Geochemistry of Archaean volcanic rocks from Iron Ore ...

Geochemis try of Archaean volcanic rocks from Iron Ore Supergroup, S inghbhum, eastern India

S SENGUPTA*, S K ACHARYYA*, J B DESMETH**

* Geological Survey of India, 27, Jawaharlal Nehru Road, Calcutta 700016 ** ITC, Kanaalweg 3 2628EB, Delft, The Netherlands

Mafic-ultramafic rocks of Archaean age constitute a significant component of the Eastern Indian Craton. These occur in two different modes. In the eastern belt these occur as a long, linear enclave within the Singhbhum granite and the primary banding in them is subvertical. In the more extensive western belt along the periphery of the Singhbhum granite, the disposition of the primary banding is subhorizontal.

The major rock type in both the belts is meta-basalt with minor peridotitic komatiite and basaltic komatiite occurring in the eastern belt. Rare ultramafic rocks with cumulate textures are present in both the belts. The larger volume of the basaltic rocks preclude the possibility of their being derived by fractional crystallization of the high-MgO components.

On the basis of trace element and REE characters the rocks may be classified into three groups. One of the groups shows a tholeiitic trend and include samples mostly from the eastern belt while the second consisting mostly of samples from the western belt shows a calc-alkaline trend. The third group includes samples having elemental ratios intermediate between these two groups. Zr/Nb ratios for the tholeiitic and calc-alkaline samples are different suggesting their sources to be different. The tholeiitic samples have been generated from a source having chondritic REE characters, while the calc-alkaline samples have been generated from a source with LREE enriched character. The high-MgO components in both the groups are suggested to represent high degrees of melting compared to the basalts in each group.

It is further suggested that the tholeiitic basalts have been generated relatively early from a chondritic source. Down-buckling of this material has added LREE enriched melts to the source, thereby changing its character into a LREE enriched one. Melting of a source with such changed character has subsequently produced the calc-alkaline melts. Rocks with variable but intermediate characters between these two groups have been generated as a result of contamination between these two groups.

1. I n t r o d u c t i o n

Mafic-ultramafic rocks with minor volcanogenic sediments and banded iron formation, together with several ovoid granitoid batholiths of Archaean age, constitute a significant component of the Eastern Indian Craton. Dunn (1940) identified the supracrustal rocks as belonging to one stratigraphic unit named as the Iron Ore Series. Presently this association is included in the Iron Ore Supergroup of rocks. Some of the granitoid batholiths intrusive into the rocks of the Iron Ore Supergroup have been dated to be 3.2 Ga and hence these are of Archaean age.

The chemical characters of these Archaean mafic- ultramafic volcanic rocks provide information about magma generation processes and the possible tectonic setting of their emplacement. Such data are vital for understanding the crustal evolution in any Archaean craton. In the Eastern Indian Craton petrogenesis of the sialic rocks have been studied in detail. (Baksi et al 1987; Sengupta et al 1983; Sengupta et al 1991; Saha 1994; Sharma et al 1994; Sengupta et al 1996). Com- pared to them, data on mafic-ultramafic rocks are meagre and only recently attention has beeen focussed on them (Acharyya 1993) but systematic chemical data on these mafic-ultramafic rocks are still lacking.

Keywords. Geochemistry; Archaean volcanic rocks; Iron Ore Supergroup; Singhbhum.

Proc. Indian Acad. Sci. (Earth Planet. Sci.), 106, No. 4, December 1997, pp. 327-342 �9 Printed in India 327

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Figure 1. Generalized geological map of the Eastern Indian Craton (modified from Iyengar and Murthy 1982), showing sample locations 1. Iron Ore Group of Badampahar-Gorumahishani area; 2. Iron Ore Group of Gua-Noamundi area; 3. Phyllite shale and BIF of Koira valley; 4. Granitoids of Singhbhum Bonai and Kaptipada batholiths 5. Ultramafic rocks of Sukinda and Nausahi; 6. Sediments and volcanics belonging to Simlipal, Dhanjori and Dalma Groups; 7. Intracratonic metasediments equivalent to Dhalbum Formation; 8. Unclassified cover sediments and gneissic rock 9. Sandstone and conglomerate occurring as younger cover (equivalent to Kolhan); 10. Intrusive granite, granophyre and associated rhyolite.

In this paper we present chemical data for mafic- ultramafic rocks from all the major supracrustal successions of the Eastern Indian Craton. Using field observations and textural data in combination with chemistry, the petrogenesis of these rocks are discussed along with the relationship between the volcanic rock suits of different composition.

