Crust–mantle interaction in the genesis of siliceous high magnesian basalts: evidence from the Early Proterozoic Dongargarh Supergroup, India

Chemical Geology 187 (2002) 21 – 37 www.elsevier.com/locate/chemgeo

Crust–mantle interaction in the genesis of siliceous high magnesian basalts: evidence from the Early Proterozoic Dongargarh Supergroup, India S. Sensarma a, H. Palme b,*, D. Mukhopadhyay c a Department of Geology, St. Anthony’s College, Shillong 793 001, India Institut fu¨r Mineralogie und Geochemie, Universita¨t zu Ko¨ln, Zu¨lpicher Strasse 49b, D-50674 Cologne, Germany c Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Calcutta 700 019, India

b

Accepted 25 January 2002

Abstract Siliceous high magnesian basalts (SHMB) represent a rock type chemically distinct from other common volcanic rocks. The petrogenesis of this magma, reported at or near the Archean – Proteorzoic transition across the world, is controversial. In this paper, we present chemical (XRF, INAA) and mineralogical data on a SHMB suite from the Early Proterozoic (2.1 – 2.5 Ga) Dongargarh Supergroup, Central India, the first of its kind reported from the Indian Precambrian. This suite of basaltic rocks is unusually high in SiO2 (54 wt.%) and enriched in incompatible elements. The SHMB melts discussed here can neither have formed by partial melting of the Earth’s mantle nor by fractional crystallisation of a mantle-derived melt. It is shown here that excess SiO2 and incompatible elements in SHMB are supplied by a crustal component to a basaltic komatiitic parent magma of mantle origin. Major and trace elements abundances and geochemical mass balance calculations suggest that a basaltic komatiite melt assimilated 15 – 20% of acid volcanics, the immediately underlying unit to these rocks in Dongargarh, before erupting as SHMB. Mantle-derived rocks have Ta/Th ratios of around 0.5 whereas crustal rocks have ratios of about 0.2. The Ta/Th ratios of Dongargarh SHMB and acid volcanics are nearly identical but both unusually low ( < 0.1) when compared to the normal upper continental crust (Ta/Th f 0.2), supporting the view that the acid volcanics are the source for incompatible elements and SiO2 in SHMB. It is also shown that there is an overall compositional similarity of such temporarily unique but spatially unrelated SHMB magmas occurring in different continents including the Dongargarh SHMB. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Dongargarh; SHMB; Contamination; Basaltic komatiite; Acid volcanics

1. Introduction The study of melts with basaltic major element composition, but with elevated SiO2 contents, is of

*

Corresponding author. E-mail address: [email protected] (H. Palme).

considerable interest (Sun et al., 1989). Siliceous high magnesian basalts (SHMB) have SiO2 contents from 53 to 55 wt.%, but their MgO contents are within or slightly above the range of ‘normal basalts’. Since the first report about SHMB from Western Australia (Readman and Keays, 1985), new occurrences were reported from several Precambrian terrains in different parts of the world, but surprisingly, in most of the

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 ( 0 2 ) 0 0 0 2 0 - 7

22

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

cases, the SHMB occur at or near the Archean – Proterozoic transition. While Sharpe and Hulbert (1985) advocated a primary siliceous picritic magma parental to the SHMB magmas in the Bushveld complex in South Africa, Sun et al. (1989) and Arndt and Jenner (1986), on the basis of trace elements and radiogenic isotopes, suggested the interaction/assimilation of mantle-derived melts depleted in incompatible elements with crustal material as source for the silica enrichment of this type of basalt. Others believe that SHMB are analogous to boninites (e.g. Sharpe and Hulbert, 1985) and the recent discoveries of aphyric andesitic primary magmas called sanukitoids (Tatsumi and Ishizaka, 1982) and high-Mg andesites (Kelemen, 1995) from a subduction setting in Japan appear to have strengthened the primary melt theory for the Sirich basaltic lavas. We have recently reported (Sensarma et al., 1999) an occurrence of SHMB lava from the Early Proterozoic Dongargarh Supergroup in the Central Indian Craton, the first report of this kind of lava from the Indian Precambrian. In this paper, we present the mineralogy and geochemistry of the Dongargarh SHMB, evaluate the extent of similarities with other SHMB occurrences of similar age across the world and seek to understand the petrogenesis of this Si-rich basalts. The occurrence of SHMB at the Archean – Proterozoic transition across the continents may also have important implication on early Precambrian crustal growth.

2. Geologic background and sampling The Central Indian Craton, in the heart of India, is separated by an east – west trending Son-Narmada lineament into a northern Bundelkhand Protocontinent and a southern Deccan Protocontinent of contrasting character (Yedekar et al., 1990) (Fig. 1). The Bundelkhand Protocontinent is represented by high-grade metamorphic rocks with a structural trend concordant to the lineament, while the Deccan Protocontinent hosts a collage of low-grade, north – south trending volcanic-sedimentary assemblages with a discordant structural relationship with the lineament. The Dongargarh Supergroup is one of the Early Proterozoic volcanic-sedimentary successions located in the Deccan Protocontinent (Fig. 1). This NNE – SSW trending

Fig. 1. Map of India (not to scale) indicating location (filled square) of the Dongargarh belt.

belt is 90-km wide and runs for about 150 km between the Chattisgarh basin in the east and the Sakoli Synclinorium in the west. The rocks are all mildly deformed and metamorphosed at low greenschist facies condition. So, a shallow crustal level is exposed in Dongargarh. Sarkar (1957 –1958) identified the major structure as the northerly plunging Sitagota Syncline and presented a stratigraphic succession for the belt. According to Sarkar (1957 –1958, 1994) and Sarkar et al. (1981), the Dongargarh rocks are unconformably overlying the Archean Amgaon Gneiss and comprise of a basal igneous suite (Nandgaon Group) and the younger sedimentary – igneous assemblages (Khairagarh Group), the two being separated by an unconformity. The major plutonic activity represented by the Dongargarh Granite predates the unconformity. The lowermost part of the Nandgaon Group is represented by acid

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

volcanics and associated pyroclastic rocks (Bijli Rhyolite), the only acid volcanic activity recorded in the area. The acidic rocks were reported to be unconformably overlain by a basic volcanic unit (Pitepani Volcanics) within the same group (Sarkar, 1957 – 1958, 1994). We could, however, not confirm this unconformable relationship anywhere in the area. The presence of dark green coloured blobs/strings of mafic enclaves within the acidic pyroclastic rocks and the close spatial relation of the two litho-units in the field could rather be indicative of the near contemporaneity of the acid and basic volcanism in the group. The Bijli Rhyolites covers an extensive area and is about 3 – 4 km in thickness at places. It yielded Rb – Sr isochron ages of 2180 F 25 to 2503 F 35 Ma (Sarkar, 1994). Recently, Neogi et al. (1996) have carried out some geochemical work on the Dongargarh volcanic rocks. The Dongargarh SHMB is a component within the Pitepani Volcanics of the Nandgaon Group. The Pitepani Volcanics is about 1-km thick and consists primarily of basalts with minor basic fragmental rocks. The basic volcanics are greenish in colour, variably foliated and crop out in the eastern part of the belt in and around the village Pitepani, about 10 km south of Bortalao (21j13V: 80j38V) as well as in the far eastern fringe of the area about 2 km west of Paniajob (21j12V: 80j41V). Rocks generally occur in boulderly outcrop. Several SHMB samples were collected from both of these areas. About 10 samples of acidic volcanics were also collected in the vicinity of Pitepani village near the exposures of the basic volcanic rocks.

