Neoarchean microblock amalgamation in southern India: Evidence from the Nallamalai Suture Zone
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Dharwar Craton
Shales are significant sedimentary units in the Archean greenstone belts and Proterozoic Cuddapah sequence of Dharwar Craton. We present the geochemical studies of the Archean shales from the Chitradurga greenstone belt and the Proterozoic Vempalle Formation of the Cuddapah Basin to evaluate their chemical composition, weathering, provenance, and depositional conditions. The Archean shales are depleted in transition metals (Ni and Co) but enriched in V, Cr, and Sc relative to upper continental crust, while the Proterozoic shales are depleted in Cr, Co, Ni, Sc, and V compared with Post‐Archean Australian shale reflecting on negligible mafic source during their deposition. The REE patterns of these shales are uniform with moderately flat LREE and negative to positive Ce anomalies reflecting on marine conditions of their deposition. The presence of both positive and negative Ce anomalies indicate fluctuating oxic and anoxic conditions. The Archean and Proterozoic shales show negative and positive Eu anomalies, respectively, along with slightly enriched HREE. Compositional variation is observed in the Archean shales, whereas the Proterozoic counterparts were affected by sediment recycling as displayed by the Th/Sc and Zr/Sc relationship. Th/U and Th along with (Gd/Yb)n ratios display significant heavy mineral enrichment in these shales. The chemical index of alteration (CIA) and chemical index of weathering (CIW) of these shales suggest moderate to intense chemical weathering during the Archean and low to moderate weathering during Proterozoic times. The overall geochemical signatures collectively indicate granitic and tonalitic provenance for the Archean and Proterozoic shales, which were deposited in an active and passive continental margins.
Dharwar Craton
Greenstone belt
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As now known, the Archean and Proterozoic appear to have been different worlds: the geology (tectonic style, basinal distribution, dominant rock types), atmospheric composition (O2, CO21, CH4), and surface environment (day-length, solar luminosity, ambient temperature) all appear to have changed over time. And virtually all paleobiologic indicators can be interpreted as suggesting there were significant biotic differences as well: (1) Stromatolites older than 2.5 Ga are rare relative to those of the Proterozoic; their biotic components are largely unknown; and the biogenicity of those older than approx. 3.2 Ga has been questioned. (2) Bona fide microfossils older than approx. 2.4 Ga are rare, poorly preserved, and of uncertain biological relations. Gaps of hundreds of millions of years in the known record make it impossible to show that Archean microorganisms are definitely part of the 2.4 Ga-to-present evolutionary continuum. and (3) In rocks older than approx. 2.2 Ga, the sulfur isotopic record is subject to controversy; phylogenetically distinctive bio-markers are unknown; and nearly a score of geologic units contain organic carbon anomalously light isotopically (relative to that of the Proterozoic and Phanerozoic) that may reflect the presence of Archaeans (Archaebacteria of earlier classifications) but may not (since cellularly preserved Archean-age Archaeans have never been identified).
Geologic record
Acritarch
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Diachronous
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A 1.5-2 m wide fine-grained undeformed acid dyke, cutting the Early Archaean Isua supracrustal succession, was found in 1978 (D.B., J.B.). Preliminary Rb-Sr isotope measurements (F.K.) of small hand samples suggested an unexpected mid-Proterozoic age. Additional material was collected in 1979 (J.B.). Owing to weight restrictions in the helicopter, some of the samples are smaller (100-500 g) than we would normally use, but we feel justified in presenting the results since they suggest Proterozoic granitic activity in the area, which has implications for the later history of the Archaean block.
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Most of central Africa is underlain by Archaean terrains (mostly below a Phanerozoic cover), represented by high-grade gneissic complexes and by low-grade granite-greenstone belts. The lowermost Proterozoic is represented either by gneisses in mobile zones or low-grade supracrustals in forelands. The remaining Lower Proterozoic is made of low-grade supracrustal metasediments in mobile zones. Such zones developed thus almost immediately after the end-Archaean cratonization. The successive mobile zones appear to have developed in a centrifugal pattern during the Lower-Proterozoic. The mineral wealth is unevenly distributed. Only some greenstone belts have given an appreciable gold output, whereas the gneissic Archaean terrains have proven to be almost barren. Iron remains an important resource of the Archaean, as manganese is for the Lower Proterozoic. Uranium and some Cu, Co has been found in the Lower Proterozoic of respectively Gabon and Uganda.
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The beginning of the Proterozoic eon is set formally by geologists at 2.5 billion years before present. However, the transition between the Archean and the Proterozoic is not a sharp one. From about 3.2 billion to 2.5 billion years ago, rocks with a modern granitic composition made a widespread appearance in the geologic record. Prior to this time, rocks making up the Archean continents had a composition different from modern granites in several important respects. Beginning around 3.2 billion years ago in what is now Africa, and extending to 2.6 billion years ago on the Canadian shield, large quantities of modern-type granites were produced. We can collect these rocks today and date them by use of radioisotopes. How did the original Archean continents form? Why was there a transition in chemical composition of the rocks roughly halfway through the Archean? What might Earth have been like today if this eruption of new rock types had not occurred? As we see, the transformation wrought on Earth's primitive continents may have been an inevitable consequence of their increasing coverage of Earth's surface.
Early Earth
Hadean
Geologic record
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