2. G e o l o g i c a l s e t u p a n d l o c a t i o n o f s a m p l e s

The mafic-ultramafic rocks together with other members of the supracrustal package occur in two different modes within the Eastern Indian Craton. One type occurs within the Singhbhum Granite batholith, near its eastern margin. A long linear belt

Geochemistry of Archaean volcanic rocks 329

of mafic-ultramafic rocks occurs as enclave extending from south of Tatanagar in the north to Badampahar in the south. This major supracrustal succession covering territories within both Bihar and Orissa state is known as the Gorumahishani-Badampahar belt. South of Badampahar this belt bifurcates and both arms continue, one to the west and the other south, towards Baula-Nansahi as trains of smaller xenoliths within the Singhbhum granite, (figure 1). The other belt, which is more extensive, occurs along the western and southern fringe of the Singhbhum Granite. This extends from Gua~Barajamda up to Sukinda where it often crops out from below an unconformable cover of grits and sandstone (figure 1). According to earlier studies, these rocks are folded into a synclinorium popularly known as the Iron Ore Synclinorium (Dunn 1940), defined by the U-shaped pattern of the banded hematite jasper (BHJ) band in the map (figure 1).

The primary banding in the mafic-ultramafic rocks of the eastern belt and the bedding in the associated banded iron formation are subvertical. The rocks are metamorphosed and often a schistosity is developed, which is always parallel to the primary banding. As a result of the subvertical nature of the primary banding a considerable part of the stratigraphic column is exposed even in the narrow width of the belt. In the Gorumahishani-Badampahar sector of the belt, three separate volcanic flows have been recorded; the bot- tom part of each flow is marked by ultramafic rocks and the top is represented by volcanogenic sediments and banded iron formation. The actual number of flows in the entire belt is likely to be much more. Complete succession is not preserved due to the intrusive nature of the granitoids on either side of the belt. At a few locations in the belt, ultramafic rocks with cumulate texture is observed which indicates that crystal settling took place in the volcanic flows.

The width of the western belt is much larger (figure 1). The BIF occurrences in this belt occupy the crests of the high hills, and overlie the mafic rocks occupying the low ground. The volume of BIF in this belt is significantly larger than in the eastern belt. The BIF is everywhere gently dipping. The mafic rocks are metamorphosed as indicated by the absence of any primary mineralogy, but a metamorphic foliation has not been noticed. At a few locations subhorizontal flow banding defined by vesicular and nonvesicular layers accompanied by systematic variation in the size of the amygdules has been recorded. As a result of the subhorizontal disposition of the supracrustal package in this belt, only a small part of the stratigraphic section of the mafic rocks is exposed and no ultramafic component is seen. The nature of compositional variation in the lava succession could not, therefore, be deciphered except near Sukinda, where samples from a few vertical boreholes have been used to get an idea of variation in the lava pile up to a certain depth.

Samples have been collected from the eastern belt to assess the compositional variation along its length and across the stratigraphic section. Seven samples have been included from Suriagora and Sulaipat. Fifteen more samples from the remaining part of the Gorumahishani-Badampahar belt have been included in this study. One sample has been selected from an isolated xenolith southwest of Badampahar for the sake of comparison with the rocks of the eastern belt.

From the western belt samples have been collected representing the entire exposed extent of the mafic rocks. Twelve samples have been collected from four boreholes drilled up to l l0 .00m, northwest of Sukinda. This displays an interlayered sequence of mafic and ultramafic rocks underlying the BIF of Daitari hill (figure 1). Ten samples have been collected from the south eastern part of the synclinorium, representing the volcanic pile underlying BIF of Gandhamardan, west of Keonjhar. Ten more samples have been collected from regions south of the synclinorium, where the best preserved subhorizontal primary layering has been recorded. Two samples have been selected from regions west of the synclinorium and underlying the BIF unit of the Gua ranges.

The differences in the general geological setting of the two belts have been interpreted by several workers to mean a difference in age of the iron ore hosting rocks of the two belts (Acharya 1984; Chakraborty and Majumdar 1986). According to another school the eastern belt is correlated with a part of the western belt, while some lava exposures in the western belt are interpreted to be o f Proterozoic age (Saha 1994). However, we have observed no difference among the lavas of the western belt. They can be mapped conti- nuously and the contact between the supposedly older and younger lavas could nowhere be deciphered. The broad petrological characters in the two belts are the same. The only significant difference is in the structural disposition of the rocks. In both the belts, at several locations, granitoid rocks of trondhjemitic and tonalitic composition are observed to intrude the supracrustals. Granitoids from three such locations have been isotopically dated to be 3.2 Ga (Moorbath et al 1986; Sengupta et al 1991; Sharma et al 1994). Thus all the rocks in the two belts are older than 3.2 Ga. Whether there is any difference in the relative age of the rocks in the two belts cannot be ascertained with the present data.