3. Sample preparation, analytical procedures and results 3.1. Sample preparation To minimize the effects of alteration and low-grade metamorphism on the chemistry of the volcanics, samples with surface contamination, fractures, amygdules and/or secondary veins were discarded. Samples with sparse phenocrysts have been given the preference for chemical analysis. Altogether eight samples were chosen for analysis. For chemical studies, the selected samples were first jaw-crushed using steel plates. Fresh chips were then milled in a rotating agate mortar. The agate mortar was

23

cleaned before and after processing of each sample, first by silica powder under the mill for 10 min followed by rinsing with distilled water and high-speed air, respectively. Powdered samples were collected in clean sample bottles. For chemical analysis, replicate samples were taken in most cases. 3.2. Analytical procedures The concentrations of major and several trace elements (Rb, Sr, Zr, Ba, Ga, Y, Ni, Cr, Co, Sc, V, Zn) were determined by XRF using fused glass discs. Glass pellets were formed by fusing 600 mg of homogenized sample powder with lithium tetraborate in the ratio of 1:3 and a small amount of ammonium nitrate. The analyses were made with a Philips PW 2400 XRF machine, equipped with an automatic sample changer. Calibration curves were obtained from regressions of more than 40 standard rocks. About 100 mg of powdered samples was irradiated in the carrousel of the TRIGA reactor of the Institut fu¨r Kernchemie, Universita¨t Mainz. Samples were irradiated for 6 h at a neutron flux of 7
1011 n/s cm2 and subsequently counted on large Ge-detectors at the Institut fu¨r Mineralogie und Geochemie, Universita¨t zu Ko¨ln. Peak fitting routines from Kruse (1979) were used to deconvolute spectra. Pure element standards irradiated under the same conditions were used. Electron microprobe analyses were carried out with a JEOL electron microprobe (JXA-8900) at the Institute fu¨r Mineralogie und Geochemie, Universita¨t zu Ko¨ln. An acceleration voltage of 20 kV and a beam current of 20 nA were used. A set of natural minerals (for Si, Ca, Na, K) and synthetic oxides (for Mg, Fe, Al, Ti, Mn, Cr, Ni) were used for standardisation. Based on the extensive comparison with standard rocks, the accuracy of the XRF data is estimated to be below 3%. For INAA, accuracies are indicated in Table 2. For elements determined with XRF and INAA, there is generally good agreement within the uncertainty (5% to 10%) of both methods (see Table 3). The accuracy of the Zr data is poor because of corrections from Eu and U decay. Nevertheless, the Zr/Hf ratios are near chondritic (f 36, see Table 2). Further confirmation for the accuracy of the XRF and INAA methods applied here comes from a recent inter-laboratory comparison (Jochum et al., 2000). The

24

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

ible elements in our basalt samples. Their overall compositional similarities with other SHMB, but dissimilarities with common volcanic rocks are shown in Table 4. In an SiO2 vs. MgO plot (Fig. 2) SHMB including the Dongargarh rocks occupy a separate field distinct from other rock-types. Boninites, though slightly overlapping with SHMB, generally contain more SiO2 at MgO contents typical of SHMB. Also, in a CaO/TiO2 vs. Al2O3/TiO2 plot (Fig. 3), boninites are clearly separated from SHMB. The low TiO2 of boninites may reflect their formation from a depleted mantle with low Ti extracted in earlier melting episodes (Falloon and Crawford, 1991). Primary high-Mg ande-

results of standard rock analyses by the two methods as applied at the Institut fu¨r Mineralogie und Geochemie, Universita¨t zu Ko¨ln are within the range of the results obtained in other laboratories on the same rocks. 3.3. Results Results of XRF analyses are shown in Table 1. INAA data for three SHMB samples and four samples of acidic volcanic rocks (see below) are given in Table 2. Representative microprobe analyses of mineral phases are shown in Table 3. XRF- and INAA-data (Tables 1 and 2) clearly reveal enhanced concentration of silica and incompatTable 1 Major and trace element data for Dongargarh SHMB rocks, Central India Sample [wt %] SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 LOI Total [ppm] Ba Rb Sr Ga Zr Y Cr Ni Co Sc V Zn Zr/Y Rb/Sr K/Rb Ti/Zr P205/TiO2 Mg*

DP1

DP2

DP3

DP6

DP101

DP102

DP103

DP106

53.98 0.50 12.56 8.64 0.17 7.34 9.57 2.12 1.66 0.13 2.26 98.93

54.77 0.51 12.93 8.50 0.17 6.89 9.13 2.00 1.78 0.13 2.21 99.02

54.65 0.52 12.22 8.52 0.17 7.44 9.59 1.96 1.53 0.14 2.39 99.13

54.43 0.50 12.44 8.48 0.18 7.39 9.70 2.00 1.28 0.13 2.58 99.11

54.11 0.50 12.55 8.66 0.17 7.38 9.59 2.12 1.68 0.13 2.28 99.17

54.67 0.51 12.92 8.58 0.17 6.81 9.06 2.00 1.77 0.14 2.51 99.14

54.62 0.53 12.18 8.58 0.17 7.57 9.68 1.95 1.53 0.14 2.09 99.04

54.30 0.50 12.62 8.62 0.18 7.46 9.85 2.03 1.30 0.13 2.02 99.01

398 59 267 34 22 6 366 111 20 33 163 49 3.67 0.22 234 136 0.26 0.60

424 70 317 28 69 11 370 86 96 28 170 103 6.27 0.22 211 44 0.25 0.59

537 60 339 26 63 10 392 131 30 29 173 96 6.30 0.18 212 49 0.27 0.61

379 57 317 28 67 9 367 74 30 31 166 90 7.44 0.18 186 45 0.26 0.61

375 63 289 26 59 11 362 76 38 34 172 88 5.36 0.22 221 51 0.26 0.60

414 67 309 25 63 11 364 89 27 29 168 91 5.73 0.22 219 49 0.27 0.59

511 61 338 24 70 10 407 89 29 29 166 102 7.00 0.18 208 45 0.26 0.61

371 54 326 29 67 11 389 83 36 32 170 95 6.09 0.17 200 45 0.26 0.61

All analyses by XRF. FeOt—total iron. * Mg = Mg-number (Mg/(Mg + Fet)).

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

25

Table 2 INAA data for Dongargarh SHMB and acid volcanics Sample

SHMB

Acid volcanics

DP3

Ca Fe Na K

Sc Cr Co Ni Zn Ga As Rb Sr Zr Mo Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Tm Yb Lu Hf Ta W Au Th U

DP106

DP2

DM3

DM 7U

wt.%

s.d.

wt.%

s.d.

wt.%

s.d.

wt.%

s.d.

wt.%

6.3 6.5 1.5 1.3

10 3 3 5

5.6 6.2 1.36 0.91

10 3 3 5

6 6 1.4 1.35

8 3 3 5

1.9 2.7 1.85 4

20 3 5 5

< 0.7 0.24 2.6 3.1

ppm 32.6 438 41 80 90 14 3.8 62 430 90

3 3 3 20 15 25 10 20 10 30

0.45 1.5 650 17.5 40

15 8 3 3 5

17 3.1 0.78 3 0.45 3.07 0.7 0.26 1.45 0.22 2.4 0.35 < 0.32 0.006 4.8 1.1

20 3 10 25 10 15 20 30 10 10 5 10 20 5 15

ppm 32 390 38 72 80 14 2.3 53 354 72 4.3 0.5 1.2 400 15.5 34 6 16 2.8 0.77 2.6 0.38 n.d. 0.54 0.22 1.25 0.21 2.1 0.33 0.9 0.0056 4.3 1.2

3 3 3 15 20 15 10 5 10 20 20 10 10 5 5 8 40 20 5 5 20 10

ppm 25.5 380 39 64 112 14 6.2 70 440 60 1.8 0.55 1.6 465 16 36

3 3 3 20 10 12 5 5 15 30 20 5 8 5 3 5

16 2.7 0.7 2.2 0.36

20 8 8 30 12

20 20 5 10 5 10 25 15 5 10

0.68 0.3 1.35 0.2 2.1 0.4 1.2 0.004 4.5 1.2

20 20 5 8 5 10 20 10 5 8

ppm 6.0 25 3.5 12 66 20 0.77 222 115 390 15 0.3 3 1020 93 200 21 73 11.5 1.4 26 1.7 7.7 2.27 0.95 5.8 1 11.7 2.3 1.5 < 0.002 38 9.5

3 5 5 30 5 10 20 5 20 15 15 20 5 5 3 3 25 10 10 5 25 5 10 20 20 5 15 3 5 20 3 8

ppm 2.8 40 0.4 6 28 15 0.31 120 60 400 6.3 0.3 0.3 1100 64 137 20 54 10 1.4 12 1.3 6.9 1.97 0.54 3.8 0.56 10.2 1.4 1 < 0.003 16 3.9

DM19

DM 21

s.d.

wt.%

s.d.

wt.%

s.d.

