C. Manikyamba

Geochemical signatures of polygenetic origin of a banded iron formation (BIF) of the Archaean Sandur greenstone belt (schist belt) Karnataka nucleus, India

Abstract Chert, ferruginous cherts, cherty banded iron formations (BIF), shaly BIF and shales are found interbedded at the eastern part of the Sandur greenstone belt. Cherts and shales are two end members in which deposition of Fe 2 O 3 and Al 2 O 3 in variable proportions has given rise to observed large-scale variation in major, trace and REE abundances. Iron, silica and REE have precipitated from the cold marine water which was enriched in these components by hydrothermal solutions from MOR vents. However, some minor amounts of Fe, Si and REE appear to have been brought in as dissolved load by rivers. Scatter in K, Mg, Ti, Cr, Ni, Zr, Hf, V, Sc, Y, Rb, Sr, Nb and Ta abundances and ratios indicate that the clastic components have both volcanoclastic and terrigenous sources. Large-scale variation in sumREE, moderate to pronounced La enrichment, positive Eu and negative Ce anomalies are characteristic of the majority of the samples of CBIF and SBIF. REE pattern shapes of CBIF are similar to hydrothermal solutions from EPR or Rs vents. Mixing of the FeO+SiO 2 rich hydrothermal solutions with ambient sea water and clastic input has resulted in an observed large variation in LREE/HREE ratio. However, total population of the samples shows the simultaneous increase of LREE and HREE from cherts to shales through CBIF and SBIF, and obliteration of hydrothermal signature in La content and the magnitude of the positive Eu anomalies. Negative Ce anomalies appear to be the result of a reaction with photosynthetic oxygen produced by bacteria which have developed stromatolites in the underlying formation. Hydrothermal and fluvial solutions provided Si, Fe and trace elements including REE to the cold marine waters of the seas. These hydrothermal solutions were emplaced in the relatively deeper part of the basin having a reducing and neutral to alkaline environment; and due to the thermal and chemical gradient convected towards the shoreline where photosynthesis was producing O 2 . Here, on the stable shelf region below the wavebase and photic zone FeO and Ce 3+ of these solutions were oxidized and mixed with classic material of divergent origin. The Ce 4+ was precipitated in the varves like SBIF and shales. Near the shoreline the environment was intermittently oxidizing but acidic most of the time and thus the precipitation of silica took place continuously, whereas precipitation of iron occurred intermittently due to the intermittent availability of oxygen or FeO or both. Our observations suggest that the BIFs of the Sandur belt are a product of hybridity between the hydrothermal emplacement of Si and Fe and divergent clastic sediments to ambient cold ocean water. The precipitation of Fe 2 O 3 was biogenically mediated; a model combining these processes explains most of the feature of BIFs of the Sandur schist belt.

Geochemistry of FeMn formations of the Archaean Sandur schist belt, India - mixing of clastic and chemical processes at a shallow shelf

