu Tb Yb l u

Fig. 5. Chondrite-normalize REE distributions in (a) marbles, (b) calciphyres, and (c) calc-silicate rocks from the Hapschan Series. Data given in Table 2.

REE distributions in Hapschan marbles are light-REE enriched with similar slopes on a chondite-normalized diagram (Fig. 5). They


may or may not exhibit Eu anomalies. As shown in the figure, the absolute REE contents do not systematically vary with A1203 or SiO2 contents indicating that REE are not entirely hosted in Al-silicates. The strong parallelism of the REE patterns, however, suggests that different proportions of various minor phases (viz., zircon, allanite, sphene) also are not to- tally responsible for the differing concentration levels. Calciphyres have similar REE patterns to the marbles, although with greater REE contents (Fig. 5). The calc-silicate REE patterns are significantlY different from either the marble or caliphyre patterns, showing variable light REE distributions, consistent negative Eu anomalies, and flat heavy-REE distributions. Since the metamorphic mineral are similar in both calciphyres and calc-silicates, it would appear that the differences in REE patterns were inherited from the protoliths. The three light-REE-enriched calc-silicates have similar REE patterns to the paragneisses (Fig. 3) suggesting a similar silicate provenance in the original sediments. The depleted light-REE patterns of samples 18 and 19 may have arisen during granulite-facies metamorphism. The abundance of scapolite in these two samples indicates that C1 may have been important in the fluid phase during metamorphism, and C1- rich fluids are capable of remobilizing light- REE (Taylor et al., t981 ).

Discussion

Carbonate geochemistry

The minimize the effect of varying amounts of silicate contaminants when comparing carbonates of different ages, the element concentrations may be normalized to corresponding A1203 contents. The element/A1203 ratio (RA) of various elements in Archean carbonates normalized to the same ratio in average PML (Re) is shown in Fig. 6. Ratios are arranged increasing from left to right for the average Hapschan marble. All elements with exception

5 r , r r I Lewis ian Dolomi te / ~ . i' % AI203

Lewision Marble"~ 7 "'~ 'I v ', Co c e / ,' ', ", '~ ~ 26

~'~ Marb e ~ /" , " ~ '~ '

,It ~ ~\ ',t' • 1 4

i ~ i~ ~ f J~HapschGn ~ Morble ,/

0 1 ~ - • ' 4 .

i 0 0 5 I i I I I I i i i I I I I R b Cr M n Ni Y i Y P b Z r Eu Fe Ce La Be Sr N b

Fig. 6. Normalized A1203 ratio distributions in average Hapschan marble. RA=(X/AIzO3)A and Rp=(X/ AlzO3)p, where X is the element concentration in % or ppm and AlzO3 is the concentration in %. A refers to Ar- chean and P to average Phanerozoic marine limestone (data from Fig. 4). Ratios arranged from left to right in order of increasing magnitude in average Hapschan marble. Also shown for comparison are average Lewisian dolomite and limestone (from Rock, 1987) and average marble from the Beit Bridge Complex in the Limpopo belt (from Boryta and Condie, 1990). A1203 contents shown along right margin.

of Sr, Ba, and Nb are depleted in the marble relative to PML, and Rb, Cr, Mn, Ni and Ti are especially depleted. At least four explana- tions need to be considered for these depletions: ( 1 ) preferential loss of trace elements during granulite-facies metamorphism; (2) silicate contaminants in the Hapschan carbonates are depleted in trace elements relative to Phanerozoic silicate contaminants; (3) Ar- chean seawater from which the carbonates were deposited was depleted in trace elements relative to Phanerozoic seawater; or (4) some trace elements in Phanerozoic limestones are hosted by minor restite phases.

During granulite-facies metamorphism in the presence ofa CO2-rich fluid phase, Rb, Cs, U, Th, and Pb may be lost (Condie et al., 1982; Condie and Allen, 1984), and the striking depletion of Rb in the Hapschan marble (RA/ Rp - 0.1 ) may reflect such a loss. Although not plotted, low Th and U contents in these marbles compared to similar ratios in Phanerozoic limestones also favor metamorphic losses. Hapschan silicate contaminants may have been