5 3 5

0.8 2.4 2.84 2.5

35 3 3 5

0.4 1.45 2.6 2.2

30 5 5 5

ppm 1.7 44 0.5 15 110 19

5 5 8 20 8 12

3 5 8 30 10 15 40 8 25 15 20 15 10 8 3 3 30 10 3 5 15 10 10 15 20 8 10 5 5 30 5 8

ppm 3.85 36 1.5 <6 124 22 < 0.6 102 82 320 5.5 0.22 0.85 775 65 155 40 58 11.8 1.71 8 1.75 9 2.3 <2 5.7 0.8 9.3 43 0.003 15.5 4.8

3 5 5 8 15 5 20 12 20 15 10 8 3 3 35 10 3 5 20 5 15 20 5 8 5 3

70 80 380 5.5 0.05 0.65 850 41 115 < 25 43 8 1.4 8 1.1 5.7 2.1 0.51 3.8 0.5 10 1.3

8 20 10 20 25 10 10 5 5 10 5 5 15 5 12 20 15 8 5 5 5

15 5 8

0.001 12.2 3.4

25 5 0

s.d.— standard deviation in %.

site represents (Kelemen, 1995) calc-alkaline andesitic rocks unlike SHMB which essentially is a basalt with narrow compositional range. Moreover, the abundances of Al2O3 ( < 13 wt.%) in SHMB are similar to Al2O3 contents in tholeiitic rocks (Philpotts, 1990). Sanukitoids, a rock type with apparent similarities to SHMB, are olivine-rich aphyric basalts and andesites

without/with sparse plagioclase and with certain geochemical differences (low FeO, higher Al2O3, low silica) compared to SHMB at a given value of MgO (f 10 wt.%). Normal basalts (both continental and oceanic types), on the other hand, have lower CaO/ TiO2 and Al2O3/TiO2 ratios than SHMB and primitive upper mantle (PUM) (Fig. 3). The higher than PUM

26

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

Table 3 Representative microprobe analyses of pyroxene and plagioclase in Dongargarh SHMB rocks Sample

Pyroxene (phenocrysts and microphenocrysts)

Pyroxene (groundmass)

Plagioclase (groundmass)

DP6

DP2

DP3

DP2

DP6

DP6

DP3

DP3

DP2

DP6

DP2

DP3

DP3

SiO2 TiO2 Al2O3 FeO MnO MgO Cr2O3 CaO Na20 K20 Total

52.99 0.19 1.42 5.87 0.19 17.45 0.00 20.79 0.11 0.01 99.02

53.23 0.12 1.06 5.16 0.19 17.72 0.45 21.28 0.03 0.00 99.24

53.05 0.17 1.13 6.80 0.25 17.74 0.24 19.34 0.13 0.03 98.88

53.08 0.06 0.53 4.80 0.19 15.99 0.27 23.95 0.19 0.00 99.06

53.21 0.19 1.50 6.04 0.17 17.67 0.00 20.74 0.23 0.00 99.77

53.37 0.17 1.24 6.08 0.19 17.70 0.00 20.49 0.11 0.01 99.38

50.12 0.50 4.57 15.84 0.31 13.18 0.03 11.98 0.76 0.31 97.60

50.87 0.26 3.66 15.49 0.29 13.45 0.01 12.31 0.82 0.24 97.40

49.67 0.14 4.34 16.09 0.35 12.67 0.02 12.17 0.81 0.30 96.56

51.40 0.04 29.41 1.01 0.01 –

52.18 0.03 29.09 1.07 – 0.03

52.74 0.02 28.55 0.84 0.02 0.05

54.49 0.06 27.45 0.79 0.01 0.13

13.29 3.90 0.10 99.16

12.50 4.46 0.09 99.45

11.77 4.76 0.32 99.07

10.28 5.29 0.48 98.98

Si Ti Al Fe Mn Mg Cr Ca Na K Total

Total cation (on the basis of 6 oxygen) 1.960 1.962 1.966 1.974 0.001 0.000 0.005 0.000 0.062 0.046 0.049 0.023 0.182 0.159 0.211 0.149 0.001 0.001 0.008 0.001 0.962 0.974 0.98 0.886 0.000 0.013 0.007 0.001 2.623 2.456 2.316 2.015 1.391 1.587 1.696 1.880 0.023 0.021 0.075 0.111 20.045 20.091 20.103 20.060

1.954 0.001 0.065 0.186 0.001 0.968 0.000 0.816 0.017 0.000 4.016

1.966 0.000 0.054 0.187 0.001 0.972 0.000 0.809 0.001 0.000 4.007

CaO/TiO2 ratios of komatiites reflect formation from a slightly depleted upper mantle.

4. Petrography and mineral chemistry The Dongargarh SHMB is primarily composed of varying proportions of clinopyroxene and plagioclase and generally shows sparsely porphyritic texture. Clustering of phenocrysts occasionally give rise to glomeroporphyritic textures. The dominant groundmass texture in the rocks is intergranular to intersertal. While pyroxene occurs as phenocrysts (including microphenocrysts) and in groundmass, plagioclase is exclusively found in the groundmass. Other phases constituting the groundmass include chlorite, apatite, Fe-oxide, and interstitial glass. Fe-oxides and Croxides also occur as inclusion within pyroxene phenocrysts. No olivine or orthopyroxene or any primary hydrous phase is present in the rock. The composition of clinopyroxene and plagioclase is presented in Table 3. The same samples were used for preparation of thin

1.927 0.015 0.207 0.510 0.010 0.756 0.001 0.494 0.057 0.015 3.990

1.957 0.007 0.166 0.498 0.010 0.771 0.000 0.508 0.061 0.012 3.989

1.936 0.004 0.200 0.525 0.012 0.737 0.001 0.508 0.062 0.015 3.998

Total cation (on the basis of 32 9.464 9.565 9.688 0.005 0.005 0.003 6.384 6.284 6.180 0.155 0.164 0.129 0.001 – 0.003 – 0.008 0.013

oxygen) 9.968 0.008 5.920 0.120 0.003 0.036

2.623 1.391 0.023 20.045

2.015 1.880 0.111 20.060

2.456 1.587 0.021 20.091

2.316 1.696 0.075 20.103

sections for studying the texture, mineralogy and mineral chemistry as for chemical analyses. The phenocrysts (f 0.5 mm) and microphenocrysts are subhedral and their modal count (including microphenocrysts) is low (f 3– 5% by vol). The phenocrystic pyroxenes are mainly of diopsidic composition (Wo41.32 – 43.60 En48.75 – 49.63 Fs6.76 – 9.78) plotting close to the diopside corner but reaching slightly into the augite field. The grains do not show any zoning. The groundmass pyroxenes, on the other hand, are Ca-poor clinopyroxene and of augitic composition (Table 3). The total of oxides of these grains, however, has consistently yielded low value despite best effort during measurements with well-calibrated standards. This may be attributed to finer grain size and/or alteration of the groundmass pyroxene. Plagioclase in the groundmass is slightly elongated to tabular in appearance and generally labradoritic in composition (Table 3). Because of alteration and finer grain size, the measurement could not be done with maximum confidence, though the standard analysis with the same calibration was perfect.