Abstract The late Archaean greenstone belts of Karnataka are unique in having manganese formations with mineable ores. An excellent example of these Mn-bearing greenstone belts is the Sandur schist belt where Fe + Mn formations are confined to the basin shelf, clearly separated from the deeper-water manganese-free BIF that accumulated at the basin margin and flanking the marine basin. Sedimentary textures and structures including stromatolites show that Fe + Mn formations have been deposited on a shallow shelf within the photic zone and above wave base. Along with quartz Fe + Mn minerals found in arenites, argillites and carbonates include ripidolite, Mg-chlorite, sericite, Mn-siderite, kutnahorite, dolomite, ferroan dolomite, psilomelane, pyrolusite, cryptomelane, haematite and magnetite. The manganese and iron contents of these rocks range up to 25% and 46%, respectively. Al 2 O 3 , TiO 2 , MgO, CaO, MnO 2 , Fe 2 O 3 (T) abundances and ratios show a wide variation. Ni, Cr, Co, Zr, V, Sc, Rb, Sr, U, Th, ΣREE, LREE/HREE, La, Ce and Eu anomalies and their binary relationships indicate that wherever the terrigenous component has increased, the concentration of elements of felsic parentage such as Zr and Hf has gone up. Elevated concentrations of Ni, Cr, Co and Sc are contributed by chlorite and other components characteristic of basic volcanic debris. The carbonates, where clastic input is minimal, are extremely depleted in REE and other trace elements. It is proposed that FeO, MnO and SiO 2 probably were added to Archaean ocean water at oceanic ridge vents. The compositional data suggest that the Fe + Mn formations of the Sandur schist belt were generated by chemical and clastic sedimentary processes on a shallow shelf that led into a deeper basin open on one side. In this environment, chemical and clastic sediments and organic matter were mixed and buried. During transgression, chemical precipitation took place at the sediment-water interface, whereas at the time of regression, these chemical sediments were buried by clastic sediments. Redox-potential differences between the watermasses on the shallower and deeper shelf, respectively, fractionated Fe and Mn. The Fe + Mn formations were deposited within the photic zone and above wave base and in a setting where the water column was oxygenated but organic material accumulated. By contrast, BIF was deposited below both wave base and photic zone in a relatively oxygen-deficient setting where formation of stable Mn-oxides was not possible. Fe + Mn formation and BIF provide an important constraint on greenstone tectonics. A shorter distance between Archaean oceanic ridge and shelf environments, namely the smaller plates model, appears to explain several features of Fe + Mn formations, BIFs and greenstone belts.

Geology and geochemistry of arenite–quartzwacke from the Late Archaean Sandur schist belt—implications for provenance and accretion processes

Abstract Detailed geological, petrological and geochemical studies have been carried out on an arenite–quartzwacke suite of rocks constituting a part of the Late Archaean Sandur schist belt in Dharwar craton, southern India for understanding the nature of provenance for these sedimentary rocks. The arenite–quartzwacke consists of rounded to sub-rounded and angular fragments of monocrystalline–polycrystalline quartz, quartzite and chert embedded in a fine-grained matrix of quartz and sericite. While arenites are more siliceous (SiO 2 , 80–92 wt.%), the quartzwacke have relatively lower silica content (ca. 69–78 wt.%). The arenites and quartzwackes have CIA values ranging from 76 to 96 which suggest intense chemical weathering. This is further corroborated by the positive correlation between Al 2 O 3 and TiO 2 in both these rock types. The ACNK modeling of arenites and quartzwackes show evidence for addition of K 2 O during later metasomatic alteration. In the ACNKFM ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during K-metasomatism and the presence of mafic rocks in the source. The high concentration of HFSE such as Zr, Hf, Nb and Ta and the trace element ratios Th/Sc, La/Sc, Th/U and Ce/Th in the arenite–quartzwacke indicate a mixed provenance. The rare earth element modeling of quartzwackes considering tonalite, granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members. A composition comprising of 25% tonalite+25% granite+50% amphibolite in the provenance appears to match with the observed range of REE patterns of quartzwackes. The presence of higher proportions of granite in the provenance is evidenced by the large negative Eu anomalies in these sediments. Field evidence and structural discordance suggest that the arenite–quartzwacke suite is an allochthonous part of the Sandur schist belt.

Late Archaean Mantle Fertility : Constraints from Metavolcanics of the Sandur Schist Belt, India