3. P e t r o g r a p h y

The ultramafic rocks from both the belts are almost invariably altered. They consist of plumose aggregate of tremolite and actinolite. The long prismatic and fibrous grains show braided habit in places. The braids are aligned to define a schistose structure in samples from the eastern belt. Though primary

330 S Sengupta et al

mineralogical characters are totally absent in all the ultramafic rocks, rare samples showing spinifex texture defined by long, bladed crystals forming triangular geometric pattern have been reported from the eastern belt (Acharyya 1993). The bladed crystals are interpreted to be original olivine/pyroxene but are at present totally replaced by tremolite and actinolite. On the basis of their texture these rocks are identified to be ultramafic volcanics. In addition to these, olivine rich rocks showing cumulate texture are occasionally present in the eastern belt and the southernmost portion of the western belt (Sukinda). These are coarse grained and relict olivine, augite and enstatite are found in them indicating their peridotitic composition.

Some relatively coarse grained mafic members are present in both the belts. They form conformable layers with the other lithological members of the sequence. These are relatively less altered and consist of augite and plagioclase as dominant minerals. The intragranular space is filled with quartz and plagioclase showing intergrowth texture. These are interpreted to be cumulate gabbro and the intergrowth assemblage is interpreted as products of crystallization from intercumulus residual liquid. Very rarely rounded grains of altered olivine are present in these gabbroid rocks.

The most dominant mafic component is fine grained. On the basis of intercalation of chert and BIF together with presence of amygdules and pillow structures in some of them, they are interpreted to be mafic volcanics. A clue to the original mineralogical composition of these rocks comes from traces of relict mineralogy. The mafic volcanic rocks are observed to be dominantly pyroxene-phyric with some plagioclase phenocrysts. The groundmass is turbid looking and is a fine grained aggregate of various secondary minerals. Part of the groundmass is possibly altered glassy mesostasis and the diverse secondary mineralogy reflects variation in the original composition of the rocks. The present mineral assemblage suggests that the rocks have been metamorphosed to low greenschist facies.

Along the road section between Keonjhar and Pallahara a variolite like feature is seen in the mafic volcanics at a few locations. In the dark coloured mafic rocks, globular bodies of relatively lighter colour occur individually or in clusters. The maximum size of the globules is up to five centimeters in diameter. Fused globules with necking are quite common suggesting that they were in a molten immiscible state at a certain stage. The globules are made up of relatively coarser grained material compared to the host mafic rock. The mineralogy is similar to that of the host rock except for the higher feldspar content in the globules, which is possibly responsible for their relatively lighter colour (Acharyya 1993). The fused clusters of felsic variolites usually taper

downward suggesting upward movement of the buoy- ant felsic liquid.

4. G e o c h e m i s t r y

4.1 Analytical techniques

The major element oxides (table 1) for fiftyseven samples were determined by XRF in the Chemical Laboratory of the Geological Survey of India, Eastern Region, Calcutta. FeO values were determined separa- tely by the wet chemical method. The elements Ba, Sc, P, Ti, V, Nb, Rb, Sr, Co, Ni, Y and Zr were determined by XRF in the Analytical Geochemical Labora- tory of Utrecht University. The rare earth elements, Ta, Hf, Th and Cr were determined by instrumental neutron activation analysis at the International Reactor Institute, Delft. Trace element data for thirty six samples are given in table 2. The Ta abundances in all the samples were too high, which may be due to the samples being powdered in a tungsten carbide mill. The Ta values are thus not discussed further. Some elemental ratios used for classifying the rocks and for petrogenetic interpretations are given in table 3.

4.2 Geochemical results

All the samples analyzed display metamorphic assem- blages characteristic of low greenschist facies. It is therefore, likely that redistribution of their chemical constituents, as recorded in many other ancient volcanic rocks (Jolly and Smith 1972), has occurred. Prior to using the chemical data for discrimination and petrogenetic interpretation, it is, therefore, necessary to identify the type and intensity of the chemical redistribution in the present samples. Following Beswick and Soucie (1978), the logarithms of different major oxide molecular proportion ratios for the samples were plotted (figure 2).

The present samples, show trends which are comparable with and parallel to those for Phanerozoic rocks, though there is considerable scatter (figure 2). In contrast to the samples of mafic volcanic rocks, all the ultramafic samples plot farthest away from the trend, suggesting that these are the most affected and their chemistry does not reflect primary characteristics. In the log S i Q / K 2 0 versus log A1203/K20 and log CaO/K20 versus log A1203/K20 plots, the present samples show minor shift from the Phaner- ozoic "bands" towards the log A1203/K20 axis. In the three other plots which do not use the A1203/K20 ratios, most of the samples plot within the Phaner- ozoic "bands". It is likely that A1203 remained immobile during metamorphic-metasomatic reactions and reflect the primary characteristics of the samples while abundances of other elements used have been disturbed.

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The SiO2 abundance in most of the samples are higher than expected in mafic-ultramafic rock (table 1). The other major elements like CaO, total FeO, MgO and MnO have also been affected uniformly. The effect on total FeO is reflected in the FeO/Fe203 ratios observed in the present samples. It is difficult to ascertain whether MgO in all the samples has been as affected as the FeO. The general range of values (table 1) suggests tha t MgO remained closer to primary abundances and the small effect on F m plots (figure 2) is likely to be contr ibuted by the FeO variation.