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

5. Geochemical character of Dongargarh SHMB 5.1. Evaluation of post-crystallisation alteration We have considered the problem of possible element mobility arising from low grade metamorphism and alteration (Brewer and Atkin, 1989). Values of LOI, appropriate indicators of hydration of mafic volcanic rocks, are low (f 2 wt.%) for the samples analysed here suggesting little aqueous alteration of the rocks. The major and trace element data also support this view. Distribution of immobile major elements (e.g. Si, Al, Mg, etc.) in the rocks are regular. With approximately chondritic Th/U ratios of around 4 (Table 2) vs. 3.6 in CI-chondrites, there is no indication of preferred mobilisation of U compared to Th. Also,

27

ratios of the mobile Rb to the immobile Th (f 13.5) (Table 2) are not far from the bulk continental crust (BCC) ratio of 9.1 (Taylor and McLennan, 1985). In addition, normalized multi-element and REE patterns (Figs. 5 and 6) are regular and consistent with magmatic processes implying that incompatible element abundances and their ratios in SHMB samples reflect original magma characteristics, i.e., are indicative of igneous processes. 5.2. Chemical characteristics 5.2.1. Major element composition The type of Pitepani lava discussed here is subalkaline, with moderately high K2O of 1.3 to 1.8 wt.% and total alkalis of 3 to 3.8 wt.% (Table 1). It is a high

Table 4 Chemical compositions (average) of SHMB-type lavas and other volcanic rocks n

1 (12)

2 (10)

3 (11)

4 (9)

5 (8)

6 (26)

wt.% MgO SiO2 Al2O3 FeO MnO CaO Na2O K2O TiO2

10.60 53.10 13.00 10.70 0.17 8.50 1.65 0.50 0.65

11.97 55.24 10.66 10.61 0.18 7.09 1.48 0.94 0.58

7.40 54.90 14.10 9.26

9.16 53.12 12.91 9.50 0.15 9.45 2.20 0.86 0.68

7.30 54.44 12.55 8.57 0.17 9.52 2.02 1.57 0.50

7.58 50.45 15.26 10.43

237 952 33.6 220.5 103.8 67.2 3.1

387 1094 32.7 213 113 83.4 5.8

150

113.1 344

91 377.0 32.3 168.5 312.8 61

149 10.7 2.6 2.8 1.9 2700 Pb – Pb

228 14.2 2.92 3.0 1.6 2241 F 4 U – Pb

ppm Ni Cr Sc V Sr Zr Nb Ta Ba La Sm Gd Yb Age (Ma) Dating method

6.30 2.00 1.60 0.60

216 229 78.3 5.2

2200 – 2300 not mentioned

271.6 9.66 2.22 1.96 1.31 2400 – 2500 U – Pb

0.34 426 16.5 2.95 2.8 1.35 2180 – 2503 Rb – Sr

11.30 2.52 0.18 1.61

149.5 41.37 – 113.2 10.71 5.7 5.47 13.9 3.9 3.75 5.08 3.9

7 (5) 8.08 60.67 12.61 7.56 0.16 7.75 1.95 0.6 0.12

152 616.5 35.3 161 84.7 25.9

32.4 1.18 0.5 0.74

8

9

3.54 59 17.4 6.63 7.08 3.5 1.44 0.78

7.41 50.51 13.45 9.59 0.17 11.18 2.28 0.49 2.63

38.6 48.4 125 648

886 19.4 3.5 3.3 2.1

10 (15) 7.33 50.20 13.57 12.03 0.18 10.23 2.57 0.55 1.73

137 361

660 280 48 2.7 350 37 10 7.62 2.16

269 169.3 11.93 0.72 17.7 6 5.64 2.42

Figures in the parentheses indicate number of samples. (1) Kambalda, Western Australia (Arndt and Jenner, 1986), (2) Vestfold Hills, Antarctica (Seitz and Keays, 1997), (3) Kapvaal Craton, South Africa (Condie and Crow (1990)), (4) Fennoscandian Shield, Russia (Lobach-Zhuchenko et al., 1998; REE data available for sample no. 18 and 868), (5) Dongargarh Supergroup, India (Present work), (6) N-MORB (Hofman, 1988), (7) Boninite (Wilson, 1989), (8) Andesite (Taylor and Mclennan, 1985), (9) OIB (BVSP, 1981; Sun and McDonough, 1989 (trace elements)), (10) CFB (Lightfoot et al., 1991).

28

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

Fig. 2. SiO2 – MgO plot for SHMB and other volcanic rocks showing a separate field for SHMB. Average SHMB compositions are plotted as given in Table 4 (Kambalda, Antarctica, South Africa, Russia, Dongargarh). Other sources of data: Komatiite (Lesher and Arndt, 1995; BVSP, 1981), MORB (Hess, 1989), CFB (Lightfoot et al., 1991; BVSP, 1981), Boninite (Falloon and Crawford, 1991; Wilson, 1989), Andesite (Wilson, 1989), UCC (Taylor and McLennan, 1985), PUM (McDonough and Sun, 1995). UCC = upper continental crust, PUM = primitive upper mantle.

SiO2 (about 53– 55 wt.%), high to moderately high MgO (f 7.5 wt.%) and low TiO2 (0.5 wt.%) type lava (Table 1), though MgO content here is at the lower end of the SHMB spectrum (this work, Sun et al., 1989) due to olivine fractionation, as discussed later. The rocks are compositionally similar to basaltic andesite in a SiO2 vs. (Na2O + K2O) diagram (Cox et al., 1979), the bulk composition of which plot in the calc-alkaline field is very near the boundary with the tholeiitic field but distinctly towards the MgO corner of the AFM diagram (Fig. 4). However, the Al2O3 content is low (f 13 wt.%) compared to calc-alkaline basalts or basaltic andesites (f 16 wt.%) (Philpotts, 1990). The rocks are low to moderately fractionated as the Mg-numbers of the samples vary between 59 and 61. 5.2.2. Trace elements composition In Fig. 5, we have plotted the concentrations of lithophile trace elements normalized to primitive

upper mantle (PUM) (McDonough and Sun, 1995) of the three SHMB samples that are analysed by INAA. For comparison, data for the bulk continental crust (BCC) (Taylor and McLennan, 1985, Ta value from Barth et al., 2000) are shown. The elements in Fig. 5 are arranged in the sequence of their enrichment in BCC relative to PUM, i.e., according to their degree of incompatibility with mantle minerals. The BCC and SHMB patterns are remarkably similar. The SHMB have somewhat higher abundances of highly incompatible elements (Rb, Cs, U, etc.) and lower abundances of the more compatible elements (Y, Ti, Lu) compared to BCC. In addition, SHMB is significantly lower in Ta (and presumably also in Nb) than BCC, while Ba is distinctly higher. While the anomaly in Ta (and Nb) is characteristic of SHMB worldwide, the excess of Ba is not, as will be discussed in more detail later in this paper. CI-chondrite-normalised REE plots (Fig. 6) show enrichment in LREE with La/Yb (12) and La/Sm (5.5)

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

29

Fig. 3. Al2O3/TiO2 – CaO/TiO2 plot for SHMB and other volcanic rocks showing the similarity in refractory element ratios of SHMB to the primitive upper mantle (PUM) and the more fractionated ratios in boninites and basalts. Source of data: same as Fig. 2.

and almost flat HREE pattern, at a level of about 10 times CI chondritic. Samples do not show a resolvable Eu-anomaly.

Fig. 4. Position of Dongargarh SHMB in an AFM plot (Irvine and Baragar, 1971). Dongargarh SHMB plot at the border of the calcalkaline and tholeitiic fields. Addition of alkaline elements by crustal contamination has moved the SHMB composition from the tholeiitic field into the calc-alkaline field.

Fig. 5. Incompatible elements normalised to PUM (McDonough and Sun, 1995) for three samples (DP2, DP3 and DP106) of Dongargarh SHMB. BCC (bulk continental crust) of Taylor and McLennan (1985) is plotted for comparison. New Ta data for BCC are taken from Barth et al. (2000). SHMB samples show positive Ba-anomaly and negative Ta-anomaly.