Abstract Late Archaean (2.7 Ga) mantle fertility and the processes of oceanic crust and greenstone formation have been inferred through a detailed geochemical study of the metavolcanics of the Sandur Belt. This belt is made up of two distinct lithotectonic assemblages. (1) The autochthonous sequence consists of Yeshwanthanagar Volcanic Block (YVB), Deogiri Block (DB), Western Volcanic Block (WVB), Central Volcanic Block (CVB) and Eastern Volcanic Block (EVB). (2) The allochthonous assemblages are divided into North Central Acid Volcanic Block (NCAVB), Sultanpura Volcanic Block (SVB) and Eastern Acid Volcanic block (EAVB). Autochthonous assemblage was formed as a pericontinental insitu sequence on a shelf to which the allochthonous blocks have been successively accreted along layer parallel faults. Lithological, structural, metamorphic and geochemical discontinuities are found across the different blocks. Volcanic components of these blocks comprise of ultramafic komatiites and/or cumulates, basaltic komatiites, high Mg-basalts, high Fe-tholeiites, tholeiitic dacites, andesites and rhyolites, metamorphosed upto amphibolite facies, but sometime preserving relictigneous mineralogy. REE abundances of most of the metabasalts are 10-40 times of those of chondrite and have generally unfractionated patterns with (La/Yb) N ∼1-5. Fractionated REE patterns are found in the felsic amphibolites of CVB (Hospet zone) and the intermediate-acid volcanic members present only in NCAVB and EAVB. Abundances and ratios of several incompatible diagnostic elements of comparable partioning coefficients such as Nb/Th, Hf/Sm, Zr/Hf, Zr/Yb, Sc/Yb, Nb/Yb, Nb/Th, Th/Ta, Th/Tb and others show large scale variation within and between different blocks. These ratios for the high Mg-komatiitic basalts are nearer to those of primitive mantle. The marked differences in geochemistry between the metabasalts of different blocks indicate that the entire volcanic sequence may not have been derived from the same mantle. Their compositions in general are not comparable to any of the basalts from modern tectonic settings. However a mix of 80 and 20% enriched and normal MORB is very close to their overall composition, which is defined as Archaean Oceanic Ridge Basalt (AORB). Geochemical data suggest that Sandur basalts were generated from a fertile and heterogenous mantle. This fertile mantle either was tapped at the Archaean ridges directly or enriched magma was fed to the ridges from nearby hotspots. Partial subduction of the AORB possibly gave rise to the mafic-felsic volcanics of the NCAVB and EAVB; the two subduction complexes having interbedded turbidites and other sediments derived from within the basin. FeO/MgO, Ce/Nb, Nb/U, Nb/Th and Nb/La ratios of samples having MgO >11% indicate that hot spot tectonics might also have played a role for the evolution of this belt, but the present level of information is equivocal. However, derivation of the Sandur metabasalts from a relatively fertile mantle in the form of oceanic ridge basalts seems to be a strong possibility as around 2.7-2.8 Ga ago only small fraction of the continental crust was extracted from the mantle. Subsequent compressional processes appears to have telescoped the two continental volcanic margins.

Geochemistry of PGE in mafic rocks of east Khasi Hills, Shillong Plateau, NE India

The mafic rocks of east Khasi Hills of the Meghalaya Plateau, northeastern India, occur as an intrusive body which cut across the weakly metamorphosed Shillong Group of rocks. Other than Shillong Group of rocks, high grade Archaean gneissic rocks and younger porphyritic granites are also observed in the study area. The studied mafic rocks of east Khasi Hills cover an area of about 4 km 2 and represent structurally controlled intrusion and varying grades of deformation. Structurally, these mafic rocks can be divided into massive type of mafic rocks, which are more or less deformation free and foliated type of mafic rocks that experienced deformation. Petrographically, this massive type can be classified as leuco-hornblende-gabbro whereas foliated type can be designated as amphibolite. On the basis of major oxide geochemistry, the investigated mafic rocks can be discriminated into high titanium (HT) (TiO 2>2 wt%) and low titanium (LT) types (TiO 2<2 wt%). Use of several geochemical variation diagrams, consideration of chondrite-normalized and mantle-normalized REE and PGE plots suggest role of magmatic differentiation (with almost no role of plagioclase fractionation) in a subduction controlled tectonic environment. The PGE trends of the studied rocks suggest relative enrichment of palladium group of PGE (PPGE) compared to iridium group PGE (IPGE). Critical consideration of Sm vs. La, Cu vs. La, Pd vs. La and Cu/Pd vs. La/Sm plots strongly favours generation of the parent magma at a columnar melting regime with batch melting of cylindrical column of the parent mantle to the tune of ∼25%. The characteristic PGE behaviours of the presently investigated mafic rocks of east Khasi Hills can be typically corroborated as ‘orogenic’ (discordant) type. These rocks have an enriched mantle affinity with a co-magmatic lineage and they have been generated by slab-dehydration, wedge-melting and assimilation fractional crystallization process at a continental margin arc setting.