The "Al tera t ion Index" suggested by Hashiguchi et al (1983) was also calculated for the present samples. The range suggested for fresh tholeiitic and arc re la ted mafic volcanic rocks are 36 • 8 and 34 + 10 respectively. All the ultramafic samples have Alterat ion Index beyond the specific range. Among the mafic rocks, samples 1, 2, 3 and 4 from Sukinda, samples 15, 26, 28 and 30 from the remaining par t of the western belt and samples 35, 38, 39, 43, 50 and 53 from the eastern belt have values higher t han the range specified for fresh samples (Hashiguchi et al 1983). The remaining samples of mafic rocks are inferred to have major element abundances close to p r imary compositions.

The corrections recommended for Fe203, to take care of the effect of metamorphism (Hughes and Hussey 1976) have been adopted for present samples. Mg-values have been calculated on the basis of the corrected values of FeO and Fe203 and are included in table 1. The same corrected values are used for the cation proport ion plot (Jensen 1976) on the basis of which mafic metavolcanic rocks may be efficiently

classified. In the Jensen cat ion plot (figure 3), the major i ty of the mafic rocks plot in the field of tholeiite showing a t rend of iron enr ichment while a minor percentage plot in the calc-alkaline field. Four of the samples plot in the field of basaltic komatii te, while the five cumulates and three o ther metavolcanics plot in the field of ultramafic komati i te (figure 3). Com- parable association of tholeii t ic and calc-alkaline mafic rocks are repor ted from late Archaean and early Proterozic volcanic rocks from Kaapvaal cra ton (Crow and Condie 1988, 1990), from late Archa~an basalts f rom Pi lbara cra ton (Nelson et al 1992) and from the late Archaean volcanics from the Abitibi greenstone belt (Blum and Crocket 1992). In the AFM diagram all the samples plot in the tholeiitic field showing two paral lel t r ends of i ron enr ichment (figure 4). Samples from the eastern and the western belt are generally separated from each other along these two different t rends of evolution.

Among t race elements, the large ion lithophile elements are potent ia l ly mobile during low grade me tamorph i sm (Hellman and Green 1979; Lesher ct al 1986). Several geochemical studies have also shown the relat ive stabil i ty under such conditions of high field s t rength elements and certain transi t ional metals (Pearce and Cann 1973; Winchester and Floyd 1976). The R E E are generally considered to be immobile, though Ludden et al (1982) and Nys t rom (1984) have demons t ra ted the potent ia l mobil i ty of light rare ear th elements (LREE). However, the REE plots for the present samples are found to be regular and smooth, features which are considered to suggest the unaffected charac ter of R E E (Sun and Nesbitt 1978). Thus the R E E d a t a for the present samples together

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wi th Ti, V, Cr, Ni, Zr, Nb, Y, Hf and T h are considered to represent their p r imary charac te r and are used for petrogenet ic in terpreta t ion.

The t ransi t ional e lements show a wide range of variat ion. Cr and Ni values are marked ly high in samples of basaltic komatiite and ultramafic komatiite. Values for both the elements in the cumulate samples are markedly higher compared to the volcanics. A general feature is the overall higher values for these elements in the eastern belt compared to the western belt. Only the samples of volcanics from

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Figure 4. A-F-M plot for samples of mafic ultramafic rocks from the Eastern Indian Craton. Symbols same as in figure 3.

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these elements with differentiation. In the case of V and Sc, the plots are scattered. On the basis of these elements, the present samples cannot be subdivided into groups. Therefore, it may be interpreted that all the samples underwent similar fractional crystallization controlling the abundance of Cr and Ni in them.

Cr, together with other immobile elements Zr, Y and Ti has been used successfully by Davies et al (1979) to recognize magma trends in volcanic rocks. The present samples are plotted in a Zr + Y, TiO2 • 100 and Cr triangular diagram (figure 5). A number of samples from the western belt plot along the Archaean calc alkaline trend while most of the samples from the eastern belt together with a few from the western belt plot along the Archaean tholeiitic-magnesian trend. The remaining samples are distributed in the field between these two trends (figure 5). This plot confirms the presence of both tholeiitic and calc alkaline trends among the present samples, which has been observed also in the major element data (figure 3). This also indicates three broad groups in the samples.