30

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

6. Comparison of different SHMB magmas Rocks with similar bulk chemical composition as those described as SHMB have been recently reported from different continents. In most of the cases, SHMB formed at or near the Archean– Proterozoic transition including the Dongargarh suite. This temporal uniqueness of SHMB of worldwide occurrences is indeed surprising. Our data along with the published ones taken from the literature are presented in Table 4. The average of 12 samples from the high-Mg basalts unit (HWB) of the Norseman– Wiluna belt (2.7 Ga), Kambalda (see Arndt and Jenner, 1986) is used in this study. Average of major and trace element data for 10 samples of the Early Proterozoic dykes from the Vestfold Hills of Antarctica (Seitz and Keays, 1997) and the average of 11 samples from the Hekpoort Formation (f 2.3 Ga) of the Transvaal Supergroup, South Africa (Condie and Crow, 1990) are also used here for comparison; though trace element data of the latter suite are not available. Lobach-Zhuchenko et al. (1998) have recently presented data of mafic intrusive (drucites) (2.4 – 2.45 Ga) from the Fennoscandian Shield, Russia. Since the authors do not classify the drucites (into both primitive and evolved type), we consider the average of nine samples of mafic rocks with Mgnumber around 0.60 (sample no. 67a, 104a, 1860, 607, 201, 591, 225, 18, 868) for comparison.

Fig. 6. CI-normalized rare earth elements for three samples (DP2, DP3 and DP106) of the Dongargarh SHMB rocks. REE patterns do not show an Eu-anomaly.

Fig. 7. CI-normalized rare earth elements pattern for SHMB samples from different parts of the world. Sources of data from Table 4.

The similarities in major element compositions of SHMB are apparent from Figs. 2 and 3. A CI-chondrite normalized REE plot (Fig. 7) of SHMB rocks from different continents reveals the similar character of the spatially unrelated but temporally unique SHMB including the Dongargarh samples. SHMB are LREE-enriched rocks with high La/Sm (f 4) and high La/Yb (6 to 10) with flat HREE patterns (Gd/Yb f 1.2). HREE are 10 to 15 times chondritic. Occasionally, samples show a weak Eu-anomaly. The remarkable similarities of REE patterns of SHMB argue for a similar petrogenetic process(es) for all these suites including the Dongargarh lava. The differences in the absolute abundances of LREE among the various SHMB are less than a factor of two. The variations in concentrations of compatible elements like Ni and Cr in SHMB from different localities (Table 4) are larger than those of the incompatible elements which argues for olivine fractionation, as olivine crystallisation would enrich the residual magma in incompatible elements and simultaneously remove Ni from the melt. With 20% olivine removal, a maximum of 20% enrichment in incompatible elements in the residual melt could be achieved, but the Ni-content could be lowered by a significantly larger factor, depending on the Ni olivine/melt partition coefficient (DNi ol/melt). A DNi ol/melt of 6 would produce a twofold depletion of Ni in the melt with 20% partial melting. Thus, the comparatively larger variations in Ni and also Cr and the uniform enrichment in REE in the

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

Dongargarh rocks are in qualitative agreement with this hypothesis.

7. Petrogenesis The petrogenesis of siliceous high-Mg basalts has been a controversial issue (Seitz and Keays, 1997; Sun et al., 1989). As already indicated, Sharpe and Hulbert (1985) considered a siliceous picritic magma for the generation of the Bushveld SHMB lava. But Arndt and Jenner (1986), on the basis of trace elements and isotopic models, argued in favor of contamination of a komatiite parent magma for the Kambalda silica-rich basalts. Later, Sun et al. (1989) preferred an assimilation and fractional crystallisation (AFC) processes starting with a komatiitic parent magma. Sun et al. (1989) also speculated that some of the Early Proterozoic SHMB suites might have formed by second-stage melting of subduction-modified refractory harzburgitic Archean lithosphere and mixing with asthenosphere-derived picrite/komatiite followed by AFC processes. 7.1. Primary melt hypothesis Anhydrous partial melting experiment of fertile peridotite (Kushiro, 1996) shows that a melt with SiO2 of about 53– 55 wt.% can be generated only at pressures between 5 and 10 kbar (stability field of

31

plagioclase lherzolite) and at a low degree of partial melting. But such a melt is not consistent with the composition of our lava (Table 5). A melt generated at higher degrees of partial melting (about 20%) with comparable silica (f 52 wt.%) also does not match with regard to FeO, Al2O3 and CaO contents (Table 5). Moreover the partial melting of a plagioclase-rich source is expected to retain Eu resulting in a negative Eu-anomaly in the SHMB samples. But no Eu-anomaly could be detected in the SHMB samples. SHMB has low Ta/La ratio (f 0.02). Low Ta/La in basalts are often considered indicative of a source with a slabcomponent. But slab-derived basalts have low Th/U (f 2.5) (Wilson, 1989) too, unlike the SHMB ( > 3.5). So partial melting of an enriched source again cannot simultaneously explain the very low Ta/La of our samples (Fig. 5) along with high Th/Ta (f 13) and Th/U (f 4) values. So a SHMB type primary melt cannot be generated by anhydrous melting of fertile upper mantle. Several investigations of hydrous peridotite partial melting have reported the generation of SiO2-rich melts (Kushiro, 1972; Mysen and Boettcher, 1975 and others). But recent experiments performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 jC, with up to f 12 wt.% H2O dissolved in the liquid (Gaetani and Grove, 1998) demonstrate that the effects of concentration of dissolved H2O on the melts in equilibrium with a spinel lherzolite mineral assemblages produce an enhanced SiO2/

Table 5 Comparison of Dongargarh SHMB with melts derived by partial melting of anhydrous mantle peridotite at 5, 10 and 15 kbar % melting a

a

[5 kbar]

wt.% SiO2 TiO2 Al2O3 FeO MgO CaO Na2O a b c d

Dongargarh SHMBc

b

[10 kbar]

[15 kbar]

6.5%

21.2%

4.4%

23%

23%

37%

55.91 1.74 16.52 5.74 5.93 8.75 2.92

52.32 1.10 15.50 7.33 9.28 11.67 1.74

53.21 1.79 18.94 5.71 5.91 7.82 3.18

50.91 0.97 13.19 9.01 12.85 10.66 1.21

53.4 1.27 6.20 7.5 22.4 7.2 0.50

49.0 0.46 12.4 8.12 16.3 12.0 0.74

Fertile mantle (Kushiro, 1996). Residual mantle (Fallon and Danyushevsky, 2000). Present work (average of eight samples). Standard deviation.

s.d.d 54.44 F 0.27 0.50 F 0.01 12.55 F 0.26 8.57 F 0.06 7.30 F 0.26 9.52 F 0.26 2.02 F 0.06

