Phase equilibria modelling of rocks from Western Australia confirms that the ancient continental crust could have formed by multistage melting of basaltic ‘parents’ along high geothermal gradients—a process incompatible with modern-style subduction Tim Johnson et al perform phase equilibria modelling of the Coucal basalts from Western Australia and confirm their suitability as parent rocks of the Archaean continental crust The authors suggest that these early crustal rocks were produced by 20–30 per cent melting along high geothermal gradients They conclude that the production and stabilization of the first continents required a protracted, multistage process When coupled with the high geothermal gradients, this suggests that the continents did not form by subduction Instead it favours a 'stagnant lid' regime in the early Archaean eon in which a single, rigid plate lay over the mantle The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1 Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 25 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3 In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 35 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8 Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal) We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9 Thus subduction was not required to produce TTGs in the early Archaean eon

Earth's first stable continents did not form by subduction

Plate tectonics is a self-organizing global system driven by the negative buoyancy of the thermal boundary layer resulting in subduction. Although the signature of plate tectonics is recognized with some confidence in the Phanerozoic geological record of the continents, evidence for plate tectonics becomes less certain further back in time. To improve our understanding of plate tectonics on the Earth in the Precambrian we have to combine knowledge derived from the geological record with results from well-constrained numerical modeling. In a series of experiments using a 2D petrological–thermomechanical numerical model of oceanic subduction we have systematically investigated the dependence of tectono-metamorphic and magmatic regimes at an active plate margin on upper-mantle temperature, crustal radiogenic heat production, degree of lithospheric weakening and other parameters. We have identified a first-order transition from a “no-subduction” tectonic regime through a “pre-subduction” tectonic regime to the modern style of subduction. The first transition is gradual and occurs at upper-mantle temperatures between 250 and 200 K above the present-day values, whereas the second transition is more abrupt and occurs at 175–160 K. The link between geological observations and model results suggests that the transition to the modern plate tectonic regime might have occurred during the Mesoarchean–Neoarchean time (ca. 3.2–2.5 Ga). In the case of the “pre-subduction” tectonic regime (upper-mantle temperature 175–250 K above the present) the plates are weakened by intense percolation of melts derived from the underlying hot melt-bearing sub-lithospheric mantle. In such cases, convergence does not produce self-sustaining one-sided subduction, but rather results in shallow underthrusting of the oceanic plate under the continental plate. Further increase in the upper-mantle temperature (> 250 K above the present) causes a transition to a “no-subduction” regime where horizontal movements of small deformable plate fragments are accommodated by internal strain and even shallow underthrusts do not form under the imposed convergence. Thus, based on the results of the numerical modeling, we suggest that the crucial parameter controlling the tectonic regime is the degree of lithospheric weakening induced by emplacement of sub-lithospheric melts into the lithosphere. A lower melt flux at upper-mantle temperatures

Subduction styles in the Precambrian insight from numerical experiments

Abstract Granitoids form the dominant component of Archean cratons. They are generated by partial melting of diverse crustal and mantle sources and subsequent differentiation of the primary magmas, and are formed through a variety of geodynamic processes. Granitoids, therefore, are important archives for early Earth lithospheric evolution. Peninsular India comprises five cratonic blocks bordered by mobile belts. The cratons that stabilized during the Paleoarchean–Mesoarchean (Singhbhum and Western Dharwar) recorded mostly diapirism or sagduction tectonics. Conversely, cratons that stabilized during the late Neoarchean (Eastern Dharwar, Bundelkhand, Bastar and Aravalli) show evidence consistent with terrane accretion–collision in a convergent setting. Thus, the Indian cratons provide testimony to a transition from a dominantly pre-plate tectonic regime in the Paleoarchean–Mesoarchean to a plate-tectonic-like regime in the late Neoarchean. Despite this diversity, all five cratons had a similar petrological evolution with a long period (250–850 myr) of episodic tonalite–trondhjemite–granodiorite (TTG) magmatism followed by a shorter period (30–100 myr) of granitoid diversification (sanukitoid, K-rich anatectic granite and A-type granite) with signatures of input from both mantle and crust. The contributions of this Special Publication cover diverse granitoid-related themes, highlighting the potential of Indian cratons in addressing global issues of Archean crustal evolution.

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Archean granitoids of India: windows into early Earth tectonics – an introduction

Archean-Proterozoic boundary represents a significant transitional phase in the Earth’s history Bastar Craton is one of the major Archean cratons in the Indian subcontinent with voluminous granites, supracrustal rocks and tectonic belts Malanjkhand, Dongargarh, and Kanker are the three major granitic plutons emplaced during the Archean-Proterozoic transition in the Bastar Craton, and this study is confined to the granites of Kanker pluton Based on geochemical systematics, the Kanker granites are classified into sanukitoids, biotite and two-mica granites and hybrid granites The compositional diversity of the Kanker granites is attributed to two end-member sources, ie, the enriched mantle and an older felsic crust, and the interactions between them The sanukitoids were derived from an enriched mantle source that was metasomatized by the subducted sediments Heat supplied by the sanukitoid magmas induced the crustal melting to form the biotite and two-mica granites The interaction between these two mutually end-member sources, ie, the enriched mantle and an older felsic crust, resulted in the formation of hybrid granites The evolution of the Kanker granites can be accounted for a transitional geodynamic environment, involving subduction and collisional tectonics during the Archean-Proterozoic transition

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Petrogenesis of the Kanker Granites From the Bastar Craton: Implications for Crustal Growth and Evolution During the Archean-Proterozoic Transition

Paleomagnetic results from the Singhbhum Craton, India: Remagnetization, demagnetization, and complication

Understanding coupled evolution of the crust-mantle system, building up of habitable continents and tectonics of evolving Earth constitute a major focus of research in Earth and Planetary Sciences. This contribution reviews the processes of the evolution of early Earth, including thermal records, mantle evolution, crustal growth, craton formation and tectonics in the first part, followed by the evolution of individual cratonic blocks in Peninsular India and their assembly into shield framework in the second part. Closely scrutinized global geochronologic and isotope database show that remnants of the Hadean-Eoarchean terrestrial record preserved in the core of cratons provide invaluable insights into planetary evolution. Multidisciplinary studies on the preserved earliest crustal remnants reveal unique features such as distinct lithological associations (tonalite-trondhjemite-granodiorite (TTG)-komatiite dominated greenstones), steeper geothermal gradients, hotter mantle, high rates of crustal growth, dome-basin patterns and plume-dominated tectonics and absence of high-pressure mineral assemblages compared to Phanerozoic Earth.

K. R. Aadhiseshan

Papers

Evolving Early Earth: Insights from Peninsular India