The high field strength elements (HFSE) usually show a small range of variation (table 2) with a few samples showing higher values. The same samples are depleted in transitional elements suggesting that these may be differentiated products. In contrast to the major elements and transitional elements, the HFSE and REE, particularly certain elemental ratios, can effectively discriminate between different cogenetic groups among the samples. The relevant ratios on the basis of which the different groups have been identified are presented in table 3. It would be observed that the different groups include samples from both the belts. One of the groups (Group I) includes samples 14, 31, 40, 44, 45, 35, 42, 38 and 32. This particular group is characterized by the lowest

total REE abundance among all the samples with the [Ce/Yb]N ratios lower than or very close to one. These samples show a fiat chondrite normalized pattern (figure 6). It would be noted that most of the samples have [La/Sm]N ratio less than one or close to one suggesting a more or less unfractionated LREE pattern. Most of the samples show a positive Eu- anomaly. These are characterized by Zr/Y ratio varying between 2.8 and 4.3, which is lower than the remaining samples. The Ti/Zr ratio for this group varies between 48.05 and 80.77 which is higher than the remaining samples. The Ti /Y ratio in this group is the highest and always exceeds 200. Samples 32 and 38 show slightly different patterns (figure 6) characterized by fractionated LREE [La/Sm]N. The [Ce/Yb]N ratios of these two samples are also significantly higher. Sample 38 also has the highest Zr/Y ratio. They have been placed in this group tentatively with the other samples in view of their comparable Zr/Nb ratio and the high Ti /Zr ratio of sample 32. One sample of basaltic komatiite is included in this group (sample 35).

The second group (Group H) includes samples 1, 2, 3, 4, 13, 23, 24, 28, 29 and 30. This is a compact group with very little variation in their normalized REE curve (figure 7(A)). They have higher total REE abundance compared to Group I samples and significantly higher [Ce/Yb]N ratios (table 3). The [La/Sm]N ratio is always much higher than in Group I indicating the fractionated LREE character of this group. All samples except the one most differentiated, have fiat HREE curves (figure 7A). Some of the samples show strong negative Eu-anomaly, a character absent in Group I, the Zr/Y ratio in this group varies between 5.2 and 6.3 which is higher than in Group I. The Ti /Zr ratio for this group is almost half of that in Group I.

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The Ti /Y ratio in samples of this group is always less than 200. The V/Zr ratio is always less than two and significantly lower than in Group I. The Zr/Nb ratios of Group II samples are generally higher than in Group I. On the basis of the elemental ratios discussed above, Groups I and II can be discriminated efficiently. In the T-Y-C plot (figure 5) the same set of samples could be separated into groups and the samples plotting along the Archaean tholeiite-magnesian trend are those belonging to Group I. Samples falling along the Archaean calc-alkaline t rend (figure 5) belong to Group II.

Samples 16, 17, 18, 19, 27 have similar total REE abundance and [Ce/Yb]N ratios as Group II rocks. The LREE fractionation of these samples is also comparable with that in Group II. In the normalized REE plot the slope in the LREE part is accompanied by flat HREE (figure 7(B)). The most primitive sample of this group has a strong positive Eu- anomaly, which decreases with increasing differentiation and finally into a negative Eu-anomaly in the most differentiated sample. The Zr/Y and Ti/Zr ratios in the samples range from values comparable to those in Group II to higher. The V/Zr ratios also match those of Group II. In contrast, the Ti /Y ratio in this group is sometimes less than and sometimes greater than 200, matching

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o 0 E

Figure 8. Chondrite normalized REE plots for samples with characters intermediate between Group I and Group II.

the character of both the groups. The Zr/Nb ratio is intermediate between those of the two groups.

Samples 26, 37, 39, 43, 47 and 48 forming the third group have total REE abundance covering the entire range from Group I to Group II. Samples 37 and 39 are two komatiite samples, which have the lowest total REE in this group. The [Ce/Yb]N ratio in this group is lower than in Group II but considerably higher than in Group I. In this aspect samples 37 and 39 differ from the other basaltic komatiite (sample 35) belonging to Group, I. In the absence of incompatible element ratios for these two samples further comparison with sample 35 is not possible. The (La/Sm)N ratios for all the samples in this group are greater than one in contrast to that of Group I. The Zr/Y and Ti/Zr ratios in this group are intermediate between values of Group I and Group II (table 3). The Ti/Y ratio shows characters of both the groups while the V/Zr ratio is similar to that of Group I. Normalized REE curve for one of the cumulates is plotted together with the two komatiites (figure 8). It shows a similar pattern but a relatively lower total REE abundance.

The remaining samples 15, 25, 33, 34 and 46 have variable total REE. The [Ce/Yb]N and [La/Sm]N ratios of all the samples except 25 is comparable to values in Group II. In the normalized REE plot they show a fractionated LREE and flat HREE (figure 9). The HREE portion of the curve for sample no. 46 is different, showing fractionated HREE pattern (figure 9). The Zr/Nb ratios in these samples vary, covering the range from Group I to Group II. Sample no. 25 is once again different from the rest and pre- serves the highest Zr/Nb ratio among all the samples. This value is significantly higher than in the remaining samples and compares with values in calc-alkaline suite of basalts (Pearce and Cann 1973). All these samples show negative Eu-anomaly and the anomaly


I 0 0

5 0

IO

Lo Ce Nd Srn Eu Tb Yb Lu

Figure 9. Chondrite normalized REE plots for samples with characters intermediate between Group I and Group II.

increases with increasing differentiation suggesting that plagioclase feldspar has been an important fractionating phase. The Zr/Y ratios range between values in Group I to values in Group II with sample no. 25 showing a value comparable to Group I values. It also has the highest Ti /Zr ratio, comparable to that in Group I while the remaining samples have values ranging between Groups I and II values. The Ti /Y ratios in the samples are variable and most are comparable with Group I while V/Zr ratios range between the two groups (table 3).