32

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

(FeO + MgO) ratio due to lower concentrations of FeO and MgO rather than an absolute increase in SiO2 concentration to make the partial melt relatively SiO2 rich. Compositions of melts that formed by partial melting of residual mantle (Fallon and Danyushevsky, 2000) have SiO2 contents broadly consistent with SHMB (Table 5), but with much higher MgO contents and lower Al2O3, CaO and particularly low TiO2 contents, typical characteristics of boninites, as discussed before (Fig. 2). In summary, generation of a primary SHMB magma from a fertile, depleted or hydrous mantle is not possible. This is supported by calculations with the MELTS program (Ghiorso and Sack, 1995). According to MELTS, the first phase to crystallize from SHMB melts on cooling is pyroxene over a pressure range of 1– 10 kbar; not olivine as expected for a primary mantle melt. However, by reducing the SiO2 content of the melt by about 4%, i.e., from 54% to 50%, at 10 kbar (crustal depth) olivine appears as the first crystallizing phase. Although, the SHMB melt is saturated with pyroxene, evidence of pyroxene fractionation is not reflected in its chemical composition. The nearly chondritic relative abundances of Ca, Al and Ti in SHMB, i.e., their similarity to PUM are evident in Fig. 2. The two compatible refractory trace elements Yb and Sc also have similar CI-normalized abundances in SHMB as Ca, Al and Ti. The relatively unfractionated compatible refractory element patterns in SHMB reflect the predominant influence of olivine in the fractionation processes that led to the formation of the parental SHMB melt (SiO2 f 50 wt.%). Fractionation of pyroxene would destroy this pattern by removing significantly more Ca and Sc from the melt. The comparatively low contents of Ni in SHMB (below 100 ppm, Tables 1 and 2) also support olivine as the major fractionating phase. So, olivine fractionation of a mantle derived melt and subsequent addition of SiO2 and incompatible trace elements could account for the observed chemical composition of SHMB as will be discussed in the following section. 7.2. Contamination hypothesis This model essentially requires plausible interaction of at least two end member components—the

mantle-derived melt and the contaminant(s). In most ‘contamination models’, a strict relationship between the material assimilated and the amount of simultaneous fractionation (Assimilation + Fractional Crystallisation, AFC) is assumed (DePaolo, 1981). But our calculations with the MELTS program suggest (see above) that silica enrichment in the Dongargarh melt has taken place after olivine fractionation. Assimilation, therefore, need not be strictly related to fractionation; rather these two processes could be decoupled (Cribb and Barton, 1996; Campbell, 1985). In such cases, assimilation approximates a simple mixing process, which is discussed in the next section. 7.3. Model Assuming a simple mixing between a parent melt and a salic contaminant, geochemical mass balance calculation was carried out. The measured concentration of a given element E in SHMB, CE (SHMB), is considered to be the sum of the concentration in a partial mantle melt CE (pmm) and in the contaminant CE (cont). Xpmm CE ðpmmÞ þ Xcont CE ðcontÞ ¼ CE ðSHMBÞ

ð1Þ

where Xpmm and Xcont are the fractions of partial mantle melt and contaminant, respectively, and Xpmm+ Xcont = 1. From the Dongargarh SHMB composition, various fractions of a contaminant were subtracted to arrive at a model parental mantle melt. Since the main characteristic of SHMB is their comparatively high SiO2 contents, the contaminant is required to be high in SiO2. If the contaminant had, for example, 75 wt.% SiO2, removal of 16% contaminant would be required to reduce the SiO2 content from 54%, typical of SHMB, to 50% characteristic of basalts. A good candidate would be the Dongargarh acid volcanics (SiO2 f 76 wt.%) which immediately underlie the litho-unit containing SHMB and occur in close spatial proximity in the field. In fact, the association of rhyolites and basalts is suggested to be of prime importance in favour of a crustal contamination model (DePaolo, 1983). In Table 6, we present the results of the mass balance calculations. From this table, it is clear that

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

33

Table 6 Geochemical mass balance calculation for origin of SHMB Dongargarh Dongargarh acid After removal of acid volcanic (contaminant) SHMB volcanics 10% 15% 20% (average) * (average)** Major elements SiO2 54.44 TiO2 0.50 12.55 Al2O3 FeOt 8.57 MnO 0.17 MgO 7.30 CaO 9.52 Na2O 2.02 K2O 1.57 P2O5 0.14 Total 96.78

76.05 0.28 11.85 2.13 0.02 0.17 0.61 3.47 3.88 0.04 98.50

Komatiitea (8999)

25%

53.88 0.54 13.07 9.61 0.19 8.38 10.88 1.92 1.36 0.16 100.00

52.48 0.56 13.14 10.06 0.20 8.87 11.50 1.83 1.20 0.16 100.00

50.90 0.58 13.21 10.57 0.22 9.43 12.19 1.72 1.03 0.17 100.00

49.10 0.60 13.29 11.14 0.23 10.06 12.98 1.60 0.83 0.18 100.00

Trace elements Compatible elements [ppm] Cr 403 25 – 44 Sc 30 1.7 – 6.0 Co 39.3 0.43 – 3.5 Na 14 200 18 500 – 28 400

445 – 442.9 33.1 – 32.7 43.6 – 43.3 13 722 – 12 622

469.7 – 466.4 35 – 34.2 46.2 – 45.6 13 441 – 11 694

497.5 – 492.8 37.1 – 36 49 – 48.3 13 125 – 10 650

529 – 522.6 39 – 38 52.3 – 51.2 12 767 – 9466

Incompatible elements [ppm] K 11 866 22 000 to Rb 61.6 70 to Ba 505 775 to La 16.3 41 to Sm 2.87 8 to Eu 0.75 1.4 to Th 4.53 12.2 to U 1.16 3.4 to Ta 0.36 1.3 to Hf 2.2 9.3 to

10 740 to 8740 10 077 to 6901 9332 to 4832 60.7 to 43.8 60.1 to 33.3 59.5 to 21.5 475 to 438.9 457.4 to 400 437.5 to 356.3 13.6 to 7.8 11.9 to 2.8 10.1 to  2.9 2.3 to 1.9 2.0 to 1.3 1.6 to 0.6 0.7 to 0.6 0.6 0.6 to 0.5 3.7 to 0.8 3.2 to  1.4 2.6 to  3.8 0.9 to 0.2 0.8 to  0.3 0.6 to  0.9 0.3 to  4.4 0.2 to  7.2 0.1 to  10.3 1.4 to 1.1 0.9 to 0.5 0.4 to  0.2

40 000 222 1100 93 11.8 1.71 38 9.5 43 11.7

8488 58.8 415 8.07 1.16 0.53 1.97 0.41 0.05 0.17

50.50 0.76 12.20 11.34 0.18 10.70 10.90 1.64 0.39 0.10 98.98

to to to to to to to to to to

2488 8.1 306.6  9.26  0.11 0.43  6.62  1.62  13.8  0.96

797 39.7 55 16 400

3238 6 148 7.01 2.29 0.746 0.99 0.203 0.15 1.41

Magnesian basaltb (ACH-9) 48.13 0.60 12.21 10.67 0.20 10.07 10.23 2.28 0.03 0.06 94.48

750 41.8 – 22 800

300 0.9 – 1.17 1.35 0.36 0.15

1.05

a

Puchtel et al. (1997). BVSP (1981). * Major elements (average eight samples, Table 1), * Trace elements (Table 2) ** Major elements (average 12 samples), ** Trace elements (Table 2). b

removal of about 20% of acid volcanics (Bijli Rhyolite) leads to compositions very similar to that of basaltic komatiites from the f 2.5-Ga Vetreny belt of the baltic Shield (Puchtel et al., 1997) or Archean high-Mg Basalt listed in Basaltic Volcanism Study Project (BVSP) (1981, p. 14). The mass balance calculations thus indicate that a basaltic komatiitic melt may have dissolved about 20% of acidic volcanics and erupted as SHMB. This model is consistent with the geologic situation and, in addition, the parent melt of komatiitic affinity postulated here is reported

to occur at the Archean – Proterozoic transition all over the world (Arndt, 1994). The mass balance calculations for compatible elements give reasonable results. Concentrations of Cr, Sc, Co, Na in SHMB and in some basaltic komatiites (Table 6) are in the same range. Removal of an acidic component, low in compatible elements, slightly increases the concentrations of compatible elements in the parental melt leading to better agreement of SHMB and the published basaltic komatiites listed in Table 6. For Na, however, removal of the Na-rich acidic