depleted in such incompatible elements as Rb, Zr, Ti, REE, and Y if crustal sources were more mafic or komatiitic than Phanerozoic sources. However, the low Cr and Ni in associated paragneisses (graywackes) do not allow significant amounts ofkomati i te or basalt in sources, thus not favoring this mechanism for trace element depletion in the carbonates. Some trace metals such as Cr, Mn, Pb, Fe, and Ni may have been depleted in Archean seawater, and thus depleted in carbonates deposited from this seawater. Such depletions could arise by deposition of BIF and related chemical sediments that scavenge these elements from seawater. However, there is no evidence for widespread deposition of BIF at 3-2.7 Ga either in Siberia or elsewhere. If some trace elements in Phanerozoic limestone are housed in minor phases (sphene, allanite, ilmenite, zircon, etc.), which were not present in the Hapschan carbonates, they would appear to be depleted in Hapschan carbonates on the A1203- normalized diagram. The chief problem with this possibility is the linear correlations of A1203 and SiO2 to many trace elements suggesting they are housed in silicates.

Also shown for comparison in Fig. 6 are three average Archean carbonates from other high- grade terranes: calcite and dolomite marbles from the Lewisian of Scotland, and a dolomite marble from the Beit Bridge Complex in the Limpopo belt in southern Africa. Normalized element distributions in these carbonates are complex compared to both the Hapschan and Phanerozoic carbonates. Relative to Phanero- zoic limestone, depletions are still evident in Rb, Cr, Ni, and Ti but in some cases Mn, Fe, Ba, and light-REE are enriched. Very striking are the depletions in Ti, Cs, La, Ba, and Sr in the Belt Bridge marbles. Clearly, granulite-facies metamorphism does not seem capable of accounting for such erratic differences in element distribution in marbles from different areas. The enrichment of metals such as Mn, Pb, and Fe in the Lewisian and Beit Bridge marbles relative to the Hapschan marbles may

reflect hydrothermal fluids associated with deposition of the Lewisian and Beit Bridge carbonates. A positive Eu anomaly in Beit Bridge marbles is consistent with this interpretation (Boryta and Condie, 1990).

Constraints on tectonic setting

Mapping in the eastern Anabar Shield shows that the Hapschan Series is composed chiefly of metasediments with a smaller proportion of enderbites and mafic granulites that represent either intrusive or volcanic rocks (Table 3). Garnet paragneisses dominate and geochemical data favor graywacke protoliths for these rocks. Their relatively high AlaO3 contents reflect clay-rich protoliths that were probably distal deposits. Major and trace element constraints as well as high 878r/86Sr ratios in the marbles (0.7046-0.7055; Rosen, 1986) indi- cate, on the whole, evolved sources in which granitoids were probably abundant. If the carbonates are biochemical deposits, they would seem to reflect shallow-water environments in which a large proportion of detrital contaminants entered the basins, perhaps as turbidity currents. In terms of modern tectonic settings, the Hapschan rock association most closely approximates passive continental margin or intra-cratonic basins. The Hapschan succession differs from early Precambrian cratonic

TABLE 3

Estimated proportions of supracrustal rocks in the Hapschan Series from the northeastern Anabar Shield.

Abundance Rock type Probable protolith (%)

40-50 Garnet para- Peliticgraywacke gneiss

20-30 Calc-silicates Carbonate-rich clastic sediments

5-12 Marbles and Limestone and dolostone calciphyres

5-10 Enderbite Tonalites and related granitoids or felsic volcanics

Mafic intrusives or /and volcanics

2-5 Mafic granulite


successions in southern Africa and western Australia by the relatively large amount of carbonate+ calc-silicate in the Hapschan Series. Also, the large volumes of quartzite and shale that characterize the Kaapvaal basin in southern Africa are missing in the Hapschan Series. The tectonic significance of these differences are not as yet clear.

The question of the nature of the basement on which the Hapschan sediments were deposited also has not been resolved. The Daldyn and Upper Anabar Series (Fig. 1 ), which are composed chiefly of metamorphosed felsic igneous rocks, may underlie the Hapschan series (Vishnevsky and Ttirchenko, 1986). As pre- viously mentioned, the oldest isotopic ages from these rocks are U - P b ion probe zircon ages of ~ 3.3 Ga from an enderbite in the Dal- dyn Series (Bibikova et al., 1988). Magnetic anomalies in Siberia can be followed south- ward over great distances from the Anabar shield suggesting that the Anabar craton may have been quite extensive. An inferred mini- mal estimate of the area of the craton from geophysical data (8-105 km 2) is greater than estimated areas of either the Kaapvaal or western Australian Archean cratons.

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