On the basis of the foregoing analysis, it may be concluded that Groups I and II basalts are distinctly different and have characteristic elemental signatures on the basis of which they may be easily discrimi- hated. These two groups represent tholeiitic and calc alkaline trends established on the basis of some other elemental parametres (figure 5). The remaining samples have geochemical characters in termediate between these two groups and preserve the entire range of variation. On the basis of REE characters these appear to be close to the Group II rocks characterized by their fractionated LREE pattern which is in contrast to the unfractionated LREE pattern characteristic of the Group I samples. The two komatiitic samples (37 and 39) and the cumulate (sample 36) appear to have characteristics closer to Group II rocks and represent the high-MgO members in the group.

5. Discuss ion

The presence of cumulates associated with the mafic rocks at least in certain sectors in the eastern belt and

in Sukinda, indicates that fractional crystallization has played a major role in the evolution of these marie rocks. The cumulate rocks of the eastern belt are closely associated with spinifex textured high magnesian volcanic rocks, and possibly represent cumulates from the lower levels of komatiite flows (Beswick 1982). In contrast the cumulates of Sukinda are associated with basaltic rocks of near primary composition as indicated by their high Cr and Ni contents (table 2).

In the eastern belt, where high-MgO volcanic members are present, they are volumetrically oversha- dowed by basalts having 6 to 8 per cent MgO. It therefore appears unlikely that the high-MgO rocks, which include the spinifex textured variety, fractionated to produce basalts with lower MgO. In the absence of crucial data on incompatible element ratios for samples of peridotitic komatiite from the eastern belt (samples 37 and 39), it was not possible to ascertain their genetic relation with the low-MgO basalts of this belt. However, the REE characters of these low- MgO basalt from the eastern belt are significantly different fl'om those of the peridotitic komatiite (figures 6 and 8).

The peridotitic komatiites represent a very high degree of partial melting of the mantle source (Green 1975; Nesbitt and Sun 1976). If neither garnet nor a large amount of clinopyroxene is left in the residue or settle out during fractional crystallization, the REE patterns of the high-MgO members could be almost identical to that of the source. The two samples of peridotitic komatiite have a fractionated LREE pattern (figure. 8) and could have been produced from a source enriched in LREE. The only cumulate sample analyzed from the eastern belt (sample 36) has an identical REE pattern with a lower total REE abundance than the peridotitic komatiite. This suggests that the dominant mineral phases separating would be olivine and orthopyroxene and the extremely low mineral-melt Kd for REE of these minerals (Kay and Gast 1973) will not cause any change in the REE character of the residual melt. Less than twenty per cent clinopyroxene separating would also not affect the REE pattern (Sun and Nesbitt 1978). The REE pattern of the peridotitie komatiite is comparable to Group I, class 3 komatiites of Jahn and others (1982) characterized by LREE enrichment. These are common in Archaean greestone belts in Canada, Finland, Western Australia and Zimbabwe-Rhodesia (Arth et al 1977; Whitford and Arndt 1978; Jahn and Sun 1979; Jahn et al 1980).

The spinifex textured basaltic komatiite (sample 35) has an REE pattern significantly different from the peridotitic komatiite. It is characterized by an unfraetionated LREE and a [La/Sm]~,- ratio of 1.06. This could neither have evolved from similar source as the peridotitie komatiite, nor could it have been evolved through fractional crystallization from those rocks. The [La/Sm]i ratio of this rock is comparable


to the ratio in Archaean spinifex textured peridotitic komatiite from different parts of the world (Jahn et al 1974; Arth and Hanson 1975). The basaltic komatiite has lower Cr and Ni abundances compared to the peridotitic komatiite, indicating it to represent com- paritively evolved melt. It is suggested that a genetically related peridotitic komatiite with [La/Sm]N ratio close to one, represents the parent from which the basaltic komatiite has evolved through fractional crystallization. The present work missed such samples.