34

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

mantle and mantle-derived melts (e.g., komatiite, picrite) and several crustal reservoirs including the acid volcanics analyzed here. Since Ta data are not available for Kambalda basalts (Jochum et al., 1991), the chondritic Nb/Ta ratio of 17.5 (Jochum et al., 1986; Green, 1995) is used to calculate Ta concentration from measured Nb contents. Mantle-derived rocks have significantly higher Ta/ Th ratios than rocks with crustal affinity. The SHMB have even lower Ta/Th ratios than the average crust, but are similar to acidic volcanics. Similar but uniquely low Ta/Th ratios of the acidic volcanics and SHMB support the contamination model suggested above. Crustal contaminants with similarly low Ta/Th ratios are also envisaged by Jochum et al. (1991) for Kambalda silica-rich basalt (Table 7). Lower Ta/Th ( < 0.1) values than the normal crust (f 2) are sometimes encountered in granulites (Jochum et al., 1991; Rudnick and Presper, 1990). We, in fact, got a better match for incompatible elements assuming average Archean granulite (Rudnick and Presper, 1990) as contaminant, though major and other trace element modeling failed. Perhaps a deeper crustal source was partly responsible for the enrichment of incompatible elements in the mantlederived basaltic komatiite melt parental to SHMB. Similar suggestions have also been made for the contamination of the Deccan Traps basalt by Peng et

component reduces the Na-contents in the resulting rocks, increasing the difference with the basaltic komatiites. This is further discussed below. The mass balance calculations for incompatible elements are, however, more uncertain. There is a considerable variation in incompatible element contents of acid volcanics as shown in Table 2. In addition, there are large variations in incompatible element contents of basaltic komatiites as well (Table 6). Some of the spread may result from contamination of the komatiites listed here with crustal components with a less enriched basaltic komatiite as has been suggested by Puchtel et al. (1997) (Table 6). Given these uncertainties, a reasonable fit for incompatible elements is obtained at 15% to 20% contamination, with the exception of Rb, Ba and K. 7.4. Further constraints on the contaminant Since the crustal contaminant is the primary source for incompatible elements in SHMB, immobile incompatible elements in SHMB should help to further constrain the contaminant in Dongargarh. Ratios of two similarly incompatible elements, such as Nb/Th or Ta/Th have been used by Condie (1994) and Jochum et al. (1991) to characterize the source region of incompatible elements. In Table 7, Ta/Th values are listed for Dongargarh SHMB along with PUM, lithospheric

Table 7 Ta, Th and Ta/Th in different terrestrial reservoirs including Dongargarh rocks Ta (ppm) Mantle Primitive upper mantle (PUM) Lithospheric mantle (Spinel Lherzolite) Komatiite (Alexo) Picrite (Caribbeans)

Th (ppm)

Ta/Th

References

0.037 0.048

0.079 0.032

0.47 1.50

McDonough and Sun (1995) Jochum et al. (1989)

0.03a 0.3

0.039 0.22

0.77 1.09

Jochum et al. (1991) Re´villon et al. (1999)

Crust Bulk continental crust

0.7

5.6

0.13

Average Archean Granulite Dongargarh acid volcanics

0.52 1.85

0.065 0.060

Rudnick and Fountain (1995), Barth et al. (2000) Rudnick and Presper (1990) this work

Si-rich basalts Dongargarh SHMB Kambalda basalt (C592)

0.34 0.29a

0.079 0.046

this work Jochum et al. (1991)

a

Ta is calculated from Nb taking chondritic Nb/Ta = 17.5.

8 27

4.55 6.32

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

al. (1994). Irrespective of the exact nature of crustal contaminant involved, however, SHMB melt formation requires crustal participation and is basically a manifestation of crust –mantle interaction where both crustal remobilisation and juvenile contribution of the mantle are responsible for new crust building at the Archean– Proterozoic transition.

8. Concluding remarks The present study leads to the following conclusions. (1) SHMB cannot be generated solely from a primary mantle-derived magma. It is a product of interaction of silicic crustal melts/rocks with parent melt of komatiitic affinity and therefore manifests participation of both crust and mantle for crustal growth at the Archean –Proterozoic transition. (2) The silicic crustal contaminants in Dongargarh are low in Ta/Th ratios which is different from the Upper Continental Crust of Taylor and McLennan (1985). (3) Assimilation and fractionation of the Dongargarh parent melt are possibly decoupled. Fractionation of olivine from the mantle-derived melt has taken place in a magma chamber located in a deeper part at or near the crust –mantle boundary (f 10 kbar) prior to interaction with the contaminant.

Acknowledgements This research was supported through a fellowship by DAAD (Deutscher Akademischer Austausdienst), Bonn to S.S. during his stay in Germany. S.S. is also grateful to Prof. Sumit Chakraborty and Dr. Gu¨nter Suhr for giving critical comments and suggestions during various stages of this work. Dr. Gerd Weckwerth introduced S.S. to INAA and provided invaluable help in evaluation of INAA spectra. Dr. Markus Klein and Dieter Wolf generously helped during EPM and XRF analyses. [RR]

References Arndt, N.T., 1994. Archean komatiite. In: Condie, K.C. (Ed.), Archean Crustal Evolution. Elsevier, Amsterdam, pp. 11 – 44.

35

Arndt, N.T., Jenner, G.A., 1986. Crustally contaminated komatiiites and basalts from Kambalda, Western Australia. Chem. Geol. 56, 229 – 255. Basaltic Volcanism Study Project (BVSP), 1981. Basaltic Volcanism on the Terrestrial Planets. Pergamon, New York. Barth, M.G., McDonough, W.F., Rudnick, R.L., 2000. Tracking the budget of Nb and Ta in the continental crust. Chem. Geol. 165, 197 – 213. Brewer, T.S., Atkin, B.P., 1989. Elemental mobilities produced by low-grade metamorphic events: a case study from the Proterozoic Supracrustals of Southern Norway. Precambrian Res. 45, 143 – 158. Cribb, J.W., Barton, M., 1996. Geochemical effects of decoupled fractional crystallization and crustal assimilation. Lithos 37, 293 – 307. Campbell, I.H., 1985. The difference between oceanic and continental tholeiites: a fluid dynamic explanation. Contributions to Mineral. Petrol. 91, 37 – 43. Cox, K.G., Bell, J.D., Pankhurst, R.J., 1979. The Interpretation of Igneous Rocks. Allen & Unwin, London. Condie, K.C., 1994. Greenstones through time. In: Condie, K.C. (Ed.), Archean Crustal Evolution. Elsevier, Amsterdam, pp. 85 – 120. Condie, K.C., Crow, C., 1990. Early Precambrian within-plate basalts from the Kaapvaal Craton in Southern Africa: a case study for crustally contaminated komatiites. J. Geol. 98, 100 – 107. DePaolo, D.J., 1981. Trace element and isotopic effects of combined wall rock assimilation and fractional crystallization. Earth Planet. Sci. Lett. 53, 189 – 202. DePaolo, D.J., 1983. Comments on ’’Columbia River volcanism: the question of mantle heterogeneity or crustal contamination’’ by R.W. Carlson, G.W. Lugmair, and J.D. Macdougall. Geochim. Cosmochim. Acta 47, 841 – 844. Falloon, T.J., Crawford, A.J., 1991. The petrogenesis of high-calcium boninite lavas dredged from the northern Tonga ridge. Earth Planet. Sci. Lett. 102, 375 – 394. Fallon, T.J., Danyushevsky, L.V., 2000. Melting of refractory mantle at 1.5, 2 and 2.5 GPa under anhydrous and H2O-undersaturated conditions: implications for the petrogenesis of high-Ca boninites and the influence of subduction components on mantle melting. J. Petrol. 41, 257 – 283. Gaetani, G.A., Grove, T.L., 1998. The influence of water on melting of mantle peridotite Contributions to Mineral. Petrol. 131, 323 – 346. Green, T.H., 1995. Significance of Nb/Ta as an indicator of geochemical processes in the crust – mantle system. Chem. Geol. 120, 347 – 359. Ghiorso, M.S., Sack, R.O., 1995. Chemical transfer in magmatic processes: IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid – solid equilibria in magmatic systems at elevated temperatures and pressures Contributions to Mineral. Petrol. 119, 197 – 212. Hess, P.C., 1989. Origin of Igneous Rocks. Harvard Univ. Press, Cambridge, MA. Hofmann, A.W., 1988. Chemical differentiation of the earth: the relationship between mantle, continental crust, and oceanic crust. Earth Planet. Sci. Lett. 90, 297 – 314.

