S. M. Naqvi

Rb-Sr ages of late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic thermotectonic event (s)

Abstract The Archean greenstone sequence of the Dharwar craton is intruded by late- and post-kinematic granites like the Chitradurga and Toranagallu (Closepet) granites with whole-rock RbSr isochron ages of 2603 ± 28 Ma (2σ) and 2452 ± 50 Ma, respectively. The whole-rock RbSr ages of their host metavolcanic rocks show a wide variation; Bababudan, 2728 ± 102 Ma; Shimoga, 2520 ± 62 Ma; Ingaldhal, 2240 ± 50 Ma; Mardihalli, 2160 ± 174 Ma; Gadag, 2456 ± 76 Ma and Sandur, 2377 ± 646 Ma. While the ages of the Bababudan and the Shimoga volcanic suites can be related to the time of the pervasive regional folding and metamorphic events (∼ 2.7-2.6 Ga), those of the other volcanic suites from the Chitradurga and Sandur supracrustal belts indicate effects of post-Archean thermal event(s). The internal isochron ages of 2.0 ± 0.1 Ga for the Chitradurga and Toranagallu granites suggest isotopic resetting on a mineral scale about ∼ 500 Ma after their emplacement and support the evidence from the metavolcanic suites for an Early Proterozoic thermal event at ∼ 2.0 Ga in the Dharwar greenstone-granite/gneiss terrain.

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.

Archaean stromatolites from the Chitradurga schist belt, Dharwar Craton, South India

Abstract Stromatolites have been found in the cherty dolomite member of the Vanivilas Formation of the Chitradurga Group belonging to > 2600 Ma old Archaean Dharwar Supergroup, South India. They occur in the Bhimasamudra, Marikanve and Dodguni areas of the Chitradurga schist belt. The stromatolites of the Bhimasamudra and Marikanve areas are described here for the first time. The stromatolites of Bhimasamudra consist of cylindrical and terete columns, as well as pseudocolumns with concave tops developed on a wavy base. Coalescence of columns and γ-type branching have been noticed. The Marikanve stromatolites are stratiform, often grading into pseudocolumnar and dome-shaped types. Naked stromatolite columns showing β-type and markedly divergent branching are found in the Dodguni area. The Bhimasamudra and Dodguni stromatolites probably developed in an intertidal environment and the Marikanve ones in a subtidal environment.

Geochemistry of the archaean greywackes from the Northwestern part of Chitradurga schist belt, dharwar craton, South India — Evidence for granitoid upper crut in the Archaean

Turbidite greywackes of the northwestern part of the Chitradurga Schist Belt constitute a part of >2.6 Ga Cbitradurga Group of Dharwar Supergroup. They consist of a detrital assemblage of mono- and poly-crystalline quartz, microcline and plagioclase feldspar. and volcanic rock fragments. Quartz content and Na 2 O/K 2 O ratios show their quartz intermediate character. In this respect and also in their high FeO(t)+ MgO content they are similar to other Archaean greywackes. The TiO 2 content. K 2 O/Na 2 O, Al 2 O 3 /CaO+Na 2 O and Al 2 O 3 /SiO 2 ratios plotted against Fe 2 O 3 (t)+MgO, although assign an island arc environment for these greywackes, do not unequivocally discriminate between continental and oceanic island arc setting. The Chitradurga greywackes are highly enriched in Zr, Cr and Ni indicating a mixed felsic and mafic source. The contribution from contemporary volcanism in the basin could be significant. The QFL proportions indicate a dissected arc and/or recycled orogenic nature of the provenance. The chemical index of alteration varying from 58 to 63 and Al 2 O 3 /Na 2 O ratio less than 6, indicate relatively unweathered nature of source rock and chemical immaturity of the sediment respectively. The REE patterns show wen-defined negative europium anomalies which reflect granitoid upper crust in the provenance during the Archaean.