Most of the low-MgO basalts of the eastern belt and one from the western belt have REE character and particularly the [La/Sm]N ratios comparable to that of the basaltic komatiite. Some of the incompatible element ratios are also comparable (table 3). It is argued that these basalts have evolved through fractional crystallization of a parent melt comparable to the basaltic komatiite. In order of progressive evolution the samples can be arranged as follows: 45 ~ 44 ~ 42 ~ 1 4 --~ 31. The Zr abundances in these samples also progressively increase from sample 45 to sample 31 confirming the role of fractional crystallization. A progressive increase in total REE abundance, without significant change in [Ce/Yb]N ratio is also noticed. At the end of the trend, the [Ce/Yb]N ratio has also increased suggesting a significant amount of clinopyroxene separation at this stage [Sun and Nesbitt 1978]. This group of samples has been designated Group I and show a tholeiitic trend.

The Ti/Zr ratios for all the basalts from the eastern belt are 0.8 to 0.7 times the chondritic values. The constancy of this ratio confirms the cogenetic nature of these samples and indicates that the incompatible elements have remained unchanged by the metamorphism affecting the rocks. The lower Ti/Zr ratio in the samples compared to chondrite suggests fractionation of Ti due to clinopyroxene and oxide phases separating from the melt (Nesbitt and Sun 1976). The Ti /Y values are close to the chondritic value of 256, which is possibly due to both elements fractionating at the same rate. The Zr/Y ratio in these samples are higher than a chondritic value of 2.5 which further confirms the control of clinopyroxene separation on Y. The Zr/Nb ratio is marginally lower than chondritic value of 16 (Graham and Mason 1972). The Zr /Nb ratio is a characteristic source indicator and is unlikely to change due to the fractionation process (Bougault et al 1980; Saunders 1984). The deviation of Zr/Nb ratio from chondritic value is, therefore, inferred to reflect a source character.

On the basis of the same elemental ratios used in the foregoing discussion, a second group of basalts designated Group II has been identified. All the basalts in this Group are from the western belt. The Ti /Zr in this group is uniform and is less than half of that in Group I. The uniformity of this ratio and other

incompatible element ratios is inferred to indicate cogenetic nature of the samples included in this group. The Ti /Y ratio in this group is consistently much lower than the chondritic ratio. The V/Zr ratio for both Group I and II basalts are lower than the chondritic ratio and it is significantly lower in Group II compared to Group I. It is interpreted that during the generation of Group II basalts, a substantial amount of V was held in a residual phase. A phase such as pyroxene with Kd for V close to one is necessary to account for such low values (Bougault and Hekinian 1974). The Zr/Nb ratio in Group II basalts are higher than in Group I and also higher than chondritic values. This suggests that Group II basalts were produced from a source different in character from that of Group I basalts.

These basalts show a common characteristic namely a fractionated LREE and they also have significantly higher [Ce/Yb]N ratio compared to the Group I basalts. The fractionated LREE character is the most striking REE feature of this group. It is difficult to envisage, how these basalts could be derived from sources similar to that producing basalts with unfractionated to marginally depleted LREE (Saunders 1984). Therefore, it may be stated with certainty that Group II basalts were produced from a different source, which is also indicated by their different Zr/Nb character, compared to Group I basalts. The two peridotitic komatiite samples from the eastern belt have fractionated LREE character and on this basis it is predicted that similar rocks might represent the parent magma from which Group II basalts evolved. Among'tl~ese basalts a progressive decrease of transitional elements, accompanied by an increase in Zr is observed. This is also accompanied by an increase in the total REE abundance with only small changes in the [Ce/Yb]N ratio. It is suggested that fractional crystallization was operative, controlling these changes and clinopyroxene fractionation played a minor role [Sun and Nesbitt 1978]. In order of progressive evolution the samples can be arranged as follows 4 ~ 3 ~ 2 ~ 29 ~ 28 ~ 13 ~ 23 --* 24.

The remaining samples have a combination of characters between Group I and II. In the T-Y-C plot (figure 5) these samples plot in the area between the tholeiitic and clac-alkaline trends followed by the Group I and Group II basalts respectively. On the basis of Zr /Nb ratio it can be observed that this group includes rock both from Group I and Group II sources, the latter being dominant. It is argued that such mixed characters have been generated either by magma mixing or by contamination of one group by the other. The Zr/Y versus Zr plot for all the samples (figure 10) show a scatter. It is observed that the two groups of basalts show different trajectories of fractional crystallization models and the remaining samples occupy fields in between, confirming that the Group I and Group II basalts are genetically


+

• ' o ' ' ' ' ' " ' ' '

50 100 200 300 400

Zr

Figure 10. Zr/Y versus Zr plots of mafic rocks from the Eastern Indian Craton. Symbols same as in figure 3.

different and follow separate trends of evolution through similar processes.

The greater volume of the basalts compared to the high-MgO volcanics precludes that the former evolved through fractional crystallization of the latter. It is therefore, suggested that in both the groups, the high MgO components represent a high degree of melting, while the low-MgO components are produced at lower temperatures and by lower degrees of partial melting. In Group II the peridotitic komatiite samples have a lower [La/Sm]N ratio compared to the basalts, confirming that they represent higher degree melts compared to the basalts.