36

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37

Irvine, T.N., Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci. 8, 523 – 548. Jochum, K.P., Seufert, H.M., Spettel, B., Palme, H., 1986. The solar-system abundances of Nb, Ta, and Y, and the relative abundances of refractory lithophile elements in differentiated planetary bodies. Geochim. Cosmochim. Acta 50, 1173 – 1183. Jochum, K.P., McDonough, W.F., Palme, H., Spettel, B., 1989. Compositional constraints on the continental lithospheric mantle from the trace elements in spinel peridotite xenoliths. Nature 340, 548 – 550. Jochum, K.P., Arndt, N.T., Hofmann, A.W., 1991. Nb – Th – La in komatiites and basalts: constraints on komatiite petrogenesis and mantle evolution. Earth Planet. Sci. Lett. 107, 272 – 289. Jochum, K.P., Dingwell, D.B., Hofmann, A.W., Stoll, B., Raczek, I., Rocholl, A., Becker, S., Besmehn, A., Besette, D., Dietze, H.-J., Dulski, P., Erzinger, J., Hellebrand, E., Hoppe, P., Horn, I., Janssens, K., Jenner, G., Klein, M., McDonough, W.M., Maetz, M., Nikogosian, I.K., Pickhardt, C., Seufert, H.-M., Simakin, S.G., Sobolev, A.V., Spettel, B., Straub, S., Vincze, L., Wallianos, A., Weckwerth, G., Wolf, D., Zimmer, M., 2000. New geological standard reference glasses for in-situ microanalysis. Geostand. Newsl. 24, 87 – 133. Kelemen, P.B., 1995. Genesis of high Mg # andesites and the continental crust. Contrib. Mineral. Petrol. 120, 1 – 19. Kruse, H., 1979. Spectra processing with computer graphics. Mayguez Conf. Proc., US Department of Energy (DOE) Rep. 78421., 76 – 84. Kushiro, I., 1972. Effect of water on the composition of magmas formed at high pressures. J. Petrol. 13, 311 – 334. Kushiro, I., 1996. Partial melting of a fertile mantle peridotite at high pressures: an experimental study using aggregates of diamond. In: Basu, A., Hart, S. (Eds.), Earth Processes: Reading the Isotopic Code. American Geophysical Union, Geophysical Monograph, vol. 95, pp. 109 – 122. Lesher, C.M., Arndt, N.T., 1995. REE and Nd isotope geochemistry, petrogenesis, and volcanic evolution of contaminated komatiites at Kambalda, Western Australia. Lithos 34, 127 – 157. Lobach-Zhuchenko, S.B., Arestova, N.A., Chekulaev, V.P., Levsky, L.K., Bogomolov, E.S., Krylov, I.N., 1998. Geochemistry and petrology of 2.40 – 2.45 Ga magmatic rocks in the north-western Belomorian belt, Fennoscandian Shield, Russia. Precambrian Res. 92, 223 – 250. Lightfoot, P.C., Sutcliffe, R.H., Doherty, W., 1991. Crustal contamination identified in Keweenawan Osler Group tholeiites, Ontario: a trace element perspective. J. Geol. 99, 739 – 760. McDonough, W.F., Sun, S.-S., 1995. The composition of the earth. Chem. Geol. 120, 223 – 253. Mysen, B.O., Boettcher, A.L., 1975. Melting of a hydrous upper mantle: phase equilibria of a natural perodotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen, and carbon dioxide. J. Petrol. 16, 520 – 548. Neogi, S., Miura, H., Hariya, Y., 1996. Geochemistry of Dongargarh volcanic rocks, Central India: implications for the Precambrian mantle. Precambrian Res. 76, 77 – 91. Peng, Z.X., Mahoney, J., Hooper, P., Harris, C., Beane, J., 1994. A

role for lower continental crust in flood basalt genesis? Isotopic and incompatible element study of the lower six formations of the western Deccan Traps. Geochim. Cosmochim. Acta 58, 267 – 288. Philpotts, A.R., 1990. Principles of Igneous and Metamorphic Petrology Prentice-Hall, New Jersey. Puchtel, I.S., Haase, K.M., Hofmann, A.W., Chauvel, C., Kulikov, V.S., Garbe-Scho¨nberg, C.-D., Nemchin, A.A., 1997. Petrology and geochemistry of crustally contaminated komatiitic basalts from the Vetreny Belt, Southeastern Baltic Shield: evidence for an early Proterozoic mantle plume beneath rifted Archean continental lithosphere. Geochim. Cosmochim. Acta 61, 1205 – 1222. Readman, B.A., Keays, R.R., 1985. Archean basic volcanism in the Eastern Goldfields Province, Yilgarn Block, Western Australia. Precambrian Res. 30, 113 – 152. Rudnick, R.L., Fountain, D.M., 1995. Nature and composition of the continental crust: a lower crustal perspective. Rev. Geophys. 33, 267 – 309. Rudnick, R.L., Presper, T., 1990. Geochemistry of intermediate/-to high-pressure granulites. In: Vielzeuf, D., Vidal, Ph. (Eds.), Granulites and Crustal Evolution. Kluwer Academic Publishing, Dordrecht, pp. 523 – 550. Re´villon, S., Arndt, N.T., Hallot, E., Kerr, A.C., Tarney, J., 1999. Petrogenesis of picrites from the Caribbean Plateau and the north Atlantic magmatic province. Lithos 49, 1 – 21. Sarkar, S.N., 1957 – 1958. Stratigraphy and tectonics of the Dongargarh System, a new system in the Precambrians of BhandaraDurg – Balaghat area, Bombay and Madhy Pradesh. Journal of Science and Engineering Research. I.I.T., Kharagpur, India 1, 237 – 268, 2, 145 – 160. Sarkar, S.N., 1994. Chronostratigraphy and tectonics of the Dongargarh Supergroup Precambrian rocks in Bhandara-Durg region, Central India. Indian J. Earth Sci. 21, 19 – 31. Sarkar, S.N., Gopalan, K., Trivedi, J.R., 1981. New data on the geochronology of the Precambrians of Bhandara-Durg, Central India. Indian J. Earth Sci. 8, 131 – 151. Sensarma, S., Palme, H., Mukhopadhyay, D., 1999. Geochemical character of a SHMB-type magma from the early Proterozoic Dongargarh Supergroup, India. Eur. J. Mineral. 11, 211. Seitz, H.-M., Keays, R.R., 1997. Platinum group element segregation and mineralization in a noritic ring complex formed from Proterozoic Siliceous High Magnesium Basalt Magmas in the Vestfold Hills, Antarctica. J. Petrol. 38, 703 – 725. Sharpe, M.R., Hulbert, L.J., 1985. Ultramafic sills beneath the eastern Bushveld Complex: mobilized suspensions of early lower zone cumulates in a parental magma with boninitic affinities. Econ. Geol. 80, 849 – 871. Sun, S.-S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in Ocean Basins. Geological Society Special Publication, vol. 42, pp. 313 – 345. Sun, S.-S., Nesbitt, R.W., McCulloch, M.T., 1989. Geochemistry and petrogenesis of Archean and early Proterozoic siliceous high-magnesian basalts. In: Crawford, A.J. (Ed.), Boninites and Related Rocks. Unwin Hyman, London, pp. 148 – 173.

S. Sensarma et al. / Chemical Geology 187 (2002) 21–37 Tatsumi, Y., Ishizaka, K., 1982. Magnesian andesite and basalt from Shodo-Shima island, Southwest Japan, and their bearing on the genesis of calc-alkaline andesites. Lithos 15, 161 – 172. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications.

37

Wilson, M., 1989. Igneous Petrogenesis. Unwyn Hyman, London. Yedekar, D.P., Jain, S.C., Nair, K.K.K., Dutta, K.K., 1990. The central Indian collision suture. Geol. Surv. India, Spec. Publ. 28, 1 – 43.