Magmatic thickening of crust in non–plate tectonic settings initiated the subaerial rise of Earth’s first continents 3.3 to 3.2 billion years ago

When and how Earth's earliest continents—the cratons—first emerged above the oceans (i.e., emersion) remain uncertain. Here, we analyze a craton-wide record of Paleo-to-Mesoarchean granitoid magmatism and terrestrial to shallow-marine sedimentation preserved in the Singhbhum Craton (India) and combine the results with isostatic modeling to examine the timing and mechanism of one of the earliest episodes of large-scale continental emersion on Earth. Detrital zircon U-Pb(-Hf) data constrain the timing of terrestrial to shallow-marine sedimentation on the Singhbhum Craton, which resolves the timing of craton-wide emersion. Time-integrated petrogenetic modeling of the granitoids quantifies the progressive changes in the cratonic crustal thickness and composition and the pressure–temperature conditions of granitoid magmatism, which elucidates the underlying mechanism and tectonic setting of emersion. The results show that the entire Singhbhum Craton became subaerial ∼3.3 to 3.2 billion years ago (Ga) due to progressive crustal maturation and thickening driven by voluminous granitoid magmatism within a plateau-like setting. A similar sedimentary–magmatic evolution also accompanied the early (>3 Ga) emersion of other cratons (e.g., Kaapvaal Craton). Therefore, we propose that the emersion of Earth’s earliest continents began during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened (∼50 km thick), buoyant, silica-rich crust. The inferred plateau-like tectonic settings suggest that subduction collision–driven compressional orogenesis was not essential in driving continental emersion, at least before the Neoarchean. We further surmise that this early emersion of cratons could be responsible for the transient and localized episodes of atmospheric–oceanic oxygenation (O2-whiffs) and glaciation on Archean Earth.

Development of Crustal-Scale Shear Zones at the Singhbhum Craton–Eastern Ghats Belt Boundary Region: A Critical Review of the Mesoarchean–Neoproterozoic Odyssey

A number of crustal-scale shear zones have developed along the southern margin of the Singhbhum Craton, in the boundary with the Neoarchean Rengali Province and the Meso-Neoproterozoic Eastern Ghats Belt. The cratonic part, evolved in a suprasubduction zone setting, bears imprints of late Mesoarchean orogenic episode (D1C) at ca. 3.1 Ga with folding and thrust imbrication of the cratonic rocks. The succeeding orogenic imprint is etched in the Neoarchean (~2.8 Ga) with development of the Sukinda thrust along the craton margin and thrust-related deformation of the rocks of the Rengali Province (D2C-D1R). The latter event remobilized cratonic fringe with development of a spectacular E-W trending transpressional belt in the Southern Iron Ore Group rocks cored by the Sukinda ultramafics. In the Eastern Ghats Belt, the major ultrahigh-temperature orogeny took place during the Grenvillian-age (~1.0-0.9 Ga) assembly of the supercontinent Rodinia. This belt eventually got juxtaposed against the expanded Singhbhum Craton in the end-Neoproterozoic time (~0.5 Ga) along the Kerajang Fault Zone. This latter event remobilized a large part of the Rengali Province (D2R) with development of an intraterrane transpressional belt bounded by the Barkot Shear Zone in the north. The northern fringe of the intruding Eastern Ghats Belt developed a complex network of strike-slip fault system under this impact, probably an outcome of tectonic activity along the Kuunga suture, which signifies the joining of greater India with East Antarctica. The present synthesis visualizes early development in the craton through formation of a typical orogenic sequence, imbricated in thrust piles, resulting from a ca. 3.1 Ga orogeny. Further cratonic expansion was achieved via repetitive accretion and remobilization, development of crustal-scale faults and transpressional belts at ca. 2.8 Ga and ca. 0.5 Ga, much in a similar fashion as documented along oblique convergent margins of all ages.

Origin of Fe-Ni-Cu (Co) sulfide and Fe-Ti oxide minerals in the c. 1.77 Ga dolerite dyke, Singhbhum Craton (eastern India)

Dolerite dyke swarms are widespread within the Singhbhum Craton (eastern India) that emplaced from the Neoarchean to Paleoproterozoic era just after the stabilization of crust before c. 3 Ga. These dyke swarms are oriented in NE - SW to NNE - SSW, NW - SE to WNW - ESE, E - W, and N - S directions. The WNW - ESE trending c. 1.77 Ga Pipilia dyke swarm is sampled from the Satkosia area of the Orissa state. The dyke shows a noticeable disparity in terms of the modal proportion and grain size of pyroxenes, plagioclase, Fe-Ti-oxide minerals and texture across the trend. At places the primary silicates are altered to secondary hydrated mineral assemblages of amphibole, chlorite and sericite. Primary silicates are clinopyroxene (augite: Mg# = 65.7 - 82.6; En37-48Fs11-17Wo36-41), orthopyroxene (clinoenstatite: Mg# = 68.5 − 78; En63-70Fs20-29Wo4-5), plagioclase (An11-39Ab44-82Or1-7) and Fe-Ti oxides are titanomagnetite (FeO = 34.38 − 39.50 wt%, Fe2O3 = 48.26 − 56.21 wt%, TiO2 = 5.05 − 9.60 wt%) and ilmenite (FeO = 40.75 − 43.79 wt%, Fe2O3 = 3.54 − 10.03 wt%, TiO2 = 47.82 − 50.87 wt%). Application of two-pyroxene thermometry yields an equilibration temperature range of 1065oC to 978oC, and coexisting titanomagnetite-ilmenite pairs reveal 731.39oC to 573.37oC at the oxygen fugacity (fO2) condition NNO+0.3 to FMQ-1.03. The dyke contains disseminated sulfides at the interstices of Fe-Ti-oxides, and silicates. Major sulfide minerals are pyrite, chalcopyrite, and vaesite; Pyrite-vaesite assemblages occur in association with secondary silicate minerals. Pyrite grains contain variable concentration of Co = 0.01 − 5.70 wt% and Ni = 0.02 − 1.95 wt%. Coexisting vaesite contains Co = 2.42 − 10.44 wt%, Ni = 26.40 − 47.88 wt%, and Fe = 7.32 − 26.55 wt%. Texture, sulfide-silicate assemblage, and presence of low metal/S sulfides such as the pyrite-vaesite assemblage indicate primary Fe-Ni- sulfides (pyrrhotite-pentlandite) that segregated from immiscible sulfide liquid at high temperature is modified by late magmatic/hydrothermal fluid activities. Numerous sulfide-bearing deposits hosted in ultramafic-mafic intrusions of Paleoproterozoic age have been recorded globally and the occurrence of Fe-Ni-sulfides in the c. 1.77 Ga Pipilia dyke swarm in the Singhbhum Craton enhances the exploration potential of this craton in eastern India.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5643989