The REE character for the Group I rocks suggests a chondritic affinity for the source. In contrast, the source for Group II basalts is distinctly enriched in LREE and other incompatible elements. For similar rocks from Onverwacht Group, South Africa, it has been suggested that the source was metasomatized by LREE-rich fluids shortly before the melting events (Jahn et al 1982). The close and distinctive spatial association of the two groups of rocks in the Eastern Indian Crato~ is important and their generation has to be explained within the framework of a common model. In a model suggested by Nesbitt and Sun (1976) it is argued that melting would increase as the crystal-melt mush moved upward and at a certain stage it is likely that a fraction of the melt would move ahead. It is also likely that this first separated melt would be enriched in LREE. After separation of this first melt, the residual mantle material would keep its upward motion and further extensive melting at shallow depth would produce the high-MgO components. The two liquid sets evolved on separate lines through fractional crystallization at shallow depths. It is appa- rent from the present samples that fractional crystallization has occurred at shallow depths because

plagioclase was a major crystal phase causing a systematic change of Eu-anomaly with fractionation.

Alternatively it may be argued that the first melt to be produced is by a high degree of melting of mantle with chondritic character. These early mafic rocks were down buckled and carried down to mantle depths in a regime comparable to present day subduction zones. Such a process added fractionated melts from the down buckled lithosphere, changing the source character into an LREE enriched one. Any melt generated from this source in the continuous event would have characters comparable to the Group II basalts. These subsequent melts would get con- taminated by the earlier high-MgO components producing basaltic rocks with mixed elemental ratios. According to this model the dominant rocks of the eastern belt would be older than those of the western belt and precision isotopic studies can only resolve the difference in age between these two groups of rocks. The variolite like globular features recorded from some part of the western belt possibly represent the differentiated more felsic liquids from the subsequent melt that stayed immiscible due to the melt composition having changed due to contamination. These felsic liquids might have pooled to form primordial sialic crust of the region. The western belt being such sites with an early generated sialic crust had no chance to downbuckle and remains a fiat dipping area.

6. C o n c l u s i o n s

The mafic-ultramafic rocks of the Eastern Indian Craton, associated with banded iron formation, are all older than 3.2 Ga. They occur dominantly in two different modes and the age relation between the two modes is uncertain. All the rocks are metamorphosed but the high field strength elements, some transitional elements and the REE have remained unaffected and they can be used for petrogenetic interpretations.

In both the modes basalts are volumetrically most dominant, overshadowing the high-MgO components occurring in the eastern belt which include peridotitic komatiite and basaltic komatiite. It is unlikely, therefore, that the voluminous basalts have evolved through fractional crystallization from these minor high-MgO parents.

Both tholeiitic and calc-alkaline trends are regis- tered among the rocks. Rocks from the eastern belt are commonly tholeiitic and rocks from the western belt are mostly calc-alkaline. Such two fold grouping is also clear in terms of their trace element parameters. Rocks showing these two trends are markedly different in their incompatible element ratios and REE characters. The Zr /Nb ratio reflecting character of the respective source is also different for the two groups. A large number of samples from both the belts have characters intermediate between those of the two

Geochemis t ry of Archaean volcanic rocks 341

g roups . I t is s u g g e s t e d t h a t such g e o c h e m i c a l f ea tu re s h a v e r e s u l t e d due to c o n t a m i n a t i o n of one g r o u p b y t h e o the r .

T h e tho le i i t i c s a m p l e s h a v e been g e n e r a t e d f rom a source h a v i n g c h o n d r i t i c R E E cha rac t e r s . T h e chemical v a r i a t i o n w i t h i n t h e b a s a l t i c m e m b e r s of t h e g r o u p is c o n t r o l l e d b y f r a c t i o n a l c ry s t a l l i z a t i on , a n d a t a c e r t a i n s t a g e c l i n o p y r o x e n e becomes a s ign i f i can t c r y s t a l l i z i ng phase . In c o n t r a s t t h e g r o u p s how ing t a l c - a l k a l i n e t r e n d has a f r a c t i o n a t e d , L R E E e n r i c h e d pattern and has been generated from an LREE enriched source. Among basaltic samples within this group the evolution is controlled by fractional crystallization of clinopyroxene and plagioclase. The high- MgO members in both the groups are products of extensive melting and retain the chemical characters of the respective source. The low-MgO basalts in the two groups have been produced at lower temperature by lower degree of partial melting.

The Group I basalts are suggested to have been generated relatively early by the melting of mantle with chondritic character. Down buckling of this material has added LREE enriched melts to the source, thereby changing its character into an LREE enriched one. Melting of the source with such changed character has produced the Group II basalts. Such a model implies that the rocks of the western belt are relatively younger than most of the rocks of the eastern belt, though the actual age difference cannot be ascertained without isotopic data. On the basis of the observed difference in source character between the two groups the alternative process suggested by Nesbitt and Sun (1976) is discounted.

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